Development of novel sorber bed heat and mass exchangers for sorption cooling systems

The current cooling systems mainly employ vapor compression refrigeration technology, which increases the electricity peak load significantly and has a high carbon footprint. One alternative solution is sorption systems, run by low-grade thermal energy, i.e. heat sources with temperature less than 100 ºC, such as waste heat, which is non-payable. Also, sorption systems have negligible carbon footprint. Despite all the promising features and benefits, current sorption systems are not ready for wide market adoption. A revolutionary approach to their design and development is needed to overcome their technical limitations such as low specific cooling power (SCP) and low coefficient of performance (COP). Graphite flakes were added to the sorbent to increase the sorbent thermal diffusivity; however, it reduces the active sorbent. The counteracting effect of graphite flake additives in the sorbent was studied using a custom-built gravimetric large pressure jump test bed. It was found that graphite flake additives can increase or decrease the sorption performance depending on the cycle time. Furthermore, 2-D analytical models were developed that consider the spatial and temporal variation of water uptake and temperature in sorber bed heat and mass exchangers (S-HMXs). Two designs of plate fin (P-HMX) and finned-tube (F-HMX) were considered because of the high SCP and COP. Using the analytical models, it was shown that the entire S-HMX components should be optimized simultaneously, and the objective functions of SCP and COP should be optimized together. Thus, an analysis of variance and simultaneous multi-objective optimization of the S-HMX components were performed using the developed analytical models. Based on the optimization study, the P-HMX and the F-HMX were specifically designed and built for sorption cooling systems. The experimental results showed that the present P-HMX achieved an SCP of 1,005 W/kg sorbent, and a COP of 0.60 for Tdes=90 °C, Tsorp= Tcond=30 °C and Tevap=15 °C. Furthermore, the F-HMX yielded an SCP of 766 W/kg and COP of 0.55. It was shown that the P-HMX provided 4.3 times higher SCP, and 3 times higher COP compared to an off-the-shelf heat exchanger coated with a similar composite sorbent consisting of CaCl2, silica gel B150 and PVA.