Pellworm is an island off the coast of northern Germany, one of the North Frisian Islands close to the border with Denmark. Its 37 km2, contains a population of 1200. During the 1980s and 90s it built up a network of renewable sources including solar, wind and biogas (the latter from farm and animal waste) that enabled the population to produce more bulk power than it consumed. Nevertheless owing to intermittency it still exchanged power with the mainland through two linking cables.

This combination of factors made it an ideal choice for E.ON’s pilot project aimed at achieving a hybrid renewables plant and storage based smart power network that was as independent as possible of the mainland supply.

The first phase of the project – a feasibility study, establishment of a ‘core grid’ and proving trials – has been running since 2012 and has been successfully completed, showing that smart grids can link renewable energy sources and battery storage systems to provide a secure and stable power supply to a region based on wind and solar energy alone, and can sustain it in the long term. For this phase E.ON teamed up with grid operator Schleswig-Holstein Netz AG, the two Fraunhofer institutes IOSB-AST and UMSICHT, the power systems manufacturer Gustav Klein, the battery manufacturer Saft, Fachhochschule Westküste, and Aachen University. In the second phase of the project, which is about to start, E.ON and its partners will explore new business models on the North Sea island to find a way of making battery-based power solutions commercially viable. There are also plans to integrate this technology into the existing virtual power plant operated by E.ON subsidiary HanseWerk Natur.

The key questions now are economic, legal and administrative – how energy storage systems will be financed, how the services they provide will be paid for, and how potential investors can be provided with planning security.

Need for storage

In the past, Pellworm produced much more wind and solar power than was needed on island, but it was often necessary to import energy from the mainland. It was therefore important to find ways of buffering the fluctuating supply of the intermittent energy sources and improve utilisation on the island. To do so, E.ON has built a dedicated power supply system comprising two large electrical batteries as well as thermal storage units which have been integrated into a novel energy management system.

Following the successful test phase, the second project phase will now focus on optimising the economics, which are still a challenge. Improved system performance, greater economies of scale and price discounts are expected to compensate for the high capex for the storage systems.

Different storage technologies have been installed to cover time-scales ranging from ‘minutes-to-hours’ and ‘hours to days’. The ‘hours to days’ storage is provided by a 200 kW, 1.6 MWh vanadium redox-flow battery. Load flexibility in terms of ‘hours’ storage is provided by a combination of night storage heaters and heat pumps, with an average energy capacity per household of 135 kWh and an average power of 17 kW.

The ‘minutes to hours’ storage is provided by a containerised Saft Intensium Max 20 lithium-ion battery system providing 560 kWh of energy storage and 1 MW power.

Smart grid

The main elements in the Pellworm smart grid are the different complementary decentralised storage technologies with a focus on innovative batteries, integration of flexible loads at the household level , central optimisation of the storage system by an EMS, automatic local substations, and smart meters with integrated load control functionality, mainly to include decentralised controllable loads in the LV grid.

System structure

In 2012, when the study started in earnest, Pellworm Island presented a vision of Germany’s renewable energy mix of the future – its share of renewables already corresponded closely to Germany’s target for 2050, and the feasibility study confirmed that the island would be the ideal location.

The annual energy production of around 21 GWh from E.ON’s hybrid power plant, including 300 kW of wind and 780 kWp of photovoltaics together with the island’s biogas plants was three times the annual consumer load of 7 GWh. There was also a high level of night storage heaters and heat pumps in place, highly suitable for demand side management. But the island still relied on its connection with the mainland grid, via two 20 kV subsea cables, for balancing local surpluses and for importing energy at times when demand exceeded supply.