Thermodynamics

Chapter 6 | 281to engineers, and the second law provides the necessary means to determinethe quality as well as the degree of degradation of energy during a process.As discussed later in this chapter, more of high-temperature energy can beconverted to work, and thus it has a higher quality than the same amount ofenergy at a lower temperature.The second law of thermodynamics is also used in determining thetheoretical limits for the performance of commonly used engineering systems,such as heat engines and refrigerators, as well as predicting the degreeof completion of chemical reactions.6–2 ■ THERMAL ENERGY RESERVOIRSIn the development of the second law of thermodynamics, it is very convenientto have a hypothetical body with a relatively large thermal energycapacity (mass specific heat) that can supply or absorb finite amounts ofheat without undergoing any change in temperature. Such a body is called athermal energy reservoir, or just a reservoir. In practice, large bodies ofwater such as oceans, lakes, and rivers as well as the atmospheric air can bemodeled accurately as thermal energy reservoirs because of their large thermalenergy storage capabilities or thermal masses (Fig. 6–6). The atmosphere,for example, does not warm up as a result of heat losses fromresidential buildings in winter. Likewise, megajoules of waste energydumped in large rivers by power plants do not cause any significant changein water temperature.A two-phase system can be modeled as a reservoir also since it can absorband release large quantities of heat while remaining at constant temperature.Another familiar example of a thermal energy reservoir is the industrial furnace.The temperatures of most furnaces are carefully controlled, and theyare capable of supplying large quantities of thermal energy as heat in anessentially isothermal manner. Therefore, they can be modeled as reservoirs.A body does not actually have to be very large to be considered a reservoir.Any physical body whose thermal energy capacity is large relative tothe amount of energy it supplies or absorbs can be modeled as one. The airin a room, for example, can be treated as a reservoir in the analysis of theheat dissipation from a TV set in the room, since the amount of heat transferfrom the TV set to the room air is not large enough to have a noticeableeffect on the room air temperature.A reservoir that supplies energy in the form of heat is called a source, andone that absorbs energy in the form of heat is called a sink (Fig. 6–7). Thermalenergy reservoirs are often referred to as heat reservoirs since theysupply or absorb energy in the form of heat.Heat transfer from industrial sources to the environment is of major concernto environmentalists as well as to engineers. Irresponsible managementof waste energy can significantly increase the temperature of portionsof the environment, causing what is called thermal pollution. If it is notcarefully controlled, thermal pollution can seriously disrupt marine life inlakes and rivers. However, by careful design and management, the wasteenergy dumped into large bodies of water can be used to improve the qualityof marine life by keeping the local temperature increases within safeand desirable levels.ONE WAYFIGURE 6–4Processes occur in a certain direction,and not in the reverse direction.PROCESS1st law2nd lawFIGURE 6–5A process must satisfy both the firstand second laws of thermodynamics toproceed.RIVERATMOSPHEREOCEANLAKEFIGURE 6–6Bodies with relatively large thermalmasses can be modeled as thermalenergy reservoirs.FIGURE 6–7INTERACTIVETUTORIALSEE TUTORIAL CH. 6, SEC. 2 ON THE DVD.Thermal energySOURCEHEATHEATThermal energySINKA source supplies energy in the formof heat, and a sink absorbs it.