Bandwidth and angle selective holographic films for solar energy applications

The objective of this research program is the development of the technology for the industrial fabrication of large format holographic optical elements (HOEs) with predetermined spectral characteristics and angular selectivity. HOEs of this type are used in a variety of technical applications, such as: holographic concentrators for photo-voltaic energy conversion and solar photo-chemistry or as integrated holographic stacks comprising several holograms operating in different ranges of the solar spectrum for daylighting, glazing and shading in buildings. The latter are required for the effective control of the transmission of solar radiation through the windows or the glass curtain wall envelopes of buildings. The HOEs (reflective or transmissive) ate recorded in dichromated gelatin layers (DCG) deposited on glass or plastic substrates. This material and the corresponding thermochemical development process facilitate the achievement of bandwidths, spectral ranges and angular selectivity that match accurately the design spectral and geometrical properties of a particular applicationl(1,2). The developed technology extends the applicability of DCG into the blue and red spectral domains and becomes a viable tool in the control and utilization of solar energy. The applicability of the technology is illustrated with design and test results obtained from various examples of HOE utilization. We will also report on the properties of asymmetric reflective holograms developed for window shading that permit unobstructed view through the window, but block the direct solar radiation for a particular angle of incidence. The angular selectivity, the bandwidth and the central wavelength can be adjusted to achieve the desired effect.