Let’s say you have a project with some beautiful, sunny, open roof space that would be perfect for harvesting solar energy. Which is better, passive solar hot water heating or photovoltaic electricity generation?
For the past five years the answer has been a pretty solid “PV all the way!” The cost of installation for PV continues to drop as panels become cheaper and more efficient, with average costs at $2.50-$3.50/watt. Additionally, wiring up PV provides better flexibility in placement and installation, as well as less risk of a maintenance headache, when compared with all the extra piping connections for solar hot water.
But what about a building with significant hot water demand? The first logical conclusion is that passive solar hot water would be the most useful, but with the right technology PV still works best. Hot water heat pumps, which are becoming much more widely available at both residential and commercial scales, are able to generate hot water so efficiently (with a COP of 3.0 or greater) that capturing “free” solar energy for hot water doesn’t give you very much added bang for your buck. In fact, Martin Holladay has done the calculations to show that in typical practice, a PV system supplying electricity to power a domestic hot water heat pump is actually more efficient and cost effective than a solar hot water system with electric backup. Actually, Holladay did the analysis twice to further make the point that PV plus heat pump is cheaper and works better than a passive solar hot water system.
Of course every project is different, and there may be particular parameters that move the needle one way or the other. However, it’s a terrific sign of how far renewable energy has come that there are a number of cost effective, high performing options available for any project.