Heronimonimo
commented
5 years ago Good points raised here.
One additional situation would be if something like the proposed North Sea Power Hub is build. In that case you would expect DC connections from the island to the mainlands and AC connections from the wind turbines to the AC-DC converter on the island.
Wouldn't the better solution to be to decide based on the length to the nearest grid node (before the simplification of the grid step) being 50km or less? In that way adding a grid node in the middle of the North Sea would automatically work.
By deciding based on grid nodes we can also guard for places that are relatively near shore but without a short distance connection to the existing grid which therefore would still be DC connected.
coroa
fneum
euronion
At the moment all sea area within 80 km of each country that is not in a Natura 2000 nature reserve (see https://nworbmot.org/natura_2000_reserves.png for the excluded areas) is allotted to AC-connected offshore wind, and then 30% of that area is actually used (to take account of other uses like shipping lanes, etc, leading to a density of 3 MW/km^2 in the allowed areas). Beyond 80 km, offshore is connected with DC, which has higher costs because of the AC-DC converter stations. These settings can be found in config.yaml. The 80 km limit was based on Figure 34 of https://ec.europa.eu/energy/sites/ener/files/documents/2014_nsog_report.pdf which plots technology choices for different parks.
This leads to tens of GW expansion potential for AC-connected offshore for Germany in the North Sea, whereas in reality Germany has very little AC-connected offshore (a few parks in the North Sea and all the parks in the Baltic Sea) but a large amount of DC-connected offshore (only in the North Sea so far), see this map of current and planned offshore for Germany and the various exclusion areas: https://upload.wikimedia.org/wikipedia/commons/7/79/Karte_Offshore-Windkraftanlagen_in_der_Deutschen_Bucht.png
There are two sources of problems: i) There are some exclusion areas we're missing. ii) Based on the map, a 50 km subsea limit for AC would better represent the existing and planned AC-DC split in the North Sea. This also fits with UK parks, which are all AC-connected, with the farthest existing park at 34 km from shore (Greater Gabbard 1). The discrepancy with the 80 km source above could be that the 80 km also included onshore. iii) The 50 km limit should be measured from the mainland, NOT from the Fresian islands or Heligoland. iv) 30% area usage may even be too strict: @fneum reports that with a 30 km limit, the potentials for Germany are 23.3 GW for AC-connected offshore wind (too high, at least for North Sea, I don't know about Baltic), and 67.0 GW for DC-connected offshore wind (seems too low to me, but I need a better reference).
Solutions: i) To find the additional exclusion areas, @fneum suggested the following references: http://www.windspeed.eu/media/publications/WINDSPEED_D2_1_revised_May_2011.pdf page 18 http://www.windspeed.eu/media/publications/WINDSPEED_Roadmap_110719_final.pdf page 26 Fig. 13 https://www.msp-platform.eu/practices/maritime-spatial-plan-german-eez-north-sea German Maritime Spatial Plan (Raumordnungsplan) http://www.esa.int/spaceinimages/Images/2009/05/NO2_map_placed_on_top_of_the_shipping_route_map ii) This is easy to change in config.yaml. iii) As far as I can tell, the country shapes in scripts/build_shapes.py include everything with an area bigger than 0.1, which may include islands. Islands should be excluded from the 50 km limit (which is applied in scripts/build_renewable_profiles.py). iv) Perhaps increase density from 3 MW/km^2 - need another reference for this.