Luminescent solar concentrator
A luminescent solar concentrator (LSC) is a device for concentrating radiation, non-ionizing solar radiation in particular, for purposes of generation of solar power. Luminescent solar concentrators operate on the principle of collecting radiation over a large area, converting it by luminescence (commonly specifically by fluorescence) and directing the generated radiation into a relatively small output target.
Current designs typically comprise parallel thin, flat layers of alternating luminescent and transparent materials, so placed as to gather incoming radiation on their broadest flat surfaces and emit concentrated luminescent radiation around their edges. However, other configurations (such as doped or coated optical fibers, or contoured stacks of alternating layers) might prove appropriate to particular applications. Commonly the device would direct the concentrated radiation onto solar cells to generate electric power.
Current luminescent solar concentrators are unsatisfactory for most practical applications, but the principle shows potential; for example, it lends itself to a wide range of configurations and to the collection and conversion of incoming light of a wide range of frequencies and intensities, whether directional or diffuse.
Structure and principles of operation
Typical structures for luminescent solar concentrators might be flat stacks of transparent material placed to gather incoming radiation over their flat surfaces and emit the output from their edges. For purposes of this discussion it does not matter whether the layers in the stack are separate parallel plates or alternating strata of different materials in a solid structure. In principle, if the effective input area of the plate surface is sufficiently large relative to the effective output area of the edges, the output would be of correspondingly higher irradiance than the input, as measured in watts per square metre. (Readers unfamiliar with the term irradiance might prefer the word intensity, or even Radiant intensity but the terms are not correctly interchangeable.) Concentration factor is the term for the ratio between output and input irradiance of the whole device.
For example, imagine a square glass sheet (or stack) 200 mm on a side, 5 mm thick. That would give an input area 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 a luminescent solar concentrator is proportional to the area of the input surfaces divided by the area of the edges multiplied by the efficiency of diversion of incoming or converted light towards the output area. Suppose that the glass sheet could divert incoming light from the face towards the edge with an efficiency of 50% (that is: 0.5). The hypothetical sheet of glass in our example would give an output irradiance of light five times greater than that of the incident light: a concentration factor of 5.
Similarly, other configurations could give attractive concentration factors. It is instructive to consider candidate examples such as a graded refractive index optic fibre 1 square mm in cross section, and 1 metre long, with a luminescent coating.
Concentration factor versus efficiency
Note the important distinction between the concentration factor and the efficiency of the device.
- The concentration factor is the ratio between the incoming irradiance and the emitted irradiance. If the input irradiance is 1 kW/m2 and the output irradiance is 10 kW/m2, that would amount to a concentration factor of 10.
- The efficiency is the ratio between the incoming radiant flux (measured simply in watts) and the outgoing wattage. In other words, the efficiency is the fraction of the incoming energy that the device can deliver as usable output energy, not necessarily light or electricity. Consider the previous example of an impressive irradiance concentration factor of 10; if that irradiance were delivered over an area of 0.01 square metre, from an input of 1 square metre, the efficiency of energy delivery would be only 10%, and 90% of the energy would be wasted, no matter how concentrated the delivered energy would be.
The concentration factor is important for several reasons, or there would be no point to concentrating the collected radiant energy. A typical reason is that most devices (such as solar cells) 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 limited ranges of frequencies, whereas input radiation tends to be diffuse and of relatively low irradiance and saturation. Concentration of the input energy accordingly is one option for efficiency and economy.
Role of luminescent components
So far this describes a wider class of concentrators (for example simple optical concentrators) than just luminescent solar concentrators. The essential attribute of the true luminescent solar concentrator is that it incorporates 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, preferably in a mode that lends itself to concentration. The narrower the frequency range, (i.e. the higher the saturation) the more efficiently for example photovoltaic cells can be designed to convert it to electricity. Suitable optical designs also can trap light that the luminescent material will emit in all directions, and redirect the output light by internal reflection and refractive index gradients, and where suitable, by diffraction, so that little escapes except to feed the photovoltaic converters. In principle such luminescent solar concentrators can use light from cloudy skies and similar diffuse sources that are of little use either for powering naked 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 disposed in the form of luminescent thin films on the surfaces of some of the transparent components.
Research in recent decades has established the theory and basic principles of luminescent concentration. Research in recent years has brought advances in the performance of various materials and design techniques.
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. If those conditions can be met, it becomes possible to configure the transparent matrix to convey the radiation to the photovoltaic conversion devices at 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 a typical window glass. Somewhat better efficiency could be achieved by using materials with higher refractive indices. Photovoltaic cells coupled to the output surfaces could use the emitted light energy to generate electric current. 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 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 absorption of light by poorly transparent materials in the transparent medium,
- the efficiency of light conversion by the luminescent components,
- the escape of luminescence beyond the critical angle due to imperfections (Plates with higher refractive index could improve the retention), and
- gross efficiency (which is the ratio of the average energy emitted to the average energy absorbed).
At the time of writing the published efficiency of luminescent solar concentrators did not exceed 7% in practice, or about a third of the potential. A major reason has been absorption of the emitted light by the luminescent dyes because the pigment is not fully transparent to the frequencies of light that the luminescent material emits; there is overlap between unproductive absorption and the dyes' frequencies of luminescence.
Practical prospects and challenges
The relative merits of various functional components and configurations are major concerns, in particular:
- organic dyes offer wider ranges of frequencies and more flexibility in choice of frequencies emitted and re-absorbed than rare earth compounds and other inorganic luminescent agents, which in turn have better durability under punishing conditions of temperature and exposure to high frequency light;
- doping organic polymers is generally practical with organic luminescent agents, whereas doping with stable inorganic luminescent agents usually is not practical except in inorganic glasses;
- luminescent agents configured as bulk doping of a transparent medium has merits that differ from those of thin films deposited on a clear medium;
- various trapping media present rival merits, for example in terms of durability, transparency, compatibility with other materials, and refractive index. Inorganic glass and organic polymer media comprise the two main classes of interest.
- the potential of photonic systems to create band gaps that prevent the escape of the trapped radiation shows interesting potential.
- the identification of materials in which practically all input light is re-emitted as useful luminescence, and in which absorption of emitted light (referred to as self-absorption) is negligible, is crucial. Attainment of that ideal depends on tuning the relevant electronic excitation energy levels to differ from the emission levels in the luminescent medium.
- an alternative to photonic elimination of self-absorption is to configure the luminescent materials into thin films that emit light into transparent passive media that can efficiently lead it away from poorly transparent media and towards the output photovoltaic conversion devices.
- it is necessary to match the sensitivity of solar cells to the maximal emission spectrum of the luminescent colorants.
- there is interest in the interaction of colorants with surface plasmons to increase their probabilities of transition from the ground state to the excited state and with it, the efficiency of absorption of solar light.
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