solar steam generation

April 2004 15

F E A T U R EF E A T U R E

Electricity generation from solar energy is cur-rently one of the main research areas in the fieldof renewable energy. Such systems require reli-able control systems to maintain desired operat-ing conditions in the face of changes in solarradiation. Parabolic trough collectors, which are

the most developed line-focus concentrating solar collec-tors, are used to feed industrial heat processes thermally.At present, parabolic reflectors can operate at tempera-tures up to around 400 C by concentrating the directsolar radiation onto a tube through which a fluid ispumped and heated (see Figure 1). Optical concentrationreduces the absorber surface area relative to the collectoraperture area and thus significantly reduces thermal loss-es. This optical concentration requires the collector torotate about a tracking axis, following the daily movementof the sun. Since only direct solar radiation is optically

concentrated, diffuse solar radiation is lost. One advantageof parabolic trough collectors is the low pressure drop ofthese systems as the working fluid passes through a single,straight absorber tube.

This article describes the DISS facility, followed by anexplanation of the main control problem. The controlscheme for one of the three operating modes, the once-through mode, is then discussed, including system models,feedforward blocks, and PI blocks. Finally, experimentalresults and conclusions are presented.

Parabolic TroughsWithin the range of 200400 C, present-day parabolictrough technology uses oil as a working fluid in theabsorber tubes, whereas a mixture of water and ethyleneglycol can be used for lower temperatures. The workingfluid is heated as it passes through the absorber tube of the

0272-1708/04/$20.002004IEEEIEEE Control Systems Magazine

By Loreto Valenzuela, Eduardo Zarza, Manuel Berenguel, and Eduardo F. Camacho

EYE

WIR

E

April 200416 IEEE Control Systems Magazine

solar collectors, thus converting the direct solar radiationinto thermal energy. The hot working fluid is then sent to aheat exchanger, where its thermal energy is transferred.

For the last 15 years a considerable effort has beenmade to develop efficient control systems for solar ther-mal power plants with parabolic trough collector fields[1]. Currently, the best commercial examples of state-of-the-art parabolic trough collectors are the eight solar ther-mal power plants operating in California. These plants,called SEGSs, use oil as the heat transfer medium betweenthe solar field and the power block. The outlet of the solarfield is connected to a heat exchanger that generatessteam to feed a Rankine cycle [2].

Solar radiation, the primary energy source in theseplants, is not controllable. The temperature of the oil atthe outlet of the solar field is controlled by modulatingthe oil mass flow at the field inlet. These systems pos-sess nonlinear dynamics and are affected by severaltypes of disturbances, mainly in the energy input vari-able, which suggests the need for advanced controlstrategies [1].

A parabolic-trough collector system prototype wasimplemented in 19961998 at the PSA in southeast Spain toinvestigate using water as the working fluid in the solarfield of a thermal power plant using a DSG process [3].From 1999 to 2001, different operating strategies and con-figurations were evaluated, taking efficiency, cost, andcontrollability into consideration, and promising resultswere obtained for the commercial implementation of thisnew system, which at present constitutes the mostadvanced plant of this type.

The main industrial application of DSG is expected tobe Rankine-cycle electricity generation, in which steam isdelivered by the parabolic-trough collectors. The DSGprocess increases overall system efficiency while reducinginvestment costs, since it eliminates the oil used at theSEGS plants as a heat transfer medium between the solarfield and the power block, and, consequently, the heatexchanger is also eliminated. The electricity generationcost will be reduced by 26%, according to current avail-able data. Furthermore, a DSG solar field can be used tofeed other industrial processes requiring thermal energyin the form of saturated or superheated steam at tempera-tures below 400 C and pressures below 100 bar. Desalina-tion is a good example of an industrial process suitable forbeing fed by a DSG solar field [4].

The main task of the control system is to provide asteady supply of live steam at the outlet of the solar fieldunder all operating conditions. In the first stage of the DISSproject, financed by CEE-Joule contract JOR3-CT98-0277,simple control structures and pragmatic approaches for thecontroller design were chosen [3], [5], [6]. The advantage ofthis approach is that the plant operators are already famil-iar with the PI or PID controllers used and are able to modi-fy controller parameters to achieve secure operation, thusallowing them to concentrate on the process itself. Afterthis training stage devoted to identifying plant dynamics

Acronyms

DISS Direct solar steam (the R + TD project)DSG Direct steam generation (the process)FFFV Feedforward function feed valve FFIV Feedforward function injector valve MISO Multiple inputs, single output PI Proportional integralPID Proportional integral derivativePSA Plataforma Solar de Almera (Spain)SEGS Solar electricity generating systemSISO Single input, single output

Figure 1. A parabolic trough collector: working principleand components. Solar radiation collected by a parabolicshape reflector is concentrated on an absorber pipe locatedin the focal line of the parabola and through which a heattransfer fluid is pumped.

Steel Structure Parabolic TroughAbsorber

Figure 2. The DISS plant. The collectors are in the trackingposition following the sun when the plant is running.

and operating modes, advanced control strategies, mainlymodel-based predictive control schemes, will be investigat-ed in an effort to optimize plant production.

The PSA DISS FacilityThe PSA DISS facility is a solar system that serves as a test-bed for investigating the DSG process in parabolic troughsolar collectors. Figures 2 and 3 show two views of thefacility; its main characteristics are listed in Table 1.

Although the solar field can operate over a wide rangeof temperatures and pressures, the three main operatingpoints investigated in the DISS project are listed in Table 2.The thermohydraulic behavior and system performance ofthree basic operating modes, namely, once-through, recir-culation, and injection modes (see Figure 4), were investi-gated under actual conditions to identify the specificadvantages and disadvantages of each mode.

In the once-through mode, feedwater is preheated,evaporated, and converted into superheated steam as itcirculates from the inlet to the outlet of the collector loop.The main disadvantage of this concept, which is the sim-plest of the three, is the controllability of the superheatedsteam parameters at the collector field outlet. A waterinjector is placed in front of the last collector to controlthe outlet steam temperature. The water injector is placedhere since, by design, superheated steam is available atthis point. The injector is not placed at the end of the col-lector row, which would be advantageous for control,because thermal losses in the last collector would signifi-cantly increase, thus reducing the energy gain. Moreover,the selective coating of the absorber pipes would bedegraded if the metal piping reached temperatures ofaround 450 C, which is possible when the system is work-ing in operating mode 3 (see Table 2).

April 2004 17IEEE Control Systems Magazine

Nomenclature

Acol Collector aperture [m]E Solar irradiance [W/m2]K Proportional gain (PID)Lcol Collector length [m]Lloop Collector loop length [m]P Pressure [bar]T Temperature [C]Tamb Ambient temperature [C]Tav Average fluid temperature in the collector

loop [C]Ti Integral time constant (PID) [s]Tin Water temperature at collector loop inlet [C]Tin c Water/steam temperature at collector inlet [C]Tinj Injection water temperature [C]Tout Steam temperature at collector row outlet

outlet [C]Tref Steam temperature reference [C]Sabs Absorber pipe area of the collector loop [m2]afv Feed valve aperture demanded [%]aiv Injector valve aperture demanded [%]apv Steam pressure valve aperture [%]em in Inlet mass flow error [kg/s]eT Temperature error [C]hin Specific enthalpy of water at collector row

inlet [kJ/kg]hin c Specific enthalpy of water at collector

inlet [kJ/kg]hinj Specific enthalpy of injection water at

collector inlet [kJ/kg]hout Specific enthalpy of steam at collector

row outlet [kJ/kg]href Specific enthalpy reference of steam at

collector row outlet [kJ/kg]min Mass water flow at collector row inlet [kg/s]min c Mass water/steam flow at collector inlet [kg/s]minj dem Injection water flow demanded at collector

inlet [kg/s]minj set Injection water flow reference at collector

inlet [kg/s]minj Injection water flow at collector inlet [kg/s]w Feed pump power [%] Incrementpfv Pressure drop across feed valve [bar]loop Global collector loop efficiency []col Global collector efficiency []

Figure 3. The Plataforma Solar de Almera. This aerialview of the research center where the DISS plant is mountedshows the CESA solar system with central receiver on theleft-hand side, the DISS collector loop in the center, and theSSPS area on the right-hand side. Located in the SSPS areaare the CRS solar system with central receiver, the ACUREXparabolic-trough collector field, a solar chemistry area, sixparabolic dishes with Stirling engines, a solar furnace, andother solar facilities for research.

In the injection mode, water is injected at severalplaces along the row of collectors. The measurement sys-tem needed for control in this mode did not work proper-ly during experiments [3], [7]. The complexity and cost ofthis operating mode made it advisable to undertake newdevelopments.

In the recirculation mode, the most conservative ofthe three modes, a water-steam separator is placed at theend of the evaporation section of the row of solar collec-tors. The amount of water fed at the inlet of the evapora-tor is greater than the amount that can be evaporated. Inthe intermediate separator, the excess water is recirculat-

ed to the collector loop inlet,where it is mixed with the pre-heated water. The excess water inthe evaporation section guaran-tees good wetting of the absorbertubes and prevents stratification.The steam produced is removedfrom the water by the separatorand is fed into the inlet of thesuperheater section. This type ofDSG system is highly controllable[6], but the excess recirculatedwater, the middle water-steamseparator, and the water recircula-tion pump all increase the para-sitic load of the system.

The preheating, evapo-ration, and superheatingsections are not preciselydefined in the once-through and injectionmodes. The length ofthese zones depends onthe inlet water flow rateand temperature, the pres-sure in the solar field, and

the radiation available. In the recirculation mode, thesuperheating process starts in the next-to-last collector, butthe length of the preheating section and, consequently, thestarting point of the evaporation section are not preciselydefined, depending in part on the operating conditions.

All three modes have advantages and disadvantages.The investment costs and complexity of the once-throughmode are lowest, and this mode has the best perfor-mance. On the other hand, the once-through mode is noteasy to control, requiring a more complex control system.One of the objectives of the DISS project has been todemonstrate that it is possible to operate the plant in theonce-through mode by guaranteeing flow stability andacceptable controllability [3].

Control ProblemSolar radiation cannot be manipulated and is subject toslow changes due to the daily cycle and in mirror reflectivi-ty caused by dust, as well as fast changes, such as passingclouds. The inlet energy of this plant is affected by thesedisturbances, as well as by changes in the temperature orpressure of the inlet water. While these effects are true of


Parameter Value

Collector row length 500 mCollector type Modified LS-3Collector aperture 5.76 mNumber of collectors 9 50-m-long collectors

2 25-m-long collectorsOrientation of the solar collectors North-SouthAbsorber pipe outer diameter 70 mm Absorber pipe inner diameter 50 mmOptical efficiency of solar collectors 73%Total mirror surface 2760 m2

Maximum pressure at the field outlet 100 barMaximum outlet temperature 400 CMaximum steam production 0.85 kg/s

Table 2. Operating points studied in the DISS solar field.

Solar Field Inlet Conditions Temperature Outlet Conditions TemperatureConditions Pressure [bar] [C] Pressure [bar] [C]Mode 1 40 210 30 300Mode 2 68 270 60 350Mode 3 108 300 100 375

Figure 4. Basic concepts for direct solar steam generation inparabolic trough collectors. In the once-through mode, wateris directly converted into superheated steam in the collectorrow. In the injection mode, water is injected at several placesalong the collector row. In the recirculation mode, there is awater-steam separator placed in the middle of the row.

Once-Through Concept

Injection Concept

Recirculation Concept

Table 1. Technical data for the PSA DISS test loop.

all solar plants with collector fields, in the particular case inhand, the control task is still more complex than in Califor-nia solar power plants [2] or others at PSA [1] because ofthe two-phase flow, complicating the engineering of the sys-tem as well as the control system that must be designed forthe solar field. As mentioned earlier, the thermal fluid cur-rently employed in other parabolic-trough solar fields is synthetic oil, theoutlet temperature of which is controlledby mass flow manipulation at the fieldinlet. In the DISS test loop, both the tem-perature and pressure of the fluid mustbe controlled to maintain the desiredsteam conditions at the outlet as deter-mined by turbine specifications.

Independent of the operating mode,the main objective of the control system is to obtainsteam at a constant temperature and pressure at the out-let of the solar field in such a way that changes producedin the inlet water conditions and in the solar radiationaffect only the amount of steam produced by the system,and not its quality.

Control Scheme for the Once-Through ModeThe control scheme for the once-through mode presentedhere has been designed, implemented, and tested withinthe framework of the DISS project [3]. As the first step, themain dynamics were approximated by linear models. Afterstudying the control scheme and analyzing possible loop

interactions, the SISO transfer functions of all relevant con-trol loops (Table 3) were experimentally investigated forthe three different operating points defined in Table 2. Sys-tem identification was used to estimate open-loop processparameters such as gains, deadtimes, and time constantsthat experimentally fit step response data. Based on the

identified low-order models, PI controller parameters werechosen by means of the process reaction method by simu-lating the closed-loop responses and by modifying theparameters when necessary to provide safe stability mar-gins [8], [9]. A final optimization of the parameter valueswas made in subsequent tests at the plant. Additionally,the control scheme designed for the once-through modeincludes mixed feedforward-cascade control schemes tocontrol the outlet steam temperature.

Some characteristics of the implemented PI functionsare as follows:

The PI output is calculated using a classical interac-tive controller. The transfer function of the con-troller has the form output = K(1 + 1/Tis)error.


Table 3. Once-through mode: Models and PI control loop parameters.

PI Parameters

Control Loop Model Kp Ti [s]

Feed pump G(s) = 0.1375s2 + 1.322s + 0.3329 1.1%/bar 10

Outlet steam pressure G(s) = 4.543103s 5.05 105

s2 + 4.976 102s + 9.693 105 5.3 %/bar 184

Outlet steam G1(s) =a

s + becs 0.0015 kg/s/C 600

temperature control viainjector valve (G1 master where a [3.12, 8.13], b [5 103, 6.6 103]loop, G2 slave loop) and c [70, 100]

G2(s) =3.2 104

s + 0.2 500%/kg/s 12

Outlet steam G1(s) =a

s + becs 8 105 kg/s/C 250

temperature control viafeed valve (G1 master where a [1.365, 2.526], b [1.8 103, 9.6 104]loop, G2 slave loop) and c [395, 750]

G2(s) =3 102s + 0.1 20% kg/s 12

The main task of the control system is toprovide a steady supply of live steam at

the outlet of the solar field under alloperating conditions.

An antireset (integral) wind-up function to compen-sate for output saturation.

A set-point modifier to define the action to be takenon a set-point change. The two options implemented

are PI and integral only on set-point change. The firstoption causes a jump in the control output due tothe proportional contribution from the error created

by a set-point change, whereas, in the second option,the proportional contribution of the error is sub-tracted from the integral contribution, thus eliminat-ing the jump in control output and resulting in

integral action only on a change in set-point. Bumpless manual-to-auto transfer.

The process diagram, including themost important feedback loops for theonce-through operating mode, isshown in Figure 5. The main controlloops for the solar field in the once-through operating mode are as fol-lows: Feed pump control loop: The rota-

tional speed of the feed pump isadjusted by a PI controller to maintain a specificpressure drop in the feed valve, creating a flow insteady state that is directly proportional to the valve


Figure 5. Diagram of the DISS test loop configured in the once-through mode. Four control loops comprise the implemented control scheme. Although outlet steam pressure and temperature loops are the main control loops, there is anadditional controller for maintaining a constant pressure drop in the feed valve.

POT

FT

FC TC

Solar Collectors Row

PDC

FeedPreheater

Feed PumpFeedwater Tank Air Condenser L.P.

EquivalentTurbineLoad

H.P

. Steam

Superheated Steam

LC

FinalSteam Separator

FC TC

FTLT

TT

PTPC

Vapor Feed

Injection LineFlashTank

LegendTT - Temperature TransmitterFT - Flow TransmitterPT - Pressure TransmitterPDT - Pressure Drop TransmitterLT - Level Transmitter

TC - Temperture Control LoopFC - Flow Control LoopPC - Pressure Control LoopPDC - Pressure Drop Control LoopLC - Level Control Loop

In the once-through mode, feedwateris preheated, evaporated, and convertedinto superheated steam as it circulatesfrom the inlet to the outlet of thecollector loop.

opening. The feed pump control loop, therefore, pro-vides a linearized flow relationship between valveposition and flow for PI control.

Outlet steam pressure control: The steam produced bythe collector row feeds a steam separator. The outletsteam pressure is kept constant by adjusting a steamcontrol valve with a PI controller.

Outlet steam temperature control loops: The outlettemperature control is achieved by inlet feed flowcontrol and water injection in the superheater. Theformer control ensures that the steady-state inletflow matches radiation conditions, whereas the lat-ter control is based on PI-feedforward control to pro-vide rapid response to sudden disturbances bymeans of water injection at the inlet of the last col-lector of the solar field.

PI functions for the first two control loops are imple-mented based on the process reaction method, and stabili-ty margins are determined by simplified linearized models

(see Table 3). Outlet steam temperature control requires amore detailed design because the process is strongly affect-ed by disturbances at the inlet and by disturbance vari-ables, and acceptable control cannot be achieved withconventional PI or PID schemes. Contrary to the recircula-tion mode illustrated in Figure 6, in the once-through modethere is no intermediate separator in the field that sup-presses disturbances occurring in the preheating and evap-oration sections, and the starting point of the superheatingsection is not precisely defined. This condition reduces thecontrollability of the once-through operating mode whencompared with the recirculation mode, in which the con-trol loops are based on the following simple PI controllers:recirculation pump control loop (recirculation flow con-trolled by PI control of the rotational speed of the recircula-tion pump), feed pump control loop (the rotational speedof the feed pump adjusted by a PI controller to maintain aspecific pressure drop across the feed valve), middle steamseparator liquid level control loop (to maintain the level


Figure 6. Diagram of the DISS test loop configured in the recirculation mode. Five control loops comprise the implemented control scheme. Although outlet steam pressure and temperature loops are the main control loops, there arethree additional controllers for settling the recirculation water flow, the pressure drop in the feed valve, and the liquid level inthe middle separator.

Solar Collectors Row

PDC

FeedPreheater

Feed PumpFeedwater Tank Air Condenser L.P.

EquivalentTurbine

Load

H.P

. Steam

Superheated Steam

LC

FinalSteam Separator

TC

LT

TT

PTPC

Vapor Feed

Injection Line

LegendTT - Temperature TransmitterFT - Flow TransmitterPT - Pressure TransmitterPDT - Pressure Drop TransmitterLT - Level Transmitter

TC - Temperture Control LoopFC - Flow Control LoopPC - Pressure Control LoopPDC - Pressure Drop Control LoopLC - Level Control Loop

FlashTank

PDT

LC LTSeparator

MiddleSteam

FT FC

RecirculationPump

FeedValve

around a nominal value, the feed flow is adjusted to controlthe aperture of the feed valve whose pressure drop is beingcontrolled by the feed pump with P-PI control), outletsteam pressure control loop (by adjusting the aperture ofthe steam control valve in the steam separator with PI con-trol), and outlet steam temperature control loop (by waterinjection in the inlet of the last collector using PI control ofthe injector valve). The last two control loops are the maincontrollers of the system in recirculation mode used toguarantee the steam quality at each instant. The remainingcontrollers are needed to improve the behavior of the over-all control system and for operational feasibility. Themethod for obtaining the PI parameters is the same asexplained for the case of the PI controllers for the once-through mode.

The solution adopted in the DISS project for the once-through mode is to control the outlet steam temperaturewith forward action. The parameters of the PI functionsappearing in these schemes are chosen using the processreaction method and by studying the closed-loop systemstability margins. Parameters a, b, and c in Table 3 are

related to the uncertainty of the models obtained. Depend-ing on the operating conditions (outlet steam flow produc-tion, temperature, pressure, and solar radiation available),the gain, time constants, and time delays vary. The differ-ent model parameters influence the PI control design.Therefore, once a set of PI parameters is chosen, closed-loop simulations are performed by varying the model para-meters to guarantee wide stability margins for the wholerange of model parameters. Consequently, the selection ofthe PI parameters is conservative. The detailed schemesare discussed in the following subsections.

Although interactions between loops exist, they aresmall because the two slave loops, which are fast com-pared to the other loops, are able to reject slow distur-bances due to the interactions caused by other loops.The outlet steam pressure loop is also faster due to asmaller time constant and no dead time and is thus ableto reject the disturbances coming from the slower tem-perature loops. Also, the temperature loops have aninherent interaction reduction mechanism. As a result,the interactions are canceled by the control strategy


Figure 7. Outlet steam temperature control based on forward action by means of a feed valve. The outer loop of the cascadestructure consists of a feedforward controller, which dictates the nominal feedwater flow, in parallel with a PI controller. Theinner control loop is a PI controller with antiwindup action.

ETin

TambTinj

minj_set

FeedforwardController

FFFV

PITrefmeTeT

mff

min_dem em_in PIAnti-Windup Plant

Tout+ afv

min

Table 4. Specific enthalpy of the fluid: Parameters of the linear regressions.

StandardPressure [bar] Phase Temperature [C] a1 a2 R-square Deviation [kJ/kg]30 Water 100 < Tfluid < 234 21 4.38 0.99989 3

Steam 234 < Tfluid < 400 +2225 2.54 0.99868 760 Water 100 < Tfluid < 276 34 4.48 0.99965 7

Steam 276 < Tfluid < 400 +1940 3.12 0.99686 11100 Water 100 < Tfluid < 312 54 4.61 0.99905 12

Steam 312 < Tfluid < 400 +1480 4.10 0.99415 15

designed. These points willbe explained more fully inthe following sections.

FeedforwardControl of OutletSteam Temperature

Feed Valve AdjustmentThe great variations in solarradiation and the long resi-dence time of the fluid in thefield call for the use of feedfor-ward action to anticipate theeffect of load changes on theoutputs to be controlled; thatis, the control system shouldcalculate the adequate value of the inlet mass flow in advanceso that the outlet steam temperature remains within therange of desirable reference values. The performance of thesystem in the once-through mode depends on the inlet flowcontrol. Changes in solar radiation and inlet fluid temperaturerequire the flow rate to change to maintain the desirable out-put. If changes involve wide oscillations, the solar field perfor-mance is strongly affected. Not only do thermal and pumpinglosses increase, but the relatively narrow margin between thedesign maximum outlet temperature and the actual tempera-ture, which triggers the alarm signal, may be bridged by wideoscillations.

To manage these instabilities, the outlet steam tempera-ture control loop is a mixed cascade-feedforward controlloop (Figure 7) aimed at guaranteeing a desired flow in theface of valve nonlinearities and changes in disturbancesaffecting the loop (see Figure 7 and Nomenclature). Thefeedforward term uses a process model to effect changesin the controller output in response to measured changesin the load before errors occur. The outer loop is com-posed of a feedforward function FFFV in parallel with a PIcontroller with fixed parameters. Block FFFV calculates anominal flow mff , and the parallel PI controller correctsthis value according to the current output Tout. In a set-point change, this PI controller uses only integral actionbecause the new temperature reference also passes to theFFFV block that calculates the nominal flow mff . The flowmin dem calculated by this master loop is the input to theinner slave PI control loop, which calculates a new aper-ture afv of the feed valve. The saturation included in frontof the PI inner control loop limits the inlet water flow to aminimum value of 0.3 kg/s; this limitation guarantees tur-bulent flow in the absorber pipes and consequently con-strains the temperature gradients in the cross-sectionalarea of the pipes that should be less than 50 C, which is atemperature gradient limit from the point of view of thepipe thermal stress.

As previously mentioned, the PI parameters of the outerand inner loops are calculated from open-loop responsesusing the process reaction method for feed pump control andpressure control loops. Conservative parameters are chosenfor the PI controller of the master loop, reducing interactionswith the temperature control loop using the injector.

The feedforward action is obtained from the simplifiedsteady-state energy balance formulation for the collectorrow [13] given by

{Enthalpy

out

}

{Enthalpy

in

}=

{Energy

collected

} {Losses}.

The collected energy is corrected using an estimated effi-ciency factor that implicitly considers the optical efficien-cy and, consequently, the optical losses. The simplifiedenergy balance equation can be written as

(min + minj)hout (minhin + minjhinj) =loop Acol LloopE Ul Sabs(Tav Tamb).

In this equation, the specific enthalpy hout at the outlet isreplaced by the outlet enthalpy reference href , and thewater flow rate minj injected in the last collector isreplaced by the nominal injection flow minj set establishedin the temperature control loop by means of the injectorvalve to avoid feeding back variations (that could be oscil-latory) dictated by the temperature control loop by meansof the injector valve in the block FFFV. Such feedback dete-riorates the temperature response. Considering these sub-stitutions, the feedforward control equation used tocalculate the nominal feedwater flow mff to achieve thedesired outlet temperature Tref is given by

mff =loop Acol LloopEUl Sabs(Tav Tamb)minj set(hrefhinj)

href hin,


Table 5. Thermal loss factor Ul in LS-3 collectors: b1, b2, and b3 values.

Fluid Average Temperature [C] b1 b2 b3Tav < 200 0.687257 0.00194 0.000026200 < Tav < 300 1.433242 0.00566 0.000046300 < Tav 2.895474 0.01640 0.000065

Table 6. Average temperature values used in the FFFV controller for the three operating points of the DISS test loop.

Pressure [bar] Inlet temperature [C] Outlet Temperature [C] Tav [C]

30 210 300 23760 240 350 277

100 280 400 316

where loop was estimated from experimental data as 0.53;Acol, Lloop, Sabs are geometric parameters (Table 1); E is afiltered value of the measured solar irradiance; href, hinj,and hin are specific enthalpy values calculated from theoutlet pressure P and the corresponding temperaturesTref, Tinj, and Tin using

hfluid|P = a1 + a2Tfluid [kJ/kg],

where a1 and a2 are coefficients estimated by linear regres-sion using the enthalpies and temperature values in ther-modynamic tables [10] (Table 4). Ul is a factor related tothe thermal losses, which for an LS-3 type collector can beapproximated by [11]

Ul = b1 + b2(Tav Tamb) + b3(Tav Tamb)2,

where b1, b2, and b3 depend on the average temperature ofthe fluid in the absorber pipes (Table 5). To simplify thecontrol-loop structure, the average temperature Tav of thefluid in the field is approximated by a constant value for thethree different operating points. Values based on inlet andoutlet conditions and conditions in the preheating, evapo-ration, and superheating sections are listed in Table 6.

Injector Valve AdjustmentThe outlet steam temperature can also be controlled byinjecting preheated water into the last collector, providinganother degree of freedom to allow a fast reaction in the

outlet temperature; however, there are regular changes inthe outlet steam temperature of the previous collector, inthe steam flow rate, and in the injection water temperatureinfluencing the behavior of this loop. These changes aremore frequent and stronger in the once-through modethan in the recirculation mode due to the lack of an inter-mediate separator.

A controller based on forward action has also beendesigned for the outlet steam loop. The feedforward blockcorrects the injection water flow rate at the inlet of the lastcollector, taking into account changes in the collector inlettemperature and mass flow, the injection water tempera-ture, and the outlet temperature reference.

The mixed cascade-feedforward control scheme isshown in Figure 8, where the cascade structure compen-sates for actuator nonlinearities. The outer loop is com-posed of a feedforward function FFIV in parallel with a PIcontroller with fixed parameters. The output mff iv of theblock FFIV corrects the PI controller output meT . Whenthe controller is set in automatic mode, a nominal injectionwater flow minj set is established at around 10% of theexpected steam mass production. The outer control loopcorrects this nominal value, and a new injection water flowvalue minj dem is calculated and dictated from the injector.This new value is the input for the inner loop, a PI controlloop, which determines a new aperture value aiv for theinjection valve. Injection valve nonlinearity detected dur-ing experiments is compensated by the cascade structure.The saturation included in the master loop avoids zero


Figure 8. Outlet steam temperature control based on feedforward action by means of an injector. The nominal injection flowis dictated manually when the controller is operating in automatic mode. This nominal value is corrected by the output of theouter loop of the cascade structure, which consists of a feedforward controller in parallel with a PI controller. The inner con-trol loop is based on a PI controller with antiwindup action.

Tinj

Tin_c

min_c

FeedforwardController

FFIV

PI

PIAntiwindup

Plant

Tref eT meT

mff_iv

minjminj

Tout

minj_set

minj_dem aiv


Table 8. Outlet steam temperature control with injector valve: FFIV parameters.

Outlet Steam StandardPressure [bar] c1 c2 c3 c4 c5 R-square Deviation [kg/s]

30 6.212 104 0.00313 1.7 106 3.0 106 0.00171 0.95035 0.0011260 8.457 104 0.00505 4.4 106 7.1 106 0.00279 0.95219 0.00146

100 8.942 104 0.00494 4.2 106 5.8 106 0.00261 0.95167 0.00117

Table 7. Design of the FFFV: Input data sets.

Input 30 bar 60 bar 100 bar

Direct solar irradiance1 [650, 1000] W/m2 [650, 1000] W/m2 [650, 1000] W/m2

Global collector efficiency1 [0.40, 0.60] [0.40, 0.60] [0.40, 0.60] Collector inlet mass flow [0.35, 0.70] kg/s [0.350.70] kg/s [0.35, 0.70] kg/sOutlet temperature reference [280, 320] C [320, 370] C [340, 400] CCollector inlet fluid temperature [250, 310] C [290, 370] C [330, 390] CInjection water temperature [180, 215] C [220, 260] C [260, 300] C1Changes in this parameter do not have significant influence on the adjusted model.

Figure 9. Control loop responses during operation at 30 bar (22 April 2002): (a) includes the feed pump control loopresponse, and (b) includes the temperature control loop response; (c) includes the outlet pressure control loop response, while(d) includes the available radiation and generated steam flow.

Outlet Steam Temperature

Flow

[kg/s

]

1.00.90.80.70.60.50.40.30.20.10.0

Flow

[kg/s

]

1.00.90.80.70.60.50.40.30.20.10.0

Pres

sure

[bar]

Pres

sure

[bar]

7

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3

2

1

0

Pressure Drop Across Feed ValveSet PointFeed Pump Power (Control Signal)

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10

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330300270240210180150120906030

Pow

er [%

]Te

mpe

ratu

re [

C]

09 10 11 12 13 14 15 16Local Time

(a)09 10 11 12 13 14 15 16

Local Time(b)

09 10 11 12 13 14 15 16Local Time

(d) 09 10 11 12 13 14 15 16

Local Time(c)

45

40353025

2015

105

0

72

645648403224

1680

1,000900800700600500400300200100

0

Aper

ture

[%]

Sola

r Irra

dian

ce [W

/m2]

Outlet Steam PressureSet PointFeed Tank PressureSteam Valve Aperture (Control Signal)

Direct Solar IrradianceOutlet Steam Flow

Set PointInlet Water Flow - Feed Valve (Control Signal)Injection Water Flow - Injector (Control Signal)

flow rate dictated from the injector valve, since zero flowrate would deteriorate the control action due to the non-linearity of the injector when this actuator is nearly closed,as was observed in real tests.

The PI parameters of the outer and inner loops werealso calculated from open-loop responses using theprocess reaction method. Linearized models detailed inTable 3 were used to simulate the closed-loop responsesand study the stability margins in the worst cases for themodel uncertainty. The final selection of values for the PIparameters was made in a conservative way to avoid insta-bility in the system and to diminish interaction with therest of controllers [8].

The feedforward action formulated for the feed valvecontrol loop is obtained from a simplified steady-stateenergy balance formulation for the collector given by

{Enthalpy

out

}

{Enthalpy

in

}=

{Energy

collected

} {Losses}.

The collected energy is corrected by the global efficiencycol of the collector, which accounts for the thermal andoptical efficiencies and thus the thermal and optical losses.The simplified energy balance equation can be written as

(min c + minj)hout (min chin c + minjhinj) = col Acol LcolE,

where the right-hand side of the equation includes theenergy collected and energy losses.

Substituting the specific enthalpy at the outlet hout bythe outlet enthalpy reference href, which is directly calcu-lated from the temperature and pressure references, the


Figure 10. Control loop responses during operation at 30 bar (26 April 2002): (a) includes the feed pump control loopresponse, (b) includes the temperature control loops response, (c) includes the outlet pressure control loop response, and (d)includes the available radiation and generated steam flow.

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sure

[bar]

45

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[bar]

09 10 11 12 13 14 15Local Time

09 10 11 12 13 14 15Local Time

09 10 11 12 13 14 15Local Time

09 10 11 12 13 14 15Local Time

70

60

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40

30

20

10

0 306090

120150180210240270300330

Pow

er

[%]

Tem

pera

ture

[C]

72

64

56

48

40

32

24

16

8

0

1,000900800700600500400300200100

0

Aper

ture

[%]

Sola

r Irra

dian

ce [W

/m2]

1.00.90.80.70.60.50.40.30.20.10.0

Flow

[kg/s

]

1.00.90.80.70.60.50.40.30.20.10.0

Flow

[kg/s

]

Outlet Steam TemperatureSet PointInlet Water Flow - Feed Valve (Control Signal)Injection Water Flow - Injector (Control Signal)

Direct Solar IrradianceEffective Radiance on AbsorbersOutlet Steam Flow

Outlet Steam PressureSet PointFeed Tank PressureSteam Valve Aperture (Control Signal)

Pressure Drop Across Feed ValveSet PointFeed Pump Power (Control Signal)

(a)

(c) (d)

(b)

corresponding injection flow in steady state is given by

minj = col Acol LcolE min c(href hin c)href hinj.

Using this equation, thermodynamic tables [10] for calcu-lating the enthalpies corresponding to each temperatureand pressure, and data obtained using the data seriesdetailed in Table 7, a regression analysis was performed toobtain the feedforward function for calculating the injec-tion flow rate correction. The multiple regression modelhas the form

mff iv = c1Tin c + c2min c + c3Tref + c4Tinj + c5.

The values obtained for the parameters c1, c2, c3, c4, and c5at the various operating points are listed in Table 8. Theseparameters depend on thermodynamic properties of thefluid as well as the geometry and global efficiency of thecollector. Table 8 includes the correlation coefficient andstandard deviation of the residual errors. A good approxi-mation is obtained within the operating ranges listed inTable 7, but outside these ranges the quality of the model isnot guaranteed due to the nonlinear characteristics of theprocess.

Both temperature control loops operate in parallel andare necessary because the temperature control based onthe feed valve adjustment calculates a nominal inlet flowrate for the field. Due to the long time delay caused by thelength of the collector loop, the temperature control loopscannot react rapidly to sudden disturbances. The tempera-ture control based on the injector valve provides a fastercontrol to compensate for sudden changes and allows theoutlet temperature to more accurately follow the refer-ence. Interactions between both controllers are avoided bythe inclusion of parameter minj set in block FFFV and bychoosing conservative PI parameters in the case of thetemperature control by means of the feed valve.

Representative Experimental ResultsThe Symphony SCADA platform [12] was used to imple-ment the controllers. Figure 9 shows the results obtainedduring an experiment in once-through mode with 30 bar ofoutlet steam pressure. During startup, the water pressurein the feed water tank varied, affecting the feed pump

response. The measured outlet steamflow oscillated due to the circulation ofwater through the steam flow transmit-ter, providing an incorrect signal. Outletsteam temperature was maintained at aconstant temperature of 300 C. Themaximum steam temperature overshootand undershoot occurred when thesuperheated steam production started ataround 12 noon; their values were 8 C

(2.7% of the reference) and 16 C (5.3% of the reference),respectively, far from the steam saturation temperature,which is 234 C at 30 bar. The steam pressure was main-tained close to the reference throughout operation withoutsignificant deviations.

Figure 10 shows the results obtained during an experi-ment in the once-through mode with 30 bar of outlet steampressure. The experiments objective was to evaluate theresponse of the control system to a defocusing of one col-lector of the evaporation section, which is equivalent toproducing a 10% step decrement in the inlet energy to thefield. Collector number 6 was defocused at 13:15 andstayed out of focus for 5 minutes. The resulting outlet tem-perature deviation was 21 C (7% of the reference). Whenthe temperature came close to the reference again, theirradiance dropped 300 W/m2, causing the temperature toapproach the saturation temperature value and changingthe outlet steam pressure around 0.8 bar (2.6% of the refer-ence). Subsequently, the nominal conditions were recov-ered in 15 min. Prior to defocusing, the irradiance haddropped around 150 W/m2 at 12:30, which mainly affectedsteam flow. The outlet steam pressure was maintainedconstant, and the maximum outlet steam temperaturedeviation was 4.5 C.

Figure 11 shows the results obtained during a once-through mode experiment with 60 bar of outlet steam pres-sure. Outlet steam temperature was maintained constantat 350 C, and the maximum deviation from this referencewas 6 C (1.7% of the reference). The temperature con-troller based on the injector valve adjustment was put inautomatic mode around 10:00, but the superheatingprocess started 45 min later. The dead time in response tothe steam temperature is due to manual control of the inletflow rate during the start-up (see also Figure 10). The oper-ator put the feed valve temperature controller in automat-ic mode at around 11:00. The experiment finished at 15:45when production was stopped.

The results show that all set points can be main-tained during steady-state conditions, even with shorttransients in the solar radiation. During longer solarradiation transients, it is more difficult to maintain thesteam temperature, since a minimum flow must be guar-anteed to avoid high temperature gradients in pipecross-sections when solar radiation recovers (in any


The DISS project demonstrated that it ispossible to produce high-pressure, high-temperature steam directly inparabolic trough solar collectors.

case, null values of inlet mass flow led to zero produc-tion, which is commercially undesirable). In this tran-sient situation, a mixed steam/water flow feeds theseparator tank where it condenses, returning to the feedwater through the separator drain valve, increasing theparasitic load of the system as well as security. A con-trol system configured to operate the plant with zeroinlet flow would have to satisfy stringent specifications,mostly under actuator saturation. Such operation wouldrequire that all the collectors be defocused to avoiddangerous conditions in the solar field.

ConclusionsThe DISS project demonstrated that it is possible to pro-duce high-pressure, high-temperature steam directly inparabolic trough solar collectors. A leading plant usingsolar technology has been operated in two differentmodes. This article describes the once-through mode,

which is the most difficult to control. Using a schemebased on PI and feedforward controllers, the controllabili-ty of the plant is guaranteed on clear days and duringshort transients in solar radiation. Longer transients insolar radiation make it difficult to maintain the steam tem-perature in favor of guaranteeing a minimum flow to avoidhigh temperature gradients in the cross-sectional area ofthe collectors absorber pipes when the solar radiationlevel is recovered.

A structure partially based on classical controllers waschosen because the plant operators are familiar with thistype of controller and can adapt the controller parametersin situations affecting plant dynamics and controller per-formance, such as modifications in plant layout or systemchanges over time. In the near future, a control strategybased on model predictive control will be investigated toimprove system performance under disturbances in thesystem inlet energy.


Figure 11. Control loop responses during operation at 60 bar (17 July 2002). (a) includes the feed pump control loopresponse, (b) includes the temperature control loops response, (c) includes the outlet pressure control loop response, and (d)includes the available radiation and generated steam flow.

8

7

6

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1

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Pres

sure

[Bar]

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sure

[Bar]

9080706050403020100

80

70

60

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40

30

20

10

0 6090

120150180210240270300330360

Pow

er [%

]Te

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C]

04

812

162024

283236 1,000

900800700600500400300200100

0

Aper

ture

[%]

Sola

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ce (W

/m2)

1.00.90.80.70.60.50.40.30.20.10

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[kg/s

]

1.00.90.80.70.60.50.40.30.20.10

Flow

[kg/s

]

09 10 11 12 13 14 15 16Local Time

09 10 11 12 13 14 15 16Local Time

09 10 12 13 14 15Local Time

09 10 11 12 13 14 15 16Local Time

Direct Solar RadianceOutlet Steam Flow

Outlet Steam TemperatureSet PointInlet Water Flow - Feed Valve (Control Signal)Injection Water Flow - Injector (Control Signal)Pressure Drop Across Feed ValveSet Point

Feed Pump Power (Control Signal)

Outlet Steam PressureSet PointFeed Tank PressureSteam Pump Power (Control Signal)

(a)

(c) (d)

(b)

AcknowledgmentsWe thank the European Commission for support of the sec-ond phase of the DISS project (contract JOR3-CT98-0277)within the framework of the E.U. JOULE Program. Wewould also thank MCYT for funding this work under grantsDPI2002-04375, DPI2001-2380, and QUI99-0663. We thankthe anonymous reviewers for their helpful comments thatimproved the article.

References[1] E.F. Camacho, M. Berenguel, and F.R. Rubio, Advanced Control ofSolar Plants. London: Springer-Verlag, 1997.

[2] G. Cohen and D. Kearney, Current experiences with SEGS parabol-ic trough plants, in Proc. 8th Int. Symp. Solar Thermal ConcentratingTechnologies, Kln, Germany, 1996, pp. 217244.

[3] E. Zarza, L. Valenzuela, J. Len, D. Weyers, M. Eickhoff, M. Eck, andK. Hennecke, The DISS project: Direct steam generation in parabolictrough systems. Operation & maintenance experience and update onproject status, J. Solar Energy Eng., vol. 124, May 2002, pp. 126133.

[4] E. Zarza, Solar Thermal Desalination Project. Phase II Results & FinalProject Report. Madrid, Spain: Editorial CIEMAT. 1995.

[5] University of Manchester (UMIST), Zentrum fr Sonnenergie- undWasserstoff-Foruschung Baden- Wrttemberg (ZSW), PSA DISS testfacility: Control scheme design studies for once-through and recircula-tion concepts, Project DISS Int. Rep., Doc. ID: DISS-EN-CD-02, Platafor-ma Solar de Almera, Aug. 1996.

[6] L. Valenzuela, M. Berenguel, E. Zarza, and E.F. Camacho, Controlschemes for direct steam generation in parabolic solar collectorsunder recirculation operation, submitted to Solar Energy J., 2002.

[7] M. Eck and M. Eberl, Controller design for injection mode drivendirect solar steam generating parabolic trough collectors, in ISESSolar World Congress, Jerusalem, Israel, vol. I, 1999, pp. 247257.

[8] B.A. Ogunnaike and W.H. Ray, Process Dynamics, Modeling, and Con-trol. New York: Oxford Univ. Press, 1994.

[9] R.N. Bateson, Introduction to Control System Technology. New York:Prentice-Hall, 1996.

[10] W. Wagner and A. Kruse, Properties of Water and Steam. Berlin-Hei-delberg-New York: Springer-Verlag, 1998.

[11] J.I. Ajona, Electricity generation with distributed collector, inSolar Thermal Electricity Generation. Madrid, Spain: Editorial CIEMAT,1999, pp. 777.

[12] Composer Series. Electronic Documentation Symphony. ElsagBailey Process Automation, Ohio (U.S.A), 19971998.

[13] D.R. Coughanowr, Process Systems Analysis and Control. New York:McGraw Hill, 1991.

Loreto Valenzuela ([email protected]) receivedthe B.S. in physics and the electronics engineering degreefrom University of Granada in 1994 and 1996, respectively.She is currently pursuing her Ph.D. in the University ofAlmera. She joined the Plataforma Solar de Almera in 1997to work as technical assistant in the Department of Parabol-ic-trough Collectors, and in 2000 she took a position withinthe research staff of the CIEMAT (the public research cen-ter leading research on renewable energy in Spain). Herresearch interests are in solar thermal concentrating tech-

nologies and control systems for solar energy systems. Shecan be contacted at CIEMAT, Plataforma Solar de Almera,Ctra. Senes s/n, E-04200 Tabernas, Almera, Spain.

Eduardo Zarza earned the industrial engineering degreefrom University of Seville in 1985, where he is a member ofthe Scientific Group of Thermodynamics and RenewableEnergies. Since 1985 he has participated in many interna-tional R&TD projects regarding solar thermal energy appli-cations. From 1990 to 1994 he was the project manager ofthe Solar Thermal Desalination Project. In 1994 he under-took the coordination and management of the DISS project,thus leading an international group of researchers involvedin the development of the DSG technology. Since 1997, hehas been the head of the Department of Parabolic-troughCollectors of CIEMAT, where he conducts research on inno-vative solar energy applications and systems.

Manuel Berenguel is an associate professor in the Depar-tamento de Lenguajes y Computacin (rea de Ingenierade Sistemas y Automtica) of the University of Almera,Spain. He earned the industrial engineering degree and doc-torate from the Escuela Superior de Ingenieros Industrialesof the University of Sevilla (Spain), where he received thePremio Extraordinario de Doctorado award (given to thebest engineering thesis of the year), and he was aresearcher and associate professor in the Departamento deIngeniera de Sistemas y Automtica for six years. Hisresearch interests are in the fields of predictive, adaptive,and robust control, with applications to solar energy sys-tems, agriculture, and biotechnology. He has been a review-er for several journals including Control EngineeringPractice; IEEE Transactions on System, Man and Cybernetics;and IEEE Transactions on Fuzzy Systems. He has authoredand coauthored more than 50 technical papers and is co-author of Advanced Control of Solar (Springer, 1997).

Eduardo F. Camacho is a professor of system engineeringand automatic control at the University of Seville. He haswritten the books Model Predictive Control in the ProcessIndustry (Springer-Verlag, 1995), Advanced Control of SolarPlants (Springer-Verlag, 1997), Model Predictive Control(Springer-Verlag, 1999), and Control e Instrumentacin deProcesos Qumicos (Ed. Sintesis). He has authored and co-authored more than a 150 technical papers, has served onvarious technical committees, and was a member of the2001 Board of Governors of the IEEE Control Systems Soci-ety. At present he is the chair of the IEEE Control SystemsSociety International Affairs Committee, vice president ofthe European Control Association, and chair of the IFACPublication Committee. He is one of the editors of ControlEngineering Practice and an associate editor of the Euro-pean Journal of Control.


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