Many people want solar hot water but are overwhelmed at the choices available. Many people know flat panels, but because of them being a fixed position, they only collect solar energy on the south of the roof. Therefore you are lucky if you are able to collect thermal energy for more than 3 hours a day. With that amount of energy collected you are lucky if you are able to heat your hot water tank. The good news is that companies and home owners now have more options for solar hot water. The newest technology on the block is called solar concentrators. These systems look like satellite dishes and they track the sun throughout the day, therefore they are 262% more efficient than flat panels. How is this calculated?
The calculation is based on the following conditions: Water Temp. Rise of 80C (176F) and Solar heat generated 7.2kW. The evacuated tubes (113 square feet) do not have a dual axis tracking system to follow the sun and they lose the efficiency as the temperature rises inside the flat panel. Imagine that the flat panel is a room with windows, the more heat that enters, the less efficient the flat panel becomes. This is called heat stagnation and is one of the fundamental problems of flat panels. As the water temperatures rise in the flat panel, the coolant temperature also rises, therefore a drop in efficiencies. See the data showing efficiency drop for a flat panel. (Thermomax).
However, the solar concentrator is a dual axis system which means it can rotate and track the sun throughout the day. The solar concentrator can focus energy from 113 square feet in to a very small heat exchanger .69 square feet (10” X 10”). From the law of the physics it is known that such a small surface cannot dissipate heat. The company, SolarTron Energy System is a manufacturer of solar concentrators and they use a special heat exchanger which has high heat absorption 98% and very low emissivity 3% As a result, the heat losses are very small around 50W.
This means that in cold temperatures the solar concentrator will only lose 2% of the heat collected. So efficiencies for this technology are very high -40 degrees and +40 degrees. The same cannot be said about flat panels which have reduced efficiency in cold climates. So if you are looking for an efficient solar hot water system, the best system is solar concentrators because they are more efficient and they have fewer problems. But please keep in mind that you need at least 20 feet of space from your commercial building or home to install a solar concentrator. The system called SolarBeam is 15 feet wide and is placed on an 8 foot post, so it is not something that can go on top of your house. However, if you do have a flat roof that can handle the pressure of a 400lb solar concentrator then you can install it on your roof. So contact your local engineer and see if this is a possibility.
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