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 has been shown to achieve the theoretical limits constrained only by thermodynamic principles. The designing principles of nonimaging optics allow a non-conventional way of thinking about and generating new optical devices. Compared to conventional imaging optics which rarely utilizes the framework of thermodynamic arguments, nonimaging optics chooses to map etendue instead of rays. This fundamental shift of design paradigm frees the optics design from ray based designs which heavily relies on error tolerance analysis. Instead, the underlying thermodynamic principles guide the nonimaging design to be naturally constructed for extended light source for illumination, non-tracking concentrators and sensors that require sharp cut-off angles. We argue in this article that such optical devices which has enabled a multitude of applications depends on probabilities, geometric flux field and radiative heat transfer while “optics” in the conventional sense recedes into the background.
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.
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.
Nonimaging optics is the theory of thermodynamically efficient optics and as such depends more on thermodynamics than on optics. Hence, in this paper, a condition for the “best” design is proposed based on purely thermodynamic arguments, which we believe has profound consequences for the designs of thermal and even photovoltaic systems. This way of looking at the problem of efficient concentration depends on probabilities, the ingredients of entropy and information theory, while “optics” in the conventional sense recedes into the background. Much of the paper is pedagogical and retrospective. Some of the development of flowline designs will be introduced at the end and the connection between the thermodynamics and flowline design will be graphically presented. We will conclude with some speculative directions of where the ideas might lead.
Nonimaging Optics has shown that it achieves the theoretical limits by utilizing thermodynamic principles rather than
conventional optics. Hence in this paper the condition of the "best" design are both defined and fulfilled in the
framework of thermodynamic arguments, which we believe has profound consequences for the designs of thermal and
even photovoltaic systems, even illumination and optical communication tasks. This new way of looking at the
problem of efficient concentration depends on probabilities, geometric flux field and radiative heat transfer while
“optics” in the conventional sense recedes into the background. Some of the new development of flow line designs
will be introduced and the connection between the thermodynamics and flow line design will be officially formulated
in the framework of geometric flux field. A new way of using geometric flux to design nonimaging optics will be
introduced. And finally, we discuss the possibility of 3D ideal nonimaing optics.
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.
Light pollution has become a prominent issue, specifically in National Parks such as Yosemite, where visitors go to
enjoy the natural ‘night sky’. In an effort to reduce light pollution, a particularly obtrusive light source has been
selected for retrofit. Using nonimaging optics and light emitting diodes (LEDs), light can be controlled to achieve a
desired prescribed illumination distribution. This distribution possesses a sharp cut-off such that light leakage is
minimal. Nonimaging optical designs are 3D printed, retrofitted into the candidate fixture, and tested in Yosemite
National Park. The end goal is to drastically reduce and even eliminate the excess light from sources around the
Proc. SPIE. 9572, Nonimaging Optics: Efficient Design for Illumination and Solar Concentration XII
KEYWORDS: Optical design, Transformers, Solar concentrators, Wavefronts, Radiative energy transfer, Black bodies, Compound parabolic concentrators, Nonimaging optics, Thermodynamics, Current controlled current source
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.
In this paper we will discuss the one-dimensional Hottel string method as it applies to symmetric, infinite sources (as in the case of constructing ideal solar concentrators) and extend the theory to asymmetric, finite sources and demonstrate that an ideal concentrator can be created in this case. Furthermore, we will discuss the concept of flowlines and explore the yet unknown relationship between strings and flowlines.
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.
In this paper we list a few problems in nonimaging optics which we believe are fundamental to further development of the subject. It is our hope that as they are solved, and crossed of the list, further progress can be facilitated.
Aplanats make great concentrators because of their near perfect imaging. Aplanatic conditions can be satisfied using two surface curves (generally mirrored surfaces) in two dimensions (see Figure 1) which are constructed by successive approximation to create a highly efficient concentrator for both concentration and illumination. For concentration purposes, having a two mirror system would be impossible because the front mirror would block incoming light (see figure 2) so the idea is to replace the front mirror with a "one-way" mirror. Light from a lower index can be transmitted, so if the aplanat surface is a higher index light is allowed to enter, and be trapped. In the Jellyfish design, TIR takes place except for light striking the surface within the range of critical angles. To combat that, a small area of reflective coating is applied to the central top part of the Jellyfish, where TIR fails (In the middle) to keep the light there from directly escaping (see figure 3). The design works in both forwards and reverse. Light entering can be focused to a collecter, or the collecter can be replaced with a light source to concentrate light out. In this case, LEDs are used for their highly efficienct properties.
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
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.