Fund Project:Supported by the National Natural Science Foundations of China (Grant No. 11365015)
Received Date:16 July 2019
Accepted Date:29 August 2019
Available Online:01 November 2019
Published Online:20 November 2019
Abstract:A model of quantum dot refrigerator driven by photon, which consists of two two-level quantum dots, a photon reservoir and two leads, is proposed in this paper. Comparing with previous studies, we consider the transitions of electrons between different energy levels in a single quantum dot, which is more practical.Based on the theory of master equation and the assumption of weak coupling, we derive the expression of the cooling rate and the coefficient of performance of the refrigerator and obtain the condition of the tight coupling of the refrigerator operation. Next, we plot numerically the performance characteristic curves between the cooling rate and the coefficient of performance in the case of the tight coupling and in the general case. We find that the curves between the cooling rate and the coefficient of performance are opened loops for tight coupling, but they are closed loops in the general case. And we gain the conclusions that the refrigerator can be reversible under the condition of the tight coupling, while it can be irreversible in the general case. Then the optimally operating range of the refrigerator is determined. Finally, the effect of the temperature of the photon reservoir, transition coefficient, and temperature ratio on the performance of refrigerator under the conditions of the maximum cooling rate are studied, and also the coefficient of performance under the maximum cooling rate, the maximum coefficient of performanceand the cooling rate under the maximum coefficient of performanceare analyzed in detail. Keywords:irreversible thermodynamics/ quantum dot/ thermoelectric refrigerator/ performance optimization
图 3 紧耦合条件下制冷率与制冷系数在不同跃迁系数${\varGamma _{n{\rm{r}}}}$下的关系 Figure3. The relation curves of the cooling rate and the coefficient of performance at different transition coefficient ${\varGamma _{n{\rm{r}}}}$ under the condition of tight coupling.
图 4 一般情况下制冷率与制冷系数在不同温度${T_{\rm{S}}}$下的关系 Figure4. The relation curves of the cooling rate and the coefficient of performance at different temperature ${T_{\rm{S}}}$ in the general case.
图 5 一般情况下制冷率与制冷系数在不同跃迁系数${\varGamma _{n{\rm{r}}}}$下的关系 Figure5. The relation curves of the cooling rate and the coefficient of performanceat different transition coefficient ${\varGamma _{n{\rm{r}}}}$ in the general case.
可以得到这四个性能优化参数${\dot Q}_{\rm{R}}^{\max }$, ${\eta ^{{Q_{\rm{R}}}}}$, ${\eta ^{\max }}$, ${\dot Q}_{\rm{R}}^\eta $的数值解. 进而在紧耦合和一般情况下讨论了光子库温度、跃迁系数、温比$\tau = {T_{\rm{R}}}/{T_{\rm{L}}}$对四个优化性能参数的影响. 图6和图7为紧耦合条件下优化性能参数${\dot Q}_{\rm{R}}^{\max }$, ${\eta ^{{Q_{\rm{R}}}}}$随着温比变化的曲线图, 温度${T_{\rm{L}}}$的变化范围在${T_{\rm{R}}} < {T_{\rm{L}}} < {T_{\rm{S}}}$时, 制冷机才能有制冷作用, 因此图6中温度${T_{\rm{S}}}$不同时, 曲线起点不同. 在做图过程中令化学势$\mu /{k_{\rm{B}}} = 0\;{\rm{K}}$, ${\varepsilon _{{\rm{r1}}}} = - {\varepsilon _{{\rm{r2}}}}$, 其他参数的选取为${\varepsilon _{\rm{g}}}/{k_{\rm{B}}} = 2\;{\rm{K}}$, $ {\varGamma _{{\rm{l1}}}} = {\varGamma _{{\rm{l}}2}} = {\varGamma _{{\rm{r}}1}} = {\varGamma _{{\rm{r2}}}} = {\varGamma _{\rm{S}}} = \varGamma $, ${T_{\rm{R}}} = 4\;{\rm{K}}$. 图6中最大制冷率${\dot Q}_{\rm{R}}^{\max }$和对应的制冷系数${\eta ^{{Q_{\rm{R}}}}}$都随着温比的增大而增大. 图7中, 随跃迁系数${\varGamma _{n{\rm{r}}}}$的增加, 最大制冷率${\dot Q}_{\rm{R}}^{\max }$增加, 但对应的制冷系数${\eta ^{{Q_{\rm{R}}}}}$减少. 因此为了达到提高制冷率的目的, 应尽可能提高跃迁系数${\varGamma _{n{\rm{r}}}}$和温度${T_{\rm{S}}}$. 图 6 在不同温度${T_{\rm{S}}}$下, 两个优化性能参数${\dot Q}_{\rm{R}}^{\max }$和${\eta ^{{Q_{\rm{R}}}}}$随温比的变化 Figure6. The curves of two optimal performance parameters ${\dot Q}_{\rm{R}}^{\max }$ and ${\eta ^{{Q_{\rm{R}}}}}$ changing with the temperature ratio at different temperature ${T_{\rm{S}}}$
图 7 在不同跃迁系数${\varGamma _{n{\rm{r}}}}$下, 两个优化性能参数${\dot Q}_{\rm{R}}^{\max }$和${\eta ^{{Q_{\rm{R}}}}}$随着温比的变化 Figure7. The curves of two optimal performance parameters ${\dot Q}_{\rm{R}}^{\max }$ and ${\eta ^{{Q_{\rm{R}}}}}$ changing with the temperature ratio at different transition coefficient ${\varGamma _{n{\rm{r}}}}$
图8—图10为一般情况下四个优化性能参数${\dot Q}_{\rm{R}}^{\max }$, ${\eta ^{{Q_{\rm{R}}}}}$, ${\eta ^{\max }}$, ${\dot Q}_{\rm{R}}^\eta $随着温比变化的曲线图, 在做图过程中令化学势$\mu /{k_{\rm{B}}} = 0\;{\rm{K}}$, 其他参数的选取为${\varepsilon _{\rm{g}}}/{k_{\rm{B}}} = 2\;{\rm{K}}$, $ {\varGamma _{{\rm{l}}1}} = {\varGamma _{{\rm{l}}2}} = {\varGamma _{{\rm{r}}1}} = {\varGamma _{{\rm{r}}2}} = {\varGamma _{\rm{S}}} = \varGamma $, ${T_{\rm{R}}} = $4 K. 图8和图9中最大制冷率${\dot Q}_{\rm{R}}^{\max }$和对应的制冷系数${\eta ^{{Q_{\rm{R}}}}}$都随着温比的增加而增加, 图10中最大制冷系数随着温比的增大而增大, 而对应的制冷率趋于0. 要使得制冷机获得最大制冷率或者最大制冷系数应使左右两导体库的温度相近, 并提高光子库的温度和跃迁系数${\varGamma _{n{\rm{r}}}}$. 图 8 在不同温度${T_{\rm{S}}}$下, 两个优化性能参数${\dot Q}_{\rm{R}}^{\max }$和${\eta ^{{Q_{\rm{R}}}}}$随温比的变化 Figure8. The curves of two optimalperformance parameters ${\dot Q}_{\rm{R}}^{\max }$ and ${\eta ^{{Q_{\rm{R}}}}}$ changing with the temperature ratio at different temperature ${T_{\rm{S}}}$.
图 10 在不同温度${T_{\rm{S}}}$下, 两个优化性能参数${\eta ^{\max }}$和${\dot Q}_{\rm{R}}^\eta $随温比的变化 Figure10. The curves of two optimal performance parameters ${\eta ^{\max }}$ and ${\dot Q}_{\rm{R}}^\eta $ changing with the temperature ratio at different temperature ${T_{\rm{S}}}$.
图 9 在不同跃迁系数${\varGamma _{n{\rm{r}}}}$下, 两个优化性能参数${\dot Q}_{\rm{R}}^{\max }$和${\eta ^{{Q_{\rm{R}}}}}$随着温比的变化 Figure9. The curves of two optimal performance parameters ${\dot Q}_{\rm{R}}^{\max }$ and ${\eta ^{{Q_{\rm{R}}}}}$ changing with the temperature ratio at different transition coefficient ${\varGamma _{n{\rm{r}}}}$.