Can we improve solar generation by opening up the light spectrum?

The recent installation of photovoltaic solar has far outstripped the growth of previous years, so says a new report published by REN21, global renewable energy policy network.

75 GW of solar was installed in 2016, outstripping 2015 installations by almost 25 GW. This is thanks, in part, to the mass production and shipping of solar panels, which has made the technology exponentially cheaper than in preceding years.

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This has created a positive feedback loop, where falling prices have driven up installation, helping to establish PV solar as a popular renewable energy source.

However, despite the increased popularity of solar, the underlying technology has not changed too much.

Solar panels have become significantly more efficient than their earliest incarnations, but they still rely on a principle called the “photovoltaic effect”.

This phenomenon was first observed in 1839 by a scientist called Alexandre Becquerel, who found that certain materials generated electric current when exposed to sunlight. Almost 200 years after the discovery of the photovoltaic effect, the technology is producing 1,327 TWh per year thanks to the development and proliferation of photovoltaic solar.

The first modern solar panel was created in 1954, and since then the technology has come on in leaps and bounds. Solar panels have had plenty of applications, from generating clean energy on earth to providing back-up power for made-made satellites in space.

 

What is the photovoltaic effect?

The photovoltaic effect takes place at the atomic level. Photons – light particles – hit a suitable material (these are called semiconductors; only particular materials such as silicon fit the bill). The electrons within that material then become excited by the light, and begin to move. This movement is the generation of electric current.

Currently, PV solar reaches average efficiencies of around 24%. Under lab conditions, some solar panels have recorded efficiencies as high as 46%, but lab conditions are often preferable to real-life conditions.

 

How can we improve solar efficiency?

However, conventional solar panels only make use of visible light, which makes up only a small section of the electromagnetic spectrum. The spectrum is made up of different types of radiation.

There are several bands of radiation in the electromagnetic spectrum, all of which move at different frequencies (or speed, measured in hertz). At one end, these are radio waves, and at the other, gamma rays.

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Visible light sits in the middle, and either side of it are ultraviolet and infrared. Visible light, as suggested by its name, is the narrow band of radiation that we can see.

Rainbows (and the cover of Pink Floyd’s Dark Side of the Moon) represent the wavelengths of light we can see, from red to violet, but there are other bands across the spectrum, all of which move at different frequencies.

Visible light sits in the middle, with infrared and ultraviolet radiation sit either side, with radio and microwaves below red and x-ray above violet (in this context, “infra” means below and “ultra” means beyond).

The energy carried in visible light accounts for less than half of the energy carried by electromagnetic radiation; as such, there is more to capture by focusing on the bands which sit outside of visible. These bands – ultraviolet and infrared, particularly, are of interest to the engineers who are developing the next generation of solar panels.

 

What techniques are being explored to improve solar generation potential?

There is a theoretical limit to how much electromagnetic radiation can be converted; the Shockley-Queisser limit dictates that traditional solar panels could convert a maximum of 33.7% of sunlight. But this could rise to 48% if other radiation from across the spectrum could be converted.

By trying to capture more of the light available in spectrum, the hope is that solar panels of the future can become much more efficient at producing electricity. There is already research underway to improve the efficiency of new solar panels.

Concentrator photovoltaics are one avenue being explored, which use several layers of silicon cells (compared to traditional PV, which uses a single layer). Concentrated sunlight is then focused on the panels, which are capable of processing several bands of the spectrum. These are already close to reaching efficiencies of almost 50%, and could represent a serious step forward.

Another developer is using new materials to improve efficiency, producing next generation panels with perovskite instead of silicon because it is a more efficient material. Another project is aiming to capture infrared radiation – specifically, infrared which is reemitted by the earth’s surface at night. If successful, this technology could help “solar” to become more attuned to daily energy demand cycles.

The cutting edge is always an exciting place to be, and it’s no different with regards to solar power.

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