Calculating for the use of sand for a thermal store.

[Marvin]

Is this right?

 

Dry sand density between 1520-1680 kg/m3 (say 1500 in the calculation below)

Course sand, dry, specific heat capacity is about 800 Joules per kg per degree of temperature change.

Course sand, dry, thermal conductivity 0.25 W/m K

1kWh equals 3,600,000 joules

 

And therefore, ignoring all factoring of energy loss due to energy conversion etc(what I call "in theory"):

 

Heating up 1kg of dry course sand would be:

 

450 degrees temperature change times 800 specific heat capacity times 1kg in weight, divided by the amount of joules in one kWh:

 

450 x 800 x 1000 / 3,600,000 = 1 kWh.

 

 

 

 

[SteamyTea]

  On 02/01/2023 at 12:27, Marvin said:

450 degrees temperature change times 800 specific heat capacity times 1kg in weight, divided by the amount of joules in one kWh

Expand  

1 kWh.

How are you going to insulate at those temperatures?

How are you going to get the energy back out?

[Marvin]

Hi @SteamyTea

 

It's good to have someone capable to review my workings. I double checked because I thought you might.

 

 

  On 02/01/2023 at 12:43, SteamyTea said:

1. How are you going to insulate at those temperatures?

2. How are you going to get the energy back out?

Expand  

Present theory:

 

1. From centre of store to outside: Sand, steel container, mineral wool, external weather protection.

2. Electric energy, to heat air, to heat sand. Heated sand to heated air to building. 

 

Storage capacity, thermal loss over time and building requirement are the next hurdles...

 

 

 

 

[SteamyTea]

  On 02/01/2023 at 13:15, Marvin said:

heat air

Expand  

You could compress air, which will warm up with entropy and then add in extra energy with an element.

Then release it back.

[JohnMo]

For extracting heat, you could use an air to water heat exchanger, such as a car intercooler to allow hot air to exchange to hot water and then feed into house.

[MikeSharp01]

  On 02/01/2023 at 13:15, Marvin said:

 

1. From centre of store to outside: Sand, steel container, mineral wool, external weather protection.

Expand  

Make it a basic cube and then use vacuum panels for insulation.

[SteamyTea]

  On 02/01/2023 at 13:42, MikeSharp01 said:

vacuum panels for insulation.

Expand  

Lots of Thermos flasks.

 

Aerogel is 0.013 W/m.K 

 

 

[Iceverge]

A very interesting project. 

 

How much space do you have to play with and how much excess energy do you have to use? Is this a budgeted project with a payback window or a fun experiment?.

 

In commercial systems they're claiming about 50% round trip efficiency and storage temps of up to 500 deg C.  

 

 

[Marvin]

  On 02/01/2023 at 13:27, SteamyTea said:

You could compress air, which will warm up with entropy and then add in extra energy with an element.

Then release it back.

Expand  

Hmm.

 

What volume of air would you need to produce the potential of 1kWh of heat?

[Marvin]

  On 02/01/2023 at 13:31, JohnMo said:

For extracting heat, you could use an air to water heat exchanger, such as a car intercooler to allow hot air to exchange to hot water and then feed into house.

Expand  

Could add the warm air to the MVHR inlet. 

[Marvin]

  On 02/01/2023 at 13:42, MikeSharp01 said:

Make it a basic cube and then use vacuum panels for insulation.

Expand  

What temperature will VIPs withstand?

[Marvin]

  On 02/01/2023 at 13:52, SteamyTea said:

Aerogel is 0.013 W/m.K 

Expand  

Aerogel is good but expensive with a 650C melting point (being made from the same sort of thing as sand (silica)?

 

What is it? about £10 a square foot 10mm thick? Comparative thickness of mineral wool 40mm?  this might blow the budget...

 

 

 

 

[SteamyTea]

  On 02/01/2023 at 14:04, Marvin said:

What volume of air would you need to produce the potential of 1kWh of heat?

Expand  

Depends on the pressure.

The SHC is better than sand at 1kJ/kg.K.

A m³ at sea level has a density of about 1.2 kg/m².

Double the pressure, halve the volume.

 

Edited January 2, 2023 by SteamyTea

[Onoff]

Throwing it out there.....could you use a literal vacuum as insulation. One big welded tank inside another?

[Temp]

Use Helium. It's specific heat capacity is over 5000 🙂

 

It's volumetric density might be an issue.

 

Edited January 2, 2023 by Temp

[JohnMo]

  On 02/01/2023 at 14:39, Temp said:

Use Helium

Expand  

Would need to be careful you did make a bomb, not sure of the expansion rate, but a temp rise of 450 degs, could give a big pressure rise in a contained volume of gas.

[JohnMo]

Here's one prepared earlier

 

https://www.caldera.co.uk/

[Nickfromwales]

  On 02/01/2023 at 15:16, JohnMo said:

Here's one prepared earlier

 

https://www.caldera.co.uk/

Expand  

So, is this filled with sand / granite ? Apples for apples?

 

  On 02/01/2023 at 13:54, Iceverge said:

claiming about 50% round trip efficiency

Expand  

 

[JohnMo]

  On 02/01/2023 at 16:30, Nickfromwales said:

Apples for apples

Expand  

Not sure apples would be that good. But they don't say the fill material, but mentions solid.

 

Looked at the datasheet stores 100kWh, it states the temp drop of 500 to 200 degs takes 19 days.  The average heat loss being 5kWh day! Pretty huge losses.  Vacuum insulated.

[SteamyTea]

£100 buys a 1.2 kWh leisure battery. £20 an inverter and about the same for a mains charger.

 

What is the aim here?

[Iceverge]

 

I'd start with something like this. 

Then line the inside with foam boards with all joints crossed and foamed. 

Then something like dense mineral wool like that which is used for EWI that would happily take hundreds of degrees of heat. 

 

Next part is tricky, Maybe a thin steel liner, tech screwed together. 

Electric heating elements embedded in the sand then. 

Run a copper coil through it back to the house to small radiator and pump to extract the heat. 

 

300mm of PIR plus 100mm mineral wool should give a U value of about 0.06w/m2K. 

 

Assuming the volume remaining inside is about 1.8mx1.8mx3.6m you would have an internal surface area of about 32m2 and a volume of 11.7m3. Sand with a specific heat capacity of sand at 800j/kg/K and a specific density of 1.6 would give about 1.5 Mj of storage per degree or 0.4KWh/K. 

 

If you were to assume the lowest useable temp to be 40 deg and you wanted to raise temp to 100 deg it would take 24kWh of input. Not undoable. 

 

The losses however ( assuming an average temp inside of 70 deg and 10 out side would be 32m2 x 0.06 W/m2k x 60 DeltaT would be 115w or 2.76kWh per day, say 3kWh with pipe losses. 

 

 

Assuming this system was charged and discharged over a 2 day period it sounds like 24kWh in and 18kWh out = 75% Efficiency. Not nearly as bad as I expected. 

 

[Roger440]

What Steamy said. Whats the aim.

 

I looked at using a milk tanker and storing and heating water. But id need 10 of them to make it remotely worthwhile!

 

This sounds interesting

[SteamyTea]

Rather than look at how much energy can be stored, look at how much energy will be lost, and how much insulation is needed to reduce those losses.

Also look up Newtons Law of Cooling.

[Onoff]

  On 02/01/2023 at 16:37, JohnMo said:

Not sure apples would be that good.

Expand  

 

McDonalds apple pies remain scalding for ages!

 

[ProDave]

Already been done   https://www.popularmechanics.com/science/energy/a41869336/sand-battery/

 

Heat the sand with cheap electricity, extract the energy as heat for heating.

 

Conversion back to electricity would have big losses.