Supercritical working fluid circuit with a turbo pump and a start pump in series configuration

A method for starting a turbo pump in a heat engine system, comprising:

• circulating a working fluid comprising carbon dioxide within a working fluid circuit by a start pump, wherein the working fluid circuit contains a first mass flow of the working fluid and a second mass flow of the working fluid and at least a portion of the working fluid circuit contains the working fluid in a supercritical state;
• transferring thermal energy from a heat source stream to the working fluid by a first heat exchanger fluidly coupled to and in thermal communication with the working fluid circuit;
• flowing the working fluid into a drive turbine of a turbo pump and expanding the working fluid while converting the thermal energy from the working fluid to mechanical energy of the drive turbine;
• driving a pump portion of the turbo pump by the mechanical energy of the drive turbine, wherein the pump portion is coupled to the drive turbine and the working fluid is circulated within the working fluid circuit by the turbo pump;
• diverting the working fluid discharged from the pump portion of the turbo pump into a first recirculation line fluidly communicating the pump portion of the turbo pump with a low pressure side of the working fluid circuit, the first recirculation line having a first bypass valve arranged therein;
• closing the first bypass valve as the turbo pump reaches a self-sustaining speed of operation;
• deactivating the start pump and opening a second bypass valve arranged in a second recirculation line fluidly communicating the start pump with the working fluid circuit;
• and
• diverting the working fluid discharged from the start pump into the second recirculation line.