Friday is here, and the 'A Taste of Science for the Weekend' corner is back — number 79.
This time: heliostats, the eutectic point, and the fascinating connection between molten salt and green energy.
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The video features a concentrated solar power plant built in Gansu Province, China.
The plant generates electricity using 30,000 large mirrors that focus sunlight onto a large tank mounted atop a 200-meter tower.
The mirrors are called heliostats, and they are mounted on motorized mounts that continuously adjust their angle to track the position of the sun.
The fluid in the tank heats up to very high temperatures and is then transferred to an insulated storage tank that retains its heat.
To generate electricity, it is pumped through a pipe into a water tank. The water boils into steam and spins a turbine that produces electricity.
Israel has a similar power plant in Ashalim in the Negev, but the Chinese model incorporates two significant changes that greatly improve its performance.
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In the past, the fluid in the heat-absorbing tank was water or oil — both of which are used in the Israeli facility.
The drawback of water is that it boils and evaporates at a relatively low temperature; to allow it to reach several hundred degrees without evaporating, it must be kept under very high pressure, which increases the risk of explosion or leakage and requires exceptionally robust piping.
Oil can reach 400°C before it begins to evaporate, so high pressure is not required as long as that temperature is not exceeded — giving it an advantage over water.
The Chinese model uses molten salt. Salt's advantages include both its ability to reach 565°C before evaporating and its particularly high energy density compared to oil and water.
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The salt used is a mixture of 60% sodium nitrate and 40% potassium nitrate.
The reason for using this specific mixture at these exact proportions is the eutectic point.
To understand what a eutectic point is, it's important to note that proper system operation has two requirements — the fluid in the tank must reach as high a temperature as possible to maximize the energy extracted, while simultaneously remaining liquid even as it cools, so it doesn't solidify in the pipes and block them.
Each salt on its own solidifies at temperatures between 308–334°C, but mixing them in precisely this ratio produces a mixture that solidifies only at 220°C.
The eutectic point of a mixture is the mixing ratio at which the components form a mixture that becomes liquid at the lowest possible temperature for those materials.
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Another interesting aspect of the Chinese model is its use of two towers instead of one.
When a single tower is used, the mirrors farther from the tower focus significantly less solar energy onto it compared to the closer ones.
Using two towers shortens the distance between each mirror and the top of a tower, and also improves the angle to capture maximum solar energy from each mirror.
The molten salt drives a single large turbine, since it generates more electricity than two smaller turbines would.
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The great advantage of this electricity-generation method is its ability to store energy for up to 6 hours ahead. Solar panels and wind turbines supply cheaper electricity, but without a built-in energy storage solution for a continuous power supply.
A power plant like this could have additional interesting applications — for example, supplying high-grade heat to factories and industrial furnaces, serving as a clean electricity source for green hydrogen production, and even as a coolant for nuclear reactor cores.
Shabbat Shalom 😊
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