  We consider, from a mathematical perspective, the power generated by a
contact-mode triboelectric nanogenerator, an energy harvesting device that has
been well studied recently. We encapsulate the behaviour of the device in a
differential equation, which although linear and of first order, has periodic
coefficients, leading to some interesting mathematical problems. In studying
these, we derive approximate forms for the mean power generated and the current
waveforms, and describe a procedure for computing the Fourier coefficients for
the current, enabling us to show how the power is distributed over the
harmonics. Comparisons with accurate numerics validate our analysis.
