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Heat transfer fluid refers to a designed mixture of chemicals that collect and transport heat. These fluids are one of the key technologies that make electrical generation possible from a concentrating solar power system (CSP). Multiple operating criteria must be determined in the selection of a suitable heat transfer fluid.
In concentrating solar power (CSP) systems, an advanced solar power technology, light energy is converted to heat. This is a distinction from photovoltaic solar power schemes, where light energy, captured by photoelectric cells, produces electricity directly. In a CSP process, light is concentrated by mirrors that focus reflected sunlight on receivers, tubing through which the heat transfer fluid travels. The hot fluids are then piped to the power generation station.
One CSP configuration uses parabolic mirrors arranged in exceptionally long rows that look like the blades of large highway snow plows. The heat transfer fluid travels down the horizontal centers of the mirrors, gaining heat as it moves from one mirror to the next. Other configurations use circular flat mirrors that focus the light on receivers strung above the mirrors. Often, the systems have a solar tracking function, where the mirrors can follow the sun’s movement across the sky.
The hot fluid is pumped to a steam-turbine power generating station. There, the fluid heats the water, taking the place of the fuel in the traditional fossil-fueled electrical station. The boiling water circuit is identical, except for the variation in the design of the heat exchanger between the heat transfer fluid and the water. There is no need for a gas manifold and exhaust mechanisms.
The use of the heat transfer fluid is remarkable for two reasons. In this scheme, no fuel was consumed; the energy came from sunlight. Therefore, there are no combustion by-products to be handled. CSP has the solar-fuel advantages of photovoltaic plants, but can potentially achieve higher efficiencies and greater electrical outputs.
Second, heat was literally piped from one place to another. Engineers typically think of heat being a waste product or a byproduct, but not the carrier of energy. Heat conducts so easily through pipe walls and duct work, it cannot be readily transported and is best used at the site of generation. The use of advanced heat transfer fluids makes the transportation of heat feasible.
Heat transfer fluids must be carefully designed to have a high heat capacity, high thermal stability, and a broad range of operating temperatures. They must either remain a liquid or maintain system-compatible properties as a gas. A typical heat transfer fluid has operating specifications of 12oC to 400oC (54oF to 752oF).