Energy storage

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Latent heat thermal energy storage system
Latent heat thermal energy storage system.

Interest in alternative energy technologies, such as solar energy, is growing due to increasing recognition of environmental considerations. The major barrier to more widespread use of solar energy is its periodic nature, as it is available only during daytime and a heat storage device is needed to store energy for use at night. The latent heat thermal energy storage system, which utilizes phase-change materials (PCMs) to absorb and release heat, is widely used for this purpose (Viskanta, 1983). The PCM in the thermal energy storage system is molten when the system absorbs heat, and it solidifies when the system releases heat. The advantages of the latent heat thermal energy storage system are that a large amount of heat can be absorbed and released at a constant temperature, and the latent heat thermal energy system is considerably smaller than its counterpart using sensible heat thermal energy storage.

A conceptual design of the latent heat thermal energy storage system is illustrated in the figure on the right (Zhang and Faghri, 1996). The structure of the thermal storage system is similar to that of a conventional shell-and-tube heat exchanger, with the PCM on the shell side and the transfer fluid flowing inside the tube. The outside of the shell is insulated to prevent heat loss to the environment. The PCM fills the space between the tube and a shell in the shape of a hollow cylinder. A transfer fluid flowing inside the tube exchanges heat with the PCM. During the heat storage process, the fluid is at a temperature higher than the PCM’s melting point, and flows through the tube to melt the initially-solid PCM. In the heat-release process, the fluid is at a temperature below the PCM’s melting point, and flows through the tube to solidify the liquid PCM. In a latent heat thermal energy storage system, these melting and solidification processes occur alternately to store and release heat.

References

Faghri, A., and Zhang, Y., 2006, Transport Phenomena in Multiphase Systems, Elsevier, Burlington, MA.

Viskanta, R., 1983, “Phase Change Heat Transfer,” Solar Heat Storage: Latent Heat Materials, edited by Lane, G.A., CRC Press, Boca Raton, FL, pp. 153-222.

Zhang, Y., and Faghri, A., 1996, “Semi-Analytical Solution of Thermal Energy Storage System with Conjugate Laminar Forced Convection,” International Journal of Heat Mass Transfer, Vol. 39, No. 4, pp. 717-724.