Infrared Capture

The idea here is to capture some of the reflected infrared energy emitted by the ground. This reflected energy has a wavelength in the range 8-13 micrometers and approximates 80% of the incident solar energy hitting the ground. In addition, the ground continues to emit infrared energy during the evening and, to a lesser extent, through the rest of the night, so this can be regarded as a continuous power source, though one which would provide peak output during the afternoon and early evening: this is a reasonable match to the daily energy demand profile in most parts of the world.

Calculations indicate that the electric output of this type of device would be about 1/10 of a typical photo-voltaic solar cell at the same location and that its operating efficiency would be around 15%.

At the time of writing a team from Harvard University have measured the infrared emissions at Lamont, Oklahoma but no energy capture devices have been built or tested.
- source: New Scientist, 23 August 2014, page 38

It is anticipated that, as thermoelectric devices are very inefficient, the most interesting energy capture device would be a rectifying antenna, also known as a rectenna. This consists of a suitably tuned antenna connected to a circuit containing a diode. The idea is that the diode is warmed by the earth's infrared radiation while the antenna radiates away heat, a process which cools the antenna. This process sets up an alternating voltage potential that the diode converts to an intermittent DC voltage which can be smoothed and used as an electric power source.

In the 1960s Raytheon used a rectenna to power an experimental drone by capturing energy from a microwave beam. Rectenna conversion efficiencies of 70% were reported. However, the problems of making the system work at a frequency some 10,000 times higher include:

• antennae capable of working at infrared frequencies can be printed on suitable substrates but there are major problems in making diodes capable of working at these frequencies and with the very low voltages that the antennae would provide.
• the IR radiation includes a range of frequencies with a chaotic phase relationship. This can lead to mutual cancellation and a consequent lack of output from an antenna.

Another factor that needs looking at is the possiblity of including this type of IR capture device in a conventional photovoltaic solar cell farm. PV solar cells work at optical frequencies and so, in theory, can coexist in a shared installation.

So far this idea has to be seen as speculative at best.