Low-grade heat engine system

A low-grade heat engine system powered by a source fluid containing low-grade heat, the system comprising:

• a primary loop containing a primary working fluid, the primary working fluid having a boiling point below the source fluid;
• a first heat exchanger configured to transfer heat from the source fluid into the primary working fluid and create a gas phase of the primary working fluid;
• a thermosiphon portion of the primary loop arranged to direct the gas phase of the primary working fluid in a direction opposite to gravitational force, a height of the thermosiphon portion being set to the Carnot efficient defined by mgh/(mgh+Enthalpy of vaporization of the primary working fluid), where m is the unit mass of the fluid g is the gravity constant and h is height of the thermosiphon;
• a second heat exchanger at an upper portion of the primary loop to extract heat from the gas phase of the primary working fluid and convert the primary working fluid into the liquid phase and
• a primary hydro turbine driven by a liquid phase of the primary working fluid as the liquid phase is driven with the gravitational force, wherein the only energy source to the system is the source fluid containing the low-grade heat.