Solar water heating has been used for hundreds of years to heat water.
With the advances in technology, solar water heating systems have become
more and more efficient, with Evacuated Tube Solar Water heating
offering efficiencies of well over 90%. That means that more than 90% of
the sun's energy landing on the surface is converted into heat which can
be used to heat water.
This is one of the cheapest renewables, with costs starting from pennies per watt required instead of dollars per watt for PV Solar.
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Evacuated Tubes are the Solution:
Vacuum or Evacuated Tubes are made from glass, typically very strong and heat resistant Pyrex with a double wall construction. The glass on the inner tube is coated on its outer surface with an absorbant coating, and on its inner surface with a reflective coating. Inside each tube all air is removed making a vacuum and a copper heat pipe is run through the centre of the tubes. Most of the infrared radiation from the sun is absorbed by this sealed heat pipe which contains an anti-freeze type liquid.
As heat rises, hot vapors from the heat transfer fluid rise up to the top of the heat pipe where its copper tip connects with a header pipe through which more fluid flows. This hot fluid is then pumped through a coil of pipe inside the hot water tank (acting as a heat exchanger) with the end result that the water gets hotter and the heat transfer fluid gets cooler.
The heat transfer fluid then continues its journey around the system and back out to the solar water heater to be reheated.
The copper at the tip of the heat tube can reach well over 200 degrees easily heating water to 90 degrees celcius on hot days and to 60 degrees celcius even in the winter. This simple system is completely sealed and needs minimal maintenance over its 20+ years life.
The advantage of using evacuated tubes is that they will work even during the coldest winter months unlike old style Flat Plate Solar Collectors. The vacuum insulates the heat tube from being cooled much by the ambient temperature which could be well below freezing. Winter sun can easily heat water to 50+ degrees even in the depths of the coldest season. Even if it is very cloudy and very cold, enough sunlight gets through to keep the tubes well above freezing and so they will still be pre-heating the water which can then be heated further by a standard immersion heater or gas burner reducing the costs of heating the water.