Overcapacity and the challenges of going 100% renewable

The installation of intermittent renewables such as wind and solar photovoltaics is growing rapidly in many advanced markets, but looking to the future, what challenges do intermittency and overcapacity pose to countries looking to move towards the holy grail of a 100% clean, renewable power system?


By many metrics, Germany’s ongoing and highly progressive campaign to rapidly shift towards renewable energy sources (RES) has been a great success. The investments made in renewable energy installations have prompted a massive surge in wind and solar presence in the German energy mix since the ‘Energiewende’ (energy transition) kicked off in earnest in 2010.

In April, the country broke its national record for daily renewable energy provision. RES covered around 85% of German electricity use on Sunday, 30 April, with sunny, breezy conditions providing a particular boost to solar and wind output.

“Most of Germany’s coal-fired power stations were not even operating on Sunday, April 30th,” said Patrick Graichen of think tank Agora Energiewende in an interview with RenewEconomy. “Nuclear power sources, which are planned to be completely phased out by 2022, were also severely reduced.”

Milestones such as this – as well as the country’s first competitive offshore wind auction, which yielded low strike prices and several subsidy-free bids – have stoked hopes that Germany, with fair winds, will be able to meet its ambitious target of sourcing 45% of its energy from RES by 2030 and as close as possible to 100% (or at least 80%) by the middle of the century.

‘Fair winds’ may be the operative phrase, however, as the progress being made in Germany and elsewhere on renewables sometimes disguise the significant problems that can be thrown up by growing RES penetration in energy systems. The primary source of these problems comes down to an issue that has plagued renewables from the beginning: intermittency and overcapacity.

Intermittency and overcapacity: a problem in Germany and elsewhere

Germany’s progressive attitude towards RES deployment makes the country a good case study for the complications that can arise from their rapid growth in energy systems. The Energiewende has certainly had some high-profile successes, but bringing the share of RES generation up to around 30% in 2015 (from 3.6% in 1990) brings its share of difficulties.

The variability of energy output from intermittent renewables such as wind and solar, based on the extent of sun and wind resources, requires hefty back-up power sources that can ramp up or down depending on the availability of RES  (which under German law are required to be used first on the grid). As Germany’s ‘Atomstop’ policy is forcing low-carbon nuclear plants to gradually shut down over the next five years, the vast majority of this back-up generation comes from coal and gas plants.

As a result, Germany’s CO2 emissions, despite a historical downward trend, have actually increased in the last couple of years as the use of brown coal and other hydrocarbon-based energy sources has increased to compensate for the loss of nuclear generation. Intermittency is an important factor here; due to poorer sun and wind conditions in 2016 compared to the year before, wind generation increased by only 1% and solar generation fell last year, despite the country adding 10% more wind turbine capacity and 2.5% more solar capacity.

"Germany’s CO2 emissions, despite a historical downward trend, have actually increased in the last couple of years."

But significant variability in RES output means that there have been many times, when the sun is shining and the wind is howling, when Germany has been pushing a massive excess of power on to its grid. This overcapacity has often been offloaded to Germany’s European neighbours, and as RES always get the first bite of the apple on the German grid, the majority of this is fossil fuel-based.

Sudden surges of surplus electricity have wreaked havoc with the grid stability of countries including Poland and the Czech Republic, which has led to threats from the countries to block transmission from Germany. Price volatility is another potential byproduct of this overcapacity, with German prices occasionally going negative when there is a large excess, meaning that large power plants have to pay commercial customers to use electricity. The German government appears to have recognised the downsides of unchecked RES growth, recently replacing the feed-in tariff subsidy with a market-responsive auction system based on pre-set RES capacity growth caps.

And it’s not only Germany that is dealing with overcapacity and intermittency problems. Other regions, such as California and South Australia, have felt the sting of intermittency without adequate back-up. South Australia, which relies on wind for around 40% of its electricity, has suffered several large-scale blackouts in recent months that have been caused or exacerbated by the lack of available wind or dispatchable back-up sources.

Going 100% intermittent renewable in Germany: massive overcapacity a necessity?

Given the issues that the early stages of Germany’s renewable revolution have been causing, the question remains: how would a country like Germany provide security of supply if it was to complete the transition to a system run on 100% intermittent renewables?

At the end of last year, Professor Freidrich Wagner of Germany’s Max Planck Institute for Plasma Physics published a study in the European Physical Journal Plus exploring exactly this question, and his conclusions are concerning.

Wagner used data taken from the German electricity system in recent years – rather than general meteorological data – to analyse renewable generation in the country and model what Germany would need to do to cover its electricity needs through intermittent renewables. He found that the country would need to install a massively oversized power supply system to achieve this, with 330GW of wind and solar photovoltaic capacity needed to meet a 100% target, and enough back-up capacity to cover an extra 89% over and above peak load. This would cause a host of problems related to overcapacity, energy spot prices and carbon-intensive back-up power when RES output falls, not to mention the landscape impact of wind turbine installation on this scale and the resistance this might cause.

“By 2022, an extremely oversized power supply system has to be created, which can be expected to continue running down spot-market electricity prices,” Wagner concludes. “The continuation of the economic response – to replace expensive gas fuel by cheap lignite – causes an overall increase in CO2 emission. The German GHG emission targets for 2020 and beyond are therefore in jeopardy.

“We have argued that overproduction by iRES [intermittent RES] may not be the right way to go. The impact on land use and the transformation of landscape, e.g. by wind convertors and transmission lines at an unprecedented density, will intensify social resistance. Therefore, it is mandatory to consider also other forms of CO2-free energy production to supplement iRES.”   

Working towards a solution

The issues over renewable energy intermittency and overcapacity have been politicised to some extent. It is certainly true that it’s in the interest of the fossil fuel industry and its advocates to highlight (and often overestimate) the challenges posed by the growing prevalence of RES. But at the same time, there is perhaps a tendency for staunch RES campaigners to label those who express concern about intermittency as ignorant anti-environmentalists or sock puppets for fossil fuel interests.

The path forward will lie in a careful balance between these two extremes, at least initially. Few would argue that completely clean, fully renewable energy systems represent the future, but threading the needle between environmental efficiency, energy security and harmonious electricity markets is a complex task that will take many decades to solve. Nuclear power could play an important role in providing low-carbon baseload or back-up power, although its suitability for dispatchable capacity is limited by how slowly nuclear plants can ramp up or down on demand when compared to gas turbines. Public and political appetite for significant nuclear fleet expansion is also low in many countries, with Germany proving particularly nuclear-averse.

"If you want to use fluctuating renewable power, you have to upgrade the grids across Europe."

Components of the solution include back-up dispatchable power, demand response and energy efficiency measures, distributed generation, data-driven smart technologies, grid-level energy storage and more integrated networks. The EU, through its harmonised policy environment, has an opportunity to take the lead on common energy markets and interconnected grids, although technical, political and financial obstacles to this are currently formidable.

“If you want to use fluctuating renewable power, you have to upgrade the grids across Europe,” Vattenfall policy advisor Daniel Genz told the MIT Technology Review in May last year.

Given the extended timespan of the challenge, breakthrough power sources such as fusion energy – which could provide emissions-free baseload power in virtually unlimited amounts – could emerge as well. Large-scale energy storage is still in the early stages of development with several competing technologies; the efficiency of dispatchable storage will be a concern moving forward, but breakthroughs in this area could also have a decisive effect on the feasibility of greater RES predominance.   

While it’s undeniable that RES intermittency and overcapacity are significant problems for the energy systems of today, a host of options can be developed to create solutions for the energy systems of tomorrow. But until fully clean energy systems become technologically and economically feasible, some level of hybridism will be needed to offset the variability of intermittent renewables.