Luminescent solar concentrator


A luminescent solar concentrator is a device for concentrating radiation, solar radiation in particular, to produce electricity. Luminescent solar concentrators operate on the principle of collecting radiation over a large area, converting it by luminescence and directing the generated radiation into a relatively small output target.

Design

Initial designs typically comprised parallel thin, flat layers of alternating luminescent and transparent materials, placed to gather incoming radiation on their faces and emit concentrated radiation around their edges. Commonly the device would direct the concentrated radiation onto solar cells to generate electric power.
Other configurations may better fit particular applications.

Structure and principles of operation

The layers in the stack may be separate parallel plates or alternating strata in a solid structure. In principle, if the effective input area is sufficiently large relative to the effective output area, the output would be of correspondingly higher irradiance than the input, as measured in watts per square metre. The concentration factor is the ratio between output and input irradiance of the whole device.
For example, imagine a square glass sheet 200 mm on a side, 5 mm thick. Its input area is 10 times greater than the output area - 40000 square mm as compared to 4000 square mm. To a first approximation, the concentration factor of such an LSC is proportional to the area of the input surfaces divided by the area of the edges multiplied by the efficiency of diversion of incoming light towards the output area. Suppose that the glass sheet could divert incoming light from the face towards the edges with an efficiency of 50%. The hypothetical sheet of glass in our example would give an output irradiance of light 5 times greater than that of the incident light, producing a concentration factor of 5.
Similarly, a graded refractive index optic fibre 1 square mm in cross section, and 1 metre long, with a luminescent coating might prove useful.

Concentration factor versus efficiency

The concentration factor interacts with the efficiency of the device to determine overall output.
Most devices for converting the incoming energy to useful output are relatively small and costly, and they work best at converting directional light at high intensities and a narrow frequency range, whereas input radiation tends to be at diffuse frequencies, of relatively low irradiance and saturation. Concentration of the input energy accordingly is one option for efficiency and economy.

Luminescence

The above description covers a wider class of concentrators than just luminescent solar concentrators. The essential attribute of LSCs is that they incorporate luminescent materials that absorb incoming light with a wide frequency range, and re-emit the energy in the form of light in a narrow frequency range. The narrower the frequency range, the simpler a photovoltaic cell can be designed to convert it to electricity.
Suitable optical designs trap light emitted by the luminescent material in all directions, redirecting it so that little escapes the photovoltaic converters. Redirection techniques include internal reflection, refractive index gradients and where suitable, diffraction. In principle such LSCs can use light from cloudy skies and similar diffuse sources that are of little use for powering conventional solar cells or for concentration by conventional optical reflectors or refractive devices.
The luminescent component might be a dopant in the material of some or all of the transparent medium, or it might be in the form of luminescent thin films on the surfaces of some of the transparent components.

Theory of luminescent solar concentrators

Various articles have discussed the theory of internal reflection of fluorescent light so as to provide concentrated emission at the edges, both for doped glasses and for organic dyes incorporated into bulk polymers. When transparent plates are doped with fluorescent materials, effective design requires that the dopants should absorb most of the solar spectrum, re-emitting most of the absorbed energy as long-wave luminescence. In turn, the fluorescent components should be transparent to the emitted wavelengths. Meeting those conditions allows the transparent matrix to convey the radiation to the output area. Control of the internal path of the luminescence could rely on repeated internal reflection of the fluorescent light, and refraction in a medium with a graded refractive index.
Theoretically about 75-80 % of the luminescence could be trapped by total internal reflection in a plate with a refractive index roughly equal to that of typical window glass. Somewhat better efficiency could be achieved by using materials with higher refractive indices. Such an arrangement using a device with a high concentration factor should offer impressive economies in the investment in photovoltaic cells to produce a given amount of electricity. Under ideal conditions the calculated overall efficiency of such a system, in the sense of the amount of energy leaving the photovoltaic cell divided by the energy falling on the plate, should be about 20%.
This takes into account:
The relative merits of various functional components and configurations are major concerns, in particular:
Luminescent solar concentrators could be used to integrate solar-harvesting devices into building façades in cities.

Advances

Transparent Luminescent Solar Concentrators

In 2013, researchers at Michigan State University demonstrated the first visibly transparent luminescent solar concentrators. These devices were composed of phosphorescent metal halide nanocluster blends that exhibit massive Stokes shift and which selectively absorb ultraviolet and emit near-infrared light, allowing for selective harvesting, improved reabsorption efficiency, and non-tinted transparency in the visible spectrum.
The following year, these researchers demonstrated near-infrared harvesting visibly transparent luminescent solar concentrators by utilizing luminescent organic salt derivatives. These devices exhibit a clear visible transparency similar to that of glass and a power conversion efficiency close to 0.5%. In this configuration efficiencies of over 10% are possible due to the large fraction of photon flux in the near-infrared spectrum.

Quantum dots

LSCs based on cadmium selenide/zinc sulphide and cadmium selenide/cadmium sulfide quantum dots with induced large separation between emission and absorption bands were announced in 2007 and 2014 respectively
Light absorption is dominated by an ultra-thick outer shell of CdS, while emission occurs from the inner core of a narrower-gap CdSe. The separation of light-absorption and light-emission functions between the two parts of the nanostructure results in a large spectral shift of emission with respect to absorption, which greatly reduces re-absorption losses. The QDs were incorporated into large slabs of polymethylmethacrylate. The active particles were about one hundred angstroms across.
Spectroscopic measurements indicated virtually no re-absorption losses on distances of tens of centimeters. Photon harvesting efficiencies were approximately 10%. Despite their high transparency, the fabricated structures showed significant enhancement of solar flux with the concentration factor of more than four.