In recent years, in the context of addressing global climate change, vigorously developing renewable energy to replace fossil fuels has become a trend for many countries in energy transformation. Among the numerous alternative types of renewable energy currently available, solar photovoltaic is undoubtedly one of the most ideal energy sources for the future, occupying an important position in the medium and long-term energy strategies of various countries.
Solar photovoltaic utilization can be divided into solar photovoltaic power generation, solar photovoltaic heating, solar photovoltaic refrigeration, solar photovoltaic seawater desalination, etc. according to the products. Among them, solar photovoltaic refrigeration is generally achieved by converting solar photovoltaic energy into electrical energy for cooling, while solar photovoltaic seawater desalination is generally achieved by converting solar photovoltaic energy into thermal energy to drive the phase change process of seawater. Therefore, this article focuses on analyzing the utilization and development trends of solar photovoltaic heating and solar photovoltaic power generation.
1. Solar photovoltaic heating utilization technology and development trends
The characteristic of small household solar photovoltaic heating devices is that they are installed and used separately by users. The system is relatively simple and does not interfere with each other. At the same time, there is no billing issue, and property management is convenient. However, the user assisted heating part consumes a lot of energy, and the overall cost is relatively high compared to central hot water systems of the same level; Due to the lack of a reliable return water system, there is a problem of water cooling in the water supply pipeline, resulting in heat energy waste. The inability to share hot water resources prevents the full utilization of system resources ; There are many system pipelines, making it difficult to coordinate with the building. The principle of solar photovoltaic hot water system is a device that uses solar radiation energy as a heat source to convert absorbed solar photovoltaic energy into thermal energy for heating water, including solar photovoltaic heat collection devices, heat storage devices, circulating pipeline devices, etc. Due to the significant impact of weather on the operation of solar photovoltaic hot water systems throughout the year, their independent applications have intermittency, instability, and regional differences. Therefore, in addition to converting solar energy into thermal energy, solar photovoltaic applications should also adopt hot water guarantee systems (auxiliary heating systems) and heat storage measures to ensure stable supply of the system.
The characteristic of large-scale centralized central solar photovoltaic heating water system is high integration degree. The advantage of centralized heat storage method is to reduce cost and heat loss, and auxiliary heating system is used for centralized heating; The hot water system has a simple supply pipeline, and a reasonable dry pipe circulation ensures the quality of water supply, achieving instant heating at each water terminal; For residential communities, centralized systems have the advantages of lower initial investment and higher integration compared to separate household systems, and modular collectors combined with buildings are also more aesthetically pleasing.
Small household solar photovoltaic heating devices mostly use flat panel solar photovoltaic collectors and vacuum tube solar photovoltaic collectors, and generally can only produce low-temperature hot water. Most large or ultra large solar photovoltaic heating devices use concentrated solar collectors, mainly including slot collectors, Fresnel collectors, disc collectors, and tower collectors.
In recent years, traditional spotlight collectors have also been continuously developed and improved, but breakthrough technological progress has not yet been achieved. Taking slot type collectors as an example, the design of the new type of slot type collectors is more suitable for the application temperature range, adopting the idea of reducing the collection temperature (the outlet temperature of the collection field is reduced from 393 ℃ to around 235 ℃), from complex to simple (the parabolic curve composed of 4 mirrors is reduced to 2 pieces), from large to small (the opening width is reduced from 5770mm to 3000mm, and the focal length is reduced from 1710mm to 950mm), and from long to short (the combination of collectors is reduced from 150m to 50m), which can appropriately reduce the configuration cost. At the same time, the reduction of temperature and pressure also reduces the safety risk of the collection system, but there is no substantial difference in energy utilization efficiency. Breakthrough.
In addition to immediate thermal efficiency, heat storage is another important technical challenge for large or ultra large solar photovoltaic heating devices. Both large and small regional heating systems and cogeneration plants should have short-term heat storage facilities, while large-scale solar photovoltaic heating systems require seasonal heat storage systems. Solar photovoltaic heating systems can produce more heat than immediately needed in summer. As winter heating demand reaches its peak, many large solar photovoltaic regional heating systems utilize large seasonal heat storage devices to extract excess heat stored in summer during winter.
2. Solar photovoltaic power generation utilization technology and development trends
Solar photovoltaic power generation systems can be divided into solar photovoltaic power generation and solar thermal power generation. Solar thermal power generation, also known as Concentrated Solar Power (CSP), uses a large number of mirrors to concentrate the direct sunlight from solar photovoltaics, heat the working fluid, and generate high-temperature and high-pressure steam, which drives the turbine to generate electricity. The schematic diagram of its principle is shown in the figure.
Solar thermal power generation, like large or ultra large solar heating devices, mainly uses concentrated solar collectors, including slot collectors, Fresnel collectors, disc collectors, and tower collectors mentioned earlier. The above technologies are relatively mature, among which the most widely used is slot type solar power generation, and the tower type solar power generation has the greatest potential for efficiency improvement and cost reduction.
Slot type solar thermal power generation is the earliest commercialized solar thermal power generation system. It mainly uses multiple trough parabolic concentrators to gather sunlight onto the heat collecting tubes of the receiving device, heat the working fluid, generate high-temperature steam, and then drive the turbine to generate electricity. The geometric characteristics of the collection device determine that the concentration ratio of trough solar power generation is lower than that of tower solar power generation, usually between 10 and 100, and the operating temperature can reach 400 ℃.
The advantages of tower solar photovoltaic power generation systems lie in their high concentration factor, energy concentration, simple process, and high thermal conversion efficiency, making them extremely suitable for grid connected solar photovoltaic power generation.
Solar photovoltaic power generation is a technology that utilizes the photovoltaic effect at the semiconductor interface to directly convert light energy into electrical energy. It mainly consists of three parts: solar panels (components), controllers, and inverters. After being connected in series and packaged for protection, solar photovoltaic cells can form large-area solar cell modules, which are then combined with power controllers and other components to form photovoltaic power generation devices.
Solar cells are generally silicon cells, mainly divided into three types: monocrystalline silicon solar cells, polycrystalline silicon solar cells, and amorphous silicon solar cells. The key to affecting photovoltaic power generation technology lies in the power generation efficiency of the battery.
At present, a large number of ground photovoltaic systems use silicon based solar cells, which can be divided into monocrystalline silicon, polycrystalline silicon, and amorphous silicon solar cells. In terms of comprehensive performance such as energy conversion efficiency and service life, monocrystalline silicon and polycrystalline silicon batteries are superior to amorphous silicon batteries.
Solar photovoltaic power generation systems can be further divided into direct solar power generation and concentrated solar power generation. A direct solar power generation system is a system in which sunlight is directed at a concentration ratio (commonly known as the concentrated ratio) to generate electricity on non concentrated solar cells. Multiple solar cells are connected to form one or several solar panels. A spotlight power generation system is a system that first focuses sunlight on a solar cell using some optical technology (transmission or reflection spotlight device), and then connects these solar cells to form a system.
The spotlight system includes concentrators, secondary concentrators, reflectors, etc. In the application of spotlight photovoltaics, the main considerations are the multiple of the spotlight and the radiation intensity, distribution, and spectral situation of the spot after spotlight. Concentrators can be divided into refractive concentrators, reflective concentrators, composite concentrators, thermal photovoltaic concentrators, etc. based on optical principles. 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.
In general, for a spotlight system with a spotlight rate exceeding 10, a tracking system must be used to ensure the spotlight effect. The tracking system is divided into two types: single axis tracking and dual axis tracking. Under the premise of comprehensive consideration of performance and cost, different forms of concentrators, solar cells, and tracking systems are adopted for engineering design as needed to form a concentrated photovoltaic system.
The most significant advantage of concentrated solar photovoltaic technology, which differs from traditional silicon crystal and thin film types, is its high photoelectric conversion efficiency. This type of solar cell chip has a photoelectric conversion efficiency between 36% and 40% when focused on sunlight by about 500 times, and the efficiency of the photoelectric module is between 22% and 28%. The efficiency of the entire system is between 18% and 20%. In terms of annual power generation, under the same conditions, the system (combined with biaxial sun chasing technology) is about 1.2-1.4 times that of traditional silicon crystal forms, which is the competitive advantage of concentrated photovoltaic power generation technology. Concentrated photovoltaic power generation technology is most suitable for application in large power plants, especially in areas with sufficient sunlight, dryness, and low humidity.