Optimizations of the organic Rankine cycle-based domestic CHP using biomass fuel

The purpose of this study is to determine the optimal operating conditions and performance for the design of ORC based biomass compact CHP for 2 kW of electric, 25 kW of thermal power productions and 60 °C warm water supply. Eight organic working fluids were selected based on thermo-physical properties and related environmental regulations: cyclopentane, isopentane, n-pentane, diethyl ether, HFO-1233zd, HFC-245fa, HFE-7000 and HFE-7100. The selected organic fluids were classified into three groups considering latent heat and boiling point. The group A fluids contained cyclopentane, isopentane, n-pentane and diethyl ether. The group B fluids contained HFO-1233zd and HFC-245fa. The group C fluids contained HFE-7000 and HFE-7100. A micro CHP system composed of a biomass boiler (200 °C heat source), an ORC power cycle and a cooling water line (20 °C cooling water supply) was modeled in four variants depending on whether post-heater and IHE were applied or not. The subcritical ORC cycle and saturated vapor state at the inlet of the expander were considered for the analysis. As a result of thermodynamic analyses and optimizations, the group A fluids have the best CHP performance because of the greatest latent heat amount. The systems using the group A fluids have the lowest mass flow rates from 0.053 kg/s to 0.081 kg/s, the lowest required heat supplies from 31.64 kW to 34.61 kW, the highest ORC efficiencies from 5.95% to 7.29% and the CHP efficiencies from 71.83% to 72.32%. The group B fluids have the mass flow rates from 0.157 to 0.215 kg/s, the highest required heat supplies from 36.98 kW to 46.41 kW, the lowest system efficiencies from 4.59% to 6.05% and the highest CHP efficiencies from 72.05% to 73.41%. The group C fluids have the highest mass flow rates from 0.213 kg/s to 0.230 kg/s, the required heat supplies from 32.30 kW to 40.54 kW, the system efficiencies from 5.07% to 6.36% and the lowest CHP efficiencies from 71.31% to 72.33%. In addition, ORC systems using the group A or group C fluids can operate at low pressure and can meet system requirements with low cooling water mass flow rate because of the high boiling points. For the group A fluids, both post-heater and IHE are very effective for the system, and the system using the group B fluids can highly improve the system through the application of the post-heater. For the group C fluids, application of the IHE significantly improves system performance.