1. Classification of Solar Focusing Technology
For the utilization of solar energy, whether it is direct utilization of photothermal energy, photothermal power generation, or photovoltaic power generation, all require the use of spotlight technology. Based on optical principles, concentrators can be divided into transparent (reflective) concentrators, composite concentrators, thermal photovoltaic concentrators, etc. In terms of specific forms, the main types of concentrators on the market are still slot concentrators, Fresnel concentrators, disc concentrators, and tower concentrators. According to the shape of the spot, the spotlight can be divided into point shaped spots and linear spots. According to the principle of focusing, the concentrator of concentrated photovoltaics mainly consists of two categories: one is the transmission mirror focusing; One type is reflective mirror focusing.
Transmission mirror concentrators are mainly divided into two types: Fresnel lenses and ordinary lenses. Among them, Fresnel lens is a planarized focusing mirror. Fresnel lens has the advantages of light weight, low cost, and simple application structure. Its specific design belongs to the category of non imaging optics, and it does not have the focal plane defined by optical components, which can produce much better light intensity gain than traditional imaging optics. Fresnel transmission mirrors can be divided into two types: arched and completely planar. Bow shaped Fresnel lenses have better optical performance, but the processing difficulty is relatively high.
The main problem with transmission concentrators is that there is a loss of about 4% of sunlight when passing through each refractive surface. The transmittance efficiency of the lens is also affected by the lens material, processing technology, and solar irradiation time, and there is also varying degrees of dispersion. In order to improve the uniformity of solar radiation on the spot, many researchers have proposed various forms of lens designs. However, in practical applications, a compromise between the complexity of the lens and the quality of the focused spot is often necessary. There are several types of reflective concentrators, including mirror, flat plate, parabolic groove, and combination parabolic surface. The reflective materials mainly include silver plated glass and aluminum plated surfaces, and there are also high reflectivity films made of polymer materials to produce reflective surfaces.
The advantage of reflective concentrator is that there is no dispersion phenomenon, the distribution of light spot irradiation is uniform, and the reflection efficiency can be close to 100%. Its disadvantage is that conventional reflective concentrators require the installation of solar cells on the reflection surface, which is more complex than refractive concentrators. Moreover, the fixed installation will produce shadows on the reflection surface, which in turn will produce shadows on the surface of the solar cell. In addition, if the reflection surface is contaminated, the reflectivity will sharply decline. A relatively special reflective concentrator utilizes the principle of Cassegrain lenses. Its advantage is that it can achieve a larger focusing ratio in a relatively small size. A small number of researchers have proposed that the splitting of sunlight in different bands can be achieved through optical coating of secondary reflection lenses. Therefore, it is possible to choose a composite photovoltaic power generation system composed of solar cells sensitive to different bands to improve the photoelectric conversion efficiency of the entire system. Its disadvantage is that the structure is relatively complex and requires high tracking accuracy.
2. Characteristics of Solar Concentrated Solar Power Generation System
- The spotlight device concentrates approximately parallel sunlight;
- A complex sunlight tracking system that tracks sunlight;
- An independent and efficient cooling and heat dissipation system ensures that the large amount of waste heat generated by photovoltaic cells is promptly taken away, and the system can operate normally.
The main research focuses on improving the utilization rate of solar energy among users under dispersed conditions, solving the problem of low efficiency in distributed solar energy utilization, and enhancing their ability to generate heat and electricity and connect to the grid. By rational allocation and utilization of light and heat energy, the energy utilization efficiency of household photovoltaic systems can be improved while reasonably reducing system installation costs, thereby meeting the needs of potential users in rural areas, pastoral areas, border outposts, and other areas for electricity and heat energy. The system designed in this article mainly consists of a concentrated solar power generation system, a cooling heat transmission system, a heat storage island, and a heating system.