What are Sand Batteries? Everything You Need to Know

News about a sand battery from Finland recently made headline news, although that system only stored and released heat.

Thermal heat storage is not a new technology, but this pilot project highlights some of the future possibilities of green energy technologies and systems.

This post looks at the technology surrounding sand for energy storage, as well as similar technologies, and what they mean for the world.

Energy From Sand?

Natural sand possesses many qualities that make it an ideal medium for thermal energy storage. You can heat it to temperatures beyond 1,000°C (1,832°F) without issues and it can hold that heat for days, weeks, and even months with minimal loss.

If you consider that a battery is a means of storing energy that was produced at a particular time, so it could be used at a different time, then sand that is heated with electrical energy for storage and later usage, is a battery.

The Viral Sand Battery From Finland

In the western Finnish district of Kankaanpää lies a patented heat storage energy system developed by Polar Night. It uses excess electrical energy from renewable sources to heat sand in a 7-meter high and 4-meter wide silo up to 600°C (1,112°F) for storage and later utilization in the district heating network.

A few things are important to keep in mind here. First, the energy used is excess generation from renewable sources such as wind and solar. This removes any objective comparisons to other energy storage systems for commercial purposes.

Secondly, the system is used solely for thermal storage and delivery – that is, the electrical energy is converted to heat and stored in the sand. Then, when needed, the heat is extracted and distributed to homes and factories where it is needed.

Thirdly, natural sand can hold quite an impressive amount of energy. For instance, this Finnish Polar Night battery holds 100 tonnes of sand at about 600 Celsius, for a total of 8MWh of stored energy at a 100kW heating capacity. This makes sand an insanely cheap energy storage medium without fancy technologies, installations, or dangerous requirements.

About Seasonal Thermal Energy Storage

Seasonal Thermal Energy Storage or STES for short, has been around for a very long time. In its simplest form, you could collect hot water from the roof during summer and save it in an underground tank, which you can then use for heating during winter.

Most STES systems, however, store heat at less than 100°C, which makes them okay for heating homes and offices, but less ideal for other industrial uses or power generation.

The method is simple, you expose any medium that can trap and hold heat to a source of radiation, such as the sun, industrial heat waste, and so on. The efficiency of the system depends on the heat-exchange method and its efficiency.

Next, you will have to store the heated medium in an insulated enclosure, so as to minimize energy loss. Some enclosures can hold heat well for many months.

Finally, the storage medium is pumped out during winter to provide heating to homes and offices by passing it through another heat exchange such as a radiator heater. Common materials used as STES storage mediums include water, oil, soil, salt hydrates, and so on.

Popular Uses Of Stored Thermal Energy

Stored thermal energy has many uses, depending on the intended application. Here are the most popular ones:

1. Heating Homes & Offices – Stored thermal can easily provide heating for living and working spaces in winter.
   
2. Hot Water – The heat can also get transferred to provide always-ready hot water for everyday uses.
   
3. Industrial Applications – Hot water is used for a wide range of industrial applications, from mixing to cleaning, food processing, making solvents, sterilization, and much more.
   
4. Electricity Production  – You can also use stored thermal energy to heat water to steam and have it drive turbines, which in turn, drive alternators that produce electric power.

The Economics of A Sand Battery

Water can store more energy compared to sand, but it becomes unstable from 100°C (212°F) upward, while sand can easily contain 600°C (1112°F) temperatures.

Water will also retain its thermal energy longer than sand, which makes water a better medium for seasonal energy storage. However, if you are considering an application that uses up the heat within hours or just a few days, then sand becomes a top option again. It is perfect for complementing intermittent energy sources such as PV solar and wind.

Back to the Finnish sand battery, the 7-meter high steel container is designed for 100 tonnes of sand, which hold up to 8MWh of energy.

To put it in perspective, the average US home uses about 10MWh of energy per year, while that number varies in Europe from around 2MWh in Romania to 9MWh in Sweden. In addition, 30-50% of energy is used for heating through winter.

This means that a 7-meter high sand reservoir can produce enough power to heat a few homes through winter, and depending on your location. But it would be an impractical application in urban centers with a dense population, given its size.

Converting its 100kW heating capacity to electricity at 30% on the other hand can produce enough power for over 20 homes during the day, and many more homes at night.

So, properly optimized, a sand battery that costs about $5 per kWh capacity can be a great alternative to the current $100+ per kWh cost for lead-acid and lithium-ion battery systems. Yes, it might be bulkier, but it’s way cheaper.

Sand Batteries For Electricity Generation

The storage of thermal energy for later use in electricity generation is a proven and reliable technology that has been implemented in Concentrated Solar Power (CSP) projects for decades.

Energy in a modern CSP system is trapped by concentrating hundreds or thousands of mirrors in a single furnace. These mirrors then track the sun through the day to guarantee constant heat at the furnace up to 565°C (1,049°F).

CSP installations are often very large, spanning millions of square feet (~1+km2) in area, with their solar receivers in the center and electrical generating capacities in the 100+ Megawatt range.

A molten salt mixture with 60% sodium nitrate and 40% potassium nitrate is used to store energy in CSP systems for night-time generation. Unlike the sand battery, however, this salt mixture melts at high temperatures to make it flow like a fluid.

Both CSP and sand battery systems convert solar power to heat energy at roughly equal efficiencies of 15-20%. But while CSP molten salt systems have about 50% efficiency in converting the stored heat to electricity, the Finnish sand battery has a theoretical 20-25% efficiency.

CSP systems are commercially viable, so if you can tweak this Finnish battery to get above a 30% heat-to-electricity conversion efficiency, then it can become a viable technology to cheaply store and supply renewable electricity.

Similar Storage Technologies

There are many other forms of energy storage, each with its pros and cons. The most popular types include:

1. Electrochemical Energy Storage – As you would find in batteries, this leverages the potential difference between two elements to store and release energy using reversible electrochemical reactions.
   
2. Mechanical Energy Storage – This involves different methods including the use of flywheels and springs, as well as gravitational systems that store energy in an object by winching it up and increasing its altitude.
   
3. Molten Salt Energy Storage (MSES) – The storage here is thermal, such as by using a combination of 60% sodium nitrate and 40% potassium nitrate.
   
4. Thermal Hot Water – This method can store up to 6kWh of energy in a 50-gallon hot water tank.
   
5. Pumped Hydro – The cheapest form of energy storage. Its major issue, however, is the limited locations where it can be implemented.
   
6. Compressed Air – Similar to hydro, this method simply compresses air to store energy. Then when you need the energy, you release the compressed air to power a turbine.
   
7. Flywheel – You simply use energy to rotate a well-balanced wheel, storing it thereby as kinetic energy which can either be used for locomotion or electric power generation.
   
8. Flow Battery – This is an electrochemical storage system where the electrolytes are in different tanks and have to flow from a fully charged tank to an empty charge tank. Then to charge the electrolytes, you simply reverse the flow. This method can produce very powerful batteries as the two electrolytes interact through a membrane that you can scale extensively.
   
9. Phase Change Materials – These materials absorb energy as they melt, then give it away as they solidify. They are ideal for storing thermal energy at precise temperatures.

Frequently Asked Questions (FAQs)

Conclusion

We have reached the end of this exploration of sand batteries and their economic potential. And as you must have realized, they offer many possibilities.

From providing heat to communities to electrical power generation, the dirt cheapness of silica sand makes it a promising medium for future energy projects.