Comparison of Solar Thermal Heating Systems and Photovoltaic Systems

Solar Consultants specializes in solar thermal (heating) systems.

We are often asked what the difference is between this and generating electricity with photovoltaic (PV) panels. There are many differences, but for most people it boils down to this: thermal delivers power at about one third the cost of PV. The first reason for this is conversion efficiency of the collector and storage. PV panels run about 8-20% efficient at converting sunlight into electricity, and this efficiency goes down as the panel gets warmer. (You may read about laboratory experimental, NASA-grade chips, or modules with mirrors or lenses to concentrate the sunlight that achieve efficiencies of 28-41%, but those are not what you can actually buy and put on your roof.) Even moderate-quality flat-plate thermal collectors operate at about 65 to 70% efficiency when the fluid running through them is not a lot hotter than ambient temperature (as it is when it first starts to heat a tank of cold water on a warm morning). And this efficiency stays high as the day heats up and your water also heats up, or if the heat is going into a radiant-heated slab, which stays about 78°. Once the heating target is hot or if the ambient temperature is very cold, collector efficiency declines according to a very predictable slope. Thermal collector ratings are performed by an independent testing agency, so you do not have to depend on any manufacturer’s or salesperson’s claims for this information. There is no equivalent third-party testing for photovoltaic equipment.

After the initial conversion of sunlight into electricity there are other inefficiencies, such as in the process of storing it in batteries or converting it into alternating current to use or send out on the grid. The overall efficiency for a grid-tied PV system (without batteries) is around 10%. In a thermal system there are, similarly, inefficiencies transferring heat from the fluid that goes through the collectors into your potable water or space heat. This varies a great deal, depending mostly on how cold the target is. The more heat you can use, the more efficiently the system runs. That’s why large families or businesses that need a lot of hot water benefit most from solar water heating. In the case of swimming pool heating, that water is usually heated directly in the collector, so there is no loss of efficiency down the line; what you heat is what you get.

The second difference is system cost. Leaving tax credits and incentives out of the figures, since they apply to both types, for each kilowatt-hour produced, a typical PV system costs $.27 while a system heating domestic water costs 9 cents. (See Home Power magazine’s article PV vs Solar Water Heating in the Nov/Dec 08 issue for details behind these figures.)

The third difference is size on the roof. A typical domestic water heating system collects power equivalent to a PV system taking up four times the space. PV is also much more sensitive than thermal collectors to shading, even from bare branches after the leaves have fallen, so you have to be much more careful about siting the panels and not letting trees grow up south of your house. According to the NC Solar Center’s Siting of Active Solar Collectors and Photovoltaic Modules, “Shading of only one cell within a PV module can reduce the module’s power output as much as 75%.”

We are not against photovoltaics – we install panels to power the glycol pump in some of our systems – and we are glad there are people willing to pay the cost of providing this clean electricity to the grid. It is just that there are clear priorities of what provides the best return on your investment and what most reduces the need for burning fuel, and PV rarely rises to the top of that list.

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Fred Stewart
Licensed plumber and solar thermal system designer

Tom Willis
Licensed plumber and NABCEP-certified system designer

Richard Copsey