How mixing during hot water draw-offs influence the thermal performance of small solar domestic hot water systems

Simon Furbo, Louise Jivan Shah

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Abstract

CFD calculations on the mixing during hot water draw-offs in vertical hot water tanks with different diameters have been carried out. The calculations, which were carried out with the same cold water inlet design, showed that the extent of mixing is strongly influenced by the tank diameter. The extent of mixing is increasing for increasing tank diameter. Further, calculations of the yearly thermal performance of small solar domestic hot water systems with hot water tanks with different mixing rates during hot water draw-offs were carried out. Both solar domestic hot water systems with mantle tanks and with spiral tanks were investigated. Tanks with different volumes, auxiliary volumes and height/diameter ratios were considered. The investigations showed that the thermal performance of systems with mantle tanks is higher than the thermal performance of systems with spiral tanks, and that a decreased auxiliary volume in the tanks and an increased height/diameter ratio of the tanks will increase the thermal performance of the systems. The investigations showed further, that mixing during hot water draw-offs decreases the thermal performance of solar domestic hot water systems. The mixing decreases the thermal performance of systems with spiral tanks more than the thermal performance of systems with mantle tanks. The mixing decreases the thermal performance stronger for systems based on tanks with low height/diameter ratios and with large auxiliary volumes than for systems with tanks with high height/diameter ratios and small auxiliary volumes. Based on the investigations, it is recommended to design hot water tanks for small solar domestic hot water systems as mantle tanks with as high height/diameter ratio as possible and with as small an auxiliary volume as possible, of course under consideration of the required hot water comfort.
Original languageEnglish
Title of host publicationProceedings of the 2005 Solar world Congress
Publication date2005
Publication statusPublished - 2005
EventISES 2005 Solar World Congress: Bringing Water to the World - Orlando, FL, United States
Duration: 6 Aug 200512 Aug 2005
http://www.swc2005.org/

Conference

ConferenceISES 2005 Solar World Congress
CountryUnited States
CityOrlando, FL
Period06/08/200512/08/2005
Internet address

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