A general framework for part-load optimization of air-cooled ORCs with screw expanders

The present work focuses on part-load optimization of air-cooled, recuperative Organic Rankine Cycle systems equipped with screw expanders driven by medium temperature (180 °C) waste heat. A series of technical operational constraints along with the off-design performance of heat exchangers and expander, pump and air-cooled condenser (ACC) for different heat source/heat sink temperatures and flow rates are considered. Two operating strategies are examined based on constant (CSH) and variable (VSH) superheating. At part-load, the cooling fluid mass flow rate and expander speed are optimized to maximize overall efficiency. The optimized part-load efficiency is far more sensitive to the cooling fluid temperature than the heat source temperature and flow rate, thanks to pressure ratio optimization achieved by varying the expander speed. The VSH operating strategy offers meaningful performance improvement over the CSH for heat source and sink temperatures exceeding the nominal. Minimum part-load overall efficiency corresponds to highest heat source and heat sink temperature and is equal to roughly 65–70 % of the nominal. The efficiency improvement thanks to expander speed optimization is significant especially at lower heat source flow rates and higher heat sink temperatures. Part-load optimization enables consistent performance across wide range of boundary conditions.