Chapter 9POWER AND REFRIGERATION CYCLES| 1Two important areas of application for thermodynamicsare power generation and refrigeration. Both powergeneration and refrigeration are usually accomplishedby systems that operate on a thermodynamic cycle. Thermo-dynamic cycles can be divided into two general categories:power cycles and refrigeration cycles.The devices or systems used to produce a net power out-put are often called engines, and the thermodynamic cyclesthey operate on are called power cycles. The devices or sys-tems used to produce refrigeration are called refrigerators, airconditioners, or heat pumps, and the cycles they operate onare called refrigeration cycles.Thermodynamic cycles can also be categorized as gascycles or vapor cycles, depending on the phase of the work-ing fluid—the substance that circulates through the cyclicdevice. In gas cycles, the working fluid remains in thegaseous phase throughout the entire cycle, whereas in vaporcycles the working fluid exists in the vapor phase during onepart of the cycle and in the liquid phase during another part.Thermodynamic cycles can be categorized yet anotherway: closed and open cycles. In closed cycles, the workingfluid is returned to the initial state at the end of the cycle andis recirculated. In open cycles, the working fluid is renewed atthe end of each cycle instead of being recirculated. In auto-mobile engines, for example, the combustion gases areexhausted and replaced by fresh air–fuel mixture at the end ofeach cycle. The engine operates on a mechanical cycle, butthe working fluid in this type of device does not go through acomplete thermodynamic cycle.Heat engines are categorized as internal combustion orexternal combustion engines, depending on how the heat issupplied to the working fluid. In external combustion engines(such as steam power plants), energy is supplied to the work-ing fluid from an external source such as a furnace, a geo-thermal well, a nuclear reactor, or even the sun. In internalcombustion engines (such as automobile engines), this isdone by burning the fuel within the system boundary. In thischapter, various gas power cycles are analyzed under somesimplifying assumptions.Steam is the most common working fluid used in vaporpower cycles because of its many desirable characteristics,such as low cost, availability, and high enthalpy of vaporiza-tion. Other working fluids used include sodium, potassium,and mercury for high-temperature applications and someorganic fluids such as benzene and the freons for low-tem-perature applications.Steam power plants are commonly referred to as coalplants, nuclear plants, or natural gas plants, depending onthe type of fuel used to supply heat to the steam. But thesteam goes through the same basic cycle in all of them.Therefore, all can be analyzed in the same manner.The most frequently used refrigeration cycle is the vapor-compression refrigeration cycle in which the refrigerant isvaporized and condensed alternately and is compressed inthe vapor phase.ObjectivesThe objectives of this chapter are to:• Evaluate the performance of gas power cycles.• Develop simplifying assumptions applicable to gas powercycles.• Review the operation of reciprocating engines.• Solve problems based on the Otto and Diesel cycles.• Solve problems based on the Brayton cycle and the Braytoncycle with regeneration.• Analyze vapor power cycles in which the working fluid isalternately vaporized and condensed.• Investigate ways to modify the basic Rankine vapor powercycle to increase the cycle thermal efficiency.• Analyze the reheat vapor power cyles.• Analyze the ideal vapor-compression refrigeration cycle.• Analyze the actual vapor-compression refrigeration cycle.• Discuss the operation of refrigeration and heat pumpsystems.99-R4232-online-chapter.qxd 3/19/07 1:32 PM Page 11■BASIC CONSIDERATIONS IN THE ANALYSIS OFPOWER CYCLESMost power-producing devices operate on cycles, and the study of powercycles is an exciting and important part of thermodynamics. The cyclesencountered in actual devices are difficult to analyze because of the pres-ence of complicating effects, such as friction, and the absence of sufficienttime for establishment of the equilibrium conditions during the cycle. Tomake an analytical study of a cycle feasible, we have to keep the complexi-ties at a manageable level and utilize some idealizations (Fig. 1). When theactual cycle is stripped of all the internal irreversibilities and complexities,we end up with a cycle that resembles the actual cycle closely but is madeup totally of internally reversible processes. Such a cycle is called an idealcycle (Fig. 2).A simple idealized model enables engineers to study the effects of themajor parameters that dominate the cycle without getting bogged down in thedetails. The cycles discussed in this chapter are somewhat idealized, but theystill retain the general characteristics of the actual cycles they represent. Theconclusions reached from the analysis of ideal cycles are also applicable toactual cycles. The thermal efficiency of the Otto cycle, the ideal cycle forspark-ignition automobile engines, for example, increases with the compres-sion ratio. This is also the case for actual automobile engines. The numericalvalues obtained from the analysis of an ideal cycle, however, are not neces-sarily representative of the actual cycles, and care should be exercised in theirinterpretation (Fig. 3). The simplified analysis presented in this chapter forvarious power cycles of practical interest may also serve as the starting pointfor a more in-depth study.Heat engines are designed for the purpose of converting thermal energy towork, and their performance is expressed in terms of the thermal efficiencyhth, which is the ratio of the net work produced by the engine to the totalheat input:(1)Recall that heat engines that operate on a totally reversible cycle, such asthe Carnot cycle, have the highest thermal efficiency of all heat enginesoperating between the same temperature levels. That is, nobody can developa cycle more efficient than the Carnot cycle. Then the following questionarises naturally: If the Carnot cycle is the best possible cycle, why do wenot use it as the model cycle for all the heat engines instead of botheringwith several so-called ideal cycles? The answer to this question is hardware-related. Most cycles encountered in practice differ significantly from theCarnot cycle, which makes it