Comments on GBA Solar Thermal is Dead Article

An Eccentric Anomaly: Ed Davies's Blog

Martin Holladay has just posted an article on the Green Building Advisor site: Solar Thermal Is Really, Really Dead.

I mostly agree but think there's a lot more to be said. It's a complicated subject and it's easy to get bogged down in detail so he's done well to simplify things for clarity but maybe he's gone a bit far and there are few other things people can usefully think about.

His big simplification is to assume that there's a certain amount of energy harvested by the systems and that this can be used to heat water. In reality the sunshine doesn't just switch on and off and it's important to consider the times when it's less bright and the air is cold.

I've previously blogged on the subject: PV, ETs and Flatties and done a little calculator from which it is easy to see that even a couple of years ago PV was beating solar thermal for domestic hot water (DHW) when the insolation (sunshine) is not very strong. Though there's little power in these conditions they tend to last a long time so producing a significant proportion of the energy available over the year.

The basic point is that PV which is sized to provide decent but not excessive amounts of hot water in the summer will provide at least some heating further into the shoulder months, and even the winter, than an equivalently sized solar thermal system.

On the other hand, there are PV to Immersion Heater Problems which I'm not sure that Martin has taken into account in his pricing. In particular, if the PV is producing a tenth of its nominal output for 10 hours you have to have a battery or net metering to run your heat pump for one hour on its output. I'd assume he's assuming net metering but he should really make that explicit - it doesn't apply in much of Europe, for example, and there is, I think, increasing resistance to it in the US.

A point in favour of a PV + heat-pump system that Martin doesn't mention is that the heat pump can also be used to contribute to the winter heating if that's from mains electricity (rather than oil, gas or wood). Instead of having to provide 37% of the hot water from mains you might have to only supply 12 or 15%, the rest coming from the air (assuming the CoP of the heat pump is fairly low in the winter for DHW-temperature outputs).

The converse of this is that if the system is sized to give decent amounts of energy for DHW in the shoulder months or winter then it will be oversized for the height of summer. With solar thermal this is a pain - you have to think about methods of dumping heat to avoid cooking your panels. With PV it's not a problem - you can leave energy in the panel if necessary but there are usually better uses for it even if you can't, for whatever mad regulatory reasons, export it to the grid. Run a slow cooker, get the freezer really cold, charge your EV, or whatever.

Essentially, electricity is a lot more useful than relatively low-grade heat. It's also more portable. I know of one case where somebody exports (dumps) excess solar-thermal to his next door neighbours' loft-mounted DHW tank but in general there's not a lot of use for spare hot water in summer.

Martin writes:

Solar thermal buffs make the argument that solar hot water systems are simple, elegant, affordable, and delightful. I sympathize with their emotional attachment to these systems, and I wish them all the luck in the world.

Indeed, some people seem to be offended by the idea of using PV to produce nice high-grade electrical energy then immediately “degrading” it to thermal energy. They should consider that a solar thermal collector degrades the even higher-grade energy of the short-wave solar radiation photons to mucky old thermal energy up on the roof - whether this awful degradation happens up on the roof or in the tank doesn't really matter.