Is a new age about to dawn for solar-powered electricity generated from the walls of skyscrapers and domestic houses? This is, in theory, a vast yet still little-tapped source of renewable energy. Adding enormously to the wattage created by roof-top solar arrays is an enticing business proposition.

But what of the science and the economics for building-integrated photovoltaic projects?  Ultra-thin film solar cells appear to be the future – they’re cheaper to make and require less photovoltaic material than crystalline silicon. A PV material such as vaporized silicon is condensed on to a substrate  – a semiconductor crystal used as base made from metal, plastic, or glass. Once deposited on something flexible, it becomes ideal for building-integrated photovoltaic use, or BIPV. 

Yet the technology hasn’t always been problem-free. Using materials such as amorphous non-crystalline silicon can cause resistance to electrical flow, making the cells less efficient. Plus, there’s the durability factor to consider. Building-integrated systems don’t have natural ventilation, unlike those on rooftops. To that tend, they tend to get hotter – and thin-film solar cells can be more prone to heat-related degradation.

Growth track

Things have come a long way since the 1990s, when prototype BIPV systems first appeared. Thin-film solar cells are accelerating the rate of change. A global analysis carried out last year by Transparency Market Research (TMR) reported that 343MW of BIPV capacity was added worldwide in 2012.¹ TMR has forecast a compound annual growth rate of 18.7% until 2019. Further, technological advances will enable the industry to reach 1.15 GW of installed global capacity, research shows. 

Europe  –  with its strict environmental laws — accounted for 41% of installations in 2012, with North America representing 27%.  In 2012, rooftop BIPV applications took 67% of the market. However, ‘curtain wall’ technology is gaining ground, and TMR expects this to take off over the next five years. One industry forecast is that the thin film segment will grow 19% by 2019.

In America, state-backing for solar research comes from US Department of Energy (DoE) Sunshot Initiative, set up in 2011. In December 2013, it invested $13 million in five projects aimed at improving industry competitiveness. Yet solar remains an industry seeking ideas – that much became clear in May, when industry professionals assembled in Anaheim, California, for SunShot's four-day summit. They were challenged to make sun-derived energy competitive in the US by 2020.

Technology trends

But what will work best?  Worldwide research is taking different directions, with the common goal of developing better technology. For instance, in Scotland, scientists at Glasgow Caledonian University are working out how low concentration PV (LCPV) systems  –  more easily made and cheaper to maintain than high concentration ones — can be designed more efficiently.

LCPV has plenty going for it, proponents say. For one, the systems don’t require tracking or cooling mechanisms. They can also capture a large part of the diffuse solar radiation , which better suits the climate of northern Europe. But the downside is they suffer losses from non-uniform illumination of their solar cells.  This means that to accommodate both the direct and diffuse components of the solar spectrum, their design must be tailored to each location.

Glasgow Caledonian has developed a 3-D optical concentrator for use in a non-tracking, wall-mounted, building-integrated system. Carefully selecting the concentrator’s field of view means that radiation can be “captured” throughout the year. Light is focused on an array of PV cells integrated in the window pane. This allows light transmission in spaces between each cell. The inventors say that power output is high because of the concentrator’s wide angle – technology ideally suited to windows, skylights or roof cladding.

BIPV has long faced the twin challenges of aesthetics and costs, along with the willingness of architects to use it. One company that thinks it may have stumbled upon the “holy grail” is US-based New Energy Technologies Ltd (NET), whose work on glass and flexible plastics has resulted in 42 US and international patent filings. 

NET’s trademarked SolarWindow see-through, electricity-generating coating technology has been evolving since 2009. The firm claims its material is less than a tenth the thickness of other thin films. Sprayed on to see-through glass at room temperature, it does not require expensive high-temperature or high-vacuum production methods, NET states.

NET is making other big claims, stating in its literature and scientific reports that SolarWindow modules outperform comparable devices by 53%, in terms of power production. Moreover, says NET, its claim that the technology produces electricity effectively is backed by a peer-reviewed study in the American Institute of Physics’ Journal of Renewable and Sustainable Energy. In March, the company announced that its six inch square SolarWindowT was being developed ‘for eventual commercial deployment in…80 million detached homes in America and more than five million commercial buildings’.

A model for estimating the energy and environmental benefits has, NET notes, been verified by the US Department of Energy’s National Renewable Energy Laboratory. NET claims that when installed on all four sides of a 50-storey building in Phoenix, Arizona, the windows could generate enough power for130 homes each year – whereas today’s rooftop systems could only produce enough for 3 to 11, the company’s literature stated.  

NET insists its product can be used on all four sides of a building — and in shade. But how efficiently would it work for skyscrapers surrounded by buildings that are not as tall? What would be the benefit of using NET’s technology on lower floors? Further, would the windows be cost-effective in shaded areas?

There’s no clear answer yet, as NET’s CEO John Conklin admits. “The company is evaluating SolarWindow’s... technology performance under different light conditions, and when exposed to shaded and diffused light conditions,” Conklin stated.

What’s the cost of covering skyscrapers with its windows at the current price of NET’s technology – and how long before such an investment is viable? Conklin knows a lot of work needs to be done before the company can start making big sales. “We must ensure the technology makes economic sense to our customers,” he explained. “It must address a need as well as use low cost, high-speed and high-volume manufacturing methods and processes.”

With development work still underway, Conklin offers no clue as to when the technology might become commercially available. That also depends on raising more capital and working with strategic partners, he explained. NET has been talking to major window and glass manufacturers; and is seeking technology and product licensing arrangements with research institutions and commercial partners.

Trust, but verify

Some industry observers are uneasy about some of the comparisons NET is making. One such individual is Scott Kelly, a specialist in green building, a LEED (Leadership in Energy & Environmental Design) fellow, and co-founder of Philadelphia-based Vision Architecture. “We feel some of the statistics are somewhat irrelevant,” he told Renewable Energy Focus magazine, citing the way NET has modelled their information on one idea.”

As Kelly explained: “The surface area of a 50-storey skyscraper has around 400 times its roof area – just 20,000 sq feet, of which around half is available for PV/ You aren’t comparing apples with apples.” 

Kelly says it’s important to understand the financial angle. With any new product, he says, you have to look at the energy it produces and offset the energy needed to make it. “We need the data for a standard array output, for example 2x5, or 3x6 panels on the roof – one roof panel versus the same amount of footage on the wall,” he explained. “You need to get the costs [of both] and multiply the difference of output.” 

There's another a major issue to consider, according to Kelly —  the toxicity in the material used in the construction process. (Note: NET says uses organic materials which are dissolved into liquid). The big challenge for building designers is, he adds, is to be smart. “We have to make sure that BIPV is the best… strategy and not just part of the great green boom,” he explained.  In other words, even if it’s flavour of the moment, that’s no reason to embrace it uncritically.

Kelly’s collaborative approach to sustainable design isn’t always in synch with fierce competition that characterises the West’s way of doing business. Moreover, Kelly, a huge support of BIPV, has a clear view about the direction he feels BIPV should take.

“Each technology has to be specific; no one size fits all,” he says. “Manufacturers shouldn’t have one solution, but many. You need the technology depth for five or ten systems, and then you can make the right design. I hope some companies making one or two designs will come together as a business solution, and have a broader offer. We need to put aside our competitive nature.”

REFERENCES

1. Building Integrated Photovoltaics (BIPV) Market: Global Industry Analysis, Size, Share, Growth, Trends and Forecast, 2013 - 2019.