A Self-Pumped Solar Hot Water System

When my fiancée and I were ready to have a solar hot water (SHW) system installed at our Washington, DC, townhouse, I figured our only choice was to buy an active system that required mechanical valves and electronic controllers, and electricity to operate them. Of the four SHW systems I have owned, two were “active” (electrically pumped) systems and two were “passive” (self-pumped) systems, the latter with no moving parts. I prefer the latter because moving parts are subject to failure.

Then I ran across a start-up company—Sunnovations—touting its “geyser pump,” which also has no moving parts. Sunnovations drew on—and then improved upon—the simultaneously famous and infamous Copper Cricket design—a passively (self-)pumped collector that was produced several decades ago (I was a satisfied owner of two of them).

Like most SHW systems of that era, the Copper Cricket was far from perfect. Despite its main advantage of having no moving parts, the Copper Cricket had two major shortcomings: it operated under a vacuum that could fail and frequently did; and it could overheat and “cook” the antifreeze solution that served as the heat-transfer fluid (HTF) between the collector and the storage tank.

Sunnovations SHW systems are currently the only “self-pumped” systems certified by the industry-standard Solar Rating and Certification Corporation (SRCC). Sunnovations has obtained SRCC “OG-300” certifications for 41 system configurations, using combinations of five different collector brands, one- to three-collector arrays of varying-sized collectors, single and double storage tanks, and gas or electric backup heat.

The Sunnovations geyser pump depends upon a vacuum (negative pressure) to reduce the boiling point of the heat-transfer fluid to about 100°F. The “geyser pump,” which relies on solar heat for its pumping action, is the heart of the system. It can be affixed to almost any collector. (The Copper Cricket was an all-in-one geyser pump and solar collector.) After an initial vacuum has been established, the pump circulates the HTF through the collectors. As the sun warms the geyser pump, pressure increases and the heated fluid flows.

Bright sun means higher operating temperatures and higher operating pressures. But even in full sun, the system operates at slightly below ambient (outside) atmospheric pressures. The maximum temperature the system may reach is 185°F, below the temperature at which the HTF breaks down. When the system pressure reaches the ambient pressure, a relief valve vents steam to a reservoir where it condenses back to liquid, and the geyser pump continues to operate. When the storage tank is fully heated, the return fluid to the collector will exceed 140°F, causing the pressure to rise rapidly. The temperature-limiting mechanism, which vents excess steam, cannot keep up, so fluid is vented out of the collector into the reservoir, thereby preventing the fluid from overheating. After the system cools, all the fluid expelled to the reservoir is drawn back into the system, which revitalizes the vacuum.

As the geyser pump system simply will not operate in extremely high temperatures, oxygen-barrier cross-linked high-density polyethylene (PEX) tubing can be used, rather than more expensive (materials and labor) copper pipe to carry the transfer fluid between the collector and the storage tank. PEX is flexible, so fewer joints are necessary and the joints are easier to make. While PEX piping can be problematic in traditional high-pressure and high-temperature active pumping SHW systems, as the Sunnovation system is self-limiting of both temperature and pressure, PEX is quite suitable.

While heated fluid flows without additional energy from electric pumps, for the passive Sunnovation system, the solar collector and geyser pump must be 33 feet or less above the storage tank. The geyser pump loses some heat to the outside air, especially on cold days. The unit must be warmed by the sun to 100°F to pump, so an active system set to come on at 90°F can be more productive on cold winter mornings. The relatively low flow rate (about 1 gallon per minute for two 4- by 8-foot collectors) will also result in some loss of heat delivered to the storage tank.