Solar Energy – Thermal or Photovoltaic ?

4 03 2008

Solar Energy – this seems to be a pretty hot topic on the street right now.

The question over what’s best to use; a solar thermal system which uses the sun’s heat to provide domestic hot for use in the house or a solar PV (photovoltaic) system which converts the sun’s light to electricity? For this discussion, we’ll stick to talking about residential projects.

There is very limited data available on this matter as far as costs go – so this past summer, while doing our Seminar series ‘Greening your home: Live Sustainably in Brooklyn’, I did a cost comparison between the two energy systems, based on the energy use & consumption of a typical Brooklyn Brownstone.

These numbers looked at the type of solar system, cost, and the return on investment to install each of these systems. The results? Solar PV panels was about a 10-14 year payback time versus a 4-6 year payback time for the solar thermal system (depending on the type of fuel used for backup domestic hot water – oil, electric or gas). Note that both of these payback periods include reductions for tax credits and rebates. Of course as the cost of electricity increases, you’ll reach the payback period faster.

Since doing these calculations, we’ve come across some more data and have spoken to contractors who have performed both types of installations, and we’ve found that these numbers are right on. You can download our calculations here. These are based on a typical Brooklyn-sized brownstone. Remember, you’ll need approximate 100 square feet per kilowatt and they add up in size quickly on a small brownstone’s roof when you consider that you also need a hatch, at least one skylight, maybe some mechanical equipment etc…

This interesting article in treehugger, The Economist misses the point on Going Solar, argues that solar PV panels, albeit expensive, can be justified if the energy usage of the house has been well managed and is low enough so that you don’t need to spend a mint to afford them.

We’re by no means opposing Solar PV panels – we’re strong advocates for any system that reduces the amount of fossil fuels being used. However, when budget is a concern, as it often is in residential projects, a solar thermal system may be a better alternative for its value and function.

The other concern I have about solar PV installations is that they seem to fairly delicate and I worry that they won’t last out the payback period. A solar thermal system, on the other hand, is constructed of durable pipe and seems much hardier. I also think that in a few years, there will be many more options for solar PV given the heavy amount of research currently being done and the promises of the presidential candidates to fund even more.

Here is a photo of a typical Solar PV Panel installation:

solar pv

This is a photo of a Solar Thermal System we saw while surveying a project in Harlem

Solar Thermal




One response

1 08 2008

I think that the most important differential between thermal and PV is that unlike electricity, heat is easy to store in a convenient and environmentally safe way, which is an advantage with the intermittancy of all microgenerated renewables.We need both thermal and PV, but at the rpesent time thermal makes sense.

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