An analytical design tool for a pin fin sorber bed heat/mass exchanger

Sorption cooling and heating systems (SCHS) have recently drawn immense attention as an alternative technology that enhances the efficiency of energy systems to reduce reliance on fossil fuels for heating and cooling. However, they have not been widely adopted mainly due to factors such as: i) the sorbents' low thermal conductivity and diffusivity; and ii) low heat and mass transfer due to the inadequacy of existing sorber bed heat exchanger designs. To address these challenges, a pin fin sorber bed design is proposed in this study as a potential alternative to conventional beds, which can provide both high Coefficient of performance (COP) and enhanced heat and mass transfer inside the sorber beds. To assess the performance of the proposed structure, a predictive model is needed to provide accurate and fast evaluation of the SCHS performance as a function of the geometric design and operating parameters. Consequently, a novel closed-form analytical model is used to predict the sorption performance of a pin fin heat/mass exchanger (PF-HMX) prototype, using the Eigenfunction expansion method to solve the governing energy equation. The proposed transient 2-D solution, includes all salient thermophysical and sorption properties, sorbent geometry, operating conditions, and the thermal contact resistance at the interface between the sorber bed heat exchanger and sorption composite. An analysis of variance (ANOVA) method is utilized to understand the percentage contribution of each parameter on the specific cooling power (SCP) and COP. It is shown that the amount of graphite flakes, sorbent thickness and fin radius on one hand and the cycle time and graphite flake content on the other have the highest level of contribution to the COP and SCP, respectively. Moreover, a parametric study found that heat/mass exchanger (HMX) geometry, sorbent properties and cycle time counteract the effects on the COP and SCP, which should be optimized simultaneously to build an optimal design. The analytical model validated successfully using the sorption data from a custom-built gravimetric large temperature jump (G-LTJ) test bed. The experimental results show that the present PF-HMX design with a relatively low mass ratio (MR) can achieve an SCP of 1160 W/kg sorbent, and a COP of 0.68 which are higher than the previously published results in the literature.