Comparison of ab- and adsorbents for low temperature heat driven sorption cooling machines
Abstract Thermally driven cooling machines allow for reduction of CO2 emissions when using waste heat from industrial processes or cogeneration, or when using solar heat. The lower the necessary driving temperature level is the more heat sources are available to be used, or the more thermal power can be drawn from a heat source, or the smaller and cheaper the heat exchanger for a given power can be. There have been debates for decades whether adsorption chillers can be driven by heat sources at lower temperatures than absorption chillers. The paper at hand intends to continue this debate on a rather fundamental level. The inorganic salt lithium bromide which is the most common absorbent and the common adsorbent RD-Silica gel are compared to organic salts (ionic liquids) as designable absorbents on the one hand, and adsorbents designed for use at low driving temperatures, on the other hand. For that purpose, functions for the vapor pressure depression of the discussed sorbents are required. Simple correlations for the absorbents are derived in this paper, whereas the correlations for the adsorbents are taken from literature. The crystallization lines of lithium bromide derived from experimental data from two different sources are also discussed shortly. From these correlations, the minimum desorption temperature at various boundary conditions is derived for five sorbents and the influence of the desorption temperature on the change of composition in the sorbents is shown. The latter is traced back to the shape of the sorption isotherms. At small temperature lifts the zeotypes and the ionic liquid [mmim][DMP] can be operated with lowest driving temperatures, but at higher temperature lifts the zeotypes are inoperable. Finally, it is concluded that the possibility to tailoring sorbents according to the customer's needs prevails for both liquid and solid sorption as a pre-requisite to operate with very low driving temperatures.