Calculating for the use of sand for a thermal store.

[SteamyTea]

3 minutes ago, Onoff said:

 

McDonalds apple pies remain scalding for ages!

 

They are a marvel, pizza bases are fire proof too.

 

 

[Marvin]

4 hours ago, Iceverge said:

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.  

 

 

Hi @Iceverge

 

Not as much space or where needed! However I calculate we have about 2000kWh available. This is at present a feasibility study.

 

I have done studies for our home for energy from wind and rain and cannot make them sensibly work.

 

Yes I need a pay back window and if I can't end up with a sensible one it's not going to be worth it.

 

M

[SteamyTea]

2 minutes ago, Marvin said:

Yes I need a pay back window

Replace that window with a foot of insulation.

[Marvin]

4 hours ago, SteamyTea said:

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.

 

what is SHC?

[Onoff]

Just now, Marvin said:

what is SHC?

 

Specific heat capacity

[Marvin]

4 hours ago, Onoff said:

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

Yes but this ups the cost and complexity

[Onoff]

All this talk of using sand as a heat store and accompanying calcs is really getting down to the nitty gritty...

[Marvin]

4 hours ago, Temp said:

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

 

It's volumetric density might be an issue.

 

Helium. I think any sealed tank causes massive expansion risks. increases the cost, now instead of an old steel bin I have to use a pressurised tank!

no this has to be cheap low risk and make a saving.

[Marvin]

4 hours ago, JohnMo said:

Here's one prepared earlier

 

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

Thanks for this. Yes this is fun but for me I need about a 4m3 storage that will last for months.

[Marvin]

4 hours ago, Onoff said:

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

True, but this will lessen the cost effectiveness.

[Marvin]

2 hours ago, Nickfromwales said:

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

 

 

Yes if you need winter heat and you have summer power this is the challenge.... 

[Marvin]

2 hours ago, SteamyTea said:

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

 

What is the aim here?

store spare energy during the summer for use over the winter (for heating only).

[Marvin]

1 hour ago, Iceverge said:

 

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. 

 

A luton vaN 18tons?? I think the tyres would burst 

[Marvin]

1 hour ago, Roger440 said:

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

I need to keep the heat for at least 2 months, which is possible, but is it practical.

[Marvin]

1 hour ago, SteamyTea said:

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.

Yes, one of the things that confuses me is the rate of heat loss difference between a tubular shape and a square shape with the same volume and same thickness of thermally resistant material. 🙂 I'm looking at a tubular shape...

[Marvin]

59 minutes ago, ProDave said:

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.

 

 

Yes, but not as a single property storage for heating over the winter being supplied by excess PV power. (as far as I can find).

[Marvin]

5 hours ago, SteamyTea said:

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.

 

Right, thanks @onoff. Specific heat recovery!  sorry. 

 

Hi @SteamyTea I have looked at air and its a lot of bottles to produce 400kW and also the increase of pressure requires more energy to produce

 

[Roger440]

11 minutes ago, Marvin said:

I need to keep the heat for at least 2 months, which is possible, but is it practical.

 

Likewise. But if using PV, it needs to be more like 4 months surely?

 

Which is where my crazy musings fall over.

 

Theres no practical way to store enough energy for heating through a winter from PV. 

[Onoff]

Can't you just chuck some pipes off the south coast of the island and wait for the next landslip? Instant GSHP?

[Iceverge]

Ah, months of storage. Not going to happen at a reasonable cost I'm afraid.

 

@tonyshouse did a borehole to dump excess solar thermal I think. Not sure of the most recent results. 

 

Scanhome and University of Ulster did a project. 

A year in the life of a passive house with solar energy store V0.9 (energyexpertise.net)

 

 

Whilst it was successful it was expensive. Stored heat was 0.79€/kWh.

 

 

 

 

[SteamyTea]

Right, rather than trying to store energy at elevated temperatures, for months  why not just use a heat pump.

Then all you need to work out is the power needed to run it, and your potential to generate that power.

 

PVGIS will be your friend here.

[pdf27]

28 minutes ago, Marvin said:

Yes, but not as a single property storage for heating over the winter being supplied by excess PV power. (as far as I can find).

If you're trying to store heat, no reason to use PV - solar thermal captures about 3x the energy per unit area, and you're going to need a liquid system to get the heat out again.

It's been done a few times (search for "Sonnenhaus" or "domestic interseasonal thermal storage"), but it's hard to make a success of it. A lot of the issue is that unless you have an exceptionally well-insulated house you're going to need a lot of heat capacity in whatever you use for storage which adds cost, and it needs to be thermally segregated from the house to avoid overheating as well as keep the store hot. You end up more or less building the house around the thermal store, with the associated costs and compromises.

The problem is really that if you are willing to design a house around minimising energy drawn from the grid, you're always going to go for a Passivhaus-esque approach to reduce demand. Once you do that, not only do you not need very much heat but overheating starts to become a significant challenge if you're storing a lot of heat energy inside the house. If you go for the heat pump + PV approach, you can buy an awful lot of PV + battery storage for the cost of building a giant thermal store into your design - and the amount you draw from the grid will be pretty minimal, with the PV helping to cover plug loads too.

 

If I'm understanding you correctly you've got 2 MWh of PV exported to grid over the course of a year (mostly summer?), and are looking to use that for winter heat given an available 4m³ storage volume. Sunamp are about the best you can do here - they're a phase change system (hence very energy dense), in industrial production and extremely well insulated. That could probably store somewhat over 250 kWh in the space you have available, with a standing loss of a few hundred watts - similar to a badly insulated hot water tank and thus probably acceptable. They also sell a large system which is probably suitable - https://sunamp.com/en-gb/products/central-bank-mini/#features .

 

Having said all that, I can't see any way of doing this cost-effectively - you aren't saving a huge amount of energy over the course of a year, and even at current energy prices that really doesn't come anywhere near to justifying the investment needed. You'd make far bigger savings investing elsewhere, either in the house or maybe in something like Ripple shares.

[SteamyTea]

5 minutes ago, pdf27 said:

solar thermal captures about 3x the energy per unit area,

Only once it has gone past the store temperature.

This means it can loose our a lot of the time.

Generally at sub 100W/m², PV is more effective, and cheaper.

 

[Roger440]

31 minutes ago, SteamyTea said:

Right, rather than trying to store energy at elevated temperatures, for months  why not just use a heat pump.

Then all you need to work out is the power needed to run it, and your potential to generate that power.

 

PVGIS will be your friend here.

 

Same problem. Sun doesnt shine much in the winter. 

 

You cant sensibly store months of electricity to run said heat pump.

[Roger440]

18 minutes ago, pdf27 said:

If you're trying to store heat, no reason to use PV - solar thermal captures about 3x the energy per unit area, and you're going to need a liquid system to get the heat out again.

It's been done a few times (search for "Sonnenhaus" or "domestic interseasonal thermal storage"), but it's hard to make a success of it. A lot of the issue is that unless you have an exceptionally well-insulated house you're going to need a lot of heat capacity in whatever you use for storage which adds cost, and it needs to be thermally segregated from the house to avoid overheating as well as keep the store hot. You end up more or less building the house around the thermal store, with the associated costs and compromises.

The problem is really that if you are willing to design a house around minimising energy drawn from the grid, you're always going to go for a Passivhaus-esque approach to reduce demand. Once you do that, not only do you not need very much heat but overheating starts to become a significant challenge if you're storing a lot of heat energy inside the house. If you go for the heat pump + PV approach, you can buy an awful lot of PV + battery storage for the cost of building a giant thermal store into your design - and the amount you draw from the grid will be pretty minimal, with the PV helping to cover plug loads too.

 

If I'm understanding you correctly you've got 2 MWh of PV exported to grid over the course of a year (mostly summer?), and are looking to use that for winter heat given an available 4m³ storage volume. Sunamp are about the best you can do here - they're a phase change system (hence very energy dense), in industrial production and extremely well insulated. That could probably store somewhat over 250 kWh in the space you have available, with a standing loss of a few hundred watts - similar to a badly insulated hot water tank and thus probably acceptable. They also sell a large system which is probably suitable - https://sunamp.com/en-gb/products/central-bank-mini/#features .

 

Having said all that, I can't see any way of doing this cost-effectively - you aren't saving a huge amount of energy over the course of a year, and even at current energy prices that really doesn't come anywhere near to justifying the investment needed. You'd make far bigger savings investing elsewhere, either in the house or maybe in something like Ripple shares.

 

 

My house is mostly 170 years old, so zero possibility of passivhaus standards. Its going to use energy, even with sensible pragmatic works applied. Having ample space, i could generate enough PV or solar thermal in summer to run it in winter.

 

Thats why i looked at milk tankers, buried if need be. But its simply economic madness. So yes, Yes, buying shares or similar seems to be a much better solution than actually solving the problem! Even at a £1 per Kwh, it still doesnt even start to stack up.