Offshore wind (OSW) energy continues to grow in the US Atlantic, complementing the national goal of deploying 30 GW of OSW energy by 2030, which will unlock a pathway to 110 GW or more capacity by 2050. The US Department of Energy (DOE) conducted the Atlantic Offshore Wind Transmission Study (AOSWTS) to understand and facilitate the transmission of electricity from OSW projects in the Atlantic Ocean. The DOE recently published the study, based on the findings of the Atlantic Offshore Wind Transmission Literature Review and Gaps Analysis 2021 and convening workshops hosted in 2022-23 by the DOE and the US Department of the Interior’s Bureau of Ocean Energy Management. the AOSWTS was funded by the DOE’s Wind Energy Technologies Office (WETO). The study guides policymakers and transmission stakeholders on the possible outcomes of a proactive, coordinated and interregional approach to transmission planning for OSW energy development in the Atlantic.
Study methodology
The study focused on the offshore space between Maine and South Carolina, and the onshore grid in those states (besides Vermont and Pennsylvania due to proximity). The entire Eastern Interconnection grid was considered in the capacity expansion, resource adequacy, production cost and reliability modelling. The AOSWTS aimed to:
- Identify and evaluate pathways to enable OSW energy deployment in the Atlantic Ocean through coordinated offshore transmission solutions in the near term (by 2030) and long term (by 2050);
- Fill gaps in prior analysis by providing a multiregional planning perspective for evaluating OSW generation development with transmission planning;
- Incorporate environmental, ocean co-use and other siting considerations in defining potential offshore transmission routes;
- Compare different multiregional offshore transmission topologies and their associated costs (using potential cable routes) and benefits (in terms of production cost savings and enhanced resource adequacy); and
- Analyse reliability impacts from a multiregional perspective.
Key findings
OSW energy is projected to be a key part of achieving a low-carbon future for Atlantic states
According to the NREL’s Regional Energy Deployment System capacity expansion model, electricity demand will be significantly higher in the low-carbon scenario because of the electrification of end-use cases such as space heating for decarbonisation. This scenario is used to compare the cost, benefits and reliability impacts of the various transmission topologies with significant long-term OSW energy development. All 2050 topology analyses of NREL leverage the low-carbon scenario and assume approximately 27 GW of OSW injection into the service area of the Independent System Operator (ISO) New England (ISO-NE) from Maine to Connecticut, 19 GW into the New York ISO (NYISO) area, 26 GW into the PJM Interconnection area from New Jersey to Virginia and North Carolina (PJM-Atlantic), and 13 GW into the SERC Reliability Corporation (SERC) area, which serves North and South Carolina (SERC-Carolinas). Interlink cables that connect platforms between these regions are considered interregional.
Offshore transmission can be planned with consideration for ocean co-use and environmental constraints
The AOSWTS has identified hypothetical cable routes that can maximise the quality of the wind resource and minimise the length of the potential export cable route to suitable points of interconnection (POIs), based on 26 data layers including shipping, military, conservation, sand borrowing and placement (for erosion management), and other considerations. These routes and POIs are not intended to be prescriptions or suggestions for precise locations, but provide a useful suite of POIs for further analysis.
OSW development provides a unique opportunity to add transmission capacity offshore to provide value to the grid
The AOSWTS has found that offshore transmission infrastructure can be leveraged by interlinking platforms to reduce overall system costs. In model estimates using radial topology for 2050, price differences between suitable POIs for OSW averaged at over $100 per MWh. This price difference is higher than the average wholesale electricity prices in recent years in some Atlantic market regions. The high price differences indicate that offshore transmission with interlinking platforms can consistently flow power from lower- to higher-price regions to benefit electricity consumers by reducing the cost of generating electricity. While evaluating the five transmission topologies, the study summarised that:
- Radial topology (and its associated export cables) is the basis for all other topologies and is the status quo today for OSW generation development in the US Atlantic;
- Intra-regional topology focuses on connections within regions that could complement (and come before) interregional solutions;
- Interregional topology is specifically designed to leverage opportunities to connect diverse regions by interlinking offshore platforms;
- Inter-intra topology combines the interlinks in interregional and intra-regional topologies; and
- Backbone topology starts with the interregional build and includes an additional cable spanning the studied portion of the Atlantic Seaboard, from Maine through South Carolina.
The benefits of networking offshore transmission include reduced curtailment, reduced usage of higher-cost generators and contributions to reliability
After studying the total economic value in 2050 for each interlinked transmission topology compared to the radial topology, and the breakdown of value by category, the study concluded that the majority of the benefits are associated with production cost savings, which include fuel, operations and maintenance (O&M), and start-up and shutdown costs.
Grid benefits from interlinked offshore transmission in 2050 under AOSWTS
In addition to economic benefits, the AOSWTS modelling showed that flows on all interlinks go in both directions every season, reducing overall OSW generation costs and curtailment. The average utilisation rate on each line was assumed to be 50 to 60 per cent of the available capacity. Under this, the study highlighted that OSW wind curtailment is about one or two percentage points lower when OSW generation is interconnected with interregional interlinks, compared to the radial topology.
Offshore transmission networks contribute to grid reliability by enabling resource adequacy and helping manage the unexpected loss of grid components (contingencies)
Improved connections between geographically diverse generation resources using offshore transmission can displace generation investment. Resource adequacy value in 2050 will accrue during winter-peaking conditions in colder, electrified Atlantic regions such as PJM, NYISO and ISO-NE when additional transmission capacity can be used to flow power from adjacent regions.
Further, a high-level assessment of the potential impacts of OSW energy and related offshore transmission infrastructure on system reliability was conducted. This effort does not represent a comprehensive system reliability analysis for 30 GW of OSW in 2030 and 85 GW in 2050, but indicates some of the challenges and opportunities for OSW transmission. For instance, the study primarily focused on voltage levels of 230 kV and above; only a limited number of operating conditions were assessed; a limited number of dynamic incidents were investigated; and a reduced set of system planning and performance criteria were monitored (such as voltage limits, branch flow limits and instability detection).
The benefits of offshore transmission networking outweigh the costs, often by a ratio of 2 to 1 or more. Offshore networks with interregional interlinks provide the highest value
The study considered the capital costs of offshore transmission infrastructure (including platform costs, circuit breakers, export cables and interlink cables) in each topology, and then compared the annual costs, benefits, net value and benefit-to-cost ratios for each interlinked topology to the radial topology in 2050. In all the networked topologies studied, the benefits outweighed the costs of adding transmission interlinks when compared to radial topology. Offshore networks with interregional interlinks provide the most value, as quantified in benefit-to-cost ratios and total net value. The benefits of offshore networking persist when mixing interregional and intra-regional interlinks, along with some radial connections.
Building offshore transmission in phases can help reduce development risk, but early implementation of HVDC technology standards is essential for future interoperability
The study team considered a possible phased approach for offshore transmission development. This order is based on interlinking projects as they are developed and available to interlink, with more favourable projects being developed earlier, considering wind resources, cable distance and state targets. This phased approach to offshore transmission development can use infrastructure development capabilities efficiently but requires a consistent (HVDC) technology standard to enable multi terminal, multi-vendor interoperability. Defining a common interoperability standard before HVDC is deployed in topologies, such as the interregional scenario, will be critical to meeting the development timelines and achieving the benefits quantified in this study.
OSW action plan
The study findings also helped the DOE finalise the Atlantic Offshore Wind Transmission Action Plan, a draft of which was first published in October 2023. The action plan is also funded by the DOE’s WETO and outlines immediate steps and extended efforts to connect the first generation of Atlantic OSW projects to the grid, as well as longer-term efforts to increase transmission over the next several decades. Over the mid-to long term, increased intra-regional coordination, shared transmission lines and an offshore network of HVDC interlinks can more efficiently bring critical, renewable OSW energy onshore. The plan’s recommendations include:
- Before 2025: Establish collaborative bodies that span the Atlantic Coast region; clarify some of the building blocks of transmission planning, including updating reliability standards and identifying where offshore transmission may interconnect with the onshore grid; and address costs through voluntary cost assignments.
- From 2025 to 2030: Simultaneously convene and coordinate with states to plan for an offshore transmission network; with industry to standardise requirements for HVDC technology; and with federal agencies, tribal nations, state agencies and stakeholders to identify and prioritise transmission paths on the outer continental shelf.
- From 2030 to 2040: Establish a national HVDC testing and certification centre to ensure compatibility when interconnecting multiple HVDC substations to form an offshore grid network and Codify updates on transmission planning through regulated interregional joint planning, transfer capacity minimums and market monitoring.
- Sustaining actions: Improve environmental review and permitting frameworks, support strong state leadership, empower permitting agencies, develop thoughtful cost allocation practices and consider the utilisation of national corridors.
Conclusion
OSW is poised to play a critical role in the US’s transition towards a clean energy future while improving the power system’s reliability and resilience, as well as providing economic opportunities. Complementing this, a significant expansion of transmission infrastructure will be needed, as no OSW transmission grid exists at present in the country. Connecting networked transmission (either offshore or onshore) across grid planning regions may be beneficial, but this introduces new challenges for planning, ownership and cost allocation. Stakeholders have expressed uncertainty regarding the tax treatment of transmission projects developed separately from generation projects by utilities, states or independent transmission developers, which may create financial disparity between different transmission approaches. There is also a need to protect the marine environment and coastal communities, and address ocean co-use conflicts through avoidance, minimisation and mitigation strategies.
These challenges demonstrate that proactive and coordinated interregional transmission planning is urgently needed to support OSW development. Coordinated planning has the potential to minimise environmental impacts associated with cable route development and onshore upgrades, improve timelines associated with permissions and construction, and lower costs by providing increased capacity and stability to the grid. To this end, the recently published OSW study and plan will guide stakeholders in collectively achieving the OSW targets.