Taking On Renewables’ AC/DC Disconnect

8:51 minutes

solar power station with two people in hard hats walking through many solar panels
Credit: Shutterstock

In the push to transition society to more renewable energy sources, there are several logistical challenges. One central question involves the best way to connect solar panels and battery storage—which both produce direct current, into an energy grid that primarily provides alternating current at the local level. 

Dr. Suman Debnath leads a project called the Multiport Autonomous Reconfigurable Solar power plant (MARS) at Oak Ridge National Lab. He and his colleagues have designed a system of advanced power electronics that allow large, utility-scale solar facilities and battery storage projects to feed either AC or DC power, as needed. The approach, Debnath says, will both allow for better integration of those electric resources into the grid, and make it more possible to transport power long distances using more efficient DC transmission lines. 

Debnath talks with Ira about the MARS project, and ways to modernize the country’s power distribution system for greater reliability and efficiency. 

Segment Guests

Suman Debnath

Dr. Suman Debnath is the project lead for the Multiport Autonomous Reconfigurable Solar power plant (MARS) at Oak Ridge National Lab in Oak Ridge, Tennessee.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. As the drive to bring solar energy and batteries into our electric grid expands, there are sometimes logistical challenges, nuts and bolts stuff like, how do you tie all the solar panels into the grid? Along with the battery storage needed to provide for times when the sun isn’t shining. And how do you feed power east and west across the US during peak times when needed?

Well, part of the solution lies in redesigning our grid to carry high-voltage direct current instead of AC, alternating current, that dates back to the time of Thomas Edison and Nikola Tesla. Someone who is working on the problem and has a solution is my next guest Dr. Suman Debnath leads a project called the Multi-port Autonomous Reconfigurable Solar Power Plant, or MARS– easier way to say it– at the Oak Ridge National Lab in Oak Ridge, Tennessee. Welcome to Science Friday.

SUMAN DEBNATH: Thank you, Ira, for that introduction. Appreciate having me on the show.

IRA FLATOW: Did I describe the challenge accurately? What is the problem that you’re trying to solve?

SUMAN DEBNATH: I think you were spot on. One of the things that we are looking at is we are trying to identify mechanisms by which we can integrate solar and energy storage to DC and AC transmission. That’s the specific challenge that we were trying to target here.

Additionally, we are trying to utilize also DC transmission to tide over some of this variability because the DC transmission provides the capability to connect over different time zones, which means that when you are seeing solar generation in one time zone, you may not see solar generation in the other time zone. One example that comes to my mind is the capability to connect East Coast to West Coast, for example.

IRA FLATOW: What does DC get you that alternating current does not?

SUMAN DEBNATH: When you’re sending power over long distances, our DC transmission is less lossy and is much more cost effective when compared to AC transmission. At the same time, DC is also able to provide controllability in the flow of power, so just like one of the examples that I had mentioned earlier that you’re trying to connect different time zones and different regions in the United States, which means that you are able to send power with controllability capability that DC brings in.

Thirdly, DC has the capability to be able to send emergency power from an unaffected region to an affected region during an extreme event. So for example, when hurricanes happen and it affects a specific region, using DC, you can send controlled power flow from an unaffected region, which is not affected by a hurricane.

IRA FLATOW: My solar panels– the solar panels that these large generating stations are producing direct current, and what comes into my house is alternating current, so you have to find a way of mixing them together. So how did you do that? I mean, what is actually inside your magical black box that allows it to run smoothly now? What did you do?

SUMAN DEBNATH: So we looked at a unique electronic architecture which is cost-effective and reliable, the black box that we see today in terms of being able to connect solar and energy storage to the power grid– and also, for that matter, at our homes, which are much smaller solar and energy storage.

The black box that is present between them is [INAUDIBLE] electronics. The difference that we were looking at is the uniqueness in the architecture that we were trying to evaluate because we were trying to go from much lower voltages, like thousands of volts I had briefly mentioned, to hundreds of thousands of votes in both DC and AC. So that’s where the uniqueness in the architecture came into picture.

IRA FLATOW: Do you have any test systems? Do you have anything up and running?

SUMAN DEBNATH: We have, at this point, laboratory tests. So this is still in research phase. We are not yet full fledged in terms of development, which is our next few steps that we have in our pipeline.

IRA FLATOW: Mm-hmm. And what will you need to get this to work? Are we going to need new transmission lines, new ways to interconnect regions and all that?

SUMAN DEBNATH: So today, it is predominantly an AC transmission power grid that we have. At the same time, there is increased interest in DC in the past decade through the more DC lines you are seeing in the regions, some of the regions, and you’re going to see much more in the next few decades as well.

So it is possible to integrate this technology in the location where there are existing DC lines, today and that was one of our case studies that we were looking at. At the same time, we were also looking at case studies where we were trying to connect to the DC lines that are coming up in the near future as well as in the next couple of decades.

IRA FLATOW: How were you able to get past the problem that Thomas Edison faced in his DC transmissions when he couldn’t get them to go more than a block? I always thought that long-range DC would lose current in the lines. Why does that not happen?

SUMAN DEBNATH: Very interesting question, Ira. I was expecting this at some point, that the war of currents will come up that happened in the late 19th century. The reason that AC transmission won over DC is the fact that there were transformers, which means that you could change from a low voltage to a high voltage and high voltage back to a low voltage with ease.

Just like you mentioned, it makes it much less lossy to be able to transmit over long distances using AC transmission because you have those transformers which could change from low voltage to high voltage. Whereas for DC back then, they didn’t have easy mechanism by which you could convert a low voltage, and so you’re transmitting at low voltage, which was more lossy.

Now, with significant enhancements that have happened in power electronics, which is the technology that will help with converting DC and AC, you are able to now convert between a low voltage DC to a high voltage DC. And you’re able to actually convert between AC and DC. So there have been significant advances that are happening and have happened [INAUDIBLE] electronics that are assisting us now.

IRA FLATOW: Are you saying then that we will be transforming our system mostly into a direct current, into a DC system for everything?

SUMAN DEBNATH: I believe it’s going to be a combination. So today’s existing system will still be prevalent in terms of an AC transmission. At the same time, DC transmission will also be increasingly being introduced. So it will be a hybrid solution that you would have– where you would have both AC and DC leveraging the benefits of DC where it becomes important to use DC while continuing to use AC where we can continue to use AC.

So when we are looking at transmission, also there are shorter distance transmission and longer distance transmission. The shorter distance transmission will still be alternating current. The longer distance would be a combination of alternating current and direct current. Because alternating current is already existing, the newer ones you’re going to see is going to be a lot of DC that is going to come in as we move forward.

IRA FLATOW: So I’m going to still be getting that AC power into that transformer on the telephone pole outside.

SUMAN DEBNATH: You are. You are going to see that still. You’re going to see that at least for some time in the future.

IRA FLATOW: Are there still problems to be worked out here, and what’s it going to take– I’m talking about a timeline here– for these to be on the grid?

SUMAN DEBNATH: The work that we were looking at, the multi-port autonomous reconfigurable solar power plant– or the acronym MARS that you mentioned– we have a commercialization timeline that we have, and we are looking at moving from laboratory-scale prototypes and laboratory-scale research that we are doing to more of controlled field environment, testing in the next 5 to 10 years for medium voltage.

So when I say medium voltage, it’s going to be in the tens of thousands of volts that you mentioned, Ira, sometime back. And at the same time, we would also be trying to look at the next step after that, which is going to be in hundreds of thousands of volts, which is the transmission grid scale that I mentioned. That will be in the next 10 to 20. Years

IRA FLATOW: There are people who say that big central power is the thing of the past. We should be doing neighborhood generation, microgrids. How do you answer that?

SUMAN DEBNATH: Very interesting question, Ira. So it is still cost effective to have large, centralized generation. At the same time, when you have micro-grids, it does provide reliability benefits. What I would envisage at the end of the day would be a combination of the two, where you have this large central generation, which is still cost effective, and having decentralized power where it is necessary and important to have much more reliability as well as have the capability to have emergency source of power.

So it’s going to be a combination of the two as we move forward.

IRA FLATOW: Dr. Suman Debnath leads a project called MARS at the Oak Ridge National Laboratory. Thanks again for being with us today.

SUMAN DEBNATH: Thank you, Ira, for hosting me. It’s been a pleasure.

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