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: santanub@iitb.ac.in (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

Krishna Priya, G.S., Bandyopadhyay, S., 2013. Emission constrained power systemplanning: a pinch analysis based study of Indian electricity sector. CleanTechnol. Environ. Policy 15, 771e782.

Kumar, K.R., Reddy, K.S., 2012. 4-E (energyeexergyeenvironmentaleeconomic)analyses of line-focusing stand-alone concentrating solar power plants. Int. J.Low Carbon Technol. 7, 82e96.

Kuravi, S., Trahan, J., Goswami, D.Y., Rahman, M.M., Stefanakos, E.K., 2013. Thermalenergy storage technologies and systems for concentrating solar power plants.Prog. Energy Combust. Sci. 39, 285e319.

Llorente Garca, I., Alvarez, J.L., Blanco, D., 2011. Performance model for parabolictrough solar thermal power plants with thermal storage: comparison to oper-ating plant data. Sol. Energy 85, 2443e2460.

N.B. Desai, S. Bandyopadhyay / Journal of Cleaner Production 89 (2015) 262e271 271given to the Development of a Megawatt-scale Solar Thermal Po-wer Testing, Simulation and Research Facility Project (15/19/2008-09/ST dated 07.09.2007).

References

Al-Soud, M.S., Hrayshat, E.S., 2009. A 50 MW concentrating solar power plant forJordan. J. Clean. Prod. 17, 625e635.

Al-Sulaiman, F.A., 2013. Energy and sizing analyses of parabolic trough solar col-lector integrated with steam and binary vapor cycles. Energy 58, 561e570.

Behar, O., Khellaf, A., Mohammedi, K., 2013. A review of studies on central receiversolar thermal power plants. Renew. Sustain. Energy Rev. 23, 12e39.

Ca

ceres, G., Anrique, N., Girard, A., Degre

ve, J., Baeyens, J., Zhang, H.L., 2013. Per-formance of molten salt solar power towers in Chile. J. Renew. Sustain. Energy5, 0531421e05314215.

Collado, F.J., Guallar, J., 2013. A review of optimized design layouts for solar powertower plants with campo code. Renew. Sustain. Energy Rev. 20, 142e154.

Desai, N.B., Bandyopadhyay, S., Nayak, J.K., Banerjee, R., Kedare, S.B., 2013. Simu-lation of 1 MWe solar thermal power plant. In: Proceedings of the ISES SolarWorld Congress, Cancun, Mexico.

Desai, N.B., Kedare, S.B., Bandyopadhyay, S., 2014. Optimization of design radiation forconcentrating solar thermalpowerplantswithout storage.Sol.Energy107,98e112.

Douglas, J.M., 1988. Conceptual Design of Chemical Processes. McGraw-Hill, NewYork.

Feldhoff, J.F., Ortiz-Vives, F., Schulte-Fischedick, J., Laing, D., Schnatbaum-Laumann, L., Eck, M., Schmitz, K., 2012. Comparative system analysis of directsteam generation and synthetic oil parabolic trough power plants with inte-grated thermal storage. Sol. Energy 86, 520e530.

Fernandez-Garca, A., Zarza, E., Valenzuela, L., Perez, M., 2010. Parabolic-troughsolar collectors and their applications. Renew. Sustain. Energy Rev. 14,1695e1721.

Flueckiger, S.M., Iverson, B.D., Garimella, S.V., Pacheco, J.E., 2014. System-levelsimulation of a solar power tower plant with thermocline thermal energystorage. Appl. Energy 113, 86e96.

Franchini, G., Perdichizzi, A., Ravelli, S., Barigozzi, G., 2013. A comparative studybetween parabolic trough and solar tower technologies in solar Rankine cycleand integrated solar combined cycle plants. Sol. Energy 98, 302e314.

Garca-Barberena, J., Garcia, P., Sanchez, M., Blanco, M.J., Lasheras, C., Padros, A.,Arraiza, J., 2012. Analysis of the inuence of operational strategies in plantperformance using SimulCET, simulation software for parabolic trough powerplants. Sol. Energy 86, 53e63.

Giostri, A., Binotti, M., Silva, P., Macchi, E., Manzolini, G., 2012a. Comparison of twolinear collectors in solar Thermal plants: parabolic trough versus fresnel. J. Sol.Energy Eng. 135, 0110011e0110019.

Giostri, A., Binotti, M., Astol, M., Silva, P., Macchi, E., Manzolini, G., 2012b. Com-parison of different solar plants based on parabolic trough technology. Sol.Energy 86, 1208e1221.

Gutierrez-Arriaga, C.G., Abdelhady, F., Bamueh, H.S., Serna-Gonzalez, M., El-Halwagi, M.M., Ponce-Ortega, J.M., 2014. Industrial waste heat recovery andcogeneration involving organic Rankine cycles. Clean. Technol. Environ. Policy.http://dx.doi.org/10.1007/s10098-014-0833-5 (in press).

Ho, C.K., Iverson, B.D., 2014. Review of high-temperature central receiver designs forconcentrating solar power. Renew. Sustain. Energy Rev. 29, 835e846.

IIT Bombay, 2012. Solar Thermal Simulator Evaluation Version 1.0.Kibaara, S., Chowdhury, S., Chowdhury, S.P., 2012. Analysis of cooling methods of

parabolic trough concentrating solar thermal power plant. In: IEEE Interna-tional Conference on Power System Technology, pp. 1e6.

Klein, S., 2004. F-chart Software. Engineering Equation Solver-V7.027.Krishnamurthy, P., Mishra, S., Banerjee, R., 2012. An analysis of costs of parabolic

trough technology in India. Energy Policy 48, 407e419.Manenti, F., Ravaghi-Ardebili, Z., 2013. Dynamic simulation of concentratingsolar power plant and two-tanks direct thermal energy storage. Energy 55,89e97.

Manzolini, G., Giostri, A., Saccilotto, C., Silva, P., Macchi, E., 2011. Development of aninnovative code for the design of thermodynamic solar power plants part B:performance assessment of commercial and innovative technologies. Renew.Energy 36, 2465e2473.

Mavromatis, S.P., Kokossis, A.C., 1998. Conceptual optimisation of utility networksfor operational variations-I. Targets and level optimisation. Chem. Eng. Sci. 53(8), 1585e1608.

Montes, M.J., Aba

nades, A., Marti

nez-Val, J.M., Valdes, M., 2009. Solar multipleoptimization for a solar-only thermal power plant, using oil as heat transferuid in the parabolic trough collectors. Sol. Energy 83, 2165e2176.

Morin, G., Dersch, J., Platzer, W., Eck, M., Haberle, A., 2012. Comparison of linearfresnel and parabolic trough collector power plants. Sol. Energy 86, 1e12.

Mller-Steinhagen, H., Trieb, F., 2014. Concentrated solar power. In: Quarterly of theRoyal Academy of Engineering (Ingenia, 2004).

Nature, 2014. Available from: http://www.nature.com/news/india-to-build-world-s-largest-solar-plant-1.14647.

Nixon, J.D., Dey, P.K., Davies, P.A., 2013. Design of a novel solar thermal collectorusing a multi-criteria decision-making methodology. J. Clean. Prod. 59,150e159.

Purohit, I., Purohit, P., Shekhar, S., 2013. Evaluating the potential of concentratingsolar power generation in Northwestern India. Energy Policy 62, 157e175.

Ramaswamy, M.A., Rao, B., Thirumalai, N.C., Suresh, N.S., 2013. Estimation of HourlyDirect Normal Irradiance (DNI) for 22 Stations in India. Center for Study ofScience, Technology and Policy, Bangalore, India.

Reddy, K.S., Kumar, K.R., 2012. Solar collector eld design and viability analysis ofstand-alone parabolic trough power plants for Indian conditions. Energy Sus-tain. Dev. 16, 456e470.

Reddy, V.S., Kaushik, S.C., Tyagi, S.K., 2012. Exergetic analysis and performanceevaluation of parabolic trough concentrating solar thermal power plant(PTCSTPP). Energy 39, 258e273.

Roget, F., Favotto, C., Rogez, J., 2013. Study of the KNO3eLiNO3 andKNO3eNaNO3eLiNO3 eutectics as phase change materials for thermal storagein a low-temperature solar power plant. Sol. Energy 95, 155e169.

Shang, Z., 2000. Analysis and Optimisation of Total Site Utility Systems. Ph.D. Thesis.UMIST, UK.

Sharma, A., 2011. A comprehensive study of solar power in India and World. Renew.Sustain. Energy Rev. 15, 1767e1776.

Sundaray, S., Kandpal, T.C., 2013. Preliminary feasibility evaluation of solar thermalpower generation in India. Int. J. Sustain. Energy 1e9.

Tian, Y., Zhao, C.Y., 2013. A review of solar collectors and thermal energy storage insolar thermal applications. Appl. Energy 104, 538e553.

Zarza, E., Valenzuela, L., Leo

n, J., Weyers, H.-D., Eickhoff, M., Eck, M., Hennecke, K.,2002. The DISS project: direct steam generation in parabolic trough systems.Operation and maintenance experience and update on project status. J. Sol.Energy Eng 124, 126e133.

Zaversky, F., Garca-Barberena, J., Sanchez, M., Astrain, D., 2012. Probabilisticmodeling of a parabolic trough collector power plant e an uncertainty andsensitivity analysis. Sol. Energy 86, 2128e2139.

Zaversky, F., Medina, R., Garca-Barberena, J., Sanchez, M., Astrain, D., 2013. Object-oriented modeling for the transient performance simulation of parabolic troughcollectors using molten salt as heat transfer uid. Sol. Energy 95, 192e215.

Zhang, H.L., Baeyens, J., Degreve, J., Caceres, G., 2013. Concentrated solar powerplants: review and design methodology. Renew. Sustain. Energy Rev. 22,466e481.

Zhu, G., Wendelin, T., Wagner, M.J., Kutscher, C., 2014. History, current state, and futureof linear fresnel concentrating solar collectors. Sol. Energy 103, 639e652.

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