CONCENTRATING SOLAR POWER

en

8 Sa

trated The facility involves solar eld, molten salts, s The results are promising and validate literatu

Mathematical optimization 260 MV. Scale-up studies reveal that the production cost can decrease by half while the investment perunit of power should become competitive with current coal based power plants if solar and coal facilities

stration plants worldwide [3] whose characteristics in terms ofpower and technology can be found in the literature [4] includingthe investment and electricity production cost [5].

In Fig. 2 we present the solar radiation over the MediterraneanSea region. It can be seen that the solar energy received is only a few

n radiation to heatelectricity produc-steam turbine andince they providenuous operation ofys, thermal energynergy is typically

So far only a few studies have dealt with the optimization of theperformance of the plant. These studies vary from focusing on thethermal cycle and using different approaches such as sensitivityanalysis for the extraction pressure [9] or the exhaust pressure fromthe turbine [10], evolutionary algorithms [11] or neural networks[12], to mathematical optimization approaches to use solar energyfor water desalinization. These last cases consider a Rankine cyclewith xed operating conditions and one expansion alone in theturbine [13], including multiobjective optimization [14]. However,

* Corresponding author. Tel.: 34 923 294479.

Contents lists available at

Applied Therma

sev

Applied Thermal Engineering 59 (2013) 627e633E-mail address: [email protected] (M. Martn).Energy consumption has increased over the last decades. So far,the use of fossil fuels has been convenient due to their availabilityand easy transformation. However, the depletion of the reservoirsand the increased need for energy demand a change in the currentenergy supply system [1]. Renewable sources are a valuable alter-native. In Fig.1 we can see the expected increase in the contributionof solar, wind and biomass. Solar energy is an option in regionswithhigh solar irradiation [2]. There are already a number of demon-

used. They are based on the concentration of suup an energy transfer uid used for steam andtion. CSP plants consist of three parts: solar eld,cooling unit. Rankine cycles are typically used sefciency advantages [8]. Moreover, for the contithese plants during the night and in overcast dafrom the heat tank or an additional source of eused [3].1. Introduction kWh/m2/day. To achieve higher intensities and high operatingtemperatures Concentrated Solar Power (CSP) technologies can bepresent similar production capacities. 2013 Elsevier Ltd. All rights reserved.a r t i c l e i n f o

Article history:Received 13 March 2013Accepted 2 June 2013Available online 27 June 2013

Keywords:EnergyConcentrated solar powerRankine cycle1359-4311/$ e see front matter 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.applthermaleng.2013.06.03team and electricity generation and cooling.re sensitive studies.

a b s t r a c t

We present the year-round optimization of the operation of a concentrated solar power facility evalu-ating the molten salts storage, the power block and cooling. We locate the plant in the south of Europe,Almera (Spain), where high solar radiation is available. The operation of the plant is a function of thesolar incidence as well as the climate and atmospheric conditions. The optimization of the system isformulated as a multiperiod Non-linear Programming problem (NLP) that is solved for the optimalproduction of electricity over a year dening the main operating variables of the thermal and coolingcycles. For a maximum of 25 MW in summer and a minimum of 9.5 MW in winter the annual productioncost of electricity is 0.15 V/kWh consuming an average of 2.1 Lwater/kWh. The investment for the plant is Plant design so far relies on process siWe optimize the operation of a concen solar power plant.mulation and only partial optimization studies.Optimal year-round operation of a concthe south of Europe

Lidia Martn, Mariano Martn*

Departamento de Ingeniera Qumica, Universidad de Salamanca, Pza. Cados 1-5, 3700

h i g h l i g h t s

journal homepage: www.elAll rights reserved.1trated solar energy plant in

lamanca, Spain

SciVerse ScienceDirect

l Engineering

ier .com/locate/apthermeng

to the best of our knowledge no one has optimized of the operatingconditions of the Rankine cycle nor evaluated the cooling toweroperation for the year-round operation of the plant.

In this paper we use mathematical programming techniques forthe conceptual optimal design and operation of a concentratedsolar power plant over a year based on molten salt technology. Weconsider the variability in the sun reception, in the freshwatertemperature and in the atmospheric conditions. The facility islocated in Almera (Spain), a region with one of the highest solarradiations in Europe, see Fig. 2. The paper is organized as follows.

investment per unit of power generated. Finally, in Section 5 wedraw some conclusions.

2. Modeling

2.1. Modeling assumptions

The plant consists of three parts, the heliostat eld including thecollector and the molten salts storage tanks, the steam turbine andthe cooling tower [3]. Fig. 3 presents the owsheet for the processwhere the heliostateld has not been included. Our process is basedon the use of a tower to collect the solar energy and a regenerativeRankine cycle, see Fig. 4. The steam is generated in a system of threeheat exchangers where it is rst heated up to saturation and thenevaporated using the total ow of molten salts. However, only afraction of the owof salts is used to superheat the steam before it isfed to the rst body of the turbine. The rest is used to reheat up thesteam before it is fed to the second body. In the second body of theturbine, part of the steam is extracted at amediumpressure and it isused to heat up the condensate. The rest of the steam is nallyexpanded to an exhaust pressure, condensed and recycled. A coolingtower is used to condensate this exhaust steam. Eachunit ismodeledusing mass and energy balances as well as thermodynamic prop-erties. The main assumptions can be seen in Table 1.

2.2. Opeating conditions

Fig. 1. Energy consumption per source of energy [6].

L. Martn, M. Martn / Applied Thermal Engineering 59 (2013) 627e633628Section 2 describes the modeling features and the operating con-ditions of the selected location. In Section 3 we present the opti-mization procedure. Next, in Section 4 the main results arediscussed such as the major operating conditions, the monthlywater consumption of the facility, an economic evaluation and astudy on the effect of the plant scale on the production cost andFig. 2. Radiation in EuropeIn Table 2 we present the monthly operating conditions of theplant including the radiation received in Almera, the sun hours, theambient temperature of the air and water and the air average hu-midity [15]. We consider as water temperature that of the Medi-terranean Sea in that region [16]. It is important to notice that thesame formulation can be used for any other region by using the(kWh m2 day1) [7].

The model is formulated as a multiperiod NLP problem written

once

L. Martn, M. Martn / Applied Thermain GAMS [17]. We maximize the energy produced, z, given by Eq.(1), over the 12 time periods:Xappropriate data and also to compare the performance of a facilityunder different ambient conditions.

3. Optimization procedure

Fig. 3. Flowsheet for the cz tp

WTurbine1;tp WTurbine2;tp WTurbine3;tp (1)

Subject to the model for the process described in Section 2.The main decision variables are the operating inlet and

discharge pressures at the three bodies of the turbine, the split

Fig. 4. Rankine regenerative cycle.fraction for the molten salts to be used at HX1 and at HX4, thefraction of steam extracted from the second body of the turbine andthe air ow rate and its temperature prole at the cooling tower.The problem consists of 3200 equations and 3500 variables. Thecomplexities due to the integration of the design of the coolingtower together with the Rankine cycle result in the need for properinitialization by using several starting points based on data fromthe literature and bounds for the variables such as owrates,temperatures and the operating characteristics of the cooling

ntrated solar power plant.l Engineering 59 (2013) 627e633 629tower, the minimum air ow and (hL/ky). CONOPT 3.0 [18] is usedto solve the problem. This formulation can also be used to evaluatedaily or weekly operation of the plant by changing the time periodsfrom a monthly basis to an hourly or daily basis monitoring theatmospheric conditions which can be useful for the integration ofthe solar energy into the grid.

4. Results

We divide this section in four parts presenting the optimaloperating conditions, the water consumption, an economic evalu-ation and a scale-up study.

4.1. Operation

The ow rate from the salts storage tank is split so that 30% ofthe ow is used in HX1 to heat up the saturated steam while therest is sent to HX4 for the reheating stage. For all the periods it turnsout that the superheated steam enters the rst body of the turbineat 125 bar and 555 C exiting it at 11 bar. This stream is reheated upin HX4 to 500 C and fed to the second body of the turbine fromwhere it exits at 6.5 bar. 15% of the stream that leaves the secondbody of the turbine is extracted and sent to HX6 while the rest isexpanded in the third body of the turbine to an exhaust pressure of0.19 bar. This stream is condensed in HX5 and the energy isremoved by the cooling tower.

In Table 3 we compare the results obtained for the Rankinecycle with those reported by different authors. The optimal inlet

Table 1Main modeling assumptions.

Equipment Main assumptions

Heliostates Design power for multiperiodP 25 MW [3]Afield P$tRadannualQcollector

3600$hs$td Es Radi$td$Afield$hfield

Afield nh$Ah$hhCosine losses (20%), shading and blocking(2% losses), heliostat reectivity(t from 0.90 to 0.95) and transmissionlosses through the atmosphere (5% loses)Field efciency (held) of 55%.Heliostat efciency (hh) of 90%.Radannual 520 kWh=m2

L. Martn, M. Martn / Applied Therma630The energy captured by a xed heliostateld at a time is transferred to the salts.

Tank 2 290 C [3]Tank 1 565 C [3]Molten salts Composition: 60%w/w NaNO3e40% w/w KNO3 [3]

Feed from tank 1 to splitter ftank1;splitter hsun24 $fCollector;tank1;pressure to the rst body of the turbine turns out to be 125 bar,which is similar to Xus et al. cases 1 and 2 [19], lower than Xuscase 3 [19] and higher than other papers such as Nezammahallehet al. [20], Morin et al. [11] or Halb et al. [10] who reported 90,

Turbine Isentropic efciency 0.9 [20]Feed pressure range 90e125 barFirst body exhaust range 11e35 barSecond body extraction range 5e10 barExhaust pressure from 0.05 bar to 0.31 barExpansions should result in a vapor phase

Heat exchangers Cooling water temperature used in HX5 increases amaximum of 10 C and the maximum cooling watertemperature should be 35 CStream output of HX6 should be liquid

Cooling tower Modeled using Mickley method [22]Water losses by evaporation (kg/s) lower than: 1.8$0.00085$f(cooling,HX5)(T(cooling,HX5)T(HX5,cooling)) [22](hL/ky) >10Exiting air cannot have a humidity higher than 95%

Operating hours 6450 hNomenclature Aeld: Heliostat eld area (m2)

Ah: Heliostat area (120 m2)f(unit 1,unit 2): mass ow from unit 1 to unit 2 (kg/s)hsun: Sun hours (h)P: Power (kW)Q(collector): Power captured by collector (kW)Es: Solar energy (J)Radannual Annual radiation (kW/m2)td: time in days in a month (days)top: Operating time (s)T(unit 1,unit 2): Temperature of the stream from unit 1 tounit 2 (C)

Table 2Plant operating conditions.

Month kWh/m2$day Day SUN(H)

Sun(h/day)

TAmb(C)

% Humidity Twater(C)

Jan 4.377 31 191 6.161 12.5 69 15.5Feb 5.125 28 191 6.821 13.2 68 15.0Mar 5.319 31 228 7.355 14.7 66 16.0Apr 6.387 30 250 8.333 16.4 64 17.5May 6.697 31 299 9.645 19.1 66 19.5June 8.587 30 322 10.733 22.7 64 25.0July 8.668 31 338 10.903 25.7 63 26.0Aug 7.342 31 312 10.065 26.4 65 27.0Sep 6.057 30 257 8.567 24.0 66 26.0Oct 4.126 31 221 7.129 20.0 68 24.0Nov 3.513 30 187 6.233 16.2 70 21.0Dec 3.326 31 176 5.677 13.7 70 17.0Average 5.794 30.4 248 8.13 18.7 66.6 20.898.7 and 100 bar respectively, see Table 3. The optimal dischargeof the rst body of the turbine occurs at 11 bar, a lower pressurecompared to most of the cases but for the results reported byMorin et al. [11]. In terms of the extractions, Palenzuelas et al.scenario 4 [9] reported a sensitivity analysis for the effect of theextraction pressure from the low pressure turbine on the plantefciency. They consider different extraction pressures, 2, 4, 6, 10and 16 bar, resulting in the fact that the efciency of the plantdecreases with the extraction pressure. Xus et al. [19] consideredone extraction at 30 bar while Nezammahalleh et al. [20]considered three extractions at 7, 2.1 and 0.5 bar respectively. Inour case, with one extraction, we obtain 6.5 bar as the optimumvalue. This result can be considered as a trade-off in the loss ofefciency presented by Palenzuela et al. [9] as the pressure of theextraction increases. Finally, the exhaust pressure we obtained,0.19 bar as saturated steam, is similar to Palenzuelas work cases 1,

Table 3Comparison of the main operating parameters of the Rankine cycle for solar plants.

T(HP) (C)

P(HP)(bar)

P(MP)(bar)

P(Ext)(bar)

P(exha)(bar)

Palenzuelaet al. 1) [9]

371 104 17 e 0.18


371 104 17 e 0.31


371 104 17 e 0.18


371 104 17 (2/4/6/10/16) 0.18

Ghobeityet al. [13]

540 40 e e 0.05

Morin et al.[11]

392.9 98.7 10.6 e 0.08

Salcedo et al.[14]

e 40 e e 1

Halb et al. [10] 373 100 18 e 0.06e0.2Xu et al. 1) [19] 552 126 e 31 0.1Xu et al. 2) [19] 552 126 e 31 0.1Xu et al. 3) [19] 552 240 48 30 0.1Nezammahalleh

[20]500 90 18 7/2.1/0.5 0.07

This work 555 125 11 6.5 0.19

l Engineering 59 (2013) 627e6333 & 4 [9], 0.18 bar, higher than the values presented by Nezam-mahalleh et al. [20], Xus et al. [19] or Ghobeity et al. [13], andlower than Salcedo et al. [14] or Palenzuelas case 2 [9] seeTable 3. Halb et al. [10] presented a sensitivity study evaluatingthe effect of the exhaust pressure on plant efciency considering arange of values from 0.06 to 0.2 bar. They found a decrease inpower efciency with the exhaust pressure pointing out that avalue of 0.073 bar is the most convenient. The value correspondsalso to Andasol solar power plant, that uses trough technology,instead of the Tower based design we considered based onGEMASOLAR plant. Furthermore, we x the exhaust to be satu-rated vapor to avoid mechanical problems in the turbine.

Fig. 5 shows the year-round production of electricity. Duringsummer we obtain a maximum of 25 MW for two consecutivemonths, June and July, while the lowest production capacities arefound in November and December, just below 10 MW. For theextreme atmospheric operating conditions, July and December, inFig. 6a and b respectively we present the cooling tower operation.The green line (In web version) shows the air temperature prolealong the column. Air is heated up along the column in Decemberwhile in July, the hotter air results in a small decrease in the tem-perature in the rst stages within the cooling tower. The averagepower generated during the year is 18 MW by distributing thehours of operation of the plant, 6450 h, proportionally to the powerproduced each month.

In spite of the use of local solvers, due to the size of the problem,

present a consumption of water in the range of 2.7e3.8 L/kWh.

Where access to water is even more restricted, two optionsarise: the use of part of the energy for the production of freshwaterout of sea water or the implementation of dry cooling technologies.Evaluating the trade-off between dry and wet technologies is notthe scope of this paper. However, it is important to point out that,according to the literature, the water consumption using drytechnologies ranges between 0.1 and 0.3 L/kWh but there is a costin terms of energy consumption. From 1% to 5% of the production ofenergy is consumed by the air fans [24]. Simulation results forplants using both technologies can be found in the literature.Palenzuela et al. [25] reported that wet cooling is more efcient.Furthermore, the power consumed by the fans in the case of drycooling is from 1.5 to 5 times the one for wet cooling [26]. Finallythe difference in the fan power consumption along the year issignicant. When dry cooling is used, the consumption duringsummer is up to 2.5 times that of winter, while wet cooling con-sumes similar power along the year and always lower than dry

Fig. 5. Energy production through the year.

L. Martn, M. Martn / Applied Thermal Engineering 59 (2013) 627e633 631Although these plants do not provide an advantage in terms ofwater consumption compared to thermoelectric ones, they stillpresent a competitive value.the consistency in the results for the different time periods andwith the different starting points suggest that good results areobtained, although no global solution can be claimed.

4.2. Water consumption

In Fig. 7 we present the water make-up needed for the plantover the year operation due to evaporative losses (Ev), computedfrom the mass balances to the plant, and blowdown (B), calculatedas Eq. (2) assuming that the number of cycles of concentration(COC) is 5 [21]. The drift is negligible compared to these two valuesand thus it is not considered [22]

B EvCOC 1 (2)

The reason for not presenting the same pattern as the energyproduction is that the upper limit for the temperature of the coolingwater is xed to 35 C. Thus the difference in the inlet and outletwater temperatures at HX5 is variable. Water consumption de-pends on the water and air temperatures and the air humidity. Wecan see that during the hottest months, the consumption is higher.The average consumption is 2.1 L/kWh, which is an interestingvalue since it is similar to US average for thermoelectric plants,1.8 L/kWh [23]. Moreover, according to the literature, CSP plantsFig. 6. Cooling tower operation at top and bottomcooling [10].

4.3. Economic evaluation

The investment includes equipment cost and installation, pipingand instrumentation, land, chemicals and administration. Thefactorial method [27] relies on the equipment cost, updated fromRef. [28]. We consider the units described in the owsheet given byFig. 3 and the heliostats. The design point for equipment sizingcorresponds to a radiation of 900 W/m2 and we assume the at-mospheric conditions of July. We obtain 2870 heliostats and amaximum power at the turbine of 62 MW. For the values obtainedin this scenario we price the equipment. The total cost for theequipment accounts for 72 MV2012. Fig. 8 presents the share of thedifferent sections to the equipment cost. More than 50% of theequipment cost is due to the solar eld and the collector while theturbine and the heat transfer system contribute with around 20%each. Finally the cooling tower burdens the equipment cost withless than 4% of the total amount.

For the evaluation of the investment cost [27], the installedequipment represents 1.5 times the equipment cost. Piping, isola-tion, instrumentation and utilities represent 20%, 15%, 20% and 10%of the equipment cost respectively. Land and buildings cost isestimated to be 8 MV, and we pay for the salts (0.665 V/kg). Theseitems add up to the x cost (191 MV). Current research is devel-oping different heat storages [29]. The fees represent 3% of the xcost, other administrative expenses and overheads and the plantlayout represent 10% of the direct costs (fees plus x capital) and 5%of the x cost respectively. The plant start up cost represents 15% ofproduction capacities (a) July (b) December.

the investment. The investment adds up to 260 MV, similar to thatreported for GEMASOLAR plant, located in Seville, cost 230 MV for20 MW gross [3].

Furthermore, we estimate the production cost of the electricity[27]. For the average annual cost, we consider the labour costs (0.5%of investment), equipment maintenance (2.5% of x costs), amor-tization (linear with time in 20 years), taxes (1% investment),overheads (1% investment) and administration (5% of labour,

values reported in the literature are in the range from $0.13 to $0.17/

Finally, we have to bear in mind that the development in thetechnology is expected to reduce the prices in themedium and long

Fig. 7. Water consumption.

L. Martn, M. Martn / Applied Thermal Engineering 59 (2013) 627e633632kWh [30] depending on the technology and location [31]. Thelevelized cost of electricity, i 0.05 [32] results in 0.30 V/kWh, inthe upper bound of the range of the ones reported for differentconcentrated solar plants [33].

4.4. Scale-up studies

Most thermal power plants are large facilities (500e1000 MW)while most solar demonstration plants have a production capacityequipment maintenance, amortization, taxes and overheads). Thetotal production costs adds up to 17 MV/year for an annual pro-duction of 117 GWh. Fig. 9 presents the breakdown of the pro-duction cost. The average production cost results 0.15 V/kWh. Halbet al. [10] reported a price, for wet cooling, of 0.15V/kWh while theFig. 8. Distribution of the equipment cost per sections.bellow 50 MW [5] so that direct comparisons are not appropriate.Based on the validated case above, we evaluate the effect of thescale of the plant on the electricity production cost and investment.

We scale up the equipment cost using the data by the companyMATCHE [28], considering that for equipment bigger than thelarger standard, a number of equipment are used in parallel.However, the land required is proportional to the number of he-liostats and so is its cost. The cost of the heliostats and that of thereceptor tower are scaled up using the common 0.6 factor for thechemical industry. In Fig.10 we present the effect of the scale on theproduction cost. We can see that the production cost decrease to0.07 V/kWh, which becomes competitive with electricity from gasor coal [34]. We can easily correlate the data obtaining Eq. (3)

CostV=kWh 1:61$PowerkW0:25 (3)In terms of the levelized cost, the scale of the plant should

decrease it by half (0.16 V/kWh) when the plant sizes reaches300 MW. In terms of investment, the plant scales with a power of0.78 and the data can be tted to Eq. (4). This reduction also makesthe use of solar energy competitive although still slightly moreexpensive per kW installed than fossil fuel energy sources [35]. i.e.for a solar power plant of 400 MW the investment decreases until7000 V/kW (1100 V/kWh for 6450 h of operation).

InvestmentMV 0:124$powerkW0:78 (4)

Fig. 9. Electricity production cost breakdown.Fig. 10. Effect of the scale of the plant on the electricity production cost.

term while other more mature technologies are already reachingtheir minimum [36].

5. Conclusions

The operation of a concentrated solar plant using a regenerativeRankine cycle has been optimized along a natural year usingmathematical programming techniques.

efciency, cost, optimization, simulation and environmental impact of energysystems 300-1, 300e314.

[11] G. Morin, P. Richter, P. Nitz. New method and software for multivariabletechnoeconomic design optimization of CSF plants. www.mathcces.rwth-aachen.de/_media/5people/richter/pascalrichter-2010-solarpaces.pdf (lastaccessed December 2012).

[12] P. Richter, E. Abraham, G. Morin, Optimisation of concentrating solar thermalpower plants with neural networks, in: A. Dobnikar, U. Lotric, B. Ster (Eds.),ICANNGA, Part I, vol. 6593, LNCS, 2011, pp. 190e199,.

[13] A. Ghobeity, C.J. Noone, C.N. Papanicolas, A. Mitsos, Optimal time-invariantoperation of a power and water cogeneration solar-thermal plant, Solar En-

L. Martn, M. Martn / Applied Thermal Engineering 59 (2013) 627e633 633The average production of energy is 18 MW but it ranges from9.5 MW during winter to 25 MW during summer as a result of thesolar radiation received. The average consumption of water is 2.1 L/kWh, which is competitive with thermoelectric plants. The in-vestment of the plant is 260 MV and the production cost 0.15V/kWh, a little high compared to fossil fuel-based electricity.However, economies of scale are expected to reduce the productioncost and the investment per kW generated by half when the pro-duction capacities reaches those of current thermal power facilities.

The formulation also allows the studyof the short-termoperationof the plant which is interesting towards the integration of solarenergy into themix of energy of the grid. Furthermore, it can also beextended to include a coal or gas furnace or a gas turbine, an inte-grated solar combined cycle ISCC plant. In both cases the energy togenerate the steamandreheat it upand/or toheat upthemolten saltsis provided either by the sun or by fossil fuels so that it is possible tomaintain the production capacity constant along the year.

Acknowledgements

The authors would like to acknowledge Salamanca Research foroptimization software licenses.

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Optimal year-round operation of a concentrated solar energy plant in the south of Europe1 Introduction2 Modeling2.1 Modeling assumptions2.2 Opeating conditions

3 Optimization procedure4 Results4.1 Operation4.2 Water consumption4.3 Economic evaluation4.4 Scale-up studies

5 ConclusionsAcknowledgementsReferences

Documents

CONCENTRATING SOLAR POWER