A vital part of a solar thermal system is storage. The sun is not always shining when you need your hot water!

On larger systems molten salt is sometimes used as the storage medium for the heat. Generally however, the heat is stored in water, especially if the purpose of the system is to provide hot water.

Water storage can take many different forms. In domestic systems by far the most common solution is to use a twin coil cylinder, with the solar connected to the bottom coil and the auxiliary heating connected to the upper coil. Ideally, your cylinder will be enlarged, a tall and thin cylinder with a solar coil at the bottom of it.

Solar coils are different to traditional boiler coils as they have a larger surface area. This is because the solar system works at a lower temperature than a fossil fuel boiler. A traditional boiler coil will have a surface area of approximately 0.6m² where the coil in a SunGift Solar cylinder has a surface area of approximately 1.62m².

The larger surface area of the coil in a solar cylinder means that the heat transfer is more efficient. A bigger surface area means that there is more opportunity for the heat from the solar to pass to the water in the cylinder. This in turn means that because the solar fluid can transfer more of its heat into the water in the cylinder, when it returns to the solar collector it will have a larger capacity to absorb heat.

When using a twin coil cylinder it is important that the cylinder is sufficiently large to ensure that there is adequate stratification within the cylinder. This will mean that when the top of the cylinder is hot from either the boiler or the solar, the bottom is still cool so that even during winter, the solar will still make a contribution. For example, if the boiler has heated the top of the tank to 60˚C the bottom of the tank may still be around 15 - 20˚C. The solar panel then only has to reach 21˚C before it can start to make a contribution. When the solar heat enters the bottom of the cylinder and rises, it becomes useful heat at the top.

Although the cylinder is larger than the hot water requirements of the household, a twin coil cylinder will allow your water to be heated throughout by the solar; a free energy source. On good sunny days, when you have produced surplus hot water, you can store it for days when the solar gain is not so good.

To enhance this process I recommend installing a thermostatic mixing valve to the cylinder. This allows the water in the cylinder to be heated up to 85˚C using the heat from the solar panels before being automatically blended with cold water from the mains to reach the desired temperature when leaving the cylinder. A thermostatic mixing valve means the solar can work for longer and the useful heat content of the cylinder increased, making the system even more efficient.

There is a lot going on behind the scenes of a solar system which is why it is vital to choose each part of the system carefully to ensure that it all works efficiently. This is why at SunGift Solar each system we install is individually designed and each component carefully chosen to ensure the you get the best out of your solar system.

About the author: Gabriel Wondrausch is founder and director of SunGift Solar, which installs solar thermal and other renewable energy systems in the South West of England.

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