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Luminescent solar concentrator

A luminescent solar concentrator (LSC) is a device for concentrating radiation, solar radiation in particular, for purposes of power generation. LSCs operate on the principle of collecting radiation over a large area, converting it by luminescence (commonly specifically by fluorescence) and diverting the generated radiation into a relatively small output target.

Though other configurations (such as doped or coated optic fibres) are possible, currently typical constructions comprise parallel thin, flat layers of luminescent and transparent materials to gather incoming radiation on their broadest flat surfaces, and emit concentrated luminescent radiation around their edges. Commonly the concentrated radiation is captured by solar cells to generate electric power.

Current LSCs are not adequate for many practical applications, but the principle shows potential; for example, it is suited to a wide range of configurations and the collection and conversion of light of a wide range of frequencies, whether directional or diffuse.

Structure and principle 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 device.

For example, imagine a square glass sheet 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 400 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 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: concentration factor of 5.

Similarly, other configurations could give attractive concentration factors. It is interesting to consider an example such as a graded refractive index optic fibre 1 square mm in cross section, and 1 metre long, with a luminescent coating.

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 fraction of the incoming energy that the device can deliver as usable output energy. 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, whereas input radiation tends to be diffuse and of relatively low irradiance. Concentration of the input energy accordingly is one option for efficiency and economy.

So far this describes a wider class of concentrators (for example simple optical concentrators) than just luminescent solar concentrators. True LSCs 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 more efficiently photovoltaic cells can be designed to convert it to electricity. Suitable optical designs also can trap light that the luminescent material emits in all directions, and redirect it by internal reflection and refractive index gradients so that little escapes except to 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 the transparent medium, or it might be disposed in the form of luminescent thin films on the surfaces of some of the transparent component.

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

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, optimal 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 medium with 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 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
• stock 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 does not exceed 7% in practice. 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 luminescent material emits; there is overlap between the dyes' frequencies of luminescence and unproductive absorption.

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. A major 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 potantial.
• 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.