Sand Battery Experement

[Ceefiveceefive]

I am about to start with my sand battery experiment.

I plan on using it as a simple heat exchanger by blowing the heated air through metal pipe(s) into a room. Dimensions of the build will be 4x8x4 to start, bigger if needed.

I've decided to bury standard stove-top heating elements like the attached photo into the insulated battery. The individual element is rated 2600 watts. I'm thinking of putting them up 12 inches apart from each other for a total of 8 buried elements. Each element will be pulled apart so they form a cone shape and will heat more sand.

I need help calculating how many panels to attach to each element to continuously keep them heating during the day. I still need to figure out how hot I should get them & how I would do that.

Thanks!

 

[defed]

i was watching a guy on youtube a little while back who was doing a lot of sand heat battery experiments....off-grid survival mike. maybe you can get some ideas there.

 

[damo green]

cool, will be following your experiment. Eventually I will be doing a water thermal battery experiment.

 

[jasonojordan]

To control the heat and how warm they get you will likely need to use some temp probes and an Temp Controller to control the heat turning on and off.

 

[efficientPV]

I hope you have another use for all that sand later.

 

[Ceefiveceefive]

I hope you have another use for all that sand later.

I live in Arizona's high desert! I'll just tip it over & it will blow away or stay.

 

[Ceefiveceefive]

To control the heat and how warm they get you will likely need to use some temp probes and an Temp Controller to control the heat turning on and off.

Good idea. Now that I recall, I think the YouTube battery theory guy talked about it.

 

[MyK3y]

I am about to start with my sand battery experiment.

I plan on using it as a simple heat exchanger by blowing the heated air through metal pipe(s) into a room. Dimensions of the build will be 4x8x4 to start, bigger if needed.

I've decided to bury standard stove-top heating elements like the attached photo into the insulated battery. The individual element is rated 2600 watts. I'm thinking of putting them up 12 inches apart from each other for a total of 8 buried elements. Each element will be pulled apart so they form a cone shape and will heat more sand.

I need help calculating how many panels to attach to each element to continuously keep them heating during the day. I still need to figure out how hot I should get them & how I would do that.

Thanks!

I'm going through the process of building one right now.

But, some caveats: it's stand-alone from the rest of the house solar. No controller - just raw DC straight to the elements. The elements I'm looking at are the largest in diameter I can find, at 2400W/240V - they typically are between 25 and 45 ohms - smaller diameter are higher resistance. I have based calculations on 30ohm.

So, a 2400W element at 240V and with a resistance of 30 ohms runs at ~800C at full power.

Given my winter panels will be delivering between 350-530W/M2 ea for ~6.5 hours in mid winter at 36V, I am looking at ~300 watts/m2 from the element for a single panel but I'm planning two panels at 72V with an area of 3.85M2 so hopefully will get ~in excess of 1000W for around 5-6 hours.

My system is planned to be a ducted warm air implementation, from a 200L/44 Gal drum sand battery with a single element, insulated on all sides and heat-broken from the ground it stands on. Under the house, it will be protected from wind and weather and thermal losses kept to a minimum by locating it directly below the duct outlet, so at most a couple of metres of duct.

If successful, I will make another for the other end of the house.

Reasons behind everything are here: Sand battery

 

[cdherman]

So water stores about 5x more heat per LB than sand. And dry sand is less dense than water. So you need a lot more space to store much heat with sand. You can buy a ready made hot water storage tank, already insulated -- its called an electric water heater. Those elements work AC and DC IIRC. You can buy any number of 12/24/48/ 120AC/240AC small pumps to move water around. Your heat exchanger is near to free. Visit a local pic and pull and a small automotive radiator from a used car does the trick.

Some ducting, which sounds easy for you, and you have a mini hydronic to air heating system.

Put some ethylene glycol in there (antifreeze) and you can even let it freeze safely, if that problem applies to your locale.

Rocks and sand don't transfer heat very well. And they don't move around on their own. Water does both. Water also integrates with a waste boiler system very well. Meaning: you build a barrel stove, wrap some loops of flexible copper around the barrel, have a diverter valve so that your flow pump can also be used to circulate water around your trash incinerator. Bingo, you have hot water on a cloudy day burning your trash.

All this stuff would be frowned upon in most urban environments. But it sounds like you are out in the open and free part of the world.

 

[MyK3y]

I don't know where you get your figures from - dry sand is 1600kg/m3, water is 1000kg/m3 - that is more dense. If sand was less dense than water, there would be no sand or beaches and making concrete would be really, really hard, no?.

Water can store 4000J/kg vs sand at 835J, but sand has more kg per L of space. so you end up with 1335J in the same amount of space as 1kg of water.

Water holds 5x more specific heat per kg but 3x specific heat/L, I showed (via link) the figures that show 1cuM of sand holding twice the kJ of water - because the water can only go to 100C in free air, vs >500C for sand. 500C is readily achievable with stove top elements, which typically run at 800-950C, depending on size and wattage.

You seem to be confusing free-air specific heat retention, which is of less interest as the vessel will be heavily insulated. The whole reason for a battery is to insulate it against uncontrolled thermal loss. The reason to use sand is because of its physical properties - it won't change state until you reach 1700C. Sand absorbing and releasing Joules at a higher transfer rate is an advantage in a battery, where you seem to think it's a negative. It would be a negative if you weren't insulating.

Or, you can go and tell the Finns they're doing it all wrong and need to convert their municipal sand batteries to water?

 

[MattiFin]

Or, you can go and tell the Finns they're doing it all wrong and need to convert their municipal sand batteries to water?

Sand batteries are sort of experimental also in here, one(1) unit in use.
Whereas large 2-3m3 water tanks are rather common and have been in use at least last 40 years.
I know half a dozen relatives and friends houses that have such a system installed. Forum member upnorthandpersonal also has one.

 

[Ceefiveceefive]

So water stores about 5x more heat per LB than sand. And dry sand is less dense than water. So you need a lot more space to store much heat with sand. You can buy a ready made hot water storage tank, already insulated -- its called an electric water heater. Those elements work AC and DC IIRC. You can buy any number of 12/24/48/ 120AC/240AC small pumps to move water around. Your heat exchanger is near to free. Visit a local pic and pull and a small automotive radiator from a used car does the trick.

Some ducting, which sounds easy for you, and you have a mini hydronic to air heating system.

Put some ethylene glycol in there (antifreeze) and you can even let it freeze safely, if that problem applies to your locale.

Rocks and sand don't transfer heat very well. And they don't move around on their own. Water does both. Water also integrates with a waste boiler system very well. Meaning: you build a barrel stove, wrap some loops of flexible copper around the barrel, have a diverter valve so that your flow pump can also be used to circulate water around your trash incinerator. Bingo, you have hot water on a cloudy day burning your trash.

All this stuff would be frowned upon in most urban environments. But it sounds like you are out in the open and free part of the world.

Thanks for your input!

I am in a very rural area with lots of free sand on my property.

I am not interested in the water version, only the sand version.

 

[Ceefiveceefive]

I'm going through the process of building one right now.

But, some caveats: it's stand-alone from the rest of the house solar. No controller - just raw DC straight to the elements. The elements I'm looking at are the largest in diameter I can find, at 2400W/240V - they typically are between 25 and 45 ohms - smaller diameter are higher resistance. I have based calculations on 30ohm.

My system will be stand-alone as well, completely separate from my main solar array.

 

[Ceefiveceefive]

So, a 2400W element at 240V and with a resistance of 30 ohms runs at ~800C at full power.

Given my winter panels will be delivering between 350-530W/M2 ea for ~6.5 hours in mid winter at 36V, I am looking at ~300 watts/m2 from the element for a single panel but I'm planning two panels at 72V with an area of 3.85M2 so hopefully will get ~in excess of 1000W for around 5-6 hours.

This is what I need help with in my calculations.

I have a total of at least ten 240W & 250W panels to use for this project. I'm not sure if I need to combine them and how many to use per element? My elements are rated at 2600W each.

 

[JJJJ]

You might need to factor in the insulation factor. Composting systems have been considered for heat generation. Basically tubes are run through composting material to pull the heat away.

It does work but as the heat is pulled away from the compost surrounding the tubes it then acts as an insulator limiting the potential exchange from compost further away from the tubes.

 

[MyK3y]

Whereas large 2-3m3 water tanks are rather common and have been in use at least last 40 years.
I know half a dozen relatives and friends houses that have such a system installed. Forum member upnorthandpersonal also has one.

Not here. Never seen such a thing.

This thread is specifically about sand storage - why has it been taken over by water heaters?

 

[MyK3y]

I have a total of at least ten 240W & 250W panels to use for this project. I'm not sure if I need to combine them and how many to use per element? My elements are rated at 2600W each.

It's really just down to maths - you need to know your solar radiation specifics for your site, your panel orientation and generation, efficiency, cable budget, voltage, etc.

I started with a solar generation spreadsheet from NIWA - our government agency that covers weather and atmosphere - you put in your exact location and orientation and panel angle and it creates a power profile of watts/m2 of collector. I did that for every panel angle from 30deg to 75deg. The sweet spot for winter was 65 deg, for summer it was 40 deg. - another reason for wanting it separate from the house solar generation - this is specifically for winter so getting the angle optimised for winter is key.

Depending where you are in the world, you may have access to the same sort of resource

The highlighted section is 'cloudless W/m2' - so what you would expect to get in perfect conditions. It's averaged for the whole month - the spreadsheet covers a whole year, by hour per month. As you can see, 3W at 7am rising to 931W at noon and back to zero at 5pm. The figures just to the left are 'cumulative kWh/m2', so how much you can expect to generate on a particular day - 2.7kWh in May, but my panels are 3.75m2 for the pair so 10.125kWh of generation for each day in May at 72V. Just as an indication, for all-year optimisation, with panels at 40deg, generation in May would be 3kWh less per day - so worth getting right.



I already had the specs from Trina for the panels - m2 of collector, voltage, amperage, etc.

I am not using an inverter/controller - just raw DC. My intended elements are 3kW at 240V, but that will be significantly reduced at the ~72V DC my panels will operate at. And that number changes during the day - they are giving ~15V at 8am, rising to 36V at peak, so from 30V to 72V in series. An element that gives 2400W at 240V will be giving 360W at 36V - your sand reservoir and how much heat you want to pull from it will dictate how much power you put in.

At 240V and 10A a 2400W element will be at 24 ohm of resistance
At 36V and 10A the same element will be giving 360W at 3.6 ohm
At 72V and 10A the same element will be giving 720W at 7.2 ohm

Look at 'Off-Grid Survival Mike's videos - he is the origin for my setup. https://www.youtube.com/@off-gridsurvivalmike8120/videos

Also look at the other thread 'Storing Heat in Sand' https://diysolarforum.com/threads/storing-heat-in-sand.27147 for more detailed reasoning and discussion - I'm going to stop posting in this thread now.

 

[GXMnow]

I am not using an inverter/controller - just raw DC. My intended elements are 3kW at 240V, but that will be significantly reduced at the ~72V DC my panels will operate at. And that number changes during the day - they are giving ~15V at 8am, rising to 36V at peak, so from 30V to 72V in series. An element that gives 2400W at 240V will be giving 360W at 36V - your sand reservoir and how much heat you want to pull from it will dictate how much power you put in.

At 240V and 10A a 2400W element will be at 24 ohm of resistance
At 36V and 10A the same element will be giving 360W at 3.6 ohm
At 72V and 10A the same element will be giving 720W at 7.2 ohm

Ideally, you will want to load the panels to close to their NOTC current at solar noon.

The resistance of a stove heating element will likely change some with temperature, but not this much. A 24 ohm element will likely still be close to 24 ohms when running at just 72 volts. The power will be much less than the rating though. When you cut the voltage in half, you only get 1/4 of the power. At just 72 volts, the power is way down.

72 volts / 24 ohms = just 3 amps. And 3 amps x 72 volts is only 216 watts. That will not take full advantage of the PV solar panels. If you run 4 of those solar panels in series, you get 144 volts. Into a 24 ohm load, that will try to pull 6 amps. The solar panels should easily be able to supply that. I think that would be a good place to start to get some idea of the heat available. 6 amps x 144 volts = 864 watts. That is only asking for 216 watts per solar panel with 4 in series. This is still well below the rating of the element, so it should not be in any danger of burning it out. You might be able to run it even a bit harder, but I would keep it to less than half of the rated power. Stove heating elements are normally run in open air, or in contact with a pan full of water to pull heat out of the element. Having the element buried in sand will pull some heat out, but it is then in an insulated casing, the temp could rise quite a bit higher than what it would do out in free air. Half power (1,200 watts) is probably still safe though.

In most areas, you are lucky to get a solid 3 or 4 sun hours of energy out of the solar panels. Using 3.5 x 864 watts means you might put a bit over 3 kilowatt hours of energy into the sand. Then blowing the air through it to make heat, it would produce similar heat to running a 1 kilowatt space heater for 3 hours.

Depending on the V/I curve on the panels, it might work better to parallel 2 of those stove heating elements. That puts you down to 12 ohms instead of 24 ohms. If the panels IMP current is 12 amps, you will get to the same 144 volts, but now at nearly double the peak power. 144 volts x 12 amps = 1,728 watts at peak solar noon power. This is asking for 432 watts per solar panel, so I think it will fall a bit below that in the real world. Are these 300 to 400 watt panels? What is the VMP and IMP? We should try to hit those as close as possible. Assuming the panels max out at only 400 watts, the voltage would fall a bit to 138 volts / 4 = 34.5 volts per panel and about 11.6 amps. Each stove element makes less heat, but you now have 2 of them pushing heat into the sand. The only problem with this setup is that the voltage will fall off a lot when the sun is not ideal. So the total energy per day might end up less. That is the problem with not using an MPPT tracker. You can only optimize a single point with a fixed resistor load.

 

[Ceefiveceefive]

You might need to factor in the insulation factor. Composting systems have been considered for heat generation. Basically tubes are run through composting material to pull the heat away.

It does work but as the heat is pulled away from the compost surrounding the tubes it then acts as an insulator limiting the potential exchange from compost further away from the tubes.

I plan on heavy insulation!

 

[MyK3y]

In most areas, you are lucky to get a solid 3 or 4 sun hours of energy out of the solar panels.

or 7-8 in winter, like we do.