Optimization On Combined Heat And Power System Powered By Moderately Low-temperature Geothermal Sources

There exist lots of low-temperature (<100°C) geothermal heat sources in China which are mainly water-dominated and mainly used for direct heating because of the low-grade characteristics. But for many cases where there exists a large temperature difference between the geothermal temperature and the heating supply temperature as well as a high exhaust temperature of the geothermal source, a lower efficiency of energy use and system techno-economy may appear. A combined heat and power system powered by moderately low-temperature geothermal sources was proposed. A power generation subsystem was incorporated aiming to avoiding the large irreversibility caused by the lager temperature difference between the geothermal temperature and the heating supply temperature and produce much electricity on the basis of satisfying the heating requirement. A heat pump subsystem was also incorporated in order to make full use of the thermal energy in the geothermal source. The proposed novel combined heat and power system has higher thermal and exergy efficiencies and becomes better with the increase of the temperature difference between the geothermal temperature and the heating supply temperature.A numerical simulation model of the combined heat and power system has been developed with methodology of pinch point analysis in order to estimate its optimum performance with optimized power cycle (ORC and transcritical ORC have been investigated) of the power generation subsystem, working fluid and cycle parameters. The screening criterion is high PPR (the ratio of power produced by the power generation subsystem to power consumed by the heat pump subsystem) values. The results show that ORC and trans-critical ORC can be chosen as the power cycle of the power generation subsystem. Working fluids and cycle parameters both play more important roles on the system performance. The optimized fluids and cycle parameters have been obtained for ORC and transcritical ORC, respectively.The variation of efficiencies of the expansion component and refrigerant pump leads to a different screening result between ORC and transcritical ORC as power cycle of the power generation subsystem. Given the geothermal source condition (geothermal source inlet/exhaust temperature of 90/25oC) and heating supply parameters (heating supply/returning temperature of 45/35oC) as well as efficiencies of the expansion component and refrigerant pump (0.6 and 0.7), C5F12, R152a/245ca(70/30wt%), R125 and CO₂/143a(20/80wt%) outstand among each fluid-cycle group, with optimized PPR value of 20%,29%,19% and 22%, respectively. It is found that the PPR value obtained in the ORC power cycle is better than that obtained in the transcritical ORC power cycle under each optimized working fluid and cycle parameters when efficiencies of the expansion component and refrigerant pump are no more than 0.6 and 0.7, respectively.The combined heat and power test system has been designed, constructed and tested. It mainly includes an R245fa-based ORC power generation subsystem and an R134a-based heat pump subsystem. A self-modified scroll expander derived from an air-conditioning scroll compressor previously used for the automobile has been used in the ORC system and a high isentropic efficiency value has been obtained. The scroll expander works reliably and stably with proper lubricant proportion mixed in the refrigerant. Variations of the experimental value of expander power output per unit mass flow rate of hot source (specific power) caused by evaporating/condensing temperature and expander speed has been investigated. Results show that isentropic efficiency of the expander was affected by expander speed and the relationship between the actual and built-in expansion ratio. There exists an optimum evaporating and expander speed pair (that is, there exists the optimum expander speed at constant evaporating temperature and exists the optimum evaporating temperature with each optimum expander speed) to lead to a largest specific power value given the condensing temperature and hot source inlet temperature. The optimum evaporating temperature is higher with higher condensing temperatures under the same hot source inlet temperature which coincides with the theoretical analysis. The optimum expander speed decreases with the increase of condensing temperatures given constant evaporating temperature. 43.5⁵7.9% of expander isentropic efficiency has been obtained according to the experimental results given the condensing temperature (31⁴5oC) and hot source inlet temperature (87⁹0oC). The PPR value of the combined heat and power system ranges from 15.2% to 13.0% given the hot source condition (hot source inlet/exhaust temperature of 90/25oC) and heating supply parameters (heating supply/returning temperature of 45/35oC).