Exergy assessment and exergy cost analysis of a renewable-based and hybrid trigeneration scheme for domestic water and energy supply

Exergy and exergy cost analyses are proposed as complementary methods for the assessment and better understanding of the efficiency of a hybrid trigeneration system based on renewable energy sources. The system combines photovoltaic/thermal collectors, an evacuated tube collector and a wind turbine and produces electricity, sanitary hot water and desalted fresh water for a single family house. The system includes two desalination technologies (reverse osmosis and membrane distillation) that consume power and heat respectively, and two kinds of energy storage devices (a hot water tank and two lead-acid batteries). The assessment is based on simulations developed by using TRNSYS software. As a first level of detail, exergy analysis is applied in ten-minute basis to selected plant components. As a second level of detail, it is proposed to apply exergy-based indicators that summarize the system behavior during a longer period of time (monthly basis). By using aggregated values, exergy accumulation terms become negligible, what allows applying symbolic thermoeconomics to calculate exergy cost and to analyze in depth the process of cost formation. The system has an exergy efficiency of 7.76% (6.68 due to electricity, 0.33 due to fresh water and 0.75 due to sanitary hot water).