Unless you have a Thermosyphon type water heating system, Solar Water Heating is an active rather than a passive system. Therefore it is necessary to pump the water heater in the solar panel around to the hot water tank, swimming pool, or hot tub etc. when the solar panel is hotter than the stored water to be heated.
A solar water heater system is a closed loop with heat transfer fluid (or water in the case of a direct solar water heating system) drive around by a low flow pump. It is not necessary to use a very powerful pump – just a small circulating pump.
For a completely renewable system, a PV solar panel with a power output well matched to the power required by the pump can be coupled so that the pump is powered when the sun is shining. The sunnier it is (and therefore the hotter the solar water heating panel is likely to be), the faster the pump will work.
A typical high-voltage powered pump requires a controller and temperature sensors to manage the speed of pumping. A direct connected PV solar panel on the other hand ensures that the flow rate of a pump will help to maximise efficient water heating.
Solar pumps are configured to circulate water as long as there is enough sunlight to cast a shadow. Best of all, they will heat your water even if you suffer a power cut or if you live off grid. In addition it can save a couple of hundred kWh of electricity usage annually.
The pumps pictured above are from Thermo Dynamics Ltd. and have flow rates from 0.3 to 2.4 Litres/min up to 2.0 to 12.0 litres/min suitable for different sizes of solar water heating system.
Other manufacturers to look out for are March, Hartell, and the El-Sid pumps from Ivan Labs Inc. All provide brushless motors and are maintenance and leak free and perfect for pumping around hot water or glycol solution. El Sid pumps for example start at wattages of just 3.5W and so can be powered directly by a small 5W solar panel.
Click here to search ebay for generic brushless 12V pumps. There are usually a few hundred to choose from with prices starting from around £10 only. Pictured above is a typical unit. Priced under £20, low power consumption, heat resistant, very simple, and strong enough to circulate heat transfer fluid around a small solar water heating system reliably for years.
When the PV solar panel is directly connected to the pump, the pump will run in the early morning when the hot water panel may still be cold and the stored ‘hot water’ may still be hot cooling it down. Similarly, if someone uses the hot water in the afternoon, the pump will stop running in the late afternoon when the solar water heating panel is still warm/hot and the stored ‘hot water’ is cold.
An alternative (and more efficient) renewable solution is to have a PV solar panel charge a deep cycle battery which in turn powers the pump, and to use a solar water heating pump controller with temperature sensors to ensure that the pump only runs when it needs to – i.e. when the water heating panel is warmer than the water to be heated.
If you would prefer to have a mains powered pump, then a standard domestic water heating system circulation pump is what you need as pictured above.
More Information about Solar Water Heating
If you are interested in making your own solar water heating panel, click here to Build Your Own Simple Solar Water Heater, or here for details of a more complex DIY Solar Water Heater. If you would like to purchase a commercial solar water heating panel, we recommend you use evacuated tubes for maximum efficiency.