FULLSPECTRUM Catches all the sun’s rays

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FULLSPECTRUM
Solar cells with the highest energy efficiency in Europe have been developed by the scientists of the FULLSPECTRUM (‘A new PV wave making more efficient use of the solar spectrum’) project. The cells are now being tested on an industrial scale in large pilot plants, paving the way for cheaper solar power in Europe and elsewhere.

FULLSPECTRUM brings together 19 partners from academia and industry from 7 EU Member States, as well as Russia and Switzerland. The project’s objective is to develop solar cells which can convert all of the sun’s rays into clean electricity.

Clean power for a clean planet

The burning of fossil fuels pumps large quantities of carbon dioxide into the atmosphere, contributing to the greenhouse effect. At the same time, rising energy prices and concerns about security of supply mean EU countries are increasingly reluctant to rely on imported energy sources.

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With this in mind, the EU has set itself the goal of producing 20 per cent of its energy from renewable sources such as the sun, wind, waves and hydropower by 2020. Today, renewables account for just 8.5 per cent of our energy. Needless to say, solar power has an important role to play in helping Europe to achieve its renewable energy goals. However, electricity produced by photovoltaic cells is still considerably more expensive than electricity produced by more conventional methods.

One of the reasons for this is the low conversion rate of today’s photovoltaic cells; the most efficient are only able to convert 24 per cent of the sun’s energy into electricity, and most commercial solar panels are even less efficient, converting just 17 per cent. The problem with these conventional cells is that they only convert a narrow portion of the sun’s spectral range into energy. FULLSPECTRUM is developing cells which are able to capture and exploit the entire solar spectrum. The project has already created cells with an efficiency of 37.6 per cent.

An introduction to photovoltaics

Conventional photovoltaic (PV) panels consist of many individual ‘cells’ made of semiconductor materials such as silicon. When photons of light collide with a PV cell, their energy is transferred to the electrons in the atoms of the cell. This energy enables the electrons to break free from their atoms and be captured to form an electric current.

So why do today’s PV cells only capture a small portion of the sun’s energy? The answer involves a phenomenon called the band gap. The band gap is the minimum amount of energy required to free an electron from its atom. Different materials have different band gaps. In the case of a silicon PV cell, for example, photons with less energy than silicon’s band gap are simply not absorbed. On the other hand, the excess energy of photons whose energy is higher than the band gap is effectively wasted.

Breaking the efficiency barrier: a European record!

FULLSPECTRUM’s innovation is the creation of a multijunction solar cell that makes better use of the entire solar spectrum. The technology uses solar cells made of different materials (including gallium, phosphorus, indium and germanium) with different band gaps stacked on top of each other. Any photons which are not absorbed by the cells at the top of the stack are absorbed by the cells beneath.

Tests have shown that these cells can convert as much of 37.6 per cent of the sun’s energy into electricity, a European record. These cells are very expensive. However, their costs can be considerably reduced by arranging them in special panels which include lenses that focus a large amount of solar energy onto the cells.

Because the cells are much more efficient, fewer PV panels are required to achieve the same power output, giving them a major advantage over conventional silicon cells. Furthermore, the project partners believe that with further research and development, conversion efficiencies of 50 per cent could be achieved. Given the limitations of silicon cells, the project partners predict that their multijunction cells will eventually come to dominate the market.

The technology is now being tested on the industrial scale at the recently opened Institute of Concentration Photovoltaic Systems (ISFOC) in Castilla La Mancha, Spain. Some industrial FULLSPECTRUM partners, have installations at the institute. With these demonstration plants, the project partners aim to provide industry with valuable information on the reliability, suitability and efficiency of the new PV concentrator technology.

In another FULLSPECTRUM achievement, the project partners provided the first ever evidence of the intermediate band effect. This refers to the absorption of photons at three different energy levels, corresponding to three different band gaps. In practical terms, this enables the system to capture low energy photons that would other wise pass through a conventional solar cell and be lost.

In the long term, intermediate band cells might substitute the complex high efficiency multijunction cells used today. This achievement has raised much attention all over the world and dozens of laboratories are now working on it.

Looking forward to a sunny future

The project’s success in advancing new concepts in photovoltaics has not gone unnoticed; and research programmes in countries as far afield as the US and Japan have been inspired at least in part by FULLSPECTRUM. The project’s coordinator, Antonio Luque of the Institute of Solar Energy at the Universidad Politécnica de Madrid in Spain, is optimistic about the project’s legacy.

‘These results are making an important contribution to significantly increasing the conversion efficiency of solar cells, which is the main route to cost reduction of solar electricity in the long term,’ he states. ‘Cost competitive photovoltaics will open the door to large-scale deployment, thus empowering Europe to produce green electricity and reduce the dependence of the EU on energy imports.’