Q&A: the case for giving solar thermal its moment in the sun

“You stand in the sun on a cold winter day, you feel the warmth – put that warmth into water, you have got solar thermal energy,” describes Christophe Williams, co-founder and CEO of Naked Energy. “It is so natural and such a pure form of renewable energy.”

Yet despite its simplicity and potential, solar thermal has long played second fiddle to its flashier cousin, solar photovoltaic (PV) energy – and more recently, to heat pumps.

In a conversation with Power Technology, Williams argues that as the world attempts to decarbonise heat – an overlooked but critical area of energy use – solar thermal deserves a front-row seat. He shares why the technology has been sidelined, how it can complement its counterparts and what is needed to scale impact.

Christophe Williams (CW): There is around 550GW of installed solar thermal capacity globally – most of this is in China. The technology is very popular in China, but in the rest of the world, it has been seen mostly as a single-family home technology or something for hobbyists.

Historically, solar thermal energy has been seen as a residential technology rather than a commercial or industrial one as policy support focused on residential applications. This wasn’t the wrong idea, but I think it was unnecessarily limited to that application. That’s why our business [Naked Energy] is focused on the commercial and industrial, where we think the opportunity is bigger because the heat demand is bigger.

It was also put on the backburner with so many subsidies going into solar PV. In the UK, solar thermal was growing at a really good rate until 2013, when there was a huge stimulus with government subsidies for PV. As a result, the cost of PV reduced dramatically, and that hurt the solar thermal industry as the whole market shifted to PV.

Lastly, I think solar thermal has been left alone because decarbonisation policy remains focused on electrification – electrifying transport, electrifying heating and so on. This has led to solar thermal being overlooked, even though it is a mature, available and low-risk technology for decarbonising heating that could and should be adopted now.

However, I think there is a bit of a renaissance now. People are starting to realise that the heat decarbonisation problem can’t be solved by electrification alone. They are also looking at issues with power outages, like the blackout in Spain and Portugal – this showed that any energy system that is 100% reliant on electricity is going to be slightly fragile. Diversifying with other clean energy sources such as solar thermal means less reliance on the power grid.

CW: I am not an engineer; I come from a creative background. The other two names on the patents are the inventors. When I first met them, they opened my eyes to the heat decarbonisation challenge, with more than half of global energy going into heat.

Coming from a non-engineering background, I saw this problem and the technologies they were looking at – thermal and PV, separately – and said: “Wouldn’t it be great if we combined the two?” We could get more energy from less space. It would be one design for the roof, one installer and one trip to the roof. There is a big opportunity in hybridisation and simplification. They hadn’t thought of that. I think from an engineering perspective, it was counterintuitive to bring PV and thermal together.

With that, we were able to develop a globally unique technology. We have gone through research and development (R&D) and have now deployed it, with major projects such as that at the British Library. It is fully certified, and now we are scaling up.

CW: Education and raising awareness are key. I think we need to stress how roof and land space are at a premium. It is about getting more from less. What we have done is take mature technologies and bring them together in an interesting package to generate more energy from less space. We also need to let the public know how solar thermal complements PV in tackling the electrification challenge.

CW: Currently, a lot of policy support is on heat pumps. In the UK, for instance, the government is putting all its hopes on heat pump rollouts and electrification of heat. The challenge with this approach is now becoming apparent – the target is 600,000 heat pumps per annum, and we are nowhere near that in terms of rollout.

So, heat pumps are getting a lot of support and expanding, while solar thermal requires more awareness and education – but costs still have to come down dramatically for heat pumps, and the same goes for solar thermal.

Heat pumps are a great technology, but the challenge is that they need electricity, and if that electricity isn’t green, then you are not really cleaning up your heat. Heat pumps are also adding more load to the already overwhelmed grid.

CW: I think they can be improved. Heat pumps take cold water – let’s say, at 10°C – and use electricity to compress and bring it to room temperature or hot water. What solar thermal does is preheat that water so that instead of starting at 10°C, the heat pump can start with warmer water, meaning it has to work less. Solar thermal improves the COP – the coefficient of performance – of heat pumps. They work in harmony.

For ground-source heat pumps, solar thermal can also put heat back into the ground, recharging it. It forms a really nice cycle.

So, I don’t see solar thermal and heat pumps as competition – I see them as co-opetition. It is not either-or. Every project we have deployed is combined with an air-source or water-source heat pump. It is all about combining the two markets to provide the best solution for the end customer.

In our commercial and industrial focus, we have some customers that are on gas and want to electrify with heat pumps, but they aren’t ready for that because of costs – they are going from low-cost gas to high-cost electricity. Thus, they start with our system, which reduces half of their gas demand while being heat pump-ready. This way, if they want to electrify [fully with heat pumps] later, they can save on capex.

CW: First and foremost, governments need to recognise the technology. So far, it has been largely overlooked. Ultimately, we need governments to be technology agnostic – they shouldn’t just put all their eggs in one basket. They need to look at solutions that work together to deliver affordable heat decarbonisation to the end customer.

One of the biggest policy changes that could help is for the government to act as a guarantor on renewable energy projects, specifically backing district heating networks or large industrial projects. If they act as a guarantor, it reduces the cost of capital for private investors. That makes the return on investment for projects much more attractive. If it is designed, integrated and maintained properly, the government wouldn’t even need to spend its own money – it would just be de-risking the project. That would accelerate deployment.

In the UK, there is huge support for heat pumps, and even that is still struggling to scale. That said, in countries such as the UK, a heat decarbonisation target would also help structure different policies to support that target. Germany has done this, and while its energy transition hasn’t worked perfectly, it has now decarbonised a lot of heating. Having a clear target pays off because then policy adjusts to meet it.

CW: Skills and training are important. We could have training centres helping to reskill people – for instance, from the oil and gas industry – that already have complementary skills to transition into renewables. We need more mechanical and electrical skills in the solar thermal industry.

We also need facilities to train people in solar thermal and heat pump integration – like Octopus Energy’s heat pump academy for heat pump training but for the full suite of technologies would be fantastic.

Solar thermal installation like heat pumps is complex and has its challenges, but many are shared with heat pumps. There is plumbing, sizing aspects, heat loss calculations – things you must do for heat pumps anyway. So, if you are training someone to do that work, why not look at the whole package, also looking at thermal storage and solar thermal integrated heat pumps?

Where I think a big difference can be made is through partnerships. Part of our strategy is a new partnership with E.ON. We are now working with it and onboarding its engineering capabilities. For instance, we had some of its engineers come to the roof of the British Library to look at our system. We are training them so that they can then train the trainers. That is how we can get the butterfly effect and reach larger volumes through knowledge sharing and skill development.

CW: I project forward with a vision of solar thermal on millions of buildings worldwide, generating affordable, clean heat on-site. The International Energy Agency estimates that we could reach 2,000GW by 2040 – that is quadrupling from current capacity.

Solar thermal will be complementary to the mission of electrification, which is rapidly progressing. One of the key advantages of solar thermal is its storage capabilities. In Europe, there are already millions of solar thermal systems, every one of them coming with storage – so by default, solar thermal is a storage technology. It is low-tech but very effective.

Water storage – used in solar thermal – is significantly cheaper than battery storage per kilowatt-hour. In Germany, Austria and Denmark, with some government support, they have brought down costs to be comparable to gas – and it is zero-carbon.

There are already 185GWh of heat storage in Europe through solar thermal systems. That is energy you don’t need from the power grid. So, if we can quadruple that capacity [of solar thermal and storage], imagine how much it can help balance the grid and reduce load.

As we electrify transport, heat and everything else – add AI’s exponential growth – global power demand is going to skyrocket in the next decade. Solar thermal will be a perfect complement to other energy efficiency policies. It can help build energy resilience and solve winter heating challenges. The more grid-independent systems like solar thermal we deploy, the better prepared we will be.

I often call solar thermal the “Cinderella” of solar. She has been stuck in the basement – now we need to get her to the ball.