Effects of geometrical parameters of a dish concentrator on the optical performance of a cavity receiver in a solar dish‐Stirling system

Dish‐Stirling concentrated solar power (DS‐CSP) system is a complex system for solar energy‐thermal‐electric conversion. The dish concentrator and cavity receiver are optical devices for collecting the solar energy in DS‐CSP system; to determine the geometric parameters of dish concentrator is one of the important steps for design and development of DS‐CSP system, because it directly affects the optical performance of the cavity receiver. In this paper, the effects of the geometric parameters of a dish concentrator including aperture radius, focal length, unfilled radius, and fan‐shaped unfilled angle on optical performance (ie, optical efficiency and flux distribution) of a cavity receiver were studied. Furthermore, the influence of the receiver‐window radius of the cavity receiver and solar direct normal irradiance is also investigated. The cavity receiver is a novel structure that is equipped with a reflecting cone at bottom of the cavity to increases the optical efficiency of the cavity receiver. Moreover, a 2‐dimensional ray‐tracking program is developed to simulate the sunlight transmission path in DS‐CSP system, for helping understanding the effects mechanism of above parameters on optical performance of the cavity receiver. The analysis indicates that the optical efficiency of the cavity receiver with and without the reflecting cone is 89.88% and 85.70%, respectively, and former significantly increased 4.18% for 38 kW XEM‐Dish system. The uniformity factor of the flux distribution on the absorber surface decreases with the decreases of the rim angle of the dish concentrator, but the optical efficiency of the cavity receiver increases with the decreases of the rim angle and the increase amplitude becomes smaller and smaller when the rim angle range from 30° to 75°, So the optical efficiency and uniformity factor are conflicting performance index. Moreover, the unfilled radius has small effect on the optical efficiency, while the fan‐shaped unfilled angle and direct normal irradiance both not affect the optical efficiency. In addition, reducing the receiver‐window radius can improve the optical efficiency, but the effect is limited. This work could provide reference for design and optimization of the dish concentrator and establishing the foundation for further research on optical‐to‐thermal energy conversion.     

Conical receiver for a paraboloidal concentrator with large rim angle

The design and optimization of novel type of receiver for a paraboloidal concentrator with 90° rim angle is carried out by means of detailed ray tracing simulations. Cylindrical, conical, and spherical geometries are compared and their dimensions optimized. The chosen design is based on a conical cavity, which differs from similar receivers developed for concentrators with smaller rim angles. In particular, the receiver is able to catch concentrated solar energy both on its outer side and on the inner walls. Water flows inside the receiver along the conical geometry, in a double layer configuration. The receiver was built and implemented in a 90° rim angle paraboloidal concentrator. Thermal efficiency of the system is evaluated for different flow rates and inlet temperatures, both in stationary and in transient regimes, and results for fluid temperatures are compared with the results predicted by a thermal model. The time constant is evaluated.     

Prediction and optimization of the performance of parabolic solar dish concentrator with sphere receiver using analytical function

Parabolic solar dish concentrator with sphere receiver is less studied. We present an analytic function to calculate the intercept factor of the system with real sun brightness distribution and Gaussian distribution, the results indicate that the intercept factor is related to the rim angle of reflector and the ratio of receiver angle to the optical error when the optical error is larger than or equal to 5 mrad, but is related to the rim angle, receiver angle and optical error in less than 5 mrad optical error. Furthermore we propose a quick process to optimize the system to provide the maximum solar energy to net heat efficiency for different optical error under typical condition. The results indicate that the parabolic solar dish concentrator with sphere receiver has rather high solar energy to net heat efficiency which is 20% more than solar trough and tower system including higher cosine factor and lower heat loss of the receiver.     

Thermodynamic optimisation of the integrated design of a small‐scale solar thermal Brayton cycle

The Brayton cycle's heat source does not need to be from combustion but can be extracted from solar energy. When a black cavity receiver is mounted at the focus of a parabolic dish concentrator, the reflected light is absorbed and converted into a heat source. The second law of thermodynamics and entropy generation minimisation are applied to optimise the geometries of the recuperator and receiver. The irreversibilities in the recuperative solar thermal Brayton cycle are mainly due to heat transfer across a finite temperature difference and fluid friction. In a small‐scale open and direct solar thermal Brayton cycle with a micro‐turbine operating at its highest compressor efficiency, the geometries of a cavity receiver and counterflow‐plated recuperator can be optimised in such a way that the system produces maximum net power output. A modified cavity receiver is used in the analysis, and parabolic dish concentrator diameters of 6 to 18 m are considered. Two cavity construction methods are compared. Results show that the maximum thermal efficiency of the system is a function of the solar concentrator diameter and choice of micro‐turbine. The optimum receiver tube diameter is relatively large when compared with the receiver size. The optimum recuperator channel aspect ratio for the highest maximum net power output of a micro‐turbine is a linear function of the system mass flow rate for a constant recuperator height. For a system operating at a relatively small mass flow rate, with a specific concentrator size, the optimum recuperator length is small. For the systems with the highest maximum net power output, the irreversibilities are spread throughout the system in such a way that the internal irreversibility rate is almost three times the external irreversibility rate. Copyright © 2011 John Wiley & Sons, Ltd.     

An optical analysis of a static 3-D solar concentrator

Concentrating technology is long established in the field of solar thermal applications. However, there is still scope for improvement due to innovation in design, materials and manufacturing methods. The optical efficiency of a solar concentrator depends largely on the geometry of the concentrator profile. This paper evaluates the optical performance of a static 3-D Elliptical Hyperboloid Concentrator (EHC) using ray tracing software. Ray tracing has been used extensively to calculate the optical efficiency of the static 3-D EHC. Performance parameters such as effective concentration ratio, optical efficiency and geometric concentration ratio are also evaluated for different aspect ratios of the elliptical profile. Optimization of the concentrator profile and geometry is also carried out to improve the overall performance; this parametric study includes the concentrator height, solar incidence angle and aspect ratio of the ellipse. The overall performance of the concentrator was assessed based on the acceptance angle, effective concentration ratio and optical efficiency. Finally, the flux distribution on the receiver area for different concentrator heights is also presented.     

基于三角腔体吸收器的透射式菲涅尔定焦线系统聚光特性分析

为解决线性菲涅尔太阳能集热系统单轴跟踪过程中出现的聚光焦线偏移以及降低系统跟踪能耗等问题,提出一种透射式菲涅尔定焦线太阳能聚光器.该聚光器采用极轴跟踪方式与线性菲涅尔透镜定期滑移调节方式相结合,可实现固定焦线聚光.将该聚光器与三角腔体吸收器所组成的太阳能集热系统,利用基于蒙特卡罗光线追迹法的TracePro光学软件分析...     

An array of directable mirrors as a photovoltaic solar concentrator

Calculations of the optics of heliostats for use in large thermal power towers have been carried out in considerable detail, chiefly by Vant-Hull et al.[1, 2]. This paper describes a simplified method for calculating the images generated by a special type of concentrator, i.e. an array of independently steered mirrors on a single frame, intended to direct the solar image onto a flat photovoltaic solar cell target. The case of interest is one in which the field of illumination on the target is as uniform as possible, and the emphasis is thus on small “rim angle” geometries (a configuration which also minimizes mirror interference effects). Calculations are presented for constructing the individual mirror target images in terms of three angles: (1) The angle between the photovoltaic target normal and the reflecting mirror (celled here the mirror position angle). (2) The angle between the target center and the sun as measured from the center of the reflecting mirror, and (3) The angle at which the plane defined by the center of the sun, the mirror center and the target center intersects the plane of the target.The overall system efficiency for various mirror configurations, charaterized by such parameters as the maximum mirror angle (i.e. “rim angle”), target-mirror plane separation, and mirror aiming accuracy is discussed in terms of the specifications desirable in an optical concentrator designed specifically to illuminate uniformly a photovoltaic solar cell target.     

Two-stage concentrator permitting concentration factors up to 300x with one-axis tracking

Economic operation of high-efficiency concentrator solar cells requires solar concentration ratios which up to now can only be achieved with two-axis tracking. In this paper we present a two-stage concentrator approaching concentration ratios up to 300X while being tracked around only one polar axis. Its principle is as follows: a parabolic trough focuses the direct solar radiation onto a line parallel to the polar tracking axis. The half rim angle of this first concentrating stage is chosen to be equal to the sun's maximum declination of 23.5°. The second stage consists of a row of dielectric, nonimaging 3-D-concentrators, which couple the concentrated light directly into square solar cells. In contrast to linear secondaries the 3-D-secondaries make use of the limited divergence of ± 23.5° in the NS-direction to achieve additional concentration. The performance of the system depends sensitively on how well the angular acceptance characteristic of the second stage matches with the square-shaped angular irradiance distribution in the focal line of the parabolic trough. A new concentrator profile has been found that exhibits an almost ideal square acceptance characteristic with a very sharp cut-off. A prototype two-stage concentrator has been constructed with a total geometrical concentration of 214X. In outdoor measurements a total optical efficiency of 77.5% was obtained.     

Performance of an intermittently tracked cylindrical parabolic trough

The distribution of local concentration ratios over a flat plate absorber used with an intermittently tracked cylindrical parabolic trough is studied for two values of the rim angle. The amount of energy collected by an intermittently tracked solar concentrator, oriented in various tracking modes, at different times of the year, for two typical latitudes, is also calculated. The results are presented graphically and discussed.     

Heat transfer in a conical cavity calorimeter for measuring thermal power of a point focus concentrator

A theoretical study of the heat transfer process that takes place in a special calorimeter of conical cavity named CAVICAL is presented. This instrument is used to measure the thermal power of a point focus solar concentrator system named DEFRAC, developed at the Center for Energy Research of the National University of Mexico. The DEFRAC concentrator has a power of 1.3 kWth and a very fine optical system. The calorimeter has a cavity opening of 8.24 cm². A detailed heat transfer study was done using FLUENT code. The heat transfer processes taken into account for the analysis were the radiative energy absorbed by the inner cavity wall, the energy transfer from the outer cavity wall to the air by natural convection, the energy transferred by conduction through the inner metallic wall of the calorimeter, and by forced convection to the fluid in the cooling system. The calorimetric information gathered allowed determining the thermal power that the concentrator is able to capture. Temperature and velocity fields have been calculated for each of the thermal fluids considered inside of the calorimeter. The analysis gave thermal losses and measured the thermal efficiency of the device. The information generated is useful to further optimize the design of the calorimeter.     

Upper bounds for the yearly energy delivery of stationary solar concentrators and the implications for concentrator optical design

Compound parabolic concentrator (CPC) type collectors have been viewed as the optimal design for totally stationary concentrators. However the CPC is ideal only for uniform incident solar flux averaged over the energy collection period. The actual yearly-averaged incident flux map turns out to be highly non-uniform, as a function of projected incidence angle, which implies that concentration can be increased markedly if optical collection efficiency is compromised. The question then becomes: what concentrator angular acceptance function is best matched to nature's radiation flux input, and how much energy can such a concentrator deliver? The recently-invented tailored edge-ray concentrator (TERC) approach could be used to determine optimal reflector contours, given the optimal acceptance angle function. We demonstrate that totally stationary TERCs can have around three times the geometric concentration of corresponding optimized stationary CPCs, with greater energy delivery per absorber area, in particular for applications that are currently being considered for stationary evacuated concentrators with the latest low-emissivity selective coatings, e.g. solar-driven double-stage absorption chillers (at around 170°C) and solar thermal power generation (at around 250°C).     

碟式/斯特林太阳能热发电系统设计Ⅰ--集热器设计计算数学模型

在碟式斯特林太阳能热发电系统没有商业运行以前，无法得到国外已有系统设计方法的情况下，提出了以能量守恒方程和集热器光学特性为基础的设计计算数学模型，并使用估算和核算相结合的方法进行了设计计算。计算结果与现有系统参数吻合良好。     

Investigation of evacuated tube heated by solar trough concentrating system

Two types of solar evacuated tube have been used to measure their heating efficiency and temperature with fluids of water and N₂ respectively with a parabolic trough concentrator. Experiments demonstrate that both evacuated tubes present a good heat transfer with the fluid of water, the heating efficiency is about 70–80%, and the water is easy to boil when liquid rate is less than 0.0046 kg/s. However, the efficiency of solar concentrating system with evacuated tube for heating N₂ gas is less than 40% when the temperature of N₂ gas reaches 320–460 °C. A model for evacuated tube heated by solar trough concentrating system has been built in order to further analyze the characteristics of fluid which flow evacuated tube. It is found that the model agrees with the experiments to within 5.2% accuracy. The characteristics of fluid via evacuated tube heated by solar concentrated system are analyzed under the varying conditions of solar radiation and trough aperture area. This study supports research work on using a solar trough concentrating system to perform ammonia thermo-chemical energy storage for 24 h power generation. The current research work also has application to solar refrigeration.     

闭式Brayton循环的太阳能热动力空间发电技术

对闭式Brayton循环太阳能热动力系统在空间站的研究作了综述，着重讨论了该循环的发电原理和主要能量转换模块-涡轮发电机-压缩机，目前国际研究达到了循环效率超达30%、系统总效率超过17%的水平，与光伏电池系统相比，SD系统具有高效率、质量轻，费用少，寿命长等特点。为提高系统的效率和减少空间环境的腐蚀，对聚能器表面和辐射散热器表面均需进行保护涂层处理，文章介绍了国际上这方面的研究成果。     

An analysis of the terminal concentrator concept for solar central receiver systems

One of the most interesting approaches to the large scale development of solar energy for electric power production is the Central Receiver System Concept. The Central Receiver System contains a large number of individually guided heliostats that reflect sunlight towards a central receiver high above the field of heliostats. The system resembles a giant Fresnel mirror and provides a substantial concentration of the solar beam. If high concentration is desired, a terminal concentrator may be included.The terminal concentrator is a device designed to increase the concentration of solar flux reflected from the collector field. Our study depends on two assumptions: (1) the beam width formula for the reflected beams and (2) the uniformly bright collector field which is a gross simplification allowing us to deal with the terminal concentrator. We obtained the necessary design relations, including a lower bound for the rim angle φ_m, the average fraction reflected $\text{?}\text{(φ}{}_{\mathrm{m}}\text{)}$, a radiative stagnation temperature for the aperature, and the concentration ratios. The temperature and concentration ratio curves determine the optimum rim angle φ_m for each of several designs. When designed to provide maximum concentration, the terminal concentrator becomes excessively large. Consequently, we consider a design which produces 90 per cent of the maximum concentration and reduces the size of the conical reflector by 5–6 times. The effectiveness of this compromise design permits us to conclude that a practical terminal concentrator of the conical variety can almost double the concentration without any appreciable loss of total power. There will be losses due to reflectivity but not due to beam spillage because of the reduced aperture. The terminal concentrator will be economically desirable for small central receiver systems if it is cheaper than the incremental cost of the heliostat field due to the additional focusing required to produce the same concentration.     

Evaluation of a large dish‐type concentrator solar lighting system for underground car park

To utilize solar energy more efficiently and reduce lighting power consumption in underground public spaces such as car park, a large dish‐type concentrator solar lighting system is put forward along with its evaluation, which is a unique design to apply a laminated layer of beam split thin‐film coating and thin‐film solar cells onto the dish reflector. The collected sunlight is split into 2 parts, one being reflected into a fiber optical bundle and transmitted for daylighting, while the rest being absorbed by solar cells for electricity generation as the other way to replenish daylighting. A set of 4 solar lighting systems using 3.28‐m diameter dish are designed to meet the lighting requirement in a 1771‐m² underground car park. A mathematical model is adopted to calculate the output power and conversion efficiency of solar cells distributed on the parabolic dish surface. The indoor illuminance distribution is given by lighting simulation. The results indicate that the average daylight illuminance in the car park can vary between 62.7 and 284 lx on February 25, 2016 and between 62.7 and 353 lx on August 17, 2016 for 2 chosen days, respectively. For the presented design, the electricity produced by solar cells is just enough to power light‐emitting diodes for lighting meeting a criterion at night. Considering about 19% conversion efficiency of solar cells and the efficacy of 129.5 lm/W of light‐emitting diodes, the hybrid solar lighting system can have about 40% utilization ratio of solar energy, so it can be concluded that a sufficient lighting provision can be provided by the proposed large dish‐type concentrator solar lighting system for applications in underground car park.     

Design and cost analysis for an ammonia-based solar thermochemical cavity absorber

A design and cost analysis is introduced for a solar thermochemical cavity absorber operated at the focus of a tracking paraboloidal concentrator and based on the ammonia dissociation reaction. The absorber design consists simply of a catalyst-filled nickel alloy tube wound into a spiral forming the inner cavity wall and shaped suitably to match the incident power density profile to the reactor heat requirements. The reactor tube is welded to a coaxial counterflow heat exchanger. The relationships among the power density profile, the reaction thermodynamics and kinetics and the heat transfer characteristics are examined in detail and it is shown that there is considerable potential that an installed cost goal of typically 10 U.S. dollars per square metre of solar collector area can be achieved through use of an appropriate high activity ammonia dissociation catalyst. There is a need for further tests of ammonia dissociation catalysts and also further nitridation tests of high-strength nickel alloys. The optimum absorber size for a given paraboloidal dish area is calculated for a system pressure of 20 MPa and it is shown that a cost effective absorber suitable for 100,000-hr operation would operate at 90 per cent efficiency at 750°C wall temperature.     

Theoretical analysis of the performance of a conical solar concentrator

This paper describes a study of the conical solar energy concentrator with tubular axial absorber. The concentrated power is evaluated, in a dimensionless form, as a function of the mirror surface quality and the absorber-to-aperture diameter ratio. The irradiated length of the absorber is determined and the axial concentration distribution along its surface is expressed mathematically. An integrated, or average, concentration ratio is used to measure the concentrating power of the reflector-absorber assembly. In addition to the mirror reflectivity, the performance is shown to be influenced by three parameters—the apex angle, the diameter ratio and the truncation ratio. The effects of these parameters on the concentrated power, the concentration profile and the reflector-surface area are investigated.     

A comprehensive study of dense-array concentrator photovoltaic system using non-imaging planar concentrator

A special modeling method using Simulink has been developed to analyze the electrical performance of dense-array concentrator photovoltaic (CPV) system. To optimize the performance of CPV system, we have adopted computational modeling method to design the best configuration of dense-array layout specially tailored for flux distribution profile of solar concentrator. It is an expeditious, efficient and cost effective approach to optimize any dense-array configuration for any solar concentrator. A prototype of non-imaging planar concentrator (NIPC) was chosen in this study for verifying the effectiveness of this method. Mismatch effects in dense array solar cells caused by non-uniform irradiance as well as sun-tracking error normally happens at the peripheral of the array. It is a crucial drawback that affects the electrical performance of CPV systems because maximum output power of the array is considerably reduced when a current–voltage (I–V) curve has many mismatch steps and thus leads to lower fill factor (FF) and conversion efficiency. The modeling method is validated by assembling, installing and field testing on an optimized configuration of solar cells with the NIPC prototype to achieve a conversion efficiency of 34.18%. The measured results are in close agreement with simulated results with a less than 3% deviation in maximum output power.     

A comparison of GaAs and Si hybrid solar power systems

Various silicon hybrid systems are modeled and compared with a Gallium Arsenide hybrid system. The hybrid systems modeled produce electric power and also thermal power which can be used for heating or air conditioning. Various performance indices are defined and are used to compare the system performance. The performance indices are: capital cost per unit electric power out; capital cost per total power out; capital cost per unit electric power plus mechanical power; annual cost per unit electric energy; and annual cost per unit electric plus mechanical work. These performance indices indicate that concentrator hybrid systems can be cost effective when compared with present day energy costs. Realistic costs and efficiencies of GaAs and Si are respectively $35,000/m² for 15 per cent efficient solar cells and $1000/m² for 10 per cent efficient solar cells based on information available at the time of this study in late 1975. The performance indices show that the limiting values for annual costs are 10.3¢/kWh and 6.8¢/kWh for Si and GaAs respectively. Results demonstrate that for a given flow rate there is an optimal operating condition for maximum photovoltaic output associated with concentrator systems. Also concentrator hybrid systems produce a distinct cost advantage over flat hybrid systems.