Aircraft propulsion systems including a closed loop-supercritical fluid system having a turbine, a cooler heat exchanger, a compressor, and a recovery heat exchanger arranged along a closed-loop flow path of a supercritical fluid. A shaft is operably coupled to the turbine and configured to be rotationally driven by the turbine. A fan is configured to generate thrust, the fan operably coupled to the shaft to be rotationally driven by the shaft. A burner is configured to combust a fuel and air from the fan to generate a combusted gas and supply said combusted gas to the recovery heat exchanger of the closed loop-supercritical fluid system and out an exhaust nozzle.

Systems and methods are provided for charging a pumped thermal energy storage (“PTES”) system. A system may include a compressor or pump configured to circulate a working fluid within a fluid circuit, wherein the working fluid enters the pump at a first pressure and exits at a second pressure; a first heat exchanger through which the working fluid circulates in use; a second heat exchanger through which the working fluid circulates in use; a third heat exchanger through which the working fluid circulates in use, a turbine positioned between the first heat exchanger and the second heat exchanger, configured to expand the working fluid to the first pressure; a high temperature reservoir connected to the first heat exchanger; a low temperature reservoir connected to the second heat exchanger, and a waste heat reservoir connected to the third heat exchanger.

Systems and methods are provided for charging a pumped thermal energy storage (“PTES”) system. A system may include a compressor or pump configured to circulate a working fluid within a fluid circuit, wherein the working fluid enters the pump at a first pressure and exits at a second pressure; a first heat exchanger through which the working fluid circulates in use; a second heat exchanger through which the working fluid circulates in use; a third heat exchanger through which the working fluid circulates in use, a turbine positioned between the first heat exchanger and the second heat exchanger, configured to expand the working fluid to the first pressure; a high temperature reservoir connected to the first heat exchanger; a low temperature reservoir connected to the second heat exchanger, and a waste heat reservoir connected to the third heat exchanger.

Extra heat in a closed cycle power generation system, such as a reversible closed Brayton cycle system, may be dissipated between discharge and charge cycles. An extra cooling heat exchanger may be added on the discharge cycle and disposed between a cold side heat exchanger and a compressor inlet. Additionally or alternatively, a cold thermal storage medium passing through the cold side heat exchanger may be allowed to heat up to a higher temperature during the discharge cycle than is needed on input to the charge cycle and the excess heat then dissipated to the atmosphere.

Aircraft power plants including combustion engines, and associated methods for recuperating waste heat from such aircraft power plants are described. A method includes transferring the heat rejected by the internal combustion engine to supercritical CO.sub.2 (sCO.sub.2) used as a working fluid in a heat engine. The heat engine converts at least some the heat transferred to the sCO.sub.2 to mechanical energy to perform useful work onboard the aircraft.

A system including: (i) a pumped-heat energy storage system (“PHES system”), wherein the PHES system is operable in a charge mode to convert electricity into stored thermal energy in a hot thermal storage (“HTS”) medium; (ii) an electric heater in thermal contact with the hot HTS medium, wherein the electric heater is operable to heat the hot HTS medium above a temperature achievable by transferring heat from a working fluid to a warm HTS medium in a thermodynamic cycle.

A system including: (i) a pumped-heat energy storage system (“PHES system”), wherein the PHES system is operable in a charge mode to convert electricity into stored thermal energy in a hot thermal storage (“HTS”) medium; (ii) an electric heater in thermal contact with the hot HTS medium, wherein the electric heater is operable to heat the hot HTS medium above a temperature achievable by transferring heat from a working fluid to a warm HTS medium in a thermodynamic cycle.

A method including: operating a pumped-heat energy storage (“PHES”) system in a generation mode to generate electricity; and responsive, at least in part, to a determination that a power generation plant will reduce supply of electricity to an electrical grid by a reduction amount of electricity, changing modes of the PHES system from the generation mode to operate in a charge mode. Operating in the charge mode can include receiving a charge amount of electricity, at least equal to the reduction amount of electricity, into the PHES system from the power generation plant and converting at least a portion of the charge amount of electricity to stored thermal energy.

Gas turbine engines are described. The gas turbine engines include a compressor section, a combustor section, a turbine section, a nozzle section, wherein the compressor section, the combustor section, the turbine section, and the nozzle section define a core flow path that expels through the nozzle section, and a waste heat recovery system. The waste heat recovery system includes a heat recovery heat exchanger arranged at the nozzle section, wherein the heat recovery heat exchanger is arranged within the nozzle section such that the heat recovery heat exchanger occupies less than an entire area of an exhaust area of the nozzle section and a heat rejection heat exchanger arranged to reduce a temperature of a working fluid of the waste heat recovery system.

Gas turbine engines are described. The gas turbine engines include a compressor section, a combustor section, a turbine section, a nozzle section, wherein the compressor section, the combustor section, the turbine section, and the nozzle section define a core flow path that expels through the nozzle section, and a waste heat recovery system. The waste heat recovery system includes a heat recovery heat exchanger arranged at the nozzle section, wherein the heat recovery heat exchanger is arranged within the nozzle section such that the heat recovery heat exchanger occupies less than an entire area of an exhaust area of the nozzle section and a heat rejection heat exchanger arranged to reduce a temperature of a working fluid of the waste heat recovery system.