A stationary solar energy concentrator

A parabolic reflector having a short focal length will keep the sun's rays focused within a small area for several hours without the employment of a heliostat mechanism. Its use as a steam generator and as a stove is described.     

太阳能集热器跟踪传感器

应太阳能集热器发展的需要提出一种新型跟踪随动系统传感器。该传感器结构新颖,构思巧妙,造价低廉,丰富了太阳能集热器跟踪随动系统传感器的类型。     

Reverse ray-tracing model for the performance evaluation of stationary solar concentrators

The yearly energy collection efficiency of stationary solar concentrators can be evaluated using reverse ray-tracing, and a solar radiation model. In reverse ray-tracing, rays originating at the receiver of the concentrator are traced towards the surrounding hemisphere. The method allows for the evaluation of the absolute energy collection: new concentrators may be optimized for location and tilt, requiring one-time ray-tracing. The tilt of existing concentrators is optimized. Only possible solar incidence is considered by our model. The method is fast and realistic; it can be modified for concentrators in tilt operation.     

Upper bounds for solar thermophotovoltaic efficiency

The main components of thermophotovoltaic (TPV) devices are the primary lens (or mirror), the absorber, the PV cell, and a photon recuperator system. A theory integrating all these components is used in this paper to analyse a particular type of TPV device (plane disk absorber and PV cell). The TPV efficiency is maximized by using three optimization parameters, namely absorber, PV cell temperatures, and cell voltage. Almost ideal operation conditions are envisaged and upper bounds are obtained for the TPV efficiency. They are strongly dependent on PV cell bandgap and radiation concentration. Preliminary results suggest the existence of an optimum solar radiation concentration ratio. The improvement in thermal design quality allows the usage of PV cells based on wide bandgap semiconductors.     

A novel procedure for the optical characterization of solar concentrators

A novel procedure for the optical characterization of solar concentrators is presented. The method is based on recording at night the light of a star reflected by the mirror. Images of the mirror taken from its focal region allow the reconstruction of the slope map. The application of this technique for the in situ characterization of heliostats is particularly simple and at very low cost. Results on first tests carried out with a heliostat of the CESA-I field at the Plataforma Solar de Almeria have shown the feasibility of this technique. Uncertainties in the reconstructed slopes of about 1.0 mrad have been estimated.     

太阳能集能器自动跟踪装置

太阳能集能器自动跟踪装置是由传感器、方位角跟踪机构、高度角跟踪机构和自动控制装置组成。自动跟踪装置驱动太阳能集能器 ,使集能器的主光轴始终与太阳光线相平行。当太阳光线发生倾斜时 ,传感器输出倾斜信号 ,指示执行器动作调整太阳能集能器的角度 ,直到太阳能集能器对准太阳 ,实现自动跟踪太阳的目的。在阴天或太阳光辐照度低于工作照度时自动关机 ,太阳光辐照度达到工作照度时自动开机。该装置适用于聚光式太阳能集能器、太阳能电池等需要跟踪太阳的装置     

Micro-structured reflector surfaces for a stationary asymmetric parabolic solar concentrator

One of the main problems in using parabolic concentrators with standard photovoltaics (PV) cells is the highly non-uniform illumination of the cells. The non-uniform irradiation causes high resistive losses in the standard cells due to their relatively high series resistance. This results in a considerably lowered efficiency. To solve the problem, we introduce three different structured reflectors that will create a more uniform illumination, and also increase the concentration ratio in certain cases. The structures were evaluated in an existing trough system by Monte Carlo ray tracing, and it was found that structures improve the system performance mainly by homogenizing the light on the cells. The yearly irradiation collected in the evaluation system is slightly lower than for a reference with smooth reflectors, but the more uniform illumination of the cells will generate a net increase of the total system performance compared to a system that was optimized with smooth reflectors. The benefit of the increased concentration ratio is increased flexibility in designing new systems with concentration ratios surpassing the limit of existing trough concentrators.     

New high-flux two-stage optical designs for parabolic solar concentrators

We present a new two-stage optical design for parabolic dish concentrators that can realistically attain close to 90% of the thermodynamic limit to concentration with practical, compact designs (e.g., at parabola rim half-angles of around 45°). For comparison, the parabolic dish-plus-compound parabolic concentrator secondary design, at this rim angle, achieves no more than 50% of the thermodynamic limit. Our new secondary concentrator is tailored to accept edge rays from the parabolic primary, and incurs less than one reflection on average. It necessitates displacing the absorber from the parabola's focal plane, along the concentrator's optic axis, toward the primary reflector, and constructing the secondary between the absorber and the primary. The secondary tailored edge-ray concentrators described here create new possibilities for building compact, extremely high flux solar furnaces and/or commercial parabolic dish systems.     

Performance model for two-stage optical concentrators for solar thermal applications

A performance model has been developed for evaluating benefits associated with the addition of a nonimaging secondary concentrator to a conventional paraboloidal solar dish. The model uses a Monte Carlo ray-trace procedure to determine the focal plane distribution as a function of optical parameters and, by evaluating the trade-off between thermal losses and optical gain, calculates the corresponding optimized concentration and thermal efficiency as a function of temperature, both with and without the secondary. These comparative optimizations, carried out over a wide range of design parameters, show that the efficiency of a two-stage concentrator is always greater than that of a single stage if all other design parameters are the same. For example, for a reference design corresponding to a dish with a focal length to diameter ratio of 0.6 and a characteristic slope error of 5 milliradians operated at a receiver temperature of 1000°C, the optimized efficiency with a secondary is 0.70 compared to 0.59 for the primary alone. At fixed focal ratio, the relative performance advantage with a secondary increases, if either the temperature or the primary slope error or both, are increased, whereas it decreases if they are decreased. However, the advantage remains significant at temperatures above 400°C, even in the “high performance limit” of slope errors <2 milliradians.     

An optical profilometer for the characterisation of parabolic trough solar concentrators

An optical profilometer has been developed based on the idea that the large panels that compose solar-power concentrators and a tolerance threshold for slope-errors to some milliradians, can allow the use of a geometric-optics framework and the investigation of ray-paths by means of a laser beam. The instrument scheme is discussed in detail together with the related problems and the solutions adopted. Data analysis also makes use of an innovative method, based on the iterative application of a criterion about the intersection between the tangents of adjacent points, which allows a realistic profile of the reflector-surface to be drawn. The methodology adopted allows us to call our instrument a profilometer rather than an instrument that gives only information about the tangent of the profile. The instrument accuracy on profile and arctangent deviations from the ideal parabola is 20 μm and 15 μrad, respectively. The results obtained for a reflecting panel are also reported.     

Design factors of sensors for the optical tracking systems of solar concentrators

Basic diagrams for the sensors of the optical tracking systems of solar concentrators are considered, the design factors that determine their accuracy are analyzed, a new sensor design is suggested, and its optimal parameters are determined.     

Optical design of a flat-facet solar concentrator

We present a procedure to design a facet concentrator for laboratory-scale research on medium-temperature thermal processes. The facet concentrator approximates a parabolic surface with a number of flat square facets supported by a parabolic frame and having two edges perpendicular to the concentrator axis. The optimum size and position of each facet are found by employing Monte Carlo ray tracing analysis to achieve desired flux characteristics in the focal plane. A 164-facet concentrator with realistic specularly-reflecting surface and facet positioning accuracy will deliver up to 8.15 kW of radiative power over a 15 cm radius disk located in the focal plane, with average concentration ratio exceeding 100.     

Principles of solar concentrators of a novel design

A new principle for collecting and concentrating solar energy, the ideal cylindrical light collector, has been invented. This development has its origins in detecting Cherenkov radiation in high energy physics experiments. In its present form, the collector is a trough-like reflecting wall light channel of a specific shape which concentrates radiant energy by the maximum amount allowed by phase space conservation. The ideal cylindrical light collector is capable of accepting solar radiation over an average ～8-hr day and concentrating it by a factor of ～10 without diurnal tracking of the sun. This is not possible by conventional imaging techniques. The ideal collector is non-imaging and possesses an effective relative aperture (f-number) =0·5. This collector has a larger acceptance for diffuse light than concentrating collectors using imaging optics. In fact, the efficiency for collecting and concentrating isotropic radiation, in comparison with a flat plate collector, is just the reciprocal of the concentration factor.     

Study on design of molten salt solar receivers for beam-down solar concentrator

Systems using molten salt as thermal media have been proposed for solar thermal power generation and for synthetic fuel production. We have been developing molten salt solar receivers, in which molten salt is heated by concentrated solar radiation, in the Solar Hybrid Fuel Project of Japan. A cavity shaped receiver, which is suitable for a beam-down type solar concentration system, was considered. In order to design molten salt solar receivers, a numerical simulation program for the prediction of characteristics of receivers was developed. The simulation program presents temperature distributions of a receiver and molten salt with the use of heat flux distribution of solar radiation and properties of composing materials as input data. Radiation to heat conversion efficiency is calculated from input solar power and heat transferred to molten salt. The thermal resistance of molten salt and the maximum discharge pressure of molten salt pumps were taken into account as restrictions for the design of receivers. These restrictions require control of maximum receiver temperature and pressure drop in the molten salt channel. Based on the incident heat flux distribution formed with a 100 MWth class beam-down type solar concentration system, we proposed a shape of solar receiver that satisfies the requirements. The radiation to heat conversion efficiency of the designed receiver was calculated to be about 90%.     

Morphogenic properties of curvilinear reflectors of solar energy concentrators

A method is proposed for determining the deformed state of the surface of a curvilinear reflector of a solar energy concentrator by approximating its generatrix with a spline curve.     

Electronic film with embedded micro-mirrors for solar energy concentrator systems

A novel electronic film solar energy concentrator with embedded micro-mirrors that track the sun is described. The potential viability of this new concept is presented. Due to miniaturization, the amount of material needed for the optical system is minimal. Because it is light-weight and flexible, it can easily be attached to the land or existing structures. This presents an economic advantage over conventional concentrators which require the construction of a separate structure to support them, and motors to orient them to intercept and properly reflect sunlight. Such separate structures must be able to survive gusts, windstorms, earthquakes, etc. This concentrator utilizes the ground or existing edifices which are already capable of withstanding such vicissitudes of nature.     

Modeling and calculation of energy characteristics of the solar radiation linear concentrators

An algorithm and program are developed for calculating and studying the characteristics of the concentration of linear cylindrical parabolic concentrators as a function of the time of day. The possibility of generalizing the concentration and flow curves at the receiver as a function of time is demonstrated.     

The design and performance of liquid filled stationary concentrators for use with photovoltaic cells

This paper discusses the design and practical application of dielectric filled stationary concentrators. The design adopted is a 3 × liquid filled lens-V trough concentrator fabricated as a thin walled plastic shell with total internal reflecting sides. The measured performance of both oil and water filled prototypes is in agreement with predicted performance. It is concluded that the dielectric filled LVT concentrator performs as well as the dielectric filled CPC in the stationary mode of operation and has advantages in relation to economy and ease of fabrication.     

Potential of a parabolic trough solar concentrator for electric energy production

At present, parabolic trough technology is considered as the most low‐cost and powerful large‐scale technology to utilize solar energy for electricity generation and produce steam for different industrial usages. This article recommends the generation of electricity by using a parabolic trough solar concentrator in the central area of the Kingdom of Saudi Arabia (KSA) at Dawadmi city. Pressurized water is used as the heat‐transfer working fluid. A computer algorithm was built using the Matlab program to simulate the performance parameters of the Euro Trough collector (ETC). The input data included the properties of the working fluid (pressurized water) and the designing parameters of ETC. The output data were the outlet water temperature, the coefficient of heat transfer, the heat loss, and the thermal, solar, and global efficiencies. The obtained results indicated the ability of this type of parabolic trough in KSA to generate electric power due to the high‐performance parameters achieved. Also, the validity of using the simulation technique was measured and it showed good conformity.