Photovoltaic cell

This podcast originally appeared on Pythagoras Trousers episode 97 on November 15th 2012

This is an interview with PhD student Christopher Emmott about his research on different types of photovoltaic cell.

Transcript:

On October 22nd, Hurricane Sandy started developing from a tropical wave in the western Caribbean sea. Since then it moved Northwards, striking Cuba, the Antilles and eventually reaching New York. The storm caused devastation where ever it went.

Weather systems like this could be due to the ongoing problem of global warming. Humans have been using fossil feuls, depleting the Earth’s resources so that we’re almost running out. To make sure we don’t, and to make sure we try to put a stop to global warming, researchers are investing time and effort into renewable energy technologies.

The Grantham institute for climate change is an institute within imperial college London that aims to bring together researchers in all areas covering issues around climate change. This covers everything from climate modelling, looking at the historic climate change and paleoclimatology, and looking at models for possible future climate change. The institute also looks at how climate change affects biodiversity: have animals change their eating habits or migratory habits during this climate change?

Another area it invests in is new mitigation technologies, and this is where Christopher Emmott fits in. Chris is a PhD student at the Grantham Institute who is researching solar technologies. His specific area is solar cell technologies, of photovoltaics.

A solar cell, or a photovoltaic panel works by taking the energy from the sun in the form of photons. When they impact on a semi-conductor material they free an electron which then passes around a circuit and creates electricity.

One of the areas I work in is looking at a new type of photovoltaic technology which is based around organic materials. So there’s a large research at Imperial crossing quite a few departments which is studying the science behind using organic materials, or carbon-based materials, rather than inorganic materials, as the semiconductor in the solar cell.

So the advantages of using organic is that they can, these carbon long chain polymers or small carbon based molecules can be printed, and can be deposited in very very thing layers, and very cheaply, is the idea. They can be processed very quickly and you could essentially have a printing press like you have at a newspaper printing press which uses the same technology to actually print solar panels. These kind of materials can also be used in lots of other devices. So, in a normal solar cell you use silicon as your semiconductor material which is the same kind of material as you get in microelectronics, or computer chips. The research here is also looking at how you can use organic materials to print microchips and to create transistors which can then be printed and integrated into very low cost microelectronics. They can also, in the same way as inorganic semiconductors, be used to create light. So people use organic materials to make organic light emitting diodes or LEDS which can be used to make displays. Those are the first type of devices which are starting to be commercialised now.

However, with so many possible applications, we still don’t see these technologies commercialised when it comes to solar cells. This is because the organic solar cells sstill seem to be rather inefficient.

So the amount of sunlight which is impacting on the cell, only a very small proportion of that is being turned into electricity. This means that you need a very large area of module to get a decent amount of electricity. The other problem is they have very low lifetimes, they only last for three or four years compared to normal solar panels you get these days on your roof which would last 25 to 30 years.

So another approach to bringing down the cost of solar and to making cheaper solar electricity is to create very very efficient cells to harness the most amount of sunlight possible. The problem with this is that you are then using multiple layers of inorganic materials which are very expensive, so using things like gallium, phosphorous and indium, which are quite expensive metals and it’s quite a complex process to grow them, to grow these crystals. The way to get around that is to the use a lens or a mirror to focus the sunlight down onto the cell, and so making the most use of the high efficiency of the cell and therefore generating very cheap, very low cost solar electricity. The problem with this approach is you then need to, because of the lens and the optics, You need to track the sun. You need to mount these solar panels on some kind of mechanism, which then faces the right way the whole time, facing directly towards the sun which doesn’t really work in terms of putting solar panels on your roof.

This means that unfortunately these types of focussing cells would be very inefficient in cloudy regions like Northern Europe. The clouds scatter the sunlight, making it impossible for the focussing mirrors and lenses to do their job properly. In order then to make the most use out of them, placing them some where like southern California, or the Sahara desert may work out better. These areas are almost constantly bathed in sunshine, so the efficiency of the cells would be greatly increased

One of the advantages of concentrated photovoltaics over normal silicon photovoltaics, which are in the normal solar panels you can get quite commonly these days, is that they actually use much less land area. So you are having a smaller impact on the land area. But also, the amount of area you need to cover over the Sahara in order to generate all of Europe’s electricity is incredibly small.

There is map done by someone…of the world you can see a small speck in the Sahara which would be the amount of land area you would need to power electricity for the whole of Europe.

As far as I know, there has been very little opposition to this whole idea because it’s a good opportunity for generating local economy, supplying jobs to what is quite a poor region in Northern Africa, apart from their Oil wealth. So it’s quite good at generating a local industry where you can export electricity to Europe. And it solves a lot of Europe’s problems. As we’ve already seen, there has been a lot of large areas, solar fields, been set up in Germany and that has faced a lot of protest from local communities who don’t like having huge areas of solar panels. If they can be placed in the Sahara for example, where there is a huge area, there would be a very small impact with respect to the whole area of the Sahara. And so it wouldn’t really impact the local communities in quite the same way as if you put them in a farm in mid Bavaria, where there is a lot of very dense population.

Aesthetic impact seems to be what is slowing down the progress of new technologies being integrated into society. But researchers in the UK and in America have come up with an ingenious plan

So one of their main potential applications is in what is known as building integrated photovoltaics. So this is where you have a glass skyscraper, and these organic materials can be tuned in terms of their colour and also their transparency so you can make transparent solar cells. And if there can be used to replace glass windows in skyscrapers and you can have solar panels where you could not notice them at all. So this is an area where new technology, and new solar panels for generating solar electricity with blending into the built environment and not having to worry about issues of local opposition to aesthetics or environmental impacts.

So there’s a couple of research groups here at Imperial and also Oxford, Cambridge and America, quite a lot in America, who have made them almost completely transparent. So they only absorb infra-red radiation which is perfect because there is a huge energy demand from airconditioning. So a lot of tall skyscrapers have an infra-red coating on the windows so you don’t let in quite so much heat in from the sun. So if you could actually use that energy which you are blocking out to generate electricity, the window would look completely transparent, there would be no colour to it. You’d be getting less heat into the building and you’d also be harnessing that excess energy.

At the moment it is still substantially more expensive to integrate completely transparent and infra-red absorbing solar panels into buildings than if you simply put solar panels on the top of the roof. In London, the architects of the Swan building have integrated solar panels into windows along one entire wall, but they are not transparent.

The problem with commercialising any technology experience is required: experience of the technology in the field.  As organic photovoltaics are still relatively new, very few people would invest in new technology for thousands of pounds if they it is not guaranteed to perform.

This is particularly the case with concentrated photovoltaics: there are a few companies which are already installing these kinds of systems. But the reasons why they are not doing so well is that there is not quite the same experience of how much electricity they’ll generate, how much maintenance they’ll require, how long they are going to last, and all these things. So people like to go with the conventional technology, stick with what they understand. So there is still a lot of room for improvement of actually experiencing new technology, but you can only really get to that stage once you’ve shown that it can be commercially viable.

So at the moment, people seem to be sticking with what they know, but this does give organic solar cells a chance to develop, and researchers to work on the technology to improve their efficiencies.

Solar electricity is an incredibly useful technology for mitigating climate change and reducing out carbon emissions. And there are brilliant prospects for emerging technologies to deliver in terms of improving the technology, lowering the cost, improving a lot of aspects to it, particularly the environmental aspects in terms of lower impact on the environment, of manufacturing the technology. However, there is still a lot of room for improvement in terms of research in this area to improve materials and lower costs, manufacturing costs of these new technologies before you will see them commercially used.

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