System architecture of Subsea Energy Storage for offshore renewables

The world is undergoing a significant energy transition, replacing fossil fuels with renewable energy sources like wind and solar power. While this shift is essential for combating climate change, it presents new challenges, primarily due to the variable nature of renewable energy. This project aims to explore how the extensive experience from the subsea oil and gas industry can be adapted to develop innovative solutions for energy storage, ensuring a stable and reliable power supply from renewable energy power plants. This industrial PhD project looks to utilize the knowledge and expertise gained from decades of subsea oil and gas operations to address the challenges of integrating renewable energy at a large scale. The objective is to study the requirements for energy storage systems that can support the transition from fossil fuels to renewable energy sources and to evaluate a proposed subsea energy storage solution to ensure it meets the energy sector's needs. Renewable energy sources like wind and solar power are inherently variable. Wind turbines generate electricity only when the wind is blowing, and solar panels produce power only during daylight hours. Since the share of renewable energy in the overall energy mix is still relatively low the effects are so far limited with increased price volatility (including negative power prices) the most notable effect. When renewables only account for a low share in the energy mix, existing dispatchable powerplants (coal, gas, hydro and nuclear based power plants) can balance the variable output from renewables, ensuring a continuous power supply. However, as the proportion of renewable energy increases and the reliance on fossil fuels decreases, the effects on the power grid will increase. Energy storage is a crucial component in mitigating the intermittency of renewable energy. By storing excess energy generated during periods of high output from renewables, it can be used during times when renewable sources are not producing power. Energy storage is not a new concept, the world already has significant energy storage capacity mainly in the form of pumped hydro storage but to solve all the issues related to the energy transition the energy storage capacity needs to increase substantially and a diverse combination of different types of energy storage capabilities is required. Each storage method comes with it's own advantages and disadvantages which needs to be evaluated. One innovative solution being explored is the Subsea Pumped Hydro Storage (SPHS). This system uses the pressure difference between the hydrostatic pressure of the oceans and a low-pressure tank located subsea to store energy. When energy is needed, water flows into the tank, driving turbines to generate electricity. During the discharge the amount of water in the tank increases, compressing the gas inside the tank and thereby increasing the pressure in the tank. To charge the system, water is pumped out of the tank, restoring the pressure difference. Benefits of SPHS: -Scalability: SPHS can be deployed in various ocean locations, unlike traditional pumped hydro storage, which requires specific topographical features. -Storage Capacity: By utilizing the hydrostatic pressure of the oceans the energy storage capacity can be enhanced. -Integration: SPHS can be co-located with offshore wind farms, reducing the need for extensive infrastructure and minimizing energy curtailment. In addition it can increase and diversify revenue streams for offshore wind farm operators. During the project, the thermodynamic efficiency limits of SPHS as well as methods for calculating/modelling the State-of-Charge, turbine and pump efficiencies have been established. Lessons learned from the oil and gas industry with regards to design and installation of subsea structures has been shared within the academic community. Academic networks has been established and further work identified. Ultimately, this project aims to create a reliable and efficient energy storage solution by repurposing subsea oil and gas expertise, addressing the challenges of renewable energy integration, and supporting the global transition to sustainable energy sources.

Outcomes - Increased Collaboration: The project has created a bridge between academia and the industry which is valuable both for academia and the industry leading to more relevant research and higher benefits for the industry. In addition, the academic network has been expanded and international connections has been made further enhancing research quality. -The project improved Phd students skills in thermodynamic analysis of Subsea Pumped Hydro Storage (SPHS) and related storage systems, through advanced modeling and system design. - Increased understanding of how the electrical power systems work and how to design solutions for the future power system with a major share of renewables. - Provided valuable insights for suppliers and operators in the renewable energy industry to optimize products. - Offered evidence-based recommendations to integrate SPHS into energy strategies, influencing policy and practice. Impacts - Environmental Benefits: SPHS supports increased use of renewable energy, reducing fossil fuel dependence and greenhouse gas emissions. - Economic Growth: Potential to create jobs and stimulate economic growth in the renewable energy sector repurposing the expertise in the Norwegian oil and gas industry. - Energy Security: Enhances energy stability, ensuring a reliable power supply and supporting economic and social stability. - Global recognition: Positions participants as experts in subsea energy storage, attracting further research and collaborations. In summary, the project enhanced skills, fostered collaboration, provided practical insights, and can potentially influence policy, with long-term benefits for the environment, economy, energy security, and society.

Many commitments and policies have been defined in order to cut CO2 emissions. For example, the European commission has stated that the EU will be carbon neutral by 2050. In order to achieve this, the renewables share of the energy portfolio needs to be increased substantially. However, one of the main drawbacks with renewables such as wind and solar power is its intermittency. In order to balance this intermittency the demand for both long term and short term energy storage solutions will increase significantly. Subsea 7 has through its technology development program developed and patented a concept for subsea energy storage. The work is based on the long experience within subsea engineering at Subsea 7 and aims to increase the utilization of the ocean resources locally and globally. Further research is needed to understand the requirements for energy storage in the energy market from 2030 and onward. First of all, an understanding of what the capacity need for power balancing and energy storage is in the period 2030-2050 is needed. Second, can the proposed energy storage system balance the variability in power output from renewables in the period 2030-2050 enough to meet the demands from the grid suppliers? These questions will be answered by a literature study and development of a model to predict the performance of an novel subsea energy storage solution. The model can be tested with various energy sources and used for hypothesis testing defined during the initial stages of the PhD study.