A cheaper, simpler way to tap solar energy

Bangalore: Scientists in the US have found a novel way to concentrate the sun's heat without using mirrors, potentially enabling a simpler and cheaper way to harness the solar energy using compact systems.

The new device has been described in a report in the journal Nanoscale Research Letters by Peter Bermel and other researchers at the Massachusetts Institute of Technology (MIT).

One of the traditional methods for harnessing solar energy involves use of the photovoltaic (PV) materials that capture sunlight and turn it into electricity.

The other solar thermal method uses an array of mirrors to focus and concentrate sunlight, enough to boil water and run a steam turbine for generating electricity.

A third less common approach is to use devices called thermophotovoltaics (TPV). These are solid-state devices that generate electricity directly from the heat emitted by the sun or any other radiant heat source such as combustible fuel.

The basic principle of their operation is similar to that of traditional photovoltaics with the difference that the radiant heat is absorbed directly not by the photovoltaic material but instead by a selective absorber in its top layer, causing it to emit light which is then converted to electricity by the photovoltaic cell.

For efficient operation of the TPV system, the material that absorbs and emits the heat radiation must operate at high temperatures, which means these solar TPV devices also require the use of mirrors to concentrate the sunlight to provide reasonable temperatures.

MIT researchers have found a way to build solar TPV devices without the need for huge and expensive mirrors for concentrating the sunlight.

They achieved this by using the so-called photonic crystal and tailoring its structure in such a way that the heat absorbed is prevented from escaping the material, thereby making it achieve very high temperatures.

This, they say, is somewhat akin to the greenhouse effect, where the infra red radiation emitted by the earth's surface is prevented (by the greenhouse gases) from escaping, thereby trapping the heat and warming up the earth.

In the case of TPV devices, this blockage is achieved by an array of precisely spaced microscopic holes in the photonic crystal that allows rays that fall within a very tiny range of angles to escape, while the rest stay in the material and heat it up.

According to researchers, the precisely designed geometry of the photonic material preferentially emits light in a direction and wavelength that is optimal for photovoltaic conversion.

This enables the device to convert as much as 36 percent of incident solar energy into electricity -- that is higher than the theoretical maximum that could ever be achieved by traditional photovoltaic solar cells.

According to the report, these TPV devices can be manufactured using standard chip-fabrication technology and, when employed on a large scale, can compete with more conventional forms of power.
 

(K.S. Jayaraman can be contacted at killugudi@hotmail.com)
 

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