Lecture EXERGY W15 EXERGY Analysis Requires
EXERGY analysis requires good understanding of the concepts we learned in Thermodynamics-I.
Below some
... [Show More] definitions are repeated.
System: A system is defined as a quantity of matter or a region in space chosen for study.
Boundary: The real or imaginary surface that separates the system from its surroundings is called the
boundary.
Surroundings: The mass or region outside the system is called the surroundings. In
Thermodynamics-I, most of the time it is assumed that the surrounding is unaffected by the change of
states of system. This is not entirely true. A portion of the surrounding closed to the system is always
affected by the state change of the system. Therefore, surrounding can be divided into two parts:
(a) Immediate surroundings: Immediate surroundings refer to the portion of the surroundings
that is affected by the state change and intermediate process,
(b) Environment: Environment refers to the region beyond the immediate surroundings whose
properties are not affected by the state change and intermediate processes.
When analyzing the cooling of a hot baked potato in
a room at 25°C, for example, the warm air that
surrounds the potato is the immediate surroundings,
and the remaining part of the room air at 25°C is the
environment. Note that the temperature of the
immediate surroundings changes from the
temperature of the potato at the boundary to the
environment temperature of 25°C. The immediate
surroundings of a hot potato are simply the
temperature gradient zone of the air next to the
potato.
Note: Any irreversibility during a process occurs within the system and its immediate surroundings,
and the environment is free of any irreversibilities. As discussed later, environment plays a
significant role during EXERGY analysis.
Dead State: A system is said to be in the dead state when it is in thermodynamic equilibrium with the
environment it is in. At the dead state,
• a system is at the temperature of its environment (in thermal equilibrium);
• a system is at the pressure of its environment (in mechanical equilibrium)
• it has no kinetic energy relative to the environment (zero velocity)
• it has no potential energy relative to the environment (zero elevation above a reference level)
• it does not react with the environment (chemically inert).
• there are no unbalanced magnetic, electrical, and surface tension effects between the system
and its surroundings
The properties of a system at the dead state are denoted by subscript zero (P0, T0, h0, u0, and s0).
Unless specified otherwise, the dead-state temperature and pressure are taken to be T0=25°C and
P0=1 atm (101.325 kPa). A system has zero exergy at the dead state.
2
Work Potential: The work potential of the energy contained in a system at a specified state is simply
the maximum useful work that can be obtained from the system. The rest of the energy is eventually
discarded as waste energy and is not useful for practical purpose. Work done during a process
depends on the initial state, the final state, and the process path. That is,
Work = f (initialstate, process path,finalstate)
Initial state is specified in an exergy analysis. Final state is the state of environment (dead state).
Therefore, the work output is maximized when the process between two specified states is executed
in a reversible manner.
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Example 2 (work potential of a thermal reservoir):
A heat engine is extracting 1000 J of heat from a 500K thermal reservoir. It is rejecting heat to a
400K low temperature reservoir. The temperature and pressure of the surrounding environment are
300K and 1atm, respectively. The net work output of the heat engine will be η maxQH (assume a
Water reservoir
T=27°C, P=1atm
V=0 m/s
T0=27°C, P0=1atm
z
z =0
Example 1 (work potential of a water reservoir): The
volume of the water reservoir is ∀=1000 m3
. Density and
specific heat of water are ρ=1000kg/m3
and Cp=4200 J/kg⋅K.
The energy content of the water reservoir is
1260000MJ
( ) 1000 1000 4200 300
=
E = mCpT = ρ∀ CpT = × × ×
However, its ability to do something useful is zero. You
cannot run a heat engine between reservoir and surrounding
because of thermal equilibrium. No way to extract kinetic and
potential energy because of zero velocity and elevation. No boundary work due to zero pressure
difference between reservoir and surrounding. Therefore, 1260000 MJ has zero work potential.
reversible operation). In this condition, this heat engine will deliver 200J or net work output and
reject 800J heat to the low temperature reservoir. As the environmental temperature is lower than the
low temperature reservoir, you can operate another reversible engine between the LTR and
surrounding environment which can produce additional 200J of net work output. The high
temperature reservoir has a potential to produce total 400J of net work output.
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Example 3 (heating potential of a furnace):
A 500K furnace is supplying continuously 1000J of heat to a room for heating purpose. The
temperature of the room is 300K. Outside temperature is 270K. The entire 1000J heat from the
furnace is used for room heating purpose. It is possible to run a heat engine between the furnace and
the room that is able to produce 400J of work output (see Example 2 for calculation) and release 600J
of waste heat to the room. This valuable 400J of work can be used to run a heat pump that will extract
QL1 amount of heat from the environment and release QH1 amount of heat to the room. Work input to
the heat pump will be 400J. Now the COP of the heat pump can be calculated from the following
−
= = . Therefore, QH1=COPHP × Win = 10×400=4000J. The
furnace has an overall heating potential of (4000+600)J=4600J.
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EXERGY: A system delivers the maximum possible work as it undergoes a reversible process from
the specified initial state to the state of its environment (the dead state). This represents the useful
work potential of the system at the specified state and is called EXERGY.
It is important to note that exergy does not represent the amount of work that a work-producing
device will actually deliver upon installation. Rather, it represents the upper limit on the amount of
work a device can deliver without violating any thermodynamic laws. There will always be a
difference, large or small, between EXERGY and the actual work delivered by a device. This
difference represents the room engineers have for improvement. [Show Less]