Opening
A nickel-titanium alloy, stretched and released inside a small prototype, recently pulled a heat sink from room temperature down below zero Celsius. No refrigerant. No compressor. Just a metal that gets cold when you stop pulling on it, described in a recent Nature paper on elastocaloric cooling.
That device is not going into a window any time soon. But it points at something the cooling industry has been quietly betting against for a decade: that the vapour-compression machine bolted to the side of half the world’s buildings is, in fact, a finished technology.
The story sold to consumers is that the air conditioner just needs a greener refrigerant and a smarter thermostat to ride out the climate century. That framing is wrong, and the engineers trying to replace the compressor entirely are the clearest evidence of it. A small but growing group of physicists and startups argue the vapour-compression cycle has hit a wall. Their pitch: rip out the refrigerant, the compressor, and the copper coils, and cool rooms with solid blocks of metal, plastic, or ceramic that change temperature when you squeeze, stretch, or magnetise them.
The promise is real. So are the doubts.
The problem the industry would rather not name
Cooling is one of the few technologies where the public health case and the climate case point in opposite directions. Air conditioning has prevented significant numbers of heat-related deaths in recent decades, particularly as extreme heat events have become more frequent. This is not a marginal benefit—it ranks among the largest public health interventions of the modern era, distributed one window unit at a time.
However, the same machines are also a major problem. Air conditioning accounts for a significant share of global electricity consumption and greenhouse gas emissions. The global stock of AC units is projected to roughly triple by 2050 as middle classes expand across South Asia, Southeast Asia, and sub-Saharan Africa.
That trajectory is locked in. The only open question is what those tripled units run on.
Why the compressor has a ceiling
Modern HVAC systems already perform better than most people realize. The coefficient of performance (COP) for a typical HVAC unit sits around 3, meaning it shifts three units of heat for every unit of electricity consumed.
That is good. But it is also close to the practical ceiling of what compressing and decompressing a refrigerant can deliver in a unit small enough to bolt to a wall.
The deeper issue is the refrigerant itself. R410A, the working fluid in many residential systems, has a global-warming potential (GWP) more than 2,000 times that of carbon dioxide. Every leak, botched recycling job, or landfilled unit represents a small atmospheric event. The industry is migrating to lower-GWP fluids, but chemistry is constrained: any substance that boils and condenses at useful temperatures tends to be either flammable, toxic, or a potent greenhouse gas.
The solid-state cooling argument begins here. If the refrigerant is the problem, get rid of the refrigerant.
Four bets on a compressor-free future
There is no single solid-state cooling technology. Instead, there are at least four promising approaches, each exploiting the same physical phenomenon from different angles: materials that get cold when you stop applying a stimulus.
Thermoelectrics
Thermoelectrics use semiconductors that pump heat when an electric current passes through them. They are reliable, silent, and already power mini-fridges in hotel rooms. However, they remain inefficient at room scale, limiting widespread adoption for larger spaces.
Magnetocaloric
Magnetocaloric systems exploit metals like gadolinium that heat up when magnetized and cool down when the magnetic field is removed. German startup Magnotherm has moved this technology from the lab to commercial refrigeration, launching a pilot with the REWE Group in German supermarkets earlier this year.
Elastocaloric
Elastocaloric materials, mostly shape-memory alloys, release heat when stretched or compressed and absorb it when relaxed. Imagine a rubber band that warms when you pull it, but scaled up and made out of nickel-titanium alloy.
Barocaloric
Barocaloric systems use pressure rather than magnetic fields or mechanical strain. A UK company called Barocal, founded on Cambridge materials research, is developing solid refrigerants intended to replace gas-based cooling fluids.
Each approach avoids fluorinated refrigerants entirely. Each one is also, today, striving to prove it can compete with the conventional compressor in the window unit.
The efficiency gap nobody can fully explain
This is where the scientific community diverges from optimistic press releases.
The open question is why solid-state coolers cannot yet consistently match the efficiency, durability, manufacturability, and cost profile of traditional thermodynamic cycles. Theoretical limits suggest they should be able to. Yet, lab results indicate the road is more challenging.
Part of the gap is engineering: heat exchangers in prototype devices remain crude, and parasitic losses diminish gains. Part is materials science: the best magnetocaloric and barocaloric compounds can be expensive, rare, or difficult to cycle reliably. Additionally, unresolved physics play a role; entropy changes promising on paper may degrade after millions of material cycles in real devices.
Researchers and startup founders often cite two to three years as a timeline for more convincing room-scale demonstrations of elastocaloric and barocaloric systems. These are not mass-market products, but demonstrations. Given that the industry timeline runs in decades, that window is tight.
Why the edges matter
Climate technology coverage tends to fixate on total replacement—either the new technology kills the old or it fails. Cooling does not work that way.
Even a small share of the global cooling market could matter if solid-state systems solve problems poorly addressed by compressors. Efficiency is not the only metric. Durability, refrigerant-free operation, quiet performance, and the ability to serve niches where compressors struggle all count.
This math becomes more interesting when considering early win areas for solid-state cooling: vibration-sensitive environments, medical cold chains, vehicle cabins where compressor weight and noise are penalties, and spaces where occupants cannot tolerate temperature swings or sudden failures common to conventional units. A pilot programme is already testing solid-state heating and cooling for people with disabilities, hypothesizing that quiet, refrigerant-free systems better serve users for whom conventional AC quirks are disabling.
None of these markets need to dethrone the window unit; they just need to exist.
The demand curve is not waiting
The urgency is tangible. Hotter summers are already reshaping attitudes toward cooling, resilience, and grid demand. Multiply that by projected AC growth, and the strain on grids designed for a cooler century becomes the central story.
India installed millions of room air conditioners last year. Indonesia, Vietnam, and Nigeria show similar growth trajectories, albeit from lower absolute numbers but with steeper rates. Each unit today almost certainly contains a fluorinated refrigerant and a compressor architecture dating back to before the moon landing.
The locked-in emissions from cooling demand over the next 25 years dwarf any plausible solid-state rollout in the same period. That is not a reason to dismiss new technologies, but a reason to be honest about their near-term role.
Who actually wants a colder room
There is a behavioral layer that engineers often overlook. Cooling demand is not just a function of outdoor temperature; it is shaped by income, building stock, cultural norms, and personal history. Thermostat preferences often track childhood economics as much as physiology.
This matters for solid-state cooling because the technology’s strongest pitch is not raw output, but granularity. Magnetocaloric or thermoelectric modules can be small, silent, and local—cooled chairs, beds, or vests in ways a compressor cannot achieve. If the future of cooling is partly about cooling people rather than rooms, efficiency comparisons with central HVAC systems shift dramatically.
You do not need to outperform a three-tonne unit if you are cooling a square meter of skin.
The investor problem
Hardware is hard. Climate hardware is harder. Cooling hardware sold into a commodity market dominated by Daikin, Carrier, Mitsubishi, and a few Chinese giants is harder still.
Startups working on solid-state cooling face financing realities that often clash with the patience science requires. Barocal has raised funds on the strength of Cambridge research and a pitch that resonates with European climate funds. Magnotherm took the supermarket refrigeration route: real customers, defined use cases, and workable cost structures today rather than hypothetical ones in 2032. That sequencing is likely the playbook—solve refrigeration first, where margins tolerate premium technology, then move toward residential cooling once volumes drop unit costs.
However, this playbook takes a decade or more. The demand curve does not slow down to wait.
What the scientists are actually saying
Skepticism in academic literature is not about whether solid-state cooling will work, but about public conversations that treat it as imminent and a silver bullet. The honest pitch is more measured: solid-state systems will likely reach commercial viability first in niches such as commercial refrigeration, specialized cooling, medical or mobility-related climate control, and high-end residential markets willing to pay for silent or refrigerant-free operation.
They are not poised to displace the window AC in a Mumbai apartment by 2050, and pretending otherwise does the technology no favors.
The scientific case continues to advance. The recent Nature paper on elastocaloric cooling described a device using low-transition-temperature nickel-titanium alloys that achieved sub-zero Celsius cooling from a room-temperature heat sink. This is not a mass-market air conditioner. It is evidence that the underlying physics is progressing toward practical machines.
That remains a long way from marketing language—but also far better than doing nothing.
The uncomfortable middle
Climate technology coverage often swings between two extremes: celebrating every lab breakthrough as the end of fossil fuels or dismissing pre-commercial tech as vaporware. Solid-state cooling sits squarely in the uncomfortable middle, which is where most critical climate technologies currently reside.
The compressor will not disappear in this decade. The refrigerant problem will not solve itself. The demand curve will not flatten simply because the physics is promising.
What can happen is slow substitution at the edges—starting in supermarkets and specialized applications—then moving toward the rooms where people live as cost curves cross. Whether that substitution becomes material by 2040 or remains a specialist niche depends on engineering breakthroughs yet to come, financing patience not yet widespread, and policy frameworks such as refrigerant pricing, building codes, and procurement standards still to be written.
It is worth asking whether the delay is truly about engineering risk. The vapour-compression cycle represents a century-old supply chain: compressor plants, refrigerant chemistries, distributor networks, certification regimes, and technicians trained to service the existing fleet. Each of these is a fixed cost amortized by incumbents but must be absorbed from scratch by competitors. What the industry calls prudence can also be seen as protecting depreciated asset bases.
The startups are not the ones who need to justify themselves. The window AC in the room already has.
