Transmission infrastructure is necessary to transport power generated in one location to the local distribution network where it is consumed. More of it is necessary for some generation sources than others because of how they are located. Because many wind and solar facilities are sited more remotely than many gas, coal, and nuclear plants, they require additional transmission infrastructure. This is also because of the intermittent nature of wind and solar. These sources ebb and flow, and because of that, backup generation is suddenly needed, and therefore a higher degree of grid interconnection and transmission tends to be required.  

A Princeton University Study “Net-Zero America: Potential Pathways, Infrastructure, and Impacts”, generated five possible pathways to achieve the target of net-zero carbon emissions by 2050. All of these pathways require significant expense and the construction of a ton of new infrastructure, but the element of this that is to be examined here is the impact on transmission infrastructure. 

The U.S. currently has about between 200,000 and 240,000 miles of high voltage transmission lines depending on which estimates one looks at. The Princeton study estimated that net-zero would require 2 to 5 times the current U.S. transmission infrastructure, with the requirement varying by pathway. But there is another layer of complexity to understanding what that expansion would require. 

Transmission lines fall into two broad categories, the vast majority of those that are already built use AC currents, the form that electricity is in when it arrives to end users. The other type, DC transmission lines, use DC currents, which are better able to transport power over long distances, but require conversion before reaching end users. Of the transmission lines currently in place, 98 percent use AC currents, but DC transmission has been suggested as a solution to the problem of delivering power from remote wind and solar installations over great distances. 

Because of this, it’s hard to accurately estimate the costs of the transmission infrastructure that the Princeton pathways would require. 

A 2019 Wood Mackenzie report estimated the costs of constructing an additional 200,000 miles of high voltage transmission lines at $700 billion. That is assuming a cost of $3.5 million per mile. This study didn’t differentiate between AC and DC transmission, and was likely extrapolating from existing data on mostly AC generation. 

A 2018 study by the EIA found that “per-mile cost of HVDC projects ranges between $1.17 million and $8.62 million”, so for a build out of 240,000 miles as the smallest Princeton scenario would require it would be between 280.8 billion and 2.07 trillion dollars. If transmission were to be quintupled instead of doubled, the costs would be astronomical, whether the new capacity came primarily from AC or from DC transmission. 

Now for how realistic this sort of build out is. In the last 5 years High voltage transmission line growth has actually decreased. For decades, from the 1970s to the 2010s transmission growth averaged 1.5 percent per year. In the last five years that has decreased to just 1 percent growth. At that rate of growth, 200,000 would only be 261,000 by 2050, and 240,000 would only be 313,970. Both of those are a far cry from doubling the current transmission mileage. 

In order to accelerate construction on transmission projects, there are significant hurdles that would need to be overcome. There are numerous issues that currently limit the pace of new projects, including permitting issues, eminent domain, and getting buy-in from states along the routes of planned development who don’t directly benefit from a project. 

Transmission is a complicated issue, and one where it’s hard to find straightforward numbers that are more readily available on other topics, but even with the more limited subset of research, it’s clear that transmission expansion on the scale that net zero requires is more of a challenge than it may at first appear.