ORC optimization for medium grade heat recovery

The ENERCO_LT (ENErgy ReCOvery from Low Temperature heat sources) project is a waste heat recovery project that aims to reduce energy consumption in industrial gas production sites by producing electricity through Organic Rankine Cycles supplied by exothermic process discharges at low and medium temperatures. Two promising thermal sources consisting of (i) almost dry gas flow at 165 degrees C and (ii) moist gas flow at 150 degrees C, were investigated. The optimal recovery solution can be identified via an appropriate system design, an adequate working fluid and suitable operating conditions. Systemic optimization coupling exergy analysis and pinch minimization have been performed to assess the potential of various working fluids within various system designs, i.e. single stage system and double stage system. The results indicate that the global exergy efficiency is strongly linked to the critical temperature of the working fluid. They emphasize the existence of an optimal critical temperature specific to the hot source temperature and the pinch value. An empirical expression of the optimal critical temperature was derived for dry and quasi-isentropic pure fluids and appears to be useful for primary selection of the working fluids. Also, recovery solutions using blends appear very attractive to meet high exergy efficiency and environmental friendly objectives. (c) 2014 Elsevier Ltd. All rights reserved.