Dynamic reference environment for exergy: A trigeneration system case study

The paper discusses the problem of reference temperature for exergy calculation using an example of a combined cold, heat and power unit integrated with biomass gasification. Depending on the ambient temperature and the demand for heating and cooling, several modes of operation are considered. The performance of the system is evaluated by exergy analysis for various reference temperatures. First, the annual average temperature of 8.5^◦C is applied. Then, monthly variation of the reference temperature between -0.5 and 18.6^◦C (Katowice, Poland) is considered. It has been shown that a constant reference temperature yields misleading results with numerical error between 5% for high-temperature media to almost 90% for cooling agents, in particular for cooling agents, where the useless return water has apparently more exergy than the cold ice water. In conclusion, a variable reference temperature is recommended for exergy analysis of systems including cooling or low-temperature heating agents.