Luminescent solar concentrator

Design

Initial designs typically incorporated parallel thin, flat layers of alternating luminescent and transparent materials, and their radiating edge. ^[1] [2] Commonly the device would direct the power of solar power.

Other configurations (such as doped or coated optical fibers , or contoured stacks of alternating layers) may be better.

Structure and principles of operation

The layers in the stack may be separated parallel flat or alternating strata in a solid structure. In principle, the output is relatively large relative to the effective output area, the output would be of correspondingly higher irradiance than the input, as measured in watts per square meter. The concentration factor is 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. Its input area (eg the surface of a single face of the sheet to the energy source) is 400 times square (200×200) as compared to 4000 square mm (200x5x4). To a first approximation, the concentration factor of such an LSC is proportional to the area of ​​the input area divided by the efficiency of diversion of incoming light towards the output area. Suppose that the glass sheet could divert incoming 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 fiber 1 square mm in cross section, and 1 meter 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.

• The concentration factor is the ratio between the incoming and the irradiated irradiance. If the irradiance input is 1 kW / m2 and the irradiance output is 10 kW / m2, it would provide a concentration factor of 10.
• The efficiency is the ratio between the incoming radiant flux (measured in watts) and the outgoing wattage, or the fraction of the incoming energy that the device can deliver as usable output energy (not the same as be usable). In the previous example, half the received power is re-emitted, implying efficiency of 50%.

Most devices (such as solar cells) for the transmission of signals and the transmission of signals and the transmission of signals at a high frequency. relatively low irradiance and saturation . Concentration of the energy input is one option for efficiency and economy.

Luminescence

The above description covers a class of concentrators (for example simple optical concentrators) than just luminescent solar concentrators. The essential attribute of LSCs is that they become luminescent materials that are absorbed by a wide frequency range, and re-emit the energy in the form of light in a narrow frequency range. The frequency range, (ie the higher saturation) the simpler a photovoltaic cell can be designed to convert it to electricity.

Suitable optical designs for light emitted by the luminescent material in all directions, the photovoltaic converters . Redirection techniques include internal reflection , refractive index gradients, and where suitable, diffraction . In this type of solar energy, they are used for the purpose of conventional solar panels or for concentrating devices.

The luminescent component may be a dopant in the material of some or all of the transparent medium, or it may be in the form of luminescent thin films on the surfaces of some of the transparent components. ^[3]

Theory of luminescent solar concentrators

Various items-have Discussed the theory of internal reflection of fluorescent light so as to Provide Concentrated issuance at the edges, both, for doped glasses ^[1] and for organic dyes incorporated into bulk polymers. ^[4]When transparent plates are doped with fluorescent materials, effective design requires that the dopants should be absorbed 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 be conveyed to the output area. Control of the internal pathway of luminescence could be relied on by the 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 indexes. ^[5] Such an arrangement using a device with a high concentration factor should offer impressive savings 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 by the energy falling on the plate, should be about 20%. ^[6]

This takes into account:

• the absorption of light in the transparent medium,
• the efficiency of light conversion by the luminescent components,
• the escape of luminescence beyond the critical angle and
• gross efficiency (which is the ratio of the average energy emitted to the average energy absorbed).

Practical prospects and challenges

The relative merits of various functional components and configurations are major concerns, in particular:

• Organic dyes offer more ranges of frequencies and more flexibility in the choice of frequencies and re-absorbed than rare earth compounds and other inorganic luminescent agents. ^[7] [8]
• Organic doping with inorganic luminescent agents is particularly useful for inorganic agents.
• Luminescent agents as a bulk doping of a transparent medium.
• Various trapping media presents different combinations of durability, transparency, compatibility with other materials and refractive index. Inorganic glass and organic polymer media including the two main classes of interest.
• Photonic systems create band gaps that trap radiation. ^[9]
• Identifying materials that re-emit more input light as useful luminescence with negligible self-absorption is crucial. Attainment of that ideal depends on the tuning of the medium level. ^[10]
• Can be configured into thin films that can be used in a transparent way.
• The sensitivity of solar cells must match the maximum emission spectrum of the luminescent dyes.
• Plasmos increases the efficiency of transition from the ground state .

Advances

Transparent Luminescent Solar Concentrators

In 2013, Michigan State University researchers demonstrated the first visibly transparent luminescent solar concentrators. ^[11] These devices were composed of phosphorescent metal halide nanocluster (gold Quantum Dot ) which displays massive amounts of Stokes shift (or downconversion) and which selectively absorbs ultraviolet and near-infrared light, allowing for selective harvesting, improved reabsorption efficiency, and -tinted transparency in the visible spectrum. The following year, these researchers demonstrated visibly transparent luminescent solar concentrators by utilizing luminescent organic salt derivatives. ^[12]These devices exhibit a clear view of a transparent 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. ^[12]

Quantum dots

In 2014 LSCs based on cadmium selenide / cadmium sulphide (CdSe / CdS) quantum dots (QD) with induced large separation between emission and absorption bands (called a large Stokes shift ) were announced. ^[13] [14]

Light absorption is dominated by an ultra-thick outer shell of CdS, while it occurs in 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 (sized in tens of centimeters) of polymethyl methacrylate (PMMA). The active particles were about one hundred angstroms across. ^[13]

Spectroscopic de la traduction 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. ^[13]

See also

• Concentrated photovoltaics
• Solar cells
• Solar cell research
• Plasmon surface
• Thin movies

References