Solar cooling plants: some characteristic system arrangements

International Journal of Low-Carbon Technologies, Oct 2007

Abstract Some schemes of operating plants are reported analysing the role of the storage and the sizing of the solar section and absorption chiller together with recorded results. After considering some schemes described in the literature, a hot and cold storage tank system designed by the author is presented. First of all layout and control logic are detailed, then experimental data recorded on the plant are illustrated concerning the seasonal performance both of the solar section and of the absorption chiller.

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Solar cooling plants: some characteristic system arrangements

International Journal of Low Carbon Technologies 2/4 Solar cooling plants: some characteristic system arrangements Renato M. Lazzarin 0 0 Dipartimento di Tecnica e Gestione dei Sistemi industriali, DTG Università di Padova , Italy Some schemes of operating plants are reported analysing the role of the storage and the sizing of the solar section and absorption chiller together with recorded results. After considering some schemes described in the literature, a hot and cold storage tank system designed by the author is presented. First of all layout and control logic are detailed, then experimental data recorded on the plant are illustrated concerning the seasonal performance both of the solar section and of the absorption chiller. solar cooling; absorption; hot storage; cold storage Nomenclature Introduction A previous paper highlighted the importance of a correct arrangement of the various parts of a solar cooling plant to determine its seasonal performance [ 1 ]. A high performance absorption chiller and very efficient solar collectors are not enough to give an efficient solar cooling system. How the chiller is controlled or how the solar energy is stored can definitely influence system performance. The role of the storage is of paramount importance in this application. Here the storage not only has the obvious function of accumulating excess solar energy to use later, but it also aids the functioning of the absorption chiller as its capacity is not easy to be modulated. The risk is that the seasonal COP is very low, even lower than 0.2 due to the ON-OFF operation of the absorption chiller [ 2 ]. This is the main reason why a cold storage should be provided and its operation must be properly planned. Three different examples of solar cooling absorption plants are reported here, equipped with hot storage only or both cold and hot storage tanks in order to better illustrate the principles described in the paper [ 1 ]. The last example refers to my personal experience and reports not only the details of the system but also seasonal performance data of the absorption chiller and solar collectors. Two different solar cooling system schemes An example of a solar cooling plant equipped with only a hot storage is at the Arizona State University (Phoenix, Arizona) [ 3 ]. The plant is comprised of solar collectors, hot pressurised storage, absorption chiller, sanitary water circuit and refrigerating circuit. One can notice in Fig. 1 pumps, heat exchangers and auxiliary boilers. Solar collectors cannot directly drive the absorption chiller. With regard to the control of solar collectors: collectors and storage pumps are turned on when a proper temperature difference is reached between the solar collectors and storage. Of course the connections can take advantage of stratification. A room thermostat turns on the refrigerating and hot water pumps. An important component is the regulator DT-4 that determines whether the absorption chiller can be driven by the storage. When the water temperature at the top of the tank is over a minimum value (say 80°C) and is higher than the hot water at the outlet of the generator, valve V-2 activates the circuit toward the storage. Control capacity is obtained through the three-way valve V-1 from which a fraction of the heating fluid is bypassed toward the storage. The valve V-1 is commanded by the refrigerating water temperature. If this increases too much, the auxiliary boiler is switched on; the hot water is soon over the storage temperature and the valve V-2 activates the boiler circuit. Control capacity is always obtained by means of valve V-1. It is worth noticing that control capacity is realised in this plant only by flow rate variation whereas the hot water temperature is at the level achieved by the storage. In principle the auxiliary boiler could operate as a booster, raising the hot water temperature from the storage, modulating through valve V-2. The control system does not allow this operating method, probably owing to the modest attainable advantages compared to higher complexity. Another interesting scheme is derived from a plant that serves the Mount Cotton Solar House in Australia [ 4 ], which gave useful suggestions for the design of the plant I myself designed. It is equipped with both hot and cold storage. As usual the connections between the tanks take advantage of stratification. Some reference values must be set. The highest acceptable temperature to the fan coils is 14°C whereas the lowest temperature produced by the chiller is 8°C. The minimum temperature to drive the chiller is 76°C. The solar collector’s circuit is controlled independently of the rest of the system by a differential controller. The system can produce heating or cooling according to a main switch. Heating operation is not considered here while summer cooling is detailed. Consider the scheme of the plant represented in Fig. 2 where the activated cooling mode connectio (...truncated)


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Lazzarin, M. Renato. Solar cooling plants: some characteristic system arrangements, International Journal of Low-Carbon Technologies, 2007, pp. 391-404, Volume 2, Issue 4, DOI: 10.1093/ijlct/2.4.391