Seasonal energy storage: vital for growth of renewables

April 30, 2018 | 08:00
Future growth of offshore wind and large scale solar will be hampered by its intermittency. To surpass the so-called ‘Dunkelflaute’ – the absence of wind as well as sun – seasonal storage is a prerequisite to balance the energy grid from 2023 onwards. Hydrogen may have the best to offer.

Sooner or later in this century the age of oil and gas will be over. Meanwhile we‘ll do have to bridge the gap between renewables and fossil fuels. PV solar has become cheaper but especially offshore wind is making cost-effective strides nowadays. Two large-scale wind farms in the North Sea have been granted concessions without subsidies: Hollandse Kust with 350 MW, 22 kilometres off the Dutch coast, and Hohe See with 497 MW, some 100 kilometres north-west of Helgoland, Germany.

The more intermittent resources - like offshore wind and, to a lesser extent, large-scale solar - will get a share in the energy mix, the more grid balancing becomes a necessity (due to curtailment – or negative pricing of wind energy – Germany alone has lost one billion euros in 2016, costs that are increasing over time). Between 2030 and 2050, grid balancing solutions such as demand-side management, increasing the number of interconnections and flexible pricing aren’t going to be sufficient to compensate for intermittancy. Moreover, battery storage such as Tesla’s huge 129 MW facility in South-Australia, cannot provide storage for longer than a couple of days.

Seasonal energy storage

Enter seasonal storage: only solutions that can store energy for weeks or even months can bridge the gap between the intermittent supply of renewables and the growing demand of an increasingly electrified society. Pumped hydro comprises of more than 95 percent of all long-term energy storage worldwide. But according to Fokko Mulder, professor of Materials for Energy Conversion and Storage at Technical University Delft in the Netherlands, that’s not nearly enough to cater for energy demand after 2023.

Compressed air energy storage (CAES), the other large-scale mechanical option, falls short on reliable reservoirs close to a sufficient amount of consumers. Also, turbines to compress air are fossil fuel-powered (even when supplied by wind power). Buoyant supports for modern wind turbines have met increasing interest but are still in their early R&D stages.

Green hydrogen

Green hydrogen may be the best offer for a future energy system that is both flexible and completely sustainable. At offshore wind farms, an electrolyser using surplus electricity splits water into oxygen and hydrogen. Huge amounts of hydrogen – up to 167 GWh of hydrogen (or 100 GWh of electricity) - can be stored in underground salt caverns or near empty gas fields. Once onshore, one can inject this hydrogen into fuel cells for transport or utilize the piping system currently used for the transportation of natural gas.

But green hydrogen has its drawbacks: first converting electricity into hydrogen and a second time into energy reduces its round-trip efficiency to 30 to 40 percent. Also, hydrogen cannot be compressed into liquid, making it difficult to transport. It is also costly: green hydrogen is five times as expensive as grey hydrogen extracted from natural gas by steam methane reformation.

The hydrogen economy

Price is especially important when looking at electrolysers that make a hydrogen system expensive. But according to Ben Madden, director of UK-based consultant Element Energy, those costs dwarf as soon as there’s economy of scale. ‘Effectively, the capital cost of electrolysers is disappearing from the problem: once you’ve got scale, the question is the price of the electricity’, he told Recharge News.

Scale can be accomplished by injecting green hydrogen into the natural gas grid. Especially in the Netherlands hydrogen offers huge possibilities after Eric Wiebes, Minister for Economic Affairs and Climate, announced a speedy phase-out of natural gas by 2022 (due to earthquakes caused by gas extraction in the North of the country). Of all EU countries, the existing national gas grid is particularly suited for that: whereas the maximum blend level of hydrogen into the grid in the UK is only two percent, in Germany and the Netherlands it’s up to fourteen percent. Moreover, the Netherlands can use its infrastructure onshore as well as offshore and modify pipelines for hydrogen.

Ongoing experiments – for instance in Delfzijl and on the island Ameland – are coupled with a comprehensive plan to make the northern part of the Netherlands leading in the hydrogen economy. The region is well suited for this: it already has large-scale renewable energy production (onshore and offshore), major chemical clusters and well-known international R&D companies (like NAM and GasTerra). In order to establish such a hydrogen economy, though, governmental policies have to favour energy storage, electrolysers need to be improved and advantages of system integration ought to be applied.

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Image: The hydrogen economy.  José Manuel Suárez CC BY 2.0
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