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Simulation of a Novel solar Assisted Combined Heat Pump – Organic Rankine Cycle System
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 61 (2014) 2101 – 2104
The 6th International Conference on Applied Energy – ICAE2014
Simulation of a novel solar assisted combined heat pump – Organic Rankine Cycle system Stefan Schimpfa*, Roland Spana a
Thermodynamics, Ruhr-Universität Bochum, 44801 Bochum, Federal Republic of Germany
Abstract A novel solar thermal and ground source heat pump system harnessing the excess heat of the collectors during summer by an Organic Rankine Cycle (ORC) is simulated. For the ORC the heat pump process is reversed. In this case the scroll compressor of the heat pump runs as a scroll expander and the working fluid is condensed in the ground heat exchanger. Compared to a conventional solar thermal system the only additional investments for the combined system are a pump, valves and upgraded controls. Systems with either flat plate or evacuated tube collectors installed in a single family house are simulated for the locations Ankara, Denver and Bochum. The ORC gains depend on the location and the installed collector type and add up to 40 – 150 kWh/a. This reduces the net electricity demand of the system by 2 – 10%. © Published by Elsevier Ltd. This © 2014 2014The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the Organizing Committee of ICAE2014 Keywords: dynamic modeling; solar heating; heat pump; Organic Rankine Cycle
1. Introduction The coupling of a ground source heat pump and solar thermal collectors in a solar combisystem providing both space heating and domestic hot water is well-established technology. As there is no demand for space heating during summer the area of the collector array becomes overdimensioned and the collectors come to a standstill whenever the maximum temperature of the storage is reached. The stagnation can however be circumvented by the application of an additional ORC which harnesses the excess heat. The domestic application of solar ORCs comprising either flat plate or evacuated tube collectors has been studied both experimentally [1-2] and theoretically [3-4]. In this study a scroll expander is used as the expansion device. The application and performance of scroll machines as expanders has been experimentally examined [5-7] but their use is not yet a prevalent and market-proven technology. Recently Quoilin et al. [8] demonstrated that it is possible to use the same scroll machine
* Corresponding author. Tel.: +49-234-32-26390; fax: +49-234-32-14163. E-mail address: [email protected].
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi:10.1016/j.egypro.2014.12.085
2102
Stefan Schimpf and Roland Span / Energy Procedia 61 (2014) 2101 – 2104
both as compressor and as expander in a reversible heat pump / ORC unit. The goal of this study is to simulate a solar combisystem providing both space heating and domestic hot water with an additional ORC using the scroll compressor of the heat pump as expansion device. The simulation results are used to evaluate the energetic and economic benefit of the ORC. 2. System description A schematic overview of the system is given in Fig. 1. The system mainly consists of flat plate or evacuated tube collectors, a multi-node storage tank, a radiant floor heating system, a reversible heat pump / ORC unit and a ground heat exchanger. The storage tank provides both domestic hot-water by a coiled tube heat exchanger and space heating. The ground-source heat pump can deliver heat for space heating purposes directly without passing the tank or charge the tank for the generation of domestic hot water. Solar energy is coupled into the tank by means of a coiled tube heat exchanger. When a predefined temperature in the storage tank is reached the ORC is started and solar heat is used to evaporate the working fluid in the condenser of the conventional heat pump cycle. The fluid is afterwards expanded in the scroll compressor / expander and condensed in the ground heat exchanger recharging the ground. The only additional investments for the ORC are a pump, valves and upgraded controls; additional investments are therefore rather low compared to investments for the conventional combined (solar thermal plus heat pump) system.
Fig. 1. Schematic overview of the combined heat pump/ORC system
3. Modeling and simulation of the combined system For the simulation models of all components are required. The solar collectors are modeled with a dynamic one-node model according to EN 12975 [9]. The radiation on the sloped collectors has been calculated using the algorithm given in the European Solar Radiation Atlas [10]. The storage tank model is similar to the one of TRNSYS type 534 [11], which is included in the TESS storage tank library. Thermodynamic properties of the working fluid of the heat pump and ORC, R134a [12], are calculated with the property database CoolProp [13]. In ORC mode heat capacities of the fluid and of the ground have a strong impact on the condensation temperature in the ground heat exchanger and consequently on ORC revenues. Therefore, the analytical short-term solution by Javed and Claesson [14] is used in this study.
2103
Stefan Schimpf and Roland Span / Energy Procedia 61 (2014) 2101 – 2104
In the simulation runs the annual behavior of a combined system installed in a single-family house with a floor space of 167 m² built according to the German low energy standard is examined for the locations Bochum, Ankara and Denver. The two latter locations offer higher solar radiation because of their lower latitude. The power required for the heat pump depends on the outdoor design temperature and lies between 4.58 kW and 6.49 kW. The absorber area of the collector array amounts to 12 m² for flat plate collectors and to 8 m² for evacuated tube collectors. The volume of the storage tank is 0.9 m³ and the daily demand for domestic hot water with a temperature of 45 °C is 0.2 m³. The domestic hot water profile was developed by Jordan and Vajen [15]. According to Quoilin et al. [8] the isentropic efficiencies of the scroll machine are 0.7 in compressor mode and 0.65 in expander mode. The goal of the simulations is to evaluate the benefit of the additional ORC energetically and financially. For the economic evaluation the electricity costs are set to 21.4 ct/kWh for Bochum, 14.7 ct/kWh for Ankara and 8.9 ct/kWh for Denver. It is assumed that the annual inflation of the electricity prices is 5 %. All simulations were performed with the software SH-PORT which has been developed during the course of the project. 4. Results Results of the simulation runs are displayed in Table 1. The results clearly show that the new combined ORC system is energetically superior compared to a conventional system. The ORC gains that can be achieved with flat plate collectors in Bochum are rather low compared to the gains at the more southern locations Ankara and Denver. The net electricity demand of a system with flat plate collectors is similar to the one of a system with evacuated tube collectors. However, the ORC gains for evacuated tube collectors are about 30 kWh higher. In this case higher collector outlet temperatures are reached more often and faster. Table 1. Net electricity demand for the novel system (ORC) and for a conventional system (conv.) as well as potential savings after 20 years for the locations Bochum, Ankara and Denver for systems with 12 m² flat plate and 8 m² evacuated tube collectors 12 m² flat plate collectors Location
Bochum
Ankara
8 m² evacuated tube collectors
Denver
Bochum
Ankara
Denver
System type
ORC
conv.
ORC
conv.
ORC
conv.
ORC
conv.
ORC
conv.
ORC
conv.
ORC gain / kWh
43.3
-
92.3
-
112.2
-
75.7
-
122.1
-
145.6
-
Net electricity demand /kWh
1906
1944
1139
1232
1419
1524
1829
1890
1101
1222
1317
1435
Savings after 20 years / €
269
-
451
-
310
-
430
-
590
-
348
-
The difference between the net electricity demand of a combined ORC system and a conventional system does not exactly equal the ORC gain. This discrepancy is caused by two effects. Due to heat injection into the ground during ORC operation the ground is regenerated, which leads to a higher COP of the heat pump and consequently to lower electricity consumption. The predominant effect is that the ORC is started when a defined temperature below the maximum temperature of the storage tank is reached. In this case a higher amount of solar thermal energy can be coupled into the conventional system. The economic analysis highlights the importance of the electricity costs. Although the highest ORC gains are reached for Denver, the savings after twenty years are lower than the ones obtained for Bochum in the case of evacuated tube collectors. The results nonetheless show that especially with evacuated tube collectors the new combined ORC system can be economically feasible because the only additional investments are a pump, valves and upgraded controls.
2104
Stefan Schimpf and Roland Span / Energy Procedia 61 (2014) 2101 – 2104
Acknowledgements The authors would like to thank the German Federal Ministry of Economics and Technology for funding this project. References [1] Wang XD, Zhao L, Wang J L, Zhang WZ, Zhao XZ, Wu W. Performance evaluation of a low-temperature solar Rankine cycle system utilizing R245fa. Solar energy 2010;84:353-64. [2] Yamaguchi, H, Zhang XR, Fujima K, Enomoto M, Sawada N. Solar energy powered Rankine cycle using supercritical CO2. Applied thermal engineering 2006;26:2345-54. [3] Delgado-Torres A, Garcia-Rodriguez L. Analysis and optimization of the low-temperature solar organic Rankine cycle (ORC). Energy Conversion and Management 2010;51:2846–56. [4] Chen Y, Pridasawas W, Lundqvist P. Dynamic simulation of a solar-driven carbon dioxide transcritical power system for small scale combined heat and power production. Solar Energy 2010;84:1103–10. [5] Lemort V, Quoilin S, Cuevas C, Lebrun J. Testing and modeling a scroll expander integrated into an Organic Rankine Cycle. Applied Thermal Engineering 2009;29:3094-102. [6] Wang H, Peterson RB, Herron, T. Experimental performance of a compliant scroll expander for an Organic Rankine Cycle. Proc. IMechE Part A: J. Power and Energy 2009;223:863-72. [7] Zanelli R, Favrat D. Experimental investigation of a hermetic scroll expander-generator. In: Proceedings of the International Compressor Engineering Conference at Purdue; 1994, p. 459-64. [8] Quoilin S, Dumont O, Lemort V. Design, modeling and performance optimisation of a reversible HP/ORC prototype. Rotterdam, ASME ORC 2013. [9] EN 12975. Thermal solar systems and components - Solar collectors - Part 2: Test methods. 2006. [10] Scharmer K, Greif J. The European solar radiation atlas. 1st ed. Paris: Les Presses de L’École des Mines; 2000. [11] Klein SA et al. TRNSYS 17: A Transient System Simulation Program. Solar Energy Laboratory, University of Wisconsin, Madison; 2010. [12] Tillner-Roth R, Baehr, H. An international standard formulation of the thermodynamic properties of 1,1,1,2tetrafluoroethane (HFC-134a) for temperatures from 170 K to 455 K at pressures up to 70 MPa. J Phys Chem Ref Data 1994;23:657-729. [13] Bell I, Quoilin S, Wronski J, Lemort V. CoolProp: An Open-Source Reference-Quality Thermophysical Property Library. Rotterdam, ASME ORC 2013. [14] Javed S, Claesson J. New analytical and numerical solutions for the short-term analysis of vertical ground heat exchangers. ASHRAE Transactions 2011;117:3-12. [15] Jordan U, Vajen K. Realistic domestic hot-water profiles in different time scales. Report for IEA SHC Task 26. Universität Marburg, Fachbereich Physik; 2001.
Biography Stefan Schimpf is research associate at the Thermodynamics institute of Ruhr-University Bochum. Roland Span is chair of this institute. Research areas of the institute include the simulation of innovative energy processes, the accurate experimental and theoretical representation of thermophysical properties, selected heat transfer and sorption problems and technical biogas processes.
ScienceDirect Energy Procedia 61 (2014) 2101 – 2104
The 6th International Conference on Applied Energy – ICAE2014
Simulation of a novel solar assisted combined heat pump – Organic Rankine Cycle system Stefan Schimpfa*, Roland Spana a
Thermodynamics, Ruhr-Universität Bochum, 44801 Bochum, Federal Republic of Germany
Abstract A novel solar thermal and ground source heat pump system harnessing the excess heat of the collectors during summer by an Organic Rankine Cycle (ORC) is simulated. For the ORC the heat pump process is reversed. In this case the scroll compressor of the heat pump runs as a scroll expander and the working fluid is condensed in the ground heat exchanger. Compared to a conventional solar thermal system the only additional investments for the combined system are a pump, valves and upgraded controls. Systems with either flat plate or evacuated tube collectors installed in a single family house are simulated for the locations Ankara, Denver and Bochum. The ORC gains depend on the location and the installed collector type and add up to 40 – 150 kWh/a. This reduces the net electricity demand of the system by 2 – 10%. © Published by Elsevier Ltd. This © 2014 2014The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the Organizing Committee of ICAE2014 Keywords: dynamic modeling; solar heating; heat pump; Organic Rankine Cycle
1. Introduction The coupling of a ground source heat pump and solar thermal collectors in a solar combisystem providing both space heating and domestic hot water is well-established technology. As there is no demand for space heating during summer the area of the collector array becomes overdimensioned and the collectors come to a standstill whenever the maximum temperature of the storage is reached. The stagnation can however be circumvented by the application of an additional ORC which harnesses the excess heat. The domestic application of solar ORCs comprising either flat plate or evacuated tube collectors has been studied both experimentally [1-2] and theoretically [3-4]. In this study a scroll expander is used as the expansion device. The application and performance of scroll machines as expanders has been experimentally examined [5-7] but their use is not yet a prevalent and market-proven technology. Recently Quoilin et al. [8] demonstrated that it is possible to use the same scroll machine
* Corresponding author. Tel.: +49-234-32-26390; fax: +49-234-32-14163. E-mail address: [email protected].
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi:10.1016/j.egypro.2014.12.085
2102
Stefan Schimpf and Roland Span / Energy Procedia 61 (2014) 2101 – 2104
both as compressor and as expander in a reversible heat pump / ORC unit. The goal of this study is to simulate a solar combisystem providing both space heating and domestic hot water with an additional ORC using the scroll compressor of the heat pump as expansion device. The simulation results are used to evaluate the energetic and economic benefit of the ORC. 2. System description A schematic overview of the system is given in Fig. 1. The system mainly consists of flat plate or evacuated tube collectors, a multi-node storage tank, a radiant floor heating system, a reversible heat pump / ORC unit and a ground heat exchanger. The storage tank provides both domestic hot-water by a coiled tube heat exchanger and space heating. The ground-source heat pump can deliver heat for space heating purposes directly without passing the tank or charge the tank for the generation of domestic hot water. Solar energy is coupled into the tank by means of a coiled tube heat exchanger. When a predefined temperature in the storage tank is reached the ORC is started and solar heat is used to evaporate the working fluid in the condenser of the conventional heat pump cycle. The fluid is afterwards expanded in the scroll compressor / expander and condensed in the ground heat exchanger recharging the ground. The only additional investments for the ORC are a pump, valves and upgraded controls; additional investments are therefore rather low compared to investments for the conventional combined (solar thermal plus heat pump) system.
Fig. 1. Schematic overview of the combined heat pump/ORC system
3. Modeling and simulation of the combined system For the simulation models of all components are required. The solar collectors are modeled with a dynamic one-node model according to EN 12975 [9]. The radiation on the sloped collectors has been calculated using the algorithm given in the European Solar Radiation Atlas [10]. The storage tank model is similar to the one of TRNSYS type 534 [11], which is included in the TESS storage tank library. Thermodynamic properties of the working fluid of the heat pump and ORC, R134a [12], are calculated with the property database CoolProp [13]. In ORC mode heat capacities of the fluid and of the ground have a strong impact on the condensation temperature in the ground heat exchanger and consequently on ORC revenues. Therefore, the analytical short-term solution by Javed and Claesson [14] is used in this study.
2103
Stefan Schimpf and Roland Span / Energy Procedia 61 (2014) 2101 – 2104
In the simulation runs the annual behavior of a combined system installed in a single-family house with a floor space of 167 m² built according to the German low energy standard is examined for the locations Bochum, Ankara and Denver. The two latter locations offer higher solar radiation because of their lower latitude. The power required for the heat pump depends on the outdoor design temperature and lies between 4.58 kW and 6.49 kW. The absorber area of the collector array amounts to 12 m² for flat plate collectors and to 8 m² for evacuated tube collectors. The volume of the storage tank is 0.9 m³ and the daily demand for domestic hot water with a temperature of 45 °C is 0.2 m³. The domestic hot water profile was developed by Jordan and Vajen [15]. According to Quoilin et al. [8] the isentropic efficiencies of the scroll machine are 0.7 in compressor mode and 0.65 in expander mode. The goal of the simulations is to evaluate the benefit of the additional ORC energetically and financially. For the economic evaluation the electricity costs are set to 21.4 ct/kWh for Bochum, 14.7 ct/kWh for Ankara and 8.9 ct/kWh for Denver. It is assumed that the annual inflation of the electricity prices is 5 %. All simulations were performed with the software SH-PORT which has been developed during the course of the project. 4. Results Results of the simulation runs are displayed in Table 1. The results clearly show that the new combined ORC system is energetically superior compared to a conventional system. The ORC gains that can be achieved with flat plate collectors in Bochum are rather low compared to the gains at the more southern locations Ankara and Denver. The net electricity demand of a system with flat plate collectors is similar to the one of a system with evacuated tube collectors. However, the ORC gains for evacuated tube collectors are about 30 kWh higher. In this case higher collector outlet temperatures are reached more often and faster. Table 1. Net electricity demand for the novel system (ORC) and for a conventional system (conv.) as well as potential savings after 20 years for the locations Bochum, Ankara and Denver for systems with 12 m² flat plate and 8 m² evacuated tube collectors 12 m² flat plate collectors Location
Bochum
Ankara
8 m² evacuated tube collectors
Denver
Bochum
Ankara
Denver
System type
ORC
conv.
ORC
conv.
ORC
conv.
ORC
conv.
ORC
conv.
ORC
conv.
ORC gain / kWh
43.3
-
92.3
-
112.2
-
75.7
-
122.1
-
145.6
-
Net electricity demand /kWh
1906
1944
1139
1232
1419
1524
1829
1890
1101
1222
1317
1435
Savings after 20 years / €
269
-
451
-
310
-
430
-
590
-
348
-
The difference between the net electricity demand of a combined ORC system and a conventional system does not exactly equal the ORC gain. This discrepancy is caused by two effects. Due to heat injection into the ground during ORC operation the ground is regenerated, which leads to a higher COP of the heat pump and consequently to lower electricity consumption. The predominant effect is that the ORC is started when a defined temperature below the maximum temperature of the storage tank is reached. In this case a higher amount of solar thermal energy can be coupled into the conventional system. The economic analysis highlights the importance of the electricity costs. Although the highest ORC gains are reached for Denver, the savings after twenty years are lower than the ones obtained for Bochum in the case of evacuated tube collectors. The results nonetheless show that especially with evacuated tube collectors the new combined ORC system can be economically feasible because the only additional investments are a pump, valves and upgraded controls.
2104
Stefan Schimpf and Roland Span / Energy Procedia 61 (2014) 2101 – 2104
Acknowledgements The authors would like to thank the German Federal Ministry of Economics and Technology for funding this project. References [1] Wang XD, Zhao L, Wang J L, Zhang WZ, Zhao XZ, Wu W. Performance evaluation of a low-temperature solar Rankine cycle system utilizing R245fa. Solar energy 2010;84:353-64. [2] Yamaguchi, H, Zhang XR, Fujima K, Enomoto M, Sawada N. Solar energy powered Rankine cycle using supercritical CO2. Applied thermal engineering 2006;26:2345-54. [3] Delgado-Torres A, Garcia-Rodriguez L. Analysis and optimization of the low-temperature solar organic Rankine cycle (ORC). Energy Conversion and Management 2010;51:2846–56. [4] Chen Y, Pridasawas W, Lundqvist P. Dynamic simulation of a solar-driven carbon dioxide transcritical power system for small scale combined heat and power production. Solar Energy 2010;84:1103–10. [5] Lemort V, Quoilin S, Cuevas C, Lebrun J. Testing and modeling a scroll expander integrated into an Organic Rankine Cycle. Applied Thermal Engineering 2009;29:3094-102. [6] Wang H, Peterson RB, Herron, T. Experimental performance of a compliant scroll expander for an Organic Rankine Cycle. Proc. IMechE Part A: J. Power and Energy 2009;223:863-72. [7] Zanelli R, Favrat D. Experimental investigation of a hermetic scroll expander-generator. In: Proceedings of the International Compressor Engineering Conference at Purdue; 1994, p. 459-64. [8] Quoilin S, Dumont O, Lemort V. Design, modeling and performance optimisation of a reversible HP/ORC prototype. Rotterdam, ASME ORC 2013. [9] EN 12975. Thermal solar systems and components - Solar collectors - Part 2: Test methods. 2006. [10] Scharmer K, Greif J. The European solar radiation atlas. 1st ed. Paris: Les Presses de L’École des Mines; 2000. [11] Klein SA et al. TRNSYS 17: A Transient System Simulation Program. Solar Energy Laboratory, University of Wisconsin, Madison; 2010. [12] Tillner-Roth R, Baehr, H. An international standard formulation of the thermodynamic properties of 1,1,1,2tetrafluoroethane (HFC-134a) for temperatures from 170 K to 455 K at pressures up to 70 MPa. J Phys Chem Ref Data 1994;23:657-729. [13] Bell I, Quoilin S, Wronski J, Lemort V. CoolProp: An Open-Source Reference-Quality Thermophysical Property Library. Rotterdam, ASME ORC 2013. [14] Javed S, Claesson J. New analytical and numerical solutions for the short-term analysis of vertical ground heat exchangers. ASHRAE Transactions 2011;117:3-12. [15] Jordan U, Vajen K. Realistic domestic hot-water profiles in different time scales. Report for IEA SHC Task 26. Universität Marburg, Fachbereich Physik; 2001.
Biography Stefan Schimpf is research associate at the Thermodynamics institute of Ruhr-University Bochum. Roland Span is chair of this institute. Research areas of the institute include the simulation of innovative energy processes, the accurate experimental and theoretical representation of thermophysical properties, selected heat transfer and sorption problems and technical biogas processes.