Roland Winston

Generalized radiance and measurement

The generalized radiance can never be measured at a given point of phase space, on account of the finite resolution of any real instrument; instead the instrument averages the radiance over some region of phase space. Thus a negative radiance is never measured, in spite of the fact that the generalized radiance can take on negative values. The relationship between the generalized radiance and the measurement process can be quantified by the instrument function, which is a property of the measurement apparatus and which allows one to calculate the response of the apparatus to any given incident wave field. The instrument function reveals a kind of reciprocity between the wave field being measured and the measurement apparatus. The theory of the instrument function is developed, and examples are discussed.

Flowline analysis of eténdue transfer of a wide-angle solar concentrator

We introduce flowlines as an analytical tool to optimize solar concentrator designing based on non-imaging optics. Comparisons were performed for multiple concentrator configurations from the same flowlines group to understand the final flux on low-cost heat pipes or minichannel absorbers for a small scale residential hybrid system. With the optical simulation results, we assemble and test a novel optical design for new low-cost, high-efficiency solar CHP collector to analyze both thermal and electric performances. By combining photovoltaic (PV) cells with heat pipes and mini channels, we further thermal energy capture while simultaneously enhance the solar cell performance.

Ray tracing and heat transfer simulation of novel glass-covered XCPC collector

The Compound parabolic concentrator used in the solar collector discussed in this paper is of the novel design, glass encased with 23% truncated reflectors and all glass receiver. Optical modeling was done in Light tools illumination design software to determine the optimum optical efficiency within a range of half acceptance angle and the heat transfer modeling and simulation was done in COMSOL Multiphysics simulation software. The Collector was built, tested and performance characterization was done. The experimental tests performed are stagnation test, water test for optical efficiency at low temperatures and closed loop oil test for thermal efficiency at high temperatures as high as 200°C. For the water and oil test, Flow rate method and Calorimetry method were used. The light tools optical modeling gave the optical efficiency of 64%. The stagnation temperature recorded at the absorber at 0% efficiency was 350°C. The water test at the temperature of 30-40°C gave the efficiency of 59%.

Nonimaging optics applications in theater lighting

It has been well established that nonimaging optics is the ideal way to concentrate or funnel light for applications ranging from solar energy to illumination. One industry where lighting is critical is the theater in which stage performers must be illuminated effectively. Historically theater lighting has been done using filament bulbs, which produce a lot of heat and provide discomfort to the actors and patrons. With the advent of LED lighting, many theaters are starting to replace the old filament bulbs with LED fixtures to provide the same effect. However, the last holdout for transition to LEDs has been the spotlight, because of the high standards of theater experts. Typical spotlights use a halogen-tungsten bulb coupled with an ellipsoid reflector to provide illumination of 10,000 lumens with a power input of 1000 watts. In this paper we present the work on the development of a nonimaging optics reflector coupled with a 100-watt LED fixture that can match the 10,000 lumens required by theaters for spotlighting, at a fraction of the power requirement and heat generation.

Two-stage 50X hybrid spectrum splitting CSP/CPV collector with InGaP/GaAs solar cells

Experimental performance of a two-stage 50X spectral beam splitting (SBS) parabolic trough collector (PTC) - incorporating double-junction epitaxial lift-off (ELO) InGaP/GaAs solar cells and using a suspended alumina particulate heat transfer media tested to 600°C - is presented.

Thermodynamic investigation of the segmented CPC

We investigate the relationship between the number of segments and the optical transmission of a CPC approximated by equal length segments whose start and end points lie along the CPC profile. We also investigate a separate method for generating CPC-like profiles by adjusting the angle of each segment to satisfy the edge-ray principle. Three variations of this method are examined where the edge-ray condition is taken from the start, mid, and end points of each segment. A flux efficiency (FE) to compare concentrators, which combines the concentration ratio and optical efficiency, is introduced and directly relates to the maximum achievable flux on the absorber. We demonstrate that the FE defined is another way to look at the compromises one makes for a geometric concentrator designed under real-world constraints.

Optical performance effects of the misalignment of nonimaging optics solar collectors

The use of non-imaging optics in the application of high temperature solar thermal collectors can be extremely advantageous in eliminating the need to track the sun. The stationary nature of non-imaging optics collectors, commonly called compound parabolic concentrators (CPC’s), present a unique design challenge when orienting them to collect sunlight. Many facilities throughout the world that adopt CPCs are not situated to orient the collectors in the ideal angle facing the sun. This East-West misalignment can adversely affect the optical and power performance of the CPC collector. To characterize how this misalignment effects CPCs, reverse raytracing simulations are conducted for varying offset angles of the collectors from solar South. Optical performance is analyzed for an ideal East-West oriented CPC with a 40-degree acceptance angle. Direction cosine plots are used to develop a ratio of annual solar collection by the CPC over the total annual solar input. From these simulations, average annual collector performance is given for offset angles ranging from 0 to 90 degrees for different Earth Latitudes in 10 degree increments.

Shaping and homogenization of LED white light using aperiodic scattering cell arrays (Conference Presentation)

Shaping of LED white light is of increasing interest for several industrial applications. There are several known design concepts available. However these concepts suffer from high uniformity errors, low efficiencies, chromatic aberrations and/or high tolerance sensitivity. To overcome these limitations we present a novel design concept which is based on the design of aperiodic scattering cell arrays. In a first design step, a unit scattering cell is designed. Afterwards this cell is periodically replicated. Finally the periodicity of the array is broken using parametric optimization. Obtained design results are compared with experimental data.

Broadband angular selectivity in solar concentrators (Conference Presentation)

In designing solar concentrator optics there are many parameters that must be optimized in order to create a useful system, such as compactness, number of elements or interfaces, and acceptance angle, among others. Using geometric optics, tradeoffs between these parameters become inevitable. For example, a lens, trough or dish may be compact but has low tolerance of angular misalignment; angular tolerance can be improved by adding secondary and tertiary optics, but this increases complexity and reduces optical throughput; nonimaging optics such as the CPC offer wide acceptance angles from as single element, but are too long to be practical, in most applications, above low concentrations. These tradeoffs can be avoided by using angle-selective photonic materials to exploit the equivalence between angular restriction and concentration. Recently, broadband angular selectivity in optical films has been demonstrated by the Soljacic group in MIT. In this collaborative work we use this material to experimentally demonstrate two visible-spectrum optical concentrators. We demonstrate that these concentrators are thermodynamically ideal when the material properties are ideal, and describe the material improvements most essential for improving device performance, and discuss how commercial solar concentrator systems could be improved by the use of angular-selective optics