Home & Garden

Solar water heaters are very simple devices, which is why their construction is not beyond the average DIY enthusiast. This article briefly describes the various components of a solar water heater.

Solar Collector
This is the part of the system that captures and exploits the heat of the sun. Their construction can vary, but the basic principle is the same. It is a set of pipes containing a liquid (Heating Medium) that is exposed to direct sunlight. When sunlight falls on the pipes, they heat up, heating the liquid inside. This liquid is then pumped to the heat-exchanger coils of a hot water tank.

Commercial units often use copper pipes mounted in evacuated glass tubes which is supposed to increase efficiency. The tubes are then mounted side-by-side on a panel fitted to a roof. The panel often has reflectors that bounce the sunlight/heat that falls between the tubes back up to the tubes.

Evacuated tubes can be bought for DIY projects, but they are expensive and are not entirely necessary. Instead, pipes are usually mounted in a shallow box that has a glass top. However, the commercial units look good and might be a good alternative if your constructional skills are not so good.

You can easily demonstrate the effect of solar heating if you have a garden and a hose-pipe. Simply ensure the hose is full of water and leave the hose lying in the sun for 30 minutes. When you come back, the water in the hose will be warm or even hot.

Heating Medium
This is the liquid that is heated by the solar collector and used to heat up your water.

In hot countries, the medium can be water, but anywhere where there is risk of frost, another medium must be used. Typically, this would be a non-toxic anti-freeze, or mineral oil.

Pipework
Temperatures in the solar heater can get very high, so it is usual to use copper pipes. Plastic pipes could melt. This is true even for plastic pipes used for modern central heating systems.

Pump
The pump is used to circulate the heating medium around the system. For even more energy saving, it could be driven from a PV solar cell.

PV Solar Cell
PV or PhotoVoltaic cells create electricity when exposed to light. A small, low-cost panel should be sufficient to power the pump.

Controller
The controller determines when the pump should operate. It measures the temperature of the hot water tank and the temperature of the solar collector. If the collector is hotter, the pump operates so that heat is transferred to the water in the tank. If the water tank is hotter, then the pump will not operate as this will have the effect of cooling the water.

Any electronics enthusiast could construct a controller, however commercial units are available at quite a low cost.

Hot Water Tank
This is a storage tank for your hot water. It is usually cylindrical and made of copper or stainless steel. Most are covered with an insulating foam. Insulation can be improved by the addition of a cylinder jacket.

Inside the water tank there are one or more heat-exchanger coils. The heating medium is pumped through the heat exchanger, thus heating the water in the tank. The heating medium, now cooled, then returns to the solar collector to be re-heated.

The heating medium is totally isolated from the water in the tank by the heat-exchanger coil.

The second heat-exchanger may be connected to an alternative heat source, such as a gas-fired boiler in the case that there is insufficient sunlight to heat the water, such as in the winter or during hours of darkness. Many tanks also have provision for the fitting of an electric immersion heater.

Safety Devices
The heater can get very hot on sunny days, and there is the danger that the water in the tank is heated to or beyond boiling point. The hot water tank will therefore require an expansion tank, and the entire system will need an additional temperature sensor that shuts down the heater’s pump if the water gets too hot.

Similarly, if the the heater has been shut down, the heating medium in the solar collector will start to get very hot. An expansion tank will therefore be needed for the heating circuit. Additionally, the heating circuit should be fitted with a self-bleeding radiator valve to expel any air that builds up in the system, otherwise the heater will fail to operate correctly.

Mounting Brackets
Your solar collector needs to be fitted in a position where it gets good incident sunlight for as long as possible. Most people have them mounted on a sloping roof that faces the sun (ie South-facing if in the northern hemisphere). Special brackets are available that hook over purlins and fit between roof tiles. This allows the collector to be fitted without impairing the integrity of the roof.

This has been an overview of the components of a solar water heater. Further details of each component, along with construction tips, may be found in other articles. Check out the list of articles in my Solar Water heater Project category, listed in the side panel of this site.