Too little, too much
Wind produces 34% of Denmark’s energy, and in Italy solar power contributes 6%. But these sources of renewable energy are intermittent and unpredictable.
To ensure that electricity is available 24/7, distributors rely on pumped hydro dams or gas-powered turbines that can quickly compensate for dips in production. Since these operational reserves work only at part of their full-load capacity, they are an expensive way of providing electricity, emphasizes University of Leeds professor Peter Taylor.
Here’s a perfect example: On 11 February 2007, wind production in Ireland plummeted from 415 MW to 79 MW in just four hours. Were a dip of proportional magnitude to occur in the UK, four nuclear power plants would have to be brought online to compensate for it.
Too much juice
Even the inverse situation – too much electricity – can be a problem: excess energy is absorbed by a frequency increase in the grid. This fluctuation can damage generators, which are optimised to run at 50 Hz. “It’s crucial to rapidly compensate for these imbalances, explains Allan Schrøder Pedersen of the Technical University of Denmark. If frequency variations accumulate, they can lead to a complete phase shift between two generators, and that can produce an explosion.” Excess energy also leads to waste: in March 2014, the operator of the British electricity grid paid wind farm operators £8.7 million to stop generating power. This is why experts agree that the growth of renewables requires a major increase in storage capacity, so that excess electricity can be saved in times of overproduction and freed during periods of high demand.
At the same time, national electricity grids need to be connected more effectively at the European level. “Denmark is very well connected with the Scandinavian grid, which has a large hydraulic capacity for absorbing fluctuations in wind energy production,” explains Taylor. “On the other hand, the British Isles are still isolated and need to increase their national storage capacity.” A renewable Europe must be able to transport solar electricity generated in the summer from south to north, and wind energy generated in winter in the opposite direction.
Building such a continental “super grid” is more of a political challenge than a technological one, explains Falko Ueckerdt, a scientist with the Potsdam Institute for Climate Impact Research. “Because electricity rates can vary from country to country, it’s often difficult to figure out how to share the cost and benefits of building a new transnational connection.” Europe has set global objectives for reducing CO2 emissions, but energy self-sufficiency remains a very sensitive issue at the national level.