Optimized sorber bed heat and mass exchangers for sorption cooling systems

Low sorbent thermal diffusivity and off-the-shelf design of sorber beds have impeded the wide market adoption of sorption systems. In this study, graphite flakes are added to the sorbent to increase the thermal diffusivity and sorber beds are specifically designed and optimized for sorption systems. First, an analysis of variance is carried out to find the key parameters of sorber beds using the developed 2-D analytical models. It is shown that all components of the sorber bed, namely the sorbent, heat exchanger and heat transfer fluid, should be optimized simultaneously. Moreover, the specific cooling power (SCP) and coefficient of performance (COP) should be optimized simultaneously due to their conflicting trends. Thus, using the developed analytical models and multi-objective genetic algorithm, an optimization study is conducted. Based on the optimization results, two new sorber beds of firmed-tube sorber bed heat and mass exchanger (F-HMX) and plate-fin sorber bed heat and mass exchanger (P-HMX) are designed, built, and tested in our custom-built two-sorber bed sorption test bed. The experimental results show that the present P-HMX can achieve an SCP of 1,005 W/kg sorbent, and a COP of 0.60 which are higher than the previously published results in the literature. Furthermore, the F-HMX design yields an SCP of 766 W/kg and a COP of 0.55. It is shown that the P-HMX, which is specifically designed and optimized for sorption cooling systems, provides up to 4.3 times higher SCP, and 3 times higher COP compared to an off-the-shelf heat exchanger, an engine oil cooler coated with a similar composite sorbent consisting of CaCl2, silica gel B150 and PVA. The present P-HMX has been tested under a wide range of operating conditions: i) desorption temperature (60-90 degrees C); ii) sorption and condenser temperatures (20-40 degrees C); iii) evaporator temperature (5-20 degrees C); and iv) cycle time (10-20 min). The SCP in the range of 320-1,230 W/kg and COP of 0.40-0.80 are measured in our test bed over the targeted operating conditions.

Automated summary

The study demonstrates improved thermal diffusivity in sorption systems by adding graphite flakes to the sorbent and optimizing sorber beds. Analyzing all components of the sorber bed simultaneously is shown to be crucial for optimization, along with optimizing specific cooling power and coefficient of performance concurrently due to conflicting trends. Optimization results in the development of new sorber bed designs achieving higher performance compared to off-the-shelf heat exchangers.