Weekly Brief

Storage Technologies for Offshore Renewable Energy

Olivier Lodeho, Head of Technology Group, Subsea 7

Olivier Lodeho, Head of Technology Group, Subsea 7

Wind offshore Fields are the preferred Renewable energy in many countries to meet their carbon reduction ambitions either to feed their domestic energy demand in electricity while limiting greenhouse gas emissions or to decarbonise their O&G facilities. However, like all renewable energy, offshore wind, despite more favourable winds offshore than onshore, are still intermittent and not pilotable. This means their output varies in intensity along the time without any control and therefore induces several challenges on grid networks and their management. We can identify 3 types of intermittences:

•Type I. Intermittences from weeks to months, which could be illustrated by seasonal variations.

• Type II. Intermittences from Hours to days, which could be represented by the day/night period

• Type III. Intermittences from second to hours: which could represent a wind fluctuation or a cloud in front of the sun

Those variations are different in time scale and in intensity and can be overcome/mitigated by different technical solutions like VSD and Batteries for Type I, Batteries, HPES for Type II, Hydrogen for Type III are some examples.

Here we will look at 2 technical solutions based on subsea pumped Hydro technology, called:

• ROPES: Repurposing Offshore Pipelines for Energy Storage

• Power Bundle: subsea dedicated pipeline system for energy storage

Subsea pumped Hydro Energy storage principle:

The principle is to charge seawater into a subsea pressured reservoir with a pump powered by the excess of energy produced by a set of offshore wind turbine and to release this water through a turbine generating power when wind does not blow or not enough. We can therefore identify 2x sub-assemblies:

• ECU system: Energy Conversion Unit composed by the pump and the turbine

• PCS Pressure containment system: which is under pressure and receive the sea water. The ECU can be installed on topside or subsea while the PCS will be subsea for the following reasons:

• No space constraints

• Minimise energy loss by being subsea with a stable cold seawater temperature and therefore limiting heat losses and maximise system efficiency

• Take advantage of the water depth for improving difference and therefore energy storage capacity of the system.

This PCS can be done in several ways and using pipelines, allow to take advantage of existing supply chain from Oil & Gas industry and therefore reach interesting cost levels.

If pipelines already exist in the vicinity of the Offshore Wind Farm, we can repurpose them; this is the ROPES Product. In that case, CAPEX will be limited as we re-use existing pipelines. This will improve NPV as this will defer their decommissioning. In other hand, amount of energy will be driven by the available volume/ pressure in pipeline which cannot reach the required storage level to optimise the electricity production of the wind field.

"PCS can be done in several ways and using pipelines, allowing to take advantage of existing supply chain from Oil & Gas industry and therefore reaching interesting cost levels"

If no pipeline exists around, an optimised Energy storage system can be installed along the wind farm: Power Bundle. This combines a wellknown O&G pipeline system called “Bundle” which consists onshore prefabricated stack of pipelines ended with structures including valves, controls and sensors; submerged towed on site and landed on seabed. This methodology does not require any heavy lift vessels and can be done on large weather windows. This is why it is widely used in North Sea locations.

Those 2 systems are developed and commercialised by Flasc and Subsea 7 companies and are utility scale energy storage solutions. They have been recently awarded by UK BEIS Grant funding for a detail engineering of a demonstrator and several engagements are ongoing with Wind Farm developers.

The benefits of this Utility scale energy storage are:

• Existing Offshore Wind Farms: increases asset utilisation without taking up onshore space