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Thermoelectric energy harvesting requires a substantial temperature difference ΔT to be available within the device structure. This has restricted its use to particular applications such as heat engine structural monitoring, where a hot metal surface is available. An alternative approach is possible in cases where ambient temperature undergoes regular variation. This involves using a heat storage unit, which is filled with a phase-change material (PCM), to create an internal spatial temperature difference from temperature variation in time. In this paper, key design parameters and a characterization methodology for such devices are defined. The maximum electrical energy density expected for a given temperature range is calculated. The fabrication, characterization, and analysis of a heat storage harvesting prototype device are presented for temperature variations of a few tens of degrees around 0 °C, corresponding to aircraft flight conditions. Output energy of 105 J into a 10- Ω matched resistive load, from a temperature sweep from +20 °C to -21 °C, then to +25 °C is demonstrated, using 23 g of water as the PCM. The proposed device offers a unique powering solution for wireless sensor applications involving locations with temperature variation, such as structural monitoring in aircraft, industrial, and vehicle facilities.