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Compared to fossil fuel energy resources, the major types of renewable energy—such as wind power, solar energy, ocean currents, and tidal energy—generally possess the innate characteristics of intermittence of availability, fluctuation of magnitude, as well as low energy density (Li, 2008). However, the utilization of energy and power in industry, living, and working often requires high energy densities, and demand may be out of phase with the period of availability of renewable energy. In other words, renewable energy is not always load following. This variability creates a demand for energy storage when people develop renewable energy technologies (Kolb, 1998). Among the several types of renewable energy, solar energy has the largest proportion of the total available and may be directly used as thermal energy in conventional thermal power plants, or converted into electrical power directly using photovoltaic panels. Although direct electrical energy storage in batteries or capacitors may have a high efficiency, it is still very challenging and expensive—particularly when storing a large quantity of electrical energy (Spiers, 1995). Electricity may be indirectly stored by pumping water to reservoirs, or by compressing air, or by electrolyzing water and making hydrogen fuel, etc. However, these methods often have low round-trip efficiency (from electricity to electricity), or are restricted by the availability of geographical conditions or suitable locations. In comparison, it has been recognized that direct solar thermal energy storage is relatively easy to approach at a reasonably low cost and high efficiency, and the energy storage capacity can be much larger than that of direct electricity storage (Price et al., 2002; Montes et al, 2009). Thermal energy storage systems use materials that can be kept at high temperatures in insulated containers. The heat retrieved can then be used in conventional thermal power plants for power generation at times when sunlight is not available or when weather conditions are not favorable (Singer et al, 2010; Laing et al, 2010).

Transient Heat Transfer and Energy Transport in Packed Bed Thermal Storage Systems