Gregory J. KolbDistinguished Member of Technical Staff,

Sandia National Laboratories,

MS1127, Albuquerque, NM 87185

e-mail: gjkolb@sandia.gov

Evaluation of AnnualPerformance of 2-Tank andThermocline Thermal StorageSystems for Trough PlantsA study was performed to compare the annual performance of 50 MWe Andasol-liketrough plants that employ either a 2-tank or a thermocline-type molten-salt thermal stor-age system. TRNSYS software was used to create the plant models and to perform the an-nual simulations. The annual performance of each plant was found to be nearly identicalin the base-case comparison. The reason that the thermocline exhibited nearly the sameperformance is primarily due to the ability of many trough power blocks to operate at atemperature that is significantly below the design point. However, if temperatures closeto the design point are required, the performance of the 2-tank plant would be signifi-cantly better than the thermocline. [DOI: 10.1115/1.4004239]

Keywords: parabolic trough, thermal storage, thermocline, molten salt, performance

1 Introduction

The Electric Power Research Institute (EPRI) recently com-pleted a study of molten-salt thermocline energy storage systemsfor application within trough and tower solar power plants. Theresults suggest that the capital cost of a thermocline storagesystem could be 33% less than an equivalent amount of storageusing the traditional 2-tank approach [1]. This cost advantageshould lead to a significantly lower levelized energy cost(LCOE) if it can be shown that the annual performance of thethermocline-plant does not degrade relative to a 2-tank plant.Thus, in support of the EPRI study, Sandia compared the annualperformance of a trough plant utilizing 2-tank and thermoclinesalt storage systems.

2 Analysis of 2-Tank Plant

The 2-tank system represents today’s baseline technology asdemonstrated at the 50 MWe Andasol plant now operating inSpain. Since the solar multiple of Andasol is �2, the excess col-lector field energy is stored within a 2-tank molten-salt system forlater use by the steam turbine. A simplified schematic and somekey design parameters [2] are shown in Fig. 1.

TRNSYS 16 was used to simulate the annual performance of this2-tank plant. A combination of standard-TRNSYS components,STEC-TRNSYS [3] components, and some new system-control com-ponents were used to create the model. The logic model isdepicted in Fig. 2. The important model parameters are listed inFigs. 1 and 2.

Rather than developing a detailed model of all components inthe steam-Rankine power block, the system was represented bytwo transfer functions; the inputs to the functions were HTF flowrate and exit temperature, and the outputs were HTF return tem-perature and turbine-generator power output. The transfer func-tions depicted in Fig. 3 were developed by scaling up (from 30 to

50 MWe) a similar empirical model developed for the SEGS VIpower block [4].

The minimum oil-inlet temperature to the steam generator isshown in Fig. 3 to be 300 �C. This is based on actual experience atSEGS VI; near noontime in the winter, the field outlet temperatureis dropped from the design value of 390 �C to �300 �C, and theRankine cycle temperature and pressure are dropped to maintain50 �C of superheat (called sliding pressure operation). The KramerJunction plant staff and Sandia’s evaluation have found thatthis operating approach maximizes the overall solar-to-electric efficiency [5]. (Establishing the minimum oil operatingtemperature is very important for the evaluation of thermoclinestorage, as described later.)

Fig. 1 Andasol-type parabolic trough plant

Contributed by the Solar Energy Division of ASME for publication in the JOUR-

NAL OF SOLAR ENERGY ENGINEERING. Manuscript received January 3, 2011; final manu-script received April 7, 2011; published online August 17, 2011. Assoc. Editor:Rainer Tamme.

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The 2-Tank TRNSYS model was run using a 3-min time step andthe hourly TMY file for Tucumcari, New Mexico.1 This weatherfile was chosen, because the annual DNI is similar to southernSpain, i.e., 2.3 MWh/m2-yr. The model predicted an annual gross

electricity output of 152 GWh. This value is similar to the valuespredicted for Andasol prior to plant startup [6],2 so the modelappears to be reasonable.

Fig. 2 TRNSYS model of an Andasol-type power plant with 2-tank storage

1The hourly DNI values were used at each 3-min time step. No interpolation wasperformed.

2Andasol is predicted to operate 3589 full load hours with 12% from fossil. Thus,solar-only performance¼ 0.88� 49.9 MWe� 3589¼ 157.6 GWh.

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3 Analysis of Thermocline-Plant

The next step in the analysis was to replace the 2-Tank modelwith a thermocline model. The thermocline chosen for study wasdefined by Black and Veatch, under a contract to EPRI [1]. Tankdesign parameters and a simplified schematic are shown in Fig. 4.

The TRNSYS logic model for this plant is depicted in Fig. 5. Onlythe storage portion of the model is shown, since the remainder isthe same as the 2-tank model. The basis of the thermocline tankmodel was the STEC Type 502 developed by Sandia; it is a varia-tion of the standard TRNSYS Type 10 component. The tank is di-vided into several equally sized control volumes (23 stackedcylinders used here), and a first-order differential equationdescribes the energy balance of each. Conduction between adja-cent control volumes is modeled and so are thermal losses to theenvironment. The thermocline model was previously validatedwith experimental data [7] obtained during the test of the180MWh thermocline at Solar One.3

The thermocline-plant TRNSYS model was run using the sametime step and the hourly TMY file as for the 2-tank. The modelpredicted an annual gross electricity output of 149 GWh. The pre-dicted thermocline profiles for several charge and discharge cycleson days near the summer solstice are depicted in Fig. 6.

4 Comparison of 2-Tank and Thermocline

Performance

The predicted annual performance of the thermocline-plant(149 GWh) is virtually identical to the 2-tank plant (152 GWh).Other previous analyses of thermoclines seem to be at odds withthis conclusion since they show a significant drop in efficiency ifthe exit temperature is not maintained near the design value (for

example, see Ref. [8]). As discussed previously, the SEGS VIpower block is designed to operate up to 90 �C below the designvalue, i.e., 300 �C versus 390 �C. It appears that previous analyseshave not taken the SEGS experience into account.

Thermocline-plants would not perform as well as a 2-tank plantif the degradation in temperature was not allowed. To estimate theeffect, the TRNSYS models were rerun assuming a minimum tem-perature of 340 �C and 360 �C. The results of this investigation aredepicted in Table 1.

5 Closing Remarks

The potential cost advantage for the thermocline is the use of1 tank (20,600 m3) versus 2 tanks (16,300 m3 each) and a muchlower volume of solar salt (�5000 m3 for thermocline and�18,000 m3 for the 2-tank). This cost advantage has been

Fig. 3 Empirical model of a 50 MWe steam-Rankine power block

Fig. 4 Andasol-type plant with a 1000 MWh thermocline stor-age system. The single tank is 12.2 m tall and 46.3 m diameter.

3A small salt thermocline (2.3 MWh) was also tested at Sandia [9]. However, thedimensions of the Solar One tank are more representative of a commercial system.

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independently estimated by EPRI [1] and Sandia [9] to reduce thecapital cost of thermal storage for trough plants by �33%.

An issue called “thermal ratcheting” has been identified thatcould potentially lead to failure of the thermocline tank. Theworry is that thermal expansion and contraction of the tank willlead to slumping of the gravel bed which will further lead to ex-

cessive pressures on the tank walls. Sandia is hopeful that thermalratcheting will not be a show stopper for three reasons: (1) the 180MWh thermocline at the Solar One power tower did not fail dueto thermal ratcheting, (2) the structural analysis of thermal ratchet-ing of the Solar One tank suggested the design was acceptable[10], and (3) although the average temperature of the Solar Onetank is 100 �C lower than the tank analyzed here, the delta Tbetween the top and bottom of the tanks is the same (�90 �C). Inthe future, Sandia will perform a detailed evaluation of the ther-mal ratcheting phenomena.

If the thermal ratcheting issue can be resolved, the thermoclineholds great promise for reducing the capital cost of thermal energystorage and lowering the LCOE of parabolic trough plants.

Acknowledgment

Sandia National Laboratories is a multi-program laboratoryoperated by Sandia Corporation, a wholly owned subsidiary of

Fig. 5 TRNSYS model of thermocline storage system. The void fraction of the tank is assumed to be24% based on experience at the Solar One thermocline tank. The density of the rock is 1940 kg/m3

and the specific heat is 880 J/kg-C.

Fig. 6 Prediction of temperature profile throughout the thermocline tank on days near the summer solstice. Node N23 is at thebottom of the tank and node N4 is near the top.

Table 1 Annual electricity produced by an Andasol-type plantas a function of storage type and minimum power block operat-ing temperature

Min oil T topower block

2-Tank plantelectricity

Themoclineplant electricity

300 �C 152 GWh 149 GWh340 �C 149 GWh 135 GWh360 �C 146 GWh 125 GWh

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Lockheed Martin Corporation, for the U.S. Department ofEnergy’s National Nuclear Security Administration under contractDE-AC04-94AL85000.

References[1] Electric Power Research Institute, 2010, “Solar Thermocline Storage Sys-

tems—Preliminary Design Study,” Electric Power Research Institute, PaloAlto, CA, Project 1019581.

[2] Sergio, R., and Gutierrez, Y., 2008, Real Application of Molten Salt ThermalStorage to Obtain High Capacity Factors in Parabolic Trough Plants, SENER,SolarPACES, Las Vegas, NV.

[3] STEC components can be downloaded from http://sel.me.wisc.edu/trnsys/trnlib/stec/stec.htm

[4] Patnode, A., 2006, “Simulation and Performance Evaluation of ParabolicTrough Solar Power Plants,” M.Sc. thesis, University of Wisconsin.

[5] Frank, L., 1995, Simulation of the Part-Load Behavior of a 30 MWe SEGSPlant, SAND95-1293.

[6] Geyer et al., 2006, Dispatchable Solar Electricity for Summerly Peak Loadsfrom the Solar Thermal Projects Andasol 1 Y Andasol 2 (sic), Solar Millen-nium, SolarPACES, Seville, Spain.

[7] Kolb, G., and Hassani, V., 2006, “Performance Analysis of Thermocline EnergyStorage Proposed for the 1 MW Saguaro Solar Trough Plant,” Proceedingsof the ISEC 2006 ASME International Solar Energy Conference, July 8–13,Denver, CO.

[8] Yang, Z., and Garimella, S., 2010, “Thermal Analysis of Solar Thermal EnergyStorage in a Molten-salt Thermocline,” Solar Energy, 84, pp. 974–985.

[9] Pacheco, J., Showalter, S., and Kolb, W., 2002, “Development of a Molten-SaltThermocline Thermal Storage System for Parabolic Trough Plants,” J. Sol.Energy Eng., 124, pp. 153.

[10] Faas, S., Thorne, L., Fuchs, E., and Gilbertsen, N., 1986, 10 MWe Solar ThermalCentral Receiver Pilot Plant: Thermal Storage Subsystem Evaluation— FinalReport, SAND86-8212, Sandia National Laboratories, Livermore, CA.

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