Green hydrogen is a key part of the energy transition. In the coming years, it’ll be made in wind farms in the German Bight. With recent technology, this process creates waste heat and brine, which are both dumped into the sea.

A new study by Helmholtz-Zentrum Hereon shows for the first time that the waste heat of 500-megawatt plant can increase the water temperature locally by up to 2°C, thereby influencing the stratification of the sea.

The authors provide new recommendations for the environmentally friendly expansion of the planned offshore hydrogen production in the North Sea in a study recently published in npj Ocean Sustainability.

Almost 80% of the energy used worldwide currently comes from fossil fuels such as oil, coal, and gas. As part of the energy transition, these are to be increasingly replaced by environmentally friendly energy sources such as climate-neutral hydrogen.

The German Offshore Wind Energy Act (WindSeeG) lays the foundation for producing hydrogen using wind energy in the North Sea in the future. The goal is to install offshore hydrogen plants with a capacity of 10 gigawatts in offshore wind farms in the German Bight. The technologies are currently being tested.

Until now, the focus has been primarily on questions of technical feasibility and economic viability. The impact on the environment has only been considered to a limited extent. Using a computer model developed in-house, the new Hereon study analyzes for the first time the potential footprint of offshore hydrogen production in the North Sea and shows how the planned expansion can be achieved in an environmentally friendly manner.

In offshore hydrogen production, seawater is first desalinated and then split into hydrogen and oxygen through a process called electrolysis. This produces waste heat and brine. According to the current state of technology, both are returned to the sea near the surface.

The authors of the Hereon study based their calculations on a thermal process in which the water is desalinated through evaporation. The results of their modeling show that, compared to brine, waste heat has a significantly greater impact on seawater. It causes the water temperature within a 10-meter radius around a 500-megawatt hydrogen plant to rise by up to 2°C on average over the course of a year.

The researchers expanded the scenario and calculated the impact for several hydrogen plants located close to each other with a total capacity of 10 megawatts. Even within a radius of 1,000 meters, there was still an average annual temperature increase of 0.1°C to 0.2°C. At a distance of 50 kilometers, it was still 0.01°C.

“The decisive temperature changes occur mainly locally and, depending on the scale of production, have an impact on the stratification of the water body,” says lead author Dr. Nils Christiansen from the Hereon Institute of Coastal Systems—Analysis and Modeling.

Stratification is the vertical division of the ocean into different water layers with varying density, temperature, and salinity. Colder, denser water with a higher salinity and many nutrients is found at the bottom. Above this is warmer, lighter water with a lower salinity. The warmer layer acts as a barrier and also influences the transport of nutrients from the bottom to the top.

The findings of the study show that this stratification intensifies when the water temperature at the surface rises due to the input of waste heat. This can alter nutrient transport and thus also the productivity of phytoplankton. Phytoplankton is found near the surface and forms the basis for the entire food chain in the sea. In order to reproduce and carry out photosynthesis, it needs nutrients from the deeper layers.

Solutions for environmentally friendly hydrogen production

To minimize the impact of hydrogen production on stratification, the authors of the study recommend distributing the input of by-products spatially, for example through decentralized solutions. This involves several small electrolyzers producing hydrogen at different locations instead of one large electrolyzer on a single platform. It also makes sense to distribute the input across the water column, from near the surface to the seabed, or to reduce waste heat through technological solutions.

“Our findings help to better understand the impact of green hydrogen production on the oceans and to develop solutions for a sustainable and nature-friendly energy transition at sea at an early stage,” says Christiansen. “Further studies are now needed to investigate other technologies, such as chemical processes, and the exact impact on ecosystems.”