Central collector solar energy receivers

Solar Energy, Vol. 18, pp. 21-30. Pergamon Press 1976. Primed in Great Britain

CENTRAL COLLECTOR SOLAR ENERGY RECEIVERSt

A. SOBIN and W. WAGNER Rocketdyne Division, Rockwell International, Canoga Park, CA 91304, U.S.A.

and

C. R. EASTON McDonnell Douglas Astronautics, Huntington Beach, CA 92647, U.S.A.

(Received 16July 1974; in revised form 3 September 1975)

Abstract--Central receivers for use with power-generating equipment require compact boiler and superheater designs to effectively concentrate the solar heat input and to improve system power cycle efficiency. Solar thermal collectors employ a central receiver that must be compact to effectively utilize the high concentration ratios of sunlight energy. The demonstration of lightweight, long-life receiver designs is a technology issue that must be resolved if the concept is to be attractive for power generation. The receiver must be lightweight to minimize the cost of the supporting tower structure. Current applied aerospace technology in compact steam boilers and rocket thrust chamber designs can be directly applied to the design and fabrication of solar central receivers.

This technology recently has been applied to a compact steam generator that uses liquid oxygen and natural gas, propane or fuel oil. The compact steam generator permits a 300:1 size reduction in boiler and superheater size. This paper discusses the application of the compact steam generator technology to the design and fabrication of central receivers for solar-energy-powered electrical powerplants. Receiver designs are discussed for tower-mounted applications where size and weight are important. The heat flux rates necessary for central solar receivers are nearly identical to the design hear fluxes for the compact steam generator. Fabrication of the central receiver is discussed as well as design details and applicable materials.

INTRODUCTION

A central collector system is an attractive method to concentrate solar radiation and convert it to heat energy in a useful form for electrical power generation. In this concept (Fig. 1) a large field of steered mirrors reflect solar radiation to a single central receiver that is mounted on a central tower. The solar radiation can be highly concentrated, and high-temperature steam can be generated with proper design for the receiver. The uso of a central receiver avoids the need for large heat transfer flow networks which are attendant when large arrays of parabolic collectors are used to deliver energy to power-generating equipment. The selection of mirror size for a given size field, the percent of total solar input that can be captured by a mirror array, and the possible tower structural designs are discussed in detail in [1]. Of particular interest is the central tower-mounted receiver (Fig. 2) where weight and output steam temperature are important to overall system cost and performance. The design considerations for the central receiver are discussed along with several design concepts that show promise of lightweight and segmentation possibilities (which would reduce subscale testing costs).

A typical central receiver was selected based upon the study results of [1]. The collector field used is approx. I mile square (3.2 km2). The location of the 450 m high tower in relation to the collector field is shown in Fig. 3a. The flat mirrors are 5 m 2. The spacing of mirrors varies over the field to give an average ground coverage of about

tThis paper was presented at the International Solar Energy Society's U.S. Section Meeting held in Fort Collins, Colorado (Aug. 1974).

0.43, which is necessary to minimize shading of adjacent mirrors. A hemispherical central receiver 20 m in diameter is used as the reference central receiver size (Fig. 3b).

The receiver is several times the size of a single mirror to account for the solar disk size and mirror-pointing inaccuracies.

The cost of the 450 m high tower is sensitive to the total weight it must carry (Fig. 4). The total central receiver weight consists of the steam-generating unit weight, the structural support weight, and the weight of water and steam carried within the receiver. There is a substantial payoff to minimize the total weight of the central receiver when used in conjunction with a guyed steel tower.

DESIGN CONSIDERATIONS

The heat flux the central receiver must be designed to handle is a function of seasonal and diurnal variations, as well as varying as a function of location on the receiver. Ref. [I] gives typical seasonal and diurnal variations of heat flux on the central receiver.

It is apparent that there are extreme circumferential variations in heat flux for the receiver seasonally and during each day. The problem of flow control to maintain constant steam output from sections of the receiver will be discussed later. The nominal design case (12:00 noon--spring) will be discussed and compared with rocket nozzle cooling technology and conventional boiler design. A nominal design solar heat flux of 230 Btu/ft2-sec is used for the receiver designs shown. This heat flux is an order of magnitude greater than the 15-20Btu/ft2-sec used in conventional steam boiler design. Thus, design operating conditions and fabrication techniques for the receiver will depart from conventional boiler techniques.

21

A. SOBIN, W. WAGNER and C. R. EASTON

Fig. 1. Solar tower concept

Fig. 2. Tower-mounted central receiver.

1 12OOm

-16OOm

2

. LATITUDE: 35’ N

.TOWER HEIGHT: 450 m *MIRROR SIZE: 5 X 5

6 m

. SPACING: S X S m tAVE)

.POINTING ERROR: 0.2’

(a)

Fig. 3. Collector field arrangement.

(b)

REFERENCE HEMISPHERICAL

RECEIVER

Central collector solar energy receivers

2.0

38 ,.o S a

0 - - 0

0 I I I 40,000 80,000

WEIGHT, POUNDS

Fig. 4. Effect of weight (of steam generating plant) on cost of guyed steel towers.

At the extreme of the demonstrated heat transfer capability spectrum are regeneratively cooled liquid rocket thrust chambers and nozzles. Heat transfer rates at the convergent throat of high-chamber-l~ressure rocket engines using hydrogen/oxygen propellants range from 2500 to 15,000Btu/fttsec. These levels are significantly above that required for the central receiver, but they demonstrate that extremely high heat loads can be cooled convectively. Most rocket nozzles, such as the Saturn V first-stage F-1 engine (Fig. 5), use thin-wall light tubular construction. Figure 6 shows the range of demonstrated throat heat flux for developed liquid rocket thrust chambers. The Space Shuttle Main Engine (SSME) is being developed by Rocketdyne with a throat heat flux of over 12,000 Btu/ft~-sec.

Tubular-type construction offers the best potential for lightweight, low-cost, efficient central receiver designs. These potential results stem from many successful designs of rocket engine thrust chambers and heat exchangers using Rocketdyne's capabilities for forming lightweight, thin-wall tubular structures of variable cross section to accommodate the high-heat-flux fluids.

The design criteria applied to the solar central

23

Fig. 5. F-I thrust chamber assembly.

receiver/boilers considered in this paper are presented below:

Overall receiver maximum diamater, feet 65 Superheated steam flowrate, lb/sec 1000 Superheated steam temperature, F 1000 Superheated steam pressure, psia 1500 Maximum solar heat flux, Btu/ft2-sec 230

RECEIVER C O N F I G U R A T I O N T R A D E O F F S

In the process of designing the central receiver, the working fluid passage configuration and the flow circuit must be selected. A tubular configuration (Fig. 7) offers the advantages of a lightweight system, more solar-

++5o+ I 12,000

20K _ ~,,-. I TUG ~ I

9000 J-2

J - 2 T P" 6000

,,r

I,- '< 3000 o OE " r I--.

o

o 500 lOOO 1500 2000 2500 3000 3500 4000 C H A M B E R PRESSURE. PSIA

Fig. 6. Heat flux for Rocketdyne thrust chambers.

24

C L O S E O U T ~ BRAZE

-T \ / I_. HOT WALL ' - - - H O T WALL - - J

CHANNEL TUBULAR

WALL CONFIGURATION CONFIGURATION


The life of the solar receiver will depend on the maximum temperature differential experienced, the hot-wall temperature, and coolant or working fluid pressure. These parameters, along with the coolant passage geometry, determine the fatigue and creep characteristics which are combined to result in life.

The central receiver can be designed for a long life even DOUBLE-WALL though it will undergo thermal cycling diurnally and

CHANNEL possibly from partial cloud cover. The compact steam CONFIGURATION generator, which will be discussed later, is designed to

handle the same heat flux as the central receiver and has a design thermal cyclic life of several hundred thousand cycles. The same technology was applied to the design of the central receiver.

Fig. 7. Working fluid passage configurations.

exposed area, and less thermal growth restraint. A channel-wall configuration, in which slots are cast or machined into a plate structure having a closeout with a brazed or electroformed back plate, could result in a lower fabrication cost. A channel-wall fabrication technique has been used successfully on high-chamber-pressure rocket engines. The channel wall provides two- dimensional fin conduction (channel land) which tends to result in a lower wall temperature than that which occurs in tubes with equivalent pressure drop.

The receiver working fluid flow circuit must optimize and trade off the following items:

1. Weight 2. Life 3. Coolant pressure drop 4. Heat flux control 5. Coolant outlet thermodynamic state

The central receiver can utilize a variety of cooling circuts such as those shown in Fig. 8. Coolant outlet quality control is envisioned as a potential problem since, as shown by the data from [1], the circumferential incident heat flux can vary more than a factor of three.A partial cloud cover would accentuate this variation. Therefore, a vertical single-pass circuit (Fig. 8) would result in a considerable circumferential variation in outlet steam state. This variation may be greatly reduced using a thermal-compensating cooling circuit, throttling the flow in segments of receiver, or providing an efficient outlet manifold mixer. A helical-spiral circuit, a thermal- compensating, two-pass circuit (where the flow traverses the high-incident-heat-flux region first, then the low-heat- flux region), or the double-panel concept shown in Fig. 7 would tend to balance the heat input to the working fluid. However, the weight is high in the double-panel concept.

SINGLE DOWN-PASS HELICAL SPLIT FLOW SPIRAL

CONVENTIONAL THERMAL TWO-PASS COMPENSATING

TWO-PASS

Fig. 8. Typical receiver flow concept s.

PARALLEL

MATERIAL SELECTION

Another factor influencing life is material selection. The influencing material properties include:

1. Yield and ultimate strength (determines minimum hot-wall thickness)

2. Thermal conductivity (this, along with the wall thickness, determines temperature differential, hot-wall temperature, and pressure drop)

3. Thermal fatigue life (determines fatigue damage portion of life)

4. Stress rupture (determines creep damage portion of life)

RECEIVER PRELIMINARY ANALYSIS

Using nickel alloy tubes at a stress level equal to one-fourth the ultimate strength, an estimate was made of the tube sizes and number for a 65 ft hemispherical receiver with a 16 ft diameter base. Figure 9 indicates that a series of tubes tapered in diameter and thickness from the 65 ft top to the 16 ft shroud diameter will provide the required surface area for the generation of saturated or superheated steam with a steam rate of approx. 10001b/see. This vertical single-pass tube array will provide reasonable pressure drops for the water and steam because the mass velocity is high for the water rising in the 16 ft cylinder and is lower for the steam in the boiling and superheat region (Figs. 10 and 11). The small-diameter, thin-wall tubes are necessary to lower the weight to acceptable levels (Fig. 12). This can be accomplished with maximum tube diameters below 1.0 in., and by tapering the diameter to 0.5 or 0.25 in. A maximum limit of 50,000 lb. (wet weight) for the receiver was chosen for this analysis.

ASOIX Z ~4000

300(;

~ 2000

~ 100~ Z

o

/ - - 65-FOOT-DIAMETER ~ RECEIVER TOP

DIAMETER BASE I I I I I I I I I I

0.4 0.8 1.2 1.6 2.0 d, INCHES (OD)

Fig. 9. Parametric tubular collector data.

Central collector solar energy receivers 25

ii t~

14 /--SATURATED

~ / S T E A M 12 ~ ' ~ t 500 F

10 ~ D E S I G N 8 .. RANGE

O = 1.28 X 106 BTU/SEC 4 SHUPERD~X,,~ I 16. FOOT.DIAMETER CYLINDE R

STEAM , ~ ' ~ 2 1000 F-- /

0 I, I i I I I 0 0.4 0.8 1.2 1.6 2.0

d, INCHES (OD)

Fig. 10. Solar heating receiver base flow velocities.

1 2.2 \ Q = 1.28 X 106 BTU/SEC

\ \ 65-FOOT-DIAMETER ~1.8 \ \ RECEIVER

\ \ ~ S A T U R A T E D ~" 1.4 \ \ / " STEAM

\ \ 500 F, 680 PSlA W = 1132 LB/SEC

< 1.0

~" ;UPERHEATED~ ~ 0.6 STEAM J ~

1000 F, 1500 PSIA ~ _ W = 902 LB/SEC

0.2 I I I I I I I I I I 0.4 0.8 1.2 1.6 2,0

d, INCHES (OD)

Fig. l 1. Solarheatinghemisphericalreceiver top velocities.

00.oool- ~ ~.o.5 / ..~..~,~ ,N. ] ASSUMED DESIGN x ~ P ~ J ~ l ~

8o,°ool- WEI6HT'IM,T / ~ ÷ ~ I\ ITUBE DIAMETER ,,~ ~ / t

"40 000 LFOR I~FOOT / / g ' / i ' I CYLINDER / / ~ . 5 IN.

-c2 IFORMEDJ/~ ' ' ' . . . . . . T . P F R ~ n ', >= . . . . [TUBES ~ ( o . 2 s ! R'O'HN'b-- ; > "° ' " ° I - n ~o:,25 IN. '~ TUB-~s

I '~L i I11" i ~ o , - ~ _ I I ~ ~ -"~

01 It II I I I I 0 0.4 0.8 1.2 1.6 2.0 d, INCHES OD (AT 65-FOOT DIAMETER)

Fig. 12. Tubeweight for taperedcircularand formed oval tubes.

The generation of superheated steam by means of a single-pass or helical-spiral circuit is feasible, based upon experimental work conducted by Rocketdyne in conjunction with the development of a compact steam generator. The compact steam generator involves the combustion of gaseous oxygen with gaseous or liquid fuels to produce high-temperature combustion gases in a single-pass firetube boiler (Fig. 13). The steam-generating circuit is once-through, beginning as a subcooled liquid, being boiled, and ending as either saturated vapor or superheated steam. The firetube boiler is designed to absorb heat flux in peak regions of 215 Btu/ft~-sec. The variation of heat transfer regimes and gas-to-water temperature differences along the circuit requires careful control of the circuit mass fluxes. This technology in superheated steam generation in lightweight tubular structures can be applied directly to the design of the central receiver.

OXIDIZER

- ~ I N J ~ ~- WATER CIRCUIT

ECT R ~-----~ COMBUSTION

WATER

Fig. 13. Boiler concept.

iTi EXHAUST._._.,..

GAS

STEAM OUT

POTENTIAL PROBLEMS

The requirements of the receiver for light weight and minimum thermostructural problems lead to compromises in tube wall thickness, manifolding and life. Thus, consideration must be given to variable tube diameter and wall thickness, segmented construction, and replaceable segments. The segmented construction, which has been used at Rocketdyne to develop full-scale rocket engines, is beneficial in providing a low-cost replaceable module for incorporation into the full-scale receiver.

These above-named techniques may be used to alleviate the potentially critical problems listed below:

1. Receiver weight and size 2. Thermal expansion, distortion, cycling 3. Pressure drop 4. Temperature 5. Fatigue and creep-rupture failure 6. Receiver servicing 7. Flow control

TUBE FORMING

Preliminary design studies of the solar receiver/boiler indicate that the use of thin wall high strength tubes would result in a low-cost, low-weight, highly efficient boiler capable of generating superheated steam from water in a single pass. Rocketdyne has developed highly sophisti- cated techniques for precision forming of tubes to fabricate high-heat-flux structures very similar in shape to the solar receiver/boiler.

A central receiver can be fabricated using formed tapered tubes with variable wall thickness, and assembl- ing them in independent segments. Segmentation provides many advantages over a one-piece unit, such as precision control of portions of the receiver, ease of assembly, and maintenance.

Rocketdyne has fabricated a variety of tube sizes and shapes for rocket engine applications. Although these tubes were made for high-heat-flux applications for rocket motors, the principles involved in the design and manufacture are applicable to a high-heat-flux receiver.

Figure 14 shows tube cross sections at various locations along one individual tube of approx. 80 in. in length. This tube was formed using high-pressure tube-forming techniques. This particular tube is utilized in a system with internal pressures of approx. 850psia and a wall temperature differential of about 350 F (other similarly

26 A. SOB1N, W. WAGNER and C. R. EASTON

TUBE SECTIONS

0 O0 0 0 o o . DETENT / - " VENT HOLE 131

~ ~ , / ~- ~ - ~ " . ~ P L U G r -cHANNEL TUBE AR R AN G E MEN T ""L,,.~r 7 ~-~ UVt't:H \ \ ]BAND

CHANNEL A L~MAX. WELD CHANNEL ~ B l , PLUG. A ,BATOB W,OTHO. " . ? u MOUNTING BAND ~ -JACKET BAND L. .~...,f~------- ..,r4

Fig. 14. H-1 thrust chamber furnacebrazed.

designed devices operate with a temperature differential up to 1000 F). Nominal tube wall thickness for this design is 0.12 in. A variable tube-wall thickness design, using the same cross-sectional tube geometry as shown in Fig. 14, was successfully developed and tested. The tube-wall thickness was increased from 0.012 to 0.018in. from cross-section location C to A.

Another Rocketdyne-fabricated, high-pressure tube design (Fig. 15) that demonstrates the precision capability is keystone in shape. Each of these tubes must be contained in a 0020 ' precut segment; no part of the mating faces of adjacent tubes can make a tolerance excursion into the boundary of its neighbors on either side. Since each tube of this bundle is fabricated from the same die, any error would be identical in each tube and cumulative in the stackup. Close control of the tube's physical dimensions and physical properties is essential. The materials for the die plates are carefully chosen for their stability in heat treating and after drawing to the desired hardness.

Tubes of large diameter and varying geometry to fabricate heat transfer structures of large dimensions (tube length of 135 in. and structure diameter of 120 in.) may require a bifurcation technique--the joining of two tubes into a larger-diameter tube. This process has proved

highly effective, and may be applied to the solar receiver/boiler. Advantages of this technique are ease of fabrication and lower weight. Figure 16 is a drawing depicting large-diameter tube design, variable geometry, and bifurcation technique in current use. The diametral dimensions of this tube are similar to those necessary for several of the receiver designs presented.

RECEIVER SEGMENTING APPROACHES

An important technique employed in the development of large rocket motors has direct application in the solar receiver design. This technique is to fabricate the receiver assembly using independent and individually controlled segments. This approach simplifies and reduces costs in the development and testing phase as well as in assembly and maintenence of the receiver. Segment width will require optimization to avoid thermal imbalance due to reflected image size.

Segmentation provides an important feature necessary in solar receiver design--precision performance control. Each segment can be controlled to provide an output temperature based on the amount of incident solar heat flux. Sensors, monitoring the amount of steam produced and the available solar heat flux, can regulate the water flow through each segment to optimize the energy output.

/ - - B I S E C T O R OF KEYSTONE ANGLE

---KEYSTONE PLANE

/ FPARALL~ L - - INNERML C L OF NOZZLE"- -~

~-~ KEYSTONE PLANE - " "

Fig. 15. Keystone tube die.

o @


@ © O 03

27

Fig. 16. Thrust chamber tube spacing.

Shutdown of a particular segment can be accomplished anytime the solar heat flux on the segment falls below the required energy necessary for production of saturated or superheated steam. To protect the segment from over- heating during this time, the particular helostats providing the reflected solar heat flux can be directed solar heat flux can be directed elsewhere.

CENTRAL RECEIVER DESIGN CONCEPTS

A preliminary feasibility study has been conducted on four design concepts of solar central receivers, all of which utilize the techniques of high-pressure tube forming and segmentation. Those concepts are:

1. Spiral tube array 2. Cruciform tube array 3. Vertical tube array 4. Free-standing, tapered-tube array

Weights for all configurations except the free-standing, tapered-tube array are expected to be within the 50,000 lb. weight target, illustrating the weight benefits derived from thin-wall tubes.

The tubular design concept, along with the segmenting of the receiver, compensates for most of the thermal changes expected. The design of the array is such that small thermal differences will exist across the tube, so that the tube and any backup will expand approximately the same amount. The shape of the tube allows for growth through a slight flexing of the tube wall. Both these factors are well understood and have been demonstrated by extensive testing.

Diametral growth capability of each segment is allowed through spacing between segments. The interface between segments can have small overlap to prevent losses between segments. The segment approach also provides additional flexibility of the receiver to take irregular shapes due to uneven heat input, or changes in expected heat flux due to temporary cloud cover.

Vertical receiver growth is accommodated by the method of attaching each segment. Fixed attachment will be made at one location in the segment only, allowing vertical growth in both directions. Rocket motor thrust chambers provide a good example of this in that they are fixed in one location and permitted to grow in the aft direction.

Spiral tube array The spiral tube array receiver design utilizes segmented

sections, grouping several tubes within each segment in an overall receiver structure. Each segment (and tube) traverses a 90 ° circumferential arc with respect to the segment's water inlet and steam outlet. The tube dimensions expected are a taper from 2 to 1/2 in., with wall thickness between 0.065 and 0.025 in. The water inlet section of the tube is smaller than the steam outlet and requires a variable tube wall with high-pressure tube forming. This type of structure provides a long tube run in the high-flux-density region of the receiver structure. The advantage of this configuration is to even out the heat loads tube to tube. It appears, however, that the additional complication presented by this configuration may be unnecessary, since the heat loads expected are well within the capability of less-complicated designs.

Independent manifolding of the water inlet and steam outlet tubes for each segment will provide precise control of each segment's performance. The Rocketdyne linear engine uses a manifolded system design for thrust chambers. This feature also enables the design concept to be scaled up or down.

Some disadvantages are evident in the spiral tube design concept. The total length as well as tube wall thickness of a segment may increase, resulting in an overall increase in weight. The size and shape of this configuration also makes handling and maintenance difficult. These problems can be solved, but the added cost and weight must be compared with the total overall performance of the boiler design. The long tube requirements may require joining of tubes of variable geometry, possibly of bifurcation design as utilized in the F-1 engine thrust chamber. Again, cost vs segment performance must be considered.

Figure 17 presents the fabrication design concepts of the spiral tube array. Figure 18 shows a segment of that array.

Cruciform tube array The cruciform tuve array concept repersents a simp-

lified approach for joining several tubes within each segment of the solar receiver/boiler. Figure 19 shows a representative sketch of the cruciform tube array concept. This configuration would have more than one segment design. The concept consists of four vertical

28 A. SOB1N, W. WAGNER and C. R. EASTON

Fig. 17. Spiral tube array.

( i~i c C

Fig. 18. Spiral tube array segment.

(YOT:X373 A-A

¢'X2(XX) B-8

C--C

Fig. 19. Cruciformtube array.

panels and an upper panel, which include segments within each panel. Tube forms are either straight or simple bends, and .represent the minimum fabrication cost expenditure. The tubes would require sufficient strength and thickness to withstand environmental loads at expected temperatures. These loads are expected to be higher for this configuration since both sides of the tubes are exposed to heat impact. No tube-to-tube brazing would be' required to prevent energy from escaping between tubes.

Solar heat flux impinges on each side of the vertical tubes to provide maximun energy capture for a given mirror array field size. The under surface of the upper panel also can be a heat-receiving surface. This configuration can, of course, also be segmented for better control and maintenance.

A deviation may be required in this configuration to allow the tube size to increase as transition from water to steam occurs. The changing shape of the tube would cause the vertical arrays to become a truncated triangle, with the short dimension oriented to the bottom of the receiver.

Vertical tube array The vertical tube array concept represents the lightest-

weight design for the solar receiver/boiler. Segmentation, tube forming and testing of this type of boiler segment would be simplified by the symmetry of the entire system. Advanced F-1 engine and aerospike thrust chambers utilize techniques applicable in the fabrication of the segments for the solar receiver/boiler. The tubes would be fabricated with thin, variable dimension walls, high- pressure-formed into the desired shapes. This configuration allows a transition from water to superheated steam even though the tube length is minimal.

Water pressure losses are reduced significantly as a result of the shorter tube length. This, in turn, reduces the wall thickness requirements and, subsequently, the overall weight. A backing plate and tube-to-tube brazing would be necessary for support, but receiver weight is expected to be lowest for this configuration. Segmenta- tion, acceptance testing, assembly of the boiler system, and replacement and maintenance of the segments installed in the boiler system would be the least difficult for this concept for all those considered in this study. Figures 20 and 21 depict the design concept of the vertical tube array and the corresponding segment. From Fig. 20 one can observe the symmetry associated with the installation of the water inlet and steam outlet valving and ducting. Precise segmented performance control is possible with this type of boiler concept.

Free-standing, tapered tube array The solar receiver/boiler concept consists of a seg-

mented structure with free-standing, tapered tube sections. No tube-to-tube brazing is required, but double- wall, steam-transport ducting of varying geometry is required. Provision is necessary to capture any energy escaping between tubes. Although similar to the vertical tuve array, the free-standing, tapered tube array would be heavier because of the larger, thicker-walled tube and


/ - - - -

Fig.20. Vertical tube array Fig.22. Free-standingtaperedtubearray.

' !U

Fig. 23. Free-standing tapered tube array segment. Fig. 21. Vertical tube array segment.

steam-transport ducting. Tube size is estimated to taper from 2 to 0.5 in., with wall thickness between 0.090 and 0.045in. Figures 22 and 23 depict the design concept of the free-standing, tapered tube array and its segment. Segmenting, installation into the receiver/boiler system, and replacement and maintenance of the free-standing, tapered tube array concept are similar to the vertical tube array. This configuration is expected to be the heaviest of those evaluated, and was considered only to get an estimate of weight-to-tube size relationship.

RECEIVER STARTUP, SHUTDOWN, AND OPERATION

A variable flow control valve can be employed in the water inlet to each segment. This valve can be designed to respond to steam state (pressure and temperature) as well as energy availability. A concept such as this can provide automatic startup and shutdown by sensing energy intensity, and can provide the timely control needed during operation in the event of partial, temporary cloud cover.

Figure 24 depicts a schematic of this concept. Start of

29

operation is triggered by a solar cell sensor array mounted on the exterior portion of each segment, and controls startup of the water flow pump. The water flow control valve is activated to an open position, corresponding to the minimum acceptable flowrate. Steam is generated within the segment, and the state of the steam is sensed at the segment outlet pipe by temperature and pressure sensors. During initial startup, a quantity of water will be injected into the steam tube. To prevent water from entering the turbine, a water separator can be installed downstream of the 20-segment outlet steam manifold system, and the collected water rerouted into the supply system. As the temperature and pressure of the steam is increased, the variable water flow control valve position is steadily opened until full operation is obtained. If the solar heat flux is interrupted by clouds, or malfunction occurs in the mirror tracking system, water flow can be decreased to the individual segment when the temperature/pressure sensors indicate a reduction in the state of the steam generated. The strategies involved in startup, i.e. full or empty tubes, rapid or slow start, are an integral part of the design requirements of the system.

30

SOLAR CELL DETECTOR ARRAY-/


I ~ SYSTEM OOCTING

. ACKPRE URE / I 1 \ SEPARATOR J H VALVE L - - ~ (TOTAL)

TEMPERATURE/PR ESSURE SOLAR [ L_J SENSOR CONTROL LOGIc RB~/N)E RECEIVERI 7 ~ O- - BOILER I / I SEGMENT I / I ,_,,,o-,-o.,z,:,:,

I///CONTROL -

I b § / ,r WATER

. I ~ - ~ CHECK PUMP | ~ VALVE l I V l I ~ 1 , ,, / WATER

I - - I r ' - - - ' - " - ~ r - - ~ - . . . . ~ XSUPPLY WATER FLOW HT' ~ ~ I . ~ , ~ P U M I ~ CONTROL VALVE ~ -

Fig. 24. Solar receiver segmented control systems.

SUMMARY

In summary, current applied aerospace technology in compact steam boilers and rocket thrust chamber designs can be directly applied to the design and fabrication of solar central receivers. A number of feasible designs have been carried through a preliminary design investigation, and a number of design and fabricat ion problem areas have been identified along with their feasible solutions. Fur ther analytical and design work is in progress on the central receiver concept directed toward a scale demonstration plant within the next several years. The solar

energy central receiver concept shows excellent promise for electrical peaking power generation in the near future.

Acknowledgements--The authors wish to thank A. D. Lucci, R. F. Sutton and B. R. McGraw for their major contribution to the central receiver design concepts, and A. T. Sutor, D. G. Hodgdon, J. M. Shoji and A. Martinez for analysis inputs.

REFERENCES

1. Solar Thermal Power Systems Based on Optical Transmis- sions, Semi-annual Progress Report, NSF Grant GI-39456, University of Houston and McDonnell Astronautics West, Report No. NSF/RANN/SE/GI-39456/PR/7314 (15 Feb. 1974).

Resumen---Los receptores centrales para en equipos generadores de energia requieren calderas compactas y disefios de sobrecalentadores para concentrar efectivamente la entrada de calor solar y mejorar el rendimiento del ciclo de poder del sistema.

Los colectores t6rmicos solares emplean un receptor central 1o mas compacto posible para usar efectivamente alto grado de concentraci6n de la radiaci6n solar. Si esta concepci6n resulta atractiva para la generaci6n de potencia se deber~i resolver la cuesti6n tecnol6gica de que el disefio del receptor sea liviano y durable. E1 receptor debe ser liviano para minimizar el costo de la estructura de la torre-soporte. La technologla aeroespacial aplicada a calderas de vapor compactas y a disefios de cfimaras de empuje de cohetes puede ser directamente usada para el disefio y fabricaci6n de receptores solares centrales.

Esta technologia ha sido aplicada recientemente a un generador compacto que usa oxfgeno lfquido y gas natural, propano o fuel oil. Este generador permite la reducci6n de 300 a I de tamafio en la caldera yen el sobrecalentador. Este articulo discute la aplicaci6n de la tecnologfa de este generador a una planta helioel6ctrica. Los diserios de receptores discutidos son para montajes en torres donde el tamafio y el peso son importantes. El flujo de calor necesario para un helioreceptor central son aproximadamente iguales que los disefio del generador de vapor compacto. Se discute la fabricaci6n del receptor central tanto como los detalles de disefio y materiales aplicables.

R6sum6---Les r6cepteurs centraux devant 6tre utilis6s avec un 6quipement g6n6rateur de puissance, n6cessitent des conceptions de chaudi&e compacte et de surchauffeurs pour concentrer efficacement la chaleur solaire incidente et am61iorer le rendement du cycle du syst~me. Les capteurs thermiques solaires emploient un r6cepteur central qui dolt 6tre compact pour utiliser eflicacement les taux de concentration 61ev6s de l'6nergie du rayonnement solaire. La d6monstration de 16g~ret6 et de dur6e de vie du r6cepteur est une voie technologique ~ laquelle on doit se r6soudre si I'on veut que la conception soit int6ressante pour la production de puissance. Le r6cepteur doit 6tre 16ger pour minimiser le coot de la structure de tour-support. La technologie courante du domaine a6ro-spatial pour les chaudi~res

vapeur compactes et les chambres de pression de fus6e peut 6tre appliqu6e directement ~t la conception et h la fabrication des r6cepteurs de centrale solaire.

Cette technologic 6tait r6cemment appliqu6e hun g6n6rateur de vapeur compact utilisant de l'oxyg~ne liquide et du gaz naturel, du propane ou du fuel-oil. Le g6n6rateur de vapeur compact permit une r6duction de 300: 1 de la taille de la chaudi~re et du surchauffeur. On 6tudie dans cet article l'application de la technologic du g6n6rateur de vapeur compact b. la conception et ~ la fabrication des r6cepteurs centraux pour installations de production de puissance 61ectrique partir de r6nergie solaire. On 6tudie les conceptions de r6cepteur destin6 b. 6tre mont~, sur une tour, oil la taille et le poids sont des facteurs importants. Les valeurs de flux de chaleur n6cessaires aux r6cepteurs centraux solaires sont presque identiques aux flux de chaleur pris en compte pour le g6n6rateur de vapeur compact. On 6tudie autant la fabrication du r6cepteur que le d6tails de conception et les mat6riaux utilisables.

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Central collector solar energy receivers