Seasonal Thermal Energy Storage Gets Hot With a 'Sand Battery'

Thermal batteries are in the news and everyone is so excited! Headlines range from the BBC's "'Sand battery' could solve green energy's big problem" to The Next Web's over-excited "What the hell is a 'sand battery?' And why am I so damn excited?" So why am I so blasé? Probably because the sand battery being discussed is basically just a form of seasonal thermal energy storage (STES)—a concept that has been around for decades. 

The basic principle is that in northern countries, there are long hot days of summer with more solar energy than one can use and short cold days in winter when lots of heat is needed and there is not enough sun. At Drakes Landing in Alberta, Canada, solar thermal collectors gather heat and dump it into an insulated sand and rock storage area underneath the park and draw out the heat in winter, covering 90% of their heating requirements. In Vojens, Denmark, the world's largest thermal battery uses water in an old gravel pit. Excess heat from district heating plants generated in summer is stored and pumped back out in winter. At Toronto Metropolitan University (where I teach) engineers are storing heat in concrete building foundations, which is really clever.

The difference with the "sand battery" in Finland from Polar Night Energy (PNE) is they use the excess electricity from solar and wind farms and run it through resistance heaters—nothing fancy, just your regular toaster-type heater—and get the sand really hot. Like up to as much as 1,000 degrees Celsius—over 10 times as hot as the storage at Drakes Landing. That means they can store more heat in less sand.

On the PNE website, they explain: "Inside the sand we build our heat transfer system that enables effective energy transportation to and from the storage. Proper insulation between the storage and environment ensures long storing period, up to months, with minimal heat losses." 

They also note they have taken everything into account, including the embodied carbon of the construction. 

"CO2 emissions of our heat storage are embedded emissions from construction materials and from the construction phase. As these embedded emissions of the PNE heat storage are minor, the emissions of produced heat result mostly from the source of electricity. It can be said that the heat taken from our storage is as clean as was the electricity fed into the storage."

It can also be said the embodied emissions could have been even lower had they dug a hole in the ground, insulated it, and then filled it with sand. Or, as they note, sand is cheap—they could have used a lot more and heated the sand to a lower temperature, and with a smaller delta T or temperature difference, needing less insulation. The district heating system and the swimming pool being supplied from this system don't want 800 degrees C water. 

If you want to pound sand and do this yourself, Roger Abdo of HydroSolar in Quebec goes through the math for building your own underground seasonal thermal energy storage (USTES) system that stores the heat from vacuum tube solar hot water heaters, which are a lot more efficient at converting sunlight to heat than solar panels powering a toaster coil. Conceptually, it is no different from what PNE is doing.

Even engineer and co-founder Markku Ylönen gets this and downplays it to Euronews: "There's really nothing fancy there. The complex part happens on the computer; we need to know how the energy, or heat, moves inside the storage, so that we know all the time how much is available and at what rate we can discharge and charge." 

None of this is to diminish Ylönen and Tommi Eronen's accomplishment here; they have taken the concept of STES and put it into a neat package of hardware, including a silo that just happens to look like a giant battery, and software that customers can order up, park anywhere and just add sand. They never once call it a battery—just heat or thermal energy storage.

On its press page, PNE notes the Polar Night Energy story went viral after the BBC picked it up; it may be that Matt McGrath of the BBC coined the "sand battery," a much sexier name and a clever analogy, especially when everyone is talking batteries these days. PNE should pay him a royalty.

Then McGrath writes that "right now, most batteries are made with lithium"—they're not; lead-acid batteries are 70% of the market—and equates storing watts of heat with watts of electricity. He carries the analogy to the extreme in the last few seconds of the video, saying "This idea still faces some challenges: can it store electricity as well as heat?" The short answer is no.

It's why I keep writing posts like "Why Every Home Should Be a Thermal Battery"—batteries are hot. Polar Night Energy is hot. But perhaps this coverage is getting a little overheated.