Performance Enhancement of a Thermally Activated Cooling System Using Microchannel Heat Exchangers

Abstract

In the current study a thermally activated cooling system that combines an ORC system with a vapor compression cooling cycle was developed and tested under laboratory conditions. This combined system can utilize waste heat or other thermal sources such as solar and geothermal to generate power and cooling. Hot oil with temperature up to 200 °C was used as the simulated heat source for its convenience. Microchannel heat exchangers were used throughout the system in order to meet the performance, size and weight challenges. Two conventional plate heat exchangers for the boiler and recuperator were also tested inside the system for comparison purposes. Compared to the plate boiler and recuperator, the integrated microchannel boiler/recuperator displayed significant improvement of the heat transfer effectiveness over the range of mass flow rates examined which translates to higher conversion efficiency and overall COP. A simplified model, used to design the microchannel recuperator, ignores the entrance and conjugate heat transfer effects, but it appears to match well the trend of measured average heat transfer coefficients for all cases tested. After taking into account the channel side walls, the measured values of the average heat transfer coefficient agree also with the predicted one. Because of this relatively conservative design approach, the actual heat transfer effectiveness for the recuperator was significantly higher than the designed value of 85%. With the integrated microchannel boiler/recuperator, this thermally activated cooling system was able to achieve over 5 kW of cooling and approached an overall COP of 0.8 under laboratory conditions.