We were discussing “

*Rankine cycle*” in our previous post, where we have seen the various components of Rankine cycle and its basic operations also. We have also discussed the “*Rankine cycle with reheat*”.
Today we will see here the basic concept of regeneration
in Rankine cycle process with the help of this post. First we will see here the
few basic concepts and terms and after that we will see the very important concept
i.e. regeneration in Rankine cycle in this post.

First we will draw here a simple Rankine cycle and after that we will try to find here the method to increase the efficiency of a Rankine cycle. Let us see here the following figure, where a simple Rankine cycle is displayed.

As we can see here that heat energy will be added
during the process 4 to 1 and heat energy will be rejected during the process 2
to 3. So let us see the efficiency of simple rankine cycle. Efficiency of a
simple Rankine cycle will be calculated by following formula.

*η= 1-[Temperature of heat rejection / Temperature of heat addition]*

Temperature of heat rejection i.e. T

_{2}will be constant because either heat energy will be rejected in to atmosphere or heat energy will be rejected at temperature lower than the atmospheric temperature but temperature of heat rejection will be constant throught out the heat rejection process.
We can also think to increase the efficiency of the
rankine cycle by decreasing the temperature of heat rejection but we must note
it here that temperature of heat rejection i.e. T

_{2}could not be decreased below a specific level.
Let us see the temperature of heat addition, as we
can see that heat energy will be added during the process 4-1 and therefore
only partial heat energy will be added at constant temperature and rest heat
energy will be added at varying temperature. So we will have one term known as
mean temperature of heat addition i.e. T

_{m1}.
Mean temperature of heat addition i.e. T

_{m1}will be termed as a constant temperature, located between T_{1}and T_{4}, 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-[T*

_{2}/ T_{m1}]
Now in order to increase the efficiency of the Rankine
cycle , we will have to increase the value of mean temperature of heat addition
i.e. T

_{m1}. We can increase the value of of mean temperature of heat addition i.e. T_{m}by increasing the maximum temperature of the Rankine cycle i.e. T_{1}.
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 T

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 T

_{1}of the Rankine cycle beyond this maximum allowable temperature.
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.

###
**Concept
of regeneration in rankine cycle**

So in order to increase the efficiency of the
rankine cycle, we will require to increase the mean temperature of heat
addition and it could be increased by increasing the amount of heat energy addition
at higher temperature.

As we can see the above Rankine cycle, considerable
quantity of heat energy will be added to the working fluid during its liquid
phase or during sensible heating or during subcooled region. Only less part of
heat energy addition will be added at maximum temperature i.e. at T

_{1}.
If we want to increase the efficiency of the cycle,
we must be aware that all heat energy must be supplied at maximum temperature
of the cycle i.e. at temperature T

_{1}in this cycle and hence we will have to think the method by which we can permit the feed water to enter in to the boiler at state 5 so that all heat energy supplied by boiler to the working fluid will be carried out at maximum temperature of the rankine cycle i.e. T_{1}in this case.So if we can use the heat energy of the high temperature steam, which is flowing through the turbine during the expansion process 1-2, to heat the feed water from 4 to 5 then all heat energy supplied by the boiler will be done at maximum temperature of the cycle.

Hence mean temperature of heat addition will be T

_{1}itself because all heat energy supplied by the boiler to the working fluid will be completed at maximum temperature T_{1}during process 5 to 1.
Therefore, feed water leaving the feed pump will be
circulated around the casing of the turbine in opposite direction of the
direction of steam flow during expansion process 1-2 in the turbine.

The basic concept of regenration in rankine cycle is
that we will have to make an arrangement to heat the feed water leaving the
feed pump by the high temperature steam flowing through the turbine during the
process 1-2.

So if feed water will reach to dry saturated liquid
state, by receiving heat energy from the hot steam flowing through turbine,
before entering to the boiler then all heat energy supplied by the boiler to
the working fluid will be done at maximum temperature of the cycle and hence
mean temperature of heat addition will be T

_{1}and therefore in that situation we will have higher efficiency of the rankine cycle.
If feed water could not reach to dry saturated
liquid state but also it is reaching at a point between 4 and 5, by receiving
heat energy from the hot steam flowing through turbine, before entering to the
boiler then all heat energy supplied by the boiler to the working fluid will
not be done at maximum temperature of the cycle.

But in that situation also we
will have higher efficiency of the rankine cycle because we will have higher
mean temperature of heat addition as compared to the simple rankine cycle
without regeneration concept.

So this is the concept of regeneration in Rankine
cycle, we will see another topic i.e. rankine cycle with regeneration

Do you have any suggestions? Please write in comment
box.

###
**Reference:**

Engineering thermodynamics by P. K. Nag

Engineering thermodynamics by Prof S. K. Som

Image courtesy: Google

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