Minjoo Larry Lee is an associate professor of electrical engineering at the Yale School of Engineering and Applied Science. He recently won a $2.5 million grant from the U.S. Department of Energy’s Advanced Research Projects Agency for Energy to develop dual-junction solar cells that are operable at temperatures above 400 degrees Celsius. Lee will collaborate with Emcore Corporation and the National Renewable Energy Laboratory to combine two major forms of solar cell technology: concentrated solar power (CSP) and concentrated photovoltaic technology. The News sat down with Professor Lee to understand the latest about his research.
Q: How would you describe CSP technology to the general public?
A CSP involves [an] optical lens — basically like a huge magnifying glass — that collects a lot of light and focus[es] it down on a small location. What you do is you take that sunlight and turn it into heat. In some sense, that seems straightforward; if I had a black piece of metal, it would get pretty hot if I put a lot of sunlight on it. But it’s actually more subtle than that. You need to find a material that can get really hot and survive being heated up and cooled down every day. The really interesting part is that you use fluids. These are high-tech materials, often synthetic oils, [that] basically transport heat away from the target where the sun is shining on. The beauty is that the fluid can be stored rather efficiently in tanks. Once you have the fluid, it must travel in very well insulated pipes. The hot fluid in the tank is later redeployed. Its heat is converted to steam, and the steam is used to spin a turbine.
Q: What are the disadvantages of CSP?
A: So far, the disadvantages have a lot to do with the specific implementation. The challenge is that for now, and for the foreseeable future, the cost [of implementing CSP] is a bit too high. It’s not for lack of trying. CSP has been around and been prototyped for decades, but to date, there are really only a few operating plants in America and Europe.
Q: What is the difference between CSP and the normal photovoltaic solar cell panels [that you see on rooftops]?
A: The difference is that photovoltaics don’t use any fluids or moving parts. They simply take light and directly convert it to electricity. One of the nice things about photovoltaic cells is that you can make small installations, and it’s not cost-prohibitive. That said, there isn’t a very easy way to store it. You can certainly store photovoltaic electricity in batteries, but it’s not very cost effective, it’s inelegant and it’s arguably not so great for the environment.
Q: You mentioned that your research bridges these two technologies. Could you describe what your research focuses on?
A: Technically, we’re bridging a related area to CSP called CPV (concentrated photovoltaics). You use a different solar cell architecture for CPV — multi-junction solar cells. These cells are really efficient, but they’re also a lot more expensive than conventional home silicon solar panels. I’m trying to bridge this world with CSP. ARPA-E asked people to marry CSP and CPV because one thing they realized is that solar cells deliberately give up some solar light. This is done in a very optimized way — it’s so difficult to convert into sunlight directly that we’re better off giving up on some of it. Some percent is [also] turned into heat [by the process]. What ARPA-E realized is that the heat is a bit of an opportunity. What if CSP could also even take the heat that was rejected by the photovoltaic conversion process? They took some of those what-ifs and realized that [by] operat[ing] a [combined CSP/CPV] solar cell at greater than 400 degrees Celsius, you could get a higher system level efficiency than either technology by itself. We [also] want what’s called “dispatchability” — the ability to store energy to use when we want it. The idea here at Yale is to take all the UV, all the visible [light] and a bit of the IR, and turn that into electricity using the photovoltaic effect. Then, we’ll use the heat generated and the IR photons to heat the fluid. It would create some electricity that we’d have to deploy right away, plus a large portion that would be storable.
Q: What applications do you see for this project in industry and for future research?
A: A small, but very interesting application is the use of electronics in extreme conditions. There’s also been a lot of interest in getting satellites closer to the sun. A really interesting project out there is trying to design a satellite that’s going to get pretty toasty — 250 degrees Celsius. If this project is successful, and the next three years go the way we and ARPA-E hope they will go, we hope this will create all sorts of other applications, too.