This paper presents a calculation of the offshore wind resource, and shows that the theoretical resource from offshore wind turbines in UK waters is approximately 2.2 TW of average (ie continuous output) of electricity. The figures are calculated across four ranges of sea depth: waters to 25 metres; 25-50 m, 50-100 m and deep waters to 700 m. The chart below shows the potential generation, with figures split out by sea depth and by distance from shore.
The available power is calculated by comparing power from an established wind farm, with the power available across the study area.
First, the calculations for an existing offshore wind farm in shallow waters; I’ll use that as a basis, to scale up the calculations for the resource in shallow, medium, deep and deepest waters.
The wind power density (the mean density of the wind’s kinetic energy, over the year) at Kentish Flats is about 713 W per square metre of vertical plane, at 80 metres above sea level. The Vestas 3 MW turbines there have a net efficiency of approximately 30%, for that wind profile: i.e. 30% of the wind’s kinetic energy is converted to electricity; (so in this case the figure is efficiency, rather than capacity factor). The turbines have a swept diameter of 90 metres, and thus a swept area of 6360 m2 (45 m x 45 m x pi). So the power generated is:
713 W/m2 x 6360 m2 x 30% = 1.36 MWe per turbine.
The turbines are spaced in a 700 m x 700 m grid, so the turbine density is
1/(0.7×0.7) = 2.06 turbines per square km of seabed,
giving a density of harvested electricity of 2.8 MWe/km2, at 100% availability. A 90% availability gives around 2.5 MWe/km2.
And now to generalise to British waters, grouping the depths into shallow, medium and deep waters: 0-25 m, 25-50 m, and 50-700 m.
80 GW in shallow waters (0-25 m)
In the (territorial + EEZ) waters around Britain & Northern Ireland, there’s roughly 40 000 km2 of seabed shallower than 25 m, with a wind power density of 579 W/m2 (that’s per square metre in the vertical plane: the area that the turbine blades sweep through). So, for the depths of 0-25 m, scaling the Kentish Flats figure accordingly, the mean potential electricity density is:
2.5 MWe/km2 x 579/713 = 2 MWe/km2
40 000 km2 x 2 MWe/km2 = 80 GWe at 0-25m depths
270 GW in seas of depth 25-50m
For the 90 000 km2 of British waters in depths of 25-50 m, the wind power density is 868 W/m2(vertical). giving mean potential electricity of:
2.5 MWe/km2 x 868/713 = 3 MWe/km2
90 000 km2 x 3 MWe/km2 = 270 GWe at 25-50m depths
790GW in deeper waters 50-100m
And then there’s about 210 000 km2 at 50m-100m, with a wind power density of 1070 W/m2(vertical)
2.5 MWe/km2 x 1070/713 = 3.75 MWe/km2
210 000 km2 x 3.75 MWe/km2 = 790 GWe at 50-100m depths
1030GW from the deepest accessible waters 100m-700m
And then there’s about 220 000 km2 at 100m-700m, with a wind power density of 1340 W/m2(vertical)
2.5 MWe/km2 x 1340/713 = 4.7 MWe/km2
220 000 km2 x 4.7 MWe/km2 = 1030 GWe at 300-700m depths
Total = 2.2 TWe
So, all together, that gives approximately 2,200 GWe, or 2.2 TWe.
For context, current UK energy demand (electricity + heat + transport) is about 0.25 TW, and current UK electricity demand is about 0.04 TWe.
The power available varies by depth, as seen above. There is also some variation, depending on distance from shore. The figures are as follows:
Caveats and other thoughts
Now, that’s just a first-order estimate of the potential.
How much of that sea is needed for other uses – shipping or military exercises, is a matter of future negotiation. Fishing boats are happy to sail between turbines at their current spacing.
Current turbine spacing is on a grid of about 7 diameters by 7 diameters, as a precaution against damage from wake/turbulence: Kentish Flats is a square grid of 7.78×7.78 diameters.
Turbine manufacturers advise that, to avoid power loss from shadowing, a grid of 5 x 3 is sufficient, which, if realised, would give potential power of more than triple the above figures. (because three grids of 5×3 diameters into less area than one grid of 7×7 diameters)
The mooring used on the Norwegian HyWind floating turbines are expected to be suitable out to depths of 700 metres, which is why I’ve chosen that depth as a cut-off for now, but I acknowledge that developments could, if necessary, open up deeper waters. But I think the figures show that deeper waters really shouldn’t be necessary!
For those of you who prefer units other than the SI standard ones, here are some alternatives. The UK offshore wind resource is:
- 2.2 TW of electricity
- 870 kWh per person per day for the UK’s 60 million residents
- 19 000 TWh per year
- 4.5 Mtoe (millions of tonnes of oil equivalent) per day
- 32 Mboe (millions of barrels of oil equivalent) per day; global oil production is about 80 million barrels per day.
Information on sea depths and wind power densities was taken from http://www.renewables-atlas.info/ which is © Crown copyright, All rights reserved. 2008.