Two friends and colleagues of mine (Carl McDaniel and David Borton) recently worked together to figure out why a solar water heating system required so much electricity to provide the modest amount of hot water used in one of their homes. After conducting a careful study that involved the solar water heating system’s installer, it was concluded that using just an on-demand water heating system was far more cost effective.
The installation and study were done at McDaniel's home (Trail Magic) in Oberlin, OH (pictured to left) with a climate very similar to ours (Both in Zone 5A per 2009 IECC, 6497 HDD vs. 6680 HDD for Albany, NY, and percent of sunshine 49% vs. 53%). Electric costs are more varied, with electricity costing $0.10/kWh in Oberlin and about $0.165/kWh in our area. While the authors were very careful not to make general assertions, it seems to me that we should consider this information carefully before investing in solar domestic hot water in our Region.
Below in Resources, you can download a full copy of the study, “Evacuated Tube Solar Hot Water Systems Are Not Energy Efficient or Cost Effective for Domestic Hot Water Heating in the Northeastern U.S. Climate.”
Here is a summary of the article:
Domestic hot water was preheated with an AP-30 evacuated tube system and preheated water was stored in 80 gallon tank before being warmed to 111˚ F, if needed, by a Bosch RP17PT on demand electric heater. Over a 6 month period the following were measured: 1) incoming water temperature, 2) the kWh to pump fluids in evacuated tube system, 3) the kWh to heat water by on demand heater, and 4) the gallons of hot water used. These data were analyzed to establish that sunshine provided ~8% of the energy to heat the hot water used annually.
Less than 1% of the annual solar heat energy harvested by evacuated tube system was in hot water used: ~9,000,000 BTUs harvested; ~81,000 BTUs used. This 2 person household annually uses ~3,000 gallons hot water while the average 2 person-household uses 9,000 gallons. Even at this higher hot water consumption only a small fraction of the harvested solar BTUs would be used.
The evacuated tube system cost, $6,589; the on demand heater cost, $499, not including cost of wiring. With the evacuated tube system saving 30 kWh or ~$3.00/year, it was a very poor in-vestment economically and equally ineffective for reducing carbon emissions.
The evacuated tube system was removed after two years. Data collected over a 3 month period indicated that heating water to 111˚ F with the on demand heater used 0.12 kWh/gallon. With an annual use of 3,000 gallons, the electrical energy required to warm water with the on demand heater is estimated to be 360 kWh annually or ~$36.00.
Howard Stoner is a retired math professor, gardener and lives in a home retrofitted for energy efficiency. He is interested in doing things by hand, the use and care of hand tools: crosscut saws, harvesting scythes, grain threshers, grinders, rollers, etc.
Editors Note: The authors are careful not to generalize the specific study to all installations; however, even so this is contrary to my way of thinking. If the article had not be written by two well respected scientists I would not have published it, without much further investigation. We need to make good decisions, based on science and specific reasons. The McDaniel-Borton study is a step in that direction. Please comment and provide your input on this interesting and important subject. Dan Gibson.