How Solid State Cooling Could Change Everything

I talk about heat pumps a lot. They really are the most energy efficient way to heat and cool things, but they’re not the only way. There’s another form of cooling that’s been getting a lot of attention in recent years that might just lie in these bendy wires.

It might seem a little abstract at first, but the study of caloric cooling represents the potential for a new, solid-state-esque approach to managing heat. Yup, I said solid state. That comes with the possibility of achieving significantly higher efficiencies without refrigerants. Caloric cooling holds massive implications for the way we cool our homes, our vehicles, and maybe even ourselves.

So, what can this metal have in store for us? How does it all work? And why does it have the potential to completely change climate control?

If you’re a regular viewer of my channel, you already know that I try to hammer on the fact that there isn’t a single technological savior coming to liberate us all. There’s more than one way to overcome the obstacles that we’re facing as we progress through the clean energy transition.¹ And here’s a new and fascinating contender.

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Why is this Propeller Getting So Much Attention?

What I have here are wires made out of nitinol, a nickel-titanium alloy or metal mixture. It’s also a type of shape memory alloy (or SMA). Basically, it can remember its shape better than I can remember where I put my car keys. That means it has a really interesting property: the ability to do this.²³

Now, I’m not just showing off nitinol’s quirks for fun (though yes, it is fun). Its unique abilities may one day be the backbone — or perhaps the wireframe — for refrigeration and air conditioning systems that are cleaner and more efficient. Welcome to the world of elastocalorics.

What is Elastocaloric Cooling?

You might be wondering: elasto-what now? And how does that wire have anything to do with the future of cooling? Well, chances are you’re familiar with the capital C Calorie associated with nutrition labels. That’s an offshoot of calorie with a lowercase c, the unit that, in the simplest sense, measures heat energy.⁴ In other words, heat is the main course on this menu. And with elastocaloric cooling, we’re looking at a new way of serving up colder air — without relying on refrigerants. This spectrum of solid-state technologies might just become the new and improved way of getting your house or your car or your leftovers to chill out.⁵

How? With the power of what a member of my scientific advisory board called ”the energy conversions of the future.” We’re already constantly converting energy in our daily lives, and not just by flicking light switches on and flipping laptops open. Your body turns the chemical energy you get out of your meat and potatoes into the mechanical energy that gets you through the day. The meat and potatoes of this channel, renewables like wind and solar, are more advanced conversions: wind’s kinetic and mechanical energy, and solar’s light and heat, are transformed into electricity. As our demand for energy grows, the race is on to find more direct, more efficient, and more safe ways of obtaining it.⁶ This means getting creative.

The Details of Vapor Compression Cooling

Before we jump deeper into elastocalorics, it helps to understand how traditional cooling systems operate. Out of the estimated 5 billion cooling units globally — including refrigerators, air conditioning, and heat pumps — most function through vapor compression cycles.⁷⁸⁹

Here’s a quick breakdown: imagine you’re plugging in your fridge for the first time. Its mission is now to get rid of all the hot air inside it. The refrigerant (often a hydrofluorocarbon, or HFC) travels through four main components: a compressor, a condenser, an expander, and an evaporator.⁷

Step one: The compressor increases the temperature and pressure of the refrigerant vapor.
Step two: The condenser changes the refrigerant from a gas to a liquid, which releases a lot of heat into the environment.
Step three: The expander depressurizes the refrigerant. As it expands, it chills, making it colder than the interior of the fridge.
Step four: The evaporator absorbs heat from inside the fridge, vaporizing the refrigerant. The cycle starts over with the compressor capturing this warm vapor.^10751112

Although the stars of the show are called “refrigerants,” this system works in a similar way for air conditioning, too. This whole process relies on phase changes and refrigerant properties to transfer heat. But what if we could avoid refrigerants altogether?

How Elastocalorics Compare

How does this relate to elastocalorics? Well, as it turns out, most alloys actually have more than one solid phase, which might sound surprising. We usually think of phases as solids, liquids, or gases, but there can be different solid phases too. A good example is carbon. When it forms a honeycomb structure, it becomes graphite, the soft stuff in pencils. But if it’s arranged in a tetrahedral structure, you get diamond, one of the hardest materials on Earth.

Now, in metal alloys, especially nitinol, two common but harder-to-picture phases are austenite and martensite. And, just like how water absorbs or releases heat when it changes from ice to liquid, these metals do something similar when they switch between austenite and martensite. It’s kind of like how refrigerants work in the vapor compression cycle we use all the time.

Now, I don’t want to compress too much information at once, but how do we practically replace the gas and liquid in our cooling systems with solid wires of fancy metal?

Obviously, we’re not going to be running metal through our compressors and cooling coils. But caloric cooling cycles can actually be broken down into four steps — just like vapor compression cycles. It all starts with the material you’re using. For elastocalorics, that could be something like natural rubber, nitinol, or other types of shape-memory alloys. Next, you apply force to the material, which stresses it. That mechanical energy turns into internal energy, making the material change shape and heat up. The heat that’s generated then flows out of the system, similar to how heat is released in the condensing stage of a vapor compression cycle.⁵

That’s a high-level explanation of elastocalorics, so let’s get down to the wire. Nitinol is superelastic. This means that it can not only shapeshift visually, but structurally — and this phase transformation is completely reversible. It’s sort of like how an untaped cardboard box is square from the front, but deforms into a parallelogram if you put force on it. It then snaps back into place once you remove that force. You can use those same shapes to visualize how nitinol moves between states, but this magic metal goes a step further.

Like a student during exam season, nitinol turns into something different under stress. When stressed, it changes phases from martensite to austenite, which causes it to heat up slightly. But when the stress is removed, the material switches back to martensite, and this phase change cools it down. As the stress is released, the wire “chills out,” pulling heat from its surroundings.

What’s interesting about nitinol is something called thermal hysteresis. That’s a fancy way of saying the temperature at which it heats up under stress is different from the temperature at which it cools down when the stress is released. For example, in high-efficiency nitinol, adding stress at 22°C can heat it up to around 49°C, but removing the stress drops the temperature all the way down to 5°C — about 17 degrees cooler than the start! ¹³ That is just one wire on a lab table, so we can imagine that with enough wires and the proper design, the wires can remove heat from a space, cooling it off.

To be refrigerant-clear, like with other solid-state tech, “solid-state” doesn’t mean there’s no liquid or no moving parts in this context. While caloric cooling doesn’t use refrigerants, you obviously can’t pump a solid material through the system. You still need mechanical input, like a linear actuator, to get things moving. So, depending on the design, a caloric cooling system might still need a heat transfer fluid to deliver that freshly cooled air where it needs to go. In fact, some elastocaloric prototypes do use fluids.⁵ Yeah, it’s confusing, I know. So let’s talk more about those prototypes.

Prototypes and Progress

If you’re like me, when I first heard about all of this I wondered how much progress there’s been on trying to make this a reality. It’s still a fairly new area, but the interest is growing fast. Within the past decade or so, the field has jumped in popularity, with more and more publications and prototypes emerging. In this bar chart from a 2023 paper, you can clearly see the spike in interest.¹⁴

According to a 2024 review, interest in elastocalorics shot up by 160% between 2017 and 2022.¹⁵ But what’s making it such a big deal all of a sudden? Well, elastocalorics are part of a broader group called caloric cooling, where things like electrical, magnetic, or mechanical fields trigger materials to absorb and release heat.

Take electrocaloric or magnetocaloric materials, for example. They use special electromagnetic properties to generate or absorb heat under certain fields. Sounds super futuristic, right? But here’s the catch: they’re really tough to control, measure, and actually put to use. Now, elastocalorics work differently. They rely on something simpler: uniaxial stress — basically pulling or pushing along one axis.⁵¹⁴

And here’s the kicker: elastocalorics perform better than other types of solid-state cooling. They have stronger cooling effects, can save more energy, and can be used in a wider range of applications. As the International Institute of Refrigeration said in 2022, “the elastocaloric effect in commercial-grade materials is already outperforming the best electrocaloric and magnetocaloric materials.”⁵ Plus, shape-memory alloys, which elastocalorics are made from, already have an existing market. So, we don’t have to reinvent the wheel with brand-new materials.⁵¹⁶

As of 2022, over 20 elastocaloric prototypes have hit the scene, and you’ve actually seen some of the action already.⁵ Remember my weird little wires? Elastocalorics’ hottest shape-memory alloy is nitinol at the moment. That’s partly because nitinol isn’t just superelastic — it’s super accessible. I bought mine on Amazon, and it was commercialized for a multitude of medical applications as early as the ‘80s.²

The last few years have seen multiple nitinol-based prototypes, but it’s important to note that they’ve started small. Literally. In a 2022 paper, researchers from Xi’an Jiaotong University and the Chinese Academy of Sciences describe a “fully integrated” elastocaloric refrigerator. It successfully demonstrates the possibility of designing these devices compactly enough for commercialization. But when the authors say “compact,” they’re not kidding. The prototype has a cooling power of 3.1 W, which is enough to cover its 0.9 L compartment.¹⁷

Later that same year, a research team at the Saarland University in Germany debuted an elastocaloric demonstrator system characterized by “artificial muscles” composed of nitinol wire bundles.¹⁸ By 2023, engineering professors at the University of Maryland had developed a device capable of producing 200 W of cooling capacity, which is enough for certain types of minifridges, like a wine cooler.¹⁹ As of 2024, researchers at Saarland had built another prototype: a refrigerator with sufficient capacity for one small bottle.²⁰

The Challenges and Future Potential

I’m sure you’re noticing a pattern here. The prototypes we’re seeing for elastocaloric systems are still fairly small, but hey, big things have small beginnings. Don’t get me wrong: small-scale appliances are pretty much what you’d expect from a lab setting.⁵ And yes, there are still some hurdles to overcome. For one, the amount of heat elastocalorics can remove is pretty small, which means you need a lot of wires and cycles to get the job done. These systems are literally hundreds of years behind the vapor-compression HVAC systems we use today, so there’s still a lot of work needed to optimize them. But there’s a huge potential performance-wise.

In heating and cooling tech, you often rate the systems efficiency with a coefficient of performance (COP). A heat pump system or refrigerator might have a COP of 3, which means for every unit of electricity you put into the system you get 3 units of heat energy back out. It’s why they’re over 100% efficient, which seems like a physics exploit.

I had a chance to speak with Dr. Ichiro Takeuchi from the University of Maryland about their work. He told me that just the materials equivalent efficiency of elastocalorics is around a COP of 20 … but you can’t directly compare that because it’s not a full system. Each component in the system for the final product will eat into that COP performance, which is why he said a final system will most likely be closer to the efficiency of vapor compression at the end of the day.

“…this process is extremely energy efficient. So there’s a good chance that if everything is put together, it could be competitive in terms of overall efficiency compared to the existing vapor compression technology.”

“…when you buy a refrigerator or air conditioner and you talk about COP, that’s a system’s final product, COP. So we are yet to be at a point where we could do one-to-one comparison. It’s difficult to… We’re beginning to be able to do that finally because after 10, 15 years, we’re able to…you know, make something that actually resembles a gadget. You know, we have a big machine in our lab, which has enough capacity to be a small scale refrigerator. But so now we’re beginning to be able to plug it into the wall and then do a one to one comparison with vapor compression.” -Dr. Ichiro Takeuchi

If you’d like to see the full interview with Dr. Takeuchi, where we go in depth on how this works and how efficient things are right now, be sure to listen or watch my follow-up podcast, Still TBD. You can find it everywhere you get podcasts, like Apple & Spotify, but also here on Youtube. I’ll put a link in the description.

Another issue is stress fatigue — the more stress on the wires, the fewer cycles they last. But if you lower the stress, there’s less cooling. Researchers are working on new alloys and techniques to improve how well the materials convert mechanical energy into heat.

Then there’s the challenge of applying stress without using tons of electricity. Actuators need to be efficient and compact, because nobody wants a fridge that’s mostly coils and little space for food. So, elastocaloric systems have to be practical in design and footprint.

Compared to other alternatives, elastocaloric have a lower Technology Readiness Level (TRL).¹⁶ TRL measures how close a new tech is to being ready for market. NASA came up with it in the ‘70s, and it’s now used across industries.²¹²² Since elastocaloric prototypes only started appearing in the 2010s, and the first dedicated conference happened just last year, it makes sense that this field is still pretty early in its development.⁵²³

So why even bother with elastocalorics? The reason is simple. No refrigerants means no hydrofluorocarbons. And that’s a way bigger deal than you might realize.

“The problem is hydrofluorocarbons, as you know, has a high global warming potential. So there is a need to pursue refrigerants that have zero global warming potential.” -Dr. Ichiro Takeuchi

Even though this technology is not yet mature, shape memory alloys like nitinol could play an important part in reducing the use of HFCs. Also, all big ideas, such as finding an alternative to the cooling systems we’ve used for nearly a century, require small steps to get started.

It’s still very early days, but this is definitely an area of study we should be keeping our eyes on. If anything, the transformations are entertaining to watch in real time.