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EU team demonstrates 'concentrated' photovoltaics

14 May 2015

EU researchers have demonstrated a new type of solar electricity generator which is twice as efficient as conventional photovoltaic panels.

One of the North African CPV installations (photo courtesy of NACIR)

Solar panels use solid-state photovoltaic (PV) cells to convert sunlight directly into electricity. But despite rapid improvement in recent years, the cells capture and convert less than a quarter of the light falling on them.

A newer type of multi-junction cell can capture a broader range of wavelengths, which makes it more efficient at converting sunlight into electricity. Efficiencies now exceed 40 percent, almost double that of conventional PV cells.

“That means we have the opportunity of generating the same amount of energy with half the collecting area used by conventional modules,” says Gabriel Sala of the Institute of Solar Energy at the Universidad Politécnica de Madrid. Sala was coordinator of the EU-funded NACIR project, which spent four years testing practical installations with the new cells.

However, multi-junction cells are very expensive and even with doubled efficiency they cannot yet compete with conventional PV technology. The answer is to use concentrating optics – lenses or mirrors – to gather sunlight from a wide area and then funnel it onto a relatively small cell only a few square millimetres in surface area. Very large concentrations are possible, more than compensating for the expense of the cell.

The drawback is that modules with concentrating optics have to be steered to follow the sun across the sky – the higher the concentration, the more accurate the tracking must be. They can also get very hot.

All this makes concentrating photovoltaic (CPV) modules relatively complicated. One objective of NACIR was to run two demonstration plants in North Africa to show that the technology was practical in real-world conditions.

One in Morocco, in the grounds of Al Akhawayn University in Ifrane, was a 30kW installation connected to the electricity grid. The other, a 40kW water pumping and desalination station at Wadi El Natrun, Egypt, pumps water for irrigation and is still operating two years after the project ended.

The partners also developed improved concentrating optics that require less precise tracking, including a novel combination of a mirror with a liquid lens.

Push for greater efficiency
CPV is ideal for large-scale power generation. But despite the promise it has not yet attracted the big investment it needs to make a real difference, with investors preferring to play safe with the cheaper and proven PV technologies.

Worldwide, CPV now has an installed base of 330MW connected to the grid – the biggest stations are 80MW and 58MW plants in China – but conventional PV still has several hundred times that capacity and is growing fast.

In this difficult climate, one of the NACIR industrial partners, Isofoton, ceased operating in this business area last year. But the other, Soitec, is now the leading European manufacturer of CPV systems and has gone on to install a 44MW plant in South Africa.

The priority now, says Sala, is to push for further improvements in efficiency. “Efficiency is key for reducing the costs,” he says. “With a more efficient cell you reduce the cost of everything because you have fewer square metres of glass and metal. The best way of reducing the cost is reducing the amount of materials.”

Sala’s group and other partners have proposed a new project within the EU's Horizon 2020 programme to further improve efficiency with new four-junction cells and cheaper concentrating optics.

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