Light-cascade technology applied to photovoltaics
The light-cascade technology may be applied to the encapsulation of solar cells, making the encapsulation an active participant in the conversion of solar energy into electrical energy. Solar panels fabricated with this technology have a lifetime of over 10 years and conversion efficiencies 20% to 40% greater than conventional solar panels, depending on the meteorological conditions. The light-cascade panels are particularly efficient under diffuse lighting conditions (i.e., cloudy days), which makes them attractive for use in regions with limited direct sunlight
Power modules using this technology have seen use in many fields, ranging from public lighting and signaling facilities to marine markers. A few examples are shown below.
Light cascade in encapsulation of photovoltaic cells
The light cascade relies on transparent materials doped with optically active molecules (OAMs) or crystal particles to redshift the spectrum of the incident light. The molecules are selected depending on the application; they absorb at a given wavelength and emit at longer wavelengths. By combining the appropriate species at the proper concentrations, a light-cascade effect is produced whereby the incident spectrum is significantly redshifted to a target absorption band, as shown at right.
This strategy of passively shifting the solar spectrum can be applied to many situations. For photovoltaic cells, the absorption is strongest in the near infrared part of the spectrum. Thus, to optimize their efficiency, the cells are encapsulated in PMMA or glass that is doped to produce a light-cascade effect that shifts the visible part of the solar spectrum to the near infrared. The result is a significant increase in efficiency of the photovoltaic cells.
The encapsulation of photovoltaic cells has other advantages: light that is not immediately absorbed by the cells reflects from the back surface of the encapsulation, and some of this light is totally internally reflected, thereby becoming trapped in the solar-cell module. This light will continue to propagate within the module until it either is absorbed by a photovoltaic cell or reaches the end of the module (see schematic at right). The effect can be seen in the photograph below of samples of PMMA doped by OAMs. The bright edges of the samples are due to light that is incident on the surface of the sample and trapped within the sample.
Another benefit of the technology is that the light in the solar-cell module is more diffuse, which reduces hot spots and increases the lifetime of the encapsulated photovoltaic cells.
Different ways to encapsulate photovoltaic cells
Cascade Light Technologies has developed several ways to encapsulate photovoltaic cells. The essence of the approach is shown at right. The photovoltaic cells are sandwiched in a supple transparent resin between a reflective back layer and a PMMA front layer. The PMMA layer is doped with optically active molecules (OAMs) that shift the incoming solar spectrum to the wavelengths most efficiently converted into electrical energy by the photovoltaic cells. Light that is not initially incident on the photovoltaic cells is reflected back by the reflection-coated rear surface to maximize the probability of absorption. Total internal reflection from the resin-PMMA interface serves to trap this light within the solar-cell module, increasing the probability of absorption by the photovoltaic cells.
Three-dimensional solar modules are also feasible, such as solar towers and double-glazed solar windows. By treating the incident surface with a dichroic coating, the light redshifted by the OAMs cannot escape but is trapped inside the three-dimensional structure. Combined with a high-reflectance back coating and the three-dimensional illumination (see schematic of solar-tower at left), the solar tower principle enables significant gains in conversion efficiency. Other possibilities include solar shelves, as shown at the lower right, or double-glazed solar windows, as shown below. The double-glazed windows are composed of a dichroic-coated, OAM-doped incident pane. The rear pane may be made of transparent or semireflective material, depending on the application. By lining the edges of the window with photovoltaic cells, energy may be produced from light that is not transmitted by the window.
The two following images show possible architectures for building-integrated photovoltaics based on LPRL technologies.