Why This Sand Battery Matters – Thermal Energy Storage Explained

As you know, efficient energy storage systems are the conundrum of making the most out of our intermittent renewable energy generation. Unless you’ve had your head in the sand, it’s a glaring problem we have to solve, which is why so many different battery technologies are being explored and developed over the last few years. To the point that some of them are now coming out of the sand … that’s what recently happened in Finland where the world’s first commercial sand battery went live this past July.¹ But how does it work and is it a viable path for storing energy? Let’s see if we can come to a decision on this.

Why storing renewable heat

Before digging into the sand battery, we should first warm up some background on thermal energy storage (TES)², which is the technology behind this new invention. If you watch my videos on a regular basis, you’ve probably noticed that I’m on a bit of a hot streak recently. I’ve talked about heat storage with phase change materials and molecules, as well as concentrated solar power plants. Chemical storage isn’t the only option we have to stock up renewable power. You can convert renewables into heat and store that instead. In other words, you would have a thermal or heat battery. But why should we care about storing zero-carbon heat? According to the Energy Information Administration (EIA), heating, along with cooling and ventilation, accounted for 46% of US buildings’ energy demand in 2021.³ Yet, this power comes from dirty sources. Over 50% of new American homes still rely on natural gas for space and water heating.⁴ Also, based on a recent study, warming up water in our houses & offices and fueling low-temperature industrial processes such as brick and food drying releases 10% of US energy-related CO2 emissions.⁵ You may see why developing more thermal storage units will reduce our reliance on fossil fuels, which can help shrink the climate impact of our heat consumption.

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So, how does the technology work? Sensible heat storage is currently one of the most widespread TES solutions.⁶ Basically, you heat up a liquid or a solid material by harvesting wind or solar energy during the day or in summer, when there’s plenty of it. The typical way of doing this is to pass electricity through a heating element in contact with your storage material. To discharge the heat, you simply lower the battery temperature by piping in cool air. Your heat transfer medium can be something as simple as water or more complex compounds like molten salts, which are typically integrated with concentrated solar power (CSP) plants, like I covered in a previous video.⁷

A thermal storage as simple as dirt

One of the main reasons we need to develop more energy storage facilities is to minimize curtailment for renewables like solar and wind. That’s when we have more energy output than demand. A good example of that is when you see wind turbines standing still on a windy day. They’ve turned them off because there’s no demand for the energy being generated. As predicted by the Australian Energy Market Operator (AEMO), over 20% of renewables will be curtailed by 2050.⁸ That’s why TES systems could play a role in getting us out of hot water. The Sweden-based utility Vattenfall will soon fill up a 45-meter high tower (around 150 feet) with water in Berlin.⁹ Fun fact: the volume of water sitting in their tank would fill 350,000 bathtubs¹⁰ … although, I don’t think I’d want to take a bath in their tank. Their battery is next to a power-to-heat (P2H) plant that harvests spare wind energy from the grid to heat up the water to just under 100 °C (212 °F). Expected to be up and running in April 2023, their giant thermos will keep water hot and supply it for up to 13 hours when needed. This sounds promising, but what if we could clean up our dirty heat by using dirt?

Recently, the Finnish startup, Polar Night Energy¹¹, added a new toy to the heated sandbox … a 23-foot tall¹² (about 7 meter) steel silo containing 100 tons of low-grade sand and a bunch of pipes.¹³ But they’re not using this storage medium to build a sand castle in the Finnish polar night sky. After running a 3 MWh pilot in Tampere to heat up a couple of buildings, the startup fine-tuned their design and scaled it up. Teaming up with Vatajankoski, Kankaanpää’s district heating network operator, a larger battery is already heating up local homes, offices and even a municipal swimming pool, serving around 10,000 people overall. And they could replicate this anywhere in the world where you have a district heating infrastructure. Just like in New York, Boston, Philadelphia, San Francisco, Denver, Minneapolis and other major US cities.¹⁴ As touted by Polar Night Energy, their latest battery model can store up to 8 MWh of energy as heat.¹⁵

Aside from environmental benefits, this storage solution allows the city to save money. Winter over there seems never-ending and Finnish temperatures can be nearly as low as those in Alaska. As you can imagine, the huge heat demand during the long cold season translates into high costs. On the other hand, the sand battery will store clean power during summer when it’s more readily available, and then reuse it during the long winter when there is little sunshine. That’s why digging into it is financially handy. Nowadays this would have a higher impact than ever. In fact, after a recent payment dispute, last May Russia froze their gas supply to Finland,¹⁶ so you can probably see why they’ll need a lot of low-cost heat to keep themselves warm. Local governments are pretty pumped on the idea and want to make it 1,000 times bigger.¹⁷

But how did Polar Night Energy turn a sand-filled tank into a heat-storing battery?¹⁸ Being connected to the grid, the battery receives excess solar and wind-produced electricity as input. This is then converted into heat and transferred to the sand. To be more specific, the renewables power a resistance heater, which heats up the air, which is then circulated around the sand through the pipes. Although it might sound complicated, their heat generation relies on resistive heating, which is basically how common appliances like toasters work.¹⁹ As claimed by the startup, their reservoir is well insulated from the outer environment, which minimizes heat losses over time. Reaching temperatures of up to 600°C (1112°F), their sand-based unit could retain heat for months.²⁰ When it’s time to deliver it, you just blow cool air through the pipes inside the hot sand bed. The end result you have coming out of the system is hot air that can produce greener steam for industrial processes, heating public water, or homes.

The storage heated battle

Clearly, leveraging something as simple and safe as sand to store green power is…hot stuff, but how does it compare to other storage technologies? Simplicity is one of the top benefits of sand-based heat batteries. As stated by the startup’s chief executive officer, “it’s really a typical
silo” which can be built in “any steel workshop”.²¹ Crazy as it sounds, they won’t need to build any factories as they scale up. Obviously, they need further components such as pipes, pumps, fans, heating elements, etc. but these are all standard equipment readily available everywhere. Same applies to their key ingredient, sand. This is a key advantage over conventional chemical batteries, which rely on hard-to-mine metals like lithium, nickel and cobalt. Although Polar Night Energy could use any type of sand from any location, the startup prioritizes upcycling the sand discarded in the construction industry, which minimizes waste. And that’s not the only environmental bonus. As estimated by a third party, Mission Innovation, based on the Avoided Emissions Framework²², Polar Night Energy’s sand battery could avoid over 100 Mt CO2e per year in 2030.²³ That’s almost twice New York City’s CO2e output from 2020.^{24 25}

Also, unlike for chemical storage, there’s no need for an electrolyte solution to shuttle ions around. And that’s a plus as the electrolyte degrades over time and lithium-ion batteries’ lifespan is around 15 years at most.²⁶ In contrast, the sand-filled insulated tank designed by Polar Night Energy can withstand high temperatures without losing its heat retention capacity and, as the company claims, can last for at least 50 years.²⁷ That claim seems very reasonable given the materials and components in use. In addition, while lithium-ion batteries’ current sweet spot for cost and energy storage is around 6 hours²⁸, the sand-based device is suitable for seasonal storage. Nonetheless, there is a scalding potato cooking underneath the heat-ridden sand. You won’t get green electricity out of these devices. I mean, in theory, you could use the stored heat to drive a steam turbine, but that would add an extra step biting into the round trip efficiency (RTE), drastically reducing it from 99%²⁹ to 25%.³⁰ This is miles away from lithium-ion batteries, facing only a 5% energy loss during their operations.³¹ So, rather than turning it into electricity, it would be more environmentally and economically sustainable to use the renewable heat as is. For instance, we could tap into it to replace natural gas-fuelled boilers for warming up our buildings. Though, this would make sense only in cities where you have district-scale infrastructure. In addition, this green P2H solution could also decarbonise heat-intensive industrial processes such as steel and cement manufacturing.³²

I can hear you already. How do heat batteries like these achieve such a high RTE? As mentioned earlier, the Polar Light Energy system relies on electric resistance heating, which is 100% energy efficient.³³

Being able to work at temperatures as high as 600°C (1112°F), sand stores more energy per unit of volume than water, which can’t go above 100 °C (212°F) for obvious reasons. Polar Night Energy said that their battery is about 3x more energy dense than water-based sensible TES.³⁴

The system would be most beneficial to district heating areas, such as some cities, universities, some industrial sites, and so on. Basically, anywhere that has an onsite steam plant could benefit from one of these sand batteries, as the infrastructure is already in place. Instead of steam running through the pipelines, forced hot air from the sand would travel through them instead.

But what about the cost? While sand is dirt cheap they’ll need loads of steel pipes buried inside it, which can inflate their expenses.³⁵ However, Polar Night Energy believe scaling up their facility 100x would only lead to a 20x higher price.³⁶ To be more specific, the startup predicted a cost of 10 euros (around $10) per kWh once their system reaches a storage capacity of 20 GWh.³⁷ Just to give you some perspective, researchers set a target cost below $15/kWh when developing new TES systems.^{38 39} Despite Polar Night Energy estimates being in line with economists’ expectations, there’s a catch. Their current storage capacity is only 8 MWh, so it will take a while for their technology to be competitive.

While sand batteries are not the panacea to a zero-carbon world, they could play a key role in decarbonising our power infrastructure when combined with other chemical and thermal storage solutions. Rather than idling wind turbines, when demand is low and supply is high, they can generate energy for storage in one of these sand batteries. I’ve said it before and I’ll say it again, it’s all about picking the right tool for the right job. There’s no silver bullet here, but a whole host of solutions.