  Motivated by the ever-growing demand for \emph{green} wireless communications
and the advantages of \emph{cell-free} (CF) massive multiple-input
multiple-output (MIMO) systems, we focus on the design of their downlink for
optimal \emph{energy efficiency} (EE). To address this fundamental topic, we
assume that each access point (AP) is deployed with multiple antennas and
serves multiple users on the same time-frequency resource while the APs are
Poisson point process (PPP) distributed, which approaches realistically their
opportunistic spatial randomness. Relied on tools from stochastic geometry, we
derive a lower bound on the downlink average achievable spectral efficiency
(SE). Next, we consider a realistic power consumption model for CF massive MIMO
systems. These steps enable the formulation of a tractable optimization problem
concerning the downlink EE per unit area, which results in the analytical
determination of the optimal pilot reuse factor, the AP density, and the number
of AP antennas and users that maximize the EE. Notably, the EE per unit area
and not just the EE is the necessary metric to describe CF systems, where we
meet multi-point transmission. Hence, we provide useful design guidelines for
CF massive MIMO systems relating to fundamental system variables towards
optimal EE. Among the results, we observe that a lower pilot reuse factor
enables a decrease of the interference, and subsequently, higher EE up to a
specific value. Overall, it is shown that the CF massive MIMO technology is a
promising candidate for next-generation networks achieving simultaneously high
SE and EE per unit area.
