We study universal aspects of fluctuations in an ensemble of noninteracting continuous quantum thermal machines in the steady state limit. Considering an individual machine, such as a refrigerator, in which relative fluctuations (and high order cumulants) of the cooling heat current to the absorbed heat current, $\eta^{ (n)}$, are upper-bounded, $\eta^{ (n)}\leq \eta_C^n$ with $n\geq 2$ and $\eta_C$ the Carnot efficiency, we prove that an { \it ensemble} of $N$ distinct machines similarly satisfies this upper bound on the relative fluctuations of the ensemble, $\eta_N^{ (n)}\leq \eta_C^n$. For an ensemble of distinct quantum { \it refrigerators} with components operating in the tight coupling limit we further prove the existence of a { \it lower bound} on $\eta_N^{ (n)}$ in specific cases, exemplified on three-level quantum absorption refrigerators and resonant-energy thermoelectric junctions. Beyond special cases, the existence of a lower bound on $\eta_N^{ (2)}$ for an ensemble of quantum refrigerators is demonstrated by numerical simulations.
Categories: cond-mat.stat-mech cond-mat.mes-hall