In a latent heat thermal energy storage system, heat is normally stored through a phase change material where it can then be supplied later to a heat transfer liquid for several applications, including concentrated solar power generation. Melting the phase change material stores the energy, which is subsequently released by solidifying the phase change material. The thermal storage system must be able to store adequate amounts of energy and release energy at sufficient rates to cope with the available supply and demand respectively. Phase change materials possess high volumetric energy densities. However, owing to their low thermal conductivity, the development of more effective configurations of latent heat thermal energy storage units is needed.
The shell and tube configuration appears to be a promising arrangement where a heat transfer fluid flows via tubes surrounded by a phase change material. An effective design should utilize the highest volume fraction of phase change material for the highest heat transfer density with the lowest temperature gradient. This in reality leads to the smallest system with a uniform solidification or melting process of phase change material in time and space without hot spots, and provides a cost effective thermal storage system.
As a part of her PhD program, Soheila Riahi at the University of South Australia conducted a numerical study on a shell and tube latent heat storage system whereby the inlet heat transfer fluid direction was reversed periodically in the course of discharging and charging. The effect of changing the boundary condition at the interface of the tubes carrying the heat transfer fluid with the phase change material on the evolution of phase change front, heat transfer area and heat transfer rate were evaluated in the course of charging and discharging. The research work is published in Applied Energy.
The main aim of the study was to illustrate a practical method, which was periodic reversal flow to impose a periodic boundary condition without changing the heat source. The authors therefore investigated the method together with its effect on the thermal characteristics of high temperature phase change material in a shell and tube latent heat thermal energy storage system.
The research results demonstrated that during the charging processes, a higher heat transfer area was developed in the early stages and amplification of a natural convection after 40% melt fraction. This led to higher heat transfer rate. Periodic flow reversal for the discharge processes led to an increase in heat transfer area for a longer period of time, which led to higher heat transfer rate specifically after 75% solidification. This effect was found to be more beneficial for the discharging processes in the absence of convection heat transfer. Heat transfer fluid periodic reversal led to lower temperature gradient in time and space, and approximately 6% increase in the time-average heat transfer rate in the charging and discharging processes.
The flow reversal approach proposed in their study therefore provides a path for the implementation of periodic boundary condition without changing the heat source; improving thermal performance of latent hear storage systems and cost effectiveness. Furthermore, the method in under experimental examination for other heat exchanger applications.
Note: The IP of flow reversal method is protected under International (PCT) Patent Application number: PCT/AU2017/000145 HEAT EXCHANGER IMPROVEMENTS.
Soheila Riahi, Wasim Y. Saman, Frank Bruno, Martin Belusko, N.H.S. Tay. Impact of periodic flow reversal of heat transfer fluid on the melting and solidification processes in a latent heat shell and tube storage system. Applied Energy, volume 191 (2017), pages 276–286.Go To Applied Energy