The USA has extensive renewable power resources. Wind is already a significant prime mover for electric generators, and solar power is steadily gaining traction as costs fall per watt. Governments and companies in Europe, China, South Korea and Japan are investing heavily in the renewable energy industries and setting ambitious targets for renewable energy use.

But in all of these countries, there is a need to upgrade the grid to cope with increasing amounts of renewable energy. For example, to meet the ambitious targets set forth by the Obama administration, the American power grid must be modernised as a matter of urgency. And underscoring the critical need for such upgrades was the recently announced suspension of plans by T. Boone Pickens to build thousands of megawatts of wind capacity in Texas’ wind-rich panhandle. Why? in large part due to a lack of transmission capacity.

The unfortunate reality is that the inability of existing transmission systems to move clean electricity generated in resource rich (but often sparsely populated) regions to concentrated urban load centres remains a primary barrier to achieving the technical, economic, and environmental benefits of renewable energy. In order for the true potential of clean energy resources in the USA and elsewhere to be realised, power grids everywhere must be modernised and re-configured to enable bulk renewable power to be transmitted economically over very long distances. This power must at the same time be precisely controlled to maximise efficiency.

However, a new option for achieving these objectives is now possible: Placed underground in existing rights-of-way, superconductor electricity pipelines could carry many gigawatts of power more efficiently than overhead lines, at a similar cost and in a pipe just three feet in diameter.

Could superconductor electricity pipelines improve today’s grid?

Current electric power grids rely on ageing overhead AC transmission lines, most of which occupy huge right-of-ways. These grids were largely designed and constructed at a time when utilities generated their own power, transmitted that power over their own system and then distributed it to their own customers. The length of transmission lines was limited, and each system was tightly integrated. Over time, longer transmission lines were built to provide connections with other utilities, but those were primarily put in place for reliability. Notably, the existing grid was designed when 'rights-of-way' were plentiful and energy losses attributable to long-distance transmission were bearable due to low fuel costs.

In recent decades, the critical need to move larger “blocks” of power over greater distances arose, requiring much higher voltages and increasingly larger rights-of-way. In addition to the obvious negative impacts of larger, unsightly towers and increased right-of-way requirements to the environmental landscape, the capability of these long-distance AC lines is also constrained by the immutable laws of physics, namely that:

• The further AC power is transmitted, the greater the percentage of electrical loss;
• The amount of power that can be moved along an overhead AC transmission line drops with distance;
• When an attempt is made to add new AC power lines, operating at a voltage higher than the existing system, much of the existing system will likely have to be modified or rebuilt to support it.

An optimal long-haul transmission solution

Superconductor electricity pipelines resolve the pitfalls associated with conventional transmission. These underground direct current (DC) voltage superconductor cables are coupled with voltage source converters (VSC) to enable multi-terminal transmission virtually without loss, over very long distances. These pipelines offer an economic, commercially-proven technology for bringing large amounts of renewable power to market.

Superconductor electricity pipelines combine conventional underground pipeline construction techniques with two highly complementary electric power transmission options: field proven superconductor cables and multi-terminal, voltage-source converter-based, DC power transmission.

The result is a high-capacity electric transmission “pipeline” that is not only out of sight, but out of harm’s way from potential damage due to severe weather or wilful attack. Superconductor electricity pipelines require minimal rights-of-way (up to 25 feet). They are much easier to site and access, highly-efficient and controllable, and offer increased power grid security compared to conventional technologies.

Superconductor power cables utilise high temperature superconductor (HTS) wires instead of the copper or aluminium traditionally used to carry electricity in overhead power lines and underground cables. HTS materials provide two major advantages. First, wires made from superconductor materials conduct approximately 150 times the amount of electricity conducted by copper or aluminium wires of the same size.

This power density advantage drives system economics and is fundamental to the reason underground superconductor cables can achieve cost parity with overhead AC power lines over long distances. When transmitting DC power, superconductors have absolutely zero resistance to the flow of electricity, making DC superconductor cables literally perfect conductors.

To exhibit their ideal electrical characteristics, superconductor materials must be refrigerated. The cables are cooled using conventional liquid nitrogen refrigeration systems that are widely used in a variety of industries. While some power is required for the refrigeration – lowering the overall system efficiency – superconductor power cable systems still have much higher overall efficiency than any other long-distance transmission system. The cables employ superconductor wires that are commercially well established and are available from multiple producers globally.

Superconductor cables have been well demonstrated at electric utilities. Three of these cables have been energised in the USA: in fact since April 2008, a 138 kV AC HTS transmission line has been operating successfully just outside of New York City on Long Island Power Authority’s (LIPA) primary transmission corridor. At full capacity, the LIPA installation is capable of transmitting up to 574 MW of electricity in a right-of-way approximately just one meter in width and is the world’s longest and most powerful superconductor cable system deployment to date.

Superconductor cable technology is also gaining traction abroad. In fact, Korea Electric Power Corporation is now in the process of deploying the first of these systems in the grid outside the city of Seoul.
Pipeline transmission: the ultimate in energy efficiency?

Power losses are typically lower for DC than with AC overhead or underground lines. Traditional high voltage DC (HVDC) transmission, however, is typically limited to the transfer of power from only one point to one other point. This “point-to-point” transmission system is commonly used when power must be moved from a large power generation source, such as a hydroelectric dam, to a single point of power consumption, such as a single metropolitan area load centre.

Overhead HVDC lines, which can be sized up to 800 kV, have right-of-way requirements similar to those of a single 765 kV AC transmission line, and as such, share the same siting and security challenges.

Conventional underground HVDC transmission has the advantage of overhead HVDC lines but with a more limited power transfer capacity. This is because DC cables are not available at the higher voltage ratings required for higher power capacities. Transmitting thousands of megawatts of power would require the use of many cables in parallel, increasing both power losses and right-of-way requirements. Superconducting cables are not limited by any of the foregoing technical constraints.

For a 1000 mile cable system, it is estimated that the cost of a superconductor electricity pipeline would be in the range of US$8-US$13 million per mile fully installed. The estimates include the cost of 7 pairs of 750 MW DC converter stations. The low end of this estimate is based on a single 5 GW pipeline while the upper end is based on a fully redundant 5 GW system (two cables).

That is in the same general range as the US$7 to US$10m cost per mile estimate for two to three 765 kV transmission lines complete with required substations, which is what would be required to carry 5 GW for 1000 miles. But, significantly, this aforementioned cost for 765 kV does not include investments that may be required to upgrade the underlying transmission infrastructure to support any type of 765 kV AC line overlay.

While conventional point-to-point overhead UHVDC transmission lines are generally less than US$5m per mile, they lack the distributed on-and-off-ramp capability, have higher losses, require a greater right-of-way, and do not alleviate the serious aesthetic, security, environmental and political issues associated with overhead lines. The longer the run, the more cost competitive superconductor electricity pipelines become. This is because the DC converters are largely a fixed cost based on the total megawatt power rating of the converters, and are not affected by line length.

Very important, superconductor electricity pipelines also offer the ultimate in 'energy efficiency' in two key respects. First, the higher efficiency of transmission and distribution with superconductor cabling results in a lower generated power requirement, resulting in lower greenhouse and other gas emissions. While T&D losses in the US grid are estimated to be 10% superconductor cables, with their increased efficiency, have substantial savings potential.

The second key environmental benefit of the pipeline concept accrues due to a feedback effect, because merely deploying the pipeline technology makes it more economically advantageous to then deploy renewable and distributed energy resources which, in turn, lower CO₂ emissions. This sets up an on-going 'virtuous cycle' because, as previously noted, the best renewable resources often are not located near load centres.

Grid modernisation: a global priority

Similar to initiatives in the USA, the European Commission (EC) has proposed a new energy policy to deal with a range of interrelated energy challenges including: reduction of CO₂ emissions and maximisation of energy efficiency and transmission of renewable energy across long distances and country borders.

One of the ways proposed to achieve the EC’s goals is to implement a vast, so-called 'super grid' using wind farms and solar power to help move Europe to a low carbon economy while securing its future energy supply by reducing dependency on imported gas and oil.

Examples of super grid projects currently being evaluated include:

• A North Sea offshore grid for wind energy;
• A Mediterranean energy network capable of accessing solar farms and natural gas reserves in North Africa; and
• A Baltic Interconnection Plan to link the grids of countries bordering the North and Baltic Seas.

Under the recently announced DESERTEC proposal, 12 major European companies additionally agreed to form a consortium that within three years aims to draft a master plan to supply vast amounts of clean renewable power to Europe. As is the case elsewhere, the success of this visionary, yet technically achievable, project will rest largely on whether adequate, environmentally compatible and cost effective transmission can be put in place.

The path forward

Implementation of advanced solutions for long-haul transmission such as superconductor electricity pipelines will require support and investment by governments and private enterprise. The US House of Representatives recently approved the Waxman-Markey Energy Bill, which provides support for the deployment of advanced superconductor transmission technologies.

In the USA and around the world, electricity derived from clean, renewable energy sources is increasingly being sought after as a means to meeting growing power demands and reducing CO₂ emissions. In light of growing environmental concerns, more stringent regulations capping greenhouse gas emissions are also likely to be enacted in the near term.

As such, enabling the transmission of renewable energy to the high-demand regions where it is needed will continue to rise as a leading priority on energy agendas globally. Unless power grids are upgraded and adequate transmission is put in place, the realisation of the optimal use of renewable energy to address these objectives will not be achievable.