What is Solar hot Water?

Before we start on what, we need to cover where. Solar Hot Water is used all over the world, it is very effective especially if you live in a warm country, this article is about the UK where solar hot water is much less effective option than in for example Spain. We also do not cover any grant schemes here (Renewable Heat Incentive, RHI), they are changed to often! With a grant scheme you will find that an installation will be more cost effective.

The terms solar energy and solar power are often used to describe technologies which collect the energy of the sun and redistribute it for our use. These terms are however a little ambiguous. Solar energy effectively drives our whole planet, wind, wave and fossil fuels could ultimately be regarded as solar energy. For the purposes of this site our only concern is for solar energy that can be collected directly by our dwelling and used to, for example, heat water or to drive our electric devices. For this article on solar thermal only heating hot water is considered. Please follow the link to take a look at Solar Photovoltaic’s

Solar Photovoltaic’s...

To collect this energy a method is required to collect photons of light and to convert the energy in this light into another form of energy for storage or use.

For domestic properties there are two main methods of collecting solar energy to heat water glazed flat plate collectors (thermal plate) and evacuated tube collectors (thermal tube), they both absorb the light energy and convert it to heat, the main difference with an evacuated tube system is that they operate in a similar way to a vacuum flask, the vacuum inside the tube collectors insulating the fluid against heat loss.

Financially are they worth it?

Over a year in the UK the amount of solar radiation hitting the surface of the earth varies enormously, we all know that! At its peak it is about 8 kWh/m² on the best day and 100 times less than that on the worst, i.e. 0.08 kWhm². Taking into account the amount of useful radiation, i.e. the radiation that the thermal collector can convert to heat and heat loss factors a total efficiency of about 30 to 40% is generally accepted. This means that only 30% to 40% of the radiation falling onto the area of the panel goes into heating the water. The grapic shows light energy availiable across Europe assuming that a panal is optimally inclined at about 38 degrees and is south oriented.

A reasonable estimate for energy use to provide a family of four with hot water is about 10 kWh each day or about 3500 kWh per year, taking a few days off for holidays. So on a sunny day taking into account efficiency factors you would get away with a 4m² panel to provide all your hot water, and on a dull winter day, you would require a 2600 m² panel. The latter is clearly impractical, on space and cost grounds, and not forgetting heat loss! In practical terms for about one third of the year the heat output from a solar thermal system is zero in the UK. It is also fairly obvious that if you have, as in the example table below, a 6m² panal an you use 10 kWh hot water each day, then you will not use all the potential of the system.

The graph just below shows the variation in light energy each day throughout the year for a point in the middle of England

Exact numbers are very difficult with solar because of the variability of the climate and personal use habits. But for a modest 6m² thermal collection system costing about £5000 installed, optimistically you may get 25% of your annual hot water requirement from the system, for the family of four this is 750kWh. If you had used natural gas to heat your water, you would have saved £54
(157.5kg) , with LPG, £91.5
(180kg) and with standart tariff electricity, £203.25
(174.8kg) .

Not much, is it? If your system cost £5000 and your main fuel is LPG it would take about 50 year to pay back the investment, not including all the maintenance cost, and will it last that long? Money is not the whole story, the CO₂ emission savings are pretty good (the numbers in brackets). But would it be your first choice to save money and reduce emissions, we would argue not. Insulate your loft or walls First.

The tables below gives more data with consideration for the different light levels in the UK, the further south you are the better. Well in terms of light levels. The value for the light energy availiable in these tables assume that the panal is optimally inclined at about 38 degrees and is south oriented. The table also assumes that all of the potential incident solar radiation is useful and this is probably not the case.