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1 Basic Introductory Thermal Aspects

      1.1 Introduction

      Thermal energy storage (TES) is one of the key technologies for energy conservation, and therefore, it is of great practical importance. One of its main advantages is that it is best suited for heating and cooling thermal applications. TES is perhaps as old as civilization itself. Since recorded time, people have harvested ice and stored it for later use. Large TES systems have been employed in more recent history for numerous applications, ranging from solar hot water storage to building air‐conditioning systems. The TES technology has only recently been developed to a point where it can have a significant impact on modern technology.

      In general, a coordinated set of actions has to be taken in several sectors of the energy system to realize the maximum potential benefits of thermal storage. TES appears to be an important solution in rectifying the mismatch between the supply and demand of energy. TES can contribute significantly in meeting society's demands for more efficient, environmentally benign energy use. TES is a key component of many successful thermal systems, and a good TES should allow the lowest thermal losses, leading to energy savings, while permitting the highest reasonable extraction efficiency of the stored thermal energy.

      There are mainly two types of TES systems, that is, sensible (e.g. water and rock) and latent (e.g. water/ice and salt hydrates). For each storage medium, there is a wide variety of choices depending on the temperature range and application. TES via latent heat has received a great deal of interest. Perhaps, the most obvious example of latent TES is the conversion of water into ice. Cooling systems incorporating ice storage have a distinct size advantage over equivalent capacity chilled water units because of the ability to store large amount of energy as latent heat. TES deals with the storing of energy, usually by cooling, heating, melting, solidifying, or vaporizing a substance, and the energy becomes available as heat when the process is reversed. The selection of a TES is mainly dependent on the storage period required, that is, diurnal or seasonal, economic viability, operating conditions, and so on. In practice, many research and development activities related to energy have concentrated on efficient energy use and energy savings, leading to energy conservation. In this regard, TES appears to be an attractive thermal application. Furthermore, exergy analysis is an important tool for analyzing TES performance.

We begin this chapter with a summary of fundamental definitions, physical quantities, and their units, dimensions, and interrelations. We consider the introductory aspects of thermodynamics, fluid flow, heat transfer, energy, entropy, and exergy.

1.2 Systems of Units

      There are two main systems of units: the International System of Units (Le Systéme International ď Unités), which is normally referred to as SI units, and the English System of Units. SI units are used most widely throughout the world, although the English System is traditional in the United States. In this book, SI units are primarily employed. Note that the relevant unit conversions and relationships between the International and English unit systems concerning fundamental properties and quantities are listed in Appendix A.

1.3 Fundamental Properties and Quantities

      In this section, we briefly cover several general aspects of thermodynamics to provide adequate preparation for the study of TES systems and their applications.

      1.3.1 Mass, Time, Length, and Force

      Mass is defined as a quantity of matter forming a body of indefinite shape and size. The fundamental unit of mass is the kilogram (kg) in SI units and the pound mass (lb_m) in English units. The basic unit of time for both unit systems is the second.

      In thermodynamics, the unit mole (mol) is commonly used and defined as a certain amount of a substance as follows:

      (1.1)

      where n is the number of moles, m is the mass, and M is the molecular weight. If m and M are expressed in units of gram and gram per mole, we obtain n in moles. For example, one mole of water, having a molecular weight of 18 (compared to 12 for carbon‐12), has a mass of 0.018 kg.

      The basic unit of length is the meter (m) in SI units and the foot (ft) in the English system.

      A force is a kind of action that brings a body to rest or changes its speed or direction of motion (e.g. a push or a pull). The fundamental unit of force is the Newton (N).

The relationship between the four aspects, for example, mass, time, length, and force is expressed by the Newton's second law of motion, which states that the force acting on a body is proportional to the mass and the acceleration in the direction of the force, as given in Eq. (1.2):

(1.2)

      Equation (1.2) shows the force required to accelerate a mass of one kilogram at a rate of one meter per second squared as 1 N = 1 kg m/s².

It is important to note that the value of the earth's gravitational acceleration is 9.80665 m/s² in the SI system and 32.174 ft/s² in the English system, and it indicates that a body falling freely toward the surface of the earth is subject to the action of gravity alone.

      1.3.2 Pressure

      While dealing with liquids and gases, pressure becomes one of the most important quantities. Pressure is the force exerted on a surface, per unit area, and is expressed in bar or Pascal (Pa). The related expression is

      (1.3)

      The SI unit for pressure is the force of one Newton acting on a square meter area (or the Pascal).

      The unit for pressure in the English system is pound‐force per square foot, lb_f/ft².

Here, we introduce basic pressure definitions, and a summary of basic pressure measurement relationships is depicted in Figure 1.1.

      (a) Atmospheric Pressure

      The atmosphere that surrounds the earth can be considered as a reservoir of low‐pressure air. Its weight exerts a pressure which varies with temperature, humidity, and altitude. Atmospheric pressure also varies

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