# Energy and sand batteries (thermal energy storage)

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• Matt27RS
I'm not familiar with the specifics of your project, but a rough calculation might be to use the following equation: Q=V*D*T, where Q is the heat transferred, V is the flow rate, D is the pipe diameter, and T is the temperature. If you want to transfer a fixed amount of heat over a fixed period of time, the best way to do this is to use a rate controller.f

#### Matt27RS

TL;DR Summary
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

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.)

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