Energy storage startup strategies for CSP plants with a dual-media thermal storage system

Ben XuDepartment of Aerospace and

Mechanical Engineering,

The University of Arizona,

Tucson, AZ 85721

Peiwen Li1Department of Aerospace and



Tucson, AZ 85721

e-mail: [email protected]

Cho Lik ChanDepartment of Aerospace and



Tucson, AZ 85721

Energy Storage Start-upStrategies for ConcentratedSolar Power Plants Witha Dual-Media ThermalStorage SystemA concentrated solar power (CSP) plant typically has thermal energy storage (TES),which offers advantages of extended operation and power dispatch. Using dual-media,TES can be cost-effective because of the reduced use of heat transfer fluid (HTF), usuallyan expensive material. The focus of this paper is on the effect of a start-up period thermalstorage strategy to the cumulative electrical energy output of a CSP plant. Twostrategies—starting with a cold storage tank (referred to as “cold start”) and startingwith a fully charged storage tank (referred to as “hot start”)—were investigated withregards to their effects on electrical energy production in the same period of operation.An enthalpy-based 1D transient model for energy storage and temperature variation insolid filler material and HTF was applied for both the sensible heat storage system(SHSS) and the latent heat storage system (LHSS). The analysis was conducted for a CSPplant with an electrical power output of 60 MWe. It was found that the cold start is bene-ficial for both the SHSS and LHSS systems due to the overall larger electrical energy out-put over the same number of days compared to that of the hot start. The results areexpected to be helpful for planning the start-up operation of a CSP plant with a dual-media thermal storage system. [DOI: 10.1115/1.4030851]

Keywords: concentrated solar power (CSP), thermal energy storage (TES), latent heatstorage, sensible heat storage, charge/discharge strategy, enthalpy-based 1D transientmodel

1 Introduction

The demand for clean and environmentally benign energyresources has been a great concern in the last two decades [1].Reduction of the use of fossil fuels by developing more cost-effective renewable energy technologies has been demanded moreand more [2]. Currently, wind energy and solar energy are themajor focus of renewable technologies. So far as solar energy isconcerned, there are still some disadvantages including highupfront costs, the requirement of a large and sunny surface area,and no night-time functionality [3]. Central to these problems isthe need to store excess energy at a relatively low cost to smoothout the short-term transients (e.g., collector shading from passingclouds) and also to extend the daily operation of solar powerplants during the late afternoon and evening hours [4].

With the technologies developed so far, a CSP system may usesolar tower, parabolic troughs, or linear Fresnel reflectors to con-centrate sunlight and produce intense heat that is carried away bya HTF to send to the thermal power plant (or power block) forpower generation. The heat carried by the HTF is stored by storingthe hot HTF itself, or by transferring the heat to another solid ther-mal storage medium. The stored thermal energy can be withdrawnusing the HTF to feed to the thermal power plant [5].

The most efficient storage method is storing the HTF at a hightemperature directly, without using any additional energy storagemediums. However, because the HTFs developed so far are very

expensive, a dual-media system is preferred since this signifi-cantly reduces the costs of energy storage. In this scenario, boththe HTF and the solid filler material store energy. By introducinga second low-cost solid thermal storage material, energy storageefficiency is sacrificed, because of the heat transfer between theHTF and the low-cost solid filler material [6]. A number ofrecently constructed commercial CSP plants use dual-media stor-age systems to cut costs.

With regards to the solid filler material, it can work on eithersensible heat (forming a SHSS) or latent heat (forming an LHSS),or a combination of both. Because the LHSS or the combinedSHSS/LHSS system offers a larger thermal storage capacity, itcan significantly reduce the storage tank volume compared to thatof using SHSS alone [7,8]. Therefore, using phase change materi-als (PCM) for thermal storage is a promising technology particu-larly for large-scale CSP plants, and it has gotten more and moreattention during the past few years [9]. Nevertheless, due to thethermal resistance caused by the possibly low thermal conductiv-ity of PCM, heat transfer between the PCM and HTF requires alarge surface area. A well-known promising approach to this prob-lem is to make a large number of small capsules filled with PCMinside [10] so that a HTF can flow through a packed bed of PCMcapsules for energy storage and extraction.

When considering the start-up operation of a CSP plant withthermal storage, either through a LHSS or a SHSS, one may exer-cise two strategies on the thermal storage [11]. The first strategy isto start the thermal storage operation on the first day with dailycharge and discharge with an initially cold tank (referred to ascold start from hereafter). In this case, the power plant can use thestored heat on the first day. The second strategy is that a thermalstorage tank is fully charged to high temperature before power

1Corresponding author.Contributed by the Solar Energy Division of ASME for publication in the JOURNAL

OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY

CONSERVATION. Manuscript received December 30, 2014; final manuscript receivedJune 11, 2015; published online June 30, 2015. Assoc. Editor: Nathan Siegel.

Journal of Solar Energy Engineering OCTOBER 2015, Vol. 137 / 051002-1Copyright VC 2015 by ASME

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generation. In this second case, first a few days may be used tocharge the thermal storage system with no power output; after thatthe power generation using stored thermal energy starts with a dis-charge process, followed by charge and discharge cycles daily.The second strategy is referred to as hot start from hereafter. Thestudy to be conducted in this work is to understand which operat-ing strategy for a CSP plant with a dual-media storage system canprovide more energy output in the same given number of days.Two examples of a parabolic trough CSP plant with 60 MW elec-trical power output were subjected to investigation. The LHSSconsiders encapsulated PCM as filler material, while granite rockswere chosen as storage filler material in the SHSS. A general vol-ume sizing strategy of thermocline storage system proposed byXu et al. [12,13] will be applied to size the storage tank volumefor both the SHSS and LHSS, which ensure that the HTF tempera-tures are above the required cutoff temperature during an entire6 hrs of energy discharge. The HTF at temperatures below the cut-off temperature is not acceptable by the power block. About 100runs or days of energy charge/discharge are considered for thestudy of the power output due to the different energy storage start-up strategies. The net extracted energy, the energy storage effi-ciency, and the influences on the operations of the CSP plant dueto the application of the two start-up strategies are to be exploredand compared. It is assumed that during the 100 days, conditionsof sunlight (insolation) and collected heat and fluid temperaturesare the same. This assumption can be made for arid areas, such asthe Southwest of the U.S. However, even if these conditions varyfrom day to day, they are applied equally to the two strategies.Therefore, the conclusions on the start-up strategies should still bevalid.

The approach employed for the studies is based on analyticaland numerical modeling. During the past 2 yrs, researchers havedone a lot of work to explore thermocline SHSS. Li et al. [14] pro-vided a generalized chart for the design and calibration of thermo-cline SHSS. Prasad and Muthukumar [15] numerically studied thetransient behavior and thermal storage capability of a sensibleheat storage unit with embedded charging tubes, by employingthree storage materials: namely, concrete, cast steel, and cast iron.Xu et al. [16] presented a comprehensive transient, two-dimensional, two-phase model for heat transfer, and fluid dynam-ics within the packed-bed thermocline sensible storage system.Yang and Garimella [17] developed a comprehensive, two-temperature model to investigate the cyclic operations of a ther-mocline with a commercially available molten salt as the HTFand quartzite rocks as the filler. In comparison, relatively feweramount of works on the analysis of thermal performance of aLHSS exist in literature. A model developed by Regin et al. [18]considered only a charge process of a tank with PCM filler for aparametric study of material properties. A model by Wu and Fang[19] applied an implicit finite-difference method to solve theequations for the case with the presence of PCM filler in the tankas a general scenario. Results from that model, however, featurednumerous oddities and oscillations in temperature distributionprofiles. To overcome the lower thermal conductivity of PCM ma-terial, Nithyanandam and Pitchumani [20] introduced heat transferaugmentation using heat pipes. Different configurations wereinvestigated by using computational fluid dynamics (CFD), andoptimal orientation and design parameters were obtained. Nithya-nandam et al. [21] also presented a transient, numerical analysisof a molten salt, single tank latent thermocline energy storage sys-tem for repeated charging and discharging cycles to investigate itsdynamic response. They also conducted parametric studies to pro-vide guidelines for designing a packed-bed PCM-based storagesystem for CSP plants.

The current study uses a model for thermocline storage systemsproposed by Tumilowicz et al. [22,23], which was called anenthalpy-based 1D transient model following the work of VanLew et al. [24] with a much needed expansion from SHSS toLHSS. The thermal resistance inside the encapsulated PCM isalso taken into consideration by incorporating a so-called effective

heat transfer coefficient, based on the work of Xu et al. [25]. It hasbeen validated that such an enthalpy-based version of Schumann’sequation [26] can accurately describe the heat transfer and energystorage/extraction between HTF and the packed-bed solid fillermaterial and allows tracking of melting/solidification interfacesthroughout the heat charge/discharge processes. The model canalso be conveniently used to study cyclic heat charge and dis-charge of any interested runs or days at a low computational load.

2 An Enthalpy-Based 1D Transient Heat Transfer

Model for HTF and Filler Material

2.1 Assumptions and Governing Equations. Shown inFig. 1 is a one-dimensional differential control volume of size dzin a packed-bed thermal storage tank. Through energy balanceanalysis, governing equations for the temperatures of HTF and fil-ler material are constructed. Several assumptions are typicallymade to simplify the analysis. First, it is assumed that there is avertical fluid flow through the tank, with a uniform fluid distribu-tion in the radial direction. This reduces consideration into a sin-gle spatial dimension z, which travels with the direction of fluidflow (top to bottom for charge and bottom to top for discharge).Constant transport properties of fluid are used. Van Lew’s analysisof the Peclet number of the convective heat transfer process foundit to be large [27], allowing heat conduction in HTF to beneglected. The heat conduction inside PCM capsules is treatedbased on lumped capacitance method, while between the capsulesthe heat conduction is negligible due to the fact that point contactbetween PCM capsules can be assumed. Finally, the storage tankis assumed to be well insulated. Also, the temperature variation ofthe thermal storage is within 100 �C, and therefore the thermalexpansion of the solid materials (rocks/PCM and tank) is not con-sidered in this analysis.

The enthalpy-based 1D transient model for the heat transferbetween HTF and encapsulated filler material was configured byTumilowicz et al. [22,23] following the work of Van Lew et al.[24]. The governing equations for HTF and filler material are

@Tf

@tþ U

@Tf

@z¼ hSr

qfCfepR2Tr � Tfð Þ (1)

d �hr

dt¼ � hSr

qr 1� eð ÞpR2Tr � Tfð Þ (2)

where Tf denotes the fluid temperature, Tr denotes the filler mate-rial temperature, �hr is the enthalpy of solid filler material, h is theheat transfer coefficient between solid filler material and HTF, qf

and Cf are the density and specific heat of HTF, respectively, qr isthe specific heat of filler material, R is the radius of the storage

Fig. 1 Schematic of a thermal storage tank and a control vol-ume for mathematical analysis

051002-2 / Vol. 137, OCTOBER 2015 Transactions of the ASME


tank, and U is the velocity of HTF based on inlet mass flow rate-and the flow area of (epR2).

The equivalent void fraction e shown in Eqs. (1) and (2) isdefined as

e ¼ Vf=Vtank (3)

where Vtank is the volume of storage tank, and Vf is the volume ofHTF within storage tank.

Also, the filler capsules are assumed to have point contact, and thusfor convective heat transfer analysis, the surface area per unit lengthof tank denoted by Sr in Eqs. (1) and (2) is defined as follows [28]:

Sr ¼ fs 1� eð ÞpR2=r (4)

where r is the radius of encapsulated filler material, and fs is thesurface shape factor, which varies depending on the particle pack-ing scheme. For encapsulated spherical particles (spheres), fs isequal to 3.0.

Based on the Colburn factor [29], the heat transfer coefficientcharacterizing the convective interaction between the primarythermal storage filler material (porous media) and HTF can befound from Nellis and Klein [30]

h ¼ 0:191 _mRe�0:278Cf Prf�2=3= epR2

� �(5)

where Re ¼ 2r � _m= 1� eð Þqf�fpR2½ �, Prf ¼ �fqfCf=kf , _m is themass flow rate of HTF. It is important to point out that Eq. (5) isonly valid for a Reynolds number varying from 90 to 2000.

Assumptions (Biot number less than 0.1) for the application ofthe lumped capacitance method are required in the above model,which allows very small internal temperature gradients in theencapsulated filler material to be encompassed in the heat transfercoefficient h. However, if the characteristic length scale of theencapsulated filler material is large, then the internal thermal re-sistance becomes significant, and a correction to the lumped ca-pacitance method is inevitable. Bradshaw et al. [31], Jeffreson[32], and Xu et al. [25] extended the lumped capacitance methodso that it could be applicable to large Biot numbers for several dif-

ferent solid–fluid configurations. To do it, the intrinsic heat trans-fer coefficient h in Eqs. (1) and (2) can be replaced by an effectiveheat transfer coefficient heff . For the encapsulated PCM inspheres, the effective heat transfer coefficient has the form ofheff ¼ 1= 1=hð Þ þ r=5krð Þð Þ, where r is the radius of PCM capsu-les, and kr is the thermal conductivity of PCM. For other types ofsolid–fluid packing configurations, readers are encouraged to finddetails from the work of Xu et al. [25].

The expressions of Eqs. (1) and (2) still retain a filler tempera-ture term, for which an equation of state is applied to relate it withthe enthalpy of filler material

Tr ¼

�hr � �hr o

Cr s

þ Tr o�hr < �hr melt

Tr melt�hr melt � �hr � �hr melt þ L

�hr � �hr melt þ Lð ÞCr l

þ Tr melt�hr melt þ L< �hr

8>>>>>>>><>>>>>>>>:

(6)

where Tr melt is the melting temperature of PCM, L is the latentheat of fusion, and �hr melt is the enthalpy of PCM at the meltingtemperature when PCM starts to melt.

Introduce the following dimensionless variables of hf

¼ Tf � TLð Þ= TH � TLð Þ, hr ¼ Tr � TLð Þ= TH � TLð Þ, z� ¼ z=H,t� ¼ t= H=Uð Þ, and gr ¼ �hr � �hr refð Þ=Cr s Tr melt � TLð Þ, and thedimensionless governing equations and the equation of state areobtained, which are

@hf

@t�þ @hf

@z�¼ 1

sr

hr � hfð Þ (7)

dgr

dt�¼ � HCR

srhr melt

hr � hfð Þ (8)

where sr ¼ U=Hð Þ qfCfepR2=heffSfillerð Þ, and HCR ¼ qfCfe=ðqrCr s 1� eð ÞÞ.

Consequently, the equations of state in dimensionless form areas follows:

hr ¼

gr � hr melt þ hr ref gr < gr melt

hr melt gr melt � gr � gr melt þ1

Stf

gr � gr melt þ1

Stf

� �� Cr s

Cr l

� �hr melt þ hr melt gr melt þ

1

Stf< gr

8>>>>><>>>>>:

(9)

where Stf ¼ Cr s Tr melt � TLð Þ=L.

2.2 Numerical Solution to the Governing Equations. Tosolve the above dimensionless governing equations, the method ofcharacteristics is applied as described in detail by Tumilowiczet al. [23]. Then, the trapezoidal rule was used to numerically inte-grate the dimensionless governing equations along the solid andfluid characteristics. In brief, however, the key steps of solution tothe governing equations are given here. First, using an equal stepsize in both time and space Dt� ¼ Dz�, a numerical grid featuringboth diagonal characteristics of t� ¼ z� and vertical characteristicsof z� ¼ constant was chosen, as shown in Fig. 2. Steps in timeprogress for j ¼ 1, 2, …, N, while steps in space progress for i¼ 1, 2, …, M. It is clear in Fig. 2 that the two characteristics inter-sect as they progress in time and space. The hyperbolic nature ofthe governing equations passes information from node to node in

a wavelike fashion. We choose two neighboring spatial nodes attime j ¼ 1, h1;1 and h2;1, which will serve as the starting points forinformation propagation through their corresponding characteris-tics. After the passing of one time step to j ¼ 2, the meeting pointof the two characteristics, h2;2, will have received informationfrom the two starting nodes. To represent this mathematically, weapply numerical integration to the equations. Along the diagonalcharacteristic, Eq. (7) becomes

hf2;2� hf1;1

¼ Dt�

sr

hr2;2þ hr1;1

2�

hf2;2þ hf1;1

2

� �(10)

Repeating the process for Eq. (8), we obtain

gr2;2� gr2;1

¼ �Dt�HCR

srhr melt

hr2;2þ hr2;1

2�

hf2;2þ hf2;1

2

� �(11)

Journal of Solar Energy Engineering OCTOBER 2015, Vol. 137 / 051002-3


Equation of state (9) is used to transform the unknown fillertemperature value at node (2, 2) to enthalpy based on local phasestate. This leaves the system of two Eqs. (10) and (11) to solve forthe two unknowns, hf2;2

and gr2;2. With these values obtained, we

step once in space to the new pair of neighboring nodes, h2;1 andh3;1, and use them identically to obtain values at node (3,2). Thisis repeated until all values have been found at j ¼ 2. We then fullyrepeat the spatial sweep at j ¼ 2 to obtain all new values at j ¼ 3.Thus, with a boundary condition provided at the inlet i ¼ 1, alongwith an initial condition in the storage tank at time j ¼ 1, solutionscan be swept through space, stepped in time, and repeated, until theentire grid is fully defined. Application of the trapezoidal rule fornumerical integration implies accuracy of the order O Dt�2ð Þ [33].

For the tracking of interfaces and their travel throughout thetime–space domain, linear interpolation is applied along the inter-face for values at its intersection with the diagonal characteristic.For thermal storage with solid spherical capsules, the 1D transientmodel with the effective heat transfer coefficient heff has beencompared to and validated by Valmiki et al. [34] with experimen-tal data. For other solid–fluid structural combinations, Li et al.[35] conducted a comprehensive CFD-based analysis to verify the1D transient model.

2.3 Validation of the Modeling. Comparison of numericalresults to experimental results by Nallusamy and Sampath [7] wasmade to demonstrate the validity of the numerical scheme of 1Dmodel. The experiment in Ref. [7] used encapsulated sphericalcapsules of paraffin with melting temperature at 60 �C as the PCMand water as the HTF. The inlet fluid temperature was maintainedat 70 �C and the mass flow rate was fixed at 2 l/min. Using theirexperimental conditions and properties, the governing parameterswere estimated as: HCR ¼ 1:008, sr ¼ 1:0269, hf inlet ¼ 1:0,hr melt ¼ 0:7368, Dt� ¼ Dz� ¼ 0:001. Figures 3 and 4 show thecomparisons of the fluid temperature and the PCM temperature asa function of time located at the middle of the tank, respectively.The agreement of the results of simulation and experiment isacceptable.

It is worth noting that the current model does not consider anyheat loss from the tank due to the assumption of perfect thermalinsulation. This may cause more or less discrepancy between thesimulation and test results. Studies about the influence of heat lossto the temperatures in the thermal storage tanks have beenreported by Modi and P�erez-Segarra [36]. The heat loss at thetank surface needs a certain length of time to penetrate and influ-ence the temperature of the center of the tank. Nevertheless, thecurrently proposed 1D model has the capability to incorporate theheat loss by adding a heat loss term on the right-hand side ofEq. (1), without sacrificing the computational efficiency.

3 Tank Volume Sizing

The approach and procedures of tank sizing proposed by Xuet al. [12,13] using the current 1D transient model are applied todecide the storage tank volume/height that can satisfy the require-ment of cutoff temperatures of a HTF in the desired dischargetime period. A typical value of diameter of the tank ispredetermined.

Procedures for thermal storage volume sizing have also beenreported by Yang and Garimella [17] and Bay�on and Rojas [37],where the energy discharge efficiency was used to determine thestorage tank height. In the current work, the criterion proposed byXu et al. [12,13] is chosen for volume sizing, which requires thatthe HTF temperature be above a cutoff temperature during thedesired discharge time period. Such a criterion is actually morepractical in a CSP plant. After obtaining a satisfactory storagetank volume/height, the start-up operation strategies will beinvestigated.

4 Start-up Operating Strategies for a Dual-Media

Storage System

4.1 Cyclic Steady-State Operation and Start-up OperatingStrategies. Considering the real operations in a CSP plant, acyclic steady-state (daily charge followed by discharge) can be

Fig. 3 Comparison of fluid temperature located at the middleof the tank

Fig. 2 Discretization grid for space and time for the method ofcharacteristics

Fig. 4 Comparison of PCM temperature located at the middleof the tank



achieved after a certain number of charge and discharge cycles.At the cyclic steady-state, the energy discharged daily from thestorage system will be independent of the most-initial conditionsinside the storage tank [35]. However, the initial condition in thethermal storage tank can affect the energy output from the CSPplant during days of the start-up period. As described before, wemay choose two projections/strategies on operating the thermalstorage and power plant from the start-up stage to a cyclic steady-state. The first strategy is to start the operation and power outputon the first day based on daily charge and discharge with an ini-tially cold tank (referred to as cold start from hereafter); the sec-ond strategy is that a thermal storage tank is fully charged beforepower generation using stored energy. After that the CSP plantgenerates electricity using stored thermal energy, starting with adischarge and then followed by charge and discharge daily.

For the cold start, the filler material in the storage tank is ini-tially at the low temperature TL, and the enthalpy of filler materialis also kept at hL associated with TL. For energy charging, the hotHTF enters a cold thermocline tank from the top by driving thecold HTF out, as shown in Fig. 5(a). One can imagine that if thedischarge starts immediately after the first charge for either SHSSor LHSS, there will be much less energy discharged from the stor-age tank compared to that in a cyclic steady-state process. There-fore, the heat supplied to the power block will not have asufficiently high temperature to support 6 hrs operation (which isa target set by the U.S. Department of Energy’s Sun Shot Initia-tive [38]). The HTF discharge will stop when its temperature islower than the minimum required temperature (cutoff tempera-ture). It is necessary to find the energy storage and discharge foreach day’s operation until a cyclic steady-state is reached.

The situation may become different if we apply the hot startstrategy to approach a cyclic steady-state. To operate a hot startprocess, the thermal storage tank is fully charged using a certainnumber of days with no energy discharge. Once the storage tankis fully charged, daily operation of thermal charge and dischargefor the power plant starts until it reaches a cyclic steady-state. Forthis period of operation, the power production needs to be pre-dicted so that a comparison with the cold start can be made. Atthe fully charged condition, the thermal storage tank has high tem-perature TH, and the enthalpy of filler material is also kept at hH

associated with TH. During thermal discharge, the cold HTF willbe pumped into the tank from the bottom to extract heat, as shownin Fig. 5(b). Because of high-temperature fluid and solid in the ini-tial conditions, when the cyclic energy discharge/charge operationstarts, the stored energy will support sufficiently 6 hrs heat of dis-charge during the hot start-up process. The total electrical energygeneration during the start-up process period can vary betweenhot start and cold start approaches. Taking a 60 MWe parabolic

trough CSP plant as an example, the two strategies will be com-pared with regard to their energy outputs during the same amountof days allotted to their start-up processes.

It is worth noting that the enthalpy-based 1D transient modelgiven in Sec. 2 is robust; it can be used to calculate the conjugateheat transfer for both sensible and latent-heat-based thermal stor-age. The numerical computations can also consider any number ofinterested charge/discharge cycles.

4.2 Energy Storage Efficiency. If the HTF in a thermal stor-age system can be withdrawn at the temperature that it was origi-nally stored, then the system has no loss of exergy and thus hasthe highest efficiency from the thermodynamics point of view[39]. Based on the work of Li et al. [14], the energy delivered inthe required time period from a dual-media storage tank withencapsulated filler material is always less than that of the energycharged to the tank. If the required heat discharging period istref;discharge, then a round-trip energy efficiency can be defined as

ntrip ¼

ðtref;discharging

0

qfCf Tfðz ¼ H; tÞ � TL½ �dt

qfCfðTH � TLÞ � tref;discharging

(12)

where the numerator represents the energy discharged from thestorage tank, and the denominator represents the energy chargedto the tank in the same time period as that of discharging.

The dimensionless form of the discharge time period is definedas

Pd ¼tref;discharging

H=U(13)

By substituting Pd into Eq. (12), the round-trip energy effi-ciency is in a dimensionless form

ntrip ¼

ðQd

0

hf z� ¼ 1; t�ð Þdt�Qd

(14)

Other than the round-trip energy efficiency, energy storage effi-ciency may be defined by accounting the energy inside the tank atthe beginning and end of energy charging and discharging. Basedon the enthalpy distributions of filler material at the end of charge/discharge in cyclic steady-state, one can easily define the energystorage efficiency as follows:

nstorage ¼

ð1

0

gr z�; t� ¼Y

c

!� gr z�; t� ¼

Yd

!" #steady

dz�

ð1

0

gr z�; t� ¼Y

c

!" #steady

dz�

(15)

where Pc is the dimensionless form of required charging time pe-riod, Pc ¼ tref;charging= H=Uð Þ.

It is important to point out that the storage efficiency defined inEq. (15) is a ratio of the energy extracted from the tank during dis-charging versus the energy stored in the tank after charging. Itneeds to be noted that the different definitions of the energy effi-ciency will give different values that should be distinguished.

5 Results and Discussion

5.1 Basic Parameters of Studied Examples. A CSP plantusing a parabolic trough with an electrical output of 60 MWe atthe thermal efficiency of 35% is studied. The HTF used in the

Fig. 5 Two different HTF charge/discharge strategies: (a) coldstart and (b) hot start



solar field is Therminol VP-1. The power plant requires high andlow temperatures of HTF of 393 �C and 293 �C, respectively. Con-stant transport properties of the HTF are used in the analysis,which are based on the average temperature, 343 �C, in the rangeof 293–393 �C. Granite rocks are used as the filler material forSHSS, and encapsulated PCM of a mixture of eutectic salts (58%NaCl and 42% KCl) is used as the filler material for LHSS.Depending on the packing scheme, the void fraction e in a packedbed with spheres (capsules) may range from 0.26 to 0.476 [28]. Avoid fraction of 0.33 is chosen for the current analysis. The diame-ter of the encapsulated PCM spheres is 4 cm, and the diameter ofthe storage tank is assumed to be 30 m [40]. The required mini-mum time period of daily energy discharge is 6 hrs, referring tothe target set by the U.S. Department of Energy’s SunShot Initia-tive [38]. The thermal storage system is assumed to operate for100 days, within the period the thermal storage operation can defi-nitely approach a cyclic steady-state. The minimum temperature(cutoff temperature) of the discharged HTF feeding to the powerblock, below which the HTF will not be used by the power block,is set to be 360 �C in a discharge process. This cutoff temperatureis based on the conclusion from Ref. [36] that no less than 30 �Cof a temperature drop will provide the largest energy storagecapacity. All the operating parameters and properties of HTF andfiller materials are listed in Tables 1 and 2.

It is worth noting that the maximum temperature of the HTF isset at 393 �C in this study. This is an appropriate setting withregards to operation requirements of solar-collecting fields usingparabolic troughs. The flow rate of the HTF in this study is alsoset at a constant, since the increase in HTF flow rate in the solarfield while keeping a designated high temperature can cause moreparasitic losses [43,44].

Based on the properties provided in Tables 1 and 2, the neededmass flow rate of the HTF is calculated from the followingexpression:

Pele

k¼ _m � Cf TH � TLð Þ (16)

The total mass flow rate of the HTF for the parabolic trough so-lar thermal power plant with an electrical output of 60 MW is

found to be 698.6 kg/s. The dimensionless parameters for deter-mining the effective heat transfer coefficient are listed in Table 3.

The Reynolds number of the flow in the packed bed is 332.8,which is within the range applicable to Eq. (5). The Biot numberfor granite rocks is 0.7, and for encapsulated PCM is 3.8, bothlarger than the criterion of 0.1 for the lumped capacitance method.Therefore, in order to consider the internal resistance of the fillermaterial, a modified effective heat transfer coefficient is neces-sary. As shown in Table 3, the effective heat transfer coefficient is43% less than the intrinsic heat transfer coefficient.

5.2 Storage Tank Volumes/Heights for SHSS and LHSS. Itis important to note that the volume sizing analysis is to obtain avirtual total volume (or total height if diameter is predetermined)for the storage system, which can be made of multiple physicaltanks connected in serial for heat charging and discharging. Themaximum height which a physical tank is limited by depends onthe bearing capacity of the soil with the typical foundation at thelocation.

Based on the parameters and properties provided in Tables 1and 2, the minimum thermal storage tank volumes for SHSS andLHSS were decided using the approach with details described inRef. [12]. The tank height and volume for SHSS areHmin¼ 27.7 m and Vmin¼ 19570.1 m3; while for LHSS they areHmin¼ 17.2 m and Vmin¼ 12151.8 m3. Here, the minimum ther-mal storage tank volumes are determined based on the assumptionthat the tank can supply the needed thermal energy for therequired 6 hrs with HTF temperature above the cutoff temperatureif the tank is fully charged.

The minimum tank height of LHSS (17.2 m) is less than that ofSHSS (27.7 m) by 10.5 m, which shows a 38% reduction of stor-age tank volume/height. The most important benefit of using PCMas the filler material is the reduction of the tank volume/heightand thus the reduction of capital cost. Xu et al. [12] showed thatemploying PCM can lower the capital cost by about 43% fromthat of a SHSS.

According to the general volume sizing strategy in Ref. [12], aminimum storage tank height is determined first, which assumesthat such a tank can be fully charged to meet the energy dischargerequirement. However, in a cyclic steady-state operation, a dailyfull charge is impossible to reach. Therefore, the actual tank sizeor height has to be increased in order to meet the required energystorage so that during the discharge period, the temperature ofHTF is not lower than the cutoff temperature. Using this storagevolume sizing strategy described in Ref. [12], the tank size wasdecided for daily cycles of heat charge and discharge, which canmeet the required energy storage with the discharged HTF temper-ature above a cutoff of 360 �C. For the SHSS, it is found that a14% increase of the minimum storage volume can satisfy therequired energy storage for 6 hrs discharge with HTF above thecutoff temperature. This leads to a storage tank height of 31.6 mfor the SHSS. For the LHSS, a 25% increase of the minimum stor-age tank volume is needed in real operation, which leads to a tankheight of 21.5 m and volume of 15,220.4 m3. The analyses

Table 1 Parameters of a 60 MW solar thermal power plant

Pele ¼ 60 MW Total electrical output k ¼ 35% Power plant thermal efficiencyD ¼ 30 m Diameter of storage tank Tcutoff ¼ 360 �C HTF cutoff temperatureTH ¼ 393 �C High temperature TL ¼ 293 �C Low temperatureDtcharge ¼ 6 hrs Time period of charge Dtdischarge ¼ 6 hrs Time period of dischargee ¼ 0:33 Void fraction dr ¼ 0:04 m Diameter of filler material

Table 2 Properties of HTF and filler materials

HTF (Therminol VP-1) properties [41]qf ¼ 761 kg=m

3 Cf ¼ 2454 J=kg Kkf ¼ 0:086 W=m K �f ¼ 2:33� 10�7 m2=s

Sensible heat filler material (granite rocks [12]) propertiesqr ¼ 2630 kg=m

3 Cr ¼ 775 J=kg Kkr ¼ 2:79 W=m K

Latent heat filler material (58% NaCl and 42% KCl [42]) properties:qr ¼ 2084:4 kg=m

3 Cr s ¼ 1180 J=kg Kkr ¼ 0:5 W=m K Cr l ¼ 1000 J=kg KL ¼ 119 kJ Tmelt ¼ 360 �C

Table 3 Dimensionless parameters and effective heat transfer coefficient

Bi—Biot number for granite rocks 0.7 Bi—Biot number for encapsulated PCM 3.8Prf—Prandtl number 5.1 h—the intrinsic heat transfer coefficient (W/m2 K) 94.8Re—Reynolds number 332.8 heff—effective heat transfer coefficient (W/m2 K) 53.9



hereafter will use the above-determined storage tank heights (forreal operation) that can satisfy the energy storage demand fordaily cyclic operations.

It is important to note that the tank sizing for LHSS is affectedby many parameters, such as latent heat of fusion, melting temper-ature, and specific heat capacities at solid and liquid phase. Inorder to explore the influences of these parameters to the volumesizing of LHSS, a general parametric study is necessary. In thisstudy, the PCM and other operating parameters are all fixed.About a 25% increase from the minimum storage tank volume isneeded in order to meet the requirement that the discharged HTFtemperature is above the cutoff temperature. The authors are alsoworking on the parametric study to LHSS, and relevant resultswill be presented in the near future.

5.3 Daily Cycles Needed to Approach Cyclic Steady-StateOperation. The first step is to find the period of days needed toreach the cyclic steady-state. Under a cyclic steady-state, the tem-perature distribution inside the storage tank at the end of one cycleshould be the same as that of another cycle. This is also the crite-rion to judge if cyclic steady-state is reached. Figure 6 shows theevolution of the dimensionless temperature distribution of the

filler material in the tank at the end of discharge cycles for SHSSand LHSS with the cold start operating strategy. It is seen fromFigs. 6(a) and 6(b) that the cyclic steady-state can be achieved af-ter a few heat charge/discharge cycles. The difference of tempera-ture distribution curves minimized significantly after the thirdcycle.

5.4 Comparisons of Two Start-up Strategies for SHSS(With a Tank Diameter of 30 m). First, it is necessary to verifythat the tank volume of SHSS satisfies the need of real operationof a 60 MWe with HTF temperature above 360 �C in 6 hrs of dis-charge in a cyclic steady-state operation. Figure 7 shows the HTFtemperature variation in a discharge process for the 100th cycle ofoperation. It is proven that the 6 hrs of heat charge allow the heatdischarge to have HTF temperatures above 360 �C during 6 hrs.

Figure 8 shows the temperature distribution of granite rocks atthe end of charge and discharge along the storage tank at severalcycles. Both the temperature curves after charge and dischargeindicate the change from the first cycle of a cold start to cyclicsteady-state. Again, it demonstrates that after about three cycles,the charge/discharge cycles approach to a cyclic steady-state.

Fig. 6 Evolution process of dimensionless temperature distri-bution of filler material at the end of discharges at differentcycles with cold start operating strategy: (a) SHSS and (b)LHSS

Fig. 7 HTF temperature in the discharge process at the exit ofSHSS storage tank at cyclic steady-state

Fig. 8 Temperature distributions of granite rocks along stor-age tank at the end of charge/discharge for the SHSS



As discussed previously, the requirement of discharged HTFbeing above a cutoff temperature is important to the power block.At the end of each discharge process, there must be some energyleft in the thermocline storage system which has a HTF temperaturelower than the cutoff temperature. Therefore, a high cutoff tempera-ture will result in lower energy storage efficiency. Also, because ofthe limit of cutoff temperature, it is understandable that during thefirst several thermal storage cycles of the cold start process, the dis-charged fluid temperature cannot be kept above the cutoff tempera-ture for the entire 6 hrs of discharge. This is a disadvantage of thecold start strategy; namely, before reaching the cyclic steady-state,the thermal storage cannot feed the demand of heat for 6 hrs whilekeeping the discharged HTF above a cutoff temperature.

Opposite to the cold start strategy, one can charge the thermalstorage system with no discharge until the system is fully charged.During this process, the thermal storage system does not provideheat for the power block. This may be viewed as a disadvantageof this start-up strategy. However, after the thermal storage isfully charged, a hot start operation can provide sufficient heat for6 hrs operation until a cyclic steady-state is approached.

Figure 9 shows the extracted energy (with the HTF temperatureabove the cutoff temperature) from the SHSS at each cycle untilcyclic steady-state is approached. It is seen that at cyclic steady-state,the extracted energy tends to have a constant value. The most impor-tant observation about Fig. 9 is the difference of the dischargedenergy due to the cold start and hot start strategies. Because the hotstart has a fully charged thermal storage tank, it discharges a largeamount of thermal energy in the first few cycles. However, theenergy discharge under the cold start strategy in the initial few cyclesis insufficient, which is opposite to that of a hot start strategy, as seenin Fig. 9. The extracted energy of each charge/discharge cycle at thecyclic steady-state has a value of 3:56� 1012J.

When comparing the hot start with cold start strategies forSHSS regarding the total extracted thermal energy (with HTFtemperatures above the cutoff temperature), it is important toremember that the operation of hot start requires the thermal stor-age system being fully charged initially. During this period, thereis no energy discharge and thus no electrical energy output basedon thermal storage. The following analysis will determine theneeded time to fully charge a storage tank.

At a fully charged status, the enthalpy of filler material at thebottom point of the tank is given as

hr fully ¼ Cr s TH � TLð Þ þ hr ref (17)

where hr ref is the reference enthalpy at the low temperature of TL.

Figure 10 shows the variation of the dimensionless enthalpy offiller material at the exit (bottom) of the thermal storage tankagainst the charging time. This figure helps determine how long itwill take to reach the fully charged status. The dimensionless en-thalpy at fully charged status is �hr ¼ 0.4831. Correspondingly, theneeded dimensionless time of charge is �t ¼ 4.83, which is equalto about 12 hrs of continuous charging. If we assume that the stor-age system can only be charged for 6 hrs per day, then it will needabout 2 days to reach the fully charged state. Therefore, for a hotstart strategy, there is no electrical power output relying on thestored thermal energy for 2 days at the beginning. This has to becounted in the comparison of the electrical energy output betweencold start and hot start strategies for 100 days.

Counting the net extracted energy in 100 days, the resultfrom cold start is compared to that of hot start, which is listed inTable 4.

As shown in Table 4, the cold start strategy can actually provideextracted energy by 2.0% more than the hot start during 100 days.Because there is no electrical power output during the first 2 days,the hot start offers overall less energy. From the above results anddiscussion, a CSP plant incorporated with SHSS is recommendedto use the cold start strategy, which can provide more energy out-put for the power generation block. If the thermal efficiency ofpower plant is 35% and the electricity retail price is assumed to be15 cents/kWh for CSP, then such a SHSS CSP plant applying coldstart can make a 0.41 million dollars profit more than that of thehot start, during 1 yr operation. This will become even more sig-nificant when a CSP plant with larger capacity is considered.

5.5 Comparisons of Two HTF Charge/Discharge Strategiesfor LHSS With a Tank Diameter of 30 m. For a LHSS, a similarstudy is conducted to compare the two start-up operation strat-egies. First, it is observed that the thermal storage volumeobtained in this study can meet the requirement that at cyclicsteady-state (at 100 cycles), the discharged HTF temperature isabove the cutoff temperature for 6 hrs. As shown in Fig. 11, the

Fig. 9 Comparison of extracted energy at different cycles forSHSS with cold start and hot start strategies

Fig. 10 Variation of dimensionless enthalpy of granite rocks atthe exit of SHSS versus the charge time to determine how longit will take to reach the fully charged state

Table 4 Comparison of net extracted energy from SHSS withcold and hot start during 100 cycles

Extracted energy during100 cycles, Qtotal

ex

Performancecomparison

Cold start 3:56� 1014 J 2.0% more than hot startHot start 3:49� 1014 J



HTF temperature can be maintained above the cutoff temperatureof 360 �C during the entire 6 hrs heat discharge.

The temperature distributions of PCM material at the end ofcharge and discharge along the storage tank for several cycles of acold start case are shown in Fig. 12. It is found that after severalcycles from cold start, the temperature distribution curves aftereach heat charge become very close, and do so after eachdischarge.

At a cyclic steady-state operation, the LHSS for the current 60MWe CSP plant has energy storage efficiency of around 72%,which is 16.9% lower than the efficiency of SHSS. However, thesize of LHSS is much smaller than the SHSS, which significantlyreduces the investment cost. Detailed comparison of cost analysisbetween SHSS and LHSS can be found in Xu et al. [12].

Similar to the SHSS, the discharged HTF temperature in theLHSS must also be above a cutoff temperature. However, duringthe first several thermal storage cycles of the cold start process,the discharged fluid temperature cannot stay above the cutoff tem-perature in the entire 6 hrs of discharge. This is again the disad-vantage of the cold start strategy; that before getting to the cyclic

steady-state, the thermal storage cannot feed the demand of heatfor 6 hrs while keeping the discharged HTF above a cutofftemperature.

With all the properties and parameters provided in Tables 2and 3, it was easy to obtain the energy that can be extracted out ateach charging/discharging cycle, while maintaining the HTF tem-perature above the cutoff temperature for cold start and hot startstrategies, as shown in Fig. 13. The two energy output curvesmerge together at the cyclic steady-state, after a number of cycles,with a constant extracted energy of 2:53� 1012J, which is lessthan the extracted energy in a SHSS by 29% at cyclic steady-state.

In order to compare the energy extraction in 100 days (with dis-charged HTF temperature above the cutoff temperature) betweentwo start-up strategies, the needed time for the full charge of thestorage tank of LHSS is calculated. Similar to SHSS, the enthalpyof PCM at the bottom of the storage tank at fully charged statusmust meet the requirement that

hr fully ¼ hr melt þ Lþ Cr l TH � Tmeltð Þ (18)

where hr melt is the enthalpy of PCM when the melting starts, L isthe latent heat of fusion, Cr l is the heat capacity of liquid PCM,and Tmelt is the melting temperature.

Figure 14 shows the change of dimensionless enthalpy of PCMat the exit (bottom) of the storage tank in the heat charge process.It can be observed that when the dimensionless enthalpy of the fil-ler material is �hr ¼ 2.9226, the dimensionless charge time is about�t ¼ 12.67, which corresponds to 18 hrs. If 6 hrs charge is appliedto the system, then it will need about 3 days to reach the fullycharged status. In this period, the LHSS will not provide any heatfor electricity output.

Finally, the total extracted energy for LHSS with cold start andhot start during 100 days was obtained, and the results are pre-sented in Table 5. The cold start strategy will be able to provide2.9% more energy than that of hot start. Again, this is becausethat the hot start strategy leaves the first 3 days of no power gener-ation using stored energy. A CPS with LHSS is, therefore, recom-mended to operate with cold start strategy on energy storage. Sincethe cold start can generate 0:07� 1014J more heat than that of thehot start, if the thermal efficiency of 35% and the electricity retailprice of 15 cents/kWh are reasonably assumed for CSP, a LHSSCSP plant with cold start strategy can make 0.41 million dollarsmore than that of the hot start strategy during 1 yr operation.

Fig. 11 Variation of HTF temperature at exit of LHSS storagetank versus discharge time at cyclic steady-state

Fig. 12 Temperature distributions of encapsulated PCM along LHSSstorage tank at the end of charge/discharge at cyclic steady-state

Fig. 13 Comparison of extracted energy at different cycles forLHSS with cold start and hot start strategies



5.6 HTF Charging/Discharging Strategy for SHSS/LHSSWith a Smaller Storage Tank Diameter of 20 m. In order toverify the conclusions from the previous analysis that a cold startcan provide more energy output than a hot start for both SHSSand LHSS, another case with a smaller storage tank diameter of20 m will be discussed. While all material properties from Tables1 and 2 are still applicable, parameters associated with the tank di-ameter will be different, which are listed in Table 6.

The Reynolds number shown in Table 6 is increased to 748.7when the diameter of storage tank is decreased to 20 m. This valueis still in the range of 90–2000 applicable to Eq. (5), which is usedto find the intrinsic heat transfer coefficient. The effective heattransfer coefficient is calculated to consider the internal thermalresistance. Following the same volume sizing procedures, theactual storage tank heights for both SHSS and LHSS with a fixeddiameter of 20 m can be found, as listed in Table 7.

For the same analysis as performed in Secs. 5.4 and 5.5, theextracted energy during the 100 days of operation was obtained aslisted in Table 8.

It is seen in Table 8 that the cold start strategy also providesmore energy output than the hot start for both SHSS and LHSS.Comparing Tables 5 and 6, the cold start strategy in a SHSSalways provides 2.0% more energy than that of hot start. For theLHSS, cold start can provide 2.9% more energy output than thatof hot start. In conclusion, it is recommended that a CSP plant usecold start as the strategy of HTF charge/discharge at the start-upprocess for daily cyclic operations.

6 Conclusions

This paper addresses the start-up heat charge/discharge strat-egies of a CSP plant which has a heat storage system. The effectsand differences of applying cold start and hot start during thestart-up process of the daily cyclic operations for a 60 MWe CSPwith a dual-media storage system were studied. The computationsare based on an enthalpy-based 1D transient model, which waspreviously proposed in Ref. [23]. A general volume sizing strat-egy was applied to find the actual storage tank volume/height thatcan maintain the output HTF temperature above the cutoff tem-perature during the entire 6 hrs discharge process in a cyclicsteady-state operation. The example uses a 60 MWe parabolictrough CSP plant incorporated with storage systems, with bothSHSS and LHSS considered. About 100 days of heat charge/dis-charge cycles were considered for the comparison of the start-upstrategies. Two cases of storage tank diameters of 30 m and 20 mwere studied. The numerical results showed that during the entire100 HTF charge/discharge cycles, SHSS applying a cold startstrategy can provide 2.0% more energy output than that of hotstart, while the LHSS adopting cold start can offer 2.9% moreenergy output than hot start. In conclusion, the cold start strategyis recommended for the daily cyclic operation in the start-up

Table 5 Comparison of net extracted energy from LHSS withcold and hot start during 100 days

Extracted energy during100 cycles, Qtotal

ex

Performanceimprovement

Cold start 2:51� 1014 J 2.9% more than hot startHot start 2:44� 1014 J

Fig. 14 Variation of dimensionless enthalpy of encapsulatedPCM at the exit of LHSS versus charge time for determination ofhow long it will take to reach the fully charged state

Table 6 Dimensionless parameters and effective heat transfer coefficient with the diameter of storage tank of 20 m

Bi—Biot number for granite rocks 1.2 Bi—Biot number for encapsulated PCM 6.8Prf—Prandtl number 5.1 h—the intrinsic heat transfer coefficient (W/m2 K) 170.2Re—Reynolds number 748.7 heff—effective heat transfer coefficient (W/m2 K) 136.8

Table 7 Storage tank sizes for SHSS and LHSS at the storage tank diameter of 20 m

Storage system Minimum volume (m3) Minimum height (m) Actual volume (m3) Actual height (m)

SHSS 19,385.0 61.7 16,399.7 52.2LHSS 12,416.0 39.5 14,154.2 45.1

Table 8 Comparison of net extracted energy and energy storage efficiency at cyclic steady-state with cold and hot start during100 days operation for SHSS and LHSS at the storage tank diameter of 20 m

System Strategy Extracted energy, Qtotalex Performance improvement Energy storage efficiency (%)

SHSS Cold start 3:55� 1014 J 2.0% more than hot start 89.0Hot start 3:48� 1014 J

LHSS Cold start 2:43� 1014 J 2.9% more than hot start 76.0Hot start 2:36� 1014 J



period. The results and discussion are believed to be beneficial forindustrial engineers to plan and operate a CSP plant with a dual-media storage system.

Acknowledgment

The authors are grateful for the support from the U.S. Depart-ment of Energy and National Renewable Energy Laboratory underDOE Award No. DE-FC36-08GO18155. Thanks are also due toMs. Ranjini Kandyil for her valuable suggestions and revisions onthe structure and organizing of this paper.

Nomenclature

C ¼ specific heat capacity (J/kg K)dr ¼ diameter of capsules (m)fs ¼ surface shape factorh ¼ intrinsic heat transfer coefficient (W/m2 K)

heff ¼ effective heat transfer coefficient (W/m2 K)�h ¼ enthalpy (J/kg)

H ¼ overall height of storage tank (m)HCR ¼ dimensionless heat capacity ratio

k ¼ thermal conductivity (W/m K)L ¼ latent heat of fusion (J/kg)_m ¼ mass flow rate (kg/s)

Pr ¼ Prandtl numberPele ¼ total electrical power output (W)QT ¼ total heat required for power generation (W)

r ¼ radius of capsules (m)R ¼ radius of storage tank (m)

Re ¼ Reynolds numberStf ¼ Stefan numberSr ¼ surface area per length scale (m)

t ¼ time (s)T ¼ temperature (�C)U ¼ axial velocity of HTF (m/s)V ¼ volume (m3)z ¼ axial tank location from reference (m)e ¼ equivalent void fraction

gr ¼ dimensionless enthalpyh ¼ dimensionless temperaturek ¼ thermal efficiency� ¼ kinematic viscosity (m2/s)

nstorage ¼ actual energy storage efficiencyntrip ¼ ideal energy storage efficiencyPd ¼ dimensionless time required for discharge

q ¼ density (kg/m3)sr ¼ dimensionless time scale

Subscripts

f ¼ HTFH ¼ highest value of a variableL ¼ lowest value of a variabler ¼ solid filler material

r l ¼ filler in a liquid phase stater melt ¼ filler melting point value

r ref ¼ filler reference valuer s ¼ filler in a solid phase state

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http://www1.eere.energy.gov/solar/sunshot/

http://www1.eere.energy.gov/solar/sunshot/



http://dx.doi.org/10.1016/j.energy.2013.10.095

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