While the United States is rich in renewable resources for the production of green electricity, expansion and modernization of the North American power grid will be required to transmit that power to where it is needed.
Superconductor Electricity Pipelines: An Optimal Long-Haul Transmission Solution
Jack McCall - American Superconductor Corp
Superconductor Electricity Pipelines are uniquely and ideally suited to address all of the requirements to move renewable energy to distant load centers:
- Highest power capacity
- Highest efficiency (lowest power losses) of any transmission technology
- Ideal for very long distances
- Capable of transferring power across the three U.S. interconnections
- Able to accept power from multiple distributed sources, and precisely deliver power to multiple distributed destinations
- All underground construction with very small (25’) right of way requirement.
- Simplified cost allocation due to precise controllability of DC terminals.
- Minimizes interaction with existing AC grid, reducing costs and increasing operational flexibility
Wind and solar power today account for a relatively small portion of electricity generation in the U.S. Those ratios, however, will rise rapidly as the country, like others around the world, increasingly turns to renewable sources to meet rising power demands and reduce CO2 emissions. The inability of America’s existing transmission system to move clean electrical energy from the country’s rural heartlands where it is generated to the high-demand urban load centers remains a primary barrier to achieving the 15-25% renewable energy objectives being outlined by the Obama Administration and Congress. In order to continue developing renewable resources, the North American electric power grid must be expanded and reconfigured to facilitate the challenge of moving large amounts of electricity over these very long distances.
The scope of this leading energy challenge will encompass the transmission of 100 gigawatts (GW) or more of green power. Networks of interstate, overhead 765 kV power lines that would require rights of way hundreds of feet wide are one solution actively being considered to achieve this objective. In addition to the significant impact to the nation’s aesthetic landscape, this traditional approach could require significant eminent domain activity and have a rather significant environmental impact.
A new option is now possible. Placed underground in existing rights-of-way, Superconductor Electricity Pipelines could carry many gigawatts of power more efficiently and at a similar cost to overhead lines, all in a pipe just three feet in diameter.
Superconducting Power Cables
As the name suggests, superconductor cables utilize superconductor materials instead of the copper or aluminum traditionally used to carry electricity in overhead power lines and underground cables. Superconductor materials provide two major advantages. First, wires made from superconductor materials conduct approximately 150 times the amount of electricity that can be conducted by copper or aluminum wires of the same size. This power density advantage drives system economics and is fundamental to the reason underground superconductor cables can achieve cost parity with overhead AC power lines over long distances. Secondly, when transmitting DC power, superconductors have absolutely zero resistance to the flow of electricity, which means that DC superconductor cables are literally perfect conductors and introduce no electrical losses of their own.
Superconductor materials must be refrigerated to exhibit their ideal electrical characteristics. The cables are cooled with conventional liquid nitrogen refrigeration systems that are widely used in a variety of industries. While some power is required for the refrigeration - lowering the overall system efficiency - superconductor power cable systems still have much higher overall efficiency than any other long-distance transmission system.
The cables employ superconductor wires that are commercially well established and are available from multiple producers globally. Superconductor cable systems are now operating in multiple in-grid sites around the world, demonstrating their reliability and performance, as illustrated in Figure 1. While all previous installations are AC applications, applying this established technology to DC is straightforward. Because superconductor cables are compact, light, and emit no heat or electromagnetic fields (EMF), they are particularly easy to install, even in close proximity to other underground infrastructure.
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Figure 1: 138 kV AC superconductor power transmission cable operating since April 2008 in Long Island Power Authority’s grid. Courtesy of American Superconductor.
Resolving Key Transmission Challenges
Conventional overhead transmission lines require new corridors hundreds of feet wide. The time-consuming and potentially litigious siting process often is a significant roadblock to developing new renewable power in the U.S. Difficulty siting new transmission lines, in fact, was one factor leading to the recent suspension of T. Boone Pickens’ 4,000 MW wind farm construction project in the Texas Panhandle.
Superconductor Electricity Pipelines offer many unique advantages. They combine the advantages of direct current (DC) superconductor cables and voltage source converters (VSC) to deliver gigawatts of electricity from multiple generation sites to multiple load centers. Superconductor Electricity Pipelines are easy to site, highly efficient, controllable and offer greater security than competing technologies.
Unlike overhead power lines, Superconductor Electricity Pipelines are deployed underground utilizing conventional pipeline construction techniques. These systems require a corridor just 25 feet wide and can be located along existing transportation rights of way, thereby eliminating or greatly reducing the need for complex, contentious and costly siting procedures. For example, railroads, gas pipelines and the medians of interstate highways can easily accommodate the co-location of DC superconductor cables.
When compared to 765 kV overhead lines for long-haul transmission projects on the order of 1,000 miles for a 5 GW transmission load, Superconductor Electricity Pipelines:
- Improve Aesthetics: Conventional high voltage towers are more than 100 feet tall and can potentially impact the aesthetics of neighborhoods, national parks and sensitive wildlife areas. Superconductor Electricity Pipelines are out of sight and out of mind. Unlike overhead power lines, they also are free from electromagnetic fields.
- Enhance Efficiency: Superconductor Electricity Pipelines are able to cut power losses by two to three times compared to conventional transmission options. This results in improved return-on-investment and reduced carbon emissions.
- Simplify Cost Allocation: Cost sharing for new AC transmission lines is a significant challenge due to the difficulty in determining the benefit each affected electric utility receives. The power supplied to and delivered from Superconductor Electricity Pipeline DC-AC on- and off-ramps enables much simpler cost allocation.
- Are Cost Competitive: When looking at the thousand-mile, multi-GW transmission runs required to transport renewable energy from America’s heartlands to its cities, Superconductor Electricity Pipelines are comparable in cost to 765 kV AC overhead transmission lines.
- Increase Security: Ice storms, hurricanes, tornadoes, vandalism and terrorism are just a few of the threats to overhead power lines. Any of these incidents can cut power supplies to distant cities for lengthy periods of time with serious impact to public security and the U.S. economy. Given their underground location, Superconductor Electricity Pipelines are out of harm’s way and safely shielded from these threats.
Courtesy of American Superconductor. Graphic shows aggregate right of way comparison to transmit 5 GW (5,000MW) for 1,000 miles with overhead AC lines and DC superconductor cables.
Building for the Future
By utilizing DC power and taking advantage of other characteristics of superconductor materials, DC superconductor cables have minimal to no external electrical or magnetic fields (EMF). The significantly higher current handling capabilities of superconductors allow operation at lower voltages (200 kV) than other high capacity transmission technologies that require voltages as high as 1000 kV. With the ability to utilize existing rights-of-way, these pipelines also minimize the impact on public lands, including national parks, wilderness areas or other environmentally sensitive areas, in addition to populated areas.
Because superconductors have no electrical resistance, it is possible to construct DC superconductor cables with practically any desired power transmission capability with little to no additional loss penalty. This essentially unlimited design capability is not possible with any other type of power transmission. Superconductor Electricity Pipelines can also be over-designed to accommodate future growth without significant increase in cost.
To request a free copy of AMSC’s white paper on Superconductor Electricity Pipelines, visit www.amsc.com/powerpipes. To view an animation of the benefits of Superconductor Electricity Pipelines, visit http://www.amsc.com/products/applications/utilities/superconductorpipeline.html.
Comments (2)
Posted by Mike Rakestraw on 08/12/09, 11:43 AM
This is real cool! (pun)... A real basic question regarding the superconductor (HTS) wire technology, what happens when the wire develops a liquid nitrogen gas leak, self healing, or ??
Posted by Robert Munck on 08/11/09, 01:49 PM
I'm curious why the article doesn't mention the possibility of pumping liquid hydrogen in the underground pipe, both to cool the superconductor and to transmit an approximately-equal amount of energy in the H2. The "SuperGrid" has been fairly widely discussed. Of course, LH2 is significantly colder and harder to handle, but its use also may make it possible to use cheaper (lower-temperature) superconductors. Also, don't you end up STORING a considerable amount of energy in a long superconductor, in its EMF? Energy storage is much needed.
