Evacuated Tubes - Solar Panels for Home Use

Evacuated Tubes

Evacuated tubes consists of a doubled walled glass tube, rather like a thermos flask, with a vacuum in the space between the walls. The inner tube has a special dark coating on it. The solar energy passes through the vacuum as radiation, and the energy is absorbed by the coating on the inner tube. This causes the temperature inside the inner tube to increase. Inside the tube there is a copper tube which itself has a vacuum in it, together with a small quantity of a fluid which because of the vacuum, boils at a low temperature.

How evacuated tubes actually work: As the sun warms the tube, the liquid situated in the heat pipe evaporates (turns into a gas) and the heat rises to the top. The top end of the heat pipe is fitted in a special manifold and is completely surrounded by water.

This extracts the heat and causes the gas to condense and return down the heat pipe and the cycle recommences. The phase changes (liquid to gas and vice versa) actually enhance the efficiency of the heat transfer considerably. A number of tubes are fitted to a manifold, and a number of manifolds can be joined together to provide the expected energy requirements of the installation.

Evacuated Tubes - Fact Sheet

 

Thermal stores are a clever way of eliminating the need for a cold water storage tank and providing mains pressure hot water. Heat energy is stored in the cylinder and a highly efficient heat exchanger transfers the energy into the incoming cold water as it is required. As the output water is not stored, it is an ideal way of minimising legionella risks. Thermal stores usually come complete with thermostatic blender valves as well as additional inputs for solar and wood burning stoves. The size of a thermal store usually needs to be larger than a typical cylinder and they benefit from a higher operating temperature. A typical 3/4 bed house should consider a 200l store. They are ideal for use with solar panels.

An accumulator is basically a large heat store. Sized from 500l to anything up to 5000 litres or more they are used to absorb and store energy for later use. They can be put underground or in outside storage but of course need high levels of insulation in order to retain the energy created.

One application for the use of an accumulator is to reduce peak energy requirements from an incoming energy source. Wood / pellet burning stoves, and ground source heat pumps can benefit from the use of these.

Another application is for use as a buffer vessel for a swimming pool. By storing heat using the solar pump, it can be transferred to the pool when the pool pump is in filtration mode. The benefit of such a system lies in the fact that if the pool pump is always used to extract the energy, it can result in more than necessary use of the pool pump.

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