Solar Energy Materials 24 (1991) 265-278 North-Holland

Solar Energy Materials

Solar testing of 2 MVVth water/s eam receiver at the Weizmann Institute solar tower

M;,ch~el Epstein , D. L iebe rmann , M. kos~, ,~,u---~ ,7,,A-'t-.'-,,.,, j . Shor

The W e i z m a n n lnst i tut~ o f S c i e n o , So iar Research Facilities Umr., , , e h a ~ , t ~ ~" "'~ ,v,~,~,,'~'ru~ ,s ,a~, ~ " ~

A water/steam ~ec.ei~cr ~ystem was designed, fabricated, assembled ano tested at the solar central receiver fac!ity at the Weizmann Institute.

The system is designed for 2 MWth input (March 21st, noon timej and generates steam "-it 15 atg. NomivM maximum energy flux on -',he evaporating panels is 300 kw/sq.m.

The receEer is built as a "C'" shaped cawty with three active panels made of 1 inch vert,ca! tubes. Cmstruction details are given. The water is forced circulated at a ratio of 1:25. Steam ~s separated in a steam drum.

The codes used for desioa and.evaluation are: MIRVAL, CAVEG, SHAPEFACT. Oi~, RADSOLVER and SAPPIt!R-WIS. 'These codes were originz, lly developed at Sandia Nationa~ Laboratories, USA, with the exception of CAVEO which wasdeveloped a~ the Weizmar.a Institute of Science (WIS), and we~e modified for cavity conditions TFJs p a c . k ~ g e proved to be useful and convenient for a paran~etric study in the assessment ot tlae bet~avior of the s:.=,;em under extreme conditions (i.e. burnoutJ and ia the evzd~,~tio~ of the test results.

A summar3' ~? design da~a, including ener~ fluxes, temperature profiles a'td s~eam generation along the ~'a'v~l~ ~s gNe~. Diffe.%~t i!ow regions in the tubes and thei~ respective transitions

*" "g. under various operati~.g conditions are indicated. TyJieal test :e,~ults oI r , . - first year are analyzed and compared to compatated resutt~. Start-up qme and sysi?m ~,aabi!ity are ghov,n. The minimiza,ion of parasitic water volume !n the syster- proved tc. be helpful in reducin~a cold start:up time t~ 50 rain acid warm start-Lp time to 35 rain.

1. lntroducti~3ri

The Solar Central Receiver Steam Production Sy~t.ern ~vas completed in early !989 ,at the Solar Facility of ,b,~ Weir, mann !nstitme of Science it, Rehovot, Israel. This project was fully supported by the Israeli Ministry of Energy and infrastruc- ture. Preliminary shakedown runs were started in March !989 and operational runs have continued to the present. The system had accumulated a total of 125 operationa I days and 500 hours of steam production. The sy~t,:m was designed for 2 M'~'th input (March 2!, sol;~r noontime) to generate saturated steam at 15 atmospheres (g) from a nominal energy flux on the evaporating panels of 3PAl k w / s q - m.

I n p u t p o w e r is p r o v i d e d by a fie~d o f up to 64 computer-c~nt io l !czd hel iostats

enc i rc l ing the n o r t h side o f the 53 m e t e r h igh " S o l a r Tower" . T h e tower provides

space at var ious levels fo r a m u ~ b e r o f test insta~!ations which L, tilize e~ther all o r

d i f f e ren t g roups o f he~iostatu focus ing on o n g o i n g paral lel expe r imen ta l targets ,

T h e s t e a m receiver is s i tua ted at the 27 -me te r -h igh exper i raen ta ~ level.

0165-1633/91/$03.50 © 19'~3 - FIsevier Science Publishers B.V. All rights reserved

266 M. Epstebz e~ al. / So lar te. ~' o) '2 M W , t, wat.er / s t e a m t :eit 'er

Fig. 1. Photo of the WlS heliostat field.

The main purposes of this project were: 1) development of a package of computer codes for the design of a wate r / s team

receiver; 2~ development of design capability for this system in Israel;

~3) evaluation and testing of the concept of separating the evaporating section from the superheating section for operational co,wcnience. Superheating can be done in a separate receiver.

4) development of start-up procedures; 5) studies on the stability of the system ove~ ~ a wide range of solar power input.

2. System description

2.1. Heliostat fieb!

MiRVAL #t and HELIOS #2 computer codes were applied to ti~e design ,3f the s " 1 . ¢ " "). he!iostat tK:ld. The mirrors are arranged in 6 rows comprising 3 focal gioups t,i 6:~,

8~ ~t aad I08.8 lr.etet.~. The heliostats are t~e Asinel-Gast model supplied by

#l P.L. Leary and LD. H~.~nkiu',, "A U~ci~'s Guide |or M|RVAL". ~2 S,~.NI~ ~A Labs, Livermore, Report SANDll-8280, Feb.. 1979.

M. Epstein etal. / So,~r test of 2 MWrh r'ater /steam receicer 267

Asine!, Spain. 'The spherically curved r,~irror surfaces totalling 54.25 m 2 per heliostat are split into r,vo halves with a b50 mm slot to permit face-down stowing. Eacb individual he!;ostat contains a local control unit and each group of 32 communicates via optical cable to a field controller. Each of four targets located at four different levels in the tower may be independently served under command of the main computer. A photograph of the heliostat field is shown in fig. 1.

2.9_. Solar tower

The tower, rectangular :.n cross-section with dimensions of I0 meters (east-we~t) by 15 meters (north-south), rises some 53 meters above ground leve!. The entire volume of the tower including the roof and some of the surrounding aree can be used as working space. The steam generator experiment lzvel has a 6 × 7 m roll-,Jp door. Behind the door is the receiver's aperture panel. Positioned around the front wall of the recziver is a partition made of ceramic fiber boards to protect the surrounding components.

Various service systems are available: A feedwater system supplying 3.5 tons per hour of treated water (reverse osmosis quality) to the steam system, instrument and ser~ ice compressed air, cooling water for circulating pump bearing, and a 600 KVA diesel generator unit for standby emergency power.

2,3. Solar steam receiver - geometric, mechanical, tiie, mohydraulic

The solar stream receiver is a he~agvnai shaped cavity, the front face consisting v,f a 2.5 meter by 2.5 meter openin!:. Opposite are three active solar absorbing panels, the central, 2.5 meter wide, the ~,,3 adjoin;ng parcels each 2 meters wide. The rcL~aining t:wo adjoining refractory panels are internal wt..:te insulation walls 2 meters wide (fig. 2). Both the ceiling and flo,.~r consist of refractory reflecting surfaces. The internal height 6f the cavity is three meters, and the depth from the front opening to the rear wai~. is 3 meters. The three active pa~e_!s accommodate a tntal 6f 246 vertical tubes of 1 inch nominal schedule 40 carbon steel pipe. Each active panel is fitted wi',h an upper ar;d lower manifol~ located outside of the cavity, the latt,~r receiving the "~;~ o ,..ocharg,~ of the _main centrifugal circulating pump wh:;,~ in turn takes suction from the bottom of the steam drum located at the highest point in the system. Each tube in the "" t" is _ e,, ,pora mg panels free to move and expand. There is no welding connection among the tubes to create a so-called "water wall" panel. There is a small space (2 n,m) between two adjacent tubes. The tubes are welded to the sides of the manifolds as shown in figs. 3 and 4. The evaporating panels are separated at their back side from the insulathlg walls by a gap of 10 cm. The three in,.;ulating walls behind the active panels can be easily dismantled to provide maintenance access to ,.he back of the panels.

~,'ig. ~ is a ~chematic dra~ktg of ~he steam generator s3,s~e, in. A pair of high capaci~.y low-differential pressure centrifugal pumps {one on standby) recircuiates

268 M. Epstein etal. / Solar test of 2 MWth water~steam receiver

\

. 2 e o B I -

~ I

'u

Fig. 2. General plan of the receiver.

water at about 70 m3/hr (a mass ratio of about 1:25 steam to water at full capacity) into ti~e three active receiver panels. The steam-water mixture passes up through the vertical tubing abr, orbing energy from the solar flux directed into the receiver by the heliostat field. The steam-water mixture is collected in the upper horizontal 10 inch pipe manifold and thence passes to the steam drum. The latter is maintained about one half full of water by a pair of high pressure centrifugal feed pumps (one on stand-by). Steam of almost 100% quality leaves the drum, after passing through entrainment separators as process steam at a maximum r~ominal rate of 2300 kg/hr at 15 atg at about 200 ° C. A control system maintains both design water level limits in the drum and the operating pressure. A two-ele- ment control system is required to simultaneously regulate feedwz.ter flow, system pressure and steam drum water level. A negative feedback algorithm was devised for the control loop to offset the rise in stear~, drum level due to boiling and the ~'esaltant decrease i1~ effective water density. Othe~-~ise feedwater flow would siaarply decrease when an actual increase would be required to maintain adequate liquid volume in the drum especially during startup and solar power input transients.

Surface thermocouples are mounted at various heights on the vertical tubes in the absorber panels to monitor variations in temperature along the tubes, espe- cially '"hot spots". System temperatures and pressures are monitored by an alarm and emergency control system to ensure safe operating limits. Operating parame-

M. Epstezn et al. / Solar test of 2 MI~ h water~steam reeeit'er 269

o

100

152

Aperture

.......... ,r ........... ~ ~ A A A A AA~A~

Fig. 3. Vertical receiver cross section of center and side panels.

ters and data are stored in a data processing system for study and analysis. The circulating water and feed water are routinely monitored for the concentration of solids and com;nvously for the presence of O, and pH, which are controlled by title addition of hydrazine to the feed water.

3. Heat transfer design codes

The configuration for thz cavity absorber above was used as a basis for radiative heat transfer computations by applying the MIRVAL and RADSOLVER #3 codes as modified for local conditions from the originals developed by the Sandia

#3 M. Abrams, "RADSOLVER - A Computer Program.~or Calculating Spectrally-Dependent Radia- tive Heat Transfer in Solar Cavity Receivers"~ SAND81-8248, Sandia National Laboratories, Livermore, CA, Sept. 1981.

270 M. Epstein et al. / Solar test o f 2 MWtj , water~steam receicer

355 [ =t i

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l___ 2632

Fig. 4. Design details of evaporating panels.

National Laboratories. The three absorbing panels are divided into 300 zones, and the two side panels into 50 zones; together with the upper and lower surfaces, a total of 377 zones were analyzed. The view factor for each zone was calculated by the code S H A P E F A C T O R #4. The highest net absorbed fluxes, 300 k W / m 2, occurred at the rear panel and generally in the upper zones as expected from the relative position of the heiiostat field and cavity aperture. The entire heat flux distribution was mapped. Fig. 6 shows a representative flux distribution for each of

#4 A.F. Emery, "Instruction M~.nual for the Program SHAPEFACTOR", SANDIA80-8024, Sandia National Laboratories, Livermore, CA, Oct. 1980.

M. Epstein et al. / Solar test of 2 l~ff ',h water~steam receiver 271

L _ _

I

Surplus steam

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Fig. 5. Process diagram of the steam syslem. (LT) Level transmitter, (PT) pressure transmitter, (FT)

f t . v . . . . . . smitter.

the 300 active zones. It ,vas estimated that during the, design day (noon, March 21) the flux entering the cavity was 1.97 MW with 90% rez'~ectivity of mirrors. The total amount of radiation losses through the aperture was 0.188 MW, resulting in a cavity radiation efficiency of 91.5% #5

The computer code SAPPHIR-WlS, a modified version of SAPPHIR by Sandia National Labs (originally developed for external receivers), proved useful and convenient for a parametric study of the assessment of the behavior of the solar steam generator system under extreme conditions (i.e. burnout) and finally in the evaluation of the experimental results. SAPPHIR-WIS calculates the ~teady state tube side heat transfer to water and to two phase, water-steam mixtures in several flow patterns in up to nine types of flow regimes and their loc~.tion inside each tube.

#5 M. Epstein, "Technology of Steam Generation in the Solar Tower', RD-15-88, Wei=mann Institute of Science, Rehovot, Israel, 1988.

272 M. Epstein etal. / Solar test 0]'2 MWa, water~steam receiver

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using one-dimensional conservatiot~ eqvations; non-equilibrium is modelled for saturated nucleate boiling.

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274 M. Epstein et al. / .Solar test of 2 mWlh water/steam receiver

Table 1 WIS steam receiver and thermodynamic profile

CENTRAL PANEL: TUBE NUMBER 4 HEAT TRANSFER CHARACTERISTICS Test Date 07 May, 10:30-11:00

Fluid outk~, temperature ( ° C) 198.2 Net input heat power on the tube (kW) 8.34 Inlet enthalpy (kW. h/kg) 224.48 Exit enthalpy (kW-h/kg) 258.78 Tube average mass flog' rale (kg/tO 250.8 Absorbed power (~W) 8.42 Convection Ios~cg in t, he tube (kW) 0.07 Maximum crown temperature ( ° C) 225.9 Eievalion cf max. crown temperature (m) 2.600 Average crown temperature (" C) 215.6 Average front surface temperature ( ° C) 209.0 Average back surface temperature ( ° C) 195.4 Exit thermodynamic steam quality (%) 4.20 Mass of stem generated in the tube (kg/h) 10.50 (*) Temperatures averaged along ti:e active height of the tube

Thermodynamic profile

Elevation Flow Net HF Tbulk Tcrn Tback X c Hin s (m) Type (kW/m 2) ( ° C) ( ° C) ( ° C) (%) (kW/m z- K)

0.000 1 14.727 190.0 195.7 190.2 0.00 2.74 0.200 1 14.766 190.3 195.9 190.5 0.00 2.75 0.500 1 15.770 190.7 196.7 190.9 0.00 2.75 0.800 1 32.354 191.3 203.8 191.7 0".00 2.75 1.100 1 59.766 192.4 215.6 193.3 0.00 2.76 1.109 i 61.198 192.5 216.2 193.4 000 2.76 1.400 2 102,26! 194,5 217.4 194.7 0.00 6,44 1,700 3 128.683 197.4 220.0 ! 9Z5 0.00 9.89 1.773 3 146.640 198.3 220.8 198.3 0.00 i2.67 2.000 4 184.829 198.3 225.1 !98.3 0,74 14.26 2,083 4 187.291 i98.3 225.4 198.3 1.04 !4.33 2.300 5 188.544 198.3 225.9 198.3 i.84 14.00 2.600 6 1°7.147 198.2 225.9 198.2 2.94 13.72 2.900 6 145.625 i98.2 220.9 198.2 3.92 12.33 3.000 6 130.452 198.2 219.0 198.2 4.19 !1.83

Fh)w Regime Key 1 - subcooled liquid (one-phase liquid convection) 2 - subcooled nucleate boiling (partial) 3 - subcooled nucleate boiling (fully-developed) 4 - saturated nucleate boiling with X,, > X e

X,, - actual steam quality X~ - thermodynamic steam e.aality

5 - saturated nucleate boiling with X~, > X e 6 - annular flow 7 - mist flow (folh:~uing dryout) with Xe < ! 8 - mist flow with X~ > 1 and X,, < 1 9 - superheaied vapor X,, = X~. > I

M. Epstein et aL / Solar test of 2 MI4~h water~steam receiver 275

Key to the Table

Elevation - height along th~ tube counted from the inlet Flow Type - as described in previous key Net HF

Tbulk Tcrn Tback X~

] ' { i n s

- net heat flux assumed to be absorbed by the fluid - fluid average temperature in the control volume - average temperature o f the control zone - average temperature at the tube back surface - thermodynamic steam quality - heat transfer o ~efficient inside the tube

- two-dimension,:!, steady state modelling of heat conduction in pipes and vessels; - coupling between fluid flow and pipe and vessels walls, heat conduction and

convection, and' surt~ce temperature profiles; - capability of modelling flow choking and refraction waves; - capability of applying to a number of constitutive models, such as heat transfer

coefficients, form loss factors, frictional and elevation loss contributions; - capability of evaluating energy losses by radiation (reflected solar and thermal)

and convection through the cavity walls; - capability of analyzing flow transitions, to predict burnout and excursion of

surface temperatures, and respective ~.|evations;

2 ~ -

STEAM RECEIVER

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276 M. Epstein et al. / Solar test of 2 MWth water~steam ~eceicer

- capability of calculating, mass quality and cumul~Ltive mass of steam generated. Typical calculation results of SAPPHIR are shown in table I. These calculations

are done for a representative single tube in the rear central panel. The table lists temperatures in the bulk of the fluid and on the surface. Also shown are the different flow regimes that exist inside the tube and the elevation of their transition points, steam quality, X e, and heat transfer coefficients. The results are also shown graphically in fig. 7.

There is a good match between the calculated product steam rate by SAPPHIR- WIS and the measured rate over a large range of power loads. Also, there is good agreement between the measured surface temperatures and the calculated pro- files.

4. Test results

4.1. Start-up-time

The system was designed to minimize start-up time. Drum holdup during standard operation is equivalent to about 10 minutes of nominal steam production rate. Furthermore, the walls of the cavity receiver are lined with low density, low heat capacity fiber insulation reaching operating temperatures usually in 30-40

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Fig. 9. ~team generation rate vs. calculated power input,

M. Epstein et al. / Solar tent of 2 MWth water~steam receiver 277

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nlinutes. Fig. 8 shows a typical time,temperature scan. Starting at 60 ° C, about 30 minutes were required to achieve operating conditions with full steam production.

4. 2. Steady-state performance

The power output at quasi-steady state may be defined as the gross input power less conduction losses from: the steam drum, connecting piping and the receiver

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278 M. Epstein et al. / Solar test of 2 MWth water~steam receiver

walls, and convective and radiative losses through the receiver aperture. This was calculated from measured steam flow and the enthalpy difference between enter- ing feed water and product steam. The gross power input was calculated for each testing date by the MIRVAL code.

The results of these calculations and measurements are shown in ~;g. 9. An almost linear relationship may be noted from the graph of input power calculated by MIRVAL vs. steam production rate per unit of direct radiation "normalized" to unity reflectivity. This may be extrapolated to give about 0.15-0.2 MWth power input 0oss) at zero output power.

Fig. 10 shows total "efticiency" defined as the ratio between the measured power output and the calculated input power. The data points were taken from tests run during the period tff September 1989 to July 1990. At high input 9ower, the total thermal efficiency of the system ranges betweer. 73-79%.

4.3. Behavior of control system

During the shakedown period, adjustments were made to the cc,ntrol system to minimize the effects of transients, typically cloud movements, on the continuity of operations. System pressures, steam production and steam driven water level, particularly the latter, require careful control to insure safe a~id reliable operation. A two-element negative feedback algorithm was thus applied to the feedwater control valve to offset the effect of decreased steam driven water density, during rapidly increasing steam production, leading to a rise in apparent water level. A negative unity gain was imposed on the water level signal and a low positive gain in the steam flow loops for control of the feedwater rate during sudden changes in input power due to cloud transients.

Fig. 11 shows the effect of a transient caused by a rapid removal Of one half of the 60 heliostats on the steam rate and on the water level in the st~;.am drum. It is noteworthy that the steam drum water level is maintained with adequate stability,. Steam production rate, despite considerable fluctuation, is brought back rapidly and safely to operating levels.

5. Summary

The overall mechanical design of the system during the operating period of approximately one and a half years has been shown to be more than adequate for the efficient production of process steam. In particular, the design of the evaporat- ing panels has been prover: '.'n that no cracking or incipient burnout has been evident. Shortenlag the startup time to 30-45 minutes by reducing the system fluid holdup, and maintaining forced circulation with relative high circulation flow rate to steam production ratios, have resulted in satisfactory transient operation and led to adequate and safe system operation over large and sudden power input variations..Although the control of the system has been adequate, further improve- ments have been indicated at lower solar power inputs.

Finally, the utilization of a model of the system involving a number of codes capped by the SAPPHIR-WlS code, has been satisfactorily validated and shown to be a convenient design and analytical tool.