Monday, September 01, 2008

Solar Cell Breakthrough


Nano-antennae "Solar Cells" printed on a Plastic Sheet

A Completely New Type
of Solar Cell

There have been several approaches over the years to gathering energy from the sun. Most concentrated on gathering the heat and using it to heat water etc. The direct conversion of solar energy into electricity has to date been achieved mainly by the use of photovoltaic (PV) cells. PV cell have been developing at a very fast rate with new types appearing almost by the month. Nano-technology is playing a big part in the developments of solar cells.

In the fairly recent past a completely different type of device as an alternative to solar cell has been suggested. But it has taken the development of nano-technology to begin to realise the concept.

A single nano-antenna with tunnelling diode rectifier at centre.

The basic idea is to use a series of microscopic tuned circuits that resonates at the infrared portion of the sunlight and in a highly efficient way these circuits or antennae can absorbs the infra-red radiation the power is in turn converted into direct current electricity.

I can remember lighting a neon tube without any contact with a power supply. This was done with a tuned coil and an aerial in fairly close proximity to a transmitter. Sufficient power was picked up by the coil to light the neon tube, and it all looked like a bit of magic.

With Nano-technology researchers have been able to construct microscopic tuned coils that exactly resonate with part of the suns spectrum. This allows a very high level of efficiency of conversion.

Tens of Billions of Tuned Circuits.

These tuned circuits can be printed on all sorts of cheap plastic materials. An 8 inch area would contain about 10 billion such tuned antennae.

In earlier experiments with microwaves, it was shown that a level of greater than 80% could be achieved. With the latest nano technology techniques, they have been able to construct efficient resonant circuits in the infrared spectrum. In the lab these new nano-antennae have show a theoretical efficiency of up to 92%. This work is being carried out in the Idaho National Laboratory under the auspices of the Department of Energy of the USA.

Solar Cells using nano-antennae show a massive improvement in efficiency over conventional non-concentrating PV solar cells which come in at around the 20% efficiency mark. The nano-antennae arrays would be very much cheaper to make than PV cells and can be printed onto many types of cheap plastic materials making them suitable for coating existing roofs and walls, or even covering the exteriors of cars etc.

Converting the Suns Energy in a New Way

The researchers have found that infrared rays create AC current in the nano-antenna. This is no good for any practical purposes - so the high frequency energy has to be converted into direct current. To do this it takes a diode or rectifier. However, any commercially available diodes are not much use at infrared frequencies. So the challenge was to find ways of rectifying these really high frequency waves into DC at reasonable cost. Current experiments are using a special “tunneling diode” and results are encouraging but more work needs to be done.

The output of the diode or rectifier can easily be dc-coupled together with “bus bars” so that large arrays can be linked efficiently together.

The present manifestations of these new devices incorporate a rectifying diode assembly built right into the nano-antenna itself. The term being used for this arrangement is a “rectenna” a combination of a receiving antenna and a rectifier. This concept was first demonstrated for microwave power by Raytheon in 1964.

Full commercial realization is still some time away but the lab experiments are very encouraging.

This is exciting stuff and has the feel of real science about it. It is another of those "discoveries" which I have a good gut feeling about.


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