Thermodynamic and feasibility analysis of air conditioning waste heat recovery via power generation cycles

Abstract

Waste heat from Air Conditioning (AC) systems has long been neglected as a heat source for power generation. With the increasing AC demand worldwide, relevant technology development are urgently required. In this study, four different combined systems have been proposed for AC waste heat conversion. Power generation modules are based on organic Rankine cycle (ORC), organic flash cycle (OFC), Kalina cycle (KCS) and transcritical CO2 cycle (T-CO2), respectively. Each of the above is integrated with a basic AC cycle to establish a combined system. Modelling studies have been carried out to evaluate the performance of the systems and it is found that an ORC-based system has a higher system performance than others throughout the entire AC condensing temperature range considered, and it results in an overall COP enhancement between 15% and 30%. The exergy analysis shows that the relatively low performance of the KCS, OFC or T-CO2-based systems is due to the higher irreversibility in the evaporator. By replacing the ORC module with a dual-pressure ORC counterpart and installing a liquid-line/suction-line heat exchanger in the AC cycle, a further COP enhancement up to 15% is achieved. Preliminary economic analysis indicates that employment of the heat recovery module becomes viable as the system cooling capacity is over 1000 kW and a payback period less than 5 years is expected.