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Control scheme for direct steam generation in parabolic troughs under recirculation operation mode
Solar Energy 80 (2006) 1–17 www.elsevier.com/locate/solener
Control scheme for direct steam generation in parabolic troughs under recirculation operation mode L. Valenzuela
a,*
, E. Zarza a, M. Berenguel b, E.F. Camacho
c
a
CIEMAT, Plataforma Solar de Almerı´a, Ctra. Senes s/n, P.O. Box 22, E-04200 Tabernas, Almerı´a, Spain Universidad de Almerı´a, Dpto. Lenguajes y Computacio´n, Ctra. Sacramento s/n, E-04120 Almerı´a, Spain Universidad de Sevilla, Dpto. de Ingenierı´a de Sistemas y Automa´tica, Camino de los Descubrimientos s/n, E-41092 Sevilla, Spain b
c
Received 22 January 2004; received in revised form 9 September 2005; accepted 20 September 2005 Available online 14 November 2005 Communicated by: Associate Editor Lorin Vant-Hull
Abstract Electricity production using solar thermal energy is one of the main research areas at present in the field of renewable energies, these systems being characterised by the need of reliable control systems aimed at maintaining desired operating conditions in the face of changes in solar radiation, which is the main source of energy. A new prototype of solar system with parabolic trough collectors was implemented at the Plataforma Solar de Almerı´a (PSA, South-East Spain) to investigate the direct steam generation process under real solar conditions in the parabolic solar collector field of a thermal power plant prototype. This paper presents details and some results of the application of a control scheme designed and tested for the recirculation operation mode, for which the main objective is to obtain steam at constant temperature and pressure at the outlet of the solar field, so that changes produced in the inlet water conditions and/or solar radiation will only affect the amount of steam produced by the solar field. The steam quality and consequently the nominal efficiency of the plant are thus maintained. 2005 Elsevier Ltd. All rights reserved. Keywords: Solar thermal energy; Direct steam generation; Process identification; PI controllers; Pressure control; Temperature control
1. Introduction Parabolic trough collectors are the best-developed line-focus concentrating solar collectors and are applied to thermally feed industrial heat processes (IHP [commonly called IPH in the US]) and * Corresponding author. Tel.: +34 950387934; fax: +34 950365015. E-mail address: [email protected] (L. Valenzuela).
thermal electricity generation. At present, parabolic troughs can operate at temperatures up to about 400 C, because the parabolic-shaped reflector (see Fig. 1) concentrates the direct solar radiation. Optical concentration reduces the absorber surface area with respect to the collector aperture area and thus significantly reduces thermal losses. This optical concentration requires the collector to rotate around an axis, the so-called ‘‘tracking axis’’, following the apparent daily movement of
0038-092X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2005.09.009
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Nomenclature BOP DISS DSG GM IHP
balance of plant (of the test facility) direct solar steam (Research & Development project) direct steam generation (the process) gain margin (dB) industrial heat processes
PI PM PSA SEGS SISO
proportional-integral phase margin () Plataforma Solar de Almerı´a (Spain) solar electricity generating system single input single output
Fig. 1. A parabolic trough collector: working principle and components.
the Sun. Since only direct solar radiation is optically concentrated, the diffuse solar radiation is lost. One advantage of parabolic trough collectors is the low pressure drop associated with these systems as the working fluid passes through a single, straight absorber tube. Also, overnight thermal losses are minimal because of the small amount of fluid within the parabolic trough receiver. For the range 200–400 C, parabolic troughs normally use oil as a working fluid in the absorber tubes, while a mixture of water and ethylene glycol can be used for lower temperatures. The working fluid is heated as it passes through the absorber tube of the solar collectors, thus converting the direct solar radiation into thermal energy. The hot working fluid is then sent to a heat exchanger where it transfers its thermal energy to the application (Zarza, 2002b). A considerable effort has been made during the last fifteen years to develop efficient control systems
for solar thermal power plants with parabolic trough collector fields (Camacho et al., 1997). At present, the best commercial exponent of the stateof-the-art of parabolic trough collectors are the eight solar thermal power plants operating daily in California (USA). These plants are called SEGS (Solar Electricity Generating System) and they use oil as a heat transfer medium between the solar field and the power block. The outlet of the solar field is connected to a heat exchanger that generates steam to feed a Rankine cycle (Cohen and Kearney, 1996). In these plants the primary energy source (solar radiation) is not controllable, but the temperature of the oil at the outlet of the solar field is controlled by acting on the oil mass flow at the inlet of the field. These systems exhibit non-linear dynamics and are affected by several disturbances (mainly in the input energy variable), which make necessary the use of advanced control strategies (Camacho et al., 1997).
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A new prototype of solar system with parabolic trough collectors was mounted at the Plataforma Solar de Almerı´a (Tabernas-Almerı´a, South-East Spain) in 1998 to investigate the direct steam generation in solar fields using water as working fluid in the collectors. This is the so-called Direct Steam Generation (DSG) process (Zarza et al., 2002a). From 1999 to 2001 different operation strategies and configurations were evaluated and promising results were obtained for the commercial implementation of this new system, which actually constitutes the most advanced plant of this type. Though the main industrial application expected for DSG processes is the production of electricity by means of a Rankine cycle thermally fed with the steam delivered by the parabolic trough collectors, it has to be pointed out here that a solar field using the DSG process can be used to feed any other industrial process demanding thermal energy in the form of saturated or superheated steam within the ranges: T 6 400 C, P 6 100 bar. Seawater distillation is a good example of industrial process suitable to be fed with a DSG solar field (Zarza, 2002b). Those regions with good solar radiation level and seawater resources could overcome the lack of good-quality water with the coupling of seawater distillation plants to DSG solar fields. In this type of technology, the main task of the control system is the provision of constant live steam conditions at the outlet of the solar field for all operating conditions. In the first stage of the DISS project, financed by CEE-Joule contract JOR3CT98-0277, simple control structures and pragmatic approaches for the controller design have been chosen (Zarza et al., 2002a). This has the advantage that the plant operators are familiar with the used proportional-integral (PI) controllers and are able to modify controller parameters to achieve secure operation, thus better dedicating their time to become familiar with the process itself. After this training stage in which plant dynamics and operational modes are mastered, advanced control strategies will be investigated (mainly model-based predictive control schemes) to try to optimise plant production. Control schemes proposed for recirculation and once through modes have been designed, implemented and tested within the framework of the DISS project (Zarza et al., 2002a). As a first step, the main dynamics have been approximated by linear models describing the behaviour of the system near the pre-fixed operating points. For the parameterisation of PI controllers, the SISO transfer func-
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tions of all relevant control loops (e.g. steam temperature, liquid level in separator, steam pressure) have been experimentally investigated for three different operating points. Based on these models the controller parameters have been designed by process reaction techniques (Ogunnaike and Harmon Ray, 1994) and final tuning was performed in subsequent tests. For once through mode it has been necessary to include mixed feedforwardcascade control schemes for controlling the outlet steam temperature (Valenzuela et al., 2004, 2005). This paper is organised as follows: Section 2 is devoted to describe the DISS facility. In Section 3, the main control problem is explained and the control scheme for recirculation mode is described in Section 4. Section 5 shows illustrative real tests performed at the plant and some conclusions are stated in Section 6. 2. Description of the PSA DISS facility The PSA DISS facility is a solar system that serves as a test bed to investigate the DSG process in parabolic trough solar collectors (see Fig. 1). Though the solar field could be operated over a wide temperature/pressure range, the three main operating points investigated in the project are listed in Table 1. In the DISS project the thermohydraulic behaviour and the system performance of three operation modes (once through, recirculation and injection modes, see Fig. 2) have been investigated under real conditions, to identify the specific advantages and disadvantages of each mode. In the once through mode feed water is preheated, evaporated and converted into superheated steam as it circulates from the inlet to the outlet of the collector loop. This concept is the simplest one, but the main technical problem is the controllability of the superheated steam parameters at the collector field outlet (Valenzuela et al., 2004). In the injection mode, water is injected at several places along the row of collectors. The measurement system necessary to assist the control scheme designed for this mode did not work properly during experiments (Zarza et al., 2002a; Eck and Eberl, 1999). The complexity and cost of this operation mode have caused us to discard it for further development. In the recirculation concept, the most conservative one, a water–steam separator is placed at the end of the evaporating section of the row of solar collectors. The amount of water fed at the inlet of
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Table 1 Operating modes investigated in the DISS solar field Solar field conditions
Mode 1 Mode 2 Mode 3
Inlet conditions
Outlet conditions
Pressure (bar)
Temperature (C)
Pressure (bar)
Temperature (C)
40 68 108
210 270 300
30 60 100
300 350 375
Fig. 2. Basic concepts for the direct solar steam generation in parabolic trough collectors.
Table 2 Technical data of the PSA DISS test loop Parameter
Value
Collectors row length Collectors type Collector aperture No. of collectors
500 m Modified LS-3 5.76 m 9 collectors 50 m length 2 collectors 25 m length North–South 70 mm 50 mm 73% 2760 m2 100 bar 400 C 0.85 kg s1
Orientation of the solar collectors Absorber pipe outer diameter Absorber pipe inner diameter Optical efficiency of solar collectors Total mirrors surface Max. pressure at the field outlet Max. outlet temperature Max. steam production
the evaporator is greater than the amount that can be evaporated. In the middle separator, the water excess is recirculated to the collector loop inlet
where it is mixed with the preheated water. The excess water in the evaporating section guarantees good wetting of the absorber tubes and makes stratification impossible. The steam produced is separated from the water by the middle separator and is fed into the inlet of the superheater section. This type of DSG system is highly controllable, but excess water that has to be recirculated, the middle water–steam separator and the water recirculation pump increase the system parasitic load. Technical data of the DISS test loop are listed in Table 2. 3. Control problem The solar radiation, which is the primary energy source in this kind of plants, cannot be manipulated and is subjected to slow changes (daily cycle and changes in mirrors reflectivity caused by dust) and
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Fig. 3. Structure of a single feedback control loop (recirculation mode control scheme).
fast ones, due to passing clouds. The inlet energy of this plant is thus affected by these disturbances, as well as by changes in the inlet water conditions (temperature or pressure). This is a feature of solar plants with collector fields, but in the case treated in this paper, the control is more complicated than that of the power plants currently working in California (Cohen and Kearney, 1996) or in the PSA (Camacho et al., 1997), as the DISS plant has two-phase flow in the solar field which complicates not only the engineering of the system but also the control system that has to be designed for the solar field. As has been pointed out in the introduction, in the case of current solar power plants the thermal fluid flowing through the solar field is synthetic oil whose outlet temperature is controlled by mass flow
manipulation at the inlet of the field. In the DISS facility not only the temperature of the fluid has to be controlled but also the pressure to maintain the desired steam conditions at the outlet (i.e. turbine specifications). Independently of the operation mode, the main objective of the control system is to obtain steam at constant temperature and pressure at the outlet of the solar field, in such a way that changes produced in the inlet water conditions and in the solar radiation will only affect the amount of steam produced by the system but not its quality. A PI control-based scheme has been designed and implemented for the recirculation mode. The structure of the single control loops, which are composing the control scheme, is shown in Fig. 3 where yr is the
Fig. 4. Schematic diagram of the DISS test loop configured in recirculation mode.
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(a) Recirculation pump control loop: Recirculation flow is maintained constant by adjusting the power input of the recirculation pump, i.e. the rotational speed (PI control). The main parameter influencing the pump behaviour, and consequently the controller behaviour, is strong change in pressure drop across the pump, i.e. solar radiation change influencing the solar field pressure level. (b) Feed pump control loop: The rotational speed of the feed pump is adjusted to maintain a specific pressure drop across the feed valve (PI control). Any change of the feed tank pressure directly affects the pressure drop controlled by the pump and those pressure changes could appear when there is a disturbance in the steam flow that feeds the DISS BOP. To avoid those changes the air condenser controls the feed tank pressure. (c) Middle steam separator liquid level control loop: The main objective of this loop is to maintain the level around a nominal value avoiding high or low levels inside the tank. The feed flow is then adjusted by controlling the aperture of the feed valve whose pressure drop is being controlled by the feed pump (P-PI control). This loop is highly affected by changes of radiation because if the radiation drops the steam production decreases and the tank level increases. Then the solar field water feed should decrease. As a consequence of the study presented in this paper, it can be observed that due to the dynamics of this control loop, some oscillations may appear. These can be avoided by acting on the recirculation pump instead of the aperture of the feed valve, thus reducing delays, which delays can cause oscillations in the controlled signal.
set point, ym(t) is the variable controlled or process variable, e(t) is error between both signals and m(t) is the control signal or output. Some characteristics of the PI functions implemented are: • The PI output is calculated using a classical interactive controller. The transfer function of such controller has the form: output = K Æ (1 + 1/(Ti Æ s)) Æ error, where K is the proportional gain and Ti is the integral time. • Anti-reset (integral) wind-up function to prevent error integral update whenever the actuator is saturated, i.e. as the PI algorithm has a term of integration, the magnitude of the integration term will increase toward infinity if the difference between the process variable and the set point remains for a long time. • Bumpless manual to auto transfer to prevent that when there is an interruption of the automatic control loop, in the transfer from auto to manual mode and vice versa the control levels would be lost. • Bumpless proportional band tuning to prevent that when there is a big change in the proportional constant with the controller running in automatic mode, the controller output would be modified drastically.
4. Control scheme for the recirculation mode The process diagram with the most relevant feedback loops for the recirculation operation mode with nine collectors [total length = 425 m] as the evaporation section and two [total length = 75 m] as the superheating section as shown in Fig. 4. The control loops for the solar field in the recirculation operation mode are: Table 3 Recirculation mode: models and PI control loops parameters Single feedback control loop
Model
PI parameters
Recirculation pump
0:0064 GðsÞ ¼ sþ0:1871
12.1% kg1 s
3.2
Feed pump
GðsÞ ¼ 0:0522310 sþ0:11765
0.95% bar1
6
Middle separator level
340 s GðsÞ ¼ 0:009 s e
Outlet steam pressure
GðsÞ ¼ 0:00170610 sþ0:00556
0.135% mm1 0.150% mm1 5.4% bar1
1300 – 184
1:65710 e100 s GðsÞ ¼ s2 þ0:016 sþ7:9105
0.39% C1
520
Outlet steam temperature (60 bar)
GðsÞ ¼
0.78% C1
700
Outlet steam temperature (100 bar)
GðsÞ ¼
1.08% C1
700
Kp
Outlet steam temperature (30 bar)
5
5
4
1:051104 e80 s s2 þ0:0169 sþ7:9105 1:381104 e90 s s2 þ0:0223 sþ1:384104
Ti (s)
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(d) Final steam separator liquid level control loop: The objective of this loop is to avoid significant liquid level in the final steam separator (On-off control). (e) Outlet steam pressure control loop: The steam produced by the collector loop feeds a steam separator; the outlet steam pressure is maintained constant by the adjustment of a steam control valve (PI control). (f) Outlet steam temperature control loop: The outlet steam temperature of the superheater is adjusted by water injection in the inlet of the last collector. The provision of injector valve control permits rapid response to sudden disturbances (PI control).
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The last two control loops are the main controllers of the system that should work in a way that guarantees the steam quality. The other four loops are required to improve the behaviour of the whole system and for operational feasibility. For the parameterization of the main controllers, transfer functions of all relevant control loops (SISO process models) have been investigated for the three operating points (Table 1), as interactions between the different control loops are low in the recirculation control mode. The identification method followed was to find in open loop (using the reaction curve method) the process parameters (gains, deadtimes, time constants, damping coefficients and natural frequencies) that fit experimentally
Fig. 5. Feed pump control: comparison between experimental results (January 31st, 2000) and closed-loop system model response. Differences between experimental and modelled pressure drop are not significant. Process gain and time constant seem to be well identified.
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open-loop step responses (see Table 3). Based on the simplified models obtained the PI controller parameters have been designed using the process reaction method and modifying adequately those controller parameters to provide safe stability margins (Ogunnaike and Harmon Ray, 1994). A final tuning of the parameters was done in subsequent tests, as a consequence of the comparison between real and simulated (using models detailed in Table 3) closed loop responses: • Fig. 5 shows a comparison for the feed pump control loop between simulated and real closed loop step responses using a PI controller with a pro-
portional constant a little bit higher than that proposed in Table 3 (that was reduced in subsequent tests). From the theoretical point of view (without plant-model mismatch), with this controller the closed loop system should have an infinite gain margin (GM) and a phase margin (PM) of 82.3, which means safe stability margins (GM > 1 and PM > 0) (Ogunnaike and Harmon Ray, 1994). • Fig. 6 shows a comparison for the separator level control loop. Simulation results using a PI control of proportional constant equal to 0.09%/mm and an integral time of 1000 s (GM = 13.3 dB; PM = 26.2), are compared with real results using the
Fig. 6. Separator level control: comparison between experimental results (June 22nd, 2000) and closed-loop system model response. Differences between experimental and modelled liquid level are due to the disturbances influencing the system during the test period, which is longer than two hours. During this period, radiation changes significantly due to the daily solar cycle but its influence is not being considered in the simulation. Anyway results corresponding to the first test hour show a good agreement between both responses.
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same controller. The set of PI parameters proposed in Table 3 (higher proportional constant and integral time) has improved the system response. • Fig. 7 shows a comparison for the outlet steam pressure control loop. Simulation results using a PI control of proportional constant equal to 1%/bar and an integral time of 50 s (closed-loop system with infinite gain margin and a phase margin of 62.8), are compared with real results using the same controller. Fig. 6 also includes the simulated response using the set of PI parameters proposed in Table 3 (GM = 1; PM = 90.5).
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• Fig. 8 shows a comparison for the outlet steam temperature control loop at 30 bar of outlet steam pressure. Simulation results using a PI control of proportional constant equal to 0.36%/C and an integral time of 520 s (GM = 10 dB; PM = 101), are compared with real results using the same controller. The behavior of this controller is conservative; subsequent readjustments led to an increase of the proportional gain (see Table 3). • Fig. 9 shows a comparison for the outlet steam temperature control loop at 60 bar of outlet steam pressure. Simulation results using a PI control of proportional constant equal to 0.5%/C and an
Fig. 7. Outlet pressure valve model: comparison between experimental results (March 3rd, 2000) and closed-loop system model response. Differences between the experimental and simulated response are not significant. Main process model parameters seem to be well identified. Using the set of PI parameters (K = 5.3%/bar, Ti = 184 s) the system response simulated improves considerably. This parameters set was selected to run this control loop in automatic mode.
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Fig. 8. Outlet temperature control (30 bar): comparison between experimental results (June 29th, 2001) and closed-loop system model response. Differences between the experimental and simulated response are not significant. Main process model parameters seem to be well identified.
integral time of 600 s (GM = 13 dB; PM = 106), are compared with real results using the same controller. The behavior of this controller is also too conservative, subsequent readjustments concluded in an increase of the proportional gain and an increase of the integral time (see Table 3). • Fig. 10 shows a comparison for the outlet steam temperature control loop at 100 bar. Simulation results using a PI controller with a proportional constant equal to 0.79%/C and an integral time of 700 s (GM = 9.4 dB; PM = 115), are compared with real results obtained using the same
controller. The step response had a long settling time, which was diminished using PI parameters finally proposed in Table 3.
5. Experimental results Some experimental results have been included and commented in the previous section, but corresponding figures do not show the complete behaviour of the system during the daily operation. In what follows, some relevant experiments performed
L. Valenzuela et al. / Solar Energy 80 (2006) 1–17
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Fig. 9. Outlet temperature control (60 bar): comparison between experimental results (July 2nd, 2001) and closed-loop system model response. Differences between the experimental and simulated response are not significant. Main process model parameters seem to be well identified.
within the DISS project are shown, where the controllers described in Section 4 were working in automatic mode. Fig. 11 shows the results obtained during an experiment in recirculation mode with 30 bar of outlet steam pressure. The responses of all the control loops described in Section 4 are detailed in this figure. During the start-up of the system the recirculation pump and feed pump control loops were working in automatic mode. The operator changed the corresponding references values according to the test requirements. The outlet pressure controller was put in automatic mode at around 11:00. An opera-
tion strategy, used in recirculation mode to reduce the start-up period, has been to configure all collectors as evaporation sections during the start-up period. When the outlet pressure desired is reached, the last two collectors are configured as the superheated section. This fact explains the change in the outlet steam flow occurring at 12:15 approximately. To change the configuration, it is necessary to manipulate hand valves and during a few minutes the final steam separator is not receiving steam. Once the final solar field configuration is established, the level and temperature control loops are put in automatic mode. From 12:00 to 21:00 the plant was operated
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Fig. 10. Outlet temperature control (100 bar): comparison between experimental results (July 4th, 2001) and closed-loop system model response. Differences between the experimental and simulated response are not significant. Main process model parameters seem to be well identified.
in automatic mode. During the operation, steps in the steam temperature reference were performed to evaluate the behaviour of the temperature controller, producing changes both in temperature and steam mass flow, while the rest of controlled variables were maintained stable around their corresponding references. The closed-loop 25 C step response has no overshoot but a settling time of approximately 25 minutes due to the use of a conservative set of PI parameters during the test. In steady state the maximum deviation from the temperature reference was equal to 5 C (1.8% of the reference value) and it occurred close to the twilight
when the solar irradiance was dropping below 400 W/m2 and the injector aperture was practically saturated. Small kick-back oscillations in the response are probably due to high frequency dynamics neglected in the modelling stage but present in this type of applications (Camacho et al., 1997), although they are not relevant from the steam generation point of view. Fig. 12 shows the results obtained during an experiment at 60 bar of outlet steam pressure. The objective of the test was to operate the system in steady state conditions for the whole day. The start-up procedure was similar to the one described
L. Valenzuela et al. / Solar Energy 80 (2006) 1–17
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Fig. 11. Plant behaviour during operation at 30 bar on June 29, 2001. Control loop responses: (a) recirculation flow control, (b) feed pump control, (c) middle separator level control, (d) steam pressure control, (e) steam temperature control, and (f) available solar radiation and steam generated during the test.
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Fig. 12. Plant behaviour during operation at 60 bar on July 3, 2001. Control loop responses: (a) recirculation flow control, (b) feed pump control, (c) middle separator level control, (d) steam pressure control, (e) steam temperature control, and (f) available solar radiation and steam generated during the test.
L. Valenzuela et al. / Solar Energy 80 (2006) 1–17
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Fig. 13. Plant behaviour during operation at 100 bar on July 5, 2001. Control loop responses: (a) recirculation flow control, (b) feed pump control, (c) middle separator level control, (d) steam pressure control, (e) steam temperature control, and (f) available solar radiation and steam generated during the test.
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Fig. 14. Plant behaviour during operation at 100 bar on July 4, 2001. Control loop responses: (a) recirculation flow control, (b) feed pump control, (c) middle separator level control that was the most influenced by disturbances in solar radiation, (d) steam pressure control, (e) steam temperature control, and (f) available solar radiation and steam generated during the test.
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before for the test at 30 bar. Once the operation conditions were established, the steam temperature reference was diminished to 330 C. The plant was working in automatic mode for around 8 h. The shutdown of the plant occurred at 19:30 approximately, when the radiation was too low to maintain the operating conditions. Fig. 13 shows the results obtained during an experiment with 100 bar of outlet steam pressure. From 11:00 to 21:00 the plant was operated in automatic mode. The pressure and temperature were maintained at the desired set points of 100 bar and 350 C. As the results presented before, the change of radiation due to the solar daily cycle influenced the steam mass flow produced but not its quality. Finally Fig. 14 also shows the results obtained during an experiment with 100 bar of outlet steam pressure, but with changes in solar radiation due to clouds passing over the field during the operation. The maximum deviation in the outlet steam temperature was of 4.2 C (1.2% of the reference value 340 C) and occurred around 18:00 when the irradiance dropped 400 W/m2. The closed-loop 10 C step response had no overshoot but a settling time of around 23 min due to the selection of conservative PI parameters. The level control loop was the most affected by the radiation transients with a maximum deviation of the water level inside the middle separator of 75 mm (5.8% of the reference value 1300 mm), when the controller was running in automatic mode. 6. Conclusions This paper has shown the development and implementation of control strategies to a new plant representing a breakthrough in technology for producing steam at high pressure and temperature directly in parabolic trough solar collectors. The DISS plant using this type of technology has been operated in two different modes. This paper has presented one of the plant configurations evaluated, the recirculation operation mode. With a PI controlbased scheme the controllability of the plant was guaranteed during clear days and even with transients in the solar radiation. A structure based on classical controllers was chosen because the plant operators are familiar with the used PI controllers and also to try to assure a secure operation in the first tests performed in this facility. These advantages would get lost if highly sophisticated control schemes would be used in the preliminary tests.
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Anyway, in the near future, a control strategy based on model predictive control will be investigated to improve system performance under disturbances in the inlet energy to the system. From the experiments performed it can be concluded that the system is more stable, and consequently, easier to control, at higher pressures (100 bar, 60 bar) than at lower pressures (30 bar), because the thermodynamic properties (specific enthalpy, thermal conductivity, etc.) of the water and the steam differ more at low pressure than at high pressure. On the other hand, there are more technical problems to operate the system at high pressure and temperature, as the risk of steam leaks and the thermal stress in the pipes is considerably higher. Acknowledgements Authors would like to thank the European Commission for the financial support given to the second phase of the DISS project (contract No. JOR3CT98-0277) within the framework of the E.U. JOULE Programme. Authors would also like to thank CICYT for funding this work under grant DPI2002-04375-C03. References Camacho, E.F., Berenguel, M., Rubio, F.R., 1997. Advanced Control of Solar Plants. Springer-Verlag. Cohen, G., Kearney, D., 1996. Current Experiences with SEGS Parabolic Trough Plants. In: Proc. of the 8th International Symposium on Solar Thermal Concentrating Technologies, Ko¨ln (Germany), pp. 217–244. Eck, M., Eberl, M., 1999. Controller Design for Injection Mode driven Direct Solar Steam Generating Parabolic Trough Collectors. ISES Solar World Congress, Jerusalem, Israel, vol. I, pp. 247–257. Ogunnaike, B.A., Harmon Ray, W., 1994. Process Dynamics, Modeling, and Control. Oxford University Press. Valenzuela, L., Zarza, E., Berenguel, M., Camacho, E.F., 2004. Direct Steam Generation in Solar Boilers. IEEE Control Systems Magazine 24 (2), 15–29. Valenzuela, L., Zarza, E., Berenguel, M., Camacho, E.F., 2005. Control concepts for direct steam generation in parabolic troughs. Solar Energy 78, 301–311. Zarza, E., Valenzuela, L., Leon, J., Weyers, D.-H, Eickhoff, M., Eck, M., Hennecke, K., 2002a. The DISS Project: Direct Steam Generation in Parabolic Trough Systems. Operation & Maintanence Experience and Update on Project Status. Journal of Solar Energy Engineering-Transactions of the ASME 124 (2), 126–133. Zarza, E., 2002b. Sistemas Termosolares con Colectores Cilindro Parabo´licos. Conceptos Ba´sicos y Generalidades. Plataforma Solar de Almerı´a, Internal Report R08/01 EZ.
Control scheme for direct steam generation in parabolic troughs under recirculation operation mode L. Valenzuela
a,*
, E. Zarza a, M. Berenguel b, E.F. Camacho
c
a
CIEMAT, Plataforma Solar de Almerı´a, Ctra. Senes s/n, P.O. Box 22, E-04200 Tabernas, Almerı´a, Spain Universidad de Almerı´a, Dpto. Lenguajes y Computacio´n, Ctra. Sacramento s/n, E-04120 Almerı´a, Spain Universidad de Sevilla, Dpto. de Ingenierı´a de Sistemas y Automa´tica, Camino de los Descubrimientos s/n, E-41092 Sevilla, Spain b
c
Received 22 January 2004; received in revised form 9 September 2005; accepted 20 September 2005 Available online 14 November 2005 Communicated by: Associate Editor Lorin Vant-Hull
Abstract Electricity production using solar thermal energy is one of the main research areas at present in the field of renewable energies, these systems being characterised by the need of reliable control systems aimed at maintaining desired operating conditions in the face of changes in solar radiation, which is the main source of energy. A new prototype of solar system with parabolic trough collectors was implemented at the Plataforma Solar de Almerı´a (PSA, South-East Spain) to investigate the direct steam generation process under real solar conditions in the parabolic solar collector field of a thermal power plant prototype. This paper presents details and some results of the application of a control scheme designed and tested for the recirculation operation mode, for which the main objective is to obtain steam at constant temperature and pressure at the outlet of the solar field, so that changes produced in the inlet water conditions and/or solar radiation will only affect the amount of steam produced by the solar field. The steam quality and consequently the nominal efficiency of the plant are thus maintained. 2005 Elsevier Ltd. All rights reserved. Keywords: Solar thermal energy; Direct steam generation; Process identification; PI controllers; Pressure control; Temperature control
1. Introduction Parabolic trough collectors are the best-developed line-focus concentrating solar collectors and are applied to thermally feed industrial heat processes (IHP [commonly called IPH in the US]) and * Corresponding author. Tel.: +34 950387934; fax: +34 950365015. E-mail address: [email protected] (L. Valenzuela).
thermal electricity generation. At present, parabolic troughs can operate at temperatures up to about 400 C, because the parabolic-shaped reflector (see Fig. 1) concentrates the direct solar radiation. Optical concentration reduces the absorber surface area with respect to the collector aperture area and thus significantly reduces thermal losses. This optical concentration requires the collector to rotate around an axis, the so-called ‘‘tracking axis’’, following the apparent daily movement of
0038-092X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2005.09.009
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Nomenclature BOP DISS DSG GM IHP
balance of plant (of the test facility) direct solar steam (Research & Development project) direct steam generation (the process) gain margin (dB) industrial heat processes
PI PM PSA SEGS SISO
proportional-integral phase margin () Plataforma Solar de Almerı´a (Spain) solar electricity generating system single input single output
Fig. 1. A parabolic trough collector: working principle and components.
the Sun. Since only direct solar radiation is optically concentrated, the diffuse solar radiation is lost. One advantage of parabolic trough collectors is the low pressure drop associated with these systems as the working fluid passes through a single, straight absorber tube. Also, overnight thermal losses are minimal because of the small amount of fluid within the parabolic trough receiver. For the range 200–400 C, parabolic troughs normally use oil as a working fluid in the absorber tubes, while a mixture of water and ethylene glycol can be used for lower temperatures. The working fluid is heated as it passes through the absorber tube of the solar collectors, thus converting the direct solar radiation into thermal energy. The hot working fluid is then sent to a heat exchanger where it transfers its thermal energy to the application (Zarza, 2002b). A considerable effort has been made during the last fifteen years to develop efficient control systems
for solar thermal power plants with parabolic trough collector fields (Camacho et al., 1997). At present, the best commercial exponent of the stateof-the-art of parabolic trough collectors are the eight solar thermal power plants operating daily in California (USA). These plants are called SEGS (Solar Electricity Generating System) and they use oil as a heat transfer medium between the solar field and the power block. The outlet of the solar field is connected to a heat exchanger that generates steam to feed a Rankine cycle (Cohen and Kearney, 1996). In these plants the primary energy source (solar radiation) is not controllable, but the temperature of the oil at the outlet of the solar field is controlled by acting on the oil mass flow at the inlet of the field. These systems exhibit non-linear dynamics and are affected by several disturbances (mainly in the input energy variable), which make necessary the use of advanced control strategies (Camacho et al., 1997).
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A new prototype of solar system with parabolic trough collectors was mounted at the Plataforma Solar de Almerı´a (Tabernas-Almerı´a, South-East Spain) in 1998 to investigate the direct steam generation in solar fields using water as working fluid in the collectors. This is the so-called Direct Steam Generation (DSG) process (Zarza et al., 2002a). From 1999 to 2001 different operation strategies and configurations were evaluated and promising results were obtained for the commercial implementation of this new system, which actually constitutes the most advanced plant of this type. Though the main industrial application expected for DSG processes is the production of electricity by means of a Rankine cycle thermally fed with the steam delivered by the parabolic trough collectors, it has to be pointed out here that a solar field using the DSG process can be used to feed any other industrial process demanding thermal energy in the form of saturated or superheated steam within the ranges: T 6 400 C, P 6 100 bar. Seawater distillation is a good example of industrial process suitable to be fed with a DSG solar field (Zarza, 2002b). Those regions with good solar radiation level and seawater resources could overcome the lack of good-quality water with the coupling of seawater distillation plants to DSG solar fields. In this type of technology, the main task of the control system is the provision of constant live steam conditions at the outlet of the solar field for all operating conditions. In the first stage of the DISS project, financed by CEE-Joule contract JOR3CT98-0277, simple control structures and pragmatic approaches for the controller design have been chosen (Zarza et al., 2002a). This has the advantage that the plant operators are familiar with the used proportional-integral (PI) controllers and are able to modify controller parameters to achieve secure operation, thus better dedicating their time to become familiar with the process itself. After this training stage in which plant dynamics and operational modes are mastered, advanced control strategies will be investigated (mainly model-based predictive control schemes) to try to optimise plant production. Control schemes proposed for recirculation and once through modes have been designed, implemented and tested within the framework of the DISS project (Zarza et al., 2002a). As a first step, the main dynamics have been approximated by linear models describing the behaviour of the system near the pre-fixed operating points. For the parameterisation of PI controllers, the SISO transfer func-
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tions of all relevant control loops (e.g. steam temperature, liquid level in separator, steam pressure) have been experimentally investigated for three different operating points. Based on these models the controller parameters have been designed by process reaction techniques (Ogunnaike and Harmon Ray, 1994) and final tuning was performed in subsequent tests. For once through mode it has been necessary to include mixed feedforwardcascade control schemes for controlling the outlet steam temperature (Valenzuela et al., 2004, 2005). This paper is organised as follows: Section 2 is devoted to describe the DISS facility. In Section 3, the main control problem is explained and the control scheme for recirculation mode is described in Section 4. Section 5 shows illustrative real tests performed at the plant and some conclusions are stated in Section 6. 2. Description of the PSA DISS facility The PSA DISS facility is a solar system that serves as a test bed to investigate the DSG process in parabolic trough solar collectors (see Fig. 1). Though the solar field could be operated over a wide temperature/pressure range, the three main operating points investigated in the project are listed in Table 1. In the DISS project the thermohydraulic behaviour and the system performance of three operation modes (once through, recirculation and injection modes, see Fig. 2) have been investigated under real conditions, to identify the specific advantages and disadvantages of each mode. In the once through mode feed water is preheated, evaporated and converted into superheated steam as it circulates from the inlet to the outlet of the collector loop. This concept is the simplest one, but the main technical problem is the controllability of the superheated steam parameters at the collector field outlet (Valenzuela et al., 2004). In the injection mode, water is injected at several places along the row of collectors. The measurement system necessary to assist the control scheme designed for this mode did not work properly during experiments (Zarza et al., 2002a; Eck and Eberl, 1999). The complexity and cost of this operation mode have caused us to discard it for further development. In the recirculation concept, the most conservative one, a water–steam separator is placed at the end of the evaporating section of the row of solar collectors. The amount of water fed at the inlet of
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Table 1 Operating modes investigated in the DISS solar field Solar field conditions
Mode 1 Mode 2 Mode 3
Inlet conditions
Outlet conditions
Pressure (bar)
Temperature (C)
Pressure (bar)
Temperature (C)
40 68 108
210 270 300
30 60 100
300 350 375
Fig. 2. Basic concepts for the direct solar steam generation in parabolic trough collectors.
Table 2 Technical data of the PSA DISS test loop Parameter
Value
Collectors row length Collectors type Collector aperture No. of collectors
500 m Modified LS-3 5.76 m 9 collectors 50 m length 2 collectors 25 m length North–South 70 mm 50 mm 73% 2760 m2 100 bar 400 C 0.85 kg s1
Orientation of the solar collectors Absorber pipe outer diameter Absorber pipe inner diameter Optical efficiency of solar collectors Total mirrors surface Max. pressure at the field outlet Max. outlet temperature Max. steam production
the evaporator is greater than the amount that can be evaporated. In the middle separator, the water excess is recirculated to the collector loop inlet
where it is mixed with the preheated water. The excess water in the evaporating section guarantees good wetting of the absorber tubes and makes stratification impossible. The steam produced is separated from the water by the middle separator and is fed into the inlet of the superheater section. This type of DSG system is highly controllable, but excess water that has to be recirculated, the middle water–steam separator and the water recirculation pump increase the system parasitic load. Technical data of the DISS test loop are listed in Table 2. 3. Control problem The solar radiation, which is the primary energy source in this kind of plants, cannot be manipulated and is subjected to slow changes (daily cycle and changes in mirrors reflectivity caused by dust) and
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Fig. 3. Structure of a single feedback control loop (recirculation mode control scheme).
fast ones, due to passing clouds. The inlet energy of this plant is thus affected by these disturbances, as well as by changes in the inlet water conditions (temperature or pressure). This is a feature of solar plants with collector fields, but in the case treated in this paper, the control is more complicated than that of the power plants currently working in California (Cohen and Kearney, 1996) or in the PSA (Camacho et al., 1997), as the DISS plant has two-phase flow in the solar field which complicates not only the engineering of the system but also the control system that has to be designed for the solar field. As has been pointed out in the introduction, in the case of current solar power plants the thermal fluid flowing through the solar field is synthetic oil whose outlet temperature is controlled by mass flow
manipulation at the inlet of the field. In the DISS facility not only the temperature of the fluid has to be controlled but also the pressure to maintain the desired steam conditions at the outlet (i.e. turbine specifications). Independently of the operation mode, the main objective of the control system is to obtain steam at constant temperature and pressure at the outlet of the solar field, in such a way that changes produced in the inlet water conditions and in the solar radiation will only affect the amount of steam produced by the system but not its quality. A PI control-based scheme has been designed and implemented for the recirculation mode. The structure of the single control loops, which are composing the control scheme, is shown in Fig. 3 where yr is the
Fig. 4. Schematic diagram of the DISS test loop configured in recirculation mode.
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(a) Recirculation pump control loop: Recirculation flow is maintained constant by adjusting the power input of the recirculation pump, i.e. the rotational speed (PI control). The main parameter influencing the pump behaviour, and consequently the controller behaviour, is strong change in pressure drop across the pump, i.e. solar radiation change influencing the solar field pressure level. (b) Feed pump control loop: The rotational speed of the feed pump is adjusted to maintain a specific pressure drop across the feed valve (PI control). Any change of the feed tank pressure directly affects the pressure drop controlled by the pump and those pressure changes could appear when there is a disturbance in the steam flow that feeds the DISS BOP. To avoid those changes the air condenser controls the feed tank pressure. (c) Middle steam separator liquid level control loop: The main objective of this loop is to maintain the level around a nominal value avoiding high or low levels inside the tank. The feed flow is then adjusted by controlling the aperture of the feed valve whose pressure drop is being controlled by the feed pump (P-PI control). This loop is highly affected by changes of radiation because if the radiation drops the steam production decreases and the tank level increases. Then the solar field water feed should decrease. As a consequence of the study presented in this paper, it can be observed that due to the dynamics of this control loop, some oscillations may appear. These can be avoided by acting on the recirculation pump instead of the aperture of the feed valve, thus reducing delays, which delays can cause oscillations in the controlled signal.
set point, ym(t) is the variable controlled or process variable, e(t) is error between both signals and m(t) is the control signal or output. Some characteristics of the PI functions implemented are: • The PI output is calculated using a classical interactive controller. The transfer function of such controller has the form: output = K Æ (1 + 1/(Ti Æ s)) Æ error, where K is the proportional gain and Ti is the integral time. • Anti-reset (integral) wind-up function to prevent error integral update whenever the actuator is saturated, i.e. as the PI algorithm has a term of integration, the magnitude of the integration term will increase toward infinity if the difference between the process variable and the set point remains for a long time. • Bumpless manual to auto transfer to prevent that when there is an interruption of the automatic control loop, in the transfer from auto to manual mode and vice versa the control levels would be lost. • Bumpless proportional band tuning to prevent that when there is a big change in the proportional constant with the controller running in automatic mode, the controller output would be modified drastically.
4. Control scheme for the recirculation mode The process diagram with the most relevant feedback loops for the recirculation operation mode with nine collectors [total length = 425 m] as the evaporation section and two [total length = 75 m] as the superheating section as shown in Fig. 4. The control loops for the solar field in the recirculation operation mode are: Table 3 Recirculation mode: models and PI control loops parameters Single feedback control loop
Model
PI parameters
Recirculation pump
0:0064 GðsÞ ¼ sþ0:1871
12.1% kg1 s
3.2
Feed pump
GðsÞ ¼ 0:0522310 sþ0:11765
0.95% bar1
6
Middle separator level
340 s GðsÞ ¼ 0:009 s e
Outlet steam pressure
GðsÞ ¼ 0:00170610 sþ0:00556
0.135% mm1 0.150% mm1 5.4% bar1
1300 – 184
1:65710 e100 s GðsÞ ¼ s2 þ0:016 sþ7:9105
0.39% C1
520
Outlet steam temperature (60 bar)
GðsÞ ¼
0.78% C1
700
Outlet steam temperature (100 bar)
GðsÞ ¼
1.08% C1
700
Kp
Outlet steam temperature (30 bar)
5
5
4
1:051104 e80 s s2 þ0:0169 sþ7:9105 1:381104 e90 s s2 þ0:0223 sþ1:384104
Ti (s)
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(d) Final steam separator liquid level control loop: The objective of this loop is to avoid significant liquid level in the final steam separator (On-off control). (e) Outlet steam pressure control loop: The steam produced by the collector loop feeds a steam separator; the outlet steam pressure is maintained constant by the adjustment of a steam control valve (PI control). (f) Outlet steam temperature control loop: The outlet steam temperature of the superheater is adjusted by water injection in the inlet of the last collector. The provision of injector valve control permits rapid response to sudden disturbances (PI control).
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The last two control loops are the main controllers of the system that should work in a way that guarantees the steam quality. The other four loops are required to improve the behaviour of the whole system and for operational feasibility. For the parameterization of the main controllers, transfer functions of all relevant control loops (SISO process models) have been investigated for the three operating points (Table 1), as interactions between the different control loops are low in the recirculation control mode. The identification method followed was to find in open loop (using the reaction curve method) the process parameters (gains, deadtimes, time constants, damping coefficients and natural frequencies) that fit experimentally
Fig. 5. Feed pump control: comparison between experimental results (January 31st, 2000) and closed-loop system model response. Differences between experimental and modelled pressure drop are not significant. Process gain and time constant seem to be well identified.
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open-loop step responses (see Table 3). Based on the simplified models obtained the PI controller parameters have been designed using the process reaction method and modifying adequately those controller parameters to provide safe stability margins (Ogunnaike and Harmon Ray, 1994). A final tuning of the parameters was done in subsequent tests, as a consequence of the comparison between real and simulated (using models detailed in Table 3) closed loop responses: • Fig. 5 shows a comparison for the feed pump control loop between simulated and real closed loop step responses using a PI controller with a pro-
portional constant a little bit higher than that proposed in Table 3 (that was reduced in subsequent tests). From the theoretical point of view (without plant-model mismatch), with this controller the closed loop system should have an infinite gain margin (GM) and a phase margin (PM) of 82.3, which means safe stability margins (GM > 1 and PM > 0) (Ogunnaike and Harmon Ray, 1994). • Fig. 6 shows a comparison for the separator level control loop. Simulation results using a PI control of proportional constant equal to 0.09%/mm and an integral time of 1000 s (GM = 13.3 dB; PM = 26.2), are compared with real results using the
Fig. 6. Separator level control: comparison between experimental results (June 22nd, 2000) and closed-loop system model response. Differences between experimental and modelled liquid level are due to the disturbances influencing the system during the test period, which is longer than two hours. During this period, radiation changes significantly due to the daily solar cycle but its influence is not being considered in the simulation. Anyway results corresponding to the first test hour show a good agreement between both responses.
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same controller. The set of PI parameters proposed in Table 3 (higher proportional constant and integral time) has improved the system response. • Fig. 7 shows a comparison for the outlet steam pressure control loop. Simulation results using a PI control of proportional constant equal to 1%/bar and an integral time of 50 s (closed-loop system with infinite gain margin and a phase margin of 62.8), are compared with real results using the same controller. Fig. 6 also includes the simulated response using the set of PI parameters proposed in Table 3 (GM = 1; PM = 90.5).
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• Fig. 8 shows a comparison for the outlet steam temperature control loop at 30 bar of outlet steam pressure. Simulation results using a PI control of proportional constant equal to 0.36%/C and an integral time of 520 s (GM = 10 dB; PM = 101), are compared with real results using the same controller. The behavior of this controller is conservative; subsequent readjustments led to an increase of the proportional gain (see Table 3). • Fig. 9 shows a comparison for the outlet steam temperature control loop at 60 bar of outlet steam pressure. Simulation results using a PI control of proportional constant equal to 0.5%/C and an
Fig. 7. Outlet pressure valve model: comparison between experimental results (March 3rd, 2000) and closed-loop system model response. Differences between the experimental and simulated response are not significant. Main process model parameters seem to be well identified. Using the set of PI parameters (K = 5.3%/bar, Ti = 184 s) the system response simulated improves considerably. This parameters set was selected to run this control loop in automatic mode.
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Fig. 8. Outlet temperature control (30 bar): comparison between experimental results (June 29th, 2001) and closed-loop system model response. Differences between the experimental and simulated response are not significant. Main process model parameters seem to be well identified.
integral time of 600 s (GM = 13 dB; PM = 106), are compared with real results using the same controller. The behavior of this controller is also too conservative, subsequent readjustments concluded in an increase of the proportional gain and an increase of the integral time (see Table 3). • Fig. 10 shows a comparison for the outlet steam temperature control loop at 100 bar. Simulation results using a PI controller with a proportional constant equal to 0.79%/C and an integral time of 700 s (GM = 9.4 dB; PM = 115), are compared with real results obtained using the same
controller. The step response had a long settling time, which was diminished using PI parameters finally proposed in Table 3.
5. Experimental results Some experimental results have been included and commented in the previous section, but corresponding figures do not show the complete behaviour of the system during the daily operation. In what follows, some relevant experiments performed
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Fig. 9. Outlet temperature control (60 bar): comparison between experimental results (July 2nd, 2001) and closed-loop system model response. Differences between the experimental and simulated response are not significant. Main process model parameters seem to be well identified.
within the DISS project are shown, where the controllers described in Section 4 were working in automatic mode. Fig. 11 shows the results obtained during an experiment in recirculation mode with 30 bar of outlet steam pressure. The responses of all the control loops described in Section 4 are detailed in this figure. During the start-up of the system the recirculation pump and feed pump control loops were working in automatic mode. The operator changed the corresponding references values according to the test requirements. The outlet pressure controller was put in automatic mode at around 11:00. An opera-
tion strategy, used in recirculation mode to reduce the start-up period, has been to configure all collectors as evaporation sections during the start-up period. When the outlet pressure desired is reached, the last two collectors are configured as the superheated section. This fact explains the change in the outlet steam flow occurring at 12:15 approximately. To change the configuration, it is necessary to manipulate hand valves and during a few minutes the final steam separator is not receiving steam. Once the final solar field configuration is established, the level and temperature control loops are put in automatic mode. From 12:00 to 21:00 the plant was operated
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Fig. 10. Outlet temperature control (100 bar): comparison between experimental results (July 4th, 2001) and closed-loop system model response. Differences between the experimental and simulated response are not significant. Main process model parameters seem to be well identified.
in automatic mode. During the operation, steps in the steam temperature reference were performed to evaluate the behaviour of the temperature controller, producing changes both in temperature and steam mass flow, while the rest of controlled variables were maintained stable around their corresponding references. The closed-loop 25 C step response has no overshoot but a settling time of approximately 25 minutes due to the use of a conservative set of PI parameters during the test. In steady state the maximum deviation from the temperature reference was equal to 5 C (1.8% of the reference value) and it occurred close to the twilight
when the solar irradiance was dropping below 400 W/m2 and the injector aperture was practically saturated. Small kick-back oscillations in the response are probably due to high frequency dynamics neglected in the modelling stage but present in this type of applications (Camacho et al., 1997), although they are not relevant from the steam generation point of view. Fig. 12 shows the results obtained during an experiment at 60 bar of outlet steam pressure. The objective of the test was to operate the system in steady state conditions for the whole day. The start-up procedure was similar to the one described
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Fig. 11. Plant behaviour during operation at 30 bar on June 29, 2001. Control loop responses: (a) recirculation flow control, (b) feed pump control, (c) middle separator level control, (d) steam pressure control, (e) steam temperature control, and (f) available solar radiation and steam generated during the test.
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Fig. 12. Plant behaviour during operation at 60 bar on July 3, 2001. Control loop responses: (a) recirculation flow control, (b) feed pump control, (c) middle separator level control, (d) steam pressure control, (e) steam temperature control, and (f) available solar radiation and steam generated during the test.
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Fig. 13. Plant behaviour during operation at 100 bar on July 5, 2001. Control loop responses: (a) recirculation flow control, (b) feed pump control, (c) middle separator level control, (d) steam pressure control, (e) steam temperature control, and (f) available solar radiation and steam generated during the test.
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Fig. 14. Plant behaviour during operation at 100 bar on July 4, 2001. Control loop responses: (a) recirculation flow control, (b) feed pump control, (c) middle separator level control that was the most influenced by disturbances in solar radiation, (d) steam pressure control, (e) steam temperature control, and (f) available solar radiation and steam generated during the test.
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before for the test at 30 bar. Once the operation conditions were established, the steam temperature reference was diminished to 330 C. The plant was working in automatic mode for around 8 h. The shutdown of the plant occurred at 19:30 approximately, when the radiation was too low to maintain the operating conditions. Fig. 13 shows the results obtained during an experiment with 100 bar of outlet steam pressure. From 11:00 to 21:00 the plant was operated in automatic mode. The pressure and temperature were maintained at the desired set points of 100 bar and 350 C. As the results presented before, the change of radiation due to the solar daily cycle influenced the steam mass flow produced but not its quality. Finally Fig. 14 also shows the results obtained during an experiment with 100 bar of outlet steam pressure, but with changes in solar radiation due to clouds passing over the field during the operation. The maximum deviation in the outlet steam temperature was of 4.2 C (1.2% of the reference value 340 C) and occurred around 18:00 when the irradiance dropped 400 W/m2. The closed-loop 10 C step response had no overshoot but a settling time of around 23 min due to the selection of conservative PI parameters. The level control loop was the most affected by the radiation transients with a maximum deviation of the water level inside the middle separator of 75 mm (5.8% of the reference value 1300 mm), when the controller was running in automatic mode. 6. Conclusions This paper has shown the development and implementation of control strategies to a new plant representing a breakthrough in technology for producing steam at high pressure and temperature directly in parabolic trough solar collectors. The DISS plant using this type of technology has been operated in two different modes. This paper has presented one of the plant configurations evaluated, the recirculation operation mode. With a PI controlbased scheme the controllability of the plant was guaranteed during clear days and even with transients in the solar radiation. A structure based on classical controllers was chosen because the plant operators are familiar with the used PI controllers and also to try to assure a secure operation in the first tests performed in this facility. These advantages would get lost if highly sophisticated control schemes would be used in the preliminary tests.
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Anyway, in the near future, a control strategy based on model predictive control will be investigated to improve system performance under disturbances in the inlet energy to the system. From the experiments performed it can be concluded that the system is more stable, and consequently, easier to control, at higher pressures (100 bar, 60 bar) than at lower pressures (30 bar), because the thermodynamic properties (specific enthalpy, thermal conductivity, etc.) of the water and the steam differ more at low pressure than at high pressure. On the other hand, there are more technical problems to operate the system at high pressure and temperature, as the risk of steam leaks and the thermal stress in the pipes is considerably higher. Acknowledgements Authors would like to thank the European Commission for the financial support given to the second phase of the DISS project (contract No. JOR3CT98-0277) within the framework of the E.U. JOULE Programme. Authors would also like to thank CICYT for funding this work under grant DPI2002-04375-C03. References Camacho, E.F., Berenguel, M., Rubio, F.R., 1997. Advanced Control of Solar Plants. Springer-Verlag. Cohen, G., Kearney, D., 1996. Current Experiences with SEGS Parabolic Trough Plants. In: Proc. of the 8th International Symposium on Solar Thermal Concentrating Technologies, Ko¨ln (Germany), pp. 217–244. Eck, M., Eberl, M., 1999. Controller Design for Injection Mode driven Direct Solar Steam Generating Parabolic Trough Collectors. ISES Solar World Congress, Jerusalem, Israel, vol. I, pp. 247–257. Ogunnaike, B.A., Harmon Ray, W., 1994. Process Dynamics, Modeling, and Control. Oxford University Press. Valenzuela, L., Zarza, E., Berenguel, M., Camacho, E.F., 2004. Direct Steam Generation in Solar Boilers. IEEE Control Systems Magazine 24 (2), 15–29. Valenzuela, L., Zarza, E., Berenguel, M., Camacho, E.F., 2005. Control concepts for direct steam generation in parabolic troughs. Solar Energy 78, 301–311. Zarza, E., Valenzuela, L., Leon, J., Weyers, D.-H, Eickhoff, M., Eck, M., Hennecke, K., 2002a. The DISS Project: Direct Steam Generation in Parabolic Trough Systems. Operation & Maintanence Experience and Update on Project Status. Journal of Solar Energy Engineering-Transactions of the ASME 124 (2), 126–133. Zarza, E., 2002b. Sistemas Termosolares con Colectores Cilindro Parabo´licos. Conceptos Ba´sicos y Generalidades. Plataforma Solar de Almerı´a, Internal Report R08/01 EZ.