Panel Discussion on Thermoelectrics
Thermoelectrics Session, MRS Memorial
Gang Chen: Let me get us started with some questions. My first question is: What are the major challenges for the field? Anyone want to take that on?
Jos Heremans: Applications — or, rather, the lack of applications. Like Jihui, I come from the automotive industry, and in 2008, there was a very nice prototype of a waste heat recovery system developed by a company that is now called GenTherm, and it made one kilowatt of power — which would have saved a considerable amount of fuel, and it was beautiful. It worked very well! But it was totally unaffordable. And so, although the whole automotive waste heat recovery also gave the thermoelectric field a boom — because that problem is a really important application in fuel economy — but it just didn’t work because it was too expensive. And that is actually a major problem for the whole field. If we’d had, back then, a material that not only had a good ZT, but was also totally affordable (and when I say ‘affordable’, I mean by two orders of magnitude; thirty dollars instead of three thousand dollars, for instance), then the field would have taken off. But that didn’t happen. Therefore, I think that major effort should really be an affordability — not just the materials, because materials costs are only about 30% of the thing — but the entire system cost. And affordability, there, means simplification.
Jihui Yang: I completely agree. We have actually very few people attending the MRS session on thermoelectrics from people who are working on engineering of modules and the systems. This is something that we may want to change, as a community. I was talking to Professor Chen from the Chinese Academy of Science, yesterday. Even on the material level, we have very few people working on lower cost thermoelectric materials — which is extremely important.
Arun Majumdar: Maybe I can give a slightly different perspective in the application area. We had a Paris agreement in 2015, and that is very well known. What is often not well known is the Kigali amendment to the Montreal Protocol, which pertains to HFCs. Now, HFCs (hydrofluorocarbons, which are used in our cooling systems) have 2000-3000 times lower Wong potential compared to CO2. So the idea in the amendment is to phase out HFCs. This means that we need new technology for cooling. Simultaneously, the demand for cooling systems in emerging economies — which are mostly in the tropical regions — is about to go up. So you’ve got this double whammy coming.
I think the use of other technologies for cooling systems is extremely important, and thermoelectrics is going to have to compete. Now, large system vapor compression is very efficient. The COPs are on the order of 5-7, or so. But if you try to take a vapor compression system and make it smaller — it doesn’t work. Your COP goes down. And the rest of the world — the emerging economies, especially — do not need refrigerators as big as the ones we use in our homes. It’s okay to make smaller ones. So I think this idea of smaller cooling systems is very important. But it will have to compete with other systems.
On the scientific side, the challenge, frankly, is overcoming that Bermuda Triangle of electrics, which I mentioned in my talk. It’s a strong one, and a very important one. And we really have to look at new science coming out of the science side. Whether you have different entropy — like the ones that you were talking about in terms of spin — or if you have correlated systems that have different kinds of degeneracy on the electronic side, that’s one. On the phonon side, we’re looking at various things we have been discussing for decades now. It’s an open area. I know that Gang is trying to do that, as well. Can we look at wave interactions of phonons and can we do Anderson localization? We have not been able to do so in a very definitive way.
So I think the introduction of new science into thermoelectrics is going to be the next big phase. And connecting it to some of the applications we’ve talked about — those are the big challenges.
Zhifeng Ren: I agree. Applications are very important. Anything we do, we’re using taxpayer money to do research. And if that’s only for our own curiosity — in the end, that will not work. We have to have something that will come back to serve the society that paid us to do the research. But right now, I think that finding more applications would definitely be urgent. If you look at the thermoelectric community, probably many more people work on materials — trying to improve materials and trying to discover new materials — but it’s still a long way to go, to get from materials to applications. And even then, even if you can find engineers who can make devices work, you need to really ask yourself... where’s the market? Right now, we do not have any end-user to really say, “I want to buy 1 million modules!” If we had that, it would trigger the whole system and materials development — better materials, better contacts, better devices. Therefore, I really feel that, at the moment, there’s no end-user. While it's true that thermoelectrics is very promising and has a lot of potential — potential for both power generation and for cooling devices — we still always need to ask ourselves, “Where is the market? Where is the buyer?” That’s the problem.
Mona Zebarjadi: I just want to add to that I completely agree. I’d also like to add that one of the applications that has, perhaps, not been the focus of our electronics thus far would be a system for the electronic cooling of chips. For things like that, there are not many options available. So maybe thermoelectrics could be an option, there.
Gang Chen: I’d also like to jump in on this discussion. I completely agree with Zhifeng. Jihui probably remembers when DOE/EERE had this automobile project — I thought to myself… ooh, automobiles! That’s the hardest application!
But at the time, I was also really optimistic, and I kept saying, “I really hope this will be successful, because if this is successful, then everything is easy!” Because, of course, automobiles have a mass market, and making a breakthrough in the automotive industry would give thermoelectrics a lot of visibility. But automobiles have a weight-cost sensitivity, and the expenses were too high.
When Zhifeng and I were trying to commercialize thermoelectric applications in our company, GMZ, we looked at the combined utilization of thermoelectrics for hot water and electrical power. We generated entropy in a home furnace, where we have such a large ΔT, but where we can basically use hot water to heat up the house. Essentially, what we’re really generating is entropy. Therefore, I thought it might be good for the thermoelectrics community to look at getting into the market for these types of things. It’s not necessary to start with the ambition of, “I’m going to conquer everything!” We need to have a product and really get into the market. That’s one.
Secondly, we know that any time we go from material to market, it takes a long time. Now, when GMZ first started, we were all very ambitious, and I remember Zhifeng saying, “Okay, we’re going to sell powders! We’re going to sell the material!” But nobody came.
It turns out that if you think about taking a material to market, you first have to realize that you need contacts. The black magic of companies are their contacts. You see, everybody in business spends a lot of time making contacts, but those who already have them don't want to tell new people how to make their own.
So while it’s true that there’s a lot of good basic science being done, and that the science we’re doing can translate to a lot of potentially interesting applications — it’s very different actually trying to create and sell that application. You have to put a lot of time and effort and money into it, and although you develop some really interesting science, trying to make it all work, it’s the kind of science that’s hard to publish in Nature or Science or any good scientific journals. The success of the applications really is defined by whether or not they sell on the open market.
So really, every step from contacts to collect the heats, reject the heat… there are a lot of challenges that need to be solved to take a material to the market. Those are really the lessons we learned.
Jos Heremans: I wanted to add a little bit to Arun’s comment. It is true that it is easier to get a high ZT in a waste heat recovery system, because of just the T. If you're going to be working at 1000 Kelvin, the T really helps you. But it is also true that the vast majority of the market is actually in cooling. It’s bismuth telluride, it's at room temperature, and the T is only 300 Kelvin. So if we look at cooling, now we have problems. Right? Because we have a lower T and, in fact, there is a demonstration possible that will show you that the ZT actually scales as T to the power of seven halves. That then gets us to these other new ideas.