Energy 29 (2004) 883893

www.elsevier.com/locate/energy

Two-tank molten salt storage for parabolic trough solarpower plants

Ulf Herrmann a,, Bruce Kelly b, Henry Price c

a FLABEG Solar International GmbH, Muhlengasse 7, D-50667 Koln, Germanyb Nexant, Inc., 45 Fremont Street, San Francisco, CA 94105-2210, USA

c National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO, USA

Abstract

The most advanced thermal energy storage for solar thermal power plants is a two-tank storage systemwhere the heat transfer uid (HTF) also serves as storage medium. This concept was successfully demon-strated in a commercial trough plant (13.8 MWe SEGS I plant; 120 MWht storage capacity) and a dem-onstration tower plant (10 MWe Solar Two; 105 MWht storage capacity). However, the HTF used instate-of-the-art parabolic trough power plants (3080 MWe) is expensive, dramatically increasing the costof larger HTF storage systems. An engineering study was carried out to evaluate a concept, whereanother (less expensive) liquid medium such as molten salt is utilized as storage medium rather than theHTF itself. Detailed performance and cost analyses were conducted to evaluate the economic value ofthis concept. The analyses are mainly based on the operation experience from the SEGS plants and theSolar Two project. The study concluded that the specic cost for a two-tank molten salt storage is in therange of US$ 3040/kWhth depending on storage size. Since the salt storage was operated successfully inthe Solar Two project, no major barriers were identied to realize this concept in the rst commercialparabolic trough power plant.# 2003 Elsevier Ltd. All rights reserved.

1. Introduction

Parabolic trough solar technology is the most proven and lowest cost large-scale solar power

technology available today, primarily because of the nine large commercial-scale solar power

plants that are operating in the California Mojave desert. These plants, developed by Luz

International Limited and referred to as solar electric generating systems (SEGS), range in size

Corresponding author. Tel.: +49-221-925970-51; fax: +49-221-2581117.E-mail address: ulf.herrmann@agsol.de (U. Herrmann).

0360-5442/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0360-5442(03)00193-2

from 14 to 80 MW and represent 354 MW of installed electric generating capacity. These plantshave been operating daily for up to 18 years, and as the year 2001 ended, these plants had accu-mulated 127 years of operational experience. These plants sell power to Southern CaliforniaEdison, the local power utility, through standard oer power purchase contracts. One of the keyfeatures of these contracts is the ability of these plants to produce energy during the periodwhen the utility has the highest demand for power. Thus, a key feature of the SEGS plants isthe ability of the plants to dispatch power to help meet the utilitys peak electricity demand. Insouthern California, the peak energy demand is during the summer afternoon and early evening,corresponding to the air conditioning load. The winter peak load is lower but more pronouncedand occurs in the early evening and corresponds to an evening lighting load.The rst SEGS plant (SEGS I), built in 1984, included 3 h of thermal storage that allowed

the plant to shift electric generation from periods when solar energy is available to the peri-ods when the utilitys peak electric demand occurs. The plant used a mineral oil HTF and atwo-tank thermal storage system: one tank held the cold oil and a separate tank held the hotoil once it had been heated to about 300

vC. This system successfully helped the plant dis-

patch its electric generation to meet the utility peak loads during the summer afternoons andwinter evenings. The mineral oil HTF is very ammable and could not be used at the later,more ecient SEGS plants that operate at higher solar eld temperatures (approximately 400vC). For these plants, the two-tank storage system used at SEGS I is not feasible becausecost of the synthetic HTF is higher, and the high vapour pressure of biphenyl-diphenyl-oxidewould require pressurized storage vessels. As a result, the later SEGS plants used fossil fuel-red backup to allow the plants to dispatch power to peak electric demand periods whensolar energy is not available. Although no new SEGS plants have been built in the last 10years, there is growing interest in the development of new trough power plants. The avail-ability of a thermal energy storage system that would allow these plants to dispatch powerand increase the plant annual capacity factor is a potential economic plus for the technologyover other renewable options.This paper reviews an engineering study that was carried out to evaluate the feasibility of

using molten salt storage in parabolic trough power plants [1]. This storage concept was suc-cessfully tested in the Solar Two project, a solar tower plant that uses molten salt as the HTF[2]. No major technical barriers were identied in this study, and thus the concept appears tohave low technical risk and could easily be used in near-term trough projects. The paperdescribes the proposed storage concept and the results of an economic evaluation. The calcu-lations were done for a 50 MW Rankine cycle and for dierent storage sizes ranging from 0 to15 h of equivalent full capacity operation.

2. Description of plant concept

Parabolic trough power plants consist of large elds of parabolic trough collectors, a heattransfer uid/steam generation system, a Rankine steam turbine/generator cycle, and optionalthermal storage and/or fossil-red backup systems. The collector eld is made up of a largeeld of single-axis-tracking parabolic trough solar collectors. A heat transfer uid (HTF) is

U. Herrmann et al. / Energy 29 (2004) 883893884

heated up as high as 393vC as it circulates through the collectors and returns to a series of heat

exchangers (HX) in the power block, where the uid is used to generate high-pressure super-

heated steam (100 bar, 371vC). The superheated steam is then fed to a conventional reheat

steam turbine/generator to produce electricity. The existing parabolic trough plants have been

designed to use solar energy as the primary energy source to produce electricity. Given sucient

solar input, the plants can operate at full-rated power using solar energy alone. During summer

months, the plants typically operate for 1012 h a day on solar energy at full-rated electric out-

put. To enable these plants to achieve rated electric output during overcast or nighttime periods,

the plants have been designed as hybrid solar/fossil plants; that is, a backup fossil-red capa-

bility can be used to supplement the solar output during periods of low solar radiation. Alter-

natively, thermal storage can be integrated into the plant design to allow solar energy to be

stored and dispatched when power is required.Fig. 1 shows a process ow schematic for a typical large-scale parabolic trough solar power

plant with a two-tank molten salt storage. In this conguration, HTF from the solar eld is

diverted through a heat exchanger that is used to charge the thermal storage system, heating

salt from the cold storage up to 385vC and storing it in the hot salt storage tank. When the

storage system is discharged, salt from the hot storage tank is sent back to the HTF to salt heat

exchanger and is used to heat cold HTF. The heated HTF is then sent to the power plant.

The cooled salt is returned to the cold storage tank. The temperature of the cold salt is about

300vC.

Fig. 1. Schematic ow diagram of a parabolic trough power plant with two-tank molten salt storage.

885U. Herrmann et al. / Energy 29 (2004) 883893

3. Storage system

The thermal storage system consists of the following principal elements: the nitrate salt inven-tory, the nitrate salt storage tanks, the oil-to-salt heat exchangers, and the nitrate salt circu-lation pumps. All main components, except the heat exchanger, were tested in large scale in theSolar Two project. Beyond it, new pumps for large salt storage tanks were developed and testedby the US company, Nagle Pumps [3].

3.1. Description of components

3.1.1. Nitrate salt inventoryInorganic nitrate salt mixtures are the preferred storage media because the salts oer a very

favourable combination of density (1880 kg/m3), specic heat (1500 J/kg-K), chemical reac-tivity (very low), vapour pressure (

3.1.4. Nitrate salt circulation pumpsMechanical pump seals, suitable for the oxidizing characteristics of the nitrate salts, have not

been identied. Thus, nitrate salt pumps are vertical turbine designs. The pump seal is providedby the combination of (1) a throttle bushing downstream of the last stage, and (2) gravity,which returns bushing leakage to the pump reservoir. The pumps draw suction from the bottomof the thermal storage tanks, and use an extended shaft which allows the pumps to be sup-ported by, and the motors to be located on, a support structure above the tanks.The principal characteristics of the thermal storage equipment are shown in Table 1 for the

range of investigated storage capacities.

3.2. Safety aspects

The thermal storage uid is a mixture of sodium nitrate and potassium nitrate, both of whichare oxidizing agents. When the nitrates are in contact with organic materials at temperaturesabove the ignition temperature, reactions may proceed quickly enough to cause ignition, com-bustion, or explosion.The heat transport uid is a synthetic organic oil, with a nominal composition of 75% by

weight diphenyl oxide/ether and 25% biphenyl. Thus, a leak in the oil-to-salt heat exchangerallows oxidizing materials to mix with a hydrocarbon oil, and the potential exists for a chemicalreaction or combustion.The most relevant reaction data between nitrate salts and hydrocarbons are probably from a

molten salt safety study conducted by Sandia National Laboratories in 1980 [4]. Liquid gasolinewas introduced into an inventory of nitrate salt at a temperature of 600

vC. The hydrocarbons

Table 1Thermal storage equipment characteristics

Item Storage capacity (h)

1 3 6 9 12 15

Active salt inventory (tons) 4778 14,096 28,192 42,288 56,384 70,480Thermal storage tanks dimensions, height diameter (mm)Cold tank 12 16:6 14 26:3 14 37:2 14 45:5 14 37:2a 14 41:6aHot tank 12 16:8 14 26:7 14 37:7 14 46:2 14 37:7a 14 42:2a

Oil-to-salt heat exchangersNumber of HX 3 3 3 4 5 6Exchanger area (m2) 8635b 8635b 8635b 8419c 9067c 9499c

Nitrate salt pumpsFlow rate (kg/s) 1189d 1189d 1189d 1546e 2.081e 2616e

Head (m) 19.2d 19.2d 19.2d 25.6e 32.0e 38.4e

Power (kWe) 316d 316d 316d 547e 921e 1389e

a Two cold tanks and two hot tanks required.b Oil-to-salt heat exchanger duty sized for discharging of storage.c Oil-to-salt heat exchanger duty sized for charging of storage (charging capacity is higher than dischargingcapacity).d Hot salt pump power rating.e Cold salt pump power rating.

887U. Herrmann et al. / Energy 29 (2004) 883893

vapourized when exposed to the nitrate salt, and burned at the surface of the inventory whenexposed to ambient air. However, the hydrocarbons did not react with the nitrate salt below thesurface of the salt inventory. In other words, a temperature of 600

vC was not high enough to

initiate a theoretical reduction reaction in which an oxygen atom was removed from a nitratemolecule.In the event of a tube or weld rupture in the oil-to-salt heat exchanger, a combustion reaction

is believed to be very unlikely, for the following reasons:

. The heat transport uid has ammability ratings of 1 from the National Fire ProtectionAgency, while gasoline has a rating of 3. As a result, it is highly unlikely that the heattransport uid will have a more energetic reaction with nitrate salt than gasoline.

. The highest temperature in the oil-to-heat exchanger is 390vC, which is 201

vC below the

exposure tests conducted by Sandia, and 220vC below the autoignition temperature of the

heat transport uid.. Oxygen is not present in the heat exchanger.

3.3. Cost estimation

Storage system costs are summarized in Table 2 for each of the cases listed in Table 1. Thesystem costs include material, installation labour, and overhead costs associated with eld con-struction, but exclude costs for engineering, procurement, construction management, and inter-est during construction. The values in the table were developed from the following:

. Nitrate salt inventory and nitrate salt pump costs were derived from supplier information onthe 15 MWe Solar Tres central receiver project in southern Spain.

. Storage tank, insulation, and foundation costs were derived from construction cost estimateson the 10 MWe Solar Two central receiver projects in southern California.

. The oil-to-salt heat exchanger unit cost was estimated to be $ 147/m2 [5].

Table 2Thermal storage system costs in US$ 1000

Item Storage capacity (h)

1 3 6 9 12 15

Nitrate salt inventory 2208 6512 13,025 19,537 26,049 32,562Storage tanks 838 2405 4638 6842 9275 11,484Tank insulation 300 608 974 1300 1947 2280Tank foundations 518 984 1653 2273 3216 3823Oil-to-salt heat exchanger 4195 4195 4195 5453 7340 9228Nitrate salt pumps 692 812 1383 1647 2063 2629Balance of system 875 1551 2587 3705 4989 6201Total 9626 17,066 28,453 40,757 54,880 68,206Unit cost, $/kWht 65.63 38.79 32.33 30.88 31.18 31.00

U. Herrmann et al. / Energy 29 (2004) 883893888

. The balance of system includes piping, valves, instruments, electric heat tracing, thermal insu-lation, electric power and control wiring, and structural steel. The costs were estimated to be10% of the identied component costs above.

The unit storage cost for the 1-h case is very high because the high cost of the oil-to-salt heatexchanger must be borne by a small storage capacity. For the remaining ve cases, the cost ofthe oil-to-salt heat exchanger becomes progressively less important.

4. Performance modelling

4.1. PCTrough performance model

To determine the electricity production from solar power plant with and without storage,annual performance calculations of the considered congurations were done using the program,PCTrough. PCTrough was developed by Flabeg Solar based on the experience gained fromsimilar programs such as SOLERGY and the LUZ model for plants of the SEGS type. It hasbeen signicantly extended to include plant congurations with combustion turbine combinedcycles, thermal energy storage and dry cooling. The computer model output has been validatedwith measured data from performance reports of SEGS plants (Price et al. [6]).From the given meteorological input values of insolation and ambient temperature, the per-

formance model calculates hourly performance values of HTF mass ow and temperatures, col-lected solar thermal energy, thermal energy fed into the storage, thermal energy taken from thestorage, heat losses of solar eld, piping and storage, dumped energy, and electric gross and netpower. The model also considers thermal inertia of the solar eld, storage, and the HTF systemunder transient insolation conditions.The energy output of the solar eld is calculated, taking the following into account: radiation,

ambient temperature, condition of solar eld, shadowing caused by other collector rows, cosinelosses, end losses of collectors, shadowing by bellows, reection losses, dirt and alignmentlosses, transmissivity of glass tube, absorption of selective layer on absorber tube, incident angleeects on the above factors and, of course, the thermal losses by radiation, convection and con-duction. For this study, the performance model was extended to account for the specic charac-teristics of a molten salt two-tank storage.

4.2. Performance modelling of SEGS plants with thermal storage

In plant congurations with integrated thermal storage, the solar eld will be oversized com-pared to congurations without storage. The solar multiple of the solar eld depends on thestorage capacity and also on the local weather conditions. The operation strategy of the storageand the whole plant can vary with the local weather conditions and the local electricity demandand tari structure. For the analyses presented here, the following operation strategy wasapplied: priority in operation always has the steam turbine; turbine operation comes beforestorage charging. Only if the thermal energy collected by the solar eld exceeds the design valueof the steam generator of the Rankine cycle, the surplus energy is fed into the thermal storage.

889U. Herrmann et al. / Energy 29 (2004) 883893

If, on the other hand, the thermal energy collected by the solar eld is lower than the design

value, additional energy will be taken from the storage, provided that enough energy was

stored. By this means, the turbine can be operated at full load with high eciency even under

low radiation conditions. Discharging of the storage and operation of the turbine continues

after sunset until the storage is discharged again. No specic operation or discharge prole was

assumed for the storage. The storage is used in a way that the turbine can be operated at full

load as much as possible. However, storage discharging can easily be shifted to periods with

higher revenues, if there are any.The storage model also considers heat losses of the cold and hot storage tanks. Heat loss

measurements of the salt storage tanks were done at Solar Two [2]. A regression analysis was

performed to develop an empirical heat loss equation from the measured values:

qloss 0:00017 Tsalt 0:012 kW=m2 (1)where Tsalt is the temperature (in

vC) of the salt in the hot and in the cold tank, respectively.

The heat loss calculation is not only necessary to determine the eciency of the storage, but

also to determine if and when freeze protection operation is required. The freezing point of the

salt is about 220vC, and it has to be guaranteed that during bad weather periods or plant out-

ages, the salt temperature stays always well above this point. Fig. 2 shows the result of a calcu-

lation for the cooling of the cold storage tank of 6 h storage, if it is out of operation for several

weeks in winter. The calculation was done with meteorological data from Barstow, CA, using

the rst 6 weeks of the year. The gure shows that after 6 weeks without charging and discharg-

ing the storage, the storage temperature will still be over 250vC and hence well above the freez-

ing point. Such a long standstill period of the system is not expected during normal operation.

Consequently, no risk of freezing of the salt exists during normal operation of the storage.This was also demonstrated in the operation of the storage tanks of the Solar Two project [2].

No danger of freezing of the tank inventory occurred in the 11/2 year of system testing. The

Solar Two tanks were considerably smaller than the tanks considered here, which leads to

higher specic heat losses. The operation temperature of the cold tank of Solar Two was

290vC, which is similar to the temperature used here.

Fig. 2. Cooling curve of cold storage tank during standby over a period of 6 weeks.

U. Herrmann et al. / Energy 29 (2004) 883893890

Table3

Resultsofperform

ance

andeconomicanalysesofthetwo-tanksmolten

saltstorageconcept

Case

50MW/0h

50MW/1h

50MW/3h

50MW/6h

50MW/9h

50MW/12h

50MW/15h

Site

Barstow

Barstow

Barstow

Barstow

Barstow

Barstow

Barstow

DNR(kWh/m2a)

2717

2717

2717

2717

2717

2717

2717

Nominalpower

(MW)

50

50

50

50

50

50

50

Storagesize

(h)

01

36

912

15

Solareldsize

(m2)

305,200

340,080

374,960

479,600

584,240

619,120

654,000

Power

block

grosse

ciency

37.5%

37.5%

37.5%

37.5%

37.5%

37.5%

37.5%

Plantperform

ance

Operatingscenario

Solaronly

Solaronly

Solaronly

Solaronly

Solaronly

Solaronly

Solaronly

Solarthermal(G

Wh/a)

378

421

484

602

718

789

817

Steam

turbine,gross(G

Wh/a)

119

133

154

196

238

267

278

Parasitics

(GWh/a)

89

12

16

20

25

27

Totalfuel(106m3/a)

00

00

00

0Fullloadhours(h/a)

2235

2480

2836

3607

4361

4840

5019

Capacity

factor(%

)26

28

32

41

50

55

57

Netelectric(G

Wh/a)

111.8

124.0

141.8

180.4

218.0

242.0

250.9

Economy

Totalprojectcost(in1000US$)

118,774

138,360

155,564

192,726

229,786

253,882

277,365

AnnualO&M

cost(in1000US$)

3514

3652

3843

4181

4498

4710

4877

LEC(U

S$/MWh)

141.7

145.2

140.9

134.0

129.9

128.2

134.0

891U. Herrmann et al. / Energy 29 (2004) 883893

Nevertheless, freezing of salt has to be avoided under all circumstances when unexpectedoperation conditions or plant outages will occur. Therefore, heat trace cable will be installed atthe salt pipes and the salt-to-oil heat exchanger, and immersion heaters will be installed in thetanks, to prevent the salt from freezing in emergency situations. Electrical heating in normaloperation is not expected to be used. Thus, no auxiliary electricity consumption of the heating isconsidered in the performance analyses. But the investment cost for the electrical heating systemis taken into account in the cost estimation.The performance calculation also takes into account that the live steam temperature is lower

during storage operation than during daytime, when steam is generated directly by the solareld. This leads to a slight decrease of power block eciency.The results of the annual performance calculation are presented below in Table 3.

5. Economy of concept

The economic value of the molten salt storage concept was assessed by a levelized electricitycost (LEC) calculation. The LEC was developed using the following equation:

LEC $=MWhe Investment cost Fixed charge rate Fuel costO&M costNet electric output

(2)

The xed charge rate is an economic factor, which converts the capital cost to an equivalentannual expense. A representative value of 0.104 is used for this study. The input data and theresults of the performance and LEC calculation are presented in Table 3. The main result of theanalysis is depicted in Fig. 3. The plot shows the LEC and the number of full load hours fordierent storage capacities and for a reference conguration without storage.According to Fig. 3, two-tank molten salt storage systems are economically attractive, if the

storage system has a minimum size. Already, the cost estimation of the storage system itself(Section 3.3) showed that the specic cost for a small storage system is relatively high because ofthe high cost of the heat exchanger. Only storage system with a capacity bigger than 3 h canachieve LECs, which are lower than those for a trough plant without storage. The lowest LEC

Fig. 3. Levelized electricity cost for trough plants with molten salt storage.

U. Herrmann et al. / Energy 29 (2004) 883893892

was calculated for a plant with a storage capacity of 12 h. The reduction in LEC is about 10%compared to the reference system.Larger storage systems again lead to higher LECs. The reason for that is not just the rising

specic cost (see Table 2). The main reason is the lower utilization period of such big storages.On good days in summer, the turbine can be operated about 12 h directly from the solar eld.Hence, the storage can only be used in the remaining 12 h and the hot tank cannot be dis-charged completely. A portion of about 20% of the storage capacity cannot be used, whichmakes the system less economic.In addition to the economic improvement, the capacity factor of the plant also increases con-

siderably. For a 9 h storage, the full-load hours are already doubled compared to a plant with-out storage, and for a 15 h storage, the plant can operate at full load for almost 5000 h.

6. Conclusion

Thermal storage can considerably improve the attractiveness of solar thermal power plants. Itallows to extend or to shift the operation of the plant from sunny periods with a high peakdemand. Thus, the plant can operate much more exibly and times of mismatch between energysupply by the sun and energy demand can be reduced. In the present study, the technical andeconomical feasibility of a two-tank molten salt storage was assessed. No major technical bar-riers were found to realize this concept. The LEC calculation has shown that this concept canimprove the economy of parabolic trough plants, provided that the storage is big enough. Astorage of 12 h full load capacity reduces the LEC about 10%. Hence, storage systems not onlyimprove the exibility of solar power plants but also help to reduce the specic electricity costand thus can support market introduction of the parabolic trough technology.

References

[1] Kelly BD, Hermann U, Kearney DW. Evaluation and performance modelling for integrated solar combined cyclesystems and thermal storage system. Final report prepared for NREL, contract number RAR-9-29442-05,Golden, CO: National Renewable Energy Laboratory; 2000.

[2] Pacheco JE, Gilbert R. Overview of recent results of the solar two test and evaluations program. Paper numberRAES99-7731. In: Hogan R, Kim Y, Kleis S, ONeal D, Tanaka T, editors. Renewable and advanced energy sys-tems for the 21st century. Proceedings of the 1999 ASME International Solar Energy Conference, Maui, HI,April 1114. 1999 [CD-Rom].

[3] Barth DL, Pacheco JE, Kolb WJ. Development of a high-temperature, long-shafted, molten-salt pump for powertower applications. Transactions of the ASME, Journal of Solar Energy Engineering 2002;124(2):1705.

[4] Martin Marietta Corporation (Denver, Colorado). Molten salt safety study. Report SAND80-8179 Albuquerque:Sandia National Laboratories; June 1980.

[5] Nexant Inc. Thermal storage oil-to-salt heat exchanger design and safety analysis, task order authorization num-ber KAF-9-29765-09. San Francisco (CA): Nexant, Inc; 2001 [March 22].

[6] Price H, Svoboda P, Kearney D. Validation of the FLAGSOL parabolic trough solar power plant performancemodel. In: Stine WB, Tanaka T, Claridge DE, editors. Proceedings of the ASME/JSME/JSES InternationalSolar Energy Conference, Maui, HI, March. 1995, p. 52732.

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