ABSTRACT Thermoelectric generator (TEG) systems are often exposed to unstable heat loads with transient temperature boundary conditions. TEG behavior recognition in thermal fluctuations is necessary to increase its efficiency and protection. This research deals with the experimental study of thermoelectric module’s performance under transient thermal boundary conditions. Therefore, three types of novel prototypes of transient heat loads including rectangular, triangular, and sinusoidal were suggested. Also, a phase change material (PCM) was applied inside an aluminum box on the hot side of the module. In the mentioned system, the effect of PCM on the module performance in the unstable heat loads was investigated. Applying PCM led to the protecting the module against temperature fluctuations, storing the energy, and generating effective voltage for a long time after turning the heat source off. Voltage generation lasted for 59.5 min with 90 W input power in 3-min periods in the rectangular heat load. Meanwhile, after switching the heaters off, 42.8- and 50-min voltages were received from triangular and sinusoidal loads, respectively, each with the power ranging from 0 to 90 W in four periods, lasted for 3 min.