The variable nature of wave power is the main reason why it is expensive and difficult to collect (see Z. L. Wang Nature 542, 159–160; 2017). Wave-power devices have much more demanding structural requirements than do offshore wind farms, for instance.

Contrary to Zhong Lin Wang's implication that wave energy is plentiful in all weathers, wave power fluctuates widely because it is created by accumulated wind stress. Monthly averages of wave power typically vary by a factor of six, compared with a factor of two for wind power. Short-term variations can be even higher (B. Sørensen Renewable Energy; Academic, 2010).

I also question Wang's contention that “waves involve slow flows and oscillations in random directions”. Wave motion is not random because it involves circular motions in the vertical plane, the direction of which is governed by earlier wind flows.

The high ratio of peak-to-average power makes wave power difficult to commercialize. High costs for Wang's frictional nanogenerators and for pneumatic or mechanical wave-power devices are inevitable.