A fluid-working machine for a renewable energy generation device, the fluid-working machine comprising a ring cam and a plurality of working chambers, the ring cam having an annular working surface extending around an axis of rotation of the ring cam, the annular working surface defining a plurality of waves, each working chamber having a piston, each piston in operative engagement with the ring cam working surface, the ring cam and working chambers being mounted to rotate relative to each other, cycles of working chamber volume being thereby coupled to rotation of the ring cam relative to the working chambers, characterized in that the individual waves of the ring cam working surface have an asymmetric profile.