This paper demonstrates a new approach using a thermoelectric generator (TEG) which converts thermal energy from ambient temperature fluctuations into electricity for the power source of portable devices. Theoretical modeling and experimental validation of a thermoelectric generator integrated with a phase change material (PCM) are reported in this paper. The proposed device converts the ambient temperature variations into a temperature difference on the TEG using a PCM, thus temperature variations can produce electrical energy. The maximum output power can reach 0.6 mW which corresponds to a power density of 37.5 W/cm2 under the ambient temperature variants from 5°C to 35°C (using the temperature-controlled oven). The electric power generated by TEG can be stored in a capacitor via a DC-DC converter. The maximum output power and the storable energy in the capacitor are 30 W and 500 mJ, respectively, under ambient temperature fluctuations near room temperature. The investigation in this work could possibly lead to high potential for battery-free wireless sensing systems.