This is a concept I never fully understood. My father even sold all kinds of renewables for ~5 years until 2016, and he just said “Thermal just isn’t worth it” but never really explained how or why. And even now when all Europe is under a strong pressure into deciding how to heat our homes this winter, it seems that no one is still considering thermal solar as a solution.
Heck some governments even decided that it was better to propose PV + Heat Pumps instead of Solar heating
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Why?
In: 4
Thermal solar heats mostly during summer, and not during winter when you need it. It doesn’t work as your only heat source, so if you do it, you also have to pay for a second device to heat your water. With PV+Heatpump you can simply buy grid electricity.
The only case where it makes a lot of sense is heating pool water, because that’s a use case where you need heat in the summer.
So basically thermal simply heats pipes with water in them and those are used for heating and hot water. Obviously in winter there usually isn’t enough sun to do this properly and you need to get the energy elsewhere.
Photovoltaic creates electricity, which can be saved in batteries and used when needed.
They’re really freaking expensive. Back in 2016 PVs were expensive enough where solar thermal plants were roughly equivalent. Before then the tech was still immature but had a lot of potential as PVs were pretty expensive.
Fast forward to today and PVs are substantially cheaper than back then. As a result the tech receives limited investment, and therefore it’s hard to develop the tech to be cheaper. You can look at some big thermal solar projects from that era and see that they struggle to turn a profit.
That’s not to say the tech doesn’t have any advantages. They are really good at storing heat for use when the sun doesn’t shine, partially addressing the storage issue with PVs. But thermal solar is a lot more complex with the need for a tower and heat storage medium (molten salt being common) and the need to control the mirrors which is a huge source of the cost.
The basic issue is that the cost pv has come down so much that for many/most locations it’s cost per unit of useful energy produced is lower than the cost for thermal. So if you are choosing how to use your roof space it almost always makes more sense to stick pv on rather than thermal.
There are hybrid pv/thermal panels which are pretty efficient but their cost is still too high to justify them over simple pv.
You mention heating homes in a European winter. Thermal solar works by using sunlight to heat up some material (usually a liquid), and then using the heat in that material. A problem is that heat can leak from this material back into the cold environment.
Photovoltaic systems work by using a photon to “bump” an electron out of place on one side of a solar cell, and then as the electron is forced to trickle the long way around to get back to the other side of the cell, we can extract energy from it. It doesn’t rely on a temperature difference, and indeed ordinary silicon PV cells are more efficient when they’re cold.
So thermal solar works best, paradoxically, when the environment is not particularly cold. Until recently, solar thermal domestic hot water systems were more popular in Australia than a lot of other places, partly for this reason. As other commenters have mentioned, economies of scale have now made PV cheaper, but have not had much effect on solar thermal. So for new installs PV plus heat pump is now attractively priced, whereas solar thermal systems cost as much as they always have.
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