The present disclosure provides an energy conversion system and method for generating electricity directly from heat by phase transformation of ferroelectric materials without any external power sources. The energy conversion system includes an electric circuit comprising a phase-changing capacitor and a charge reservoir. The phase-changing capacitor has a dielectric layer comprising a phase-transforming ferroelectric material. When the phase-changing capacitor is initialized and subjected to thermal cycles through a transformation temperature of the phase-transforming ferroelectric material, the polarization of the dielectric layer undergoes an abrupt change between a ferroelectric phase and a paraelectric phase such that a current flow between the phase-changing capacitor and the charge reservoir via a load thereby converting heat into electrical energy. The present energy conversion method does not require any external bias fields during the energy conversion.