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The prototypes in question were designed to use waste heat or solar energy as the main heat sources. The applications in focus are ice making and air conditioning. The air conditioning systems comprise those with closed cycles and those with open cycles, such as in desiccant systems. The alternatives already studied to increase the performance of the machines are also presented. They mainly include the use of advanced type sorption cycles to improve the internal heat management and heat transfer intensification within the adsorber to improve the SCP. 2. SOLAR POWERED ADSORPTION ICEMAKERS Places with high insolation usually have a large demand for cooling to preserve food, drugs and vaccines, and much research has been devoted to develop machines that could employ solar energy efficiently for such purpose. The development of sorption refrigeration systems powered by solar energy emerged in the late 1970s following the pioneering work of Tchernev [52], who studied a basic solid sorption cycle with the working pair zeolite-water. Since then, a number of studies have been carried out, both numerically and experimentally, but the costs of these systems still make them non-competitive for commercialisation. Therefore, the focus of some research is placed on cost reduction and on the increase of the efficiency of the machines, and promising results have already been obtained. Based on the results of a previous study [53], Pons and Guilleminot [54] concluded that the solid sorption systems could be the basis for efficient solar powered refrigerators, and they developed a prototype with the pair activated carbon-methanol. This machine produced almost 6 kg of ice per m2 of solar panel when the insolation was about 20 MJ day-1, with a solar COP of 0.12. This rate of ice production remains one of the highest obtained by a solar powered icemaker. Critoph [55] mentioned a solar vaccine refrigerator studied in his laboratory in the early 1990s [56]. Such machine, shown in Fig. 1, could keep the cold box at 0 °C during the daytime, after one adsorption cycle performed during the previous night. According to this author, although the COP and ice production of this machine, that used the pair activated carbon-ammonia, were smaller than those produced by a machine with the pair activated carbon-methanol, the former is less sensitive to small leakages, which makes it more reliable to be applied in remote areas where the maintenance is not readily available. Fig. 1. Solar cold box for vaccine preservation [55]. Li et al. [57] performed experiments with the solar ice maker that is shown schematically in Fig. 2. This icemaker that used carbon-methanol as working pair had a COP ranging from 0.12 to 0.14, and produced between 5 and 6 kg of ice per m2 of collector. Analysing the temperature gradient within the adsorbent bed, the authors concluded that in order to improve the performance of this system, the heat transfer properties of the adsorber needed to be enhanced. This could be achieved by increasing the number of fins or using consolidated adsorbent. Fig. 2. Scheme of the solar adsorption icemaker: 1) adsorbent bed; 2) glass cover; 3) damper; 4) insulation; 5) pressure gauge; 6) temperature gauges; 7) valves; 8) evaporator; 9) condenser; 10) refrigerant reservoir; 11) ice box [57]. Based on the previous prototype, Li et al. [58] developed a simpler solar powered icemaker without valves, as shown in Fig. 3. The authors decided to produce the adsorber from stainless steel, instead of cooper or aluminium alloys. This was done since according to experiments done by Hu [59], at temperatures higher than 110 °C, methanol will decompose into dimethyl-ether, which would reduce the efficiency of the system with the time. The adsorber was placed inside an insulated case, which was covered by two transparent plastic fibre sheets. This kind of plastic fibre is more suitable to cover the adsorber than glass because its permeability to solar radiation is higher. To ensure better heat transfer between the front side of the solar collector and the adsorbent, several fins (also made of stainless steel) were placed inside the adsorber. The distance between these fins was approximately 0.1 m and the thickness of the adsorbent layer was 0.04 m. According to the authors, these dimensions were decided according to previous experimental results and optimisation studies. The experiments with this prototype were performed both under indoor (insolation simulated with quartz lamp) and under outdoor conditions. Under indoor conditions, with an insolation from 17 to 20 MJm-2, the ice production was between 6.0 and 7.0 kg per m2 and the COP between 0.13 and 0.15. Under 2PDF Image | International Sorption Heat Pump Conference
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