According to Germany Trade and Invest (GTAI), more than 335GW of existing wind capacity could add energy storage between 2017 and 2035. This figure equates to the amount of wind capacity that will no longer be paid the FIT subsidy as the 20-year contracts come to an end over the period.
GTAI forecasts that from 2017 to 2020, 10-12GW of onshore wind capacity will emerge from their FIT repayment contracts.
The rationale for co-locating energy storage with wind is that if wind farms continue to operate beyond their 20-year FiT contractual terms, they will need to seek a new revenue streams.
"Installing energy storage with wind farms straightens out the intermittency, enabling operators to bid into wholesale power markets in 10-15 minute time slots," said Stornetic project manager Thilo Englemann.
To do so, the wind resource needs to achieve a high level of predictability, beyond what the latest forecasting technologies can manage.
Stornetic, a developer of flywheel-based energy storage, is targeting the wind industry since flywheels are able to match the rapidly fluctuating and unpredictable profile of wind.
Like lithium ion batteries, flywheels can absorb and release energy within seconds and minutes. Unlike lithium ion batteries, flywheels do not degrade even after thousands of cycles, so their operational lifetimes can match that of wind and solar farms.
Without the level of predictability energy storage can provide, a wind farm operator would need a substantial safety margin to trade electricity in the wholesale power market. In theory, the operator might be able to sell a maximum of 20MWh, but in reality they could only sell 16MWh, missing out on revenue. Operators are penalised if they do not produce enough output.
Adding storage enables wind farm operators to ensure their output better matches trading slot timeframes. Secondly, the battery can also enable the wind farm to provide frequency control services, to help balance the grid.
These duel benefits of wind firming and ancillary services provision that co-locating energy storage with wind can unlock requires minimal storage. According to Engelmann: "To firm output into these 15-minute tradable slices, your storage component needs to be about 6-8% of the wind farm’s capacity, which is enough to compensate for, or eliminate 90% of fluctuations. So a 20MW wind farm would require 1.4MW of energy storage."
But this innovative deployment of energy storage by the German wind industry to try and continue generating income from existing wind farms faces regulatory hurdles. Under the country's latest Renewable Energy Act (EEG), double fees are applied to energy storage, which is treated both as a generator and a consumer of electricity.
German energy storage association Bundesverband Energiespeicher (BVES), of which Stornetic is a member, is working on amendments to the law. But the law will not be changed until after the next general election in Germany, which is slated for the second half of 2017, so any further changes to EEG are not expected before 2018.
Engelmann says payback models vary depending on individual wind farms and their business model and potential revenue streams. Engelmann says existing wind farms and those undergoing repowering can potentially benefit from adding energy storage.
Italian utility Enel Green Power has developed some experimental projects to integrate battery energy storage with renewable energy generation plants. The utility has installed a 2MW/2MWh storage facility with its 18MW Potenza Pietragalla wind farm in Italy. Results from the pilot already show the battery is able to enhance wind forecasting by around 10%.
The challenge with wind output, compared with output from solar PV, is that the fluctuations are less predictable. Within a matter of seconds or minutes wind farm output can send large amounts of power to the grid, often without the demand to absorb it, which makes it harder to predict than solar PV.