Exergy analysis of an isothermal heat pump dryer

A numerical simulation of a plate contact-type isothermal heat pump dryer (HPD) is used to examine the energy efficiency improvement obtainable from this system compared with a conventional HPD. While we consider this system design to be entirely feasible, we are not aware of any existing practical applications of the design. The simulation incorporates a detailed plate, product and air flow model, solving the mass, momentum and energy balances within the drier, into a pre-existing model of the remaining HPD components. The accuracy of an idealised drier-duct model used in a previous analysis is assessed. Although the accuracy of the idealised model is found to be sensitive to local system temperature variations, this is found not to lead to significant error when it is integrated into the whole-system HPD model. The energy efficiency benefit associated with the isothermal contact HPD is confirmed to be a factor of between 2 and 3. An exergy analysis is used to determine the causes of this perfomance gain. Contact heat transfer in isothermal HPD is found to reduce irreversibility within the refrigerant cycle by roughly the same amount as that occurring in heat transfer from the condenser to the product.

► Numerical simulation is used to assess energy efficiency benefit of a novel heat pump drying (HPD) configuration, called ICHPD.
► Validation of the model includes comparison with measured data and idealised analytical solutions for the standard and ICHPD configurations.
► The energy efficiency benefit associated with ICHPD is shown to be a factor of between 2 and 3.
► Exergy analysis shows that ICHPD reduces refrigerant-cycle irreversibilities and condenser-product heat transfer irreversibilities by roughly equal amounts.
► An economic case study shows that ICHPD provides an opportunity to reduce economic risk associated with uncertain energy prices.