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We propose an Otto heat engine that undergoes processes involving a special class of feedback and analyze theoretically its response. We use stochastic thermodynamics to determine the performance characteristics of the heat engine and indicate the possibility that its maximum efficiency can surpass the Carnot value. The analytical expression for efficiency at maximum power, including the effects resulting from feedback, reduces to that previously derived based on an engine without feedback. [ABSTRACT FROM AUTHOR]

Theoretical thermoelectric nanophysics models of low-dimensional electronic heat engine and refrigerator devices, comprising two-dimensional hot and cold reservoirs and an interconnecting filtered electron transport mechanism have been established. The models were used to numerically simulate and evaluate the thermoelectric performance and energy conversion efficiencies of these low-dimensional devices, based on three different types of electron transport momentum-dependent filters, referred to herein as kx, ky, and kr filters. Assuming the Fermi-Dirac distribution of electrons, expressions for key thermoelectric performance parameters were derived for the resonant transport processes, in which the transmission of electrons has been approximated as a Lorentzian resonance function. Optimizations were carried out and the corresponding optimized design parameters have been determined, including but not limited to the universal theoretical upper bound of the efficiency at maximum power for heat engines, and the maximum coefficient of performance for refrigerators. From the results, it was determined that kr filter delivers the best thermoelectric performance, followed by the kx filter, and then the ky filter. For refrigerators with any one of three filters, an optimum range for the full width at half maximum of the transport resonance was found to be <2kBT. [ABSTRACT FROM AUTHOR]

The irreversibilities resulting from the finite-rate heat transfer between the working substance and the external heat reservoirs and the regenerative losses in regenerative processes are considered in the cycle model of a magnetic Ericsson refrigerator. The fundamental optimum relations between the cooling rate and the coefficient of performance and between the power input and the coefficient of performance are derived, based on a class of heat-transfer laws and the Curie law. The general characteristic curves of the cycle are presented. The effect of the different forms of the regenerative time and heat-transfer laws on the performance of the cycle is discussed in detail. The results obtained here will be useful to further understand the general optimum performance of real magnetic Ericsson refrigerators. [ABSTRACT FROM AUTHOR]

Based on quantum thermodynamic processes, we make a quantum-mechanical (QM) extension of the typical heat engine cycles, such as the Carnot, Brayton, Otto, Diesel cycles, etc., with no introduction of the concept of temperature. When these QM engine cycles are implemented by an ideal gas confined in an arbitrary power-law trap, a relation between the quantum adiabatic exponent and trap exponent is found. The differences and similarities between the efficiency of a given QM engine cycle and its classical counterpart are revealed and discussed. [ABSTRACT FROM AUTHOR]