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Second law analysis of an experimental domestic scale co-generation plant incorporating a heat pump
Applied Thermal Engineering 21 (2001) 93±110
www.elsevier.com/locate/apthermeng
Second law analysis of an experimental domestic scale cogeneration plant incorporating a heat pump M.A. Smith a, P.C. Few b,* a
Institute of Energy and Sustainable Development, De Montfort University, Leicester, LE1 9BH, UK Department of Mechanical and Manufacturing Engineering, De Montfort University, Leicester, LE1 9BH, UK
b
Received 21 December 1999; accepted 27 March 2000
Abstract The incorporation of a heat pump within a domestic scale combined heat and power plant (CHP/HP) overcomes many problems associated with domestic scale CHP. This paper discusses the development of the CHP/HP concept and describes the construction of an experimental plant. Exergy analysis is employed to examine the plant performance to indicate areas of improvement that could not be identi®ed by ®rst law analysis alone. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Exergy analysis; Combined heat and power (CHP); Heat pump (HP)
1. Introduction To realise the full environmental potential of combined heat and power (CHP) in the UK, it is required that co-generation be implemented in the residential sector. The use of district heating systems, as used in the majority of residential co-generation schemes, is compromised by the low-density suburban nature of British towns and cities. The installation of district heating systems to supply low-density suburban housing would be prohibitively expensive. A solution would be to install co-generation plants in individual dwellings [1,2]. The authors have previously demonstrated that conventional forms of co* Corresponding author. Tel.: +44-116-255-1551; fax: +44-116-257-7052. E-mail address: [email protected] (P.C. Few). 1359-4311/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 0 ) 0 0 0 5 1 - X
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Nomenclature E I NCV T h m . m s w x e j C
exergy (kW or W) irreversibility (kW) net calori®c value (kJ/kg) temperature (K or 8C) speci®c enthalpy (kJ/kg) mass (kg) mass ¯ow (kg/s) speci®c entropy (kJ/kg K) work or electrical output (kW) species or constituent of a mixed ¯ow speci®c exergy (kJ/kg) chemical exergy to NCV ratio for industrial fuel second law eciency
Subscripts 1±9 points within prototype plant air air or air ¯ow e electrical ehe exhaust heat exchanger engine engine thermal output or losses ex refers to exhaust gas thermal availability or losses f fuel hp heat pump i state or mode under consideration n mode within a system or a time period 0 initial or reference state wt water or LPW system parameter
generation may not be applicable to domestic installations due to low part load eciency and their electrical/thermal rating [3]. The authors propose incorporating a vapour compression heat pump into a conventional gas engine co-generation plant and to demonstrate the advantages over conventional co-generation for domestic applications [3,4]. Initial modelling exercises yielded favourable results, which led to the design and construction of an experimental domestic scale CHP plant incorporating a heat pump (CHP/HP) for laboratory evaluation. On completion of the laboratory testing, the results from the experimental plant were analysed in both ®rst law and second law terms. The exergy analysis was fundamental to the understanding of plant behaviour and to the acquisition of practical and theoretical knowledge to design commercial plants. As exergy analysis assesses the qualitative aspects of energy
M.A. Smith, P.C. Few / Applied Thermal Engineering 21 (2001) 93±110
95
systems, the preservation of exergy (or the minimisation of entropy production) has a direct eect on the economics of the energy plant. This paper introduces the CHP/HP concept and describes its use in an experimental plant. Exergy analysis is developed for a number of operating conditions, using experimentally acquired data. A comparison is made between ®rst law analysis and second law exergy analysis. 2. The CHP/HP concept The incorporation of a vapour compression heat pump aords a high degree of ¯exibility to a CHP plant. The theoretical operation of a domestic CHP/HP plant is discussed at length by Smith et al. [4]. Three operating modes of the CHP/HP plant are considered as follows: CHP mode HP mode
CHP/HP mode
when domestic electrical demand exceeds the electrical rating of the CHP/HP plant, the heat pump will not operate and the plant functions as a conventional CHP plant. when domestic electrical demand is negligible and domestic thermal demand is high, the CHP/HP plant can be eectively con®gured to be a gas driven heat pump, where all the electrical demand is used to drive the heat pump compressor. when the domestic electrical demand is lower than the electrical rating of the CHP/HP plant, surplus electrical power of the plant is used to drive the heat pump. This enhances plant thermal delivery while avoiding part load eciency penalties, thus reducing costs and emissions due to the lower energy consumption of the dwelling.
3. The experimental plant The experimental plant consisted of a single cylinder air-cooled gas engine and generator. A purpose designed exhaust gas heat exchanger (EHE) was ®tted to the exhaust port of the engine, where exhaust gas was cooled by a low-pressure hot water system (LPH). An electrically driven heat pump, capable of air/water and water/air heat transfer, was integrated into the LPH water system and acted as a pre-heater to the EHE. The LPH water system also consisted of a simulated heat load, a pressurisation unit and a circulation pump. The experimental plant was constructed from commercially available components where possible in order to demonstrate the practical feasibility of domestic scale co-generation. The main aim of the research was to investigate the CHP/HP concept and not to concentrate on individual components, as this would duplicate previous work. The single cylinder engine /generator set was a modi®ed commercial petrol four stroke engine (Briggs and Stratton 4IC). After conversion to natural gas fuel, the maximum electrical output of the engine/generator set was 1 kWe. Previous modelling exercises [4] indicated that
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the economic optimum rating for a CHP/HP plant in a typical domestic application would be 1.1 kWe and hence the experimental plant was rated suitable. A modi®ed Versatemp VV204 heat pump (Temperature Ltd) formed the heat pump sub-system of the experimental plant. This particular model had the best matched compressor rating (0.9 kWe) for the engine/ generator set of any water/air heat exchanger commercially available. As the VV204 unit was designed to be compatible with conventional low-pressure hot water heating systems the unit was ideal. Water ¯ow rate, pressure and system temperatures were set according to manufacturer's speci®cations.
4. Analysis The following sections will explain the exergic analysis of the experimental CHP/HP plant for the three dierent modes of operation. Second law analysis was carried out to examine aspects of plant operation that could not be dealt with using ®rst law techniques. By assessing the maximum available work (exergy) that could be obtained from a mass ¯ow or energy transfer, the quality of an energy transfer can be valued. As the second law of thermodynamics implies that work energy has a greater value than thermal energy, the analysis of energy quality has practical and economic implications. The exergy method [5] examines the properties of a mass transfer and calculates the maximum work that could theoretically be carried out by the mass transfer by bringing it to a reference state. The exergy analysis was carried out using the following assumptions: . Changes of potential, kinetic, electromagnetic, and electrostatic energies are negligible and will not be included in the analysis. . The reference state will be assumed to be the atmospheric environment at a pressure (P0) of 1 bar and at a temperature (T0) of 298.15 K. The engine and heat pump will be assumed to be steady ¯ow devices, as detailed examination of their internal cycles will not contribute to the understanding of the CHP/HP concept. 4.1. Second law analysis of CHP operation The approach taken in the second law plant analysis is to calculate the maximum work availability (exergy) of energy transfers and ¯uid streams throughout the plant. Once exergy values have been ascertained, exergy balances can be completed and second law eciencies calculated. Fig. 1 shows the typical conditions of plant energy transfer and ¯uid condition throughout the plant in CHP operation. These data were obtained for steady state conditions during laboratory tests of the experimental plant. The exergy content of the fuel input (Ef ) is equivalent to the maximum work that could be carried out by ignition of the fuel under perfect reversible conditions. To simplify the analysis, the relationship identi®ed by Szargut and Styrylska [5] will be employed, where it is assumed the ratio of fuel exergy content to net calori®c value is the same as the pure chemical constituents, i.e.:
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97
ef NCV
1
Ef jQf
2
j
. Rearranging and multiplying both sides by the mass (mf ) gives:
Kotas [5] gives j 1:04, hence: Ef 1:04 6:53 6:78 kW The combustion air induced into the engine is assumed to be in the same condition as the environmental reference state and hence: Eair 0:00 kW As exergy is a measure of work availability, the exergy content of an engine/generator's electrical output (Ee) will be pure exergy, hence: Ee we Ee 1:0 0:98 0:98 kW
3
To analyse the second law operation of the EHE it is necessary to examine the exergy values of the LPW system at points 4 and 5 (see Fig. 1). Under assumptions stated in the introduction to Section 4, the maximum work available in a ¯uid, per unit mass, can be shown to be:
Fig. 1. Thermal condition of plant under CHP operation.
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ei hi ÿ h0 ÿ T0 si ÿ s0
4
The exergy values for points 4 and 5 were calculated from: Ei ei m_ wt
5
Entropy and enthalpy values for the LPW system at points 4 and 5 were found from property tables [6]. Table 1 tabulates the calculation of E4 and E5. The exergy delivery rate to the LPW system is given by: E5 ÿ E4 0:261 ÿ 0:109 0:152 kW
6
The exergetic conditions of the exhaust gas at points 6 and 7 must be considered for a full analysis and are given by the combined exergy content of the constituent gaseous species within the exhaust mass stream, i.e.: e
X ein x n
7
n
where xi is the mass fraction of each species. Substituting into Eq. (8) gives: Ei m_ n
n X ÿ 0
hin ÿ h0n ÿ T0 sin ÿ s0n x n
8
The eect of pressure on entropy values of exhaust gases is negligible and will be ignored. The calculation of the speci®c exergy content of the exhaust gas at points 6 and 7 is shown in Table 2. The exergy transfer from the exhaust gas is represented by: E6 ÿ E7 0:993 ÿ 0:022 0:971 kW Once exergy values have been ascertained, exergy balances for individual plant components (i.e., engine/generator set and EHE) can be completed to identify the thermodynamic losses. Considering the exergy balance for engine/generator set: Exergy inputs Exergy output Irreversible losses In this case, with reference to Fig. 2: Ef Eair E6 we Iengine Table 1 Exergy condition of LPW system for operation Point
Ti (K)
hi (kJ/kg)
h0 (kJ/kg)
si (kJ/kg K)
s0 (kJ/kg K)
ei (kJ/kg)
Ei (kW)
4 5
311.4 319.6
160.78 194.81
105.41 105.41
0.550 0.658
0.369 0.369
1.32 3.14
0.109 0.261
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Table 2 Exhaust gas exergy content at points 6 and 7 for CHP mode
6±N2 6±H2O 6±CO2 6total 7±N2 7±H2O 7±CO2 7total
Ti (K)
xi
hi (kJ/kg)
h0 (kJ/kg)
si (kJ/kg K)
s0 (kJ/kg K)
ei (kJ/kg)
758.9 758.9 758.9 758.9 331.0 331.0 331.0 331.0
0.769 0.104 0.127 1.000 0.769 0.104 0.127 1.000
491.96 913.19 472.18
000.00 000.00 000.00
07.821 12.294 05.783
06.836 10.476 04.855
034.21 061.81 029.28
000.00 000.00 000.00
06.935 10.655 04.940
06.836 10.475 04.855
152.45 038.59 024.85 215.88 003.55 000.85 000.50 004.90
6:78 0 0:99 0:98 Iengine
mi (kg/s)
Ei (kW)
0.0046
0.993
0.0046
0.022
9
Irreversible losses are divided into avoidable and intrinsic losses. Intrinsic losses are due to eects such as heat transfer over a ®nite temperature dierence or the irreversibility of real gases. Avoidable losses comprise of mechanical, turbulence and electrical losses that could practically be reduced within the individual component. Identifying irreversible losses within the engine/generator set would require extensive analysis of individual components and would not contribute to the examination of the CHP/HP concept. At this stage, the exergy transfer via exhaust gas (E6) will not be considered to be a loss, as it can be potentially recovered by the EHE. The exergy balance is illustrated in Fig. 2. An exergy balance for the EHE can be completed, ascertaining the irreversible loss (see Fig. 3). As heat transfer takes place over a wide temperature dierence, the quality of the energy being transferred is signi®cantly degraded. This degradation accounts for the intrinsic losses. As the exhaust gas leaves the system at point 7, E7 can be considered to be an avoidable loss. In order to reduce the value of E7 (and hence the exergy loss), the LPW system would have to operate at a lower temperature. This would reduce the exergy content of the system and hence, increase intrinsic losses.
Fig. 2. Exergy balance for engine/generator in CHP mode.
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Fig. 3. Exergy balance for EHE in CHP mode.
The entire plant, when running in CHP mode, can now be assessed in terms of second law eciencies. It must be noted that second law eciencies examine the availability of an energy transfer, thus the values for second law eciencies are lower than for their ®rst law counterparts. Plant performance is summarised in Table 3 and Fig. 4. 4.2. Second law analysis of HP operation Second law analysis of the prototype plant in HP operation was carried out in a similar manner to that of the CHP operation. Exergy values throughout the plant are ®rst assessed and then the exergy balances are completed. The method of ascertaining exergy values from entropy and enthalpy values was validated for CHP operation and will be used in all subsequent analysis. The typical steady state thermal conditions of HP operation, obtained from experimental testing, are illustrated in Fig. 5. In HP operation, the entire generator electrical output is eectively consumed by the heat pump compressor: hence, we whp :
Table 3 Second law eciencies for CHP operation Description
Function
Second law electrical eciency
ce
Second law EHE eectiveness Second law CHP thermal eciency Second law total CHP eciency
we Ef E5 ÿ E4 cehe E6 ÿ E7 E5 ÿ E4 cth Ef E5 ÿ E4 we cthchp Ef
Value (%)
First law equivalent (%)
14.5
15.0
15.1
±
2.2
44.3
16.7
59.3
M.A. Smith, P.C. Few / Applied Thermal Engineering 21 (2001) 93±110
Fig. 4. Grassman diagram for CHP operation.
Fig. 5. Thermal conditions of HP operation.
101
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Fig. 6. Exergetic conditions of HP operation.
Fig. 7. Engine/generator exergy balance for HP operation.
Fig. 8. Exergy balance for EHE in HP operation.
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103
For the analysis of CHP operation, the Szargut and Styrylska relationship [5] will be employed: Ef jQf Ef 1:04 7:00 7:28 kW
2a
The exergy values for ¯uids within the plant were calculated in the same manner as for the CHP operation. Fig. 6 summarises the exergetic condition of the plant operating in HP mode. Figs. 7 and 8 illustrate the completed exergy balances for the engine/generator set and for the EHE. An exergy balance for the heat pump must take account of the work input (whp) and the exergy values of ¯uids entering and exiting the boundary (see Figs. 6 and 9). The irreversible losses within the heat pump will be partly due to some avoidable losses such as compressor friction. However, the low exergy values associated with the LPW system imply a signi®cant degradation of output energy. A detailed analysis of losses would involve the exergy analysis of the refrigerant system, which is beyond the scope of this analysis, as stated in the introduction to this section. It can be assumed that most of the irreversible losses experienced within the refrigerant system will be due to temperature drops across heat exchangers and expansion losses in the capillary tubes, and are, therefore, intrinsic in nature. Table 4 summarises overall plant performance in second law terms. Although in the HP mode no electrical output leaves the plant, the second law electrical conversion eciency for the engine/generator set has been included for reference with other modes of operation. The second law thermal eciency of the plant considers the combined heat pump and exhaust exergy delivery to the LPW system. In the HP mode, this is identical to the total plant second law eciency. Fig. 10 illustrates the exergy ¯ows through the plant. The actual exergy values for E8 and E9 are negligible, owing to the air temperature being approximately the same as the reference
Fig. 9. Exergy balance of heat pump for HP operation.
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Table 4 Second law eciencies for HP Operation Description
Function
Second law electrical eciency
ce
Second law EHE eectiveness Second law heat pump thermal eciency Second law plant thermal eciency Second law total plant eciency
we Ef E5 ÿ E4 cehe E6 ÿ E7 E3 ÿ E2 chp whp E3 ÿ E2 E5 ÿ E4 cth Ef E3 ÿ E2 E5 ÿ E4 cchphp Ef
Value (%) 12.4
First law equivalent (%) 13.0
32.8
±
13.7
265.0
5.04
70.0
5.04
70.0
temperature, but are represented on Fig. 10 to illustrate the exergy ¯ow. Due to transducer inconsistencies, a discrepancy exists between E3 and E4, as these values should be equal. This is accounted for in Fig. 10, by stating their individual values at the appropriate control boundary. The transducer inconsistencies result in a 0.4% error in the overall plant exergy balance, shown in Fig. 10.
Fig. 10. Grassman diagram for HP operation.
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Fig. 11. Thermal conditions of CHP/HP operation.
4.3. Second law analysis of CHP/HP operation The analysis of the CHP/HP operation is similar to that for HP mode, with the exception that some electrical generation is exported from the plant (we ') and the remainder is eectively used by the heat pump, (whp), i.e.:
Fig. 12. Exergetic condition of plant in CHP/HP operation.
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Fig. 13. Engine/generator exergy balance for CHP/HP mode.
Fig. 14. EHE exergy balance for CHP/HP mode.
Fig. 15. Heat pump exergy balance for CHP/HP operation.
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Table 5 Second law eciencies for CHP/HP operation Description
Function
we 16.5 Ef E5 ÿ E4 EHE eectiveness cehe 16.5 E6 ÿ E7 E3 ÿ E2 heat pump thermal eciency chp 9.5 whp E3 ÿ E2 E5 ÿ E4 plant thermal eciency cth 3.6 Ef 0 E3 ÿ E2 E5 ÿ E4 we total plant eciency cchphp 6.8 Ef
Second law electrical eciency Second law Second law Second law Second law
Value (%) First law equivalent (%)
ce
we whp we0
17.1
254.0 69.6 72.9
10
The experimentally obtained steady state thermal conditions of the plant operation during the CHP/HP mode are shown in Fig. 11. The exergy values for ¯uid ¯ows throughout the plant are shown in Fig. 12. Subsequent analysis will take the form of exergy balances to estimate irreversible plant losses. Figs. 13±15 represent the completed energy balances for the engine/generator set, EHE and heat pump, respectively.
Fig. 16. Grassman diagram for CHP /HP.
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Exergy value of the fuel is again calculated using: Ef jQf Ef 1:04 6:65 6:916 kW
2b
Second law eciencies for individual sub-systems and overall plant performance are summarised in Table 5. The overall second law eciency for the plant includes the eective plant electrical output we0 we ÿ whp ). The exergetic transfers throughout the plant are illustrated in Fig. 16. As with HP analysis, inconsistencies between E3 and E4 are accounted for.
5. Discussion The following section discusses the analysis of the prototype plant in relation to practical issues. It must be noted that second law eciencies report a lower ®gure than ®rst law results. This is a consequence of exergy being a function of available work and of the practical restrictions placed on the plant. The use of exergy analysis allows for the identi®cation of losses that degrade the quality of energy transfer. Table 6 summarises the irreversible losses for each analysed component, for each mode of operation, in terms of a percentage of the total fuel exergy input. The greatest losses are experienced in the engine/generator set and are comprised of intrinsic and avoidable losses. First law analysis has previously shown that the engine/generator set has poor performance: this point is reiterated by second law analysis. Little could be done to increase the performance of the engine used, hence, reduction of irreversible losses would require replacement of the engine/generator set. Water cooling of the engine would result in a small reduction of exergy loss, as the exergy recovery would take place at a relatively low temperature. First law analysis does not take into account the quality of energy transfer and therefore, would show a large reduction in losses with water cooling. The heat pump has good ®rst law performance, with COPs in the region of 2.5±3. This represents the best performance that could be practically obtained from a production unit. First law analysis has indicated that compressor losses are small and so it must be concluded that irreversible heat pump losses (Ihp) are largely intrinsic in nature. The electrical energy
Table 6 Irreversible losses of plant components
Engine/generator EHE Heat pump
CHP (%)
HP (%)
CHP/HP (%)
73.7 9.3 ±
79.1 4.9 10.6
71.1 9.2 12.0
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consumed by the heat pump (considered to be pure work) is degraded to a larger amount of low quality thermal energy, giving rise to the intrinsic losses. The reduction of heat pump exergy losses would require a higher LPW system temperature, requiring an alternative refrigerant, as discussed previously. Exergy values are dependent on temperature and as the temperatures within the LPW system are close to the reference value, the exergy values are highly sensitive to relatively small changes in temperature. By comparing EHE performance for the CHP and HP modes, the eects of temperature are apparent. The lowest EHE irreversible losses are recorded for HP operation (see Table 6), where LPW system temperatures are relatively high and the exhaust temperature is relatively low. Heat transfer takes place over a relatively narrow temperature range, which reduces the degradation of thermal energy (giving rise to lower irreversible losses). The heat transfer takes place over a wider temperature range in CHP operation, with relatively high exhaust temperatures and low LPW system temperatures. This wider range degrades the transferred thermal energy to a greater extent and produces higher irreversible losses than for the HP mode. To reduce exergy losses, the temperature range across the heat exchanger must be reduced, by either increasing LPW system temperatures or by reducing exhaust temperatures. However, narrowing the temperature range will decrease ®rst law performance. In the HP operation, the lower exhaust temperature was due to the lower engine load. This was accompanied by an increased exergy loss within the engine Ð the highest engine/generator losses were recorded during HP operation. The overall eect is a decrease in plant performance, as the increased exergy delivery to the LPW system is negated by increased engine exergy losses.
6. Conclusions and future work The exergetic analysis of the experimental plant enhanced the conventional ®rst law analysis. As stated, the main aim of the research was to develop the CHP/HP concept and examine CHP/HP interaction: the use of exergy analysis contributed to the research by identifying issues that could not be addressed by ®rst law analysis. First law analysis identi®es signi®cant energy losses as a result of the air cooling of the engine. When these losses are analysed using the exergy method, the actual exergy loss is insigni®cant compared to thermodynamic losses within the engine. This indicates that improving engine mechanical eciency is of more importance than the recovery of low-grade losses. First law analysis alone could have led to modi®cations being made to recover low-grade energy (of low ®nancial value) rather than improvements in engine eciency (which has greater ®nancial value). Exergy analysis suggested that by using an alternative refrigerant, operation at higher temperatures would reduce the exergy loss within the heat pump, with a marginal detriment to the ®rst law performance of the EHE. Exergetic analysis of a co-generation system must be placed in context, as often low grade heat is used. The use of exergy analysis in this paper signi®cantly contributed to the development of the CHP/HP concept and will aid design of future CHP/HP plants. Future work will include the use of exergy analysis in domestic scale CHP/HP modelling (validated by experimental results), including the examination of fuel cells for domestic cogeneration application. Future work will commercially develop the CHP/HP concept, with the
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installation of a second generation CHP/HP plant in an actual dwelling. Applied exergy analysis is now included in courses at De Montfort University, as it is seen, due to the CHP/ HP project, as a valuable tool for energy systems analysis. Acknowledgements The authors acknowledge the following support: Temperature Ltd (Southampton, UK) Ð Versatemp VV 204 heat pump; The School of Engineering and Manufacture, DeMontfort University Ð postgraduate bursary; The Institute of Energy and Sustainable Development, DeMontfort University Ð equipment costs. References [1] M.A. Smith, The economic and commercial feasibility of domestic CHP, M.Sc. Thesis, University of Wales College of Cardi, 1994. [2] W.R. Agar, M. Newborough, Implementing micro-CHP systems in the UK residential sector, Journal of the Institute of Energy, December 1998. [3] M.A. Smith, P.C. Few, J.W. Twiddle, Modelling of a domestic CHP plant incorporating a heat pump, International Journal of Energy 22 (7) (1997). [4] M.A. Smith, Small scale and micro combined heat and power, Ph.D. Thesis, De Montfort University, May 1999. [5] T.J. Kotas, The Exergy Method of Thermal Plant Analysis, reprint edition, Krieger, New York, 1995. [6] G.F.C. Roger, Y.R. Mayhew, Thermodynamic and Transport, Properties of Fluids, 5th ed., Blackwell, Oxford, 1995.
www.elsevier.com/locate/apthermeng
Second law analysis of an experimental domestic scale cogeneration plant incorporating a heat pump M.A. Smith a, P.C. Few b,* a
Institute of Energy and Sustainable Development, De Montfort University, Leicester, LE1 9BH, UK Department of Mechanical and Manufacturing Engineering, De Montfort University, Leicester, LE1 9BH, UK
b
Received 21 December 1999; accepted 27 March 2000
Abstract The incorporation of a heat pump within a domestic scale combined heat and power plant (CHP/HP) overcomes many problems associated with domestic scale CHP. This paper discusses the development of the CHP/HP concept and describes the construction of an experimental plant. Exergy analysis is employed to examine the plant performance to indicate areas of improvement that could not be identi®ed by ®rst law analysis alone. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Exergy analysis; Combined heat and power (CHP); Heat pump (HP)
1. Introduction To realise the full environmental potential of combined heat and power (CHP) in the UK, it is required that co-generation be implemented in the residential sector. The use of district heating systems, as used in the majority of residential co-generation schemes, is compromised by the low-density suburban nature of British towns and cities. The installation of district heating systems to supply low-density suburban housing would be prohibitively expensive. A solution would be to install co-generation plants in individual dwellings [1,2]. The authors have previously demonstrated that conventional forms of co* Corresponding author. Tel.: +44-116-255-1551; fax: +44-116-257-7052. E-mail address: [email protected] (P.C. Few). 1359-4311/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 0 ) 0 0 0 5 1 - X
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M.A. Smith, P.C. Few / Applied Thermal Engineering 21 (2001) 93±110
Nomenclature E I NCV T h m . m s w x e j C
exergy (kW or W) irreversibility (kW) net calori®c value (kJ/kg) temperature (K or 8C) speci®c enthalpy (kJ/kg) mass (kg) mass ¯ow (kg/s) speci®c entropy (kJ/kg K) work or electrical output (kW) species or constituent of a mixed ¯ow speci®c exergy (kJ/kg) chemical exergy to NCV ratio for industrial fuel second law eciency
Subscripts 1±9 points within prototype plant air air or air ¯ow e electrical ehe exhaust heat exchanger engine engine thermal output or losses ex refers to exhaust gas thermal availability or losses f fuel hp heat pump i state or mode under consideration n mode within a system or a time period 0 initial or reference state wt water or LPW system parameter
generation may not be applicable to domestic installations due to low part load eciency and their electrical/thermal rating [3]. The authors propose incorporating a vapour compression heat pump into a conventional gas engine co-generation plant and to demonstrate the advantages over conventional co-generation for domestic applications [3,4]. Initial modelling exercises yielded favourable results, which led to the design and construction of an experimental domestic scale CHP plant incorporating a heat pump (CHP/HP) for laboratory evaluation. On completion of the laboratory testing, the results from the experimental plant were analysed in both ®rst law and second law terms. The exergy analysis was fundamental to the understanding of plant behaviour and to the acquisition of practical and theoretical knowledge to design commercial plants. As exergy analysis assesses the qualitative aspects of energy
M.A. Smith, P.C. Few / Applied Thermal Engineering 21 (2001) 93±110
95
systems, the preservation of exergy (or the minimisation of entropy production) has a direct eect on the economics of the energy plant. This paper introduces the CHP/HP concept and describes its use in an experimental plant. Exergy analysis is developed for a number of operating conditions, using experimentally acquired data. A comparison is made between ®rst law analysis and second law exergy analysis. 2. The CHP/HP concept The incorporation of a vapour compression heat pump aords a high degree of ¯exibility to a CHP plant. The theoretical operation of a domestic CHP/HP plant is discussed at length by Smith et al. [4]. Three operating modes of the CHP/HP plant are considered as follows: CHP mode HP mode
CHP/HP mode
when domestic electrical demand exceeds the electrical rating of the CHP/HP plant, the heat pump will not operate and the plant functions as a conventional CHP plant. when domestic electrical demand is negligible and domestic thermal demand is high, the CHP/HP plant can be eectively con®gured to be a gas driven heat pump, where all the electrical demand is used to drive the heat pump compressor. when the domestic electrical demand is lower than the electrical rating of the CHP/HP plant, surplus electrical power of the plant is used to drive the heat pump. This enhances plant thermal delivery while avoiding part load eciency penalties, thus reducing costs and emissions due to the lower energy consumption of the dwelling.
3. The experimental plant The experimental plant consisted of a single cylinder air-cooled gas engine and generator. A purpose designed exhaust gas heat exchanger (EHE) was ®tted to the exhaust port of the engine, where exhaust gas was cooled by a low-pressure hot water system (LPH). An electrically driven heat pump, capable of air/water and water/air heat transfer, was integrated into the LPH water system and acted as a pre-heater to the EHE. The LPH water system also consisted of a simulated heat load, a pressurisation unit and a circulation pump. The experimental plant was constructed from commercially available components where possible in order to demonstrate the practical feasibility of domestic scale co-generation. The main aim of the research was to investigate the CHP/HP concept and not to concentrate on individual components, as this would duplicate previous work. The single cylinder engine /generator set was a modi®ed commercial petrol four stroke engine (Briggs and Stratton 4IC). After conversion to natural gas fuel, the maximum electrical output of the engine/generator set was 1 kWe. Previous modelling exercises [4] indicated that
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the economic optimum rating for a CHP/HP plant in a typical domestic application would be 1.1 kWe and hence the experimental plant was rated suitable. A modi®ed Versatemp VV204 heat pump (Temperature Ltd) formed the heat pump sub-system of the experimental plant. This particular model had the best matched compressor rating (0.9 kWe) for the engine/ generator set of any water/air heat exchanger commercially available. As the VV204 unit was designed to be compatible with conventional low-pressure hot water heating systems the unit was ideal. Water ¯ow rate, pressure and system temperatures were set according to manufacturer's speci®cations.
4. Analysis The following sections will explain the exergic analysis of the experimental CHP/HP plant for the three dierent modes of operation. Second law analysis was carried out to examine aspects of plant operation that could not be dealt with using ®rst law techniques. By assessing the maximum available work (exergy) that could be obtained from a mass ¯ow or energy transfer, the quality of an energy transfer can be valued. As the second law of thermodynamics implies that work energy has a greater value than thermal energy, the analysis of energy quality has practical and economic implications. The exergy method [5] examines the properties of a mass transfer and calculates the maximum work that could theoretically be carried out by the mass transfer by bringing it to a reference state. The exergy analysis was carried out using the following assumptions: . Changes of potential, kinetic, electromagnetic, and electrostatic energies are negligible and will not be included in the analysis. . The reference state will be assumed to be the atmospheric environment at a pressure (P0) of 1 bar and at a temperature (T0) of 298.15 K. The engine and heat pump will be assumed to be steady ¯ow devices, as detailed examination of their internal cycles will not contribute to the understanding of the CHP/HP concept. 4.1. Second law analysis of CHP operation The approach taken in the second law plant analysis is to calculate the maximum work availability (exergy) of energy transfers and ¯uid streams throughout the plant. Once exergy values have been ascertained, exergy balances can be completed and second law eciencies calculated. Fig. 1 shows the typical conditions of plant energy transfer and ¯uid condition throughout the plant in CHP operation. These data were obtained for steady state conditions during laboratory tests of the experimental plant. The exergy content of the fuel input (Ef ) is equivalent to the maximum work that could be carried out by ignition of the fuel under perfect reversible conditions. To simplify the analysis, the relationship identi®ed by Szargut and Styrylska [5] will be employed, where it is assumed the ratio of fuel exergy content to net calori®c value is the same as the pure chemical constituents, i.e.:
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97
ef NCV
1
Ef jQf
2
j
. Rearranging and multiplying both sides by the mass (mf ) gives:
Kotas [5] gives j 1:04, hence: Ef 1:04 6:53 6:78 kW The combustion air induced into the engine is assumed to be in the same condition as the environmental reference state and hence: Eair 0:00 kW As exergy is a measure of work availability, the exergy content of an engine/generator's electrical output (Ee) will be pure exergy, hence: Ee we Ee 1:0 0:98 0:98 kW
3
To analyse the second law operation of the EHE it is necessary to examine the exergy values of the LPW system at points 4 and 5 (see Fig. 1). Under assumptions stated in the introduction to Section 4, the maximum work available in a ¯uid, per unit mass, can be shown to be:
Fig. 1. Thermal condition of plant under CHP operation.
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ei hi ÿ h0 ÿ T0 si ÿ s0
4
The exergy values for points 4 and 5 were calculated from: Ei ei m_ wt
5
Entropy and enthalpy values for the LPW system at points 4 and 5 were found from property tables [6]. Table 1 tabulates the calculation of E4 and E5. The exergy delivery rate to the LPW system is given by: E5 ÿ E4 0:261 ÿ 0:109 0:152 kW
6
The exergetic conditions of the exhaust gas at points 6 and 7 must be considered for a full analysis and are given by the combined exergy content of the constituent gaseous species within the exhaust mass stream, i.e.: e
X ein x n
7
n
where xi is the mass fraction of each species. Substituting into Eq. (8) gives: Ei m_ n
n X ÿ 0
hin ÿ h0n ÿ T0 sin ÿ s0n x n
8
The eect of pressure on entropy values of exhaust gases is negligible and will be ignored. The calculation of the speci®c exergy content of the exhaust gas at points 6 and 7 is shown in Table 2. The exergy transfer from the exhaust gas is represented by: E6 ÿ E7 0:993 ÿ 0:022 0:971 kW Once exergy values have been ascertained, exergy balances for individual plant components (i.e., engine/generator set and EHE) can be completed to identify the thermodynamic losses. Considering the exergy balance for engine/generator set: Exergy inputs Exergy output Irreversible losses In this case, with reference to Fig. 2: Ef Eair E6 we Iengine Table 1 Exergy condition of LPW system for operation Point
Ti (K)
hi (kJ/kg)
h0 (kJ/kg)
si (kJ/kg K)
s0 (kJ/kg K)
ei (kJ/kg)
Ei (kW)
4 5
311.4 319.6
160.78 194.81
105.41 105.41
0.550 0.658
0.369 0.369
1.32 3.14
0.109 0.261
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99
Table 2 Exhaust gas exergy content at points 6 and 7 for CHP mode
6±N2 6±H2O 6±CO2 6total 7±N2 7±H2O 7±CO2 7total
Ti (K)
xi
hi (kJ/kg)
h0 (kJ/kg)
si (kJ/kg K)
s0 (kJ/kg K)
ei (kJ/kg)
758.9 758.9 758.9 758.9 331.0 331.0 331.0 331.0
0.769 0.104 0.127 1.000 0.769 0.104 0.127 1.000
491.96 913.19 472.18
000.00 000.00 000.00
07.821 12.294 05.783
06.836 10.476 04.855
034.21 061.81 029.28
000.00 000.00 000.00
06.935 10.655 04.940
06.836 10.475 04.855
152.45 038.59 024.85 215.88 003.55 000.85 000.50 004.90
6:78 0 0:99 0:98 Iengine
mi (kg/s)
Ei (kW)
0.0046
0.993
0.0046
0.022
9
Irreversible losses are divided into avoidable and intrinsic losses. Intrinsic losses are due to eects such as heat transfer over a ®nite temperature dierence or the irreversibility of real gases. Avoidable losses comprise of mechanical, turbulence and electrical losses that could practically be reduced within the individual component. Identifying irreversible losses within the engine/generator set would require extensive analysis of individual components and would not contribute to the examination of the CHP/HP concept. At this stage, the exergy transfer via exhaust gas (E6) will not be considered to be a loss, as it can be potentially recovered by the EHE. The exergy balance is illustrated in Fig. 2. An exergy balance for the EHE can be completed, ascertaining the irreversible loss (see Fig. 3). As heat transfer takes place over a wide temperature dierence, the quality of the energy being transferred is signi®cantly degraded. This degradation accounts for the intrinsic losses. As the exhaust gas leaves the system at point 7, E7 can be considered to be an avoidable loss. In order to reduce the value of E7 (and hence the exergy loss), the LPW system would have to operate at a lower temperature. This would reduce the exergy content of the system and hence, increase intrinsic losses.
Fig. 2. Exergy balance for engine/generator in CHP mode.
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Fig. 3. Exergy balance for EHE in CHP mode.
The entire plant, when running in CHP mode, can now be assessed in terms of second law eciencies. It must be noted that second law eciencies examine the availability of an energy transfer, thus the values for second law eciencies are lower than for their ®rst law counterparts. Plant performance is summarised in Table 3 and Fig. 4. 4.2. Second law analysis of HP operation Second law analysis of the prototype plant in HP operation was carried out in a similar manner to that of the CHP operation. Exergy values throughout the plant are ®rst assessed and then the exergy balances are completed. The method of ascertaining exergy values from entropy and enthalpy values was validated for CHP operation and will be used in all subsequent analysis. The typical steady state thermal conditions of HP operation, obtained from experimental testing, are illustrated in Fig. 5. In HP operation, the entire generator electrical output is eectively consumed by the heat pump compressor: hence, we whp :
Table 3 Second law eciencies for CHP operation Description
Function
Second law electrical eciency
ce
Second law EHE eectiveness Second law CHP thermal eciency Second law total CHP eciency
we Ef E5 ÿ E4 cehe E6 ÿ E7 E5 ÿ E4 cth Ef E5 ÿ E4 we cthchp Ef
Value (%)
First law equivalent (%)
14.5
15.0
15.1
±
2.2
44.3
16.7
59.3
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Fig. 4. Grassman diagram for CHP operation.
Fig. 5. Thermal conditions of HP operation.
101
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Fig. 6. Exergetic conditions of HP operation.
Fig. 7. Engine/generator exergy balance for HP operation.
Fig. 8. Exergy balance for EHE in HP operation.
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For the analysis of CHP operation, the Szargut and Styrylska relationship [5] will be employed: Ef jQf Ef 1:04 7:00 7:28 kW
2a
The exergy values for ¯uids within the plant were calculated in the same manner as for the CHP operation. Fig. 6 summarises the exergetic condition of the plant operating in HP mode. Figs. 7 and 8 illustrate the completed exergy balances for the engine/generator set and for the EHE. An exergy balance for the heat pump must take account of the work input (whp) and the exergy values of ¯uids entering and exiting the boundary (see Figs. 6 and 9). The irreversible losses within the heat pump will be partly due to some avoidable losses such as compressor friction. However, the low exergy values associated with the LPW system imply a signi®cant degradation of output energy. A detailed analysis of losses would involve the exergy analysis of the refrigerant system, which is beyond the scope of this analysis, as stated in the introduction to this section. It can be assumed that most of the irreversible losses experienced within the refrigerant system will be due to temperature drops across heat exchangers and expansion losses in the capillary tubes, and are, therefore, intrinsic in nature. Table 4 summarises overall plant performance in second law terms. Although in the HP mode no electrical output leaves the plant, the second law electrical conversion eciency for the engine/generator set has been included for reference with other modes of operation. The second law thermal eciency of the plant considers the combined heat pump and exhaust exergy delivery to the LPW system. In the HP mode, this is identical to the total plant second law eciency. Fig. 10 illustrates the exergy ¯ows through the plant. The actual exergy values for E8 and E9 are negligible, owing to the air temperature being approximately the same as the reference
Fig. 9. Exergy balance of heat pump for HP operation.
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Table 4 Second law eciencies for HP Operation Description
Function
Second law electrical eciency
ce
Second law EHE eectiveness Second law heat pump thermal eciency Second law plant thermal eciency Second law total plant eciency
we Ef E5 ÿ E4 cehe E6 ÿ E7 E3 ÿ E2 chp whp E3 ÿ E2 E5 ÿ E4 cth Ef E3 ÿ E2 E5 ÿ E4 cchphp Ef
Value (%) 12.4
First law equivalent (%) 13.0
32.8
±
13.7
265.0
5.04
70.0
5.04
70.0
temperature, but are represented on Fig. 10 to illustrate the exergy ¯ow. Due to transducer inconsistencies, a discrepancy exists between E3 and E4, as these values should be equal. This is accounted for in Fig. 10, by stating their individual values at the appropriate control boundary. The transducer inconsistencies result in a 0.4% error in the overall plant exergy balance, shown in Fig. 10.
Fig. 10. Grassman diagram for HP operation.
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105
Fig. 11. Thermal conditions of CHP/HP operation.
4.3. Second law analysis of CHP/HP operation The analysis of the CHP/HP operation is similar to that for HP mode, with the exception that some electrical generation is exported from the plant (we ') and the remainder is eectively used by the heat pump, (whp), i.e.:
Fig. 12. Exergetic condition of plant in CHP/HP operation.
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M.A. Smith, P.C. Few / Applied Thermal Engineering 21 (2001) 93±110
Fig. 13. Engine/generator exergy balance for CHP/HP mode.
Fig. 14. EHE exergy balance for CHP/HP mode.
Fig. 15. Heat pump exergy balance for CHP/HP operation.
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Table 5 Second law eciencies for CHP/HP operation Description
Function
we 16.5 Ef E5 ÿ E4 EHE eectiveness cehe 16.5 E6 ÿ E7 E3 ÿ E2 heat pump thermal eciency chp 9.5 whp E3 ÿ E2 E5 ÿ E4 plant thermal eciency cth 3.6 Ef 0 E3 ÿ E2 E5 ÿ E4 we total plant eciency cchphp 6.8 Ef
Second law electrical eciency Second law Second law Second law Second law
Value (%) First law equivalent (%)
ce
we whp we0
17.1
254.0 69.6 72.9
10
The experimentally obtained steady state thermal conditions of the plant operation during the CHP/HP mode are shown in Fig. 11. The exergy values for ¯uid ¯ows throughout the plant are shown in Fig. 12. Subsequent analysis will take the form of exergy balances to estimate irreversible plant losses. Figs. 13±15 represent the completed energy balances for the engine/generator set, EHE and heat pump, respectively.
Fig. 16. Grassman diagram for CHP /HP.
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M.A. Smith, P.C. Few / Applied Thermal Engineering 21 (2001) 93±110
Exergy value of the fuel is again calculated using: Ef jQf Ef 1:04 6:65 6:916 kW
2b
Second law eciencies for individual sub-systems and overall plant performance are summarised in Table 5. The overall second law eciency for the plant includes the eective plant electrical output we0 we ÿ whp ). The exergetic transfers throughout the plant are illustrated in Fig. 16. As with HP analysis, inconsistencies between E3 and E4 are accounted for.
5. Discussion The following section discusses the analysis of the prototype plant in relation to practical issues. It must be noted that second law eciencies report a lower ®gure than ®rst law results. This is a consequence of exergy being a function of available work and of the practical restrictions placed on the plant. The use of exergy analysis allows for the identi®cation of losses that degrade the quality of energy transfer. Table 6 summarises the irreversible losses for each analysed component, for each mode of operation, in terms of a percentage of the total fuel exergy input. The greatest losses are experienced in the engine/generator set and are comprised of intrinsic and avoidable losses. First law analysis has previously shown that the engine/generator set has poor performance: this point is reiterated by second law analysis. Little could be done to increase the performance of the engine used, hence, reduction of irreversible losses would require replacement of the engine/generator set. Water cooling of the engine would result in a small reduction of exergy loss, as the exergy recovery would take place at a relatively low temperature. First law analysis does not take into account the quality of energy transfer and therefore, would show a large reduction in losses with water cooling. The heat pump has good ®rst law performance, with COPs in the region of 2.5±3. This represents the best performance that could be practically obtained from a production unit. First law analysis has indicated that compressor losses are small and so it must be concluded that irreversible heat pump losses (Ihp) are largely intrinsic in nature. The electrical energy
Table 6 Irreversible losses of plant components
Engine/generator EHE Heat pump
CHP (%)
HP (%)
CHP/HP (%)
73.7 9.3 ±
79.1 4.9 10.6
71.1 9.2 12.0
M.A. Smith, P.C. Few / Applied Thermal Engineering 21 (2001) 93±110
109
consumed by the heat pump (considered to be pure work) is degraded to a larger amount of low quality thermal energy, giving rise to the intrinsic losses. The reduction of heat pump exergy losses would require a higher LPW system temperature, requiring an alternative refrigerant, as discussed previously. Exergy values are dependent on temperature and as the temperatures within the LPW system are close to the reference value, the exergy values are highly sensitive to relatively small changes in temperature. By comparing EHE performance for the CHP and HP modes, the eects of temperature are apparent. The lowest EHE irreversible losses are recorded for HP operation (see Table 6), where LPW system temperatures are relatively high and the exhaust temperature is relatively low. Heat transfer takes place over a relatively narrow temperature range, which reduces the degradation of thermal energy (giving rise to lower irreversible losses). The heat transfer takes place over a wider temperature range in CHP operation, with relatively high exhaust temperatures and low LPW system temperatures. This wider range degrades the transferred thermal energy to a greater extent and produces higher irreversible losses than for the HP mode. To reduce exergy losses, the temperature range across the heat exchanger must be reduced, by either increasing LPW system temperatures or by reducing exhaust temperatures. However, narrowing the temperature range will decrease ®rst law performance. In the HP operation, the lower exhaust temperature was due to the lower engine load. This was accompanied by an increased exergy loss within the engine Ð the highest engine/generator losses were recorded during HP operation. The overall eect is a decrease in plant performance, as the increased exergy delivery to the LPW system is negated by increased engine exergy losses.
6. Conclusions and future work The exergetic analysis of the experimental plant enhanced the conventional ®rst law analysis. As stated, the main aim of the research was to develop the CHP/HP concept and examine CHP/HP interaction: the use of exergy analysis contributed to the research by identifying issues that could not be addressed by ®rst law analysis. First law analysis identi®es signi®cant energy losses as a result of the air cooling of the engine. When these losses are analysed using the exergy method, the actual exergy loss is insigni®cant compared to thermodynamic losses within the engine. This indicates that improving engine mechanical eciency is of more importance than the recovery of low-grade losses. First law analysis alone could have led to modi®cations being made to recover low-grade energy (of low ®nancial value) rather than improvements in engine eciency (which has greater ®nancial value). Exergy analysis suggested that by using an alternative refrigerant, operation at higher temperatures would reduce the exergy loss within the heat pump, with a marginal detriment to the ®rst law performance of the EHE. Exergetic analysis of a co-generation system must be placed in context, as often low grade heat is used. The use of exergy analysis in this paper signi®cantly contributed to the development of the CHP/HP concept and will aid design of future CHP/HP plants. Future work will include the use of exergy analysis in domestic scale CHP/HP modelling (validated by experimental results), including the examination of fuel cells for domestic cogeneration application. Future work will commercially develop the CHP/HP concept, with the
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installation of a second generation CHP/HP plant in an actual dwelling. Applied exergy analysis is now included in courses at De Montfort University, as it is seen, due to the CHP/ HP project, as a valuable tool for energy systems analysis. Acknowledgements The authors acknowledge the following support: Temperature Ltd (Southampton, UK) Ð Versatemp VV 204 heat pump; The School of Engineering and Manufacture, DeMontfort University Ð postgraduate bursary; The Institute of Energy and Sustainable Development, DeMontfort University Ð equipment costs. References [1] M.A. Smith, The economic and commercial feasibility of domestic CHP, M.Sc. Thesis, University of Wales College of Cardi, 1994. [2] W.R. Agar, M. Newborough, Implementing micro-CHP systems in the UK residential sector, Journal of the Institute of Energy, December 1998. [3] M.A. Smith, P.C. Few, J.W. Twiddle, Modelling of a domestic CHP plant incorporating a heat pump, International Journal of Energy 22 (7) (1997). [4] M.A. Smith, Small scale and micro combined heat and power, Ph.D. Thesis, De Montfort University, May 1999. [5] T.J. Kotas, The Exergy Method of Thermal Plant Analysis, reprint edition, Krieger, New York, 1995. [6] G.F.C. Roger, Y.R. Mayhew, Thermodynamic and Transport, Properties of Fluids, 5th ed., Blackwell, Oxford, 1995.