Solar energy is currently captured by two means – either by converting solar energy to electricity (photovoltaic systems), or by converting it to thermal energy (solar-thermal systems). The current technology enables both of these conversions to become more efficient, by using a reflective cavity to reduce energy lost through infrared radiation.
In solar-thermal systems, a high temperature is needed in order for the solar energy to be efficiently converted into thermal energy. However, at these high temperatures, the amount of energy lost through infrared radiation is also high, reducing overall efficiency. Several strategies have been proposed to counter this radiation loss – one of which is the use of an angularly-selective surface, that can limit the angles at which infrared radiation can be lost from a surface, while still allowing a large angle for sunlight to be captured. However, physical devices for doing this have thus far not been proposed.
As for photovoltaic systems, one major source of inefficiency is the fact that not all of the wavelengths in solar energy can be captured by a photovoltaic cell – a photon can only be absorbed if its energy falls within the bandgap of a photovoltaic material. This leads to what is known as the Shockley-Queisser (SQ) limit – where photovoltaic systems generally have a limiting efficiency of 31%.
The current technology proposes a physical device capable of achieving angular selectivity. It comprises a circular vacuum enclosure with a solar absorber suspended at its center. Sunlight is able to enter through an aperture in the circular cavity, but infrared radiation emitted from the absorber is reflected back to it because of the cavity's geometry. This device can be optimized for energy efficiency by balancing the ratio between the diameter of the cavity and size of the aperture; using a sphere as the cavity as opposed to a cylinder; using a wavelength-selective solar absorber which has high solar absorptance but low infrared emittance; or using a material at the aperture that can transmit sunlight but reflects infrared emission back into the cavity.
A device of this nature also creates an opportunity to increase the efficiency of photovoltaic systems. A solar-thermal upconvertor (which increases the energy of photons it absorbs) is placed at the device's center. The absorbing surface of the upconvertor faces the cavity's aperture, while the other side emits upconverted photons towards a photovoltaic cell. In this configuration, the entire device is able to channel both within-bandgap and below-bandgap photons towards the photovoltaic cell.
Significantly higher efficiency for solar-thermal systems (up to 90% efficiency, more than twice that of blackbody absorbers)
- Significantly high efficiency for photovoltaic systems (up to 76% efficiency, as opposed to SQ limit of 31%)
- Reduced requirements for optical concentration of sunlight, thus reduced cost of optics required