The influence of periodic operation on the characteristics of adsorption devices

In this paper, we elaborate on the improvement of the design of adsorption heat pump cycles by combining economic and thermodynamic information. A method is used, which results in a relation between the performance and the specific cost of the device. The performance, of course, is given in terms of COP. The cost is approximated by a figure for the required area for heat exchange, which includes all the cost for the heat exchange equipment that is required to exchange heat with the environment (hot and cold heat source, heat sink). Because these costs represent a major part of the investment cost for the device, we use it our calculations. The resulting relation between the COP and the area cost is representative for the specific device. It is represented in the "optimal operational plot", which is the maximal efficiency (COP) against the required heat exchange area. The study is performed on a standard, single stage, zeolite-water adsorption device with two adsorbers and an internal heat recovery. Usually, continuously operating devices perform better than discontinuously operating ones. The cyclic behaviour has some consequences, e.g. same design for adsorber and desorber, thermal cycling of adsorbers, importance of inactive thermal mass and different approaches to internal heat recovery. The effect of these items on the optimisation results is presented in this study. It is shown how the periodic operation results in a decrease in COP and an increase in the specific heat exchange area, i.e. the cost. The increase in exchange area depends mainly on the duration of the phase, where no cooling energy is produced, while the decrease in COP depends mainly on the inactive thermal mass. (c) 2006 Elsevier Ltd. All rights reserved.