Solar Water Heating Explained

by LJ Martin

With many people now focussed on reducing their carbon footprint, there has been increasing interest in renewable energy. Arguably the best and most cost-effective way to do this is to install a solar-thermal water heating system. Solar-thermal means using the heat of the sun directly, rather than turning the sun's energy into electricity then using the electricity to do work. Most people in the developed world use lots of hot water for washing and cleaning, so this is an excellent place to start reducing carbon emissions.

Let's look at a diagram showing how a typical solar-thermal water heating system works. This is the type of system that would commonly be used in Northern Europe or the Northern parts of North America. It relies on pumping pressurized water through a closed loop between a solar panel and a hot water tank.

Diagram of solar water heating system

From the diagram we can see that a solar water heating system is really very simple. On the roof of the house there is a solar panel. This is usually a metal panel painted black, with a network of pipes running through it, and a glass cover on top. The panel absorbs the sun's heat, which is then absorbed by the water. The glass cover allows the sun's rays through, but traps a layer of air to insulate the panel from the atmosphere outside (you could say it uses the greenhouse effect!) This means that the panel can produce hot water even in cold air temperatures, so long as the sun is shining. More expensive panels may take the form of glass tubes with metal pipes inside, and they can be more efficient than flat panels in some circumstances. Good quality solar panels will last at least 25 years without maintenance.

The water in the panel is moved through a closed loop of pipework by a water pump. The pump uses very little power, generally around 25 to 50 Watts of electricity. This type of system therefore does consume a little electrical power in order to harness a much larger amount of power from the sun. Depending on the strength of the sun's rays, the system can harness more than ten times as much free solar power as the amount of electrical power taken to run the pump. More expensive systems might use a photovoltaic panel to generate electricity from the sun, which is then used to run the pump.

As the water in the system heats up and cools down, it expands and contracts. To keep the pressure in the system fairly constant, somewhere in the loop there will be an expansion vessel which compensates for changes in volume due to temperature. The pressure used is only around 2 atmospheres, which is not high enough to worry about.

Finally, the hot water generated by the solar panel must be transferred to the hot water tank. This is done using a hot water tank with a special heat transfer coil. This usually takes the form of a flat coil in the bottom of the tank. This is the coldest part of the tank, so the system can transfer heat into the tank even if the sunshine is not very hot. On days with only a little sunshine, your electric or gas powered water heating will still be needed, but they will have less work to do as the water will have been pre-heated somewhat by the solar heating system. Modern hot water tanks with these special solar heating coils are very efficient at transferring heat into the water, and only a little more expensive than a standard tank.

The water that comes out of your hot water tap has not been through the solar panel, it remains completely separate. This is partly because a pressurized, closed system is more efficient at transferring heat, and partly because the closed system must contain antifreeze to protect the panel from damage in cold temperatures. You might be worried about the coil becoming damaged and leaking antifreeze into the hot water tank. Fortunately, this is not as big a concern as you might think. Solar water heating systems use a special kind of antifreeze called Propylene Glycol, which is a common non-toxic food additive. (Older systems used Ethylene Glycol, which is toxic when consumed, though minimally toxic through skin contact. The antidote is alcohol!) It also contains a lurid dye, usually luminous pink, green or yellow, which will alert you in the unlikely event of a leak. It may contain a special bitter chemical to stop you drinking it, although you shouldn't drink out of the hot tap anyway!

The whole system will be run by an electronic controller. This uses temperature sensors to work out when the panel is hotter than the water in the tank, so it knows when to switch the pump on and off.


Solar-thermal water heating systems are becoming more and more cost effective due to recent rises in energy prices. Depending on the levels of sunshine where you live, and how much hot water you use, the system will generally pay for itself within 15 years, and often less than 10. In addition, houses with eco-friendly features tend to stand out in the market and sell more quickly. You should notice an immediate fall in your fuel bills, particularly in summer. Costs may be offset even further by government grants which help people install solar water heating. People with the right DIY skills can save money by installing their own solar-thermal water heating, as it involves general plumbing and wiring tasks, although you do have to work safely on the roof as well. If you are looking for a way to reduce your carbon footprint and save money in the long term, solar thermal water heating could be perfect for your needs!

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