Requirements for pipeline materials in different solar thermal power plants
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The defocus temperature in CHP plants primarily determines the maximum operating temperature of the system. Since biphenyl-biphenyl ether begins to decompose when its temperature exceeds 400 degrees, this temperature must not go above 396°C. At 396°C, the heat collection field needs to be defocused in order to prevent the heat transfer oil from reaching excessive temperatures; this is why the maximum temperature of the heat collection field in such projects is set at 396°C. For pipes and materials, the selection is primarily based on their temperature-related fluid properties as well as cost considerations. For heat transfer oils that use biphenyl and biphenyl ether as their components, carbon steel is generally used, as the temperature does not exceed 400°C; therefore, ordinary carbon steel is sufficient to meet the requirements. Previously, both the hot pipes and cold pipes in the design process were made of carbon steel pipes. Regarding silicone oil, the temperature of the silicone oil used by Yumen Longteng has now reached 425°C, which falls within the range suitable for such applications. Hot pipes can reach a temperature of 425°C; therefore, alloy steel is used for hot pipes. However, cold pipes remain at around 300°C, so carbon steel can still be used for them. In trough systems that use molten salt, the temperature is higher; the Axee project can reach around 565°C. Similar to tower-based systems, the molten salt pipes in such systems must be made of stainless steel. When selecting the pipe diameter, attention must be paid to pipeline design; a step-by-step design process is required, taking into account factors such as pressure, temperature, flow rate, and various physical properties. The pipe diameter is calculated step by step based on these physical properties. All slot circuits are parallel circuits; as one section of a slot circuit is passed through, the flow rate of the heat transfer oil decreases. Therefore, the diameter of the main pipes also decreases gradually, and expansion bends are installed every two or three circuits.
This is the same as the fact that different processes and media have varying requirements for pipeline materials and auxiliary facilities
In the 1930s, Dow Chemical produced biphenyl-biphenyl ether heat transfer oil for the first time in the world. This heat transfer oil, composed of 73.5% biphenyl ether and 26.5% biphenyl, was first commercially used in the SEGS tank-type solar thermal power plants developed in the United States in the 1980s. And even 30 years later, today, biphenyl-biphenyl ether heat transfer oils, represented by DowthermA and Shounuo Therminol®VP-1, remain the most commonly used heat transfer media in trough-type photothermal power generation systems. But is biphenyl-biphenyl ether heat transfer oil truly the most suitable heat transfer medium for trough-type photothermal power generation systems? The fact that traditional trough systems have a maximum operating temperature of no more than 400°C has actually hindered the further development of trough technology ； Biphenyl biphenyl ether heat transfer oil will undergo cracking and degrade under long-term operation at 400°C, and its high crystallization point of 12°C may also cause problems in the cold climate of western China during power plant operation. Although these shortcomings are not sufficient to affect the actual operation of trough power plants, we still hope to address them as much as possible. To improve the efficiency of trough solar power plants, changing the heat transfer medium is the most direct and effective method; as a result, molten salt trough technology has been promoted to a certain extent, yet its commercial application still faces numerous obstacles. So, apart from molten salts, is there a more suitable heat transfer oil product that can operate over a wider temperature range? Thus improving the operational efficiency of trough systems. Is there a more stable heat transfer oil that can withstand long-term high temperatures above 400°C without deteriorating easily? Is there a heat transfer oil product with a lower freezing point that can better suit the special development conditions of solar thermal power plants in western China? The answer remains unknown so far. However, a type of silicone-based heat transfer oil is set to see its first commercial application in China. The Longteng Solar Thermal Power Plant in Yumen, Gansu, which is one of the first 20 solar thermal power demonstration projects, plans to use a new generation of silicone oil as the heat transfer medium. If successful, this could mark a milestone in the innovative use of heat transfer media in solar thermal technology.
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In different industries, there are differences between sectors such as renewable energy in the power industry and the chemical industry. By the same logic, in all of these fields, there is the issue of selecting materials suitable for operation under different temperature and pressure conditions; the principles remain the same regardless of the industry