Elpipes are polymer-insulated underground HVDC conductors based on low cost extruded metal conductors. Elpipes are designed for higher efficiency than is practical for overhead power lines. For a 325-800kV DC elpipe, we have selected a design basis of 1% loss per 1000 km, about three times better than an overhead 800kVDC line, similar to “high temperature” superconducting (HTS) lines after accounting for the energy HTS lines consume for cryogenic cooling. Elpipes use 3-18 times as much metal/ampere as a conventional transmission line, yet conductor cost never goes above 20% of total project cost in our modelling. The lower power density in an elpipe gives them exceptional overload capacity for an underground line.
Elpipes are not wrapped on a reel for transport, and so can use far more conductor than cables. Elpipes are composed of pipe shaped rigid segments mounted on powered, controllable wheeled carriages that run inside a pipeline. The pipe-like segments are linked through flexible splice modules, and the segments snap together. Elpipes can be fully underground, installed at the surface, subsea, or above ground (basically anywhere that gas pipelines are practical). There are many options for elpipes; Figure 1 illustrates a particular elpipe design with aluminum pipe conductor, insulated by a multi-layer crosslinked polyethylene (XLPE) pipe within a steel conduit. Figure 2 shows the major features of elpipes that make them in effect, part train, part power line.
By combining features of a train with a major power line, elpipes solve three critical problems that have previously prevented any underground power line from carrying the power levels that will be needed for building a supergrid (5-30 GW). First, the segmented nature of elpipes allows far more conductor to be used than is feasible for any wire or cable. Second, the mass-manufactured standard modules that snap together in a clean room environment at one end of the line, and then crawl into the pipe under power and control (like a train) can achieve the extremely high reliability required to enable elpipes (with thousands of splices) to be reliable. Third, the train-like feature of elpipes combined with regularly spaced maintenance vaults and side tracks (Figure 3) allows for rapid repairs and also for routine maintenance (which is completely impossible for conventional buried cables).