The speed with which solar PV has taken off has surprised most analysts, with the cost curve coming down far faster than expected as materials technology develops. When people think of solar PV, they tend to think about roof-top panels or large installations in fields or deserts, but imagine if you could integrate photovoltaic cells into ordinary windows…the many buildings that are unsuitable for rooftop panels would suddenly have the ability to generate their own electricity, with enormous potential for modern glass-and-steel office buildings.
Clearly (pun intented!), such a proposition would only be viable if the windows continued to fulfil their original function, ie being transparent / providing daylight (otherwise energy gains would be wasted on lighting, and few people want to live or work in windowless environments). So how real is this technology, and can we expect to be seeing it in practice any time soon?
What is transparent solar PV?
Traditional solar cells generate electricity by absorbing photons and converting them into electrons. By definition, this process is incompatible with transparency because the incoming photons are absorbed by the process. As a result, only low levels of transparency have been achieved with traditional solar PV technology, although even at these levels, they can still be suitable for use in building facades and curtain walls. Spanish company Onyx Solar, has a range of products with transparency levels between 10 and 30% and has installed PV curtain walls, windows, skylights, floors and even furniture in projects around the world, and has an impressive track record with some projects achieving very short payback periods of a year or less and impressive IRRs.
Germany company Heliatek has developed a PV thin film with up to 50% transparency which can be applied to glass building facades, as well as concrete, metal and other substrates. The company claims to hold the world record for cell efficiency for non-transparent organic solar cells, at 13.2%, and has installed its product at a number of sites in Germany, Italy, China, Singapore and Egypt, and is now developing its production processes.
Researchers at Michigan State University and MIT have been working on developing even higher levels of optical transparency in PV cells. In order to achieve a solar cell which is also transparent, the electricity must be generated from parts of the electromagnetic spectrum that are not visible to the human eye, so ultra-violet and infra-red wavelengths are absorbed by the cell, and converted into electricity, while visible wavelengths continue to be transmitted.
These solar cells are then applied onto glass as thin coatings, and produce power at low cost per watt, functioning even on overcast days and at low sun angles. The MIT team believes that a quarter of the energy needs of a typical skyscraper could be met through this means.
MIT-spin-out, Ubiquitous Energy, is also developing a near-transparent product, targeting mobile telephony and Internet of Things applications as well as use in buildings, with efficiency levels above 10%, which compares with traditional commercial solar PV which typically has efficiency levels of around 12%.
Another American company SolarWindow has developed a fully transparent PV glass which transmits the electricity generated through its thin polymer PV cells through connectors that are only 50 micrometers wide and therefore almost invisible to the naked eye. The company, which yet to put its products into production, claims that payback periods as short as 1 year would be possible with its system.
Elsewhere, research is continuing into alternative photovoltaic materials, including perovskite, a relatively inexpensive mineral composed of calcium titanate (a titanium-oxygen salt), originally discovered in Russia. Scientists at Nanyang Technological University in Singapore discovered that in addition to absorbing light, the material also emits light at different wavelengths (colours) giving it the potential for use in touch-screens.
Perovskite cells have been developed with an impressive 21% efficiency, but there are issues with stability that must be resolved before the technology can be commercialised. The outer shell of the panel, which conducts the electric charge, is made from organic compounds that quickly degrade outside clean-room conditions, reducing the cell’s life to mere months. However, scientists at the Swiss Federal Institute of Technology in Lausanne believe they have developed an alternative material which will remove this issue, opening the possibility for cheap, reliable and efficient perovskite panels to be developed.
Building the future
The market for Building Integrated Photovoltaics (“BIPV”) is forecast to grow from US$2.4 billion in 2014 to nearly US$6 billion by 2017 and almost US$23 billion by 2021. It is likely that the market will be worth substantially more as the various technologies develop…incorporating PV technology into the fabric of buildings will generate significant energy savings, and avoid the need for the bulky and unattractive solar panels that currently dominate the market.
Ambitious buildings like the iconic Dubai Frame which is clad in gold-coloured PV glass from Onyx Solar, highlight the potential of BIPV. The building consists of two 150 metre towers, each 9 3 metres wide, connected with a 100 square metre bridge. The structure is clad with 1,200m2 amorphous silicon photovoltaic glass with a transparency level of 20% and a peak installed power capacity of 38 kW.
Less iconic, but still innovative is the UK’s first solar-powered bus stop, just round the corner from me in London’s Canary Wharf (pictured above). The bus stop can generate up to 2,000 kWh of electricity per year and is used to power smart signs and other infrastructure on the Canary Wharf estate. Designed by UK solar company Polysolar, the technology has 6-12% conversion efficiency depending on the film’s level of transparency, and operates in low and ambient light.