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? I’m Matt Ferrell … welcome to Undecided. This video is brought to you by Factor.

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. And  here’s a new fascinating contender.

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. 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. 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. 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. 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. This means getting creative.

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. 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. 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. 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 does this relate to elastocalorics?

Well,  as it turns out, most alloys 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?

Speaking about cooling and things like  refrigerators is making me think about food. Seems like now is a good time to take a quick  snack break with today’s sponsor, Factor. My schedule is so hectic it’s made it challenging  to prepare nutritious meals at lunch time.

I was taking shortcuts and eating the easy, not so  healthy stuff … way too much. Factor really helped keep the stress out of planning and preparing  what I eat, while keeping it nice and healthy. Fun fact, I actually signed up for Factor well before  they were ever a sponsor … two years ago.

One of my favorite things is the variety. Factor offers  35 wholesome meals every week to choose from, like Gourmet Plus, Keto, Calorie Smart, or Vegan +  Veggie options. It’s easy to find something you’ll love.

I’m kind of obsessed with their Peanut  Buddha Bowl. Factor isn’t just flexible with what you eat each week, but you can pause or reschedule  your deliveries anytime. No prep, no mess, never frozen, and ready in just 2 minutes.

Head  to FACTOR75 dot com or click the link below and use code 50UNDECIDED to get 50% off and free  shipping on your first Factor box! That’s code 50UNDECIDED at FACTOR75 dot com to get 50% off and  free shipping on your first box! Thanks to Factor and to all of you for supporting the channel.

So,  how do we use solid wires in our cooling systems? 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. 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 an exam, 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!

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.

Yeah, it’s confusing, I know.  So let’s talk more about those prototypes. 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 idea, 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.

According to a 2024 review, interest in  elastocalorics shot up by 160% between 2017 and 2022. 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.

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.

” 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. As of 2022, over 20 elastocaloric  prototypes have hit the scene, and you’ve actually seen some of the action  already.

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. 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.

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. 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. As of 2024, researchers  at Saarland had built another prototype: a refrigerator with sufficient  capacity for one small bottle.

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.

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 that needs to be done 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  a vapor compression system 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. 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).

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.  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.

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.

Even though this technology is not yet mature,  shape memory alloys like nitinol could play an important role 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. But what do you think? Would you be interested  in an elastocaloric fridge or air conditioner?

Jump into the comments and let me know and be sure  to listen to my follow up podcast Still TBD where we’ll keep this conversation going. Thanks  as always to my patrons for your continued support and a big welcome to new Supporter +  members Tayler Busalacchi and Richard Campbell, as well as Producer Michael R. Koch.

I  hope I didn't butcher any of these names. You really help to keep this channel  going. I’ll see you in the next one.