Thermal performance of brazed metalfoam-plate heat exchanger as an evaporator for organic Rankine cycle

Compact-sized organic Rankine cycle (ORC) power generators call for higher performance and down-sized heat exchangers. Heat exchangers in ORC systems, especially evaporators contribute to a big portion of the system size as well as the capital cost, and their price is also directly related to their size. Since the heat transfer area plays a direct role in the performance of heat exchangers, high-porosity metal foams are proposed for insertion into heat exchanger channels to enhance the heat transfer mechanism in evaporators. Their high surface area to volume ratio makes them a good candidate for manufacturing high-performance heat exchangers. The metalfoams are being considered to improve performance while keeping the size of heat exchangers small. In this experimental study, the performance of a 100 kW class heat exchanger with the channels brazed with nickel based metal foam and stainless steel sheets was investigated. A hot water loop was designed for heat input. The cold side of the heat exchanger works with R245fa as the working fluid. The phase-change heat transfer experiments were performed with different mass flow rates ranging from 0.32 to 0.51 kg/s while the operating pressure was at 10 to 13 bar with hot water inlet temperature was 133℃. Furthermore, the heat transfer performance is compared with a commercial plate heat exchanger (produced by Alfalaval, AC120EQ) which was custom-made as a 100kW class evaporator. In the comparison test, refrigerant side has 0.5kg/s of mass flow rate at 44℃ and heat source side has 1.23 kg/s mass flow rate at 132℃. Although the pressure drop in the metal foam plate heat exchanger is increased compared to that of the conventional plate heat exchanger, increase of the recovered waste heat from the heat source is much higher due to higher overall heat transfer coefficient. As a result, the energy density of the new heat exchanger is about 2.5 times higher than that of the conventional plate heat exchanger.