Setting Flow Rates on Solar Panels

Before we discuss setting actual flow rates, it is necessary to look at the function of the solar controller and it's limitations. The diagram below shows a typical solar set-up.


Solar controllers are sometimes known as differential controllers. When the panel becomes warmer than the bottom of the cylinder by a certain number of degrees (normally 4°C to 8°C) it switches the Solar pump on. The panel then begins to cool, when the panel temperature has dropped to about 2°C to 4°C warmer than the cylinder the pump is turned off allowing the panel temperature to rise again. The Controller then repeats the cycle.

However, this method is not without its problems. The temperature sensor is normally in contact with the copper manifold, and it can take up to 30 seconds to correctly resister that the water (or antifreeze mix) temperature. This can lead to the panel not performing at it's optimum efficiency.

This problem is overcome to some extent if the panel flow is restricted. This means that under full sunlight conditions the energy being collected by the panel raises the temperature of the water flowing through the panel by between 6°C and 15°C. This means that the ON/OFF controller, stays ON throughout the day, preventing temperature hunting. To achieve this, many manufacturers suggest setting a flow rate of between 0.5 l/min to 1 l/min for each M² of panel.

A better way of course is to use a modulating controller, which automatically adjusts the flow rate to keep a constant temperature differential between the collector and the cylinder. This is explored in more detail later.



The reason for the hysterisis in switching levels (the higher switch on differential and the lower switch off differential) is to maximize the solar yield without drawing heat unnecessarily from the bottom of the cylinder and to avoid running the pump when there is no heat to be collected. The simplified temperature graph below illustrates how the differential switch occurs in practice. As the collector heats up its temperature rises above that at the bottom of the cylinder by more than the set differential, often about six degrees.



This temperature difference is chosen to so that the solar system will come on only when there is enough energy arriving at the collector to ensure that it stays on. Too low a differential means that the collector will quickly cool, allowing the temperature of the collector to drop below the lower differential set-value and causing the pump to switch off. This on-off cycle generally draws energy unnecessarily from the bottom of the cylinder and allows it dissipate in the solar loop pipework. As can be seen from the graph cooling of the collector tends to occur initially as the cooler water from the solar loop pipework enters the collector. As long as a time delay in the collector holds the pump on for a few minutes, the collector temperature will recover and the system will stabilize without any unwanted hunting on and off. Setting the differential value too high will mean that there will be a longer delay in bringing on the pump at a time when there is energy to be collected, this lowers the collector efficiency. The lower switch-off differential is set at about two degrees typically, as below this the pump will be running while no net energy is being stored in the cylinder. Electrical costs will rise and the marginal heat rise from the collector is dissipated in heat losses in the return pipework. The graph below illustrates the system switching off as the cylinder temperature rises to within two degrees of the collector temperature. The system would also turn off if heavy cloud obscured the collector causing it temperature to be cooled below the switch-off set-point by the cooler water from the cylinder.