When Tonny Dam Jensen took over a modest district heating plant in a Danish town of 750 homes, no one expected him to become a pioneer in one of the world’s most critical energy transitions. But with a bold mindset, a heat pump, an e-boiler, and a whole lot of thermal storage, he is now showing the world what is possible. Here is how Sønder Felding became a global showcase of smart, flexible, and future-proof heating.
Morten Jordt Duedahl
Tonny Dam Jensen
Ebbe Nørgaard
Just a few years ago, the idea of large-scale heat pumps being deployed across Denmark’s district heating networks would have drawn skepticism. “I would have thought you didn’t understand much about our systems,” says Morten Jordt Duedahl, host of DBDH’s District Heating Podcasts. But that was then.
Now? “I travel around the world telling people electrification through heat pumps and electric boilers is the new super green,” he proclaims.
And at the heart of that transformation is Sønder Felding, a small town with a big idea.
From biomass to brilliance
Tonny Dam Jensen has been managing Sønder Felding’s district heating plant for over a decade. What once relied entirely on an old biomass boiler has been completely reinvented. Today, the system runs on a 10 MW electric boiler, a 2.5 MW air-source heat pump, and a massive 3,300 m³ thermal storage tank. The old 4 MW biomass unit? It’s just for backup, and it hasn’t been needed in over a year.
This setup not only provides heat reliably and affordably to the town’s residents but also supports Denmark’s entire electrical grid.
“We’ve transitioned to electrification with overcapacity and energy storage,” Tonny explains. “It gives us flexibility – and that’s where the value is.”
The power of flexibility
The podcast dives deep into a topic that may sound technical but is actually quite intuitive: using energy when it’s cheap, storing it when you don’t need it immediately, and being responsive to the needs of the grid. This responsiveness – called grid balancing – is where things get really interesting.
Denmark’s transmission system operator, Energinet, operates several markets to keep the electricity grid stable. One of them is the electricity price market, where prices typically drop in the afternoon when solar power production is high — that’s when flexible consumers like Tonny’s system can take advantage of cheap electricity.
In parallel, Energinet also runs a balancing market, which ensures real-time grid stability. Here, consumers can be activated to quickly increase or decrease their electricity use when supply and demand suddenly diverge — for example, when a cloud temporarily reduces solar production or wind output surges unexpectedly.
With AFRR (Automatic Frequency Restoration Reserve), the heat pump can be turned off within five minutes if required, absorbing those surplus megawatts when needed, and being rewarded for doing so. The e-boiler is much faster.
Storage: The unsung hero
Ebbe Nørgaard, Sales Director at heat pump manufacturer Fenagy, underscores that heat pumps alone don’t make this possible. “If you only have something that consumes electricity and produces heat, you have no flexibility. You need accumulation storage.”
And Sønder Felding has plenty. Their thermal tank can store up to 14 days’ worth of summer heat or two days in winter. This allows the system to decouple energy production from heat delivery – buying power when it’s cheap and using the produced heat when it’s needed.
“This is where the business case is hidden,” Ebbe says. “And this is where Denmark is very unique.”
It’s not rocket science – It’s just smart
What is striking about the Sønder Felding setup isn’t its complexity – it’s how simple it is. As Morten puts it, “This is Power-to-X, and in this case, X is heat. It works now. It’s cheap. And it’s simple.”
There is no need for futuristic inventions or massive subsidies. The core ingredients are already available: a heat pump, an electric boiler, thermal storage, and a digital link to grid markets.
And the cost savings? Astounding.
By strategically avoiding just the top 20% of the most expensive electricity hours — based on spot market prices — Tony has reduced average production costs by over 50%. During some periods with extremely low electricity prices, he has even been able to produce heat at no cost. Literally, free heat.
From heat planner to electricity strategist
Tonny says his job has changed dramatically. “I used to be a heat planner. Now I’m an electrical planner,” he laughs. “Heat is almost a waste product of electricity.”
But make no mistake – this shift is where the real opportunities lie. By thinking like a grid participant, not just a heat provider, Tonny has unlocked a whole new business model. In 2024, his plant ran 85% on the heat pump and 15% on the electric boiler. And since entering the AFRR market in October, that balance has become even more dynamic, with more frequent start-stop cycles tailored to grid needs.
But what about the hardware?
One concern that often comes up with such responsive systems is wear and tear. After all, starting and stopping a compressor 40 times a day sounds like a recipe for failure.
Tonny and Ebbe both acknowledge the risk, but they’re optimistic. The CO₂-based heat pumps used in Sønder Felding are designed to handle frequent cycling. “As long as it stays warm, there is not too much stress,” Ebbe explains. “These compressors are used to start and stop six times an hour.” And adds, “Let’s see how it works, and in the meantime, make a lot of savings for our end users.“
A call to policymakers: Let others do the same
Perhaps the most important takeaway from the podcast is that this model is replicable. District heating networks across Europe – and the world – could benefit from the same strategy. But they need the right market conditions.
“This isn’t just good for the heating company or the consumer,” Morten says. “It’s good for society.”
The podcast ends with a heartfelt plea to policymakers and transmission system operators: Make this possible elsewhere. Support flexible, storage-enabled systems. Let them participate in grid markets. Enable the green transition – not with complexity, but with courage.
Be Brave; Buy Big, Robust, and Agile
Tonny’s final words ring like a rallying cry: “Be brave. Invest in overcapacity and energy storage. Buy something big, flexible, robust, and agile – because that’s what’s necessary for the green transition.”
Ebbe agrees: “Rethink your business case. It’s not just about average electricity prices and COP. It’s about flexibility, storage, and market opportunity.”
And that’s the message to take home: this isn’t a pie-in-the-sky concept. It’s happening now. In Sønder Felding. In Denmark. And, if we’re bold enough, maybe in your town next.