We were discussing “The basic concept of Rankine cycle in a steam power unit ” in our recent post, where we have discussed the
various components of steam power cycle and their basic operation too.
As we know that largest source of production of
electric power in the world is steam power plants and these steam power plants
work on the principle of Rankine cycle or we can say that steam power units are
designed to work on Rankine cycle.
If we can try to figure out the methods for
increasing the efficiency of Rankine cycle then we will be able to reduce the
consumption of fuels or in other words we can say that we will have more work
output from the cycle.
Therefore let us see here the various methods by which we can increase the efficiency of the Rankine cycle
Let us first draw here one simple Rankine cycle to
refresh the basic concept of heat addition process and heat rejection process
in simple Rankine cycle and then we will go ahead to understand the various
methods for increasing the efficiency of the Rankine cycle on which steam power
units are based.
As we have studied in our previous post that heat
energy will be added in simple Rankine cycle during the process of 4-1 and heat
energy will be rejected during the process of 2-3. Let us recall the Carnot cycle,
we have discussed there that heat energy will be added isothermally and heat
energy will be rejected isothermally too. Here in simple Rankine cycle, heat
energy will be rejected isothermally but heat energy will not be added to the
working fluid isothermally.
We can see from simple Rankine cycle that heat
energy will be added partially at constant temperature and rest of heat energy
will be added to the working fluid at varying temperature and this is the main
deviation of Rankine cycle with the concept of Carnot cycle.
Temperature at which heat energy will be
rejected could be lowered by lowering the operating pressure of condenser. Thermal
efficiency of the Rankine cycle, as we have discussed earlier, will be
increased by reducing the temperature of heat energy rejection.
Hence we will have lower efficiency for a Rankine
cycle as compared with the Carnot cycle efficiency. So we will have one term
known as mean temperature of heat addition i.e. Tm1.
Mean temperature of heat addition i.e. Tm1 will be termed as a constant
temperature, located between T1 and
T4, at which if same quantity of heat energy will be added then we
will have same change in entropy as we were having changes in entropy during
the process 4-1.
Therefore, Efficiency of a simple rankine cycle will
be calculated by following formula.
η=
1-[T2 / Tm1]
Now we will have basic concept behind each
modification for enhancing the efficiency of a steam power plant. There will be
basically two basic concepts from above formula of efficiency of a steam power
cycle.
First concept, we will have to increase the mean
temperature of heat addition i.e. Tm1. Second concept, we will have
to decrease the temperature of heat rejection i.e. T2.
There is one more way to increase the thermal
efficiency of the Rankine cycle by increasing the boiler pressure.
Now we will discuss these three basic concepts, with
the help of this post, for increasing the thermal efficiency of a steam power
cycle or Rankine cycle.
Lowering
the condenser pressure and temperature of heat rejection
Temperature at which heat energy will be
rejected could be lowered by lowering the operating pressure of condenser. Thermal
efficiency of the Rankine cycle, as we have discussed earlier, will be
increased by reducing the temperature of heat energy rejection.
We can see here, in following TS plane, the effect
of lowering the condenser pressure on cycle efficiency. We have maintained temperature
of the working fluid constant i.e. T1 at the inlet of the turbine in
order to determine the effect of lowering the condenser pressure on cycle
efficiency.
We have reduced the condenser pressure from P2
to P2’ and we can easily observe here in TS plane the increase in
net work output from the Rankine cycle. We have displayed the increment in net
work output of the cycle by the colored area.
Heat energy input will also be increased as a result
of lowering the condenser pressure but this increment in heat energy input will
be very small and final result will be increase in the efficiency of the cycle
as result of lowering the condenser pressure.
We can not reduce the operating pressure of condenser below the saturation pressure corresponding to the temperature of the
cooling medium and this pressure will be termed as lower limit on condenser
pressure and we will never reduce the condenser pressure below this lower
limit.
We have seen the positive point of lowering the
condenser pressure i.e. increases in thermal efficiency of the Rankine cycle. Now
we must note it here the negative points too from lowering the condenser pressure
in Rankine cycle.
As we can see that at the exit of the turbine,
working fluid will be at dry saturated vapour state at point 2 at condenser pressure
P2. Now let us see, for same inlet temperature of the working fluid i.e.
T1 at the inlet of the turbine, the quality of steam at the final
stage of the turbine after reducing the condenser pressure to P2’.
We can see that quality of working fluid is reduced
after reducing the condenser pressure to P2’. Hence we can say that
lowering the condenser pressure will increase the moisture content in steam at
the final stage of the turbine. This increased moisture content will surely
deteriorate the turbine blade by the process of corrosion and hence efficiency
of the turbine will be reduced.
Superheating the steam to high temperature
As we have discussed earlier that we can increase
the thermal efficiency of the steam power cycle by increasing the mean temperature
of heat energy addition. Mean temperature of heat energy addition could be
increased by increasing the temperature of the steam by heating the working
fluid to a high degree of superheat.
We can see the effect of heating the steam to a high
degree of superheat over the efficiency of the steam power cycle i.e. Rankine
cycle in following figure.
Heat energy input and net work from the cycle will
be increased as a result of increasing the temperature of the steam by heating
the working fluid to a high degree of superheat. Overall effect will be
increase in the thermal efficiency of the Rankine cycle as a result of increasing
the temperature of the steam by heating the working fluid to a high degree of
superheat.
We will have one more positive result that moisture
content in the steam will be reduced at the final stage of the turbine as a
result of increasing the temperature of the steam by heating the working fluid
to a high degree of superheat.
We must note it here that temperature of heat
addition could be increased up to a limit only as it will be restricted by
various practical parameters such as material properties of turbine blades.
Turbine blade material will not work satisfactory if we increase the maximum
temperature of Rankine cycle i.e. T1 above a certain level and that level of
temperature will be termed as maximum allowable temperature and we could not
increase the temperature T1 of the Rankine cycle beyond this maximum
allowable temperature.
Increasing the boiler pressure
We can also increase the boiler pressure in order to
increase the efficiency of the Rankine cycle because range of temperature for
heat energy addition will be increased.
We can see the effect of increasing the boiler
pressure over the efficiency of the steam power cycle i.e. Rankine cycle in
following figure.
We have maintained temperature of the working fluid constant
i.e. T1 at the inlet of the turbine in order to determine the effect
of increasing the boiler pressure over the efficiency of the steam power cycle.
We can see that content of the moisture in the steam
will be increased as a result of increasing the boiler pressure and therefore this
increased moisture content will surely deteriorate the turbine blade by the
process of corrosion and hence efficiency of the turbine will be reduced.
We will see another topic “Difference between Rankine cycle and Carnot Cycle” in our next post.
Do you have any suggestions? Please write in comment
box.
References
Thermodynamics an engineering approach by Y. A. Çengel and M. A. Boles
Engineering thermodynamics by P. K.
Nag
No comments:
Post a Comment