Inventor believes emerging technology could boost underground HVDC transmission
TransmissionHub: You call your invention “elpipes” for “electric pipelines.” Can you describe them?
Faulkner: An elpipe is essentially a rigid pipe of either solid or segmented construction, wrapped in insulation, and mounted on wheels inside buried conduit. Bipole pairs of elpipes would be placed in side-by-side conduits with a ground potential return, so that half the rated power can still be delivered in case of a fault on one pole (this "ground potential return" is standard practice on most major HVDC lines). Mounting the insulated conductors on wheels allows the elpipes to move within the conduit, making assembly and maintenance easier, and de-risking the cost of installation (because conduit construction is essentially identical to constructing a gas pipeline).
THub: Why underground?
Faulkner: Greater efficiency, for one. Elpipes are designed for higher efficiency than is practical for overhead power lines. A 325- to 800-kV DC elpipe has a design basis of 1% loss per 1000 km, about three times better than an overhead 800-kV DC line (the current best state of the art). This is high enough efficiency to make coast-to-coast transmission practical in the US for example. Another advantage is that elpipes would assuage concerns around the aesthetics of overhead power lines, at lower cost than the two viable underground HVDC alternatives, underground cables and gas-insulated lines. The cost of the elpipes per se will be higher than overhead transmission lines, but will be significantly less than current-technology buried cable. For many routes, the lower right-of-way (ROW) cost for elpipes and the anticipated reduction of project delay due to vociferous NIMBY opposition to overhead lines, means that the all-in costs for elpipes though crowded corridors will generally be less than for a comparable capacity overhead line. The full price of a six gigawatt (GW) elpipe project, excluding only the converter stations would be about $5m/km. That’s a bargain compared to cost projections for cables or underground superconducting lines, or gas-insulated lines with equivalent capacity. Depending on ROW costs, 6-GW elpipes can be less expensive than overhead on some routes.
THub: Aside from cost, what other advantages will elpipes have over existing underground technologies?
Faulkner: Conventional paper/oil underground cables have a maximum voltage capacity of 500-kV, while the less expensive XLPE cables are currently limited to 325-kV. Land cables are also severely limited by the fact that they must generally be wrapped on truck transportable reels. Such cables can carry no more than 500 megawatts (MW). Subsea cables, because they are wrapped on much larger reels, can go to both higher voltage (600-kV) and can use thicher conductive cores, and so can reach 1100 MW (1.1 GW) capacity. This does not compare well with the currently proven 7.2 GW capacity of overhead 800-kV HVDC lines. Gas-insulated lines (GIL) can carry up to 800-kV and in principle, like elpipes, can to go to capacity greater than 12 GW; however, a 12 GW GIL would have to be ~1.5 meters in diameter, twice as large as an equivalent elpipe. In addition, one of the gases used for GIL insulation, sulfur hexafluoride, has a very strong greenhouse gas effect approximately 23,000 times greater than carbon dioxide. Another advantage that elpipes have over cables is that the hollow pipe shape of the conductor gives more surface area through which to dissipate the waste heat. However, the biggest advantage versus underground cable is that the design enables the use of 10 to 50 times more conductor than is even feasible for a cable.
T Hub: With a 10- to 50-fold increase in the amount of conductor, how much current would an elpipe be capable of transmitting?
Faulkner: Directly buried elpipes can be designed to handle current up to at least 10-kA, which implies greater than 15 GW power transmission at 800-kV, and still be passively cooled underground. Simple strategies such as using more conductor or backfilling with thermally conductive sand can boost the capacity of directly buried elpipes significantly. Compare that to underground cables, where waste heat removal limits maximum transfer capacity to about 0.5 GW.
THub: You mentioned the aesthetic advantage over aboveground power lines. Are there other advantages?
Faulkner: Elpipes are designed for higher efficiency than is practical for overhead power lines. A 325- to 800-kV DC elpipe has a design basis of 1% loss per 1000 km, about three times better than an overhead 800-kV DC line. The higher efficiency is achieved simply by using a lot more conductor than other types of transmission lines, in the form of inexpensive aluminum extrusions. Like all underground lines, elpipes are less susceptible to the elements, but unlike other underground lines, their wheeled, position controllable nature speeds repairs compared to directly buried lines. Elpipes are well suited to follow pipeline rights of way or overhead power lines. In many areas, burying the elpipe in fairly shallow coastal waters has major advantages in terms of permitting and right-of-way, and can be implemented faster than onshore installation.
In fact, I believe that siting elpipes will be more like siting a gas pipeline than overhead power lines. That would overcome one of the more commonly raised objections in the siting of large power lines: their visual impact, but I also expect it will ease the regulatory process of siting them, which is a major hurdle for transmission developers.
THub: Elpipes, as you envision them, are on wheels. Why?
Faulkner: For ease of construction and maintenance. The elpipe rolls into the conduit from one end of the line; there’s only one point of assembly. If you ever need to fix anything, you move the train of elpipes until the faulted section is in a maintenance vault. The rigid nature of the elpipes limits the maximum curvature of the conduit, similar to the limits on a rail line or highway. The length of the elpipe sections is limited only by the length of the trucks that will transport them, the conduit diameter, and the maximum allowed curvature.
THub: What challenges lie ahead?
Faulkner: One challenge is the development of an effective way to connect the segments. Since a large number of splices are required, they must be simple, very reliable, and cheap. This is really the crux of the technology, the hardest part. Another is the development of circuit breakers capable of handling the large amount of current elpipes will be capable of transmitting. And then, there is the political climate. There are many specific places in the U.S. and European Union where elpipes would enable high capacity transmission with low visibility, but I believe though that Western countries are too risk averse to be first adopters. As with the first 600kV HVDC line - which was installed in Brazil - or the first 800kV lines – which were installed in China - I believe the first installation will be outside Europe and North America.
About the Author
Carl Dombek, senior editor for TransmissionHub, is an award-winning journalist with nearly two decades of experience as a broadcast journalist on radio and TV, and as a writer for newspapers, magazines, and the Web.Prior to joining TransmissionHub, Carl spent five years in the U.S. power industry, including positions at the North American Electric Reliability Corporation and the Midwest ISO.
I have so far found that US-based venture capital investors will not take an interest in the elpipe because it is "too big, too long term." I have therefore put my startup Electric Pipeline Corporation onto Wefunder: https://wefunder.com/elpipes
in hopes that I can get started on this important (and expensive) development soon. It would help me move this forward if you will "follow" me on Wefunder; I expect the US Securities and Exchange Commission will be issuing rules by May that will allow me to raise funds though Wefunder (this is not legal yet, but by following EPC, you will help move me to the front of the queue when the final rules are announced).
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