First, I don’t want anyone to think I have been disparaging distributed, rooftop solar in some of my recent posts. Having solar on every roof would be a fantastic thing, and I think it’s a logical first step toward carbon-free power. PV panels are affordable, roof space is already present, and photovoltaic arrays don’t lose efficiency when they’re installed in smaller arrangements. But my point, every time distributed solar comes up, is that even if every rooftop were covered, this wouldn’t produce the amount of renewable power that we will need, and there isn’t enough rooftop space in cities to even begin to produce enough power to meet the demand of the people there. But, this doesn’t doesn’t mean it wouldn’t be worth doing—rooftop space in the sun is rooftop space in the sun. However, in cities, photovoltaics might not be the best use of this valuable real estate.
So, to back up a bit, let me sing some praises for solar hot water. In terms of efficiency, solar hot water collectors dramatically outperform solar photovoltaic panels. Capturing heat from solar insolation is just a fairly efficient process, compared to converting light into electricity. And, until just recently it didn’t make any sense, in terms of price, to install photovoltaics to make electricity that would then used to heat water. In recent years, however, heat pumps have become efficient enough, and PV systems have become cheap enough, that in many situations it might be a better choice to use photovoltaics and a heat pump to make hot water, instead of thermal collectors. (Good article here.)
However, regardless of the price of photovoltaics, it is likely that the best use of rooftops on large, tall buildings in the city will be thermal solar hot water, at least in any building where hot water or space heating is required. Though prices between the two systems are close to equitable today, the higher efficiencies of solar hot water make the physical footprints of such systems much smaller than that of PV panels. My rough estimate is that the PV panels required to run heat pumps would take up four times the area of modern, evacuated-tube solar hot water collectors.
So, I suppose I have a rather small point to all of this—that down the road, we might need to use rooftops in cities for solar hot water. The footprints of such systems are smaller, and we could maximize gain from that limited rooftop real estate. Electricity can be produced remotely and then brought in on transmission lines; such a task with hot water would be fairly unworkable.
My second point might be that for some people in some situations, that advances in PV systems and heat pumps have made it less of a clear choice whether installing thermal solar hot water systems is the best way to use solar to heat water. But, as I was walking through small-town Vermont last week and pondering the fact that many people in town have quite-limited roof space with appropriate southern exposure, it could be that thermal systems, due to their smaller footprints, might remain a pretty good choice for many, whether they live in the city or not.
Blog note: Welcome, Australia, Canada, and Great Britain! My post about perennial agriculture seems to have been spread all over facebook, and I have been getting hundreds of views from all over the world, with clear groupings from these places. I write about my corner of the United States sometimes, but my focus is always on ideas and systems for the whole world, so it’s great to have you on-board. We’ve got a whole planet to fix, and we’re going to need people from every corner of it to get it done.