Analysis of Organic Rankine Cycle

Assignment I Name: xxxxxx Student ID: xxxxxxx
Part 1: Analysis of organic Rankine cycle
AIM
The main objective of this report to get information about organic rankine cycle, its
components, process, operations used in it.
Introduction
The thermodynamic system generally studied to get information about heat engine cycles .Heat
engine is the device that convert heat into work and part of heat into surroundings .It work on
the principle of 1st and 2nd laws of thermodynamics which states that “energy can neither be
created not be destroyed but it can be transferred into another form” and “Heat energy can’t
completely converted to work as there is some heat loss in surroundings .The thermodynamic
cycle is a continuation process under which on which heat engine work .This cycle takes heat
from source and converted to work .Rankine cycle and organic rankine cycles both are
thermodynamic heat engine cycle in which steam or organic fluid is heated to vapors state then
expands in the turbine .Here turbine or expander is work generating device and boiler or
evaporator is heat consuming device .
BASIC RANKINE CYCLE
It is the thermodynamic cycle that is used to convert heat into work .It is close loop cycle that
uses water (steam) as the working fluid .
Figure 1.1 ( Steam rankine cycle)
Working cycle of rankine cycle shown above .There are four major components of cycle. Boiler
,steam turbine , condenser and pump .In boiler , high pressure water from pump is converted to
superheated steam having high enthalpy .Boiler containing multiple tubes from where water
flows , heated is produced in boiler by using coal ,oil or natural gas .Superheated steam then
passes through steam turbine which rotates the turbine cause reduce in enthalpy of steam .After
turbine ,mixture of water and steam enters into condenser in which steam is completely
condensed . Condensation process of steam is done to pump the condensate back to boiler as
pumping of steam is more difficult than pumping water .Then again water enters in boiler and
this cycle continues .Power output obtained from turbine work which further used for electricity
generation in generator .Heat is supplied to boiler for steam generation .Some part of turbine
work is used to power the pump (Huber, McLinden , 2013) .
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Assignment I Name: xxxxxx Student ID: xxxxxxx
Figure 1.2 ( Temperature –Entropy Rankine cycle)
ORGANIC RANKINE CYCLE
Steam rankine cycle is uneconomical to use because it need source temperature above 400
Deg C and very low pressure requirements due to which size of components becomes very
large and system efficiency becomes very less . Steam rankine cycle upgraded to organic
rankine cycle which can be used for lower temperature range in small scale .Organic rankine
cycle was developed by two physicist Lucien Bronicki and Harry Zvi Tabor in late 1950. Lots
of scientist tried experiment on organic rankine cycle and obtain superior results from using
heat input from biomass with recuprator system .It is also considered that efficiency of organic
rankine cycle is high in winters as compared to from summers .This is because in summers ,
the temperature of working fluid also high , so working fluid having less heat carrying
capacity but during winters , working fluid temperature is less , due to which difference
between heat source temperature and working fluid becomes high therefore fluid having more
heat carrying capacity , this increase the efficiency of cycle .This phenomena works on daily
basis as well , during the day the temperature of atmosphere increases during sunrise due to
which temperature of fluid also increases which reduce the efficiency but at sunset time
temperature of atmosphere again decreases due to which heat carrying capacity of fluid
increases so efficiency also increases .The efficiency of organic rankine cycle is highly depend
on the effectiveness of heat exchanger used in it .As fluid exit from the expander have some
heat left in it , this heat can be utilized to heat the working fluid enters in the evaporator .More
will be the heat transfer occurs therefore less head need to be provided to working fluid in
evaporator therefore for same power output , heat input becomes less so efficiency improves .
Organic rankine cycle use geothermal, biomass and industrial waste heat as the heat source to
evaporate the working fluid .Working fluid for this cycle are organic fluids which are better
than steam for working in low temperature (Macián, Sánchez, 2013). The working fluid for
organic ranking cycle are organic fluids like R-245fa ,R-134a .Although process for organic
rankine cycle is similar to ordinary stream rankine cycle but some components are changed.
There are six major components in this cycle. Evaporator ,Expander , condenser , recuperator,
storage tank and pump (Broek, Dewallef, 2013).Organic rankine cycle can also use dry fluid
coupled with regenerator to increase the cycle efficiency .As dry fluids do not form liquid
vapour mixture (2 phase) after expansion in expander , due to this temperature of dry fluid is
comparatively high at exit of expander from other working fluid used which help the liquid
fluid before entering in evaporator .So a heat exchanger system should be assembled between
condenser and expander to exchange heat between hot vapour exit to expander and liquid
enter in evaporator and hence increase the efficiency .The ORC can be widely used in small
scale power plants ,industrial system and furnaces where waste heat recovery need to be
done .
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Assignment I Name: xxxxxx Student ID: xxxxxxx
Model of ORC
Figure 1.3 ( Organic Rankine cycle (Orosz, Hemond, ,2009))
The organic rankine cycle is upgradation of steam rankine cycle as the boiler is replaced by
evaporator, high pressure organic fluid from pump is converted to vapours having high enthalpy
.Evaporator is act as heat exchanger where geothermal, biomass and industrial waste heat is
extracted by organic fluid flowing through multiple tubes in evaporator. Vapours of organic
fluid then passes through expander (turbine) which rotates the turbine cause reduce in enthalpy
of organic fluid .After expander , partial mixture of liquid and vapor enters into recuperator
where heat is exchanged with pumped organic fluid and rest of mixture of liquid and vapor
enters in condenser where mixture of liquid and vapor are completely liquefy
(Quoilin ,Lebrun2010) .Condensation process is done to pump the condensate back to
evaporator as pumping of vapor is more difficult than pumping of liquid .Then again organic
fluid enters in evaporator and this cycle continues .Power output obtained from expander work
which further used for electricity generation in generator .Heat is supplied to evaporator for
vapor generation .Part of expander work is used to power the pump. Storage tank contains
working fluid which is used to recover any loss in working fluid due to frequent evaporations
(Oralli, E. 2010).The ORC cycle is more efficient than ordinary steam rankine but cycle
efficiency can be further improved by considering the superheating and sub cooling
phenomena .In superheating , the working fluid is heating far above it vaporization point so that
it may get extra heat before entering to turbine .In sub cooling ,after expansion in turbine the
working fluid is cooled below its limit so that it may easily gain the heat .
Figure 1.4 (Temperature –Entropy Organic Rankine cycle)
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