Air-cooled condenser systems incur a high cost of operation during peak power demand due to hot ambient temperatures, resulting in poor thermal performance. To shift the thermal performance of air-cooled condenser systems closer to that of wet cooling towers, a salt hydrate phase change material (PCM)-based supplemental thermal energy storage system is proposed. Low-grade waste heat from saturated steam in a power generation cycle is stored in a “cold” sink of PCM during the day and rejected at night during cooler ambient air temperatures. Passive loop thermosyphons are used to transfer heat in and out of the PCM heat exchanger. A 30MJ prototype was designed, fabricated, and tested to demonstrate the longevity of the system. Repeated thermal cycling tests have been completed for 50 cycles. The measured latent heat capacity results have shown that the salt hydrate solution segregates into different hydration levels over time but can stabilize by remixing the bulk solution.