Page 261 - Alternative Energy Systems in Building Design

SOLAR COOLING AND AIR-CONDITIONING 237

The branch of physics that deals with the mechanics of heat transfer is called
thermodynamics. There are two principal universal laws of thermodynamics. The first
law concerns the conservation of energy and states that energy can neither be created nor
destroyed; however, it can be converted from one type to another. The second law
deals with the equalization and transfer of energy from a higher state to a lower one.
Simply stated, energy is always transferred from a higher potential or state to a lower
one until the two energy sources achieve exact equilibrium. Heat is essentially defined
as a form of energy created as a result of the transformation of another form of
energy. A common example of this is when two solid bodies are rubbed together,
resulting in friction heat. In general, heat is energy in a transfer state because it does
not stay in any specific position and constantly moves from a warm object to a cooler
one until such time that, as per the second law of thermodynamics, both bodies reach
heat equilibrium.
It should be noted that the volume, size, and mass of the objects are completely
irrelevant in the heat-transfer process. Only the heat energy levels are factors in the
energy-balance equation. With this principle in mind, heat energy can flow from a
small object, such as a hot cup of coffee, to one with much larger mass, such as your
hand. The rate of travel of heat is directly proportional to the difference in temper-
ature between the two objects; size plays no role in this matter.
Heat travels in three forms: radiation, conduction, and convection. As radiation, heat
is transferred as a waveform similar to radio waves, microwaves, or light. For example,
the sun transfers its energy to earth by rays, or radiation. In conduction, heat energy
flows from one medium or substance to another by physical contact. Convection, on
the other hand, is the flow of heat between air, gas, liquid, and a fluid medium. The
basic principles of refrigeration are based on the second law of thermodynamics, that
is, the transfer or removal of heat from a higher-energy medium to a lower one by
means of convection. Figure 6.11 illustrates the evaporation and condensation cycle in
refrigeration.

TEMPERATURE
Temperature is a scale for measuring heat intensity with a directional flow of energy.
Water freezes at 0°C (32°F) and boils at 100°C (212°F). Temperature scales are simply
temperature differences between freezing and boiling water temperatures measured at
sea level. As mentioned earlier, based on the second law of thermodynamics, heat trans-
fer or measurement of temperature does not depend on the quantity of heat.

MOLECULAR AGITATION

Depending on the state of heat energy, most substances in general can exist in vapor,
liquid, and solid states. For example, depending on the heat-energy level, water can
exist as solid ice when it is frozen, as a liquid at room temperature, and as a vapor
when it is heated above its boiling temperature of 212°F (100°C). In each of the states,
water is within or without the two boundary temperatures of 32°F (0°C) and 212°F
(100°C).