| Absolute temperatures | are temperatures measured on the Kelvin scale or Rankine scale, and these temperatures vary between zero and infinity.
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| 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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| Carnot cycle | was first proposed in 1824 by French engineer Sadi Carnot. It 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 can have when operating between the two thermal energy reservoirs at temperatures TL and TH; ηth, 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 measure of performance of refrigerators and heat pumps. It is expressed in terms of the desired result for each device (heat absorbed from the refrigerated space for the refrigerator or heat added to the hot space by the heat pump) divided by the input, the energy expended to accomplish the energy transfer (usually work input).
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| Condenser | is a heat exchanger in which the working fluid condenses as it rejects heat to the surroundings. For example, the condenser in a steam power plant in which steam leaving the turbine as a vapor condenses to the saturated liquid state as the result of heat transfer from the vapor to a cooling medium such as the atmosphere or water from a lake or river.
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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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| 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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| Friction | is a familiar form of irreversibility associated with bodies in motion which results from the force that opposes the motion developed at the interface of the two bodies in contact when the two bodies are forced to move relative to each other.
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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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| 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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| Heat transfer | is defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference.
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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 scale | is the thermodynamic temperature scale in the SI and is named after Lord Kelvin (1824-1907). The temperature unit on this scale is the kelvin, which is designated by K (not °K; the degree symbol was officially dropped from kelvin in 1967). The lowest temperature on the Kelvin scale is 0 K.
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| kelvin | is the temperature unit of the Kelvin scale in the SI.
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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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| Quality of energy | is a measure of how much of the energy can be converted to work. More of energy at high temperatures can be converted to work. Therefore, the higher the temperature, the higher the quality of the energy.
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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 adiabatic compression | is the process in which a working fluid is compressed (decreases in volume) reversibly and adiabatically.
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| Reversible adiabatic expansion | is the process in which a working fluid expands (increases in volume) reversibly and adiabatically.
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| Reversible isothermal compression | is the process in which the temperature is held constant while a working fluid is compressed (decreases in volume) reversibly.
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| Reversible isothermal expansion | is the process in which the temperature is held constant while a working fluid expands (increases in volume) reversibly.
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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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| Second law efficiency | measures how well the performance of actual processes approximate the performance of the corresponding reversible processes. This enables us to compare the performance of different devices that are designed to do the same task on the basis of their efficiencies. The better the design, the lower the irreversibilities and the higher the second-law efficiency.
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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 | is defined as an amount of energy produced by the combustion of natural gas and is 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 efficiency of 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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| Unrestrained expansion of a gas | is the process of the free expansion of gas, unrestrained by a moving boundary such as the rapid expansion of air from a balloon that has just been burst.
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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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