Catalyst: Super Solar Cells

In this March 2015 episode of Catalyst, Dr Graham Phillips investigates new technology that is able to convert more than 40 per cent of the sun’s light into electricity. This is more than double the efficiency of today’s domestic rooftop solar panels, and could eventually lead to cheaper sources of renewable energy.

Super Solar Cells

Catalyst, March 2015

Watch online here: http://www.abc.net.au/catalyst/stories/4194517.htm

Transcript

At the centre of our solar system lies great power. The Sun bathes our tiny blue world with an immense amount of solar energy… many thousands of times the world’s current energy usage. Exploiting solar makes so much sense. In space, solar power has been used for a long time. Satellites have advanced photovoltaic cells based not on silicon, like our rooftop cells, but on gallium arsenide. These are more efficient but generally too expensive for use on Earth.

Dr Graham Phillips
But that may be changing. The sky is clear, the sun’s shining and today we’re going to find out about a new kind of solar cell. At the moment, it’s the most efficient cell in the world and it was invented right here in Australia. It brings space technology down to the surface. The story starts at the University of New South Wales.

Dr Graham Phillips
Hi. I’m Graham Phillips.

Prof Martin Green
Good to meet you, Graham.

NARRATION
Professor Martin Green has been researching solar power since 1974. He’s seen some huge changes.

Prof Martin Green
The early ’70s, people realised that we needed to do something apart from continuing to burn fossil fuels. The whole crises of that era really got people alerted to the fact that we mightn’t be able to depend on fossil fuels for ever for our energy supply, and that’s when the interest in photovoltaic started taking off.

NARRATION
Solar power has always had a great challenge to improve the efficiency of sunlight being converted into electricity. Your average rooftop panel converts about 15% of the sun’s light.

Prof Martin Green
We were the first to get a 20% efficient solar system, you know, back in the late 1980s, so that was one of our early highlights of the work we did here. There’s a fundamental limit on how efficient you can convert with just a single cell, and that’s about 25% for silicon. If you want to go higher than that, you’ve got to do something different.

NARRATION
Martin and his team did something different. They came up with a system where one photovoltaic cell was replaced by two and the sun’s light was split into two beams – one going to one cell and one to the other.

Prof Martin Green
What works the best is if you split the sunlight up into its different colours, much the same as a rainbow does, then you send each different colour to a cell that’s specialised in doing that conversion.

NARRATION
Standard silicon cells are most efficient for longer wavelengths, or colours, of light and gallium arsenide cells are better for shorter wavelengths.

Dr Graham Phillips
The key to this device is this, the filter. Now, the sunlight comes from this direction, some of the wavelengths pass straight through to the silicon-based cell over here. Other wavelengths are reflected back to the gallium arsenide-based cell. The gallium arsenide cell is a triple stack of photovoltaic layers, unlike the silicon cell, which has only one. Splitting the sunbeams and using the double cell system produced a staggering 40% efficiency for converting sunlight to electricity – the highest for any commercially viable solar power system.

Prof Martin Green
I think we can go further, but that the first time anyone’s ever exceeded the magic 40% landmark.

NARRATION
Another key to this system is to concentrate the sunlight onto the cell. By focusing the sun’s rays, a curved mirror can turn the power of one sun into the power of hundreds.

Dr Mark Keevers
By doing that you can use more expensive cells and also, they give a higher performance. It focuses and concentrates the light onto the cells at about 365 suns.

Dr Graham Phillips
365! That’s a decent amount of heat going into those cells.

Dr Mark Keevers
That’s right. And what happens if you sort of misalign things and you’ve got that kind of power lurking around?

Dr Mark Keevers
If we do misalign, we can start cooking some of the rubber and plastic parts.

Dr Graham Phillips
Have you done that?

Dr Mark Keevers
Once.

NARRATION
To develop their technology further, Martin and Mark partnered with another veteran of Australian solar power, John Lasich. He’s built a test facility for concentrated solar cell technology. And he’s already solved one of its biggest challenges – cooling the solar cells. While the sunlight hitting the cells needs to be intense, the excess heat created has to be taken away or it causes problems.

John Lasich
One of the very big challenges has to be manage that heat and extract the heat so that we can keep the cells happy and running cool so that they have maximum efficiency. This is a secret that we can demonstrate it, we can’t tell you precisely how. But we’ve developed an ultra-efficient heat sink that’s able to extract rather a large of heat from the cells and keep them below 60 degrees.

NARRATION
60 degrees is the maximum operating temperature for these cells. Any higher and they become less efficient. Much like the UNSW prototype, John’s system uses mirrors to direct and focus sunlight onto a solar cell. They’re called ‘heliostats’ and they concentrate the sun up to 700 times, in other words, 700 suns. The cooling system must work extremely well. Back in the lab, John shows me just how good it is.

John Lasich
OK, I think it’s time to introduce the blowtorch.

Dr Graham Phillips
The blowtorch?

John Lasich
The blowtorch.

Dr Graham Phillips
That’s a piece of high-tech equipment I think, isn’t it?

John Lasich
Indeed it is, but it needs to be able to extract the heat that is applied by this blowtorch and that’s equivalent to what it needs to do when it’s actually operating.

Dr Graham Phillips
OK, it’s yours, not mine, I guess.

NARRATION
If there was no cooling present, the blowtorch would do serious damage to the chip. But sure enough…

Dr Graham Phillips
This is stone cold.

John Lasich
Stone cold.

Dr Graham Phillips
That is an impressive cooling system. And I guess you need that if you’ve got the power of 600 or 700 suns hitting those cells.

John Lasich
Indeed, and that’s one of the secrets that we’ve cracked – how to do that effectively, cheaply, easily and reliably.

NARRATION
John’s system uses only a single type of solar cell, not the dual system developed at UNSW. But his cell is made of gallium arsenide and capable of generating a lot of power.

Dr Graham Phillips
On a typical home solar panels, you know, the panels are so big, metre and a half by a metre wide. Now imagine ten of those. Well, the power they generate could be generated with just one of those. So to power an entire home, which would take about 20 conventional panels… ..the equivalent amount of power would come from two.

NARRATION
John has even built a pilot power station in central Victoria, which is fully operational.

Dr Graham Phillips
Gee, it’s an impressive set of mirrors you’ve got here.

John Lasich
Yeah, it’s pretty exciting.

Dr Graham Phillips
How many?

John Lasich
There are 56 heliostats here and you’re looking at the world’s first wireless photovoltaic power station. So there are no wires in this field at all.

NARRATION
Each heliostat is programmed to track with the sun, so their collective light is always reflected to the top of the tower. There sits one square metre of John’s gallium arsenide cells. Cooling all that focused sunlight onto the cell requires a super-sized version of that top-secret cooling system.

Dr Graham Phillips
When this is up and running, how much power will it generate?

John Lasich
This will generate almost 220 kilowatts, so that’s almost a quarter of a megawatt.

NARRATION
That’s enough to power 75 homes.

John Lasich
This system could be multiplied as many times as we like, so if you just want a megawatt, we would just use four of these. If you want 100 megawatts, we would use 400 of these. The efficiency is high regardless of the scale of the project.

NARRATION
And the system could be made even more efficient by simply replacing John’s gallium arsenide cells with even more efficient ones, like the University of New South Wales’ dual cells.

Prof Martin Green
RayGen have demonstrated the concept of focusing light to a central tower and then converting using solar panels there. They were the first to demonstrate that at a reasonable scale. So we’re hoping that our improvement will get added as a way of supercharging the performance of that type of system.

John Lasich
It’s a very good match for what we do. Because the receiver is very small, it’s possible to do sophisticated things in there to enhance the efficiency. That type of technology would work very well. This type of technology and perhaps several others in parallel could indeed mean that a large portion of the world’s energy is provided from clean, renewable sources.

NARRATION
It’s another small but important step to tapping into one of the greatest clean sources of energy in the solar system.

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