Energy and the Environment

Energy and the Environment

Fall 2014Instructor: Xiaodong Chu

Email ： [email protected] Tel.: 81696127, 13573122659

mailto:[email protected]

Thermodynamic Principles: Ideal Heat Engine Cycles

• The Otto cycle (奥托循环 ) – The Otto cycle is associated with fossil-fueled engine (reciprocating

internal combustion engine (ICE)) (往复式内燃机 ) in the automobile, which does not depend upon heat transfer to the working fluid from an external combustion source

– The fuel is burned adiabatically inside the engine, and the products of combustion produce more work during the expansion stroke (膨胀行程 ) than that is invested in the compression stroke (压缩行程 ) , giving a net power output

– The combustion products exhausted to the atmosphere are replaced by a fresh air-fuel charge to start the next cycle, which is termed an open cycle



• Multiple-cylinder design to prevent the engine from running jerkily


– The net cyclic work, equivalent heat added, and thermodynamic efficiency for the Otto cycle are

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– The Otto cycle efficiency is a monotonically increasing function of the volumetric compression ratio and the thermodynamic properties of the working fluid

– In gasoline engines, the compression ratio is limited by the tendency of the fuel-air mixture to combust spontaneously; in diesel engines, a higher compression ratio is available since the fuel is injected after the air is compressed

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– For the Otto cycle, the amount of equivalent heat is limited by the amount of fuel that can be burned in the fuel-air charge in the cylinder at the end of the compression stroke

• The combustible fuel is maximum when the air/fuel ratio is stoichiometric• The maximum temperature at the end of combustion is the adiabatic combustion temperature of the

compressed mixture in the cylinder

– The thermodynamic efficiencies of automobile engines are less than the ideal efficiency of the Otto cycle (the best thermal efficiencies are about 28% and 39% for the gasoline and diesel engine, respectively)

• Friction of pistons and bearing• Power required to operate valves, cooling pump, and the fuel supply system• Pressure losses in intake and exhaust systems• Heat loss to the cylinder during the power strokes

Thermodynamic Principles: The Vapor Compression Cycle

• The vapor compression cycle (蒸气压缩循环 ) is the use of mechanical power to move heat from a lower temperature source to a higher temperature sink – Refrigerators– Air conditioners– Heat pumps

• The process is the reverse of a heat engine in that power is absorbed rather than being produced, but it still observes the restrictions of the first and second laws of thermodynamics


• Reversed Carnot cycle – The cyclic refrigeration device operating between two constant

temperature reservoirs – Recall that in the Carnot cycle heat transfers take place at constant

temperature




• The refrigeration equipment consists of an evaporator (蒸发器 ), a vapor compressor (蒸气压缩机 ), a condenser (冷凝器 ), and an expansion valve (膨胀阀 ) connected in series in a piping loop filled with the refrigerant fluid (制冷液 )– The refrigerant undergoes a change of phase between liquid and vapor at the

temperatures and pressures within the system– The fluid flows through these components, being propelled by the pump


• For refrigerators and air conditioner, the desired output (heat removed from the refrigerated space) is not necessarily less than the input ( compressor work), of which the ratio is called the coefficient performance (COP) (性能系数 )

• The work required to compress a unit mass of refrigerant equals its change in its enthalpy (1->2) and the heat absorbed by the refrigerant from the refrigerated space is equal to its change in enthalpy (5->1) equaling that of (4->1)

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• Deviations from the ideal vapor compression refrigeration Cycle

– Pressure drop across the evaporator is caused by fluid friction: P1 < P5

– Mechanical and fluid friction in the compressor cause the entropy of the working fluid to increase

– Pressure drop across the condenser is caused by fluid friction: P4 < P2

– Friction losses and heat exchange with the surroundings also occurs in the piping that connects the four processes that make up the cycle, which results in pressure drops and increases or decreases in temperature depending on the temperature of the working fluid and the temperature of the surroundings


• How can we make the cold space even colder (reduce TC) and reject heat at an even higher temperature (increase TH)?– Two refrigeration cycles that use two different refrigerants are linked by a

heat exchanger– The lower cycle is colder and it absorbs heat from the refrigerated space; it

rejects heat into the upper cycle through the heat exchanger– The upper cycle absorbs heat from the lower cycle through the heat

exchanger; it is hotter and can reject heat to a very hot reservoir– Can use the same refrigerant in both cycles or use a refrigerant with a low

vapor pressure in the upper cycle and one with a relatively high vapor pressure in the lower cycle


• Comparison between a simple vapor-compression refrigeration cycle and a cascade simple vapor-compression refrigeration cycle with the same boundary conditions: TC , TH,

PL , PH

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Increase in qc

Decrease in qh


• A heat pump is a refrigerator operating in reverse– It delivers heat to an enclosed space by transferring it from

environment having a lower temperature– Heat pumps are commonly used to provide wintertime space heating

in climates where there is need for summertime air conditioning for which the same refrigeration unit is used for both purposes by redirecting the flow of air (or other heat transfer fluid) between the condenser and evaporator: how to?

• By simply allowing the evaporator and condenser to trade places, readily accomplished by a slide or reversing valve


• The coefficient of performance of a heat pump is defined as which is always greater than unity

• COP of the reversed Carnot cycle– Refrigerator

– Heat pumpch

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Documents

Energy and the Environment