Heat engines with cascade cycles

A heat engine system, comprising:

• one or more waste heat exchangers thermally coupled to a source of waste heat, the one or more waste heat exchangers being configured to heat a first flow of working fluid;
• a power turbine fluidly coupled to the one or more waste heat exchangers, the power turbine being configured to receive the first flow from the one or more waste heat expanders and to expand the first flow;
• a first recuperator fluidly coupled to the power turbine, the first recuperator being configured to receive the first flow from the power turbine and to transfer heat from the first flow to a second flow of working fluid;
• a second turbine fluidly coupled to the first recuperator, the second turbine being configured to receive the second flow from the first recuperator and to expand the second flow;
• a second recuperator fluidly coupled to the second turbine, the second recuperator being configured to receive the second flow of working fluid from the second turbine and to transfer heat from the second flow to a combined flow of the first and second flows of the working fluid;
• a condenser fluidly coupled to the first and second recuperators, the condenser being configured to receive the first and second flows from the first and second recuperators, respectively, as the combined flow and to at least partially condense the combined flow;
• and
• a pump fluidly coupled to the condenser and to the second recuperator, the pump being configured to receive the combined flow from the condenser and pump the combined flow into the second recuperator.

A method for extracting energy from a waste heat, comprising:

• transferring heat from the waste heat to a first flow of working fluid in a heat exchanger;
• expanding the first flow in a first expander to rotate a shaft;
• transferring heat from the first flow to a second flow of working fluid in a first recuperator;
• expanding the second flow in a second expansion device to rotate a shaft;
• transferring heat from the second flow to at least one of the first and second flows in a second recuperator;
• at least partially condensing the first and second flows with one or more condensers;
• and
• pumping the first and second flows with a pump.

A heat engine system, comprising:

• a working fluid circuit configured to flow a working fluid therethrough and comprising:
• a pump configured to circulate the working fluid through the working fluid circuit, wherein the working fluid is split into a first portion and a second portion downstream of the pump;
• a first loop comprising a waste heat exchanger configured to transfer heat from waste heat to the first portion of the working fluid, a first expansion device configured to expand the first portion of the working fluid, and a first recuperator downstream of the first expansion device and configured to transfer heat from the first portion of the working fluid to the second portion of the working fluid;
• and
• a second loop comprising the first recuperator, a second expansion device disposed downstream of the first recuperator and configured to expand the second portion of the working fluid, and a second recuperator configured to transfer heat from the second portion of the working fluid to at least one of the first portion and the second portion of the working fluid downstream of the pump.