Polar Night Energy is designing a sand-based thermal storage system

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When we try to study nature objectively, we are often reminded of how natural forces affect us personally. We can sit at the desk and look at the heat in its various forms, but we might get distracted if our toes are cold! When we raise the temperature in our homes and workplaces, we must balance our personal need for warmth with the global impact of burning fossil fuels such as oil, gas, coal, and biomass. Anthropogenic climate change presents humanity with a challenge: How can we keep warm now while trying to prevent our world from warming in the future?

It’s a daunting question that a startup called Polar Night Energy, in the small, cold country of Finland (Fig. 1), is trying to answer. In a region known for its long, dark winter nights, Polar Night Energy is building a system in the city of Tampere that can heat buildings using stored solar energy – all day, all night and all winter. The apparent contradictions do not end there. In an era of complex cleantech solutions, often made of rare and expensive materials, Polar Night Energy’s heat storage and distribution system consists of simple ducts, pumps, valves and sand. The new system shows the potential for addressing global problems in a patient, measured, human-sized way.

Small country with big heating needs

Big problems require big solutions, and there is perhaps no bigger problem in the 21st century than climate change. To address this challenge, many governments and organizations are investing in new technology to help reduce fossil fuel use. These initiatives have largely focused on the generation, distribution and storage of renewable electrical energy.

“When you ask people about clean energy, they think of electricity,” says Tommy Ironen, CEO of Polar Night Energy. “But we also have to cut emissions from heating.” Of energy-related emissions in Finland, 82 percent comes from domestic building heating (ref 1). “We want to replace all of that if we are to have any hope of meeting our global climate goals,” says Eronen.

Think globally, heat locally

The ethos of “Think Global, Act Local,” a slogan associated with the 1960s, is still alive with the team of innovators at Polar Night Energy. Their journey began with a question posed by its founders, Tommi Eronen and Markku Ylönen, when they were classmates at the university: “Is it possible to build an energy-self-sufficient, cost-effective hippie community for engineers who use only solar energy?” After graduating, the project they called “Hippie Commune” became Polar Night Energy, with Eronen as CEO and Ylönen as Technical Director.

What started as a fun (but serious) student project has led to the construction of a 3MW/100kW pilot plant in the Finnish city of Tampere, which became operational during the winter of 2020-2021. The system uses electricity to heat the air, which is then circulated through an exchanger that heats the water and distributes it to several buildings in the city’s Haidanranta district (Fig. 2).

Within the system, electrically powered resistance heating elements heat the air to over 600°C. Hot air is circulated through a network of tubes inside a sand-filled heat storage vessel. The heated air then flows back from the vessel into a heat exchanger, where it heats the water which is then circulated through the building’s heating systems. The thermal sand’s storage capacity ensures that even when the resistive elements are cold, the circulating air remains hot enough to keep the water (and buildings) warm.

“We only have pipes, valves, a fan, and an electric heating element. Nothing special here!” Iron says with a laugh.

A heat battery made of sand

Famed chemical engineer Donald Sadway is quoted as saying, “If you want to make a cheap battery, you have to make it out of the dirt.” The Polar Night Energy System faces the same fundamental challenges as any other energy infrastructure. It must provide power to people when they need it, where they need it, and at a reasonable price. This means that storing and distributing energy is just as important as generating it. Existing infrastructure meets these challenges in familiar ways. For combustion-based heating, fuels such as oil and gas are stored and transported to where they can be burned. The electrical grid also supports the efficient distribution of energy and takes advantage of the energy generated through renewable means such as wind and solar energy. However, the intermittent nature of daylight and strong winds is an intractable problem. Energy storage is needed to maintain constant energy production throughout the peaks and valleys of renewable inputs. But even with recent advances in battery technology, storing electrical energy remains relatively expensive, especially on the scale required to heat buildings. What if, instead of storing electricity, a “battery” could store heat instead?

Markku Yl\u00f6nen stands in construction equipment inside a mound of sand-like dirt.

Figure 3. Markku Ylönen with a representative sample of cheap heat storage medium from Polar Night Energy.

Many conventional heating systems already store and distribute heat by retaining and circulating warm water. Eronen and Ylönen recognized the benefits of water-based heat storage as well as its limitations. “There’s a lot of heat you can add to the water before it turns into steam,” Ironen says. “Vapor can distribute heat efficiently, but it is not cost-effective for large-scale storage.” To avoid the drawback of storing heat in water, they instead turned to sand — 42 metric tons of it! (Fig. 3) After the sun goes down, the stored sand heat is gradually released back into the circulating airflow. This keeps the air hot enough to maintain constant temperatures in the water that flows through the customers’ radiators. In this way, the sand enables solar energy to warm people, even on the darkest and coldest Finnish nights. “Sand provides four times the energy storage capacity of water,” says Eronen. “Sand is effective, non-toxic, portable and cheap!”

“We need predictive modeling to answer as many questions as possible, before we commit to assembling all this equipment – and all this sand!”
—Tommi Ironen, CEO of Polar Night Energy

Sophisticated analysis behind a simple solution

Cost efficiency is the foundation of Polar Night Energy’s value proposition. “Once we decided to pursue this idea, we were trying to figure out what the finances looked like,” says Eronen. In their quest to do more with less, Polar Night Energy has long relied on numerical simulation tools. Eronen and Ylönen began using COMSOL Multiphysics as students and it remains an integral part of their design process. For example, Irunen mentions specifications for an expanded thermal storage system that will serve more buildings in Tampere. The team calculated that supplying heat to an area of ​​35,000 people would require a sand-filled storage cylinder 25 meters high and 40 meters in diameter. How did they arrive at these dimensions? “It is easy to calculate the approximate amount of material required, because we know how much heat we can store in a cubic meter of sand,” explains Eronen. “We also had to determine the area required for efficient heat transfer between the sand and our air circulation system (Fig. 4). This is much trickier! We used COMSOL to model and evaluate different design options.”

Tommi Eronen wears construction equipment mounts near the company's heat storage system.

Figure 4. Tommi Eronen (foreground) and Ylönen inspect the ducts of a Polar Night Energy heat storage vessel.

Multiphysics simulation software helped shape the design of the Polar Night Energy heat exchanger (Figs. 5-6). Eronen says, “We built a particular model to explore a design idea: What if we created a superheated core of sand surrounded by heating channels around the perimeter?” By modeling the effects of fluid flow and heat transfer in COMSOL Multiphysics software, the Polar Night Energy team can determine the relative design advantages and disadvantages. “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 distribute the hot air ducts evenly throughout the sand storage vessel,” he explains.

The sheer scale of Polar Night Energy’s sand-based heat storage system makes the simulations indispensable. “We can’t 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!” says Iron. “It is imperative for us to use these very powerful tools.”

Adapting new ideas to existing infrastructure

By separating the task of storing heat from generating and distributing heat, Polar Night Energy has made its system more efficient and adaptable. There is great potential for modification of sand-filled heat storage and transfer systems into existing infrastructure (Fig. 7). Tampere, an inland Finnish industrial city of nearly 250,000 people, is an ideal testing ground for this new technology. “Tampere, like many European cities, already has a local heating system that circulates water through entire neighborhoods,” says Ironen. “This enables us to quickly convert many buildings to a renewable heat source,” he says. The Polar Night Energy pilot station in Tampere can also draw on power from the existing electrical grid, along with electricity generated from new solar panels. Reliable heat storage enables the city to generate or purchase power when it is most affordable and then distribute heat when it is needed most.

Photo of the company's heat transfer system, which consists in part of large horizontal and vertical metal tubes.

Figure 7. Part of the heat transfer system installed by Polar Night Energy in Tampere, Finland. The vertical tubes on the left are part of the heat exchanger, while the resistive heater elements are wrapped in white insulation on the right. Among these components is a radial air blower.

Today: Finland; Tomorrow: the world

Since the Tampere system went live during the winter of 2020-2021, the Polar Night Energy team has been collecting data to compare with their models. “Our simulations have proven to be very accurate, which is encouraging,” says Ironen. And as the Polar Night Energy team continues to develop their ideas locallyThey aim to work worldwide also. The same technology that warms Finland’s long, cold nights could also provide better energy management options for the rest of the world. Affordable heat storage could help industries and cities capture currently wasted heat, as well as balance out the discrepancies in wind and solar energy production. But while Polar Night Energy is eager to work directly with potential clients, they know that the challenges ahead are too great for them to handle alone.

A man in a red jacket stands near Polar Night Energy's heat transfer system.

Tommy Ironen, CEO, Polar Night Energy.

We want to license this technology. If you operate a power plant, please contact us. And on a more serious note, he adds, “We need to stay away from all kinds of burning, even biomass. We need to protect and restore forests so they can continue to remove carbon from the air. Because climate change is happening so quickly, we want our ideas to spread faster.” Available time “.


Finland Statistics“More than half of Finland’s electricity was produced from renewables in 2020,” November 2021.

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