Review 正式发布 Versions 1 Vol 28 (5) : 929-947 2019
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Review on Development of Small Point-Focusing Solar Concentrators 小型点聚焦太阳能聚光装置的研究进展综述
: 2019 - 09 - 05
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Abstract & Keywords
Abstract: The technology of small point-focusing concentrator of solar energy has been developing rapidly in recent years owing to its compact structure and high collecting efficiency. This report presents important developments of small point-focusing concentrator in the past decade. This kind of solar concentrator refers to the parabolic dish concentrator, the point-focusing Fresnel lens, and the Scheffler reflector. Technological advances of these concentrators and the related performances have been presented. There are three main mirror fabrication technologies for dish concentrator, which are high polishing metal, silver-glass mirror and vacuum-membrane. Polymethyl methacrylate is widely used as material in Fresnel lens. Many scholars have proposed new lens shape to improve the uniformity of focusing. The Scheffler reflector has a characteristic of fixed focus, but its design parameters are not perfect so current research focuses on the theoretical calculation of the mirror. In addition, typical applications of the small point-focusing concentrator in photovoltaic system, solar thermal system, solar chemical system, and day-lighting system are summarized. Upon listing the important publications in open literature, a category of main applications of such kind of solar collector is provided based on the working characteristics of the system. 近几年来,小型点聚焦太阳能聚光技术由于其紧凑性和高效率在世界各国得到了快速发展。本文回顾了三种小型点聚焦聚光技术,即抛物型碟式、点聚焦菲涅尔透镜和舍弗勒反射镜的近十年来的发展情况。针对每种SPFC存在的问题,学者们已经提出了很多新的结构设计以提升它们的性能。其中,碟式聚光镜主要有三种镜面制造技术,分别为采用高抛光金属,镀银玻璃及抽真空膜。菲涅尔透镜普遍使用PMMA作为材料,很多学者已经提出了新的透镜形状来改善其聚光均匀性。舍弗勒反射镜具有固定焦点的特点,不过其设计参数还没有完善,目前研究重点为镜面的理论计算。此外,SPFC在不同领域内的应用也得到了最全最新的回顾。最后,我们总结了不同类型SPFC的工作特点及研究趋势,以给其它该领域的研究人员提供参考。
Keywords: point-focusing concentrator, Scheffler, dish, Fresnel, application
1. Introduction
Because of the reserve limit of fossil fuels and their negative effects on the environment, it becomes a hot topic to use effectively the renewable energy sources in the world [1]. Solar energy is one of the most important renewable energies. With advantages such as the wide distribution, the pollution free, and inexhaustible exploitation, solar energy is considered to face a huge potential market in the future [2]. Nevertheless, using the solar energy efficiently needs to solve some special problem. To solve the problem of the low energy density, the concentration technology must be taken into account, which is usually coupled with the solar-tracking technology. In the manufacturing, the concentrator should have a high processing precision, otherwise it may cause an uneven heating on the absorbor and then damage its life. Traditional solar concentrating systems can use different types of collector, such as the parabolic trough, the linear Fresnel, the parabolic dish, or the tower system [3]. The parabolic trough and the linear Fresnel are usually known as line focusing system, while the other two types are known as the point focusing system. As compared with the heliostat type and the linear concentrating type, the small point-focusing concentrator (SPFC) has a more compact structure and a higher concentration ratio. Therefore it has been paid more and more attention today.
In 1864, a French named Augustin Mouchot designed some solar engines that were driven by dish solar collectors. His thermal dynamic devices were shown at the Paris International Exhibition in 1878 [4]. As the longest researching of the small spot-focus solar collector, the parabolic dish concentrator (PDC) was considered to be a promising system due to its high power density, high efficiency, easy modularity, and long lifetime etc [5]. Today, there have been quite a few solar power stations in the world that applied the PDC system [6]. The highest efficiency for such kind of power generation system was reported to be 31.25% [7].
The other two kinds of widely used SPFC systems are the point-focusing Fresnel lens (PFFL) and the Scheffler reflector (SR). The PFFL is a kind of compact point-focusing concentrator. It was invented in 1822 by the French mathematician and physicist Augustin Jean Fresnel [8]. The early PFFL were made of glass, which was heavy and costly. Nowadays most of the Fresnel lens are made of the lighter and cheaper polymethyl methacrylate (PMMA) material that is optically similar to glass [9]. With the merits of small size, light weight, and good optical characteristics, Fresnel lenses made of PMMA material have been commercialized and widely applied in the engineering of solar energy [10]. The SR device was first designed by the German Engineer Scheffler et al. It was a new type of concentrator with a fixed focus point [11]. Keeping a relative stationary position of the dish to the sun, Scheffler reflector was able to provide a fixed focus on the rotating axis. The reflector can track the solar daily movement and the seasonal movement [12]. In recent years, SR systems have been widely used in engineering cases of the medium temperature [13].
In the current report, we will present a review on the latest progress in the past decade of the three types of the small solar concentrator mentioned above. The emerging technologies of different SPFCs will be introduced, and the developing trends of the SPFC collectors will be summarized. In addition, main application areas of the SPFC systems will also be discussed.
2. Emerging Technologies for SPFC System
The SPFC system generally consists of the following parts:
(1) Solar concentrator. It collects the sunlight and reflects it onto a focal point. As a 2-D symmetric reflector, the parabolic dish collector features with a variable solar concentration ratio that can change from hundreds to thousands. Correspondingly, a heat source of high temperature can be generated at the focal point.
(2) Receiver. It is installed at the focus of the collector, and responsible for the conversion of the solar energy into thermal energy. The thermal energy is then transferred to the working medium in the cycling loop or directly to the photovoltaic panel.
(3) Tracking system. It supports the device to involve around the solar-tracking axis. The system is driven by an electrical stepper motor. Through the revolution of the axis, the supporting rig is able to catch up with the solar movement in time. The revolving speed of the axis corresponds to the change of the solar height angle and azimuth angle, which need to be determined by the celestial law in advance.
In the past decade, some new concepts have been proposed to improve the performance of SPFC system. The general classification of SPFC collectors is shown in Fig. 1. The PDC system is divided into two types according to its structure, i.e. the single-dish concentrator and the multi-dish concentrator. On the other hand, the PFFL system is grouped to the flat plate type and the curved type according to the shape. All of them will be discussed in the following sections.


 
Fig. 1 Classification of SPFCs
2.1   Two kinds of parabolic dish concentrator
When the PDC system is in operation, it must be controlled by a biaxial solar-tracking device. The axis is timely advancing with the solar movement in the sky. The mirror surface of the dish should have a good optical precision, and the dish structure must remain stable under the effect of the wind or the gravity load [14]. There are many factors that need to consider as design a dish concentrator. Hafez et al. [15] summarized 10 factors to affect the design. Those factors included the material characteristic of the reflector, the shape of the reflector and the receiver, the solar radiation intensity, the dish diameter, the aperture area, and the focal length of the dish, etc.
2.1.1   The single-dish concentrator
The shape of a single dish concentrator is similar to that of a parabolic antenna. To facilitate the processing and the maintenance, it is usually to cut a paraboloid off to a number of pieces of the same size. Through connecting the small pieces, a large parabolic solar reflector is formed.
In recent years, some scholars proposed a new method for designing the PDC of high performance. Pavlović et al. [16] put forward a concept of a solar parabolic collector consisted of flat square facets. They optimized the size and the location of each square facet by the Monte Carlo ray tracing method. The dish concentrator could deliver 13.6 kW of the solar radiation power onto the receiver of 250 mm in radius. The focal length of the concentrator was 1.5 m and the concentration ratio was over 1200.
The Greek scientist Bakos and Antoniades [17] considered a new design of the dish collector with the elastic film. The proposed solar concentrator consisted of two layers of 10 mm rubber film to form a hermetic volume. The pressure difference of inside and outside the hermetic volume caused the elastic film to form a concave profile, which acted as the flexible reflecting mirror, see Fig. 2. The primary mirror concentrated the sunlight onto the secondary mirror, and then reflected it to the receiver on the Stirling engine. Eccher et al. [18] developed a process to build modular solar dish reflectors by using plane mirrors and a sandwich structure and applied them to 0.8 m2 parabolic sectors. The spatial distribution of the irradiance at the focal plane has been evaluated by means of a thermopile mounted on an electrical plotter. Measurement showed a peak concentration ratio of about 870 X and an overall optical efficiency of 80%±4%.


 
1–primary mirror, 2–supporting springs, 3–secondary mirror, 4–receiver (Stirling engine), 5–reflective mirror, 6–supporting structure
Fig. 2 The solar concentrator system with a secondary mirror [17]
To improve the concentrating efficiency of the dish concentrator, Xiao et al. [19] proposed a model-based approach to assess the optical performance of a solar dish. The slope error was considered as random following Rayleigh distribution, and the alignment error was considered as a systematic error. Photogrammetry combined with 3D laser scanning was used to measure the dish surface. Following the model-based realignment recommendation, the concentration ratio of the collector was increased from about 500 to above 1500 with the intercept factor increasing from 0.66 to 0.9.
Although the PDC collector features with the high thermal and optical efficiency, its cost is usually higher by per unit area as compared to the parabolic trough or the linear Fresnel concentrator. To reduce the manufac- turing cost of the PDC, Li and Dubowsky [20] developed a petal-shaped PDC collector as shown in Fig. 3. The device was made of a few flat metal petals that were covered with a highly reflective surface layer. The petal shape had been optimized in such a way that all the petals formed an ideal parabola as they were pulled together by rods or cables.


 
Fig. 3 Petal shaped dish concentrator [20]
In [21], Cameron and Noor presented another novel solar dish collector of high concentration factor that used a split-recombined focus arrangement. The author thought that the solar concentration factor could reach over 7000 suns (1 sun = 1 kW/m2) after optimization of the design by improving the structure rigidity and reducing the solar ray’s divergence. At the same time the manufacture cost could be kept at the middle level. Pavlović et al. [22,23] examined a dish type of collector that used a spiral absorber. With a light weight and low cost, this solar dish collector was operated mainly at medium temperature levels. It consisted of 11 curvilinear trapezoidal reflective petals of the PMMA material that were covered with a silver coated glass layer. A thermal model was established and solved with the software of Engineering Equation Solver. The result was also validated with the experimental study to determine the optimum operating condition. The results showed that when the working temperature was close to 200°C, the exergy efficiency reached the maximum. A similar study related to the cost reduction of the solar dish was conducted by Hijazi et al. [24], but their study was performed aiming to the optimization of the mechanical perspective.
In order to distribute evenly the radiant heat flux onto the receiver, Zhou et al. [25] proposed two types of non-imaging dish reflectors, in which the solar ray passed through different routes. By a numerical simu- lation and theoretical analysis, they found that the both type reflectors could get uniform heat flux as long as the solar ray irradiated in a collective manner. The Type I reflector showed the characteristic of the less sensitivity to the solar incident angle than that of the Type II, meaning the Type I was more suitable for use in practical engineering.
2.1.2   The multi-dish concentrator
The multi-dish concentration collector uses many small curved mirrors as the unit reflector. Compared with the single-dish collector, the multi-dish collector is stronger to stand up to the wind and the gravity load. However, due to the inevitable gap between the adjacent mirrors, the area efficiency of the whole concentrator is relatively low.
The solar heat flux distribution for a solar concentrator is always important. Xia et al. [26] once measured the solar heat flux at a sixteen-dish concentrator by using the infrared reflection method and the Monte Carlo ray tracing method. The PDC collector, as shown in Fig. 4, consisted of sixteen identical small dishes that were mounted on a girder grillage of two metal circles. The small dish was 1.05 m in diameter and 3.25 m in focal length. Its surface reflectivity was above 0.90 and the solar-tracking error was less than 2 mrad. When the equivalent radius of the aperture of the multi-dish concentrator was 2.5 m, the peak value of the collected solar power was over 10 kW. Afterwards, Huang et al. [27] improved the design of the sixteen-dish concentrator system and established an analytical model. It was found though the new model produced a better concentration ratio, the pointing error of the concentrator affected the heat flux distribution seriously.


 
Fig. 4 The sixteen-dish concentrator system [26]
Chang [28] proposed a confocal concentrator that was composed of some spherical facets with the same aperture and curvature radius. The results of the numerical simulation showed that the heat flux on the receiving plane exhibited a local maximum when the focal length of the concentrator was around 4 m. The author thought that such kind of concentrator with the confocal configuration could reduce the cost by about 30% in comparison to the splicing dish type. Perez- Enciso et al. presented a simple method to obtain uniform flux distribution on the receiver of the multi-dish concentrating system [29]. By moving the receiver from a focal plane and enlarging the solar spot impinging on it, the uniformity of the heat flux distribution was improved. This was fulfilled by adjusting the aiming point of each mirror and superimposing the mirrors one another. To evaluate the effect of the technology, a prototype of eighteen spherical mirrors was modeled with a profe- ssional software. Using the mirror of aluminized glass with the reflectivity of 0.92, the multi-dish concentrator generated a concentration factor of 150–900 suns as the receiver diameter was equal to 7 cm.
Careful selection to the reflective material for the dish surface can help to promote the performance and prolong the serving life of the concentrator. Zanganeh et al. [30] studied a dish concentrator that was made based on the aluminized polyester membrane. An array of the poly- ester membrane facets were clamped along their elliptical edges, and a slight vacuum condition was kept under- neath the membrane so that it formed an ellipsoidal shape. The simulation results showed that such a concentrator of 10.9 m in radius, 11 m in focal length, and comprised of 121 facets of different curvatures, had reached to a peak solar concentration ratio of 23546 suns. Following the design of Zanganeh, Schmitz et al. developed a 18-dish concentrator (see Fig. 5) [31]. The diameter of the unit mirror was 0.55 m, and the whole concentrator’s area was 4.28 m2. It took the silvered aluminum sheet as the membrane material. This kind of material was stronger and not easy to be damaged as compared to the original polyester membrane. During the onsite test, it achieved the peak concentration ratio of 3140 suns and the average ratio was 897 suns on a 60 × 60 mm area. The best concentrated solar power was 4.1 kW. Schmitz and his colleagues also presented a multi-focus photovoltaic- thermal cogeneration system [32]. As the core of the system, there was a novel solar dish that could achieve a high concentration and compactness by subdividing the parabolic dish into the identical modules. Each module comprised an individual focal point and was arranged symmetrically around the central axis. To avoid the environmental impact, a transparent protective membrane was deployed on the dish top. The solar concentrator had a geometric concentration ratio of 1733X and achieved 1353 suns of average solar radiative flux.
Ancona et al. [33] proposed another solar concentrator model for PV application on the basis of non-imaging optics. This concentrator could be embedded in a multi-junction solar cell of high efficiency. They used seven mirrors to replace the traditional segmented dish and focused the light to the same point. Therefore, the light spot pattern on the receiver was resulted from the superposition of the single illumination produced by each mirror. The diameter of the single mirror was 2.6 m, and the size of the total system was about 7.8 m with the total optical area over 35 m2. In the test condition of 1 kW/m2


 
Fig. 5 Prototype of the vacuum facet multi-dish concentrator by Schmitz et al. [31]
(1 sun) of the solar radiation, the output of the working power had reached around 35 kW. In 2015, Pavlović et al. [34] reported a offset-type parabolic concentrator. Unlike the conventional paraboloid concentrator, the offset type was only a lateral part of a paraboloid dish. The system consisted of three identical offset satellite dishes of 2400 mm in diameter, which directed solar rays toward the target area. The inner surface of the offset dishes was covered with an aluminum foil of the reflectivity ranging from 58% to 60%. The authors only developed a geome- trical and optical model for the analysis but nothing experimental result was provided.
2.2 Solar concentrator of Fresnel lens of point focus
The PFFL is usually simple, portable and easy to install. It has attracted more and more scholars to study today. Different from the conventional lens, the PFFL type is a laminar lens that possesses a series of discontinuous curved surfaces that are arranged in parallel or coaxial pattern. Kumar et al. [35] discussed the factors that affected the performance of the PFFL collector. Those factors included the facet corner rounding, the spectral absorption, the draft angle, etc. Xie et al. [36] grouped the PFFL collectors into the imaging system and the non-imaging system.
2.2.1   The flat plate type of Fresnel lenses
The Flat plate Fresnel lens is easy to manufacture and install. Usually it is hoped that the Fresnel lens has a high concentration ratio and a uniform solar heating flux, so that the overheated spot in the heating area could be avoided. Aiming to this goal, Pan et al. designed a homogenized Fresnel concentrator that consisted of a set of refraction prisms [37]. Each prism was focused at a different position of the receiver. Using Pan’s method, Zhuang and Yu [38] proposed a hybrid Fresnel concen- trator to improve the uniformity of the irradiance distri- bution on the solar cell, as shown in Fig. 6. It could reduce the Fresnel loss and improve the conversion efficiency of the solar cell. This hybrid concentrator was composed of inner and outer parts. The inner part was a conventional point-focus Fresnel lens, while the outer part was a double total-internal-reflection lens. The simu- lation results showed that such a Fresnel concentrator could obtain a good distribution of the solar light on the cell. The spatial non-uniformity was less than 16.2%, and the geometrical concentration ratio reached to 1759.8.


 
Fig. 6 Hybrid Fresnel-based concentrator by Zhuang and Yu [38]
Li and Xuan [39] introduced a planar Fresnel lens composed of square or rectangular lenses for solar concentration. They proposed the practical way to realize a square light spot and the receiving area. The analytical results revealed that the spot uniformity and the concentration ratio of the new lens were insensitive to the wavelength change. Languy et al. [40] designed and manufactured a solar concentrator composed of achro- matic Fresnel doublets. The achromatic Fresnel doublet was able to keep the advantages of the plastic lens and not subject to chromatic aberrations. The lens could achieve an extremely high concentration factor with a high fault tolerance. Thus it was suitable to be operated in a high temperature condition that might cause the refractivity or the mirror shape to change. Michel et al. [41] introduced a new design of planar solar concentrator that could be used in space engineering. The concentrator focused the light onto two spatially separated PV cells so that the output power of each cell could be independently controlled. Due to the superposition of the blazed diffraction grating on the Fresnel lens, the advantage of the spectral splitting and the light focus was combined together. With the solar tracking error below ±0.9°, the optical efficiency was about 75%, and the losses were less than 10%.
Ke proposed an accuracy method to design and optimize the flat Fresnel lens with large aperture [42]. They tested the convergent light intensity by measuring the light spot intensity on the focal plane. The size of the Fresnel lens was 1000 mm × 1000 mm and the focus length was 1200 mm. When a beam of 550 nm wave- length was irradiated, the optimized lens could converge the sunlight at the efficiency of 78.7%. Zou and Yang [43] proposed a design method of Fresnel lens concentrator for the use in PV modules, and analyzed the optical performance numerically. In the design, a pair of confocal parabolic reflectors and a Fresnel lens with matching focal length were employed to achieve high irradiance uniformity on the solar cell. The maximum optical efficiency could reach to 82%.
Some scholars have studied the effect of processing parameters on the quality of the flat Fresnel lens. Vallerotto et al. [44] discussed the improvements about the manufacturing process to obtain the achromatic doublet of Fresnel lenses. First, an adhesion promoter had been used in order to enhance the adhesion between the plastic and the elastomer interface. Second, the whole lamination process had been almost completely automatized. Finally, a new injection mold based on the nickel stamper technology has been manufactured that improved the geometrical characteristics of the plastic element of the lens. The optical efficiency of the samples of the new mold was increased 1.2%. Kuo et al. [45] evaluated the processing parameters of the Fresnel lens according to the power output, the efficiency, and the error of the groove filling. The results showed that when the melt temperature was 270°C, the mold temperature 100°C, the packing pressure 1040.4 bar, and the injection speed 60 mm/s, a high light conversion efficiency and low error of groove filling could be achieved.
2.2.2   The curved type of Fresnel lenses
Although the fabrication and processing of the plane Fresnel lens are relatively simple, its concentration efficiency is usually not high. Therefore, the type of curved Fresnel lens was developed to obtain a high efficiency. Compared with the flat plate type, the curved Fresnel lens not only has a higher concentration ratio, but also the mechanical stability is improved because of the convex shape [46]. In addition, as used in outdoor situations, the convex shape has the advantages of low wind resistance and the minimal accumulation of dust and rainwater. Ma et al. [47] analyzed the concentrating principle of the curved Fresnel lens and then proposed the optimal transmittance condition of the prism. They illustrated that the curved Fresnel lens has a higher transmittance than the flat plate Fresnel lens, especially on the two flanks of the lens.
In recent years, the study on the curved Fresnel lens became more and more. Pham et al. [48] proposed a curved Fresnel lens by combining the edge ray theorem, the Snell’s law, and the conservation of optical path length. The structure of such curved Fresnel lens could improve significantly the uniformity of the sunlight distribution over the solar cell. Its concentration ratio could reach 900. The design idea was based on the uniform sunlight distribution of the every groove of the lens so that the whole lens also distributed the sunlight uniformly. The structure of the lens was composed of two surfaces: the inner surface (or upper surface) as a part of spherical surface, and the outer surface (or lower surface) that consisted all grooves of the lens, as shown in Fig. 7.


 
Fig. 7 The curved Fresnel lens in three dimensions [48]
Zamora et al. [49] designed a dome-shaped Fresnel – the Köhler concentrator. It was claimed to be of durable work and of a high concentration factor. A simulation to this concentrator showed some outstanding results. Without any anti-reflective coating applied in the system, the effective concentration–acceptance product reached 0.72 and the optical efficiency reached 85%. Moreover, the concentrator could provide uniform irradiance on the cell surface with low spectral aberrations. As a result, a higher optimal efficiency of the solar cell was achieved. Akisawa et al. [50] developed dome-shaped non-imaging Fresnel lenses with a certain acceptance half angle. The designed lense was a deep dome shape. The optical efficiency of the new design was better than that of the conventional one with the incident angle equal to the acceptance half angle. The maximized optical efficiency was as high as 0.9.
In addition to the dome-shaped Fresnel lens, some scholars have also studied the elliptical point-focus Fresnel lens. Based on the optical geometry and the light-tracing technique, Yeh [51] studied an elliptical Fresnel lens concentrator. To form different mixed colors on the target plane, the author incorporated the solar spectrum with the refractive indices of the lens materials. The model identified the solar spectrum distribution patterns, which would facilitate the study of the wavelength matching for different solar energy applications. Yeh. N and Yeh. P also delved into the main parameters that were related to the system design of the point-focus and non-imaging Fresnel concentrator [52].
The manufacture technology of the curved Fresnel lens has also been studied. Hsu et al. [53] proposed a molding technology to implement the hybrid 3D microlens. The curvature of the lens could be tuned by the hydraulic pressure and the surface tension in the molding process. Advantages of this molding technique were as follows, (1) the initial lens curvature was defined by the polymer volume and the surface tension, (2) the final lens curvature was tuned by the hydraulic pressure, and (3) the optical pattern (Fresnel zone) could be defined by the molding. The biconvex, the biconcave, and the convex-concave lens, as well as the curved Fresnel lens, were all demonstrated in the application. On the other hand, though with the excellent features mentioned above, the curved Fresnel lens is not easy to be manufactured. Especially, the requirement for the grinding tool is highly strict in the machining process. If the machining process could be more simple and reliable, the curved Fresnel lens would be more widely used than the flat plate type.
2.3   Scheffler reflector
Scheffler reflector (SR) is a new type of the SPFC collectors. Though Scheffler collector has the advantage of fixed focus, it is complex to design. The mirror needs to change constantly the curvature according to the solar declination angle so that the working accuracy is guaranteed. Munir et al. [54] presented a complete description of the design principle and the construction details of a SR of 8 m2 surface area. They demonstrated that it was possible to construct a concentrator that could provide the fixed focus for all days of the year. The report presented the mathematical calculations how to design the reflector parabola curve and the reflector elliptical frame. The reflector assembly was composed of flexible crossbars and a frame, which satisfied the required change of the parabola curves. Distributions of the crossbar on the reflector, the depth and the arc length of different crossbars were calculated. For daily tracking, these concentrators rotated along the axis that was parallel to the polar axis of the earth with the help of self-tracking devices, as shown in Fig. 8. For seasonal tracking, the reflector rotated at the half of the solar declination angle with the help of a telescopic clamp mechanism. The design procedure was simple, flexible, and did not need any special computational setup.
To make the design process of SR simpler, Bajaj et al. [55] used the MATLAB software to further study the variation of the concentrator performance in different seasons. Reddy et al. [56] made an attempt to simplify the design methodology. Design charts were prepared for the manufacturer to determine various design parameters of the SR. The parameters included the angle of solar inclination, the radius of curvature, the length of crossbars, and the focal length, etc. Furthermore, a general equation for the seasonal change in the parabolic profile of the SR has been proposed. A simplified equation for the concentration ratio has been developed to ease the design.
In order to avoid the heat loss of the thermal radiation to the absorber, Dib et al. [57] analyzed the image formed by the SR. Taking into account the optical characteristic, an analytical geometry was proposed. The author summarized the variations of the aperture area and the concentration ratios of the collector in a year. The results suggested that there could be a drastic variation of the energy flow at the receiver entrance. Ruelas et al. [58,59] analyzed the opto-geometric performance of the Scheffler concentrator. By their results the fixed-focus concentrator tended to increase the diameter of the solar image at the receiver by 1–5 times compared with the PDC type. This means that the fixed receiver has a bigger solar image and then decreases the concentration.
In Table 1, the main publications related to SPFC advances in the past decade are summarized in category. Through investigations of the above papers, it has been found that the dish concentrating mirror has many manufacturing methods, including thin flat metal [20,24,34], silver-glass mirror [18,22,23,29,32] and vacuum-membrane [17,19,30,31]. The multi-dish


 
Fig. 8 Schematic of theScheffler reflector by Munir et al. [54]
Table 1 Overview of the important publications of the SPFC system
 
SPFC TypeReferencesStudy methodHighlightsMain performance
Single-dish concentratorPavlovic et al.[16]SimulationPresented a procedure to design a square facet concentrator.The dish concentrator could deliver 13.6 kW of the solar radiation power.
Bakos and Antoniades [17]SimulationStudy on a solar collector with primary and secondary mirror using elastic film.Concentration ratio 100.
Eccher et al. [18]Simulation and experimentalDeveloped a manufacturing procedure about solar reflector modules based on the sandwich approach.A peak concentration ratio of about 870 and overall optical efficiency of 80%± 4%.
Xiao et al. [19]Simulation and experimentalProposed a model-based approach to assess the optical performance of a solar dish.Concentration ratio was increased from 500 to 1500 with intercept factor increasing from 0.66 to 0.9.
Li and Dubowsky [20]Numerical and experimentalDeveloped a petal-shaped dish concentrator to reduce the manufacturing cost.Focal length 338 mm, energy efficiency 62.5%.
Cameron and Noor [21]SimulationPresented a novel solar dish which attained a high concentration factor with low manufacturing cost.Solar concentration factor could reach over 7000 suns.
Pavlović et al. [22,23]Experimental and numericalExamined a dish solar concentrator of low cost and light weight.Concentration ratio 28.36, mirror reflectance 0.7.
Hijazi et al. [24]SimulationDesigned a low cost parabolic solar dish concentrator of small or moderate size.The collector can deal with various applied forces and weights to provide the minimum possible cost.
Zhou et al. [25]NumericalDesigned a non-imaging concentrating reflector for providing uniform irradiance on the receiver.The Type I reflectors present more uniform flux maps than Type II reflectors.
Multi-dish concentratorXia et al. [26]Experimental and numericalStudied the concentrated solar flux of a sixteen-dish concentrator by using measuring infrared reflection and MCRTM.Surface reflectivity was above 0.90, peak collected solar power was over 10 kW.
Huang et al. [27]SimulationDeveloped a model for designing of a multi-dishes concentrator system.Pointing error of the concentrator affected the heat flux distribution seriously
Chang [28]NumericalProposed a concentrator model of confocal configuration composed of spherical facets with identical aperture and curvature.The 8 m radius of curvature of identical facets is the optimal choice.
Perez-Enciso et al. [29]SimulationProposed a simple design method to achieve uniform flux distributions on the receiver of a solar multi-faceted concentrator.Mirror reflectance 0.92, concentration factor of 150–900 suns.
Zanganeh et al. [30]SimulationDesigned of a solar PDC with polyester membrane facets.Peak solar concentration ratio of 23,546 suns.
Schmitz et al. [31]Experimental and simulationDeveloped 18-dish concentrator, it achieved the peak concentration ratio of 3140 suns with the best concentrated solar power output 4.1 kW.Peak concentration ratio of 3140 suns and the average ratio was 897 suns.
Schmitz et al. [32]ExperimentalPresented a multi-focus photovoltaic-thermal cogeneration system.A geometric concentration ratio of 1733X and 1353 suns of an average solar radiative flux.
Ancona et al. [33]SimulationProposed a PV solar concentrator model on the basis of non-imaging optics that could be embed in a multi-junction solar cell of high efficiency.Working power around 35 kW.
Pavlović et al. [34]NumericalPresented a new offset-type parabolic concentrator.Irradiance for absorbed rays on receiver was from 3.88×109 W/m2 to 3.25×105 W/m2.
The flat plate Fresnel lensesPan et al. [37]SimulationDesigned a single concentrator to homogenize and concentrate the solar energy.The uniformity was 14.6 and the concentration ratio was 1018.
Zhuang and Yu [38]NumericalProposed a novel hybrid Fresnel-based solar concentrator to improve the uniformity of the irradiance distribution on the solar cell.The spatial non-uniformity was less than 16.2%, and the geometrical concentration ratio reached to 1759.8.
Li and Xuan [39]SimulationStudied the uniformity of the light spot of a square concentration lens.Optical concentration ratio reached the maximum value of 157.
Languy et al. [40]Experimental and simulationProposed a theoretical design of achromatic Fresnel lens that combined the advantage of mirrors and plastic lenses with good tolerance to manufacturing errors.Concentration factor remained above 1600.
Michel et al. [41]SimulationProposed a design of a solar concentrator with light splitting on two cells, which has wide application in space engineering.The optical efficiency was about 75%, and the losses were less than 10%.
Ke et al. [42]SimulationProposed an accuracy method to design and optimize flat Fresnel lens with large aperture in MATLAB.Optical efficiency 78.7%.
Continued Table 1
 
SPFC TypeReferencesStudy methodHighlightsMain performance
The flat plate Fresnel lensesZou and Yang [43]NumericalProposed a novel Fresnel lens concentrator design for PV modules.Maximized optical efficiency of about 82%.
Vallerottoet al. [44]ExperimentalPresented improvements in the manufacturing process to obtain Achromatic Doublet on Glass Fresnel lenses.Optical efficiency increased of 1.2%.
Kuo et al. [45]Experimental and simulationAnalyzed parameters of the Fresnel lens according to the ration of power efficiency and the error of the groove filling.High light conversion efficiency and low error of groove filling.
The curved Fresnel lensesPham et al. [48]SimulationProposed a curved structure Fresnel lens which has a uniformity of sunlight distribution and high concentration ratio.Concentration ratio 900.
Zamora et al. [49]SimulationDesigned a dome-shaped Fresnel–Köhler concentrator, which was of durable work and uniform irradiance.The effective concentration–acceptance product reached 0.72 and the optical efficiency reached 85%.
Akisawa et al. [50]SimulationDeveloped dome-shaped non-imaging Fresnel lenses with a certain acceptance half angle.Maximized optical efficiency of 90%.
Yeh. P [51]NumericalFormulated an elliptical-based Fresnel lens concentrator system using optical geometry and ray tracing technique.Optimum performance occurred at the lenses with the focal ratio falls between 0.4 and 0.55.
Yeh.N and Yeh.P [52]NumericalInvestigated the parameters that facilitate the Fresnel concentrator system design.Discussed the mechanism of spectral distribution patterns under the lens.
Hsu et al. [53]ExperimentalProposed a molding technology to implement the hybrid 3D microlens.The biconvex, the biconcave, and the convex‒concave lens, as well as the curved Fresnel lens, were all demonstrated in the application.
Scheffler ConcentratorMunir et al. [54]NumericalPresented a complete description about the design principle and construction details of an 8 m2 surface area SR.Demonstrated that SR could provide the fixed focus for all days of the year.
Bajaj et al. [55]NumericalUsed the MATLAB simulation to study variations in the concentrator during seasonal changes.7 m2 reflector for equinox, summer and winter solstice is 0.227 m, 0.223 m and 0.229 m respectively.
Reddy et al. [56]NumericalPresented design charts that are useful for the manufacturer to determine various design parameters of the SRs.Scheffler reflector preferable over other parabolic concentrators when solar intensity is low.
Dib et al. [57]NumericalSummarized the image formed variations of the aperture area and concentration ratio of SRs during the year.A general equation for seasonal change and concentration ratio in the parabolic profile of the SR has been proposed.
Ruelas et al. [58]Simulation and experimentalPresentd a new method for testing focal image of a Scheffler-type Solar Concentrator.Solar image geometry of elliptical shape and area of 0.0065 m2 on average
Ruelas et al. [59]SimulationStudied on opto-geometric performance of the Scheffler concentrator.Pointed out the loss maybe reach as high as 1–5 times as compared with the PD technology.
Table 2 Advantages and disadvantages for different SPFC type
 
Type of SPFCAdvantagesDisadvantages
Single-dish concentrator- Entire surface can be utilized.
- Simple design process
-Weak wind resistance
-high machining precision
-High installation and transport costs
Multi-dish concentrator-High concentration ratio
-Strong wind resistance
-Simple installation
-The concentrator area is not fully utilized.
Flat plate Fresnel lens-Simple manufacturing process
-Low cost, commercial production has been realized.
-Compact structure
-Low optical efficiency
-Easy to accumulate ash
Curved Fresnel lens-Strong wind and dust resistance
-High concentration ratio
-Complex processing
Scheffler Concentrator-Fixed focus point, suitable for specific occasions-The technology is not yet mature.
-The mirror needs to be adjusted frequently when used.
concentrator has a higher concentration ratio and radiative flux than the single-dish concentrator. The current optimization of Fresnel lenses focuses on obtaining a more uniform radiant flux on the absorber surface [37-39,43,48-49]. Compared to the flat Fresnel lenses, the curved Fresnel lenses have higher optical efficiency and environmental adaptability. The Scheffler reflector has a short history of research, and the main research direction is the theoretical calculations of mirrors which are used to achieve uniform design standards [54-57]. The features for different types of the SPFC are presented in Table 2.
3. Recent Applications of SPFC
Today the application of SPFC for solar energy has been paid more and more attention by researchers. In this section, we will summarize the important applications of the SPFC in recent years. The category of the main application area is shown in Fig. 9. These applications are related to the PV, the solar thermal power, the solar chemical engineering, and the day lighting.


 
Fig. 9 Category of the SPFC by the application
3.1   Concentrating PV system
As one of the renewable energy technologies, solar PVs have received much attention recently due to their environmental and economic benefits [60]. The concen- trating PV system can transform the solar radiation into electricity usually in an efficiency higher than the conventional PV cells. Schmitz et al. [61] mentioned a PV-thermal dish system that used a micro-channel heat exchanger as the heat generator. The generated hot fluid medium was delivered to the secondary heater for temperature upgrading. The test result of the prototype showed that the single PV mode reached the solar-to- electricity efficiency of 28.5% and the power output of 12.1 kWe. In cogeneration mode, the solar-to-electricity efficiency was 26.6% and the power output was 11.3 kWe/21.5 kWt.
A solar concentrator of deep dish type was developed in [62] to serve the joint power generation system of concentrating PV and wind energy, see Fig. 10. A wind turbine was installed at the center of the base duct of the concentrator. When the wind went through the device, the wind speed would be increased to promote the power output of the turbine. The system was considered to be superior to the conventional PV system. Under the usual level of solar radiation, the power generation efficiency of the module was 18.8%. The wind augmentation through the solar concentrator had enabled the short circuit current to be increased by 10% averagely, and the start wind speed to be reduced by about 1.5 times.


 
Fig. 10 Deep dish concentrated photovoltaic system [62]
Chen et al. [63] studied experimentally a high concen- trating photovoltaic/thermal (HCPV/T) module. Com- pared with the conventional HCPV module, the HCPV/T module had a better performance with 26.5% electrical efficiency and additional 49.3% thermal efficiency. Analysis of the system efficiency indicated that the overall efficiency was not sensitive to the coolant temperature. Xu et al. [64,65] set up a highly concen- trating photovoltaic-thermal (HCPV/T) system based on the PFFL principle. The geometric concentration ratio of the device was 1090. The experimental result of the system in a clear day showed that the highest electrical efficiency reached 28% and the thermal efficiency reached 54%. In addition, they found that the electrical efficiency was mainly affected by the solar radiation intensity rather than the cell temperature. The thermal efficiency increased with the solar radiation, the ambient temperature, and the cooling water flow. On the contrary, the thermal efficiency decreased as the water temperature or the wind speed increased. Using a PFFL collector, Renno and Petito [66] evaluated the performance of a concentrating PV and thermal system about the electrical characteristic, the concentration factor, the cell tempera- ture, and the fluid temperature. A sunny day of averaged solar irradiance of 920 W/m2 resulted in a mean power output of 1.72 W in contrast to the theoretical value of 2.27 W.
3.2   SPFC in solar thermal engineering
The application of SPFC in solar energy is either for obtaining thermal energy directly, such as in the cooking and the water heating system, or for the energy transfor- mation such as that used in solar heat engines and the solar power generation system.
3.2.1   Thermal power generation
Besides the PV power system, SPFCs are also widely used in a system of solar thermal power generation. Wu et al. proposed a novel solar power system that involved the PDC and the alkali metal for thermal to electric conversion [67]. The PDC was cascaded with the alkali metal through a coupling heat exchanger. The theoretical analysis revealed that the thermal to electric conversion efficiency of the system could reach to 20.6% with a power output of 18.54 kW at the operating temperature of 1280 K. Zhang et al. [68] designed a double-acting thermo-acoustic heat engine for dish solar power generation. With a special-shaped heater of a bundle of tubes on the top, the device could produce more power due to its multi-cylinder configuration. The simulation results showed that when the heating temperature was 650℃, the highest thermal efficiency of the engine could reach 51.37% and the acoustic power output reached 1.6 kW. Nevertheless, the experimental result of the system showed the maximum thermal efficiency less than 10%. It was speculated that too much heat loss and the uneven heating to the tube bundle caused the drastic deterioration.
A cogeneration system combining the PFFL and the thermoelectric module was proposed in [69]. The main components of the system included a mono-axial adjustable structure, a thermoelectric generator, and a Fresnel lens of 0.09 m2. The experimental results revealed that under the solar radiation intensity of 705.9 W/m2, the power output was 1.08 W with 51.33% efficiency. Hussain et al. [70] studied the heat transfer characteristics of the cavity absorber of a Stirling engine with Fresnel lens. The research parameters were the aperture ratio AR = d/D (the ratio of the aperture diameter d to the cavity inner diameter D) and the aperture position AP = H/D (the ratio of aperture position H to the cavity inner diameter D). The heat loss analysis and the temperature profile of the absorber showed that the cylindrical cavity receiver of AR = 0.5 and AP = 0.53 was the best to absorb the direct solar radiation. Aksoy and Karabulut tested a small Fresnel/Stirling solar energy conversion system [71]. The system used a manual tracking device of two-axis. The experimental system is shown in Fig. 11. They tested the performance of the absorber as it made of the aluminum, the copper, or the stainless steel. By the test result the aluminum absorber exhibited the best performance, with the maximum engine shaft power of 64.4 W and the thermal conversion efficiency of 5.64%.


 
Fig. 11 Small Stirling heat engine system by Aksoy and Karabulut [71]
3.2.2   Solar cooker and water heater
Using solar energy to cook meals is a proper way to save the wood or the fossil fuel, and it also provides a feasible cooking way in the situation of open country. Lecuona et al. [72] designed a portable PDC solar cooker, which consisted of two coaxial cylindrical cooking pots. The space between the two coaxial pots was filled with some phase change material. The stored heat in the phase change material was not only enough to cook the evening dinner, but also sufficient to warm the breakfast in the next day. Kumar et al. [73] suggested a new parameter to characterize the efficiency of a PDC solar cooker. This parameter established the relationship between the useful solar energy absorbed by the cooking pot and the concentration ratio of the dish solar collector.
It was considered as the dish solar heating system was automatically tracking the solar movement, the thermal efficiency of the dish water heater could reach to 52%–56% [74]. Badran et al. improved the configuration of a PDC water heater by covering the dish surface with a layer of aluminum foil of high reflectivity [75]. Valmiki et al. [76] used a large PFFL device to concentrate the solar light for cooking. It could be used either indoors or outdoors, see Fig. 12. The prototype was tested in a sunny day in south Arizona, USA. The temperature of the outdoor solar stove could reach to 300°C. Such a high temperature could meet the requirements for cooking or for frying food. The solar heat collected by the stove could also be delivered to the indoor stove through the circulating oil pipeline. The temperature of the indoor cooking stove reached to 150°C. The thermal efficiency of the stove achieved 83% at the maximum. Such a solar stove was easy to fabricate, low cost, highly safe, and convenient to operate.
An experiment setup of the separated solar water- heating system was designed by Patil et al. [77]. The collector dish rotated synchronously with the sun and reflected the sunlight onto the secondary reflector beneath the pot in the kitchen house. The pot was of the size of 20 L water. The system was able to work in both summer and winter. At the solar radiation level of 742 W/m2, the maximum temperature in the storage tank had achieved 98°C in a clear day.


 
Fig. 12 A prototype of solar stove by Valmiki et al. [76]
A solar cooking system of 8 m2 collector was installed in the community of Altiplano in Argentina for bread bakery [78]. The solar oven had a volume of 200 L and could reach to a temperature of 350°C. The solar oven was working for about 6 hours every day to bake 60 kg bread. Adoption of such a solar cooking system had resulted in savings of 60 kg of firewood for a family every week. Dafle et al. [79] studied the performance of 16 m2 SR collector for water heating and cooking. It was found that the instantaneous efficiency declined as the solar radiation increased. The maximum steam tempera- ture was 107°C at the outlet of the boiler, and the overall efficiency was 57.41%.
3.2.3   Solar desalination of distilled water
The technology of solar desalination can effectively alleviate the shortage of fresh water resources around the world. To evaluate the performance of the PDC for desalinating the sea water, Prado et al. [80] conducted careful experiments and theoretical calculations. Their results showed that the optical efficiency of the glass evaporator coated with matte black paint was 0.273. The maximum yield of the distilled water was 4.95 kg/(m2·day). An improved solar desalination system consisted of the triple-basin glass still, the cooling cover, the PDC device, and the PV panel [81]. The experimental result of the new system revealed that the distillated water yield was 16.94 kg/(m2·day), much higher than that of the traditional system. A simple solar collector with a modified boiler for brackish water desalination was presented in [82]. The system used a solar-tracking system of open-loop control, see Fig. 13. Using the glass mirror as the surface of the PDC, the system had the average yield of distillate water 6.7 L/(m2·day).
Chandak et al. [83] designed multistage evaporation system for production of distilled water by using solar energy. The system consisted of 2 Scheffler dish of 16 m2 area for generation of steam. The pressure was 8 bar in the first stage, and then decreased gradually to 1 bar. The temperature drop in every stage was designed to 25°C. The sensible heat of the condensation in each stage was made use for preheating the water in the next batch. Total yield of the distilled water in the project was 2.3 times that of the single stage distillation. Chandrashekara and Yadav [84] developed a low-cost and efficient method based on the SR collector. They used exfoliated graphite coating and paraffin wax in a solar desalination system. The latent heat thermal energy was stored by using paraffin wax in a receiver, and the exfoliated graphite coating played a key role in increasing the absorptivity of the concentrator to raise the temperature of the receiver. Experimental results revealed that the system could produce 6.67 L/day of distilled water, which was 13% higher than that of the system without graphite coating.


 
Fig. 13 Solar desalination system in [82]
3.2.4   Solar drying
Solar drying is an inexpensive method of drying materials that contains moisture. Hanif et al. [85] designed an efficient solar dryer of the dish type. A small solar air heater was connected to the drying chamber to dry grapes. The efficiency of the solar air heater increased as the airflow rate was increased. After 21 hours of the solar air drying the moisture content of the grape was reduced to less than 10 percent. Hadzicha et al. [86] presented the design of a solar coffee roaster. A SR device was used to concentrate the solar light onto a roast drum. This device with 2.7 m2 solar concentrator area could roast 1 kg of coffee in 24 minutes. The rotation of the drum ensured the coffee’s effective mixing to guarantee the homogeneous roasting. In the summer of Nagel, Scheffler concentrators were used to dry clean clothes [87]. The solar system of total 15 dishes was used to generate a surrounding of 150°C. The system generated heat/steam that was used for drying and cleaning the cloths for 100110 people every day.
3.3   SPFC in solar chemical
Many chemical reactions are endothermic, thus using solar energy to provide the heat of reaction instead of fossil fuels is a sensible choice. At present, an important way to utilize the solar energy in chemical engineering is the thermochemical hydrogen production and pyrolysis heated by the SPFC solar collecting system.
3.3.1   Thermochemical hydrogen production
Nowadays, the main method of hydrogen production is steam methane reforming or the CO2 methane reforming, which are represented by the following equations
CH4+H2O→CO+3H2 ΔH1 =206 kJ/mol (1)
CH4+C2O→2CO+2H2 ΔH2 =247 kJ/mol (2)
Wang et al. [88] studied numerically the effect of the radiative heat loss and the thermal conductivity of the porous matrix on the thermochemical reaction in the solar driven steam methane reforming process. The thermochemical reactor of porous medium was placed vertically in the focal plane of a PDC device. The numerical results revealed that the hydrogen production was a 3 order polynomial relation with the thermal conductivity of the porous matrix. Zhao et al had demonstrated the feasibility of the solar hydrogen generation through the reformation of the methanol vapor in a special reactor at the temperatures of 200°C to 300°C [89]. Using a square PFFL collector of 330×330 mm, the authors tested the hydrogen generation rate of the device at the solar irradiation of 1000 W/m2. The system achieved a hydrogen generation rate up to 68.1 g/(m2·h). The solar-to-hydrogen efficiency reached to 41.3% and the total energy efficiency (considering the solar and the methane fuel input together) was up to 76.6%.
Rathod et al. [90] investigated the catalytic reforming method of biogas to hydrogen rich syngas by using solar energy. The syngas can be used as the clean fuel in the fuel cell, the methanol production, as well as the I.C. engine applications. The author designed a foam type Ni-based Al2O3 catalyst reactor, with a 16 m2 SR collector for supplying the reaction heat. In the experi- ment the input biogas contained CH4/CO2 = 60/40 and a flow rate of 5 L/minute was maintained at the atmos- pheric pressure. Results showed that the highest yield of H2 and CO was around 16% and 10% respectively. It was also found that the CH4 and CO2 conversions were increased with the rising temperature.
In addition, Bicer et al. [91,92] studied experimentally the effect of the solar intensity on the efficiency of the photochemical hydrogen generation systems (Fig. 14). In the experiment, a PFFL collector was used to concentrate the sunlight. The sunlight was spectroscopic after passing through a light splitter. A part of the spectroscopic light was used to generate electricity in the PV module, another part was for the photoelectrochemical production of hydrogen in the reactor.
3.3.2   Solar pyrolysis
Pyrolysis is a well-known thermochemical process used to treat various types of solid waste. In order to avoid the environmental pollution caused by the traditional pyrolysis method, the small focusing solar concentrator was suggested to provide the heat for the


 
Fig. 14 Solar concentrated photoelectrochemical hydrogen production system [91,92]. PEC representing the photoelectrochemical reactor.
reaction in pyrolysis process [93]. Zeaiter et al. designed and built an automated solar reactor system to carry out the catalytic pyrolysis of scrap rubber tires at 550°C [94]. To maximize the solar energy concentration, a PFFL device was integrated with an automated solar-tracking system. With the H-beta catalyst, the experimental system of the scrap rubber pyrolysis gave the highest gas yield of 32.8%. The gasoline-like components of the gas product were mainly the low chain hydrocarbons of which the propene and the cyclobutene were dominant.
Biomass is one of the important renewable energy sources. By using solar energy to convert biomass efficiently into fuel or chemical stock receives more and more attention. The product of the conversion can be the bio-fuels, bio-oils, biogases and chars [95]. Zeng et al. [96] investigated the solar pyrolysis of beech wood to determine the optimal pyrolysis parameters for maximizing the lower heating values (LHVs) of the gas products by using PDC heating way. The results indicated that the total gas LHVs greatly increased with increasing the temperature (from 600 to 1200°C) and increasing the heating rate (from 5 to 50°C/s). A maximum gas production of 62% with the LHV of 10376±218 (kJ/kg of wood) was obtained under the heating condition of 1200°C, 50°C/s, 0.85 bar and 12 NL/min. This heating value was almost identical to that of the initial beech wood.
Chintala et al. [97] used a SR collector of 16 m2 area to study the conversion of non-edible Jatropha seeds to biofuels via solar thermochemical pyrolysis process. The experimental system is shown in Fig. 15. It was said that 20% maximum bio-oil yield was obtained with the average reactor temperature of 250–320°C. The pyrolytic zone for the biomass was identified in the range of 203– 508°C. The ultimate analysis of the bio-oil revealed that the oil was rich in carbon (58.3%) and hydrogen (8.7%).


 
Fig. 15 Schematic of integrated Scheffler collector and pyrolysis reactor by Chintala et al. [97]
Thus it could be employed as the fuel candidate for diff- erent applications such as in the engine or in the boiler.
3.4   SPFC in daylighting
Solar lighting technology that uses the natural light for illumination in day time has received attention in recent years. Han et al. [98] introduced a lighting system of high performance. It included a PDC device, a biaxial solar-tracking device, and light-guiding optical fiber cables. The author also discussed the way how to combine the natural sunlight with the electric lighting effectively so that the dimming control was readily conducted. Song et al. [99] designed a fiber-optic daylighting system based on the parallel mechanism. Their system was composed of 49 concentrating cells arranged in a 7×7 array. One of the concentrators was a sun position sensor, and the other 48 were all PFFL collectors with the same diameter of 10 cm. The system reached the light transmission efficiency of 25% and the luminous efficiency of 250 lm/W. Their further study suggested that the lens with a big ratio of the focus length to the diameter would be helpful to achieve a high transmission efficiency [100].
To assess the photometric characteristics in collecting and delivering the solar rays into an office space for better lighting, Kim et al. [101] compared the PDC type and the PFFL type concentrators to analyze the performance of active fiber daylighting systems. With the aperture area of the concentrator 0.073 m2, the study demonstrated the system using the Fresnel lens performed better than that using the parabolic concen- trator due to the structural simplicity of its operation.
At the end of this section, we summarize the important literature for SPFC collectors in Table 3. Different types and sizes of the SPFC concentrators, together with the research highlights, are all presented in Table 3. It is hoped this will facilitate the information access for those who intend to delve into the related study.
4. Conclusions
How to effectively convert the solar energy into other energy forms is an important issue in the solar energy engineering. In this article, we have reviewed that the recent development and the important applications of three types of small point-focus solar collector. Various studies have shown the following:
(1) For the PDC, if the design requirement is to simplify the production process and reduce the cost, the metal sheet can be used as the mirror material. When the lenses are made of silver-glass mirror or vacuum- membrane, it has a high optical property.
(2) By modulating the geometric parameters of the PFFL (such as acceptance angle [37] and groove [48]), or using a lens made of an achromatic Fresnel doublet [38] and hybrid Fresnel concentrator [40] can improve the uni- formity of the irradiance distribution. In addition, using an injection mold based on the nickel stamper technology [53] or hybrid 3D microlens molding technology [44] can also further improve optical efficiency of Fresnel lens.
(3) The SR has advantages such as flexible surface curvature, fixed focal area and shadow less concentration over other solar concentrators. A comprehensive design methodology is required for manufacturing the Scheffler reflectors at large-scale. It is also necessary to design a device that automatically changes the shape of the mirror with the number of days.
Most applications of the SPFC device referred to the concentrating PV cell, the solar thermal system, the solar chemical system, and the daylighting system. Generally speaking, PDC is the most popular type and widely used in many aspects. PFFL collectors are commonly used in small and compact devices, such as the micro Stirling engine, the solar stove, and the fiber-optic day lighting system. With the advantage of the fixed focus, SR collectors are used mostly in the solar cooking, the water desalination, the thermochemical reaction, and the biomass pyrolysis, etc.
Table 3 Summary of main publications on the application of SPFC system
 
ApplicationSPFC typeSize of the collectorHighlightsAuthorsType of study
Solar energy photovol- taic powerPDC38.7 m2 in total areaPresented a novel 6-focus PV-thermal solar poly generation system.Schmitz et al. [61]Experimental
the radius of the front inlet was 500 mm and the overall height 800 mmPresented a novel combined solar concentration/ wind augmentation system.Tao et al. [62]Experimental
PFFL33.02×33.02 cm2 areaStudied a high concentrating photovoltaic/thermal (HCPV/T) module.Chen et al. [63]Experimental
33.02×33.02 cm2 areaInvestigated a high concentration photovoltaic/ thermal (HCPV/T) module.Xu et al. [64,65]Experimental and theoretical
32 mm in diameterEvaluated the performance of a concentrating PV and thermal system.Renno and Petito [66]Experimental and theoretical
Solar thermal power generationPDC100 m2 in areaProposed a parabolic dish/AMTEC solar thermal power system.Wu et al. [67]Theoretical
/Designed a double-acting thermo-acoustic heat engine.Zhang et al. [68]Experimental and theoretical
PFFL0.09 m2 in areaInvestigated cogeneration solar system using thermoelectric module.Nia et al. [69]Experimental
/Studied the heat transfer characteristics of the cavity absorber of a Fresnel lens Stirling engine.Hussain et al. [70]Experimental
1.4 m2 in areaTested a small Fresnel/Stirling solar energy conversion system.Aksoy and Karabulut [71]Experimental
Solar cooker and water heaterPDC1.5 m2 in areaProposed a portable utensil solar cooker including heat storage.Lecuona et al. [72]Experimental
/Introduced a new parameter to characterize the efficiency of a PDC solar cooker.Kumar et al. [73]Experimental
2.19 m2 in areaDescribed a design of a PDC solar water heater for domestic hot water application.Mohammed et al. [74]Experimental
150 cm in diameterTested a portable solar water heater was designed.Badran et al. [75]Experimental
PFFL0.95 m × 1.25 mPresented a novel prototyped solar cooking stove which used a large Fresnel lens.Valmiki et al. [76]Experimental
SR8 m2 in areaTested a SR water heater.Patil et al. [77]Experimental
8 m2 in areaUsing SR to bake breads.Müller et al. [78]Experimental
16 m2 in areaStudied the performance of 16 m2 SR for water heating and cooking.Dafle et al. [79]Experimental
Desalina- tionPDC0.3312 m2 in areaEvaluated the performance of a dish solar concentrator for desalinating.Prado et al. [80]Experimental and theoretical
1.25 m in diameterTested a stand-alone triple basin solar desalination system.Srithar et al. [81]Experimental
1 m in diameterPresented a simple dish collector and modified boiler for brackish water desalination.Omara et al. [82]Experimental
SR16 m2 in areaDesigned multistage evaporation system for production of distilled water.Chandak et al. [83]Experimental
2.7 m2 in areaDeveloped a low-cost and efficient method based on SR, exfoliated graphite coating and paraffin wax.Chandrashekara and Yadav [84]Experimental
DryingPDC/Designed an efficient dish solar dryer to dry grapes.Hanif et al. [85]Experimental
SR2.7 m2 in areaPresented detailed a design of solar coffee roaster.Hadzicha et al. [86]Experimental
16 m2 in areaStudied performance of SR used for Solar Dry Cleaning.Bhasme et al. [87]Experimental
Hydrogen productionPDC1.4 m in aperture radiusStudied effects of key factors on chemical reaction for solar methane reforming are studied.Wang et al. [88]Theoretical
PFFL330×330 mmResearched the possibility of effective hydrogen generation via methanol steam reforming inside a novel solar collector/reactor.Zhao et al. [89]Experimental
0.8761 m2 in areaStudied the effect of the solar intensity on the efficiency of the photochemical hydrogen generation systems.Bicer et al. [91,92]Experimental
SR16 m2 in areaInvestigated catalytic reforming of biogas to produce hydrogen rich syngas using solar energy source.Rathod et al. [90]Experimental
PyrolysisPFFL69×91 cmDesigned an automated solar reactor system to carry out catalytic pyrolysis of scrap rubber tires.Zeaiter et al. [94]Experimental
Continued Table 3
 
ApplicationSPFC typeSize of the collectorHighlightsAuthorsType of study
PyrolysisPDC2 m in diameterDetermined the optimal pyrolysis parameters of beech wood.Zeng et al. [96]Experimental
SR16 m2 in areaStudied the conversion of non-edible Jatropha seeds biomass to biofuels via solar thermochemical pyrolysis process.Chintala et al. [97]Experimental
DaylightingPDC30 cm in diameterIntroduced the applicability and functional effectiveness of a daylighting system.Han et al. [98]Experimental
PFFL48 lens with 100 mm diameterDesigned a fiber-optic day lighting system based on the parallel mechanism.Song et al. [99]Experimental
48 lens with 100 mm diameterStudied the optic characteristics of daylighting system.Song et al. [100]Experimental
PDC and PFFLBoth are 0.073 m2 in areaCompared dish and Fresnel lens concentrators to analyze the performance of active daylighting systems.Kim et al. [101]Experimental
Acknowledgements
This work was sponsored by the National Key Basic Research Program of China (No. 2015CB251303).
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Article and author information
Springer Link
WANG Lu
YUAN Zhongxian*
zxyuan@bjut.edu.cn
ZHAO Yan
GUO Zhanquan
Publication records
Published: Sept. 5, 2019 (Versions1
References
Journal of Thermal Science