The main advantage of using DC power to run a solar thermal installation is that you can draw that power from a PV panel and, at the same time, eliminate the need for a controller.

Since PV-based power will only switch on when the sun shines, the solar thermal installation's pump will only run when there is heat to be gathered. When the sun disappears, both the PV power and the pump will switch off.

If the day is particularly cloudy, the PV panel will produce power at a reduced rate, so the solar thermal pump will run at a reduced rate. No matter what happens, the solar thermal and PV systems run in concert without any need for control.

Solar thermal systems get heavier use in winter, so it's important to locate the PV panel in an accessible location if your region suffers from heavy snowfalls. You will need to adjust the panel's tilt angle to vertical to ensure it keeps operating. Set it back to the same angle as the collector array for the rest of the year.

DC circulating pumps

It is important to choose a pump that is designed specifically to run on PV power , which usually means a brushless model. You can use one with brushes, but you should consider adding a linear current booster in case the pump has trouble starting up on low power.

It is also important to choose the right size wire when using a PV power supply. There is a direct relation between the wire size and the flow of current, so do your research and choose the correct size for a low-voltage PV supply.

Siting the pump

AC systems usually have the circulating pump installed by the heat exchanger and storage tank. DC systems tend to locate the pump much closer to the PV panel and solar collectors to avoid a very long wire run – since the wire size is usually much bigger than for an AC setup, a long run can get expensive!

DC pumps are most often installed under the roof, in the attic space. If this is not possible, you can site it directly behind the PV panel itself, though you will need to place the pump in a weatherproof, ventilated box. For ground mountings, the same setup works: install the pump behind the collector array in a weatherproof, ventilated box. It is important to ensure good ventilation or the pump can overheat when running.