Two-Phase Expander Model for Energy Recovery Applications

The present study considers a new approach in improving the cost-benefit analysis of waste energy recovery systems by using two-phase expanders. Two-phase expanders have the potential to increase the overall conversion efficiency of organic Rankine cycles (by limiting superheat generation requirement during off-design conditions) and liquefied natural gas plants (by offering an alternative approach to the Joule-Thompson valve). A semi-empirical vapour expander model was adapted for a two-phase piston expander by considering: inlet pressure drop, heat transfer losses (Shah correlation), condensation during expansion and friction losses (Chenn-Flynn). Engine exhaust heat recovery using the organic Rankine cycle was considered. At the design point, the superheated expander produced 9 kW of net power, amounting to 5.7% of engine crankshaft power. The expander was examined with a reducing inlet vapour quality at the typical and the extreme engine speed-load conditions. The two-phase expander at the extreme off-design condition offered a 16% increase in the net power output compared to the reference superheated state. By selecting the suitable two-phase and superheated expander inlet conditions, efficiency was maintained between 58-86% over a wide operating range, hence, improving the cost-benefit analysis of heat recovery systems.