Tom Gill Date published: 1 August 2022 8 min read Renewable energy isn’t available 100% of the time. The sun isn’t always shining and the wind isn’t always blowing, unlike fossil fuels which can be burned whenever there is need.But using fossil fuels puts pollutants into the atmosphere, accelerating the climate crisis, so we need a way to store the energy generated from renewables.Giant lithium batteries already exist, but what about sand batteries? Finnish company Polar Night Energy hopes these will help solve the problem of heating homes in the long winters the country experiences.The Eco Experts spoke with Markku Ylönen, one of the company's founders, about what this exciting technology could mean for the future of energy storage.What's on this page? 01 What is Polar Night Energy? 02 How can sand batteries help heat homes in winter? 03 How many homes can sand batteries provide heat for? 04 How long does it take to build a sand battery? 05 How much energy do sand batteries produce? 06 Can sand batteries generate electricity? 07 Could I install a sand battery in my home? 08 Has Polar Night Energy had much commercial interest? 09 Summary What is Polar Night Energy?“Polar Night Energy was founded in 2018. Tommi Eronen (CEO) and I first got the idea in 2011/12, during our studies in Power Plant engineering and Applied Mechanics respectively.“We started looking at energy systems in general, looking at how to make a system that’s as energy independent as possible, and in a cold country like Finland, the question of energy storage comes to the forefront quite often.“First, we looked at water storage. At some point we realised that the electricity market is really going in a direction where sometimes you have a lot of available and cheap electricity.“For example in Finland today, it’s super windy and the electricity prices are something like €10 (£8.40) per megawatt hour (mWh), or even less most hours.“It’s clean energy, but you don’t have anywhere to put any excess power generated. So if you have electricity as the primary energy source it makes sense to start building solid materials.“In the end, we came up with the idea of using sand as a large-scale, bulk-energy storage system.“When there’s a lot of clean electricity available, we want excess electricity generated to be used later on as heat and in heat networks.“Basically, the bulk-energy storage system functions like a giant sand battery, where the energy stored can be used when it’s needed.” How can sand batteries help heat homes in winter?“Using clean electricity in the summer, we heat up sand in the storage system to around 500°C. We can then use this heat to warm homes in the winter months, when the price of energy is at its most expensive.“The process of heating the sand is done via resistive heating, which is actually 100% efficient — 100% of the electricity is converted into heat. We use a heat exchanger to circulate hot air in the sand.“This hot air is then discharged by the battery to warm water for a district heating system, which is then pumped through a series of pipes into homes, offices, and whatever else needs to access the heating.” How many homes can sand batteries provide heat for?“Our currently existing silo has the capacity to heat around 100 homes, but that’s quite small in our terms. The silos scale better upwards rather than down, so we want to build silos that are 100 times larger, that can store enough energy to heat 10,000 homes.“Obviously it depends on where you are and how much electricity can be used to heat the sand batteries. Also, the energy consumption per home is highly dependent on where you are in the world. Homes in Finland or even the UK will inevitably require more heating in the winter than homes in Spain.” It’s more efficient to build bigger silos then?“Much more efficient if we increase the size. So the technology remains the same, we just put in more pipes, use a bigger silo, the volume-to-surface area ratio gets bigger — basically, the relative heat loss gets lower the larger we build.” How long can sand batteries store heat?“Heat dissipates quite slowly — the silos can store heat for months at a time. The only issue is if your storage period is too long, you won’t use the storage so much.“Cycling any kind of battery or storage makes the economics look better, so typically we aim for two to three weeks of heating, then recharge using whatever renewable energy is available at the time.“A major problem with adding more renewables to the grid is that you need to increase other energy sources to balance the network. Otherwise you run the risk of having either too much power or too little, which can cause the grid to collapse.“Being able to store excess energy generated in low-cost storage systems like our sand silos will help.” How long does it take to build a sand battery?“The active installation time for our first storage system was something like five months, with a testing period of one to two months. Smaller systems in the future will take no more than one or two months to get up and running.“Systems with a one gigawatt hour (gWh) capacity will likely take anywhere between six and 12 months to build. But as with any new technology, processes get streamlined and we imagine the installation time for these systems to shrink.“It’s always a totally different discussion building the first one compared to the tenth one. The silo design is quite standard, but a lot of the components aren’t available off the shelf.“And of course now with the current component crisis, even the basic automation components might have a three-month waiting time.“If the component crisis is solved, we expect the building of even the larger, one-gWh systems to shrink to weeks — again, assuming a streamlined process.” How many sand batteries are currently operating?“We have one commercial one, and a demonstration plant that has been running for two years now.” How much energy do sand batteries produce?“The storage system, installed in the Vatajankoski power plant, has an energy capacity of 10 mWh, but we’re looking to scale up to a one gWh capacity.” To put that into perspective, the average UK home uses 12,000 kWh of heat per year, or 32.9 kWh per day. A sand battery with a one gWh capacity could provide heat to 30,417 UK homes every day, on average. Can sand batteries generate electricity?“Yes, technically it is possible, but when you convert from heat to power, you always have losses. It’s one of the key rules of thermodynamics. The efficiency would be quite low — around 20% — so we prioritise direct heat use.“If there is a business case, adding a turbine to the system is not a big issue. It’s a high-temperature storage, but not high enough to make generating electricity economically sound in most cases.“Our storage systems are technically batteries, because they store energy, but generating electricity with them is not our main goal.” Could I install a sand battery in my home?“Producing these silos on a residential scale introduces both efficiency and price issues. If it was a super-small storage, it wouldn’t be feasible — our technology is currently optimised for larger scales.“It could be someday that we do a private home solution, but even then it’s unlikely it would work optimally.“In terms of conserving enough heat and making the installation large enough, it’s doable, but the price would be quite high. Also, there is a lot of competition in the private home sector — for example, heat pumps are quite efficient. It’s not in our current scopes, at least.” Has Polar Night Energy had much commercial interest?“Yes, especially with the recent media coverage, the interest has been huge. We have many more queries than we are able to answer at the moment! The interest was large even before the coverage, with industries and businesses interested in finding solutions to the energy storage problem.“We have negotiations ongoing globally, pretty much in every direction. A lot of companies and governments are interested in this kind of technology — especially with the ongoing energy crisis.“What we want is for technology like ours to become part of a mix of green energy storage solutions.” SummaryFossil fuels need to be phased out, make no mistake, so seeing companies like Polar Night Energy develop creative solutions to energy storage is encouraging..Governments around the world must commit to funding initiatives like this, and stop subsidising oil companies looking to further exploit fossil fuels. We’re already living through the damaging effects of climate change and the time for action is now.Imagine being able to provide warmth to an entire town, or even a city, using nothing but sand heated with renewable energy — an exciting prospect indeed. Tom Gill Writer Tom is a big fan of all things eco and has a passionate interest in how technology and localised projects can work together to make the world greener.