Performance improvement of organic rankine cycle for power generation by photovoltaic-assisted heat pump

Abstract

This study proposed a new technique of solar photovoltaic-cascade heat pump assisted ORC (PV-HP-ORC) system integration from combined cooling, heat and power (CCHP). The typical power outputs of the R245fa-ORC were 7 and 20kWe at 40oC-ORC condensing temperature. The application of the heat pump was to reduce the ORCcondensing temperature (40 to 30oC) for a higher ORC power output by cooling down the condenser-cooling water temperature and to generate heat in terms of hot water (80- 90oC). This meant that some/whole part of heat rejected at the ORC condenser served as the cooling load of the heat pump. Some part of the generated heat was used for assisting the ORC boiler by preheating ORC heat source water condensate after it supplied heat to the ORC evaporator. Therefore, the boiler biomass consumption could be saved. In addition, the other part of heat (hot water) could also be used for other thermal applications such as drying processes. Due to that the heat pump consumed electrical energy, a set of PV modules, each of 320Wp, was integrated to save electricity transmitted from the grid of the power plant. Moreover, from the annual exergy generated by the system due to ORC generated power output and heat pump generated heat, system investment cost, and system energy saving, the unit costs of exergy (UCEs) of the system were determined based on exergy-costing method and GHG CO2 emission of the system was also defined. As mentioned above, the new technique of HP-ORC system was proposed, and the results of the system performances and economic and environmental assessments were obtained. The ORC thermal and exergy efficiencies increased around 15 and 16.5% when the ORC condensing temperature was reduced by 10oC under the heat pump assistance. In addition, the overall HP-ORC system thermal and exergy efficiencies also increased from 7.25 to 50.5% and from 7.25 to 12.21%, respectively. From the annual generated exergy and total investment costs of the systems, ORC and HP-ORC, the unit costs of exergy (UCEs) were determined. The cassava biomass gave the highest UCEs while the palm fruit bunch gave the lowest UCEs of the systems. The bigger system with 20kWORC also gave the lower UCEs compared to those of the system with 7kW-ORC. At 40oC ORC-condensing temperature, the UCEs of the bigger system were 8.47 and 4.91Baht/kWh, respectively, with cassava and palm fruit bunch compared to 9.11 and 6.5Baht/kWh of the small system. The UCEs of the HP-ORC could be reduced to lower than the UCEmarket due to that the system could generate heat and higher power output as the ORC condensing temperature was reduced. The UCEs of the HP-ORC dropped below the market price (4Baht/kWhe) when the ORC condensing temperatures were equal or below 37 and 38oC for the systemwith 7 and 20kW-ORC, respectively. The suitable lowest ORC condensing temperature to be reduced was 33oC and the payback periods were, respectively, 6.69 and 6.35 years with palm fruit bunch biomass. The total daily CO2 emissions were also increasing from 575.1 to 971.7kg.CO2 eq./day and from 1564.2 to 2696.7kg.CO2 eq./day, respectively, for the system with 7 and 20kW-ORC when the ORC condensing temperature was reduced from 40 to 30oC. In contrast, the CO2 emission factors were decreasing from 4.11 to 0.46kgCO2 eq./kWh and 3.91 to 0.45kgCO2 eq./kWh as the ORC condensing temperature was reduced from 40 to 30oC. Therefore, the bigger system also possessed the better environmental performance. The heat-to-power energy ratio of the HP-ORC system was also given corresponding to the reduced ORC condensing temperature. From this ratio, the capacity of the heat pump could be designed and the UCE and the payback period of the combined HP-ORC system could be determined. Furthermore, with a set of the PV modules, the UCEs and the amount of CO2 emission of the PV-HP-ORC were lower compared to those of the HP-ORC. The UCEs of the HP-ORC with palm fruit biomass were reduced from 6.5 and 4.91Baht/kWh to 2.51 and 2.48Baht/kWh, respectively, for the system with 7 and 20kW-ORC when the ORC condensing temperature was reduced from 40 to 30oC. With 60 PV modules, the UCE values of the PV-HP-ORC dropped from 6.5 and 4.91Baht/kWh to 2.43 and 2.45Baht/kWh. With lower UCEs, the payback periods of the PV-HP-ORC were also slightly shorter with specific number of PVs at ORC condensing temperature range of 37- 40oC. Below 37oC, the payback periods increased due to higher costs of PVs and inverter then the integration of PVs was not good. Similarly, the integration of the PVs reduced the CO2 emission of the system. With 50 PV modules, the CO2 emission factors dropped 0.43 and 0.44kgCO2 eq./kWh, respectively, for the system with 7 and 20kW-ORC at the condensing temperature of 30oC. It could be noticed that without PV the system with 20kW-ORC environmentally performed better than the small one due to less CO2 emission factor. The opposite result was found when specific number of PVs was integrated due to less heat pump electricity consumption of the small system. Sensitive analysis of the system UCEs with affecting parameters such as prices of palm fruit bunch biomass and PVs, daily operating time, discount rate, and stack gas waste heat recovery was also performed in the last section of the study.