Concentrated photovoltaic (CPV) systems achieve the highest level of solar conversion efficiency of all photovoltaic (PV) technologies by combining solar concentration, sun tracking, and high-efficiency multi-junction PV cells. Although these design features increase the overall efficiency of the device, they also dramatically increase the cost and physical volume of the system and make the system fragile and unwieldy. In this paper, we present recent progress towards the development of a robust, reduced form-factor CPV system. The CPV system is designed for portable applications and utilizes a series of low profile optical and optomechanical components to concentrate the solar spectrum, enhance energy absorption, and track the sun throughout the diurnal cycle. Based on commercial off-the-shelf (COTS) single-junction PV cells, the system exploits the efficiency gains associated with tuning the wavelength of the incoming light to the band-gap of a PV material. This is accomplished by spectrally splitting the concentrated incident beam into multiple wavelength bands via a series of custom optical elements. Additional energy is harvested by the system through the use of scavenger PV cells, thermoelectric generators, and biologically inspired anti-reflective materials. The system’s compact, low-profile active solar tracking module minimizes the effects of wind-induced loads and reduces the overall size of the system, thus enabling future ruggedization of the system for defense applications. Designed from a systems engineering approach, the CPV system has been optimized to maximize efficiency while reducing system size and cost per kilowatt-hour. Results from system tests will be presented and design trade-offs will be discussed.
Electrochromic materials are of great interest, owing to their potential application in large area displays, active camouflage and energy saving smart windows. The effectiveness of devices fabricated for most applications depends in part on the ability to tailor the observed color in a predictable manner. Several color-tailoring strategies such as polymer blends, copolymers, and layered composites have been investigated recently. Another technique utilizing patterns of electrochromes is currently under development in our labs and affords a false way to modify a device's observed color. The pattern is composed of materials that have different observed colors; at least one of which is an electrochrome. When the pattern is viewed at a distance, the observer perceives a different color than those of the materials comprising the pattern. This `confusion' is due to diffraction. By knowing the color of the patterning materials the observed color can be predicted in a straight forward manner by color subtraction theory. The patterns are produced by screen-printing the electrochrome and other materials onto a Mylar/ITO substrate that is then used as the working electrode in a device. Following this strategy, geometrical patterns composed of thick stripes, fine stripes, small dots, and checkerboards were studied using different materials as the foreground and background colors. We will report on the fabrication of these patterned devices and their characterization by spectrocolorimetry.
A novel approach to tailor the electrochromic properties of polythiophenes utilizing a silyl bridge to form discrete electrochromes is described. The monomer bis[2-(5,2'- dithienyl)]dimethyl silane was chemically synthesized and electrochemically polymerized. Electrochromic properties of devices incorporating this polymer as active material were studied and analyzed following CIE L*a*b* formalism. This polymer exhibits a yellow to green electrochromism from neutral to oxidized state.