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Sustainable futures

It’s all solar, really

Almost all of our energy supply (with the exception of nuclear) is solar power in one form or another. Coal and oil are little more than sunlight stored in carbon captured from a prehistoric era that we can no longer afford to release during our own time. Natural hydro is solar energy stored in rainwater deposited at high ground on a timescale of months or years. Wind turbines use solar energy stored in the movement of warm air around the globe on a timescale of days.

However, photovoltaics take the energy of sunlight and use it to directly excite electrons to higher energy (remember this) to flow through a device – almost instantaneously. Photovoltaics operate in a truly zero-carbon way, but as with all other energy technologies, we had to make them in the first place. Most solar panels that you can buy today are made from silicon, the second most abundant element in the Earth’s crust (after oxygen).

But it’s not easy to melt - and melt it you must if you want to grow crystalline silicon – at just over 1400°C, that’s 290 kWh/kg of physics that you simply can’t get around regardless of what else you do in production. Nevertheless, even considering other steps in the process, you’re still looking at less than two years of energy payback.

Considering silicon solar panels can produce electricity for over 50 years, that’s an incredible investment from a climate change perspective, even when it’s not an attractive financial one. But, remember, all a solar cell does is excite an electron from a low energy level to a higher one and allow it to go off and do something useful. That’s not that difficult to do and provides a huge space for innovation and new ideas. Some materials do this much better than silicon but might be more difficult to make, and there are molecular systems that are not as efficient but have such low embedded energy that their payback time is measured in days rather than years.

Take dye-sensitized solar cells for example. These devices take a wide bandgap semiconductor such as titanium dioxide (white paint pigment) and sensitize it to visible light with a dye molecule. Not only can this type of cell reach efficiencies around half that of the best silicon cells, but they’re also translucent – think stained-glass window that produces electricity while providing decorative coloured shade to your home or office.

There are more types of photovoltaic solar cells than most people realise. There are at least 20 besides silicon, with new technologies emerging frequently over the last decade, such as perovskites and their tandem variations and continued advances in multi-junction compound semiconductors.

The University of Nottingham’s Energy Institute (UoNEI) brings together researchers working on all aspects of solar energy, from new chemistries for organic solar cells with energy payback times on the order of hours, to understanding the integration of photovoltaics in the UK and global communities. There is of course a downside to the instantaneous nature of solar power – what happens when the sun goes down? This is why researchers at UoNEI are also investigating new battery chemistries to conveniently store energy in the short term and thermal solutions for long-term energy storage.

But perhaps we don’t always want to convert sunlight into electricity at all. Sometimes we need fuel, so researchers at UoNEI are also investigating ways to use solar cells to directly split water to produce hydrogen that can be used for transport, heat, or power when needed.

With so many innovative solar cell technologies, why are we not embracing these and applying them wherever we have the opportunity? Why for example, is that surface over there, bathed in light, not photovoltaic?

"Current research building on decades of solar innovation and energy storage is making it possible to think seriously about integrating solar power generation into the stuff that surrounds us"
Dr James O’Shea

Sometimes the argument is an aesthetic one, but more often a financial one. Different types of solar cell will suit different applications. Silicon lends itself particularly well to solar farms and roof-top installations. Translucent dye-sensitised and perovskite cells can be embedded into the windows of buildings and vehicles, while compound semiconductors provide power for satellites and high-tech equipment. Buildings, vehicles and devices with integrated solar power reduce the demand on external sources.

When was the last time you had to change the battery in your calculator? I’m guessing never. I have a small solar array on the roof of my vehicle, which coupled with a lithium ion battery for storage keeps a fridge and auxiliary devices running without ever connecting to the engine or grid. One day, hydrogen filling stations might split water directly using photons from the sun.

Current research building on decades of solar innovation and energy storage is making it possible to think seriously about integrating solar power generation into the stuff that surrounds us.


Dr James O’Shea

Dr James O’Shea is an Associate Professor and Reader in Physics in the School of Physics and Astronomy and a director of the university’s Energy Institute

Further reading

Scanning photocurrent microscopy of 3D printed light trapping structures in dye-sensitized solar cells

A Knott, O Makarovskiy, J O’Shea, Y Wu, C Tuck

Solar Energy Materials and Solar Cells 180, 103-109

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