Performance Of Heat Pump Cycle Research Articles

Improved model for the isosteric heat of adsorption and impacts on the performance of heat pump cycles

The isosteric heat of adsorption strongly associates with the energetics of the adsorption processes. Being a direct function of the uptake amount, accurate modelling of the isosteric heat of adsorption is crucial in the design and development of adsorption systems. The isosteric heat of adsorption is often calculated using the Clausius-Clayperon equation with the perfect gas assumption and negligible adsorbed phase volume. We present a thermodynamic model of the isosteric heat of adsorption that addresses the non-ideal behaviour and the variation in the adsorbed phase density. The model is first validated using experimental data in comparison with the available models in the literature that invoke the ideal gas assumption with negligible adsorbed phase volume. Using these models, the isosteric heat for adsorption processes is examined from partial vacuum to high pressure as well as adsorption temperatures above the critical point. The impact of the prediction by each model on the system performance is then assessed in terms of the theoretical COP. For the realistic evaluation of system COP, an improved equilibrium cycle model is developed and the system performance is investigated using various models for the isosteric heat of adsorption. For adsorption at partial vacuum conditions, the COP values predicted by the ideal model are up to 1.8% higher when compared to the calculations using the present model. For high-pressure adsorption below the critical temperature, for example, Maxsorb III + R134a pair, the present model predicts 13% lower COP than the models available in the literature. The variation in the adsorbed phase density addressed in the present model leads to a higher value of isosteric heat (3.8%–8.6%) for adsorption above the critical temperature.

General method for the thermodynamic evaluation of heat pump working fluids

Worldwide experience with Rankine cycle prime movers demonstrates that organic fluids have the potential of meeting future needs of high temperature heat pumps. Many promising candidate fluids exhibit a complex molecular structure, which imparts a peculiar character to the thermodynamics of the heat pump cycle. Since reliable tables of thermodynamic properties are unavailable for most of the fluids of potential interest, a general method was worked out which requires a small amount of data to evaluate the energy performance of different fluid classes. Cycle quality, defined as the ratio of actual to ideal COP, is recognized to be a function of the following main parameters: complexity of the fluid molecular structure, reduced temperature at which evaporation is performed; fractional temperature lift, ΔT/ T C. For fluids having rather complex molecules both technical feasibility (owing to condensation during compression) and energy performance are unsatisfactory unless regenerative precooling of the liquid prior to expansion is applied. If this is done, the same good performance can be obtained from all fluids. In any case the reduced evaporation temperature should be moderate, preferably below 0.7. Knowledge of the aforesaid main parameters, together with the set of graphs provided, allows the energy performance of heat pump cycles to be estimated.