A storage system that is still standing neatly in the garden after twenty years but only reaches sixty per cent of its original capacity is not sustainable. It simply has not broken yet.
That distinction is systematically underrated in energy engineering. When service life comes up, attention usually goes to whether a system still functions. The question that matters sits a layer deeper: after twenty, thirty or fifty years, does it still deliver roughly what it promised on handover? For many storage systems the answer is far from self-evident.
This is where technically interesting systems and economically usable systems part ways.
Worn out without breaking down
Degradation rarely announces itself. Nothing smokes, nothing fails, and no engineer is called out. It is the slow process by which a storage medium is still present and takes an ever smaller active part in charging and discharging.
In low-temperature systems using phase change materials this is a known risk. On paper such materials store heat efficiently. In practice, repeated heating and cooling can cause structural changes: settling, clumping, stratification, incomplete melting, incomplete solidification. The material is then still sitting neatly in the system and only half participating thermally.
In the accounts that goes by the name of capacity loss.
Why seasonal storage is more sensitive to this than daily storage
With an electric home battery, degradation is common knowledge. It is in the datasheet, in the warranty terms and in the calculation. Nobody pretends a battery performs exactly as it did on day one after ten years.
With seasonal storage it is more easily glossed over, while the problem lands harder. Capacity is the core of a seasonal storage system: the buffer is filled during the warm half of the year and has to last through winter. Fall twenty per cent short of capacity in January and you are missing heat at the moment you need it most.
Capacity loss in July is annoying. Capacity loss in January is a system failure.
The real design requirement
For a storage system meant to last fifty years, the absence of degradation belongs in the specification, on the same line as pressure, temperature and volume.
That translates into concrete conditions. A storage medium that retains its thermal properties across the entire service life. No phase transition that becomes unpredictable after a few thousand cycles. No material that slowly drops out of the active process. No storage capacity quietly declining while the housing stands there immaculately.
Caldum’s storage mass was selected from that starting point. It sounds unspectacular, and that is the intention: a system for the built environment earns its place only if it still delivers decades from now. A house is not a test rig. A resident has to be able to count on the installation doing in year 25 what it was installed for.
What this means financially
The energy world likes to talk about purchase price. Understandable, and rarely decisive. What counts in the end is lifetime cost.
A system that keeps working at close to its original capacity for thirty or fifty years has a fundamentally different business case from one that has to be replaced sooner or falls back in capacity. The difference sits in the depreciation profile: the same investment is spread across more years and across more usable kilowatt-hours.
That difference rarely appears in the quotation. The resident notices it fifteen years later all the same.
The wrong question
“How long does the system last?” is the wrong question.
“How long does the system remain thermally usable at the performance level it was bought for?” comes closer.
That is the question that ought to be central to seasonal storage. Whether the vessel, the housing and the pump physically still exist says little; whether the installation still does what it was meant to do says everything.
For seasonal storage, the absence of degradation counts as the lower limit. Caldum was developed from exactly that requirement: storage capacity that does not slowly disappear while the system apparently still stands there working.
This article looks at the economics of service life. The underlying mechanism — why the storage mass itself does not deteriorate — is covered in Heat storage without degradation: why doesn’t a thermal buffer wear out?