  Vertical-axis wind turbines (VAWT) are excellent candidates to complement
traditional wind turbines and increase the total wind energy capacity.
Development of VAWT has been hampered by their low efficiency and structural
unreliability, which are related to the occurrence of dynamic stall. Dynamic
stall consists of the formation, growth, and shedding of large-scale dynamic
stall vortices, followed by massive flow separation. The vortex shedding is
detrimental to the turbine's efficiency and causes significant load
fluctuations that jeopardise the turbine's structural integrity. We present a
comprehensive experimental characterisation of dynamic stall on a VAWT blade
including time-resolved load and velocity field measurements. Particular
attention is dedicated to the dilemma faced by VAWT to either operate at lower
tip-speed ratios to maximise their peak aerodynamic performance but experience
dynamic stall, or to avoid dynamic stall at the cost of reducing their peak
performance. Based on the results, we map turbine operating conditions to one
of three regimes: deep stall, light stall, and no stall. The light stall regime
offers VAWT the best compromise in the dynamic stall dilemma as it yields
positive tangential forces during the upwind and downwind rotation and reduces
load transients by 75% compared to the deep stall regime.
