How do you exchange the heat stored in a Sand Battery into a water heating system.

[deema]

So, @Sideways and I have been mulling over making a sand battery, Sideways is a long way down the off grid self sufficient energy path and I’m seriously considering following in his foot steps. The big issue with solar is how to economically store the excess electricity and a sand battery seems a good solution.

Making and insulating the sand battery seems fairly straight forward. However, getting the heat out safely when the sand is say 200, 400 or 700C which seem to be the three max temp step points commercial ones run at is a puzzle we cannot get our heads around. Letting hot air rise out of the sand into a chamber with water pipes running through seems an obvious solution, but how do you regulate the heat such that the water doesn’t go above 80C and become dangerous steam / super heated steam and explode? Now, there may be another way of achieving the heat exchanger but we haven’t come up with anything and this sort of stuff is way outside our areas of expertise. Does anyone know a way in which it is done?

 

[sploo]

There are temperature controlled switches around (and quite cheap). Possibly there are physical temperature controlled valves available; and that could be used to switch a flow of water? I.e. you have a pipe that has cool water passing by the sand battery, and when the flow is under a certain temperature it opens a valve to direct some/all of that flow through a pipe that passes within in the battery.

 

[Fitzroy]

In the oil and gas processing industries we would likely use the following
- 200degC, use a low pressure (1-2barg) water/glycol mixed system such that a sensible gap between max temperature and liquid boiling point is maintained.
- 400degC. either a liquidd based heating medium system (water/glycol mix at 5-10brg) or an oil based system at lower pressure 1-2barg.
- 700degC, use a pressurised steam based system, so the heat transfer medium is always in the vapor phase.

A critical concern of all these systems would be pressure relief to manage the risk of boiling, with appropriately sized relief valves to manage the worst case scenario, which is not a trivial design exercise.

Thinking about something more home built. I could picture a system where a blower is used to move air through the sand battery, the outlet air is then fed to a gravity based (open) water system where any boiling is low risk as you have a nice big vent. Hot water in a break-tank is then pumped round a conventional domestic heating system. The only control you need is on blower speed, the faster you blow air the quicker you heat the water in the break tank.

Something like this.

Thinking more on this and you could likely repurpose an old combi boiler, where the hot air outlet from the sand battery enters the combustion chamber on the combi boiler (the combustion chamber is rated to likely 600-800deg C), the relief on the system will be designed for the max steam the heat exchanger can develop. The problem will be making the control system function, i think you'd have to rip it out and replace it with something custom built in raspberry pi or similar.

Fitz.

 

[Sideways]

To add a little context for anyone who hasn't looked into these things before, an oversize solar array can deliver 20+ kW hours of energy on quite a lot of days of the year.
There seems to be logical hierarchy to using it up:

1. Do energy intensive jobs like cooking and laundry while the sun is out if you can
2. Charge your battery if you have one
3. Charge your car if you have an EV
4. Heat up your water tank

But then it may only take a couple of sunny days and everything is topped up, and the challenge becomes how to use the surplus since you get paid so little for exporting it. Heat batteries of one sort or another are a possibility.

The problem with water thermal stores is they need to be big and heavy to be worthwhile and a lot of us don't have space. Water is great at absorbing energy but limited by boiling.

Thermal storage based on phase change materials is available tech but they are focussed on saving space, not increasing the amount of storage that you can get. The largest sunamp unit is a more compact alternative to a 300 litre water tank but at more than twice the price.

Thermal storage in hot sand is interesting. There's a new to market gizmo called the ZEB (Zero Emissions Boiler) by Tepeo that is just 2 feet square x 3 feet tall. The bottom half of a fridge freezer. It will store 40kWh of energy as heat and give it back as hot water. It charges at 9kW and gives back at 15kW max. It is aimed at heating mid sized houses (3 bed) using the heat for hot water and driving the existing radiators at normal high temperatures. No retrofit of underfloor heat or large bore pipes and big radiators. It doesn't have the 2-3x energy gain that a heat pump provides, but if you could charge it from free solar energy or an overnight low cost electricity, then the economics might work out vaguely similar.
But the Zeb is really new and costs about £6k plus installation. Yes they have figured out all this heat exchanger stuff and it will be using high temperature vacuum insulated panels so that it's safe to have 400C temperatures inside your house, but otherwise it is made with heating elements and high density bricks basically the same as an overnight storage heater. It must be possible to DIY this, and on a larger scale.

Another startup - Caldera - began promoting a larger version of the same thing. Theirs is a rounded "Dalek" of a thing that installs outdoors, stores 100kW of energy and will hold it for longer than the 1 week that the smaller Tepeo block can manage. Sadly, Caldera have announced that they will not be commercialising this for the forseeable future. They've realised that there is more money to be made applying their ideas to improving the storage of industrial process heat. I can't fault their logic but it's a shame that the domestic market doesn't have this option.

Here's a little calculation for interest. Scale this up or down however you wish to get an idea of how much energy can be pushed into sand ....

Image a cube, 2 feet on each side. 600mm x 600mm x600mm
That has a volume of 6x6x6 = 216 litres
Filled with sand it weighs appx 335 to 345 Kg
The specific heat of sand is 830Joules per KG degree C,
To increase the temperature of the block by 100C will require 8.4kWh of energy

So if you heated it to 400C and insulated it well enough you could store 33kWh as heat. That's enough to heat a small house for a day. Move daytime energy to the evening, or overnight energy to all day heat.
Make it 1.8 metres tall like a fridge freezer, plus the insulation, and you have the Caldera prototype at 100kWh.
Put this across the back wall of your shed and you could be storing half a Megawatt if you don't burn the place down and extending the season of your solar PV at least part way into the 2-3 lousy months of winter.

The combination of very simple, basic and cheap materials, with what is really just industrial standard heat management and the new technology of high temperature vacuum insulated panels sounds credible and I think we'll see a lot of development in this area as we all try to manage the energy crisis.

 

[Spectric]

but how do you regulate the heat such that the water doesn’t go above 80C and become dangerous steam / super heated steam and explode?

use the excess energy to heat another sand battery,

Water is great at absorbing energy but limited by boiling.

So use oil instead, now you have removed the 100° C boiling point.

I think the problem with sand might be transfer of heat, a liquid like oil makes good contact and so has good conduction, water is similar but I do recall using wetting agents in cooling systems to improve this, look up water wetter.

 

[Fitzroy]

Nothing is new! Having not heard of sand batteries before a bit of google-fu and I found this image that looks very similar to mine. Only difference is I put a break tank in the water system to manage the boiling risk, which with a decent control system this could be eliminated. There would be a relief valve in the system below.

This one is another interesting picture:

• Highlights the need for stainless steel as the system is hot and plain old carbon steel will not live long.
• Shows something akin to a small unit in the house, 'combi-boiler'esk but with a single air circuit that can be used to charge the battery via heat generator or recover heat via the air water exchanger.
• A distribution header and return header to feed and recover air to/from the sand battery but with air flowing directly through the bed. This would work very well with the correct sand (uniform particle size of say 0.1-0.2mm) excellent heat exchange would occur due to the very high surface area and with a big flow area pressure drop would be ok.

 

[Fitzroy]

use the excess energy to heat another sand battery,


So use oil instead, now you have removed the 100° C boiling point.

I think the problem with sand might be transfer of heat, a liquid like oil makes good contact and so has good conduction, water is similar but I do recall using wetting agents in cooling systems to improve this, look up water wetter.

High temperature stable oils are pretty expensive, if you can design a system to use air or water I'd always recommend. Heat transfer from sand is easy as it has such a high surface area, when you step barefoot on a hot beach this is quickly clear, blowing clean air through a uniform particle size sand bed would be super effective. Blowing dirty air through a sand bed would however quickly result in a blockage, or a fire if that contamination is organic and the bed at 400degC.

 

[Sideways]

So use oil instead, now you have removed the 100° C boiling point.

Interestingly if you used extra virgin olive oil, you could heat upto about 180 C to stay comfortably below the smoke point. As the specific heat of olive oil is double that of sand, it would come close to storing the same energy at a much lower temp.

But the idea of a leak with 180 degree oil feels scarier than the higher temp of the sand

 

[Ozi]

On the days you get excess sun will you want heat? Or am I missing how long the sand battery can hold the heat?

 

[Jameshow]

How about an aluminium heat exchanger where there is a physical disconnect between the water like a plate of cement board which is replaced by alu plate as required!

Old school I know!!

 

[Fitzroy]

To add a little context for anyone who hasn't looked into these things before, an oversize solar array can deliver 20+ kW hours of energy on quite a lot of days of the year.
There seems to be logical hierarchy to using it up:

1. Do energy intensive jobs like cooking and laundry while the sun is out if you can
2. Charge your battery if you have one
3. Charge your car if you have an EV
4. Heat up your water tank

But then it may only take a couple of sunny days and everything is topped up, and the challenge becomes how to use the surplus since you get paid so little for exporting it. Heat batteries of one sort or another are a possibility.

The problem with water thermal stores is they need to be big and heavy to be worthwhile and a lot of us don't have space. Water is great at absorbing energy but limited by boiling.

Thermal storage based on phase change materials is available tech but they are focussed on saving space, not increasing the amount of storage that you can get. The largest sunamp unit is a more compact alternative to a 300 litre water tank but at more than twice the price.

Thermal storage in hot sand is interesting. There's a new to market gizmo called the ZEB (Zero Emissions Boiler) by Tepeo that is just 2 feet square x 3 feet tall. The bottom half of a fridge freezer. It will store 40kWh of energy as heat and give it back as hot water. It charges at 9kW and gives back at 15kW max. It is aimed at heating mid sized houses (3 bed) using the heat for hot water and driving the existing radiators at normal high temperatures. No retrofit of underfloor heat or large bore pipes and big radiators. It doesn't have the 2-3x energy gain that a heat pump provides, but if you could charge it from free solar energy or an overnight low cost electricity, then the economics might work out vaguely similar.
But the Zeb is really new and costs about £6k plus installation. Yes they have figured out all this heat exchanger stuff and it will be using high temperature vacuum insulated panels so that it's safe to have 400C temperatures inside your house, but otherwise it is made with heating elements and high density bricks basically the same as an overnight storage heater. It must be possible to DIY this, and on a larger scale.

Another startup - Caldera - began promoting a larger version of the same thing. Theirs is a rounded "Dalek" of a thing that installs outdoors, stores 100kW of energy and will hold it for longer than the 1 week that the smaller Tepeo block can manage. Sadly, Caldera have announced that they will not be commercialising this for the forseeable future. They've realised that there is more money to be made applying their ideas to improving the storage of industrial process heat. I can't fault their logic but it's a shame that the domestic market doesn't have this option.

Here's a little calculation for interest. Scale this up or down however you wish to get an idea of how much energy can be pushed into sand ....

Image a cube, 2 feet on each side. 600mm x 600mm x600mm
That has a volume of 6x6x6 = 216 litres
Filled with sand it weighs appx 335 to 345 Kg
The specific heat of sand is 830Joules per KG degree C,
To increase the temperature of the block by 100C will require 8.4kWh of energy

So if you heated it to 400C and insulated it well enough you could store 33kWh as heat. That's enough to heat a small house for a day. Move daytime energy to the evening, or overnight energy to all day heat.
Make it 1.8 metres tall like a fridge freezer, plus the insulation, and you have the Caldera prototype at 100kWh.
Put this across the back wall of your shed and you could be storing half a Megawatt if you don't burn the place down and extending the season of your solar PV at least part way into the 2-3 lousy months of winter.

The combination of very simple, basic and cheap materials, with what is really just industrial standard heat management and the new technology of high temperature vacuum insulated panels sounds credible and I think we'll see a lot of development in this area as we all try to manage the energy crisis.

I was dubious of the insulation side of the problem, but maths wins again. With some of the new VIPs you would only need c. 3cm of insulation around a hot 400K storage block to reduce the losses from a 2foot square block to a few hundred watts, the cooling 'half life' would then be 60+hrs so negligable losses in a 24hr cycle. To be useful over a longer period could be a challenge, even a 4' cube with 15cm of external VIP would loose half its 230kW hrs of stored heat in c. 26days, so it may be possible to pull some of that spare heat into the winter months, in the end sand is cheap so scale up costs are small. Effective distribution of air within the block will be a growing challenge of scale up.

Feels good to exercise the engineering brain after nothing but management and people issues since new year.

 

[Sideways]

On the days you get excess sun will you want heat? Or am I missing how long the sand battery can hold the heat?

We are getting 19-20kWh off our roof on a good day at the moment (UK in early Feb), but this can hammer in at 5.5kW for a few hours between say 10am and 3pm. The gas central heating then turns on a couple of hours later.

All of this is very new, it's my first winter with the PV. It was pitiful during Dec and January with lots of days only making 3-6 kWh. The irraditaion curves say that it will ramp up quickly. In sizing the system, I just installed as much as we could get on the roof. The idea was for it to be as near self sufficient as possible spring and autumn, winter was always going to depend on the grids and summer will be stupidly overpowered. But in future maybe we'll want to be out and about in the summer in an EV vs hiding from the weather in winter...

 

[Sideways]

I was dubious of the insulation side of the problem, but maths wins again. With some of the new VIPs you would only need c. 3cm of insulation around a hot 400K storage block to reduce the losses from a 2foot square block to a few hundred watts, the cooling 'half life' would then be 60+hrs so negligable losses in a 24hr cycle. To be useful over a longer period could be a challenge, even a 4' cube with 15cm of external VIP would loose half its 230kW hrs of stored heat in c. 26days, so it may be possible to pull some of that spare heat into the winter months, in the end sand is cheap so scale up costs are small. Effective distribution of air within the block will be a growing challenge of scale up.

Feels good to exercise the engineering brain after nothing but management and people issues since new year.

Finding high temp VIP's that won' melt at 500C may be an issue. I've found a German manufacturer who wrap the porous tiles in thick aluminium foil - a commercial high temp vacuum insulated panel - but it's rated maybe 100C less than I'd like it to be.
I suppose an advantage of sand is that it could be shovelled out and different patterns of air circulating tubes tried out.
Underground seems like the obvious way to go with this, but then accessibility becomes tricky, tree roots have to be worried about, heat leakage drying out our clay soil has to be worried about as I don't want to affect the foundaions of house or garage ....

The idea of something a metre thick, above ground and full height / width across the back wall of the brick garage or in a lean-to outside might be less optimised but more achievable as DIY.

 

[deema]

I like the idea of burying the sand battery in the garden, @Sideways is at some stage going to extending his workshop / garage and that might provide an opportunity to bury a big block under the extension. Really outside my knowledge base but I seem to remember that surface area ratio to volume decreases with size, so the losses decrease the larger you go.

Im looking at adding solar to my next house, but hate the idea of battery’s, they just seem silly expensive. In an ideal world I’d rather have a large sand battery, heat it up over summer and have a toasty house over winter. That to me seems a good use of the power that is available, but, I’m not sure how feasible it is.

 

[Spectric]

But the idea of a leak with 180 degree oil feels scarier than the higher temp of the sand

What about people working in fish and chip shops or anywhere they use deep fat fryers, it may not be as bad as first thoughts would suggest. Then you could also use the heat store as a fryer so saving more energy.

 

[Jameshow]

What about people working in fish and chip shops or anywhere they use deep fat fryers, it may not be as bad as first thoughts would suggest. Then you could also use the heat store as a fryer so saving more energy.

What's for dinner tonight dear fish and chips again....!!!

 

[Inspector]

The heat losses of a sand battery in the driveway would be nice for keeping the snow melted.
Never mind if you don't know what snow is.

Pete

 

[Jameshow]

The heat losses of a sand battery in the driveway would be nice for keeping the snow melted.
Never mind if you don't know what snow is.

Pete

Even better underneath your workshop!!

 

[Sideways]

Im looking at adding solar to my next house, but hate the idea of battery’s, they just seem silly expensive. In an ideal world I’d rather have a large sand battery, heat it up over summer and have a toasty house over winter. That to me seems a good use of the power that is available, but, I’m not sure how feasible it is.

Forget grid-tied, forget export, forget microgeneration certification, forget inverters. Just hard wire the PV panels to the heating elements at 600V DC. Anything under 1kV is "low voltage".

 

[aet1]

I like the idea of burying the sand battery in the garden, @Sideways is at some stage going to extending his workshop / garage and that might provide an opportunity to bury a big block under the extension. Really outside my knowledge base but I seem to remember that surface area ratio to volume decreases with size, so the losses decrease the larger you go.

Im looking at adding solar to my next house, but hate the idea of battery’s, they just seem silly expensive. In an ideal world I’d rather have a large sand battery, heat it up over summer and have a toasty house over winter. That to me seems a good use of the power that is available, but, I’m not sure how feasible it is.

Find a house built on a sandy beach?