What is Solar hot Water?
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. For example, 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 hot water is considered. Please follow the link to take a look at Solar Photovoltaic’s
| If you are interested, all light is made up of photons, which might be best described as tiny little packets of energy. There is a whole range of different energies or “packet sizes” which if you put them all together from small to large would constitute the spectrum of light. |
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.
| The light photons are absorbed by electron transitions in the material of the solar panel, these excited states decay releasing the energy as heat, this heat is conducted to a fluid in the panel. |
Financially are they worth it?
Over the 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. 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 £ 23, with LPG £ 41 and with electrical £ 89.
Not much is it! If your system was £ 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?
Data table
| Thermal Tube | Thermal plate | ||||||||
| Light energy avaiable to you see graphic kWh/m2 per year | 900 | 1000 | 1100 | 1200 | 900 | 1000 | 1100 | 1200 | |
| Light energy incident on a 6m2 panel * kWh | 5400 | 6000 | 6600 | 7200 | 5400 | 6000 | 6600 | 7200 | |
| Light capture efficiency, or Gross efficiency over full area of solar collector | 50% | 50% | 50% | 50% | 70% | 70% | 70% | 70% | |
| Energy captured by the collector kWh/m2 per year | 2700 | 3000 | 3300 | 3600 | 3780 | 4200 | 4620 | 5040 | |
| Working efficiency how much of this light energy converts to usable hot water | 70% | 70% | 70% | 70% | 60% | 60% | 60% | 60% | |
| Energy converted to useable hot water kWh | 1890 | 2100 | 2310 | 2520 | 2268 | 2520 | 2772 | 3024 | |
| Equivalent Price if you use:- (£ ) | Gas | £ 59 | £ 65 | £ 72 | £ 78 | £ 70 | £ 78 | £ 86 | £ 94 |
| Electric | £ 223 | £ 248 | £ 273 | £ 297 | £ 268 | £ 297 | £ 327 | £ 357 | |
| LPG | £ 102 | £ 113 | £ 125 | £ 136 | £ 122 | £ 136 | £ 150 | £ 163 | |
| Indicative installed cost (£ ) | 5000 | 5000 | 5000 | 5000 | 5000 | 5000 | 5000 | 5000 | |
| grant (£ ) | 400 | 400 | 400 | 400 | 400 | 400 | 400 | 400 | |
| Payback period (years) if you use:- | Gas | 79 | 71 | 64 | 59 | 65 | 59 | 54 | 49 |
| Electric | 21 | 19 | 17 | 15 | 17 | 15 | 14 | 13 | |
| LPG | 45 | 41 | 37 | 34 | 38 | 34 | 31 | 28 | |
| Payback period (years) if energy doubles in prices and if you use:- | Gas | 39 | 35 | 32 | 29 | 33 | 29 | 27 | 25 |
| Electric | 10 | 9 | 8 | 8 | 9 | 8 | 7 | 6 | |
| LPG | 23 | 20 | 18 | 17 | 19 | 17 | 15 | 14 | |
| The value for the light energy availiable assumes 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, so this table is an optomistic presentation of the potential of solar thermal in the UK. | |||||||||
