3. Energy and exergy analysis
Exergy is a generic term for a group of concepts that define the
maximum work potential of a system, a stream of matter or a heat
interaction; the state of the (conceptual) environment being used
as the datum state. In an open flow system there are three types of
energy transfer across the control surface namely work transfer,
heat transfer, and energy associated with mass transfer or flow. The
work transfer is equivalent to exergy in every respect as exergy is
maximum work, which can be obtained from that form of energy.
The exergy ExQ of heat transfer Q from the control surface at
temperature T is determined from maximum rate of conversion of
thermal energy to work Wmax. The Wmax is given by
Wmax ¼ ExQ ¼ Qð1 Ta=TÞ (1)
Exergy of steady flow stream of matter is sum of kinetic,
potential and physical exergy Exph. The kinetic and potential energy
are again equivalent to exergy. The physical specific exergy j
depends on initial state of matter and environmental state and is
given by b ba, where b ¼ h Tas, suffix ‘a’ refers to the environmental
state, h is specific enthalpy; s is specific entropy and Ta is
environmental/ambient temperature.
Energy analysis is based on the first law of thermodynamics,
which is related to the conservation of energy. Exergy analysis is
a method that uses the conservation of mass and conservation of
energy principles together with the second law of thermodynamics
for the analysis, design and improvement of energy systems. Exergy
analysis is a useful method; to complement not to replace energy
analysis. Unlike the mass and energy the exergy is not conserved.
The first law of thermodynamics or energy balance for steady flow
process of an open system is given by
XEi þXn
j ¼ 1
Qj ¼ XEo þ Wnet (2)