I Energy and sand batteries (thermal energy storage)

AI Thread Summary
The discussion revolves around the feasibility of building a sand battery and the calculations needed to heat 1000 kg of sand to 400 degrees Celsius. Initial calculations indicated that 332,000 kJ of energy would be required, leading to a misunderstanding of the time needed for a 4 kW solar array, which was corrected to approximately 83,000 seconds, or about a day. The user then inquired about calculating the heat transfer rate for a copper pipe running through the heated sand, seeking guidance on flow rate, temperature, and pipe dimensions. They are particularly interested in using a 28mm pipe with an 8m length for their setup. The conversation highlights the importance of accurate calculations and understanding heat transfer dynamics in thermal energy storage systems.
Matt27RS
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How much energy is required to heat a sand battery
Hi,
I'm new here, so apologies if this is covered somewhere else.

I'm just playing with the notion of building a sand battery.

Sand has a spec heat cap. of 830j/kg degrees K.

If I want to heat 1000kg of sand up to 400 deg c, by my rough calculations that will require 332 000 Kj of energy, or 332,000 KWatts of electrical energy.

If I haveva solar array producing 4Kw, this will take around 83 000 hours!

Is this correct? Or amI completely wrong?

Many thx
 
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Matt27RS said:
If I want to heat 1000kg of sand up to 400 deg c, by my rough calculations that will require 332 000 Kj of energy,

If I haveva solar array producing 4Kw, this will take around 83 000 hours!

Is this correct? Or amI completely wrong?
You're not completely wrong, but it's 83,000 seconds (not hours). Which is about a day (neglecting heat lost in the process.)
 
PeroK said:
You're not completely wrong, but it's 83,000 seconds (not hours). Which is about a day (neglecting heat lost in the process.)
Doh! Thanks very much. I knew it was a bit wrong somewhere!

My next question....does anyone know a quick and dirty calc for understanding the rate of heat transfer for a copper pipe running through a heated source? In this instance it would be the sand.

I'm sure there must be a graph which plots flow rate, temp,pipe length etc for a given pipe size? I'm looking to hear water flowing through the heated sand battery. Flow rate will be the control method for modulating the temperature transfer from the sand to the water in the pipe. The other variable will be the pipe diameter. Ideally I would like to use pipe sizes of 28mm.

The length of the pipe run within the sand is also variable, but I would start with a length of 8m.

Any thoughts....

Thank you
 

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