Sand battery, thermal energy storage

Some excerpts:

How do you heat the sand?

With electricity from the grid or from local production, in both cases from fluctuating sources such as wind and solar. We charge it when clean and cheap electricity is available. The electrical energy is transferred to the heat storage using a closed loop air-pipe arrangement. Air is heated up using electrical resistors and circulated in the heat transfer piping.

How long does the sand stay hot in the winter?

It can stay hot for months if needed, but the actual use case of the heat storage in Kankaanpää is to charge it in about 2-week cycles. The heat storage has its best range of use when it is charged and discharged 20 to 200 times per year, depending on the application.

You can read more about the efficiency of the thermal energy storage from this blog post: Sand Battery’s Efficiency Explained – Polar Night Energy’s ‘Sand Battery’ Has Efficiency Up To 95 Per Cent.

Can it store electricity?

Not as such, as it stores energy in the form of heat. The heat can be converted back to electricity using turbines like the ORC-turbine or a steam turbine. This requires additional investments to the turbine technology, and the conversion to electricity has inherent losses, thus complicating the economical side.

Is this a new technology?

Well, yes and no. The idea of heating sand to store energy is not new. Our way of doing things and commercializing it in large scale applications is.

Can I buy your heat storage for my house?

No. We don’t have products for individual homes yet.

Been thinking about something like this ever since I move here in the high desert.
Almost every day here is sunny, and while temps in winter approach or hit freezing at night, by 10:00 AM you are sweating if out in the sunshine. Daily winter high temps are normally in the 50’s and 60’s degrees
Fahrenheit. So what usually comes to mind for me is some way to directly store the solar heat during the day, by heating maybe sand, gravel, soil, etc. using sunlight, possibly using a glass covering to help contain the heat. By the way, we have a lot of black sand around here mixed in with the regular sand, which separates in streambeds. It’s actually an iron ore that can be used to make iron using a charcoal furnace with bellows. Anyway, it seems like there might be a way to use such materials to directly store heat energy in the day, for use at night and early morning, for heating. Literally an almost stone-age concept, but somehow we’ve missed taking advantage of it, except for the fact that here, a well-insulated house seldom gets below 50 degrees Fahrenheit inside, even without heating, so the demand is not huge. Anyone have any ideas on how to make something like that work?

I thought of something similar. But:

As we have learned in the past few weeks, light leaves a source such as the Sun and travels in all directions, (including) to the sand and to the water. Sand is a poor conductor of heat so when the Sun warms the sand, the heat is not distributed evenly, making the surface hot. Also the specific heat of sand is lower than that of water so when both are exposed to the same amount of light, the sand at the surface will have a higher change in temperature.

Water is a good conductor of heat so the heat from the sunlight is evenly distributed throughout the water. Also water has a higher specific heat than sand so it will not warm as much when exposed to the same light.

Conversely, water cannot exceed 100 degrees Celsius, while according to the designer, sand can reach or even exceed 500 degrees Celsius.

About the insulation of sand battery, there is an interesting description:

1. Since sand is solid material, the heat is transferred inside the storage only by conduction. As the heat conductivity of the sand is rather low, the outer parts of the storage act effectively as insulators for the core and thus there always is considerably steep radial temperature profile inside the storage.

Simply put, unlike for water-based storages that have constant temperature everywhere, the outer layers of a sand-based heat storage have temperatures much below the average temperature of the system and the heat does not flow effectively from core to the outer layers and finally to the ambient space.

So is there a way to heat the sand battery using desert surface heat instead of electricity, perhaps using the principle of heat pump?

There is Passive solar building design - Wikipedia and Solar thermal collector - Wikipedia

Good vacuum insulated thermal collectors can get you water close to boiling temperature I think. I’d use those, or use ones you make yourself, to heat up a water tank to heat up radiators in the morning.

Or use a heat pump. Or lower your thermostat and put on more layers.

Oil based vacuum solar thermal collectors. I’d be interested to know what temperatures those can reach and what the losses are, and so on. The higher the temperatures the higher the losses probably. Maybe they are more impacted by cloud cover? Would they even work at higher latitudes? Sounds like a hassle and you’re more likely to get something working and ready for sale if you just put a resistive heater in your sand box. Which would also allow your system be charged in those cases where you would need the heat, like when there is no sun.

What about

  1. sand immersed in water? (But would the sand even be relevant at that point?)
  2. Water with dissolved black ink
  3. Sand, gravel, with a low-power circulating fan to distribute the heat to lower layers, or in the case of black ink water, or water in a black tank, a circulating pump to mix the thermal layers?
  4. maybe just a black water tank, which many are?
    Only problem is what parts to insulate and what parts to not insulate, so as to allow the heat in without losing it when the sun goes down or even moves across the sky…

Maybe a blanket or insulated door that covers the hot material when the sun goes down.

Seems like it would not exactly be rocket science to get something like that working though…

At this altitude (3600 feet ASL) I can place a thermometer under a roof at my entry, still totally outdoors, and it may read, say, 36 degrees F. at 6:00 AM. I can then move it out where it is facing the dry, open sky, and within a few minutes it will read 31 or 32 degrees, simply by radiating its heat to outer space.

Also, frost will form on grass, vehicles, and other plants or objects in areas open to the sky, but not on outdoor plants or objects below a roof.

If it is cloudy, with no open sky, the thermometer will read the same in both locations.

Infrared radiation is a huge, yet largely unappreciated factor in how warm or cold we feel, or even what a thermometer will read.

Adding additives to the water would just gunk everything up.

3 - You could use metal rods or (water or oil filled) pipes to distribute the heat through the sand box. That’s probably how it’s done anyway. Inside an insulated water tank you don’t want or need to add to heat loss by adding forced convection.

4 - is standard and easy. You add thermal mass, like concrete or black water tanks exposed to sunlight, somewhere indoors. See the Wikipedia article.