# Hydrogen Production from Offshore Wind Parks: Current Situation and Future Perspectives
## 3 System Configurations
### Offshore Electrolyzer Scenario  
One of the significant costs in an offshore wind farm is the equipment to bring the electricity to shore, namely the cables, transformers, and power electronics. Considering a High Voltage Alternating Current (HVAC) transmission system, losses are around 1% to 5% for wind farms with nominal power from 500 to 1000 MW and located 50-100 km from shore [21; 22]. For a HVDC system, losses range from 2% to 4%, depending on nominal power and distance [21; 22]. However, hydrogen travelling through a pipeline has considerably lower losses, under 0.1% [2; 3], along with reduced initial costs for an underwater pipeline compared to underwater electrical cables and the power electronics needed. Figure 2 contains an overview of the centralized offshore electrolyzer system.  
Figure 2: Offshore electrolyzer system.  
PEMELs represent the best choice for this system due to the smaller footprint and easier maintenance [11], which in an offshore scenario means the platform can be smaller and the maintenance trips can be further apart.  
Since the output pressure of a PEMEL is around 30 bar [11; 17], additional compression is required to export the hydrogen to shore. The hydrogen compressor and export pipeline must be sized according to the distance to shore, operating pressure of the electrolyzer, flow of hydrogen, and pressure drop along the pipeline. A study done by North Sea Energy [23] estimated the required pipeline diameter and pressure, assuming an output pressure of 68 bar and 20 m/s maximum travel speed. The results show that for a 1 to 2 GW wind farm located 50 to 200 km from shore, the minimum diameter of the pipeline ranges from 0.25 to 0.41 m, while the minimum input pressure ranges from 83 to 100 bar.  
To size the PEMEL, the nominal power does not need to be equal to the wind farm's nominal power, since the wind farm might not spend large periods of time at nominal power. From an economic point of view, the most interesting approach might be to slightly undersize the electrolyzer, since the revenue lost when the wind farm is at nominal power could be lower than the additional cost of a more powerful electrolyzer [6]. Furthermore, the energy used in purifying the water and compressing the hydrogen for transmission, along with the wake and array losses, lowers the actual available power for the electrolyzer [24].  
A backup power source must be provided for the electrolyzer during periods of shutdown, when the electrolyzer must consume a small amount of power to remain in stand-by mode [11]. Periods of shutdown are not common and do not last long, due to the PEMEL capability of being able to start operating at 1% nominal power, although with low efficiency.  
Two electrolyzer configurations are possible: a unique centralized electrolyzer fed by the whole wind park or individual electrolyzers, one per wind turbine. The details of each configuration are given below. The main components for the centralized electrolyzer system are the same as for the individual electrolyzer system, since the operating principle is similar. The components are  
* PEMEL and the supporting electronics
* AC-DC rectifiers (possibly already included in electrolyzer)
* Desalination unit and reservoir for desalinated water
* Seawater pumps
* Export pipeline
* Backup power source
* Communication equipment