Solar Energy Materials 24 (1991) 707-719 North-Holland

Solar Energy Materials

Performance of a CRS with stretched fiaembrane heliostats for steam reforming of methane

Manuel Romero, Francisco Sobr6n and Luis F. Puebla

C!EMAT-IER, Acda. Complutense, 22, E-28040 Madrid, Spain

This paper compiles some conclusions after modelling a solar CRS for a steam reforming of methane application. An amount of 24 differer~t plants are optimized for different plant sizes, heliostat beam dispersion errors, number of receiver apertures and focusing or flat heliostats. All of them were conceived with 12~J m 2 str~:chcd me_rnbrane heliostats and convective reformers for a re,zeivcr/reactor separated solution. For such a system, commercial competitiveness can be expected,

1 . I n t r o d u c t i o n

Solar reforming of methane is being extensively worked out in a number of SSPS-IEA projects, a~ a first and conservative approach to solar fuels and chemicals. Even though reforming i~ only partially i~ateresting for solar energy technology, it has the advantage of being a well known process. Therefore, it is specially indicated for short-terrn research because main effort of scientists can be focused on solar !echnology competitiveness and CRS/rc~,~r~er coupling. Appli- cations like synthesis gas production in chemical industries [1] an~.! thermochemical pipes for process heat transport with chemical pro&~cts at ambient tecaperature [2] are suggested.

Solar fuels and chemicals for CRS will be transferred to eammercial stage, only if key systems of the plant, like heliostat field and sol~r receiver/reactor configura- tion are improved and well optimized. For this purpos,:, an exercise of economic optimization, by taking into account the last innovations and expected efficiencies will provide previous information about the feasibility of commercial plants.

Two specific tasks are o.hosen for a whole definition of the system: Optimization of the CRS a~!d theoretical simulation of chemical conversions and heat transfer within the tubular reformer. For such an optimization, the following plant assump- tions were selected: - Thermal Power: 100, 2~30 and 300 MWt - Operation mode: 24 h/day - Hybrid plant with auxiliary fossil faei ~'

An amount of 24 plants is optimized fnr different powers, heliostat beam dispersion errors, receiver apertures and focusing or flat heliostats.

0165-1633/91/$03.50 © 1991 ' - Elsevier Science Publishers B.V. All rights reserved

7O8

Table 1 List of options optimixed

M. Romero et al. / Performance o f a CRS

Output Receiver Beam dispersion Focusing mode

power aper~+ures error

100 MWt 1 aperture or = 2.6 mrad Active vacuum

200 MWt 2 apertures cr = 3.8 mrad 300 MWt U~focused (flat)

~ol~rr Rerel~,~r

hat. nlrlhell~m ('~O ~C} O

T . n ~ l l h ~ n ~ r

t ! ~i~+el ! ~ r, anvaet ! ve r~O.C r~formr.P

Lit 4 • I Ipo ~ynthentll R~n

a lO .e l ' 4o bar :~OQC

Fig. 1. Scheme of the CRS suggested. Rece iv / : r / reac tor independent . Receiver: 1 or 2 ~pertures with a gas heated cavity (air or heliumJ. Reactor: Convective reformer with tubes filled ~¢ith a Ni catalyst.

Heliostats: Dual facet stretched membrane heliostats with 120 m 2 of reflectnnt surface each.

2. Stretched membrane heliostats

Stretched membrane hcli~,~ats (both passive flat and vacuum focusing), are used for optimization, since they present promisi]r!~ cost trends for mass p roduc tion against ctmventional gi~ss-metai heiiostats. A dual facet unit with 120 m 2 fr~.~m a design currently going on i+~ CIEMAT [3], is used foa" iield layout configuration. For those b, eliostats a range of beam dispersion errors between 2.6 m:d 3.8 mrad are assumed from SANDIA ~4] and CIEMAT experiences.

/t,1. Ro ~ero et al. / Performance o f a CRS 709

j~,~,~%__ DlsmotQr round fscot ~ 9 m.

g o f l s c t 2 v o £11m m gCP-300 (3M)

[ X ~ X ~ ' \ ' ~ ' " I " , X ' , ~ ~":"2"\" ,.?~ ~ Membr.no: , , l v s n [ . o d s t o o l ,

~\ ~ \ \ • ' . . . \ \ ¢'t~\% \ \ \ d \ ",,\ To:slonS=~ to ,o ld . ~z3z-3o4: \ \ , , O ' ~ \ \ ' . X ' C \ ¢ " "xX ' , ' x , . . x \ \ , no. c l , , ~ , . 24 e . , , . . . b , s n .

Ring: .qT44; th Ick .s 4 ~ . c r o s s 8 o c t . ~ 200xlO0 mm

~ ~ ~ j no . o5 r i n g s u p p o r t s - S.

r t ng /mombreno s t t s c h . = Su u ~ l d i n g

Focus c o n t r o l : R o c ~ _ i l t n s s r p o t s n -

t l o n o t o r + S p s o d vsrtstc.r+vec,:u~+ ~.~n

Fig. 2. Dual-facet stretched membrane prototype (120 m2).

Mass production cost estimated for 50000 units per year is 126.4 D M / m 2 for vacuum focusing and !24.8 D M / m 2 for passive flat heliostats. Concerning optical performance, for:

Day 80 and solar time = 12.00 Direct irradiation = 950 W / m 2 Clean reflectivity = 0.9 Slaet range = 100-500 m

resu!ts a:;e similar to conventional heliostats: Thermal efficiency (r/) = 82-87% Peak flux = 4 ,-60 k W / m 2 Diameter (90%)= 2-12 m.

STRETCHED ~4EMBRANE 120 m2 ($1G.=3.8 ¢~rod)

~ 6 ¢ l

~"~ 24"

.::~ 16". 0

o . . . . . . . . | i ~ 1 1 i I ' M I I I ~ [ I I I I I I I I

loo 200 400 50o

DisLance H - B (m. ) Fig. 3. Peak flux versus slant range for vacuum focusi,:g and passive flat heliostats.

710 M. Romero et al. / Performance o f a CRS

~'IRETCHED MEMBI~-J~E 120 m2 (SIG.=3.8 mrod)

~2- . . ~

10. n~r f,~.avt

0 8' ~ ~ , ~ I ~

~- VACUUM FOCUSI

" ~ 4'

E " o 2 2

2 0 0 ,'500

DisLance H-B (m.) Fig. 4. D iamete r of the spot conta in ing 90% of incident energy over the larget .

3. Convective reformer

There are two main chemical reactions in the s team reforming process:

CH4 + H 2 0 (v) ~ 3H 2 + CO

AH o = 206.2 k J / m o l T* = 1059 K

CO + H 2 0 (v) ~ C O 2 + H 2

AH ° -= - 4 1 . 2 k J / m o l T* = 979 K

* T = 700-900 o C

* P = 0 .1-4 MPa

* Ratio H 2 0 / C H 4 = 3 -4

The experience gathered in the Nuclear Research Center of JiJlich [5] showed that helium convective reformers could achieve similar heat fluxes than conven- tional ones ( = 70-80 kW/m2) , getting additional advantages like more uniform heat fl~;x around the circumference of the tube and because of convection, a very compact heat exchanger can be arranged (30-40 t u b e s / m 2) [6]. I l l 'order to analyze chemical and thermal performance ipsi:ie the tubes, a variational model was established (see fig. 5).

For simplification, two relevant conclusions from Jiilich experience have been extracted: a) Due to the large reaction rate, the process is only limited by the heat transfer

possibilities and theoretical eqailibria are fulfilled. b) T, Tw and heat flu,~ ~ ~orofile.~ are homogeneous and smooth, therefore a similar

performance can be expected from the portion of heat used for the chemical reaction. With these assuml3tions, mass balance is avoided and enly energy balance for

heating gas, process gas a~-ld catalyst are used.

~,1. Romero et aL / Performance o f a CRS 711

r--Re < Tw Tw °

r=Ro

r = R i

T ° T,Ts > r=O

< - - Tw

Fig. 5. Oounterfiow reformer configuration. T: temperature of the process gas, T~: temperature of the catalyst, T,~: temperature of helium/air.

Energy balance to the internal fluid:

0-[' ¢ 02T ! 0T uspgCP'~z = ' k ' [ - ~ r 2 + - - - ) , r or + a v h f ( T ~ - T ) .

r:

Energy balance to the catalyst:

[ 02Ts 1 0Ts ( 1 - e)k~l[ ~ + ~ ~ J = a~h~(' K - T),

r Or

where av = m 2 catalyst /m 3 bed = 370 m - ; E = bed porosity = 0.6; o- = Fraction of wall surface in contact with the fluid = 0.8; FR = Fraction of heat consumed in the chemical reaction: optimized by iteration.

Energy balance to the external fluid:

UWpwC pw dT W 2Rihw d----z = Re 2 - Ri 2 (°'(Tw - TIRi) + (1 -- t r ) (Tw- T.~ I Ri))

Boundary conditions:

z = 0 : T = T ° z = L : T w = T ~

OT I OTs r = 0 : -~'r o T 0 0

0 T I' = - . . . _ r = R i : , 'R i ' k r -~ - r lR i (1 FR) _ . R o hw(Tw TI :~)

0T I (1 - , ) • Ri" k~-~/ Ri = (1 -- FR) " (1 - tr) " Ro" hw(Tw - Ts ] Ri)

The partial differential equations system is solved by approximating the solu- tions to Fourier series, converting it in a matricial ordinary differential equation [7]. This so-called "method of the constants", provides a final expression able to be solved by a quasi-linearization procedure at equally spaced increments [8]. Using

712 M. Romero et al. / Performance o f a CRS

Table 2 Productions predicted for lalznts between 100 and 300 MW

Power Synth. gas Number of tubes

?~]W T m / h Air Helium

io~ 16.2 250 166 200 32.4 5G0 332 300 48.6 750 498

an iteratiee, a complete scanning of the parameters distribution could be worked OUt.

The following base case was adopted for optimization:

A i r o r Hel ium 97301( < , < ~ - 1223°K

U Reformer t u b e L? j 7730K - - > . . . . . . . > 10830K

P r o c e s s gas

Tube:

D i re former tube = ,0 .10 m

D,, re former tube = 0.12 m

D e gu ide tube = 0 .138 m

L tube = ! 0 - 1 5 m

Process gas:

Ratio v a p o u r / m e t h a n e = 4

Pin = 46 a t m

Pout = 40 atm

m m

wim~ • ° _ J - ~ ° ~ ° e ° ° ~ ° ° e o ° ' ~ . , . ~ . ~ ,

~ l l ~ l ~ m ~ ~ ~ ,. • • . • ,, . • • • • -~ l~u~dkn~=,~ ~ - • • • • . • • . . . % * . : . . o . . i ~ , . . , . . . ~ . . , , . . . j . ~...-,.:- . ~

v ~ m @ l o o o e o o a ~ e ~ * • @ v @ l o o o e o o a ~ e ~ e ~ ~ . . . . .

n m"

0 • I .

F i g , 6, A v e r a g e p r g c e s s gas c o m p o s i t i o n v e r s u s t u b e l e n g t h ,

M. Romero et al. / Performance o f a CRS 713

80000.

7®oo,

E

o u

V

X 7.)

20000

RESULI~3 WITH HEUUM (Br=0.55)

..,e

===; : : ; : f i r = : =1 : ; ~ ! I v . .M , ~ , . l lWV mv ,w l= ! l s vww. . ! m~ '~ .1 ~m Bw~ , ms . ,m I

, 10 I I 12 13 14 IS 18 TUBE LENGTH (m. )

Fig, 7. tieat flux versus reformer tube length tor a helium cooling efficiency of 0.55.

i 20000 ]

.,. CONVE NTIO,.'-'~,_

I 0 C 0 0 0 "-

• t C,I "

E \x - oooo._

0 . ~', HELIUM 0

v 6 0 0 0 0 , x X ~ x Z ) AIR

L L 4 0 0 0 0 '

<[: LLJ "T" 2 0 0 0 0

0 0.00

\

I v v vm v ~ n l i l T~m l vw~ in 1 T ~ v w w I~ l V~ IW lW~WVW V I

0 , 2 0 0.40 - 0.60 0 .80 I .CO

AXIAL COORDINATE Z/L Fig. 8. Comparison of heat fluxes versus reformer tube axial coordinate in convective heated tubes with

helium and air.

714 M. Romero et al. / Perjor~iance ,~f a CRS

9ITIt VACUL~t FOOJ~.,~NG HELI,_~,TAT~

19.~ t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . _ . F~_n_ cost O~ST IW"I/G J

~?

~MJtl, h~L, 7000 -I .. "

([000 " " f ..~'J

' .- .- -~'~" j ,2 j ,~ , _ ~ •

:'000 -!

~.'2L TD--i AF

~A'.-,vL --2 AP

TO~R HEI@Iff ( H , ) 4 ..¢~. - ~,

.,....I-" .~_~'-~ - ~ - ~ - - - -

T .=gL !D--"_ AP t~C-N. - 2

iO0 ~ ,J , , ~ ,

m

t O 0 i ~ O ~ 0 ~ L ~ O ~ 0 0

OI.Y~UT POWER '~Mt, J)

Fig. 9. Res u l t s for p r o d u c t i o n cost , n u m b e r of he! ios ta t s a n d tower heigl , i in d i f f e r e n t o p t i m i z e d p lan ts . cons ide r ing v a c u u m focus ing " " o n e , o . . t o t s (120 m a) wi th 2.6 a n d 3.3 m r a d a n d rece ive r s w i th 1 a~d 2

ape r tu re s .

M. Romero et at / Performance of a CRS

C-~'S i',iIFH P~,SSi~E FLAT ~'~LlO$i~ii,

t9.~ ] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

DM/GJ 19~ x ~ .. "

p<.:'.-..."..

,o. 1 "21-'-.4"-.

17.~ -T ~ ~ -- ICv-' t-~. - ~00 ~5-0

FUEL CG$T

50L!~=[ .'?

300

715

gO00 ,

~.lFh 8OL~ 1 ,Lt~_L,

7000 t

-~000,

~000

3000

ZOO0 iO0

)

h?-:'>- 50LI£,--I t ,p

i [ '~ ,~ I , =~ ,~

i~o ~oo ~.o 7oo

6C~ ]

TO!,~[R ~ - l HEICFff 4_. -

4-00- .,,~-'~-'__~. -

"I' .---I.-- ~ ' - . - - " - .-

:~0. "I ..I'" ....I "I .-- ~ . ~

~r~O~.~iJ'.--"~ ,~--~ .~'20.,L'[~--~ A I~ ~ PA9% =2 ~,P

200 iO0 tSO 200 ~0 TOO

t]LITPffT Pt:IWE~ { '~) 5~.lAc'~.= 2,G (3,8) MEAD TRlrN~IG,= 3,8 (4,8) HP~

Fig. 10. Results for production cost, number of heliostats and tower height in different optimized plants, consid':ring passive flat heliostats (i20 m 2) with 3.8 and 4.8 mrad (including wind effects) and

receivers with 1 and 2 apertures.

"p~p!suo3 o~e s . ~ ! a ~ o~n~,~d~ ~l~U!S ,~luO ~pe~m 8'E pue 9"~ jo slezso!la q ]e~ o^!ssed pue ~u!sn~oj '~mn~e^ ql!A~ slueld p~z!~!Ido ~o~ $q~!~q ~,:)] pue slelso!l~q jo ~qmnu 'lso~ uo!l~npo~d jo uos!.~ed,~_~o::) "( i "~I

OVt~l 9 " £ ='OIWINI ~VtJN ~'~ r-_~n0~ (HN) ~ Ifl£tf£

ooz o,, F. o~ ~, oo~ , ~ ~ - ~ 0o,

OOZ

00£

J.~-13= 'l~l

_ . m . , " " ~ " ..'~ 000~

~ " . , , " 000~

ep

s , O 0 0 t . B,"

. - ~

"GI4

'~=°,,°~ ~_ ]" ~- --~...~-~..

~ ----._ .; - .'~..:~

S~D v Jo aauvm~o~ d / "! v 1~ oJocuo~ "~ 9 [ L

M. Romero et al. / Performance o f a CRS 717

12OO

1080

OOO

6OJ

40B

200.

-2N.

-4BO'

- 6 0 0

.eBB

-)egO,

I~mB

100~

050.

~OB.

4O0

-- aOB' E

N " -200'

-40~

-6BO

"OOO

- l ~

- 1 2 N "qo0 - 6 U -50B g' 390 60B go0 12~0 -h?.M -ql~o -60J -3OB iS ~BO 600 qO0 l:tOt~ I:'JE X (m.I [JE X Im, I

Fig. 12. Field layouts f o r optimized plants with vacuum focusing heliostats of 2.6 mrad. (Left) Fie!~ shape for l aperture receiver. (Right) Field shape for 2 aperture receiver.

For such temperatures, we obtained the results depicted in figs. 6 and 7. After an optimization of mass flows, the following heat fluxes were obtained:

(He) --: 80.70 k W / m 2

(air) = 53.62 k W / m 2

(cor~ventional) = 81.40 k W / m 2

Helium yield is 34% higher tb~.~ air, and gets similar heat fluxes than conven- tional reformers.

4. Plant optimizatim,

*-'-~ 3 h "" ASPOC (A Solar Plant Optimization ..... ~ a s been uUhzed for economic - ~ ' * ~ " ~ " . ~ . n ~ m . . . .

optimization of the plant. This code was developed by Aslnei Ior the GAST project [9]. This program looks for electrical energy production at minimum cost. Cost of kWh produced is expressed as a function of heliostat characteristics, heliostat spacing in the field, field lay-out, tower size, receiver dimensions, etc., and this function is optimized by a stepping mode.

Optimization was performed with heliostat geometry as described in fig. 2, average reflectivity of 85% including dust and availability, and or = 2.6 and 3.8 mrad for vacuum focusing heliostats arid or = 3.8 and 4.8 mrad for the same helios'~ats when considered passive flat. This increase is calculated because of wind defocusing effect, which produces axi-symmetri~; deformations. Estimation of or is calculated taking into account SERI results [10].

718 Itt Romero et aL / Performance of a CRS

Table 3 Summary of results for optimized plants

Thermal power

100 MW 200 MW 300 MW

Cost DM/GJ (total) 18.93 18.06 17.78 (solar) 16.60 13.23 12.17

Number ~f heliostats 2006 3832 5676 Tower height (m.) 187 284 357 Aperture radius (m) 5.04 7.20 8.79 Annual efficiency (%) 37.8 39.5 40.1 Synthesis gas (Tm/day) 388.8 777.6 1166.4 Energy cost (DM/Tm) 420.7 401.3 395.1 Conventional (DM/Tm) 439.0 439.0 439.0

Main conclusions of the plant optimization fgr 24 systems were: - For power sizes between 100 and 300 MW, is more efficient (5%), and

economical (1.3%), a plant with a single aperture receiver. Cosine factor and blocking and shaaowing effects improve with a ~ing~e a0erture. Difference between 1 and 2 apertures diminishes when power size of ~,~e plant increases, but increases if heliostat beam dispersion error decreases.

- The option of 2 apertures can be selected when tower height should be minimized. This benefit is even higher if we have bad i~q~,ostats. On the other hand, the field layout needs more heliostats when &; :-aperture receiver is chosen.

- Vacuum focusing neliostats are necessary, since a l.~//o rdduction in the number of heliostats and 8-18% in the tower height are. obtained against fiat passive collectors.

- The difference between 2.6 and 3.8 mrad heliostats is 11% in tower height and number of heliostats, but only 1-5% in thermal efficiency. Efficiency difference increases for big plants.

- For optimum plants with 2.6 mrad vacuum fi~cusing heiiostats and single aperture receivers, the synthesis gas production estimated is 16.2 T m / h , with a net energy saving of 18.2 D M / T m (100 MW), 37.6 D M / T m (200 MW) and 43.8 D M / T m from conventional plants. A 100 MW CRS will need 2006 heliostats of 120 m: each, a 187 m tower and a i0 m circular aperture receiver. Annual average efficiency expected is 37.8% and at design point is 42.9%.

R e f e r e n c e s

[1] L.G. Radosevich, C.W. Pr*tzel, E.H. Carrel[ and C.E. Tyner, SAND86-8019, NTIS, Oct. 1986. [2] H.B. Vakil and J.W. Flock, COO-2676-1, NTIS, 1978. [3] M. Romero, E. Cone.jero and J.M. Figarola, Proc. ISES Solar World Congress-1989, Kobe, 5ep.L

4-8, Japan, Pergamon Press. [4] D.L Alpert, R.M. Ho'aser, A.A. Heekes and W.W. Erdman, SANDg0-0183, NTIS, Feb. 1990.

M. Romero ct at / Performance of a CRS 719

[5] K. Kugeler, M. Kugeler, H.F. Niessen and K.H. Hammelmann; Nuclaar Engineering and Design, 34(i975), pp.i29-145.

[6] J. Singh, H.F. Niessen, R. Harth, H. Fedders, H. Reutler, W. Panknin, W.D. MiiUer and H.G. Arms; Nuclear Engineering and Design, 78(1984), pp. 179-194.

[7] L.F. Puebla; Aplicaci6n de las Series Trigonom6tricas de Fourier a la resoluci6n de ecuaciones diferenciaies, Thesis ef Licenciature, Univ. of Valladolid, Spain, 1989.

[8] E. Carrera and F. Sobr6n; An. Quire., 85(1989), pp., 253-262. [9] F. Ramos and M.A. Muro; Proc. final presentation GAST project, May 30-31 1989, Lahnstein,

Germany, Ed. Springer Verlag, 1989, pp.115-124. [10] L.M. Murphy, J.V. A)~der~o~ W. Short and T. Wendelin; SERI/TR-253-2694, NTIS, Dec. 1985.