Photovoltaics set to join building mainstream

It’s late afternoon on a gorgeous August day in London and, from the high vantage point of a roof terrace in Waterloo, the city is looking at its best. The golden cross atop the dome of St Paul’s Cathedral glitters in the distance and the sun sparkles off the glass panels of Waterloo Station’s sawtooth skyline. On this terrace, the sun shines off some neat rows of hedging, pebbly pathways and several sections of angled roof, all of which have different types of solar panels nestled among their tiles or slates.

‘This is partly a test rig and partly a sales suite,’ said Alan South, chief technology officer of Solarcentury, a UK supplier of building-integrated photovoltaics (BIPV). ‘We can show off our whole range of solar cells for different applications and types of building. Everything we do is building integrated, but the solar industry gets very tied up with what is and what isn’t BIPV; it isn’t at all well understood.’

The most accepted definition for BIPV is that the photovoltaic panels, rather than being simply bolted onto a building, replace some of the conventional building materials, forming an integral part of the structure of a building. The most obvious application is for roofing; photovoltaic cells can be built into panels that do the same job as conventional roof tiles while also generating electricity and, because the roof is exposed to more sunshine than any other part of the building, this makes sense.

Other applications for BIPV also exist, however, and new forms of photovoltaic cells are beginning to make them more practical. Solar cladding on the facades of buildings, particularly on the sunny south-facing walls, are now beginning to be seen, especially on modern commercial buildings with large wall areas.

Meanwhile, the newer thin-film photovoltaics may be about to make a splash in the BIPV market. Not as efficient as the current market-leading cells, which are made from crystalline silicon, thin films are nevertheless lighter and cheaper and can be made transparent; some industry observers believe they could kick-start a new chapter in the story of solar power.

It is a fast-growing sector, especially in Europe. Worth around €143m (£126m) in 2007, growth rates are accelerating in Germany, France and Italy as their governments set up subsidies for homeowners to install solar systems. With similar subsidies set to become available in the UK next year, the sector is expecting to see demand take off.

Solarcentury’s experience with BIPV is extensive; among its projects is the roof of a new and striking education centre at the Eden Project in Cornwall. South explained that, in order to offer BIPV, the company had to develop its own range of products. ‘We were forced into that, really, because the laboratory’s thinking of a solar wafer doesn’t work so well with the construction industry’s thinking of the materials you need to put up a building,’ he said.

However, South also considers that the concept of BIPV is a slippery one. ‘If you imagine a field of solar panels, a solar farm, that’s one thing,’ he said. ‘But if you imagine a large industrial installation, with many panels all tilted up at 10° to catch the maximum amount of sunlight, you might think that it was just a bolt-on. But a considerable amount of integration has to be done, in engineering terms; you have a massive aerodynamic load from all those panels and it had to be well integrated with the building’s structure, otherwise the panels will fly off or they’ll pull the building down.’

For Solarcentury’s installations, there are two further levels of integration, said South. ‘The second type is architectural integration, where the solar panels look more like part of the building, but still have to be installed by specialists; solar cladding is a good example of that,’ he explained. ‘And the third is what we call process integration, where the panels are not only an integral part of the building, but they can be installed by someone without specific qualifications or experience. Our solar roof tiles are an example of that; they can be installed by a roofer on the same battens as a standard tile and you just have to screw them down and connect two plugs.’

Integrating renewable generation into buildings is contentious. The government is calling for increasingly ambitious targets for renewables generation and for buildings to generate their own power; wind turbines are becoming more commonplace and can even be bought at DIY stores. However, there is some debate over how effective such measures are. Payback times for wind turbines, especially domestic ones, are long and many measures are dismissed as ‘greenwash’.

Solar, however, is different, at least in terms of its potential. Imperial College photovoltaics expert Ned Ekins-Daukes claims that the UK has sufficient roof space to generate a large amount of power. ‘The most efficient solar modules right now, which use crystalline silicon, have a 19 per cent energy conversion rate; that is, they convert 19 per cent of the solar energy that hits them into electricity,’ he said. ‘And if you covered every roof in the UK with those, you’d have 35GW of generating capacity, but the capacity doubles in the summer and it dives in winter because of the difference in day length and the intensity of the sun. We’ll get huge amounts of electricity in the summer, when we use less of it, and much less in the winter — and that brings us into the discussions of how we can manage demand, rather than just meeting it.’

For a household connected to the National Grid, BIPV could make a lot of sense, according to Ekins-Daukes. ‘If you assume an average of 40m2 of roof area per household and if that house doesn’t have any unusual energy demands, then if you install a 3-4kW peak BIPV system, they’ll break even over the year, selling electricity to the grid in the summer and buying it during the winter. But the crucial issue here is the cost of the electricity.’

South added: ‘The acid test is grid parity — if the energy you make is the same cost or cheaper than what you get off the grid and that depends a lot on the cost of the system.’ Countries such as Germany have ‘feed-in tariffs’, government grants that subsidise renewable power so that it attains grid parity. The UK will bring one in this year.

Ekins-Daukes said that this has been very successful in Germany. ‘The way the Germans see it is like it’s purchasing a government bond,’ he explained. ‘You make an investment, you get your money back in a guaranteed time and then you’re in profit. The banks have been very cooperative in providing the finance to buy and install the systems, so it’s a very safe investment. And if that happens here, then I think people will be very happy to generate their own electricity, especially in new-builds. But it depends very much on how the feed-in tariff is adopted and presented, whether the banks support it and what non-economic barriers there are, such as the availability of reliable, competent installers.’

So can developing technology help? Crystalline silicon is still the leading product, mainly because of its efficiency. Thin films tend to be less than 10 per cent efficient, with the highest efficiencies, from amorphous silicon cells and rare-earth metal mixtures, approaching 12 per cent. Indeed, a recent project to install BIPV on a model sustainable home at the UK’s Building Research Establishment specified crystalline-silicon roof panels for just this reason. Katherine Holden of Arup, which built the house, said: ‘The design was heavily influenced by the solar panels. We had to opt for a monopitched roof, with the entire roof south facing, and it had to overhang the building to get enough photovoltaics and a suitable output.’

However, other thin-film technologies are knocking on the door, or rather the window; organic-based photovoltaics, with efficiencies of up to eight per cent, are a possibility for surfaces that crystalline silicon cannot reach.

‘The research on crystalline silicon is concentrating on reducing cost rather than increasing efficiency,’ said South. ‘The physical limit — the highest efficiency possible — for a silicon solar cell is 29 per cent; the units we offer are 21 per cent. My strong belief is that we aren’t going to get better efficiency in my lifetime, but there are huge efforts going in to maximise the output per pound of capital investment, rather than the output per unit area.’

In part, this is because organic thin-film photovoltaics is inherently cheaper than silicon, which requires a large amount of energy to refine the element from sand. ‘Organic photovoltaics is going to win on cost — there’s no doubt about that,’ said Ekins-Daukes. ‘Its problem is that it’s lower efficiency and, therefore in the case of BIPV, not so architecturally useful as silicon.’

Massachusetts-based organic solar pioneer Konarka is trying to address this with a particularly low-cost, low-energy product that it calls Power Plastic. Capable of being made transparent and in large volumes, Power Plastic can make any surface a solar energy converter, even windows.

The technology, originally developed as a material to provide solar power for troops in the field, works by mimicking the structure of a semiconductor with organic materials. ‘We work with polymer organic photovoltaics, using a conducting polymer,’ explained Srini Balasubramian, co-founder and research and development director of Konarka. ‘Into this, we put donor and acceptor species,’ he said. ‘The donor would typically be something like polythiophene and the acceptor would be a fullerene-based material. We mix that into the polymer and coat them onto the base sheet in layers. When light hits these blends, we have charge transfer from donor to acceptor. We finish the device by putting blocking contacts on the interface, so electrons can go through one side and positive charges through the other.’

This approach allows the company to build its photovoltaics material by using a simple coating technique. ‘We’ve formulated these semiconductor materials as a fluid, so you can think of it as an ink, so we can use a printing technology,’ said business development director Stuart Spitzer. ‘The temperature is never higher than boiling water and, with a wide enough machine, you can make great quantities very quickly.’ Other organic photovoltaics techniques involve vacuum deposition or crystal growth, which require far more energy, time and expensive equipment, he claimed.

The conversion efficiency of Konarka’s material is 6.4 per cent, according to Balasubramian, although there is a trade-off between the thickness of the coatings and the amount of electricity that can be produced. This is an important point, because one of the most interesting applications of Power Plastic for BIPV applications is to make the coating thin enough to be transparent so that the material can be bonded to glass to make a solar window.

‘Because we’re using organic materials, we can tune the energy absorption spectrum of the polymers to get different colours; red, green and blue,’ said Balasubramian. ‘And we can also have different intensities and saturations of the colour.’

Spitzer added: ‘We’re planning to launch these products this year, selling to window manufacturers, and they would then supply solar-collecting window systems.’

As transparency is important, output will be limited and this means that the systems will be best suited to buildings with very large areas of glass: commercial and office buildings, in particular, according to Balasubramian. ‘My assumption would be that these would provide supplemental power, although, for a roofing installation, we can produce an opaque version that produces more power,’ he said.

However, Balasubramian’s ambition is to produce a totally transparent, yet still high-output, system. ‘It’s not possible yet, but we could conceivably fine-tune the polymers to absorb in the long-wavelength region and not in the visible region at all,’ he said. ‘It would look transparent, but it would still generate a lot of power from the solar light you can’t see.’

Ekins-Daukes sounds a note of caution, however; in terms of reducing the carbon emissions from power generation, solar can never be the full answer. ‘You have to keep it in perspective,’ he said. ‘The average family doesn’t actually use that much electricity, in comparison to the gas central-heating boiler, the fuel for their car, the energy that goes into producing their food or the fuel burned to get them on their holiday to Spain. We can provide the present electricity needs of domestic buildings and some commercial buildings through BIPV and that’s an important potential, but that’s just a fraction of our energy use. As with all energy questions at the moment, there is no single silver bullet.’


Sidebar: Fluorescence concentrators

While crystalline silicon and thin-film photovoltaics fight it out in the market, a new type of photovoltaics cell, currently under development, could prove to be particularly effective. Known as fluorescent solar concentrators, they use significantly less photovoltaic material, but can still generate useful amounts of electricity.

In a normal photovoltaics cell, the active material faces the sun, but in a fluorescent solar concentrator the sun-facing component is a polymer sheet about 5mm thick doped with a material that fluoresces when hit by sunlight. Around three quarters of this emitted light — which, due to the properties of fluorescence, is all of the same wavelength — cannot escape from the polymer sheet; instead, it bounces around in the same way a light beam bounces inside an optical fibre. When the light reaches the edge of the sheet, it is absorbed by thin strips of photovoltaics cells, the same width as the thickness of the sheet, mounted perpendicular to the sun-facing surface.

‘Photovoltaics cells are most efficient at certain wavelengths and, because we generate a common wavelength by fluorescence, we can tune that so we produce the optimum wavelength for the cells,’ said Amanda Chatten, a physicist at Imperial College who is working on fluorescence concentrators with researchers from the Netherlands, Germany, Switzerland and Ireland. ‘The fluorescent material can be a dye, which was originally used when this technology was first developed in the 1970s, but they tend to degrade when they’re exposed to ultraviolet. We’re now looking at quantum dots, which are much more stable, re-emit virtually all the light they absorb and can be tuned quite simply.’

Fluorescent concentrators are particularly suitable for building-integrated photovoltaics as they are both lighter and cheaper than conventional solar panels, she said. ‘The polymer sheet is essentially transparent, so you can use them as a window; the photovoltaics cells are part of the frame,’ said Chatten. ‘The sheet would be tinted and we could tune the colour.’

She added that these panels would be ideal for use in the UK and elsewhere in northern Europe as they concentrate diffused light on a cloudy day just as well as direct sunlight. The angle of the light on the panel makes no difference.