Why don't we use sand to store the energy from the Sun to use at night? (physics, transfer)
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Would it be possible? I suppose so, but it would be incredibly inefficient.
Whether a substance remains solid or not is irrelevant to storage at scale. To the contrary, phase change (from solid to liquid, liquid to solid, liquid to vapor, etc.) usually stores or releases large amounts of thermal energy. Boiling water to create pressurized vapor (steam) is an example. It isn't that water vapor is magic, what makes it worthwhile is that there can be huge amounts of it easily produced under pressures sufficient to push turbine wheels around or pistons in steam engines.
Something akin to what you suggest for thermal storage was proposed years ago using sodium sulfate:
What's the heat capacity, per volume, or per mass, of sand vs. water?
I believe your answer resides right there.
You need to take in account the fact that water will become a gas after it's heated above 100 degrees Celsius, and storing a gas is more difficult than storing a solid like sand. Sand will stay a solid even at 900 degrees Celsius.
We could use heating elements like those in electric showers to heat sand using electricity from photovoltaic solar panels, during day hours. That super hot sand could be stored in some tanks with thermal insulation, to be used at night time for boiling water to generate steam and turn a turbine to generate electricity.
Sand is so cheap, and can be heated to temperatures above 500, 600, 700 degrees Celsius without melting.
The installation you're describing is expensive enough that the price of the storage medium is close to irrelevant. As harry chickpea points out, using phase conversion (solid-to liquid and back, so as not to have to deal with rapid volume change) lets you store way more energy per cubic meter of medium. On anything resembling industrial scale, this becomes really important.
The installation you're describing is expensive enough that the price of the storage medium is close to irrelevant. As harry chickpea points out, using phase conversion (solid-to liquid and back, so as not to have to deal with rapid volume change) lets you store way more energy per cubic meter of medium. On anything resembling industrial scale, this becomes really important.
The big problem with water is that if you heat it above 100 degrees Celsius on normal atmospheric pressure it becomes a gas, and dealing with a gas is much more complicated. Water at 98 or 99 degrees Celsius could be a thermal storage medium, but with sand you can have much higher temperatures, even one thousand degrees maybe....
It is not enough to identify the maximu temperatur sand can be heated to without changing states. The key is, how effient is it to heat sand, how much lost energy does it take, and how much energy does it return.
Water has the highest specific heat capacity by far. It is extremely efficient. For the use you are discussing, it just a matter of maching the volume of water ot the amount of energy you want to store. But the water will easily accept the heat, it will return a great amount of energy back.
You would have horrific energy losses trying to heat the sand. So you would waste an enormous amount of energy. So now only are you wasting boat loads of energy relative to water, but you have to scale up the volume of the sand by multiples to store the same amount of energy as you would the water.
Finally, since the sand has much lower specific heat capacity than water, it would be very inefficient in giving back the energy.
It sounds like a lose-lose-lose to me. Storing energy as hot water is not every useful. Storing energy as hot sand is mostly useless IMHO.
What's the heat capacity, per volume, or per mass, of sand vs. water?
I believe your answer resides right there.
You're real close: it's a matter of energy density. For instance-- the energy stored in gasoline contained in a 20 gal tank (~1'x1'x2') is about double the energy stored in the best EV battery pack (~1'x3'x5'). ...I don't know the exact figures for sand storage, but I bet you'd need Waikiki beach to get a range of 200 miles for your sand powered car.
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