Low-temperature waste heat recovery from internal combustion engines and power output improvement through dual-expander organic Rankine cycle technology

Energy, being the driving force behind the economic development, there has been continued efforts to develop alternative energy-utilisation strategies. Due to lower heat utilisation efficiency of internal combustion engines, they provide a huge potential of waste heat recovery by organic Rankine cycle technology. The present work is devoted towards the waste heat recovery from engine coolant. This study provides the complete thermodynamic analyses, cycle configuration, system size design and selection of working fluid for the development of the stand-alone engine-coolant based ORC. The system performance at fluctuating flow of jacket cooling water with engine speed and load is investigated. The effect of changing mass flow rate of heat source, which leads to varying heat input available to be recovered from the engine coolant is considered with fixed temperature of 90°C. Methodology has been presented to enhance the cycle thermal efficiency with the use of dual expander at part load conditions, by parallelising the flow to attain the favourable input conditions for the expanders over wider variation of heat input. R245fa showed the highest performance characteristics giving cycle efficiency and work output of 4.72% and 0.97 kW, respectively at dual expander operating mode. The control strategy to switch to the dual expander is based upon its evaluation of actual isentropic efficiency and optimum working range. The present study establishes the importance of considering the continually varying heat-input conditions and its effect on the key components, unlikely assuming its performance to be constant. And to capture their varying characteristics and integrate its effect on the overall system to provide operational controlling at off-design conditions to attain flatter efficiency at wider operating conditions for system optimisation.