What are the environmental impacts of offshore wind substations?

Offshore wind turbines positioned more than 30 miles from the shore cannot efficiently transmit the generated power to the onshore grid without significant power loss. Offshore substations (1) (OSS) are platformed infrastructures built on the sea that convert electrical power allowing for efficient long distance power transmission. Offshore turbines produce Alternative Current (AC) which is then converted to Direct Current (DC) energy through an Offshore substation, before traveling to the onshore grid. 

The processes surrounding Offshore substations holds some environmental impact (2) concerns. The AC-to-DC conversion generates significant amounts of heat. As a cooling solution, sea water is drawn upwards and pumped through the engine as a means to absorb the heat. Energy cables also generate small amounts of heat. As a method of mitigation, the energy cables are installed deep underground where the heat generation ranges from 0.15-2.5 degrees Celsius. The water is then discharged back into the sea. During this process some smaller organisms are at risk of impingement (3), where species are sucked into the inflow, become trapped, and eventually die. Reducing seawater intake velocity has shown to be a successful method in preventing species from entering inflows. 

Determining siting for offshore substations is critical for preventing ecological harms. Avoiding sensitive areas such as coral habitats, breeding grounds, migration routes, and areas of high biodiversity are prioritized. The dischargement of seawater during the cooling process may contain heat and particulates which add to the substation location importance. Companies such as Sunrise Wind utilize hydrothermal modeling (4) for determining the best optimal depth for discharge (5) of the heated effluent, which contributes to siting decisions. 

Offshore Substation Mitigation Infographic. Source: National Wildlife Federation (2)

Offshore wind transmission elements. Source: National Wildlife Federation (3)

References​

1. ​Rougeot, J. B., & McLean, S. (2024, July 31). Offshore substations. Spinergie. https://www.spinergie.com/blog/offshore-substations-and-what-to-expect-in-the-future

2. ​National Wildlife Federation. (2023). Offshore Wind Energy Cooling Systems. https://offshorewind.nwf.org/wp-content/uploads/2025/05/Substation-Cooling-Factsheet.pdf

3. Steingard, S., Ung-Kono, V., & Thomas, S. (2024). Offshore transmission development and the Benthos. National Wildlife Federation. https://www.nwf.org/-/media/Documents/PDFs/NWF-Reports/2024/Wires-and-Wildlife-Offshore-Transmission-Development-and-the-Benthos.pdf

4. Bureau of Ocean Energy Management. (2023). Sunrise Wind Environmental Impact Statement - Appendix Q. BOEM. https://www.energy.gov/sites/default/files/2024-09/final-eis-0566-sunrise-wind-vol-2-app-q-2024-09.pdf 

5. U.S. Department of the Interior Bureau of Ocean Energy Management Office of Renewable Energy Programs. (2022). Supporting National Environmental Policy Act Documentation for Offshore Wind Energy Development Related to High Voltage Direct Current Cooling Systems. tethys. https://tethys.pnnl.gov/sites/default/files/publications/BOEM_2022b.pdf