Exergy analysis is a useful concept for ecology and sustainability because it can used as a common measure of resource quality along with quantity.

1. Zin Eddine Dadach
Higher Colleges of Technology,UAE

2.  Introduction
 Energy = Quantity + Quality
 Issues about energy balance
 Advantages of exergy analysis
 Comparing energy and exergy flows in power
plants
 Equations for exergy destructions in OCGT
 Recommendations

3. Exergy analysis is a useful
concept for ecology and
sustainability because it can
used as a common measure of
resource quality along with
quantity.

4. Quantity (1st law: Energy is conserved)
Quality (2nd law: Exergy is destroyed).
The exergy of an energy form or a
substance is a measure of its
usefulness or work potential
 Work has higher energy quality than heat. 1 kJ of
electricity is fully useful but not 1 kJ of heat.

5.  Plant efficiency management in the first instance is
concerned with thermal efficiency. Ultimately a plant
wants to produce as much output for the least
expenditure of fuel.
 When plant efficiency deteriorates, the challenge is first
to locate the cause and then cost the value of repairs.
 Energy balance is not useful because the real plant
inefficiencies are not related to energy loss but to exergy
destruction.

6. Determining the exergy destruction of each
equipment of the plant.
By analyzing the exergy destroyed by each
component in a process, we can see where
we should be focusing our efforts to improve
system efficiency.

7.  Exergy of an energy stream can be lost to the
surroundings (flue gas, cooling water and heat
loss),
 However, the primary contributors to exergy
destruction are irreversibilities associated with
chemical reaction, heat transfer, mixing, and
friction
 No exergy destruction during a reversible process.
Reversible work is then the maximum amount of
useful work output

8.  The initial and final energy and exergy
are similar
 Large amount of heat but small amount
of exergy leave the process . Exergy
destruction is due mainly to
irreversibilities in the process ( exergy
flow very narrow).
 Furnace losses represent small
percentage of the total energy
conversion. From the exergy diagram that
something drastic happens
(Irreversibilities due to combustion).
 Energy flows, the losses are heaviest in
the condenser. However, very low
temperature has a very low quality
 Energy and exergy loss similar in the
turbine. Exergy destruction due to energy
loss through the boundaries

9.  Irreversibilities (chemical reaction, heat transfer,
mixing, and friction ) are invisible in energy
balance.
 Irreversibilities destroy the quality and then the
economic value of energy flows. Therefore
increasing their environmental impact.
 Exergy analysis distinguish between the exergy
destroyed by irreversibilities and those lost
through the boundaries of the system.

10. Furnace has the highest exergy destruction .
It is critical to use a fuel that meets the
original specifications for optimum
combustion.
Continuous measurement of both O2 and CO
leaving the combustor provides also
information needed for effective combustion
for significant benefits in energy savings

12. 1) Fuel composition, flow rate and (P,T)
2) Air composition (including humidity), flow rate and (P, T).
3) Air to fuel ratio or excess air
4) Flow, P,T of air leaving compressor
5) Power input to the compressor
6) Power generated by the turbine
7) P,T, composition, flow of flue gas before and after
turbine

13. Exergy destruction and exergy efficiency of
the power plant and its components are
found by solving the three balance
equations applied at any control volume, in
other words, at any component of the plant:
Mass balance: Σmin =Σmout
Energy balance: Σmout.hout −Σmin .hin =Qnet,in −Wnet,out
Exergy balance: Σmin.exin −Σmout .exout - W +Σ Q (1- T0/T) = (EX)destroyed

14. Exergy of streams is evaluated with respect
to a reference environment (i.e. dead state).
 for a component of a stream at rest: ek=ep=0
(ex)t= (ex)tm + (ex)ch
= (h-h0 ) –T0 (s-s0)- RT0 ln (x)0
 In our study, the atmospheric pressure, annual average
temperature and relative humidity in Abu Dhabi will be
selected as the parameters of the “dead state”

15.  Compressor:
(ex)D,K= ma .(ex2-ex1)-WK
 Combustor:
(ex)D,CC= ma.ex2 +mf.(ex)f – mfg. ex3
 Gas turbine:
(ex)D,GT = mfg. (ex4-ex3) - WGT

16.  Work from a gas turbine can be defined as the product of
mass flow, heat energy in the combusted gas (Cp), and
temperature differential across the turbine
WT= mfg. Cp,fg. (T3-T4)
 The mass flow of flue gas is the sum of compressor
airflow and fuel flow.
 The heat energy of the flue gas is a function of the
elements in the fuel and the products of combustion.

17. Energy output and efficiency of gas turbine
vary according to operating conditions.
The main parameters to be considered are:
 Ambient temperature,
 Compressor pressure ratios
 Fuel temperature
 Turbine inlet temperature
 Air to fuel ratio
.

18.  With the exergy analysis of power plants , we can
detect causes to irreversibilities:
 Recoverable loss which can be rectified by water
washing or, more thoroughly, by mechanically
cleaning the compressor blades and vanes after
opening the unit.
 Non-recoverable loss. Because this loss is caused by
reduction in component efficiencies, replacement of
affected parts are recommended during inspection
intervals.

19.  In Abu Dhabi, Ambient temperature is much
higher than design value of 150C.
 Plant energetic and exergetic efficiencies could
increase by 20% and 12% by decreasing air
temperature by 10oC.
 Wet air compression technology increases power,
reduces NOx emissions, and improves heat rate
and is not ambient temperature dependent.

20. If the fuel consists only of hydrocarbons
with no inert gases and no oxygen atoms,
work output increases as LHV increases
Heating fuel will result in higher turbine
efficiency due to the reduced fuel flow
required to raise the total gas temperature
to firing temperature.
But reducing fuel flow will have slight
decrease in turbine output.

21. Energy and Exergy efficiencies increase to
maximum value as compressor pressure
ratio (rp) increases.
However, increasing the compressor
pressure ratio has less improvement when
it is over 12.5.
Maximum Compressor pressure ratio
should be estimated.

22. Higher temperature at the exit of
the combustor increases the
output of the power plant.
The constraint is the
metallurgical thermal limitations
of the turbine.

23.  Minimize the use of excess air by appropriate
process control system but continuous
measurement of both O2 and CO leaving the
combustor provides also information needed for
effective combustion for significant benefits in
energy savings.
 Results indicate that the thermal efficiency of a
gas turbine will increase by about 2.4% when the
air to fuel ratio decreases from 50 to 30.