Green hydrogen and the cable-pipeline dilemma


A group of scientists from the Nanyang Technological University in Singapore has examined the challenges of bringing green hydrogen to a distant location and whether it is better to transport renewable electricity there via a long-distance electric cable to produce the clean fuel on-site or to first produce it and transport it to the distant location via a gas pipeline.

In the paper “Long-distance renewable hydrogen transmission via cables and pipelines,” recently published in the International Journal of Hydrogen Energy, the researchers assessed the economical feasibility of both options. “The cost only considered the ‘transmission' portion but the upper stream cost is also added as a ‘lump sum,'” research co-author Miao Bin told pv magazine. “So the cost is doorstep hydrogen from a renewable source to energy importer’s terminal.”

The academics started from the assumption that large-scale batteries are still unviable in storing grid electricity and that the capacity factor of renewable energy sources is still incapable of sustaining a stable grid operation.

Analyzing the electric line option, they stressed that the high voltage alternating current (HVAC) cables and the high voltage direct current (HVDC) transmission lines that are usually built for the transport of electricity to distant locations suffer from losses that reach 6.7% and 3.5% per 1000 km, respectively. “The AC cable possesses higher dissipative losses and higher material cost over high voltage and long-distance energy transmission due to the skin effect of the alternating current,” they explained. “The longest power transmission projects are HVDC overhead transmission cables and are mainly located in the continent countries.” In the cost assessment for an HVDC line, they considered the renewable sources, AC transmission lines, an AC to DC converter station, HVDC transmission lines, a DC to AC converter station, and AC transmission and distribution lines.

As for the second option, the research group considered the cost for setting up the power-to-gas facilities, gas buffer storage tanks, compressors, the gas pipeline itself, and receiving infrastructures. “The hydrogen pipeline cost was assumed to be similar to that of natural gas pipelines with an additional 10% increase,” it specified.

The electricity cost is assumed to be $78.67/MWh, which the scientists described as the simple average from solar PV, onshore and offshore wind costs, while the averaged capacity factor is assumed at 32.7%. Power transmission losses for the cable line are estimated at 3.5% per 1000 km and the fugitive emission from the high-pressure gas pipeline is assumed to be between 0.02% and 0.05% per 1000 km per year. One gigawatt of renewable power was assumed to be transmitted in the cable or pipeline.

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Their analysis showed that, in general, onshore transport is less expensive than offshore transmissions and that cables are more expensive than pipelines in the offshore option. “For the onshore cases both cable and pipeline have similar costs at 4000 km transmission distance,” they emphasized. “The cables are relatively maintenance-free and undertaking more transmission loss, which has been reflected in the higher electricity cost terms.” The costs for transporting hydrogen to a distant location for downstream applications such as power generation or town gas usage were defined as too high, as the levelized cost of energy (LCOE) of hydrogen was found to be around $10.0/kg. 

Long-distance transport gas pipelines were found to be cheaper than cables, although the latter could benefit from scaling up and higher utilization. “The increase of capacity factor is always beneficial to both cables and pipelines,” the paper notes. For both cables and pipelines transmitting renewable hydrogen, low utilization remains one of the main reasons for the high cost. “Unfortunately, it is challenging to further increase the renewable energy capacity factor beyond 50% in the short-term.”

“From characteristics of the transmission technology, cables have lower cost barrier in terms of per unit gigawatt but the bandwidth is much lower than pipelines,” Bin told pv magazine. “The implementation of which technology highly depends on the exporters/importers long-term strategy based on their future perspective. If the energy supply/purchase agreement is long enough and large enough, pipelines win over the cables.” From the energy carrier’s point of view, electricity is much more popular than hydrogen if it can be supplied stably,” he added. “If the fuel cell age is here, then hydrogen may play a bigger role as chemical energy can be stored easily,” he affirmed. Currently, however, hydrogen prices do not seem to justify immediate action to build huge pipelines or cables. “On the other hand, for cable/pipeline hydrogen, there is another question to answer: which sector is capable of bearing the expensive hydrogen?”

According to another recent study, a research team comprising scientists from the Norwegian University of Science and Technology (NTNU) and Spain's Technical University of Madrid sought to establish which form of energy vector between hydrogen transport and high-voltage submarine cables was the most economical solution for energy transport among countries separated by water. According to their findings, the hydrogen shipping alternative does not present very good prospects of applicability in the future, unless some disruptive technological breakthroughs are made.

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