Kok-Keong Chong

Non‐imaging Focusing Heliostat

A non-imaging focusing heliostat for effective use of thermal solar energy is proposed. The heliostat consists of a number of grouped slave mirrors, which are able to move according to a proposed formula to eliminate the first order aberration. The master mirror tracks the sun by a proposed rotation-elevation mode to project solar rays together with the rest of slave mirrors into a fixed target. The merit of this design is that it may benefit the use of solar energy in high temperature applications by allowing a single stage collector to replace a conventional double stage structure; it may also benefit high concentration applications, e.g., solar powered Stirling engines, solar pumped lasers, etc. The feasibility and a reliability test of the proposed method by a prototype heliostat in the University of Technology, Malaysia is reported.

Comparison of Two Sun Tracking Methods in the Application of a Heliostat Field

The basic mathematics and structure of heliostat have remained unchanged for many decades. Following the challenge first made by Ries et al., the non-imaging focusing heliostat recently proposed by Chen et al. provides an alternative in the field of concentrated solar energy. This paper investigates the performance of a heliostat field composed of the newly proposed heliostats. In contrast to the dynamic curvature adjustment proposed in our previous work for a solar furnace, a fixed asymmetric curvature is used here with the spinning-elevation tracking method. This restriction is intended to equalize the manufacture cost of the new heliostat with that of traditional heliostats with azimuth-elevation tracking and spherical curvature. Fixing the curvature results in only partial aberration correction, compared to full correction using the dynamic adjustment of curvature. Nevertheless, the case studies presented in this paper show that the new heliostat design can reduce the receiver spillage loss by 10-30%, and provide a much more uniform performance without large variations with time of day.

Study of residual aberration for non-imaging focusing heliostat

Instead of using a specific focusing geometry, a non-imaging focusing heliostat has no fixed geometry but is composed of many small movable element mirrors that can be manoeuvred to eliminate the first-order aberration. Following our previous publication on the principle of non-imaging focusing heliostat, this paper further explores higher order residual aberration that limits the size of the focusing spot. The residual aberration can be partially corrected by offsetting the pivot point of mirrors and pre-setting the tilting angles of mirrors.

Optical Characterization of Nonimaging Planar Concentrator for the Application in Concentrator Photovoltaic System

The design and construction of miniature prototype of nonimaging planar concentrator, which is capable of producing much more uniform spatial irradiance and reasonably high concentration ratio, were presented in the previous paper. In this paper, we further explore the optical characteristics of the new concentrator that is specially designed to be incorporated in concentrator photovoltaic systems. For this study, we have carried out a comprehensive analysis via numerical simulation based on all the important design parameters, i.e., array of facet mirrors, f/D ratio, receiver size, and the effect of suntracking error, which lead to the overall optical performance of the new concentrator.

Optical characterization of nonimaging dish concentrator for the application of dense-array concentrator photovoltaic system

Optimization of the design of a nonimaging dish concentrator (NIDC) for a dense-array concentrator photovoltaic system is presented. A new algorithm has been developed to determine configuration of facet mirrors in a NIDC. Analytical formulas were derived to analyze the optical performance of a NIDC and then compared with a simulated result obtained from a numerical method. Comprehensive analysis of optical performance via analytical method has been carried out based on facet dimension and focal distance of the concentrator with a total reflective area of 120 m2. The result shows that a facet dimension of 49.8 cm, focal distance of 8 m, and solar concentration ratio of 411.8 suns is the most optimized design for the lowest cost-per-output power, which is US

Performance optimization of dense-array concentrator photovoltaic system considering effects of circumsolar radiation and slope error.

1.93 per watt.

Optimization of nonimaging focusing heliostat in dynamic correction of astigmatism for a wide range of incident angles

This paper presents an approach to optimize the electrical performance of dense-array concentrator photovoltaic system comprised of non-imaging dish concentrator by considering the circumsolar radiation and slope error effects. Based on the simulated flux distribution, a systematic methodology to optimize the layout configuration of solar cells interconnection circuit in dense array concentrator photovoltaic module has been proposed by minimizing the current mismatch caused by non-uniformity of concentrated sunlight. An optimized layout of interconnection solar cells circuit with minimum electrical power loss of 6.5% can be achieved by minimizing the effects of both circumsolar radiation and slope error.

Temperature effects on the performance of dense array concentrator photovoltaic system

To overcome astigmatism has always been a great challenge in designing a heliostat capable of focusing the sunlight on a small receiver throughout the year. In this Letter, a nonimaging focusing heliostat with a dynamic adjustment of facet mirrors in a group manner has been analyzed for optimizing the astigmatic correction in a wide range of incident angles. This what is to the authors knowledge a new heliostat is not only designed to serve the purpose of concentrating sunlight to several hundreds of suns, but also to significantly reduce the variation of the solar flux distribution with the incident angle.

Solar flux distribution analysis of Non-Imaging Planar Concentrator for the application in concentrator photovoltaic system

The performance of dense-array concentrator photovoltaic (CPV) is studied in detail by considering temperature distribution pattern generated from a water-cooled copper cooling block. Using water flow rate 0.400 kg/s and inlet temperature 30°C, the temperature distribution is simulated and the temperature values are used as a means to predict each CPV cells operating temperature. It is observed that a solar cell located at the central region of the array experiences the highest temperature of 58.3°C, while CPV cells located furthest to the center are operating at 7°C lower than the center region cell. For comparison purpose, we also investigated the effect of uniform temperature distribution for all the cells at 55.2°C. It is found that the output power varies by less than 1W compared to the case of non-uniform temperature distribution where each solar cell is experiencing a different temperature value. On the other hand, the output power increases to 462.70 W when the array temperature is reduced to 40°C, while the output power dropped to 418.60 W when the array temperature is increased to 100°C. Through better understanding of temperature effects to dense array CPV performance, a suitable cooling system can be designed to minimize power loss.

Novel Optical Scanner Using Photodiodes Array for Two-Dimensional Measurement of Light Flux Distribution

The design and construction of Non-Imaging Planar Concentrator (NIPC), capable of producing much more uniform spatial irradiance and reasonably high concentration ratio, have been presented in our previous research paper. In this study, we would carry out a comprehensive analysis through the numerical simulation on solar flux distribution at the target by considering all the important criteria to improve the overall performance of dense-array concentrator photovoltaic system, which are the maximum solar concentration, uniform illumination area, spillage loss etc. Maximum solar concentration ratio and percentage of energy in uniform illumination area are plotted for different cases. In general, the simulated results have shown a reasonably good uniformity of solar irradiance and high concentration ratio at the receiver plane.