Solar Hot Water Heating

In the early days of solar, the 1970’s,  solar heating was done with a copper absorber inside an insulated box covered with glass (a glazing). Trapping the heat in with the glazing and insulating the box allows the collector to perform better in colder windier conditions along with higher water temperatures being heated. Solar hot water heating was aimed at domestic water heating where cold water at about 10C was heated to hot water tank temperature of typically 60C.  The thinking was always that we had to tackle the complete load. We had to efficiently heat water at 60C when the outside air temperature was low. This worst case design analysis lead to the demise of the industry gradually over the decades until last decade finally it was declared that solar thermal was dead. The idea that displacing a large portion of the load from all the hot water heaters in the world would make a significant dint in the world’s fossil fuel use was overshadowed by the fact that the energy produced by one of these small systems operating on relatively small loads was usually less than 1/100th of the upfront cost.  

In 1995 we at Hot Sun developed a solar domestic hot water heating system using our unglazed collector.  For 1/5 the upfront installed cost we could deliver a much larger solar collector along with a better insulated tank with a more effective heat exchanger inside. The drainback concept was employed so that no antifreeze or glycol was required. It worked great and was many times more cost effective. Our concept was simply that the unglazed collector was actually more efficient per area than the far more expensive glazed options as long as we focussed on the colder half of the typical dhw load. . When heating water from 10C to 60C, half the load is the first half where water is preheated from 10C to 35C. Realize that the way solar collectors work is the water flow through them is relatively high such that the collector temperature is close to the incoming water temperature which is the storage tank temperature (or swimming pool).  Solar collectors are more efficient when the difference between the water temperature and the outside air temperature are close.  You don’t need to insulate a pipe if it’s carrying water that is  the same temperature as the outside air.

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