Image: SeaTwirl

New analysis for offshore floating wind developer SeaTwirl indicates that vertical axis turbines can be sited closer together to increase the power density.

The study by University of Manchester researcher Pablo Ouro finds that the wake effects of the vertical axis turbines are much reduced compared with the conventional horizontal axis turbines, which would enable them to be sited closer together and provide more power generation from the same area.

The study considers different spacing configurations adjusted both streamwise and spanwise of 25 10MW turbines placed in five rows of turbines each. In all wind speed and direction scenarios the vertical axis turbines were found to outperform the horizontal axis turbines due to the faster wake recovery.

Moreover, the study found that the aspect ratio of the vertical axis turbine rotor is essential to increase the performance, with a taller and more slender (lower diameter/height, D/H) aspect giving the greater output.

For example, with a turbine density of 1.39/km2 over a total area of 18.05km2 the annual energy generation of the vertical axis turbines was estimated between 1,151-1,242GWh (for D/H respectively 1.25 and 0.5) compared with 1,028GWh for horizontal axis turbines.

At a lower density of 0.36 turbines/km2 over an area of 69.3km2, the vertical axis turbine generations were estimated between 1,268-1,310GWh and the horizontal axis turbines at 1,103GWh.

Similar differences are reflected in the calculated efficiencies, i.e. the mean yield compared with the rated power, for the different combinations.

In the higher density configuration, the efficiencies were estimated at 53.3-57.5% for the vertical axis turbines compared with 47.6% for the horizontal axis turbines. In the lower density example, the respective figures were 58.7-60.6% versus 51.1%.

Notably, in one of the configurations, the power output for the vertical axis turbines reached almost 19MW/km2, while the highest for the horizontal axis turbines was 6.5MW/km2.

Jonas Boström, CTO of SeaTwirl, explains the findings in terms of the turbulence intensity, comparing a propeller in a stream of water with stirring the water with a spoon, with the turbulence created with the latter subsiding much faster.

For SeaTwirl, which has its technology under development with a demonstration due in Norway, the findings add to its value proposition.

“There are discussions in many countries to increase the power density demands for coming offshore wind farms to avoid building in eco conservation areas. This white paper shows that SeaTwirl can be placed in very dense wind farms without losing much efficiency due to wake effects,” says CEO Peter Laurits.

Development of the demonstration is advancing with the first test sections of the moulds for the blades having come off the line in early September and production of the blades due to start before yearend.

SeaTwirl also has entered into a cooperation with the University of Tokyo to adapt the technology to withstand the typhoons that are common there, typically during the summer months.