Sustainable Energy and Smart Grids: Breakthrough in Thinking, Modelling, and Technology

Wind energy is the production of electrical energy by using the kinetic energy of moving air (wind). Nowadays, wind energy is mainly generated with three-bladed horizontal axis wind turbines. Wind energy is the third-largest source of sustainable electrical energy (after hydro energy and biomass). From 1980 to 2010, wind energy grew at roughly 30 per cent per year. Since 2010, growth has been lower because of the economic crisis, but its potential is still very large. In densely populated countries in Western Europe, there is a trend to place wind farms offshore. The power produced by a wind turbine can be expressed as

where $ρ$ is the mass density of air, $Cρ$ is the aerodynamic efficiency, $r_r$ is the rotor radius, and $v_w$ is the wind speed. From this expression a few important conclusions can be drawn. The fact that the power is proportional to the cube of the wind speed has a number of important implications.

If there is no wind or the wind speed is low, the turbine produces no or little power. On average, a wind turbine operates at a capacity factor (average produced power divided by maximum power) in the order of 30 per cent offshore and 20 per cent onshore. And while it is mainly attractive to develop wind farms at locations with high wind speed, the very low spatial efficiency of wind farms requires huge areas covered by widely spaced wind turbines to avoid negative wake turbulence effects.

Thus to obtain significant electricity production hundreds of square kilometers are needed (example: London Array Installation on 100 km² for 600 MW, i.e. a very low ratio of 6 MW/Km²). This drawback is severely threatening the expansion of large ground wind farms in highly populated developed countries, especially in northern Europe (such as Germany and the Netherlands).

A less attractive property of wind energy is that there is hardly any energy storage. Some energy is stored in the rotating mass of the blades. That is enough to filter power pulsations with frequencies above around 1 Hz, but lower frequency power variations have to be compensated for in another way.