Loviisan Lämpö Invests in Polar Night Energy''s Sand Battery in Pornainen –
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Finnish startup Polar Night Energy is developing thermal energy storage system known as "sand batteries" for warming up buildings
Polar Night Energy, a startup in Finland, has developed technology for warming up buildings with solar-generated heat stored in sand. The team uses thermal modeling to optimize the design of their heat storage and distribution systems, which are helping Finnish cities reduce their consumption of nonrenewable heating fuels.
This sponsored article is brought to you by COMSOL.
As we try to objectively study nature, we are often reminded of how natural forces affect us personally. We can sit at a desk and consider heat in its various forms, but we might be distracted if our toes are cold! When we turn up the heat in our homes and workplaces, we must balance our personal need for warmth with the global impact of burning fossil fuels like oil, gas, coal, and biomass. Anthropogenic climate change confronts humanity with a challenge: How can we keep warm now as we try to prevent our world from overheating in the future?
It is a daunting question that a startup called Polar Night Energy, in the small and chilly nation of Finland (Figure 1), is attempting to answer. In a region known for long, dark winter nights, Polar Night Energy is building a system in the city of Tampere that can heat buildings with stored solar energy — all day, all night, and all winter long. The apparent contradictions do not end there. In an era of complex cleantech solutions, often made from rare and expensive materials, Polar Night Energy''s heat storage and distribution system consists of simple ducts, pumps, valves, and sand. The novel system shows potential for tackling global problems in a patient, thoughtful, and human-scaled way.
Figure 1. The nation of Finland, part of which is above the Arctic Circle. Polar Night Energy's heat storage systems are currently installed in the cities of Tampere and Kankaanpää.
Big problems demand big solutions, and there is perhaps no bigger 21st-century problem than climate change. To meet this challenge, many governments and organizations are investing in new technology to help lessen the use of fossil fuels. These initiatives have largely focused on renewable electric power generation, distribution, and storage.
"When you ask people about cleaner energy, they think of electricity," says Tommi Eronen, CEO of Polar Night Energy. "But we also have to cut emissions from heating." Out of Finland''s energy-related emissions, 82 percent come from heating domestic buildings (Ref. 1). "We want to replace all of that if we are to have any hope of meeting our global climate goals," Eronen says.
The spirit of "Think Globally, Act Locally", a mantra associated with the 1960s, lives on with Polar Night Energy''s team of innovators. Their journey began with a question posed by its founders, Tommi Eronen and Markku Ylönen, when they were university classmates: "Is it possible to build an energy-self-sufficient and cost-effective hippie commune for engineers using only solar power?" After graduation, the project they codenamed "Hippie Commune" became Polar Night Energy, with Eronen as CEO and Ylönen as CTO.
What began as a lighthearted (but serious) student project led to a 3 MWh/100 kW pilot plant in the Finnish city of Tampere, which began operation during the winter of 2020–2021. The system uses electricity to heat air, which is then circulated through an exchanger that heats water and distributes it to multiple buildings in the city''s Hiedanranta district (Figure 2).
Figure 2. A schematic of the components and operating cycle of the Polar Night Energy system.
Inside the system, electrically powered resistive heating elements heat air to more than 600°C. The hot air is circulated through a network of pipes inside a sand-filled heat storage vessel. The hot air then flows back out of the vessel into a heat exchanger, where it heats water that is then circulated through building heating systems. The sand''s heat storage capacity ensures that even when the resistive elements are cool, the circulating air is still hot enough to keep the water (and buildings) warm.
"We only have pipes, valves, a fan, and an electric heating element. There is nothing special here!" Eronen says, laughing.
Figure 3. Markku Ylönen with a representative sample of Polar Night Energy's dirt-cheap heat storage medium.
Figure 4. Tommi Eronen (foreground) and Ylönen inspecting the ductwork of a Polar Night Energy heat storage vessel.
Multiphysics simulation software helped shape Polar Night Energy''s heat exchanger design (Figures 5–6). Eronen says, "We built a particular model to explore a design idea: What if we created a super hot core of sand surrounded by heating ducts around the perimeter?" By modeling fluid flow and heat transfer effects in the COMSOL Multiphysics software, the Polar Night Energy team could quantify its design''s comparative advantages and drawbacks. "The simulation confirmed that the ''hot core'' design was good at storing heat for very long periods of time," says Eronen. "But for our intended operational cycle, it makes more sense to evenly distribute hot air ducts throughout the sand storage vessel," he explains.
Figure 5. Simulation images showing temperature changes inside a proposed sand–air heat storage vessel design over a 100-hour period.
Figure 6. Simulation image of natural convection effects through ductwork inside the sand storage vessel.
The sheer scale of Polar Night Energy''s sand-based heat storage system makes simulation software indispensable. "We cannot possibly build full-size prototypes to test all of our ideas. We need predictive modeling to answer as many questions as possible, before we commit to assembling all this equipment — and all this sand!" Eronen says. "It is essential for us to use these immensely powerful tools."
Figure 7. Part of the heat transfer system installed by Polar Night Energy in Tampere, Finland. The vertical pipes at left are part of the heat exchanger, while the resistive heater elements are wrapped in white insulation at right. Between these components is the air-circulating radial blower.
About Polar night sand battery
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