In an article posted on the website of Green Building Advisor, author, Martin Holladay explains that due to the ongoing reduction in costs for photovoltaic (PV) systems, it makes no financial or practical sense to install a solar hot water system for the supply of domestic hot water in residential applications.
There are a number of interesting points in the article, but the most compelling is that even a very modest PV system, consisting of an electric resistance hot water heater and a grouping of PV panels, can provide domestic hot water about 25% cheaper than the cost of a solar hot water system. Yes, even a good old, off the shelf, electric resistance hot water heater can work more efficiently than a solar hot water system. Mind you, the PV array required in this electric resistance system is getting pretty big at 1.71 kW, requiring an area of about 10′ x 13′. And remember, this is just the PV area required for domestic hot water. If someone is looking at offsetting the remaining energy used in their home, for all other day to day functions, the PV array is going to get rather large.
Although the option for electric resistance heating is a simple and straightforward option, there is another PV system variation that truly “kills” solar thermal. If a PV array were coupled with a heat-pump water heater, the cost would be approximately half that of solar hot water. The PV array required for such a system would consist of only two 40″x 65″ (280 watts) PV panels. In addition to being cheaper, the PV system is a simpler system and therefore has the advantage of requiring less operating and maintenance costs over time.
Interestingly, the results of these residential industry changes may have possibilities in the commercial design and construction world. Could a series of these smaller PV and heat-pump systems be used for larger projects, such as multi-residential, institutional or office buildings? Will PV systems soon be cheap enough to provide all of the offsetting power requirements for a building, everything from heating to cooling to illumination? What will be the next building system to be considered “dead” because it no longer makes financial sense to use energy directly supplied from fossil-fuels, or to install complicated and maintenance intensive HVAC systems, in an attempt to make buildings more efficient?