Will Pipelines Be Repurposed For HVDC Transmission? – CleanTechnica

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This question is asked regularly of me these days. After all, high-voltage direct current (HVDC) is the new pipeline, doesn’t care if it’s run underwater, underground or through a steel pipe, and the millions of kilometers of pipelines in the world are going to be decommissioned at an accelerating rate over the coming decades.

Further, at least in the developed world, transmission towers get as much NIMBY pushback as wind turbines, and unlike wind turbines have to march through cities and towns in many cases. Then there’s the health paranoia of the people who think that electromagnetic spectrum radiation is harmful to their family’s health because they are incompetent to assess medical evidence.

The combination leads people reading pieces such as my recent Forbes article on the subject to ask, “Why don’t we run HVDC lines through the pipelines! Out of sight, out of mind, existing conduit. Win win!”

Well, not so fast. HVDC is more expensive than high-voltage alternating current (HVAC) transmission still. Because its magnetic fields aren’t oscillating backward and forward 50 to 60 times a second but staying stable, they interact a lot less with water and soil. As such, the breakeven point for HVDC being cheaper than HVAC is only about 50 kilometers undersea, while it’s 800 kilometers on overhead lines. HVAC losses can be reduced by increasing voltage on overhead lines, but not underwater.

That said, there’s an emerging medium voltage direct current (MVDC) set of technologies from firms like Hitachi ABB and Siemens. HVDC runs from 100 kilovolts (kV) to 800 kV usually, and there are UHVDC deployments that run at 1,100 kV. MVDC is intended to run at 10 to 33 kV, more in the range of the primary distribution grid, not the transmission grid.

A 2021 study by a French research team under a grant and supervision from the French National Research Agency found that breakeven for medium voltage alternating current could be under 10 kilometers to a few dozen kilometers. Use cases where MVDC is being proposed include offshore wind farms, large and inherently direct current solar farms and electrification of railways.

Much of the value of this can be ascribed to the relatively new direct current transmission technology of voltage source converter (VSC) AC to DC conversion. It recreates the sine wave of alternating current in tiny pixelated steps that are fine enough to be indistinguishable to the analog HVAC grid. With that new ability, it’s possible to provide more voltage and frequency control to local distribution grids via MVDC. One UK example, the Angle DC MVAC to MVDC upgrade project, is expected to increase delivered electricity across the lines by 23%.

That VSC technology and its predecessor line commutated converter (LCC) do bring losses with them of about 2% of the electricity converted through them, hence the distance based break even point. I became aware of the material while struggling to find data on how much HVDC there was globally and where it was going. RTE International publishes a monthly snapshot of HVDC globally, but only for the VSC side. Wikipedia has a VSC and LCC list, but it’s Wikipedia. I assume the global interconnection agency China set up under UN provenance, GEIDCO, with its 140 or so member countries has a complete list, but it’s private.

Cornelis Plet, head of the power systems advisory practice at DNV helped me understand this space better. Before his current role, where he spends a lot of time consulting related to HVDC, he was global practice lead for HVDC for the same firm. Much of his career since his PhD in power electronics and power systems from Imperial College in the UK has been focused on this technology, so it was great to sit down with him for 90 minutes. The couple of podcasts that resulted from that will be live in January on Redefining Energy – Tech, so subscribe now if you are interested in deeply nerdy conversations about where the world is actually going (and why so many people seem to not understand or refuse to understand where it is going). Plet too has a DNV confidential and more complete list.

When I analysed where HVDC was and wasn’t, one of the things that leapt out at me from Europe’s deployment was how many segments of HVDC transmission routes were underground. Often, HVDC would come ashore from wind farms or UK interconnectors, run above ground for a while, and then dive underground, or vice versa. Given the additional costs of burying transmission or any electrical lines, clearly there was a requirement or constraint being met. To be clear, burying electrical infrastructure is much more prevalent in Europe than in North America, part of the reason why their grid is much more resistant to disruption.

As for pipelines, the era of moving molecules for energy is coming to an end. Obviously we aren’t going to be moving oil, natural gas, diesel and gasoline long distances through pipelines to anywhere near the same extent. Peak demand for coal, oil and gas are this decade per Fatih Birol, executive director of the International Energy Agence, and China’s giant refiner and distributor Sinopec announced that peak gasoline demand in that country had already been reached. After this, the decline, and it will be much more rapid than the oil and gas industry wishes to believe. It won’t, however, be instantaneous, and the rate will vary by geography. Some pipelines will have longer lifetimes, even at diminishing volumes, for longer, while others will become uneconomic and close down sooner.

For example, I consider the likelihood that Canada’s Trans Mountain Pipeline will be bankrupt and defunct by 2040 to be very high. The well over C$30 billion spent on tripling it will have been wasted. The combination of peak oil demand and increased costs of low-carbon hydrogen essential for hydrocracking, hydrotreating and desulfurizating Alberta’s heavy, high-sulfur crude will increase the quality discount and knock if off the market first. The distribution cost discount the Trans Mountain Pipeline was theoretically going to have addressed won’t be helped because of the construction budget explosion, only small AFRAMax tankers can get into the Port of Vancouver, and China won’t be interested as it rapidly moves away from oil for ground transportation and inland water shipping.

As I noted in 2022 when commenting on the various weak studies that purported to show that moving hydrogen through pipelines was more efficient with higher energy transmission, they all made a few common mistakes. They drew the system boundaries at the end of the pipelines as opposed to more broadly, assuming that there would be very large amounts of low-carbon hydrogen where the pipeline started, something only true if the source is fossil hydrogen. They assumed very cheap hydrogen, something which is increasingly apparent even to hydrogen for energy proponents isn’t going to be a thing. They made the primary energy fallacy, assuming that all of the gigajoules of heat in the hydrogen were equally useful to the MWh of electricity in the cables, when that’s only true for heat and not for any of the other myriad things we use energy for like transportation. Even at that, they ignored heat pumps which can’t be run with hydrogen. They ignored the relatively low efficiency of turning electricity into hydrogen. And they tended to downplay hydrogen pipeline compression costs.

So we have a confluence of technologies. We have a declining pipeline industry with a bunch of to-be-abandoned assets, both underwater and underground. We have a growing demand for HVDC and perhaps MVDC transmission. We have opposition to unsightly overhead transmission lines which is often due to people who have their hearts in the right place, think globally and resist everything locally.

This leads a lot of people to ask me and others, why not run the HVDC through the pipelines?

My answer up until this article has been:

I really need to write a solid answer to this. But until then, the 800 km break even distance overland and the relative lack of pipelines running between electricity generation and demand sources combine to make it unlikely to pencil out in most cases.

And so, the fuller answer.

There’s little likelihood of repurposing offshore pipelines and little need. HVDC is trenched into the bottom of the ocean when anywhere near shore, and this is undoubtedly cheaper than feeding a cable into a pipeline and trying to drag it through.

Onshore, pipelines rarely run from large penetrations of new generation directly to centers of electricity demand. There are exceptions with the North Sea wind and North Sea natural gas extraction, but globally massive wind and solar farms and hydroelectric dams tend to be located in places more distant from oil and gas fields. The same problem of having lots of low-carbon hydrogen conveniently located at the mouth of a pipeline to put into it applies to green electrons.

Next up is the challenge of running heavy, thick and relatively unbending cables through hundreds of kilometers of pipeline. HVDC cables run from 30 kg to 60 kg per meter. That means that an 800 kilometer cable has a total weight of 24,000 to 48,000 metric tons. Trying to pull segments of that through a pipeline is a high energy, high friction job. HVDC transmission cables have a maximum pulling force that they can withstand of around 20 tons, but that includes not just the weight of the cable but also the rapidly increasing friction with distance. It would require digging up pipeline segments at frequent intervals and at a certain point that gets expensive.

Pipelines tend to have a lot of sharp bends in them. Gases and liquids go around corners much better than HVDC cables, which have a minimum turning radius of 1.5 to 2 meters. Pipeline thermal expansion loops have three sharp bends in a few meters to enable the pipeline to expand and contract with heat and cold without destroying itself, or at least not quickly. Those expansion loops would still be required if pipelines are repurposed as HVDC conduit, but they would have to be ripped out and replaced with gentler curves, ones calculated to not decrease their radius to the point of breaking the cable.

Lastly, the need for transmission is mostly discontiguous with the sunsetting of pipelines. We need a lot of transmission in the next two decades in order for pipelines to be sunset in the 2030s through 2050s. Pipelines that would pencil out if empty often won’t be empty when the transmission run is required to move forward.

The answer changes somewhat for MVDC transmission. The distances are much shorter. The MVDC cables are thinner and more bendy. The pipelines are also distribution pipelines which have come and gone for decades. As one correspondent noted, the Netherlands repurposed some small pipelines for fiber optic runs and milk shipping. There are over five million kilometers of pipelines in the USA, and many of them aren’t massive transmission pipelines. It’s more likely as MVDC progresses that reusing some smaller pipelines will make sense. The expansion loops would likely still require reengineering, but there are fewer on tiny pipelines than big ones, so the problem could be avoided more easily.

There’s another potential use case, although I think it’s less likely as well. There’s nothing technical that prohibits running an HVDC line through a big natural gas transmission line that’s operating. It would reduce the volume of the often one meter diameter pipeline slightly, by about 2% in the example I just worked out for the Baltic Pipe and a 1,000 MW HVDC cable (if my numbers are correct), but radically increases energy carrying capacity and future proofs the pipeline. Where that pencils out, something possible in the North Sea to central Europe corridor, it’s worth looking at. But it’s unlikely as pipeline builders and operators are fighting a rearguard action against transmission and the death of their business model. It doesn’t strike me as something that they would willingly embrace, but would need to be forced into, with much governmental money for their ‘losses’ as the pipeline is reengineered.

Another correspondent suggested that it wasn’t the pipelines themselves that could be repurposed, but their right of ways. That’s fine when they are above ground, but the majority of pipelines are buried to give them additional protection from all sorts of natural and human-caused damage. It doesn’t help much when they are buried in landslide and flooding prone areas, something the residents of Satartia, Mississippi discovered in close to the worst possible way a couple of years ago. A carbon dioxide pipeline carrying liquified CO2 ran afoul of heavy rains and land slides and ruptured explosively. The CO2 immediately flashed to gas and as it’s heavier than the air, pooled in low-lying areas until it diffused naturally. Before that occurred, dozens of people were in convulsions on the ground, a couple of hundred were evacuated and internal combustion emergency vehicles couldn’t get close to the victims because they need oxygen too. It’s likely that the Satartia residents who were most impacted will suffer some long term organ and brain damage.

The larger point is that pipelines go under things and don’t have rights of way in the same way that roads or railroads do. As such, repurposing the right of way is less possible.

And to be clear, when I had the briefest version of this conversation with Jesse Jenkins, Princeton Andlinger Center assistant professor and deep energy analyst, on that social media platform that is collapsing and flailing a few months ago, he was more optimistic than I was. As he pointed out, there’s an awful lot of pipeline that we’ve buried to date, over five million kilometers in the USA alone. Some of it will be able to be repurposed.

All in all, I think it’s likely we’ll see some pipelines repurposed for MVDC for distances of a few tens of kilometers, but think it’s unlikely we’ll see HVDC in more than one or two pipelines. The option should be included in transmission routing scenarios however, and investigated. And to be clear, this is a rough first principles with large error bars. If someone is actively working to repurpose a pipeline for transmission, please reach out to let me know. That I’m unaware of it doesn’t mean it isn’t happening.