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

About

Energy storage is the key to make renewable energy consumption independent from energy production, allowing for flexibility and reducing the waste of energy. The FLASC Hydro-Pneumatic Energy Storage (HPES) solution specifically targets offshore applications, a crucial energy sector, where existing solutions for onshore applications are not able to feasibly address this problem due to safety and reliability issues.

The solution uses compressed air and pressurised seawater in a patented, pre-charged accumulator concept, resulting in an energy storage device that is inherently safe, reliable and also cost-effective thanks to a +30 year lifetime.

Key Benefits

The FLASC solution brings significant environmental benefits by facilitating the integration of renewables into mainstream consumer grids as well as energy-intensive processes and critical infrastructure in offshore oil & gas. Energy storage in general is a key enabler in this context. However, as a storage device, the FLASC solution has particular advantages.

The most prevalent electrochemical solutions, lithium-ion batteries, face numerous adoption challenges in such applications due to safety issues and fire hazards. Therefore, the environmental benefit associated with the integration of renewables in offshore applications cannot be sufficiently reaped using battery technologies

Batteries also tend to suffer depleted lifetimes when used with intermittent energy sources such as wind. The harsh charging-discharging cycles associated with such applications have been shown to cause a significant drop in battery lifetime resulting in the need for frequent replacements. This creates two sets of problems: the environmental cost of replacement, and the disposal of the spent cells. To replacement a depleted battery in a remote offshore application results in significant fuel consumption and emissions associated with the deployment of vessels and other logistical aspects. Once the battery has been replaced, the spent cells must be discarded in a sustainable manner. Particularly for lithium-ion batteries, this is highly problematic, since these cells are notoriously difficult to recycle.

The FLASC hydro-pneumatic technology can be designed for a 30-year lifetime with no replacements, only requiring standard maintenance associated with offshore infrastructure. Moreover, the primary material is steel, which can be easily recovered and recycled at end of life.

Finally, the installation of a storage device with a subsea component brings interesting local ecological benefits. It has been shown in a number of published studies that subsea infrastructure enhances the abundance and biodiversity of local marine species, to the extent that it can also contribute to the fisheries industry.

Applications

FLASC is an energy storage system based on a hydro-pneumatic liquid piston concept, whereby electricity is stored by using it to pump seawater into a closed chamber to 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 unique, 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 makes up around 85% of the stationary energy storage market in terms of capacity, and is one of the most established, best understood methods of storing energy. The main difference is that in pumped-hydro the pressure head is provided by a body of water at height, whereas for the FLASC solution, it is provided by a pre-charged volume of compressed air.

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 exist, but these rely on external hydro-static pressure, and therefore require very deep water (+1000m) to be feasible. Most offshore renewable energy applications are not anticipated to be installed at such water depths, so the FLASC technology has a significant advantage.

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

1) Compressed air reservoirs installed on 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 part of a TRL 7 demonstrator of a multi-purpose floating platform to be deployed in Greece in 2022. The overall project is funded through an H2020 grant [H2020 BG-05-2019].

3) The full system can 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.

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. The storage capacity can cater for multiple hours of stable power delivery, and provide a number of services, including: (i) Bulk Energy Services (energy time-shifting, power supply capacity); (ii) Ancillary Services (operating reserves, voltage support, black start); (iii) Transmission Infrastructure Services (transmission upgrade deferral); (iv) Renewables Integration (ramp rate control , generation peak shaving, capacity firming).

There is also the flexibility to customize the energy conversion system. For example, the system can be charged using electricity, but during discharging, can provide a combination of electricity and cold, pressurized 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) green hydrogen production. Other applications could include Carbon Capture and Storage and enhanced oil recovery using water-injection.

The core energy storage technology has received a Statement of Feasibility from DNV, and is covered by patents in the US (US10344741B2), Europe (EP3256716B1), China (CN107407248B) and Japan (JP6709225B2).

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