Jizhou He

Coefficient of performance for a low-dissipation Carnot-like refrigerator with nonadiabatic dissipation.

We study the coefficient of performance (COP) and its bounds for a Carnot-like refrigerator working between two heat reservoirs at constant temperatures T(h) and T(c), under two optimization criteria χ and Ω. In view of the fact that an adiabatic process usually takes finite time and is nonisentropic, the nonadiabatic dissipation and the finite time required for the adiabatic processes are taken into account by assuming low dissipation. For given optimization criteria, we find that the lower and upper bounds of the COP are the same as the corresponding ones obtained from the previous idealized models where any adiabatic process is undergone instantaneously with constant entropy. To describe some particular models with very fast adiabatic transitions, we also consider the influence of the nonadiabatic dissipation on the bounds of the COP, under the assumption that the irreversible entropy production in the adiabatic process is constant and independent of time. Our theoretical predictions match the observed COPs of real refrigerators more closely than the ones derived in the previous models, providing a strong argument in favor of our approach.

Efficiency at maximum power output of an irreversible Carnot-like cycle with internally dissipative friction.

We investigate the efficiency at the maximum power output (EMP) of an irreversible Carnot engine performing finite-time cycles between two reservoirs at constant temperatures T(h) and T(c) (<T(h)), taking into account the internally dissipative friction in two adiabatic processes. The EMP is retrieved to be situated between η(C)/2 and η(C)/(2-η(C)), with η(C) = 1-T(c)/T(h) being the Carnot efficiency, whether the internally dissipative friction is considered or not. When dissipations of two isothermal and two adiabatic processes are symmetric, respectively, and the time allocation between the adiabats and the contact time with the reservoir satisfy a certain relation, the Curzon-Ahlborn (CA) efficiency η(CA) = 1-sqrt[T(c)/T(h)] is derived.

Efficiency at maximum power of a quantum Otto cycle within finite-time or irreversible thermodynamics.

We consider the efficiency at maximum power of a quantum Otto engine, which uses a spin or a harmonic system as its working substance and works between two heat reservoirs at constant temperatures T(h) and T(c) (<T(h)). Although the behavior of spin-1/2 system differs substantially from that of the harmonic system in that they obey two typical quantum statistics, the efficiencies at maximum power based on these two different kinds of quantum systems are bounded from the upper side by the same expression η(mp)≤η(+)≡η(C)(2)/[η(C)-(1-η(C))ln(1-η(C))] with η(C)=1-T(c)/T(h) as the Carnot efficiency. This expression η(mp) possesses the same universality of the CA efficiency η(CA)=1-√(1-η(C)) at small relative temperature difference. Within the context of irreversible thermodynamics, we calculate the Onsager coefficients and show that the value of η(CA) is indeed the upper bound of EMP for an Otto engine working in the linear-response regime.

Coefficient of performance under maximum χ criterion in a two-level atomic system as a refrigerator.

A two-level atomic system as a working substance is used to set up a refrigerator consisting of two quantum adiabatic and two isochoric processes (two constant-frequency processes ω_{a} and ω_{b} with ω_{a}<ω_{b}), during which the two-level system is in contact with two heat reservoirs at temperatures T_{h} and T_{c}(<T_{h}). Considering finite-time operation of two isochoric processes, we derive analytical expressions for cooling rate R and coefficient of performance (COP) ɛ. The COP at maximum χ(=ɛR) figure of merit is numerically determined, and it is proved to be in nice agreement with the so-called Curzon and Ahlborn COP ɛ_{CA}=sqrt[1+ɛ_{C}]-1, where ɛ_{C}=T_{c}/(T_{h}-T_{c}) is the Carnot COP. In the high-temperature limit, the COP at maximum χ figure of merit, ɛ^{*}, can be expressed analytically by ɛ^{*}=ɛ_{+}≡(sqrt[9+8ɛ_{C}]-3)/2, which was derived previously as the upper bound of optimal COP for the low-dissipation or minimally nonlinear irreversible refrigerators. Within the context of irreversible thermodynamics, we prove that the value of ɛ_{+} is also the upper bound of COP at maximum χ figure of merit when we regard our model as a linear irreversible refrigerator.

Universality of maximum-work efficiency of a cyclic heat engine based on a finite system of ultracold atoms

We study the performance of a cyclic heat engine which uses a small system with a finite number of ultracold atoms as its working substance and works between two heat reservoirs at constant temperatures Th and Tc(<Th). Starting from the expression of heat capacity which includes finite-size effects, the work output is optimized with respect to the temperature of the working substance at a special instant along the cycle. The maximum-work efficiency ηmw at small relative temperature difference can be expanded in terms of the Carnot value

Thermal entanglement in two-atom cavity QED and the entangled quantum Otto engine

{{boldsymbol{eta }}}_{{boldsymbol{C}}}={bf{1}}-{{boldsymbol{T}}}_{{boldsymbol{c}}}/{{boldsymbol{T}}}_{{boldsymbol{h}}}

Efficiency at maximum power of a heat engine working with a two-level atomic system.

ηC=1−Tc/Th,