Cold current: using superconducting cables to carry electricity within cities has many advantages, but the difficulties are also considerable
A steady stream of liquid nitrogen will next year begin flowing beneath the city of Essen in the Ruhr region of Germany. Its purpose: to cool the world’s longest superconducting cable, part of a trial to replace the city’s high-voltage transmission system with a safer, smaller and cheaper alternative.
Underground high-voltage cables are commonly used to carry 110kV of electricity or more beneath urban areas, connecting the national transmission grid to local distributions networks, where the voltage is reduced before entering people’s homes and businesses. But in the last 10 years a new alternative has emerged – implemented first in the US and due to be installed in Germany in 2013 – where superconductors are used to transmit energy at lower voltages using less material and requiring smaller trenches and fewer transformers.
A superconductor is a material that carries electricity with virtually no resistance when cooled to very low temperatures, usually below -200°C, meaning less energy is lost as heat. Some materials behave as superconductors at relatively higher temperatures and using these as electrical cables allows power to be cost-effectively transmitted at lower voltages, which would usually produce energy losses that were prohibitively expensive.
The €13.5m “AmpaCity” project in Essen will see German utility company RWE, working with cable firm Nexans and the Karlsruhe Institute of Technology, install a 1km underground high temperature superconductor (HTS) transmitting electricity at 10kV. Because it will connect to other medium voltage parts of the grid, it will allow RWE to reduce the number of urban transformer stations needed to step down the power from the long-distance transmission voltage of 110kV, freeing up valuable space in the city.
‘This makes the application of superconductors very attractive,’ said Mark Stemmle, project manager for superconducting cable systems for Nexans, which has designed the cable that will form the core of the AmpaCity project. ‘It’s not really an attractive application at the moment for areas in the country because normally you have a lot of space. But if you go inside the cities, you find there are often constraints in building space.’
Superconducting cables are larger than conventional power lines but only one is needed to carry the same amount of power as five traditional medium voltage cables. They don’t produce as much heat so need less insulation, nor do they create external magnetic fields, unlike conventional cables that can sometimes induce currents in adjacent underground pipes.
The smaller space needed for the cables frees up the distribution company to develop simpler network configurations, further reducing the amount of land used. A study the AmpaCity partners conducted last year found that a typical urban network of 20 transformers could be reduced to 15 using superconducting cables. Having fewer transformers is cheaper, and also reduces risks in the event of a fire in the city.
The study also found that superconductor cables – despite needing a flow of liquid nitrogen to cool them – would be cheaper both to install and run over a 40-year period than conventional high voltage lines, which require high levels of maintenance as well as the additional network infrastructure.
The superconducting cable Nexans will produce is the product of the company’s decade of experience since their involvement in the world’s first superconductor installation on Long Island in New York. It features three concentric insulated circles of cable made from bismuth strontium calcium copper oxide (BSCCO), cooled to 68K (-205°C) using liquid nitrogen that flows one way through the centre of the cable and back around the outside to be recooled. ‘The reason we chose this design is because it’s the most material-efficient and therefore also relatively cheap, especially when you look at superconducting material, which is quite expensive still,’ said Stemmle.
As well as enabling superconduction, the nitrogen cooling is also what allows the cable to use the concentric arrangement. It means you don’t need three separate wires (unlike conventional AC transmission systems) and cancels out the cables’ magnetic fields. But pumping two to three litres per metre of cable presents one of the biggest challenges. ‘Since you have the different flows of the liquid nitrogen within one cable, there’s also a kind of heat exchange between the inner and outer flow,’ said Stemmle. ‘So you need to make sure that works because otherwise the concept won’t work.’
Although the superconductor allows energy losses to be reduced enough to make medium voltage transmission cost-effective, it still leaks more energy than a high voltage cable would – it’s the reduced cost of installation and maintenance that makes it the cheaper option. This is because to reduce the voltage but maintain the power you must increase the current, which in this case leads to an estimated average 20 per cent increase in energy losses over a one-year period.
How does Nexans expect to reconcile this with the current pervasive trend for energy efficiency? ‘There’s a reason why they originally designed the grids like they did, with high, medium and low voltage, where you transport very large amounts of power at very high voltage,’ said Stemmle. ‘Even if you’re not really more efficient in terms of energy, you’re more efficient in terms of materials. It also depends on the loading: if you have high loading in the high voltage cable it could turn in favour of the superconducting cables.’
The cable is due to be installed by the third quarter of 2013 and its use studied for the following two years. ‘We’ve already showed in other projects that it’s technically possible,’ said Stemmle. ‘This project is important to demonstrate it in a real application, we are connecting two substations within a city and this has never been done before. I think this will help the technology to gain some more trust by the utilities, which most of the time are quite conservative.’
This will be vital if superconducting technology is to become more widely used in grid systems. In the UK, no utility firms have announced any plans for similar trials and National Grid sees the technology is not sufficiently advanced for longer distance transmission, although it says it is monitoring the situation.
‘If we look far in to the future, the trend we are seeing is that metals like copper and aluminium are getting more expensive and this will be an advantage for superconducting solutions because it can be even more cost-competitive,’ said Stemmle. ‘At the moment the cooling system is still quite expensive but as we use this type of system more and more you could have a totally new concept for a city grid because you could have three or four cooling systems shared between all the cables and this would increase the efficiency.’
- 1 km superconducting cable between two urban substations
- 10 kV system voltage
- 40 megawatts capacity
- 2.3 kA operating current (40 MVA)
- Total cost €13.5m
- German Federal Ministry of Economics and Technology funding €6m
- Installation completion Q3 2013