Since the introduction of the RHI, there has been a huge increase in the number of people installing solar thermal in their homes. Solar thermal in layman terms turns the suns energy into hot water – which is great because during the summer months you never have to pay for any hot water you use in the house and even during the colder months it still helps since it allows you to minimise the usage of your boiler.
If you decide to install solar thermal in your home you will need a hot water tank to store the hot water produced from your collector – the problem though, is that you can’t plumb one of these systems into the older hot water tanks that are historically found with boilers.
Twin Coiler Cylinders
The reason for this is that inside the hot water tank there needs to be a separate coil for each ‘hot water source’. In this case you would need a coil for the solar thermal and you would need a coil for the hot water. Normally in a residential solar store (i.e. a hot water tank with a solar coil), the solar is connected to the lower coil and the boiler (or main heating source) is connected to the top coil.
Solar coils are much larger than traditional boiler coils because they need a far bigger surface area to transfer their heat into the water compared to a boiler. The reason is that the hot water travelling through the solar thermal coil is at a much lower temperature than the water travelling through a boiler coil.
As a guide, the surface area of a solar thermal coil needs to be in excess of 1.5m2, while a boiler coil can be as little as 0.6m2 – this increased surface area maximises the opportunity for heat transfer and is a must based on the lower water temperature flowing through the coil.
If you cast your mind back to your GCSE science, you will know that heat rises and therefore within a hot water tank, the water at the top of the tank is far warmer than the bottom of the tank.
In a solar thermal store, it is important that this temperature differential is maximised and this is achieved by making the hot water tank rather large and tall. So while the top of the tank could achieve temperatures of 600C plus, the water at the bottom of the tank might be as low as 150C degrees. What this means is that even if the solar thermal is only producing water to 200C degrees, it will still contribute to the hot water demand of the property.
Storing the hot water you produce on sunny days
Since the hot water tanks used for solar thermal systems tend to be big, they tend to be able to store far more hot water than is actually required by most families that install one of them. Since solar thermal is intermittent, i.e. it produces much more hot water when the sun is shining, this oversized heat tank allows you to store the hot water – thereby taking advantage of favourable conditions a day or two later to help minimise the need to use the boiler.