  Cell-free massive MIMO (CF-mMIMO) is expected to provide reliable wireless
services for a large number of user equipments (UEs) using access points (APs)
distributed across a wide area. When the UEs are battery-powered, uplink energy
efficiency (EE) becomes an important performance metric for CF-mMIMO systems.
Therefore, if the "target" spectral efficiency (SE) is met, it is important to
optimize the uplink EE when setting the transmit powers of the UEs. Also, such
transmit power control (TPC) method must be tested on channel data from
real-world measurements to prove its effectiveness. In this paper, we compare
three different TPC algorithms using zero-forcing reception by applying them to
3.5 GHz channel measurement data featuring ~30,000 possible AP locations and 8
UE locations in a 200mx200m area. We show that the max-min EE algorithm is
highly effective in improving the uplink EE at a target SE, especially if the
number of single-antenna APs is large, circuit power consumption is low, and
the maximum allowed transmit power of the UEs is high.
