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.  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.  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 
Chang  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 . 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.  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) . 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 . 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.  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. 
(1 sun) of the solar radiation, the output of the working power had reached around 35 kW. In 2015, Pavlović et al.  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.  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.  grouped the PFFL collectors into the imaging system and the non-imaging system.