China is rich in solar energy resources, with theoretical reserves reaching 1.700 billion tons of standard coal per year. In most areas, the annual average daily radiation is above 4kWh/m2, and Tibet’s daily radiation is as high as 7kWh/m2. Compared with other countries at the same latitude, it is similar to the United States and has more advantages than Europe and Japan. As a clean and inexhaustible energy source, solar energy should be fully utilized.
Solar thermal power generation can not only generate electricity, but also realize heating and cooling at the same time, forming a co-generation of heat, electricity, and cooling. The solar thermal power generation system is composed of a heat collection system, a heat transmission system, a heat storage and energy storage system, a heat engine, and a generator. After the heat collection system collects solar energy, it transfers the collected solar heat energy to the heat engine through the heat transmission system. The heat engine generates power and drives the generator to generate electricity. The heat source of the entire system comes from solar energy, so it is called a solar thermal power generation system.
Solar thermal power generation uses heat collectors to convert solar radiant energy into heat energy, and then generates electricity through steam turbines and generators. According to the temperature of the heat collection, solar thermal power generation can be divided into two categories: high-temperature thermal power generation and low-temperature thermal power generation. Divided by solar energy collection methods, there are three main types of solar thermal power plants: tower type, trough type and pan type.
High-temperature solar thermal power generation, also known as tower solar power generation, is a large-scale solar power generation. The United States, Japan, and Europe have built some solar test power stations of thousands to tens of thousands of kW. Tower-type solar power generation is to place the energy collector on the top of the tower, and its components mainly include reflector arrays, high towers, energy collectors, heat accumulators, generator sets, etc. The mirror array is composed of many mirrors (also known as heliostats) arranged in a certain pattern. These mirrors automatically track the sun, and the reflected light can be accurately projected into the window of the energy collector. The tower can be built in the center or south of the mirror array. The energy collector is designed to receive light on one side or all around. When sunlight is projected on the energy collector and is absorbed and converted into heat energy, the medium (water or other medium) flowing in the heating coil generates steam. Part of the heat is used to drive the steam turbine generator set to generate electricity, and the other part of the heat is stored in the heat accumulator to prepare for electricity generation when there is no sunlight. The key technology of tower solar power generation has 3 aspects:
(1) Mirror and its automatic tracking.
Because this kind of power generation requires high temperature and high pressure, it must have a large concentration ratio for the focus of sunlight, and hundreds of mirrors are required. And there must be a reasonable layout so that the reflected light can be concentrated to the smaller collector window. The reflectivity of the reflector should be 80% or even more than 90%, and the automatic tracking of the sun should be synchronized, which is generally controlled by an electronic computer.
(2) Energy collector. Also called “solar boiler”, it requires small size and high energy conversion efficiency. The existing energy collectors have various forms, such as cavity type, disc type, cylindrical type and so on. It is generally believed that the disc-type solar boiler developed in Italy is better, which can generate 150 equivalent atmospheric pressure and 600 ℃ superheated steam, which can be used for 350 kW generator sets.
(3) Thermal storage. As the intensity of solar radiation changes all the time, in order to ensure relatively stable power generation, heat storage measures must be taken. This is an indispensable part of tower thermal power generation, but the most ideal heat storage material has not yet been found. According to the evaluation of 300 kinds of various salt mixtures with melting points of 262-321 ℃ by the American Nevre Company, 10 kinds of better heat storage materials were selected.
In 1950, the former Soviet Union designed the world’s first solar tower power station and built a small test device. In the 1970s, solar cells were expensive and low in efficiency. Relatively speaking, solar thermal power generation had high efficiency and relatively mature technology. Therefore, at that time, many industrially developed countries focused on solar thermal power generation and invested in the construction of a number of experimental solar thermal power stations. According to incomplete statistics, from 1981 to 1991, there were about 20 solar thermal power stations (above 500 kW) built in the world, with a maximum generating power of 80MW.