Hydro-Pneumatic Energy Storage System by Flasc BV

FLASC is developing an energy storage technology tailored for offshore applications. The solution is primarily intended for short- to medium-term energy storage in order to convert an intermittent source of renewable power into a smooth and predictable supply.

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The technology is based on a hydro-pneumatic liquid piston concept, whereby electricity is stored by using it to pump seawater into a closed chamber and compress a fixed volume of pre-charged air. The energy can then be recovered by allowing the compressed air to push the water back out through a hydraulic turbine generator. Thanks to a patented, pre-charged, dual-chamber design that uses the ocean itself as a natural heatsink, the system is capable of very high thermodynamic efficiencies (+95%) that when combined with off-the-shelf hydraulic machinery, result in a competitive round-trip efficiency. The core principle is analogous to pumped-hydro, which is one of the most well-established, best understood methods of storing energy.

The FLASC system is built around well-proven technologies with established supply chains that are familiar and well understood by the offshore industry. The closed, pre-charged concept is a crucial innovation, since it allows the system to have a high energy storage capacity even in relatively shallow water (down to 20-30m). Other subsea concepts for energy storage typically rely on external hydro-static pressure, and therefore require very deep water (+1000m) to be feasible.

The solution is highly flexible and can be delivered in various embodiments:

  1. Compressed air reservoirs installed within the floating platform with an external subsea hydro-pneumatic module. This embodiment was the first that was tested in 2017-2019 as part of an experimental campaign. The up-scaled version was the scope of a techno-economic feasibility assessment by DNV-GL, based on which the technology was granted a Statement of Feasibility.
  2. The full system can be installed on the floating platform. This embodiment will be installed as a pilot system within a large-scale multi-purpose floating platform, to be deployed in 2022.
  3. The full system can also be installed as a completely subsea assembly. This embodiment is presently being explored with a leading subsea engineering service company, targeting oil & gas applications and fixed-foundation offshore wind.

There is also the flexibility to customise the energy conversion system. For example, the system can be charged using electricity, but during discharging, can provide a combination of electricity and cold, pressurised seawater. Based on this principle, on-going feasibility studies have so far shown the potential of the technology in the context of (i) liquefaction of natural gas, (ii) reverse osmosis desalination and (iii) offshore green hydrogen production. Other applications could include Carbon Capture and Storage and enhanced oil recovery using water-injection.


The solution addresses a fundamental problem related to the integration of large-scale renewable energy production into conventional energy systems: the mismatch between energy supply and demand.

Balancing supply and demand is quickly becoming the biggest obstacle to the increased uptake of renewable energy. Due to the intermittent nature of the resource, a reliable and sustainable supply of energy can only be achieved if sufficient flexibility is introduced into the energy system. An excellent source of flexibility is energy storage.

Conventional battery technologies are not ideal for the sharp charging-discharging cycles associated with wind power, particularly in offshore applications. In this context, the lifetime quickly degrades and the logistical cost of replacement is significant. Moreover, there are significant safety and environmental concerns and existing standards and certification requirements make it difficult to deploy large-scale stationary battery storage offshore. Finally, batteries operating in applications where they require more frequent replacements are a problem in themselves, since it is difficult to recycle the spent components and materials in a sustainable manner.

An energy storage device with a long lifetime translates into direct economic benefits. Based on cumulative damage modelling techniques, a lithium-ion battery bank installed to mitigate wind turbine intermittency will require 4 replacements throughout a 25-year project lifetime. An equivalent FLASC system can last throughout the entire project lifetime using standard offshore maintenance practices. So in this case, a FLASC system would deliver a lifetime cost saving of roughly €500,000 per MWh of installed capacity, when compared with the equivalent lithium ion battery bank. This excludes the logistical cost of replacing the battery systems, and any associated downtime.

The benefits of the FLASC technology go beyond the economic incentive. By using sustainable principles and recyclable materials, the solution does not place any additional burden on the environment throughout its lifetime. In some applications, such as remote offshore applications, it is simply just not possible to utilise renewables without some form of energy storage to stabilise the supply. In these cases, the right storage technology can unlock a bountiful supply of clean energy.

Ongoing research is presently evaluating the potential for FLASC working in conjunction with green hydrogen production. However, an electrolyser directly connected to the intermittent output of a wind turbine could poses operational challenges, and its efficiency could suffer. A storage device such as FLASC could be ideal as interface between the two systems, providing short-term storage and constant power supply to the electrolyser. Batteries have also been proposed in this context, however, with no inherent flammability issues, a hydro-pneumatic solution presents a much safer alternative.

How will this innovation contribute to the industry?

Decarbonisation initiatives in the offshore sector are becoming ever more prominent, and projects coupling offshore wind with oil & gas production are already taking shape (eg. Hywind Tampen). Moreover, governments are encouraging offshore wind farm developers to start thinking about flexibility already at tender stage. The recent Hollandse Kust Nord tender in the Netherlands gave a specific advantage to bids incorporating innovations that contribute to supply flexibility. With ambitious climate targets and renewable penetration rates set to increase substantially, this is only the beginning.

Innovations that contribute flexibility to the supply of intermittent renewable energy, such as FLASC, will be fundamental enablers for the industry to continue developing in a sustainable manner. Specifically it will bring two key advantages to the sector:

  • Increasing the value of offshore wind. The FLASC solution allows for the supply of schedulable power from offshore wind farms directly to shore. Numerous studies on energy economics show that intermittent renewables can leverage energy storage to increase the value of each unit of electricity they produce by selling at the right moment and in the right market segment. As favorable subsidised tariffs are now giving way to low-margin subsidy-free projects, increasing the value of each delivered kWh can significantly improve the profitability of a grid-connected offshore wind farm project. By co-locating the right storage technology in an intelligent manner, this benefit can be readily derived within the envelope of an offshore wind farm project.
  • Decarbonisation of offshore oil & gas. A significant quantity of energy-intensive infrastructure already operates offshore, typically co-located with abundant renewable resources such as offshore wind. This presents an opportunity for the next generation of offshore oil & gas projects to operate in a cleaner and more sustainable manner by using these resources as a local source of power. Oil & gas majors are already evaluating these types of projects, leveraging the rapid developments in floating wind technologies. Most of these projects are located in remote locations, far from conventional electrical grids. A direct connection to a variable renewable power supply is therefore not feasible, and to offset a substantial part of the energy demand, coupling the right storage technology will be fundamental. The FLASC solution is an excellent fit for this application, it uses most of the same supply chains as the co-located infrastructure, well understood installation and operational principles, and is suited to the harsh offshore environment. Moreover, it uses compressed air and pressurised seawater, and therefore poses no additional environmental or flammability hazards.