Steamback Shows Promise for Solar Water Overheating
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ReVision Energy co-founder and solar engineer, Fortunat Mueller, P.E., recently had a technical article of his regarding the merits of steamback as a method of solar hot water overheat protection published in Renewable Energy World. Steamback is an alternative to drainback overheat protection, which uses pure water as a heat transfer fluid and requires that the pipe run be pitched so that water does not stay inside solar collectors in freezing weather.
Below is some information from the article:
There are multiple design options to ensure systems don’t overheat, but as our industry grows and matures, installers know that customers demand a robust solution. Few customers want to invest significant sums to mitigate a relatively infrequent situation. Historically most closed-loop solar hot water systems have relied on control strategies that either dump excess hot water from the tank into a drain, or on dump zones which dump heat from the solar loop itself once the tank has reached a certain set point. Those strategies work, but the first wastes water and the latter wastes electricity, and both introduce additional costs and failure points to the system.
… In 2008, having experienced the limitations and liabilities associated with these other methods, we set out to find a better way to protect our installations from overheating and power failures. In researching literature, we discovered a method called steamback that can resolve many of the challenges of traditional systems. This simple method is widely used in closed-loop systems in Europe, especially ‘combi’ systems that provide both heat and hot water and thus are necessarily oversized during the non-heating season.
Unlike the traditional options, steamback is a passive overheat protection and doesn’t require additional hardware or introduce additional failure points to the system; it simply involves an understanding of the details of piping and collector geometry plus expansion tank sizing and location to capture and protect the coolant during an overheating situation.
Steamback systems work as follows: when the solar hot-water tank reaches its maximum temperature – typically around 180 degrees – the solar controller automatically switches the pump off. If the sun is still out and producing heat in the collector, the collector temperature will continue to climb. When the temperature of the water/antifreeze mixture in the collector rises above the boiling point (typically around 240-250 degrees depending on system pressure), it will start to boil. Since water vapor (steam) occupies about 1,600 times more volume than an equivalent amount of water as a liquid, it only takes a tiny amount of evaporation to fill the collectors with steam and push all of the remaining antifreeze mixture down into the expansion tank.
Read more at Renewable Energy World.