Bennett Widyolar

Thermodynamics and the segmented compound parabolic concentrator

Abstract. Compound parabolic concentrator (CPC) reflector profiles are complex and can be difficult to manufacture using traditional methods. Computer numeric control machines, however, can approximate complex profiles by bending a series of small flat segments. 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.

Hybrid solar collector using nonimaging optics and photovoltaic components

The project team of University of California at Merced (UC-M), Gas Technology Institute, and Dr. Eli Yablonovitch of University of California at Berkeley developed a novel hybrid concentrated solar photovoltaic thermal (PV/T) collector using nonimaging optics and world record single-junction Gallium arsenide (GaAs) PV components integrated with particle laden gas as thermal transfer and storage media, to simultaneously generate electricity and high temperature dispatchable heat. The collector transforms a parabolic trough, commonly used in CSP plants, into an integrated spectrum-splitting device. This places a spectrum-sensitive topping element on a secondary reflector that is registered to the thermal collection loop. The secondary reflector transmits higher energy photons for PV topping while diverting the remaining lower energy photons to the thermal media, achieving temperatures of around 400°C even under partial utilization of the solar spectrum. The collector uses the spectral selectivity property of Gallium arsenide (GaAs) cells to maximize the exergy output of the system, resulting in an estimated exergy efficiency of 48%. The thermal media is composed of fine particles of high melting point material in an inert gas that increases heat transfer and effectively stores excess heat in hot particles for later on-demand use.

Nonimaging optics maximizing exergy for hybrid solar system

The project team of University of California at Merced (UC-Merced), Gas Technology Institute (GTI) and MicroLink Devices Inc. (MicroLink) are developing a hybrid solar system using a nonimaging compound parabolic concentrator (CPC) that maximizes the exergy by delivering direct electricity and on-demand heat. The hybrid solar system technology uses secondary optics in a solar receiver to achieve high efficiency at high temperature, collects heat in particles and uses reflective liftoff cooled double junction (2J) InGaP/GaAs solar cells with backside infrared (IR) reflectors on the secondary optical element to raise exergy efficiency. The nonimaging optics provides additional concentration towards the high temperature thermal stream and enables it to operate efficiently at 650 °C while the solar cell is maintained at 40 °C to operate as efficiently as possible.

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.

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.

The error tolerance of nonimaging optic systems (Conference Presentation)

In real world applications the nonimaging optics has the advantage of high error tolerance compared to conventional imaging optics. In this paper we present the result of a solar collector constructed with the nonimaging optics principles and the effect of off-positioning its absorbers on its optical efficiency. Thermal analysis of such effects are also presented.

Nonimaging optics heating up Mongolia's harsh winter

One of the world’s oldest civilizations – with the worst air pollution and the coldest capital city – will employ cutting-edge technology from the newest UC campus starting in February. Professor Roland Winston, who leads the UC Merced-based UC Solar Institute, just returned from a trip to Ulaanbaatar (UB), Mongolia’s capital. He met with the owner of Mongolia National University (MNU), a 15-yearold institution with about 9,000 students, to discuss installing a solar-thermal unit on one of the campus buildings to generate 3 kilowatts of steam heat for a portion of the campus