A power system comprising: a working fluid circuit having a high pressure side and a low pressure side and configured to flow a working fluid therethrough; a heater configured to circulate the working fluid in heat exchange relationship with a hot fluid to vaporize the working fluid; serially arranged first expander and second expander fluidly coupled to the working fluid circuit and disposed between the high pressure side and the low pressure side thereof, configured to expand working fluid flowing therethrough and generating mechanical power therewith; a driveshaft (9A; 11A) drivingly coupled to one of said first expander and second expander, and configured to drive a device with mechanical power produced by said drivingly coupled expander; a pump fluidly coupled to the working fluid circuit between the low pressure side and the high pressure side thereof, configured to rise the pressure of the working fluid in the working fluid circuit, and drivingly coupled to the other of said first expander and second expander and being powered thereby; a cooler arranged and configured to remove heat from the working fluid in the low pressure side of the working fluid circuit; characterized in that at least one of said first expander and second expander is provided with a by-pass valve, configured and controlled to cause at least part of the working fluid circulating in the working fluid system to by-pass said expander, wherein the by-pass valve is arranged in a by-pass line parallel to the one of said first expander and second expander, which is drivingly connected to the driveshaft and a regulating valve is arranged in the working fluid circuit, between the outlet of the first expander and the connecting point of the by-pass line between the first expander and the second expander. a working fluid circuit having a high pressure side and a low pressure side and configured to flow a working fluid therethrough; a heater configured to circulate the working fluid in heat exchange relationship with a hot fluid to vaporize the working fluid; serially arranged first expander and second expander fluidly coupled to the working fluid circuit and disposed between the high pressure side and the low pressure side thereof, configured to expand working fluid flowing therethrough and generating mechanical power therewith; a driveshaft (9A; 11A) drivingly coupled to one of said first expander and second expander, and configured to drive a device with mechanical power produced by said drivingly coupled expander; a pump fluidly coupled to the working fluid circuit between the low pressure side and the high pressure side thereof, configured to rise the pressure of the working fluid in the working fluid circuit, and drivingly coupled to the other of said first expander and second expander and being powered thereby; a cooler arranged and configured to remove heat from the working fluid in the low pressure side of the working fluid circuit; characterized in that at least one of said first expander and second expander is provided with a by-pass valve, configured and controlled to cause at least part of the working fluid circulating in the working fluid system to by-pass said expander, wherein the by-pass valve is arranged in a by-pass line parallel to the one of said first expander and second expander, which is drivingly connected to the driveshaft and a regulating valve is arranged in the working fluid circuit, between the outlet of the first expander and the connecting point of the by-pass line between the first expander and the second expander.