Thermo generators deliver more electrons

See also: Government funds thermovoltaic research using gallium antimonide materials

Thermophotovoltaic (TVP) generators are photovoltaic (PV or solar) cells for infrared light.

Like all solar cells, only photons above the bandgap energy can be absorbed. As such, they have to be made from low bandgap materials – mostly gallium antimonide or germanium.

According to MIT, a GaSb PV cell has a bandgap energy of 0.7eV so will convert light of 1.7µm or shorter wavelength into electron-hole pairs – visible light stretches roughly from 700 (red) to 400nm (blue).

Temperatures of around 1,000°C are required to provide suitable photons.

The Ioffe Physico-Technical Institute in Russia has a 10W generator made by surrounding a burner, looking much like a backpacking stove, with 30cm2 of GaSb thermophotovoltaic cells.

“Efficiency as high as 12% has been achieved in the developed Ge and GaSb TPV cells illuminated by emitter with temperature of 1200-1300°C,” said the Institute.

Ioffe presented a paper late last year on a small combined solar-fuel thermophotovoltaic generator using a photon source heated by focussed sunlight or a burner and producing 3.1W and 4.9W respectively.

This week the UK Government revealed it is funding research into gallium antimonide-based thermovoltaics.

Led by Milton Keynes-based Wafer Technology, the £2m project includes Lancaster University and QinetiQ, with cash from the Technology Strategy Board.

Alloys including InAsSb and InGaSbN will be lattice matched to 100mm GaSb substrates from Wafer Technology.

“Such cells will exhibit significantly higher efficiencies than existing devices and will more effectively generate electricity from waste heat sources at temperatures below 1,000°C,” said Wafer Technology.

Lancaster and QinetiQ will study epitaxial growth of the alloys, and QinetiQ will fabricate devices.

The slightly clumsy term thermophotovoltaics differentiates infra-red solar cells from visible light photovoltaics, and from Seebeck-effect (thermocouple) generators for which the term thermovoltaics is generally reserved.

To get useful power, Seebeck generators have many thermocouples connected together in structures that balance the competing characteristics of low thermal conductivity – to prevent heat passing straight through – and high electrical conductivity.

Unlike thermophotovoltaics, easily-constructed thermoelectric generators can deliver power from any temperature difference – from a few degrees to a few hundred degrees C.

Seebeck generators in particular are proposed for energy scavenging as tiny generators can produce a few microwatts from body heat.

Thermoelectric technology is central to North Carolina-based firm Nextreme.

It uses a combination of bismuth and antimony tellurides, Bi2Te3 and Sb2Te3 in its Seebeck devices, and released a power generation evaluation kit in December.

Earlier this month, Belgian research lab IMEC, at the International Solid-State Circuits Conference, described a thermo-electric power conditioner chip.

Aimed at wireless sensor nodes requiring between 50 and 100µW, it wastes only 1.4µA.

Another phenomenon that converts heat directly to electricity, and was the first such effect to be discovered, has been dubbed pyroelectricity.

So far this has not been widely exploited.

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Tags: bandgap, cell, electrons, energy, MIT

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