Lun Jiang

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.

Performance of Organic Luminescent Solar Concentrator Photovoltaic Systems

Organic luminescent solar concentrator (LSC) photovoltaic (PV) systems are investigated as low concentration systems in harvesting solar energy. The prototypes are fabricated by embedding red and green organic dyes into PMMA plastic sheets. High efficiency mc‐Si PV cells are attached at the edges of the fabricated concentrators. The performance of the fabricated system is characterized based on the spectral properties and the outdoor electrical gain of the system. Properties of single‐ layer LSCs are compared with properties of stacked LSCs. The largest prototype LSC PV system we fabricated yields the concentration factor of 4.3. The output power is improved up to 2× by using optical optimization methods. The tested results for the prototypes as “smart” windows show that LSCs can perform very well for concentrating both direct and diffuse light. These results can be applied for further optimal design of LSC PV systems.

Solar receiver with integrated optics

The current challenge for PV/Thermal (PV/T) systems is the reduction of radiation heat loss. Compared to solar thermal selective coating, the solar cells cannot be used as an efficient thermal absorber due to their large emissivity of the encapsulation material. Many commercial PV/T products therefore require a high concentration (more than 10x) to reach an acceptable thermal efficiency for their receivers. Such a concentration system inevitably has to track or semi-track, which induces additional cost and collects only the direct radiation from the sun. We propose a new PV/T design using a vacuum encapsulated thin film cell to solve this problem. The proposed design also collects the diffuse sun light efficiently by using an external compound parabolic concentrator (XCPC). Since the transparent electrode (TCO) of thin film cell is inherently transparent in visible light and reflective beyond infrared, this design uses this layer instead of the conventional solar cell encapsulation as the outmost heat loss surface. By integrating such a vacuum design with a tube shaped absorber, we reduce the complexity of conducting the heat energy and electricity out of the device. A low concentration standalone non-tracking solar collector is proposed in this paper. We also analyzed the thermosyphon system configuration using heat transfer and ray tracing models. The economics of such a receiver are presented.

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.

Asymmetric design for compound elliptical concentrators (CEC) and its geometric flux implications

The asymmetric compound elliptical concentrator (CEC) has been a less discussed subject in the nonimaging optics society. The conventional way of understanding an ideal concentrator is based on maximizing the concentration ratio based on a uniformed acceptance angle. Although such an angle does not exist in the case of CEC, the thermodynamic laws still hold and we can produce concentrators with the maximum concentration ratio allowed by them. Here we restate the problem and use the string method to solve this general problem. Built on the solution, we can discover groups of such ideal concentrators using geometric flux field, or flowline method.

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.

Optical simulation-based on flowline method

Nonimaging optics is focused on the study of techniques to design concentrators or illuminators systems. The flowline optical design method, based on the definition of the geometrical flux vector J, is one of these techniques. The main property of flowline method is, its ability to estimate how radiant energy is transferred by the optical systems using the concepts of vector field theory, like field line or flux tube, overcoming traditional raytrace methods. This method has been developed only at an academic level, where characteristic optical parameters are ideal and the studied geometries are simple. The main objective of the present paper is the extension of the flowline method to the analysis and design of real 3D concentration and illumination systems by means of simulation. Using the concept of vector potential we can generalize flowline computations to real 3D systems. This new computation methodology provides, traditional simulations results like irradiance maps with higher precision and lower computation time, and new information as vector field maps produced by the system.

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.

The Design and Analysis of Spectrum-Splitting Hybrid CSP-CPV Solar Collectors

Summary: Sponsored by the Arpa-E agent under Department Of Energy (DOE), we developed a novel kind of solar receiver for parabolic troughs. The technology uses novel secondary optics in a solar receiver to achieve high efficiency at high temperature, collects heat in particles for high temperature and low fire danger, stores heat in particles instead of molten salt for low cost, and uses double junction (2J) photovoltaic (PV) cells with backside infrared (IR) reflectors on the secondary optical element to raise exergy efficiency. It delivers significant enhancement to established, bankable trough technology, and exceeds the heliostat power tower molten salt temperature limit. The use of inert particles for heat transfer fluid make parabolic trough safe near population centers and valuable for industrial facilities at even higher temperatures than today.