Potential of a Regenerative Transcritical Series Two Stage Organic Rankine Cycle for Dual Source Waste Heat Recovery

orc2019 Tracking Number 161

IC engine waste heat recovery is the most common application of Organic Rankine Cycle (ORC) systems, which were mostly pre-heated or dual loop[1]. Recent studies on dual pressure organic Rankine cycle (ORC) architectures shows series two stage ORC (STORC) to have improved exergetic efficiency compared to single pressure ORC[2][3]. Studies have also shown that adopting partial evaporation in ORCs architecture improves exergetic efficiency than the simple ORC[4]. So far, only a few studies have focussed on the use of two stage evaporation architectures for dual source heat recovery in which maximum heat source utilization is desired[5]. Adopting a supercritical evaporation processin the high pressure (HP) stage combined with partial evaporation in the low pressure(LP) stage could lead to improved thermal matching and increased heat source utilization. The regenerative use of superheated vapour from the exit of HP turbine could also lead to improved thermal efficiency. In this study, a regenerative transcritical STORC (RT-STORC) is analysed for waste heat recovery from a 2.97 MW natural gas IC engine. The HP and LP evaporator extracts heat from the exhaust gas (primary heat source) and jacket water(secondary heat source)respectively. A thermodynamic model of the system in developed in MATLAB, for which the HP stage pressure, vapour outlet temperature and LP stage evaporation temperature are specified. Based on this, the model optimizes the vapour quality in the LP stage evaporator, so as to satisfy the saturated vapour condition in the LP turbine inlet. The effect of vapour outlet temperature and evaporation pressure in HP stage is analysed for a given LP stage evaporation temperature. For the HP stage, higher evaporator pressures and lower vapour outlet temperatures leads to increase in net work output.There exists an optimum LP stage evaporation temperature that maximises the utilization rate of secondary heat source. The optimum vapour fraction in the LP stage increases with decrease in HP stage pressure and outlet temperature. At the engine design point, the RT-STORC delivers 419kW, which is 13.6% and 25.8% higher than the sub critical STORC and pre-heated ORC respectively.