Yunus A. Çengel,
University of Nevada, Reno
Michael A. Boles,
North Carolina State University
| Air conditioners | are refrigerators whose refrigerated space is a room or a building instead of the food compartment.
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| Air-source heat pumps | use the cold outside air as the heat source in winter.
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| Annual fuel utilization efficiency, or AFUE | accounts for the combustion efficiency as well as other losses such as heat losses to unheated areas and start-up and cool-down losses in buildings.
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| Carnot cycle | was first proposed in 1824 by French engineer Sadi Carnot, is composed of four reversible processes-two isothermal and two adiabatic, and can be executed either in a closed or a steady-flow system.
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| Carnot efficiency | is the highest efficiency a heat engine operating between the two thermal energy reservoirs at temperatures TL and TH can have, hth, rev = 1 - TL / TH.
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| Carnot heat engine | is the theoretical heat engine that operates on the Carnot cycle.
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| Carnot heat pump | is a heat pump that operates on the reversed Carnot cycle. When operating between the two thermal energy reservoirs at temperatures TL and TH the Carnot heat pump can have a coefficient of performance of COPHP, rev = 1/ (1- TL / TH) = TH /( TH - TL).
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| Carnot principles | are two conclusions that pertain to the thermal efficiency of reversible and irreversible (i.e., actual) heat engines and are expressed as follows:
- The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs.
- The efficiencies of all reversible heat engines operating between the same two reservoirs are the same.
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| Carnot refrigerator | is a refrigerator that operates on the reversed Carnot cycle. When operating between the two thermal energy reservoirs at temperatures TL and TH the Carnot refrigerator can have a coefficient of performance of COPR, rev = 1/ (TH / TL - 1) = TL /( TH - TL).
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| Clausius statement of the second law | is expressed as follows: It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body.
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| Coefficient of performance (COP) | is the efficiencyof a refrigerator or heat pump.
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| Combustion efficiency | combustion equipment is defined as the amount of heat released during combustion divided by the heating value of the fuel. A combustion efficiency of 100 percent indicates that the fuel is burned completely and the stack gases leave the combustion chamber at room temperature, and thus the amount of heat released during a combustion process is equal to the heating value of the fuel.
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| Condenser | is a heat exchanger in which the working fluid condenses as it rejects heat to the surroundings.
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| Efficiency | is one of the most frequently used terms in thermodynamics, and it indicates how well an energy conversion or transfer process is accomplished.
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| Efficiency of a cooking appliance | can be defined as the ratio of the useful energy transferred to the food to the energy consumed by the appliance.
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| Efficiency of a water heater | is defined as the ratio of the energy delivered to the house by hot water to the energy supplied to the water heater.
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| Efficiency of resistance heaters | is 100 percent as they convert all the electrical energy they consume into heat.
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| Energy efficiency rating | (EER) is the performance of refrigerators and air conditioners, and is the amount of heat removed from the cooled space in Btu's for 1 Wh (watt-hour) of electricity consumed.
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| Evaporator | is a heat exchanger in which the working fluid evaporates as it receives heat from the surroundings.
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| Externally reversible | process has no irreversibilities to occur outside the system boundaries during the process. Heat transfer between a reservoir and a system is an externally reversible process if the surface of contact between the system and the reservoir is at the temperature of the reservoir.
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| Generator | is a device that converts mechanical energy to electrical energy.
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| Generator efficiency | is the ratio of the electrical power output to the mechanical power input.
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| Geothermal heat pumps | (also called ground-source heat pumps) use the ground as the heat source.
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| Heat engines | are devices that convert heat to work.Heat engines differ considerably from one another, but all can be characterized by the following:
- They receive heat from a high-temperature source (solar energy, oil furnace, nuclear reactor, etc.).
- They convert part of this heat to work (usually in the form of a rotating shaft).
- They reject the remaining waste heat to a low-temperature sink (the atmosphere, rivers, etc.).
- They operate on a cycle.
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| Heating value of a fuel | is the amount of heat released when a specified amount of fuel (usually a unit mass) at room temperature is completely burned and the combustion products are cooled to the room temperature.
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| Heat pumps | are cyclic devices which operate on the refrigeration cycle and discharge energy to a heated space to maintain the heated space at a high temperature.
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| Heat pump coefficient of performance | is the efficiency of a heat pump, denoted by COPHP, and expressed as desired output divided by required input or COPHP = QH/Wnet, in.
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| Heat reservoir | is a thermal energy reservoir since it can supply or absorb energy in the form of heat.
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| Heat sink | is a heat reservoir that absorbs energy in the form of heat.
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| Heat source | is a heat reservoir that supplies energy in the form of heat.
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| Higher heating value | or HHV, is the heating value of the fuelwhen the water in the combustion gases is completely condensed and thus the heat of vaporization is also recovered. Efficiencies of furnaces are based on higher heating values.
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| Internally reversible process has | no irreversibilities that occur within the boundaries of the system during the process. During an internally reversible process, a system proceeds through a series of equilibrium states, and when the process is reversed, the system passes through exactly the same equilibrium states while returning to its initial state.
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| Irreversible processes | are processes which, once having taken place in a system, cannot spontaneously reverse themselves and restore the system to its initial state.
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| Irreversibilities | are the factors that cause a process to be irreversible. They include friction, unrestrained expansion, mixing of two gases, heat transfer across a finite temperature difference, electric resistance, inelastic deformation of solids, and chemical reactions.
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| Kelvin-Planck statement of the second law of thermodynamics | is expressed as follows: It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work. This statement can also be expressed as no heat engine can have a thermal efficiency of 100 percent, or as for a power plant to operate, the working fluid must exchange heat with the environment as well as the furnace.
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| Kelvin unit | magnitude was established at the International Conference on Weights and Measures in 1954. The triple point of water (the state at which all three phases of water exist in equilibrium) was assigned the value 273.16 K (0.01°C). The magnitude of a kelvin is defined as 1/273.16 of the temperature interval between absolute zero and the triple-point temperature of water. The magnitudes of temperature units on the Kelvin and Celsius scales are identical (1 K, 1°C). The temperatures on these two scales differ by a constant 273.15.
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| Lighting efficacy | is defined as the amount of light output in lumens per W of electricity consumed.
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| Lower heating value | or LHV, is the heating value of the fuel when the water in the combustion gases is a vapor. Efficiencies of cars and jet engines are normally based on lower heating values since water normally leaves as a vapor in the exhaust gases, and it is not practical to try to recuperate the heat of vaporization.
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| Motor efficiency | is the ratio of the mechanical energy output of a motor to the electrical energy input. The full-load motor efficiencies range from about 35 percent for small motors to over 96 percent for large high-efficiency motors.
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| Overall efficiency | for a power plant is the ratio of the net electrical power output to the rate of fuel energy input. Overall efficiencies are about 25 to 28 percent for gasoline automotive engines, 34 to 38 percent for diesel engines, and 40 to 60 percent for large power plants.
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| Perpetual-motion machine | is any device that violates either the first or second law of thermodynamics.
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| Perpetual-motion machine of the first kind | (PMM1) is a device that violates the first law of thermodynamics (by creating energy).
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| Perpetual-motion machine of the second kind | (PMM2) is a device that violates the second law of thermodynamics.
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| Refrigerant | is the working fluid used in the refrigeration cycle.
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| Refrigerators | are cyclic devices which allow the transfer of heat from a low-temperature medium to a high-temperature medium.
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| Refrigerator coefficient of performance | is the efficiency of a refrigerator, denoted by COPR, and expressed as desired output divided by required input or COPR = QL/Wnet, in.
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| Reversed Carnot cycle | is the result of reversing all the process that comprise the reversible Carnot heat-engine cycle, in which case it becomes the Carnot refrigeration cycle.
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| Reversible process | is defined as a process that can be reversed without leaving any trace on the surroundings. Reversible processes are idealized processes, and they can be approached but never reached in reality.
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| Steam power plant | is an external-combustion engine in which steam (water) is the working fluid. That is, combustion takes place outside the engine, and the thermal energy released during this process is transferred to the steam as heat. A turbine in the power plant converts some of the energy of the steam into rotating shaft work.
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| Therm | of natural gas is an amount of energy equal to 29.3 kWh.
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| Thermal efficiency | is a measure of the performance of a heat engine and is the fraction of the heat input to the heat engine that is converted to net work output.
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| Thermal efficiency of a heat engine | is the fraction of the thermal energy supplied to a heat engine that is converted to work.
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| Thermal efficiencyof a power plant | is defined as the ratio of the shaft work output of the turbine to the heat input to the working fluid.
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| Thermal energy reservoir | or just a reservoir is a hypothetical body with a relatively large thermal energy capacity (mass specific heat) that can supply or absorb finite amounts of heat without undergoing any change in temperature.
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| Thermodynamic temperature | scale is a temperature scale that is independent of the properties of the substances that are used to measure temperature. This temperature scale is called the Kelvin scale, and the temperatures on this scale are called absolute temperatures.On the Kelvin scale, the temperature ratios depend on the ratios of heat transfer between a reversible heat engine and the reservoirs and are independent of the physical properties of any substance.
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| Ton of refrigeration | is a measure of the rate of energy transfer in the amount of 12,000 Btu/h or 211 kJ/min.
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| Totally reversible process | or simply reversible process, involves no irreversibilities within the system or its surroundings. A totally reversible process involves no heat transfer through a finite temperature difference, no non-quasi-equilibrium changes, and no friction or other dissipative effects.
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| Vapor-compression refrigeration cycle | is the most frequently used refrigeration cycle and involves four main components: a compressor, a condenser, an expansion valve, and an evaporator.
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| Working fluid | is the fluid to and from which heat and work is transferred while undergoing a cycle in heat engines and other cyclic devices.
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2002 McGraw-Hill Higher Education