The photovoltaic revolution

22 October 2007 (Last Updated October 22nd, 2007 18:30)

It is no surprise that electricity from sunlight holds great appeal for residential and commercial designers. As the desire for photovoltaic technology grows, Jim Banks reports on how surprisingly few architects are working together with the industry.

The photovoltaic revolution

Solar technologies are becoming increasingly popular, both as a feature of the design or improvement of domestic residences, or for larger applications in commercial and office buildings. This is no surprise, given the current urgency around climate change, especially in the UK.

LOWERING CARBON EMISSIONS

The UK government has made carbon emissions reduction a flagship policy but the country lags behind Europe in implementation, despite growing demand. Monthly Low-Carbon Buildings Programme (LCBP) grants of above £250,000 are massively oversubscribed.

In 2005, PV capacity in Germany had reached 1,429MW peak, the UK's 10.9MW peak equalling less than 1% of that. A 1KWh peak PV system generates around 800KWh of power each year, while energy-efficient homes consume around 1,960KWh.

According to PV system vendor Solar Century representative Charlotte Webster, this appetite for PV is due largely to the 'feed-in tariff' (FIT), which rewards renewable power generation with payments of four times the market price over 20 years. FIT makes PV more cost-effective, and has been implemented in nearly 50 countries.

"There is the Low Carbon Building Programme, which is stimulating demand to some extent, but the UK would benefit from larger incentives, like the feed-in tariff as seen across Europe," Webster says.

"However, local planning reforms are affecting the on-site use of micro-renewables by large developers. The momentum is coming from local government."

THE POWER OF PV

Tariff or no tariff, solar solutions are popular, but there are those more favourable than others. Active solar panels heat water, while photovoltaic (PV) cells produce electricity. PV systems are seen to be among the most promising green technologies and are fast building a big fan base.

PV is 'fit and forget' technology, requiring little maintenance, consuming no natural resources and generating no harmful emissions. With no moving parts it operates silently and is resilient in outdoor applications.

The principles of PV technology remain the same but efficiency – the ratio of light absorbed to power generated – improves steadily. PV cells absorb photons, which stimulate a flow of electrons to create direct current (DC) which is converted into the required alternating current (AC).

"Active solar panels heat water, while photovoltaic (PV) cells produce electricity."

Buildings use power from PV cells first, pulling extra power from the grid if required. If PV generation exceeds the building's demand it can become a net producer of energy by exporting power to the grid.

Various PV technologies exist to suit different applications. Monocrystalline PV cells, which use a single crystal of silicon, are highly efficient and suited to small surface areas such as domestic roofs.

They require about 7m² of panels per KW peak. Polycrystalline or multicrystalline cells offer slightly lower yield and suit larger areas of external cladding.

Cells from German PV manufacturer Kyocera – the first to commercialise multicrystalline technology – now deliver about 18.5% efficiency, thanks to optimised grid-line configuration and texturing of the cell surface to reduce reflectivity.

Meanwhile, amorphous systems will require more space – 15m² to 20m² per KW peak – but these absorb a greater proportion of the light spectrum. This technology suits overcast skies and can be deployed as a fine layer on glass, stainless steel or plastics making it versatile for large roof area applications. Hybrid systems combining monocrystalline and amorphous technology deliver the highest yield.

Surprisingly, the cost is roughly the same for each system. Manufacturers have had to balance efficiency gains against the cost and scalability of production according to German PV manufacturer Q-Cells AG spokesperson Stefan Dietrich.

"Three years ago we produced polycrystalline cells with an efficiency of little more than 14%. Now we are in between 15% and 16% with the best cells up to nearly 17%. We see a short to medium-term potential of 18% for polycrystalline and 21% for monocrystalline on an industrial scale. Thin-film technology is just beginning but you can expect something from 8% to 12%," Dietrich says.

THE ARCHITECT'S ROLE

Cost is key to architects' and clients' photovoltaic technology choices. Desired generation capacity and emissions reductions must be weighed against clients' budgets. PV works with other energy-saving measures such as use of natural daylight, insulation, reduced cooling load and passive ventilation, which must all be accounted for.

Architects can, however, improve the productivity of PV panels. For instance, in the UK, south-facing panels deployed at 30° tilt and panels collecting reflected light from water features have higher yield. PV panels also replace materials in a building's façade.

"PV is 'fit and forget' technology, requiring little maintenance and consuming no natural resources."

These factors improve the cost profile of PV, which can be further enhanced if it is considered in the context of building-integrated photovoltaics (BIPV). This rapidly spreading concept promotes the incorporation of PV in the design and construction phases to make modules in the façade and roof integral to the building.

Gerber Architekten's proposal for Dubai's Burj al-Taqa – the Energy Tower – uses PV as a vital design element. It is will produce all of its own power through a large wind turbine and solar PV arrays, some of which will form a floating island in the sea.

PV technology can also be fitted to existing buildings, as with Manchester's CIS tower – Europe's largest vertical solar array. The three sides of its 25-storey service tower each house 7,244 solar PV panels. Along with roof-mounted wind turbines these produce 10% of the building's power.

SUBTLE PHOTOVOLTAICS

The CIS tower's arrays have a strong aesthetic impact, but PV can be subtler. At the Print House in Dalston, London, Solar Century's panels are barely visible from ground-level, yet represent one of the largest and most efficient installations of its kind in the UK.

Aesthetic considerations and cost reduction are likely to define the development of PV technology in the foreseeable future according to Solar Century's Webster.

"The technology will certainly change in the next five to ten years, but the efficiency will not necessarily improve. 20% efficiency is about as good as it gets at the moment. Thinner film technologies may reduce the cost of PV cell production, but may require larger areas for installation. The aesthetic qualities are important, particularly in the UK, and these are improving, which is good news for architects," Webster says.

At the cutting edge, however, efficiency is still improving. Spectrolab, part of Boeing, recently revealed a cell twice as efficient as typical rooftop solar panels with over 40% efficiency.

Most manufacturers, however, recognise that cost-effectiveness is the most important factor in encouraging greater use of PV in new buildings, though some feel that architects need to understand the technology's aesthetic properties, as panels are much more versatile and varied in their appearance.

"PV technology works with other energy-saving measures such as use of natural daylight, insulation."

"PV will become cheaper, which is probably the most important point. Efficiency will rise, and thin film technology will find its place in the market, mainly in PV power plants and BIPV. Price decreases, first of all, then a good legislative framework will encourage greater use of PV," notes Dietrich of Q-Cells.

"Better ways of integrating PV into buildings and a more aesthetic approach are needed in order to persuade architects of the benefits of PV. Thin film could play a major role, especially in large projects," he adds.

PV IN THE FUTURE

Even if clients can't afford to invest heavily in PV now, architects can still prepare buildings for installation later. Creating room for solar panels on the roof, for instance, enables a building to more easily accommodate PV systems later on. Architects can also help clients understand the additional value of PV.

"The upfront cost may seem expensive, but it requires very little maintenance and reduces demand for grid electricity, so it has more value than may initially be understood," believes Webster.

The key for architects, however, will be to get solar PV experts involved in new projects as early as possible to ensure the technology and the building design are symbiotic.