How keeping cool just went green, thanks to solid refrigerants

Refrigerants, (the chemicals that are used in air conditioners to reduce temperatures), have a high global warming potential (GWP)l. For the uninitiated, GWP is a measure of how much energy the emissions of 1 ton of a gas will absorb over a given period of time, relative to the emissions of 1 ton of CO2. The larger the GWP, the more that a given gas warms the Earth compared to CO2 over that time period.

When these leak out into the atmosphere, it massively impacts the climate. This is because they are very efficient at absorbing infrared radiation and trapping heat inside the atmosphere. The hydrofluorocarbon, hydrochlorofluorocarbons and other refrigerants used in these and other cooling devices are potent greenhouse gases and major drivers of climate change. The GWP of refrigerants is thousands of times greater than CO2, and some are 13,850 times more potent.

So, we need environmentally-friendly solutions before it’s not too late. Step forward Adam Slavney Ph.D. and his team at Harvard University’s Center for the Environment, who have created a prototype device that could someday replace existing “A/Cs”.

Their new version is more environmentally friendly and uses solid refrigerants to cool a space, in what is claimed to be a world first.

The advantage of this is that, unlike gases, they won’t leak into the environment. When looking into this, Slavney and his team found that barocaloric materials, a class of solid refrigerants, work similarly to traditional gas-liquid cooling systems – they use pressure to go through heat cycles, driving a solid-to-solid phase change.

To find out how exactly this came about, the potential the discovery has in real-world terms, and the challenges the team will have to overcome before this is a commercially-viable option, IE took the opportunity to catch up with Adam Slavney, before the next phase of the project gets going in earnest.

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Interesting Engineering: What prompted this research in the first place?

Adam Slavney Ph.D.: My colleague, Jinyoung Seo, was originally looking to use these materials as passive thermal energy storage media because they undergo solid-solid phase transitions with very large latent heats.

This means they can store a lot of heat during hot parts of the day or year and release it during colder periods. Then Jinyoung realized that the phase transition temperature of these materials was very sensitive to pressure which unlocked this whole world of barocaloric cooling for us.

Did you achieve what you set out to achieve?

This certainly wasn’t where we were originally intending to go, but, once we focused in on barocaloric cooling we started to make a lot of progress, particularly in terms of improving the fundamental thermodynamic performance of our materials.

Are there any other aims to the work that you have yet to achieve?

Ultimately, we want solid-state refrigerants, either our materials or others, to completely replace the existing gaseous hydrofluorocarbon refrigerants in use in pretty much every air conditioner and refrigerator today.

That’s a really big task since, until our work, there hasn’t really been any demonstration of barocaloric cooling in the real world and there certainly isn’t a commercial product someone can go out and buy. So, I think we still have a long way to go.

What barriers stand in the way of you making your solid refrigerant findings commercially available?

Currently, I think the biggest challenges are in device engineering and in further reducing the operating pressure of the system. We need to figure out ways of efficiently transferring heat to and extracting heat from our barocaloric materials as well as find new materials which can operate at even lower pressures than we have currently achieved (ideally in the 5 – 30 bar range).

Beyond that, while our materials so far have been able to endure thousands of cycles without much loss in efficiency, in a commercial system they would need to be able to last for millions of cycles and we need to show that they will hold up over that time period.

What sort of time-scale do you anticipate before (if) you can make that happen?

We are still at the pretty early stages in thinking about the best toward commercializing our technology. Given the increasing importance of sustainable cooling technologies, we are optimistic that we can rapidly advance this technology to a commercially viable product.

Are there any barocaloric materials in particular that you feel optimistic about in this regard?

We’ve made a lot of progress here actually and continue to find new promising materials which are improving on our existing ones. A lot of those haven’t been disclosed yet, so I’m afraid I can’t provide you with any details.

What are you and the team working on now, as a result of these findings?

So as I mentioned, I think the biggest challenges are in designing a device which can transfer heat efficiently and on lowering our operating pressures. Those are the main things we are working on currently.

Tell us more about your custom barocaloric prototype device and what it might help you to achieve.

For the most part, I think we’ve already achieved what we wanted to with this first prototype. We were able to demonstrate that barocaloric cooling is actually possible in the real world and gained a lot of important practical knowledge.

Getting the prototype working required solving a thousand little problems, most of which I hadn’t anticipated. Learning to solve those problems will make the second generation prototype a lot easier, which is what we are working on now.

Has there been any recent progress or new challenges that have arisen?

We’ve made some progress on both materials and on device design, but nothing I can talk about in detail yet I’m afraid.

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Quickfire questions

What or who inspires you?

I’ve become increasingly concerned about climate change over the past decade or so. Currently, I think we are in a narrow window in time where both the effects of climate change are obviously all around us and we still have a chance to do something about it.

I’d love to contribute to that. It’s an exciting and scary time to be alive.

What gets you out of bed in the morning?

I love working on science and engineering problems. Sometimes I wake up in the middle of the night thinking about them and can’t get back to sleep.

What makes you smile?

My dog Abel. He’s a cute, silly pup.

What is your greatest achievement to date?

My (now) wife and I did five years of long distance while I was in graduate school. Making it through that feels like a pretty big deal.

What is your biggest regret?

I have a lot of little regrets, but I don’t know if I have any big ones.

What would you say to someone wanting to follow in your footsteps?

Start working on real-world implementation of your technology earlier than you think you’re ready to. There will always be things you didn’t anticipate and it’s nice to find them out sooner rather than later.

What advice would you give your younger self?

Get a dog earlier.

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