Pumps

Pumps move the heat transfer fluids through their piping circuits. The pump associated with the collector fluid will be exposed to the highest temperature and the greatest pressure. To minimize the high temperature in this loop, the pump is placed in the pipe going to the collectors. But even here there will be short term periods when the pump is handling temperatures greater than the design collector discharge temperature. Such a time is when the hot heat transfer fluid has been forced out of the collector into the recapture tank due to stagnation. If using an anti-freeze solution as the collector heat transfer fluid, the pump components will need to be compatible with the water/glycol mixture. These considerations will affect the materials used in the construction of the pump.

The pump will also need to operate through the flow range that is designed for the system. In some solar thermal systems, the heat transfer fluid flow is slowed when the solar radiation is not at its peak. At this lower flow the pump must still provide the required pressure to obtain fluid movement through the pipe system. Having clean fluid strainers/filters, a minimum of valves, and properly operating sensors will aid in keeping the system pressure drop as low as possible. In large systems, the piping system flow is modeled to define the pump pressure needed for proper operation. It is important to take into account that glycol/water mixtures have a different viscosity than water alone. The manufacturer of the heat transfer medium should provide the necessary data. The energy needed to pump the heat transfer fluid is considered parasitic energy; it must be kept as low as possible. The pump should thus not be oversized.