  We study work extraction from the Dicke model achieved using simple unitary
cyclic transformations keeping into account both a non optimal unitary
protocol, and the energetic cost of creating the initial state. By analyzing
the role of entanglement, we find that highly entangled states can be
inefficient for energy storage when considering the energetic cost of creating
the state. Such surprising result holds notwithstanding the fact that the
criticality of the model at hand can sensibly improve the extraction of work.
While showing the advantages of using a many-body system for work extraction,
our results demonstrate that entanglement is not necessarily advantageous for
energy storage purposes, when non optimal processes are considered. Our work
shows the importance of better understanding the complex interconnections
between non-equilibrium thermodynamics of quantum systems and correlations
among their subparts.
