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Control concepts for direct steam generation in parabolic troughs
Solar Energy 78 (2005) 301–311 www.elsevier.com/locate/solener
Control concepts for direct steam generation in parabolic troughs Loreto Valenzuela b
a,*
, Eduardo Zarza a, Manuel Berenguel b, Eduardo F. Camacho c
a CIEMAT, Plataforma Solar de Almerıa, P.O. Box 22, Tabernas (Almerıa) E 04200, Spain Departmento de Lenguajes y Computacion, Escuela Politecnica Superior, Universidad de Almerıa, Crta. Sacramento s/n, Almerıa E04120, Spain c Departmento de Ingenierıa de Sistemas y Automatica, Escuela Superior de Ingenieros, Universidad de Sevilla, Camino de los Descubrimientos, s/n, Sevilla E41092, Spain
Received 21 July 2003; received in revised form 14 May 2004; accepted 17 May 2004 Available online 15 June 2004 Communicated by: Associate Editor David Mills
Abstract A new prototype parabolic-trough collector system was erected at the Plataforma Solar de Almerıa (PSA) (1996– 1998) to investigate direct steam generation (DSG) in a solar thermal power plant under real solar conditions. The system has been under evaluation for efficiency, cost, control and other parameters since 1999. The main objective of the control system is to obtain steam at constant temperature and pressure at the solar field outlet, so that changes in inlet water conditions and/or in solar radiation affect the amount of steam, but not its quality or the nominal plant efficiency. This paper presents control schemes designed and tested for two operating modes, ‘‘Recirculation’’, for which a proportional-integral-derivative (PI/PID) control functions scheme has been implemented, and ‘‘Once-through’’, requiring more complex control strategies, for which the scheme is based on proportional-integral (PI), feedforward and cascade control. Experimental results of both operation modes are discussed. 2004 Elsevier Ltd. All rights reserved. Keywords: Direct steam generation; Parabolic troughs; Feedforward control; PI control
1. Introduction For the last 20 years, a considerable effort has been exerted in the control of solar thermal power plants with distributed parabolic-trough collector fields (Camacho et al., 1997). Up to now, commercial plants currently working in the US have been using synthetic oil as the heat transfer medium. The outlet of the solar field is connected to a heat exchanger that generates steam to feed a turbine. As the primary energy source, solar
*
Corresponding author. Fax: +34 950365015. E-mail address: [email protected] (L. Valenzuela).
radiation, is not controllable, the main control variable is the temperature of the oil at the outlet of the solar field, and the variable controlled is the oil mass flow at the field inlet. The dynamics of these systems are nonlinear and affected by several disturbances (mainly in input energy), which make the use of advanced control strategies necessary. A new parabolic-trough system to investigate direct steam generation (DSG) in a solar field using water inside the collectors as the heat transfer medium was erected at the Plataforma Solar de Almerıa (Tabernas, Spain) from 1996 to 1998. Since 1999, different operating strategies and configurations have been evaluated with promising results for system commercialisation.
0038-092X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.05.008
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Nomenclature Aab Ac Kp Lc Lr Pout Pw Ref Tamb Tav Ti Tin Tin c Tinj Tout Tref afv aiv apv hin hin
c
hinj hout
ref
mFFfv min
absorber tube surface area of collector row (m2 ) collector width (m) proportional constant of P-PI-PID control collector length (m) collector row length (m) steam pressure at collector row outlet (bar) feed pump power demanded by PI control (%) effective solar irradiance on collector (W/m2 ) ambient temperature (C) average temperature in collector row (C) integral time of PI-PID control (s) water temperature, collector row inlet (C) fluid temperature, last collector inlet (C) injection water temperature (C) steam temperature, collector row outlet (C) steam temperature reference (C) feed valve aperture dictated by PI control (%) injector valve aperture dictated by PI control (%) pressure valve aperture dictated by PI control (%) specific enthalpy water, collector row inlet (J/kg) specific enthalpy fluid, last collector inlet’ (J/ kg) specific enthalpy of water injected at the inlet of last collector (J/kg) specific enthalpy reference of steam, collector row outlet (J/kg) inlet water flow rate dictated by FFfv (kg/s) water flow rate, collector row inlet (kg/s)
The main task of the control system to be designed is to maintain constant live steam conditions at the end of the solar field in all operating modes. For the DISS (DIrect Solar Steam)-phase II project, financed by EC (European Commission)-Joule contract JOR3-CT980277 (1998–2001), structures using classical control and pragmatic approaches were chosen, as this has the advantage that the plant operators are familiar with the use of PI and PID controllers and in case of plant modification or system changes over time affecting system dynamics, and, thereby, controller performance, the plant operator is able to adapt the controller parameters to the new situation. This advantage would be lost if highly sophisticated control schemes were used.
min min
c ref
minj minj
ref
minj
set
DmFFiv Dminj pref DTin gc gr h
fluid flow rate, last collector inlet (kg/s) inlet water flow rate dictated by FFfv-PI (kg/s) water flow injected at last collector inlet (kg/ s) injection water flow rate dictated by outer loop of temperature controller using the injector (kg/s) injection water flow reference settled by the outer loop of the temperature controller via injector injection flow rate correction dictated by FFiv injection water flow rate correction dictated by PI (kg/s) pressure drop reference in the feed valve (bar) change of inlet temperature at last collector in two consecutives sample times (C) global collector efficiency (–) efficiency value for the entire collector row (–) incident angle of direct solar radiation ()
Acronyms DISS dIrect solar steam DSG direct steam generation EC European Commission FFfv feedforward function feed valve FFiv feedforward function injector valve IAM incident angle modifier PSA Plataforma Solar de Almerıa PI proportional-integral PID proportional-integral-derivative SCADA supervisory control and data acquisition
This article presents control schemes designed, implemented and tested for the Recirculation and Oncethrough operating modes. For the parameterisation of the controllers, low-order models of all relevant control loops (e.g., steam temperature, steam pressure) were studied in experiments under different operating conditions. This article is structured as follows. Section 2 describes the DISS facility. Section 3 discusses the main control problem in this system and the control schemes for Recirculation and Once-through modes are described in Sections 4 and 5, respectively. Section 6 shows illustrative experiments performed at the plant and some conclusions are presented in Section 7.
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2. Description of the DISS facility The DISS parabolic-trough solar plant is the leading DSG test facility in the world. Although the solar field can be operated over a wide temperature/pressure range, the three main operating points investigated in the DISS project are listed in Table 1. In the DISS project, thermal-hydraulic behaviour and system performance were investigated in three operating modes (Once-through, Recirculation, and Injection, see Fig. 1) under real working conditions, to identify the advantages and disadvantages of each mode (Eck et al., 2003). 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 row. This concept is less complex and requires a lower investment, but the main technical problem is process control. In the Injection mode, water is injected at several different points along the row of collectors. The measurement system necessary to implement the control
scheme designed for this mode did not work properly during experiments (Zarza et al., 2002), and its complexity and cost have resulted in this operating mode being discarded in new developments (Zarza, 2002). In the Recirculation mode, which is the most conservative of the three, a water-steam separator is installed at the end of the evaporator section of the collector loop. The amount of water fed to the evaporator is greater than the amount that can be evaporated. In the intermediate separator the excess water is recirculated to the collector loop inlet, where it is mixed with preheated water. The excess water in the evaporator
Table 2 DISS test facility: main parameters
Table 1 Operating modes studied in the DISS solar field Solar field conditions
Inlet P /bar
T /C
Outlet P /bar
T /C
Mode 1 Mode 2 Mode 3
40 68 108
210 270 300
30 60 100
300 350 375
303
Parameter
Value
Collector row length Collector type Collector width Number of collectors
500 m Modified LS-3 5.76 m Nine 50 m-long collectors Two 25 m-long collectors North–South 70 mm 50 mm 73% 2760 m2 100 bar 400 C 0.85 kg/s
Row orientation Absorber pipe outer diameter Absorber pipe inner diameter Mirrors optical efficiency Total mirror surface Max. outlet pressure Max. outlet temperature Max. steam production
Once-Through Concept
Injection Concept
Recirculation Concept
Fig. 1. Basic concepts of direct solar steam generation using parabolic-trough technology.
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In contrast to the single phase (liquid) flow used in the solar power plants currently working in the Mojave Desert in California (Cohen and Kearney, 1997) or at the PSA, this new system involves two-phase flow in the solar field, which complicates not only the engineering of the system, but also solar field control. In current solar power plants, the thermal fluid flowing through the solar field is a synthetic oil whose field outlet temperature is controlled by mass flow manipulation at the inlet. In the DISS solar field, not only the fluid temperature, but also the pressure must be controlled to maintain the desired steam conditions at the outlet (i.e., for the turbine). Therefore, regardless of the operating mode, Oncethrough, Injection or Recirculation, the main objective of the control system is to maintain steam at constant temperature and pressure in the outlet of the solar field. So changes produced in the inlet water conditions as well as in the solar radiation affect the amount of steam produced by the system but not its quality. A PI control scheme was designed and implemented for the Recirculation mode. For the Once-through mode, it was necessary to select more sophisticated control loops for controlling the outlet steam temperature, and controllers based on forward action were designed.
section guarantees good wetting of the absorber tubes and makes stratification impossible. The steam produced is separated from the water by the separator and fed into the inlet of the superheating section. This type of system is highly controllable, but the excess water that must be recirculated, the intermediate steam separator and the recirculation pump increase the system parasitic load. Some technical data for the DISS test facility are given in Table 2.
3. Control problem As the primary energy source, solar radiation, cannot be manipulated, the heating process output must be controlled by mass flow manipulation. The energy input is affected by several types of disturbances. These disturbances can be slow, such as those produced by variations in the solar radiation on a clear day, or they can be fast and strong, such as those due to clouds or changes in the inlet water conditions (temperature or pressure).
TT
Solar Collectors Row
PC PT
FT
LT
TC
Middle Water Steam separator
Final Water-Steam Separator
FC
Recirculation pump
Superheated steam
LC
Injection line
Feed preheater
H.P. steam
Flash Tank
Vapor feed
Feed pump
Feed Water Tank
Air Condenser
Equivalent Turbine Load L.P.
Legend PDT
PDC
TT - Temperature transmitter FT - Flow transmitter PT - Pressure transmitter PDT - Pressure drop transmitter LT - Level transmitter
TC - Temperature control loop FC - Flow control loop PC - Pressure control loop PDC - Pressure drop control loop LC - Level control loop
Fig. 2. Schematic diagram of the DISS test loop configured in Recirculation mode.
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Control schemes implemented and tested for the two operating modes are presented below.
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lating closed-loop responses, and adjusting these values, if necessary, to provide stability safety margins. Final optimisation of the parameter values was done in subsequent testing of the plant.
4. Recirculation mode control scheme The Recirculation mode process diagram is shown in Fig. 2 with the most important feedback loops for a configuration with nine evaporating collectors (425 m) and two superheating collectors (75 m). The control loops for the solar field in the recirculation mode are: • Recirculation pump control loop: Recirculation flow is maintained constant by adjusting the power input of a recirculation pump, i.e., the rotation speed (PI control: Kp ¼ 12:1%/kg/s, Ti ¼ 3:2 s). • Feed pump control loop: The rotation speed of the feed pump is adjusted to maintain a specific pressure drop in the feed valve (PI control: Kp ¼ 0:95%/bar, Ti ¼ 6 s). • Middle steam separator liquid level control loop: The main purpose of this controller is to maintain the nominal level, neither too high nor too low, inside the tank. The feed flow is adjusted by controlling the feed valve aperture where the pressure drop is controlled by the feed pump (P-PI control: Kp ¼ 0:150–0.135%/mm, Ti ¼ 1300 s). • Outlet steam pressure control loop: The steam produced by the collector row feeds a steam separator; the outlet steam pressure is maintained constant by adjusting the aperture of a steam control valve (PI control: Kp ¼ 5:4%/bar, Ti ¼ 185 s). • Outlet steam temperature control loop: The steam temperature at the superheater outlet is adjusted by injecting water into the inlet of the last collector. An injector valve controller provides rapid response to sudden disturbances (PI control: Kp ¼ 0:78%/C, Ti ¼ 700 s). The last two control loops are the main system controllers and must guarantee steam quality at every instant. The rest of the controllers are required to improve the behaviour of the whole control system and for operational feasibility. For parameterisation of the main controllers, transfer functions of all relevant control loops were studied in Single-Input Single-Output (SISO) models at the three operating points given in Table 1. Measured transfer functions were estimated by first-order lag models and first-order lag plus dead-time models. The identification method was to find the open-loop process parameters (gains, dead times and time constants) that experimentally fit step response data. Based on these models, the controller parameter values were chosen using the process reaction method (Bateson, 1996), studied by simu-
5. Once-through mode control scheme A complete description of the control system developed for this mode can be found in (Valenzuela et al., 2004). The process diagram showing the most relevant feedback loops for the Once-through operating mode is shown in Fig. 3. The main control loops in the Once-through mode are: • Feed pump control loop: The feed pump rotation speed is adjusted to maintain a specific pressure drop in the feed valve. In this operating mode, the feed valve is not the same as the one used in the Recirculation mode, therefore the model and, consequently, controller parameters are different (PI control: Kp ¼ 1:0%/bar, Ti ¼ 10s). • Outlet steam pressure control: This control loop has the same input and output as in the Recirculation mode. (PI control: Kp ¼ 5:4%/bar, Ti ¼ 185 s). • Outlet steam temperature control loops: Outlet temperature control is achieved by controlling both inlet feed flow rate and water injection in the superheater. The first ensures that the steady-state inlet flow rate matches the insolation conditions, whereas the second controller, at an injection point in the next to the last collector from the end of the row, provides rapid response to sudden disturbances (PI-Feedforward control-based loops). The outlet steam temperature control loops for this operating mode required a more sophisticated design than the rest, as this process is more sensitive to disturbances in the input variables and acceptable control with conventional PI or PID schemes was not possible. The subsections below describe the two temperature control loops designed for Once-through mode in detail. 5.1. Outlet steam temperature control by injector valve adjustment: regulation using a forward-action controller In this operating mode, the outlet steam temperature is mainly influenced by the injector aperture and, consequently, by the water injected before the last collector, but also by changes in the outlet steam temperature of the previous collector, the steam flow rate and the injection water temperature. Contrary to the Recirculation mode, in the Once-through mode there is no intermediate separator that muffles disturbances occurring in the preheating and evaporating sections.
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FC
TC
FT
TT
PC PT
Solar Collectors Row TC
FT
Final Water-Steam Separator
Superheated steam
FC PDT
Injection line Flash Tank
Air Condenser
Feed Water Tank
Feed pump
Equivalent Turbine Load L.P.
H.P. steam
Vapor feed
Feed preheater
Legend PDC
TT - Temperature transmitter FT - Flow transmitter PT - Pressure transmitter PDT - Pressure drop transmitter LT - Level transmitter
TC - Temperature control loop FC - Flow control loop PC - Pressure control loop PDC - Pressure drop control loop LC - Level control loop
Fig. 3. Schematic diagram of the DISS test loop configured in Once-through mode.
∆ pref
+
-
PI
Pw
Feed Pump
Water flow rate
Pressure drop across valve Feed flow rate
R dir Tin TambTinj m inj_offset
Tout_ref
FFfv
+
m in_ref +
+
-
PI a fv
PI
Feed Valve
+
Outlet steam pressure
-
Pout_ref
PI
-
PI
a pv
Outlet Pressure Valve
m inj_offset
T in_c m in_c Tinj
FFiv
+
+
+
∆m inj
+
+
m inj_ref
+
-
PI
a iv
DISS Collector Outlet steam temperature row
Injector Valve
Injection flow rate
Fig. 4. Block diagram of the multi-loop feedforward–feedback control system for the Once-through mode.
L. Valenzuela et al. / Solar Energy 78 (2005) 301–311
A forward-action controller has been designed to correct the injection flow rate at the inlet of the last collector based on changes in the collector inlet temperature and mass flow rate, injection water temperature and outlet temperature reference. The Once-through mode control scheme implemented is shown in Fig. 4. The feedforward control uses a process model to make changes in the controller output in response to changes measured in a major load variable without waiting for the error to occur. The feedforward function works in parallel with a PI controller whose parameters were calculated in the same way as for Recirculation mode (Kp ¼ 4 105 kg/s/C, Ti ¼ 250 s). The sum of these two outputs, Dminj , corrects the injection flow reference, minj offset , for current system conditions. This master loop dictates a new injection flow rate, minj ref , which is the reference for the inner control loop (PI control: Kp ¼ 20%/kg/s, Ti ¼ 12 s) that calculates the corresponding injection valve aperture, aiv . The cascade structure compensates for the nonlinearity of the injection valve. The feedforward function implemented to calculate the steady-state injection flow rate correction, DmFFiv , is DmFFiv ¼ ðða1 þ ða2 a3 Tout ref Þ Tinj Þ min–c a4 Þ DTin–c ð1Þ The values calculated for parameters a1 , a2 , a3 , and a4 at the different operating points are listed in Table 3. Eq. (1) was obtained by applying the energy balance equa-
Table 3 Outlet steam temperature control with injector valve: feedforward function FFiv parameters values Pressure/bar
a1
a2
a3
a4
30 60 100
7.35e)4 6.32e)4 4.64e)4
4.700e)6 7.945e)6 8.928e)6
8.000e)9 1.035e)8 1.024e)8
0.0 2.5e)5 1.0e)4
307
tions, linear regressions and input data detailed in Table 4. The simplified energy balance equation applied was: minj ¼
gc Ac Lc Ref min c ðhout hout ref hinj
ref
hin c Þ
5.2. Outlet steam temperature control by means of feed valve adjustment: regulation using a forward-action controller The wide variations in solar radiation require the control system to predict the inlet mass flow rate so that the outlet steam temperature remains within the required range. In the Once-through mode, system performance is very dependent on inlet flow control. Changes in radiation or inlet fluid temperature require the flow rate to change in such a way as to maintain the desirable output. However, changes involving large amplitude oscillations strongly affect solar field performance. Thermal and pumping losses increase, and the margin between the design and real outlet temperatures, which triggers the alarm signals, could also be bridged by large amplitude oscillations. The outlet steam temperature control loop designed and tested to manage these instabilities is also a forwardaction controller (see Fig. 4). The feedforward function works in parallel with a PI control block (PI parameters for 60 bar outlet pressure: Kp ¼ 6:5 103 kg/s/C, Ti ¼ 400 s) that corrects the steady-state flow rate calculated by the feedforward function, avoiding steady-state errors due to inaccuracy of the model used. This calculated value, min ref , is the set point for the inner loop (PI control: Kp ¼ 500%/kg/s, Ti ¼ 12 s), which calculates the feed valve aperture, afv . The feedforward control equation used to calculate the steady-state feed flow rate is given by mFFfv g Ac Lr Ref Ul Aab ðTav Tamb Þminj ðhout–ref hinj Þ ¼ r hout–ref hin ð3Þ
Table 4 Feedforward function design FFiv: input data sets Input
30 bar
60 bar
100 bar
Direct solar irradiancea Collector efficiencyb Collector inlet mass flow ratec Outlet temperature referenced Injection water temperatured
800 W/m2 0.5 0.35–0.8 kg/s 280–320 C 150–210 C
800 W/m2 0.5 0.35–0.8 kg/s 330–370 C 225–260 C
800 W/m2 0.5 0.35–0.8 kg/s 350–400 C 260–300 C
a
Variation in this parameter does not significantly effect coefficients calculated. Variations <10% of this parameter do not significantly effect coefficients calculated. c Mass flow rate modified in steps of 0.05 kg/s between the limits indicated. d Temperatures modified in steps of 10 C between the limits indicated. b
ð2Þ
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The collector row efficiency factor gr includes mirror reflectivity and peak optical efficiency. It is estimated using a constant value equal to 0.54. Ref is the effective solar irradiance on the collector and is calculated as follows
6. Experimental results
Ul is a factor related to the thermal loss coefficient of the collector absorber tube area. For an LS-3-type collector this can be estimated by Ajona (1999)
Controllers were programmed using a commercial SCADA system installed for DISS plant monitoring and control. Results from some of the experiments performed in the facility are discussed below. The graphs presented in Section 6.1 do not include all the signals related to the complete Recirculation mode control scheme, but only the outlet steam temperature and pressure, which are the main variables controlled in the solar field, the inlet water temperature, the direct solar irradiance available during the experiments, and the steam flow rate. The graphs presented in Section 6.2 also include the feed water flow rate at the collector row inlet and the water flow rate through the injector, which are control signals in the Once-through mode control scheme.
Ul ¼ b1 þ b2 ðTav Tamb Þ þ b3 ðTav Tamb Þ2
6.1. Recirculation mode results
Ref ¼ Rdir IAM cosðhÞ
ð4Þ
where Rdir is the measured direct solar irradiance, h is the incident angle and IAM is the incident angle modifier which, for a 50 m-long DISS collector, can be estimated by Eickhoff (2002) IAM ¼ 1 1:88 103 h 1:49206 104 h2
ð5Þ
ð6Þ
where the values of b1 , b2 and b3 which depend on the average temperature of the fluid in the absorber tubes, are listed in Table 5. Aab is the total area of the solar field absorber tubes (see Table 2). To simplify the control scheme structure, the average temperature of the fluid in the solar field, Tav , was estimated using a constant value at each operating point (see Table 6). The values of the specific enthalpies, hout ref , hin , and hinj , are calculated from the corresponding temperatures and the outlet pressure reference using thermodynamic water and steam tables (Wagner and Kruse, 1998).
Table 5 Thermal loss factors Ul in LS-3 collectors Average fluid temperature/C
b1
b2
b3
Tav < 200 200 < Tav < 300 300 < Tav
0.687257 1.433242 2.895474
0.001941 )0.00566 )0.01640
0.000026 0.000046 0.000065
Table 6 Average temperature values used by feedforward function FFfv for the three operating points investigated in the DISS project Pressure/bar
Inlet temperature/C
Outlet temperature/C
Tav =C
30 60 100
210 240 280
300 350 400
236.6 277.1 315.2
Fig. 5 shows the results obtained during an experiment with 100 bar outlet steam pressure. During plant start-up, the recirculation pump and feed pump control loops were working in automatic. The rest of the controllers were running in manual until approximately 11:00 am. One of the DISS project Recirculation mode operating strategies is to configure all the collectors as if they were in the evaporating section during start-up to reduce the start-up time. When the outlet pressure desired is reached, the last two collectors are reconfigured as the superheating section. This explains the change in the outlet steam flow rate at approximately 11:00 am. To change the configuration, manual valves must be manipulated, and therefore, for a few minutes, the final steam separator does not receive steam. Once the final solar field configuration is established, the level and temperature control loops are put into automatic. The plant was running in automatic mode until 9:00 pm. The pressure and temperature were maintained at the desired set points of 100 bar and 350 C and the change in radiation due to the daily solar cycle influenced the steam flow produced, but not its quality. During the experiment, the recirculation ratio (recirculation flow rate to feed valve flow rate) was approximately 1. Fig. 6 below also shows the results obtained with 100 bar outlet steam pressure. During this experiment, there were more drastic changes in solar radiation compared to the results in Fig. 5 due to passing clouds. The maximum deviation in the outlet steam temperature was 4.2 C (1.2% of the reference 340 C) at around 6:00 pm, when the irradiance dropped to 400 W/m2 . The closedloop 10 C step response did not overshoot, but had a settling time of around 23 min due to the conservative PI parameters. The control loop most affected by the disturbances was the level control loop. The maximum
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1000
1.0
800
0.8
600
0.6
400
0.4
200
0.2
350 100
200 40 150 20
outlet steam pressure inlet water temperature outlet steam temperature
0 10
12
14
16
18
-1
direct solar irradiance outlet steam flow rate
100
0 08
flow rate/(kg·s )
60
o
pressure/bar
250
temperature/ C
-2
80
solar irradiance/(W·m )
300
20
0.0 08
local time, GMT+02:00
10
12
14
16
18
20
local time, GMT+02:00
Fig. 5. Plant behaviour in Recirculation mode on July 5, 2001. The graph on the left shows the outlet steam pressure and temperature, which were controlled automatically, and the inlet water temperature. The graph on the right shows the available solar radiation during the test and the steam mass flow generated.
120
1000
1.0
800
0.8
600
0.6
400
0.4
200
0.2
350 100
200 40 150 20
outlet steam pressure inlet water temperature outlet steam temperature
0 10
12
14
16
18
local time, GMT+02:00
20
-1
direct solar irradiance outlet steam flow rate
100
0
flow rate/(kg·s )
-2
60
o
250
temperature / C
80
pressure/bar
solar irradiance/(W·m )
300
0.0 10
12
14
16
18
20
local time, GMT+02:00
Fig. 6. Plant behaviour in Recirculation mode on July 4, 2001. The graph on the left shows the outlet steam pressure and temperature, which were controlled automatically, and the inlet water temperature. The graph on the right shows the available solar radiation during the test, which varied widely during the day, and the steam mass flow generated.
L. Valenzuela et al. / Solar Energy 78 (2005) 301–311
320
80
280
70
240
60
200
50
160
40
120
30
80 outlet steam pressure inlet water temperature outlet steam temperature
20
800
0.8
600
0.6
400
0.4
40
10 11
12
13
14
15
0.2
0
0 10
direct solar irradiance outlet steam flow rate inlet water flow rate injection p. water flow rate
200
16
-1
90
flow rate/(kg·s )
360
1.0
-2
100
1000
solar irradiance/(W·m )
400
o
110
temperature/ C
pressure/bar
310
0.0 10
local time, GMT+02:00
11
12
13
14
15
16
local time, GMT+02:00
Fig. 7. Plant behaviour in Once-through mode on May 21, 2003. The graph on the left shows the outlet steam pressure and temperature, which were controlled automatically, and the inlet water temperature. The graph on the right includes the solar radiation available during the test, the steam mass flow generated, the feed water flow (variable manipulated by the temperature control loop) and the water flow injected in the superheating section (variable manipulated by the temperature control loop).
28
350
24
300
20
250
16
200
12
150
8
100
1000
1.0
800
0.8
600
0.6
400
0.4
direct solar irradiance outlet steam flow rate inlet water flow rate injection p. water flow rate
200 outlet steam pressure inlet water temperature outlet steam temperature
4
50
0
0 12
13
14
15
local time, GMT+02:00
16
flow rate/(kg·s-1)
temperature/oC
pressure/bar
-2
400
solar irradiance/(W·m )
32
0.2
0.0
0 12
13
14
15
16
local time, GMT+02:00
Fig. 8. Plant behaviour in Once-through mode on May 15, 2003. The graph on the left shows the outlet steam pressure and temperature, which were controlled automatically, and the inlet water temperature. The graph on the right shows the available solar radiation during the test, the steam mass flow generated, the feed water flow (variable manipulated by the temperature control loop) and the water flow injected in the superheating section (variable manipulated by the temperature control loop).
L. Valenzuela et al. / Solar Energy 78 (2005) 301–311
deviation of the water level inside the middle separator, with the controller running in automatic, was 75 mm (5.8% of the reference 1300 mm). During the experiment, the recirculation ratio was around 1.2. Instabilities observed during start-up (from 10:00 to 11:00 am.) in the outlet steam flow are due to water passing through the steam flow transmitter placed at the solar field outlet, producing an incorrect measurement of the steam flow rate. 6.2. Once-through mode results Fig. 7 shows the results obtained during an experiment with 100 bar outlet steam pressure. During startup, the feed-valve-aperture temperature control was in manual until around 11:30 am., when it was put into automatic. The outlet steam temperature was maintained close to the reference of 375 C throughout operation without significant deviation. The maximum overshoot of 11 C (2.9% of the set point) and undershoot of 5 C (1.3% of the set point) in the steam temperature, far from leading the steam to the saturation temperature, which is 311 C for 100 bar, occurred when superheated steam production began, before solar noon. Steam pressure was also maintained close to the set point without significant deviations. Fig. 8 shows the results obtained during an experiment with 30 bar outlet steam pressure. Around solar noon, a cloud transient covered the solar field for a few minutes, reducing available irradiance from 875 to 300 W/m2 approximately (66%). This disturbance made the outlet steam temperature drop 16 C (6% of the set point) and the outlet steam pressure drop 1bar (3% of the set point). However, the steam produced during this transient did not reach saturation. If the disturbances in the solar radiation had continued for more than 15 min the inlet flow rate would have reached the minimum set value and the fluid produced would have been saturated steam. This minimum flow rate is established to avoid high temperature gradients in the cross section of the absorber tubes when the radiation level is recovered.
7. Conclusions The DISS project has demonstrated that it is possible to produce steam at high pressure and temperature directly in the parabolic-trough solar collectors. This paper presents the prototype plant operated and evaluated in two different modes, Recirculation and Oncethrough. For the first experiments in the facility, control structures were mostly based on classical controllers. Even with transients in solar radiation, it is possible to control the solar field running in Recirculation mode with a PI-based scheme. For the Once-through mode it
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was necessary to design a more complex control scheme using feedforward and cascade control to maintain the outlet steam temperature constant. However, strong transients in solar radiation make it difficult to maintain the steam temperature with the plant configured in Once-through mode, because a minimum feed flow rate must be guaranteed in the solar field to avoid high temperature gradients in the cross section of the absorbers tubes when the radiation level is recovered. Model predictive control strategies are currently being studied to improve system performance.
Acknowledgements We thank the European Commission for the financial support given to the DISS project (C.No. JOR3-CT980277) under the EC JOULE Programme and the Spanish CICYT for funding under Grant DPI2002-04375C03. We also appreciate the anonymous reviewers’ helpful comments, which improved the article.
References Ajona, J.I., 1999. Electricity generation with distributed collector. In: Solar Thermal Electricity Generation. Ed. CIEMAT, Madrid. Bateson, R.N., 1996. Introduction to Control System Technology. Prentice-Hall, Inc., New York. Camacho, E., Berenguel, M., Rubio, F.R., 1997. Advanced Control of Solar Plants. Springer-Verlag, London. Cohen, G., Kearney, D., 1997. Current experiences with SEGS parabolic trough plants. In: Proceedings of the 8th International Symposium on Solar Thermal Concentrating Technologies, K€ oln. C.F. M€ uller Verlag, Heidelberg, pp. 217– 244. Eck, M., Zarza, E., Eickhoff, M., Rheinlander, J., Valenzuela, L., 2003. Applied research concerning the direct steam generation in parabolic troughs. Solar Energy 74 (4), 341– 351. Eickhoff, M., 2002. Incident angle modifier of the LS-3-DISSCollectors. Internal report of the DISS project. Doc. ID: DISS-SC-SF-30. February, 2002. Valenzuela, L., Zarza, E., Berenguel, M., Camacho, E.F., 2004. Direct steam generation in solar boilers. IEEE Control System Magazine 24 (2), 15–29. Wagner, W., Kruse, A., 1998. Properties of Water and Steam. Springer-Verlag, Berlin-Heidelberg-New York. Zarza, E., 2002. DISS Phase II Final Report, EU Contract No. JOR3-CT98-0277. Ed. CIEMAT, Madrid. Zarza, E., Valenzuela, L., Le on, J., Weyers, H.-D., Eickhoff, M., Eck, M., Hennecke, K., 2002. The DISS Project: direct steam generation in parabolic trough systems. Operation & maintenance experience and update on project status. Journal of Solar Energy Engineering––Transactions of the ASME 124 (2), 126–133.
Control concepts for direct steam generation in parabolic troughs Loreto Valenzuela b
a,*
, Eduardo Zarza a, Manuel Berenguel b, Eduardo F. Camacho c
a CIEMAT, Plataforma Solar de Almerıa, P.O. Box 22, Tabernas (Almerıa) E 04200, Spain Departmento de Lenguajes y Computacion, Escuela Politecnica Superior, Universidad de Almerıa, Crta. Sacramento s/n, Almerıa E04120, Spain c Departmento de Ingenierıa de Sistemas y Automatica, Escuela Superior de Ingenieros, Universidad de Sevilla, Camino de los Descubrimientos, s/n, Sevilla E41092, Spain
Received 21 July 2003; received in revised form 14 May 2004; accepted 17 May 2004 Available online 15 June 2004 Communicated by: Associate Editor David Mills
Abstract A new prototype parabolic-trough collector system was erected at the Plataforma Solar de Almerıa (PSA) (1996– 1998) to investigate direct steam generation (DSG) in a solar thermal power plant under real solar conditions. The system has been under evaluation for efficiency, cost, control and other parameters since 1999. The main objective of the control system is to obtain steam at constant temperature and pressure at the solar field outlet, so that changes in inlet water conditions and/or in solar radiation affect the amount of steam, but not its quality or the nominal plant efficiency. This paper presents control schemes designed and tested for two operating modes, ‘‘Recirculation’’, for which a proportional-integral-derivative (PI/PID) control functions scheme has been implemented, and ‘‘Once-through’’, requiring more complex control strategies, for which the scheme is based on proportional-integral (PI), feedforward and cascade control. Experimental results of both operation modes are discussed. 2004 Elsevier Ltd. All rights reserved. Keywords: Direct steam generation; Parabolic troughs; Feedforward control; PI control
1. Introduction For the last 20 years, a considerable effort has been exerted in the control of solar thermal power plants with distributed parabolic-trough collector fields (Camacho et al., 1997). Up to now, commercial plants currently working in the US have been using synthetic oil as the heat transfer medium. The outlet of the solar field is connected to a heat exchanger that generates steam to feed a turbine. As the primary energy source, solar
*
Corresponding author. Fax: +34 950365015. E-mail address: [email protected] (L. Valenzuela).
radiation, is not controllable, the main control variable is the temperature of the oil at the outlet of the solar field, and the variable controlled is the oil mass flow at the field inlet. The dynamics of these systems are nonlinear and affected by several disturbances (mainly in input energy), which make the use of advanced control strategies necessary. A new parabolic-trough system to investigate direct steam generation (DSG) in a solar field using water inside the collectors as the heat transfer medium was erected at the Plataforma Solar de Almerıa (Tabernas, Spain) from 1996 to 1998. Since 1999, different operating strategies and configurations have been evaluated with promising results for system commercialisation.
0038-092X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.05.008
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Nomenclature Aab Ac Kp Lc Lr Pout Pw Ref Tamb Tav Ti Tin Tin c Tinj Tout Tref afv aiv apv hin hin
c
hinj hout
ref
mFFfv min
absorber tube surface area of collector row (m2 ) collector width (m) proportional constant of P-PI-PID control collector length (m) collector row length (m) steam pressure at collector row outlet (bar) feed pump power demanded by PI control (%) effective solar irradiance on collector (W/m2 ) ambient temperature (C) average temperature in collector row (C) integral time of PI-PID control (s) water temperature, collector row inlet (C) fluid temperature, last collector inlet (C) injection water temperature (C) steam temperature, collector row outlet (C) steam temperature reference (C) feed valve aperture dictated by PI control (%) injector valve aperture dictated by PI control (%) pressure valve aperture dictated by PI control (%) specific enthalpy water, collector row inlet (J/kg) specific enthalpy fluid, last collector inlet’ (J/ kg) specific enthalpy of water injected at the inlet of last collector (J/kg) specific enthalpy reference of steam, collector row outlet (J/kg) inlet water flow rate dictated by FFfv (kg/s) water flow rate, collector row inlet (kg/s)
The main task of the control system to be designed is to maintain constant live steam conditions at the end of the solar field in all operating modes. For the DISS (DIrect Solar Steam)-phase II project, financed by EC (European Commission)-Joule contract JOR3-CT980277 (1998–2001), structures using classical control and pragmatic approaches were chosen, as this has the advantage that the plant operators are familiar with the use of PI and PID controllers and in case of plant modification or system changes over time affecting system dynamics, and, thereby, controller performance, the plant operator is able to adapt the controller parameters to the new situation. This advantage would be lost if highly sophisticated control schemes were used.
min min
c ref
minj minj
ref
minj
set
DmFFiv Dminj pref DTin gc gr h
fluid flow rate, last collector inlet (kg/s) inlet water flow rate dictated by FFfv-PI (kg/s) water flow injected at last collector inlet (kg/ s) injection water flow rate dictated by outer loop of temperature controller using the injector (kg/s) injection water flow reference settled by the outer loop of the temperature controller via injector injection flow rate correction dictated by FFiv injection water flow rate correction dictated by PI (kg/s) pressure drop reference in the feed valve (bar) change of inlet temperature at last collector in two consecutives sample times (C) global collector efficiency (–) efficiency value for the entire collector row (–) incident angle of direct solar radiation ()
Acronyms DISS dIrect solar steam DSG direct steam generation EC European Commission FFfv feedforward function feed valve FFiv feedforward function injector valve IAM incident angle modifier PSA Plataforma Solar de Almerıa PI proportional-integral PID proportional-integral-derivative SCADA supervisory control and data acquisition
This article presents control schemes designed, implemented and tested for the Recirculation and Oncethrough operating modes. For the parameterisation of the controllers, low-order models of all relevant control loops (e.g., steam temperature, steam pressure) were studied in experiments under different operating conditions. This article is structured as follows. Section 2 describes the DISS facility. Section 3 discusses the main control problem in this system and the control schemes for Recirculation and Once-through modes are described in Sections 4 and 5, respectively. Section 6 shows illustrative experiments performed at the plant and some conclusions are presented in Section 7.
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2. Description of the DISS facility The DISS parabolic-trough solar plant is the leading DSG test facility in the world. Although the solar field can be operated over a wide temperature/pressure range, the three main operating points investigated in the DISS project are listed in Table 1. In the DISS project, thermal-hydraulic behaviour and system performance were investigated in three operating modes (Once-through, Recirculation, and Injection, see Fig. 1) under real working conditions, to identify the advantages and disadvantages of each mode (Eck et al., 2003). 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 row. This concept is less complex and requires a lower investment, but the main technical problem is process control. In the Injection mode, water is injected at several different points along the row of collectors. The measurement system necessary to implement the control
scheme designed for this mode did not work properly during experiments (Zarza et al., 2002), and its complexity and cost have resulted in this operating mode being discarded in new developments (Zarza, 2002). In the Recirculation mode, which is the most conservative of the three, a water-steam separator is installed at the end of the evaporator section of the collector loop. The amount of water fed to the evaporator is greater than the amount that can be evaporated. In the intermediate separator the excess water is recirculated to the collector loop inlet, where it is mixed with preheated water. The excess water in the evaporator
Table 2 DISS test facility: main parameters
Table 1 Operating modes studied in the DISS solar field Solar field conditions
Inlet P /bar
T /C
Outlet P /bar
T /C
Mode 1 Mode 2 Mode 3
40 68 108
210 270 300
30 60 100
300 350 375
303
Parameter
Value
Collector row length Collector type Collector width Number of collectors
500 m Modified LS-3 5.76 m Nine 50 m-long collectors Two 25 m-long collectors North–South 70 mm 50 mm 73% 2760 m2 100 bar 400 C 0.85 kg/s
Row orientation Absorber pipe outer diameter Absorber pipe inner diameter Mirrors optical efficiency Total mirror surface Max. outlet pressure Max. outlet temperature Max. steam production
Once-Through Concept
Injection Concept
Recirculation Concept
Fig. 1. Basic concepts of direct solar steam generation using parabolic-trough technology.
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In contrast to the single phase (liquid) flow used in the solar power plants currently working in the Mojave Desert in California (Cohen and Kearney, 1997) or at the PSA, this new system involves two-phase flow in the solar field, which complicates not only the engineering of the system, but also solar field control. In current solar power plants, the thermal fluid flowing through the solar field is a synthetic oil whose field outlet temperature is controlled by mass flow manipulation at the inlet. In the DISS solar field, not only the fluid temperature, but also the pressure must be controlled to maintain the desired steam conditions at the outlet (i.e., for the turbine). Therefore, regardless of the operating mode, Oncethrough, Injection or Recirculation, the main objective of the control system is to maintain steam at constant temperature and pressure in the outlet of the solar field. So changes produced in the inlet water conditions as well as in the solar radiation affect the amount of steam produced by the system but not its quality. A PI control scheme was designed and implemented for the Recirculation mode. For the Once-through mode, it was necessary to select more sophisticated control loops for controlling the outlet steam temperature, and controllers based on forward action were designed.
section guarantees good wetting of the absorber tubes and makes stratification impossible. The steam produced is separated from the water by the separator and fed into the inlet of the superheating section. This type of system is highly controllable, but the excess water that must be recirculated, the intermediate steam separator and the recirculation pump increase the system parasitic load. Some technical data for the DISS test facility are given in Table 2.
3. Control problem As the primary energy source, solar radiation, cannot be manipulated, the heating process output must be controlled by mass flow manipulation. The energy input is affected by several types of disturbances. These disturbances can be slow, such as those produced by variations in the solar radiation on a clear day, or they can be fast and strong, such as those due to clouds or changes in the inlet water conditions (temperature or pressure).
TT
Solar Collectors Row
PC PT
FT
LT
TC
Middle Water Steam separator
Final Water-Steam Separator
FC
Recirculation pump
Superheated steam
LC
Injection line
Feed preheater
H.P. steam
Flash Tank
Vapor feed
Feed pump
Feed Water Tank
Air Condenser
Equivalent Turbine Load L.P.
Legend PDT
PDC
TT - Temperature transmitter FT - Flow transmitter PT - Pressure transmitter PDT - Pressure drop transmitter LT - Level transmitter
TC - Temperature control loop FC - Flow control loop PC - Pressure control loop PDC - Pressure drop control loop LC - Level control loop
Fig. 2. Schematic diagram of the DISS test loop configured in Recirculation mode.
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Control schemes implemented and tested for the two operating modes are presented below.
305
lating closed-loop responses, and adjusting these values, if necessary, to provide stability safety margins. Final optimisation of the parameter values was done in subsequent testing of the plant.
4. Recirculation mode control scheme The Recirculation mode process diagram is shown in Fig. 2 with the most important feedback loops for a configuration with nine evaporating collectors (425 m) and two superheating collectors (75 m). The control loops for the solar field in the recirculation mode are: • Recirculation pump control loop: Recirculation flow is maintained constant by adjusting the power input of a recirculation pump, i.e., the rotation speed (PI control: Kp ¼ 12:1%/kg/s, Ti ¼ 3:2 s). • Feed pump control loop: The rotation speed of the feed pump is adjusted to maintain a specific pressure drop in the feed valve (PI control: Kp ¼ 0:95%/bar, Ti ¼ 6 s). • Middle steam separator liquid level control loop: The main purpose of this controller is to maintain the nominal level, neither too high nor too low, inside the tank. The feed flow is adjusted by controlling the feed valve aperture where the pressure drop is controlled by the feed pump (P-PI control: Kp ¼ 0:150–0.135%/mm, Ti ¼ 1300 s). • Outlet steam pressure control loop: The steam produced by the collector row feeds a steam separator; the outlet steam pressure is maintained constant by adjusting the aperture of a steam control valve (PI control: Kp ¼ 5:4%/bar, Ti ¼ 185 s). • Outlet steam temperature control loop: The steam temperature at the superheater outlet is adjusted by injecting water into the inlet of the last collector. An injector valve controller provides rapid response to sudden disturbances (PI control: Kp ¼ 0:78%/C, Ti ¼ 700 s). The last two control loops are the main system controllers and must guarantee steam quality at every instant. The rest of the controllers are required to improve the behaviour of the whole control system and for operational feasibility. For parameterisation of the main controllers, transfer functions of all relevant control loops were studied in Single-Input Single-Output (SISO) models at the three operating points given in Table 1. Measured transfer functions were estimated by first-order lag models and first-order lag plus dead-time models. The identification method was to find the open-loop process parameters (gains, dead times and time constants) that experimentally fit step response data. Based on these models, the controller parameter values were chosen using the process reaction method (Bateson, 1996), studied by simu-
5. Once-through mode control scheme A complete description of the control system developed for this mode can be found in (Valenzuela et al., 2004). The process diagram showing the most relevant feedback loops for the Once-through operating mode is shown in Fig. 3. The main control loops in the Once-through mode are: • Feed pump control loop: The feed pump rotation speed is adjusted to maintain a specific pressure drop in the feed valve. In this operating mode, the feed valve is not the same as the one used in the Recirculation mode, therefore the model and, consequently, controller parameters are different (PI control: Kp ¼ 1:0%/bar, Ti ¼ 10s). • Outlet steam pressure control: This control loop has the same input and output as in the Recirculation mode. (PI control: Kp ¼ 5:4%/bar, Ti ¼ 185 s). • Outlet steam temperature control loops: Outlet temperature control is achieved by controlling both inlet feed flow rate and water injection in the superheater. The first ensures that the steady-state inlet flow rate matches the insolation conditions, whereas the second controller, at an injection point in the next to the last collector from the end of the row, provides rapid response to sudden disturbances (PI-Feedforward control-based loops). The outlet steam temperature control loops for this operating mode required a more sophisticated design than the rest, as this process is more sensitive to disturbances in the input variables and acceptable control with conventional PI or PID schemes was not possible. The subsections below describe the two temperature control loops designed for Once-through mode in detail. 5.1. Outlet steam temperature control by injector valve adjustment: regulation using a forward-action controller In this operating mode, the outlet steam temperature is mainly influenced by the injector aperture and, consequently, by the water injected before the last collector, but also by changes in the outlet steam temperature of the previous collector, the steam flow rate and the injection water temperature. Contrary to the Recirculation mode, in the Once-through mode there is no intermediate separator that muffles disturbances occurring in the preheating and evaporating sections.
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FC
TC
FT
TT
PC PT
Solar Collectors Row TC
FT
Final Water-Steam Separator
Superheated steam
FC PDT
Injection line Flash Tank
Air Condenser
Feed Water Tank
Feed pump
Equivalent Turbine Load L.P.
H.P. steam
Vapor feed
Feed preheater
Legend PDC
TT - Temperature transmitter FT - Flow transmitter PT - Pressure transmitter PDT - Pressure drop transmitter LT - Level transmitter
TC - Temperature control loop FC - Flow control loop PC - Pressure control loop PDC - Pressure drop control loop LC - Level control loop
Fig. 3. Schematic diagram of the DISS test loop configured in Once-through mode.
∆ pref
+
-
PI
Pw
Feed Pump
Water flow rate
Pressure drop across valve Feed flow rate
R dir Tin TambTinj m inj_offset
Tout_ref
FFfv
+
m in_ref +
+
-
PI a fv
PI
Feed Valve
+
Outlet steam pressure
-
Pout_ref
PI
-
PI
a pv
Outlet Pressure Valve
m inj_offset
T in_c m in_c Tinj
FFiv
+
+
+
∆m inj
+
+
m inj_ref
+
-
PI
a iv
DISS Collector Outlet steam temperature row
Injector Valve
Injection flow rate
Fig. 4. Block diagram of the multi-loop feedforward–feedback control system for the Once-through mode.
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A forward-action controller has been designed to correct the injection flow rate at the inlet of the last collector based on changes in the collector inlet temperature and mass flow rate, injection water temperature and outlet temperature reference. The Once-through mode control scheme implemented is shown in Fig. 4. The feedforward control uses a process model to make changes in the controller output in response to changes measured in a major load variable without waiting for the error to occur. The feedforward function works in parallel with a PI controller whose parameters were calculated in the same way as for Recirculation mode (Kp ¼ 4 105 kg/s/C, Ti ¼ 250 s). The sum of these two outputs, Dminj , corrects the injection flow reference, minj offset , for current system conditions. This master loop dictates a new injection flow rate, minj ref , which is the reference for the inner control loop (PI control: Kp ¼ 20%/kg/s, Ti ¼ 12 s) that calculates the corresponding injection valve aperture, aiv . The cascade structure compensates for the nonlinearity of the injection valve. The feedforward function implemented to calculate the steady-state injection flow rate correction, DmFFiv , is DmFFiv ¼ ðða1 þ ða2 a3 Tout ref Þ Tinj Þ min–c a4 Þ DTin–c ð1Þ The values calculated for parameters a1 , a2 , a3 , and a4 at the different operating points are listed in Table 3. Eq. (1) was obtained by applying the energy balance equa-
Table 3 Outlet steam temperature control with injector valve: feedforward function FFiv parameters values Pressure/bar
a1
a2
a3
a4
30 60 100
7.35e)4 6.32e)4 4.64e)4
4.700e)6 7.945e)6 8.928e)6
8.000e)9 1.035e)8 1.024e)8
0.0 2.5e)5 1.0e)4
307
tions, linear regressions and input data detailed in Table 4. The simplified energy balance equation applied was: minj ¼
gc Ac Lc Ref min c ðhout hout ref hinj
ref
hin c Þ
5.2. Outlet steam temperature control by means of feed valve adjustment: regulation using a forward-action controller The wide variations in solar radiation require the control system to predict the inlet mass flow rate so that the outlet steam temperature remains within the required range. In the Once-through mode, system performance is very dependent on inlet flow control. Changes in radiation or inlet fluid temperature require the flow rate to change in such a way as to maintain the desirable output. However, changes involving large amplitude oscillations strongly affect solar field performance. Thermal and pumping losses increase, and the margin between the design and real outlet temperatures, which triggers the alarm signals, could also be bridged by large amplitude oscillations. The outlet steam temperature control loop designed and tested to manage these instabilities is also a forwardaction controller (see Fig. 4). The feedforward function works in parallel with a PI control block (PI parameters for 60 bar outlet pressure: Kp ¼ 6:5 103 kg/s/C, Ti ¼ 400 s) that corrects the steady-state flow rate calculated by the feedforward function, avoiding steady-state errors due to inaccuracy of the model used. This calculated value, min ref , is the set point for the inner loop (PI control: Kp ¼ 500%/kg/s, Ti ¼ 12 s), which calculates the feed valve aperture, afv . The feedforward control equation used to calculate the steady-state feed flow rate is given by mFFfv g Ac Lr Ref Ul Aab ðTav Tamb Þminj ðhout–ref hinj Þ ¼ r hout–ref hin ð3Þ
Table 4 Feedforward function design FFiv: input data sets Input
30 bar
60 bar
100 bar
Direct solar irradiancea Collector efficiencyb Collector inlet mass flow ratec Outlet temperature referenced Injection water temperatured
800 W/m2 0.5 0.35–0.8 kg/s 280–320 C 150–210 C
800 W/m2 0.5 0.35–0.8 kg/s 330–370 C 225–260 C
800 W/m2 0.5 0.35–0.8 kg/s 350–400 C 260–300 C
a
Variation in this parameter does not significantly effect coefficients calculated. Variations <10% of this parameter do not significantly effect coefficients calculated. c Mass flow rate modified in steps of 0.05 kg/s between the limits indicated. d Temperatures modified in steps of 10 C between the limits indicated. b
ð2Þ
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The collector row efficiency factor gr includes mirror reflectivity and peak optical efficiency. It is estimated using a constant value equal to 0.54. Ref is the effective solar irradiance on the collector and is calculated as follows
6. Experimental results
Ul is a factor related to the thermal loss coefficient of the collector absorber tube area. For an LS-3-type collector this can be estimated by Ajona (1999)
Controllers were programmed using a commercial SCADA system installed for DISS plant monitoring and control. Results from some of the experiments performed in the facility are discussed below. The graphs presented in Section 6.1 do not include all the signals related to the complete Recirculation mode control scheme, but only the outlet steam temperature and pressure, which are the main variables controlled in the solar field, the inlet water temperature, the direct solar irradiance available during the experiments, and the steam flow rate. The graphs presented in Section 6.2 also include the feed water flow rate at the collector row inlet and the water flow rate through the injector, which are control signals in the Once-through mode control scheme.
Ul ¼ b1 þ b2 ðTav Tamb Þ þ b3 ðTav Tamb Þ2
6.1. Recirculation mode results
Ref ¼ Rdir IAM cosðhÞ
ð4Þ
where Rdir is the measured direct solar irradiance, h is the incident angle and IAM is the incident angle modifier which, for a 50 m-long DISS collector, can be estimated by Eickhoff (2002) IAM ¼ 1 1:88 103 h 1:49206 104 h2
ð5Þ
ð6Þ
where the values of b1 , b2 and b3 which depend on the average temperature of the fluid in the absorber tubes, are listed in Table 5. Aab is the total area of the solar field absorber tubes (see Table 2). To simplify the control scheme structure, the average temperature of the fluid in the solar field, Tav , was estimated using a constant value at each operating point (see Table 6). The values of the specific enthalpies, hout ref , hin , and hinj , are calculated from the corresponding temperatures and the outlet pressure reference using thermodynamic water and steam tables (Wagner and Kruse, 1998).
Table 5 Thermal loss factors Ul in LS-3 collectors Average fluid temperature/C
b1
b2
b3
Tav < 200 200 < Tav < 300 300 < Tav
0.687257 1.433242 2.895474
0.001941 )0.00566 )0.01640
0.000026 0.000046 0.000065
Table 6 Average temperature values used by feedforward function FFfv for the three operating points investigated in the DISS project Pressure/bar
Inlet temperature/C
Outlet temperature/C
Tav =C
30 60 100
210 240 280
300 350 400
236.6 277.1 315.2
Fig. 5 shows the results obtained during an experiment with 100 bar outlet steam pressure. During plant start-up, the recirculation pump and feed pump control loops were working in automatic. The rest of the controllers were running in manual until approximately 11:00 am. One of the DISS project Recirculation mode operating strategies is to configure all the collectors as if they were in the evaporating section during start-up to reduce the start-up time. When the outlet pressure desired is reached, the last two collectors are reconfigured as the superheating section. This explains the change in the outlet steam flow rate at approximately 11:00 am. To change the configuration, manual valves must be manipulated, and therefore, for a few minutes, the final steam separator does not receive steam. Once the final solar field configuration is established, the level and temperature control loops are put into automatic. The plant was running in automatic mode until 9:00 pm. The pressure and temperature were maintained at the desired set points of 100 bar and 350 C and the change in radiation due to the daily solar cycle influenced the steam flow produced, but not its quality. During the experiment, the recirculation ratio (recirculation flow rate to feed valve flow rate) was approximately 1. Fig. 6 below also shows the results obtained with 100 bar outlet steam pressure. During this experiment, there were more drastic changes in solar radiation compared to the results in Fig. 5 due to passing clouds. The maximum deviation in the outlet steam temperature was 4.2 C (1.2% of the reference 340 C) at around 6:00 pm, when the irradiance dropped to 400 W/m2 . The closedloop 10 C step response did not overshoot, but had a settling time of around 23 min due to the conservative PI parameters. The control loop most affected by the disturbances was the level control loop. The maximum
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outlet steam pressure inlet water temperature outlet steam temperature
0 10
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0 08
flow rate/(kg·s )
60
o
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80
solar irradiance/(W·m )
300
20
0.0 08
local time, GMT+02:00
10
12
14
16
18
20
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Fig. 5. Plant behaviour in Recirculation mode on July 5, 2001. The graph on the left shows the outlet steam pressure and temperature, which were controlled automatically, and the inlet water temperature. The graph on the right shows the available solar radiation during the test and the steam mass flow generated.
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800
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600
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Fig. 7. Plant behaviour in Once-through mode on May 21, 2003. The graph on the left shows the outlet steam pressure and temperature, which were controlled automatically, and the inlet water temperature. The graph on the right includes the solar radiation available during the test, the steam mass flow generated, the feed water flow (variable manipulated by the temperature control loop) and the water flow injected in the superheating section (variable manipulated by the temperature control loop).
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deviation of the water level inside the middle separator, with the controller running in automatic, was 75 mm (5.8% of the reference 1300 mm). During the experiment, the recirculation ratio was around 1.2. Instabilities observed during start-up (from 10:00 to 11:00 am.) in the outlet steam flow are due to water passing through the steam flow transmitter placed at the solar field outlet, producing an incorrect measurement of the steam flow rate. 6.2. Once-through mode results Fig. 7 shows the results obtained during an experiment with 100 bar outlet steam pressure. During startup, the feed-valve-aperture temperature control was in manual until around 11:30 am., when it was put into automatic. The outlet steam temperature was maintained close to the reference of 375 C throughout operation without significant deviation. The maximum overshoot of 11 C (2.9% of the set point) and undershoot of 5 C (1.3% of the set point) in the steam temperature, far from leading the steam to the saturation temperature, which is 311 C for 100 bar, occurred when superheated steam production began, before solar noon. Steam pressure was also maintained close to the set point without significant deviations. Fig. 8 shows the results obtained during an experiment with 30 bar outlet steam pressure. Around solar noon, a cloud transient covered the solar field for a few minutes, reducing available irradiance from 875 to 300 W/m2 approximately (66%). This disturbance made the outlet steam temperature drop 16 C (6% of the set point) and the outlet steam pressure drop 1bar (3% of the set point). However, the steam produced during this transient did not reach saturation. If the disturbances in the solar radiation had continued for more than 15 min the inlet flow rate would have reached the minimum set value and the fluid produced would have been saturated steam. This minimum flow rate is established to avoid high temperature gradients in the cross section of the absorber tubes when the radiation level is recovered.
7. Conclusions The DISS project has demonstrated that it is possible to produce steam at high pressure and temperature directly in the parabolic-trough solar collectors. This paper presents the prototype plant operated and evaluated in two different modes, Recirculation and Oncethrough. For the first experiments in the facility, control structures were mostly based on classical controllers. Even with transients in solar radiation, it is possible to control the solar field running in Recirculation mode with a PI-based scheme. For the Once-through mode it
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was necessary to design a more complex control scheme using feedforward and cascade control to maintain the outlet steam temperature constant. However, strong transients in solar radiation make it difficult to maintain the steam temperature with the plant configured in Once-through mode, because a minimum feed flow rate must be guaranteed in the solar field to avoid high temperature gradients in the cross section of the absorbers tubes when the radiation level is recovered. Model predictive control strategies are currently being studied to improve system performance.
Acknowledgements We thank the European Commission for the financial support given to the DISS project (C.No. JOR3-CT980277) under the EC JOULE Programme and the Spanish CICYT for funding under Grant DPI2002-04375C03. We also appreciate the anonymous reviewers’ helpful comments, which improved the article.
References Ajona, J.I., 1999. Electricity generation with distributed collector. In: Solar Thermal Electricity Generation. Ed. CIEMAT, Madrid. Bateson, R.N., 1996. Introduction to Control System Technology. Prentice-Hall, Inc., New York. Camacho, E., Berenguel, M., Rubio, F.R., 1997. Advanced Control of Solar Plants. Springer-Verlag, London. Cohen, G., Kearney, D., 1997. Current experiences with SEGS parabolic trough plants. In: Proceedings of the 8th International Symposium on Solar Thermal Concentrating Technologies, K€ oln. C.F. M€ uller Verlag, Heidelberg, pp. 217– 244. Eck, M., Zarza, E., Eickhoff, M., Rheinlander, J., Valenzuela, L., 2003. Applied research concerning the direct steam generation in parabolic troughs. Solar Energy 74 (4), 341– 351. Eickhoff, M., 2002. Incident angle modifier of the LS-3-DISSCollectors. Internal report of the DISS project. Doc. ID: DISS-SC-SF-30. February, 2002. Valenzuela, L., Zarza, E., Berenguel, M., Camacho, E.F., 2004. Direct steam generation in solar boilers. IEEE Control System Magazine 24 (2), 15–29. Wagner, W., Kruse, A., 1998. Properties of Water and Steam. Springer-Verlag, Berlin-Heidelberg-New York. Zarza, E., 2002. DISS Phase II Final Report, EU Contract No. JOR3-CT98-0277. Ed. CIEMAT, Madrid. Zarza, E., Valenzuela, L., Le on, J., Weyers, H.-D., Eickhoff, M., Eck, M., Hennecke, K., 2002. The DISS Project: direct steam generation in parabolic trough systems. Operation & maintenance experience and update on project status. Journal of Solar Energy Engineering––Transactions of the ASME 124 (2), 126–133.