Sunrise promises a brighter future

The UAE has huge potential to diversify its power production by harvesting solar power. The biggest challenge is how to do it more efficiently, and that corner may be about to be turned with the harnessing of the little-known mineral type known as perovskites, Robert Matthews writes

The Shams 1 solar power station near Madinat Zayed is able to produce only about 10 per cent of the power of a typical power station. New research suggests perovskites could hugely improve this outputShow more

It’s a cruel joke of cosmic proportions. Each morning the solution to one of UAE’s biggest challenges hoves into view – and each evening it disappears again.

In the quest to diversify the UAE’s sources of energy, sunlight is unbeatable. It’s safe, clean, and there’s plenty of it. The nation’s entire annual energy demand could be met by harvesting the solar energy that falls on its land area in a single day.

The problem, of course, lies in capturing and bottling that sunlight. Even the UAE’s world-beating 2.5-million-square-metre solar array at Shams, 120km from Abu Dhabi, produces only about one tenth the power of a typical power station.

It uses so-called concentrated solar power (CSP) technology, where long parabolic mirrors focus sunlight onto tubes containing liquid that drives turbines, producing electricity.

Plans to create a similar site that converts sunlight to electricity directly via silicon photovoltaic (PV) solar cells have, however, suffered years of delay.

But the proposed Nour-1 PV plant may yet benefit from the recent rash of solar energy breakthroughs centred on materials known as perovskites.

Until recently, few scientists had even heard of these dull-looking minerals, first uncovered in Russia’s Ural Mountains in 1839.

But appearances have proved astoundingly deceptive. It turns out that the way perovskites’ atoms are arranged gives them a host of bizarre properties.

Squeeze them and they’ll produce electric fields; chill them, and they become electrical superconductors. Swap the types of atoms around, and they’ll generate electricity from heat – or even act like lasers.

Now research teams are creating their own perovskites, with results that are transforming the prospects for solar energy.

Sunlight may be an abundant form of energy, but it’s also pretty diffuse, with each square metre of the Earth getting on average less than half a kilowatt.

The most direct way of turning sunlight into electricity is via PV cells. Long dismissed as hopelessly expensive, advances in production methods have brought costs down to a point where – in some markets, at least – they’re comparable with small-scale fossil fuel-based generation.

But at the scale of power plants, PV still can’t compete. For that, they need to be able to turn far more sunlight into electricity than the 20 per cent they currently manage.

And it seems perovskites could drastically improve that figure – at little extra cost.

Adding a perovskite layer to standard PV cells allows them to convert more energy at different wavelengths, making them more efficient.

Last month, a team at a British solar energy research company, Oxford Photovoltaics, announced that it had turbocharged the performance of existing silicon cells by a fifth – a leap that would normally take years.

Meanwhile, research teams are racing to make pure perovskite solar cells. These promise to be much easier – and thus cheaper – to mass-produce than silicon cells.

The focus of research is currently on driving up the energy conversion rate. When the first such cell was built by a Japanese team in 2009, it had an efficiency of barely 4 per cent. After three years, that more than doubled to 10 per cent. Earlier this year, it doubled again, to about 20 per cent. It took silicon PVs decades to reach this level of performance.

Yet no matter how the Sun’s energy is captured, all the techniques face the same problem: it gets dark at night. If solar energy is ever to become a serious player, some way must be found of storing its energy for night-time use.

New research now suggests perovskites may even hold the key to solving this conundrum.

The trick lies in a method of energy storage known as solar water splitting. Put simply, the energy from sunlight is used to tear apart the oxygen and hydrogen atoms that make up water molecules, with the hydrogen gas being stored as a source of energy.

This is akin to nature’s own approach to the problem of solar energy storage, better known as photosynthesis. By exploiting the properties of perovskites, however, the efficiency of the process can be pushed far higher than the 1 per cent achieved by plants.

Dr Jingshan Luo and his colleagues have now demonstrated the possibilities in experiments reported last month in the journal Science.

Separating the hydrogen from the oxygen in water requires about 1.8 volts of electricity – much higher than the voltage achievable by a conventional PV solar cell.

But use a solar cell based on perovskites, and their strange atomic structure turns solar energy into a decent fraction of the required voltage, making water-splitting viable.

According to the team, conversion rates of more than 12 per cent are already possible – exceptional for this energy storage method. Cheap, highly efficient solar cells that produce their own fuel for night-time use … it seems too good to be true.

Certainly there is work to be done before the technology is ready for prime time.

Researchers talk of achieving the “golden triangle” of low cost, high efficiency and stability.

The cost of perovskite-based solar cells is looking very promising, with figures below $0.20 per watt – less than half that of today’s best silicon cells – being mooted. Efficiencies are also expected to continue to rise.

But the third arm of the triangle – stability – remains a concern.

Ironically, the sheer speed with which perovskite-based devices have come on to the scene has made this the key issue. There just hasn’t been enough time on the clock to know how they’ll perform.

This could yet prove a major brake on the current rate of progress. For if perovskites are to reach their full potential, they must be able to cope with years of exposure to extremes of temperature.

Just last week, the International Energy Agency said solar energy could be the leading source of electricity by 2050.

If the perovskite revolution continues at its current pace, that day may arrive far sooner.

Robert Matthews is visiting reader in science at Aston University, Birmingham