My system does not yet have an active monitoring system to be able to quantitatively track performance. However, I can share some observations. On sunny days, the HTF going into the storage tank is about 140°F. My summer gas consumption fell from an average of 6.6 therms per month to 0.6 therms (91% less), as it is now just used for cooking.
To get a quantitative sense of system efficiencies, it is useful to compare SRCC ratings for similar systems. The “SHW System Comparison” table (opposite page) depicts the best available approximations of three configurations: a Sunnovations geyser pump system with an AET collector; a geyser pump system with Kioto collectors; and a comparable active pump system with an AET collector.
While this SRCC OG-300 comparative analysis is interesting to technical types, it is not the most appropriate metric to evaluate SHW systems. Most interesting to the consumer is whole-system cost-effectiveness—measured in levelized dollars per kBtu—which is more appropriate, since it also factors in maintenance costs over the system’s life. PEX rather than copper pipe means simpler and less expensive installation costs. No moving parts likely means fewer service calls. Lower—but hot enough—operation means no potential HTF overheating.
I tend to have an early-adopter personality (not among the very first, but ahead of most everyone else), so it’s no surprise that I chose to be one of the first to have a Sunnovations geyser pump when it came time to invest in an SHW system. Although its self-pumped system goes against the grain of current conventional SHW systems by operating at significantly lower temperatures and pressures, moving parts and electronics can fail, and I favor systems without them. I also don’t like having to periodically change out “cooked” HTF.
I was willing to accept a lower total system performance for a passive, rather than active, system (see “Comparison” table). The production penalty (assuming we use every Btu of heat produced) for this particular system would be 19% less hot water than for a comparable active system. Given the significantly lower installation costs (PEX, etc.) and lower expected operating costs, the lower cost made the 19% production penalty worth it.
Now it turns out that the theoretical production penalty for using a Sunnovations passive system in my area (it varies based on the amount of annual insolation) need only be 4% for Washington, DC. Since our installation in December 2011, Kioto collectors are now available in North America. The Kioto collectors have a narrower riser diameter, which make them a more optimum fit with the Sunnovations geyser pump. They are also less expensive than AET collectors.
When one runs the numbers on these same three system configurations as if they were in Albuquerque, New Mexico (see table), it turns out the production penalty for my configuration over an the active system would be 21%. If the Sunnovation Kioto system were used, that penalty would be reduced to 8%.
Will the Sunnovation geyser pump go the distance? Did I make the right choice? I think I did, but only time, data, and experience will tell.
Andy Kerr is a frequent contributor to Home Power and a renewable energy and efficiency blogger (andykerr.net). He splits his time between Ashland, Oregon, and Washington, DC.
Solar Rating and Certification Corporation (SRCC) • solar-rating.org
Sunnovations • sunnovations.com