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Hybrid waveguides for highly efficient planar solar concentrators
As the world’s need for energy increases every year, solar power represents one of the greatest sources of clean energy available. As noted by Dr Gerhard Knies while he pondered alternative sources of clean energy in the wake of the Chernobyl disaster, However, the challenge remains of how to harness this energy efficiently and cheaply.
The two preeminent technologies used to convert sunlight into electrical power are photovoltaics (PV) and concentrated solar power (CSP). The concept behind a CSP plant is relatively simple – light is collected with thousands of mirrors (which are mechanically moved to track the position of the sun) and focused into one place where solar energy is converted into thermal energy. This energy is then converted rst into mechanical energy and then into electricity using a heat engine and an electrical power generator. The efficiency of state-of-the-art CSP plants is fairly high — about 30 per cent — but the maintenance costs and difficulties of working in the desert environment mean that they are often too expensive.
Photovoltaic solar cells use a different approach, directly converting solar power into electrical power. Their conversion efficiencies are comparable to that of CSPs plants, although they are more expensive toproduce than mirrors and so their cost per area is larger. However, significantly, their maintenance costs are much lower than what is required for CSP plants. Reducing the production cost of solar cells would thus make them the more attractive option.
Efforts have been made to increase the efficiency of photovoltaic solar cells using solar concentrators that take sunlight from a wide area and bunch it together into a specific, smaller location – similar in concept to CSP, but with no expensive moving parts. This combination of concentrating sunlight onto solar cells has the potential to push the efficiencies of solar power beyond what has been achieved before. “The concept can revolutionise photovoltaics,” says Professor Patrick Görrn of the University of Wuppertal. “Concentrated sunlight can be efficiently divided up into different wavelengths. This would allow for effective bandgap matching, in which different wavelengths of the sunlight are guided to solar cells optimised for that wavelength. The theoretical efficiency limit of such a device would be increased from 34 per cent to 86 per cent.”
To make these devices a reality, planar collectors that both efficiently collect the light hitting them and transport it over large planar distance are needed. However, efforts to achieve this have revealed new challenges. “Essentially, there is a trade- off between efficient collection of sunlight and guiding it over large distances with low optical loss,” says Görrn. “At present, the most studied approach to getting around this has been to use luminescent solar concentrators (LSCs). However, work on these has been going on for 30 years and they are still a long way off being practically useful.”
Görrn is now suggesting an entirely new approach to solar concentration. “Our idea uses a planar concentrator which has a new type of planar waveguide.” says Görrn. “Ideally, our device will take the form of a black plastic foil that that concentrates the solar energy to a point at which the resulting immense light intensity can be used for efficient small- area solar cells or solar-to-fuel reactions. We hope to eventually be able to produce this foil for under €1/m2. Unlike CSP, the device will work with light shining at any angle and so will remove the need for expensive moving parts.”
The proposed device could theoretically concentrate incident solar power to a concentration factor of more than 104. “This level of light concentration would be a game-changer,” says Görrn. “These devices could be created cost efficiently and used in households for passive lighting and even passive heat transfer.”
Unlike LSCs, which use energetic separation to get around the trade-off between efficiently collecting sunlight and efficiently guiding it, Görrn’s idea uses spatial separation. The planarity ( flatness) of the concentrator is broken using structures that are separated from the guided modes, thus allowing for both efficient concentration and propagation. Waveguides have discreet modes, some of which have nodes that exhibit “blindspots” for the light inside the wave guide.When scattering bodies at the point of the node excite a wave laterally, the wave doesn’t “see” the scattering bodies and so propagation is not affected.
The project, which began in March 2015, has seen exciting developments in recent months. “We started with collection efficiencies of 10-8 (meaning 10-6 per cent of the light is excited laterally). We have now reached 10 per cent, propagating over a distance of 8mm. These are good signs that what we thought we could do is achievable.”
The film used to excite the waves laterally is made up of silver nanoparticles, which have the strongest interaction with light of any particle known today, due to plasmonic interactions. “This film — which appears to the eye as being extremely black — is then placed in the middle of the waveguide at the point of the node. The film is special because it is able to excite efficiently but is also thin enough to fit within the node.”
Part of the work has been to find the optimal material to make the waveguides (which should be as transparent as possible). This has produced some unexpected and welcome findings. “Our original plan was to use dielectrics produced by atomic layer deposition – an expensive and lengthy process. In fact,what we have found is that one of the best materials for making the waveguides is actually a commercially produced polymer. So not only have we managed to make the device more efficient, we have also managed to make it more cost efficient.”
Research now continues with trying to optimise the excitation efficiency, which is what Görrn believes will be the crux of their work. “Once we can show that we are quickly achieving high excitation and propagation, then we can start to get excited that we have found something that can really change the world of solar power.”
The PDF version of this article is availble here.
Published: Wednesday, 5th April 2017
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