Night-Time Solar Energy.

Devices employing billions of heat collecting nanoantennas (“nantennas”) are under development, which may eventually provide a solar energy collector that is amenable to mass-production using flexible sheets, and will produce electricity at night. It is not presently possible to convert the energy collected to electricity but it is envisaged that once this hurdle is overcome, lightweight "skins" could be made to power all kinds of electrical devices from i-Pods to electric cars, at a higher efficiency than is possible with traditional PV cells. The nanoantennas also have the potential to cool buildings or electronics by collecting background infra-red (heat) energy which could be used to make electricity that could provide further cooling by powering air-conditioning units. Since they target mid-infrared rays, which the Earth continuously radiates as heat after absorbing energy from the sun during the day they could be used to produce electricity at night, in contrast with PV cells which are useless after dark. I.R.-driven PV cells are another route to providing night time solar electricity.

A nantenna is an electromagnetic collector designed to absorb specific wavelengths that are proportional to the size of the nantenna. Currently, Idaho National Laboratories has designed a nantenna to absorb wavelengths in the range of 3-15 μm. Since around 85% of the solar radiation spectrum contains light with shorter than infra-red wavelengths, in the range 0.4-1.6 μm it would be ideal to make nantennas of these dimensions to harvest more energy than is possible with PV. Nantennas work in practically the same way as rectifying antennas: namely that Incident light drags electrons in the antenna material back and forth at the same frequency as the incoming light, in consequence of the oscillating electric field component of the electromagnetic light wave. The refractive index of a material has a similar origin.

The oscillating electrons generate an alternating current (AC) in the antenna circuit, which must be rectified to convert it into DC power usually with a diode device of some kind, and the DC current can then be used to power an external load. Since the wavelengths in the solar spectrum lie in the approximate range 0.3-2.0 μm, a rectifying antenna needs to be on the order of hundreds of nm in size to provide an efficient energy collector. Since the oscillating (AC) frequency from the nantenna array is around 10 THz, converting it to the 50-60Hz power that the world uses poses a challenge in terms of using the technology to generate real usable power. The main problem with rectifying diodes is that they have a finite recovery time which limits their operating frequency. Commercially available ultrafast diodes presently have an upper limit of the order of several GHz, and so they need to be made to work faster. This seems to be the principal hurdle to the success of generating electricity using nantenna.

There have been many affirmations to the effect that the theoretical efficiency of nantennas is > 85%, which in comparison with the theoretical efficiency of single junction solar cells (30%) looks very impressive. There is some ambiguity over this, however, depending on exactly how the efficiencies are calculated for the two kinds of device.

The most obvious advantage of nantennas over semiconductor photovoltaics is that the nantenna arrays can be scaled to absorb any frequency of light. Since resonance frequency is in direct proportion to the size of the antenna, the array may be tuned by simply varying the size of the nantenna in the array to absorb specific light wavelengths. In the case of PV the frequency of absorbed light depends almost entirely on the band gap energy, and so the semiconductor material must be changed to vary the latter. Indeed, this aspect of dimensional engineering is in some ways reminiscent of nanotube and quantum dot devices. Although the latter work in quite different ways the point is made that it is not only the chemical composition of the material but the size of its assembly that provides a tuning to the absorption of light that is possible by a device.

Related Reading.

http://solarpowerpanels.ws/solar-panels/solar-panels-work-at-night