Solar Power Industry Application
In the field of solar photovoltaics, valve applications fall into the following two parts. First, control cooling water, circulating water, and other media in the solar power generation system to ensure normal operation of the system; and a photovoltaic panel cleaning system. Second, solar power station.
Photovoltaic panels allow us to generate electricity in a sustainable manner by converting solar energy into electricity. However, to ensure high-energy operation of photovoltaic systems, the flow of liquid through the system needs to be accurately controlled to maintain temperature and cool panels and other critical components.
Photovoltaic cooling system:
In photovoltaic cooling systems, valves are used to control the flow of cooling liquid; this helps maintain the temperature of the photovoltaic panels within a safe range, ensuring high-performance operation of the system.
Temperature adjustment:
The temperature of photovoltaic panels has a significant impact on their performance, and valves can be used to regulate the flow of cooling liquid to maintain a constant temperature, thereby maximizing the efficiency of the panels.
Cleaning system:
Photovoltaic panels will accumulate dust and dirt during operation, affecting their performance. The valve is used to control the spraying of cleaning liquid to ensure that the photovoltaic panels remain clean and maximize power generation efficiency.
Valves must provide constant heat transfer and fluid flow and withstand extreme temperatures, pressures and flow rates that may be exacerbated by low outdoor temperatures. Materials swell, bend and warp, requiring maintenance and reducing factory output.
From a performance and reliability perspective, thermomechanical stress will be the most important consideration
1. Corrosion resistance
Able to adapt to long-term use in sunlight, humidity, and rainy environments without being damaged; stainless steel material is the first choice.
2. High temperature tolerance
Photovoltaic panels generate high temperatures during the power generation process, so a cooling system is required to keep the temperature within a suitable range. Valves need to be able to operate stably in high-temperature environments to ensure the effectiveness of the cooling system.
3. Energy-saving
Electric valves are usually powered by electricity, which allows them to achieve high-energy control and reduce energy consumption. Compared with traditional manual valves, electric valves can regulate flow more accurately, thus reducing energy waste.
4. Remote control
Valves equipped with remote control functions can make operation and monitoring more convenient. The status of the valve can be monitored and controlled anytime and anywhere through a remote system to better manage the operation of the photovoltaic power station.
1.1 Trough Solar Thermal Power Station
The trough solar thermal power station is a series and parallel arrangement of multiple trough-shaped parabolic concentrating collectors to heat thermal oil. The high-temperature thermal oil transfers heat to steam through a heat exchanger, thereby driving the steam turbine to generate electricity.
Due to the maturity and reliability of trough CSP technology, currently about 90% of solar CSP plants in the world are trough power plants.
The working medium of trough power station is generally thermal oil. Since heat transfer oil will decompose at high temperatures, its maximum operating temperature is generally limited to about 395°C. After exchanging heat with steam, the temperature of the heat transfer oil will drop to about 295°C.
Since thermal oil is flammable and explosive, the external sealing of the equipment is generally strict, and thermal oil is strictly prohibited from leaking into the environment to avoid the risk of combustion and explosion.
In order to achieve continuous power generation on rainy days or at night, trough power stations are generally equipped with molten salt heat storage systems. The thermal storage system mainly consists of hot tanks, cold tanks, molten salt working fluid and other equipment. When the weather is fine during the day, the high-temperature thermal oil heated by solar energy heats the molten salt to about 385°C and stores it in a hot salt tank. At night or when the weather is cloudy, the high-temperature molten salt in the hot salt tank is used to heat the steam to generate electricity. Therefore, a solar thermal power station equipped with a heat storage system can achieve 24-hour continuous power generation.
1.2 Tower-type Solar Thermal Power Station
The biggest difference between tower-type solar thermal power stations and trough-type power stations lies in the different structures of the solar islands.
The tower power station consists of thousands of sun-reflecting plane mirrors that focus sunlight onto the heat-absorbing tower and heat the working fluid. The principle is shown in Figure 2. The heated working fluid exchanges heat with steam to increase the energy of the steam.
Tower solar thermal power stations generally use molten salt as a working fluid, and their maximum operating temperature can reach 565°C. Compared with trough solar thermal power stations, tower solar power stations have higher steam temperatures and higher rankine cycle power generation efficiency.
Due to the low technical maturity of tower power plants, there are currently relatively few successfully constructed tower power plants in the world. However, compared with the trough type, the tower type has the advantages of low initial investment and high efficiency. At this stage, tower-type solar thermal power plants have replaced the trough trend.
In addition, in order to achieve continuous power generation, tower power stations are generally equipped with a heat storage system. The structure of the heat storage system is similar to that of the trough type.
2.2 Valve Working Conditions
The operating conditions of CSP plants with different technical types are relatively clear. The operating parameters of CSP plants with different unit capacities will be different.
For tower power stations, due to the high height of the heat collecting tower, the working pressure of the medium inside the solar island is generally higher than the medium pressure of the heat storage system. The working pressure of the heat storage system is between 0.7 and 1.1MPa.
The general working pressure of the heat transfer oil in a trough power station is designed according to the length of the heat transfer oil pipeline. For example, for a 100 MW trough power station, the heat transfer oil valve generally chooses Class 600 lb.
2.3 Valve Requirements
Since the working fluid inside the valve of a solar thermal power plant is different from that of ordinary valves, the structure and materials used in the valve are also different from those of conventional valves.
For the trough power station heat collection system, the circulating medium is heat transfer oil with flammable characteristics, so the leakage requirements of the valve are relatively high. The sealing of the packing is generally a double-sealing structure of bellows and graphite packing.
For molten salt valves, due to the high working temperature of molten salt and the strong oxidizing property of molten salt at high temperatures, it will cause strong corrosion to metals and non-metals. In addition, binary molten salt will solidify below 220°C. Once the salt is not completely drained in the valve, it will not be able to open and close.
Molten salt valves have certain special requirements compared to ordinary valves:
(1) Normal operation of the valve can be ensured when the valve is frequently opened and closed, and salt should not accumulate in the valve body during salt removal to prevent the molten salt from freezing and causing the valve to be unable to seal.
(2) The material of the molten salt valve body can resist the corrosion of molten salt.
(3) The non-metallic gaskets and packings of the valve can withstand the corrosion of high-temperature molten salt.
(4) The heat collection system of the solar thermal power station is affected by sunlight (when clouds cover the sun), so the equipment will operate intermittently, so the valve can withstand the requirements of alternating hot and cold temperatures.
(5) In order to prevent the molten salt valve from freezing, the molten salt valve is generally equipped with electric heating requirements.
2.4 Valve Selection
In view of the fact that molten salt has a high freezing point, the impact of solidification of molten salt on the valve needs to be taken into consideration when selecting the valve.
Therefore, valves that are prone to liquid accumulation in the middle cavity (such as ball valves and gate valves) are not suitable for molten salt systems.
Based on actual use experience analysis, the general selection of molten salt valves is divided into two types:
1. Goble bellows valve, which is mainly aimed at valves with smaller diameters.
2. Butterfly valve: For large-diameter molten salt pipelines, butterfly valves are generally chosen.
In addition, taking into account the temperature variability of actual working conditions and the safety of on-site molten salt leakage, molten salt valves generally use welded end connection methods, and it is not suitable to use flange structures to avoid leakage at the flange due to temperature changes.
