Sand Batteries

What is a sand battery? What do you think it is would be my answer as people always assume (incorrectly)that is stores electricity and any battery is simply an energy store and it doesn’t really define the type of energy it is storing and it can technically and precisely be any form of energy. You cannot store electricity as you can only store the chemicals for the chemical reaction to create electricity; imagine going to the filling station with your 20L metal jerry can and asking for 20 litres of electricity, would that be unleaded or super unleaded electricity?

What is thermal mass should be the next question and thermal mass is anything which can be heated quickly or slowly and it stores the thermal energy which by any other name is heat and the best stores of thermal mass are sand, clay and concrete and concrete is only sand and cement, so thermal mass is any material which can be heated up and it slowly releases this heat for hours. Is thermal mass confined to sand and concrete? No because these are the best as they can be heated up to over 650°C and they will hold heat at over 500°C and they will release this heat for many hours. Can water be used? Actually yes but it has many limitations and the first is maximum heat capacity as water boils at 100°C so your thermal capacity is severely restricted unless you generate steam to heat your sand battery and to get anywhere near the 650°C you would like it would have to be superheated steam and that is dangerous and limited in industry now and certainly shouldn’t be used at home as you cannot see it and it will cut limbs off.
How much energy can a sand battery store? According to my maths if you can get the sand to a temperature of 650°C it can store over 4.5Kw of heat per kilo of sand so it is not limited by temperatures the same as water and the quantity of sand you can heat means you can store more heat for longer as the more heated sand you have, the longer it takes the sand to release the heat.

Can it be used to heat your home, actually it is possible but would take a lot of work and practically it would take up a lot of space and cost a lot of money and if you intend building something such as a rocket stove using a small 15/18kg gas bottle they should hold a 20Kg bag of dried sand to act as the insulation material in the cavity between the burner tube and the outer housing and 20Kg X 4.5Kw = = 90 Kw of stored energy or 90Kw of stored heat means it will be released slowly for at many hours. This makes it economically viable for heating workshops, sheds, and greenhouses and any outside building and working in your garage during the winter and a sand battery heater will mean heating for around 1 hours burning time you can generate many hours of heat which will be released long after the flames are extinguished and you can spray paint without fear of a vapour explosion making this viable.

Imagine your greenhouse fire being lit at dusk and burning for an hour and after going out it has produced so much energy for your sand it will slowly release this heat all night and instead of having enough fuel in the form of logs you can use much thinner and smaller material and thin branches and thick twigs become viable fuels and you need to store much less of it meaning you gather much less of it.

Up to now we have looked at timber to create our energy and this is not the only option as we can modify this to make it clean and free from fires and powered totally by solar panels and the upside is the battery will be hotter in summer and cooler in winter when we really need more heat and this assumes you have spare solar panels and basic electrical knowledge. If you get a new heating element for a kettle and you note its resistance in ohms Ω you can work on the premise that you fill a metal bucket with sand and connect the kettle element to your solar panels this can be used as your thermal source of heat for your battery.

Other thermal mediums can be used and with crushed rocks and molten salts as the thermal mass you can achieve well over 1000 °C using these materials but for our uses we will confine this to sand as it is readily available and cheap and the ideal medium for experimenting with for the home user and the ideal material for rocket stoves as they can achieve the 650°C.

Realistically many people won’t want this amount of heat and they can experiment with lower temperatures such as 200°C sand batteries and use the lower temperatures more efficiently such as a small resisted wire sand battery driven solely by solar panels through the day and it would store enough heat to see it through the night. To do this you need a larger sand battery and the heat and time to heat it all up sufficiently to allow it to operate throughout the night.

Finland installed the world’s first commercial solar battery in 2022which consisted of a steel box containing 100 tonnes of sand and the idea was to heat it throughout the summer sufficiently and allow it to work through their cold winters and it worked, harvested heat all summer and ran through sub zero temperatures all winter.

There is nothing new about thermal mass as they were used commercially as storage heaters in which the old Economy 7 gave very cheap night-time electricity which heated up an element in the storage heater and this was immersed in oil which had lots of rocks submerged in it and the element heated the oil which in turn heated the rocks which reverse cycled. They emitted their heat back into the oil which heated the radiator body and this emitted warm air into the room for several hours throughout the day and they worked, but not cheaply and unlike gas they could not be turned up for instant heat.

How does gas central heating work? Basically the boiler heats up water passing through the combustion chamber to 80°C and this water is pumped to your radiators which emit this heat at around 80°C as this is the ideal temperature for the constant and instant heat so your home will be warm throughout the day and as you come in from work you crank the temperature up and within 10 minutes your house heats up to your desired temperature.
Why 80°C? because your heating pipes and pump are designed to pump water at this temperature to the radiators which ae also designed to emit their heat at this temperature as this is the optimum temperature to fulfil the functions we desire. If we get up at 6.30am and we set the temperature as the background temperature of 16°C it will maintain that all night as we are in bed and covered with blankets or a quilt and we don’t need the normal living temperature and at 6.00am we set the timer to crank the heat up to 20°C and it will fire up to 20°C within about 10 minutes and if we leave it set to this temperature for 1 hour it will be set at 20°C until 7.00 o clock which normally gives you time to get up, have a shower if that is your routine and remain warm. If you have your breakfast after this shower you will be dressed and go for your breakfast and if you have it at around 7.00 O clock the heating will switch back to the background heat of 16°C and it will normally take 30-60 minutes to cool down and if you leave for work at 7.30 your home will still be warm. Recommended minimum home temperatures are 18°C and remember this is the recommended minimum and it is higher for elderly, disabled, or those with limited movement which is why hospitals and old people’s homes are always warm. When these adverts claim you can turn your gas boiler output temperatures down to 60°C you are pumping water around your system at 60°C and at this temperature it takes around 3 times as long to raise the temperature to the 20°C you require and while this suggests you are saving gas and money it is a total lie and as it takes around three times as long to warm up to 20°C and while you are saving gas heating to only 60°C it balances out that you are using around twice as much gas to do the same job and we know who the winners are.
Radiators and heating systems are designed to work at 80°C and this includes everything from the flow rate of the water and the radiator size and if you turn it down to 60°C your radiators are now too small and the flow rate is too low and to be anywhere near efficient would require radiators twice their current size.

Thermal mass is nothing new as man has lived below ground and in caves and the temperatures are consistent and he has dug pits and filled them with rocks and set fires on them and let the fire go out and cooked on the hot rocks and thermal mass has been firmly established for centuries.

How does this affect us? Because we can experiment and make our own sand batteries and I am currently running one using a rocket stove in an 18Kg gas bottle with an 80mm square burner and this is insulated with sand, this flue runs into a second gas bottle and the flue bolts to the first pipe and runs nearly to the top of the second bottle and exits into a 4” round flue and up the chimney and the second bottle is filled up to just below the flue and in total it took 65Kg of sand to fill it and running it for around 1 hour heats he sand up to over 350°C in the bottom bottle and slightly warmer in the top bottle and this heats his 32’ X 12’ greenhouse and keeps it at a high temperature for around 14 hours and it burns twigs and old cuttings so he doesn’t have the problem of storing a lot of wood and it is fired with a lot of willow.

very similar concept to yours!

This is fantastic ,

I as I am sure many will be am dreading the uncertain future that is quickly unfolding faster than I for one envisaged ,

So as a basic way of extending your wood burner output on a night time would be literally be a cake tin , metal bucket etc on top or on a trivet throughout the day . I'm thinking as I sell them and they are relatively cheap and once purchased can be used over and over , The clay pebbles used for hydroponics would these have the same if not as hot effect ?

Hi memegirl , clay pebbles are mostly air and very porous ,so not ideal for thermal mass . The more dense the better , rocks,sand,cement/concrete,gravel,metal or even water will work well.

we have a 20 ltr pot with lid on our stove which provides constant hot water that can be used in the kettle and boils in a min or two instead of 5 to 10 for a full kettle which reduces our daily leccy use a fair bit. It also acts as thermal mass and gives out its heat for a fair time after the fire goes out and is often still warm enough for a wash the next morning.

Thanks lopsum ,
I was wondering wether they may explode ,
I used to sell fire bricks and know that some blocks can explode hence fire bricks are used ,
I've dug out some old stock and they're more lumps of clay rather than the balls , I'm giving them a go 2 kg fills a big metal bowl ,
We used to have an enamel kettle on the stove all winter but it went a bit manky inside , we use a coffee machine from the charity shop as it heats just enough and keeps it warm for a few hours for top ups while we are working in the polytunnels .

Fire bricks are actually insulated and sand is used as Lopsum states, because it excludes air, ballast is better for longer burn times as this had ballast stones and sand inbetween so it excludes the air and conducts the heat through the sand and into the stones and this can retain heat for days.

Lopsum

Water does not work well as its limit is 100 degrees whereupon it turns into steam.

Rocks and sand can easily go up to over 650 degrees C and this aditional heat is vital for longevity.

maybe not for greenhouse heating, but if you can utilise the hot water like we do it is fine.

Lopsum

You have identified something and this is usage or intended usage and if you apply yourself you can have both, one of the prototypes I have built actually has a copper coil around the combustion chamber and the sand surrounds the while lot and as this is thermo syphon it pumps hot water all day and still retains its thermal mass and accordin to my calculations it should heat a bungalow easily.