The Danes are finally going nuclear. They have to, because of all their renewables

Green grids need the resilience of synchronous turbines

A nuclear power plant generating turbine. Making this kind of machinery spin faster or slower takes time, and that keeps power grids resilient Credit: Milan Jaros/Bloomberg

The Danish government plans to evaluate the prospect of beginning a nuclear power programme, this week lifting a ban imposed 40 years ago. Unlike its neighbours in Sweden and Germany, Denmark has never had a civil nuclear power programme. It has only ever had three small research reactors, the last of which closed in 2001.

Most of the renewed interest in nuclear seen around the world stems from the expected growth in electricity demand from AI data centres, but Denmark is different. The Danes are concerned about possible blackouts similar to the one that struck Iberia recently. Like Spain and Portugal, Denmark is heavily dependent on weather-based renewable energy which is not very compatible with the way power grids operate.

Conventional generators produce alternating current, creating a stable output of current and voltage that alternates at a frequency which is directly – synchronously – linked with the rotating turbines which drive the generators in gas, coal, nuclear or hydropower plants. All of these turbines rotate at a speed of 3000 revolutions per minute, so producing electricity with current and voltage that varies in a sine wave shape with a frequency of 50 cycles per second (ie 50 Hz).

Electrical equipment is highly sensitive to this frequency and can break if it deviates too much from 50 Hz. For this reason, power stations, substations, switching equipment and other devices in the grid have fail-safes which will cause them to trip out should frequency fall outside acceptable bounds.

This frequency property is also connected to the balance between supply and demand: if there is more generation than consumption, frequency increases and the turbines speed up, and if there is more demand than generation, the opposite happens.

Fortunately, conventional generators are large heavy lumps of metal whose speed of rotation is hard to change. They oppose changes to their speed of rotation, providing resistance to changes in grid frequency, a property known as “inertia”.

However, wind and solar do not produce synchronous alternating current. Although wind turbines rotate, they do not do so at a constant speed, and solar has no moving parts at all. They produce direct current which is converted to alternating current using electronic devices known as inverters. Wind and solar also have no inertia.

The situation in Iberia highlighted the difficulties with running low inertia power systems: an as yet unidentified grid fault caused a disturbance and frequency changed. There was not enough inertia to contain the frequency shift and inverters dropped out, widening the gap between supply and demand and worsening the frequency deviation. This caused other equipment to disconnect, quickly leading to a cascading blackout. The process was too fast for grid operators to take action to contain it, due to the lack of inertia. If there had been more time, generators could have been shut down or started up in order to match supply to demand and get the grid frequency back to 50 Hz.

That the incident happened just days after the Spanish grid operator boasted about running with 100 per cent renewables suggests a desire for net zero virtue signalling trumped concerns over grid security.

It was notable that when the fault propagated into France, it was quickly contained and power was rapidly restored to the small region that experienced an outage. This is because the French grid operates with very high inertia, since almost all of its electricity is generated by big, heavy nuclear and hydroelectric turbines.

The Danish government is worried about how it will continue to decarbonise its power grid if it closes all of its fossil fuel generators leaving minimal inertia. There are only three realistic routes to decarbonisation that maintain physical inertia on the grid: hydropower, geothermal energy and nuclear. Hydro and geothermal depend on geographic and geological features that not every country possesses.

While renewable energy proponents argue that new types of inverters could provide synthetic inertia, trials have so far not been particularly successful and there are economic challenges that are difficult to resolve.

Denmark is realising that in the absence of large-scale hydroelectric or geothermal energy, it may have little choice other than to re-visit nuclear power if it is to maintain a stable, low carbon electricity grid.

Original article here