Controls

The controller controls the flow of the heat transfer fluid in the collectors by modifying the pump operation. Normally the pump is just turned on/off in small systems. The most common pump controller used in solar thermal systems is the “differential controller.” This controller requires two different temperature settings, one for “on” (upper band) and one for “off” (lower band). The system temperatures are measured on an absorber in a collector (usually one whose flow is a short distance to the storage tank) and in the storage tank (near the tank outlet to the collectors) or in this discharge pipe adjacent to the tank. If the collector temperature exceeds the storage tank temperature plus the upper band the pump will turn on. If the collector temperature drops below the store temperature plus the lower band the pump will turn off. A common upper band (“on”) temperature difference is 41–46 °F (5–8°C) and for the lower band (“off”) 36–39 °F (2–4 °C). Proper location of these sensors is very important. Good placement guarantees that the pump only runs when this is beneficial for collecting solar energy that can be stored in the tank. Failing to measure the temperature difference correctly will affect the overall system performance significantly; this requires care and verification at the installation.

The controller also must turn off the pump when the maximum storage temperature (e.g., 194 °F (90 °C) is reached, or when the collector exceeds the maximum flow temperature (e.g., 68.00 °F [20 °C]).

Some pump-controller combinations offer different power settings (often step-wise) to be able to match the mass flow rate with the amount of solar radiation available. This limits the temperature increase in the solar circuit and prevents unnecessary pumping.

More advanced controllers offer additional functionality: insolation and heat measurement, datalogging, error diagnostics, and auxiliary heater (boiler) regulation, and a graphical user interface.