Solar Hot Water

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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.

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.

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 £ 36, with LPG £ 43.5 and with electrical £ 115.5.

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 £ 90.72 £ 100.8 £ 110.88 £ 120.96 £ 108.86 £ 120.96 £ 133.06 £ 145.15
Electric £ 291.06 £ 323.4 £ 355.74 £ 388.08 £ 349.27 £ 388.08 £ 426.89 £ 465.7
LPG £ 109.62 £ 121.8 £ 133.98 £ 146.16 £ 131.54 £ 146.16 £ 160.78 £ 175.39
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 51 46 41 38 42 38 35 32
Electric 16 14 13 12 13 12 11 10
LPG 42 38 34 31 35 31 29 26
Payback period (years) if energy doubles in prices and if you use:- Gas 25 23 21 19 21 19 17 16
Electric 8 7 6 6 7 6 5 5
LPG 21 19 17 16 17 16 14 13
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.

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+1 # solar heatingdavid Karniewicz 2014-01-29 13:01
If anyone is considering the choice between solar pv and solar heating,choose solar pv and fit a device that recognises when the pv sustem is exporting back to the grid and then feeds this surplus electricity through your immersion heater,free solar heating for an extra outlay of £500 for the device
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0 # Silicon SolarSunny 2013-04-22 16:19
Solar thermal energy id definitely worth it! No matter what part of the country you are from you can always save money by investing in solar thermal energy. There is a company called silicon solar and they now have offices on both the East and West coast and they have sold their products all across the country because they are simply the best and most high end solar manufacturer in the market. Definitely check out their website and take a look at their competitive prices!www.sili consolar.comwww .sunmaxxsolar.c om
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0 # Solar Thermal Calculations or potential YieldMatt 2013-03-31 16:01
Hi there,I'm enjoying looking around your excellent web site - however I'm interested to understand the calculation parameters you've used to calculate the Solar gain. The figures suggested in Energy converted to usable hot water in kWh are considerably higher than in any SAP calculation in fact the figures mentioned above would almost certainly be unachievable according to SAP for any standard twin coiled hot water cylinder. Therefore if possible would it be possible to see the calculations behind the figures above?We are currently going for SAP Appendix Q recognition with our unique and patented Solar 3C solution and are in regular communication with BRE regarding low temperature heating with Solar Thermal, so if possible I would very much welcome chatting through the benefits of Solar 3C and how it may add clarity to the Solar Thermal/PV tables plus it may change the negative downward thumb to a positive thumbs up in terms of payback.Histori cally and as confirmed by BRE, Solar Thermal has always suffered from the restriction of the medium used to hold the energy collected and then in the vast majority of cases this is only used for hot water via twin coil cylinders. Thermal Stores accept solar energy however due to their very high standing temperatures (just to be able to deliver any useable hot water) the solar contribution annually is virtually worthless.Solar 3C is different - it's a 3-coiled unvented hot water cylinder (2 input coils middle & bottom + 1 output coil at the top) any/all spare energy instead of either sitting in the cylinder all day doing nothing or slowly passing through the cylinder wall (heat loss) can be used to contribute towards space heating. By way of an example, if there was just 35degrees available on the roof/at the thermal collector, this would roughly equate to a maximum achievable cylinder temperature of around 31degrees C not hot enough for washing, bathing or showering however ideal for use with an Underfloor Heating system - therefore in this example when there would not have been enough energy to provide hot water and where the cylinder would require a back-up source to top up the temperature it would have been able to contribute potentially all day to the UFH and providing space heating (end users largest energy consumption costs) - at the end of the in this example the back-up souce would still be required to top up the cylinder for hot water just as it would have had the energy available on the roof not been used for space heating. Solar 3C therefore extends the annual window of solar yield significantly especially when installed in New Build properties with Underfloor Heating - Solar 3C would contribute more to the space heating than it could ever do for hot water because of the cooler running temperatures required for the UFH. Solar 3C is not limited to UFH because the weather compensator that makes up the Solar 3C controller will call for low grade or temperature heating even when installed with radiators or indeed in a retrofit heating system.The use of Solar 3C is extended further by the use of the ImmerSun switch whereby any Solar PV export is sent to an immersion heater (instead of being exported back to the national grid) this energy can then either be used to pasteurise the cylinder helping to protect against legionella or be used directly for space heating rather than just hot water - this potentially trebles the value of the PV export. With all of the above in mind I would very much welcome the opportunity to chat through the figures in your Solar chart and see if we can change the negative down pointing thumb for Solar Thermal to a positive THUMBS UP?? Just as hot water cylinders have evolved through time, from 0 coil (direct) to 1 coil for just boiler and no Solar, 2 coil boiler/heat pump and solar but no space heating to, 3 coil boiler/heat pump and solar PLUS space heating!! It's as obvious and easy as 1, 2, Solar 3C!!Thank you in advance for taking the time to read the aboveMatt (m 07850 741213)www.sola
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