Fluid pumps

The type pump to be used is a centrifugal type that is used in building heating systems. If the loss of fluid to the environment is a concern a seal-less magnetic drive centrifugal pump should be considered. The pump components (seals, gaskets, bearings, etc.) must be able to withstand hot temperatures that could reach may reach 300 °F (149 °C) for short periods. Basically, three types of pumps are available:

• constant flow pumps
• electronic pressure controlled pumps (variable flow)
• high efficient pressure controlled pumps (variable flow).

Variable speed wet rotor circulators are preferred since they can operate the collectors either at a setpoint, or the most efficient range, while minimizing the electricity used to drive the pump. The use of variable flow is a method for achieving the desired temperature rise of the heat transfer fluid in the solar collectors. If the solar radiation is not very intense, then the fluid flow is reduced, causing the fluid to spend more time in the collector. Figure 5.10 shows the difference in energy use of these different methods of pump operation.

Sizing of the pump is accomplished in the normal way of heating pump selection. An estimate of the flow rate can be made using the common flow rate for the collector field of 0.020m gpm per sq ft of collector area (when using water as the collector fluid). If a 40% glycol mixture at 140 °F (60 °C) is the collector fluid, then the flow rate would be ~10% more than a water solution. For a more accurate value, use of the computer programs identified in Section 4.3.2.

If water is the circulated fluid and the system is open to the atmosphere or the water is potable then the pump wetted components should be made from stainless steel or bronze to minimize corrosion. Solar collector pumps should be placed in locations where leakage would not cause serious damage