A proposed hub for connecting the three independent electricity grids that span the continental United States could make it easier to ramp up production of renewable electricity.
The project, called the Tres Amigas Superstation, would use superconducting “pipelines” and converter stations to connect three grids: the Western, Eastern, and Texas Interconnections. Connections between the grids have been limited because the grids aren’t synchronized–the AC power is out of phase. Special stations that convert AC power into DC power and then back into AC power in the correct phase are needed to move power from one grid to another.
Only a fraction of 1 percent of the electricity generated in the United States can currently be transferred between the grids, and there is no direct connection between Texas and the Western grid. The Tres Amigas station, which will connect all three grids together in one place for the first time, will initially more than double the ability to transfer power between them, providing five gigawatts of capacity. Eventually, the station is expected to transfer as much as 30 gigawatts of power.
The station will “solve a host of problems” related to renewable energy, says Phil Harris, CEO of the Tres Amigas company, based in Santa Fe, NM. Primarily, it will help address a key problem with renewable sources of energy–their intermittency. Because wind comes and goes and clouds block the sun from time to time, wind and solar power can destabilize the electric grid.
One way to compensate is to make sure that no renewable source accounts for too much of the total power mix–so that other sources can easily fill in when there’s a drop in power. In Texas, however, this strategy would quickly limit the size of wind farms, since the grid there is relatively small. By connecting to the rest of the country, Tres Amigas removes the limit on the size of these farms.
Intermittency can also be addressed by gathering renewable energy over a wide area. That way, a drop in solar power due to a cloudy day in one region could be offset by wind or solar elsewhere. Connecting the three grids makes it possible to draw on a wider variety of renewable sources, especially in the Southwest, which is divided by the borders between them. The station will also provide between 50 and 150 megawatts of battery storage to smooth out power fluctuations on the grid to help prevent outages.
The project could also be a valuable testing ground for direct current superconducting transmission lines, which could have significant advantages over conventional power lines for delivering large amounts of power over long distances, says Steven Eckroad, a project manager at the Palo Alto, CA-based Electric Power Research Institute (EPRI), which is studying the potential of superconductors for long-distance transmission. Such transmission lines could collect wind power from the Midwest, where it is abundant, and transmit it to the South, which has fewer renewable resources.
To connect all three grids required a place where they were geographically close together–in the case of Tres Amigas, a 60-square-kilometer swath of land near Clovis, NM. As with conventional connections between the grids, the system converts AC to DC. But unlike conventional two-way connections, at Tres Amigas that DC power will then circulate in superconducting cables that form a triangular electrical pipeline. Any of the three grids can draw power from this, as needed.
Providing five gigawatts, and eventually 30 gigawatts, of transfer capacity between all three grids required the use of superconducting DC lines, which greatly reduced the number of cables needed to carry the power–a single superconducting cable can carry the same power as nine or 10 sets of conventional copper cables. If the conventional cables were suspended overhead, they, along with the incoming and outgoing transmission lines, would have created a “rat’s nest” vulnerable to weather and sabotage, says Jack McCall, a director of business development at Devens, MA-based American Superconductor, which is supplying the superconducting cables.
Burying conventional cables would add to the complexity and size of the project, since the cables would need to be kept several meters apart to avoid overheating. At first, only one superconducting cable would be needed–greatly simplifying the system compared to using conventional cables. As Tres Amigas is expanded, more cables will be needed, but these can be buried close to each other.
In addition to connecting the three grids Tres Amigas will serve as a demonstration of the type of DC superconducting lines and AC-to-DC converters that would be needed for high-power, long distance transmission lines–the superconductor “pipeline” EPRI has studied. Such a system could be easier to site than conventional overhead high-power transmission lines: the superconducting cables can be buried along existing rights of way, such as along interstate highways. Convincing land owners to allow large overhead transmission towers is one of the biggest obstacles to installing conventional transmission lines, and it could stymie efforts to develop a system for distributing wind power from the Midwest. Superconducting transmission lines could also be easier to integrate with the existing grid, since the amount of power converted from DC to AC power at stations along the line could be precisely controlled.
According to the EPRI analysis, a superconductor pipeline would cost about as much as a conventional transmission line, if the superconductor system were designed to transmit high amounts of power (greater than five gigawatts) over long distances (around 1,000 miles). This is in part because the cost for refrigerating superconducting lines (required to maintain their superconducting properties) becomes a small share of the total system costs. Also, at long distances, the higher efficiency of superconducting systems is important–about half as much energy is wasted as with conventional high-power, long-distance power lines.
But it is a technology that companies aren’t familiar with, and so they may be reluctant to roll it out at the scale required for it to be cost-competitive, Eckroad says. The Tres Amigas project could be important to demonstrating that the technology works, he says.
McCall says that Tres Amigas plans to file with the Federal Energy Regulatory Commission in the coming weeks–clearing that regulatory hurdle is the next step for the company. If that goes well, the initial five-gigawatt system could be completed by the end of 2014.
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