Small-scale trigeneration – A theoretical and experimental study

Combined heat and power (CHP) plants typically achieve efficiencies of 80 to 90%, but their overall economic and environmental benefits can be limited when on-site demand for heat is reduced. Trigeneration, which is the production of cool, heat and power from one system, can increase these benefits by using the excess heat from the CHP to drive a cooling cycle. Such systems have been used for several years at ratings from tens of kWe, but they still pose economic and technological challenges at ratings of a few kWe. This paper presents the results of a theoretical and experimental investigation of small-scale trigeneration systems. An ejector cooling cycle designed to operate off the excess heat from a gas engine mini CHP unit (5.5 kWe) was designed and tested. A model of the trigeneration system was developed using the simulation software TRNSYS, in order to study the influence of operating parameters such as heat exchanger size, heating capacity and driving temperature on the cooling capacity and overall efficiency of the system. The analysis shows that systems delivering simultaneous heating and cooling have the best overall efficiency, allowing savings around 15% compared to conventional systems. While using part of the CHP electrical output to drive the cooling cycle increases the cooling capacity, it does not improve the overall efficiency and will usually increase CO2 emissions.