In more and more places in the Netherlands, solar panels and wind turbines temporarily have nowhere to send their electricity. The grid is full. Installations are curtailed or switched off, generated energy evaporates, and in some cases owners of generating capacity will eventually pay for the privilege of feeding in.
In the media, grid congestion usually appears as an infrastructure shortfall: billions need to be spent and cables need to go into the ground. That is true enough. It only describes half of what is going on. During certain hours more is generated than is consumed at that moment, and there is too little capacity to move that energy elsewhere or hold on to it. Behind the grid problem sits a storage problem.
What actually happens during congestion
Congestion comes in two flavours. With demand congestion, an area draws more power at peak times than the cables can carry — the classic variant, where large consumers end up on a waiting list. With feed-in congestion the opposite applies: generators want to push more onto the grid than it can absorb, and it is the feed-in parties that get curtailed. This article is about the second variant.
When generation and grid capacity fall out of step, an odd scene unfolds: renewable electricity is produced and then not allowed into the system. Solar panels are throttled, installations deliver less than they can, and the owner’s return evaporates during precisely the sunniest hours of the year.
That goes to the heart of the business case. Capital invested in generation earns the least when conditions are at their best. The reflex is understandable: the grid needs to get bigger. There is a second shortage in play, though, and it concerns flexible offtake — parties that can absorb extra at exactly those moments.
Three routes when there is a surplus
With a local electricity surplus there are essentially three options.
You can curtail generation. Technically that works fine and economically it is dismal: you invested in capacity and then switch it off.
You can raise consumption at that moment. That calls for appliances that can wait until it suits, and most households have a limited supply of those.
Or you store the energy locally. That is the most interesting route, provided the storage system responds quickly enough and can absorb the supply immediately. This is where thermal storage comes in.
Why a thermal buffer fits here
A thermal buffer needs little persuading to accept electricity. Surplus power goes straight in via heating elements and comes out as stored heat. No intermediate step, no delicate chemistry, no debate about charging windows or cell temperatures.
The control logic can stay correspondingly simple. If there is a surplus and there is room in the buffer, charging is the right call. A decision like that costs no computing power and above all no time, and speed is exactly what a congestion peak asks of an offtaker.
That makes a thermal buffer a concrete flexible offtaker. When the grid overflows, it can simply switch on.
What that changes economically
For owners of solar panels the whole calculation shifts. Electricity that would otherwise go onto the grid at a low — and these days sometimes negative — price is stored as heat that later replaces gas, electricity or purchased heat.
A congestion moment becomes a charging moment. What the grid operator sees as an unwelcome peak in the graph is the most favourable hour of the day for the buffer.
What that means at neighbourhood level
The appeal of thermal buffers is not confined to the individual home. A street or neighbourhood full of buffers behaves as a distributed demand-response layer: many small offtakers that can absorb extra at precisely the peak moments.
That makes them interesting to grid operators, municipalities and neighbourhood developers. Grid reinforcement remains necessary alongside; these buffers meanwhile take the pressure off, flatten peaks and make better use of local generation. An energy system with a lot of variable generation needs absorption capacity as well as cable capacity.
The calculation for a single home
A home with 25 solar panels comfortably produces 20 kWh or more on a good summer’s day. During exactly those hours household consumption is usually low: the dishwasher has already run, the washing is out on the line, and not much else is happening. Without a buffer that electricity goes onto the grid, or gets curtailed as soon as congestion occurs.
With a thermal buffer the pattern changes. Production goes straight into the buffer as long as there is room, and that energy comes back out later as heat. Congestion turns from a blockage into an opportunity to charge.
The heart of it
Behind grid congestion sits a shortage of infrastructure and a shortage of smart, fast, local offtake. Anyone thinking only in terms of thicker cables has answered half the question. The other half reads: where can that electricity go right now?
A thermal buffer has a short answer: over here.
Caldum is designed to charge quickly at exactly those surplus moments and convert that electricity into usable heat for later.