A Arteconi

C Brandoni

F Polonara
email: f.polonara@univpm.it

Distributed generation and trigeneration: energy saving opportunities in

Keywords: CHP plant, absorption chiller, supermarket, distributed generation, photovoltaics CHP plant, trigeneration, supermarket, absorption chiller, photovoltaics

This paper presents an analysis of the potential for introducing distributed generation systems in the supermarket sector in the light of Italian legislation (inasmuch as concerns tax incentives and regulations) with a view to arriving at some generally applicable criteria. The energy users in question are characterized by a strong demand for energy for refrigeration for food preservation and for ambient air-conditioning during the summer. This makes supermarkets particularly suitable for trigeneration applications with the prime mover coupled with absorption systems. This study analyses the feasibility of implementing trigeneration systems for the combined production of electricity and ambient heating and airconditioning energy or, alternatively, for the combined generation of electrical energy and refrigeration for the preservation of food. Finally, the hypothesis of combining trigeneration systems with photovoltaic systems aimed at maximizing the energy saving achievable was also considered. This paper analyses the various technologies from a technical, economic and environmental standpoint, enabling advantages and disadvantages to be identified in relation to a real case.

Introduction

The term distributed generation (DG) includes various fossil fuel or renewable energy generation and/or cogeneration technologies located in the vicinity of the point where the energy is used. Using DG systems offers a number of advantages, such as a reduction in the energy costs for the user and for the domestic and international economies as a whole, fewer losses in transmission, fewer carbon dioxide emissions, a better quality electrical energy generation and a less vulnerable electrical system [START_REF] Chicco | Distributed multi generation: A comprehensive view[END_REF].

Based on data presented by Terna, the Italian national grid owner, [2], the proportion of electrical energy consumption deriving from public and commercial services in Italy amounts to 25% and, according to an investigation conducted by the International Energy Agency [START_REF] Baxter | Summary of Advanced Supermarket R&D Activities Conducted Under International Energy Agency[END_REF],

supermarkets represent the principal consumer in the commercial services sector. It is consequently useful and interesting to analyze potential energy saving alternatives for this area. A supermarket's energy requirements are characterized by a strong demand for electrical energy, which represents a mean 80% of its total demand, and the need for ambient heating energy that is concentrated in the winter months. Based on research conducted by the Canadian Energy Efficiency Office [START_REF]Canada, Commercial and Institutional Retrofits -Guide -Saving Energy Dollars in Stores[END_REF] on energy consumption in supermarkets, Figure 1, the refrigerated food counters are one of the main energy consumers, followed by lighting and the electrical energy needed for air-conditioning in summer.

The present paper assesses the feasibility of using trigeneration systems to guarantee the generation of electrical, heating and air-conditioning energy. The system analyzed consists of a natural gas powered primary motor, an internal combustion engine or a micro-turbine system, coupled with a lithium-bromide(LiBr)/water absorption system able to cope with the air conditioning load. Using the absorber has a useful boosting effect on the generation of heat and electricity, affording a considerable improvement in the profitability of the system. [START_REF] Macchi | La microcogenerazione a gas naturale[END_REF] Given the great demand for refrigeration for foodstuff preservation, as a second solution combining cogeneration systems with absorption systems for refrigerating the chilled food units was considered as well. Ammonia/water refrigeration systems are the only solution capable of guaranteeing low temperatures (0°C to -40°C), but although they are the oldest type of refrigeration technology known, they are less widespread and standardized than LiBr/ water systems, and are generally only installed in large-scale industrial plants [START_REF] Colonna | Industrial trigeneration using ammonia-water absorption refrigeration systems (AAR)[END_REF].

This article therefore analyses also the feasibility of replacing the compression refrigerators with the ammonia/water absorption systems for the chilled food units while keeping compressor refrigeration for the frozen food units [START_REF] Tassou | Application of tri-generation systems to the food retail industry[END_REF].

Finally, with a view to polygeneration in order to maximize the energy saving, combining trigeneration systems with photovoltaic systems has also been considered. In accordance with the provisions of the EU decision makers (Directive 2001/71/EC [8]), Italy is promoting the generation of electrical energy from solar energy, which is considered strategic for its great potential, especially in consideration of the climatic features of the Country, and southern Italy in particular.

The photovoltaic system is assessed on the basis of the "feed-in-tariff" (DM 19/2/2007 [START_REF]Criteri e modalita' per incentivare la produzione di energia elettrica mediante conversione fotovoltaica della fonte solare[END_REF]), a scheme of trading account incentives for promoting the use of electricity from solar sources that awards an economic incentive as a function of the kWh generated by a plant.

2.

Technical and economic assessment: case study

Baseline case

The analysis was developed on the basis of the consumption of a typical supermarket situated in central-northern Italy, characterized by a sales area of 10,000 m². Table 1 shows the characteristic parameters of the energy user in question.

The electrical energy demand for all the other uses and electrical energy demand for food is characterized by a major demand for electricity for refrigeration, aimed at food preservation (Figure 2), which is almost constant throughout the year, while the demand for central heating is concentrated in the winter months.

There is also a clearly evident increment in the electrical energy demand during the summer months due to the need for air-conditioning and to the worse performance of the refrigerators because of the higher condensing temperatures in summer.

For a proper sizing of the distributed generation systems, we cannot fail to consider the daily distribution of the energy consumption. Figure 3 shows the results of monitoring daily electrical energy consumption for air-conditioning and refrigerated food preservation in various months of the year. There is a clearly different time distribution of the energy consumptions recorded, with the electrical energy demand for air conditioning concentrated during the supermarket's opening hours, whereas the consumption of the refrigerators for food preservation is continuous, around the clock.

The present energy procurement system is managed separately, electrical energy being purchased from the national grid, while the refrigeration energy is produced by the vapor compression type refrigerators and the heating energy by a condensing boiler. The parameters for the current management system are given in Table 2.

Adopting the methodology presented in [START_REF] Maidment | Application of combined heat-andpower and absorption cooling in a supermarket[END_REF][START_REF] Maidment | Combined cooling heat and power in supermarkets[END_REF][START_REF] Maidment | Combined cooling and heating using a gas engine in a supermarket[END_REF], in the present study 4 possible distributed generation solutions for the supermarket in question are analyzed, using for comparison the current energy procurement system.

Starting from the energy needs, the ideal solution for each scenario, assessed from a technical and economic point of view, has been identified.

The economic analysis was conducted on an existing supermarket, with the baseline system already in operation. The investment costs considered in the analysis consisted therefore in the cogeneration and absorption system to be superimposed on the existing plant.

Financial analytical methods [START_REF] Blank | Engineering Economy[END_REF] were used considering the Net Present Value (NPV) [START_REF] Chicco | Distributed multi generation: A comprehensive view[END_REF] and the Pay Back Period (PBP), based on a calculation of the cash flows, F k , generated by the use of DG systems. The cash flows (2) take into account the characteristic costs, C k , incomes, Re k , and cost of interests, I k , of the single applications. The expenditure for the initial investment, E k (k=0), was assumed to have been financed completely with third-party capital at an interest rate of 6%.

( )

0 1 n k k k F N P V r = = + ∑ (1)
F k =(Re k -C k -I k ) -E k (2)

Case 1_Combined generation of electrical energy and heating/cooling for air conditioning purposes

The trigeneration system analyzed comprises a methane gas cogenerator combined with a LiBr/water absorption system fed with the heat produced by the cogenerator and used for air conditioning. As far as the choice of prime mover is concerned, the dual possibility of installing internal combustion engines (ICE) or micro gas turbine systems (µGT) was considered.

As shown in the graph of a typical supermarket's energy needs (figure 2), the demand for electricity is considerably greater than the demand for heat. Cogeneration systems have an electrical/thermal capacity ratio coming between 0.6 and 0.9 [START_REF] Cardona | Part I: Assessing economic and technical potential. Energy saving in airports by trigeneration[END_REF] so, if we were to size the system on the basis of the electricity demand, following a logic of keeping up with the electrical energy load, this would coincide with an excessive waste of thermal energy, with a consequent reduction in the global efficiency of the system. For the particular type of user analyzed, it is consequently more advisable to follow a logic of covering the thermal load and sizing the cogenerator on the basis of said thermal energy requirement.

Reciprocating engines represent the most widespread technology used today for the combined generation of electrical energy and heat from fossil fuels. This is for economic reasons, relating to a limited initial investment and a high performance in terms of the output of electrical energy, thanks to the great reliability typical of a mature technology that has been applied in various sectors for some time [START_REF] Macchi | La microcogenerazione a gas naturale[END_REF]. There are ICE available in sizes ranging from a few dozen kW el up to MW el , and that is why they are extremely versatile in a number of different applications [START_REF] Sun | Energy efficiency and economic feasibility analysis of cogeneration system driven by gas engine[END_REF].

Micro-turbine systems represent nowadays a potential alternative to ICE, particularly in the context of mini cogeneration. These are systems based on the Brayton regenerative cycle and they use high-speed revolving centrifugal turbo machines (at approximately 100,000 rpm).

The recovery of the hot gases leaving the turbine enables the air to be preheated before it enters the combustion chamber, increasing the system efficiency by approximately 30%.

By comparison with ICE, though they have a lower electrical efficiency and a higher initial cost, they are more compact and lightweight, they require less maintenance and they have a longer working life, thanks to a more straightforward architecture. For trigeneration applications, they offer the great advantage of generating a high-temperature thermal energy:

the hot gases released can reach temperatures of 250°C, and are therefore directly suitable for powering absorption systems. [START_REF] Pilavachi | Mini-and micro-gas turbines for combined heat and power[END_REF] The absorbers adopted operate on LiBr/water: these systems have a higher COP than ammonia/water systems, coming between 0.7 and 0.8, though they cannot achieve sub-zero temperatures like the latter. The presence of a condensing tower is needed to keep the temperature of the condenser low, which is essential to avoid lithium bromide crystals forming and obstructing the heat exchanger piping, causing a deterioration in performance or a machine stoppage [START_REF] Macchi | La microcogenerazione a gas naturale[END_REF].

Figure 4 shows the diagram of the equivalent mean thermal capacity (3), eq Q , calculated as the sum of the mean thermal capacity effectively required for heating purposes and the thermal capacity that would result from satisfying the demand for air conditioning with an absorption system having a COP of 0.75.

Clearly, if we were to size the cogenerator according to the thermal demand in summer, the cogenerator would have to work under a partial load for the rest of the year with a consequent loss of performance. The optimal choice is therefore to size the system to suit the winter thermal demand, which would mean that a percentage of the refrigeration demand for air conditioning is supplied by compression type refrigerators (this percentage amounts to 27% if

the prime mover is ICE and to 19% if it is µGT) COP R Q Q eq + = (3)
The system chosen (Figure 5) comprises a cogenerator which produces 330 kW el and 480 kW th coupled -in summer -with a 300 kW c LiBr/water absorber type refrigerator powered by the heat recovered from the exhausts and from the water used to cool the engine.

The heat available for heat recovery in summer operation is 407 kW th because the thermal energy recoverable from oil cooling cannot be used due to its too low temperature (80°C compared to 88°C required to supply the absorption).

The characteristic parameters of this option are given in Table 4.

As for the choice of operating hours, based on the supermarket's opening times (8:00-21:00), and on the assumption that the cogenerator is switched off when there is no simultaneous thermal and electrical load, the total number of operating hours would amount to about 4000 per year.

The items used to calculate the cash flows and thus assess the investment are listed below.

Capital costs:

initial cost of the cogenerator including the cost of installation and commissioning initial cost of the absorption system including the cost of installation and commissioning

Running costs:

cost of maintenance on the system cost of purchasing methane gas cost of purchasing electrical energy not produced by the cogeneration system Revenues:

revenue deriving from not needing to purchase electrical energy revenue deriving from the sale of white certificates revenue deriving from tax deductions on methane gas

The revenue from cogeneration relies on selling, or not purchasing, electrical energy and on the tax benefits established by national legislation. In Italy cogeneration systems defined as "high-performance" enjoy a series of incentives such as a tax rebate on methane gas purchased, dispatching priority and the awarding of Tradable White Certificates, TWC.

A cogeneration system is defined as high-performance when it complies with the "primary energy saving index" (PES) and "thermal limit" (L th ) [START_REF] Luigi | Experimentation on a cogenerative system based on a microturbine[END_REF] parameters defined by the Italian Authority of Energy and Gas [18]. The PES index (4) enables the primary energy saving obtained with cogeneration to be compared with a separate electric generation/boiler system quantified in relation to a separate management.

The thermal limit is the ratio between the thermal power annually produced by the cogenerator and the total useful effect given by the sum of electrical and thermal energy produced. For the power ratings in question, these parameters must be higher than 10% and 33%, respectively. If PES>0,10 and L th >0,33 a quantity of methane gas amounting to 0.25 Nm 3 per kWh produced is liable to a tax reduction, which is further rebated if the electrical energy is being used for self-supply [19,[START_REF]Verifica di congruità dei parametri per la determinazione dell'onere termico per il primo trimestre[END_REF].

ACCEPTED MANUSCRIPT

f i el,ref th,ref m K PES 1 W Q η η ⋅ = - + (4) Q W Q L th + = (5)
It can be demonstrated [START_REF] Macchi | La microcogenerazione a gas naturale[END_REF] that if the cogenerator has an electrical efficiency of 40% no fuel tax has to be paid. In the case studied, not all the gas methane used is rebated because the electricity cogeneration efficiency is in both cases lower than 40%. On the gas amount liable of tax reduction, the tax payed is € 0,000135 €/ Nm 3 instead of 0,012498 €/ Nm 3 . TWC, implemented by some European Union Member (Italy, UK, France), are tradable certificates used to achieve energy efficiency in end-use sector. A TWC scheme involves achieving a mandatory energy-saving target against the "business-as-usual" scenario [START_REF] Mundaca | Transaction costs of Tradable White Certificate schemes: The Energy Efficiency Commitment as case study[END_REF].

TWCs attesting to the saving achievable can be exchanged on the basis of bilateral agreements, or using the market managed by the administrator of the electrical energy market.

Each Ton of Oil Equivalent, TOE, of electrical and/or thermal energy that is saved corresponds to a TWC, the value of which has been set, for the purposes of simulations, at 100 €/TOE. The calculation of the TOE has been based on a specific table established by the AEEG.

According to current legislation, TWCs are issued annually (subject to an audit on the system's compliance with the PES and L th parameters) for a period of 5 years as of the first year of its application. It is worth noting here that the mechanism for issuing TWCs is currently under review.

The outcome of the financial analysis on the investment is the NPV shown in Figure 6, which shows a Pay Back Period of less than 5 years, demonstrating the potential for applying this solution.

Figure 6 shows the PBP for the present solution also in the case that no economic incentives are provided (dotted line). As can be seen also in this hypothesis the PBP is a still acceptable value of 5 years and 3 months.

Finally, in order to assess the emission reduction from trigeneration it has been calculated the TCO 2 ER indicator (6) defined by Mancarella and Chicco expressed in terms of emission factors [START_REF] Chicco | Assessment of the greenhouse emissions from cogeneration and trigeneration systems. Part 1:Models and indicator[END_REF][START_REF] Chicco | Assessment of the greenhouse emissions from cogeneration and trigeneration systems. Part 2:Analysis Techniques and application cases[END_REF] and reported in Table 5.

2 2 2 2 2 ( ) 1 1 ( ) ( ) ( )( ) f th Q f CO Q W W SP SP th CO CO CO SP Q TCO ER W Q R COP µ µ µ µ = - + + (6)
This indicator compares the emissions produced from trigenerative plant, which depend on the fuel input, to the emissions produced from separate production systems and gives the percentage of emissions reduction.

As suggested in [START_REF] Chicco | Assessment of the greenhouse emissions from cogeneration and trigeneration systems. Part 1:Models and indicator[END_REF][START_REF] Chicco | Assessment of the greenhouse emissions from cogeneration and trigeneration systems. Part 2:Analysis Techniques and application cases[END_REF] it has been assumed

2 f Q CO µ = 202 [g/kWh], W CO 2 µ = 525 [g/kWh], Qth CO 2 µ = 2 , f Q CO SP th µ η [g/kWh], with , SP th η =0.9.
As mentioned previously, the alternative hypothesis of using micro turbine systems as prime mover to be coupled with a direct fumes absorption system was also assessed. The micro turbine systems currently on the market have power ratings coming between 30 and 200 kW el .

To assess the technical and economic feasibility of this option, a 100 kW el model was chosen. The ideal solution emerged as an installation with three micro turbines (figure 7) coupled during the summer with three 100 kW c direct fumes absorption systems, the technical and economic parameters of which are given in Table 4.

The cost items used to calculate the cash flows are the same as for the ICE systems.

The analysis (figure 6) indicates a PBP of 7 years and 6 months, because of the higher initial cost of the micro turbine systems and their lower electrical efficiency, though the systems entail lower maintenance costs and are more reliable (guaranteeing 60,000 hours of operation with maintenance costs of 0.014 €/kWh each).

In this case the absence of any incentives (dotted line) give a PBP of 8 years.

Table 5 shows the PES, L th and TCO 2 ER values and the percentage of primary energy saved for the solutions analyzed. Despite their lower PES, due to their lower electrical performance, the micro turbine systems can guarantee a higher L th because, coupled with an absorber, they can reuse all the heat being generated, which has a high enthalpic content.

It is common knowledge that the advantage of using trigeneration systems with natural gas depends strictly on the price of methane gas vis-à-vis the price of electricity. Figure 8 shows the trend of the PBP as a function of said ratio: it is clear that the PBP is strongly influenced by the mutual cost of natural gas and electricity.

For the present case the ratio is 0.247. It is evident that for ratios of 0.3 and above the PBP rises sharply, especially for the micro turbine systems, which are characterized by a greater fuel consumption. The analysis shows that any feasibility study must keep this ratio's influence in proper account. Moreover, during the fuel contract negotiation it is of paramount importance trying to obtain a fixed link between cost of natural gas and electricity.

Case2_Combined generation of electrical energy and refrigeration for chilled food preservation

The hypothesis of using cogeneration systems to generate electrical energy and refrigerate chilled food products has been analyzed within the Case 2. At the supermarket under investigation the percentage of refrigeration energy demanded for the chilled food counters amounted to 70% of the total requirement for food preservation.

The system hypothesized (figure 9) consists of two micro turbine systems coupled with ammonia/water absorption units. The micro turbine systems are similar to those used in Case 1, while the absorption unit chosen is a small 11 kW c [START_REF] Tassou | Application of tri-generation systems to the food retail industry[END_REF]. The technical and economic parameters characteristic of this solution are given in Table 6. Having seen that the demand for cooling capacity for food preservation is constant throughout the year and for 24 hours every day (Figure 3), and since it is known that the profit is maximized when the electrical energy is used for self-supply, the cogenerator and the absorption units were sized so as to guarantee that all the electrical energy generated at night could be used by the supermarket.

To reach this goal all the frozen food cabinet are maintained in the compression type refrigeration and part of the chilled food counters as well. As a whole the absorption refrigeration covers the 43% of the food refrigeration load for the supermarket.

With this arrangement the operating hours of the system amount to 8000 hours/year.

The following is a list of the items used to calculate the cash flows needed to assess the investment.

Capital costs:

initial cost of the cogenerator including the cost of installation and commissioning initial cost of the absorption system including the cost of installation and commissioning Running costs: cost of plant maintenance cost of purchasing methane gas cost of purchasing the electrical energy not generated by the cogeneration system Revenue: revenue deriving from not needing to purchase electrical energy revenue deriving from tax deductions on methane gas revenue deriving from the sale of white certificates

The assessment gives rise to a PBP of less than 5 years (figure 10), an extremely worthwhile result for future applications. The PES, L th and TCO 2 ER values for the application are shown in Table 5, they are higher than in Case1 since all the energy generated can always be exploited.

A problem that cannot be neglected, given the greater number of operating hours involved, is the need to overhaul or replace the machine after approximately 7 years of operation.

Case 3_Combined generation of electricity and heating/cooling for air-conditioning, associated with electrical energy generation using photovoltaic panels

As demonstrated previously, the electrical energy generated by the trigeneration systems does not saturate the demand for electricity so, with polygeneration in mind, we hypothesized being able to exploit the roof of the building for the installation of photovoltaic panels.

The PVSOL [START_REF]PVSol 3.0 -Computer simulation software for design and calculation of photovoltaic systems[END_REF] dynamic simulator was used to design the grid-connected photovoltaic system. Table 7 shows the results of the sizing and the characteristics of the system installed.

The following is a list of the items used for the economic assessment of the PV system.

Capital costs: initial cost of the system including installation and commissioning

Running costs:

cost of plant maintenance Revenue: revenue deriving from not needing to purchase electrical energy revenue deriving from the 2007 energy account funding for semi-integrated systems revenue deriving from the sale of unused energy Figure 11 shows the NPV for the investment in the solar system and in the solar system associated with trigeneration using ICE. It is worth emphasizing that, having established an extra-maintenance agreement for the ICE that includes a total number of 96,000 operating hours, the investments are compared for a working life of at least 15 years.

As it is well known, even if they show great perspectives for the future, PV systems still need Government subsidies to reach profitability. The Italian Government support the production of electric energy from solar source on the basis of "feed in tariff" (DM 19/2/2007 [START_REF]Criteri e modalita' per incentivare la produzione di energia elettrica mediante conversione fotovoltaica della fonte solare[END_REF]), a scheme of trading account incentives for promoting the use of electricity from solar sources that awards an economic incentive as a function of the kWh generated by a plant.

It is interesting to note that the integration of the two distributed energy technologies presents a NPV lower than the phothovoltaic system alone thank to the profitability of the trigenerative plant. Table 5 shows the CO 2 ER (7) calculated as the sum of TCO 2 ER and carbon dioxide emission reduction derived from photovoltaics. CO 2 ER= TCO 2 ER+CO 2 ER pv (7)

Case4_Combined generation of electricity and refrigeration for chilled food preservation, associated with electrical energy generation using photovoltaic panels

Table 5 shows the CO2ER for the solution analyzed, calculated as in the previous case.

To hypothesize the investment relating to the combined production of electricity and refrigeration energy for food preservation and of electrical energy by means of photovoltaic panels, one needs to consider the different working lives of the systems. In the cost items used for the analysis of the trigeneration system, an item for extraordinary maintenance has been added, which includes the replacement of the micro turbine and absorber systems at the end of their working life. Figure 11 shows the trend of the NPV for the photovoltaic system and the solution combined with trigeneration.

Although the need to replace the equipment after seven years induces a delay in the PBP with respect to the previous solution, the Pay Back Period is always lower than the photovoltaic system alone.

CONCLUSIONS

This article analyses four different solutions for applying Distributed Generation and trigeneration to the supermarket sector, which emphasize an important primary energy saving opportunity.

As a first case, the generation of electricity and heating/cooling energy for airconditioning was considered. Using ICE combined with a LiBr/water absorption systems gives a PBP of less than 5 years, whereas using a micro-turbine-system, though potentially interesting, is still not competitive because of the higher initial cost and lower electrical efficiency of the machine. It is important to emphasize, however, that the profitability of the investment strongly depends on the ratio between the prices of natural gas and electrical energy.

As a second solution, using trigeneration systems for the combined generation of electricity and refrigeration energy for the chilled food products was considered, which is particularly interesting because the refrigeration energy demand is one of the main cost items for supermarkets. The PBP in this case is less than 5 years, assuming the congenerating equipment has 7 years of operative life. This option enables a PES of 56%.

The drawbacks of this solution relate to the difficulty of obtaining indirectly powered ammonia/water absorption systems on the market, and to the high initial outlay.

Finally, with a view to maximizing the energy saving, the hypothesis of combining photovoltaic systems with the trigeneration systems was also analyzed. The investments, exploiting the fiscal incentive mechanisms, gave a PBP of approximately 10 years.

Out of 4 options, the first two were analyzed both in the case an economic incentive is provided by the Government (the Italian situation has been applied here) and in the case no economic incentive at all is provided. Absence of incentives showed only a reduced penalty in PBP, therefore demonstrating that the technology is intrinsically viable not only on energetic terms but also on economic terms. On the other hand, when the PV system is added to the trigeneration, economic incentives still remain a must in order to achieve a reasonable profitability for the investment.

NOMENCLATURE

  where f m is the flow rate of the fuel delivered to the cogenerator, K i the net heating value of the fuel used, W the electrical energy generated, Q the useful thermal energy and ref el , η and ref th, η the electrical and thermal reference efficiencies that, for the case in point, are respectively 0.4 and 0.9.

Figure 1 .Figure 2 .Figure 3 .

 123 Figure 1. Electrical energy demand according to end-uses in a supermarket [4] Figure 2. Monthly energy consumption at the investigated supermarket Figure 3. Daily distribution of electrical energy used for air-conditioning and refrigeration

Figure 4 .Figure 5 .Figure 6 .Figure 7 .Figure 8 .Figure 9 .Figure 10 .Figure 11 .

 4567891011 Figure 4. Equivalent mean thermal capacity (Case_1) Figure 5. Layout of a cogeneration plant with ICE (Case_1) Figure 6. NPV for Case 1, solutions with ICE and µGT (the dotted lines represent the NPV in case no economic incentives are provided) Figure 7. Layout of a cogeneration plant with µGT (Case_1) Figure 8. Variation in the PBP as a function of the ratio between gas and electricity prices Figure 9. Layout of a trigeneration plant with ammonia/water absorbers (Case 2) Figure 10. NPV for Case 2, combined generation of electricity and refrigeration for chilled food preservation (the dotted line represents the NPV in case no economic incentives are provided) Figure 11. NPV solution with a PV system combined with a trigeneration plant

Figure 7 .Figure 8 .Figure 9 .Figure 10 .Figure 11 .

 7891011 Figure 7. Layout of a cogeneration plant with µGT (Case 1)

Table captions Table 1 :

 captions1 Characteristic parameters of the supermarket under study

Table 2 :

 2 Characteristic parameters of the current energy procurement and separate management system

Table 3 :

 3 Distributed generation solutions analyzed

Table 4 :

 4 Technical and economic parameters of the trigeneration plant with ICE and µGT

	(case 1)

Table 5 :

 5 PES, L th and carbon-dioxide emission reduction of the solutions analyzed

Table 6 :

 6 Technical and economic parameters of the trigeneration plant with ammonia/water absorbers (case 2)

Table 7 :

 7 Technical and economic parameters of the PV system

		Other_electrical
	Lighting 21%	equipment 16%
		Bakery
		6%
	Food Preparation
	7%
		HVAC
		12%
	Refrigeration
	38%
	Figure 1. Electrical energy demand according to end-uses in a supermarket [4]

Table 1 :

 1 Characteristic parameters of the supermarket under study

	5,093

Table 2 :

 2 Characteristic parameters of the current energy procurement and separate

	management system

Table 5 .

 5 PES, L th and carbon-dioxide emission reduction of the solutions analyzed

	Parameters
	Absorber
	Number of units installed	15
	Unitary Cooling capacity [kW c ]	11
	Unitary Input thermal capacity [kW th ]	22
	COP	0,5
	Initial cost [€/kW]	813
	Cost of O&M [€/kW-year]	57,21

Table 6 .

 6 Technical and economic parameters of the trigeneration plant with ammonia/water absorbers (Case 2)

	Parameters
	Manufacturer	Sharp Corporation
	Power rating [W]	125
	Performance STC	13.3%
	Electrical efficiency	38.9%
	Orientation	45°I
	nclination	30°P
	lant
	Energy produced	856,000 [kWh]
	Initial cost of plant	3.4 M€
	Cost of O&M	2% of Initial cost of plant
	Guaranteed life	20 years

Table 7 .

 7 Technical and economic parameters of the PV system

Acknowledgements

The authors are grateful indebted to Inres Coop, Milan, which monitored and elaborated the energy consumptions of the supermarket analyzed. This work has been supported by the Italian Ministry of Environment and by Marche Regional Government (Ancona, Italy) within the the framework of the project "Ricerche energetico-ambientali per l'AERCA di Ancona Falconara e bassa valle dell'Esino".

The authors thank the anonymous reviewers for their insightful comments and precious advice given on how to improve this paper.


Luis M Romeo
email: luismi@unizar.es

Irene Bolea

Yolanda Lara

Jesús M Escosa

Optimization of intercooling compression in CO capture systems

Keywords: intercooling compression, CO 2 capture, energy optimization

Compression of CO 2 is an essential process in the development of carbon capture and storage (CCS) technologies. In spite of power requirements for CO 2 compression could be as much as 100 kWe per tonne CO 2 , the minimization of energy requirements has received little attention in the literature. Although intercooling compression reduces power requirements, it introduces important cooling necessities that could be minimized.

The aim of this paper is the integration of intercooling compression into the low-pressure part of a steam cycle to take advantage of the intercooling heat and analyse the energetic and economical results under different assumptions. Simulation and optimization have been performed in order to evaluate the intercooling configuration, energy requirements and the most cost-effective integration.

Results have shown reduction in compression power requirement around 40% and reduction of the incremental COE around 23%. Proposed integration could be used to increase the efficiency of CO 2 capture processes and, therefore, to reduce the CO 2 capture cost.

W extraST extra power by steam turbine (kW)

INTRODUCTION

Recently, there has been a growing interest in the study of CO 2 capture processes as an option to reduce greenhouse gasses emission and mitigate the global warming. Many recent studies have focused on the analysis of post combustion capture, oxyfuel and pre combustion capture schemes. In particular, the development of zero (or near zero) emissions power plant technologies is gaining rapid momentum worldwide and some large demonstration projects are expected in the coming decade for new plants [START_REF]Special Report on Carbon Dioxide Capture and Storage, Intergovernmental Panel Climate Change[END_REF]. Total capital requirement (TCR), cost of electricity (COE), CO 2 avoided and cost per ton of CO 2 avoided are the essential variables used to compare different options, configurations and new developments.

Although considerable research has been devoted to capture processes, rather less attention has been paid to CO 2 compression. As an intermediate and essential process between capture from flue gases and a safe storage, CO 2 treatment for transport plays an important role in efficiency penalty and total capital requirement associated with capture processes. It is required temperature and pressure over the critical point due to CO 2 specific volume at supercritical conditions is more than 500 times smaller compared with values at gas phase (0,8 m 3 /kg at 150ºC and 1bar; 0,00158 m 3 /kg at 40ºC and 100bar). Geologic storage requires a high density CO 2 stream with a suitable pressure for safe storage and capacity maximization. These temperature and pressure values are also necessary for pipeline transport because it reduces pipe diameter (although increasing thickness). For tank transport, conditions differ completely, 6.5 bar and approximately -50ºC are needed in order to be economically feasible and remain in liquid phase [START_REF] Aspelund | Gas conditioning-The interface between CO 2 capture and transport[END_REF].

Around 25% of the total capital requirements are due to compression equipment cost. Energy requirements for gas conditioning are typically 90-120 kWh/tonne CO 2 [START_REF] Aspelund | Gas conditioning-The interface between CO 2 capture and transport[END_REF]. For an existing sub-critical power plant with 38% efficiency, 0.9 tonneCO 2 is emmited per MWh produced, so around 100-133 kWh are needed for compression per MWh produced. With COE around 35 /MWh the additional of detailed analysis in order to minimize both, energetic requirements of the compression and additional capital requirement for the capture system Compression process includes condensation of water before CO 2 conditioning and generally dehydration in the CO 2 compression intercooling is also required [START_REF] Birkestad | Separation and Compression of CO 2 in an O 2 /CO 2 -fired Power Plant[END_REF][START_REF] Andersson | Process evaluation of CO 2 free combustion in and O 2 /CO 2 power plant[END_REF][START_REF] Heggum | CO 2 conditioning and transportation[END_REF][START_REF] Oryshchyn | Developments in integrated pollutant removal for low-emission oxy-fuel combustion[END_REF][START_REF] White | Purification of oxyfuel derived CO 2 for sequestration or EOR[END_REF][START_REF] Aspelund | Ship transport of CO 2 -technical solutions and analysis of costs, energy utilization, exergy efficiency and CO 2 emissions[END_REF][START_REF] Aspelund | A study of the interface between CO 2 capture and transport[END_REF]. A recent research also includes the removal of other components in CO 2 rich-stream [START_REF] Aspelund | Gas conditioning-The interface between CO 2 capture and transport[END_REF]. This interesting work focused on the necessity of different process flow schemes for CO 2 compression depending on transport specifications, ship or pipeline, and took into account the gas quality (depending on capture process) before the removal of water, other liquids and volatile gases in the gas conditioning.

Gas conditioning in oxyfuel applications have deserved major effort at research [START_REF] Birkestad | Separation and Compression of CO 2 in an O 2 /CO 2 -fired Power Plant[END_REF][START_REF] Andersson | Process evaluation of CO 2 free combustion in and O 2 /CO 2 power plant[END_REF][START_REF] Oryshchyn | Developments in integrated pollutant removal for low-emission oxy-fuel combustion[END_REF][START_REF] White | Purification of oxyfuel derived CO 2 for sequestration or EOR[END_REF].

Detailed descriptions of the compression process for a 900 MWe oxyfuel capture system have been previously presented [START_REF] Birkestad | Separation and Compression of CO 2 in an O 2 /CO 2 -fired Power Plant[END_REF][START_REF] Andersson | Process evaluation of CO 2 free combustion in and O 2 /CO 2 power plant[END_REF]. Research was focused on system analysis, equipment sizing and economic evaluation. Due to the high moisture content in the gases (German lignite), a flue gas condenser was included to take advantage of condensation heat in the feed-water preheating.

Compression was designed in some stages with intercooling, a two-stage compression increases pressure up to 30 bar, a Tri Ethilene Glycol (TEG) unit eliminates moisture and avoid sweet corrosion.

Another compression stage increased the pressure up to 58 bar. The stream is then cooled down and MWth for the sub cooling of CO 2 previous to the final compression. However, it remains unclear whether it is possible to recover energy from the compression process in order to reduce the COE of capture systems and hence the cost per ton of CO 2 avoided.

The aim of this paper is to analyze and optimize the design of a CO 2 intercooling compression system taking advantage of the low temperature heat duty in the low-pressure heaters of a steam cycle. The novelty of this work includes the separation of the intercooling in two heat streams, one to be used in a steam cycle and another to be released in a cooling system. Integration of this energy in a steam cycle has been previously reported [START_REF] Romeo | Integration of power plant and amine scrubbing to reduce CO2 capture costs[END_REF] for amine scrubbing in an existing power plant. The objective functions for optimization are energy and exergy requirements, and total cost, including Total

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Capital Requirement (TCR) plus operation and maintenance cost. The paper is organized as follows:

A description of the intercooling compression is given in section 2. The equations and the optimization problem are described in section 3. Results and discussion are included in section 4 and, finally, conclusions are given in section 5.

CO2 INTERCOOLING COMPRESSION

The basic configuration of a CO 2 intercooling compression system is illustrated in Figure 1.

The CO 2 -water stream coming from a post combustion capture process or from oxyfuel combustion is firstly cooled down in order to condensate and separate the water content. Compression process is divided in several stages. Assuming a maximum total pressure relation of five and a final CO 2 pressure of 100 bar, three or four radial compressor steps are necessary [START_REF] Aspelund | Gas conditioning-The interface between CO 2 capture and transport[END_REF]. Compression up to 30 bar is usually designed with a three-stage process, compressors with the same pressure ratio and an isentropic efficiency around 80%. The net work required by each compressor can be reduced by means of multistage intercooling. Since the solubility of water in CO 2 gas decreases with pressure [START_REF] Diamond | Solubility of CO 2 in water from -1.5 ºC to 100 ºC and from 0.1 to 100 MPa: evaluation of literature data and thermodynamic modelling[END_REF] a TEG is recommended for drying process at the pressure of 30 bar [START_REF] Birkestad | Separation and Compression of CO 2 in an O 2 /CO 2 -fired Power Plant[END_REF]. Adsorption with molecular sieves or silica gel have been also proposed in the literature [START_REF] Aspelund | Gas conditioning-The interface between CO 2 capture and transport[END_REF]. A water concentration of 60 ppm can be reached using any of these systems. After the dehydration stage, one additional compression stage is required to make the CO 2 stream pressure reach 100 bar.

The positive effects of intercooling are the reduction of power needs for gas compression and therefore the compressor size and TCR. In general this heat is rejected to low temperature cooling equipment in order to reduce compression penalty. This strategy is beneficial for operation, especially in cold locations, but TCR could increase due to the necessity of larger heat exchangers for gas cooling in locations with higher temperatures.

Three variables influence the compressor power consumption and cooling necessities:

compressor efficiency, pressure ratio (i.e. compression stages) and CO 2 inlet temperature. Figure 2 shows the effect of these variables in power and cooling demands. Approximately the effect of varying inlet temperature from 30 to 50ºC is an increase of 10% in power requirements due to suction volume augmentation with temperature. Increment of outlet temperature is higher that inlet temperature, so cooling necessities also increases up to 30%. A reduction in the compressor isentropic efficiency, from 90 to 75%, causes an important increase (around 20%) in power and cooling necessities. The effect of increasing pressure ratios, or reducing the number of compressor stages from 4 to 3 (increasing pressure ratio from 3.1 to 5.5), is more important than those previously mentioned for a single compressor. Evidently, with pressure ratio of 5.5, one compressor stage is eliminated.

The CO 2 compressor outlet temperatures are shown in Figure 3. For a pressure ratio of 3.1 temperature ranges are 110-165ºC depending on efficiency and inlet temperatures. Values increase with pressure ratio of 5.5 up to 160-225ºC. This energy, with important temperature level, could be integrated into the capture process or a power plant steam cycle in order to reduce energy penalty associated with CO 2 compression. In this case, the main disadvantage is that the temperature level of the water coming from condenser is higher than cooling water temperature. As a consequence, the CO 2 compressor inlet temperature increases and also the power necessities, reducing or cancelling some of the benefits of energy integration.

With the aim of studying the configuration and values that optimize the intercooling compression in CO 2 capture system, i.e. minimization of the incremental COE associated with CO 2 compression, a proposed intercooling layout is presented in

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number of stages is reduced, compressor power requirements increase but it is possible to take advantage of more energy (heat) with higher temperatures in a medium temperature section of steam cycle. The same effect is observed when reducing the compressor efficiency.

The independent variables that influence the system are the compressor efficiencies, pressure ratios (i.e. compression stages), CO 2 intermediate temperatures and compressor inlet temperatures.

OPTIMIZATION OF INTERCOOLING COMPRESSION

To study the performance of the system under different situations and implementing a optimization, a process simulation has been developed [12]. The objective function for optimization was the incremental COE associated with CO 2 compression. This function includes the capital investment for main components and net compression power consumption.

The effects of compressor efficiency, pressure ratio, intermediate temperature and compressor inlet temperature have been studied in order to minimize the incremental COE.

For reference purposes a COE of 36. For compressors cost estimation, the experience of manufactures in calculating this capital cost as a function of their capacity and efficiency is essential. Such experience is not always available

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( ) i _ in i _ out i h h m W - * = (3) ( ) c i _ in s _ i _ out i _ in i _ out h h h h η - + = (
or reliable in open literature. In some cases, there are important differences in investment costs equations [START_REF] Duc | CO2 capture by hydrate crystallization -A potential solution for gas emission of steelmaking industry[END_REF][START_REF] Ieaghg | Pipeline Transmission of CO2 and Energy[END_REF][START_REF] Fox | Long distance transportation of CO2[END_REF]. One of the recent proposals for the function cost for compression depends on the power requirements, compressor type and Marshall & Swift index [START_REF] Peters | Plant design and economics for chemical engineers[END_REF] and it is given by [START_REF] Duc | CO2 capture by hydrate crystallization -A potential solution for gas emission of steelmaking industry[END_REF], equation The function cost for heat exchangers is taken from [START_REF] Olsommer | Méthode d'optimisation thermoéconomique appliqué aux centrales d'incinération d'ordures à cogénération avec appoint énergétique[END_REF] adapted from [START_REF] Frangopoulos | Comparison of thermoeconomic and thermodynamic optimal designs of a combined-cycle plant[END_REF], equations ( 8

Q 3650 = C             ( 8
)
Where Q is the heat exchanged between cold and hot streams in kW, ∆ T lm the logarithmic mean temperature difference in K.

The cooling necessities (Q cooling ) and heat to the steam cycle (Q heating ) are given by equations ( 9) and ( 10):

( ) ( ) ( ) ( ) ( )
h h m h h m h h m h h m h h m Q Q Q Q Q Q - * + - * + + - * + - * + - * = = + + + + = ∑ (9) ( ) ( ) ( ) ( ) [ ] ( ) ( ) ( ) ( )
h h m h h m h h m h h m h h m h h m Q Q Q Q Q Q - * + - * + - * + + - * + - * + - * = = + + + + = ∑ (10)
The cost function for active dehydration with TEG, equation [START_REF] Diamond | Solubility of CO 2 in water from -1.5 ºC to 100 ºC and from 0.1 to 100 MPa: evaluation of literature data and thermodynamic modelling[END_REF], is derived from the values in [START_REF] Birkestad | Separation and Compression of CO 2 in an O 2 /CO 2 -fired Power Plant[END_REF]. Where C TEG,ref is 2 million euro and CO 2 mass flow at reference is 205.86 kg/s.

S & M 6 . 0 ref , 2 CO 2 CO ref , TEG TEG f m m C C ⋅             ⋅ = ⋅ ⋅ (11)
In order to compare the thermal energy that is possible to take into account with the proposed process diagram, it is suggested to evaluate the electrical energy produced by means of the integration in the low-pressure section of a steam cycle. Two or three low-pressure heaters could be eliminated and, consequently, will increase net power output. As an example of a general power plant, it is supposed that the pressure of these extractions are 1.

h h * m h h * m h h * m Q (12)
Left-hand side of the equation is the heat taken from the intercooling compression, and the final enthalpy of the extraction is saturated liquid at 0.3 bar (lowest pressure heater outlet). Extra power in steam turbine, equation 13, is the power produced in the expansion of these steam bleedings from the original conditions to the condenser inlet (assuming a condenser pressure of 0.045 bar).

        - +         - +         - = bar 045 , 0 ste _ sat bar 3 , 0 3 bar 045 , 0 C º 110 bar 7 , 0 2 bar 045 , 0 C º 160 bar 5 , 1 1 extraST h h * m h h * m h h * m W ( 13
)
The configuration requires the constraints equations displayed in table 1. Constrains include the water content in CO 2 stream (X water ) and CO 2 properties (enthalpy, pressure and temperatures) at first compressor inlet, TEG and last compressor outlet. Constraints also include energy and mass balances for each equipment and negligible pressure drop through intercoolers. up to 10.4 MW. The net power consumption is 16.5 MW, slightly lower that previous case but 10.2 MW lower than base case. Compressor cost is higher than analysed cases due to higher consumption and specific volume. Heat exchangers costs are lower than before due to higher temperature difference.

As a consequence the COE is 39.21 /MWh, 0.02 /MWh lower than previous case and a reduction of 23.5% with the base case.

Increasing temperatures in compressor inlet with the objective to reduce heat exchanger cost do not improve results. For a compressor inlet temperature of 60ºC the COE is 39.34 /MWh higher that previous values. In this case the reduction of heat exchanger cost is not compensated by the increase in compressor cost and compressor power necessities. integration is not able to compensate the increase of compressor power when low efficiency equipment is used. Consequently, for this efficiency there are no differences in the incremental COE between the options of cooling CO 2 down to 30ºC or eliminate the second stage heat exchanger, 3.17 /MWh.

The effect of increasing the pressure ratio (reducing the number of stages from four to three) is showed in table 5. In general COE values increase, but the best option is reducing the compressor inlet temperature to 30ºC with the second heat exchanger. COE is 0.02 /MWh lower than the option of 45ºC and 0.89 /MWh lower than base case.

Figure 6 shows a possible integration scheme of intercooling compression and low-pressure section of a steam cycle for the case of 45/45ºC, 80% efficiency and four stages.

With data from previous optimization, table [START_REF] White | Purification of oxyfuel derived CO 2 for sequestration or EOR[END_REF], the reduction in each steam extraction mass flow is 8.7 kg/s for the 1.5 bar pressure bleeding and 9.0 kg/s for the 0.7 and 0.3 bar pressure bleedings, and therefore the extra power in steam turbine is 10.4 MWe.

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Present results provide support for the proposed integration, with a reduction of the incremental COE of compression from 3.54 to 2.71 /MWh. Anyway, specific studies are needed in order to fully integrate the intercooler heat streams into the capture system or steam cycles and to obtain a detailed compressor cost data that improve the cost analysis of the overall system.

CONCLUSIONS

CO 2 compression introduces an important energy and efficiency penalty in CCS technologies.

Intercooling compression is used to reduce compression power requirements. This heat is usually dissipated in the cooling equipment.

A CO 2 intercooling compression process flow diagram has been proposed. Intercooling has been divided in two-stages. The heat transferred in the first one is a useful energy and it could be used in the low-pressure part of the steam cycle for water preheating. The second one has to be dissipated in the cooling system. The second stage increases the cost of heat exchangers but compressors cost and power necessities decrease. A thermal simulation has been presented to calculate the minimum incremental COE due to CO 2 compression under several assumptions.

Results from energetic and economical analysis of different compression options have been suggested: firstly, the integration of CO 2 intercooling waste energy into the steam cycle reduces 23% the incremental COE associated with compression (approximately 0.8 /MWh), also there is a TCR augmentation around 2 M but power necessities reduce around 40% (10 MW as average); secondly, the compressor efficiency has an important influence in economical results, it has been calculated a reduction of 0.3-0.4 /MWh when results of 90% and 80% compressor efficiency are compared;

thirdly, the reduction of compressor stages does not improve energetic or economic results; and finally the interest of proposed two-stage intercooling has been demonstrated for high pressure ratio compressors (three compressor stages). With four compressor stages it is not necessary the second heat exchanger to obtain a minimum incremental COE.

Proposed integration results could be used to reduce the energy and efficiency penalty of CO 2 capture processes and, therefore, to reduce the CO 2 capture cost. However a detailed integration and optimization is required for each singular CCS application.

LIST OF FIGURES AND TABLES

  stream enthalpy from compressor (kJ/kg) h out exit stream enthalpy from compressor (kJ/kg) m mass flow feeding the compressor (kg/s) m i water mass flow (kg/s) m co2 CO 2 mass flow through compression (kg/s) m h2o water mass flow before compression (kg/swater outlet temperature (ºC) W compressor power (kW)

CO 2

 2 is liquefied to install a pump for increasing the pressure up to 100 bar. Cooling necessities for compressor intercooling were approximately as high as power demands, 53.3 MWth for cooling and 58 MWe for power. Moreover the CO 2 condensation demanded 35.2 MWth in a condenser with an important heat transfer area (mean logarithmic temperature difference of 3ºC) and additionally 3.6

Figure 4 .

 4 The novelty is that the cooling process is divided in two-stages. The heat extracted from the first one is a useful energy and it could be used in the low-pressure part of the steam cycle for water preheating (Q heating ). This strategy could reduce steam turbine bleedings mass flow and increases the turbine gross power output. The heat extracted from the second one has to be dissipated (Q cooling ), for example in the power plant cooling system (cooling tower). This stage would decreases CO 2 temperature in order to reduce compressor power requirements. The use of heat from intercooling in the preheating section of steam cycle enlarges the problem with new questions to solve:-Firstly, regarding intermediate and compressor inlet temperatures. Intermediate temperature has a limitation depending water from condenser temperature plus a temperature difference in heat exchangers. It is evident that lower compressor inlet temperatures means a reduction of compressor power needs, but it increases the cooling requirements and heat transfer area.-Secondly, the number of compressor stages and compressor isentropic efficiency. With four intercooler stages the compressor outlet temperatures are maintained in low-medium values and heat qualities are also low (i.e. it is only possible to transfer this energy to the very low temperature section of steam cycle that is condenser outlet). However, if the

5 /

 5 MWh has been considered. It has been supposed a 400 MW(net) power plant with specific emissions of 720 kg/MWh and 80% of plant capacity factor (CF). CO 2 mass flow to compression is assumed 72 kg/s, plus a water content of 15% (w). The objective function includes the capital investment and compressor consumption, 0.1, the annuity factor for an interest rate of 8.78% and 25 year project life. The new net power production, MWh, depends on the reference production, compressor power requirements and extra power in steam turbine (equation 2). The total compression power as the sum of the work from different compression stages (equation 3). The effect of taking advantage of intercooling heat is the reduction of steam turbine bleedings in the low-pressure part of the steam cycle. Consequently, mass flow in this part increases and steam turbine could be designed with an extra power production. The compressor output temperatures-enthalpies are calculated with compressor isentropic efficiencies and assuming negligible heat transfer with the ambient and no appreciable kinetic and potential energy variations (equation 4).

4 )

 4 Total capital requirement (TCR) is the sum of main components cost: Compressor, TEG unit and heat exchangers. Cooling process has been divided in two-stages. Heat exchanger cost is separated in the cost for equipment that heat condensate from steam cycle (HEheat) and the cost of cooling heat exchanger (HEcool).

( 7 )

 7 assuming a centrifugal, reciprocating compressor.

  5 and 0.7 bar of superheated steam at 160ºC and 110ºC, and 0.3 bar of saturated steam (69.1ºC). It is also supposed that extractions have the same mass flow, which is approximately correct as last supercritical cycle designs suggest. With the energy balance shown in equation (12) it is possible to calculate the reduction in extractions mass flow.

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

 123456 Figure 1. Basic configuration of the CO 2 intercooling compression Figure 2. Effect of compressor pressure ratio, efficiency and inlet temperature in power consumption and cooling requirements Figure 3. Effect of compressor pressure ratio, efficiency and inlet temperature in compressor outlet temperatures Figure 4. Proposed configuration of the CO 2 intercooling compression for energetic integration Figure 5. Composite curves. 5(a) compressor inlet temperature (30ºC) lower than intermediate temperature (45ºC). 5(b) compressor inlet temperature (45ºC) equal to intermediate temperature (45ºC) Figure 6. Integration of intercooling compression with steam cycle low-pressure section

  Tables 4-5 illustrate the effect of isentropic efficiency and pressure ratio on the incremental COE. In table 4 a compressor efficiency of 90% has been assumed. The base case has a compression total cost of 25.7 M mainly due to compressors (22.8 M ). Compressors power necessities are 28.0 MW and the COE is 39.65 /MWh. Therefore, the incremental COE due to CO 2 compression is 3.15 /MWh. This value is 0.39 /MWh lower than less efficient compressors. For this efficiency, differences in the incremental COE between options increase. The COE for the best option is 38.86 /MWh, 0.04 /MWh lower than the case of cooling CO 2 down to 30ºC. This represents a reduction of 25.1% with the base case of 90% efficiency. For compressor efficiency of 70% heat

Table 1 . Optimization constraintsTable 2 . Energetic comparison between base case and integration for different intermediate/compressor inlet temperatures (80% compressor efficiency and 4 stages)Table 3 . Economical comparison between base case and integration for different intermediate/compressor inlet temperatures (80% compressor efficiency and 4 stages)Table 4 . Economical comparison between base case and integration for different intermediate/compressor inlet temperatures (90% compressor efficiency and 4 stages) Table 5. Economical comparison between base case and integration for different intermediate/compressor inlet temperatures (80% compressor efficiency and 3 stages)

 1234

	h	co	2	_	i	=	f	p (	i	,	T i	)	2 T =	3 T	P 14 =	bar 30
			T 1 =	110	º	C	4 T =	5 T	14 T =	15 T
				P 1 =	bar 1				T 3 , 7 , 10 , 13 , 17	≥	45	º	C	P 18 =	bar 100
	X	water	_	1	=	0	.	15	T 5 , 8 , 11 , 15	≤	45	º	C	T 18 =	60	º	C

Table 1 . Optimization constraints

 1

Acknowledgements

The authors are grateful for the financial support from the Spanish Government, without which, this work could not have been undertaken. The work described in this paper was supported by the R+D Spanish National Program from the Spanish Ministry of Science and Education under project ENE2004-06053, Cuasi-zero CO 2 emissions power plant technologies research. The Spanish case.

The authors are thankful for the comments and advice given by the reviewers. They have improved the content of this work and pointed out some research directions of particular interest.

The process simulation was implemented with one objective function, 252 optimization variables and 252 constrain equations. Independent variables include: output temperature from first heat exchanger in each stage (T 3 = T 7 = T 10 = T 13 ), compressors inlet temperature (T 5 = T 8 = T 11 = T 15 ) and compressors pressure ratios and efficiencies.

For a specific integration of this scheme into a power plant an additional constraint is needed, that is, the total heat transferred to the steam cycle must not exceed the maximum specified. In parallel with solving the optimization problem, it is necessary to know the possibility of integrating the energy from compression in a steam cycle. An analysis using pinch methodology has been implemented to calculate the final condensate temperature.

The grand composite curves for some relevant cases are shown in Figure 5. The heat integration only presents a pinch point when compressor inlet temperature is lower than steam cycle condensate temperature plus a temperature difference. Additional cooling is only required in this situation, Figure 5a. The excess of heat above the pinch shows the possibility of integrating the lowpressure part of a steam cycle. The hot stream is the CO 2 coming from compressor, the heat is exchanged with low pressure water substituting some low pressure heaters.

Final water temperature depends on integration and variables affecting compressor performance. In order to have the same reference to compare results, an external heat source maintains this temperature in a constant value (T ref ). The heat source is the 1.5 bar pressure bleeding of the steam turbine. Equation 14shows the bleeding mass flow necessary to obtain a constant water temperature T ref .

( )

RESULTS AND DISCUSSION

Under usual optimization objectives, compression work should be minimized. That leads to reduce inlet compressor temperatures with the limit of cold reservoir plus a temperature difference in cooling equipment (e.g. 10ºC). For example, for cold reservoir temperature (ambient temperature or water) around 20ºC and a temperature difference of 10ºC the minimum compressor inlet temperature could be 30ºC. For the reference case, the energy from CO 2 intercooling is not integrated into the steam cycle, therefore the compressor inlet temperature is 30ºC. Assuming four compression stages

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with an isentropic efficiency of 80% the compression total cost is 28. There is a minimum COE when the second heat exchanger for cooling is eliminated. The compressor inlet temperature is equal to the intermediate temperature and higher than the base case.

This causes an important increase in power demand from 25.4 to 26.9 MW but there is also an important increase in available energy (see Figure 1) that increases the extra power in steam turbine