Performance of discrete heat engines and heat pumps in finite time

Abstract

The performance in finite time of a discrete heat engine with internal friction is analyzed. The working fluid of the engine is composed of an ensemble of noninteracting two level systems. External work is applied by changing the external field and thus the internal energy levels. The friction induces a minimal cycle time. The power output of the engine is optimized with respect to time allocation between the contact time with the hot and cold baths as well as the adiabats. The engine's performance is also optimized with respect to the external fields. By reversing the cycle of operation a heat pump is constructed. The performance of the engine as a heat pump is also optimized. By varying the time allocation between the adiabats and the contact time with the reservoir a universal behavior can be identified. The optimal performance of the engine when the cold bath is approaching absolute zero is studied. It is found that the optimal cooling rate converges linearly to zero when the temperature approaches absolute zero.

Quantum four-stroke heat engine: Thermodynamic observables in a model with intrinsic friction

“…In frictionless engines, where the internal Hamiltonian commutes with the external control field, the energy observable is still sufficient to characterize the engine's cycle [5,6]. In the general case additional variables have to be added.…”

Speed and efficiency limits of multilevel incoherent heat engines

“…The foregoing features and the lack of quantum (coherence-assisted) effects are consequences of a driving Hamiltonian H S (t) that commutes with itself at all times (see however bath-induced persistent quantum coherence in a degenerate multilevel WF [40]). By contrast, HE whose H S (t) does not commute with itself [3,[12][13][14][15] may face unwarranted friction effects.…”

“…In the Otto cycle, attempts to allow for strokes of finite duration have been primarily confined, for both classical and quantum-mechanical HEs, to slow operation, as in the Curzon-Ahlborn analysis, which shows that efficiency drops as the speed (cycle rate) increases [4][5][6]. Likewise, for a driven three-level working fluid (WF) the speed of continuous-cycle operation has been shown to be detrimental, leading to friction, i.e., loss of work at the expense of wasted heat production [3][4][5][6][7][8][9][10][11][12][13][14][15].…”

“…Such operation avoids possible friction [3,7,[12][13][14][15]. These HEs are comprised of a frequency-modulated N -level WF described by, e.g., molecular angular-momentum giant spin or harmonicoscillator models, i.e.…”

Quantum lubrication: Suppression of friction in a first-principles four-stroke heat engine

“…This term represents the power invested against friction therefore it vanishes when J = 0 oṙ ω = 0 [14].…”