Storing heat in sand?

[MyK3y]

Your numbers are all of the place ...

600°C .. even more .. 1000°C, build your system and show me the numbers you're at ..

No, they're not.

600C is easily achievable with basic domestic stove elements.

1600C is achievable with more advanced elements, such as industrial kiln elements.

Thus, an additional 1000C of storage as a hypothetical to show the potential.

But, 600C is perfectly adequate for my needs and, as multiple YouTube videos can attest, is being attained with simple basic setups.

I'm not sure what it is you are setting out to prove - but I'm not interested in your willy-wagging game. I didn't join this forum to have arguments, but to gain and share knowledge.

This is a thread about sand energy storage, not a soapbox for you to try to prove your superiority . either contribute to sand storage or start your own 'water is better than sand' thread.

I don't need to prove myself to you or anyone else and I certainly don't need to accede to the demands of some random ego on the internet

 

[Mart Hale]

Perhaps we might want to branch off into multiple threads? I think I might create a new thread for wood powered sand battery, or thermal mass heat storage.....

 

[MyK3y]

This would be the case if your application is to melt ice floating in water (0C).
If it is to keep a room at comfortable 25C, 99C - 25C = 74C delta, 310kJ storage.
If heating water (from whatever incoming temperature) to 49C (120F), 50C delta from storage tank, it is 210kJ
Of course, if potable you could use the water directly. And if incoming water is colder than 25C (but it is likely not below 0C), you energy storage capacity between the two figures. For the specific purpose of heating incoming water.

Instead of 0C, if you compare it to 0K (-273C), then delta of 372C and 1559kJ.
Perhaps that would be useful if you ever needed to thaw a block of solid hydrogen (liquid is 25K)

Point being that you must consider delta between temperature of storage tank and lower temperature you are letting the energy flow to.
Which swings the needle in favor of higher storage temperatures.

With 99C water heating 25C room, the modest temperature delta doesn't allow efficient extraction of mechanical energy.
I think the theoretical limit is (Tstorage - Tsink)/Tstorage, where temperatures are in Kelvin, but I'm not finding it right now.

Carnot cycle - Wikipedia

en.wikipedia.org

With your sand at several hundred degrees Celsius, you could allow heat to flow though a steam engine, releasing heat to the room while extracting some energy with a mechanical shaft. That could spin a generator or otherwise power a heat pump, providing additional heat energy to the room if ambient isn't too low.

Yeah, It was the end of a long day and I was working out with a basic calculator - will spreadsheet it today and come back with something more sensible. I was off on the panel voltage by a bunchanumbers, too. 34Vx2 is a long way from 96V... dunno where I got that from - I was planning 3x panel setup originally and didn't change the numbers.

I need a DC voltage sensor that can handle >50V so my task for today is to do that, terminate some cable with MC-4s, wire up a test bed with my enormous Longi panels and start to gather data. I have a few high-powered shunts with bluetooth-enabled meters that I can probably put to use and mqtt the data via an ESP. so I don't need to be close to capture data.

Today is predicted to be sunny and 20C - a perfect spring day with no cloud or wind. Time to get busy - it's nearly 7am!

Thermal storage of power plants is a thing, whether it is cost-effective or not.

Thermal Storage System Concentrating Solar-Thermal Power Basics

One challenge facing solar energy is reduced energy production when the sun sets or is blocked by clouds. Thermal energy storage is one solution.

www.energy.gov

Yeah, I know - molten salt, water, mineral oil, glycol, and a bunch of products from chemical companies have high thermal transfer numbers and can be used for energy storage, but this is specific to my requirements. I've settled on sand as it meets almost all of my needs. At scale water is eminently cost-effective as we see with various arctic-circle heat storage projects.

We used to live in Borneo - this project would not see the light of day there, as you probably only get two days a year where the temperature is below 15C. It's specific to this location, this house type, and my specific needs. Cost comparison was in relation to those specifics.

The guy I bought my barrel off had 1000L caged IBCs (Intermediate Bulk Containers) for only a few dollars more which would be perfect for water storage



But, my access to under my house is via a pair of 700mm x 700mm doors.

The thermal properties of water are well known and it is inherently safe for use in a wooden house. The cost, the thick polypropylene sides and a metal cage makes using IBCs a natural fit - if I had a house that could utilise them.

I'm not inherently against water as a medium, but it isn't practical in my specific environment.

 

[MyK3y]

Perhaps we might want to branch off into multiple threads? I think I might create a new thread for wood powered sand battery, or thermal mass heat storage.....

I found something the other day where a guy used a rocket-type stove to heat sand for energy and I forgot to bookmark it for you.

I'll see if I can find it in my history.

And, I had a think - projects like the Finnish one use heat circulating through pipes in the sand to add energy and separate pipes to pull the heat out - your rocket stove could heat a coil filled with a high-temperature transfer fluid which ends up in the sand, rather than trying to heat the sand directly.

Mineral oil is effective up to 300C, after that you are looking at silicone oils. Mineral oil is cheap and safe, silicone oils are nasty, environmentally.

 

[Hedges]

I need a DC voltage sensor that can handle >50V so my task for today is to do that, terminate some cable with MC-4s, wire up a test bed with my enormous Longi panels and start to gather data. I have a few high-powered shunts with bluetooth-enabled meters that I can probably put to use and mqtt the data via an ESP. so I don't need to be close to capture data.

I just make a resistor divider, or a pair of resistor dividers. That lets me read voltage across a sensor that lives on a different voltage plane (at the expense of attenuated signal, 10:1 or 20:1 when I drop from 50V offset to a 5V system.)

 

[Honuz]

No, they're not.

600C is easily achievable with basic domestic stove elements.

1600C is achievable with more advanced elements, such as industrial kiln elements.

Thus, an additional 1000C of storage as a hypothetical to show the potential.

But, 600C is perfectly adequate for my needs and, as multiple YouTube videos can attest, is being attained with simple basic setups.

I'm not sure what it is you are setting out to prove - but I'm not interested in your willy-wagging game. I didn't join this forum to have arguments, but to gain and share knowledge.

This is a thread about sand energy storage, not a soapbox for you to try to prove your superiority . either contribute to sand storage or start your own 'water is better than sand' thread.

I don't need to prove myself to you or anyone else and I certainly don't need to accede to the demands of some random ego on the internet

Even the industrial test that are barely commercial are at 600°C, there is maybe space for improvment but it certainly comes with drawbacks, so it's better to stay on the actual parameters instead planning even further.

The thread is "Storing heat in Sand ?" and even if you do not like it, my post are exactly in the subject, it is about defining what is worth and what is not, just with scientific thinking, i give you proofs that your own numbers and deduction are wrong, you gave me no counter argument. Maybe my logic is wrong but you still change subject and talk about ego ..? Maybe you got a problem here i do not, the goal is to get a scientific analyse of the situation or maybe like many guys that keep trying to build perpetual movement machines you want to believe... i dunno.

So your points where all wrong..? The 2000x ... the energy needed to heat a "house" ...?

With your numbers if my calculus are correct i got a volumetric energy density of sand that is twice the one of water, that's an advantage for sand.
200L sand => 200*801kJ = 160MJ
200L water => 200*414kJ = 82MJ

- Still the problem of temperature delta between interior and exterior => higher delta mean higher loses => round trip efficiency decrease => this system will lose energy faster then a water storage system => mainly because water specific heat capacity is higher it can keep more energy with a smaller increase of temperature.

A Sand Battery: Not obviously a great idea.

Friends, a few weeks ago I was asked by several people whether I had heard about the latest ‘sand battery’? I had never heard of any such thing. Most people asking me had come across th…

protonsforbreakfast.wordpress.com

- An other problem is the dificulty to get heat in and out of sand with efficiency and speed.

- The advantage i see for sand storage is industrial, some industry can take advantage of it, not far from my house ther is a compagny that make ceramic tiles, they use a lot of energy to heat their giant stove, the exhaust of those stoves is sent to a ceramic storage. after the cycle is finished, they can used the stored heat to feed stove with preheated air. In this kind of case, water could not be used because of the lower temperatures.

In a house i still can't see the advantage of sand storage over water appart the volumetric energy density, but 2x is not enough to counterweight the disadvantages.

 

[Hedges]

In a house i still can't see the advantage of sand storage over water appart the volumetric energy density, but 2x is not enough to counterweight the disadvantages.

Energy stored relative to 25C room temperature, I came up with silica at 600C 3.09x the volumetric thermal energy storage of water at 100C.
But that was solid silica, not considering air space. Sand, I came up with 2.09x

Insulation will have to be thicker, and higher temperature at least at the center. Water, you could use foam. Sand, could have higher temperature inner layer and lower temperature outer, but would have to design carefully to ensure gradient doesn't overheat the foam.

Sand would be slower to transfer heat, but that could be OK if you have all summer to heat it and all winter to draw heat from it. I would expect there to be slow convection as air circulates in sand. Air will expand and contract with temperature change, can probably just be allowed to vent without much energy loss. May need to filter to avoid introducing anything undesireable.

Electric heating element at the bottom or vertically through center. Tubes to collect heat and carry it out in air at top or vertically.

The larger the container, the better it gets. A shrew has to eat its body weight every day, an elephant much less.
50 gallon drum or electric water heater would be good for experimenting. Water heater, need to determine max operating temperature given urethane foam or whatever. The heating element could be used at reduced voltage. Best to arrange a temperature sensor. A different element with positive TCR would serve the purpose, but elements in many electrical appliances have low TCR.

Such a drum/tank might provide some benefit (perhaps night time storage). Scale up for seasonal.

Impact of losses could be mitigated if enclosed in an air jacket. During summer heating cycle, lost heat would just be lost, but as soon as heating is needed in the fall, domestic air circulating through the jacket would harvest it. Forced or convective circulation through pipes in the storage tank would be regulated to contribute when more heat needed. Well insulated baffles, I'm thinking.

I know of a few disadvantages for water. I'm not coming up with disadvantages for sand except slow transfer rate, which I think will not be a problem.

 

[Honuz]

Energy stored relative to 25C room temperature, I came up with silica at 600C 3.09x the volumetric thermal energy storage of water at 100C.
But that was solid silica, not considering air space. Sand, I came up with 2.09x

Insulation will have to be thicker, and higher temperature at least at the center. Water, you could use foam. Sand, could have higher temperature inner layer and lower temperature outer, but would have to design carefully to ensure gradient doesn't overheat the foam.

Sand would be slower to transfer heat, but that could be OK if you have all summer to heat it and all winter to draw heat from it. I would expect there to be slow convection as air circulates in sand. Air will expand and contract with temperature change, can probably just be allowed to vent without much energy loss. May need to filter to avoid introducing anything undesireable.

Electric heating element at the bottom or vertically through center. Tubes to collect heat and carry it out in air at top or vertically.

The larger the container, the better it gets. A shrew has to eat its body weight every day, an elephant much less.
50 gallon drum or electric water heater would be good for experimenting. Water heater, need to determine max operating temperature given urethane foam or whatever. The heating element could be used at reduced voltage. Best to arrange a temperature sensor. A different element with positive TCR would serve the purpose, but elements in many electrical appliances have low TCR.

Such a drum/tank might provide some benefit (perhaps night time storage). Scale up for seasonal.

Impact of losses could be mitigated if enclosed in an air jacket. During summer heating cycle, lost heat would just be lost, but as soon as heating is needed in the fall, domestic air circulating through the jacket would harvest it. Forced or convective circulation through pipes in the storage tank would be regulated to contribute when more heat needed. Well insulated baffles, I'm thinking.

I know of a few disadvantages for water. I'm not coming up with disadvantages for sand except slow transfer rate, which I think will not be a problem.

Agreed on 25°C, we can still heat the slab at something like 20°C, under 25°C it's nearly unusable. Allright, we seems to agreed on 2x thermal capacity for sand at equal volume.

To mitigate the loses of high temperature medium, insulation will need to be bigger and high temperature resistant => we will lose on this 2x factor. On my 1000l i got 10cm of polyuréthane, i do not know what would be needed on a 600°C sand vessel to meet the same loses then my tank... but i'm pretty sure il will be far more then 20cm... and it can't be polyuréthane which is one of the most efficient insulation => rock wool, glass wool ... it will then increase further those 20cm ....

let's try a small evaluation : my tank is 200*80cm = 1000l, 10cm of insulation = 570l (side)+ 78l x2 (top and bottom) = 726l of insulation .. ! It already nearly DOUBLE the usable volume.

I found this (https://www.watlow.com/blog/posts/heat-loss-factors) => at 200°F (100°C) the losses in W/in2 = 0.2, at 1200°F (600°C) they are at something between 9 and 30 W/in2.
That's a rought estimation and there is certainly someone that could evaluate this in a better way, but clearly we will need more then double efficiency then the 10cm of polyuréthane which will already add 768 of volume .. and like that .. the 2x factor is lost.
So in the end we will have a sand vessel twice smaller then a water vessel .. but with 4 times the insulation (number 4 is taken from my hat just to show the problem).

To be fair... if the goal is to heat the house, then those loses are not that important... just that it will need a special ventilated vessel to get this loses elsewhere in the house (as you explained later).

Slow transfer speed : agreed that this is not that important for a house, sand battery have enough heat output for this use.

To heat medium there is not much problems .. a resistive element .. ok, now let's say your battery is empty in the middle of winter... you need to charge it... you need electricity ? I fire my stove which got a water exchanger => i charge the water tank super easily, heating this sand tank would not be that easy. => You do not have a lot of options to heat this battery, electricity or direct fire which is possible, my father got a water tank that is heated with gas, there is a central chimney (as you said) and water around it .. this chimney is full of tubes to increase efficiency.

The problem is more to get this heat back, i do not like to move energy iun a house by air... it's a bad idea, it convey dust, you need powerfull fans.. .filters .. etc... so maybe injecting water, getting vapor ... then an exchanger to a water circuit ..? I can only see complicated things ... while i just have at tube coming out of my thermal tank.. a circulator... and that's all.

What is TCR ?

 

[Hedges]

Thermal Coefficient of Resistance.

Light bulb filament increases 10x in resistance at operating temperature. Most heating elements do not.
I've made use of filament TCR in a vacuum instrument to detect pressure loss and degradation of emission current. Also for a Pirani sensor.

For heating element in sand, the issue is to avoid getting it too hot. If it had significant TCR you could detect temperature rise by reduction in current. Without that, need a separate temperature sensor. Maybe just a wrap a nichrome or other resistance wire around it (but avoid getting nicked by sand.)

 

[MyK3y]

Thermal Coefficient of Resistance.

Light bulb filament increases 10x in resistance at operating temperature. Most heating elements do not.
I've made use of filament TCR in a vacuum instrument to detect pressure loss and degradation of emission current. Also for a Pirani sensor.

For heating element in sand, the issue is to avoid getting it to hot. If it had significant TCR you could detect temperature rise by reduction in current. Without that, need a separate temperature sensor. Maybe just a wrap a nichrome or other resistance wire around it (but avoid getting nicked by sand.)

The TCR for nichrome, for example, is 0.000112 ohm at 300C compared to 20C which seems to be the baseline temperature they use for measurements.

Negligible.

 

[MyK3y]

I just make a resistor divider, or a pair of resistor dividers. That lets me read voltage across a sensor that lives on a different voltage plane (at the expense of attenuated signal, 10:1 or 20:1 when I drop from 50V offset to a 5V system.)

That was my original idea. Hence the 'I will make one today' comment.

I'm becoming impatient with the men from Aliexpress... where are my thermocouples???

 

[MyK3y]

Yesterday was reasonably productive.

I wired up the panels in series and to the element and in free air it hit over 100C in short order.

Buried it in around 15kg of sand and left it for the rest of the day and it got to over 200C

And, I worked out that if I want efficiency at 60V, 240V stove-top elements just aren't going to cut it, so I ordered 50M of Nichrome 80 in 1.0mm

Just running through some rough calculations I can wind three elements that suit 60V and stay well within my 10A panel limit that will also keep within the limits of nichrome.

I can wind spirals of nichrome easily enough using a drill and a basic wooden block with a rod through it. I've got a length of 20mm carbon fibre that will make nice big loops so I can get a lot of wire in a small physical area.

 

[OzSolar]

Buried it in around 15kg of sand and left it for the rest of the day and it got to over 200C

How many usable BTU's did you collect?

 

[Hedges]

Nichrome TCR 0.04% vs. copper 0.4% per degree C.
A poor choice for me to suggest. Copper wrapped in mechanically protective cover would be better.

I'd rather have a heating element that was itself the sensor.

And, I worked out that if I want efficiency at 60V, 240V stove-top elements just aren't going to cut it, so I ordered 50M of Nichrome 80 in 1.0mm

Efficiency will be near 100% (less what's lost in external wiring.
Question is what wattage you want, what temperature rise will occur surrounded by hot, insulating, sand.
I'd think 60V into a 240V element could be about right, just a SWAG.

 

[MyK3y]

How many usable BTU's did you collect?

What's a BTU?

 

[MyK3y]

Efficiency will be near 100% (less what's lost in external wiring.
Question is what wattage you want, what temperature rise will occur surrounded by hot, insulating, sand.
I'd think 60V into a 240V element could be about right, just a SWAG.

I'm leaving so much power on the table.

Without a controller, I want to use the maximum my panels can deliver - so as close to 80% of 10.8A (I think - without referring to the brochure).

The element I have is drawing about an amp and a half.

The answer to the question of what wattage I want is easy - all of them

I'm not getting hung up on such matters - it's going to be a simple equation - does it keep the house warm? If it doesn't it will be recycled into a workshop heater. I found a watt meter - I'll wire it up and see what ti says.

I get that people love numbers, but I'm more primitive than that - I hate cold. If I can be not cold for cheap, that's the equation I see as solved.When I have it working how I want, I'll publish the figures.

 

[Hedges]

The element I have is drawing about an amp and a half.

The answer to the question of what wattage I want is easy - all of them
.

My point is you have to operate the heater at a temperature it (and the sand) can handle.
If only raising to 100C maybe 200C, fixed power could be OK.
Raising sand to 600C, depending on conductivity, temperature of the element may rise as the drum of sand gets hotter, so tapering to a lower final wattage would allow faster heating earlier. Otherwise, a constant reduced power.

You can experiment, see if you wreck elements. My guess is they will get too hot if powered with 240V and buried in sand. It might come out encased in glass, and/or it may fracture.

The stove control may simply be a dimmer switch. That or a light dimmer would be a good way to test power vs. temperature.

 

[brum]

The stove control may simply be a dimmer switch. That or a light dimmer would be a good way to test power vs. temperature.

For AC - yes. But the OP is considering DC as source.

Just running through some rough calculations I can wind three elements that suit 60V and stay well within my 10A panel limit that will also keep within the limits of nichrome.

Fixed load works in fixed conditions, but the power you produce is not a constant. That's why there are MPPT trackers. Even such with DC output for heating elements. They use PWM to reduce the power on load side and dynamically control the load to get the maximum of the panels.

 

[Hedges]

While experimenting and determining what element to run at what voltage, AC makes it easy to modulate power levels.

When powering a resistive load, you can achieve MPPT with just switching and a capacitor, no inductor needed. There is a unit which does that for water heaters, I think that's what brum is referring to. Might be worthwhile if goal is to get "all of them" watts.

 

[JoeyJoystick]

Maybe some of you saw David Poz build a water tank that holds 3 days of hot water . My concept it to use sand instead for a few reasons. I think a sand tank would have no maintenance & last for ever. It would not be a possible source of humidity &/or mold. Sand can get hotter than water. No leaks.

How to plumb all that is where I am going to need some advice. Main question is how do I circulate the water (yes copper coil)? But how do I plumb to use the same water to heat the mass & then get that heat out when I need it. My best guess is some kinda valve system. Maybe I should watch Davids video again lol. But I'd also like to know what ppl think of the sand.

Hi Guda,

I like your idea for many reasons, but one statement appears incorrect and is also the one that worries and concerns me. The specific heat for sand is 5 times lower than that of water by mass. While its density is only about 1.7 times higher. So there where you can heat water comfortable to let's say 90C, the same amount of energy input in the sand would likely give you a temp well in access of 200C. Now industrially, this would be great! I love the energy efficiency you get at higher temperatures. But do you really want to mess with that at home? And, this will cost you big time, I think. The safety aspects here can not be ignored. High Temperatures and High Pressure --> Auch.

Having said that, it still sounds like a great project and a lot of fun. Please be careful.

Joey