Performance analysis of biomass boiler-organic Rankine cycle with assisted cascade heat pump for combined heat and power generation including exergy-costing

Performance analyses and economic evaluations based on exergy-costing of a biomass-boiler organic Rankine cycle coupled with a cascade heat pump system for combined heat and power generation were investigated. The net power outputs of the Rankine cycles were 7 and 20 kWe at the condensing and evaporating temperatures of 40 and 100oC, respectively. The heat pump generated chilled water (10-28oC) for reducing the Rankine cycle condensing temperature to improve the cycle performance and to generate hot water (80-90oC) for preheating the boiler water condensate after supplying heat at the Rankine cycle evaporator and for other thermal applications. When the Rankine cycle condensing temperature was reduced from 40 to 30oC, the cycle net power output and its first law and second law efficiencies could be enhanced by 22, 15 and 16.5%, respectively. The combined heat pump-organic Rankine cycle thermal and exergy efficiencies also increased with the Rankine cycle condensing temperature reduction. The heat-to-power energy ratio of the thermal and electrical energy generated by the system was also given corresponding to the Rankine cycle condensing temperature reduction. From the study, the highest ratio of 11 could be obtained at the Rankine cycle condensing temperature of 32oC. From economic assessments based on exergy-costing, the unit costs of exergy by the combined cycle were lower than the power grid electrical charge of 0.128USD/kWhe for Thailand, when the Rankine cycle condensing temperature was lower than 38oC. As the condensing temperature was 33oC or below, the unit cost of exergy was nearly steady. At this temperature, the annual generated exergies were 593.1×10^3 and 1692.84×10^3kWhe, the unit costs of exergy were 0.08 and 0.079 USD/kWh, respectively, for the systems with 7 and 20kW Rankine cycles and the payback periods were also found to be 6.69 and 6.35 years with palm fruit bunch biomass.