A hybrid energy system has been proposed, consisting of an intercooled-recuperated gas-turbine-cycle engine coupled with a pneumatic motor (compressed-air engine), using reciprocating linear motion of a piston or a rotary vane motor, with quasi-isothermal heat addition process. Both gas-turbine (GT) engine and pneumatic motor have their own and separate combustion processes, occurring serially one after another, owing to the fact that in the GT engine exhaust gas there is still enough oxygen needed for combustion of fuel (liquid or gaseous) in an internal combustion engine. The quasi-isothermal nature of the heat addition process within the pneumatic motor is a result of averaging of the two thermodynamic processes, simultaneously interconnected within a cylinder-piston motor: isobaric heat addition and adiabatic gas expansion. The final expansion pressure at the end of the pneumatic-motor quasi-isothermal heat addition/expansion process is considered to correspond to the initial specific volume of inlet ambient air. Three (3) possible configurations of such a hybrid energy system were analyzed, differing only in the sequence of equipment connecting in the direction of air/working-gas flow (GT-cycle combustor, GT, pneumatic motor with combustor and recuperator). The results showed that the most efficient cycle configuration is No. 2, in which quasi-isothermal heat addition / gas expansion in the pneumatic motor occurs right after the GT-cycle heat addition in the associated GT-cycle combustor, then the partly expanded combustion gas first cools down in the GT-cycle recuperator, prior to its final expansion in the GT and exhaust to atmosphere. Estimated overall cycle thermal efficiency for the system configuration #2 ranges from ∼62% for a maximum GT/pneumatic motor inlet temperature of 1500 K (1227°C or 2240°F) to ∼66% for a maximum GT/pneumatic motor inlet temperature of 1700 K (1427°C or 2600°F), assuming a purely isothermal heat addition/expansion process in the pneumatic-motor cylinder. This is likely due to the fact that there is no necessity to cool the low-temperature GT of this configuration. Overall cycle thermal efficiency increases with the ambient temperature decrease for any cycle configuration.

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