Phase change materials (PCMs) are used in applications where temperature regulation is important because they absorb and release a large amount of energy at a fixed temperature. In the experimental part of this investigation, PCM was placed in the annular region of a double-pipe heat exchanger with water circulated in the inside pipe. Experiments were performed in which the PCM would absorb (charge) and then release (discharge) energy at various temperatures and water flows. Two materials, Climsel 28 (C28) by Climator and microencapsulated Thermasorb 83 (TY83) by Outlast Technologies, were each tested in smooth and spined annuli to observe which configuration facilitated heat transfer. The latent heats and thermal conductivities of C28 and TY83 are $126kJ∕kg$ and $186kJ∕kg$ and $0.6W∕m∕°C$ and $0.15W∕m∕°C$, respectively. The experimental data were analyzed to verify which PCM transferred more heat. The effect of different water flow rates on the heat transfer rate was also examined. In the theoretical part of this investigation, heat transfer theory was applied to C28 in the smooth-piped heat exchanger in order to better understand the phase change process. The presence of spined fins in the phase change material accelerated charging and discharging due to increased fin contact with the outer layers of the PCM. The spined heat exchanger charged and discharged in $180min$ and $120min$, respectively, whereas the temperature in the smooth heat exchanger remained below the fully charged/fully discharged asymptote by about $3°C$ and thus failed to fully charge or fully discharge. Also, higher water flows increased heat transfer between the PCM and water. TY83 in the spined heat exchanger transferred more heat and did it faster than C28 in the spined heat exchanger. The heat transfer rate from the water to TY83 while charging was 25% greater during the transient period than in C28. While discharging, the heat transfer from TY83 to the water was about 20% greater than in C28. There was generally good agreement $(±1.5°C)$ between theory and experimental data of C28 in the smooth-piped heat exchanger in terms of the trends of the temperature responses. The differences are expected to be a result of approximations in boundary conditions and uncertainties in how the temperature variation of the specific heat is formulated.

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