Purist Energy
solar hot water systems design guide

Purist Energy SPS and SPC solar thermal appliances use the closed loop antifreeze method of operation. In this type of solar hot water system, water or non-freezing fluid circulates to the collectors and back to a heat exchanger, where the heat is transferred to the stored potable water. The loop is completely filled with fluid, and therefore requires a non-freezing fluid such as Propylene Glycol (not automotive antifreeze) in areas prone to freezing. The heat exchanger may be integral to the tank requiring only one pump, or external to the tank, requiring a second pump for the potable water. External heat exchanger systems, when operated properly, can be more efficient, recovering more heat from the solar collector(s). Closed loop antifreeze systems are of the most common installed in the US, with a time proven track record and good performance.

Purist Energy manufactures only the piping and controls segment of a solar hot water system. Solar collectors are available from other sources and vary greatly in design. The two most commonly found are "flat plate" and "evacuated tube", but other systems are also available, such as low temperature roof integrated systems. The SPS and SPC systems reduce the time and costs involved with on-site installation, and ensure a uniform, quality-controlled system.

Selection of an SPS or SPC system involves several parameters:
Collector size: The amount of hot water to be heated will determine the number of collectors required and if an SPS or SPC system is appropriate for the application. The SPS system is sized for a maximum water volume of 240 gallons in the Northeastern US. A common rule of thumb is that the typical household uses 20 gallons per person per day for the first two people, and 15 per additional person.

Storage tank type: solar heated hot water can be stored in conventional hot water heaters or in special solar storage tanks with internal or wrap-around heat exchangers. Solar storage tanks are fitted with special ports for solar hot water, but are more expensive. It is possible to use conventional water heaters as storage tanks by using a few tricks, such as cutting the cold diptube, or attaching special fittings to the drain valve ports. SPS-1 systems are for use with special solar tanks with internal heat exchangers, and SPS-2 systems are for use with conventional water heaters.

Installation of an SPS system typically involves insulated tubing runs with flexible copper tube. Two lines are run from the SPS to the collectors, and two short lines are run to the connections on the storage tank. The AC units plug into a standard outlet, while the DC units require power from a PV panel set in the sun.