An energy harvesting system includes a composite structure that includes a base spring and a first piezoelectric element. The first piezoelectric element is mounted on the base spring. The base spring has a base spring thickness, a base spring length, and a base spring width. The base spring is for vibrating around an axis parallel to the base spring width. The base spring thickness varies from position to position along a direction parallel to the base spring length. The base spring thickness varies to provide the first piezoelectric element with constant strain when the base spring vibrates around the axis parallel to the base spring width.