-
rm
Pow
(CSth
omiinel ef
variation in optimal turbine inlet pressure with turbine inlet
temperature, design radiation, plant size,
tively. The optimum pressure is observed to be a weak function
of design solar radiation. The overall
is onlogiesainly forough
ically viable option (Zarza et al., 2002).The second most
installed CSP technology after PTC is SPT
(Zhang et al., 2013). SPT plant uses DSG (Mller-Steinhagen
andTrieb, 2014) or molten salt as HTF (Caceres et al., 2013).
Franchiniet al. (2013) have presented the comparative analysis of
CSP
with PTC and LFRleast applied CSP).e the stability andusing
molten saltsalt and quartzitee materials (Rogetd in literature.
A
detailed review on thermal energy storage technologies for
CSPplants have been presented by Kuravi et al. (2013) as well as
Tianand Zhao (2013). Dynamic simulation model with thermal
energystorage has also been developed by Llorente Garca et al.
(2011).
Selection of type and size of solar eld, power cycle
parameters,and sizing of power block are the most important aspects
indesigning a CSP plant. Several studies on optimization of
differentparameters for PTC based CSP plant are reported. Economic
opti-mization of design radiation, the direct normal irradiance
(DNI) at
* Corresponding author. Tel.: 91 22 25767894; fax: 91 22
25726875.
Contents lists availab
Journal of Clean
els
Journal of Cleaner Production 89 (2015) 262e271E-mail address:
[email protected] (S. Bandyopadhyay).based heat transfer uid
(HTF), is the most established andcommercially attractive
technology (Purohit et al., 2013). In such aplant, the temperature
limit is about 400 C with a resulting steamtemperature, at turbine
inlet, of about 370 C (Al-Soud andHrayshat, 2009). However, if
molten salt is used as a workinguid then the steam temperature up
to 540 C is achievable, whichmay lead to higher steam turbine
efciency (Zaversky et al., 2013).Direct steam generation (DSG) in
the PTC eld is also an econom-
sented the comparative analysis of CSP plantstechnologies. A
paraboloid dish system is thetechnology for power generation
(Sharma, 2011
Heat storage is an important option to improvreliability for a
CSP plant. Analysis of CSP plant(Manenti and Ravaghi-Ardebili,
2013), moltenrock (Flueckiger et al., 2014), and phase changet al.,
2013) based storage have been reporteFresnel reector (LFR), solar
power tower (SPT) and paraboloiddish. Among these technologies, PTC
with synthetic or organic oil
has a lower optical efciency compared to PTC eld (Zhu et
al.,2014). Giostri et al. (2012a) and Morin et al. (2012) have
pre-OptimizationCost of energyTurbine inlet pressureEfciency
1. Introduction
Concentrating solar power (CSP)among renewable energy
technoBandyopadhyay, 2013). There are mable CSP technologies:
parabolic
thttp://dx.doi.org/10.1016/j.jclepro.2014.10.0970959-6526/ 2014
Elsevier Ltd. All rights reserved.efciency increases and levelized
cost of energy decreases with increase in turbine inlet
temperature,plant size and various modications of the Rankine
cycle.
2014 Elsevier Ltd. All rights reserved.
e of the viable options(Krishna Priya and
ur commercially avail-collector (PTC), linear
plants with PTC and SPT technologies. A detailed review on
helio-stat layout design (Collado and Guallar, 2013), central
receiverdesign (Behar et al., 2013), and SPT technology based CSP
plants (Hoand Iverson, 2014) have been reported in literature. LFR
eld withDSG has been proposed as a cheaper alternative because of
atmirrors and structural advantages (Nixon et al., 2013). However,
itConcentrating solar powerParabolic trough collectorKeywords:and
various modications of Rankine cycle are also analyzed. Energy and
cost optimal turbine inletpressures for 1 MWe plant (with basic
Rankine cycle) are about 4.5e7.5 MPa and 3.5e7.5 MPa,
respec-Optimization of concentrating solar theparabolic trough
collector
Nishith B. Desai, Santanu Bandyopadhyay*
Department of Energy Science and Engineering, Indian Institute
of Technology Bombay,
a r t i c l e i n f o
Article history:Received 21 May 2014Received in revised form31
October 2014Accepted 31 October 2014Available online 7 November
2014
a b s t r a c t
Concentrating solar powerbased heat transfer uid isextensive
energy and econturbine inlet pressure, turbof Rankine cycle on
overal
journal homepage: www.al power plant based on
ai, Mumbai 400 076, India
P) plant with parabolic trough collector (PTC) using synthetic
or organic oile most established and commercially attractive
technology. In this paper,c analysis of PTC based CSP plants,
without storage, are reported. Effects ofinlet temperature, design
radiation, plant size, and various modicationsciency as well as
levelized cost of energy are studied. Furthermore, the
le at ScienceDirect
er Production
evier .com/locate/ jc lepro
-
hx heat exchanger
of Cwhich plant produces the rated power output, has been
presentedby Montes et al. (2009). Effects of design radiation on
capacityfactor and dumped energy, for a PTC based CSP plant without
hy-bridization and thermal storage, have been demonstrated by
Nomenclature
Ap aperture area of the collector (m2)C Cost ($)d discount rateE
annual electricity generation (kWh/y)h specic enthalpy (J/kg)I
aperture effective direct normal irradiance (W/m2)m mass ow rate
(kg/s)n life time (y)P power (W)Pr pressure (MPa)Q heat owrate (W)T
temperature (C)Ul heat loss coefcient based on aperture area
(W/
(m2$K))x dryness fraction
Greek symbolsD differenceh efciencyq incidence angle ()
AbbreviationsCSP concentrating solar powerDNI direct normal
irradianceDSG direct steam generation
N.B. Desai, S. Bandyopadhyay / JournalSundaray and Kandpal
(2013). Recently, Desai et al. (2014) reporteda methodology to
determine the optimum design radiation for CSPplant without
hybridization and thermal storage.
Garca-Barberena et al. (2012) have evaluated different
opera-tional strategies using SimulCET computer program. Reddy
andKumar (2012) have presented modeling of PTC eld as well
asfeasibility study of stand-alone PTC based CSP plant with HTF
andDSG for various places in India. Kumar and Reddy (2012)
havecarried out energy, exergy, environmental, and economic
analysesof stand-alone DSG based CSP plant of different sizes.
Giostri et al.(2012b) have compared the PTC based CSP plants using
conven-tional HTF, molten salt, DSG, DSG-HTF, and DSG-molten salt
asworking uid and reported annual overall efciency of
15.3%,16.2%,17.9%, 16%, and 17.8%, respectively. Probabilistic
modeling of PTCbased CSP plant has also been reported by Zaversky
et al. (2012).
Conventional steam Rankine cycle is the most widely used po-wer
generating cycle in CSP plants. Many researcher have evaluatedthe
performance of steam Rankine cycle in PTC based CSP plants(e.g.,
Manzolini et al., 2011; Desai et al., 2013). Fernandez-Garcaet al.
(2010) have presented a survey of CSP plants with steamRankine
cycle for power generation. Kibaara et al. (2012) haveanalyzed the
dry and wet cooled steam Rankine cycle based CSPplants and
concluded that in case of a dry cooled plant, compared toa wet
cooled plant, the capital cost and the levelized cost of
energy(LCOE) are increased by 5% and 15%, respectively. Reddy et
al. (2012)have reported increase in energetic and exergetic
efciencies by1.49% and 1.51% with increase in turbine inlet
pressure from 90 barto 105 bar, respectively. It may be noted that,
the dryness fraction ofsteam at the outlet of low pressure turbine
(LPT) decreases withincrease in turbine inlet pressure.
Subsequently, the isentropic ef-ciency of the LPT also decreases.
However, the isentropicefciency of turbine has been kept constant
during the analysis(Reddy et al., 2012). Al-Sulaiman (2013) has
presented energyanalysis of a typical 50 MWe PTC based CSP plant
using a steamRankine cycle as well as with steam Rankine cycle as a
topping cycle
in inletis isentropicm meanmax maximummin minimumo
opticalO&M operation and maintenanceopt optimumout outletth
thermodynamicu usefulHTF heat transfer uidLCOE levelized cost of
energyLFR linear Fresnel reectorLPT low pressure turbinePTC
parabolic trough collectorSPT solar power towerTAC total annualized
cost
Subscriptsa ambientAR annual replacementCL collectorD designHTF
heat transfer uid
leaner Production 89 (2015) 262e271 263and an organic Rankine
cycle as a bottoming cycle. The effects ofdifferent design
parameters on the size of solar eld have beenstudied.
In this paper, extensive energy and economic analysis of a
PTCbased CSP plant, without storage, is carried out. Effects of
turbineinlet pressure, turbine inlet temperature, design radiation,
plantsize, and various modications of Rankine cycle on overall
ef-ciency as well as LCOE are studied. Variations in turbine
isentropicefciency with turbine inlet pressure, temperature and
mass owrate as well as dryness fraction at the outlet of turbine
are modeledappropriately in this paper. There is no such analysis
reported in theliterature. The analysis is useful for sizing of
solar eld, sizing ofpower block and deciding power cycle
parameters.
2. Effect of turbine inlet pressure on overall efciency
andlevelized cost of energy
Simplied schematic of a PTC based CSP plant is shown in Fig.
1.PTC eld heats HTF to a high temperature using concentrated
solarradiation (from state 1 to state 2) and then high temperature
HTF isfed into a heat exchanger to produce steam (from state 4 to
state 5).The cold HTF coming out of heat exchanger (state 3) is
re-circulatedback into the PTC eld using HTF pump. The high
temperature andhigh pressure steam is used to generate power
through a conven-tional steam turbine (from state 5 to state 6).
Finally, steam fromthe turbine exhaust is condensed in a condenser
(from state 6 tostate 7). The collector eld useful heat gain (Qu)
and collector ef-ciency (hCL) are given by,
Qu mHTF$h2 h1 hCL$I$Ap (1)
-
of ChCL ho Ul$Tm Ta
I
(2)
where ho is optical efciency of collector eld, Ul is heat loss
co-efcient based on aperture area of collector eld (W/(m2$K)), Tmis
mean temperature of collector eld (C), Ta is ambient temper-ature
(C), I is the DNI corrected by cosine of incidence angle
(i.e.,DNI$cos q) which is also known as aperture effective DNI
(Feldhoffet al., 2012),mHTF is mass ow rate of HTF (kg/s), Ap is
aperture areaof collector eld (m2), hi and Ti are specic enthalpy
and temper-ature at i-th state point.
Denoting the difference between Tm and Ta as DT,
hCL ho Ul$DTI
and hCL;D ho Ul$
DTID
(3)
where hCL,D and ID are collector efciency and aperture effective
DNIat design condition. Neglecting heat losses through pipes and
po-wer input to pumps,
Qu mHTF$h2 h1 m$h5 h7 (4)
where m is mass ow rate of steam (kg/s).From Equation (1) and
Equation (4).
m$h5 h7 hCL$I$Ap (5)Substituting the collector efciency from
Equation (3) in Equa-
tion (5),
m$Dh h Ul$DT
$I$A (6)
Fig. 1. Simplied schematic of a PTC based CSP plant.
N.B. Desai, S. Bandyopadhyay / Journal264o I p
mAp
ho$I Ul$DTDh
andmDAp
ho$ID Ul$DTDh
(7)
where mD is steam mass ow rate at design condition, Dh is
thedifference between h5 and h7 (see Fig. 1).
The aperture specic design power output can be calculatedfrom
the relation given below,
PDAp
mDAp
$Dhis$his;D (8)
where Dhis is the isentropic enthalpy change in the turbine and
his,Dis the isentropic efciency of the turbine at design condition.
FromEquation (7) and Equation (8),PDAp
ho$ID Ul$DT$Dhis$his;DDh
(9)
and the aperture specic power output at any aperture
effectiveDNI (I) can be given as,
PAp
ho$I Ul$DT$Dhis$hisDh
(10)
The turbine isentropic efciency can be calculated usingfollowing
correlation (Mavromatis and Kokossis, 1998),
his
65$B
$
1 A
Dhis$mD
$1 mD
6$m
(11)
At design condition (mmD), the turbine isentropic efciency
isgiven as follows:
his;D 1B
$
1 A
Dhis$mD
(12)
where A and B are the isentropic efciency parameters, depend
onturbine inlet pressure and turbine size.
A a1 a2$Tsat;in (13)
B b1 b2$Tsat;in (14)
where a1, a2, b1, b2 are turbine regression coefcients, and
Tsat,in isturbine inlet saturation pressure. Values of these
coefcients arereported by Mavromatis and Kokossis (1998) for back
pressureturbines and by Shang (2000) for condensing turbines.
The net power output is calculated by subtracting power inputto
pumps from turbine output. Therefore, the aperture specic netpower
output at design condition can be calculated as,
Pnet;DAp
PD PHTF PFeedWaterAp
(15)
The overall efciency (solar to electric energy efciency)
atdesign condition is given as follows:
hoverall;D Pnet;DID$Ap
(16)
The annualized cost (CAnnual) and levelized cost of energy
(LCOE)can be calculated as,
CAnnual$=y CCapital$d$1 dn1 dn 1 (17)
LCOE $=kWh P
CAnnual CO&M CAREAnnual
(18)
where d is discount rate, n is lifetime (y), CO&M is annual
operationand maintenance cost ($/y), CAR is annual component
replacementcost ($/y), and EAnnual is annual electricity generation
(kWh/y).
From Equation (9) it may be noted that, the aperture
specicdesign power output is directly proportional to the product
ofenthalpy difference ratio and isentropic efciency of the turbine
atdesign condition (i.e., (Dhis/Dh)$his,D). Typical T-h diagrams
for a PTCbased CSP plant for two different turbine inlet pressures
are shownin Fig. 2. It should be noted that the enthalpy difference
ratio (i.e.,Dhis/Dh) increases with increase in turbine inlet
pressure. On the
leaner Production 89 (2015) 262e271other hand, the dryness
fraction at outlet of the turbine decreaseswith increase in turbine
inlet pressure (see Fig. 2), resulting in
-
used for simulations and the results are shown in Fig. 4. It may
be
lant for two different values of turbine inlet pressure.
N.B. Desai, S. Bandyopadhyay / Journal of Cleaner Production 89
(2015) 262e271 265lower turbine isentropic efciency. Typical
variation in the productof enthalpy difference ratio and isentropic
efciency of the turbineat design condition, as a function of
turbine inlet pressure, is shownin Fig. 3. It may also be noted
that, the power input to HTF pumpand feed water pump increases with
increase in turbine inletpressure. Moreover, the heat exchanger
outlet temperature in-creases with increase in turbine inlet
pressure (see Fig. 2) and thecollector outlet temperature is
typically kept constant. This leads toincrease in the mean
collector temperature difference (DT) andsubsequently the collector
eld efciency decreases. This justiesthe existence of a
thermodynamically optimal turbine inlet pres-sure, for which the
net design power output is the maximum.Consequently, at that
pressure the overall efciency at design
Fig. 2. Typical T-h diagrams for a PTC based CSP pcondition is
also the maximum (Equation (16)).For demonstration the simulations
are carried out using Engi-
neering Equation Solver (Klein, 2004). The data given in Table 1
are
Fig. 3. Typical variation in the product of enthalpy difference
ratio and isentropic ef-ciency of the turbine at design condition
((Dhis/Dh)$his,D) as a function of turbine inletpressure.observed
that the optimal turbine inlet pressure is about 7.5 MPa. Itmay
also be noted from Fig. 4 that the nature of the net designpower
output curve is not very sharp near the maximum. The po-wer output
remains within 1% of the maximum, for the turbineinlet pressure
range 4.5e11 MPa. The total annualized cost per unitaperture area
of the collector increases with increase in pressure.This implies
that the cost optimal turbine inlet pressure should bealways lesser
than the thermodynamically optimum. Therefore, thethermodynamically
optimal turbine inlet pressure is about4.5e7.5 MPa. It may also be
noted that, the aperture specic netTable 1Data used for the
simulation.
Input Parameter Value/Type Reference
Collector eld Parabolic Trough Collector (PTC) eCollector eld
efciency
model parametersho 0.7; Ul 0.1 W/(m2$K) Desai
et al. (2013)Collector tracking mode Focal axis NeS
horizontal
and EeW trackinge
Collector eld HTF Therminol VP-1 eCollector outlet
temperature390 C (controlled) e
Ambient temperature 30 C (design value) eTurbine isentropic
efciencymodel parameters
A a1 (a2$Tin,sat);B b1 (b2$Tin,sat)For turbine size 1.5 MW:a1
0.0981 (MW);a2 0.001 (MW/C)b1 1.2059; b2 0.0006 (1/C)For turbine
size > 1.5 MW:a1 0.0376 (MW);a2 0.0014 (MW/C);b1 1.1718;b2
0.0003 (1/C)
Shang (2000)
Turn down ratio of theturbine (Pmin/Pmax)
0.2 e
Temperature drivingforce (DTmin)
10 C e
Isentropic efciencyof the pump
0.6 e
Condensing pressure 0.1 bar e
-
in Fig. 5. Results show that the cost optimal turbine inlet
pressure is
results in low capacity factor of the plant and very low design
ra-diation results in excessive unutilized energy (Desai et al.,
2014).Therefore, there exists an optimal design DNI for a CSP plant
whichminimizes the LCOE. Fig. 7 demonstrates the effect of design
radi-ation on LCOE as function of turbine inlet pressure. It may be
notedthat LCOE is the lowest for design radiation of 600 W/m2. It
mayalso be observed that, there is no signicant change in the
ther-modynamically as well as cost optimal turbine inlet pressure
withFig. 4. Aperture specic net design power output as function of
turbine inlet pressure
Table 3Financial parameters, operation and maintenance data for
economic analysis.
Operation and maintenanceAnnual solar eld component replacement
cost 2.5% of solar eld costAnnual operation and maintenance cost 4%
of equipment costFinancial parametersDiscount rate (%) 10Lifetime
(years) 30
N.B. Desai, S. Bandyopadhyay / Journal of Cleaner Production 89
(2015) 262e271266about 6 MPa which is lesser than the
thermodynamically optimalvalue (7.5 MPa), as explained. It may be
observed that for turbineinlet pressure within 3.5e10 MPa, the LCOE
remains within 1% ofthe maximumvalue. However, the higher pressure
is limited by thethermodynamically optimal value. Therefore, based
on the as-sumptions of equipment characteristic parameters and cost
data,the cost optimal turbine inlet pressure is about 3.5e7.5
MPa.
3. Effect of design radiation on overall efciency andlevelized
cost of energy
The effect of design radiation on aperture specic net
designpower output as function of turbine inlet pressure is shown
inFig. 6, which demonstrates that the power output increases
withincrease in design radiation. This is expected because higher
designdesign power output increases with increase in turbine inlet
tem-perature (Tmax). This is expected because higher turbine
inlettemperature increases the Rankine cycle efciency. However,
itsmaximum value is limited by the maximum HTF temperature.
The effect of turbine inlet pressure on LCOE is studied using
thecost data given in Table 2 and Table 3. DNI data for the
simulationsare taken from Ramaswamy et al. (2013) and the results
are shown
at different turbine inlet temperature.radiation decreases the
collector aperture area, for the xed designpower output
requirement. However, very high design radiation
is about 3.9% with regeneration (at Prcost,opt 8 MPa) compared
to
Table 2Equipment cost data for economic analysis.
Equipment Cost correlation
PTC eld and HTF system eTurbine a$kWbGenerator a$kWebCondenser
a$kWthbBoiler feed pump FP$a b$kW c$kW2
Heat exchanger a$Area0:65HX $Fc 2:29Fc Fd Fp$Fm
Civil works a$kWe b$kWe2Miscellaneous costLand and site
development cost
Parameters for cost correlations have been updated to 2014 using
Chemical EngineeringCSP plant without regeneration (at Prcost,opt 6
MPa).
Variable of cost correlation Reference
280 ($/m2) Krishnamurthy et al. (2012)a 31,093; b 0.41
Gutierrez-Arriaga et al. (2014)A 2447; b 0.49 Gutierrez-Arriaga et
al. (2014)A 597; b 0.68 Gutierrez-Arriaga et al. (2014)variation in
design radiation.
4. CSP plant with regenerative Rankine cycle
Regenerative feed-water heating is commonly used forincreasing
the thermal efciency of the steam Rankine cycle.Simplied schematic
of a PTC based CSP plant using regenerativeRankine cycle is shown
in Fig. 8. It should be noted that steam, atsome intermediate
pressure, is withdrawn from the turbine (state9). This is mixed
directly with feed water (at state 8) in a directcontact heater and
the resultant mixture (at state 10) is fed tosecond feed water
pump. The other state points are same asexplained earlier.
Variations of net design power output and LCOE as function
ofturbine inlet pressure, for basic and regenerative Rankine
cycles,are shown in Fig. 9. This gure demonstrates that the cycle
modi-cation increases the net design power output and decreases
theLCOE, as expected. In case of regenerative Rankine cycle, the
ther-modynamic and cost optimum range (for 1 MWe plant) is
about6.2e10MPa and 4.5e10MPa, respectively. It may also be noted
that,the thermodynamically optimal as well as cost optimal
turbineinlet pressure increases with regeneration. The increase in
aperturespecic net design power output is about 7.9% with single
regen-eration (at Prth,opt 10 MPa) compared to CSP plant
withoutregeneration (at Prth,opt 7.5 MPa). Moreover, the decrease
in LCOEa 6607; b 485;c 0.417; FP 2.12
Gutierrez-Arriaga et al. (2014)
a 533;Fd 1.35 (kettle type),0.85 (U-tube);Fm 1 (CS/CS
material);Fp 0.25 (pressure 2.5 MPa),0.52 (pressure 5.5 MPa),0.55
(pressure > 6.9 MPa)
Douglas (1988)
a 169; b 0.00053 Krishnamurthy et al. (2012)183 ($/kWe) IIT
Bombay (2012)20 ($/m2) IIT Bombay (2012)
Plant Cost Index.
-
Fig. 5. Levelized cost of energy as function of turbine inlet
pressure at different turbineinlet temperature.
Fig. 6. Aperture specic net design power output as function of
turbine inlet pressureat different design DNI.
Fig. 7. Levelized cost of energy as function of turbine inlet
pressure at different designDNI.
Fig. 8. Simplied schematic of a PTC based CSP plant using
regenerative Rankine cycle.
Fig. 9. Aperture specic net design power output and LCOE as
function of turbine inletpressure for basic and regenerative
Rankine cycles.
Fig. 10. Aperture specic net design power output as function of
turbine inlet pressurefor different plant size.
N.B. Desai, S. Bandyopadhyay / Journal of Cleaner Production 89
(2015) 262e271 267
-
Fig. 11. Aperture specic net design power output as function of
turbine inlet pressurefor PTC based CSP plant using molten salt as
HTF.
Fig. 12. Levelized cost of energy as function of turbine inlet
pressure for different plantsize.
Fig. 13. Simplied schematic of a PTC based
N.B. Desai, S. Bandyopadhyay / Journal of Cleaner Production 89
(2015) 262e2712685. Effect of plant size on overall efciency and
levelized costof energy
Isentropic efciency of the turbine increases with the size
ofturbine. The effects of plant size and resulting higher
isentropicefciency of the turbine on aperture specic net design
poweroutput are shown in Fig. 10, which demonstrates that the
poweroutput increases with increase in plant size. It may also be
notedthat the optimal turbine inlet pressure increases with
increase inplant size. Increase in turbine inlet pressure increases
the overallthermal efciency of the Rankine cycle, and
simultaneously, in-creases the moisture content of steam, at the
outlet of the turbine,to an unacceptable level (see Fig. 2).
Typically, the minimum dry-ness fraction at the outlet of a turbine
is kept around 80e90%. Tosatisfy theminimum dryness fraction at
outlet of turbine, the steamat inlet of turbine should be
superheated to high temperature.However, in a PTC based CSP plant
with synthetic or organic oil
Fig. 14. Aperture specic net design power outputs as function of
turbine inlet pres-sure for different plant size with reheat
Rankine cycle.based HTF, the temperature limit is about 400 C with
a resultingsteam temperature at the turbine inlet about 350e370 C.
There-fore, the optimal turbine inlet pressure is limited by the
minimumdryness fraction at outlet of turbine (a limit for 88%
dryness fractionis shown in Fig. 10).
CSP plant using reheat Rankine cycle.
-
Fig. 15. Levelized cost of energy as function of turbine inlet
pressure for different plantsize with reheat Rankine cycle.
Fig. 17. Aperture specic net design power output as function of
turbine inlet pressurefor different plant size with
reheat-regenerative Rankine cycle.
N.B. Desai, S. Bandyopadhyay / Journal of Cleaner Production 89
(2015) 262e271 269With the use of molten salt as HTF, the steam
temperature (Tmax)up to 540 C is achievable. As a result, higher
steam turbine ef-ciency, higher dryness fraction at the outlet of
turbine, and lowerLCOE can be achieved. Fig. 11 shows the variation
of aperture spe-cic net design power output as function of turbine
inlet pressure,for PTC based CSP plant using molten salt as HTF. It
may beobserved that, the thermodynamically optimal turbine inlet
pres-sure increases with the plant size and typical operating
pressure(about 18 MPa) of a conventional steam power plant can
beachieved.
The effect of plant size on LCOE is shown in Fig. 12; LCOE
de-creases with increase in plant size. The cost optimal turbine
inletpressure also increases with plant size. Typically, in larger
plantsthe steam is expanded in two stages, and reheating the steam
inbetween these two stages of the turbine helps in achieving
theminimum desirable dryness fraction at the outlet of the last
stage ofthe turbine.Fig. 16. Simplied schematic of a PTC based CSP
plant using reheat-regenerative Rankine cycle.
Fig. 18. Levelized cost of energy as function of turbine inlet
pressure for different plantsize with reheat-regenerative Rankine
cycle.
-
schematic of a PTC based CSP plant using
reheat-regenerativeRankine cycle is shown in Fig. 16. All the state
points are same asexplained earlier.
Fig. 17 demonstrates the variation of aperture specic netdesign
power output as function of turbine inlet pressure, fordifferent
sizes of the plant. It may be noted that the thermody-namically
optimal turbine inlet pressures for 5 MWe, 25 MWeand 50 MWe plants
are 9e13 MPa, 10.5e15 MPa and10.5e15 MPa, respectively. Fig. 18
illustrates the variation of LCOEas function of turbine inlet
pressure, for different sizes of theplant. The cost optimal turbine
inlet pressures for 5 MWe,25 MWe and 50 MWe plants are 7e13 MPa,
9e15 MPa and
temperature, plant size, and various modications of Rankine
cycle.
N.B. Desai, S. Bandyopadhyay / Journal of Cleaner Production 89
(2015) 262e2712705.1. CSP plant with reheat Rankine cycle
Concentrating solar power plant with reheat Rankine cycle
cantake the advantage of increased efciency as well as it avoids
thelow-quality steam at turbine exhaust. Simplied schematic of a
PTCbased CSP plant using reheat Rankine cycle is shown in Fig. 13.
Itshould be noted that steam is expanded up to some
intermediatepressure in the rst stage turbine (state 11). This is
reheated in areheater (from state 11 to state 12), using a small
fraction of HTF(state 13) from outlet of the collector eld. The
reheated steam (atstate 12) then expands in the second stage of
turbine to thecondenser pressure. The HTF coming out of the
reheater (state 14)is mixed with the main line HTF and the
resultant mixture (at state15) is fed to HTF pump. The other state
points are same as explainedearlier.
The effect of plant size and resulting higher isentropic
efciencyof the turbine on aperture specic net design power output
isshown in Fig. 14. It may be noted that the
thermodynamicallyoptimal turbine inlet pressures for 5 MWe, 25 MWe
and 50 MWeplants are 7.5e11.5 MPa, 9e13.5 MPa and 9e13.5 MPa,
respectively.Fig. 15 illustrates the variation of LCOE as function
of turbine inletpressure, for different sizes of the plant. It may
be noted that the
Fig. 19. Levelized cost of energy as function of turbine inlet
pressure for differentplaces with reheat-regenerative Rankine
cycle.cost optimal turbine inlet pressures for 5 MWe, 25 MWe and50
MWe plants are 6e11.5 MPa, 7.5e13.5 MPa and 8e13.5
MPa,respectively.
5.2. CSP plant with reheat-regenerative Rankine cycle
Reheating and regeneration are the most commonly usedmodications
in the basic steam Rankine cycle. Simplied
Table 4Summary of results.
Plant size (MWe) Energy optimrange (MPa)
CSP plant with basic Rankine cycle 1 4.5e7.5CSP plant with
regenerative Rankine cycle 1 6.2e10CSP plant with reheat Rankine
cycle 5 7.5e11.5
25 9e13.550 9e13.5
CSP plant with reheat-regenerative Rankine cycle 5 9e1325
10.5e1550 10.5e15Additionally, there is a cost optimal design
radiation thatminimizesthe cost of electricity generation.
The estimated minimum LCOE is about Rs. 11.3 per kWh (18.8
/kWh). This cost is higher compared to coal (Rs. 2.5 per kWh),
nu-clear (Rs. 3 per kWh) as well as natural gas (Rs. 5.5 per kWh)
basedthermal power plants in India (Nature, 2014). The LCOE for
PTCbased CSP plant may further decrease with higher plant
size,multiple extractions from the steam turbine, thermal storage
as
um Maximum aperture specicnet design power output (W/m2)
Cost optimumrange (MPa)
Minimum levelizedcost of energy (/kWh)
87.9 3.5e7.5 33.694.9 4.5e10 32.3
108.2 6e11.5 22.9115.6 7.5e13.5 20.4116.5 8e13.5 19.9115.7 7e13
21.9123.5 9e15 19.49.5e15 MPa, respectively.In case of a 50MWe CSP
plant with reheat-regenerative Rankine
cycle, increase in aperture specic net design power output is
about7% and decrease in LCOE is about 5.3% compared to reheat
Rankinecycle. Fig. 19 shows the comparison of LCOE as function of
turbineinlet pressure, for three different places in India. It may
be notedthat the minimum LCOE of 11.3 Rs./kWh (18.8 /kWh) is
estimatedfor Jodhpur, India.
6. Conclusions
In this paper, effects of turbine inlet pressure, turbine
inlettemperature, design radiation, plant size, and various
modicationsof Rankine cycle on overall efciency as well as LCOE for
the PTCbased CSP plant, without hybridization and storage, are
presented.Moreover, the variation in optimal turbine inlet pressure
withturbine inlet temperature, design radiation, plant size, and
variousmodications of Rankine cycle are also determined. The
importantobservations are summarized in Table 4. In case of a PTC
based plantwith basic Rankine cycle, thermodynamically and cost
optimalturbine inlet pressures for 1 MWe plant are about 4.5e7.5
MPa and3.5e7.5 MPa, respectively.
The optimal turbine inlet pressure is a weak function of
designradiation. However, the optimum value increases with plant
sizeand various modications of Rankine cycle. The optimal
turbineinlet pressures for 5 MWe, 25 MWe and 50 MWe plants
(withreheat-regenerative Rankine cycle) are 7e13 MPa, 9e15 MPa
and9.5e15 MPa, respectively. The aperture specic net design
poweroutput increases and LCOE decreases with increase in turbine
inlet124.7 9.5e15 18.8
-
well as clean development mechanism benets. Moreover, the useof
molten salt as HTF has the potential of decreasing the
LCOEsignicantly.
Acknowledgments
Authors would like to thank the Ministry of New and
RenewableEnergy (MNRE), Government of India for the nancial
support
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Optimization of concentrating solar thermal power plant based on
parabolic trough collector1. Introduction2. Effect of turbine inlet
pressure on overall efficiency and levelized cost of energy3.
Effect of design radiation on overall efficiency and levelized cost
of energy4. CSP plant with regenerative Rankine cycle5. Effect of
plant size on overall efficiency and levelized cost of energy5.1.
CSP plant with reheat Rankine cycle5.2. CSP plant with
reheat-regenerative Rankine cycle
6. ConclusionsAcknowledgmentsReferences