Q&A: PV Controls for a Solar Thermal System?
Question: “Can you control a solar thermal system using a solar radiation meter or UV meter?” – Stephanie
Using a radiation detector as the control element for a solar thermal system has been tried and it works – sometimes. Usually the radiation detector is a small PV panel. While a radiation meter will tell us if there is solar radiation available, it won’t tell us if we “ought” to turn the system on. We want to make sure that the collectors will add heat to the tank, not dump heat from the tank. However, the PV panel knows nothing about temperatures, so it will turn the system on whether on not the collectors are hotter than the tank.
That is why we do a differential controller that compares two temperatures. One reads the hottest part of the collectors, and the other reads the temperature at the bottom of the solar tank, where the cold water will go to the collectors. When the collectors are about 18F hotter than the tank, the pump turns on. At the end of the day when the collectors fall to less than 5F hotter than the tank, the pump turns off. We leave a little difference to account for losing temperature in the water pipes from the collectors to the tank.
This kind of control will never run the system when the tank is hotter than the collectors. A PV panel will.