Solar thermal collectors and applications
Adsorption units
Porous solids, called adsorbents, can physically and reversibly adsorb large volumes of a vapour, called the adsorbate. Though this phenomenon, called solar adsorption, was recognised in the 19th century its practical application in the field of refrigeration is relatively recent. The concentration of adsorbate vapours in a solid adsorbent is a function of the temperature of the pair, i. e. the mixture of adsorbent and adsorbate, and the vapour pressure of the latter. The dependence of adsorbate concentration on temperature, under constant pressure conditions, makes it possible to adsorb or desorb the adsorbate by varying the temperature of the mixture. This forms the basis of the application of this phenomenon in the solar-powered intermittent vapour sorption refrigeration cycle.
An adsorbent-refrigerant working pair for a solar refrigerator requires the following characteristics:
1. A refrigerant with a large latent heat of evaporation.
2. A working pair with high thermodynamic efficiency.
3. A small heat of desorption under the envisaged operating
pressure and temperature conditions.
4. A low thermal capacity.
Water-ammonia has been the most widely used sorption-refrigeration pair and research has been undertaken to utilise the pair for solar-operated refrigerators. The efficiency of such systems is limited by the condensing temperature, which cannot be lowered without introduction of advanced and expensive technology. For example, cooling towers or desiccant beds have to be used to produce cold water to condensate ammonia at lower pressure. Amongst the other disadvantages inherent in using water and ammonia as the working pair are the heavy gauge pipe and vessel walls required to withstand the high pressure, the corrosiveness of ammonia, and the problem of rectification, i. e. removing water vapour from ammonia during generation. A number of different solid adsorption working pairs such as zeolite-water, zeolite - methanol, and activated carbon-methanol, have been studied in order to find the one that performed better. The activated carbon-methanol working pair was found to perform the best [19].
Because complete physical property data are available for only a few potential working pairs, the optimum performance remains unknown at the moment. In addition, the operating conditions of a solar-powered refrigerator, i. e. generator and condenser temperature, vary with its geographical location [19].
The development of three solar/biomass adsorption air conditioning refrigeration systems is presented by Critoph [141]. All systems use active carbon-ammonia adsorption cycles and the principle of operation and performance prediction of the systems are given.
Thorpe [142] presented an adsorption heat pump system which uses ammonia with granular active adsorbate. A high COP is achieved and the cycle is suitable for the use of heat from high temperature (150-200 °C) solar collectors for air conditioning.
