Thermal energy harvesting from ambient heat into electricity is of interest due to its wide potential applicability. This paper demonstrates a moving beam thermal energy harvesting mechanism exploiting both pyro- and piezo-electric effects simultaneously, for use adjacent to a heat source with small temperature variations at low frequency (below 0.1 Hz). For the first time, the relative contributions of these two mechanisms in such a device is established both theoretically and experimentally, and a dynamic model is provided. The relative phase of the contributed currents is shown to be a critical factor, and methods are introduced to optimise this phase relationship, particularly by selection of the mechanical configuration. The reported prototype achieves around 0.4 for a temperature difference of around 15 K at frequency 0.02 Hz with an optimal condition in the fixed-fixed configuration about 90% above the fixed-free end configuration.