M&V Solar Water Heating Guideline - Eskom

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Measurement and Verification Standard Guideline for Low Pressure Solar Water Heating Systems v1r1 strategy of encouraging and assisting all sectors of South African society to reduce electricity use and to relieve pressure on the electricity network. [1] 3 OVERVIEW OF RESIDENTIAL SOLAR WATER HEATING TECHNOLOGIES 3.1 General A conventional Solar Water Heating System operation is based on the physical phenomenon known as natural convection or thermo siphon effect. This effect manifests in circulating the water (or other designated heath transfer fluid) through the solar collector and hot water storage tank. It is essential that the hot water storage tank is positioned above the solar collector to use the natural flow of the water being heated. As water in the collector heats, it becomes lighter and naturally rises into the tank above, thus creating a circulation flow between collector and hot water storage tank. To eliminate the dependence of the position of the storage tank and to increase the rate of circulation /heat transfer a forced circulation may be employed, thus allowing the hot water storage tank to be placed in a convenient place at the house. 3.2 Solar Collectors 3.2.1 Main Types of Collectors Used in South Africa To obtain maximum utilisation of the solar irradiation various collectors are in use. The collector performance characteristics are shown in Figure 1. and illustrate the efficiencies of the main three types solar collectors use presently in South Africa, namely: standard non selective-coated flat-plate collectors selective coated flat-plate collectors evacuated glass tube collectors. From the graphs shown in Figure 1, it is evident that the different collectors should be used for different purposes, namely: The tested non-selective coated flat-plate glazed collectors perform poorly at high operating temperatures. Their normal operational temperatures are in the limits of 30 – 80 o C, heat loss coefficient is above 5 W/m 2 .K, and optical efficiency of 0.8; thus the applications for these collectors are limited for domestic hot water. The tested selective coated flat-plate glazed collectors perform well at high operating temperatures. Their normal operational temperatures are in the limits of 40 – 90 o C, heat loss coefficient is above 3 W/m 2 .K, and optical efficiency of 0.80. Thus the applications of this collectors is extended for domestic and commercial hot water large systems. The higher operational temperatures allow reducing the size of the collector when compared with non-selective coated collectors. 8
Measurement and Verification Standard Guideline for Low Pressure Solar Water Heating Systems v1r1 The tested evacuated tubes collectors have high operating temperatures, namely between 90 – 130 o C, heat loss coefficient is above 2 W/m 2 .K, and optical efficiency of 0.65. These collectors are ideal for space cooling and heating and for process heat. Figure 1:. Collector Output Efficiency Curves measured for several types of solar collectors available in South Africa as a function of (Tm – Ta ) where: Tm - average collector temperature and Ta - ambient temperature. Source [3] Low Pressure Solar Water Heater Roll-out as supported by ESKOM and the South African Government, initially will be based on mainly imported SWH systems [2]. According to the Energy Minister Ms. Dipuo Peters on the opening of the one of the first roll-outs at Winterveldt (Tshwane) , “the units installed there were imported, as would be the case with the first 200 000 units installed through the programme, as the country gears up its manufacturing capacity towards SWH” [2]. The SWH units installed at Winterfeldt were based on evacuated tubes collectors and should perform as shown in Figure 1. One could expect that the rest of systems installed initially will be similar.[5] 3.2.2 Collector Orientation To achieve maximum performance of the SWH system the collector should be facing ideally in the equator i.e. in the North direction in South Africa. Roofs and houses position toward North does not always allow this rule to be observed. Deviations from North up to 45 0 will reduce the solar harvesting insignificantly (less than 10%). This is illustrated on the chart shown in Figure 2., where positioning of a house is shown. The size of the suns pointed at 9
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M&V Solar Water Heating Guideline - Eskom

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