Sand for Thermal Energy Storage

[@SimpleTek]

I need your input on piping, insulation, and distribution of heat for a sand battery! Help!

[@tonyarnez5193]

From what I saw, they don't even use steel pipes, I think they just used flat rocks, steel furnace pipe would probably rot out fast.

[@jeffstout6567]

R50 is zero energy from a starting point. Fiberglass, foam and air Crete.

[@SimpleTek]

@@jeffstout6567 nice

[@dvr102]

maybe lower temperature, but a bigger area with sand?

[@SimpleTek]

@@dvr102 that's possible

[@MichaelJohnson-jt5cu]

When I worked in the Solar Industry decades ago we used a saturated water/salt solution that would turn solid at room temperature and turn to liquid when heated. This liquid was stored in plastic stackable trays stored in an insulated room, air flowed through the trays to charge them with heat or extract heat at night using a conventional HVAC duct system in an attic or space for the thermal heat storage.

[@SimpleTek]

Interesting! Thank you for the comment!

[@brianevolved2849]

any chance you can give us more details

[@SuperJLTube]

Phase change material. I wanted to use a layer of this in my design

[@AlxGontijo]

So interesting! Awesome! Can you share any technical information about? Any financial information?

[@Kontorotsui]

What salt?

[@nathanielmccormick7230]

Hello I'm an hvac journeymen. I believe air is your best medium. Using soft copper you can store heat in your sand. Then when you want move your heat from the sand with air to a tube in shell style heat exchanger. A tube in tube heat exchanger would work as well. You then can control water temp by simply controlling flow rate of air around the water tubes. And from Here you have your radiators and such.

[@SimpleTek]

Interesting

[@keithduffield5239]

Agree - instead of sand the medium can be small granite rocks and you heat air in the parabolic reflectors and discharge at the bottom of the rock pile. In a closed system with non-return valves I am sure circulation could be done with convection/Pressure difference flow.

[@davidhoff3829]

I would also be in favor of the air return. This should be the cheapest option.

[@shanewestphal1557]

I was a boilermaker specializing in heat exchangers with high temp, high pressure. I agree with your answer, was thinking the same thing essentially. Any liquid you use with a high boiling point is going to be expensive and/or and pressure. High heat and high pressure is not safe for most people and expensive.. A combination of water/air/sand would be relatively simple, diy friendly, low maintenance, low pressure.

[@mrogormand1]

@@shanewestphal1557 I've been thinking, would it be possible to remove the burner from a domestic gas boiler, and just feed hot air from your thermal battery to the heat exchanger instead? What would the air temperature at the heat exchanger need to be to make this viable?

[@nicolas_poli]

let's do a little bit of math here. before that I just want to point out that water usable working range is closer to 30-90°C rather than 60-85°C. With 30°C water you definitely can heat a room equipped with a costly heating floor OR more smartly a fan-equipped water radiator to replace old high temperature radiators. For water, this gives a DeltaT of 90-30=60K So the volumetric heat capacity for water over that range is E(water)=Cp(water)*rho(water)*DeltaT(water)=4186*1*60=251kJ/L Cp being the thermal capacity in J/K/kg Now for sand : E(sand)=Cp(sand)*rho(sand)*DeltaT(sand) So if we want E(sand) > E(water) we need DeltaT(sand)>251kJ/Cp(sand)/rho(sand) Google says rho(sand)=1.6kg/L and Cp(sand)=835J/K/kg This gives DeltaT(sand)> 187°C Since we took a minimal working temperature of 30°C for water we need to take the same for sand which eventually gives a minimal heating temperature of 217°C. Now we need to remember that at this temperature sand is just as good at storing heat as water is in a given volume. We need it to be a lot better to compensate for its inability to be easily pumped like water. Let us asume that we want sand to be 30% better per volume unit than water. Then sand will have to be heated at 273°C. This means that our free heat source will have to be at least that temperature, which is clearly not that certain for domestic use. The only places where sand can be heated continuously over 250°C is in those concentrated solar farms where a thin curtain of black sand falls across the supper bright focused hotspot on top of the tower. On top of that, those temperature are very close to the auto inflamation of most materials like wood, dust, plastics, etc. So the fire hazard would be extremely high in residential use. With all that said I think that, for domestic use, the better way to consume less wood throughout the year is to throughly insulate your house, use a heat pump that only work during the daylight and store the daily heat in a water tank or a massive wall or soil using simple PEX tubes, it is less sexy (I love exploring tech ideas too...) but it is a long term solution.

[@SimpleTek]

Thank you for all the detail!

[@shingiraidjongwe1703]

Really some good thinking

[@markhemsworth2670]

How do you ensure the water doesn't boil? At least for sand you can just keep pumping in more heat. Good point on flammability though.

[@nicolas_poli]

@@markhemsworth2670 the simplest way would be to activate a circulator at a given temperature threshold. It would send water to radiators or any heat diffusers. An even simpler way is to make the water buffer big enough.

[@MinnesotaBeekeeper]

Minnesota. We worked on one house that stored their heat with 1 inch riverstone instead of sand. Never thought about it before but the stone would allow full airflow from the heat source. Eliminating piping. Hum. The stone was in insulated below ground bunkers.

[@SkySailor75]

Interesting. I just bought a property where they pushed the old stone house into a hole, so I definitely have the stones to do this.

[@off-gridsurvivalmike8120]

Great concept, I am building prototypes now for demonstration and to work out the bugs. So your idea of the larger stones will work great. So my thoughts are to have a heating element buried in sand at the bottom of a barrel or some container and now I will add larger stones at the top with maybe some grating between the two mediums at that point I won't need internal piping, like you said the air will flow around the larger stones. Perfect, fantastic commen.

[@kawaiisenshi2401]

​@@off-gridsurvivalmike8120how's it going???

[@billdevany3303]

I love that you have more questions than answers. It breads intelligent conversations and innovation from the "crowd". there are more intelligent and well thought out responses in your comments that I wont even through in my two bits. keep us up on how it goes.

[@SimpleTek]

Soo happy I could confuse you

[@TheGijzzz]

Here in the Netherlands there is already an housing project that uses a big isolated basalt stone mass as an heating battery for heating in the winter. During the summer regular solar panels collect electricity that is stored with piping as heat is the basalt. In the winter this stored heat is used to heat a whole housing block ( name of the project: cesar-project and basalt-batterij)

[@SimpleTek]

That’s awesome thank you for sharing

[@joergsonnenberger6836]

In Hamburg Siemens-Gamesa has an upscaled grid-level version of this using basalt as well: ETES, it can store 130MWh of heat in about 1000t of stone.

[@AlxGontijo]

Really nice! That's idea is really awesome... Do you have any information about the ROI of this? Any financial information?

[@AlxGontijo]

@TheGijzzz

[@MikeH-sg2ue]

Hi there, as a Canadian, I understand how sand is a nice thought in the Winter! Tahiti has some lovely sandy beaches! I am very interested in sand batteries. In the past I’ve done a couple of small scale experiments. In buckets, & large SS saucepans. Bought 3 older electric kettles from thrift stores. Hooked directly to a 100w solar panel. They all worked. The bucket worked best, but I was concerned about heating the galvanized bucket that hot in the house. Fumes might be an issue. The saucepan worked well, but wasn’t big enough. After watching your video, I now have a bunch of new ideas! Who knows what I’ll come up with? Keep your smiles on!

[@SimpleTek]

That’s awesome!

[@zeoxbg]

I would say that electric heaters, fed by photovoltaics are a great way to store a lot of heat in a closed and well insulated sand container, that has built in pipes for heat exchange. You could easily get the sand to 300-400 degrees C. Stainless steel should be a good material for the container/heat exchanger. Insulation should be very good, maybe a first layer of perlite/cement, second layer of hempcrete or rock wool (or maybe both). And the whole thing put underground. Using a loop of high temp oil, you can easily transfer heat from the sandbox to a water boiler. And relying on photovoltaics, rather than solar heat collectors, means that even in very dim and short winter days, you could get at least some heat. And in the summer you can use the extra power for other stuff.

[@SimpleTek]

interesting

[@paullehto2294]

Check out Finnish company polar doing it

[@t9358]

Thank you for post. is high temp oil vs high temp air better and less cost. i think Polar Night Energy does hot air and thought it strange

[@zeoxbg]

@@t9358 Hot air is a nice concept too. But I think it would be harder to achieve as a DIY project. High temp oil can be achieved very easily using old water heater tanks, copper or steel serpentines... Basically junk that can easily be obtained.

[@thedoctorjobabyjo9105]

Maybe you dont need that high temperatures if the size of the container fits to the consume to have enough storage just by having enough thermal mass. Thats my problem with all that stuff. Nobody is setting this up with some sensors and exact numbers of the projects.

[@ronlabe5487]

So say you use some sort of rocket mass stove to heat a large volume of sand to maybe 500 c. Then used in auger system to move hot sand to an insulated storage bunker. Augers are cheap to make then work fairly reliably. What I'm saying is basically you'd like to have your thermal storage like in your basement or crawl space but you don't want to have a wood stove there heating sand there, so you could do something sort of like a outdoor central boiler just super heat the sand and then move it to a spot that was more beneficial to have it like an ultra insulated dump tank in your basement. Sand is the fluid.

[@SimpleTek]

good ideas

[@johnbutterworth1369]

Made me think of smaller sand batteries in cylinders for less heat and larger ones of more heat. Rolling along a track under the house as needed. Even spheres would work. They could seek out the cold spots.

[@SimpleTek]

@@johnbutterworth1369 interesting

[@acmefixer1]

What you need to be more concerned about is how you will pump the fluid through the system without melting or burning up the pump, pipes or valves. The way I would do it is to use hundreds of inexpensive flat mirrors to reflect the sunlight through a glass covered hole in the sand box to heat the sand directly. Then circulate water through the sand for use wherever.

[@SimpleTek]

yep

[@NwoDispatcher]

How about fiber optic?

[@l0I0I0I0]

@@NwoDispatcher fiber optics? Is that only used for the visible range? Does it also transfer infrared?

[@NwoDispatcher]

@@l0I0I0I0 well it's all electromagnetism. I'm sure different materials could be used for different wavelengths

[@l0I0I0I0]

@@NwoDispatcher as long as heat doesn't escape through the fibers as well. Idk, sounds interesting. 52% of heat from solar is IR. Small amount UV. I've studied this in the past on my own including black Body emissivity etc. It's not easy to find a material that is like a diode and heat energy in all it's forms goes through a material without blocking 5-10% even in thin materials. I've seen lights however going through fiber optics straight from the sun to light a space indoors, however they are expensive and probably filter out IR radiation. Curious to see what you come up with? Love your idea of heating a hole in the ground. The glass covering the space and it's emissivity to heat radiation can be a challenge. I've been working on this at home for a long time now. Found one possible solution but the state of the art material is not in production yet.

[@unclepete100]

Use the heat stored in the sand to heat air and pump the air to the dwelling / greenhouse. Eliminate the complexity and expense of liquid handling altogether. Used aircon ducts are usually available in metal scrapyards. Cheers

[@SimpleTek]

I like it

[@frankfrederick5158]

A little simplified, but the sand must be heated first. Then the heat can be distributed. Air alone will not create enough heat in a compost pile to heat the sand/rock bed uniformly. Also taking too much heat or a a fast rate from the compost pile decreases the amount of "energy" created. Distributing heat via an air plenum also lowers the efficiency and is adequate for a small confined space but not a larger area. Think "Rocket Mass Heater" concept but will a large mass heat storage area. My greenhouse is 30' x 60' and I will be adding hydroponic growing space next year.

[@shaun6828]

@@frankfrederick5158 Air doesn't create any heat. It's just a medium for transporting it. Move the air and you move the heat energy it contains. Works pretty much the same as with fluids. Fermentation inside the compost pile will produce heat, but only a small amount. You can extract some of that, but if you take too much, the fermentation process will slow. Air would work fine for warming sand just like any typical household heater or heat gun does. The issue, is having enough input heat to charge the battery at a rate faster than it loses heat. Even with insulation, heat will be leaking out of the battery.

[@frankfrederick5158]

@@shaun6828 Thanks Shaun, I do believe we are saying the same thing, just in different ways. Your comment on fermentation is correct. I might add it take "air" to create the fermentation process. Starve the process of air, deplete the fermentation (heat) created. Also, for the compost pile to last (up to) 18 months small mounts of water, air, and bio digester are added regularly and the pile "turned" to help the process complete.

[@shaun6828]

@@frankfrederick5158 I saw a video of a farm that used sealed tanks for the process. They tapped it for methane emissions. I guess it also depends on what is being composted, but ethanol is another possible byproduct...

[@ulfcarlfinnes2820]

In Sweden, for many decades, they have made and used tiled stoves that are made of ceramic tiles according to a certain system that forces the smoke to make a round trip inside the stove before it enters the chimney. These stoves are only fired up once a day and they slowly give off their heat during the day and night.

[@SimpleTek]

very cool!

[@enigmaapoc403]

Are you talking about "Rocket Mass Heaters?" I've been looking into those quite a bit. I have several project ideas inside and out that I'm planning; from cooking to space heating.

[@SimpleTek]

@@enigmaapoc403 rocket mass heaters work great

[@CandidZulu]

@@enigmaapoc403 en.wikipedia.org/wiki/Masonry_heater

[@garyweber6413]

that's great in all but the whole idea is to never have to feed that god dam wood stove anymore

[@yellowbird5411]

As a primitive heat storage for those who have had to use this method throughout history, rocks and I imagine, sand, would hold their heat for long periods of time. I am thinking of the very old fashioned methods of heating specific areas with hot stones, heated by a fire, then transported to where they were needed. They not only used them for heating beds in freezing bedrooms, but also in outdoor carriages, under the seats, and in their homes where needed. These rocks would get switched out for newly heated stones brought in from either outdoor fires or indoor fireplaces. In this day of fuel shortages throughout many parts of the world, it seems we must inform ourselves of lifesaving methods of survival. When the weather thrust Texas into a deep freeze a couple of years ago, it was reported people froze to death, and pipes broke from freezing also. It seems the electric was down in many areas, and people apparently were not able to stay warm. It pays to know basic survival techniques, and sand used for heat storage makes sense to me. I had been thinking of getting stones, as in Florida we don't have "stones" as much as limestone, inland. But I thought of sand as an option, because if nothing else, Florida has all the sand one needs. Of course, it is mixed in with organic materials, but it is not hard to wash sand to the pure white state. I have done that. Once that is done, dry sand should be good for heating by the fire outside in metal containers that can be safely moved indoors to a metal or fireproof platform to warm small areas, should our grid go down. We dip down into the 30's here, and while no one would necessarily freeze to death, it can get very bitter during the nights, with days usually in the 50's. Thank you for this video, and I just wanted to recognize sand as a great potential heat storage/insulation resource.

[@SimpleTek]

Great info

[@OktoPutsch]

Hey mate, what you need to study are simply "heat exchangers" this is a well known domain and largerly used in the industry, and this is the convergence point between heat storage and its use. The good thing here is that you know which temperature range you'll have to work with, so you can start projecting and dimensioning your installation. Even simpler to design and order well sized thermo-electric generators. Have fun !

[@SimpleTek]

cool

[@inner-mate5089]

Hi, Would you be able to recommend any sites where I could size efficient 12-24 volt thermo electric generators. Thanks do much.

[@edschultheis9537]

I believe that the best working fluid for transferring heat to/from your sand battery may be MINERAL OIL. It seems to fit all of your requirements, while also having advantages over other fluids. As an alternative, you may also check into using cottonseed oil or rapeseed oil (also known as Canola oil) as these last two choices are more biodegradable. I saw a couple of suggestions to use water with antifreeze, but that will start to boil at about 226 F (108 C) at atmospheric pressure w/ a 50/50 mix of water and antifreeze. You wanted a fluid that would handle about 300 C. So I don't think this is a good choice. Some facts about MINERAL OIL: > It is already used as a working heat-transfer fluid in products like electrical radiators, power pole transformers and high-voltage applications. > The maximum operating temperature is 316 C (600 F) > Boiling point at atmospheric pressure is 349 C (657 F) > It is essentially non-toxic and more importantly essentially non-toxic to humans/animals > It is odorless and colorless > It does not conduct electricity > It can be purchased in both industrial grades and food-safe grades Sources: www.multitherm.com/multitherm-pg-1.html en.wikipedia.org/wiki/Mineral_oil Ed Schultheis, PE Mechanical design engineer for 35 years My consulting business Schultek Engineering & Technology, Inc. schultek.com

[@SimpleTek]

interesting

[@edschultheis9537]

@@SimpleTek I have another thought about heat transfer to the sand thermal battery... Heat transfer between one material and another is maximized by making sure that there is intimate contact between the materials. If this were my project I think that I would experiment a bit with the following setup: 1) Use mineral oil for the working fluid in a closed-loop system, pumping the oil through aluminum or copper tubing from the solar source to the sand battery. 2) Fill the box with sand as you originally planned. 3) Pour additional mineral oil directly into the box of sand so that it fills all the tiny voids between the sand particles. This setup would ensure very intimate contact between the outside of the tubing and the mineral oil in the sand box. This setup would also ensure very intimate contact between the mineral oil in the sand box and the sand particles. This would ensure a very good heat transfer to the sand. I'm not sure how much mineral oil would be required to fill the tiny voids between the sand particles in the box. Hopefully it would not be much. I would do a small test with container of sand of known volume. Then fill the container with water until no more will fit. Then you will know how much additional oil would be required to fill the tiny voids between the sand particles. Ed Schultheis

[@jeffreydustin5303]

@@edschultheis9537 Mineral oil is a very interesting idea. What about burying a kiln in sand and somehow solar heating the sand to heat the kiln? Inside the kiln is a rockwool blanketed tubing of some heat resistant material that carries mineral oil or salt water to the desired heating endpoint. It is a loop so the cool fluid goes back to the kiln. The sand is a thermal mass and can slowly release heat...buried treasure. The insulation is more about slowly cooking the fluid, because insulation just slows heat transfer, it doesn't prevent it.

[@zedostenso3069]

@@jeffreydustin5303 A thermos bottle arrangement around the storage would help to slow the heat loss. Now me are talking big bucks.

[@stevenjohnson1409]

@@edschultheis9537 What are the environmental considerations of mineral oil in case of a spill/leak?

[@patrickforbes6745]

Air is probably the best choice for moving the heat into the sand. I’m looking to build a rocket stove and blowing cool air with a small fan across the top in a 4” pipe and then down into the bottom of the sand. The pipe then splits out as a manifold into smaller pipes that have holes in their bottom so the air can get into the porous sand. A similar manifold at the top could bring warm air to where it is needed.

[@SimpleTek]

That’s awesome

[@SimpleTek]

@palipali4264 wise words

[@nickrowe9221]

schedule 80 steel pipe, used full synthetic motor oil (it is free from your local garage) it will not burn until it gets to 450*F not as high as you could heat sand but it is the most cost effective thing I have found so far, consider using sea shipping containers for your storage, and rockwool insulation (the only insulation that can take 1500*F ) Ive been giving this system a lot of thought for a central heating system for my farm. Resistive heat is horribly inefficient. Im thinking a heat pump would work better... if I charge the sand in the summer the wast cold could be used to cool the house and green houses. Im in NH and we will build deep winter greenhouses that are insulated on all but the south side, so will only need added heat for 3 months. We are still working on design and collecting building materials and will start building summer of 2023. best of luck to everyone trying to figure out this problem!

[@SimpleTek]

Sweet

[@thebandplayedon..6145]

So far, I have my greenhouse pit dug, aprox. 5' deep. After insulation with ridged foam I plan on filling the floor in with two feet thick adobe/cob slab, then add another foot or so on top of that but with a 6" diameter channel running thru it to direct and collect heat from a woodstove before heading out thru the chimney. This will operate similar to a cob bench often attached to a rocket mass heater in a home. Only requiring a very minimal amount of wood and effort in the day/evening to create enough radiant heat to get thru our cold Canadian winter nights. And, in summer, the cool floor mass will help regulate the interior temps with at most a dc fan for circulation. I've come to appreciate systems with as few movable, motorized, and electrified parts as possible- less to break, more reliable, easy/free to DIY fix, etc.

[@SimpleTek]

sounds awesome!

[@SinMihai]

Use as tubing, glass tubes. glass melting point approximately 1400 C to 1600 C

[@SimpleTek]

Nice

[@baldyetichronicles]

Transfer the heat from the parabolic collectors with air. It's free and no need to worry about leaks. Keep the blower fans on the cooler inlet side. Water coils in the sand can transfer heat to the house. The collected air from the top of the thermal sand storage can flow through the greenhouse before is goes back to the solar collectors, or just circulate the warm air to the greenhouse on cloudy days. Water coils in the hotter parts of the sand can be used to power steam generators for electricity.

[@graveseeker]

That eliminates the 'rainy day' backup that the sand provides.

[@SimpleTek]

ok

[@johanventer5730]

Good idea with the steam generator for electricity

[@rowanmurphy5239]

Boiler technician here. Water reaching those temperatures to me sounds like it would be a good idea to get a bladder tank, to give the heating and expanding water somewhere to expand into so it doesn't drive up the pressure and burst your copper. Also, if anyone is going to solder this together, they should remember to wipe their joints with a rag afterwards. So common is a situation where they don't do this and the joints all leak within a couple of years, because the flux is acidic. I really don't know about the longevity of ProPress joints. They are working like a charm for my boiler team so far, but it is probably a better idea to not have any joints or fittings inside your thermal battery; just continuous unbroken loops of copper, so that if one should burst on you, you can more easily repair it.

[@SimpleTek]

@@rowanmurphy5239 wise words

[@whosonfirst1309]

What if you took a new empty 500 gal septic tank and fill it w/ sand/water mix (water added for better thermal transfer) added copper/pex/ stainless steelcoils for both the primary and secondary exchanger and used that? The tank cold be wrapped in insulated board both inside and out. You could manifold the tubing for better transfer also there’s a thing called Glauber salt that phase changes with heat. Just a few thoughts I had, have a good one

[@venim1103]

My idea is stupid (very inefficient) but I’ve been trying to think maybe the solar heater could produce steam to run a generator which stores electricity into an array of supercapacitors (crude, cheap homemade) which in turn after having enough energy stored could directly use heating element to heat the sand (so like buffering the heat first trough electricity back into heat). Haven’t done any calculations, but most likely an useless approach. At least this would not require complicated/expensive equipment. Not sure how high temperature can you reach with the cycle of charging and discharging a certain size supercapacitor array and what type of heating element would be good for such a setup. The amount of energy losses with this approach will be very high but maybe this way of buffering would enable the sand to reach higher temperatures? Ps. If someone asks why not just store the electricity directly to batteries, the whole idea of storing energy to sand etc. is to find an alternative to current battery technology storage systems.

[@SimpleTek]

Cool idea

[@Sol-En]

I think that the best thermal exchange fluid for sand heating is air at high pressure in pipes. You can circulate this air by high temperature compressor between concentrator and sand heat storage

[@SimpleTek]

maybe...

[@Sol-En]

@@SimpleTek Turbo high temperature compressor is not difficult to manufacture. You just need long shaft between impeller and electric engine

[@frankfrederick5158]

Radiant in floor heating also ups the anty. I have using a compost pile heater in the cold, cold winter for a greenhouse using radiant heat. Going on two years, it has served me well.

[@SimpleTek]

Thanks for sharing, that's awesome!

[@GetOutsideNZ]

I am looking into this at the moment. Can I ask what size the pile is and how you extract and distribute the heat? Cheers.

[@frankfrederick5158]

Look at "Compost Pile Heat" here on KZbin. The videos should answer most of your questions. I use a water based system (twice), one loop of actual water for use in a hot tub and other areas, and second with a glycol mix for radiant floor heating. My in floor heating system is not a normal radiant floor. I did the usual insulated base of gravel/sand/insulation then the in-floor tubing run and covered that with sand, then 2" of concrete. I have a small "boiler" for very cold nights and the usual manifold and pumps. A water heater tank (large) serves as a reserve tank for the hot water. AT a -30 F temp outside over winter, the inside greenhouse temp was 68 F.. Although a bit of trouble to create, my energy costs this year were next to nothing. Summer ideas include a better hydroponic system and an HHO generator for electricity/heat/cooking/cooling/pumping/etc. Good luck.

[@jeffstout6567]

@@frankfrederick5158 what inlet and outlet temperatures were you able to maintain throughout the winter months. Usual infloor supply temps are 110 degrees F and 90-100F outlet for residential, garage and commercial bldg. what’s your thoughts using your compost heating system.

[@frankfrederick5158]

@@jeffstout6567 Thoughts? No. Facts, Yes. Average 108 to 114 degrees F into the system. Out between 80 and 90 F. It gets cold where I am - VERY cold, so the floor can suck up a lot of heat.

[@smjones4238]

I am now subbed... I was going to try synthetic oil, can get hot. Have always wanted to use a trough collector. Need auto adjusting capability however, Bright days can call the fire department. In and out loops into sand use the oil. Run thru a plate to plate exchanger to loop in the house. Pump the water in a loop thru the coils in the house. Use 3way valves so loop is always flowing. Have a 3 way mixing valve on the water side of the exchanger to keep loop speed up while adjusting the percentage of hot water to recirculated water. You could probably even pipe the sand/oil side of the plate to thermo siphon when the demand is low, but have a boost pump in the circuit. Random thoughts: Might want to check the stratification of heat in a big pile of sand. You might use the coils and pumps that introduce the heat and take it out to recirculate the heat by pumping the top coil through the bottom one to keep heat saturation fairly equal. Heat always gets away at the top. I'm sure that you can see that I'm a controls guy. Retired from 30 years of being paid to be a control freak. I also ran the Building Management System where I was. A LOT of analog mixing control stuff with fussy people that like their offices just so. When you put in a lot of control, you can either make it work or it will SHOW YOU the weak spot. 70yo retired with 65yo wife on 2.3ac 45mi ne of Metro Denver limits. 35X70X25 shop just gettin 'restarted'!

[@SimpleTek]

interesting!

[@terrywolsey8741]

Iam using foam crete to insulate my 55 gallon drums and heat element to heat the sand to 600degrees I have to put it in play this spring but have some problems to solve in the process. This way I can do foced air for the cabin and green house and heat water as needed

[@SimpleTek]

I like it

[@randycrowe4978]

Here are a few thoughts for you. 1. oils and other chemicals are costly and in the case of a spill, may cost a fortune to clean up. If you used oil for pumping the heat into the sand, simplest solution for retrieving the heat is obviously air. Liquid salts are very abrasive and will destroy most piping quickly if I understand that correctly. If you are storing heat, you could even use a Stirling engine to convert some of the heat to electric.

[@SimpleTek]

interesting!

[@umardentras7251]

I have such a thought bt don't knw hw practical and efficient it is

[@peteraquablue]

where can i get a decently sized stirling engine from?

[@mariusmossum2923]

Assuming one is using sand already; and reaching temperatures well above 100C, then I guess using a steam-engine for generating electricity would be better? Not that I have built one myself. But my research leads me to believe that it will be way more compact, give higher output, will be easier to build, and probably cheaper. Not a steam-turbine though. Would have to be a piston-system, like an old four-stroke engine or something.

[@Omnesum]

Video just popped up again, haven't seen any follow up but a few channels fail to notify. Anyway, consider how much energy your 300C sand actually holds per volume and then look at water in reverse: what is the local groundwater temp and how much water would be required to absorb your heat to below boiling? Depending on how you plan to access/ use the stored heat you could set up a steam turbine generator for electric which could also absorb some energy transfer. You could stall the steam in tanks buried in sand with valves that release the steam/ pressure through other turbines/ tanks until it's just hot water of the desired temperature to heat the house and fill the water heater. This would also distill the water in process.

[@Omnesum]

Let me simplify this ramble. If you have a sand tank in the ground, the temperature around it will decrease with distance. Water pipes in the ground where it's around 100C can then be piped through the floor for heating then into the water heater. Same distance diffusion should work with oil or antifreeze

[@SimpleTek]

Interesting

[@kickeryou]

For charging use electric Resistive Heating and solar. Vehicle Glow plugs can heat to 1400C. Stove elements can heat to 900C. For extracting heat look at copper pipe and air to a holding water tank like you discussed. I wonder what a regular hot water tank filled with sand would do if the aquastats are modified for high heat?

[@SimpleTek]

Thank you! very interesting ideas!!!!!!!!

[@jeebus6263]

More volume would be more efficient, a well insulated water tank may be a good test bed tho.

[@ai4px]

Glow plugs and resistive elements get heat in. How to get it out?

[@Swampwild1]

Great video. I’m in Hadashville. Here’s my main point- if I buy a system that can meet my needs in the winter then I’m producing 10-15x the power I need in spring/summer. So efficiency is irrelevant, it’s all about cheap storage and retrieval. Especially with panels price dropping. The panels are probably 1/4 of the total system cost.

[@Swampwild1]

I like the idea of sand heated with glow plugs. Simple reliable. Imagine using TEG to convert that heat back to electricity. I know it’s very very inefficient. But someone do the math for me. How much sand? You could create a 2 stage sand cell for just the hydronic heat. But you probably also need electricity because we get so little in December? I use wood in a soapstone fireplace now, hope to do the Chinese greenhouse maybe.

[@TheWickerShireProject]

We plan on using MINERAL OIL ( it's Inert ) 350F / 180C. Run through Pex using a Solar style water heater/collector with a 1 watt Car water pump. Its simple. System is buried in IBC totes under ground wrapped ( set ) in aircrete. Return closed loop is water ( easy clean up should there be any future failures to the system ) run through hydro radiators. Great Preheater to add to any Hot water system.

[@SimpleTek]

cool!

[@1978jra]

Some back of the envelope calculations. 4,19 kJ/(K·kg) *100kg*100K=41 900kJ 0,84 kJ/(kg*K)*100kg*1000K=84 000kJ So yes you can store more heat in sand if you heat it to 1000C but I still would say that water is better choice. 1000C is good forging temperature and I suspect materials that can handle that are quite expensive (at least for small scale). Still sand is far better choice than I anticipated. You might even be able to do it without any piping in the sand itself, just pump air trough the sand.

[@SimpleTek]

thank you for the insight!

[@plinble]

Direct solar PV for the heating. May need some electronics and relays to optimise the balance between input and output, definitely not a big converter. To move heat I've used advection. Basically move the sand from the hot store, to the house, to the cool store. 15kg blocks, 440C max, so mild steel is still good? About 1.5kWh each, that's all you need in a smaller room to take the autumn chill off for an evening.

[@SimpleTek]

@@plinble sounds difficult

[@plinble]

​@@SimpleTek For small amounts of heat it does work, e.g. 18 litres of solar heated water to 50C. 100,000 btu/ hour, not practical, that's like 30kW.

[@SimpleTek]

@@plinble lots of things work, but is it affordable

[@nigelwilliams7920]

Well a cubic metre of sand stores 105 MJ for a heat difference of 30C, while a cubic metre of water stores 125 MJ. So water is better up to 100 C. The advantage of sand/rock is that a much higher temperature differential is available. But, as you say, and as my own contemplations have found, the complexity of dealing with temperatures over 100C on a domestic set up are too much. I can buy at 30 cubic metre plastic or concrete water tank for about $3000, and if that is heated to say 60 degrees above ambient, then I have 7560 MJ of energy stored there. Foam concrete insulation, plus fibre glass batts would slow heat loss enough and such a system would be affordable and simple enough to build. Hot rock/sand with say high pressure air to transport heat from the solar concentrator is just toooooo hard for the ordinary person. A bit of warm water plumbing and a few circulator pumps is a simple job.. 7560 MJ is equivalent to 70 days heating at 30 kWh per day. Even assuming 50% lost via heat losses, and another 50% lost via poor transfer effeciencies, that is 35 days at 15 kWh per day. If your heat capture panels are able toget anything like 30 kWh input on a normal winter day, then you are safe for the full year's space heating providing you don't get more than 15 days rubbish weather in a row. You would see the storm coming anyway, and button up tight to conserve the resource. That sounds viable to me. Rocks or sand? Too hot to handle, IMHO..

[@SimpleTek]

interesting! thank you for the reply

[@myounges]

I fully agree on domestic DIY scale you need to get as simple as possible because even the easiest setups bring unexpected challenges... :) Check out a guy called Sergiy Yurko he has a channel about a lot of solar collectors and thermal storage. Really interesting.

[@SimpleTek]

@@myounges I actually have a channel link exchange with Sergiy. He does INCREDIBLE work with solar collectors and water thermal batteries! I absolutly LOVE his work.

[@myounges]

@@SimpleTek great minds think alike... Haha yes that guy really takes his job seriously. 👍

[@graveseeker]

How about lights, cooking or cooling for those of us in the deep South? Shouldn't the system provide electricity?

[@joemurphy4517]

I'm about to build a high performance house. We are using geothermal hvac and solar hot water. We are running our hot water in the roof rain screen with metal roof and radiant barrier. It will be a continuous Ling with no breaks being pumped to an insulated pre tank in the attic. We are in climate zone 3 so our cooling is more of an issue than heating. Non the less we will try it to see how it works out. Good luck with your project 👍

[@SimpleTek]

I hope your house works awesome! thank you for the kind words!

[@ronson-natsarim]

The flow rate of the liquid through the sand determines contact time which regulates thermal energy absorbed by the liquid. You could run water through 1,000° sand and adjust your flow rate so that the output temperature is 70° (or whatever you desire).

[@SimpleTek]

nice

[@kyleburdick8771]

Water run through 1000 degree sand would be steam...

[@SimpleTek]

@@kyleburdick8771 depends on speed, length of run and contact time

[@kenhilving276]

How do you get around having some water left in the pipe where the sand continues to heat it to super steam?

[@SimpleTek]

@@kenhilving276 good point

[@sunworksco]

Would love to speak to you. I’ve been a designer of hydronic solar thermal heating exchangers, greenhouse and geothermal. I have some very good designs for the sand battery. Cheers! Giovanni

[@SimpleTek]

We’ll chat soon!

[@lynnebalzer6689]

We have a fireplace insert with a huge fire pit - so large that when we fill it with wood, things get too hot, and there is a risk of chimney fire. We came up with the idea of putting large concrete blocks in the back. They will heat up from the burning coals, and stay hot for a long time. Two fans will blow this heated air around the house.

[@SimpleTek]

Thermal mass!!! Good idea

[@k1ng401]

I imagine a large insulated storage container with two sets of copper tubing coiled inside surrounded by sand. One set of tubing circulates heated air from a standard electric heater through the sand and heats it, the other tube also full of air which is heated from the sand is ducted to a fan in your house or greenhouse. The sand heater operates during the day, usually powered by excess solar. The fan that circulates the heated air inside the house can run at any time but in daytime will usually be solar as well. I’m just not sure if the air will be too hot straight out of the sand battery and the best way to manage that. If it is is too hot then I’d siphon off some of the super hot air, mix it with outside air to cool it to the right temp before releasing it in the house.

[@SimpleTek]

Cool idea

[@tmbrye2]

Have you looked at Russ Finch's methods for growing things in cold climates? If I had a big enough property that is the way I would heat things up like a green house. I'm just starting to look at sand heating and storage for some around the house ideas so I'm just beginning my experimentation. My first challenge is to see how hot I can get sand with just the sun.

[@SimpleTek]

Yes, he has a much higher ground temperature than I do. I have a video that addresses how to fix that though

[@reuse_or_die]

If you want really simple and low tech that uses upcycled materials it makes sense to me to build solar walls with old windows or glass doors, especially those from shower units or vehicles, which are very tough and harder to melt down for recycling. I built a rocket mass heater which uses mostly sand to store the heat from my wood burner, reducing the wood used by two thirds

[@SimpleTek]

Great idea

[@hughmanatee7433]

The best system I have seen is much simpler and does use sand. My friend built a concrete building 10x10x10 he used concrete blocks but I would use the styrofoam concrete forms that lock together. Down the middle of the building is a wood stove custom made 2’x2’x9’. The stove is ruggedly built to withstand the weight of the sand. He also used a 16 foot length of 6 inch steel well casing as a chimney welded into the top of the stove. The building has styrofoam placed on the inside of the block walk to keep the heat in and also on the top. The stove is completely covered in sand and a system of pipes are distributed through the sand which goes all the way up to the top of the building. Water is pumped through the sand with ordinary circulator pumps out to his house. He actually rigged the plumbing to his existing oil boiler so he can heat his home with existing piping and radiators. He lights a small fire once every couple of days to maintain the temperature in the heat sink. He has antifreeze in the lines so it won’t freeze if he isn’t using it in the winter. The thing that is better about this system is that there is no creosote buildup because the water doesn’t contact the firebox directly. Also those water jacket heaters only hold so much water and don’t store the heat. My friend also has a domestic water coil going through the heat sink. If he goes away during cold weather he has to blow out the domestic coil because it has no antifreeze. This system can be adapted to use in greenhouses and other uses.

[@SimpleTek]

very cool thank you for sharing

[@patrickcleburneuczjsxpmp9558]

@John Pino That's very similar to something I'm interested in doing. Can you recommend ways to get more information/details on what you described, not just that set-up but any sort of related information or links or people...? Thanks!

[@hughmanatee7433]

@@patrickcleburneuczjsxpmp9558 I don’t have any more info. I know he did buy the plans from an engineer but the plans are outdated. The pipe used was 1/2” pvc and copper. I would think that pex or soft copper coil pipe would be a more modern choice. I know that there was no pipe within 6” of the top of the stove, I would stay away a foot to be safe. I don’t know how hot the sand gets you could melt the pex. I know he doesn’t make a big fire, just enough to maintain the heat sink. If your project requires a lot more heat you could use iron pipe and a larger building. I think my friend’s system can hold a million BTU’s and release 100,000 an hour. I also wonder if a rocket stove would be more efficient. I have also thought about modifying this model to include a charcoal oven so that you could make bio char for the farm.

[@patrickcleburneuczjsxpmp9558]

@@hughmanatee7433 Thanks very much for the reply. You're saying more of the same things I'm thinking. I'm currently using an outdoor water stove (and the firebox is big enough that I'm also making charcoal in a modified standard size (5 gal?) propane tank), and that system has worked great for me, but the stove is about at the end of its life -- it has a small leak already -- and I'm trying to figure out what I want for a replacement. One thing I'm pretty sure about is that I want to continue to heat with wood and I want to continue with an outdoor system. (I think I prefer a system like I have now that is completely separate from the house, but I might consider a stove that was in/attached to the house if I could feed it from outside.) If the stove itself were all masonry, I wouldn't have to worry about leaks or any water-metal issues with the stove itself, but then if I had some kind of water tank above the masonry I could store BTU's in the tank and use the water to transfer the heat to the house (and maybe use the water to indirectly heat my domestic hot water like I do now.) But if the tank started to leak I could replace it without replacing the whole stove. So that's more or less my idea. I don't know if a rocket type batch firing system (like traditional central/eastern European masonry stoves) is what I'd want or not -- I like that I can currently use very large pieces of wood, that I'm not strictly limited to ideally seasoned wood, and that I can keep a fire going rather than having to start lots of fires, but I realize there's a trade-off in efficiency and how cleanly it burns -- but I guess an all masonry firebox would allow for long-slow or fast-intense burns. I guess a masonry firebox would have to be smaller than what I have now, but I might be able to incorporate a baking oven or warming chamber into the the design. I don't think I've ever even heard of anyone else that has an outdoor masonry stove with water to circulate the heat to the house, but I'm not sure why. Obviously steel water stoves can be mass manufactured and delivered ready to use, but I really like the idea of a more modular set-up that might be able to last a lot longer and be relatively cheap and easy to fix (especially compared to water stoves which rust out or leak for other reasons.) Thanks!

[@hughmanatee7433]

@@patrickcleburneuczjsxpmp9558 I would avoid any water tank used to save heat. The sand is the key and it is about mass. Go big.

[@SuperJLTube]

Have you considered using steel black pipe to make your heat exchanger. I was just trying to research if slag would make a better storage medium than sand as it usually still has a good content of iron in it.

[@SimpleTek]

Good idea

[@SuperJLTube]

@Simple Tek I noticed after someone else commented on the gas pipe. What are your thoughts on the slag. I feel like I'm getting conflicting info? Do you have any formula to figure out how much heat needs to be stored for your project, projected heat available from different sources etc? My house is about 3000 sqft built in 1900. I was thinking of building my battery under my driveway. I was thinking slag because it's even cheaper than sand, probably in line with pit run. It also packs down nicely and provides a more stable base. I don't think air would flow through that aggregate well though unless I used nut slag. I also wanted to incorporate a PCM, maybe directly beside the exchanger so that I could get heat rapidly when needed and insulate it from heat pump pipes

[@SimpleTek]

@@SuperJLTube I don’t have a way to give exact amounts of heat needed, only general estimate guesses

[@georgeyjungle]

Build the sand storage box with an air gapped insulated compartment on top and control the temperature of the air so that you can heat a closed loop water system directly via salvaged radiators or copper pipe. You may need an exhaust in the air box as protection to prevent over heating of the water.

[@SimpleTek]

interesting

[@markhemsworth2670]

As an amateur enthusiast, I would really analyze the cost of using PV panels instead, and using a direct heating element...rather than complex pumping and piping...and rather than worrying about your mirrors lining up right all the time.

[@SimpleTek]

I like the idea

[@markhemsworth2670]

@@LaurenBurger thanks for the tip. Have you seen a financial life cycle comparison for the two options?

[@taith2]

I've been researching subject for long time, use ceramic medium, best guess is chimney prefabs. Use nitrogen as heat transfer medium and air turbines to move heat around, can place turbine on cold side. I'm aiming for temperatures reaching 700+C here, borderline stainless steel heat, because it's in later parts in my project where stainless is inevitable at one point of it. Liquid salt and liquid sodium is too corrosive, for your small scale I'd suggest using nitrogen as neutral medium. Can use existing evacuated tubes and parabolic mirrors. For your use best is steel water pipes that are stick weld and have nitrogen running trough it. Obvious side note, you need air pump engine well separated from blades and any regular bearings.

[@SimpleTek]

very cool!

[@PyroEvil]

I did the calculation on my side ( with density and specific heat value of sand ) and I need about 60 000 liters of sand ( 60 cubic meter ) at a temperature of 500 degres celcius to cover my heating needs during winter in Canada. That's enormous ! It's about 10 000kWh of heating energy needed and perfectly isulated. So probably to cover the heat loss , need more than that.

[@SimpleTek]

Ok

[@esatecnic]

Eso es unos 15 metros horizontales y 2 metros de profundidad , es perfectamente posible y barato. Si ademas consiges 50 grados , el salto termico a 60 0 70 grados centigrados sería muy economico para la casa . Un saludo

[@tarawood-bradley2119]

As a proof of concept for sand I would personally start with your boiler and building an enclosure around it and filling it with sand. Then the system can grow with solar collectors.

[@tarawood-bradley2119]

thinking about this, we need to change the boiling point of water, an anti freeze/boil additive. Interesting question is if you saturate water with as much salt as it can hold then how much does that increase boiling point by.

[@tarawood-bradley2119]

vegetable oils also have a flash point of 406 degrees C. drum.lib.umd.edu/handle/1903/11333

[@tarawood-bradley2119]

Sand is the storage medium I am trying to think of a way to move that heat from external storage to be usable in the house. An advanced method would be to convert the heat to steam to drive a turbine connected to a generator.

[@SimpleTek]

great ideas

[@michaeljames5936]

I'm hoping there is a series to follow, where you build this. I've been thinking of building something like this for a while. I suspect that is the famous 1% Inspiration, and the years of refinement ahead the 99% perspiration. The pattern of pipes through your sand, whether you heat/extract from the whole mass evenly, or use it as a series of units, with maybe colder areas still maybe having use for a greenhouse, using air flow, and saving some areas, so you still have piping hot water by spring. Control systems are probably the make or break in a project like this. Where I'm from, PV is the only real solar option, but powering resistive, or radiany heat with those, should be very efficient. I wouldn't get hung up on only collecting solar-thermal energy.

[@SimpleTek]

@@michaeljames5936 ok

[@1202Sid]

I'm Canadian situated in Quebec and I've been looking into this for a long time. Like said before we need to follow the KISS principal. I think that using a heat pump to move heat from a lower temp source to a high temp storage might be the way to go. I'll be looking at a 4 port system soon where I can use 2 ports to store/retrieve heat from the storage and 2 ports for house HVAC. Might be even better with only one circuit to control heat transfer both ways from the battery.

[@SimpleTek]

great ideas!

[@donalddalley7274]

Depending on the amount of storage mass, a heat pump is going to, typically, manage low temperatures. The advantage of sand is that it can deal with very high temps. The greater the mass, the higher the temps, the greater the energy storage capacity. If you have a well sealed house with in-floor heating, the demand for space heat may be low. Water heating is different.

[@1202Sid]

@@donalddalley7274 Of course, for maximizing the low limits.

[@kamra99a]

Here is a question for people with experience using sand in tanks: I have heard that if you put round balls into a container and cycle the temperature, because of gravity pushing down, the balls will expand when heated and settle down when cooled. The next time they are heated, they tend to break the walls with thermal expansion. Is this something that also happens with sand? Will the sand tear the tank apart with repeated thermal expansion and contraction?

[@SimpleTek]

Interesting question

[@esatecnic]

No, porque en la arena hay espacios vacios y facilitan la expansion .

[@TheGijzzz]

For a small room/ green house i would say: buy an green egg /ceramic bbq, fill it with sand/ basalt. Stick some 12v ceramic heating elements ptc in it on and connect them the solar pannels. Use the lid to regulate the heat.

[@SimpleTek]

Expensive but might work

[@adytech5788]

This green egg BBQ has alway burn my steak after 1 min ,i like it blue :)

[@rolandroka2082]

Sand battery is probably one of the most interesting idea to store heat energy for a long time. I am thinking about to realize that in a small scale, just for an alternative heating source for my house. Interesting question is, how to transfer heat out of the battery (in my case, intake goes trough electric heating element attached to a solar system)? Well, i think the cheapest version is the air (free available, heat resistant) . Also, you can use it in some kind of heat exchanger to heat up oil, watter or even another separated airflow like room heating with air.

[@SimpleTek]

It is a great idea

[@DanSolowastaken]

Used and spent motor oil might be an affordable solution in some areas. Switching to an oil burner isn't the most eco friendly. However that also allows you the opportunity to meet with more mechanics that might be able to help. Hydraulic fluid is pretty useful as it can take a ton of heat and is designed to be pumped. However you heat it, you can contain it in 55 gallon motor oil drums and cycle it through a radiator. Heliostats are great in sunnier places than Manitoba. Honestly an automated pellet stove hooked up to wifi for the auger might be the simplest solution to what you're trying to manage. Just elevate the hopper that feeds the augur and maintain it as necessary.

[@SimpleTek]

cool ideas

[@rodneymoore7270]

I would run it thru a good filter first and remember that spent motor oil is also a good base to make diesel fuel

[@DaveBuildsThings]

After reading many of the comments, my 2 cents for what it's worth. If you have enough sand in a sufficient volume and insulated then my thought would be to use a copper tube coil buried into the sand at regular intervals. Copper has a melting point of 1,085 °C so the temperature would not affect it so long as it was as solid run. (No solder) I would make the size of the piping about 1/2" to 3/4" to maximize the contact area. A simple blower system can be used to push the outside air into the solar collector and feed the heated air into the pipes buried in the sand. No need for a special air pump as it only blows the outside air in and not the warmed air. Also, if the sand temps are right you could use a few peltier wafers to utilize the heat to run the air pump after the mass has warmed. Peltier wafers or chips are used in thermoelectric coolers. When these wafers are powered they transfer heat from one side to the other, which side the heat or cold goes to depends on the DC polarity applied to them. That's why they can also heat the contents of a cooler. But if they are heated on one side and cooled on the other they produce electricity and maybe enough to run an air blower if combined in a large enough scale. Think wood stove fans with no battery that run on the heat generated. That's how they work. Electrical generation was the intent in their original design. The cooler/heater part was found out later. My idea would not be a fast transfer of heat but in IMO it really doesn't need to be if you're collecting heat throughout the day, week, month, etc. Super efficient? Maybe not 100% but I figure it's a good starting point. Thanks for getting me thinking about this. It might be a great way to warm my large chicken coop (24 chickens) in Ontario Canada in the winter. 👍

[@SimpleTek]

Cheers!

[@Ed_Stoddard]

I created a scaled down version of a mirror concentrator from a Mother Earth News article in the mid 80's. I used a 10 x 10 array of 12" x 12" mirrors that tracked the sun using simple photo voltaic cells, a relay and a car starter motor to maintain focus on the sun. I raised our 24' above ground pool by about 17 degrees F. I only tracked across the sky and had to adjust every couple of weeks for longitudinal changes. Adding a second motor and sensors would be simple, especially with arduino systems available. I like your idea of using oil instead of water to avoid the thermal limit that steam introduces. The article is entitled "Make a Solar Furnace" under the renewable energy category. Using oil instead of water through a system like that would give you the temperatures you are looking for. Using oil in the trough design would allow a simple thermal gravity flow if aligned right. The cost of mirrors like the article do add up quickly. Keep us up to date on your plans. I like it.

[@SimpleTek]

Thank you for the info! I enjoyed that!

[@danf4447]

how did you build or make the frame ? strength..windproofing..allignment?

[@katim3111]

I've seen fans that run on heat so that attached where the heat source feom the sun is and blowing the heated air into pipes that go into sand will heat the sand or water barrels/tanks so heat is held for when the sun goes down. Just a thought.

[@SimpleTek]

Actually a good thought…,

[@clydewxi]

I would look at conducting tubing both electrically and thermally. It serves as both electrical heating element and air conduit. Bury such a tubing array in the sand. Use the output of the solar panels to power electrical heater. Run an air pump to circulate air through the tubing.

[@SimpleTek]

good idea

[@HawkinsMutes]

This is what I was just coming around to. Another option is to mix some carbon black in with the sand and the medium becomes the heater. Would still need copper pipe to harvest the hot air/water though...

[@jonthegman]

I would think a multi-level system would be necessary to utilize the all the heat actually available. I do however have a question... What is the most efficient way to extract the heat once stored?

[@SimpleTek]

That’s a good question… with heat delivery it’s radiant floors, but extraction… I don’t know

[@dustind3502]

Air under the sand should rise through the sand while heating so long as it remains dry and you have coarse sand. Delivery of the air could be as simple as metal piping to molded holes in the air create.

[@SimpleTek]

interesting

[@l0I0I0I0]

I would be afraid of air quality issues with the air flowing through the sand. Yes it would rise and heat but still. Cheaper than piping. If the air could heat separate air or surface UNDER an air barrier then perhaps.

[@dustind3502]

@@l0I0I0I0 if air quality was a problem, a simple heat exchanger would nullify the problem. Dirty air on one side, house clean air on the other. Or wash the sand and use an air filter until the fine particles are gone.

[@l0I0I0I0]

@@dustind3502 yes heat exchanger would be ideal for this build. Also as few engineers seem to know, water vapor also rises slowly through the whole building, and wood framing would not respond well but mold and other nastys would love it! Therefore you would have to install a robust dehumidifier. So as you stated, it would need a robust vapor barrier and heat exchanger which are sold commonly on the market. TY

[@zedostenso3069]

@@l0I0I0I0 Takes a lot of energy to operate a dehumidifier.

[@zeneszpeter]

I am thinking of somethint like this. The main problem with the soil is the heat transfer and the poor heat capacity. If you use a liquid with 300 celsius of maximum temperature, you can heat the sand also maximum to this temperature. The volumetric heat capacity of the soil is about 1.8MJ/m3. Water can store 4.18MJ/m3. The energy you win by the higher temperature, you loose by the poorer heat capacity. What about to build a well isolated huge water tank above the ground? 10 or 20 cubuc meter capacity? You could heat it all year with a simple water based heat collector, then you could take the energy out by a heat pump.

[@SimpleTek]

I agree water is better

[@z4zuse]

The Know You Know channel had an item about Polar Night Energy, 2 weeks ago

[@SimpleTek]

I saw it - but they don't really talk about how to charge and uncharge the sand battery - just that sand hold heat lol

[@H2Dwoat]

Hi, do you have a series of videos following on from this one. Couldn’t find a playlist.

[@SimpleTek]

Not really but lots of other videos in my archives

[@H2Dwoat]

@@SimpleTek cheers, I’ve been checking them out. A bit disappointing the idea has struck a chord with me and the video raised a lot of questions 8 was looking forward to exploring with future videos. Hint: just in case you’re looking for video ideas 🤓🤓.

[@SimpleTek]

@@H2Dwoat I’ll see what I can do this winter on exploring the idea more, thank you for the input

[@scott.ballard]

I wonder if blowing heated air bubbles through the sand from the bottom up would help distribute the heat throughout the sand better than running pipes through the sand.

[@SimpleTek]

interesting

[@jarrettvick4571]

My sentiments exactly! Fluidized sand…..

[@LetsrepLiftingLeague]

Hey I run an induction furnace and a rolling mill. So we manufacture rebar rods. So after its final stage the rebar is about 300-400 degree on the cooling bed. Is there a way to store that much of heat and then translate in to an energy.

[@SimpleTek]

I’m sure there is!

[@LetsrepLiftingLeague]

@@SimpleTek can you point me into right direction.

[@SimpleTek]

@@LetsrepLiftingLeague is need a lot more information but I’m thinking some kind of metal piping using a high temp liquid could extract a lot of the heat

[@robertclark8928]

You could use photovoltaics to run an electric heater to heat air and blow it through tubes in a sand battery. The same hot air would be recycled around in a closed loop and continuously reheated by the electric elements of a heater. I don't know how hot you could get it but I would think you could get it to at least 500 degrees like an oven.

[@SimpleTek]

I like it

[@mattstaat2646]

This is what I am investigating. Right now I'm uncertain on how to calculate the area of heat an individual pipe would transfer into the sand.

[@edclark6543]

Couldn't you just use heating elements embedded in the sand?

[@NeilStansbury]

I've just finished lining the interior wall of my house with 20mm corrugated cardboard filled with very fine sand to act as acoustic barriers and a thermal heat sink, we'll see how well it works later this year. My suggestion would be to buy 9 cheap copper electric immersion heaters. Remove the immersion units and fill each one with your copper piping and sand. Then sink each one of those units into a concrete "jacket" in a 1, 2, 3, 2, 1 layout and surround that jacket with the water. Concrete has excellent thermal mass itself, is easy to insulate all around, has high heat tolerance, and it's water tight. With individual "heating units" units you can easily replace/upgrade when needed.

[@SimpleTek]

I like it!

[@NeilStansbury]

@@SimpleTek Btw, the reason I suggested a 3x3 layout of the immersion units rather than in a circle was the XPS insulation that is designed for below grade insulation comes in large 2.4x1.2m sheets (in the UK anyway), so insulating a square concrete jacket rather than a round one would be much easier. I also suspect running the water through a square jacket would be simpler too.

[@michietn5391]

Talk about this... to transfer heat from wood stove to sand, try air, thru steel pipe. Wood exhaust contains VOCs so we'll need to use a filter or transfer device to keep our hot air clean. For pumping hot air, use the hot end of auto turbocharger, cold end replaced with electric motor. Other thoughts: supercharge (compressed air) the wood combustion chamber, electrostatic exhaust filter at other end.

[@SimpleTek]

Great idea

[@garyd5622]

A 70s solar system used air blowers from a solar collector to an insulated gravel heat storage running year long collecting heat during the summer and releasing it in the winter. Gravel was used to make the air pass through it easier to move throughout.

[@SimpleTek]

Cool! Thank you for sharing

[@matttaimuty5397]

I did some research on this very idea. I learned that asphalt will hold more heat, up to a certain point, than any other earthly material.

[@SimpleTek]

interesting

[@jeffstout6567]

The denser the material the better the heat holding capability

[@michaeljoncour4903]

stinky

[@rodneymoore7270]

In looking into this it appears to be advantageous to put rock on the top of the sand before the aircrete enclosure. In that enclosure heat of course tends to rise and the rocks can store additional heat before the aircrete insulation comes into play .....

[@SimpleTek]

interesting

[@jeffv968]

Could you run steam pipes through the heated sand into a steam turbine and make electricity?

[@SimpleTek]

steam dangerous

[@justinweatherford8129]

Steam can be dangerous if it isn't handled properly, but the same thing is true about extremely hot sand. Lol

[@brianevolved2849]

you exactly

[@ttaylor7777]

The Cp of Sand is about 0.8 KJ/(kg*K). So if you heat it from 0 to 500C you get about 400 KJ/Kg. Water taken from 0 to 100C will give you 418 KJ/kg. About equal to sand with only 1/5th of the temperature change. If you can use steam the value goes up to 2776 kJ/kg. Most good storage media use phase change to provide much great energy density in a small range. If you want to use something for heating or cooling you are better off using palm or coconut oil with a phase change temperature near what you want to use for room temperature. Sand will require too much mass, space, and infrastructure to be worthwhile.

[@SimpleTek]

wow

[@matthewnewton9129]

But you aren’t generally going to be heating water from 0-100. More likely from ambient temperatures to about 80c to avoid pressure problems. So a lot less energy storage. Also you will have steam/evaporation problems with water or degradation problems over time using oil.

[@mbseeking6570]

Which is not a lot of heat. 1kWh = 3600 kJ...

[@victorpresher3661]

What about diverting the heat from the heat source via a culvert type pipe buried underground. You might need to control the temperature of the air as it enters the structure being heated.

[@SimpleTek]

Maybe

[@danlds17]

You're worried about 300 F deg air being too hot for your house. So at a convenient point after you've extracted the heated air from your sand-mass, mix it with a variable amount of external air to achieve a reasonable temp. for your home. The engineering for the mixing area might be expensive (in order to not lose your original stored energy), but I'm not an expert. Or another idea, just pump the 300 F deg right into an old-fashioned floor radiator (the kind originally built for steam).

[@SimpleTek]

Well said

[@tigra-f9q]

For heating the house, I think hot air can be used. That will be easier then setting up additional hot water system.

[@SimpleTek]

@@tigra-f9q ok

[@donalddalley7274]

David Poz has a series of videos regarding his water storage tank and solar water heating panels system. It's just a bit small for his usage, but it gives him up to three days of autonomy. And that is what thermal storage is all about, right? The more days of autonomy the better.

[@SimpleTek]

I know his channel, great stuff

[@BenKoren]

As a thermal mass, the problem with sand is that energy cannot efficiently conduct between granules due to the air gaps between them, so then energy needs to transfer via radiation, which is much slower. This is why sand is known as a great insulator - the pockets of air. If you’re looking for cheap thermal mass, look to “road rock” or “road base”. The specific composition varies by region, but it’s what’s used under roads. It has a healthy composition of large, medium, and small aggregates, and compacts very, very well - meaning that energy will flow through it via conduction, not radiation.

[@SimpleTek]

Interesting… thank you

[@Bucksighter]

What has a better thermal coefficient for a heat battery medium, clay or sand?

[@SimpleTek]

I don't know!

[@rainewalker532]

The problem with clay is that any moisture contained in the clay at the high temperatures would turn to steam which would cause pockets to form in the clay or worse could cause a steam explosion as it heated up in a tight space.

[@wocookie2277]

What if you use sand as the medium? Vertical steel like a grain auger with the worm lifting the sand up the auger surrounded by a solar collector like a parabolic mirror. Use a electric motor powered by solar to run a group of them on a south facing wall. Hot sand falls back into the battery built like a grain hopper to spread the heat. Then collect it in liquid like you described. Gear it so the sand gets good and hot.

[@SimpleTek]

Great idea

[@anomikak1062]

Have you thought about moving the sand? My knee jerk reaction is to say it would cost too much to move the sand except in a separate battery project moving sand is the actual solution to a physical electric battery. I wonder if you could combine those 2 systems. The sand is slowly moved upwards by (excess) solar panel power via a standard motor. But move it upward on a parabolic mirror trough. As it goes up it is storing both potential energy and heat. Once it's hot, drop it off the trough and into your aircrete tank which is 20 feet off the ground or on a hill or cliff if available. When your batteries need charging or you are cold you drop the hot sand down a pipe and collect the electric via inverter and the hot sand lands inside your green house where you blow a fan it to extract the heat into the air. The sand sits in some sort of "sand radiator" until it's cold and you use a microcontroller to open the flap at the bottom and the sand drops out into the base holding tank where it waits for a sunny day to make it's trip back up the mirror trough. I like this idea so much I'm going to try it. Thanks for the inspiration.

[@SimpleTek]

interesting

[@tedrookstool6627]

MOVE THE HEAT NOT THE SAND.

[@necspenecmetu-24]

One major pojnt that seems to be missing in this interesting discussion is how long heated sand would retain its heat before start releasing, and then how long will it take to drain out after heat is being used. How large would be the sand storage to maintain heat during several months, compared to the dimension of the ambient to be heated? I wonder if the sand close to the draining tubes would cool faster and so there would be the need to periodically remix the sand to spread the remaining heat more evan? obviously there are many factors coming into play (speed of draining, dimension, insulation of sands...)... which are the most important? any clue on how to calculate them?

[@SimpleTek]

Heated sand retains heat based on volume and insulation

[@necspenecmetu-24]

@@SimpleTek yes indeed that is clear. but where can I find some more actual detail on how to make some calculations? on the questions I asked the web seems rather vague. partly might be due to the obvious high number of variables, but how to get some starting point? or else, does anyone started some small scale experiment to start digging out these problems? thanks

[@SimpleTek]

@@necspenecmetu-24 good question, deep googling maybe?

[@OyvindSOyvindS]

Hello, and thanks for the video. I am planning a similar project. You mention Aircrete, maybe look at foamcrete instead, as aircrete are prone to water permetation and consequently cracking.

[@SimpleTek]

Thanks for the tips!

[@wildfun496]

Make a manifold out of concrete or any metal or high temp pipe, bury it in an insulated sand box with pipes, and run hot air through it, and at night it should hold heat, also you could just put the whole thing under the house, just use separate pipes that lay on top of the block and go back into the house for heat at night, the out side could have a temp thermostat ,on, off fan could be solar with small battery back up. Also you have radiators, you cold bury a tank or manifold in the sand box, circulate the water through everything, have a high pressure safety valve outside and in. The sand should take a lot of the heat out, just have a shut off for high heat that way the temp won't go too high, or loop the water that is in the radiators somehow, till the temp drops, separate from the solar setup, Just ideas that may or may not work! but good video, and I also subscribed!

[@SimpleTek]

Good idea

[@Poolboy28560]

You move from vac tubes to other solar very quickly. I reciently filled a vac tube with salt. Installed a thermocouple and stuffed the top with insulation. I don’t know how hot it got because the thermocouple quit working at 500F - 260C. These will work year round for heating oil but seals may be the concern.

[@SimpleTek]

very cool

[@willm5814]

What about storing it in a rectangular hole in the ground - similar to what is done when installing a backyard pool - line it with insulating type firebrick on all 4 sides - then fill with sand???? With such a large container the temp can be a bit lower (maybe keep it to 150 C max - enabling the use of less exotic materials to get the heat to your dwelling.

[@SimpleTek]

Good idea

[@garywillis5790]

The higher the temperature the higher the losses. The best way to my mind would be to pump the heat into the ground in the summer, then extract in the winter. The heat wouldn't heat your home for long without the aid of a heat pump, that's the only issue! Greenhouse would perhaps keep it ice free year round.. although the losses from a greenhouse are massive.

[@SimpleTek]

good insight

[@beaurobinson4518]

@@SimpleTek hho

[@beaurobinson4518]

@@SimpleTekhydrogen on demand It burns at very high temps and is easy to generate using solar And you do not have to store hydrogen

[@DavidPaulNewtonScott]

OK retired physics teacher here. As my old friend the late actor Jeff Rudom would say herswachagaddado: Forget water altogether you have done the maths too volumetric basically. Water as a heat transfer medium also sets a system temperature limit of 100Centigrade. Let's get a bit more ambitious here and go for 800 to 1000centigrade. So we need some sort of really exotic heat transfer material like liquid sodium huh? No we don't we just need low pressure super heated steam. You are right about yhe aircrete container but it should be spherical in shape and sealed on the outside against water. Option two get solar panels and a load of old night storage heaters. Remove the cores and put the heaters all together in said aircrete boxes. I am going to set up an offgrid house in Portugal and I am doing something similar to heat my house but I will just use a rocket stove thermal mass heater for house heating. It uses about one tenth of the wood you would need for a wood stove with no pollution. Wood should be the heating choice for all of North America you have so much pine beatle dead wood that if you don't get it sorted you will have forest fires on an apocalyptic scale SOON. So consider this what do you want electricity for at night well you want lights (LEDS) nothing, TV (12V ) nothing radio/music nothing. Even if you do all of this together it doesn't amount to a hill of beans. So you say I want to run a hoover, washing machine fridge etc. These can all be done in the daytime when solar electricity is OVER abundantly avaliable. Modern fridges and freezers will tolerate power outages for up to 17hrs by the way. This is called demand side management. So I am going to set up the thermal mass rocket stove but I am going to do a small size solar powered high temperature night storage heater running winter and summer but I am going to extract heat from it at night to drive a tiny heat engine probably a Stirling engine to drive a generator for my tiny night time electricity demand. This is the basis of ongoing research for renewable grid energy storage heating concreted and has a high efficiency and a charge discharge lifetime running into the millions it also costs peanuts. I am a bit tied down at the moment with covid restrictions but my channel will contain all of this and more so subscribe in readiness it won't hurt a bit. Where's David-Paul?

[@SimpleTek]

that's really awesome, thank you!

[@edclark6543]

What's your channel

[@SemperBlood]

Going in blind. I would go with Iron piping in a sand pit. Just use air to charge it in the summer. And air to siphon out the heat. The higher the temperature the less air you need to move throughout the system.

[@SimpleTek]

interesting idea

[@kimmer6]

Air is not that good of a heat transfer medium. Hydrogen conducts heat 7 times faster than air. That's the lifeblood of every refinery and its in almost every generator over 100 megawatts but it finds every crack and escapes. Liquids are a better heat exchange medium especially if a phase change occurs...gas to liquid, solid melting, etc. Water is the cheapest most available heat transfer medium. I was going to try filling a 275 gallon IBC tote with road base gravel with 6 inches of sprayed rigid urethane foam as an insulator and water as the transfer medium. Ain't got there yet....

[@redstone1999]

@@kimmer6 I thought the same thing. But the plastic of ibc tote needs to be below 160* F (preferably 140* F or less). My plan was using it to regulate water temperature of fish tank at 80*F year round in my insulated fish-house. The plan is down-sized to a 55 gal steel drum with PEX and sand-gravel mix as water heat exchange/bank. The ibc is better used as another 25 kgs of fish raising and water volume is a heat bank also. The floor is 6 " of sand & river gravel and polypipe over 4 " of insulation board (R-20) base. ( I wanted to go 2-3 feet deep and do STHC system. But time frame & finances did not allow it. )

[@kimmer6]

@@redstone1999 I was planning on having a roof company spray 6'' of rigid urethane foam on all sides and top and bottom embedding the steel frame. Then it wouldn't really matter how hot the IBC tank got as it is encapsulated. Make a top hatch to load it with sand/gravel. I had 2 inches of foam sprayed on my roof 18 months ago and the stuff is wonderful. I can walk all over it and not damage anything. My attic fan rarely comes on at all any more. The foam insulates really well. We had 106 degrees F the other day and the house bottled up got to 79F. The attic was 88F. I saw 119 degrees F in the attic with the laser thermometer 2 years ago, and 106 degrees on my bedroom ceiling. Roofing foam is way more rigid than foam in the can. You have a good plan. My aquarium hobby went down the tubes decades ago. My son was 3 years old and turned the thermostat to full. I came home and $1000 worth of tropical fish were dead.

[@redstone1999]

@@kimmer6 Kids ??? Lovable little lords of destruction :D I like the idea of insulating my fish tanks with roofing foam.

[@Bird-0]

Try lining the bottom with a structural foam like foamglas. You may require reinforcing the bottom of the structure through areas of the insulation to take some weight off the foam (it depends on the vertical axis of your tank and the specific density of the sand you're using (I don't imagine any variety you'd want to use being too different from any other). I'm thinking there could be benefit to designing the battery with a larger, flatter footprint (potentially, a roof mounted "sand panel" could work but thickness will increase weight while adding more total thermal storage potential per sq ft of panelling or a wide silo on the ground for larger applications) would allow an air channel to be added. This could be used to preheat incoming air to the HVAC system during the winter by "boiling" air through the sand inside the channel from bottom to top. The channel or pipe would be filled with sand and completely surroubded by sand, so it will easily regain heat lost through use simply through its metal housing being a part of the battery.

[@SimpleTek]

I think aircrete might be better and way cheaper, but good idea

[@Bird-0]

@@SimpleTek AFAIK aircrete requires pressurized vessels to attain the required air pocket stratification throughout the concrete substrate. I'd guess there could be methods to create a similar material on-site but it may not have the same structural properties as the aircrete which is produced in factories. I, personally, wonder how a method using a dissolved gas in the water used during the later stages of the mixing phase could produce the necessary gas for such a material but whether or not those hold during the curing process is something I can't figure out in my head so I can't say whether the idea has merit. I do know that the acidity introduced via the carbonization of water could be a problem for the reactions involved in the setting of the concrete, so another gas may be more viable. Nitrogen doesn't dissolve nearly as readily as CO² but wouldn't lower the Ph as significantly as well as remaining an entirely inert gas regarding oxidation of the concrete's consitiuants during curing (air's oxygen content could make this an issue but I don't have the reactions memorized for concrete so I can't say for certain). Other inert gasses would likely be too expensive (helium makes me wonder how well the pockets would form due to its molecular mass being so low requiring a higher density gas for expansion to occur properly but helium isn't exactly cheap anyway) so I'd actually say CO² might be the best option, though I'd say it would have questionable long-term integrity when compared to factor-made aircrete.

[@SimpleTek]

@@Bird-0 lot of big words there, tons of people make air Crete at home on KZbin.

[@tonyarnez5193]

Check out Korean traditional floor heating system ... basically a giant rocket stove under the ground that could heat the sand which will radiat a constant temperature all night !

[@SimpleTek]

very interesting!!!!! Thank you

[@michaelglenning5107]

Sounds like the Russian system as well.

[@SimpleTek]

@@michaelglenning5107 which Russian system?

[@michaelglenning5107]

@@SimpleTek Russians would build a slightly off horizontal chimney in the ground. The fire box would be below ground. Seasonal solar heat storage is the same concept. Heat from the summertime is pumped through a horizontal underground air duct system. In winter the heat works it's way up. I see numerous people venting excess heat buildup in the greenhouse outdoors when in fact it should be transfered to the ground underneath it.

[@michaelglenning5107]

kzbin.info/www/bejne/bYG7l6mcj5KVY9E

[@yager4092]

What about air Creek I haven't searched the temperature of concrete concrete but I think I think we can do something with that that might work

[@SimpleTek]

It’s a great possible solution for insulation!!!

[@metatechnologist]

Regular store bought PVC pipe can go to 140 F or 60 C. While it won't be the primary "coil" it certainly might work in a return/transfer circuit.

[@SimpleTek]

need more heat!!!!

[@Dominick13777]

Could I hook up the exhaust of a mini split to help charge the battery and if so how?

[@SimpleTek]

interesting idea

[@kristalmckinstry2095]

Consider a concentrated reflector system directly heating an air-crete box of sand with one window wall lined on the interior with cupric-oxide. An air gapped box around that box (thus two layers of window) could collect heat losses of the sand box into a lower temp storage like water.

[@SimpleTek]

interesting idea!

[@jimargeropoulos8309]

I've been thinking about this all day. 1. Take a page from Sergio(so.?) And test as much as you can before committing to expensive endeavors 2. Stick to simple Tek, even if it means you can't capture as much heat. The cost and likely the risk will be far lower. To me that means store lower temperatures and use simpler more common parts. 3. Start small. Don't soon to heat your house on the first go. Maybe just a room. That way if you fall, not much is lost

[@SimpleTek]

nice

[@andrewb9595]

Love the idea of using aircrete (which I was thinking of doing myself) for the insulation on your storage tank. You could build the tank entirely out of aircrete and rebar OR you could buy a metal water tank (like a livestock tank) and insulate it by filling aircrete around it. Storing heat that will be used for heating is easy enough, but storing heat and transferring it into electricity to cover the other 40% of your homes electricity demands when the sun isn't out is the real challenge.

[@SimpleTek]

great ideas!

[@l0I0I0I0]

Andrew, love the idea of storing water in livestock tank and insulating with aircrete. May need to waterproof aircrete as water may seep through it and exposed parts may crack with ice allowing water and thus corrosion as long term issues. Great idea however. Water even at lower temps under 200c is still great at storing heat just requires more space to meet total needs. Yes storing heat does not account for the rest of electric bill but it's a great start and reduces the rest. If one uses high temp solution, the stored heat can be used for cooking! That's cool! May be more practical to have a dedicated system for cooking to reduce cost such as piping, building etc. and for simplicity. May be easier to have solar panels and batteries for the remaining electrical needs. That's my guess. Turbines for electricity are cost prohibitive to my knowledge.

[@andrewb9595]

@@l0I0I0I0 Batteries are also pretty cost prohibitive if you have grid access. As for cooking and/or energy generation when the sun isn't out, a methane digester (piped directly to the stove or to a generator) wouldn't be a terrible idea. Also instead of solar panels, I've been looking into the idea of using a parabolic trough collector to generate electricity via a sterling engine and then collect the water after the sterling engine as stored hot water which can be recycled in a closed loop. This would be one hell of a project, but it'd be totally off grid depending on your energy demand and geographic location.

[@l0I0I0I0]

@@andrewb9595 keep me in the loop! Want to see what you come up with. Biogas is a thing. It's common in India I have been told and perfectly clean. Big or cow poop may work better than human.

[@ericslayton4779]

@@l0I0I0I0 hydrogen sulfide is not clean but can be filtered out with activated charcoal and lime water.

[@riverhouseparadiseproject3298]

I've seen videos of simple electric ceramic heating nodes. They seemed to get to incredible heat over time with little electricity. If i recall they could be placed under a copper sheet to transfer a larger area of heat up into the sand. From there I don't know if simple air tubing or liquid tubing would be better. I would think air that could then be moved to a 2nd area for conversion as you mentioned.

[@SimpleTek]

Well said

[@CS-uc2oh]

12v ptc heaters they are called

[@jeebus6263]

Put resistive coils in there, even if you don't plan to use them at the moment.

[@SimpleTek]

humm, good idea!

[@jeebus6263]

@@SimpleTek I'll give credit where due, Dirt Simple Energy Storage | In Depth kzbin.info/www/bejne/qp6apKODfJWWnpo