System for producing electrical or mechanical energy comprising a fluid circuit, wherein a working fluid circulates, comprising a plurality of members passed through by the working fluid and among which: - at least one heat exchanger configured to be thermally coupled to at least one first heat source, - an expander, an inlet of which is fluidically connected to an outlet of the first exchanger, - a second heat exchanger configured to be thermally coupled to a second heat source, colder than the first heat source, - at least one pump configured to move the working fluid in the fluid circuit, - at least one reservoir receiving the working fluid and arranged between the second exchanger and the pump on a tapping duct, the fluid circuit being configured such that the working fluid passes successively through at least the pump, the first exchanger, the expander and the second exchanger, then the pump again, where the system comprises a device for regulating the fluid load circulating in the fluid circuit configured to operate in a closed circuit with the fluid circuit and for maintaining saturation conditions in the reservoir, such that the reservoir contains the working fluid simultaneously to the liquid state and to the gaseous state, the regulation device comprising thermal energy input elements (5, 6, 7) in the reservoir intended to increase the pressure of the reservoir (Pr) and to induce the injection of the working fluid of the reservoir in the fluid circuit and energy removal elements (8, 9, 10) of the reservoir intended to decrease the pressure of the reservoir (Pr) and to induce the suctioning towards the reservoir of the working fluid outside of the fluid circuit, and where the system comprises a mechanically operating self-regulation module intended to regulate the pressurising and the depressurising of the reservoir and comprising a casing receiving a regulator associated with a regulator spring and a valve associated with a valve spring. - at least one heat exchanger configured to be thermally coupled to at least one first heat source, - an expander, an inlet of which is fluidically connected to an outlet of the first exchanger, - a second heat exchanger configured to be thermally coupled to a second heat source, colder than the first heat source, - at least one pump configured to move the working fluid in the fluid circuit, - at least one reservoir receiving the working fluid and arranged between the second exchanger and the pump on a tapping duct, the fluid circuit being configured such that the working fluid passes successively through at least the pump, the first exchanger, the expander and the second exchanger, then the pump again, where the system comprises a device for regulating the fluid load circulating in the fluid circuit configured to operate in a closed circuit with the fluid circuit and for maintaining saturation conditions in the reservoir, such that the reservoir contains the working fluid simultaneously to the liquid state and to the gaseous state, the regulation device comprising thermal energy input elements (5, 6, 7) in the reservoir intended to increase the pressure of the reservoir (Pr) and to induce the injection of the working fluid of the reservoir in the fluid circuit and energy removal elements (8, 9, 10) of the reservoir intended to decrease the pressure of the reservoir (Pr) and to induce the suctioning towards the reservoir of the working fluid outside of the fluid circuit, and where the system comprises a mechanically operating self-regulation module intended to regulate the pressurising and the depressurising of the reservoir and comprising a casing receiving a regulator associated with a regulator spring and a valve associated with a valve spring.