Rodríguez Hidalgo

Rodríguez Aumente

Izquierdo Millán

Lecuona Neumann

Salgado Mangual

ENERGY AND CARBON EMMISION SAVINGS IN SPANISH HOUSING AIR-CONDITIONING USING SOLAR DRIVEN ABSORPTION SYSTEM

Keywords:

Air Conditioning (AC) is spreading in Spain, where summer weather reaches temperatures above 35ºC. This causes the increase, not only of electricity consumption, but also organic refrigerant leakage due to the use of vapourcompression machines.

An experimental research on solar absorption cooling has been carried out at

Universidad Carlos III de Madrid (UC3M) in order to evaluate alternative lower impact AC techniques. The facility is based on an on-Campus field of 50m 2 flat

plate solar collectors driving a single-effect commercial LiBr/H 2 O absorption machine through a hot water storage tank.

Experimental operating parameters have been recorded during 2004 summer season in periods of 10 minutes showing all the relevant figures of merit.

Results show that absorption machine cooling power reaches 6-10kW, with a generator driving power input of 10-15kW, achieving a mean cooling period of 6.5 hours of complete solar autonomy on a seasonal average day.

In order to measure the market applicability of this setup, the instantaneous thermal load and demand of a single detached housing of total floorspace of 90m 2 were calculated using standard techniques. Its peak ranged from 6 to 8kW. Seasonal solar fraction of AC provided by the facility reached 56%.

This work shows results on system performance, economic investing, energy and economic savings and includes environmental impact reduction, when comparing the solar absorption system with a conventional vapour-compression machine. A sensitivity study to solar field size has been performed to yield the reasonable share of the absorption machine in the seasonal cooling demand. The result indicates that the dimensioning of the facility is correct. Keywords: Solar energy; Absorption cooling; Environmental impact reduction; Solar air conditioning; Energy saving; Sustainability.

NOMENCLATURE:

A : Total housing area [m 2 ].

INTRODUCTION

In the past century, scientific community has devoted much effort to procure energy sustainability of housing in two main directions: reducing external energy supply, and using Renewable Energy (RE) for the remaining. In both ways solar resources are gaining popularity because they increase energy independence and sustainability, at the same time offering nearly zero impact to the environment [START_REF] Demirbas | Global Renewable Energy Resources[END_REF]. This paper aims to contribute to the development of techniques that reduce external electricity consumption for air conditioning in housings. For this goal an experimental research has been performed using current technology to shed some light on the possibilities and barriers for its application.

In Europe, there is a strategic objective of sustainability, reduction of primary energy consumption, as well as increasing the RE fraction in heating and cooling of buildings, whose share is 40% of the total energy consumption [START_REF]Council of 16 December 2002 on the energy[END_REF]. An ongoing political action promotes heating and cooling from RE, as stated in the EU Commission's Impact Assessment [START_REF] Mercier | Results of the Public Consultation on the EU Initiative on heating and cooling from Renewable Energy Sources[END_REF]. In Spain, although housing primary energy share is lower, around 20%, there is a maximum peak in electricity consumption in summer season every year [START_REF]Informe mensual[END_REF] due to the use of electrically powered AC, with risk to overload the grid and eventually leading to blackouts. Peak-power increases yearly because AC is being installed at a high pace. In 2003 the peak value was over 34GW; in 2004 it reached 36GW and in 2005 about 38GW [START_REF]Informe mensual[END_REF]. In 2005 1.3 million AC units were sold (mainly mini split-type residential systems) and about 0.6 million new housings were built. This indicates that the problem will keep on growing [START_REF] Mercado De Climatización | Asociación de Fabricantes de Equipos de Climatización AFEC[END_REF].

Another problem is that conventional vapour-compression systems, using R407C refrigerant, offer a null Ozone Depletion Potential (ODP), but their direct Global Warming Power (GWP) in 20 years horizon is very high, 3,200 [START_REF] Rodríguez | Decentralised cooling of electronic equipment using the thermal conversion of solar energy[END_REF], posing a threat to greenhouse effect when leaking.

The current technologies in the market for cold water production, using thermal Solar Energy (SE) are: Absorption machines, solid and liquid desiccant and solid adsorption.

The most commercially developed are the absorption systems, [START_REF] Hans-Martin | Solar assisted air conditioning of buildings -an overview[END_REF].

One of the advantages of these technologies is that AC load is simultaneous to the availability of solar radiation, so that heat storage does not need to be large. It is also a solution to Solar Thermal Collector (STC) overheating during summer season, when there is no need of space heating and at the same time Domestic Hot Water (DHW) demand is reduced on average down to 14%, according to [8].

Nowadays technology demonstration actions are dominated by absorption in a fraction of 78% of the reviewed projects (75% using LiBr/H 2 O and 25% using NH 3 /H 2 O). They are used in combination with Flat Plate Solar Thermal Collector (FPSTC), which are employed in 63% of the reviewed projects, delivering water at temperatures below 97ºC [START_REF] Balaras | Overview of solar assisted air-conditioning systems and applications[END_REF].

Under nominal conditions absorption machines offer an instantaneous COP in the range of 0.5-0.73, of which LiBr/H 2 O systems average 0.66 and NH 3 /H 2 O systems average 0.60 [START_REF] Balaras | Overview of solar assisted air-conditioning systems and applications[END_REF]. The COP of double-effect absorption systems can be up to 1.3, but requiring a relatively high driving temperature, difficult to reach with standard SC technologies [START_REF] Hans-Martin | Solar assisted air conditioning of buildings -an overview[END_REF].

LiBr/H 2 O absorption machines show the best performance, but they are not competitive with vapour-compression systems unless a minimum threshold of RE is supplied [START_REF] Rodríguez | Decentralised cooling of electronic equipment using the thermal conversion of solar energy[END_REF].

This has been studied in a previous work [START_REF] Izquierdo | Energetic, economic and environmental viability of absorption air-conditioning systems in Spain[END_REF], concluding that single effect LiBr/H 2 O absorption units using fossil-fuels are not competitive from the energy, economic and environmental points of view. They are only competitive when using waste or renewable heat as part of the driving energy. Similar conclusion can be found after the simulation of an analogous system with TRNSYS, [START_REF] Florides | Modelling, simulation and warming impact assessment of a domestic-size absorption solar cooling system[END_REF]. Biomass or fossil fuel could be used for cold production complementing SE. In this case competitiveness could increase, as will be shown in this paper.

One of the constraints for a wider use of this technology in housings is the unavailability of air-condensed absorption machines with a low enough capacity [START_REF] Safarik | Solar powered H 2 O/LiBr Absorption machine with low capacity[END_REF].

Recently there have been made available some machines with low nominal power. One of such is the 4kW ROTARTICA 045v from ROTARTICA S.A. ® , [START_REF] Rotartica | [END_REF] indicating this that further study is worthwhile.

Local climate characteristics have a significant influence on collector performance.

Moreover, like in the vapour compression machines, the absorption ones suffer a limiting effect on the cold production, stagnating when the external temperature increases. As a consequence, it seems important to use local real weather data for analysis purposes and in order to avoid an over/underestimation of AC yield. As solar irradiation varies considerably from year to year, a large variation in the STC heat supply has to be expected [START_REF] Adsten | The influence of climate and location on collector performance[END_REF]. Following the results obtained from a previous work [START_REF] Izquierdo | Thermal Solar Energy for Spanish houses comfort: energy-saving and environmental impact reduction[END_REF] 47% of heat, cooling and DHW demands can be provided with 50m 2 of collector area, for a representative housing in central Spain, Madrid region. Accordingly the chosen experimental facility is a field of FPSTC of such area. It feeds a 35kW singleeffect LiBr/H 2 O absorption machine. This configuration shows the lowest cost and is one of the most used, according to [START_REF] Hans-Martin | Solar assisted air conditioning of buildings -an overview[END_REF]. The absorption machine is oversized as there was no model of smaller nominal power commercially available at the time of starting the project. Fortunately this machine incorporates specific devices to improve COP at partial load or at operating temperatures different from nominal, thus being representative of a lower capacity machine.

The first objective of this paper is to describe the solar facility and to show the relevant working parameters; secondly to make an application of this cooling system to a numerically simulated cooling load demand of a representative contemporary Spanish housing; and thirdly to study the influence of the size and some component efficiency of the solar cooling facility on performance.

FACILITY DESCRIPTION

Overview

The experimental facility consists on the usual components of a thermal solar plant, plus an absorption machine for chilled water production for AC purposes, see Figure. The facility includes a meteorological station which records local relevant data such as solar radiation, air temperature, wind velocity and direction as well as relative humidity.

Monitoring and Instrumentation

The data acquisition program developed using National Instruments Labview ® collects, processes and displays data. It takes all sensors signals every 2 seconds and outputs the mean value at each 10 minutes forming time series. Real time experimental data are available "on-line" at the following website: http://termica.uc3m.es/solar.htm

Hot water tank operation

According to current practice, the storage-tank was configured to work under stratification regime, believing that this would be the best for the AC application, but this was not the case. The cooling period achieved producing cold water was not satisfactory because it finished early in the evening when AC was still necessary due to the high outdoor temperature. Eliminating stratification, by forcing hot water to enter at the bottom of the tank, a nearly homogeneous temperature was achieved. This configuration has given better results for the AC application because a larger autonomous cooling time was achieved, as well as a higher cool production. A delay in the cooling capacity was evident, being able to supply chilled water late in the evening.

Nevertheless, lower collector efficiency has been detected using this configuration.

As seen in Figure 1, the system is operated with no supplementary heating from a boiler. This way, the cooling capabilities of the solar facility can be clearly shown; that is, absorption machine works at partial-load and under non steady conditions, using only the SE available. Better results are expected applying supplementary heating, as a higher absorption machine COP could be attained; being this a result of the higher hot water temperature and higher load operating conditions.

EXPERIMENTAL RESULTS

Data gathered from the solar AC facility during the summer 2004 season are described in this section.

Instantaneous operation

The instantaneous data allow obtaining the power curves through the following expressions.

( )

cw cool w icw ocw Q m c T T = ⋅ ⋅ - (1)
•

solar p Q I A = (2)
( )

wt generator w wgo wgi Q m c T T = ⋅ ⋅ - (3)
The energy balance results of a representative summer day, corresponding to July 28 th , are depicted in Figure 2. Solar radiation shows a Gaussian profile this day with the typical shadow effects due to the clouds over the solar field. The collector heat power curve follows the solar radiation. There is a delay between the collector and the cooling power curves. In the day chosen, due to the presence of the storage tank, a significant cooling power was obtained for more than 2h in the evening (18:00h to 20:00h), after the collectors power is null. According to the outdoor air temperature curve, it can be concluded that this is a useful delay. In fact, as it is shown in the next section, the maximum value of instantaneous cooling load of the selected house is attained at 18:00h approximately. During the cooling period of this day, an average value of nearly 3kW cooling power was obtained. For a more detailed description a previously published work is recommended [START_REF] Syed | A novel experimental investigation of a solar cooling system in Madrid[END_REF].

Generator temperature was below 80 ºC, shown in Figure 3, precluding the machine to reach the maximum efficiency.

Daily operation

The facility worked during 2004, from June 16 th to October 14 th with a seasonal average cooling period value of 6.5 hours, with a fan-coil output air temperature of 15ºC.

Daily operation energy balance is calculated by integrating the instantaneous value for each day, as shown in Figure 4. The cooling energy is a small amount of the total SE available, in part due to the facility heat losses.

Averaging the daily cooling energy values, the seasonal cooling energy obtained was 23 kWh per day.

Data, corresponding to 2004 summer season, are raw from Data Acquisition System.

Gaps in data, which have not been removed in Figures 4 to 8, are due to maintenance operation.

Efficiency parameters

The efficiency parameters of the solar AC facility are:

cool generator Q COP Q = ( 4
) cool solar Q SCOP Q = (5)
The seasonal value for COP during the already specified 2004 summer season was 0.33, daily values shown in Figure 5. This low value is attributed to the low hot water temperature in the generator of absorption machine during a significant fraction of the daily cooling period. It seems advisable to increase COP up to the nominal value of 0.7 with supplementary heating from a boiler.

SCOP shows the efficiency of the energy conversion for the total solar absorption system, from the solar radiation over the collector field to the chilled water in the absorption machine evaporator. The average value for SCOP was 0.07 while the seasonal value of si η , defined as follows, was 0.22.

generator si solar Q Q η = (6)
This parameter takes into account not only collector's efficiency (i.e. thermal losses in the collector itself) but also thermal losses in the solar facility (piping, pumps, storagetank and heat-exchangers). At this point it is worthwhile to indicate that higher than normal heat losses characterize this facility as the length of pipes is substantially longer than in a compact design for a single detached housing.

An application to a single detached housing in Spain is presented in the next section in order to compare the cooling demand and the cooling power of the system. Some findings of the above described experimental analysis can be explained from this comparison.

APPLICATION TO A SINGLE DETACHED HOUSING

Evaluation of solar AC technology to Mediterranean market is of much interest. For this aim a Spanish single detached housing was selected as representative of the recent way of living. Its instantaneous cooling load and demand have been calculated using conventional methodology [17] and local data.

Despite the internal distribution, the house is enclosed by four main flat walls and roof; all of them are in contact with the atmosphere. It includes two superimposed floorspaces, each of 45 m 2 , making a total occupied floorspace of 90 m 2 . All the external walls are made of double hollow bricks with a thermal insulator layer in between. The windows are double glazed with air space separation (Climalit ® ).

Construction elements are the typical in Spanish contemporary buildings and their properties are well known, so that obtaining the housing cooling load is straightforward.

The Spanish regulations [17] allow calculation of a harmonized value of w U for each house wall, floor and roof. Outdoor air temperature and solar radiation over the four oriented walls follow experimental data. The solar radiation received on the roof was not considered because its net value is negligible as the STC are installed there.

Sunny wall temperature is different from air temperature accordingly to [START_REF] Ojer | Manual de climatización Tomo II Cargas Térmicas[END_REF], so that:

w w w w a in e I Q A U T T h α   ⋅ = ⋅ ⋅ + -     (7)
α depends on wall colour and finishing, in this case the value considered was 0.6.

A constant, homogeneous indoor air temperature of 24 ºC has been applied to cooling load calculations, according to [START_REF][END_REF].

Floor heat transfer coefficient depends only of the walls thermal insulator thickness, according to [17]:

f insulator insulator f A L e U ⋅ ⋅ = λ (8)
The calculation for the external wall overall heat transfer coefficient is as follows:

1 1 1 w i i e i i U e h h λ = + + ∑ (9)
Repeating the process for every external wall, an overall heat transfer coefficient U can be obtained for the whole enclosure. The total cooling load can be computed depending on the temperature difference:

( )

a in T T A U Q - ⋅ ⋅ = (10)
There are several additional cooling loads, such as four people sensible and latent heat (about 100 W per person) cooling load from house lighting, ventilation load (amount equal to one air change per hour) and infiltration, which were considered as a 10% of total cooling load.

The calculated average value for the overall house U•A was 360 W/ºC.

As an example, the results of daily cooling load evolution corresponding to July 28 th are shown in Figure 6.

According to [START_REF]Atlas de la demanda eléctrica Española. Proyecto INDEL[END_REF], the typical AC demand in Spain is centred between 10:00 and 22:00 hours. Keeping only this frame of time from the instantaneous cooling load, the daily cooling demand can be obtained, as shown also in Figure 6.

By integration of this instantaneous cooling demand throughout the day, a daily cooling demand has been obtained for each day of the season, giving the results shown in Figure 7.

Cooling demand increases rapidly during the first summer months from June to July, but since August it slowly decreases to end of season, October. Weather data of other years result in a different behaviour but research demonstrated that this one is fairly representative.

One important parameter is the fraction of AC demand that is supplied with SE; it is called solar fraction:

cool demand Q f Q = (11)
Figure 8 shows the daily values of f provided by the solar facility. As can be seen, it changes from day to day, depending on the weather conditions. The seasonal value of f obtained was 56% of the total AC demand. The remaining demand could be fulfilled using Natural Gas (NG).

ECONOMIC AND ENVIRONMENTAL COMPARISON WITH CONVENTIONAL COOLING

ACCEPTED MANUSCRIPT

In order to validate the solar cooling technology for the proposed application, a comparison with a conventional cooling system has been performed as follows, both attending to energy cost and environmental aspects. The conventional vapourcompression machine uses R407C refrigerant and operates in the same local weather conditions as the absorption machine. The seasonal COP chosen is 2.4 according to [START_REF] Apread | An evaluation of R22 substitutes performances regulating continuously the compressor refrigeration capacity[END_REF]. For the absorption machine the COP used in this section was the nominal value of 0.7. The reason is that the total cooling demand is fulfilled by powering with supplementary heating, using a NG boiler, boosting water up to 90 ºC.

Economic Impact

-Solar cooling:

Solar contribution, in terms of energy, is 1.6 MWh of the total cooling demand of 2.9

MWh. Total heat supply needed to provide the remaining demand is 1.8 MWh. NG cost is estimated at 0.034 /kWh according to [22], summing up a total expense of 62 .

-Conventional cooling:

Supplying the total AC demand with a conventional vapour-compression machine electric energy needed reaches a value of 1.2 MWh. Total expense, estimating 0.135 /kWhe according to [23], amounts 165 .

Comparing vapour-compression machine vs. absorption machine, energy cost saving reaches 62%, i.e. 103 for the overall summer season.

Facility Cost

Sustainability using RE is an obvious concept. But reality is that up-front investment is high, so the payback time is very long, according to [START_REF] Mercier | Results of the Public Consultation on the EU Initiative on heating and cooling from Renewable Energy Sources[END_REF], due to the high price of the main components of the facility. A previously performed research [START_REF] Aumente | Home Solar Absorption-Based Air Conditioning in Spain[END_REF] indicates that it results in longer than the operational life of some of the components, using current prices and no subsidies to RE. This shows the need of developments that introduce an economy of scale in manufacturing, marketing and distribution to make this technology competitive with fossil fuels, in agreement with [START_REF] Tsoutsos | Solar cooling technologies in Greece. An economic viability analysis[END_REF].

There is a effect of a future increase in fossil fuel cost and in addition to this; there are government incentives and subsidies to RE relying on the principle of compensating their lower external costs. And, still to come, the Kyoto agreement application date 2008-2012 and beyond will imply cost savings as a consequence of carbon emission reduction.

Environmental Impact

-Solar cooling:

For summer 2004 total NG consumption to supply the remaining demand, estimating 0.22 kg/kWh of CO 2, according to [START_REF]Plan de Energías Renovables en España 2005-2010[END_REF], yields a total emission of 400 kg of CO 2 .

-Conventional cooling:

According to the Spanish National Energy Balance data, there is 0.52 kg of CO 2 emitted for each kWh of electric energy produced in the Spanish electric network during a representative year. This gives that CO 2 emission would be around 632 kg of CO 2 to supply the house cooling demand with current technology.

As a result, the savings achieved by the use of absorption are 62% for NG consumption and 36% for CO 2 .

SENSITIVITY TO f AND TO COP.

This section presents the study that has been performed to obtain the reasonable figure The conventional vapour-compression machine driven with electricity mentioned in section 5 is used as a reference for this study. The NG consumption in order to complete the fraction of cooling demand not supplied with SE has been calculated. The overall primary energy needed and the CO 2 emitted have been compared with the ones that will be generated with the vapour compression machine that supplies the total cooling demand.

Figure 9 shows results from this study. A value of COP equal to 0.7 is assumed, considering that machine is working under nominal operating conditions. Cooling fraction determines the relative size of the solar facility for the specific geographical location of Madrid.

As it can be seen, the total primary energy, the total energy cost and the CO 2 emission savings are always positive and considerable, even if the cooling fraction is reduced to a minimum value.

Figure 10 shows the results that would be obtained throughout the season, where the seasonal COP value was 0.33, as experimentally obtained in the facility. Results in this case change considerably. Total primary energy and energy cost savings together with CO 2 emission reduction become negative when the renewable fraction decreases under a threshold value.

In order to better describe the sensitivity of savings to COP, keeping f fixed seems convenient. Figure 11 shows the sensitivities to absorption machine COP for a reasonable f of 56%. It indicates that reaching a high enough COP value is of paramount importance.

As a conclusion to this study, the best choice seems to select the adequate size of the absorption machine, to make it work at nominal operating conditions, so that both COP reaches the nominal value and f , consequently, is higher. The figures presented validate the size of the solar cooling system selected for the application under study.

CONCLUSIONS

• An experimental facility with 50 m 2 FPSTC, has been developed and fully monitored for housing AC applications using LiBr/H 2 O absorption technology.

• During 2004 summer season it has produced a daily average cooling power of 3 kW for 6.5 hours.

• A low efficiency from solar to cooling (SCOP) has been obtained. Further work will reduce facility thermal losses and increase absorption cooling machine COP. This can be achieved by selection of machine size and control strategy, so that it operates more time at the suitable hot water temperature and full load conditions.

• Instantaneous cooling load and demand of a single detached Spanish housing have been calculated using local environmental data. These data and the experimental results of the solar facility indicate that 50m 2 of collectors are necessary to provide this representative housing with 56% cooling demand, using only solar energy.

• Storage-tank volume of 40 l/m 2 collector area seems to be satisfactory according to experimental results. It allows adjusting daily cooling period to the housing AC demand, especially later in the afternoon. Non stratified storage tank configuration resulted as the best choice.

• Satisfying 100% of the housing AC demand, (56% by SE and 44% with a NG boiler), the energy cost would be 62% and the CO 2 -emission savings would be 36% respectively, according to the experimental results obtained.

pA:

  Solar collectors' effective area [m 2 ].

wA:

  House outdoor wall area [m 2 ].

fA:

  First floorspace area [m 2 ].

wc:

  Water specific heat [J/ (kg• ºC)]. COP : Absorption machine coefficient of performance [].

  i e : i component wall thickness[m]. f : Solar fraction [].

eh:

  External side of wall convective heat transfer coefficient [W/(m 2 •ºC)].

ih:

  Internal side of wall convective heat transfer coefficient [W/(m 2 •ºC)].

wI:.:

  Instantaneous solar radiation on the outside wall tilted surface [kW/m 2 ]. I : Instantaneous solar radiation on tilted surface (40º) [kW/m 2 ]. T ∆ : Instantaneous outdoor to indoor air temperature difference [ºC]. L: floor perimeter [m] cw m Cooled water mass flow rate [kg/s].

.

wtm:

  Tank to generator water mass flow rate [kg/s]. w Q : Thermal load due to solar radiation over a sunny wall surface [kW]. Q : Instantaneous house cooling load [kW].

coolQ:

  Instantaneous cooling power [kW]. demand Q : Instantaneous house cooling demand [kW]. generator Q : Instantaneous heat power driving the absorption machine generator [kW].solar Q : Instantaneous solar power available on collectors surface [kW]. SCOP : Solar Coefficient of Performance []. a T : Instantaneous outdoor air temperature [ºC]. icw T : Instantaneous cooled water absorption machine inlet temperature [ºC]. in T : Instantaneous indoor air temperature [ºC]. ocw T : Instantaneous cooled water absorption machine outlet temperature [ºC].wgi T : Instantaneous water generator inlet temperature [ºC]. wgo T : Instantaneous water generator outlet temperature [ºC]. U : Overall heat transfer coefficient [W/( m 2 •ºC)]. w U : External wall overall heat transfer coefficient [W/(m 2 •ºC)].f U : Floor overall heat transfer coefficient [W/(m 2 •ºC)].

  1. It has been designed to operate in a flexible way, so that different applications can be reproduced. The facility includes 20 Vitosol 100 FPSTC produced by Viessmann Werke GmbH and Co KG ® . Water is pumped from collectors to two Alfa Laval Corporate AB ® plate heat-exchangers in a closed loop. They are needed because the water flowing through collectors incorporates antifreeze fluid for winter operating purposes. Water is pumped from the 2,000 l capacity storage tank through the other side of the heat-exchangerswhere it is heated and sent to the absorption machine as the driving heat input to the generator. This water returns to the storage-tank to be heated again. The absorption machine is a Yazaki ® pumpless WFC10 model. The condenser and the absorber are cooled by water, using an open cooling-tower. The water chilled at the evaporator is pumped to fan-coils to gain the desired output of cool air inside the AC zone. The experimental setup delivers all the cold water that is capable of.

  for the share of the absorption machine in the AC cooling demand, by varying the size of the solar system. It has been carried out by taking into account the experimentally obtained mean values of the three studied parameters: COP, primary energy and CO 2 emission savings. In modifying f and COP the same calculations of the previous section are repeated. Results are shown in Figures 9 to 11

  .

Figure 1 :

 1 Figure 1: Configuration of the solar facility at UC3M.

Figure 2 :

 2 Figure 2: Instantaneous thermal power and outdoor temperature corresponding to July 28 th 2004.

Figure 3 : 7 -

 37 Figure 3: External hot water inlet to the absorption machine generator and external

Figure 4 :

 4 Figure 4: Daily solar energy Production for 2004 summer season. The daily mean of

Figure 5 :

 5 Figure 5: Daily average efficiency parameters of the solar cooling system. The daily

Figure 6 :

 6 Figure 6: Numerically calculated total instantaneous cooling load and demand period

Figure 7 :

 7 Figure 7: Numerically calculated daily cooling demand of a representative single

Figure 8 :

 8 Figure 8: Daily solar-fraction of AC provided by the solar facility corresponding to

Figure 9 :

 9 Figure 9: Sensitivity of the economic and environmental impacts to the solar fraction

Figure 10 :

 10 Figure 10: Sensitivity the economic and environmental impacts to the solar fraction

& & & &

ACKNOWLEDGMENTS

This work has been partially funded by the Spanish government research grant M. C. y T. CLIMABCAR project DPI 2003-01567, ENE 2005-08255-CO2-01 and MINICOM project (FIT 0204-2004-68 and FIT 020100-2003-233), Project UC3M-IME-05-011 by the Local Government of Madrid. Their contribution is greatly appreciated.

Renewable energy. SC: Solar Collectors. SE: Solar Energy. STC: Solar Thermal Collectors.


Vollrad Kuhn
email: kuhn@berliner-e-agentur.de

Jiří Klemeš
email: klemes@cpi.uni-pannon.hu

Igor Bulatov
email: i.bulatov@manchester.ac.uk

Igor Klemeš

Bulatov

Klemeš

MicroCHP: Overview of Selected Technologies, Products and Field Test Results

Keywords: Technologies, Products and Field Test Results Micro CHP, Stirling engine, field tests, market implementation

come   L'archive ouverte pluridisciplinaire

Introduction

Decentralized power generation combined with heat supply (CHP) is an important technology for improving energy efficiency, security of energy supply and reduction of CO 2 emissions. The EU and national governments encourage microCHP systems deployment to able meet international and domestic targets on carbon emissions. The UK government has lowered VAT from 17.5% to 5% for households that install microCHP systems and set out the targets for households to reduce carbon emissions by 60% by 2050. The Dutch government is backing microCHP deployment with similar initiatives and made public funding available for companies developing mass-market CHP systems. By 2050 microCHP systems could provide 30-40 % of the UK electricity needs [1] The larger and mini systems, for industrial application and for use in small organisations such as hospitals, schools and community centres or grouped households, have got proven track records. The microCHP systems applicable to domestic users in individual households still need further development to be widely acceptable. MicroCHPs are especially interesting for market of SFH (small family houses) and MFH (medium family houses), smaller buildings and SME (small and medium scale enterprises) due to their technical and performance features: • A high overall energy conversion efficiency (e.g. in excess of 90% for Stirling engines) • Low maintenance requirements equivalent to a domestic gas boiler • Very low noise and vibration levels for installation at home • Very low emissions of NO x , CO x , SO x and particulates Early adoption of CHP by utilities and manufacturers could lead to sales and service contracts worth over £1.5bn/a ( 2.2bn) across Europe by 2010 [1]. Across the EU there is a potential for up to 50 million installations; with the key markets being the UK, the Netherlands, Germany, France, Italy, Belgium, Demark and Ireland. Manufacturers are already actively marketing systems in the UK, the Netherlands and Germany [START_REF]COGEN[END_REF]. There are a few assessments of microCHP impact on the environment, see eg Fig. 1.

FIGURE 1 HERE

Only practical tests and field trials with statistically meaningful sets of data can give A proper understanding and prediction of how efficient these technologies are at present.

Methods

The paper benefits several sources:

• The German experience in Stirling machines and field tests of a special Stirling engine (power range 2 to 9.5kWel) of a German manufacturer, • Field tests of a Stirling engine (about 1kWel) of a manufacturer from New Zealand in France and the Netherlands • The UK field tests of a number of CHPs.

The market studies (Berger [4]) and the description of experiences with field tests in Germany (Forster [START_REF] Forster | Ideal solutions with warm-up[END_REF]) and UK (The Carbon Trust [START_REF] Carbon | The Carbon Trust's Small-Scale CHP field trial update[END_REF]) were evaluated to get comprehensive information for this study. They focused on producers and conversion techniques with a high development status. These products are close to market introduction and either undergoes EC certification procedures or have been already matured. They are expected to enter the market soon.

The study used the following classification:

• Stirling engines,

• Hot air engines • Combustion engines.

Fuel cells and micro gas turbines were not considered because of their problematic application in the small scale housing sector.

An output temperature close to 100 o C should be designed for a microCHP system as the minimum required temperatures in buildings may vary between 40 o C and 80 o C. There are several technologies in the micro scale that can be used for CHP generation:

(i) Reciprocating engines, (ii) Micro turbines (electric power below 250 kW), (iii) Stirling engines, (iv) Fuel cells.

(i) Internal combustion -Reciprocating engines

A typical reciprocating engine (diesel, gas, multiple fuel) and a generator linked to the engine are quite efficient in producing electricity, have a large power range and choice of fuels. The applicability of gas engines is best in back-up systems. Diesel engines are recommended for continuous use. Their drawback is relatively noisy operation. This may render them unattractive for residential applications. The moving parts require regular maintenance which further contributes to the Carbon Footprint (CFP) [START_REF] Perry | Integrating renewable energy sources into energy systems for the reduction of carbon footprints of buildings and building complexes, PRES'07, Ischia[END_REF] The type of fuel used affects CO 2 and SO 2 emissions. CHP based on reciprocating engines are more applicable to larger buildings with less peaked electricity and heat consumption profiles [START_REF] Alanne | Sustainable small-scale CHP technologies for buildings: the basis for multi-perspective decision-making[END_REF]. The most efficient performance can be achieved by the proper selection of the size of the internal combustion engine, the capacity of thermal and electrical storage systems and the operation scenario on the energy performance of the entire micro-CHP system [START_REF] Onovwionaa | Modeling of internal combustion engine based cogeneration systems for residential applications[END_REF].

(ii) CHPs using microturbines use gas turbines with electrical power generation from 25 to 250 kW. Such a plant generally consists of a generator, a compressor, a combustion chamber, and a turbine connected to each by a shaft. The high exhaust gas outlet temperature (450-550 o C) enables considerable heat generation. Among other advantages are their low noise levels, small size and lower emissions level (especially NO x ) compared with reciprocating engines [START_REF] Soares | Microturbines Applications for Distributed Energy Systems[END_REF]. Gas and liquid fuels are suitable for micro-turbines. Their low electrical efficiency, especially on part load, capital and maintenance costs are rather high. CHPs with microturbines are most applicable for steam production and similar applications. However they are not applicable for residential purposes because they are expensive and inflexible to load changes.

(iii)External combustion -Stirling engines

The Stirling Engine is a reciprocating engine with its cylinder closed and combustion taking place outside of the cylinder. Stirling Engines are characterised by rather low emissions (especially NO x ) and lower noise levels. External combustion also requires less maintenance which favourably influences the carbon footprint of the technology. The Stirling Engine is usually quiet because the combustion is not explosive and, it can use almost any combustible fuel and any source of heat, including biomass. This type of CHP has rather low electrical efficiency, about 25-30 % when natural gas is used as a fuel. When solid fuels (e.g. biomass) are used, the efficiency can be as low as 15 %. The total efficiency is not significantly lower than that of other CHP applications. Stirling engines are applicable to residential buildings, because the electricity/heat ratio. Their low efficiency supports their use as backup power supplies rather in continuous use [START_REF] Peacock | Impact of micro-CHP systems on domestic sector CO 2 emissions[END_REF].

(iv) Fuel Cells (FC)

A FC produces electricity electrochemically, by combining hydrogen and atmospheric oxygen. If the fuel is not available as pure hydrogen, it can be released from various fuels by means of a reformation process. The electrical efficiency of these systems can be as high as 45-55% [START_REF] Alanne | Sustainable small-scale CHP technologies for buildings: the basis for multi-perspective decision-making[END_REF]. If pure hydrogen is used, the only emission is water. If reformation is used, CO 2 and a minimal amount of oxides of sulphur and nitrogen are formed, depending on the fuel. Other benefits are noiselessness, reliability, modularity, and rapid adaptability to load changes.

According to assessment made by [START_REF] Peacock | Impact of micro-CHP systems on domestic sector CO 2 emissions[END_REF] of the carbon footprint of the Stirling engine and FCs applied for a single dwelling in Central England, the Stirling engine yields daily savings of 2.5+ kg CO 2 in winter and less than 1 kg CO 2 in summer. The reduced thermal output of the 1 kW FC system causes significantly less seasonal variation and yields daily savings of 3+ kg CO 2 in winter and 2+ kg CO 2 in summer. Micro-CHP systems can offer considerable CO 2 emissions reduction compared to a condensing boiler and network electricity. The promising advantages of FCs still remain as potential benefits because of the current stage of the technology development and the problems it faces:

• The important drawback is the investment cost. There is a very strong competition in the power generation market, and the costs of generation as well as risk and availability are the key factors in technology selection. So far, FC cannot compete with a modern conventional generating plant [START_REF] Knowles | Micro Energy Systems: Review of Technology, Issues of Scale and Integration[END_REF]. Costs should be driven down through technology development and economies of scale. • The current economy is heavily carbon-based, so the inertia has to be taken into account which presumes that shift to FC en masse will not happen fast until the technology is developed sufficiently to force change.

• FC are more demanding in respect of fuel production, storage, and transportation.

Therefore, a serious barrier to the wider spread of FC is poorly developed hydrogen infrastructure, and on the other hand, a hydrogen infrastructure will not develop until there is a sufficiently large adoption of hydrogen technologies.

Results

German market study

Within the market study for the market launch of microCHP between 1 to 5kWel (Berger [START_REF] Berger | Market analysis for Micro CHP-application of Micro CHP in the power range between 1-5kWel[END_REF]) 15 relevant microCHP products were identified and analyzed. To study feasibility and application of micro-cogeneration the following applications were chosen: 1. Lion Powerblock (free-piston steam engine) 2. WhisperGen MicroCHP (Stirling engine) 3. Microgen M-CHP (Stirling engine) 4. Ecowill (combustion engine) 5. Dachs (combustion engine) 6. Ecopower Mini-CHP (combustion engine)

Table 1 provides selected technical data of the chosen cogeneration products. Subsequent to the study the profitability was analysed considering a cost benefit calculation of the product application in single consumption mode different buildings in Germany, as SHF, semidetached houses, and MFH as well as in commercial-objects. The analysis included the viewpoint of different client-groups and manifested enormous differences between them. They were in most of the cases negative, a larger number of the case studies was analysed as cost inefficient. The electricity generation costs varied in a range from 38.0 to 9.8 Ct/kWh.

TABLE 1 HERE

But some of the CHP-products are already cost efficient regarding module costs and energy prices. The results for SFH vary from extra costs of 520 /y to cost minimisations of 255 /y. An essential criteria for the choice of a product is besides a high "number of hours in full usage" (equal total generated electricity p.a./max. electrical output capacity) the technical implementation of the monovalent operation mode. The "number of hours in full usage" differs according to client group and CHP product between 1,250 to 8,000 h. The small commercial companies could not attain a satisfying result of the chosen CHP technique despite optimal disposal structures. Reason for that are the low energy and electricity costs in small-scale companies.

For Stirling engines natural gas-fired WhisperGen microCHP and natural gas-fired Microgen microCHP savings of 10% of energy costs can be reached in SFH. Because of the less heat consumption (50%) in new build SFHs the feasibility of microCHP is much lower than in older buildings. For a newly-build SFH the Stirling engine is at its feasibility limit. The free-piston-steam engine natural gas-fired Lion Powerblock is the least cost efficient with electricity costs of 38 Ct/kWh, which derogates the yearly energy costs. All the figures are summarized in Table 2. In the variation of investment costs and energy price development it is clearly to see, that ample changing of the economic frame conditions are necessary to have cost efficient microCHP application. A good example are the better conditions for the feed-in tariffs for electricity which is produced by CHP applications with the use of bio-fuel in Germany.

Demand site the application of microCHP was estimated in a potential and scenario analysis for Berlin with the conclusion that the demand for microCHP will raise to 8,000 to 15,000 installations p.a. till 2030. This increase results from potential substitution and exchange of gas supplied objects during refurbishments and newly build houses including changes of energy used e.g. gas fired apartment heating. Table 3 shows the annual potential of microCHP in Berlin by different client groups with an average life span 20 years.

TABLE 2 HERE TABLE 3 HERE

Taking into consideration these figures as well as the estimations of a Berlin gas utility company the CO 2 reduction potential is considerable. Up to 200,000 t/a of CO 2 can be reduced by the application of microCHP.

Conclusion of the study is, that under certain consumption structures micro-CHP turned out to be a comprehensive solution for energy supply. The feasibility is dependent on the operational mode and engine application according to the demand structure. Single task oriented products have a better feasibility, as modules designed with an additional "back-up" like a boiler or a gas fired thermae. For the launch to a mass market a lot of work in the technology development is still to be done. This lack could be overcome by the support of energy utilities.

German field tests with Stirling engines (Forster [5])

The manufacturer SOLO Stirling GmbH in Sindelfingen had started a serial production of a gas fired Stirling 161 engine in 2004. Over 120 units were sold. Unfortunately the manufactures had to close the company at in the beginning of 2007 because of funding problems. The new owner from Switzerland absorbed the technology rights, patents and assets and is preparing the full commercial launch of the Stirling 161 together with an own-developed 1kW Stirling module. Whereas currently natural gas is used as fuel, pellets might be used in the near future.

Berlin

One of the Stirling 161 engines was tested by the Berlin gas utility GASAG. A first Stirling CHP unit is used to power a state public institution building -the Kreuzberg fire station in the district Friedrichshain-Kreuzberg of Berlin. They have been checking the capability and client acceptance in a three years test (since November 2005). The first engines from the serial production run well, yet with some small problems. The electrical output has been underachieved, and a frequent faults lead to higher maintenance costs. Overall the gas distribution company is positive as the Stirling is a very efficient way to convert energy. It is a well-engineered technology on the way of market implementation, in comparison to fuel cells. This Stirling 161 (2 to 9.5kWel; 8 to 26kWth) was integrated in the existing heating system of the building. It is parallel spliced with the central heating boiler.

When it was no requirement for the heat the boiler was not used. The whole Stirling CHP unit can be taken offline with an automatic powered shut-off valve. As this fire station has a constant demand of energy and heat it is perfectly suitable for the test-run. The Stirling 161 can be operated at a full load about 5,800 h and part-load 7,800 h. The feed temperature 68° C in the heating circulation is judged as less efficient which reduces the electric power. The producer recommends a maximum feed temperature of 65 °C. After to serial production in 2004 several problems with material occurred. The company Solo investigated the reason for the defect of the connection rod. Meantime a new method was developed for precise measurement and evaluation of tolerances. There was also a change of a subcontractor and the piston rod is available with the requested quality and has been be refitted. The technical and performance data are summarised in Tab 4.

Fürth

Another testing run of Solo production Stirling 161 was proceeding in Fürth. In cooperation with the energy agency EAM and a local district heating provider a station was chosen, which is supplying heating for a residential area (79 buildings). For the combination of the district heating station (max. 4.5MWh) and the Stirling CHP unit, various modes can be measured, without having an impact on the supply of the client. Solo Stirling achieved a degree of efficiency of over 90 % and its optimum in part-load. Because of the high feed temperature from the district heating station the pre-conditions can be considered as suboptimal. Because of suboptimal conditions of the heating system the forecasted degree of efficiency could not be complied. Nevertheless the EAMs appraisal is positive stating that fundamental problems did not occurred but the testing of a larger number of Stirling engines is needed.

An EAM project manager has been questioning if the malfunctions like the defect of the connection rod were system systematic or occasional. The Solo maintains that the major part of the 120 Stirling engines runs without any problems.

Ditzingen-Hirschlanden

An ESCO (part of a large French utility) reports just satisfactory results using the Stirling 161. In the Swabian town of Hirschlanden a Stirling CHP unit integrated in a district heating station is supplying with heat 100 SFHs, a home for old people and a nursing home. The electricity produced by the Stirling CHP unit is predominantly used to provide the station supply of the boiler house.

The correlation between the high feed temperature and the derogation of electric efficiency has occurred also in this test-run. At a feed temperature of 60 °C an electric power of 6.5kWel is measured in Ditzingen-Hirschlanden. The total efficiency amounts up to 79 %. Because of the external combustion the interior of the Stirling engine remains free of residues. The pollutant emissions are much lower than the emission from the other engines.

The ESCO specified the following disadvantages:

• A limited operation experiences,

• A lack of a mass production • A still high specific investment costs.

The technical potentials are still not taped fully. The service and support should be optimized. If the positive experiences will proceed, further projects with the Stirling will be encouraged.

Other locations

In

UK market studies

The UK market for micro CHP is assessed to be about 400,000 households (Carbon Trust [6]) within ca. 1 million households and SMEs which displace boilers per annum (Envocare [15]). Unlike mini CHP (which is regarded as a mature technology and have been applied for quite a long time), micro CHP technology is not yet available commercially and there are rather few data about its performance in UK. Mini systems used in industrial and commercial applications show advantages of their performance for quite a long time. On the other hand, the direct projection of mini-CHP experience to micro-CHP level turns counterproductive due to extra complexity at the micro level and lesser tried technologies (for example fuel cell technology) which still needs to get mature.

Overall CHP are intrinsically more efficient in comparison with other generating facilities and the grid with its 30-40% efficiency (Ecpower [16]) Potentially micro-CHP can have the competitive edge over condensing boilers, but still considerable work has to be done to implement this into real economic and environmental benefits.

The latest research shows that microCHP have to improve their performance to become widely accepted in the market. Since the potential market is quite large in terms of units, there are good incentives for improvements. By 2006, not much progress was made in microCHP penetration into the market. There are still very few micro-systems available, the public, architects and civil engineering community en mass are not much familiar with the technology.

Another factor of a rather poor microCHP market penetration was a successful campaign to promote energy-saving condensing boilers.

The UK government in 2001 stated that CHP was one of important ways to implement the Kyoto commitments. The UK target for installations set for the end of the year 2000 was 5,000 MWel and by 2010 a further target of 12,000 to 19,000MWel was set (Envocare [15]). A significant result of this trial is that the first two performance indicators differ very significantly for micro and small CHPs. The microCHP units assessed show very different performance characteristics in different environments. The carbon footprint and savings being important characteristics, the capital costs needed to be evaluated as well. So far the units are not manufactured at the scale sufficient enough to a give a final conclusion on the capital cost. The preliminary results of the Carbon Trust trial indicate that the performance of microCHPs was not as impressive as expected. Some of the reasons are:

UK Carbon Trust field tests

• Actual efficiencies of was lower than assumed by existing models • The amount of electricity generated is much lower than the forecast

The poorer than expected efficiency is caused by the current stage of the design and operation. The thermal inertia of the microCHP units seems to be still too high compared with the conventional boilers for a fluctuating demand for heat in buildings.

In many houses the demand is to a large degree irregular and the heat is required only for short periods but many times a day. The repeated warm-ups cause waste of energy which is not re-released in a useful way which reduces the efficiency. This makes a vast scope for improvements focused on reduction of thermal inertia and/or number of cycles. Another problem for microCHP performance is the assumption of constant average electricity demand.

The trial has proved that for the most of the time demand is much lower than average and also lower than typical micro-CHP output. This is also combined with short periods of very high demand (when some appliances are running, e.g. kettles, electric showers, etc). So far the significance of peaks and troughs has been neglected by the designers.

With new design of low voltage networks which could cope with high levels of export, in addition to full load import when the units are not running, the microCHP industry needs to design units with the ability to modulate electrical output much more widely than currently. Summarising the preliminary test the Carbon Trust indicates that at the current stage of technology microCHP have limited contribution to CO 2 reductions and considerable improvements are required (Fig. 5).

FIGURE 5 HERE

Boatie et al [START_REF] Boait | Optimisation of consumer benefits from microCombined Heat and Power[END_REF] show that the financial benefit to consumers from microCHP is sensitive not only to the thermal properties of dwellings, but also to lifestyle and consequent choice of heating time and temperature settings. Because financial benefit is closely tied to local consumption of the electricity generated, the carbon mitigation benefits -to the extent that they are improved by reduction of national peak demandhave the same sensitivity. They also show that import and export profiles will vary substantially between households with microCHP.

Field tests with Stirling engines in France and the Netherlands DEO Domestic Energy Optimisation

A DGTREN funded project DEO (Priaulx [START_REF] Priaulx | Domestic Energy Optimisation[END_REF]) compiled a comparative study on innovative technologies of power generation in buildings, analysing pilot projects in 2001-2003. Partners were several European gas utilities. The study focused on the application of innovative gas technologies as micro-cogeneration, gas heat pumps; gas fired household appliances and combination-systems e.g. with solar heat.

The DEO project studied SFHs or apartments in for the period of a year. To compare the energy saving potential, other objects without the innovative technology were taken into consideration. MicroCHP units were installed by leading gas companies in France, Gaz France and the Netherlands, Gasunie. A WhisperGen module powered a test-run apartment in France and a SFH in the Netherlands.

In the Dutch test the microCHP unit was supported by applications of other innovative energy saving technologies, as gas heat pumps, gas fired household appliance and electronic thermostatic valves etc.

The French demonstration-run achieved satisfying results. No operational malfunctions appeared. The specific performance of the heat supply met the producer data. The electricity was close to the predicted performance 82 % efficiency (75 % thermal and 7 % electric). During the monitored test 55 % of the power demand was satisfied. The average value for the available electrical power was 0.2 kW considered over the year. Savings 13% of primary energy (gas) and 7% of final energy (electricity, heat) were achieved in comparison to a reference building.

The Dutch test example was in contrast disappointing. Several problems were observed in operation and with the start/stop mode and noise-emissions. The operation time was limited. Only 278kWh were generated overall by the micro-CHP -just 6% of the energy demand. By combination of actions savings of 13% primary and 12% end use energy were achieved.

Discussion and Conclusions

MicroCHP is already a maturing and comprehensive solution for energy supply for certain consumers according to the present market study (Berger [4]). In certain consumption structures micro CHP is already a comprehensive solution for energy supply, according to the present market study. As shown in the Berlin study potential customers for microCHP units exist and their attitude is in principle positive. The market introduction of microCHP has to be supported actively with the help of energy utilities, research institutes, producers and networks.

The installation and service networks are supposed to take a key function in designing a microCHP service package in the phase of market launch. The lack of these networks hindered the application of microCHP installations in the UK and in the Netherlands.

Supply side the design of a service package is promising due to the fact that the product microCHP installations ties services and binds the customer to a long-term usage.

Regarding the political framework it is to mention that legal regulations for power supply support an accelerated market implementation for CHP technologies, as in Germany the KWK ModG.

The need of further stimulation is the outcome of tests (Forster [5], Carbon Trust [START_REF] Carbon | The Carbon Trust's Small-Scale CHP field trial update[END_REF]).

The conclusions are that with positive experiences of microCHP and the elimination of the deficiencies a mass production and full market implementation would start in the future.

Stirling engines are characterised by low electrical efficiency and high overall efficiency. They are typically designed to operate in a heat led mode where they dispatch at full output capacity for any period where there is a space heating demand. Minimum up-time is typically about 1 h. Stirling engines usually have a set of predetermined operating points [START_REF] Whispergen | User manual: Model PPS24-ACLG[END_REF]. Eg a 1.1kWe system could be controlled to dispatch at 500, 900, and 1,100 We. A warm Stirling engine is able to change electrical output level almost instantly. On-off operation according to a predefined program is possible [START_REF] Hawkes | Cost-effective operating strategy for residential microcombined heat and power[END_REF].

Gas engines have higher electrical efficiency than Stirling engines. Minimum start up is similar to a Stirling engine These systems are also typically driven by heat demand [START_REF] Hawkes | Cost-effective operating strategy for residential microcombined heat and power[END_REF].

Gas engines appear to be more inclined to ''follow'' an electrical load, and can ramp up and down rapidly. On-off operation according to a predefined program is possible.

FC have high electrical efficiency. They may benefit from avoidance of thermal cycling which creates stress on component parts and could reduce the lifetime of the stack, and are more inclined to operate continuously. Once operating they are able to respond rapidly to load fluctuations. No statistically meaningful field trials have been carried out regarding this type of technology.

A valuable idea for this review extension would be to include beside field test also related software and its implementation as eg Ameli at al [START_REF] Ameli | Integrated distributed energy evaluation software (IDEAS): Simulation of a micro-turbine based CHP system[END_REF]. The issues covering the trigeneration would be beneficial as well, see Klemeš and Friedler [START_REF] Klemeš | Recent novel developments in heat integration-total site, trigeneration, utility systems and cost-effective decarbonisation. Case studies waste thermal processing, pulp and paper and fuel cells[END_REF], Hennian at al [START_REF] Henning | Micro tri-generation system for indoor air conditioning in the Mediterranean climate[END_REF]. They are obviously more test and pilots needed covering more countries' specific conditions and emerging products from various manufacturers [START_REF] Klemeš | Heat integration, energy management, CO 2 capture and heat transfer enhancement[END_REF] and renewables related options [START_REF] Pavlas | Integrating renewable sources of energy into an existing combined heat and power system[END_REF]. This contribution is targeting to raise this point and encourage more overviews for exchanging data and experiences in the filed.

Acknowledgement

The support from an EC TREN/05/FP6EN/S07.54455/020114 "Network for promotion of Eco-building technologies, small Polygeneration, and renewable heating and cooling technologies for buildings -ProEcoPolyNet" (PEP-Net) has been gratefully acknowledged.

Nomenclature

  Fig 2 visualises the influence of flow temperature on performance and efficiency related to the load (machine pressure) of the SOLO Stirling 161 engine.

  Carbon Trust launched a trial in 2003 aimed to a comprehensive analysis of the current situation and development of recommendations to overcome the barriers that impede the implementation of this technology. The trial is expected to finish by mid/late -2007 and results will be published. Only preliminary data have been available so far. The technologies assessed were Stirling engines, Organic Rankine Cycle Machines, Fuel Cells and Internal Combustion Engines. In total about 40 units (of those 31 microCHP, mostly in homes) have been assessed. The performance indicators recorded were: a) Overall thermodynamic efficiency, b) The amount of electricity generated the carbon intensity of the electricity displaced from the grid.

Figure 2 .

 2 Figure2. The influence of flow temperature on performance and efficiency related to the load (machine pressure) of the SOLO Stirling 161 engine[START_REF] Opet Chp | Small and micro scale CHP[END_REF]

Figure 3 .Figure 4 .Figure 5 .

 345 Figure 3. Power supply of WhisperGen, field test in SFH (Berlin/Brandenburg)[START_REF] Gasag | Experiences with field tests in Berlin and the federal state Brandenburg[END_REF]

TABLE 4

 4

	HERE
	FIGURE 2 HERE

  2006 the Berlin gas utility started the second two year test-run in a SFH and the embassy of New Zealand, where gas operated Stirling CHP units WhisperGen from a New Zealand manufacturer (1kWel; 7.5kWth) were installed. The same CHP unit was chosen by another German service company in Mannheim. Over 20 SFH started with voluntarily participation in the test. Furthermore a 2-year WhisperGen field test started in Berlin in summer 2007. Out of 1,600 candidates 20 housing and single-house units have been chosen. The Figs 3 and 4 are showcase of the degree of efficiency and the power supply of the WhisperGen filed test.

	FIGURE 3 HERE
	FIGURE 4 HERE

Table 1 :

 1 Chosen micro-CHP products, by Berger[START_REF] Berger | Market analysis for Micro CHP-application of Micro CHP in the power range between 1-5kWel[END_REF]

Table 2 :

 2 Total electricity generation costs, Berger[START_REF] Berger | Market analysis for Micro CHP-application of Micro CHP in the power range between 1-5kWel[END_REF]

Table 3 :

 3 Annual potential for microCHP (Berlin), Berger[START_REF] Berger | Market analysis for Micro CHP-application of Micro CHP in the power range between 1-5kWel[END_REF]

Table 4 :

 4 SOLO Stirling 161, by Berger[START_REF] Berger | Market analysis for Micro CHP-application of Micro CHP in the power range between 1-5kWel[END_REF]

Table 1 :

 1 Chosen micro-CHP products, by Berger[START_REF] Berger | Market analysis for Micro CHP-application of Micro CHP in the power range between 1-5kWel[END_REF]

	Technology Free-piston	Stirling	Stirling	Combustio	Combustio	Combustio
		steam	engine	engine	n engine	n engine	n engine
		engine
	Electrical	0.2 -3	1	1	1	5,5	1.3 -4.7
	output
	capacity
	(kW)
	Thermal	2 -16	12	15, 24, 36	3.25	10.3 -12.5	4 -12.5
	output
	capacity
	(kW)
	Costs/ unit	12,500	6,000	4,500	5,600	15.000	15,000
	( )
	Specific	4,170	4,500	4,090	4,170	3.272	3,829
	cost of unit
	( /kWel)

Table 2 :

 2 Total electricity generation costs, Berger[START_REF] Berger | Market analysis for Micro CHP-application of Micro CHP in the power range between 1-5kWel[END_REF]

		Microgen	WhisperGen Lion	Ecowill	Ecopower	Dachs
				Powerblock
	SFH	10.6	11.0 ct/kWh	22.9 ct/kWh 22.9	-	-
	refurbished	ct/kWh			ct/kWh
	SFH new build	16.1	17.3 ct/kWh	38.0 ct/kWh 31.9	-	-
		ct/kWh			ct/kWh
	Semi detached	9.8 ct/kWh 10.5 ct/kWh	20.9 ct/kWh 21.8	24.5	22.8
	house				ct/kWh	ct/kWh	ct/kWh
	MFH	-	9.9 ct/kWh	12.1 ct/kWh -	12.5	12.7
	refurbished					ct/kWh	ct/kWh
	Commerce	-	9.8 ct/kWh	12.2 ct/kWh -	11.2	11.7
						ct/kWh	ct/kWh

Table 3 :

 3 Annual potential for microCHP (Berlin), Berger[START_REF] Berger | Market analysis for Micro CHP-application of Micro CHP in the power range between 1-5kWel[END_REF]

	Client group	Refurbishmen	Change	Newly build	Total
		t
	Central building heating (gas-	5,700	4,000	0	9,700
	fired)
	SFH, semidetached houses	1,500	1,100	1,000	3,600
	MFH (incl. housing and	500	400	100	1,000
	commerce)
	Business/ trading companies	100	200	100	400
	Total	7,800	5,700	1,200	14,700


PhD Ben Costelloe
email: ben.costelloe@dit.ie

PhD D P Finn

Heat Transfer Correlations for Low Approach Evaporative Cooling Systems in Buildings

Keywords: Free Cooling, Low Energy Cooling, Evaporative Cooling, Cooling Towers, Chilled Ceilings, Thermal Performance, Heat Transfer

Meteorological enthalpy analysis of temperate and maritime climates above 45˚N latitude suggests that the water-side evaporative cooling technique has considerable unrealised potential with contemporary "high temperature" building cooling systemssuch as chilled ceilings. As low approach conditions are the key to fully exploiting the cooling potential of the ambient air, thermal performance at such conditions needs to be investigated. To address the research issues a test rig, based on an open cooling tower and plate heat exchanger and designed to maximize evaporative cooling potential, has been constructed. A combination of experimental measurement and analysis is used in the investigations. The thermal performance of open cooling towers, resulting from experimental research, is usually correlated, as a function of the water and air flow rate, in terms of the cooling tower coefficient, or number of transfer units (NTU) achieved. A new correlation has been developed for the low approach experimental tower, which shows a significant increase in the NTU level, at the lower water to air flow rate ratios of interest. As the cooling tower in this application is predominantly a mass transfer device in summer, the evaluation of the total volumetric heat and mass transfer coefficient (kg a .s -1 .m -3 ) is of particular interest. This coefficient has also been determined for the experimental tower and provides a key parameter for the design of this low energy form of heat dissipation in buildings.

Nomenclature

Symbol a heat transfer area per unit volume (m 2 .m -3 ) C pw specific heat of water at constant pressure (kJ.kg -1 .K -1 ) C T cooling tower constant, dimensionless h elemental rise in the air enthalpy across a cooling tower element (kJ.kg -1 ) t drop in cooling tower water temperature across a cooling tower element (K) G air flow rate in the tower (kg.s -1 ) G' air flow rate flux in the tower (kg.s -1 .m -2 ) h a enthalpy of the bulk air stream in the cooling tower (kJ.kg -1 ) h asw enthalpy of air in the cooling tower saturated at the bulk water temperature (kJ.kg -1 ) K total heat transfer coefficient (kg a .s -1 .m -2 ) Ka product of total heat transfer coefficient and heat transfer area (kg a .s -1 .m -3 ) L primary water flow rate in tower (kg.s -1 ) L' primary water flow rate flux in tower (kg.s -1 .m -2 ) n exponent for G' t water temperature in the cooling tower (˚C) T as ambient adiabatic saturation temperature (AST) (˚C) T pa primary approach temperature (PAT) (K) T pr primary loop return temperature (˚C) T ps primary loop supply temperature (˚C) T sa secondary approach temperature (SAT) (K) T sr secondary loop return temperature (˚C) T ss secondary loop supply temperature (˚C) V heat transfer volume in tower (m 3 ) x exponent for cooling tower L/G ratio in context of experimental correlations Subscript a with reference to the general air stream as adiabatic saturation asw air saturated at water temperature i with reference to an individual heat transfer element N final cooling tower heat transfer element pa primary approach pr primary return ps primary supply sa secondary approach sr secondary return ss secondary supply T with reference to the cooling tower Abbreviations AST adiabatic saturation temperature DBT dry bulb temperature NTU number of transfer units PAT primary approach temperature SAT secondary approach temperature WBT psychrometric wet bulb temperature

Introduction

Traditionally interest in evaporative cooling, as an effective cooling technique for buildings, has focused on hotter dry latitudes [START_REF] Watt | Evaporative air conditioning handbook[END_REF], where it was seen as being mainly applicable. Up to quite recently this focus has persisted [START_REF] Bom | Evaporative Air-Conditioning -Applications for Environmentally Friendly Cooling[END_REF]. Recent work however on air-side [START_REF] Barnard | Low Energy Cooling -Annex 28 Subtask 2[END_REF] and water-side [START_REF] Costelloe | Indirect evaporative cooling potential in air-water systems in temperate climates[END_REF] evaporative cooling, has demonstrated the considerable potential of the technique in temperate and maritime European regions.

While the water-side evaporative cooling technique can be exploited with any water based building cooling system, the technique is particularly advantageous when used in conjunction with a chilled ceiling system, due to the higher cooling water temperatures (14-20˚C) which are employed and hence the higher cooling water availability levels which result.

Figure 1 shows a simplified schematic of a water side indirect evaporative cooling system, with the key operating parameters indicated. The natural governing parameter is the adiabatic saturation temperature (AST), approximated by the psychrometric wet bulb temperature (WBT) of the ambient air. With indirect systems the significant performance parameter is the secondary approach temperature (SAT) which is equal to T ss -T as . It has been shown that cooling water availability levels heavily depend on the approach conditions achieved in European locations and that SATs as low as 3 K and primary approach temperatures (PATs) as low as 1 K are technically feasible and viable [START_REF] Costelloe | Experimental energy performance of open cooling towers used under low and variable approach conditions for indirect evaporative cooling in buildings[END_REF] with contemporary cooling tower packing surface densities of 200m 2 .m -3 and low approach plate heat exchangers [START_REF] Costelloe | Thermal effectiveness characteristics of low approach indirect evaporative cooling systems in buildings[END_REF].

There are two basic approaches to this form of indirect cooling system (i) the closed wet cooling tower and (ii) the open tower with external plate heat exchanger. Each arrangement has advantages in particular circumstances and locations [START_REF] Costelloe | The design and performance of an evaporative cooling test rig for a maritime climate[END_REF]. While much research has been done on the closed tower in this application [START_REF] Facao | Thermal behaviour of closed wet cooling towers for use with chilled ceilings[END_REF][START_REF] Hasan | Performance investigation of plain circular and oval tube evaporatively cooled heat exchangers[END_REF] there is a need to investigate the thermal performance of the open tower in operating conditions well outside those encountered in refrigeration condenser heat rejection, with range and approach conditions as low as 1-4 K, cooling water temperatures of 12-20˚C and ambient conditions of < 20˚C AST. These conditions result in much smaller levels of enthalpy difference, the key driving force in the tower, and therefore smaller associated heat and mass transfer rates with, crucially, resulting higher air and water flow rates than those encountered in more conventional applications.

To address these issues an experimental research facility has been developed at the Dublin Institute of Technology and is described in detail elsewhere [START_REF] Costelloe | The design and performance of an evaporative cooling test rig for a maritime climate[END_REF]. The test rig consists of an open counter-flow cooling tower and counter-flow plate heat exchanger, both with enhanced heat transfer areas for the purpose of minimizing the approach conditions. The tower has 195m 2 of wave-form packing with a surface density of 200m 2 .m -3 , while the plate heat exchanger has a design overall heat transfer coefficient of 4691 W.m -2 .K -1 . The cooling tower has a high degree of inbuilt operating flexibility with an air and water flow rate range of 0.8-2.8 m 3 .s -1 and 0.8-2.4 L.s -1 respectively, giving a possible L/G mass flow rate ratio range of 0.25-3.0. The cooling tower fan motor is inverter controlled while the 24 kW electric cooling load heater is thyristor controlled. SATs as low as 2 K have been measured in the rig at an AST of 17˚C and 20kW heat rejected.

Theoretical analysis applied to low approach evaporative cooling

The key measure of open cooling tower performance is the cooling tower coefficient (KaV/L) or number of transfer units (NTU) achieved. The performance of open cooling towers, determined by experimental research is typically correlated in terms of the NTU level achieved as a function of the water to air flow rate ratio (the L/G ratio) as follows:

x T KaV L C L G - = (1)
In this equation C T (the cooling tower constant) and the exponent x are constant for a specific correlation and are determined from the experimental results. ASHRAE [START_REF] Finn | ASHRAE Handbook: Systems and Equipment[END_REF] gives the following general correlation for cooling towers:

0.6 KaV L L G - ∝ (2)
Bernier [START_REF] Bernier | Cooling tower performance: theory and experiments[END_REF] measured the thermal performance of an experimental, semi-industrial scale cooling tower in a laboratory along the lines described in ASHRAE [START_REF] Finn | ASHRAE Handbook: Systems and Equipment[END_REF]. The resulting correlation was: The overall heat transfer coefficient, Ka (kg a .s -1 .m -3 ) can be determined from Equation 1as NTU(L/V). The area "a" is included with K because of the difficulty of determining the area involved in isolation from the coefficient. In this application the dominant mode of heat transfer in the tower is latent as the ambient dry bulb temperature of the air in summer (16-26˚C) in temperate climates is close to the water temperature (12-20˚C) and hence there is often little heat transfer by sensible means.

While the water temperature required in this application with chilled ceilings is constant through out the year, the ambient dry bulb temperature falls in winter and hence the portion of the total heat transferred which is sensible, increases. In summer, however, the process of the air through the tower is effectively isothermal in temperate climates when cooling water is being generated for chilled ceilings (the air dry bulb temperature variation in the tower does generally not exceed 3K). This process can be contrasted with the traditional applications for cooling towers in building cooling systems, such as in water-cooled condenser heat rejection in which typically, sensible and latent heat rejection occurs in near equal measure, the air process following an approximate 45˚ diagonal on the psychrometric diagram. Hence in this particular application the correlation for Ka, the overall heat transfer coefficient, is particularly interesting and is a key parameter in the design of this form of heat dissipation in buildings.

The idea of seeing the heat and mass transfer in a cooling tower in terms of enthalpy potential is attributed to Merkel [START_REF] Merkel | VDI Forchungarbeiten[END_REF], who proposed that the total heat transfer taking place at any point in a cooling tower is proportional to the difference between the total heat of the air saturated at the water temperature at that point and the total heat of the unsaturated bulk air at that point. The determination of the NTU level is commonly undertaken by applying Merkel's equation in the following form:

1 ( ) ( ) i N i pw i asw a t KaV C L h h = = ∆ = - (5)
The technique is based on dividing the counter-flow tower into a series of horizontal heat transfer elements, each with an equal incremental drop in the water temperature (typically 0.1K). Numerical integration is used in the form of a summation of the discrete values prevailing in "N" sections of the tower (beginning at the tower known outlet water temperature and known inlet air adiabatic saturation temperature (AST)

or wet bulb temperature and working upwards). The enthalpy of the saturated air film (h asw ) at the mean water temperature (t) of the element is determined from the following equation attributed to Stoecker and Jones [START_REF] Stoecker | Refrigeration and air conditioning[END_REF] h asw = 4.7926 + 2.568t -0.029834t 2 + 0.0016657t 3 [START_REF] Costelloe | Indirect evaporative cooling potential in air-water systems in temperate climates[END_REF] In each element, the heat lost by the water is equal to the heat gained by the air, hence:

G h = LC pw t (7)
therefore, for each element;

h = (L/G)C pw t or h = (L/G)0.418 (8)
Hence as the inlet air enthalpy is known, the enthalpy of the bulk air in the tower can be determined at each element by incrementing the rise in enthalpy across the element for a specific L/G ratio. It follows therefore that the NTU level is a function of the L/G ratio. An example of this procedure is shown in Table 1 for an inlet water temperature 19˚C, an exit temperature of 16˚C (a condition which would be applicable to a chilled ceiling), an ambient condition of 12˚C AST and an L/G ratio of 1.0. It is seen that the NTU level on the basis of this analysis is 1.43. The analysis is based on dividing the counter-flow tower into 30 horizontal elements each with drop of 0.1 K in the water temperature.

Required NTU analysis in this application

Cooling towers can operate over a wide range of water flow rates, air flow rates, and heat load rejection rates, with variation in the primary approach temperature. A distinction can be drawn between the achieved (or available) NTU level and the required NTU level. The NTU level required is function only of the operating conditions. The NTU level achieved depends on the operating conditions and the performance of a particular tower and is obtained from the experimental results. For any particular tower the achieved NTU level must at least equal the required NTU level for the operating conditions if the tower is to perform successfully.

As the NTU level varies with the ambient AST and with the L/G ratio it can be determined, using the template contained in Table 1, for a wide range of L/G ratio and for the ambient ASTs commonly encountered in temperate climates. This analysis has been completed and the results indicated in Figure 2. for traditional applications. However for this work, in which a low approach is a key requirement, it might be expected that the NTU level would exceed 2.5. Given that the water flow rate is generally dictated by the load rejected and that a low range condition is required in low approach conditions, the tower water flow rate is relatively high in this application. If also the L/G ratio is maintained below 1.0 (i.e., G > L), the conclusion that the process inherently requires a relatively high air flow rate seems inescapable.

It is also of interest, in this context, to examine the conditions described in Figure 2 from the point of view of the approach conditions. The data contained in Figure 2 can be expressed in terms of the NTU required as a function of the approach condition achieved (see Figure 1). This is done in Figure 3 for L/G ratios ranging from 0.2 to 2.2.

It is clear from Figure 3 that the NTU level required has a low dependence on the approach condition while the approach is in excess of 4K and the and L/G ratio is at or below 1.0. If, as reasoned above, the NTU level required has a practical limit of 4.0, then the L/G level is limited to a maximum of 1.2, if the approach is not to exceed 2 K at design conditions.

When the data presented in Figure 3 is examined for an approach condition of 1-4 K the implications of the low approach condition are brought into focus as shown in Figure 4. It seems that an L/G ratio in the range 0.2 to 1.0 is, perhaps, an appropriate range in this application as it allows an approach of less than 2 K at an NTU level of 4 and makes very low approach conditions possible at more practical NTU levels of < 4.

Also on this basis if a very low approach condition of the order of 1K is to be possible, ensuring full exploitation of the ambient cooling potential, then L/G ratios of less than 0.6 would seem to be required, to achieve this.

Results and discussion of experimental tests

Tests were conducted in which the following five cooling tower operating variables were measured (i) the inlet water temperature (ii) the exit water temperature (iii) the ambient AST (iv) the water flow rate and (v) the air flow rate. These measurements enable the performance of the tower to be analysed by determining the difference in enthalpy between the saturated air film and the unsaturated air at each element of the tower in accordance with Merkel's method.

As there is some evidence from the work of Bernier [START_REF] Bernier | Cooling tower performance: theory and experiments[END_REF] that the NTU level achieved may have some slight dependence on the ambient AST and the inlet water temperature for a particular tower, the tests are more accurate when selected at similar AST levels and similar inlet water temperatures. This process was carried out for 10 selected tests. Furthermore it was established previously that the combination of a high L/G ratio (>1.0) and a high ambient AST (>14˚C) results in required NTU levels which are impractically large (> 4.0) even for this low approach application. Also it was seen that the preferable range for the L/G ratio was 0.2-1.0. Hence tests were also selected with L/G ratios in this range and as tests with primary cooling water in the range 12-20˚C are required for chilled ceilings, the tests selected are also within this range. The experimental results are shown in Table 2.

These results can also be expressed graphically, as shown in Figure 5. For the purpose of comparison also shown in Figure 5 are the results obtained when the general correlation of Kuehn et al. [START_REF] Kuehn | Thermal Environmental Engineering[END_REF] and Bernier [START_REF] Bernier | Cooling tower performance: theory and experiments[END_REF] is applied to the L/G ratios used in the tests. As indicated in Figure 5 the experimental results produced the following correlation:

77 . 0 3 . 1 - = G L L KaV (9)
The comparison with the experimental work of Bernier [START_REF] Bernier | Cooling tower performance: theory and experiments[END_REF], carried out at at WBTs of approx 16˚C and an approach condition of 5 K, is perhaps more appropriate to the current work. The comparison with Kuehn's work [START_REF] Kuehn | Thermal Environmental Engineering[END_REF] is perhaps less appropriate, as it is based on model studies of Braun [START_REF] Braun | Effectiveness models for cooling towers and cooling coils[END_REF] and introduces some simplifications.

It must be borne in mind, that established correlations in this field relate the primary objective of effective heat rejection, rather than, as in this work, a low approach and hence effective availability as the primary concern. Therefore, it is not necessarily to be expected that the correlation for this work will produce similar constants to established correlations. Nevertheless, in order to set this work in context, it is informative, to compare the measured results with those of other researchers in the general field.

The constant obtained in the experimental work (1.3) is equal to that given by Kuehn [START_REF] Kuehn | Thermal Environmental Engineering[END_REF] and less than that given by Bernier (1.42) [START_REF] Bernier | Cooling tower performance: theory and experiments[END_REF]. However the exponent, (-0.77) is significantly less than that given by either Kuehn (-0.6) or Bernier (-0.43). Hence the NTU level, at the lower L/G ratios of interest, is above that indicated by the existing correlations. This is to be expected as the experimental tower is capable of achieving exceptionally low approach conditions and as the NTU level achieved rises as the approach falls, a tower with a low design approach condition would be expected to achieve a high NTU level.

At the low L/G levels of interest (< 1.0) in low approach evaporative cooling, in maritime temperate climates the test rig gives significantly higher NTU levels than conventional towers. In fact at the low limit L/G ratio of 0.25, the test rig NTU level of 4.5 represents a 30-60 % increase on previously reported performance, for more conventional towers as shown in Figure 5. This indicates that a building heat rejection system, designed on a similar basis to the test rig will have the ability to produce exceptionally low approach temperatures at a low water flow rate flux. This is due to two test rig design decisions (i) the use of high area of packing fill (200m 2 .m -3 ) and

(ii) a low water flow rate flux, both of which combine to significantly increase the residence time of the water droplets in towers, and thereby decrease the approach, provided the air flow rate is at a level to absorb the water vapour in semi-humid to humid martime ambient conditions and thereby maintain an enthalpy difference driving force at significant levels. Hence a lower rate of water flow rate per unit of air flow rate (a low L/G ratio) is essential in these climates.

Heat transfer coefficient

As the cooling tower is predominantly a mass transfer device in this application, the evaluation of the volumetric total heat transfer coefficient (kg a .s -1 .m -3 ) is of particular interest. This coefficient is usually determined in terms of Ka, not K due to the difficulty of isolating the relevant area from the transfer coefficient. The heat transfer area is not necessarily equal to the packing surface area as heat and mass can also be transferred from water droplets in suspension in the air stream.

Due to the requirement in this work to achieve a low approach condition, the volume of the test rig tower packing for the cooling load rejected (20 kW), is considerably larger than in traditional applications. As the ratio of heat rejected to volume of packing is low, it would therfore be expected that the volumetric heat transfer coefficient is also low in comparison with more conventional applications such as in refrigeration condenser heat rejection, where the design approach condition is often a multiple of that required in this application.

The quantity Ka is usually correlated as follows (as described by Coulson and

Richardson [START_REF] Coulson | [END_REF] ):

( ) ( ) ) 1 ( ' ' n n L G Ka - ∝ (10)
where G' and L' are the flow rate flux (kg.s -1 .m -2 ). Coluson and Richardson [START_REF] Coulson | [END_REF] give the following general correlation for traditional industrial scale towers in conventional applications:

( ) ( ) 26 . 0 72 . 0 ' ' 95 . 2 L G Ka = (11)
Other, more specific work by Goshayshi [START_REF] Goshayshi | Investigation of smooth and rough corrugated cooling tower packings in various arrangements[END_REF] with reference to experimental work on a model laboratory tower (however with a packing density of 200m 2 .m -3 , similar to the semi-industrial scale test rig used in this work) resulted in a correlation of :

( ) ( ) 45 . 0 6 . 0 ' ' 75 . 1 L G Ka = (12)
This indicates that Goshayshi [START_REF] Goshayshi | Investigation of smooth and rough corrugated cooling tower packings in various arrangements[END_REF] found that the proportionality constant for the model tower was considerably lower but that the water flow rate has a greater impact and the air flow rate a lesser impact on heat transfer than with the industrial scale tower general behaviour described in Equation 11. For the work described in this paper Equation 9 can be re-written as follows:

23 . 0 77 . 0 ) ( ) ( 3 . 1 L G V Ka = (13)
As the crosssectional area of the tower is 0.84m 2 it follows that L = 0.84(L') and G = 0.84(G') and therefore:

Conclusions

Initially an analysis of the NTU level required with low approach cooling towers operating at conditions typically encountered in chilled ceiling applications, in temperate climates, was conducted. Subsequently the thermal performance of an experimental open cooling tower, at a series of low water to air flow rate ratios, which are required in low approach "high" water temperature cooling, was measured. The The correlation for the cooling tower coefficient in this work was: the L/G ratio falls with an increase of 30%-60%, over that indicated for traditional towers, at a L/G ratio of 0.25. This indicates that building heat dissipation systems, designed on the same basis as the test rig, have an ability to produce very low approach temperatures at low water to air flow rate ratios.

3. The correlation for the heat transfer coefficient was:

( ) ( ) 23 . 0 77 . 0 ' ' 12 . 1 L G Ka =
In this correlation the of the exponents are very similar to those quoted in the literature for industrial scale towers, however the experimental results constant (1.12) is considerably less. This indicates that while the pattern of the variation in the volumetric heat transfer coefficient with air and water flow rate flux is similar to that for traditional towers the actual volumetric heat transfer coefficient achieved is relatively low, due to the high volume of cooling tower fill employed, per unit of heat rejected. Required NTU levels at the low approach conditions of interest in this application with an L/G ratios ranging from 0.2 to 1.0.

  . (1998)[START_REF] Kuehn | Thermal Environmental Engineering[END_REF] gives a general correlation, based on the model studies of[START_REF] Braun | Effectiveness models for cooling towers and cooling coils[END_REF] [START_REF] Braun | Effectiveness models for cooling towers and cooling coils[END_REF] and when L < G as follows: of Lowe and Christe[START_REF] Lowe | Heat transfer and pressure drop in cooling tower packings in[END_REF] is considered seminal. The results of this work demonstrate that enhancing the fill arrangement by the use of corrugated sheets of various forms significantly improves the value of the tower constant over that obtained with flat sheets but has no significant impact on the value of the exponent. Hence the rationale underlying the use of corrugated fill in most modern packings.

Figure 2

 2 Figure 2 indicates that the NTU level required is strongly dependent on the L/G ratio

  measaured results have been analysed in terms of the tower coefficient achieved and a new correlation has been developed from this analysis which is applicable to low water to air flow rate ratios. Using this correlation a further correlation has been derived for the volumetric heat transfer coefficient, based on the air and water flow rate flux in the tower. Both correlations have been compared with established correlations in the literature for open towers in more traditional applications and have been found to differ considerably from existing correlations. The correlations proposed in this work provide a key parameter for the design of this form of heat dissipation in buildings. Specifically the following conclusions can be drawn:1. The results of the theoretical analysis indicate that the ratio of the water flow rate to the air flow rate in the tower should be in the range 0.2-1.0 for this application. As a low approach condition requires a relatively low range condition the water flow rate is itself relatively high. Hence it can be concluded that a relatively high cooling tower air flow rate, per unit of load rejected, is required in this application.

2 .

 2 At the low L/G ratios of interest (< 1.0) the coefficient rises significantly as

Figure 1 :

 1 Figure 1: Simplified schematic of a water-side indirect evaporative cooling system

Figure 4 .

 4 Figure 4. Required NTU levels at the low approach conditions of interest in this application with an L/G ratios ranging from 0.2 to 1.0.

  Figure 3.Required NTU levels at the low approach conditions of interest in this application with an L/G ratios ranging from 0.2 to 2.2. (based on an inlet water temperature of 19˚C and an outlet temperature of 16˚C).

	5	2.6
					0.2	0.4	0.6
	fan open counter-flow 0.6 cooling tower 1 1 0 1.4 1.8 2.2 1 NTU required 2 3 NTU required 4	primary circuit pump T ps Approach tem perature (K) 4 5 7 T pr temperature (K) 3 temperature (PAT) Tpa 2 primary range primary approach 0.8 1.4 2	plate heat counter-flow 8 1 1.6 2.2	chilled ceiling or other cooling load (Q) temperature (K) circuit pump range secondary 10 secondary AST 2C 9 AST 4C AST 8C AST 10C AST 12C AST 14C AST 6C 1.2 1.8
			secondary approach			exchanger
			temperature (SAT) Tsa	T ss	T sr	typically 18 O C
		adiabatic saturation temperature (AST) 0.2 0 0.4 0.8	1.2	1.6	2	typically 15 O C 2.4
		or wet bulb temperature of ambient air at intake to tower Tas Ratio of w ater to air flow rate (L/G)
	Figure 2. Required NTU levels for a range of L/G ratios and a series of ambient
		AST conditions. (based on an inlet water temperature of 19˚C and an outlet
			temperature of 16˚C).

Table 1

 1 Comparison of NTU achieved in the tests with two of the established correlations for more conventional applications. Range of L/G ratio is the range of interest in this work. Cooling tower NTU analysis based on an inlet water temperature of 19°C, an outlet temperature of 16°C, an AST of 12°C, an L/G ratio of 1.0 with 30 elements each raising the mean water temperature by 0.1 K.

	y = 1.2933x -0.7703
	R 2 = 0.9707

Table 2 :

 2 NTU level achieved in the tower for a series of test results. The inlet water temperature is within the range 15.4˚C+/-0.8K and the AST is within the range10.4 +/-0.8K. The heat rejected is constant at 20kW for all tests.

	Inlet water	Exit water	Ambient	L/G ratio	NTU level
	temperature	temperature	AST	for test	achieved
	˚C	˚C	˚C
	15.06	12.89	9.75	0.88	1.39
	16.21	13.37	10.12	0.71	1.55
	14.92	13.02	11.15	0.69	1.77
	14.61	12.69	10.80	0.69	1.77
	14.93	12.70	10.84	0.60	1.91
	15.30	13.00	11.17	0.60	2.00
	15.11	12.35	10.60	0.48	2.25
	15.16	12.36	10.61	0.51	2.32
	15.55	12.35	10.91	0.39	2.64
	15.97	11.91	10.69	0.30	3.18

Acknowledgements

This work was supported by research grants from CIBSE, Enterprise Ireland and the Dublin Institute of Technology. Many private companies donated equipment and services, the support of whom is gratefully acknowledged. The support of the laboratory technical staff of the Department of Building Services Engineering is also acknowledged.

The test cooling tower packing volume can be see in terms of the packing volume (0.97 m 3 ) or the total volume of the space between the nozzle layer and the the water surface in the reservoir (1.52m 3 ) i.e. the volume associated with the formal packing surface and total possible surface respectively. Hence in terms of the packing volume

The values of the exponents (0.77 and 0.23) are remarkably similar to those in Equation 11 (0.72 and 0.26), which is probably explained on the basis that both are industrial scale towers. The experimental results constant, (1.12) however is very much lower at 40% of that quoted in this equation.

The correlation (Equation 15) can be expressed graphically as the variation in Ka with water flow rate flux for a series of air flow rates flux. Figure 6