Thermodynamics of Active Refrigerators

In the past decade, a lot of studies have been done on cyclic heat engines for small systems with large fluctuations within the modern framework of stochastic thermodynamics. A paper published in 2022 has established a generalized second law for active cyclic heat engines that contain hidden dissipative degrees of freedom. Based on the generalized second law, it has been shown that the pseudo-efficiency of such active heat engines is able to exceed the Carnot bound. In our recent work, thermodynamic properties are studied for active cyclic systems that transfer heat from cold reservoirs to hot reservoirs, which are called "active refrigerators." Our studies show that the pseudo-coefficient of performance(COP) of such active refrigerators can exceed the traditional thermodynamic bound. Furthermore, such systems can transfer heat like refrigerators without input of work from the environment, which is called "Maxwell's demon phase." Even work can be extracted from such systems while heat is transferred like refrigerators, which is called the "hybrid phase." Phase diagrams are also plotted for these active cyclic systems.