Energy and the Environment

Energy and the Environment

Fall 2013Instructor: Xiaodong Chu

Email ： [email protected] Tel.: 81696127

mailto:[email protected]

Flashbacks of Last Lecture

• The various forms of energy that can be possessed by a material body can be added together to define a total energy E

• Thermodynamics deals with the interaction of a thermodynamic material system and its environment and there are two different modes of interaction, i.e., the work interaction and the heat interaction

magelnucchem EEEEUPEKEE

Flashbacks of Last Lecture• The first law of thermodynamics is an energy conservation

principle

• The second law of thermodynamics states that it is not possible to devise a cyclic process in which heating supplied from a single source is converted entirely to work

• The third law of thermodynamics states that the entropy of all thermodynamic systems is zero at the absolute zero of temperature

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f

iif dWdQEE

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Flashbacks of Last Lecture

• Thermodynamic properties include intensive properties, extensive properties and specific extensive properties

• Enthalpy is defined as

• Gibbs’ free energy is defined as

• For a reversible process with dq = Tds

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dfsdTdh

vdpdh

pdvdeTds

Thermodynamic Principles: Heat Transfer and Heat Exchange

• The time rate of exchange of work and heat quantities determine the mechanical or thermal power that can be produced

• In most cases of steady heat transfer from a hot to a cold environment, the time rate of heat transfer

where u is the heat transfer coefficient, A is the surface area, and uA is called the thermal conductance

• To obtain high value of u, one should use a thin layer of good heat conductor; to get low value of u, one should use a thick layer of thermally insulating material

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Thermodynamic Principles: Heat Transfer and Heat Exchange

• Heat exchangers are passive devices accomplishing a transfer of heat, usually between two streams of fluids, one hot and the other cold

• The functioning of heat exchangers involves loss of mechanical power, reduced thermodynamic efficiency, and increased economic cost

• The transfer of heat at finite rates in thermodynamic systems inevitably incurs performance penalties that cannot be reduced to zero except by the expenditure of infinite amounts of capital

Thermodynamic Principles: Combustion of Fossil Fuel

• The most common fossil fuels are hydrocarbons, i.e., mixture of molecules composed of carbon and hydrogen– Upon their complete combustion, the carbon is oxidized to

carbon dioxide and the hydrogen to water vapor – The energy made available in this oxidation is the net

amount released when the carbon and hydrogen atoms are separated from each other and combined with oxygen to form carbon dioxide and water


• Denoting a hydrocarbon fuel molecule as CnHm, the molecular rearrangement can be represented by the reaction

• The ratio of the number of oxygen molecules to the number of fuel molecules, n + m/4, is called the stoichiometric ratio and it can be expressed as a mass ratio as

which lies in the range between 8/3 (for pure carbon) and 7.937 (for pure hydrogen)

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mn

mnmn

mn

mn

008.112

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mass fuel

massoxygen


• The stoichiometric ratio is more usefully expressed in terms of the ratio of air mass to fuel mass by multiplying the ratio of the mass of air to the mass of oxygen in air, which is 4.319

• If less air is available than is required, not all of the carbon

or hydrogen will be fully oxidized and some amount of CO, solid C, or H2 may be present while not all the available chemical energy is released in the combustion process

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mass fuel

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• Fuel heating value (燃料热值 )– When a mixture of fuel and air is burned, the temperature of

the combustion products formed is much higher than that of the fuel–air mixture

• Heat may be transferred from the hot combustion products to a colder fluid

• The amount of heat available for this purpose is called the fuel heating value and is usually expressed in energy units per unit mass of fuel


– Consider a combustion chamber of a gas turbine power plant that is supplied with a steady flow of a fuel-air mixture (reactant)

• If fuel is burned at constant pressure and no heat is lost, i.e., adiabatic and workless, by recalling the equation

• The product gas enthalpy hp{Tp, pr} will equal the reactant stream enthalpy hr{Tr, pr}, where Tp is called the adiabatic combustion temperature

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},{},{ rrrrpp pThpTh


• If the hot product gases are cooled at constant pressure to the reactant temperature, then the heat removed per unit mass of product gas will be equal in magnitude to the reduction in enthalpy hp{Tp, pr} - hp{Tr, pr} = hr{Tr, pr} - hp{Tr, pr}

• Multiplying by the mass flow rate of products divided by the mass flow rate of fuel, we obtain the fuel heating value

• If the H2O formed in the combustion product is in the vapor phase, then the fuel heating value is called the lower heating value whereas it is called the higher heating value if the H2O is in the liquid phase

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Thermodynamic Principles: Ideal Heat Engine Cycles

• Generating mechanical power from fossil fuel must utilize the combustion process to change the temperature and/or pressure of a fluid and then find a way to use the fluid to make mechanical work by moving a piston or turning a turbine– The first and second laws of thermodynamics limit the amount of

work that can be generated for each unit mass of fuel used, and those limits depend upon the details of how the fuel is used to create power

– Analyze ideal devices in which a fluid is heated and cooled, and produces or absorbs work, as the fluid moves through a cycle and such a device can be called a heat engine


• The Carnot cycle– The Carnot cycle is illustrative of the second law limits on the simplest

of heat engine cycles– It is sustained by two heat reservoirs, a hot one of temperature Th and

a cold one of Tc

– Consider the heat engine to be a cylinder equipped with a movable piston and enclosing a fluid of unit mass

– The cycle consists of four parts: two expansions and two compressions

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• Reversible isothermal expansion of the gas at the "hot" temperature (1 to 2)

• Reversible adiabatic expansion of the gas (2 to 3) • Reversible isothermal compression of the gas at the "cold"

temperature (3 to 4) • Isentropic compression of the gas (4 to 1)


– The thermodynamic efficiency of the Carnot cycle to be

the thermodynamic efficiency depends only upon the temperatures of the two reservoirs and not at all upon the properties of the fluid used in the heat engine

– The amount of net work w that the heat engine delivers does depend upon the fluid properties and the amount of expansion

the net work is equal to the area enclosed by the cycle path in the T –s plane

h

c

hth T

T

q

w 1

w pdv Tds


– For Carnot cycle, the thermodynamic efficiency is improved by supplying the heat qh to the engine at the highest possible temperature Th

– For a fuel burning in ambient air and supplying this heat to the hot reservoir, Th could not exceed the adiabatic combustion temperature Tad and only a fraction of the fuel heating value could be added to the hot reservoir, that fraction being approximately (Tad − Th)/(Tad − Tc)

which has a maximum value of

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h

c

TT

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T

T1

1/

1/

had

cad

TT

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– A possible plan for increasing the efficiency would be to employ a large number of Carnot engines, each operating at a different hot reservoir temperature Th but the same cold reservoir temperature Tc

– The combustion products of a constant-pressure burning of the fuel would then be brought into contact with successively cooler reservoirs, transferring heat amounts dh from the combustion gases to produce work amounts dw, where

and by integrating it, we obtain

)(1 sThddhT

Tdw c

h

c

1)/(

)/ln(1

cad

cad

TT

TT


• An example of a Carnot cycle of a steam power plant– If the temperature of steam produced by the boiler is 600oC and that

of water from the condenser is 25oC, neglecting the work conducted on the pump, what is the efficiency?



Adiabatic Expansion

Isothermal Compression

AdiabaticCompression

Isothermal Expansion

Hurricane as a Carnot cycle

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