NOVELTY - The apparatus comprises: a device such as thermal power cell provided with a first electrically conducting electrode (11), a second electrically conducting electrode (12), where the and second electrodes are spaced apart, a pyro-electric material (13) to which the electrodes are attached/applied, and a heat-exchanging structure (14) thermally coupled to the pyro-electric material; and an electric oscillator circuitry (20). The device is electrically connectable to the electric oscillator circuitry to provide an oscillation of the pyro-electric material.    USE - The apparatus is useful for converting heat such as off-heat, solar heat, geothermal heat, ground heat, water heat and ambient heat, into electricity.    ADVANTAGE - The apparatus is capable of efficiently converting the heat into electricity in a robust manner.    DETAILED DESCRIPTION - The apparatus comprises: a device such as thermal power cell provided with a first electrically conducting electrode (11), a second electrically conducting electrode (12), where the first and second electrodes are spaced apart, a pyro-electric material (13) to which the electrodes are attached/applied, and a heat-exchanging structure (14) thermally coupled to the pyro-electric material; and an electric oscillator circuitry (20). The device is electrically connectable to the electric oscillator circuitry to provide an oscillation of the pyro-electric material. The device further comprises: a first inductor coupled to the device to form a first oscillator; a transistor arranged between the first oscillator and a second oscillator; a supercapacitor; and a rectifier. The electric oscillator circuitry comprises the second oscillator that comprises a capacitor and a second inductor. The first oscillator and the second oscillator are coupled by conductive connections so that the two oscillators are caused to jointly oscillate. The first oscillator and the second oscillator are arranged in series. The first oscillator has a first resonance frequency, and the second oscillator has a second resonance frequency. The first resonance frequency and the second resonance frequency are different and both in a range of 5-500 kHz. A difference of the resonance frequencies is 0.1-5%. The first oscillator and the second oscillator oscillate with a beat frequency of 10-100 times per second. The transistor is switched so that current flows from the first oscillator into the second oscillator. The rectifier is arranged to provide a direct current (DC) output signal. The DC output signal is applied to the supercapacitor. The supercapacitor is coupled to the device so that the supercapacitor is enabled to provide energy for the oscillation. The electric energy is made available at electric nodes when exposing the device to a heat source or flow. The electric oscillator circuitry and the device are electrically coupled to cause the pyro-electric material to oscillate. The pyro-electric material is caused to oscillate at a frequency of above 50 kHz. The electric oscillator circuitry drives the device in a non-linear mode by providing an ascending slope of an amplitude of the oscillation that brings the device in the non-linear mode. A solid state crystal serves as the pyro-electric material.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross sectional view of a device coupled to an oscillator.    First electrically conducting electrode (11)    Second electrically conducting electrode (12)    Pyro-electric material (13)    Heat-exchanging structure (14)    Electric oscillator circuitry. (20)