In July your PV panels work overtime and there isn’t much to do with all that energy. In January it’s the other way around: little sun, plenty of heat demand, and the grid creaks because half the country switches on the heating at the same time.
That isn’t a detail. That’s the entire problem.
Seasonal heat storage means holding energy not for a few hours or days, but for weeks to months. You use a summer and autumn surplus later in the year, when demand exceeds supply.
Why specifically heat?
Most storage discussions are about electricity. Fair enough — you see it on the meter. But in an average home, the biggest chunk of energy goes to heat: space heating and hot water. That demand isn’t spread evenly across the year. It’s concentrated in winter.
If the end use is heat, there’s no need to route it through the electrical chain first. A home battery is excellent for daily shifting: solar at two in the afternoon, used at eight in the evening. But a battery sized to carry summer energy through to January gets unwieldy. Too large, too expensive, too material-heavy for most buildings.
Thermal storage asks a different question. Not how do we keep all the electricity, but what energy do we actually need later, and in what form. For heating and hot water, the answer is usually heat.
How does a seasonal thermal buffer work?
The buffer is charged with energy — from solar panels, or from the grid when supply is abundant. That energy becomes heat and goes into a storage medium. Later, the buffer releases the heat in a controlled way to the heating system or hot water supply.
The principle isn’t new. An ordinary boiler is a heat buffer too; just sized for hours, not months. Try doing seasonal storage with water and you’ll end up with a tank the size of a lorry. That’s why Caldum uses a denser storage medium than water. Same amount of heat, far less volume.
Why now?
The energy system is becoming more volatile. Lots of solar and wind get added, but they don’t produce when we want them to. Solar mostly midday and summer, wind unpredictable, heat demand mainly in the evenings and winter. Without storage, you solve that with grid reinforcement, gas plants, imports or large heating networks. Sometimes necessary, often clumsy.
Local storage shifts energy in time at the point where it’s generated. A house, a block, a small heating network can then organise a large part of its own heat supply. Fewer peaks, less grid dependency, and heat available when it’s needed.
Is that the same as off-grid?
Not necessarily. A Caldum system can run fully autonomously in off-grid energy systems or island-mode applications. In most cases, near off-grid is more practical: the building stays connected, but uses the grid far less.
In a new installation, Caldum usually replaces the heat pump. Hybrid operation is possible — alongside a heat pump, an existing gas boiler or another heat source — for example as a back-up, or in a refurbishment where the existing setup still has years of life in it. The point isn’t ideological disconnection. The point is having heat available at the right moment.
Who is this for?
Seasonal storage gets interesting when heat is a large share of the energy demand and there’s a clear mismatch between supply and demand. In practice that’s homes going off gas, new-build with lots of PV, housing blocks where individual heat pumps are awkward, small collective systems, commercial buildings with predictable heat demand, and locations where grid congestion is slowing electrification.
The right design depends on the situation. A passive house behaves differently from a 1970s terrace. A row of homes is different from a school or a workshop. Sizing therefore doesn’t start with the buffer — it starts with the question: how much heat is needed, when, and how much energy can be generated locally?
Where is Caldum right now?
The storage medium and charging method have been validated in the lab. The next step is the field. We’re looking for pilot partners for the first installations — not as a non-committal demonstration, but to run the technology under real conditions, measure it, and refine it. On paper, a lot is possible. In practice you find out how a system behaves under variable use, how it fits in existing installations, and what residents make of it.
Not a silver bullet. It doesn’t automatically replace every heat pump, every district heat scheme, every boiler. There are situations where something else fits better. But between summer surplus and winter demand sits a gap you can’t close with cables, batteries and good intentions alone. Sometimes you just need to keep the heat until you need it.