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Development of a prototype low-temperature Rankine cycle electricity generation system
Applied Thermal Engineering 21 (2001) 169±181
www.elsevier.com/locate/apthermeng
Development of a prototype low-temperature Rankine cycle electricity generation system V.M. Nguyen, P.S. Doherty*, S.B. Riat Institute of Building Technology (IBT), School of the Built Environment, Nottingham University, University Park, Nottingham NG7 2RD, UK Received 18 September 1999; accepted 28 March 2000
Abstract This paper describes the development of a small-scale system designed to generate electricity from low temperature heat (e.g., solar energy). The system operates on the Rankine cycle and uses n-pentane as the working ¯uid. A prototype system has been designed, constructed and tested. It is capable of delivering 1.5 kW of electricity with a thermal eciency of 4.3%. Laboratory test results and a cost estimate for the prototype unit are presented in the paper. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Rankine cycle; Low-temperature electrity generation; Prototype; Laboratory test; n-Pentane
1. Introduction
In 1998 the total installed electricity generation capacity in the UK was approximately 68 GW with over 93% of the electricity supply obtained from fossil fuel and nuclear power generating plants [2]. Fossil fuel combustion leads to the emission of environmental pollutants such as carbon dioxide (CO2), sulphur dioxide (SO2) and oxides of nitrogen (NOx) while nuclear ®ssion products are radioactive and present challenging disposal problems. It is therefore clear that electricity generation from fossil fuel and nuclear power has serious * Corresponding author. Tel.: +44-115-951-3134; fax: +44-115-951-3159. E-mail address: [email protected] (P.S. Doherty). 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 2 - 1
170
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
detrimental eects on the environment and these include global warming, acid rain and contamination of land and seas. Traditionally, centralised fossil fuel and nuclear generation plants have provided the UK with electrical power. Coal ®red generating plants currently operate with average fuel to electricity conversion eciencies of 36%, combined cycle gas turbines (CCGT) with average eciencies of 47% and nuclear power plants are around 37% ecient. In 1998 an estimated 37% of the energy content of fuel input (912 TWh) to fossil and nuclear generators was converted to useful electricity resulting in 63% of the energy input going waste at the generation site [3]. In addition to the conversion losses, transmission losses occur due to ohmic heating in the conductors and this results in a further reduction in the generation eciency from the fuel to the end-user. Lower emission of pollutants per kW of energy produced as well as higher conversion eciencies has brought about a rapid increase in the installation and use of CCGT's with a corresponding rapid decrease in the use of coal ®red electricity generators. Also, the installation of combined heat and power (CHP) systems in the UK has increased with a government target installed capacity of 5000 MW by the year 2000 [2]. The CHP systems provide much higher overall eciency by utilising a proportion of the waste energy resulting from the electricity generating process to provide heat on site [1,5]. Because of the relatively high cost of transporting heat as compared with transporting electricity, de-centralised CHP systems are located closer to the demand for heat, thus minimising heat transport costs whilst satisfying the electricity requirements. A unique form of small scale CHP system that directly heats ventilating air for buildings or process air for industry (Air CHP) can achieve energy conversion eciencies of 95% or more. Around 2.6% of the total electricity generated in the UK during 1998 was obtained from renewable sources. The Government's aim is to meet 5% of the UK's electricity requirements from renewable sources by 2003 and 10% by 2010. To date 650 MW of electricity generation from renewable sources has been commissioned through the Non-Fossil Fuel Obligation schemes [4]. Solar thermal electricity generating plants have been developed and can now operate at eciencies approaching that of conventional generation techniques. Examples of this technology include the 10 MWe S2 power tower installation in the Mojave Desert, CA, which operates on a steam turbine cycle. The turbine inlet temperature is 3908C and the thermal eciency (Eq. (1)) for this system can exceed 25% with a solar to electricity conversion eciency of up to 14% [7]. Highest eciencies are achieved using parabolic dish solar collectors and Stirling engines. A solar to electricity conversion eciency of up to 29.4% has been achieved by the 25 kWe system built by the Advanco Co. The collector temperature for this system can reach 7508C [7]. A mathematical modelling [8] and a `proof of concept' work [6] conducted at the Institute of Building Technology resulted in a research study on a small-scale electricity generation system for domestic applications. A prototype operating on the Rankine cycle using n-pentane as the working ¯uid has been developed. The system is driven by heat sourced from a gas-®red boiler. Heat is rejected to the environment through an air-cooled condenser. The main advantage of this system is that it can be totally independent of the grid and could, therefore be marketable in hot climates and areas remote from mains electricity supply.
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
171
2. Description of the electricity generation system Fig. 1 illustrates the layout of the developed electricity generation system. Liquid n-pentane boils in the compact brazed plate heat exchanger (CBE) as heat is transferred from the hot water circuit. Hot water is provided by a gas-boiler. The resulting n-pentane vapour passes to a droplet separator through line 2. From here liquid droplets drain back to the boiler through line 3. The dry pentane vapour passes via line 4, through a control valve, to the turbine inlet. Turbine inlet pressure and hence power output can be controlled using this valve. The turbine is a radial ¯ow device that provides full rated power at 65,000 rpm. The turbine is directly coupled to a high-speed alternator. The rated output is three phase, 1100 Hz, 170 V at 5 A, thus giving a power output of 1.47 kW. The electrical output is converted to direct current using a three-phase recti®er bridge and smoothed using a capacitor. Water-cooling is provided for the alternator that also features thermistor protection to prevent damage from overheating. Low-pressure n-pentane vapour exhausted from the turbine passes through line 5 to the aircooled condenser. Condensation and a small degree of sub-cooling occur. Liquid is returned to the boiler through lines 6 and 1 using a compressed air-powered diaphragm pump. Also shown in Fig. 1 is the boiler pressure relief valve. This valve vents n-pentane vapour to the condenser should the boiler pressure rise above its preset limit. In this way any risk of boiler rupture is avoided and no loss of n-pentane should occur. Fig. 2 shows the assembled system. The speci®cation for the turbine±generator unit is as listed below: Power output Fluid
1.5 kW AC n-Pentane (C5H12)
Fig. 1. Electricity generation system.
172
Inlet pressure Discharge pressure Vapour ¯ow
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
4 bar (a) 0.982 bar (a) 432 kg/h
The gas-®red boiler is a propane boiler with a maximum output of 60 kW (Hamworthy Heating; model P60). The compact brazed heat exchanger is supplied by Alfa Laval Thermal Ltd. model CB76-90H. The air-cooled condenser is a 120 kW fan condenser unit supplied by NRS, model MDE 133-4D. The compressed air diaphragm pump was supplied by Totton pumps. The vapour±liquid separator was designed and manufactured in-house. 3. Testing of the assembled system The system has been tested using hot water provided by a Hamworthy P60 propane boiler with a maximum output of 60 kW. The hot water circuit operates on a closed loop which is
Fig. 2. Electricity generation system.
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
173
pressurized at 2.5 bar (gauge) to allow operation above 1008C. An expansion tank, automatic air vents and safety valves are provided for the hot water loop. The system is instrumented comprehensively. Pressure transducers provide pressure measurement for the boiler and condenser, and thermocouples provide temperature measurement around the system. A turbine wheel type ¯ow meter is used to measure the npentane circulation rate. These instruments are connected to a PC through a Datataker 500 series datalogger allowing rapid simultaneous interrogation of all instruments. A bank of resistance heaters provides the electrical load for the turbine. By adjusting the total number of heaters and their method of connection, the rated output voltageÐcurrent combination of the generator can be achieved. A voltmeter and an ammeter are provided for voltage and current measurement. The results from the laboratory testing are presented in Section 4. 4. Results and discussion Typical test results are shown in Figs. 3±7. A typical set of values for the measured parameters are tabulated in Table 1. The thermal eciency of this system, Zthermal , is de®ned as: Zthermal
electrical output kW 100% boiler heat input kW
1
A voltmeter and an ammeter are used to measure the voltage and current generated and hence the electrical output is deduced. The boiler heat input is determined from the product of the hot water ¯ow rate, the speci®c heat transfer capacity of water and the temperature dierence of the hot water supply to the prototype unit. The average thermal eciency for the test results shown in Fig. 3, Zthermal , is 4.3%. The Carnot eciency, ZCarnot , for a heat engine operating between the boiler saturation temperature of 818C and the condenser saturation temperature of 388C is:
Table 1 Laboratory test results Boiler saturation temperature Boiler pressure Hot water inlet temperature Hot water outlet temperature Hot water ¯ow ratea Boiler heat input Pentane ¯ow rate Turbine inlet temperature Turbine outlet temperature a
Derived values.
818C 3.8 bar(a) 938C 838C 0.80 kg sÿ1 33.6 kW 0.10 kg sÿ1 818C 628C
Condenser inlet temperature Condenser saturation temperaturea Condenser outlet temperature Condenser pressure Ambient temperature Generator output voltage Generator output current Generator power outputa Thermal eciencya
678C 388C 318C 1.1 bar(a) 248C 160 V 9A 1.44 kWe 4.3%
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
Fig. 3. Electricity generation system performance.
174
175
Fig. 4. Electricity generation system performance.
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
Fig. 5. Electricity generation system performance.
176
177
Fig. 6. Electricity generation system performance.
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
Fig. 7. Electricity generation system performance.
178
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
ZCarnot
condenser temperature K 100% 12:1% 1ÿ boiler temperature K
179
2
The theoretical cycle eciency, Ztheoretical , for this system operating isentopically using n-pentane as the working ¯uid can be determined as: ÿ specific isentropic turbine work kJ kg ÿ1 44:6 ÿ 100% Ztheoretical 100% ÿ1 422:5 specific boiler heat input kJ kg 10:6%
3
Values for the speci®c isentropic turbine work and boiler heat input have been obtained from the computer program developed by the National Institute of Standards and Technology, NIST reference database 23, version 4.01. Assuming a 5% heat loss through the walls of the boiler, separator and connecting pipework, a mechanical eciency of 95% and a generator eciency of 90%, the isentropic eciency of the turbine, Zisentropic , can be estimated as: Zthermal 1 1 Zisentropic 1:05 100% 49:8% 4 0:95 0:9 Ztheoretical This turbine's isentropic eciency is low compared to that of a high-pressure steam turbine as used in conventional centralised electricity generation. Improvements in the turbine design could improve the turbinec's isentropic eciency substantially and hence improve the overall thermal eciency.
5. Cost estimate The estimated costs for the prototype unit is £21,560. Table 2 shows a breakdown for the major items. The major capital cost item is the turbine±generator representing over 37% of the total cost of the prototype unit. The capital recovery factor (crf), an economic parameter determined from the lifetime of the system and the discount rate, has been used to estimate the cost per kWh of electricity generated. A discount rate of 10% and a lifetime of 5 years have been assumed in the evaluation. The initial cost estimate of electricity generated is 64 p/kWh for the 1.5 kW prototype unit. For a mass-produced commercial unit, the cost of electricity generation would be reduced signi®cantly.
6. Conclusions An electricity generation system operating on the Rankine cycle using a hydrocarbon working ¯uid has been developed and tested. It is able to operate using low-temperature heat at 818C.
180
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
The thermal eciency of this system is low (typically 4%). The maximum achievable eciency for a heat engine operating between the boiler saturation temperature of 818C and condenser saturation temperature of 388C is 12% (Carnot eciency). The low isentropic eciency of the turbine (49.8%) and ¯uid imperfections reduce this eciency further. To improve the viability of this system, the thermal eciency needs to be increased. This could be achieved in a number of ways: 1. Increasing the isentropic eciency of the turbine and reducing heat losses around the system by better insulation; 2. Increasing the boiler±condenser saturation temperature dierence thereby increasing the Carnot eciency for the cycle. Further work should be conducted towards increasing the turbine isentropic eciency (issue 1). Increasing the boiler saturation temperature will increase the Carnot eciency for the cycle (issue 2). Currently this system operates with a boiler saturation temperature of 808C enabling Table 2 Initial estimate of electricity costs per kWh generated Estimate of econimic costs of prototype unit Lifetime of system (years) Discount rate (%) Capital recovery factor CAPITAL COSTS Turbine±generator unit Gas-®red boiler CBE generator Air-cooled condenser Vapour±liquid seperator Pipework etc Instrumentation Computer + Datalogger Labour (10% total cost)
Percentage of total capital costs 5 10 0.26
Annualized capital costs
8000 1500 1500 3800 200 1500 600 2500 1960 21560 5687.47
OPERATING COSTS (% of capital) n-Pentane, propane, etc.
5 1078
ANNUAL COSTS (£)
6765.47
Rated output (kW) Load factor Electricity supply (kWh)
1.5 0.8 10512
Economic costs (p/kWh)
64.36
37.11 6.96 6.96 17.63 0.93 6.96 2.78 11.60 9.09 100.00
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181
it to use low-temperature heat. A higher boiler saturation temperature would require a higher grade of heat input. The economic viability looks promising, as the set-up cost of this prototype does not represent that of a mass-produced system. Future work will be conducted to develop a system of higher generating capacity and eciency, and lower capital costs. This system is ideally suited to hot climate and remote locations away from mains electricity supply. Potential markets for this system have been identi®ed. In developed countries, the unit could operate from waste heat from a variety of industries including petrochemicals, timber and metal processing. In southern latitudes where the solar resource is more dependable, this system could provide electricity for outlying areas that are not grid connected. Acknowledgements The authors would like to acknowledge the ®nancial support provided by the European Union for this project under contract JOR3-CT97-0183. References [1] W.R. Agar, M. Newborough, Implementing micro-CHP systems in the UK residential Sector, Journal of the Institute of Energy 71 (1998) 178±189. [2] DTI, Digest of United Kingdom Energy Statistics, ISBN 0-11-515465-5, 1999a. [3] DTI, UK Energy Sector Indicators, DTI/Pub 4567/2.5k/12/99/NP.URN 99/193, Dec. 1999b. [4] DTI, New and Renewable Energy-Prospects for the 21st Century, DTI/Pub 4024/3k/3/99/NP.URN 99/744, March 1999c. [5] K. Harvey, Development of combined heat and power in the UK, Energy Policy 22 (2) (1994) 179±181. [6] V.M. Nguyen, S.B. Riat, A novel combined heat and power system driven by solar energy, in: Proceedings of 97 North Sun Conference, 1997, pp. 99±106. [7] W.B. Stine, R.B. Diver, A Compendium of Solar Dish/Stirling Technology, Contract No. 67-3678, Sandia National Laboratories, Livermore, CA 94550, USA, 1999. [8] J.L. Wolpert, S.B. Riat, M. Nguyen, Solar-powered Rankine system, in: Proceedings of the International Conference Exhibition for Village Electri®cation CASE, Delhi, India, 1997, pp. 674±678.
www.elsevier.com/locate/apthermeng
Development of a prototype low-temperature Rankine cycle electricity generation system V.M. Nguyen, P.S. Doherty*, S.B. Riat Institute of Building Technology (IBT), School of the Built Environment, Nottingham University, University Park, Nottingham NG7 2RD, UK Received 18 September 1999; accepted 28 March 2000
Abstract This paper describes the development of a small-scale system designed to generate electricity from low temperature heat (e.g., solar energy). The system operates on the Rankine cycle and uses n-pentane as the working ¯uid. A prototype system has been designed, constructed and tested. It is capable of delivering 1.5 kW of electricity with a thermal eciency of 4.3%. Laboratory test results and a cost estimate for the prototype unit are presented in the paper. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Rankine cycle; Low-temperature electrity generation; Prototype; Laboratory test; n-Pentane
1. Introduction
In 1998 the total installed electricity generation capacity in the UK was approximately 68 GW with over 93% of the electricity supply obtained from fossil fuel and nuclear power generating plants [2]. Fossil fuel combustion leads to the emission of environmental pollutants such as carbon dioxide (CO2), sulphur dioxide (SO2) and oxides of nitrogen (NOx) while nuclear ®ssion products are radioactive and present challenging disposal problems. It is therefore clear that electricity generation from fossil fuel and nuclear power has serious * Corresponding author. Tel.: +44-115-951-3134; fax: +44-115-951-3159. E-mail address: [email protected] (P.S. Doherty). 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 2 - 1
170
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
detrimental eects on the environment and these include global warming, acid rain and contamination of land and seas. Traditionally, centralised fossil fuel and nuclear generation plants have provided the UK with electrical power. Coal ®red generating plants currently operate with average fuel to electricity conversion eciencies of 36%, combined cycle gas turbines (CCGT) with average eciencies of 47% and nuclear power plants are around 37% ecient. In 1998 an estimated 37% of the energy content of fuel input (912 TWh) to fossil and nuclear generators was converted to useful electricity resulting in 63% of the energy input going waste at the generation site [3]. In addition to the conversion losses, transmission losses occur due to ohmic heating in the conductors and this results in a further reduction in the generation eciency from the fuel to the end-user. Lower emission of pollutants per kW of energy produced as well as higher conversion eciencies has brought about a rapid increase in the installation and use of CCGT's with a corresponding rapid decrease in the use of coal ®red electricity generators. Also, the installation of combined heat and power (CHP) systems in the UK has increased with a government target installed capacity of 5000 MW by the year 2000 [2]. The CHP systems provide much higher overall eciency by utilising a proportion of the waste energy resulting from the electricity generating process to provide heat on site [1,5]. Because of the relatively high cost of transporting heat as compared with transporting electricity, de-centralised CHP systems are located closer to the demand for heat, thus minimising heat transport costs whilst satisfying the electricity requirements. A unique form of small scale CHP system that directly heats ventilating air for buildings or process air for industry (Air CHP) can achieve energy conversion eciencies of 95% or more. Around 2.6% of the total electricity generated in the UK during 1998 was obtained from renewable sources. The Government's aim is to meet 5% of the UK's electricity requirements from renewable sources by 2003 and 10% by 2010. To date 650 MW of electricity generation from renewable sources has been commissioned through the Non-Fossil Fuel Obligation schemes [4]. Solar thermal electricity generating plants have been developed and can now operate at eciencies approaching that of conventional generation techniques. Examples of this technology include the 10 MWe S2 power tower installation in the Mojave Desert, CA, which operates on a steam turbine cycle. The turbine inlet temperature is 3908C and the thermal eciency (Eq. (1)) for this system can exceed 25% with a solar to electricity conversion eciency of up to 14% [7]. Highest eciencies are achieved using parabolic dish solar collectors and Stirling engines. A solar to electricity conversion eciency of up to 29.4% has been achieved by the 25 kWe system built by the Advanco Co. The collector temperature for this system can reach 7508C [7]. A mathematical modelling [8] and a `proof of concept' work [6] conducted at the Institute of Building Technology resulted in a research study on a small-scale electricity generation system for domestic applications. A prototype operating on the Rankine cycle using n-pentane as the working ¯uid has been developed. The system is driven by heat sourced from a gas-®red boiler. Heat is rejected to the environment through an air-cooled condenser. The main advantage of this system is that it can be totally independent of the grid and could, therefore be marketable in hot climates and areas remote from mains electricity supply.
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
171
2. Description of the electricity generation system Fig. 1 illustrates the layout of the developed electricity generation system. Liquid n-pentane boils in the compact brazed plate heat exchanger (CBE) as heat is transferred from the hot water circuit. Hot water is provided by a gas-boiler. The resulting n-pentane vapour passes to a droplet separator through line 2. From here liquid droplets drain back to the boiler through line 3. The dry pentane vapour passes via line 4, through a control valve, to the turbine inlet. Turbine inlet pressure and hence power output can be controlled using this valve. The turbine is a radial ¯ow device that provides full rated power at 65,000 rpm. The turbine is directly coupled to a high-speed alternator. The rated output is three phase, 1100 Hz, 170 V at 5 A, thus giving a power output of 1.47 kW. The electrical output is converted to direct current using a three-phase recti®er bridge and smoothed using a capacitor. Water-cooling is provided for the alternator that also features thermistor protection to prevent damage from overheating. Low-pressure n-pentane vapour exhausted from the turbine passes through line 5 to the aircooled condenser. Condensation and a small degree of sub-cooling occur. Liquid is returned to the boiler through lines 6 and 1 using a compressed air-powered diaphragm pump. Also shown in Fig. 1 is the boiler pressure relief valve. This valve vents n-pentane vapour to the condenser should the boiler pressure rise above its preset limit. In this way any risk of boiler rupture is avoided and no loss of n-pentane should occur. Fig. 2 shows the assembled system. The speci®cation for the turbine±generator unit is as listed below: Power output Fluid
1.5 kW AC n-Pentane (C5H12)
Fig. 1. Electricity generation system.
172
Inlet pressure Discharge pressure Vapour ¯ow
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
4 bar (a) 0.982 bar (a) 432 kg/h
The gas-®red boiler is a propane boiler with a maximum output of 60 kW (Hamworthy Heating; model P60). The compact brazed heat exchanger is supplied by Alfa Laval Thermal Ltd. model CB76-90H. The air-cooled condenser is a 120 kW fan condenser unit supplied by NRS, model MDE 133-4D. The compressed air diaphragm pump was supplied by Totton pumps. The vapour±liquid separator was designed and manufactured in-house. 3. Testing of the assembled system The system has been tested using hot water provided by a Hamworthy P60 propane boiler with a maximum output of 60 kW. The hot water circuit operates on a closed loop which is
Fig. 2. Electricity generation system.
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
173
pressurized at 2.5 bar (gauge) to allow operation above 1008C. An expansion tank, automatic air vents and safety valves are provided for the hot water loop. The system is instrumented comprehensively. Pressure transducers provide pressure measurement for the boiler and condenser, and thermocouples provide temperature measurement around the system. A turbine wheel type ¯ow meter is used to measure the npentane circulation rate. These instruments are connected to a PC through a Datataker 500 series datalogger allowing rapid simultaneous interrogation of all instruments. A bank of resistance heaters provides the electrical load for the turbine. By adjusting the total number of heaters and their method of connection, the rated output voltageÐcurrent combination of the generator can be achieved. A voltmeter and an ammeter are provided for voltage and current measurement. The results from the laboratory testing are presented in Section 4. 4. Results and discussion Typical test results are shown in Figs. 3±7. A typical set of values for the measured parameters are tabulated in Table 1. The thermal eciency of this system, Zthermal , is de®ned as: Zthermal
electrical output kW 100% boiler heat input kW
1
A voltmeter and an ammeter are used to measure the voltage and current generated and hence the electrical output is deduced. The boiler heat input is determined from the product of the hot water ¯ow rate, the speci®c heat transfer capacity of water and the temperature dierence of the hot water supply to the prototype unit. The average thermal eciency for the test results shown in Fig. 3, Zthermal , is 4.3%. The Carnot eciency, ZCarnot , for a heat engine operating between the boiler saturation temperature of 818C and the condenser saturation temperature of 388C is:
Table 1 Laboratory test results Boiler saturation temperature Boiler pressure Hot water inlet temperature Hot water outlet temperature Hot water ¯ow ratea Boiler heat input Pentane ¯ow rate Turbine inlet temperature Turbine outlet temperature a
Derived values.
818C 3.8 bar(a) 938C 838C 0.80 kg sÿ1 33.6 kW 0.10 kg sÿ1 818C 628C
Condenser inlet temperature Condenser saturation temperaturea Condenser outlet temperature Condenser pressure Ambient temperature Generator output voltage Generator output current Generator power outputa Thermal eciencya
678C 388C 318C 1.1 bar(a) 248C 160 V 9A 1.44 kWe 4.3%
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
Fig. 3. Electricity generation system performance.
174
175
Fig. 4. Electricity generation system performance.
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
Fig. 5. Electricity generation system performance.
176
177
Fig. 6. Electricity generation system performance.
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
Fig. 7. Electricity generation system performance.
178
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
ZCarnot
condenser temperature K 100% 12:1% 1ÿ boiler temperature K
179
2
The theoretical cycle eciency, Ztheoretical , for this system operating isentopically using n-pentane as the working ¯uid can be determined as: ÿ specific isentropic turbine work kJ kg ÿ1 44:6 ÿ 100% Ztheoretical 100% ÿ1 422:5 specific boiler heat input kJ kg 10:6%
3
Values for the speci®c isentropic turbine work and boiler heat input have been obtained from the computer program developed by the National Institute of Standards and Technology, NIST reference database 23, version 4.01. Assuming a 5% heat loss through the walls of the boiler, separator and connecting pipework, a mechanical eciency of 95% and a generator eciency of 90%, the isentropic eciency of the turbine, Zisentropic , can be estimated as: Zthermal 1 1 Zisentropic 1:05 100% 49:8% 4 0:95 0:9 Ztheoretical This turbine's isentropic eciency is low compared to that of a high-pressure steam turbine as used in conventional centralised electricity generation. Improvements in the turbine design could improve the turbinec's isentropic eciency substantially and hence improve the overall thermal eciency.
5. Cost estimate The estimated costs for the prototype unit is £21,560. Table 2 shows a breakdown for the major items. The major capital cost item is the turbine±generator representing over 37% of the total cost of the prototype unit. The capital recovery factor (crf), an economic parameter determined from the lifetime of the system and the discount rate, has been used to estimate the cost per kWh of electricity generated. A discount rate of 10% and a lifetime of 5 years have been assumed in the evaluation. The initial cost estimate of electricity generated is 64 p/kWh for the 1.5 kW prototype unit. For a mass-produced commercial unit, the cost of electricity generation would be reduced signi®cantly.
6. Conclusions An electricity generation system operating on the Rankine cycle using a hydrocarbon working ¯uid has been developed and tested. It is able to operate using low-temperature heat at 818C.
180
V.M. Nguyen et al. / Applied Thermal Engineering 21 (2001) 169±181
The thermal eciency of this system is low (typically 4%). The maximum achievable eciency for a heat engine operating between the boiler saturation temperature of 818C and condenser saturation temperature of 388C is 12% (Carnot eciency). The low isentropic eciency of the turbine (49.8%) and ¯uid imperfections reduce this eciency further. To improve the viability of this system, the thermal eciency needs to be increased. This could be achieved in a number of ways: 1. Increasing the isentropic eciency of the turbine and reducing heat losses around the system by better insulation; 2. Increasing the boiler±condenser saturation temperature dierence thereby increasing the Carnot eciency for the cycle. Further work should be conducted towards increasing the turbine isentropic eciency (issue 1). Increasing the boiler saturation temperature will increase the Carnot eciency for the cycle (issue 2). Currently this system operates with a boiler saturation temperature of 808C enabling Table 2 Initial estimate of electricity costs per kWh generated Estimate of econimic costs of prototype unit Lifetime of system (years) Discount rate (%) Capital recovery factor CAPITAL COSTS Turbine±generator unit Gas-®red boiler CBE generator Air-cooled condenser Vapour±liquid seperator Pipework etc Instrumentation Computer + Datalogger Labour (10% total cost)
Percentage of total capital costs 5 10 0.26
Annualized capital costs
8000 1500 1500 3800 200 1500 600 2500 1960 21560 5687.47
OPERATING COSTS (% of capital) n-Pentane, propane, etc.
5 1078
ANNUAL COSTS (£)
6765.47
Rated output (kW) Load factor Electricity supply (kWh)
1.5 0.8 10512
Economic costs (p/kWh)
64.36
37.11 6.96 6.96 17.63 0.93 6.96 2.78 11.60 9.09 100.00
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it to use low-temperature heat. A higher boiler saturation temperature would require a higher grade of heat input. The economic viability looks promising, as the set-up cost of this prototype does not represent that of a mass-produced system. Future work will be conducted to develop a system of higher generating capacity and eciency, and lower capital costs. This system is ideally suited to hot climate and remote locations away from mains electricity supply. Potential markets for this system have been identi®ed. In developed countries, the unit could operate from waste heat from a variety of industries including petrochemicals, timber and metal processing. In southern latitudes where the solar resource is more dependable, this system could provide electricity for outlying areas that are not grid connected. Acknowledgements The authors would like to acknowledge the ®nancial support provided by the European Union for this project under contract JOR3-CT97-0183. References [1] W.R. Agar, M. Newborough, Implementing micro-CHP systems in the UK residential Sector, Journal of the Institute of Energy 71 (1998) 178±189. [2] DTI, Digest of United Kingdom Energy Statistics, ISBN 0-11-515465-5, 1999a. [3] DTI, UK Energy Sector Indicators, DTI/Pub 4567/2.5k/12/99/NP.URN 99/193, Dec. 1999b. [4] DTI, New and Renewable Energy-Prospects for the 21st Century, DTI/Pub 4024/3k/3/99/NP.URN 99/744, March 1999c. [5] K. Harvey, Development of combined heat and power in the UK, Energy Policy 22 (2) (1994) 179±181. [6] V.M. Nguyen, S.B. Riat, A novel combined heat and power system driven by solar energy, in: Proceedings of 97 North Sun Conference, 1997, pp. 99±106. [7] W.B. Stine, R.B. Diver, A Compendium of Solar Dish/Stirling Technology, Contract No. 67-3678, Sandia National Laboratories, Livermore, CA 94550, USA, 1999. [8] J.L. Wolpert, S.B. Riat, M. Nguyen, Solar-powered Rankine system, in: Proceedings of the International Conference Exhibition for Village Electri®cation CASE, Delhi, India, 1997, pp. 674±678.