Mr Dejan Brkic

Dejan Sincerely
email: dejanrgf@tesla.rcub.bg.ac.yu

Toma Brkic

Tanaskovic

Dejan Brkić

Toma I Tanasković

Systematic Approach to Natural Gas Usage for Domestic Heating in Urban Areas

Keywords: Natural Gas, Settlement, Gas Distribution, District Heating, Urbanism Natural Gas, Settlement, Gas Distribution, District Heating, Urbanism

Natural gas can be used for satisfying population needs for heating, either directly by bringing the gas to the dwellings through the gas distribution system and combusting it in the domestic boiler (gas distribution system-G), or indirectly by combusting the natural gas in the heating plant and distributing the heat energy to the dwellings through the district heating system (district heating system-DH). The selection of a certain type of heating system is made according to the disposition of buildings in the area, their number, size, insulation quality, etc. Based on these characteristics, calculations of investments and exploitation costs have been made for both heating systems and a comparison has been made for all of the 96 presented cases. Almost each type of real settlement can be represented by one of the types of the conditional urban area which are introduced in the paper. The main goal of this paper is to establish a general model to achieve coordinated development of centralized energy supply systems fueled by natural gas, based on defined and accepted criteria. A structure analysis of centralized systems for energy supply has been done with accent on their pipelines.

Dear Prof. Lior, We are grateful to referees for their suggestions. According to referees' comments, we have modified our paper "Systematic Approach to Natural Gas Usage for Domestic Heating in Urban Areas"-EGY-D-08-00110R1. We are returning our rewritten manuscript. This paper has not been published previously, it is not under consideration for publication elsewhere, and if accepted it will not be published elsewhere in substantially the same form, in English or in any other language, without the written consent of the Publisher. Also, don't forget we want to attach Electronic annex refer to our work -MS Excel file (annex DB ver3). We hope that English expression in our paper is now improved.

Introduction

If a gas based system in a settlement is planned, the decision can be done among two conflicted options: 1. Indirect system; natural gas is being combusted in a heating plant and household heat supply is provided by a District Heating System (DH), 2. Direct system; dwellings are being heated by natural gas brought through a gas distribution system and then combusted in domestic gas boilers in each dwelling, individually (G).

The initial decision on choosing one of two systems is based on the number and size of buildings in a settlement, the size of the settlement itself and the heating insulation of buildings [1]. In boundary cases, if it is possible to achieve both options it is also possible to introduce a sort of hybrid system which is not considered in this paper. The economic analysis of the renovation of small-scale district heating systems in Lithuania is available [START_REF] Dzenajaviciene | Economic analysis of the renovation of small-scale district heating systems-4 Lithuanian case studies[END_REF]. District heating systems using cogeneration, as well as the local fuel-based and electric heating systems for detached houses, are analyzed by L. Gustavsson and A. Karlsson [3]. Their analysis includes the whole energy system, from the natural resource to the end user, with respect to the primary energy use, emission and cost. They found that natural gas based systems are less expensive than the corresponding wood-fuel based systems, except the matter of ecology. In the future, green energy sources or fossil energy sources such as oil and natural gas will be more used in industrial processes in order to decrease the ratio of greenhouse gases released from the coal-based local and industrial processes [4].

The goal of the model introduced in this paper is to determine the connections among urban and energy characteristics of settlements in cities and to be benefit from more rational usage of natural gas as non-renewable fossil fuel. Energy demands for heating are shown for the Canadian case [5], from the economical point of view. The introduction of natural gas in the Greek energy market has broadened the options in the field of space heating [6]. The paper by C. Dinca, A. Badea, P. Rousseaux, and T. Apostol [7] aims to select the optimal energetic scenario applied to a consumer with 100 000 inhabitants from the residential-tertiary sector in Romania (series of seven scenarios based on natural gas have been analyzed). The natural gas in optimized bivalent heating systems is shown in the paper of SI. Gustafsson and BG. Karlsson [8]. The study of MS. Torekov, N. Bahnsen, and B. Qvale [9] is in correlation with this paper and strives to clarify to what extent the improved insulation of new buildings affects the economically rational choice of heating supply. District heating should be advocated only for areas with a strong heat demand, primarily for areas with apartment buildings [9]. R. Lazzarin and M. Noro [10] have done analyses of local or district natural gas heating from energetic, environmental and economic points of view. The legal and policy aspects of the utilization of different energy supply systems in households sector can also be found in the available literature [11]. Some German studies with subject relevant for development of district heating systems in urban environment are also useful and highly evaluated, but this literature is available only in German language [12][13][14], with related papers presented in scientific journals [15]. The main achievement of these German researches is the establishing of interaction between heating systems, settlement structure and urban planning at the local level. The study analyses options for heat supply in up to 10 types of built up areas [12] -from densely populated urban areas to villages. A new German study 20 years later refers to this work [13], with projection to 2020 [14].

Considering the selection of heating systems and the utilization of the existing capacities in the systems for centralized energy supply, the present practice in many cities is that every single case must be considered separately and, very often, the selection is done without clear criterions. The proposed model could be useful to urban planners, municipal officials, public utility companies, etc., as a first step in system selection (see Electronic Annex in the online version of this article).

The most suitable option for satisfying heating demands in urban areas is by using a centralized system. The centralized energy supply from heating plants has many advantages: saves primary energy (due to the modern construction of boilers in heating plants as well as the utilization of modern energetic and ecological ways of combustion, the primary energy sources are better used during the transformation of primary energy into heat energy), the distribution of hot water consumption (the centralized hot water distribution is the way to avoid the transformation of primary energy, mostly from heat to electric energy, and then again, from electric to heat energy), the possible utilization of low quality fuel, the possible utilization of some alternative kind of fuel, the centralized storage for fuel, less expenses for the standard discontinuous transportation of fuel (saving motor vehicles' fuel), due to the centralized and highly controlled heating, there is less danger from fire. There is also a well organized, professional fire protection.

There are also some negative aspects of the heating systems from heating plants: high investments during the initial phase of building of heating sources and pipeline structure, possible quitting with heat energy supply caused by the damage in heating plant or distribution network, in some cases, heating expenses are measured by squaring, not by consumption. The advantages of the systems for the individual consumption of gas in households by using the gas distribution network are the following: the gas consumption is being measured separately for each apartment and the paying of costs depends on consumption (which is not always the case when DH systems are being used), gas saving for hot water supply and cooking (these demands are being satisfied directly by the transformation of chemical energy of natural gas into heat energy, that is how the gas used for transformations of primary energy into electric energy is saved), there is no need for storehouses in households, less costs for the standard fuel distribution (the fuel is saved for motor vehicles), relatively small investments in the construction of distribution network in relation to thermal network, less possibility for quitting of supply. The disadvantages of the centralized natural gas supply systems are: an increased fire danger, explosions, or possibility of suffocation caused by damaged installations for different reasons, the combustion is taking place in the apartment, the possible lack of gas or an interrupted distribution pipeline, etc.

Life comfort [16] is the same in both options; every individual dwelling has the same network of conduits and radiators. The main intention of this model approach is to find a way to distribute heat energy in each dwelling using the existing capacities (not to make strategy for a city planning, but to exploit most possible rationally existing capacities). The primary goal of this paper is not to investigate district heating or gas distributive infrastructure, but to compare investments in both systems with their specific details (pipelines with included costs of domestic boilers for the G system or costs of heat exchanger for the DH system, investment in new capacities in heating plant, etc.). The main subject of the examination is in the "Conditional Urban Areas". The comparison of investments in pipelines for both systems is the most important parameter of this analysis. In the most detailed heating analyses in one town, the other types of fuel for heating plants, the alternatives for heating in the cases of the lack of natural gas, etc. must also be taken into consideration.

The model which is presented here is developed as a tool for solving some of the misunderstandings in the strategy of urbanism correlated to natural gas heating in Serbian towns.

This project has been supported by the Ministry of Science of Serbia. All the values in this paper refer to conditions in Serbia, but readers also have an access to the relevant file (see Electronic Annex 1 in the online version of this article), and can change all the values (green tones in excel file can be changed). According to that possibility, this model can be applied for conditions anywhere in the world.

The strategy for heating of dwellings in urban areas of Serbia, since the communist period, had been made to favor district heating systems in towns. The consumer area of Belgrade is being supplied with thermal energy by district heating system consisting of 15 heating plants which use gas (83%=265•10 6 m 3 /year [1]) and crude oil as basic fuel. Statistical data show that 38% of the buildings are connected to the district heating system, which represents 240000 flats and 7500 business offices heated in that way. The construction projects of thermal network and gas distribution network in downtown areas take an important place in the scope of the Belgrade environmental protection program, so approximately 800 individual solid fuel boilers have been shut down so far. Nowadays, most of these plants are being fueled by natural gas, but in the past they were fueled by liquid fuels or by coal (in some smaller and obsolete plants). All of these mini plants have to be closed. According to the new strategy, the heat supply for relevant dwellings will be provided by some sort of natural gas heating. The government has the strategy to connect almost all dwellings to some form of natural gas heating system. The goal is not to use solid fuel heating, especially not electric energy. They should be used only in some rare cases. There are 42 city heating plants in Serbia with heat energy capacity of 5.5GW. However, Serbia does not have sufficient energy production or funds for their procurement. The main characteristics of Serbia's heating plants are low operating readiness due to insufficient maintenance and outdated equipment, financial exhaustion and an inability to perform urgent intervention on sources and grids. Heating is poor and there is a need for additional capacity, mostly fueled by natural gas. Serbia doesn't have enough gas production reserves from its own fields or to satisfy the demands (the annual peak of the production was 600•10 6 m 3 and now it is several times lowers (285•10 6 m 3 ) [1]). The imported gas is available for Serbia since 1979. from one direction (from north, through Hungary). Serbia also has the EU perspective [17,18], and the Government's strategy is to make Serbia a transient country for the export of Russian gas to the western countries of EU countries (from the second direction, through Bulgaria). Due to the European obligation to reduce greenhouse gas emissions in the framework of the Kyoto Protocol, the trend towards the use of natural gas is expected to continue in the future. The increased consumption and comparably low indigenous gas resources within Europe are expected to increase the Europe's dependency on gas imports from abroad in the future. In addition to the existing supply sources from Russia [19] and Algeria, gas resources from the Middle East and the Caspian and Central Asian regions could be the possible supply options to cover Europe's gas demand in the future. Today, natural gas heating in Serbia has a great perspective [20][21][22][23].

A hypothetical equivalent within a model has been made for every type of settlement. The investment costs were calculated and the comparison has been made for each (of a limited number) of hypothetical settlements. The system with the smallest investments (including exploitation and the maintenance in the next 25 years), depending on the city planning parameters, is more cost effective and adopted. This creates a direct link between the city planning parameters and the choice of one of the systems.

The concept of conditional urban areas

The model upon which the decision on choosing one of two systems is made (DH vs. G) is based on the introduction of hypothetical urban settlements. By introducing this practice, it is not necessary to perform the entire calculation for both systems and after which the choice of a heating system in the settlement is made. The application of this model makes easier the job for energy and city planners. People with higher living standard often do not take economic parameters into consideration when deciding between gas or district heating system. The decision is based on a personal affinity (and often, prejudice) [24].

The parts of a city with residential and other buildings within real settlements are called "Real Urban Area". They can be divided into several zones based on the same or similar urban characteristics. The zones divided like this, for the model purposes, need to have an area of 0.05 km 2 -"Basic segment" (Fig. 1.). Further on, real urban parameters of a real settlement can be copied onto the selected "Basic segment" (Fig. 2.). "Basic segment" with rectangular size with dimension approximately 160 m•315 m=0.05 km 2 is adopted for this purpose [1]. Based on the spatial disposition within the model, there are 6 different versions of "Basic segments": 4, 8, 16, 32, 64 and 128 buildings per "Basic segments": (Fig. 1). "Examination segment" (Fig. 3.) consists of ten "Basic segments" with an added pipeline (gas and district heating). Only one type of "Basic segment" can exist in one "Examination segment". Real zone formed like this can be joined with one of the 96 "Conditional Urban Areas"(Fig. 4. and Table 1.). Each particular "Conditional Urban Area" consists of ten "Basic segments" (all the ten belong to one of the six types shown in fig 1 .), buildings (all the buildings are the same in one regarded segment) and pipeline (district heating pipeline or gas distribution pipeline) -Table 2. Every real situation in town has to be joined to one of the best fitted "Conditional Urban Areas". A different heat load (peak load density) can be given to each one of these basic types of "Examination segment", which is based on the size of buildings. There are 8 different heat loads in this model: 10 MW/km 2 , 20 MW/km 2 , 30 MW/km 2 , 40 MW/km 2 , 50 MW/km 2 , 75 MW/km 2 , 100 MW/km 2 or 125 MW/km 2 (Fig. 2). These values are chosen for the analysis in order to include a wide range of possible real urban situations. Every settlement, found in reality, corresponds to one of the 96 hypothetical settlements included in the model (Table 1). They are called "Conditional Urban Area" (six "Basic segment" multiply by eight "Heat Loads" multiply by two types of insulation) [1]. According to a variety of possible settlements (in density, size and layout of buildings) a model which has the ability to represent their different characteristics is formed. With six types of "Basic segments" (Fig. 1) all the possible densities of built up areas in settlements can be described. Different sizes of buildings can be found in each particular urban area from small houses to skyscrapers, i.e. with eight types of buildings graduated by size all situations can be described (eight different "Heat Loads"). Each of the 96 proposed hypothetical settlements can be joined with different city planning parameters such as: the number of buildings reduced to the unit value of surface, the size of buildings, the number of dwellings within the buildings, the number of floors within buildings, the spatial disposition of buildings within the settlement, the quality of heat insulation of dwellings etc [25][26][27]. If there are several types of buildings or density of built up areas within the settlement, settlements need to be divided into several "Conditional Urban Areas". Every single type of building can be very good or poor insulated (older buildings versus newer buildings) [28]. Numbers of "Conditional Dwellings" per building are shown in Table 1.

For every "Conditional Urban Area", it is possible to calculate the entire investment costs for the implementation of gas distribution and district heating systems (Table 3). The investment's calculations are based on the detailed estimate of distribution network for both systems with all the equipment included as well as the labor expenses and the spare parts for replacement in the first 25 years. Thus, the investments for all the 96 cases can be calculated (one of these cases is shown in Table 4). After that, the values of gas distribution costs are being subtracted from the investment costs of the district heating system (Table 5) and then they are being discounted (Table 6). For cases with positive values the option of gas distribution is more favorable than the district heating system (negative values).

The disposition of networks for all cases is shown in figure 3 ("Examination segment"). The disposition designates the pipeline length, but not structure of diameters of conduits in the pipeline composition. The determination of structure of diameters of conduits the pipeline composition can be done only after the "Conditional Urban Area" is formed. A structure of pipes diameters depends on building size. The "Examination segment" has ten times bigger area surface than the "Basic segment" because of the network sensitivity exploration. Note that in fig. 3 ("Examination segment") pipeline network exists, while in Fig. 1 or in Fig. 2 it doesn't ("Basic segment"). So, the similarity between "Conditional Urban Area" and "Real Urban Area" can be determined by two different independent quantities [1].

1) Number of buildings in an urban area (the number of buildings on 0.05 km 2 -5 hectares);

(Fig. 1), 2) A heat demand [29] of an urban area ("Heat Load" or peak load densities of all buildings heated in a zone divided by the size of an area), MW/km 2 ; (Fig. 2),

The term "building" is used here for family houses also, as well as for the similar smaller constructions with the same meaning as e.g. skyscrapers. In all analyses, "Conditional Residential Unit" [1], i.e. "Conditional Dwelling" [1] with net heating surface area of 60 m 2 is observed. "Conditional Dwelling" has, for the purpose of the model approach, a heat demand of 142 W/m 2 (heat peak load for lower insulated dwelling) in case of a low (bad) insulation, and in case of better (good) insulation it has a heat demand of 95 W/m 2 (heat peak load for better insulated dwelling). Each combination of a defined number of buildings and peak load density corresponds to a different number of average dwellings in the building (Table 1). An average dwelling (60 m 2 ) is practically "Conditional Dwelling".

The concept of the "Examination segment" is regarded only for the purpose of exploration on realistic values of diameters in the pipeline structure, but values of N-number of buildings presented in this paper are nominally per "Basic segment". "Heat load" is expressed in MW/km 2 (not in MW/0.05 km 2 or in MW/0.5 km 2 ) and it is nominally equal for both "Examination segment" and "Basic segment".

The model of rational natural gas usage based on city planning parameters

Based on the introduced "Conditional Urban Area", a techno-economical model of rational natural gas usage has been made. For each of the 96 cases investment, a calculation has been made in both of the proposed heating systems (Gas Distribution [30] vs. District Heating [31]) including the exploitation in the next 25 years (investments). For each case, a comparison of costs has been made so that the heating system with the smallest cost has an advantage in the implementation. The number of dwellings per buildings, i.e. the identification of all the 96 cases shown here is shown in Table 1.

Both of the heating types in the model have special costs since both of them have special elements; e.g. the district heating system is made of steel conduits, pumps and heat exchangers, on the contrary, the gas distribution system is made of cheaper polyethylene conduits and has stations for measuring and regulation with internal gas equipment (each dwelling has domestic gas boiler etc). The investment in new capacities for heating plant is included in the model in a directly i.e. by increasing the price of natural gas for district heating, or can be added by including the new cost indirectly. The investments in a new heating plant fueled by natural gas are: 80000 €/MW (for heat plant capacity <50MW), 65000 €/MW (for heat plant capacity 50-100 MW) and 52000 €/MW (for heat plant capacity 100-200 MW) [1]. That means additional costs of 450-680 € per "Conditional dwelling". It implies that this kind of additional costs is not essential for this kind of analyses (Fig 5 .). The changes in the slope of borderline in the model diagram 2 are caused the discount rate changes (Fig 5 .) or by differentiation in the structure of diameters of conduits in the pipeline 3 . Same conclusion can be made with price variations of domestic boiler (see Table 5 andFig 5). L. Gustavsson and A. Karlsson [3] estimated the DH investment and maintenance costs. An increasing price of domestic gas boilers simultaneously with the introduction of the same amount of investments in a heating plant are to be annulled (Fig 5 .) (for detailed analyses consult electronic annex 1).

A relative amount of investments (per "Conditional Dwelling" included annual costs) in district heating -DH and in local gas heating system -G (each "Conditional Dwelling" is equipped with domestic boiler fueled by natural gas) can be calculated after following eqs. (1 and 2 (Seasonal Efficiency of Domestic Boilers in United Kingdom) [32] is very useful (Fig. 6 and7).

District heating is often less efficient with respect to modern heating technologies using natural gas. Condensing boilers guarantee higher efficiencies with respect to ''traditional'' district

heating.

An investment in a new heating plant or the increasing domestic gas boiler's price is included in the calculation, with the introduction of such costs or investments on both G and DH sides simultaneously, caused the equal impact in all of the six "examination segments" (does not change slope of borderline in Fig. 5 5 ).

In the equations above, the common costs for both systems are not shown. Different investments in these two opposite systems can be compared for the purpose of a model (Fig 8).

All the previous investments are considered for the present conditions in Serbian energy sector.

Of course, this model is applicable for conditions and particular cases all over the world, but diagrams (figures in this paper) are generated for prices ratio in Serbia. Detail calculation for one case of 96 is shown in table 4.

Values of initial costs (shown in Table 3) for DH subtracted by initial costs of G are shown in table 5.

Generally, each project task has a time component. Every particular element of the two systems shown here has a lifetime, and it has a price on the market (expressed in €). If one of the elements has reached the end of its lifetime (e.g. 26 years), it has to be replaced. Thus, in a project, during the first and the next several years, costs for provision, maintenance and replacement of elements exist (e.g., some elements must be replaced after 14 years and some after 25 years). A sum of costs calculated for each year (t) for gas distribution system is labeled here as G, and for district heating system as DH. "Present Value of Costs" is one of the most useful criterions for project analyses for a whole lifetime of every particular system element. In that way, discount flows reduced on "Present Value of Costs" can be evaluated. "Present Value of Costs" is, by default, cumulative cost for all the elements of the system in present and in future expanded for discount rate. "Future Value of Costs" has to be reduced to present value and to be added to real present costs. Thus, the generated value is called "Present Value of Costs".

For the reduction of "Future Value of Costs", an appropriate "Discount Rate" (dr) has to be adopted. In the case of possible risks for the safety of investments, higher value of "Discount Rate" is being calculated. "Discount Rate" could be equalized with "Interest" on the market or for realized credit.

In our case, an economical evaluation is realized during the comparison of two "Present Values of Costs", for gas distribution system (G) and for district heating system (DH) [33]. "Net Present Value of Costs" (NPV) (eq. 3) is the result of subtraction of the "Present Value of Costs" calculated for district heating system (DH) and the "Present Value of Costs" calculated for gas distribution system (G).

( )

∑ = + - = n 1 t t r D 1 G DH NPV (3)
The value of "Discount Rate" or capital cost rate in this case is rated and adopted as dr=10%.

The value of discount rate has great impact on the optimal choice of quantitative conclusions (Fig. 5). The changes of calculated values for dr=5% and dr=7% are shown in Table 6. Annual gas consumption calculated for one "Conditional Dwelling" is 857 m 3 for heating only, and this amount is 10% higher for district heating system. Besides, a statistical approach based on nonlinear regression principles can be made to natural gas consumption estimation of individual residential and small commercial customers [34]. The annual costs of electrical energy for running the pumps for district heating system are estimated to 250 kWh per "Conditional Dwelling".

Analyses are done for all the 96 cases (six "Conditional Urban Area" multiplied by eight "Heat

Loads" and by two conditions of insulation -bad or good insulation) for the period of t=26 years. In table 6, case with a bad insulation is shown.

The values in certain cases in table 6 vary more or less from zero. For example, for "Conditional Urban Area" with 16 buildings and with "Heat Loads" 50 MW/km 2 , the calculated "Net Present Value of Costs" (NPV) is only -5 per "Conditional Dwelling" for a period of 26 years. Therefore, the realization of gas distribution system for that "Conditional Dwelling" is only 5 € for 26 years in advantage versus district heating system. This case and the other similar cases are in the "gray zone" (Fig. 9). In the "gray zone", both systems are payable, especially for a period of 26 years.

If a considered value for a certain housing block exceeds the recommended limiting value, more or less, the decision to recommend a particular centralized heating system is more reasonable.

Changing the structure and the price ratio (i.e. the state political decision to subsidize the gas price in a system or to issue a price, which would disturb the current price ratio of gas used for district heating system and individual consumers), the conception and manner of a construction of certain system and its elements may cause these parameters to vary. Currently, plasticpolyethylene conduits (cheep) are used for distribution to individual consumers while steel conduits (expensive) had been used before. It led to the considerable cuts in network construction prices, while maintaining the same level of safety and endurance.

For the same "Heat Loads", when there are many smaller family houses located on a "Conditional Urban Area" e.g. N=32, sometimes only with one "Conditional Dwelling", gas distribution system is more reasonable. On the contrary, when there is a smaller number of skyscrapers located on "Conditional Urban Area" e.g. N=4, it is more reasonable to use district heating system (Fig. 9).

By means of techno -economic analysis for a certain area, it is possible to determine the advantages of one system over the other (district heating over gas). However, considering the situation on the ground, it may turn out that the system is inaccessible in that part of the city (heating plant does not have enough capacity or, in the first stage, the areas closest to the plant could have a priority in system installation). In that case, if the installation costs of the other available system are not too high, it is rational to install that type of heating system (thus saving the consumption costs of electric energy and reducing the amount of pollution if the object is coal).

If, after the analysis, it turns out that one system has more significant economic advantages than the others, but it is unavailable, some form of hybrid system should be considered. For instance, if the installation of district heating system has a lot of economic advantages but it is not available, it is possible to build a local boiler room which would be gas operated. This hybrid solution would demand additional economic and ecologic analyses [35] and estimations of which good features of district heating system would be kept and which would be discarded.

Case study; Application under real conditions

The characteristics of the "Real Urban Area": number of buildings, disposition and size of buildings, construction type, etc. are the factors of influence. The adoption of a type of centralized heating system can be done according to the conclusion established by the model shown. That is possible only if both systems are available near the settlement.

Today, social help habitants have a discount for district heating price of up to 50%. For the gas consumers who use gas distribution systems directly, such discounts are not available. Besides, the payment for the DH is per m 2 of heated surface area (proportional to the size of the house), and in the G system payment is per m 3 of used gas, in Serbia. In the DH bills are fixed, and any sort of economizing in that way is discouraged.

In that case (Belgrade's settlement Karaburma is taken here into consideration), a type of the chosen system depends on urban characteristic of the settlement only. Because of that, the demonstrative settlements are parceled (divided) into eight "Real Urban Areas" with similar buildings on each particular parcel; (Fig. 10). That way, the determined "Real Urban Area" can be associated with the "Conditional Urban Area". The characteristic points for each of the eight parcels (intersection of number of buildings and heat load of parcel) can be plotted into the characteristic model diagram; (Fig. 10). The types of insulation of buildings in the settlement are mixed; old buildings have bad and new buildings have good insulation. Both old and new buildings have heterogeneous spatial disposal.

Two boundary cases have been treated, because of heterogeneity of insulation quality of buildings [27,28,[36][37][38]; (Fig. 11):

-Maximal "Heat Load", all buildings have bad insulation (142 W/m 2 ), -Minimal "Heat Load", all buildings have good insulation (95 W/m 2 ),

The value of fuel consumption depends on heating insulation of the building. Energy consumption in the residential sector can be determined according to various methods shown in an available literature [39]. Therefore, the "Heat Load" depends on heating insulation of the building. The number of buildings on each particular parcel is constant. "Gray zone" is the zone where decision on the type of the system depends, in a great deal, on the type of insulation of the building; (Fig. 9. and 10). In the "Gray zone", the costs for both systems (gas distribution system and district heating system) are very similar. Characteristic points for each of eight particular also depend on the quality of insulation. Small change in slope of borderline for good insulation "Real Urban Area" No 8 includes types of small buildings or family houses which can contain only one "Conditional Dwelling". Only for that "Real Urban Area" the gas distribution system has very payable advantages versus district heating system. "Real Urban Area" No 3 includes school, kindergarten, local office, shops. These kinds of buildings can contain twenty or more "Conditional Dwellings". Only for that "Real Urban Area" the district heating system has great advantages. All the other zones are in the "gray zone". In the "gray zone", one system is more payable in comparison with the other, no more than 200 € per "Conditional Dwellings". This amount cannot be crucial for decision. Some illustrations of investigation of price changes (natural gas price, or domestic boilers price) are shown in figure 13.

(K 1 =tgθ 1 )
If large areas are covered by agricultural terrain, by excluding it, the above-mentioned parameters obtain unrealistic values.

Note that pipeline is one of the most important parameters of the analyses shown in this paper.

Every particular type of the "examination segment" has a different length of a pipeline route. The length of a pipeline route is fixed by choosing of one six "examination segments". This selection has to be done to present density of built up areas. Further, for detailed examination, when certain type of "examination segment" is chosen, the length of the route is determined, at the same time. Now, the next factor to be varied is the heat load. The heat load is directly correlated with the size of buildings, i.e. diameters can vary for the fixed length of a pipeline and its structure. Larger diameters of conduits in pipeline structure are correlated to larger buildings. So, the length of a pipeline route depends on the density of built up areas, and structure of the pipe diameters in a pipeline depends on the size of buildings. Therefore, in city conditions, the diameters of pipes are more sensitive (have a greater impact) in some cases, apropos to the length of a pipeline for decision between heating systems. In the conditions of densely populated urban areas, when one type of the "examination segment" is considered, the decision for implementation of certain heating system has to be done according to the above mentioned criterions. The lengths of pipelines and the structures of diameters of conduits are the main factors for establishing these criterions. Accordingly, in villages, the distances between houses are larger, so the length of the dwelling's pipeline has greater impact then the pipeline diameters.

The analysis of the distribution of district heat in sparse areas by C. Reidhav and S. Werner [40,41] shows that such distribution can be profitable for Swedish district heating companies, if wisely implemented. Sparse district heating is a label for district-heating systems located in the areas of low heat densities. However, a profitable sparse district heating presupposes a favorable combination of certain factors. The boundaries of two such factors have been identified for Swedish conditions; linear heat density and annual use of district heat/house. An annual use of district heat higher than 50 GJ/house and a linear heat density greater than 2 GJ/m are required for profitability. In Sweden, the general competitiveness of sparse district heating is facilitated by the high consumption taxes for fuel oil, natural gas, and electricity. Hence, it should be more difficult to introduce sparse district heating in other countries with low energy taxes. Such areas are unfavorable, since revenues from heat sold are low compared with the investment costs for the local distribution network. This situation results in high distribution costs. Distribution heatlosses are also higher at low heat-densities.

All the previous values of limiting parameters are calculated for the present gas prices for heating plants, for households, for domestic gas boilers' price, the costs of pipes, labor, etc.

Regarding the variation of all parameters, readers can consult electronic annex (Microsoft Excel file). Valuable information for the energetic situation in the city of Belgrade is shown in the paper of M. Jovanović, N. Afgan, P. Radovanović and, V. Stevanović [42] (Belgrade's settlement Karaburma is shown in Fig. 10).

A city is considered to be sustainable if it establishes the balance between economic and sociocultural development, on one side, and the progress in environmental protection with active participation of its citizens, on the other side. When using one of the power resources (in this case natural gas) in various systems, the difference in the amount of pollution is very small. Big differences cannot occur in the overall effect on the city level, but in certain areas they can.

While heating plant is a concentrated pollutant which disperses harmful combustion products evenly on a wide area depending on the wind, gas lined consumption disperses locally (consumer pollutes his nearest neighborhood) [43]. Globally, the biggest gas consumer is the biggest city polluter. For including environmental impacts of different centralized energy supply systems in a model, it is necessary to estimate the annual environmental costs for both systems and include them in related equations. Compared to all of the fossil fuels, natural gas is a minor pollutant. It burns without a solid residue and has the least coefficient of CO 2 emission of about 56 kg/GJ (which is significant considering the limitations imposed by The Kyoto Protocol) [44][45][46][47] . M.S. [46]. The comparison of environmental impacts of two residential heating systems in Canada is shown in paper of L. Yang, Zmeureanu R., and Rivard H. [48].

The key advantage of installing gas or district heating system is not in their mutual differences, but in the substitution of by the far more expensive (in terms of energy and ecology [49]) and the highest quality form of energy -the electric energy, whose usage for heating is by far less rational; (Fig. 14). The introduction of competition to electricity generation and commercialization has been the main focus of many restructuring experiences around the world.

The open accesses to the transmission network and a fair regulated tariff have been the keystones for the development of the electricity market [50]. Parallel to the electricity industry, the natural gas business has great interaction with the electricity market in terms of fuel consumption and energy conversion. Given that the transmission and distribution monopolistic activities are very similar to the natural gas transportation through pipelines, economic regulation related to the natural gas network should be coherent with the transmission counterpart [51]. Electricity and natural gas use for residential space heating in USA is shown in paper by D. Bodansky [52].

Today, discussions about district heating systems are not rare in scientific literature [53][54][55][56][57][58][59][60][61][62][63].

Conclusions

The latest increase in gas prices turned all eyes once again to the space heating problem in Europe. However, economic concern is not the only factor pushing the authorities to rethink about the suitability of the currently existing sources. Cities are the biggest consumers of the country's energy production. The increase in annual consumption of total primary energy is 3%

and its largest part is used for lighting, cooking, heating, cooling, and transport of freights and passengers. The importance of reducing the energy consumption level, by changing the forms of consumption and making improvements in technology and lifestyle, should be noted. Sustainable forms of energy production, distribution and usage represent the goals of a sustainable development. A city is considered to be sustainable if it establishes the balance between economic and socio-cultural development, on one side and the progress in environmental protection with active participation of citizens. On the other side, the economics of the G and the DH systems very much depend on the specific circumstances [64,65]. If installed in an area with only scattered buildings, the length of the pipes which are necessary to supply households will be higher, relative to the number of buildings. Installation costs will be shared by fewer consumers.

The DH system is the capital intensive; in particular due to the distribution system of insulated pipes. The G system is another form of energy which is being distributed for domestic heating purposes. The dimensions of pipes are small compared to the DH pipes and no insulation is needed, thus the distribution system is less capital intensive.

District heating is often less efficient with respect to modern heating technologies using natural gas [66][67][68]. Condensing boilers guarantee higher efficiencies with respect to the "traditional" district heating. In this study, a noncondensing boiler with average efficiency has been chosen from following database: "Seasonal Efficiency of Domestic Boilers in United Kingdom" [32].

The efficiency of domestic boiler and heating plant has equalized by efficiency in this model.

The typical efficiencies of plants fueled by natural gas are reported in paper of R. Lazzarin and M. Noro [10]. ). The same conclusion is made for the investments in new capacities in heating plant, but it moves borderline in direction of the gas distribution system. On the contrary, changes in value of discount rate have a great impact (Fig 5 .). Changes in the price of domestic boilers or investment in new capacities in heating plant has the equal impact in all of the six "examination segments", changes of discount rate does not.

The conducted analyses confirm literature and empirical information:

-District heating system is a better option in areas with small built up density, but with skyscrapers.

-Gas system is a better option in areas with high density of individual houses (in this case gas distribution system is significantly in advance).

These two boundary cases are illustrative. But in the case with small density of built up areas with individual houses, the G system is a cheaper option (but not necessary according to some Swedish studies [40,41]). With high density of built up areas with skyscrapers, the G system is also a cheaper solution. But, safety regulation is a limiting factor for the implementation of gas system directly in such a high building. Anyway, in this case the G system has a slight advantage, and by the increasing price of domestic gas boilers, the DH system became the cheaper option (Fig 8 . and Table 5).

In more details, if a certain number of objects N exist in a "Conditional Urban Area", it can be concluded that (Fig. 8.):

1. N = 4, district heating system has an advantage over gas if an average building has over 29 rarely appears in practice. The gas distribution system is advantageous because all 128 buildings must be equipped with expensive heat exchangers. That is highly unpractical and expensive for individual houses. For skyscrapers, gas system, theoretically, has a slight advantage, but with an increased price of domestic gas boilers, district heating system can be applied in larger buildings.

Besides, the implementation of the gas system in skyscrapers is forbidden according to safety regulations.

Acknowledgment

This paper is a part of the research done within the project of Ministry of Science NP EE 34-406A, Serbia, Belgrade and the authors would like to thank for financial support.

Appendix

The algorithm for initial decision for selection of a natural gas heating system in settlements is shown in figure 15: Abstract: Establishing of proper criteria for economical and energy efficient operation of district heating i.e. natural gas supply system for fulfilling low temperature heat demands in domestic use, is necessary condition for optimal and mutual persuasive operation of these systems. The main goal of this paper is to establish general model to achieve coordinated development of centralized energy supply systems, based on defined and accepted criteria. An analysis of structure for centralized systems for energy supply has been done with accent on their pipelines.

8.
Investment and exploitation costs are evaluated considering specified area of town as a function of systems' technical characteristics. Model for evaluation of economy for different solutions is established, backed up by computer spreadsheet. This model for different heat loads and number of objects on conditional urban area (as parameters of real energy and urban situation) with defined prices and costs, suggests one of systems as optimal for the area. General model for harmonization and optimal development of district heating and natural gas supply systems represents analytical approach in considering development of centralized energy supply systems in towns. The goal function of this model besides component that considers costs, involves energetic, techno economic, social, ecological and safety constraints. Natural gas can be used for satisfying population needs for heating, either directly by bringing the gas to the dwellings through the gas distribution system and combusting it in the domestic boiler (gas distribution system-G), or indirectly by combusting the natural gas in the heating plant and distributing the heat energy to the dwellings through the district heating system (district heating system-DH).

The selection of a certain type of heating system is made according to the disposition of buildings in the area, their number, size, insulation quality, etc. Based on these characteristics,

calculations

Introduction

If a gas based system in a settlement is planned, the decision can be done among two conflicted options: 1. Indirect system; natural gas is being combusted in a heating plant and household heat supply is provided by a District Heating System (DH), 2. Direct system; dwellings are being heated by natural gas brought through a gas distribution system and then combusted in domestic gas boilers in each dwelling, individually (G).

The initial decision on choosing one of two systems is based on the number and size of buildings in a settlement, the size of the settlement itself and the heating insulation of buildings [1]. In boundary cases, if it is possible to achieve both options it is also possible to introduce a sort of hybrid system which is not considered in this paper. The economic analysis of the renovation of small-scale district heating systems in Lithuania is available [START_REF] Dzenajaviciene | Economic analysis of the renovation of small-scale district heating systems-4 Lithuanian case studies[END_REF]. District heating systems using cogeneration, as well as the local fuel-based and electric heating systems for detached houses, are analyzed by L. Gustavsson and A. Karlsson [3]. Their analysis includes the whole energy system, from the natural resource to the end user, with respect to the primary energy use, emission and cost. They found that natural gas based systems are less expensive than the corresponding wood-fuel based systems, except the matter of ecology. In the future, green energy sources or fossil energy sources such as oil and natural gas will be more used in industrial processes in order to decrease the ratio of greenhouse gases released from the coal-based local and industrial processes [4].

The goal of the model introduced in this paper is to determine the connections among urban and energy characteristics of settlements in cities and to be benefit from more rational usage of natural gas as non-renewable fossil fuel. Energy demands for heating are shown for the Canadian case [5], from the economical point of view. The introduction of natural gas in the Greek energy market have broadened the options in the field of space heating [6]. The paper by C. Dinca, A.

Badea, P. Rousseaux, and T. Apostol [7] aims to select the optimal energetic scenario applied to a consumer with 100 000 inhabitants from the residential-tertiary sector in Romania (series of seven scenarios based on natural gas have been analyzed). The natural gas in optimized bivalent heating systems is shown in the paper of SI. Gustafsson and BG. Karlsson [8]. The study of MS.

Torekov, N. Bahnsen, and B. Qvale [9] is in correlation with this paper and strives to clarify to what extent the improved insulation of new buildings affects the economically rational choice of heating supply. District heating should be advocated only for areas with a strong heat demand, primarily for areas with apartment buildings [9]. R. Lazzarin and M. Noro [10] have done analyses of local or district natural gas heating from energetic, environmental and economic points of view. The legal and policy aspects of the utilization of different energy supply systems in households sector can also be found in the available literature [11]. Some German studies with subject relevant for development of district heating systems in urban environment are also useful and highly evaluated, but this literature is available only in German language [12][13][14], with related papers presented in scientific journals [15]. The main achievement of these German researches is the establishing of interaction between heating systems, settlement structure and urban planning at the local level. The study analyses options for heat supply in up to 10 types of built up areas [12] -from densely populated urban areas to villages. A new German study 20 years later refers to this work [13], with projection to 2020 [14].

The present practice in many cities, for heating systems selection and utilization of existing capacities in systems for centralized energy supply, includes separate consideration of every single case or very often selection without clear criterions. Proposed Considering the selection of heating systems and the utilization of the existing capacities in the systems for centralized energy supply, the present practice in many cities is that every single case must be considered separately and, very often, the selection is done without clear criterions. The proposed model could be useful to urban planners, municipal officials, public utility companies, etc., as a first step in system selection (see Electronic Annex in the online version of this article).

In urban areas, the most suitable option for satisfying the heating demands is by using a centralized system. The most suitable option for satisfying heating demands in urban areas is by using a centralized system. The centralised energy supply from heating plants has many advantages: saves primary energy (due to the modern construction of boilers in heating plants as well as the utilisation of modern energetic and ecological ways of combustion, the primar energy sources are better used during the transformation of primar energy into heat energy), the distribution of hot water consumption (the centralised hot water distribution is the way to avoid the transformation of primary energy, mostly from heat to electric energy, and then again, from electric to heat energy), the possible utilisation of low quality fuel, the possible utilisation of some alternative kind of fuel, the centralised storage for fuel, less expenses for the standard discontinuous transportation of fuel (saving motor vehicles' fuel), due to the centralised and highly controled heating, there is less danger from fire. There is also a well organised, professional fire protection. There are also some negative aspects of the heating systems from heating plants: high investments during the initial phase of building of heating sources and pipeline structure, possible quitting with heat energy supply caused by the damage in heating plant or distribution network, in some cases, heating expenses are measured by squaring, not by consumption. The advantages of the systems for the individual consumption of gas in households by using the gas distribution network are the following: the gas consumption is being measured separately for each appartment and the paying of costs depends on consumption (which is not always the case when DH systems are being used), gas saving for hot water supply and cooking (these demands are being satisfied directly by the transformation of chemical energy of natural gas into heat energy, that is how the gas used for transformations of primary energy into electric energy is saved), there is no need for storehouses in households, less costs for the standard fuel distribution (the fuel is saved for motor vehicles), relatively small investments in the construction of distibution network in relation to thermal network, less possibility for quitting of supply. The disadvantages of the centralised natural gas supply systems are: an increased fire danger, explosions, or possibility of suffocation caused by damaged instalations for different reasons, the combustion is taking place in the appartment, the possible lack of gas or an interrupted distribution pipeline, etc.

If a gas based system in a settlement is planned, decision can be done among two conflicted options:

1. Indirect system; natural gas is being combusted in a heating plant and households heat supply is provided by a District Heating System (DH), 2. Direct system; dwellings are being heated by natural gas brought through a gas distribution system and then combusted in domestic gas boilers in each dwelling individually (G).

Life comfort [16] is the same in both options; every individual dwelling has the same network of conduits and radiators. Main The main intention of this model approach is to find a way how to deliver distribute heat energy in each dwelling using the existing capacities (not to make strategy for a city planning, but to exploit most possible rationally existing capacities). Primarily The primary goal of this paper is not to investigate whole district heating or gas distributive infrastructure, but to compare investments in both systems with their specific details (pipelines with included costs of domestic boilers for the G system or costs of heat exchanger for the DH system, investment in new capacities in heating plant, etc.). Main The main subject of the examination is in the "Conditional Urban Areas". Comparisons The comparison of investments in pipelines for both systems are is the most important parameter in of this analysis. The most detailed analyses of the heating in one town must take into consideration other types of fuel for heating plant, alternatives for heating in the cases of the lack of natural gas, etc. In the most detailed heating analyses in one town, the other types of fuel for heating plants, the alternatives for heating in the cases of the lack of natural gas, etc. must also be taken into consideration.

Model The model which is presented here is developed as a tool for solving some of the misunderstandings in the strategy of urbanism correlated to natural gas heating in Serbian towns.

This project has been supported by the Ministry of Science of Serbia. All the values in this paper refer to conditions in Serbia, but readers also have an access to the relevant file (see Electronic

Annex 1 in the online version of this article), and can change all the values (green tones in excel file can be changed). According to that possibility, this model can be applied for conditions anywhere in the world.

Strategy The strategy for heating of dwellings in urban areas of Serbia, since from the communist period, had been made to favor district heating systems in towns. Consumer The consumer area of Belgrade is being supplied with thermal energy by district heating system consisting of 15 heating plants which use gas (83%=265•10 6 m 3 /year [1]) and crude oil as basic fuel. Statistical data show that 38% of the buildings are connected to the district heating system, that is, which represents 240000 flats and 7500 business offices are heated in that way. In the scope of the Belgrade environmental protection program, construction of thermal network and gas distribution network in downtown area takes an important place The construction projects of thermal network and gas distribution network in downtown areas take an important place in the scope of the Belgrade environmental protection program, so approximately 800 individual solid fuel boilers have been shut down so far. Most of these plants nowadays are fueled by natural gas, but in the past they were fueled by liquid fuels or some smaller and obsolete by coal. All of these mini plants have to be closed, and heating for relevant dwellings, according to new strategy, will be provided by some sort of natural gas heating. Nowadays, most of these plants are being fueled by natural gas, but in the past they were fueled by liquid fuels or by coal (in some smaller and obsolete plants). All of these mini plants have to be closed. According to the new strategy, the heat supply for relevant dwellings will be provided by some sort of natural gas heating.

Government strategy is The government has the strategy to connect almost all dwelling dwellings to some form of natural gas heating system. Goal The goal is not to use solid fuel heating, and especially not electrical electric energy. They should be used only in some rare cases. There are 42 city heating plants in Serbia with heat energy capacity of 5.5GW. However, Serbia does not have sufficient energy production or funds for their procurement. The main characteristics of Serbia's heating plants are low operating readiness due to insufficient maintenance and outdated equipment, financial exhaustion and an inability to perform urgent intervention on sources and grids. Heating is poor and there is a need for additional capacity, mostly fueled by natural gas. Serbia doesn't have enough gas production reserves from its own fields or to satisfy the demands (the annual peak of the production was 600•10 6 m 3 and now it's several times lower (285•10 6 m 3 ) [1]). Note, that The imported gas from import is available for Serbia since 1979. from one direction (from north, through Hungary). Also, Serbia Serbia also has the EU perspective [2, 3 17, 18], and government the Government's strategy is to be make Serbia a transient country for the export of Russian gas to the western countries of EU countries (from the second direction, through Bulgaria). Due to the European obligation to reduce greenhouse gas emissions in the framework of the Kyoto Protocol, the trend towards the use of natural gas is expected to continue in the future. The increased consumption is faced with and comparably low indigenous gas resources within Europe, so that the dependency of Europe on gas imports from abroad will rise in the future are expected to increase the Europe's dependency on gas imports from abroad in the future. In addition to the existing supply sources from Russia [4 19] and Algeria, gas resources from the Middle East and the Caspian and the Central Asian regions may be could be the possible supply options to cover Europe's gas demand in the future.

Today, natural gas heating in Serbia has a great perspective [5-8 20-23].

For every type of settlement a hypothetical equivalent within a model was made. A hypothetical equivalent within a model has been made for every type of settlement. For each (of limited number) of hypothetical settlements investments were calculated and comparison was made. The investment costs were calculated and the comparison has been made for each (of a limited number) of hypothetical settlements. The system with the least smallest investments (including exploitation and the maintenance in the next 25 years), depending on the city planning parameters, is more cost effective and it is adopted. This creates a direct link between the city planning parameters and the choice of one of the systems.

The initial decision on choosing one of two systems is based on the number and the size of buildings in a settlement, it's size and heating insulation of the buildings [1]. In boundary cases, if it is possible to achieve both options it is also possible to introduce a sort of hybrid system, not considered in this paper. Economic analysis of the renovation of small-scale district heating systems in Lithuania is available [9]. District heating systems using cogeneration, as well as local fuel-based and electric heating systems for detached houses, are analyzed by L. Gustavsson and A. Karlsson [10]. Their analysis includes the whole energy system, from the natural resource to the end user, with respect to primary energy use, emission and cost. They found that systems based on natural gas are less expensive than the corresponding wood-fuel based systems, except the matter of ecology. In the future, green energy sources or fossil energy sources such as oil and natural gas will be more greatly used in industrial processes in order to decrease the ratio of greenhouse gases released from coal-based local and industrial processes [11].

Goal of model introduced in this paper is to determine the connections among urban and energy characteristics of settlements in cities, and to be of benefit for more rational usage of natural gas as non-renewable fossil fuel. Energy demands for heating are shown for Canadian case [12] from the economical point of view. The introduction of natural gas in the Greek energy market broadened the options in the field of space heating [13]. Paper by C. Dinca, A. Badea, P.

Rousseaux, and T. Apostol [14] aims to select the optimal energetic scenario applied to a consumer with 100 000 inhabitants from the residential-tertiary sector in Romania (series of seven scenarios based on natural gas has been analyzed). Natural gas in optimized bivalent heating systems is shown in paper of SI. Gustafsson and BG. Karlsson [15]. Study of MS.

Torekov, N. Bahnsen, and B. Qvale [16] is in correlation with this paper and strives to clarify to what extent the improved insulation of new buildings affects the economically rational choice of heating supply. District heating should be advocated only for areas with a high concentration of heat demand, primarily areas with apartment buildings [16]. R. Lazzarin and M. Noro [17] have done analyses of local or district heating by natural gas from energetic, environmental and economic point of views. Legal and policy aspects of different energy supply systems utilization in households sector can also be found in available literature [18].

Concept The concept of conditional urban areas

The model upon which the decision on choosing one of two systems is made (DH vs. G), is based on the introduction of hypothetical urban settlements. By introducing this practice, performing it is not necessary to perform the entire calculation for both systems is avoided and after which the choice on of a heating system in a the settlement is made. Application The application of this model makes easier the job for energy planners and city planners. People with higher living standard, often do not take economic parameters into consideration economic parameters when deciding between gas or district heating system. The decision is based on a personal affinity (and often, prejudice) [ 19 24].

Parts The parts of a city with residential and other buildings within real settlements are called "Real Urban Area". They can be divided into several zones based on the same or similar urban characteristics. Zones The zones divided like this, for the model purpose purposes, need to have an area of 0.05 km 2 -"Basic segment" (Fig. 1.). Further on, real urban parameters of a real settlement can be copied onto the selected "Basic segment" (Fig. MW/km 2 (Fig. 2). These values are chosen for the analysis in order to include a wide range of possible real urban situations.

Every settlement, found in reality, corresponds to one of the 96 hypothetical settlements included in the model (Table 1). They are called "Conditional Urban Area" (six "Basic segment" multiply by eight "Heat Loads" multiply by two type types of insulation) [1]. According to a variety of possible settlements (in density, size and layout of buildings) a model which has the ability to represent their different characteristics is formed. With six types of "Basic segment segments" (Fig. 1) all the possible densities of built up areas in settlements can be described. In each particular urban area, buildings with different sizes can be found; Different sizes of buildings can be found in each particular urban area from small houses to skyscrapers, i.e. with eight types of buildings graduated by size all situations can be described (eight different "Heat Loads"). Each of the 96 proposed hypothetical settlements can be joined with different city planning parameters such as: the number of buildings reduced to the unit value of surface, the size of buildings, the number of dwellings within the buildings, the number of floors within buildings, the spatial disposition of buildings within the settlement, the quality of heat insulation of dwellings etc [20-22 25-27]. If within the settlement there are several types of buildings or density of built up areas, If there are several types of buildings or density of built up areas within the settlement, settlements need to be divided into several "Conditional Urban Areas". Every single type of building can be very good or poor insulated (older buildings versus newer buildings) [ 23 28].

Numbers of "Conditional Dwellings" per building are shown in Table 1.

For every "Conditional Urban Area", it is possible to calculate the entire investment cost of costs for the implementation of gas distribution and district heating system systems (Table 3).

Investments are calculated The investment's calculations are based on the detailed estimate of distribution network for both systems with all of the equipment included, as well as the labor expenses and the spare parts for replacement in the first 25 years. Thus, the investments for all the 96 cases can be calculated (one of these cases is shown in Table 2 4). After that, the values of gas distribution costs are being subtracted from the investment cost costs of the district heating system (Table 3 5) and then they are being discounted (Table 4 6). For cases with positive values the option of gas distribution is more favorable than the district heating system (negative values).

Disposition The disposition of networks for all cases is shown in figure 3 ("Examination segment"). Disposition means length of pipeline The disposition designates the pipeline length, but not structure of diameters of conduits in composition of pipeline the pipeline composition.

Determination The determination of structure of diameters of conduits in composition of pipeline the pipeline composition can be done only after the "Conditional Urban Area" is formed. A structure of pipes diameters depends of on building size. The "Examination segment" has ten times bigger area surface than the "Basic segment" because of the network sensitivity exploration. Note that in fig. 3 ("Examination segment") exist pipeline network pipeline network exists, while in Fig. 1 or in Fig. 2 don't it doesn't ("Basic segment").

So, the similarity between "Conditional Urban Area" and "Real Urban Area" can be determined by two different independent quantities [1].

1) Number of buildings in an urban area (the number of buildings on 0.05 km 2 -5 hectares);

(Fig. 1),

2) Heat A heat demand [24 29] of an urban area ("Heat Load" or peak load densities of all buildings heated in a zone divided by the size of an area), MW/km 2 ; (Fig. 2), Term The term "building" is used here also for family houses also, as well as for the and similar smaller constructions with the same meaning as e.g. skyscrapers. In all analyses, "Conditional Residential Unit" [1], i.e. "Conditional Dwelling" [1] with net heating surface area of 60 m 2 is observed. "Conditional Dwelling" has, for the purpose of the model approach, a heat demand of 142 W/m 2 (heat peak load for lower insulated dwelling) in case of lower a low (bad) insulation, and in case of better (good) insulation it has a heat demand of 95 W/m 2 (heat peak load for better insulated dwelling). Each combination of a defined number of buildings and peak load density corresponds to a different number of average dwellings in the building (Table 1). Average An average dwelling (60 m 2 ) is practically "Conditional Dwelling".

Concept The concept of the "Examination segment" is regarded only for the purpose of exploration on realistic values of diameters in structure of pipeline the pipeline structure, but values of N-number of buildings presented in this paper are nominally per "Basic segment".

"Heat load" is expressed in MW/km 2 (not in MW/0.05 km 2 or in MW/0.5 km 2 ) and accordingly it is nominally equal for for both "Examination segment" as for and "Basic segment".

Model The model of rational natural gas usage based on city planning parameters

Based on the introduced "Conditional Urban Area", a techno-economical model of rational natural gas usage was has been made. For each of the 96 cases investment investment, a calculation was has been made in both of the proposed heating systems (Gas Distribution [25 30] vs. District Heating [26 31]) including the exploitation in the next 25 years (investments). For each case, a comparison of costs was has been made so that the heating system with the least smallest cost has an advantage in the implementation. Number . The number of dwelling dwellings per buildings, i.e. the identification of all here shown 96 cases the 96 cases shown here is shown in Table 1.

In the model, both types of heating Both of the heating types in the model have special costs since both of them have special elements; e.g. the district heating system is built with made of steel conduits, pumps and heat exchangers, on the contrary, the gas distribution system is built with made of cheaper polyethylene conduits and has stations for measuring and regulation with internal gas equipment (each dwelling has domestic gas boiler etc). Investment The investment in new capacities in for heating plant are is included in the model in a direct way directly i.e. by increasing the price of natural gas for district heating, or can be added by including the new cost Annual costs of maintenance The annual maintenance costs calculated as a percentage of investment (eq. 1), in network (2,5%); lifetime of 25 years, heat exchanger station (1,5%); lifetime 12 years, annual natural gas consumption -10% more than in the system with domestic boiler in each "Conditional Dwelling") (942,7 m 3 per "Conditional Dwelling" -0.12 €/m 3 ) and the annual electricity consumption for pumps drive (250 kWh -0,035 €/kWh 2 ). Assumption of the The estimated heat losses in the district-heating network is are 10%. per "Conditional Dwelling" -0.12 €/m 3 ).

The efficiency of the NG boiler is the best during the winter when space heating is needed. In During summer, when only hot water is required the boiler is often operated at a very low load and the average efficiency is reduced. For examination in this paper, on-line database SEDBUK 3 (Seasonal Efficiency of Domestic Boilers in United Kingdom) [32] is very useful (Fig. 6 and7).

District heating is often less efficient with respect to modern heating technologies using natural 3 available from www.sedbuk.com gas. Condensing boilers guarantee higher efficiencies with respect to ''traditional'' district heating.

Investment An investment in a new heating plant or the increasing of price of domestic gas boiler boiler's price is included in the calculation, with the introduction of such costs or investments on both G and DH sides simultaneously, caused the equal impact in all of the six "examination segments" (does not change slope of borderline in Fig. 5 4 ).

In the equations above costs common above, the common costs for both systems are not shown.

Different investments in these two opposite systems can be compared for the purpose of a model (Fig 6 8).

All the previous investments are considered for the present conditions in Serbian energy sector.

Of course, this model is applicable for conditions and particular cases all over the world, but diagrams (figures in this paper) are generated for prices ratio in Serbia. Detail calculation for one case of 96 is shown in table 2 4. Generally, each project task has a time component. Every particular element of the two systems shown here has a lifetime, and it has a price on the market (expressed in €). If one of the elements has reached the end of its lifetime (e.g. 26 years), it has to be replaced. Thus, in a project, during the first and the next several years, costs for provision, maintenance and replacement of elements exist (e.g., some elements must be replaced after 14 years and some after 25 years). A sum of costs calculated for each year (t) for gas distribution system is labeled here as G, and for district heating system as DH. "Present Value of Costs" is one of the most useful criterions for project analyses for a whole lifetime of every particular system element. In that way, discount flows reduced on "Present Value of Costs" can be evaluated. "Present Value of Costs" is, by default, cumulative cost for all the elements of the system in present and in future expanded for discount rate. "Future Value of Costs" has to be reduced to present value and to be added to real present costs. Thus, the generated value is called "Present Value of Costs".

Values of initial costs (shown in

For the reduction of "Future Value of Costs", an appropriate "Discount Rate" (dr) has to be adopted. Higher value of "Discount Rate" is calculated when risks for the safety of investments exist In the case of possible risks for the safety of investments, higher value of "Discount Rate" is being calculated. "Discount Rate" could be equalized with "Interest" on the market or for realized credit.

In our case, an economical evaluation is realized during the comparison of two "Present Values of Costs", for gas distribution system (G) and for district heating system (DH) [ 27 33] . "Net Present Value of Costs" (NPV) (eq. 3) is the result of subtraction of the "Present Value of Costs" calculated for district heating system (DH) and the "Present Value of Costs" calculated for gas distribution system (G).

( )

∑ = + - = n 1 t t r D 1 G DH NPV (3)
Value The value of "Discount Rate" or capital cost rate in this case is rated and adopted as dr=10%. The value of discount rate has great impact on the optimal choice of quantitative conclusions (Fig. 5). Changes The changes of calculated values for dr=5% and dr=7% are shown in Table 4 6. Annual An annual gas consumption calculated for one "Conditional Dwelling" is 857 m 3 for heating only, and for district heating system, this amount is 10% higher and this amount is 10% higher for district heating system. Also, Besides, a statistical approach based on nonlinear regression principles can be made to natural gas consumption estimation of individual residential and small commercial customers [ 28 34]. Annual The annual costs of electrical energy for running the pumps for district heating system are estimated to 250 kWh per "Conditional Dwelling".

Analyses are done for all the 96 cases (six "Conditional Urban Area" multiplied by eight "Heat

Loads" and by two conditions of insulation -bad or good insulation) for the period of t=26 years. In table 4 6, case with a bad insulation is shown.

The values in certain cases in table 4 6 vary more or less from zero. For example, for "Conditional Urban Area" with 16 buildings and with "Heat Loads" 50 MW/km 2 , the calculated "Net Present Value of Costs" (NPV) is only -5 per "Conditional Dwelling" for a period of 26 years. Therefore, in this case, the realization of gas distribution system for that "Conditional Dwelling" is only 5 € for 26 years in advantage versus district heating system. This case and the other similar cases are in the "gray zone" (Fig. 7 9). In the "gray zone", both systems are payable, especially for a period of 26 years.

If a considered value for a certain housing block exceeds, more or less, the recommended limiting value the recommended limiting value, more or less, the decision to recommend a particular centralized heating system is more reasonable. Changing the structure and the price ratio (i.e. the state political decision to subsidize the price of gas gas price in a system or to issue a price, which would disturb the current price ratio of gas used for district heating system and individual consumers), the conception and manner of a construction of certain system and its elements, may cause these parameters to vary. Currently, plastic-polyethylene conduits (cheep) are used for distribution to individual consumers while steel conduits (expensive) had been used before. It led to the considerable cuts in network construction prices, while maintaining the same level of safety and endurance.

For the same "Heat Loads", when there are many smaller family houses located on a "Conditional Urban Area" e.g. N=32, sometimes only with one "Conditional Dwelling", gas distribution system is more reasonable. On the contrary, when there is a smaller number of skyscrapers located on "Conditional Urban Area" e.g. N=4, it is more reasonable to use district heating system (Fig. 7 9).

By means of techno -economic analysis for a certain area, it is possible to determine the advantages of one system over the other (district heating over gas). However, considering the situation on the ground, it may turn out that the system is inaccessible in that part of the city (heating plant does not have enough capacity or, in the first stage, the areas closest to the plant could have a priority in system installation). In that case, if the installation costs of the other available system are not too high, it is rational to install that type of heating system (thus saving the consumption costs of electric energy and reducing the amount of pollution if the object is coal).

If, after the analysis, it turns out that one system has more significant economic advantages than the others do, but it is unavailable, some form of hybrid system should be considered. For instance, if the installation of district heating system has a lot of economic advantages but it is not available, it is possible to build a local boiler room which would be gas operated. This hybrid solution would demand additional economic and ecologic analyses [29 35] and estimations of which good features of district heating system would be kept and which would be discarded.

Case study; Application under real conditions

Characteristics The characteristics of the "Real Urban Area": number of buildings, disposition and size of buildings, construction type, etc. are the factors of influence. Adoption The adoption of a type of centralized heating system can be done according to the conclusion established by the model shown. That is possible only if both systems are available near the settlement.

Today, social help habitants have a discount for district heating price of up to 50%. For consumers of gas the gas consumers who use gas distribution systems directly, such discounts are not available. Also, in Serbia, Besides, the payment for the DH is per m 2 of heated surface area (proportional to the size of the house), and in the G system payment is per m 3 of used gas, in Serbia. In the DH bills are fixed, and any sort of economizing in that way is discouraged.

In that case (Belgrade's settlement Karaburma is taken here into consideration), a type of the chosen system depends only on urban characteristic of the settlement on urban characteristic of the settlement only. Because of that, the demonstrative settlements are parceled (divided) into eight "Real Urban Areas" with similar buildings on each particular parcel; (Fig. 8 10). That way, the determined "Real Urban Area" can be associated with the "Conditional Urban Area".

Characteristic The characteristic points for each of the eight parcels (intersection of number of buildings and heat load of parcel) can be plotted into the characteristic model diagram; (Fig. 8 10). The types of insulation of the buildings in the settlement are mixed; old buildings have bad and new buildings have good insulation. Both old and new buildings have heterogeneous spatial disposal.

Two border boundary cases have been treated, because of heterogeneity of insulation quality of buildings [22, 23, 30-32 27, 28, 36-38]; (Fig. 9 11):

-Maximal "Heat Load", all buildings have bad insulation (142 W/m 2 ), -Minimal "Heat Load", all buildings have good insulation (95 W/m 2 ),

The value of fuel consumption depends on heating insulation of the building. Energy consumption in the residential sector can be determined according to various methods shown in an available literature [33 39]. Therefore, the "Heat Load" depends on heating insulation of the building. The number of buildings on each particular parcel is constant. "Gray zone" is the zone where decision on the type of the system depends in great deal depends, in a great deal, on the type of insulation of the building; (Fig. 7 9. and 8 10). In the "Gray zone", the costs for both systems (gas distribution system and district heating system) are very similar. Characteristic points for each of eight particular also depend on the quality of insulation. Small change in slope of borderline for good insulation (K 1 =tgθ 1 ) and for bad insulation (K 2 =tgθ 2 ) in Fig 9 11 is generated only because of differentiation in structure of diameters of conduits in pipeline (Fig 10 12).

"Real Urban Area" No 8 includes types of small buildings or family houses which can contain only one "Conditional Dwelling". Only for that "Real Urban Area" the gas distribution system has very payable advantages versus district heating system. "Real Urban Area" No 3 includes school, kindergarten, local office, shops. These kinds of buildings can contain twenty or more "Conditional Dwellings". Only for that "Real Urban Area" the district heating system has great advantages. All the other zones are in the "gray zone". In a the "gray zone", one system is more payable in comparison with the other, no more than 200 € per "Conditional Dwellings". This amount cannot be crucial for decision. Some illustrations of investigation of price changes (natural gas price, or domestic boilers price) are shown in figure 11 13.

If large areas are covered by agricultural terrain, by excluding it, the above-mentioned parameters obtain unrealistic values.

Note that pipeline is one of the most important parameters of the analyses shown in this paper.

Every particular type of the "examination segment" has a different length of a pipeline route. The length of a pipeline route is fixed by choosing of one six "examination segments". This selection has to be done to present density of built up areas. Further, for detailed examination, when certain type of "examination segment" is chosen, the length of the route is determined, at the same time. Now, the next factor to be varied is the heat load. The heat load is directly correlated with the size of buildings, i.e. diameters can vary for the fixed length of a pipeline and it's structure. Larger diameters of conduits in pipeline structure are correlated to larger buildings. So, the length of a pipeline route depends on the density of built up areas, and structure of the pipe diameters in a pipeline depends on the size of buildings. Therefore, in city conditions, the diameters of pipes are more sensitive (have a greater impact) in some cases, apropos to the length of a pipeline for decision between heating systems. In the conditions of densely populated urban areas, when one type of the "examination segment" is considered, the decision for implementation of certain heating system has to be done according to the above mentioned criterions. The lengths of pipelines and the structures of diameters of conduits are the main factors for establishing these criterions. Accordingly, in villages, the distances between houses are larger, so the lenth of the dwelling's pipeline has greater impact then the pipeline diameters.

The analysis of the distribution of district heat in sparse areas by C. Reidhav and S. Werner [40,41] shows that such distribution can be profitable for Swedish district heating companies, if wisely implemented. Sparse district heating is a label for district-heating systems located in the areas of low heat densities. However, a profitable sparse district heating presupposes a favorable combination of certain factors. The boundaries of two such factors have been identified for Swedish conditions; linear heat density and annual use of district heat/house. An annual use of district heat higher than 50 GJ/house and a linear heat density greater than 2 GJ/m are required for profitability. In Sweden, the general competitiveness of sparse district heating is facilitated by the high consumption taxes for fuel oil, natural gas, and electricity. Hence, it should be more difficult to introduce sparse district heating in other countries with low energy taxes. Such areas are unfavorable, since revenues from heat sold are low compared with the investment costs for the local distribution network. This situation results in high distribution costs. Distribution heatlosses are also higher at low heat-densities.

All the previous values of limiting parameters are calculated for the present gas prices for heating plants, for households, for domestic gas boilers' price, the costs of pipes, labor, etc.

Regarding the variation of all parameters, readers can consult electronic annex (Microsoft Excel file). Valuable information for the energetic situation in the city of Belgrade is shown in the paper of M. Jovanović, N. Afgan, P. Radovanović and, V. Stevanović [42] (Belgrade's settlement Karaburma is shown in Fig. 10).

A city is considered to be sustainable if it establishes the balance between economic and sociocultural development, on one side, and the progress in environmental protection with active participation of its citizens, on the other side. Using When using one of the power resource resources (in this case natural gas) in various systems, the difference in the amount of pollution is very little small. Big differences cannot occur in the overall effect on the city level, but in certain areas they can. While heating plant is a concentrated pollutant which disperses harmful combustion products evenly on a wide area depending on the wind, gas lined consumption disperses locally (consumer pollutes his nearest neighborhood) [43]. Globally, the biggest gas consumer is the biggest city polluter. The key advantage of installing gas or district heating system is not in their mutual differences, but in the substitution of by the far more expensive (in terms of energy and ecology [39 49]) and the highest quality form of energy -the electric energy, whose usage for heating is by far less rational; (Fig. 12 14). The introduction of competition to electricity generation and commercialization has been the main focus of many restructuring experiences around the world.

The open accesses to the transmission network and a fair regulated tariff have been the keystones for the development of the electricity market [50]. Parallel to the electricity industry, the natural gas business has great interaction with the electricity market in terms of fuel consumption and energy conversion. Given that the transmission and distribution monopolistic activities are very similar to the natural gas transportation through pipelines, economic regulation related to the natural gas network should be coherent with the transmission counterpart [ 40 51]. Electricity and natural gas use for residential space heating in USA is shown in paper by D. Bodansky [41 52].

Today, discussions about district heating systems are not rare in scientific literature [53][54][55][56][57][58][59][60][61][62][63].

Conclusions

The latest increase in gas prices turned all eyes once again to the space heating problem in Europe. However, economic concern is not the only factor pushing the authorities to rethink about the suitability of the currently existing sources. Cities are the biggest consumers of the country's energy production. The increase in annual consumption of total primary energy is 3%

and its largest part is used for lighting, cooking, heating, cooling, and transport of freights and passengers. It should be noted how important it is to reduce the energy consumption level, by changing the forms of consumption and making improvements in technology and lifestyle The importance of reducing the energy consumption level, by changing the forms of consumption and making improvements in technology and lifestyle, should be noted. Sustainable forms of energy production, distribution and use usage represent the goals of a sustainable development.

A city is considered to be sustainable if it establishes the balance between economic and sociocultural development, on one side and the progress in environmental protection with active participation of citizens. On the other side, the economics of G and DH very much depends depend on the specific circumstances [64,65]. If installed in an area with only scattered buildings, the length of the pipes which are necessary to supply households will be higher, relative to the number of buildings. Installation costs will be shared by fewer consumers. The DH system is the capital intensive; in particular due to the distribution system of insulated pipes.

The G system is another form of energy which is being distributed for domestic heating purposes. The dimensions of the pipes are small compared to the DH pipes and no insulation is needed, thus the distribution system is less capital intensive.

District heating is often less efficient with respect to modern heating technologies using natural gas [66][67][68]. Condensing boilers guarantee higher efficiencies with respect to the "traditional" district heating. In this study, a noncondensing boiler with average efficiency was has been chosen from following database: "Seasonal Efficiency of Domestic Boilers in United Kingdom" ). The same conclusion is made for the investments in new capacities in heating plant, but that it moves borderline in direction of the gas distribution system. On the contrary, changes in value of discount rate have a great impact (Fig

5.). Changes in the price of domestic boiler boilers or investments investment in new capacities

in heating plant has the equal impact in all of the six "examination segments", changes of discount rate does not.

The conducted analyses confirm literature and empirical information:

-District heating system is a better option in areas with small built up density, but with skyscrapers.

-Gas system is a better option in areas with high density of individual houses (in this case gas distribution system is significantly in advance).

These two boundary cases are illustrative. But in the case with small density of built up areas with individual houses, the G system is a cheaper option (but not necessary according to some Swedish studies [43, 44 40, 41]). On the contrary, with With high density of built up areas with skyscrapers, the G system is also a cheaper solution. But, safety regulation is a limiting factor for the implementation of gas system directly in a building so high such a high building. Anyway, in this case the G system has a slight advantage, and by the increasing the price of domestic gas boiler boilers, the DH system became the cheaper option (Fig 6 8. and Table 3 5).

In more detail details, if a certain number of objects N exist in a "Conditional Urban Area", it can be concluded that (Fig. 6 8.):

1. N = 4, district heating system has an advantage over gas if an average building has over 29 is appeared rare rarely appears in practice. Gas The gas distribution system is advantageous because all 128 buildings must be equipped with expensive heat exchanger exchangers. That is highly unpractical and expensive for individual houses. For skyscrapers, gas system, theoretically, has a slightly slight advantage, but with an increased prize price of domestic gas boiler boilers, district heating system can be applied in larger buildings. Also, Besides, the implementation of the gas system in skyscrapers is forbidden during according to safety regulations.

If large areas are covered by agricultural terrain, by excluding it, the above-mentioned parameters obtain unrealistic values.

The above-mentioned limiting values for use of gas are consistent in case of an average heatinsulated apartment of 60m 2 . In other cases, additional corrections should be made, or different input values must be entered in the model.

Note that, pipeline is one of the most important parameters in the analyses shown in this paper.

Every particular type of "examination segment" has a different length of pipeline route. Length of pipeline route is fixed by choosing of one six "examination segments". This selection has to be done to present density of built up areas. Further, for detailed examination, when certain type of "examination segment" is chosen, in the same time length of route is determined. Now, next factor to be varied is heat load. Heat load is directly correlated with the size of buildings, i.e. for fixed length of pipeline, structure of pipeline diameters can vary. Larger diameters of conduits in pipeline structure are correlated to larger buildings. So, length of pipeline route depends on the density of built up areas, and structure of pipe diameters in pipeline depend on the size of buildings. Therefore, in city condition diameters of pipes are more sensitive in some cases (has greater impact) apropos to length of pipeline for decision of heating system. In the conditions of densely populated urban areas, when one type of "examination segment" is considered, decision for implementation of certain heating system has to be done according to above mention criterions. Lengths of pipeline and structures of diameters of conduits are main factor for establishing of these criterions. Accordingly, in villages, distances between the houses are larger, so dwelling's length of pipeline has greater impact then the pipeline diameters. The analysis of the distribution of district heat in sparse areas by C. Reidhav and S. Werner [43,44] shows that such distribution can be profitable for Swedish district heating companies, if wisely implemented. Sparse district heating is a label for district-heating systems located in areas of low heat densities. However, profitable sparse district heating presupposes a favorable combination of certain factors. The boundaries of two such factors have been identified for Swedish conditions; linear heat density and annual use of district heat/house. An annual use of district heat higher than 50 GJ/house and a linear heat density greater than 2 GJ/m are required for profitability. In Sweden, the general competitiveness of sparse district heating is facilitated by the high consumption taxes for fuel oil, natural gas, and electricity. Hence, it should be more difficult to introduce sparse district heating in other countries with low energy taxes. Such areas are unfavorable, since revenues from heat sold are low compared with the investment cost for the local distribution network. This situation results in high distribution costs. Distribution heatlosses are also higher at low heat-densities.

All previous values of limiting parameters is done for present prices of gas in for heating plants, for households, for price of domestic gas boilers, costs of pipes, labor, etc. But for variation of all parameters, readers can consult electronic annex (Microsoft Excel file).

Some German studies with subject relevant for development of district heating systems in urban environment are also useful and highly evaluated, but this literature is available only in German language [45][46][47], with related papers presented in scientific journals [48]. Main achievement of these German researches is in establishing of interaction between heating systems, settlement structure and urban planning at the local level. The study analyses options for heat supply in up to 10 types of built up areas [45] -from densely populated urban areas to villages. A new German study 20 years later refers to this work [46], with projection to 2020 [47]. Valuable information for the energetic situation in city of Belgrade is shown in the paper of M. Jovanović, 1. New abstract is in the paper.

2. The introduction is reorganized as requested (first literature review and after basic assumption and main objectives of our paper.

3. Explanation is added in lines 121-148; alternative option is to delete sentence 'The most suitable option for satisfying heating demands in urban areas is by using a centralized system.' to avoid further explanations 4. In our opinion text is now reorganized to explain basic assumption in better way (this request maybe we have not understood clear and we will be grateful for further explanations).

5. We hope that English expression in our paper is now improved. "Discount Rate" = 10,0 % "Net Present Value of Costs"; NPV = 24,88 m Small differences in sum are generated by omission of decimal places a Year of project, b District heating pipeline and construction of new heating plant, c Heat exchanger, d District heating system maintenance, e Cost for additional gas (10%), f Cost for electrical energy g Gas Distribution pipeline, h Cost for regulation station, i Cost for household connection set, j Cost for Gas Boiler (variant Iprice of domestic boiler is 500€), k Gas distribution system maintenance l one of cases shown in Table 5., m one of cases shown in Table 6. Number of buildings per 0,05 km 2 ("Basic segment"; see Fig. 1 and2), note that all examination are done on ten time larger segment ("Examination segment"; see Fig. 3) because this size of segment is more suitable for examination of pipeline costs; more representative pipe diameters are included in model c Price of domestic boiler; I-500 € (value also used in Table 6, see also Fig. 6.), values for I are the same in case of increasing the prize of domestic gas boiler for 500 € (sum 1000 € for boiler), but with simultaneously adding of investments (500 € per average dwelling) in new heating plant on the other side II-1000 €; dr=10%;

Table(s)

  and for bad insulation (K 2 =tgθ 2 ) in Fig 11 is generated only because of differentiation in structure of diameters of conduits in pipeline (Fig 12).

Figure 5

 5 Figure 5 is essential for the full understanding of the model presented here. All dwellings in the

Figure 1 .Figure 2 .Figure 3 .Figure 4 .Figure 5 .Figure 6 .Figure 7 .Figure 8 .Figure 9 .Figure 10 .Figure 11 .Figure 12 .Figure 13 .Figure 14 .Figure 15 .

 123456789101112131415 Figure 1. "Real Urban Area" associates with several types of "Basic segments" according to the

  of investments and exploitation costs have been made for both heating systems and a comparison has been made for all of the 96 presented cases. Almost each type of real settlement can be represented by one of the types of the conditional urban area which are introduced in the paper. The main goal of this paper is to establish a general model to achieve coordinated development of centralized energy supply systems fueled by natural gas, based on defined and accepted criteria. A structure analysis of centralized systems for energy supply has been done with accent on their pipelines.Keywords: Natural Gas, Settlement, Gas Distribution, District Heating, Urbanism Nomenclature y -number of "Conditional Dwellings" per building [-] N -number of buildings per "Conditional Urban Area" [-] x -Peak load densities or "Heat Load" [MW/km 2 ] DH -costs of district heating system [€] G -costs of local gas heating system (domestic boiler in each dwelling) [€] 3 DHN -costs of district heating network, i.e. costs of building/civil works, costs of materials (insulated pipes, pumps, accessories, etc.) and telemetry systems, etc [€] HE -costs of heat exchanger stations located in buildings [€] HP -investment in new heating plant [€] DHOC -annual costs of maintenance calculated as percentage of investment, in network, heat exchanger station, annual natural gas consumption and annual electricity consumption for pumps drive [€] MPRS -costs includes costs of main pressure reduction stations [€] PRS -costs of pressure reduction stations [€] DN -costs of natural gas distribution network [€] DS -costs of domestic measurement sets [€] B -costs of domestic boilers [€] GOC -annual costs of maintenance calculated as percentage of investment; in gas distribution network, in pressure reduction station, in measurement set, domestic boiler and annual natural gas consumption [€] dr -"Discount Rate" [%] NPV -"Net Present Value of Costs" [€] t -Time [years]

  indirectly. Investments The investments in a new heating plant fuel fueled by natural gas are: 80000 €/MW (for heat plant capacity <50MW), 65000 €/MW (for heat plant capacity 50-100 MW) and 52000 €/MW (for heat plant capacity 100-200 MW) [1]. That means additional cost costs of 450-680 € per "Conditional dwelling". That It implies that this kind of additional cost costs is not essential for this kind of analyses (Fig 5.). Changes The changes in the slope of borderline in the model diagram 1 are caused by changes in discount rate the discount rate changes (Fig 5.) or by differentiation in the structure of diameters of conduits in the pipeline 2 .Same conclusion can be made with variation in price price variations of domestic boiler (see Table 3 5 and Fig 5). L. Gustavsson and A. Karlsson [10 3] estimated DH investment costs andDH maintenance costs the DH investment and maintenance costs. Increasing of price of domestic gas boiler simultaneously with introducing of investments in heating plant in the same amount are to be annulled An increasing price of domestic gas boilers simultaneously with the 1 See alsoFig 11.

2

  See alsoFig 12 in case study introduction of the same amount of investments in a heating plant are to be annulled (Fig 5.) (for detailed analyses consult electronic annex 1).Relative A relative amount of investments (per "Conditional Dwelling" included annual costs) in district heating -DH and in local gas heating system -G (each "Conditional Dwelling" is equipped with domestic boiler fueled by natural gas) can be calculated after following eqs. there are: DH -costs of District Heating System [€], DHN -costs of District Heating Network, i.e. costs of building/civil works, costs of materials (insulated pipes, pumps, accessories, etc.) and telemetry systems, etc [€], HE -costs of Heat Exchanger stations located in buildings [€], HP -investment in new heating plant [€], DHOC -annual costs of maintenance calculated as a percentage of investment, in network, heat exchanger station, annual natural gas consumption and annual electricity consumption for pumps drive [€] and y -number of "Conditional Dwellings" per building[-].

  Where there are: G -costs of Local Gas Heating System (domestic boiler in each dwelling)[€], MPRS -costs include costs of Main Pressure Reduction Stations [€], PRS -costs Pressure Reduction Stations [€], DN -costs of natural gas Distribution Network [€], DS -costs of Domestic measurement sets [€], B -costs of domestic boilers [€],GOC -annual costs of maintenance calculated as a percentage of investment; in gas distribution network, in pressure reduction station, in measurement set, domestic boiler and annual natural gas consumption [€] and and y -number of "Conditional Dwellings" per building [-]. Annual costs of maintenance The annual maintenance costs calculated as a percentage of investment (eq. 2); in gas distribution network (2,25%); lifetime of 25 years, in pressure reduction station (2,25%); lifetime of 25 years, in measurement set (2,25%), lifetime of 12 years, domestic boiler (2,25%), lifetime of 12 years and the annual natural gas consumption (857 m 3

[

  42 32]. Efficiency The efficiency of domestic boiler and efficiency of heating plant on that way has equalized by efficiency in this model. Typical The typical efficiencies of plants fueled by natural gas are reported in paper of R. Lazzarin and M. Noro[17 10].

Figure 5

 5 Figure 5 is essential for fully the full understanding of the model presented here. All dwellings in

Figure 1 .Figure 2 .Figure 3 .Figure 4 .Figure 5 .Figure 6 .Figure 7 .Figure 6 .Figure 8 .

 123456768 Figure 1. "Real Urban Area" associates with several types of "Basic segments" according to the

Figure 7 .Figure 9 .Figure 9 .Figure 10 .Figure 10 .Figure 11 .Figure 11 .Figure 12 .Figure 12 .Figure 13 .Figure 14 .Figure 15 .

 799101011111212131415 Figure 7. Figure 9. Identification of some characteristic cases in model diagram

6 . 4 :

 64 Figure(s) Click here to download high resolution image

d

  Example from Table2, e NPV (II)-NPV (I); dr=10%

  Torekov, N. Bahnsen and B. Qvale[9] found that DH system realized 78-93 kg/MWh CO 2 , 0.1

kg/MWh NO x and 0.06 kg/MWh SO 2 , G system with individual furnace realize 205 kg/MWh CO 2 , 0.07 kg/MWh NO x and 0.001 kg/MWh SO 2 , and electrical heating system 559 kg/MWh CO 2 , 88 kg/MWh NO x and 44 kg/MWh SO 2 . 1 MWh of delivered energy is 61% cheaper in natural gas in comparison with electrical energy

Table 1 :

 1 Identification of all of the 96 considered case (number of average dwelling per building)

Table 2 :

 2 Structure of pipe diameters for one of the cases (example)

Table 3 :

 3 Investments in both systems per dwelling; G and DH [€]

Table 4 :

 4 Costs for gas distribution system and district heating system (example)

Table 5 :

 5 Initial costs a -€ per "Conditional Dwelling"

Table 6 :

 6 "Net Present Value of Costs" (NPV) a -€ per "Conditional Dwelling"

	1

Systematic Approach to Natural Gas Usage for Domestic Heating in Urban Areas Dejan Brkić a* , 1 Toma I. Tanasković b

  Faculty of Mining and Geology, University of Belgrade, ðušina 7, 11000 Beograd, Serbia Received: 22 February 2008; received in revised form:

a Ministry of Science and Technological Development, ðušina 7, 11000 Beograd, Serbia, b

Table 3 )

 3 for DH subtracted by initial costs of G are shown in table3 5.

  For including environmental impacts of different centralized energy supply systems in a model, it is necessary to estimate the annual environmental costs for both systems and to include them in related equations. Compared to all of the fossil fuels, natural gas is a minor pollutant. It burns without a solid residue and has the least coefficient of CO 2 emission of about 56 kg/GJ (which is significant considering the limitations imposed by The Kyoto Protocol)[34-37 44-47] . M.S. Torekov, N. Bahnsen and B.

	Qvale [16 9] found that DH system realized 78-93 kg/MWh CO 2 , 0.1 kg/MWh NO x and 0.06
	kg/MWh SO 2 , G system with individual furnace realize 205 kg/MWh CO 2 , 0.07 kg/MWh NO x
	and 0.001 kg/MWh SO 2 , and electrical heating system 559 kg/MWh CO 2 , 88 kg/MWh NO x and
	44 kg/MWh SO 2 . 1 MWh of delivered energy is 61% cheaper in natural gas in comparison with
	electrical energy [36 46]. Comparison The comparison of environmental impacts of two
	residential heating systems in Canada is shown in paper of L. Yang, Zmeureanu R., and Rivard
	H. [38 48].

Table 1 :

 1 Identification of all of the 96 considered case (number of average dwelling per building)

Table 2 :

 2 Costs for gas distribution system and district heating system Structure of pipe diameters for one of the cases (example)

Table 3 :

 3 Initial costs a -€ per "Conditional Dwelling" Investments in both systems per dwelling;

	G and DH [€]

Table 4 :

 4 "Net Present Value of Costs" (NPV) a -€ per "Conditional Dwelling" Costs for gas distribution system and district heating system (example)

Table 5 :

 5 Initial costs a -€ per "Conditional Dwelling"

Table 6 :

 6 "Net Present Value of Costs" (NPV) a -€ per "Conditional Dwelling"

	Reviewer 1:

Table 1

 1 Identification of all of 96 considered case (number of average dwelling per building)

	Number of buildings -N per basic segment
		"Heat Loads", MW•km -2 W/m 2 125 100 75 50	40	30 20 10
	N=4	95 a 142 b	274 c 219 164 110 88 181 145 109 72 58	66 44 22 43 29 14
	N=8	95 142	137 90	110 82 72 54	55 36	44 29	33 22 11 22 14 7
	N=16	95 142	69 45	55 36	41 27	27 18	22 14	16 11 5 11 7 4
	N=32	95 142	34 23	27 18	21 14	14 9	11 7	8 5	5 4	3 2
	N=64	95 142	17 11	14 9	10 7	7 5	5 4	4 3	3 2	1 1
	95 N=128 142	9 6	7 5	5 3	3 2	3 2	2 1	1 1	1 0 d
	a Good insulated dwelling, b Poor insulated dwelling c 274 good insulated dwellings per building, 4 buildings per "Basic segment" d less than 1 average dwelling (<60 m 2 ) per building; poor insulated, 128 houses per "Basic
	segment"

Table ( s ) Table 2

 ()2 Structure of pipe diameters for one of the cases (example) Number of buildings -N=8 per basic segment -bad insulation (example; 1 of 96 cases), pipe length 6676.25 m for G, and (2•6676.25)= 13352.5 m for DH

		"Heat Loads", MW•km -2
		125		100		75		50		40		30		20		10
	Pipe diameter [mm] DH	G	DH	G	DH	G	DH	G	DH	G	DH	G	DH	G	DH	G
	25	0	0	0	0	0	3200	0	3200	0	3357.5 0	3987.5	0	4302.5	6400 5798.75
	32	0	3200	0	3200 0	630	0	787.5	0	945	6400 787.5	6400	1496.25 1575 157.5
	40	0	630	0

Table 3

 3 Investments in both systems per dwelling; G and DH [€]

	Bad insulation		"Heat Loads", MW•km -2
	Number of buildings per basic
	segment		125	100	75	50	40	30	20	10
	N=4 (DH)	∑	752	798	850	988	1059	1208	1453
	-pipeline	147	174	189	267	291	374	504	914
	-heat exchanger	105	124	161	221	268	334	449	498
	-heat plant	500	500	500	500	500	500	500	500
	N=4 (G)	∑ 1427 1429	1434	1442	1450	1461	1484
	-pipeline	15	16	20	25	29	34	47	78
	-regulation station	12	13	14	17	21	27	37	52
	-connection set and domestic boiler 1400 1400	1400	1400	1400	1400	1400
	N=8 (DH)	∑	857	919	1010	1184	1296	1480	1665
	-pipeline	169	198	234	325	381	473	667
	-heat exchanger	188	221	276	359	415	507	498	691
	-heat plant	500	500	500	500	500	500	500	500
	N=8 (G)	∑ 1429 1433	1437	1446	1456	1470	1496
	-pipeline	17	20	23	29	35	43	59	105
	-regulation station	12	13	14	17	21	27	37	52
	-connection set and domestic boiler 1400 1400	1400	1400	1400	1400	1400
	N=16 (DH)	∑ 1033 1110	1247	1427	1573	1787	2076
	-pipeline	212	251	305	429	520	623	885
	-heat exchanger	321	359	442	498	553	664	691
	-heat plant	500	500	500	500	500	500	500	500
	N=16 (G)	∑ 1431 1436	1443	1456	1468	1486	1520
	-pipeline	19	23	29	39	47	59	83	152
	-regulation station	12	13	14	17	21	27	37	52
	-connection set and domestic boiler 1400 1400	1400	1400	1400	1400	1400
	N=32 (DH)	∑ 1283 1351	1448 a	1758	1987	2324	502.405
	-pipeline	296	353	417	594	727	902	1.299
	-heat exchanger	487	498	531	664	760	922	1.106
	-heat plant	500	500	500	500	500	500	500	500
	N=32 (G)	∑ 1438 1445	1455 a	1474	1490	1517	1565
	-pipeline	26	32	41	57	69	90	128	243
	-regulation station	12	13	14	17	21	27	37	52
	-connection set and domestic boiler 1400 1400	1400	1400	1400	1400	1400
	N=64 (DH)	∑ 1456 1545	1722	2046	2556	2663	3700
	-pipeline	319	381	485	661	812	1057	1541
	-heat exchanger	637	664	737	885	1244	1106	1659
	-heat plant	500	500	500	500	500	500	500	500
	N=64 (G)	∑ 1442 1449	1461	1483	1501	1529	1583
	-pipeline	30	36	47	66	80	102	146	286
	-regulation station	12	13	14	17	21	27	37	52
	-connection set and domestic boiler 1400 1400	1400	1400	1400	1400	1400
	N=128 (DH)	∑ 1783 1846	2274	2679	3209	4079	5489
	-pipeline	398	461	594	852	1050	1367	1989
	-heat exchanger	885	885	1180	1327	1659	2212	3000
	-heat plant	500	500	500	500	500	500	500	500
	N=128 (G)	∑ 1450 1458	1471	1496	1518	1555	1626
	-pipeline	38	45	57	79	97	128	189	371
	-regulation station	12	13	14	17	21	27	37	52
	-connection set and domestic boiler 1400 1400	1400	1400	1400	1400	1400

a see

Table 4 Table ( s)

 4(

Table 4

 4 Costs for gas distribution system and district heating system (example) € per "Conditional dwelling" N=32 buildings per 0,05km 2 , x="Heat Load"=75 MW/km 2 , y=14 "Conditional Dwellings" per building

		District Heating System				Gas Distribution System
	a	DH	b	c	d	e	f	G	g	h	i	j	k	DH-G
	1	1447,5 916,6 530,8	0	0	0	1454,5 40,7 13,7 400 1000		-7,0
	2	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	3	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	4	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	5	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	6	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	7	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	8	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	9	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	10	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	11	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	12	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	13	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	14	568,2	0	530,8	18,38 10,2 8,75	921,7	0	0	100 800		-353,5
	15	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	16	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	17	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	18	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	19	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	20	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	21	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	22	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	23	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	24	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	25	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
	26	37,4	0	0	18,38 10,2 8,75	21,7	0	0	0	0		15,7
		Σ 2913	917	1062	459	255 219	2897	40,7 13,7 500 1800		16,0 l

Table 5

 5 Initial costs a -€ per "Conditional Dwelling" Number of buildings -N b (bad insulation) gas distribution system is more expensive Positive values: district heating system is more expensive a DH-G (see eq. 1 and 2 and Table3)

			"Heat Loads", MW•km -2
		c	125	100	75	50	40	30	20	10
	N=4	I II -1,174 -1,131 -1,083 -674 -631 -583	-454 -954	-390 -890	-253 -753	-30 -530	381 -119
	N=8	I II -1,071 -1,013 -926 -571 -513 -426	-261 -761	-159 -659	10 -490	168 -332	810 310
	N=16	I II -898 -398	-325 -825	-195 -695	-28 -528	106 -394	301 -199	556 56	1.608 1,108
	N=32	I II -656 -156	-94 -594	-7 d -507	284 -216	497 -3	807 307	1.339 839	2.963 2,463
	N=64	I II -486 14	96 -404	262 -238	563 63	1.056 556	1.134 634	2.117 1,617	4.714 4,214
	I N=128 II -167 333	388 -112	1.183 303	1.183 683	1.691 1,191	2.524 2,024	3.863 3,363	5.438 4,938
		|II-I| 500	500	500	500	500	500	500	500
		e 689	689	689	689	689	689	689	689
	Negative values:

b

Table ( s)

 (

Table 6 "

 6 Net Present Value of Costs" (NPV) a -€ per "Conditional Dwelling" .504 2.135 3.171 4.820 6.614 Negative values: gas distribution system is more expensive Positive values: district heating system is more expensive Variant I-gas boiler price: 500€ (see Table5) c Value of "Discount Rate"

	Number of buildings (bad insulation)
	I b	dr c	"Heat Loads", MW•km -2 125 100 75	50	40	30	20	10
		5% -1,010 -946 -870	-679	-576	-371	-34	529
	N=4	7% -911	-852 -784	-609	-517	-329	-22	499
		10% -802	-749 -689	-532	-451	-283	-9	467
		5% -846	-757 -624	-378	-223	34	243	1,161
	N=8	7% -763	-682 -562	-337	-196	38	233	1,078
		10% -671	-599 -492	-291	-166	42	223	985
		5% -573	-462 -262	-23	174	472	822	2,357
	N=16	7% -516	-416 -234	-15	166	437	761	2,168
		10% -454	-364 -203	-5	157	398	695	1.959
		5% -197	-111 16	446	762	1,229 1,999 4,336
	N=32	7% -176	-97	20	413	701	1,127 1,834 3,980
		10% -152	-80	25 d	377	635	1.015 1.652 3.587
		5% 79	196 441	891	1,664 1,722 3,211 7,103
	N=64	7% 72	180 404	814	1,514 1,576 2,931 6,479
		10% 65	163 363	730	1.348 1.415 2.623 5.791
		5% 584	657 1,305 1,858 2,642 3,931 5,971 8,062
	N=128	7% 528	596 1,182 1,689 2,400 3,569 5,423 7,374
		10% 467	530 1.048 1

a see eq. 3. b d Example from Table 4

Table ( s) Gas distribution system vs. District heating system

 (

		Note: calculations are in hidden layers
	Investments per dwelling	€	year-replacement% maintenance
	Pressure Reduction Stations -G (€ per flat)	12	25	2.3%		MW/km 2	125	100	75	50	40	30	20	10
	Domestic measurement sets -G (€ per flat)	400	12	2.3%
	Domestic gas boiler -G (€ per flat)	1000	12	2.5%	N/0.05km 2	4	DH	DH	DH	DH	DH	DH	DH	G
	Heat exchanger -DH (see hidden layers for used prize)	******	12	1.5%		8	DH	DH	DH	DH	DH	G	G	G
	Gas Distribution network	c22	25	2.3%		16	DH	DH	DH	DH	G	G	G	G
	District Heating network	f22	25	2.5%		32	DH	DH	G	G	G	G	G	G
	Annual gas consumption -G	857	m 3 /year per flat for heating in G system		64	G	G	G	G	G	G	G	G
	Annual gas consumption -DH	1.10	DH/G			128	G	G	G	G	G	G	G	G
	Investment in heating plant	500	€/flat
	Annual electric energy consumption	250	KWh/

year per flat for pumps in DH system Natural gas price 0.12 €/m 3 (here is equal price for gas in both system for Energy readers) (NPV) -€ per "Conditional Dwelling"

	INPUT PARAMETERS (
	Price of el. Energy	0.035	€/KWh				MW/km 2	125	100	75	50	40	30	20	10
	Discount rate	10.0%
	*****fixed and locked for Energy readers in hidden layers	G				N/0.05km 2	4	-802 -1,335 -689 -532 -451 -283	-9	467
	note: calculation only for poor insulated dwellings for Energy readers DH					8	-671 -599 -492 -291 -166	42	223	985
							16	-454 -364 -203	-5	157	398	695 1,959
		Price of network conduits				32	-152	-80	25	381	635 1,015 1,652 3,587
							64	65	163	363	730 1,348 1,415 2,623 5,791
		Diameter	€/m	Diameter	€/m		128	467	530 1,048 1,504 2,135 3,171 4,820 6,614
		25	8.1	20	40
		32	11.0	25	43
		40	13.0	32	49		initial costs -€ per "Conditional Dwelling"
		50	13.6	40	52		MW/km 2	125	100	75	50	40	30	20	10
		63	19.3	50	55.5
		75	20.0	65	58.5	N/0.05km 2	4	-674 -1,131 -583 -454 -390 -253	-30	381
		90	27.0	80	74		8	-571 -513 -426 -261 -159	10	168	810
		110	30.0	100	93		16	-398 -325 -195	-28	106	301	556 1,608
		125	37.0	125	118		32	-156	-94	-7	288	497	807 1,339 2,963
		140	39.5	150	124		64	14	96	262	563 1,056 1,134 2,117 4,714
		160	47.5	200	158		128	333	388	803 1,183 1,691 2,524 3,863 5,438
		180	51.5	250	224
		200	55.5	300	254
		225	62.5	350	295
		250	69.4	400	313
				450	322
				500	380

edit only green cells) Main results Gas distribution pipeline District heating pipeline

@BCL@340BF02D.xls 11/08/2008 12:55

Table ( s)

 (

available from www.sedbuk.com

or in case study inFig 11

or in case study inFig 11

N. Afgan, P. Radovanović and, V. Stevanović [49] (Belgrade's settlement Karaburma is shown in Fig. 8.).

Today, discussions about district heating systems are not rare in scientific literature [50][51][52][53][54][55][56].

Acknowledgment

The This paper is a part of the research done within the project of Ministry of Science NP EE 34-406A, Serbia, Belgrade and the authors would like to thank for financial support.

Appendix

Algorithm The algorithm for initial decision for selection of a natural gas heating system in settlements is shown in figure 13 15: