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International Journal of Environmental Science and Development, Vol. 2, No. 6, December 2011 to sensible heat storage in applications with a small temperature swing because of its nearly isothermal storing mechanism and high storage density. Phase change materials are employed in different fields of thermal engineering like energy storage, thermal conditioning of buildings, waste heat recovery, off peak power utilization, heat pump systems, space applications, laptop computer cooling, and telecom shelters. When a temperature peak occurs, PCM absorbs the excessive energy by going through a phase transition and releasing the absorbed energy later when the peak has passed off. The possibility of varying conditions during the design process urges us to design a reliable PCM model which would enable parametric studies to be conducted at speed and would also enable the comparison of several alternatives. This would also preclude the need to build experimental platforms for measurements. II. THEORY Heat transfer in PCM storage is characterized by a moving solid–liquid interface, generally referred to as the “moving boundary” problem. It is a transient, non-linear phenomenon. Analytical solutions for phase change problems are only known for a couple of physical situations which possess a simple geometry and simple boundary conditions, as nonlinearity poses major difficulties in moving boundary problems. Neumann originated the most well-known precise analytical solution for a one-dimensional moving boundary problem, called the Stefan problem. Some analytical approximations for one-dimensional moving boundary problems with different boundary conditions are the quasi-stationary approximation, perturbation methods, the Megerlin method and the Heat-balance-integral method. It has been assumed here that the melting or solidification temperature is constant. However, for example technical grade paraffin has a wide temperature range at the points where melting and solidification occur. Phase change problems are usually solved with finite difference or finite element methods in accordance with the numerical approach. The phase change phenomenon has to be modeled separately due the non-linear nature of the problem. A wide range of different kinds of numerical methods for solving PCM problems exist. The most common methods used are the enthalpy method and the effective heat capacity method. III. THERMAL ENERGY STORAGE SYSTEMS (TESS) The demand and supply gap for energy sources is widening day by day. Moreover, the fact that the energy can neither be created and nor destroyed has resulted in focusing of scientific research in the direction of storing the different forms of energy using diverse devices. Thermal energy is one such energy which is of interest to researchers worldwide. Thermal energy could have several geneses but storage of solar thermal energy is one of the principal areas of investigation. In recent years, various conventional and unconventional materials are investigated for their capability to store thermal energy. These thermal energy storage devices (TESD) are selected on the basis of some crucial physical, chemical and economic properties. Melting point, heat of fusion, density, heat capacity, thermal conductivity, compatibility with container and cost of production are the chief parameters for selection of TESD. It is a genuine challenge to find out an ultimate TESD as the overall suitability of materials to be used as TESD is governed by a multifaceted interplay between several properties of those materials. IV. PROPERTIES AFFECTING TESS A. Melting Point Melting point is the temperature at which the first crystal of the material collapses. It is imperative to have the melting point of TESD within the temperature range of application. The melting point as such does not affect the energy storage capacity of a material. However, as a phase change is involved in melting, the inclusion of melting point in temperature range of application can permit the use of phase change as an on-off switch. Melting point lower as well as higher than the temperature range of application prohibits the use of the material in TESD. B. Heat of Fusion (ΔH) Heat of fusion (ΔH) also known as enthalpy of fusion or latent heat of fusion is a very important property useful in selecting a TESD. It refers to the amount of thermal energy that a material must absorb or evolve in order to change its phase from solid to liquid or vice versa. Large values of heat of fusion aid in increasing the efficiency of TESDs. C. Heat Capacity Heat capacity refers to the amount of energy per molecule that a compound can store before the increase in its temperature. This energy is generally stored in translational, vibration and rotational modes. Thus materials with greater number of atoms in its composition are expected to have higher heat capacity. D. Thermal Conductivity (K) Thermal Conductivity (k) measures the ability of a material to conduct heat. Greater values of k imply an efficient heat transfer. Thermal conductivity is a property which needs to be optimized. Since thermal conductivity is phase dependant property, it is important to know values of k in both the solid as well as molten phases. It has been observed that most of molten materials exhibit much higher values of thermal conductivity as compared to that in their solid state. Higher values of thermal conductivity in molten state can facilitate an efficient heat transfer and smooth operation of the thermal circuits in TESD. E. Density (ρ) Density (ρ) of a material refers to its mass per unit volume. Density values can readily be measured using densitometers. Materials with higher density thus occupy less space which in turn increases the energy storage capacity. Materials with high density obviously possess higher energy storage capacity but many of them show a significant decrease in density in their molten state. This is due to the expansion of 438PDF Image | Analysis of PCM Material in Thermal Energy Storage
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