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LiBr absorption chillers with post-combustion
Applied Thermal Engineering 25 (2005) 87–99 www.elsevier.com/locate/apthermeng
Performance analysis of combined microgas turbines and gas fired water/LiBr absorption chillers with post-combustion Joan Carles Bruno *, Anton Valero, Alberto Coronas CREVER––Universitat Rovira i Virgili, Autovıa de Salou, s/n, 43006 Tarragona, Catalunya, Spain Received 23 November 2003; accepted 14 May 2004 Available online 25 June 2004
Abstract The integration of microgas turbines (MGT) and absorption chillers is an emerging technology that uses a wide range of fuels to produce electricity, cooling and heating simultaneously for small scale distributed generation in grid connected or isolated locations. This paper studies the performance of MGTs of different power capacities directly coupled to doubleeffect water–LiBr absorption chillers. In these systems the MGT exhaust gas is the heating medium to drive the chiller. Also post-combustion natural gas is used to increase the cooling capacity of the system. The paper analyses the effect of the post-combustion degree on the integrated system performance of four MGT power sizes. Two cases are considered. In the first, fresh air is added together with the post-combustion natural gas and in the second it is not added. In the latter case the oxygen necessary for the combustion reaction is extracted from the MGT exhaust gas stream. For the sake of comparison a study is also made of the performance of a more conventional system consisting of an MGT and a hot water heat exchanger to drive an absorption chiller. The main advantages of the new technology over this conventional system are that the COP of the chillers is higher because they are driven by higher temperatures, the production of electricity and chilled water is decoupled and there is a wider range of chilled water production capacity. 2004 Elsevier Ltd. All rights reserved. Keywords: Microgas turbines; Absorption chillers; Heat integration
*
Corresponding author. Tel.: +34-977-540205; fax: +34-977-542272. E-mail address: [email protected] (J.C. Bruno).
1359-4311/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2004.05.002
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1. Introduction and objectives The new distributed generation equipment coming onto commercial and industrial markets provides waste heat that can be applied to absorption cooling. This significantly increases overall system efficiencies. On the other hand, expanding power demands, increasingly restrictive environmental regulations and the deregulation of the electricity market are providing an opportunity for developing these new on-site distributed generation technologies such as microgas turbines (MGT). MGT technology can be integrated in distributed power generation systems to produce electricity, heating and cooling simultaneously for air conditioning and industrial applications although such uses as vehicle propulsion are also currently under development. The main advantages that they have over other competing technologies are the fuel flexibility, low emissions, quiet operation and low maintenance [1,2]. The electrical efficiency of the current regenerative MGTs, which are also characterised also by low pressure ratios and modest turbine inlet temperatures, is in the range of 25–30% depending on the MGT size. Thus if the overall system efficiency is to be competitive the waste heat available in the MGT exhaust gas must be effectively recovered. In warm climates such as the Mediterranean where there can be long hot periods, so-called trigeneration systems (power, hot and chilled water) are a better alternative than the simple cogeneration system for producing only power and heat. Absorption refrigeration systems can be used in combination with MGT to recover MGT waste heat so that chilled water can be produced and the electric load demanded by the more conventional compression refrigeration systems reduced or eliminated. Nowadays the only commercially mature technology for cooling using absorption chillers for small power applications in combination with MGT is the one using an intermediate gas/water heat exchanger that produces hot water to drive the absorption system. Because of the low temperatures, only single-effect absorption chillers can be used. So the existing MGT/absorption systems are too limited: their COPs are too low, the range of chilling capacities are not wide enough, and electricity and cooling capacity depend completely on each other, etc. For these reasons it was decided to analyse the possibility of using the exhaust gas in direct fired absorption machines with the option of natural gas post-combustion. Direct-fired chillers are more efficient and better positioned in the refrigeration market than their hot water driven counterparts. Some research projects are being carried out into direct MGT exhaust gas fired absorption systems although not all of them use cofiring. The most well known are the CEEE project at the University of Maryland [3], Southern California Gas Co., Takuma/Capstone and Ingersoll–Rand/ Hussman all in the USA, the University of Pisa project in Italy [4] and the Icogen S.A. development in Spain. Some new small ammonia/water generator–absorber heat exchange (GAX) chillers are promising because they can be driven directly by exhaust gas and do not require a cooling tower [5]. These GAX systems are entering the market now (2004) [6]. The objective of this paper is to study the integration of MGTs and double-effect direct-fired absorption systems. The exhaust gases from the MGT are directed to the absorption chiller with the option of additional natural gas post-combustion. Two cases are considered. In the first, fresh air is added together with the post-combustion natural gas. In the second, fresh air is not added and the oxygen necessary for the combustion reaction is extracted from the MGT exhaust gas stream. A comparison is made with a similar system that uses a hot-water driven absorption system. Before the modelling characteristics and the results are presented, the current technology
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in MGT and absorption cooling suitable for MGT/absorption refrigeration coupling will be briefly reviewed. Also the special features of the various MGT/absorption refrigeration integrated systems will be presented.
2. Current status of available microgas turbine technology Microgas turbines are small high-speed turbo-alternators of up to 200 kW, which consist of a centrifugal compressor, a radial turbine and a permanent magnet alternator rotor operating as a Brayton cycle. Their main feature is that a high-speed generator is directly coupled to the turbine rotor and that they use power electronics instead of a gearbox and a conventional generator to adapt the power produced to the grid power quality. For a complete description on the working principles, main features and benefits of the microgas turbine technology, the reader can refer to the available literature on this subject [7–9]. The electrical efficiency of current MGTs, which are also characterised by low-pressure ratios and modest turbine inlet temperatures, is between 25% and 30%, depending on the size of the MGT. When biogas is available and the average power output is in the order of magnitude of a few hundred kilowatts, MGTs could be the process of choice although the biogas burning option is not yet available in all the commercialised MGT models. Table 1 shows a list of the current manufacturers of MGTs below 200 kWe .
3. Available absorption chiller technology Today absorption chillers are a well-proven technology powered by hot water, steam or combustion gases unlike the more conventional compression chillers, which consume electricity. Absorption chilling is a key technology in distributed generation systems because it makes it possible to transform waste heat into cooling. At the same time it can reduce the electrical demand, increase the gas turbine capacity by inlet air-cooling and provide the necessary space cooling in summer. Table 2 summarises the main features of available water–LiBr chillers. Another interesting feature is that some direct-fired double-effect absorption systems can work as chiller/heaters: that is, they can heat and cool simultaneously. Table 1 Current manufacturers of microgas turbines (less than 200 kWe ) Manufacturer
Range of models
Capstone Turbine Corporation Elliot Energy Systems Inc. Turbec AB Bowman Power Ltd. Ingersoll–Rand Energy Systems
30, 60 kWe (next 200 kWe ) 80 kWe 100 kWe 50, 80 kWe 70 kWe (next 250 kWe )
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Table 2 Summary of available water–LiBr absorption chillers Type of chiller
Cooling capacity
High temperature source
COP
Single effect
350–5200 kW
0.6–0.7
Double effect
350–5000 kW
Steam at low pressure or hot water (as low as 90 C) Steam from 6 to 10 bar g or direct fired
1.0–1.2
16–400 kW (only direct fired)
Ammonia/water chillers are designed primarily for industrial refrigeration applications, such as freezing food or process refrigeration, with evaporator temperatures as low as )55 C. These systems are not designed to be driven by direct combustion of natural gas so they will not be considered in this study. There are also some air-cooled small-capacity units based on advanced single effect cycles (GAX cycles) directly driven by hot gases. Cerezo et al. [5] presents a study of a MGT of 30 kWe coupled with a GAX chiller cycle concept developed at the UNAM (Mexico).
4. MGT/absorption refrigeration integrated systems 4.1. Description of indirectly coupled MGT/absorption systems Nowadays the only commercially mature technology that produces cooling using absorption refrigeration for small power applications in combination with an MGT is an intermediate gas/ water heat exchanger that produces hot water to drive the absorption system. Because of the low temperatures, only single-effect absorption chillers can be used. So the existing MGT/absorption systems are too limited: their COPs are low, the range of chilling capacities is too restricted, the electricity and cooling capacity depend on each other completely, etc. Therefore, it is very interesting to analyse the possibility of using the exhaust gas in direct fired absorption machines with the option of natural gas post-combustion. Direct-fired chillers are more efficient and better positioned in the refrigeration market than their hot water driven counterparts. 4.2. Description of directly coupled MGT/absorption systems In the direct coupling MGT/absorption chiller configuration, the MGT exhaust gases directly drive the absorption chiller. In this way an intermediate heat exchanger is not required to produce hot water and the temperatures of the heating medium are higher so it is possible to drive a more efficient absorption system (for example, a double-effect chiller). The MGT exhaust gas, which contains a high concentration of O2 , can be used as combustion air in the direct-fired absorption chiller. In this case, as the chiller still uses its own burner, it can run independent of the MGT. So in this way the refrigeration and electric production are decoupled. Another important feature of this coupled system is that the burner can act as a postcombustion burner. In this case there is no longer any need for the capacity of the absorption chiller to match that of the MGT exactly. An MGT and an oversized direct-fired chiller can be
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Fig. 1. Diagram of a directly coupled MGT and absorption chiller.
used, driven by a mix of exhaust gases and additional combustion with fresh air if it is required. Among other benefits the exhaust gas from the MGT will help to reduce the chiller’s fuel expenditure. The gases at the outlet of the absorption chiller are often still hot enough to produce hot water (Fig. 1). As can be seen in Fig. 1, the system is very flexible and can provide electricity, chilled water and hot water at different decoupled loads. So the system can produce electricity, heating and cooling simultaneously, or only electricity and cooling, or electricity and heating. The flexibility and load decoupling is even higher if an absorption direct-fired chiller/heater is used instead of a chiller.
5. Modelling of the combined MGT/absorption chiller systems The complete combined system was modelled using the software package Engineering Equation Solver (EES). The advantage of this software is that users can easily build their own model equations and use the built-in thermodynamic properties for the most common substances, including water–LiBr properties. 5.1. Microgas turbine Four MGTs of 30, 60, 80 and 100 kWe have been modelled based on commercialised systems. The main model parameters for each microturbine are the following: • • • • • • •
pressure ratio; air inlet pressure and temperature; compressor and turbine isentropic efficiency; combustion chamber heat losses; turbine exit temperature; regenerator heat exchanger efficiency; regenerator exit temperature of the exhaust gas.
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The values of these parameters were obtained or calculated from the information provided by the manufacturers: electrical and thermal efficiencies, net power, temperatures at some points in the cycle, mass flow rates, etc. in ISO conditions. The weight composition of the natural gas has been taken as 80.1% of CH4 and 19.9% of C2 H6 . The natural gas is also considered to burn with an air excess of 600% in the combustion chamber. The regenerator efficiency (gx ) has been defined using the enthalpies of each stream as shown in Eq. (1), where hx , h2 and h4 are the specific enthalpies for the air stream leaving the regenerator and at the inlet of the combustion chamber, the air at the compressor outlet and the combustion gases at the turbine outlet before entering the regenerator, respectively. gx ¼
ðhx h2 Þ ðh4 h2 Þ
ð1Þ
The temperature of the combustion chamber exhaust gas is calculated using the energy balance given by Eq. (2) where the enthalpy of all the reacting components (Hr ) has to be equal to the enthalpy of all the combustion products (Hp ) minus a certain quantity to account for energy losses.
Energy losses Hr ¼ Hp 1 100
ð2Þ
5.2. Post-combustion chamber Two situations have been considered in the study of the post-combustion process: 1. The amount of fresh air added is equal to the stoichiometric air needed for the combustion of the natural gas considered in each case for post-combustion. 2. No fresh air is added so the microgas turbine exhaust gas is used as the only source of combustion air. To calculate the gas exit temperature in the post-combustion chamber in each situation, a parameter is used to account for heat losses using an equivalent expression to Eq. (2).
5.3. Absorption chillers The single- and double-effect absorption chillers were modelled with the usual assumptions made in the literature [10]. The most important parameters are the following. The inlet and outlet chilled water temperatures are 12.2 and 6.7 C, respectively. For the direct-fired chiller, the exhaust gas at the outlet of the chiller is set at 170 C and the temperature at the exit of the heat exchanger downstream of the chiller is considered to be 100 C. The inlet and outlet hot water temperatures considered for the hot water driven chiller are 93 and 87.7 C, respectively.
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6. Results 6.1. Performance of the modelled microgas turbines Four systems are modelled using four different regenerative MGT of 30, 60, 80 and 100 kWe . The MGT modelling parameters are based on the maximum number of data available from the information provided by the manufacturers of these systems. Table 3 shows the simulation results of the four studied MGTs modelled at full load and ISO conditions. These results will be used in the following sections to couple these MGT with absorption chillers. 6.2. Analysis of an MGT/hot water driven single-effect absorption chiller system The coupling of an MGT and a hot water single-effect absorption chiller can be considered as the conventional way for coupling MGTs and absorption chillers. As MGTs usually have a regenerative cycle configuration, the gas exhaust temperatures are only appropriate for the production of hot water at relatively low temperatures. At these temperatures only single-effect chillers can be used. Below we report the performance of conventional MGT/hot water driven absorption chillers. The results were obtained by modelling the system with Equation Engineering Software (EES). The single-effect chiller was set to provide chilled water at 7 C and driven by hot water at 90 C. As can be seen in Fig. 2, the maximum cooling capacity that can be obtained with each of the four MGTs considered in this study ranges from 36 kW, for the MGT with the smallest capacity, to 90 kW for the biggest. It is important to notice that only a few commercial chillers are available in this capacity range. Fig. 2 also shows the hot water production capacity, which corresponds to the heating power required to drive the absorption chiller.
Table 3 Calculated performance of the microgas turbines studied Variable
MGT 1 (30 kWe )
MGT 2 (60 kWe )
MGT 3 (80 kWe )
MGT 4 (100 kWe )
Temperature compressor inlet (C) Temperature compressor outlet (C) Temperature combustion chamber inlet (C) Temperature turbine inlet (C) Temperature turbine outlet (C) Temperature microturbine outlet (C) Natural gas mass flow rate (kg/h) Compressor isentropic efficiency (%) Turbine isentropic efficiency (%) Combustion chamber pressure (bara )
20a 225 533 867 593 294 8.5 80 73.8 4.8
20a 215 574 905 635 286 16 84.2 70.6 4.8
15 205 556 884 615 274 20.8 85 71.4 4.8
15 195 587 916 650 269 24.4 90 69 4.8
a
Increased temperature due to the air cooling of the electric generator.
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J.C. Bruno et al. / Applied Thermal Engineering 25 (2005) 87–99 Cooling
Heating
140
Power (kW)
120 100 80 60 40 20 0 20
40
60 80 Electric Power (kW)
100
Fig. 2. Production of cooling and heating power as a function of MGT electric power for the four MGTs studied.
6.3. Analysis of a MGT/direct-fired absorption chiller system This section presents the performance results of the direct coupling of an MGT and a doubleeffect water/LiBr absorption chiller connected to a heat recovery boiler arranged in the same way as in Fig. 1. Like the conventional configuration evaluated in the section above, the four most common MGT sizes in the present MGT market are considered for analysis. The performance parameter that is used to evaluate the overall system efficiency in this case is the Fuel utilisation factor (FUF), defined as FUF ¼
Electrical Power þ Cooling Power þ Heating Power 100 Fuel Consumption
ð3Þ
It is interesting to analyse the behaviour of the integrated system for different natural gas postcombustion contribution ratios with respect to the total gas consumption. This analysis was carried out with a parameter called PCF (post-combustion factor). It is defined (Eq. 4) as the ratio of the fuel burned in the MGT itself (QFMGT) with respect to the total quantity of fuel consumed, that is, including the fuel used for cofiring (QFPSC). PCF ¼
QFMTG QFPSC þ QFMGT
ð4Þ
Thus, a PCF value of 1 indicates that the system does not use cofiring, and an extreme value of PCF ¼ 0 indicates that only the post-combustion chamber drives the chiller and that the MGT is not running. 6.3.1. Analysis of an MGT/direct-fired absorption chiller with post-combustion and additional fresh air This subsection analyses the performance of several MGT/direct-fired absorption chillers with post-combustion and additional fresh air. The added air is just the stoichiometric amount of air required for natural gas combustion. Exhaust gases and additional air are mixed and used as combustion air.
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130
FUF (%)
120 110 100 90 80 70 0
0.2
0.4
0.6
0.8
1
PCF MGT 1 (30 kWe)
MGT 2 (60 Kwe)
MGT 3 (80 kWe)
MGT 4 (100 kWe)
Fig. 3. Evolution of the fuel utilisation factor (FUF) of the MGT/absorption chiller as a function of the post-combustion contribution to the total heat required to drive the chiller.
600
120
500
100
400
80
300
60
200
40
100
20
0
0
10
20 QFPSC (kg/h)
Heating Power
30
Cooling Power
40
FUF (%)
Power (kW)
It has been seen that increasing the post-combustion contribution to the total heat available for use in the absorption chiller increases the fuel utilisation factor of the integrated MTG/absorption system, as can be seen in Fig. 3. However, a point of maximum efficiency (FUF) appears because as the post-combustion fuel increases, the new fresh air for the post-combustion also increases and dilutes the favourable effect of the hot gases from the MGT on the FUF. As a result, at high postcombustion rates the cooling capacity is greater but the efficiency lower and there comes a time that the cooling capacity also starts to decline as can be seen in Fig. 4. The PCF value that produces the maximum efficiency is almost the same for all the MGTs analysed and is about 0.25– 0.28 (Fig. 3). This point of maximum FUF exactly matches the point of maximum temperature of the hot gas entering the chiller. Fig. 4 shows the effect of the natural gas post-combustion on a 30 kWe system. The results for the other MGT capacities considered (60, 80 and 100 kWe ) were similar so they have not been included for reasons of space. As mentioned above, an increase in natural gas post-combustion considerably increases cooling capacity and modestly increases heating capacity. Initially the efficiency increases too because the post-combustion contributes up to a maximum point at which the fresh air required for post-combustion has a dilution effect and starts to cool the exhaust gas entering the chiller. Beyond this point the efficiency decreases and the increase in cooling capacity is reduced. Finally, if the post-combustion consumption is further increased, the cooling capacity of the system begins to decrease because less heat is recovered by the double-effect chiller and at the same time the heating capacity of the downstream boiler increases significantly.
0
FUF
Fig. 4. Effect of the post-combustion natural gas flow rate on the performance of a 30 kWe MGT (MGT 1).
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Cooling Power (kW)
1800 1600 1400 1200 1000 800 600 400 200 0
0
20
MGT 1 (30 kWe)
40
60 80 QFPSC (kg/h)
MGT 2 (60 kWe)
MGT 3 (80 kWe)
100
120
MGT 4 (100 kWe)
Fig. 5. Cooling capacity of a double-effect cofired absorption chiller directly coupled to different MGTs as a function of the post-combustion natural gas with additional fresh air.
For the 30 kWe MGT the highest FUF is 109% (Fig. 4). At this point the PCF is 0.25 (25 kg/h of cofired and 8.35 kg/h of MGT natural gas consumption) and the cooling and heating capacities are 427 and 42 kW, respectively. Fig. 5 shows the cooling capacity of a chiller integrated in an MGT for different electric power capacities with post-combustion. The maximum cooling capacity goes from 500 kW to more than 1600 kW depending on the size of the MGT and the degree of post-combustion. The heat recovered from the exhaust gas leaving the chiller also depends on the degree of postcombustion but in the reverse sense (Fig. 6). With the initial increase in the post-combustion natural gas the heating capacity goes up slowly at first and then faster at higher natural gas consumption levels. At the same time the FUF and cooling capacity decrease. 6.3.2. Analysis of an MGT/direct-fired absorption chiller with post-combustion and no additional fresh air This subsection analyses the performance of several MGT/direct-fired absorption chillers with post-combustion but without fresh air. The oxygen necessary for the combustion reaction is taken from the MGT exhaust gas stream.
Heating Power (kW)
400 350 300 250 200 150 100 50 0
0
20 MGT 1 (30 kWe)
40
60 80 QFPSC (kg/h)
MGT 2 (60 kWe)
MGT 3 (80 kWe)
100
120
MGT 4 (100 kWe)
Fig. 6. Heating capacity of an MGT/chiller system produced by the heat recovery of the gas leaving the chiller as a function of the post-combustion natural gas with additional fresh air.
Post-combustion outlet temp. (Cº)
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1200 1000 800 600 400 200 0
0
5
10
MGT 1 (30 kWe)
15
20 25 30 QFPSC (kg/h)
MGT 2 (60 kWe)
35
MGT 3 (80 kWe)
40
45
50
MGT 4 (100 kWe)
Fig. 7. Post-combustion outlet temperature without adding fresh air to the MGT exhaust gases used as combustion air.
In this case, as no additional air is used, the MGT hot exhaust gases are not diluted and the temperatures achieved with the same amount of post-combustion are higher than those obtained with additional air. So the gas temperature reached in the post-combustion process limits the maximum amount of natural gas used for post-combustion. The material’s resistance to high temperatures sets this maximum temperature. Fig. 7 shows the temperatures reached with different amounts of post-combustion natural gas for each MGT. With additional fresh air the maximum temperature of the gas at the post-combustion outlet is always lower than 775 C for all MGT/absorption systems. As can be seen in Fig. 8, even though no fresh air is added there is no limitation because of the quantity of oxygen available. The system reaches its maximum temperature long before the oxygen available is exhausted. Fig. 9 shows the maximum cooling capacity for each MGT with post-combustion and without adding fresh air. If these results are compared with those obtained when fresh air is added (Fig. 5), it can be seen that the maximum cooling power is lower. Nevertheless, below the maximum attainable point, the differences are not very significative when fresh air is not added. For example, for MGT 2 (60 kWe ) and using 30 kg/h of post-combustion natural gas with fresh air, the cooling capacity is 599 kW. With additional air it is only 570 kW. The heating capacity in the case without fresh air changes only slightly with the post-combustion natural gas flow rate consumption. This is because the inlet and outlet gas temperatures from the boiler are fixed at 170 C in the absorption chiller outlet and at 100 C in the boiler outlet. Also the exhaust gas flow rate changes are very small because fresh air is not added. So the 20
% O2
18 16 14 12 10 8
0
10 MGT 1 (30 kW)
20 30 QFPSC (kg/h) MGT 2 (60 kW)
MGT 3 (80 kW)
40
50
MGT 4 (100 kW)
Fig. 8. Oxygen content at the post-combustion outlet without adding fresh air to the MGT exhaust gases used as combustion air.
J.C. Bruno et al. / Applied Thermal Engineering 25 (2005) 87–99
Cooling Power (kW)
98
1000 900 800 700 600 500 400 300 200 100 0 0 MGT 1 (30 kWe)
10
20 30 QFPSC (kg/h) MGT 2 (60 kWe)
MGT 3 (80 kWe)
40
50
MGT 4 (100 kWe)
Fig. 9. Cooling capacity of a double-effect cofired absorption chiller directly coupled to different MGTs without additional fresh air as a function of post-combustion natural gas consumption.
heating production is around 21, 39, 51 and 60 kW for the four MGTs studied. The maximum value of FUF is lower than when fresh air is added, although it is quite high (around 106% for the four MGTs considered). When designing an MGT/absorption chiller with a post-combustion system, some practical considerations for the specific burner, absorption chiller and gas exhaust duct system must be taken into account. For example, the maximum air combustion temperature that the type of burner can accept or the maximum and minimum gas flow rate that the selected absorption chiller can handle. Likewise, the two possibilities of adding fresh air or not can be included in the same design, depending on the specific electricity, heating and cooling site demands.
7. Conclusions The main advantages of a directly driven absorption chiller with a post-combustion system over the more conventional single-effect hot-water driven system are a higher COP, a decoupling between electricity and chilled water production and a wider range of chilled water production capacity. Two directly fired cases have been studied: in one case extra fresh air was added for postcombustion and in the other it was not. With additional fresh air, the cooling capacity of a directly fired double-effect chiller integrated in an MGT with an electric power capacity between 30 and 100 kWe increases from 500 to more than 1600 kW depending on the MGT size and the degree of post-combustion. In the MGT/absorption chiller system with post-combustion and fresh air there is an optimum amount of post-combustion represented by a PCF ratio of about 0.25 that produces the optimum global operation point of the system. This means that at this ratio value, the operational costs will be lowest. This point of maximum efficiency exactly matches the maximum temperature of the gases entering the chiller. As the amount of post-combustion natural gas increases so does the cooling capacity but the fresh air required for cofiring also increases and dilutes the heat provided by the gases coming from the MGT. For this reason the cooling power decreases after a certain optimum value of post-combustion is exceeded.
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When no additional fresh air is added for the post-combustion, the MGT exhaust gas is directly used as combustion air. In this case, the amount of post-combustion natural gas is limited by the maximum temperature that the post-combustion chamber can withstand, not by the oxygen content in the MGT exhaust gas. The maximum cooling capacity in this case is lower than when additional fresh air is used. Below this maximum point, the cooling capacities with and without additional air are not very different but higher when fresh air is added. Acknowledgements The authors acknowledge the funding of this project by the Ministerio de Ciencia y Tecnologıa of Spain through the Programa PROFIT, FIT-020100-2002-268 and the collaboration of Mr. Jordi L opez Launes. References [1] B. Peters, K. Dielmann, Capstone Microturbines: experiences with different gaseous and liquid fuels, Workshop on Energy Efficiency and Emissions Reduction with MicroGas Turbines––Technology and Operating Experience, Tarragona, 22nd October 2002. [2] J.C. Bruno, Ll. Massagues, A. Coronas, Power quality and air emission tests in a microgas turbine cogeneration plant, in: Proceedings of the International Conference on Renewable Energy and Power Quality (ICREPQ’03), Vigo (Spain), 2003. [3] M. Cowie, Microturbine based CHP system testing, Third Annual Workshop on Microturbine Applications, Calgary, Canda, 2003. [4] S. Banetta, F. Paganucci, R. Giglioli, System description and test planning for a combined heat and power (CHP) plant composed by a microgas turbine and an absorption chiller/heater, in: Proceedings de ASME Turbo Expo 2001, New Orleans, USA, 4–7 July 2001. [5] J. Cerezo, J.C. Bruno, W. Rivera, A. Coronas, Energy analysis of microgas turbine/absorption chiller combined systems, International Congress of Refrigeration, Washington DC, 2003. [6] W. Ryan, New developments in gas cooling, ASHRAE Journal 44 (4) (2002) 23–28. [7] M.J. Moore (Ed.), Microturbine Generators, Professional Engineering Publishing Ltd., Institution of Mechanical Engineers, England, 2002, ISBN 1-86058-391-1. [8] P.A. Pilavachi, Mini- and Micro-gas turbines for combined heat and power, Applied Thermal Engineering 22 (2002) 2003–2014. [9] J.C. Bruno, L.L. Massagues, A. Coronas, Stand-alone and grid-connected performance analysis of a regenerative microgas turbine cogeneration plant, Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 218 (2004) 15–22. [10] K.E. Herold, R. Radermacher, S.A. Klein, Absorption Chillers and Heat Pumps, CRC Press, New York, 1995.
Performance analysis of combined microgas turbines and gas fired water/LiBr absorption chillers with post-combustion Joan Carles Bruno *, Anton Valero, Alberto Coronas CREVER––Universitat Rovira i Virgili, Autovıa de Salou, s/n, 43006 Tarragona, Catalunya, Spain Received 23 November 2003; accepted 14 May 2004 Available online 25 June 2004
Abstract The integration of microgas turbines (MGT) and absorption chillers is an emerging technology that uses a wide range of fuels to produce electricity, cooling and heating simultaneously for small scale distributed generation in grid connected or isolated locations. This paper studies the performance of MGTs of different power capacities directly coupled to doubleeffect water–LiBr absorption chillers. In these systems the MGT exhaust gas is the heating medium to drive the chiller. Also post-combustion natural gas is used to increase the cooling capacity of the system. The paper analyses the effect of the post-combustion degree on the integrated system performance of four MGT power sizes. Two cases are considered. In the first, fresh air is added together with the post-combustion natural gas and in the second it is not added. In the latter case the oxygen necessary for the combustion reaction is extracted from the MGT exhaust gas stream. For the sake of comparison a study is also made of the performance of a more conventional system consisting of an MGT and a hot water heat exchanger to drive an absorption chiller. The main advantages of the new technology over this conventional system are that the COP of the chillers is higher because they are driven by higher temperatures, the production of electricity and chilled water is decoupled and there is a wider range of chilled water production capacity. 2004 Elsevier Ltd. All rights reserved. Keywords: Microgas turbines; Absorption chillers; Heat integration
*
Corresponding author. Tel.: +34-977-540205; fax: +34-977-542272. E-mail address: [email protected] (J.C. Bruno).
1359-4311/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2004.05.002
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1. Introduction and objectives The new distributed generation equipment coming onto commercial and industrial markets provides waste heat that can be applied to absorption cooling. This significantly increases overall system efficiencies. On the other hand, expanding power demands, increasingly restrictive environmental regulations and the deregulation of the electricity market are providing an opportunity for developing these new on-site distributed generation technologies such as microgas turbines (MGT). MGT technology can be integrated in distributed power generation systems to produce electricity, heating and cooling simultaneously for air conditioning and industrial applications although such uses as vehicle propulsion are also currently under development. The main advantages that they have over other competing technologies are the fuel flexibility, low emissions, quiet operation and low maintenance [1,2]. The electrical efficiency of the current regenerative MGTs, which are also characterised also by low pressure ratios and modest turbine inlet temperatures, is in the range of 25–30% depending on the MGT size. Thus if the overall system efficiency is to be competitive the waste heat available in the MGT exhaust gas must be effectively recovered. In warm climates such as the Mediterranean where there can be long hot periods, so-called trigeneration systems (power, hot and chilled water) are a better alternative than the simple cogeneration system for producing only power and heat. Absorption refrigeration systems can be used in combination with MGT to recover MGT waste heat so that chilled water can be produced and the electric load demanded by the more conventional compression refrigeration systems reduced or eliminated. Nowadays the only commercially mature technology for cooling using absorption chillers for small power applications in combination with MGT is the one using an intermediate gas/water heat exchanger that produces hot water to drive the absorption system. Because of the low temperatures, only single-effect absorption chillers can be used. So the existing MGT/absorption systems are too limited: their COPs are too low, the range of chilling capacities are not wide enough, and electricity and cooling capacity depend completely on each other, etc. For these reasons it was decided to analyse the possibility of using the exhaust gas in direct fired absorption machines with the option of natural gas post-combustion. Direct-fired chillers are more efficient and better positioned in the refrigeration market than their hot water driven counterparts. Some research projects are being carried out into direct MGT exhaust gas fired absorption systems although not all of them use cofiring. The most well known are the CEEE project at the University of Maryland [3], Southern California Gas Co., Takuma/Capstone and Ingersoll–Rand/ Hussman all in the USA, the University of Pisa project in Italy [4] and the Icogen S.A. development in Spain. Some new small ammonia/water generator–absorber heat exchange (GAX) chillers are promising because they can be driven directly by exhaust gas and do not require a cooling tower [5]. These GAX systems are entering the market now (2004) [6]. The objective of this paper is to study the integration of MGTs and double-effect direct-fired absorption systems. The exhaust gases from the MGT are directed to the absorption chiller with the option of additional natural gas post-combustion. Two cases are considered. In the first, fresh air is added together with the post-combustion natural gas. In the second, fresh air is not added and the oxygen necessary for the combustion reaction is extracted from the MGT exhaust gas stream. A comparison is made with a similar system that uses a hot-water driven absorption system. Before the modelling characteristics and the results are presented, the current technology
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in MGT and absorption cooling suitable for MGT/absorption refrigeration coupling will be briefly reviewed. Also the special features of the various MGT/absorption refrigeration integrated systems will be presented.
2. Current status of available microgas turbine technology Microgas turbines are small high-speed turbo-alternators of up to 200 kW, which consist of a centrifugal compressor, a radial turbine and a permanent magnet alternator rotor operating as a Brayton cycle. Their main feature is that a high-speed generator is directly coupled to the turbine rotor and that they use power electronics instead of a gearbox and a conventional generator to adapt the power produced to the grid power quality. For a complete description on the working principles, main features and benefits of the microgas turbine technology, the reader can refer to the available literature on this subject [7–9]. The electrical efficiency of current MGTs, which are also characterised by low-pressure ratios and modest turbine inlet temperatures, is between 25% and 30%, depending on the size of the MGT. When biogas is available and the average power output is in the order of magnitude of a few hundred kilowatts, MGTs could be the process of choice although the biogas burning option is not yet available in all the commercialised MGT models. Table 1 shows a list of the current manufacturers of MGTs below 200 kWe .
3. Available absorption chiller technology Today absorption chillers are a well-proven technology powered by hot water, steam or combustion gases unlike the more conventional compression chillers, which consume electricity. Absorption chilling is a key technology in distributed generation systems because it makes it possible to transform waste heat into cooling. At the same time it can reduce the electrical demand, increase the gas turbine capacity by inlet air-cooling and provide the necessary space cooling in summer. Table 2 summarises the main features of available water–LiBr chillers. Another interesting feature is that some direct-fired double-effect absorption systems can work as chiller/heaters: that is, they can heat and cool simultaneously. Table 1 Current manufacturers of microgas turbines (less than 200 kWe ) Manufacturer
Range of models
Capstone Turbine Corporation Elliot Energy Systems Inc. Turbec AB Bowman Power Ltd. Ingersoll–Rand Energy Systems
30, 60 kWe (next 200 kWe ) 80 kWe 100 kWe 50, 80 kWe 70 kWe (next 250 kWe )
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Table 2 Summary of available water–LiBr absorption chillers Type of chiller
Cooling capacity
High temperature source
COP
Single effect
350–5200 kW
0.6–0.7
Double effect
350–5000 kW
Steam at low pressure or hot water (as low as 90 C) Steam from 6 to 10 bar g or direct fired
1.0–1.2
16–400 kW (only direct fired)
Ammonia/water chillers are designed primarily for industrial refrigeration applications, such as freezing food or process refrigeration, with evaporator temperatures as low as )55 C. These systems are not designed to be driven by direct combustion of natural gas so they will not be considered in this study. There are also some air-cooled small-capacity units based on advanced single effect cycles (GAX cycles) directly driven by hot gases. Cerezo et al. [5] presents a study of a MGT of 30 kWe coupled with a GAX chiller cycle concept developed at the UNAM (Mexico).
4. MGT/absorption refrigeration integrated systems 4.1. Description of indirectly coupled MGT/absorption systems Nowadays the only commercially mature technology that produces cooling using absorption refrigeration for small power applications in combination with an MGT is an intermediate gas/ water heat exchanger that produces hot water to drive the absorption system. Because of the low temperatures, only single-effect absorption chillers can be used. So the existing MGT/absorption systems are too limited: their COPs are low, the range of chilling capacities is too restricted, the electricity and cooling capacity depend on each other completely, etc. Therefore, it is very interesting to analyse the possibility of using the exhaust gas in direct fired absorption machines with the option of natural gas post-combustion. Direct-fired chillers are more efficient and better positioned in the refrigeration market than their hot water driven counterparts. 4.2. Description of directly coupled MGT/absorption systems In the direct coupling MGT/absorption chiller configuration, the MGT exhaust gases directly drive the absorption chiller. In this way an intermediate heat exchanger is not required to produce hot water and the temperatures of the heating medium are higher so it is possible to drive a more efficient absorption system (for example, a double-effect chiller). The MGT exhaust gas, which contains a high concentration of O2 , can be used as combustion air in the direct-fired absorption chiller. In this case, as the chiller still uses its own burner, it can run independent of the MGT. So in this way the refrigeration and electric production are decoupled. Another important feature of this coupled system is that the burner can act as a postcombustion burner. In this case there is no longer any need for the capacity of the absorption chiller to match that of the MGT exactly. An MGT and an oversized direct-fired chiller can be
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Fig. 1. Diagram of a directly coupled MGT and absorption chiller.
used, driven by a mix of exhaust gases and additional combustion with fresh air if it is required. Among other benefits the exhaust gas from the MGT will help to reduce the chiller’s fuel expenditure. The gases at the outlet of the absorption chiller are often still hot enough to produce hot water (Fig. 1). As can be seen in Fig. 1, the system is very flexible and can provide electricity, chilled water and hot water at different decoupled loads. So the system can produce electricity, heating and cooling simultaneously, or only electricity and cooling, or electricity and heating. The flexibility and load decoupling is even higher if an absorption direct-fired chiller/heater is used instead of a chiller.
5. Modelling of the combined MGT/absorption chiller systems The complete combined system was modelled using the software package Engineering Equation Solver (EES). The advantage of this software is that users can easily build their own model equations and use the built-in thermodynamic properties for the most common substances, including water–LiBr properties. 5.1. Microgas turbine Four MGTs of 30, 60, 80 and 100 kWe have been modelled based on commercialised systems. The main model parameters for each microturbine are the following: • • • • • • •
pressure ratio; air inlet pressure and temperature; compressor and turbine isentropic efficiency; combustion chamber heat losses; turbine exit temperature; regenerator heat exchanger efficiency; regenerator exit temperature of the exhaust gas.
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The values of these parameters were obtained or calculated from the information provided by the manufacturers: electrical and thermal efficiencies, net power, temperatures at some points in the cycle, mass flow rates, etc. in ISO conditions. The weight composition of the natural gas has been taken as 80.1% of CH4 and 19.9% of C2 H6 . The natural gas is also considered to burn with an air excess of 600% in the combustion chamber. The regenerator efficiency (gx ) has been defined using the enthalpies of each stream as shown in Eq. (1), where hx , h2 and h4 are the specific enthalpies for the air stream leaving the regenerator and at the inlet of the combustion chamber, the air at the compressor outlet and the combustion gases at the turbine outlet before entering the regenerator, respectively. gx ¼
ðhx h2 Þ ðh4 h2 Þ
ð1Þ
The temperature of the combustion chamber exhaust gas is calculated using the energy balance given by Eq. (2) where the enthalpy of all the reacting components (Hr ) has to be equal to the enthalpy of all the combustion products (Hp ) minus a certain quantity to account for energy losses.
Energy losses Hr ¼ Hp 1 100
ð2Þ
5.2. Post-combustion chamber Two situations have been considered in the study of the post-combustion process: 1. The amount of fresh air added is equal to the stoichiometric air needed for the combustion of the natural gas considered in each case for post-combustion. 2. No fresh air is added so the microgas turbine exhaust gas is used as the only source of combustion air. To calculate the gas exit temperature in the post-combustion chamber in each situation, a parameter is used to account for heat losses using an equivalent expression to Eq. (2).
5.3. Absorption chillers The single- and double-effect absorption chillers were modelled with the usual assumptions made in the literature [10]. The most important parameters are the following. The inlet and outlet chilled water temperatures are 12.2 and 6.7 C, respectively. For the direct-fired chiller, the exhaust gas at the outlet of the chiller is set at 170 C and the temperature at the exit of the heat exchanger downstream of the chiller is considered to be 100 C. The inlet and outlet hot water temperatures considered for the hot water driven chiller are 93 and 87.7 C, respectively.
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6. Results 6.1. Performance of the modelled microgas turbines Four systems are modelled using four different regenerative MGT of 30, 60, 80 and 100 kWe . The MGT modelling parameters are based on the maximum number of data available from the information provided by the manufacturers of these systems. Table 3 shows the simulation results of the four studied MGTs modelled at full load and ISO conditions. These results will be used in the following sections to couple these MGT with absorption chillers. 6.2. Analysis of an MGT/hot water driven single-effect absorption chiller system The coupling of an MGT and a hot water single-effect absorption chiller can be considered as the conventional way for coupling MGTs and absorption chillers. As MGTs usually have a regenerative cycle configuration, the gas exhaust temperatures are only appropriate for the production of hot water at relatively low temperatures. At these temperatures only single-effect chillers can be used. Below we report the performance of conventional MGT/hot water driven absorption chillers. The results were obtained by modelling the system with Equation Engineering Software (EES). The single-effect chiller was set to provide chilled water at 7 C and driven by hot water at 90 C. As can be seen in Fig. 2, the maximum cooling capacity that can be obtained with each of the four MGTs considered in this study ranges from 36 kW, for the MGT with the smallest capacity, to 90 kW for the biggest. It is important to notice that only a few commercial chillers are available in this capacity range. Fig. 2 also shows the hot water production capacity, which corresponds to the heating power required to drive the absorption chiller.
Table 3 Calculated performance of the microgas turbines studied Variable
MGT 1 (30 kWe )
MGT 2 (60 kWe )
MGT 3 (80 kWe )
MGT 4 (100 kWe )
Temperature compressor inlet (C) Temperature compressor outlet (C) Temperature combustion chamber inlet (C) Temperature turbine inlet (C) Temperature turbine outlet (C) Temperature microturbine outlet (C) Natural gas mass flow rate (kg/h) Compressor isentropic efficiency (%) Turbine isentropic efficiency (%) Combustion chamber pressure (bara )
20a 225 533 867 593 294 8.5 80 73.8 4.8
20a 215 574 905 635 286 16 84.2 70.6 4.8
15 205 556 884 615 274 20.8 85 71.4 4.8
15 195 587 916 650 269 24.4 90 69 4.8
a
Increased temperature due to the air cooling of the electric generator.
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Heating
140
Power (kW)
120 100 80 60 40 20 0 20
40
60 80 Electric Power (kW)
100
Fig. 2. Production of cooling and heating power as a function of MGT electric power for the four MGTs studied.
6.3. Analysis of a MGT/direct-fired absorption chiller system This section presents the performance results of the direct coupling of an MGT and a doubleeffect water/LiBr absorption chiller connected to a heat recovery boiler arranged in the same way as in Fig. 1. Like the conventional configuration evaluated in the section above, the four most common MGT sizes in the present MGT market are considered for analysis. The performance parameter that is used to evaluate the overall system efficiency in this case is the Fuel utilisation factor (FUF), defined as FUF ¼
Electrical Power þ Cooling Power þ Heating Power 100 Fuel Consumption
ð3Þ
It is interesting to analyse the behaviour of the integrated system for different natural gas postcombustion contribution ratios with respect to the total gas consumption. This analysis was carried out with a parameter called PCF (post-combustion factor). It is defined (Eq. 4) as the ratio of the fuel burned in the MGT itself (QFMGT) with respect to the total quantity of fuel consumed, that is, including the fuel used for cofiring (QFPSC). PCF ¼
QFMTG QFPSC þ QFMGT
ð4Þ
Thus, a PCF value of 1 indicates that the system does not use cofiring, and an extreme value of PCF ¼ 0 indicates that only the post-combustion chamber drives the chiller and that the MGT is not running. 6.3.1. Analysis of an MGT/direct-fired absorption chiller with post-combustion and additional fresh air This subsection analyses the performance of several MGT/direct-fired absorption chillers with post-combustion and additional fresh air. The added air is just the stoichiometric amount of air required for natural gas combustion. Exhaust gases and additional air are mixed and used as combustion air.
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130
FUF (%)
120 110 100 90 80 70 0
0.2
0.4
0.6
0.8
1
PCF MGT 1 (30 kWe)
MGT 2 (60 Kwe)
MGT 3 (80 kWe)
MGT 4 (100 kWe)
Fig. 3. Evolution of the fuel utilisation factor (FUF) of the MGT/absorption chiller as a function of the post-combustion contribution to the total heat required to drive the chiller.
600
120
500
100
400
80
300
60
200
40
100
20
0
0
10
20 QFPSC (kg/h)
Heating Power
30
Cooling Power
40
FUF (%)
Power (kW)
It has been seen that increasing the post-combustion contribution to the total heat available for use in the absorption chiller increases the fuel utilisation factor of the integrated MTG/absorption system, as can be seen in Fig. 3. However, a point of maximum efficiency (FUF) appears because as the post-combustion fuel increases, the new fresh air for the post-combustion also increases and dilutes the favourable effect of the hot gases from the MGT on the FUF. As a result, at high postcombustion rates the cooling capacity is greater but the efficiency lower and there comes a time that the cooling capacity also starts to decline as can be seen in Fig. 4. The PCF value that produces the maximum efficiency is almost the same for all the MGTs analysed and is about 0.25– 0.28 (Fig. 3). This point of maximum FUF exactly matches the point of maximum temperature of the hot gas entering the chiller. Fig. 4 shows the effect of the natural gas post-combustion on a 30 kWe system. The results for the other MGT capacities considered (60, 80 and 100 kWe ) were similar so they have not been included for reasons of space. As mentioned above, an increase in natural gas post-combustion considerably increases cooling capacity and modestly increases heating capacity. Initially the efficiency increases too because the post-combustion contributes up to a maximum point at which the fresh air required for post-combustion has a dilution effect and starts to cool the exhaust gas entering the chiller. Beyond this point the efficiency decreases and the increase in cooling capacity is reduced. Finally, if the post-combustion consumption is further increased, the cooling capacity of the system begins to decrease because less heat is recovered by the double-effect chiller and at the same time the heating capacity of the downstream boiler increases significantly.
0
FUF
Fig. 4. Effect of the post-combustion natural gas flow rate on the performance of a 30 kWe MGT (MGT 1).
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Cooling Power (kW)
1800 1600 1400 1200 1000 800 600 400 200 0
0
20
MGT 1 (30 kWe)
40
60 80 QFPSC (kg/h)
MGT 2 (60 kWe)
MGT 3 (80 kWe)
100
120
MGT 4 (100 kWe)
Fig. 5. Cooling capacity of a double-effect cofired absorption chiller directly coupled to different MGTs as a function of the post-combustion natural gas with additional fresh air.
For the 30 kWe MGT the highest FUF is 109% (Fig. 4). At this point the PCF is 0.25 (25 kg/h of cofired and 8.35 kg/h of MGT natural gas consumption) and the cooling and heating capacities are 427 and 42 kW, respectively. Fig. 5 shows the cooling capacity of a chiller integrated in an MGT for different electric power capacities with post-combustion. The maximum cooling capacity goes from 500 kW to more than 1600 kW depending on the size of the MGT and the degree of post-combustion. The heat recovered from the exhaust gas leaving the chiller also depends on the degree of postcombustion but in the reverse sense (Fig. 6). With the initial increase in the post-combustion natural gas the heating capacity goes up slowly at first and then faster at higher natural gas consumption levels. At the same time the FUF and cooling capacity decrease. 6.3.2. Analysis of an MGT/direct-fired absorption chiller with post-combustion and no additional fresh air This subsection analyses the performance of several MGT/direct-fired absorption chillers with post-combustion but without fresh air. The oxygen necessary for the combustion reaction is taken from the MGT exhaust gas stream.
Heating Power (kW)
400 350 300 250 200 150 100 50 0
0
20 MGT 1 (30 kWe)
40
60 80 QFPSC (kg/h)
MGT 2 (60 kWe)
MGT 3 (80 kWe)
100
120
MGT 4 (100 kWe)
Fig. 6. Heating capacity of an MGT/chiller system produced by the heat recovery of the gas leaving the chiller as a function of the post-combustion natural gas with additional fresh air.
Post-combustion outlet temp. (Cº)
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1200 1000 800 600 400 200 0
0
5
10
MGT 1 (30 kWe)
15
20 25 30 QFPSC (kg/h)
MGT 2 (60 kWe)
35
MGT 3 (80 kWe)
40
45
50
MGT 4 (100 kWe)
Fig. 7. Post-combustion outlet temperature without adding fresh air to the MGT exhaust gases used as combustion air.
In this case, as no additional air is used, the MGT hot exhaust gases are not diluted and the temperatures achieved with the same amount of post-combustion are higher than those obtained with additional air. So the gas temperature reached in the post-combustion process limits the maximum amount of natural gas used for post-combustion. The material’s resistance to high temperatures sets this maximum temperature. Fig. 7 shows the temperatures reached with different amounts of post-combustion natural gas for each MGT. With additional fresh air the maximum temperature of the gas at the post-combustion outlet is always lower than 775 C for all MGT/absorption systems. As can be seen in Fig. 8, even though no fresh air is added there is no limitation because of the quantity of oxygen available. The system reaches its maximum temperature long before the oxygen available is exhausted. Fig. 9 shows the maximum cooling capacity for each MGT with post-combustion and without adding fresh air. If these results are compared with those obtained when fresh air is added (Fig. 5), it can be seen that the maximum cooling power is lower. Nevertheless, below the maximum attainable point, the differences are not very significative when fresh air is not added. For example, for MGT 2 (60 kWe ) and using 30 kg/h of post-combustion natural gas with fresh air, the cooling capacity is 599 kW. With additional air it is only 570 kW. The heating capacity in the case without fresh air changes only slightly with the post-combustion natural gas flow rate consumption. This is because the inlet and outlet gas temperatures from the boiler are fixed at 170 C in the absorption chiller outlet and at 100 C in the boiler outlet. Also the exhaust gas flow rate changes are very small because fresh air is not added. So the 20
% O2
18 16 14 12 10 8
0
10 MGT 1 (30 kW)
20 30 QFPSC (kg/h) MGT 2 (60 kW)
MGT 3 (80 kW)
40
50
MGT 4 (100 kW)
Fig. 8. Oxygen content at the post-combustion outlet without adding fresh air to the MGT exhaust gases used as combustion air.
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Cooling Power (kW)
98
1000 900 800 700 600 500 400 300 200 100 0 0 MGT 1 (30 kWe)
10
20 30 QFPSC (kg/h) MGT 2 (60 kWe)
MGT 3 (80 kWe)
40
50
MGT 4 (100 kWe)
Fig. 9. Cooling capacity of a double-effect cofired absorption chiller directly coupled to different MGTs without additional fresh air as a function of post-combustion natural gas consumption.
heating production is around 21, 39, 51 and 60 kW for the four MGTs studied. The maximum value of FUF is lower than when fresh air is added, although it is quite high (around 106% for the four MGTs considered). When designing an MGT/absorption chiller with a post-combustion system, some practical considerations for the specific burner, absorption chiller and gas exhaust duct system must be taken into account. For example, the maximum air combustion temperature that the type of burner can accept or the maximum and minimum gas flow rate that the selected absorption chiller can handle. Likewise, the two possibilities of adding fresh air or not can be included in the same design, depending on the specific electricity, heating and cooling site demands.
7. Conclusions The main advantages of a directly driven absorption chiller with a post-combustion system over the more conventional single-effect hot-water driven system are a higher COP, a decoupling between electricity and chilled water production and a wider range of chilled water production capacity. Two directly fired cases have been studied: in one case extra fresh air was added for postcombustion and in the other it was not. With additional fresh air, the cooling capacity of a directly fired double-effect chiller integrated in an MGT with an electric power capacity between 30 and 100 kWe increases from 500 to more than 1600 kW depending on the MGT size and the degree of post-combustion. In the MGT/absorption chiller system with post-combustion and fresh air there is an optimum amount of post-combustion represented by a PCF ratio of about 0.25 that produces the optimum global operation point of the system. This means that at this ratio value, the operational costs will be lowest. This point of maximum efficiency exactly matches the maximum temperature of the gases entering the chiller. As the amount of post-combustion natural gas increases so does the cooling capacity but the fresh air required for cofiring also increases and dilutes the heat provided by the gases coming from the MGT. For this reason the cooling power decreases after a certain optimum value of post-combustion is exceeded.
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When no additional fresh air is added for the post-combustion, the MGT exhaust gas is directly used as combustion air. In this case, the amount of post-combustion natural gas is limited by the maximum temperature that the post-combustion chamber can withstand, not by the oxygen content in the MGT exhaust gas. The maximum cooling capacity in this case is lower than when additional fresh air is used. Below this maximum point, the cooling capacities with and without additional air are not very different but higher when fresh air is added. Acknowledgements The authors acknowledge the funding of this project by the Ministerio de Ciencia y Tecnologıa of Spain through the Programa PROFIT, FIT-020100-2002-268 and the collaboration of Mr. Jordi L opez Launes. References [1] B. Peters, K. Dielmann, Capstone Microturbines: experiences with different gaseous and liquid fuels, Workshop on Energy Efficiency and Emissions Reduction with MicroGas Turbines––Technology and Operating Experience, Tarragona, 22nd October 2002. [2] J.C. Bruno, Ll. Massagues, A. Coronas, Power quality and air emission tests in a microgas turbine cogeneration plant, in: Proceedings of the International Conference on Renewable Energy and Power Quality (ICREPQ’03), Vigo (Spain), 2003. [3] M. Cowie, Microturbine based CHP system testing, Third Annual Workshop on Microturbine Applications, Calgary, Canda, 2003. [4] S. Banetta, F. Paganucci, R. Giglioli, System description and test planning for a combined heat and power (CHP) plant composed by a microgas turbine and an absorption chiller/heater, in: Proceedings de ASME Turbo Expo 2001, New Orleans, USA, 4–7 July 2001. [5] J. Cerezo, J.C. Bruno, W. Rivera, A. Coronas, Energy analysis of microgas turbine/absorption chiller combined systems, International Congress of Refrigeration, Washington DC, 2003. [6] W. Ryan, New developments in gas cooling, ASHRAE Journal 44 (4) (2002) 23–28. [7] M.J. Moore (Ed.), Microturbine Generators, Professional Engineering Publishing Ltd., Institution of Mechanical Engineers, England, 2002, ISBN 1-86058-391-1. [8] P.A. Pilavachi, Mini- and Micro-gas turbines for combined heat and power, Applied Thermal Engineering 22 (2002) 2003–2014. [9] J.C. Bruno, L.L. Massagues, A. Coronas, Stand-alone and grid-connected performance analysis of a regenerative microgas turbine cogeneration plant, Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 218 (2004) 15–22. [10] K.E. Herold, R. Radermacher, S.A. Klein, Absorption Chillers and Heat Pumps, CRC Press, New York, 1995.