Could a superconducing grid connecting solar concentrators in North Africa to wind farms in northern Europe help the EU meet its green energy targets?
When asked to reflect on the life of Thomas Edison for a New York Times article on the inventor’s death in 1931, Nikola Tesla had few kind words to say about his former boss. ’His method was inefficient in the extreme, for an immense ground had to be covered to get anything at all unless blind chance intervened and, at first, I was almost a sorry witness of his doings, knowing that just a little theory and calculation would have saved him 90 per cent of the labour.’
It was the final insult after a long-drawn-out public spat between the two men who were on opposing sides of the great AC/DC debate dubbed the ’War of Currents’ at the turn of the 20th century.
By all accounts Tesla had emerged the winner with his crowning achievement, the Niagara generation project. Backed by his commercial partner George Westinghouse, Tesla was the first to harness the Falls’ potential to generate hydroelectric power through his alternating-current plant, which began transmitting electricity to New York industries in 1896. Tesla had demonstrated that a directcurrent power network – Edison’s chosen option – would never be economical because DC was too difficult to transmit over long distances. Tesla was right at the time, but the ghost of Edison may finally have a chance to prove him wrong.
This summer, the French government announced that energy giant EDF would lead an industrial initiative to study the feasibility of an underwater high-voltage direct-current (HVDC) electricity network connecting both sides of the Mediterranean. The network is one of the major puzzle pieces needed to form the concept European supergrid – a bold idea championed by environmentalists as a way to ensure the European Union (EU) meets its future commitment to energy from renewable sources.
A supergrid would interconnect Europe and regions around its borders with a HVDC power grid. The area map of the grid has yet to become any more official than a drawing on the back of an envelope, but proponents see the possibility of connecting widespread renewable energy sources from Iceland to the Sahara desert – bringing together geothermal, hydroelectric, wind, wave, tidal and solar into one constant, weatherproof electricity supply system.
The challenges facing the supergrid developers do not differ much from Edison’s
Dragan Jovcic, University of Aberdeen
The French initiative, labelled Transgreen, has extended its hand to the similar German-led consortium Desertec, which has already pulled together 30 blue-chip companies, including E.ON and Siemens, to invest in solar and wind-power production facilities across North Africa and the Middle East.
Transgreen’s backers insist their initiative is not competing with Desertec, considering one of its partners is also Siemens, but the French government is not blind to the commercial opportunities in the Sahara sun. According to the French Embassy’s estimates, the Sahara could generate 20 gigawatts (GW) of renewable energy production capacity – particularly solar – by 2020, and a quarter of this could be fed into the European market.
Transgreen’s partners, such as French cable manufacturer Nexans, say their speciality lies in getting the electricity produced by desert solar and wind farms to the areas where it will be used. This will require new transport and interconnection infrastructures linking production sites to the local grids and to Europe, most importantly by undersea HVDC cables.
Currently, Europe is connected to Africa only through a 400kV AC link installed across the Mediterranean, under the Strait of Gibraltar, with a capacity of 1,400 megawatts (MW). France’s transmission system operator RTE, a Transgreen partner, estimated installing a higher-power DC connection there could cost at least €1bn (£847m) for every gigawatt of capacity. The cost of installing new connections from Tunisia to Sicily and Libya to Malta will likely be higher.
While the Transgreen team does not foresee any major technical hurdles standing in the way of installing these new connections, turning DC transmission lines into a functional supergrid across Europe remains an ambitious engineering task.
Dr Dragan Jovcic, an HVDC expert from the University of Aberdeen, said the challenges facing supergrid developers do not differ much from those faced by Edison more than 100 years ago.
Jovcic explained Edison was correct that DC is in principle better than AC for electricity transmission because it is possible to transmit more power for the same copper size and it has no reactive power flows, which is beneficial for long distances. But in Edison’s network, DC was only transmitted from shorter distances at a lower voltage because there was no way to step it down to a level that was safe for the end user.
Tesla’s AC transmission system, on the other hand, had transformers. Unfortunately for Edison, a transformer can only work with AC because its principle of operation, a phenomenon known as mutual inductance, relies on changing magnetic fields, and DC can only produce steady magnetic fields.
There are ways to design the HVDC grid without transformers. Dr Steve Finney, an HVDC expert from the University of Strathclyde, said some concept studies depict the European supergrid as a multi-terminal DC network all at the same voltage. This would connect to each country’s national network at DC to AC nodes. Once it is converted to AC, the voltage would be stepped down.
If you want to pull together the resources of renewables, you’re pushed toward DC transmission
Steve Finney, University of Strathclyde
’That’s quite challenging because no one has actually built that kind of DC network yet,’ he said. ’There are various bits of the building-block technology about, but it’s not been up at voltage and power levels that you’d require for that type of thing.’
Finney is currently working with Alstom’s power transmission unit, a division formally owned by Areva, to design and analyse the technical issues raised by HVDC grids through the €56.81m EU-funded TWENTIES energy demonstration project.
Among the long list of concerns for the project is figuring a way to clear a fault on a multi-terminal DC network. While faults on AC grids can be tripped with a simple mechanical circuit breaker, this technique does not work with DC because of its steady-state current.
If plotted on a graph, an alternating current moves in a wave motion, hitting zero at each half-cycle. In order to isolate a fault, a signal is sent to interrupt the current at its next zero crossing. This is not possible with direct current because it steadily increases at a rapid pace and never crosses zero.
With typical point-to-point HVDC links, such as those that connect offshore wind farms to land, this is not a problem because an entire line can be cut off to isolate a fault without further implications. In a multi-terminal HVDC grid, this could lead to a costly shutdown of an entire network and complications on connected AC systems.
Finney’s research colleague Dr Keith Bell, a senior lecturer in electronic and electrical engineering at the University of Strathclyde, said many of the major power technology companies are developing a DC circuit breaker for a multi-terminal network, but no one has successfully managed to trade-off conduction losses and speed of operation. ’For example, some people have toyed with the idea of big solid-state switches in series with the main current path, but that would raise the total losses significantly,’ he said. ’Other ideas are hard to make work fast enough.’
Finney said there is reason to be optimistic with high-voltage, low-loss semiconductor technology such as silicon carbide or diamond on the horizon. These technologies could be ripe enough for use on something like the European HVDC supergrid, which by most calculations would not be completed until 2050. They will come at a high price though, making the European supergrid a possibly more expensive venture than originally estimated.
A 2009 European Wind Energy Association report estimates the North Sea grid, which will link up offshore clean energy projects from nine countries, including the UK, could cost up to €30bn over the next 20 years. Adding to this is Europe’s connection to the Sahara, which will likely require 20 transmission lines of 5GW each. The German Aerospace Centre (DLR), which carried out the feasibility studies into the Desertec idea, reckon these lines will add up to €45bn. The additional costs of beefing up Europe’s existing interconnections could throw the entire supergrid scheme into the hundreds of billions over the next 40 years.
Energy companies are unlikely to be deterred by the upfront costs and, without surprise, consumers will bear the financial burden. RTE warned there would be higher electricity bills across Europe in the medium to long term.
But it may be the supergrid is the only choice for a sustainable European energy infrastructure. If the EU is to meet its targets for 20 per cent of energy from renewable sources by 2020 and possibly a decarbonisation of the electricity supply by 2050, it will need to think big.
For the engineers involved, Finney said the challenges laid out by a DC network are a necessary burden. The energy captured from a far-offshore wind turbine or a remote desert solar farm would dissipate before it ever reached mainland Europe using AC lines.
’If you want to do this, to pull together the resources of renewables, you’re pushed toward DC transmission,’ he said.
That whirring sound could be the whine of a turbine, or it could be Nikola Tesla, spinning in his grave.
sustainable supply making a connection
Greenpeace has offered suggestions as to how Europe can push the supergrid forwards
There is much to do before Europe integrates more renewable energy into its supply. Greenpeace’s energy grid study ’Renewables 24/7: Infrastructure Needed to Save the Climate’ outlines several suggestions that will cost Europe €209bn or €5.225bn per year until 2050.
- Strengthening 34 HVAC interconnections between neighbouring countries in Europe: 5,347km of upgrades at a cost of approximately €3bn
- 17 new or strengthened HVDC interconnections within Europe: 5,125km of upgrades at a cost of approximately €16bn
- Up to 15 new HVDC ’supergrid’ connections, with 11 connections in Europe totalling up to 6,000km at a cost of approximately €100bn
- Between Europe and Africa, the capacity of interconnections needed will depend on the amount of imported concentrated solar plant electricity and availability of storage capacity within Europe. Without further optimisation and storage capacity, Greenpeace suggests four HVDC connections with a total length of 5,500-6,000km at a cost of approximately €90bn