Solar windows: the future of zero-carbon buildings?

An Innovate UK-funded industry group is working on a new generation of transparent solar panel technology that matches the costs and performance of standard high-performance glazing while delivering clean, renewable energy to the buildings in which they are installed. Developer Polysolar is collaborating with chemical giant Merck and the Centre for Process Innovation (CPI), to promote a new age of zero-carbon buildings.


Polysolar windows

Buildings will no longer have to compromise on window transparency when installing solar panels. Polysolar, based in Cambridge, has a clear vision for the future of solar power, and has launched a replacement for traditional glass window panes that also produce clean, renewable energy.

Polysolar has been around since 2007, developing next-generation photovoltaics based on organic polymers. The company has been supplying solar panels to the market for about five years.

The printing process

Photovoltaic (PV) glass uses the same basic principle as solar panels that you see on roofs, but it is transparent. The technology used is known as thin-film, which simply means that the active PV layer is applied very thinly. Unlike conventional solar panels where silicon monocrystals are grown and sliced into wafers, thin-film technology vacuum-deposits a film onto a conducted glass layer.

Polysolar CEO Hamish Watson explains the process, which has been developed in collaboration with the German chemical group Merck.

"[Merck] is producing and developing our active material; a colourless absorbent layer which can be liquid-printed," he says. "So it's a printing process rather than the more traditional use of a big vacuum chamber.

"You print onto a piece of glass and you have an electrical grid printed on. You then put on different layers, but it's all printed on top of each other, to make your cell."

The other partner on the project is the Centre for Process Innovation (CPI), a UK government-funded research institute with an innovation centre in Sedgefield, County Durham that specialises in the field of scaling up technology for manufacturing.

The advantage of the printing element is that any size, shape and variation of the glass a customer wants can be produced.

Why PV glass trumps the competition

Watson says that there are many benefits to using PV glass instead of traditional panels, in addition to being able to see through it.
"It works at lower light levels, so it'll work down to less than 10% of sunlight," says Watson. "It doesn't change its efficiency depending on the amount of light it's getting."

This means that these panels can be placed where others wouldn't normally work, such as on vertical facades on any side of a building. Ambient and reflective light are all absorbed, so the position of the panels isn't as paramount as it is for original panels that have to soak up as much direct sunlight as possible.

Also, when one panel is shaded, the others are not affected, which can be the case when there are lots of solar panels together in a series string.

"Polysolar windows don't change efficiency with the amount of light they're getting."

"With our technology, you link in parallel to already high-voltage panels," Watson says. "That means that if one is shaded, the whole lot don't go down.

"Our [panels] are always working at their optimum performance across an array, rather than basically being at the worst performing level of any panel in a series string."

Conventional solar panels are less efficient the hotter they get, and thus have to be ventilated to keep them cool. Polysolar panels aren't affected by temperature because they can be insulated directly, and therefore can be added into double glazing units.

PV glass also has the advantage of reducing both the thermal gain into a building and its heat losses.

"Our panels, the double glazed unit, are the equivalent in terms of their performance to a triple glazed unit," says Watson. "It does away with having to put on things like brise soleil and that sort of thing to shade buildings."

Trying to keep buildings cool is expensive, so assisting this will probably have an impact on the amount of money companies spend on air conditioning and shading elements.

"It's actually also taking some of the heat out and turning it into electricity, so some of the infrared light is converted into electricity," explains Watson. "One of the other key elements is that the conductor layers that we use to make up the cell are exactly the same as what you use in low-e glass, so you put on a transparent conductor outside layer to reflect the heat back."

Polysolar windows

Is PV glass a realistic alternative?

Watson explains that the average office building power requirements are pretty huge, due to a high dependence on computers and cooling systems.

"Having said that, yes you could make a building zero-carbon by covering it in photovoltaics," he adds.

Each panel produces 60 to 70 watts per square metre, so buildings with large surface areas, such as The Shard in London, will produce significantly more power than those with small surface areas. The amount of power produced will vary greatly depending on the building shape and position, but Watson says that significant levels have been reached in most case studies.

For example, Polysolar put up a building at the Building Research Establishment in Watford, which has a three-storey glass atrium that produces all of its power requirements.

"Equally, some of the other buildings we've got produce between about a third and a half of the power requirements," Watson says.
In terms of affordability, Watson says that Polysolar panels are more expensive than conventional glazing, but it's only a marginal additional cost, and the team is working on making the panels more efficient and cost-effective all the time.

"If you're putting up a curtain wall on a building, you're looking at between £700 and £1000 per square metre," he says. "The PV side is likely to cost £150 to £200 per square metre, so it's only a reasonably small percentage."

With the associated energy savings, Watson says that generally users can expect to see payback within eight to ten years of the installation.

Improving efficiency and future proofing

The latest Polysolar panels are entirely transparent, whereas previous versions had an orange tint. This improvement makes them considerably more attractive, and also means that there is flexibility with regards to making them in varying transparencies, colours and sizes.

Watson says that the technology is progressing, and the company is working on the efficiency and appeal of the glass.
"The more efficient you can make them the better," he says. "Clearly if you're absorbing all the light to turn it into power, there is a limit to how far you can go on the efficiency front.

"So our main drive at the moment is aesthetics, to make them as close and as similar and easier to install and as cheap as conventional glass."

"The main drive at the moment to make them as easy to install and as cheap as conventional glass."

He says that if Polysolar glass ultimately costs the same as a regular pane, there would be no reason not to use it for future developments, especially if energy costs are reduced in the long term.

Watson says that this technology has a role to play in the trend towards zero-carbon buildings, especially if the installation is just as simple and accessible as regular materials.

"There's been spluttering starts and stops in terms of building regulations dictating zero-carbon buildings and the move towards zero-carbon buildings," he says. "But generally speaking, that is the way the whole world is heading."

Despite the fact that the UK government is not being particularly proactive in terms of making buildings zero-carbon, Watson believes that the movement will only accelerate.

"The building trade itself is taking the lead because they're trying to future-proof buildings," he says. "It's an established process in getting there. The European regulation comes in in a few years, which will dictate it."