Energy–exergy analysis and economic investigation of a cogeneration and trigeneration ORC–VCC hybrid system utilizing biomass fuel and solar power

Abstract

The purpose of the present work is the thermodynamic modeling and economic analysis of a micro-scale tri/co-generation system capable of combined heat and power production and refrigeration, based on the joint operation of an Organic Rankine Cycle (ORC) and a Vapor Compression Cycle (VCC). The ORC expander, VCC compressor and electricity generator are connected to the same shaft. The condensation of both cycles takes place under a common pressure in a single condenser. A biomass boiler together with a module of Parabolic-Trough Collectors (PTC) provides heat to the ORC via two distinct intermediate pressurized water circuits. In trigeneration mode (summer operation), a portion of the power produced by the ORC expander is consumed by the VCC compressor, while any surplus power is converted to electricity. The heat generated in the condenser of the system is used to meet hot water demand. In cogeneration mode (winter operation) the VCC is disconnected, since no refrigeration is necessary.The performance of the system is assessed for subcritical operation pressures for the organic medium R245fa. Investigations on the effect of various parameters, such as condensation and evaporation temperatures on the system performance are carried out. The impact of superheating and installing a recuperator is also examined. In a base case scenario (evaporation temperature at 90°C without superheating) assuming an overall 50kWth heat input and a cooling load of 5kWth (during the summer), the net electric efficiency is 2.38%, with an electricity output equal to 1.42kWe and a heating output of 53.5kWth. The exergy efficiency of the ORC was estimated at about 7%.An economic analysis of the system is carried out for a case study considering a typical apartment block on a Greek Island, assuming PTC area of 50m2. The savings in fuel oil and electricity consumption account for an IRR around 12%, with a payback period of 7years.