A Look at the Future of Transmission: Getting Clean Electrons to Where They Need To Be

NREL Senior Engineer Jarrad Wright Describes Transmission Expansion Needs and Trade-Offs in a Low-Carbon Future

April 30, 2024 | By Jarrad Wright | Contact media relations

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This installment of the National Renewable Energy Laboratory's (NREL's) Tell Me Something Grid series features Jarrad Wright, a member of NREL's Grid Planning and Analysis Center (GPAC) and researcher who is part of the National Transmission Planning Study. Wright discusses transmission technologies, transmission needs, and how transmission might operate in the future to support a low-carbon grid.

Studies of clean electricity futures in the United States have shown that transmission is likely to play a substantial role in a cost-effective transition to a decarbonized grid. In some cases, transmission capacity may need to increase by more than double to move the large quantities of low-cost renewable electricity to where people need it. That shift in resource mix and level of transmission expansion would completely change how transmission systems are planned and operated.

I have a general interest in electricity access and affordability, considering my South African heritage where electricity access levels were historically very low. I have also undertaken generation and transmission planning in South Africa and on the African continent, so I suppose lighting up Africa has been and still is a big passion of mine.

I'm currently on a team at NREL that is studying how a transformed power system with significant levels of transmission infrastructure might operate. We're specifically focusing on the potential of interregional transmission, which, like it sounds, carries electricity super long distances across regions that have historically largely planned for only their own geographic footprint.

This work is part of the National Transmission Planning Study (NTP Study) led by the U.S. Department of Energy's Grid Deployment Office and done in partnership with the Pacific Northwest National Laboratory (PNNL). Results will be shared later this year, but I'm excited about our work so far and I couldn't wait to share some thoughts and insights. We've modeled hundreds of scenarios with various constraints to understand trade-offs of different transmission options and ultimately inform regional and national transmission planning strategies.

Detailed Technical Rigor

Our team applies a range of modeling tools to study future transmission needs, starting with NREL's flagship Regional Energy Deployment System (ReEDS) model that co-optimizes current and estimated future electricity supply, demand, storage, and transmission needs in 134 zones across the country.

What is cool and unique about the NTP Study is that we dig even deeper than these zones and model down to the nodal level, or substations where transmission lines, underground cables, and transformers intersect. I am a lead researcher of this part of the study, alongside a number of my colleagues at NREL and PNNL. As part of this, we developed a set of techniques to disaggregate the zonal outcomes and expand transmission at a nodal level for further analysis in production cost tools (for example, NREL's Sienna) and selected commercial power flow tools.

From there, we can examine many questions: how power wants to flow; how the future transmission system performs with a given amount of wind, solar, hydro, storage, existing and augmented thermal generation, and new high-voltage transmission lines; and what happens on the system under particularly stressful conditions. All these questions are answered with models that are nodal, with a few hundred thousand substations and transmission lines, to reflect the rigor used in industry planning practices. We have seen some very interesting outcomes in our modeling results.

More Interregional Transmission Enables Lower Costs

Building interregional transmission is difficult for several reasons. To name a few, regions tend to plan by themselves, it is difficult to split costs with neighbors in a way that seems fair to both sides, and rights-of-way across several jurisdictions (federal, state, county, private land, etc.) can be difficult or impossible to obtain. However, we are finding in our model results that the overall system costs are generally lower when there are no constraints on building out interregional transmission. This is particularly true in systems with high amounts of wind and solar. Watch a quick video on interregional transmission with my colleague and principal investigator of the NTP Study, David Palchak.

If we limit building interregional transmission, wind and solar get built in locations that don't have the absolute best resource, and we tend to see a shift in preference for less wind and more solar nationwide. This constraint on building transmission can also lead to the deployment of other technologies, like nuclear and gas plants retrofitted with carbon capture and storage. Overall, wind and solar in less-than-ideal locations and the increase in other technologies costs more than just building the interregional transmission.

Is the Era of Ubiquitous HVDC Upon Us?

Almost all of today's transmission system consists of high-voltage alternating current (AC) infrastructure. However, the role of high-voltage direct current (HVDC) infrastructure and specifically multi-terminal HVDC grids could increase in futures with a lot of wind and solar plants located where there are the best resources, sometimes far from the big cities where electricity is consumed. Major investments in HVDC deployment and research have started trending upward around the world because of these dynamics. The reason is that HVDC lines are more efficient and cost-effective than high-voltage AC lines at moving larger amounts of electricity (2,000–5,000 megawatts) across longer distances (greater than 500 kilometers, as a general rule of thumb).

Another benefit of HVDC is that it can utilize underground cables over long distances, whereas equivalent AC lines over similar distances experience technical challenges. This advantage could assuage some of the right-of-way challenges with all transmission lines. Because of these benefits, we start to see in our model results more HVDC infrastructure being preferred over strictly AC transmission expansion in futures that are highly decarbonized and where we let the models build large amounts of HVDC.

Power Flows in All Directions, at Larger Scales

Power flow patterns in a future with lots of interregional transmission—driven by a generation resource mix of mostly wind and solar—are also quite different than today. For example, in some scenarios we see power flow into an area during certain days and seasons and flow out to neighboring regions during other days and seasons. This is fundamentally different than traditional systems, where power flows almost exclusively from large thermal or hydro generation plants toward demand. The changes in direction and magnitude are largely driven by the range of different resource mixes in some of the scenarios we are analyzing and the increased ability to transfer power across large geographical regions and lean on neighbors in times of need.

Interim Steps To Increase Transmission Capacity

Transforming the transmission system is complicated. The land where transmission is deployed has many uses, and siting and building new transmission lines can take many years—especially with building interregional transmission in new corridors, which requires permitting and regulatory approvals that can take several years alone to obtain. Lots of individuals, communities, and organizations are trying to figure out this challenge, many of whom have engaged in the NTP Study, and we are incorporating as much of their perspectives as possible in our analysis and findings.

We can also start to meet some of the interregional transmission capacity needs by better utilizing existing infrastructure and rights-of-way—the designated area/corridor where infrastructure is installed to transmit electricity. Enhancing current infrastructure might include upgrading lines to carry more power; adding more transmission lines within the same corridor (the tract of land that is owned or leased by a transmission provider); upgrading lines to higher voltages; optimizing the transmission network topology; or implementing dynamic line ratings (enabling grid operators to adjust power transfer in real time based on ambient environmental conditions). Increasing the utilization of existing transmission via the use of battery storage placed strategically on either side of a set of large transmission lines can also increase utilization of transmission infrastructure. You can watch an NREL video to learn more about some of these options for increasing transmission capacity.

Although options like these are not the full answer to expanding our transmission system, they can address some immediate needs and therefore are an important part of the solution. Please keep an eye out for the full study results from the NTP Study later this year that will help guide the future of transmission in the United States.

Stay tuned for more articles in the Tell Me Something Grid series, and sign up for NREL's energy analysis newsletter. Learn more about the National Transmission Planning Study.

Tags: Energy Analysis,Grid Modernization