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Solar hydrogen production by hybrid process of water thermolysis and electrolysis
Solar& Wind Technology Vol. 6. No 3. pp. 183-188. 1989
0741-983X/89 $3.00+.00 Pergamon Press pie
Pnnted in Great Britain.
SOLAR H Y D R O G E N P R O D U C T I O N BY H Y B R I D PROCESS OF WATER T H E R M O L Y S I S A N D ELECTROLYSIS S. Z. BAYKARA* a n d E. BILGENt * Marmara Research Institute, Scientificand Technical Research Council of Turkey, P.O. Box 21 Gebze, Kocaeli, Turkey ; 1"Ecole Polytechnique, C.P. 6079 Succ. A Montreal P.Q. H3C 3A7, Canada
(Receit'ed 18 May 1988; accepted 4 October 1988) A~tract--A hybrid process for hydrogen production by direct solar thermolysis and electrolysisof water is de~ribed and rated thermodynamicallyand economicallyutilizing appropriate models. The results of a feasibility analysis performed for an industrial size plant module of 324 GJ/year capacity (172 GJ/year thermolysisand 152 GJ/year electrolysis)are presented. Utilizingcurrent technologyand economicassumptions applicable for power utilities,over 13o module efficiencyand $39 per GJ hydrogen cost are estimated.
I. INTRODUCTION Solar water thermolysis allows chemical storage of solar energy in the form of transmittable hydrogen and direct utilization of solar energy as process heat. Furthermore, production of hydrogen as such, and its use as a fuel, constitute a completely renewable and non-polluting cycle. On the whole, solar water thermolysis implies solar process heat-production by a system comprised of a solar collector, i.e. a solar furnace, a dish type concentrator, and a chemical process for decomposition of water and treatment of the product gas to obtain hydrogen. So far water thermolysis has been taken up experimentally, theoretically and from preliminary feasibility and engineering points of view. In addition, an overall engineering assessment of hydrogen production by water thermolysis based on thermodynamic and economic modelling and simulation was completed for process rating for a plant of 600 G J/year capacity equipped with a dish type single reflection system [I]. The process in which the product gas is quenched with water and hydrogen is separated at low temperature by solid diffusion schemes was found to be thermodynamically the most feasible. Earlier thermodynamic and engineering analyses indicate that the optimum decomposition temperature is about 2500 K and that only less than 10% of the collected high temperature solar energy is used directly for the thermolysis process [2]. The rest is usually available as thermal energy for heat matching within the process and for conversion to mechanical energy. If the main aim is to produce solar hydrogen it would be best to maximize the hydrogen production
by converting the remaining mechanical energy to electrical energy and then using it in an electrolyser. Therefore, in the following sections a hybrid process consisting of water thermolysis with water quenching and electrolysis will be developed, analysed and evaluated. 2. THERMOLVSIS REACTIONS It is quite probable that the decomposition takes place on the hot surface of the reactor in a thin boundary layer and that the rest of the steam does not participate in the reaction. If this is the case, and if the reactor is designed in such a way that the nonparticipating part of the medium stays relatively cool with respect to the part in the boundary layer, this part can serve to quench the product gases once the decomposition reaction takes place in the boundary layer.
2.1. Decomposition o f water The following chemical reactions are believed to constitute the thermolysis process at the temperature levels of interest : H20 --, OH + H
(1)
OH ---,H + O
(2)
2H ~ H2
(3)
2 0 ~ 02
(4)
and the overall reaction may be represented as : H20 --* xT(xl H20 + x2OH + x3H + x40
183
-{'-X5 H 2 --~,-x 6 0 2 ).
(5)
1~4
~Z. tL'OgKAR.'~ u n d
.g
|:.
I h l (d \
;%"O
:..',:,
K H~O
:%"3
• .'
')'~'
Q:' ~,,
'3 '~
"2 'c)
'(.i
"'?
:iv
'~*
MoLe frachor' 0 ~ product gases
Im I Iquilibrium composition m thernlol.vnis process al I aim
T h e e q u i l i b r i u m c o m p o s i t i o n o f the resulting gas m i x t u r e is then d e t e r m i n e d by o b l a i n i n g the unknov, n mole fractions, x, from the following s i m u l t a n e o u s relations via n u m e r i c a l c a l c u l a t i o n s (i.e. N e w t o n R a p h s o n m e t h o d ) , w h e r e .x- is the total n u m b e r o1 m o l e s o f gas :
It can be seen, lbr c x a m p l c , that the p e r c e n t a g e o l w a t e r d e c o m p o s i t i o n is a b o u t 13% at 2500 K and 4 4 % at 3000 K.
3. METHOD FOR PROCESS EVAi,I:ATION 3.1.
]'[2 a t o n l c o n s e r v a t i o n balance.
['h¢'rmod.l'rt¢lmic ~'t'~ilZt~llion
( i ) O t ' e r a l l pro~'e vs q { l i t i ~ ' m l .
x, f-(l.2).x,+(1.2).~,
÷ ~~ -- I..x..
(6) q .... ~I, q •
( I3
O, a t o m c o n s e r v a t i o n balancc. q, = q,,IJh, q:,q/,.tl,, ~l.,
(I..2)xt F ( I . 2 ) x : + ( l 2 ) x z - - . ~ ,
-
2x.
.: A l l
Q,
114)
(7) q .... A l l
(Q:-~Q~)
115)
Mole flaction constraint. ),l .l v . - . . v ~ - F v 4 t ~, k . \ ~ . Equilibrium
18)
relations.
K, ~ t ' \ . ~ :
~,
(9)
K, '
~,
(10)
/'~:.~.~
K, - \ , K4
=
/'.v;
(11)
. v . ' l ' ~ ;i
(12)
w h e r e K, (i - 1 4) are the e q u i l i b r i u m c o n s t a n t s for the r e a c t i o n s I. 4 a n d P is the total presst,re in a t m o spheres. O n c e the m a s s b a l a n c c a r o u n d tlae r e a c t o r is o b t a i n e d , the energy r e q u i r e m e n t o f the d e c o n l p o sition p r o c e s s is d e t e r m i n e d from the enthalp~ balance, a n d is totally a c c o m m o d a t e d by the solar system. Figure I from [3] s h o w s a typical e x a m p l e o f equilibrium c o m p o s i t i o n at I alto. pressure ill ptlre Mean1.
'1,..,
-
q,,ll,
I],.q.,.,
(16)
v, here q, - s o l a r %vstcm efficient',, c a l c u l a t e d a c c o r d m g to cq. (14) ulili>'ing the c o m p u t e r p r o g r a m described in [4] (.'.--0.7) and q, , = c h e m i c a l plato
efficiency. ~1.,, : c o n c e n t r a t o r optical eflicicnc.x. •h, -- heliostat reltection efficient,,. )h. = ..'oncenlrator rcflcction efficiency, q,,, = c o r r e c l i o n lernl for o c c a s i o n a l s h a d o w s , )M • c o r r e c t i o n term for surface i m p e r f e c t i o n s a n d d u s t i n g o f m i r r o r s and tracing c r r o , s , q , , - t r a n s m i s s i o n ['actor r e p r e s e n t i n g tl;.m,',p a r e n t m e d i a in the p a t h o f beam r a d i a t i o n . )1,:, : c(>r. rcction factor for s h a d o w s causcd by o b s t r u c t i o n ' , in the p a t h p r e s e n t e d b~ the system. )/,,J. = receiver efficiency ( g e o m e t r y - r e l a t e d ) . q,. = t h e r m a l efficient3 o f the receiver al high t e m p e r a t u r e . #,,, - c o r r e c t i o n term to a c c o u n t for the fraction o f radiation o b s t r u c t e d by the recei~cr itself. A l l -= higher hcating value o f h y d r o g e n by c o n v e n t i o n ( - 286 k ] . m o l c ) . Q, --- solar energy inpul to the s',stem. (_), : total thor-
Solar hydrogen production mal energy input of the chemical process, QM = thermal energy equivalent of total mechanical energy input of the chemical process with a conversion efficiency of ~/.~ (~0.3), all three Q on molar basis. (ii) Total heat input.
Q, = rl,O.~-Qr¢~
(17)
where Qr~,¢= heat recuperated on molar basis from the process. (iii) Total work input.
QM
=
Q,,#IM = ( W , + IV,.+ Wp)#l M
(18)
W, = work of separation = - (nRT#h)Zxi In x, (19)
185 Table I. Economic assumptions Item
Value
Plant life (y) Discount rate (D) Inflation rate (I) Capital charge rate (FCR) Interest during construction (IDC) Construction period (n) Total direct cost coefficient (C I) Indirect cost coefficient (C2) Contingencies coefficient (C3) Variable costs (VCI) Initial operation and maintenance rate (OM R)
20 years 0.05 0.00 0.0 to 0.30 0.05 I month (1/12 year) 1.00 0.15 0.12 0.01 of equipment cost 0.01
IV,. = isothermal compression work
=-(nRT#Ic)In(V2/V~) Wp = pump work = AH/tlp
(20)
4.1. Total capital investment
(21)
Total capital investment (TCI) in current dollar values is composed of fixed capital investment (FC1) and the variable capital investment (VCI) which is also referred to as the working capital :
where rh ~ 0.5, r/c ~ 0.7, and ~/p ~ 0.7. 3.2. Engineering evaluation This type of rating can be done at two levels based on a conceptual process flow sheet containing only the basic equipment (this can be a more advanced thermodynamic study taking into account irreversibilities) or, a detailed and complete process flow sheet showing all the equipment involved. Here the characteristic process parameters such as various reflux ratios, heat-exchanger surfaces, construction materials, equipment efficiencies and other specifications are available. 3.3. Economic evaluation A reliable economic evaluation is based on an engineering evaluation of the second type. Here, the basis of comparison is generally expressed as $/GJ of hydrogen produced. The cost of hydrogen obtained by water thermolysis is a function of two different sets of capital and operating costs pertaining to the solar plant and chemical plant. Despite the availability of reliable cost estimation methodology, cost of hydrogen may be difficult to calculate with confidence due to the fact that some of the equipment involved must be manufactured according to individual designs.
4. ECONOMIC MODEL The aim of the economic model is to determine the total capital investment for the plant and the cost of hydrogen. The methodology for capital cost estimates is based on the installed equipment cost.
TCI = FCI + V C I .
(22)
The main components of FCI are the total direct cost (TDC), total indirect cost (TIC) and contingencies during construction (CONT) : FCI = T D C + TIC + C O N T .
(23)
T D C is expressed as percentages of the installed equipment cost (IEC), TIC as percentages of T D C and C O N T as those of T D C plus TIC : T D C = CI (IEC)
(24)
TIC = C2 (TDC)
(25)
C O N T = C3 ( T D C + TIC).
(26)
Where C values represent the total percentage factors given in Table 1. Total capital investment at startup time (TCS) is determined considering an interest rate IDC, an inflation rate I, and the duration of construction, n years : TCS = (1 + IDC)(I + I)n(TCI).
(27)
4.2. Total annual cost and product cost The total annual cost (TAC) is composed of fixed charges (FC) and the sum of various operation related costs (OC) : TAC = FC+OC.
(28)
Fixed charges are expressed as a percentage of TCS, which is referred to as the fixed charge rate (FCR) : FC = (FCR)(TCS).
(29)
S. Z, BA'tKARA and I(. BIl.(il:x
186
Table 2. Parameters for unipolar dec trolyser technology used in cq. (311 updated to 1987S from [5] Parameter
Value
/~ I~ .v~ (S.kW DC) i, ( m A c m " )
(Lg{) (~45 171,80 I ~,4.(10 I S2 7181)
t', (v)
i(mAcm')
.
;,( e ,
"
/_
F-
'
g
.....
I !
,LF-I: :: [. ....:I_ [ th
:.i.i:;
( )"()('~
tl R,
. ' ~ [ _.,
t'Jg 2 I'lo'.'. sheet of the h,. brid ~ater decolnp,,~MIl[)ll plo,..-c~,,
Cost of operation is expressed as :
5, (ASI'~ s l l
OC = (FL)(OMR)(T(S)
(30)
where Fi. is the levelized factor. OM R the initial operation and maintenance rate. v phmt life and I) discount rate. Finally, the levelizcd product cost (PC) is expressed in terms of T A C and annual production capacity (APROD) : PC = ( T A ( ' ) " ( A P R O D ) .
(32)
The values tbr various parameters discussed above arc listcd in Table 1. 4.3. Inslalh'd equipment cost ('heroical phmt. As seen in the abo',c sections. annual, and cspccially capital costs, arc heavily dcpcndent on the installed equipment cost. Consequcntl.,,, as mentioned prcviousl}, realistic cnginccring evaluation and process flow sheets are essential in this economic approach. Once a sufficiently dctailcd process dcsign is cstablishcd depending on the plant capacity, subprocesses and the basic properties of the equipment involved arc specilicd. Then, the pertinent cost data are compiled from the published reports and from the manufacturers and the equipment costs are estimated. The cost data arc updated, when neccssary, employing the Marshall and Swift all-industry cost index. Electrolyser. The cost of clcetrolyser is cstintated using the following expression, the paramcters of which are listed in Table 2 [5]: ('e-
777(APROD..3()0).',[f,.i4-(I
L).i, lx,,
•t O . 5 f . [ l l i + l . . i , Jx,,,',
(33)
where x,, = t,i,&:100 and the coefficient has the dimension of (GJ..day). (mA.cm ? ).
1)~
5. I. I'r,,,c.sA d(',S(Til)llOlt A self-sufficient solar water thernmlysis planl module pov..ered by a 230 kW capacity parabolic dish type reflector is studied thermodynamically from the engineering point of vie,a and economically. In the process shown, Fig. 2. v, atcr to be decomposed is introduced into the sectioned reactor cavity R as steam at 453 K, superheated tip to 1700 K and tinally undergoes thermolysis reaching the central section of the cavity where the temperature goes up to 2500 K The cavity is sized with respect to the radiant flux and temperature distribution available at the reflector, to produce 172 GJ..year equivalent of thcrmol}tic hydrogcn. The product gases are quenched with watcr m quencher Q, and very rapidly cooled to 473 K. The resulting gas is a mixture of molecular hydrogen, oxygen and steam. Water vapor is removed by passing the gas through condenser ('1 where the rtuect heat is utilized to produce steam at 453 K and 10 atm.. part of which is fed to the reactor, remaining part expanded to 0.1 arm. in steam turbine T, to produce 21.2 kW clectricit_,, which is enough to cater I\)r the total electricit,, requirement of the process and power a Stuart ('ell t(" produce 152 GJ.year equivalent of electrol,,..tic hydro-. gen. Thus the total plant capacity is increased b\ near[,, 90%. Hydrogen resulting from thermolysis is separated from oxygen diffusing through a Pd membrane and is compressed to atmospheric pressure. The condensed steam in ('1 is pumped back to quencher Q through pump PI. The water from condenser ("2 is mixed with make-up ,.,,:tier and pumped through P2 to condenser CI. Process characteristics arc given in f a b l e 3. 5.2. Process evaluation and re.su/t.~ The process of Fig. 2 with the characteristics given in Table 3 is evaluated using a computer code
Solar hydrogen production Table 3. Process characteristics Item
187
20 --
Value
',00
Sotor hydrogen cost. ÷
80 --
Reflector (parabolic dish) Diameter Focal plane diameter ~/(concentrator efficiency)
~. . 6o
17.00 m 0.60 m 0.95
+ ~
.~
-
$ 5 3 4 5 / m e concentrator 40
20 O0 2o
Recei~er (concentric cavities) Central section (reactor) Outer sections (steam superheater) Efficiency
0.90
Chemical plant Operating time Thermolysis H2 production rate Reactor temperature Reactor pressure Mole quenching H20/mole feed water Conversion rate Electrolysis H, production rate
3000 h 172 G J/year 2500 K I arm. 2 37.4 mole H20/H2 152 G J/year
I 005
L Ci
I 0,5
i 02
I 025
i 03
Cap,tat charge rote
Fig. 3. Parametric study and cost results of solar hydrogen production by the hybrid process.
developed earlier [1]. The thermodynamic and engineering results are reduced to obtain energy loads in various equipment and are presented in Table 4. The installed equipment costs for the module described in Section 4 are obtained from [6] and various publications [7]. They are updated when necessary to 1987 $ as described earlier in Section 4.3 and listed also in Table 4. Using the economical assumptions listed in Table 1 and costs in Tables 3 and 4, the overall solar hydrogen cost is calculated using the methodology described in Section 4 and the above code [1]. The capital charge rate and concentrator Table 4. Energy types and loads in the process and installed equipment costs updated to 1987 $ Equipment
Energy type
Energy load (M J/h)
Cost ($1987)
Concentrator Receiver Reactor Steam superheater Condenser CI Separator Compressor Turbine Condenser C2 Pump PI Pump P2 Pump P3 Electrolyser
Thermal Thermal Thermal Thermal Thermal Thermal Mechanical Mechanical Thermal Mechanical Mechanical Mechanical Electrical
828.00 828.00 439.20 388.80 (1044.00) 1.30 7.20 (76.32) (648.00) 0.43 0.01 1.80 64.80
14368 222 --21571 11300 14500 6540 13700 1178 1280 710 4370
Total
÷
--
89739
Values in parentheses indicate energy yield from equipment.
costs are taken as variable. The results are presented in Fig. 3. The total energy supplied to the system is 828 MJ/h which is used in the decomposition process to superheat steam and decompose water. The available thermal energy shown in parentheses is used to heat water and generate steam. The mechanical energy produced from the available thermal energy amounts to 76.32 MJ/h a small portion of which is used to power the compressor and the pumps. The major part is supplied to the electrolyser. The module overall thermal efficiency calculated using eq. (13) is found as 13%. 6. DISCUSSION AND CONCLUSIONS The hybrid module produces 200 mole H2/h by thermolysis and 177 mole H2/h by electrolysis. Therefore, the overall thermal efficiency can also be estimated from Table 4 as (200+177)(286 kJ/mole H2)/(828,000 k J/h) = 0.13. The earlier studies on the thermolysis process [I, 2] indicate a thermal efficiency of about 8-10%. It can be seen that the thermal energy is better utilized in the hybrid process and for the same amount of high temperature solar energy, the hydrogen production capacity is increased by about (152 GJ/year)/(172 G J/year) = 90%. For the base case (53.45 $/m 2 concentrator) the total module cost at start up is $115,344. This becomes $107,242 for 30.00 $/m 2 concentrator and $103,356 for 20.00 $/m 2 concentrator. The effect of this improvement on the hydrogen cost is negligible as can be seen in Fig. 3. The other big cost items such as Pd membrane, condensers and compressor do not lend for further improvements in cost, however they constitute study areas for alternative solutions. For example Pd membrane can be replaced by cheaper, however less efficient materials such as vycor glass and polymers. The economical calculations are carried out for 1987 constant dollar. The results in Fig. 3 indicate that for a typical capital charge rate of 10%, the solar
18g
S Z. II¢YK.'~RA and [ . IJll(;t:x
hydrogen cost is about 39 $..'(iJ. This can bc compared to the solar hydrogen costs obtained by other systems : P.V. electrolytic hydrogen of 60 115 $..GJ [8], hybrid thermochemical solar hydrogen ol'15 70 $(i.! [9]. It can be seen that the solar hydrogcn I'rom the hybrid thermolysis electrolysis systenl is quite ect~llolllicul and competitive with other solar schemes. The systenl will
bc m o s t
suitable
ltn
nlodtliar
hydrogen
d u c t i o n in s u n n y r e m o t e a r e a s ~ here the d e m a n d
pronla\
be ['ton1 300 GJ..year to se,,eral Jlundred nll_llliples o1" a module. In c o n c l u s i o n it m a y he s l a t e d t h a i the p r c s e n l p l a n l module is technologically possible, rcllcclors of 20 ill diameler being comrnerciallx available. Thermolytic hydrogen is also getting less cxpensive ,,'iih rcccnl breakthrough in materials and manufacturing technologies. If the determined research programs aiming at $20..m ~ succeed, then the present gas cost of S3q.(LI can be reduced to $35..(7J. which would render hydrogen production by water thermolysis cvcn more attractive. It is possible to expand plant capacity hundreds of times simply bv increasing the number ~ i modules.
"['he linancial supporl o1" thc Natural Sciences ~tnd l-ngineering Research ('ouricil of CInada and .IrkmJlrled.qeme#lt.~'
lilt: N A N )
('ollaboratl~.C Research (il'aills !~rogralllnlc . arc
acknowledged. R I ' } l " l l I.'N('I.Ts
I S. /.. |'l~l)'k~.ml, I { y d l o g c n pr~duclj~;I b,. v..aler thermol,.sis Ph.D. Thesis. [ c o l e I)olytcchniquc. Montreal. (':In,ida 2. [!. liilgct!. Solar h).dr~gcv] protlklctlon b. dirc~.l x~,~ll~..r dc'u'Olllpl~sition proc'css : .,~. prciilninar) cn,ginccrilliZ assu's,,, n]ent. Int..L II.rdr~.¢'n /:)~r,'q.v 9. 53 flc)144) 3 I';. Ililgen and ('. Itilgen. Solar synthetic I'tlcl plodllciloii /Ill .Z Hrdroqen Fn,,'r¢' 6, 349 3(2 (l<,lgl) -1 .I. ( l a l i n d o lind I! Bilgen, I"Iux tilld Icnlpcr/.ilurc disi r i b u l i o n in the roe'eider of" pclrabo]lC ~,~ltlr l'urnaccs .'~'t>/,, t:',rr.q.r 3,t. 125 I.t5 l l¢441 $ I4 1.. I.cRo) ~lnd ..~, K ~lu;.irl. I.mpt>]~il clcdroi),,Cl Icchnology. A compctili~c Icct]nolt~2) I'r,, _*rid If'Ill.'('. pi t J59 375(1~)7~) (,. ~'l S. Peters zlnd K. 1) ]llnnlcril~tu:-;. t~lattl .Or.w!l. ~t##~! I:rottotnirs / . r ('/trmir, l/ t:'#l~llll~'d', \It(|raw-Hill. Nov, Y o r k (198(I) 7 DI':VI It brochure for I 7 in dianlcici COllC¢lllrat~,r s~,Icil! I<1647: K. rzihonl:.llli allcl 13. i]utlcr. ,\d\ant_'tkt conlp<~sitcn b r qrt.'s,'-;t'd-lllelllbrilnu, hclioslals t.'lld'¢tl I~. ['45 7ql) (1<)~7). ~..,\. I|~inlmachc and IL Bii~2cn../tis:gcsnnlcnl tl[ mlar h)ell (~ gL'n production by pkoto~oltaic cleciroly/cr ,;).s;Ic'nis Solar ~7 />mr Ig~,'" |.S'li3; ..tnm~a! ..ll(,eti#~q. I(1~ 1I"
(19b17) <). li Ililgen. Solar h)'drl>gell prodliclloil bv hbbrid Ih¢l nlochc'nllcal processes .'~',,l<," I:'tt(>r:ll 41. I~)tj ( 19XX I
0741-983X/89 $3.00+.00 Pergamon Press pie
Pnnted in Great Britain.
SOLAR H Y D R O G E N P R O D U C T I O N BY H Y B R I D PROCESS OF WATER T H E R M O L Y S I S A N D ELECTROLYSIS S. Z. BAYKARA* a n d E. BILGENt * Marmara Research Institute, Scientificand Technical Research Council of Turkey, P.O. Box 21 Gebze, Kocaeli, Turkey ; 1"Ecole Polytechnique, C.P. 6079 Succ. A Montreal P.Q. H3C 3A7, Canada
(Receit'ed 18 May 1988; accepted 4 October 1988) A~tract--A hybrid process for hydrogen production by direct solar thermolysis and electrolysisof water is de~ribed and rated thermodynamicallyand economicallyutilizing appropriate models. The results of a feasibility analysis performed for an industrial size plant module of 324 GJ/year capacity (172 GJ/year thermolysisand 152 GJ/year electrolysis)are presented. Utilizingcurrent technologyand economicassumptions applicable for power utilities,over 13o module efficiencyand $39 per GJ hydrogen cost are estimated.
I. INTRODUCTION Solar water thermolysis allows chemical storage of solar energy in the form of transmittable hydrogen and direct utilization of solar energy as process heat. Furthermore, production of hydrogen as such, and its use as a fuel, constitute a completely renewable and non-polluting cycle. On the whole, solar water thermolysis implies solar process heat-production by a system comprised of a solar collector, i.e. a solar furnace, a dish type concentrator, and a chemical process for decomposition of water and treatment of the product gas to obtain hydrogen. So far water thermolysis has been taken up experimentally, theoretically and from preliminary feasibility and engineering points of view. In addition, an overall engineering assessment of hydrogen production by water thermolysis based on thermodynamic and economic modelling and simulation was completed for process rating for a plant of 600 G J/year capacity equipped with a dish type single reflection system [I]. The process in which the product gas is quenched with water and hydrogen is separated at low temperature by solid diffusion schemes was found to be thermodynamically the most feasible. Earlier thermodynamic and engineering analyses indicate that the optimum decomposition temperature is about 2500 K and that only less than 10% of the collected high temperature solar energy is used directly for the thermolysis process [2]. The rest is usually available as thermal energy for heat matching within the process and for conversion to mechanical energy. If the main aim is to produce solar hydrogen it would be best to maximize the hydrogen production
by converting the remaining mechanical energy to electrical energy and then using it in an electrolyser. Therefore, in the following sections a hybrid process consisting of water thermolysis with water quenching and electrolysis will be developed, analysed and evaluated. 2. THERMOLVSIS REACTIONS It is quite probable that the decomposition takes place on the hot surface of the reactor in a thin boundary layer and that the rest of the steam does not participate in the reaction. If this is the case, and if the reactor is designed in such a way that the nonparticipating part of the medium stays relatively cool with respect to the part in the boundary layer, this part can serve to quench the product gases once the decomposition reaction takes place in the boundary layer.
2.1. Decomposition o f water The following chemical reactions are believed to constitute the thermolysis process at the temperature levels of interest : H20 --, OH + H
(1)
OH ---,H + O
(2)
2H ~ H2
(3)
2 0 ~ 02
(4)
and the overall reaction may be represented as : H20 --* xT(xl H20 + x2OH + x3H + x40
183
-{'-X5 H 2 --~,-x 6 0 2 ).
(5)
1~4
~Z. tL'OgKAR.'~ u n d
.g
|:.
I h l (d \
;%"O
:..',:,
K H~O
:%"3
• .'
')'~'
Q:' ~,,
'3 '~
"2 'c)
'(.i
"'?
:iv
'~*
MoLe frachor' 0 ~ product gases
Im I Iquilibrium composition m thernlol.vnis process al I aim
T h e e q u i l i b r i u m c o m p o s i t i o n o f the resulting gas m i x t u r e is then d e t e r m i n e d by o b l a i n i n g the unknov, n mole fractions, x, from the following s i m u l t a n e o u s relations via n u m e r i c a l c a l c u l a t i o n s (i.e. N e w t o n R a p h s o n m e t h o d ) , w h e r e .x- is the total n u m b e r o1 m o l e s o f gas :
It can be seen, lbr c x a m p l c , that the p e r c e n t a g e o l w a t e r d e c o m p o s i t i o n is a b o u t 13% at 2500 K and 4 4 % at 3000 K.
3. METHOD FOR PROCESS EVAi,I:ATION 3.1.
]'[2 a t o n l c o n s e r v a t i o n balance.
['h¢'rmod.l'rt¢lmic ~'t'~ilZt~llion
( i ) O t ' e r a l l pro~'e vs q { l i t i ~ ' m l .
x, f-(l.2).x,+(1.2).~,
÷ ~~ -- I..x..
(6) q .... ~I, q •
( I3
O, a t o m c o n s e r v a t i o n balancc. q, = q,,IJh, q:,q/,.tl,, ~l.,
(I..2)xt F ( I . 2 ) x : + ( l 2 ) x z - - . ~ ,
-
2x.
.: A l l
Q,
114)
(7) q .... A l l
(Q:-~Q~)
115)
Mole flaction constraint. ),l .l v . - . . v ~ - F v 4 t ~, k . \ ~ . Equilibrium
18)
relations.
K, ~ t ' \ . ~ :
~,
(9)
K, '
~,
(10)
/'~:.~.~
K, - \ , K4
=
/'.v;
(11)
. v . ' l ' ~ ;i
(12)
w h e r e K, (i - 1 4) are the e q u i l i b r i u m c o n s t a n t s for the r e a c t i o n s I. 4 a n d P is the total presst,re in a t m o spheres. O n c e the m a s s b a l a n c c a r o u n d tlae r e a c t o r is o b t a i n e d , the energy r e q u i r e m e n t o f the d e c o n l p o sition p r o c e s s is d e t e r m i n e d from the enthalp~ balance, a n d is totally a c c o m m o d a t e d by the solar system. Figure I from [3] s h o w s a typical e x a m p l e o f equilibrium c o m p o s i t i o n at I alto. pressure ill ptlre Mean1.
'1,..,
-
q,,ll,
I],.q.,.,
(16)
v, here q, - s o l a r %vstcm efficient',, c a l c u l a t e d a c c o r d m g to cq. (14) ulili>'ing the c o m p u t e r p r o g r a m described in [4] (.'.--0.7) and q, , = c h e m i c a l plato
efficiency. ~1.,, : c o n c e n t r a t o r optical eflicicnc.x. •h, -- heliostat reltection efficient,,. )h. = ..'oncenlrator rcflcction efficiency, q,,, = c o r r e c l i o n lernl for o c c a s i o n a l s h a d o w s , )M • c o r r e c t i o n term for surface i m p e r f e c t i o n s a n d d u s t i n g o f m i r r o r s and tracing c r r o , s , q , , - t r a n s m i s s i o n ['actor r e p r e s e n t i n g tl;.m,',p a r e n t m e d i a in the p a t h o f beam r a d i a t i o n . )1,:, : c(>r. rcction factor for s h a d o w s causcd by o b s t r u c t i o n ' , in the p a t h p r e s e n t e d b~ the system. )/,,J. = receiver efficiency ( g e o m e t r y - r e l a t e d ) . q,. = t h e r m a l efficient3 o f the receiver al high t e m p e r a t u r e . #,,, - c o r r e c t i o n term to a c c o u n t for the fraction o f radiation o b s t r u c t e d by the recei~cr itself. A l l -= higher hcating value o f h y d r o g e n by c o n v e n t i o n ( - 286 k ] . m o l c ) . Q, --- solar energy inpul to the s',stem. (_), : total thor-
Solar hydrogen production mal energy input of the chemical process, QM = thermal energy equivalent of total mechanical energy input of the chemical process with a conversion efficiency of ~/.~ (~0.3), all three Q on molar basis. (ii) Total heat input.
Q, = rl,O.~-Qr¢~
(17)
where Qr~,¢= heat recuperated on molar basis from the process. (iii) Total work input.
QM
=
Q,,#IM = ( W , + IV,.+ Wp)#l M
(18)
W, = work of separation = - (nRT#h)Zxi In x, (19)
185 Table I. Economic assumptions Item
Value
Plant life (y) Discount rate (D) Inflation rate (I) Capital charge rate (FCR) Interest during construction (IDC) Construction period (n) Total direct cost coefficient (C I) Indirect cost coefficient (C2) Contingencies coefficient (C3) Variable costs (VCI) Initial operation and maintenance rate (OM R)
20 years 0.05 0.00 0.0 to 0.30 0.05 I month (1/12 year) 1.00 0.15 0.12 0.01 of equipment cost 0.01
IV,. = isothermal compression work
=-(nRT#Ic)In(V2/V~) Wp = pump work = AH/tlp
(20)
4.1. Total capital investment
(21)
Total capital investment (TCI) in current dollar values is composed of fixed capital investment (FC1) and the variable capital investment (VCI) which is also referred to as the working capital :
where rh ~ 0.5, r/c ~ 0.7, and ~/p ~ 0.7. 3.2. Engineering evaluation This type of rating can be done at two levels based on a conceptual process flow sheet containing only the basic equipment (this can be a more advanced thermodynamic study taking into account irreversibilities) or, a detailed and complete process flow sheet showing all the equipment involved. Here the characteristic process parameters such as various reflux ratios, heat-exchanger surfaces, construction materials, equipment efficiencies and other specifications are available. 3.3. Economic evaluation A reliable economic evaluation is based on an engineering evaluation of the second type. Here, the basis of comparison is generally expressed as $/GJ of hydrogen produced. The cost of hydrogen obtained by water thermolysis is a function of two different sets of capital and operating costs pertaining to the solar plant and chemical plant. Despite the availability of reliable cost estimation methodology, cost of hydrogen may be difficult to calculate with confidence due to the fact that some of the equipment involved must be manufactured according to individual designs.
4. ECONOMIC MODEL The aim of the economic model is to determine the total capital investment for the plant and the cost of hydrogen. The methodology for capital cost estimates is based on the installed equipment cost.
TCI = FCI + V C I .
(22)
The main components of FCI are the total direct cost (TDC), total indirect cost (TIC) and contingencies during construction (CONT) : FCI = T D C + TIC + C O N T .
(23)
T D C is expressed as percentages of the installed equipment cost (IEC), TIC as percentages of T D C and C O N T as those of T D C plus TIC : T D C = CI (IEC)
(24)
TIC = C2 (TDC)
(25)
C O N T = C3 ( T D C + TIC).
(26)
Where C values represent the total percentage factors given in Table 1. Total capital investment at startup time (TCS) is determined considering an interest rate IDC, an inflation rate I, and the duration of construction, n years : TCS = (1 + IDC)(I + I)n(TCI).
(27)
4.2. Total annual cost and product cost The total annual cost (TAC) is composed of fixed charges (FC) and the sum of various operation related costs (OC) : TAC = FC+OC.
(28)
Fixed charges are expressed as a percentage of TCS, which is referred to as the fixed charge rate (FCR) : FC = (FCR)(TCS).
(29)
S. Z, BA'tKARA and I(. BIl.(il:x
186
Table 2. Parameters for unipolar dec trolyser technology used in cq. (311 updated to 1987S from [5] Parameter
Value
/~ I~ .v~ (S.kW DC) i, ( m A c m " )
(Lg{) (~45 171,80 I ~,4.(10 I S2 7181)
t', (v)
i(mAcm')
.
;,( e ,
"
/_
F-
'
g
.....
I !
,LF-I: :: [. ....:I_ [ th
:.i.i:;
( )"()('~
tl R,
. ' ~ [ _.,
t'Jg 2 I'lo'.'. sheet of the h,. brid ~ater decolnp,,~MIl[)ll plo,..-c~,,
Cost of operation is expressed as :
5, (ASI'~ s l l
OC = (FL)(OMR)(T(S)
(30)
where Fi. is the levelized factor. OM R the initial operation and maintenance rate. v phmt life and I) discount rate. Finally, the levelizcd product cost (PC) is expressed in terms of T A C and annual production capacity (APROD) : PC = ( T A ( ' ) " ( A P R O D ) .
(32)
The values tbr various parameters discussed above arc listcd in Table 1. 4.3. Inslalh'd equipment cost ('heroical phmt. As seen in the abo',c sections. annual, and cspccially capital costs, arc heavily dcpcndent on the installed equipment cost. Consequcntl.,,, as mentioned prcviousl}, realistic cnginccring evaluation and process flow sheets are essential in this economic approach. Once a sufficiently dctailcd process dcsign is cstablishcd depending on the plant capacity, subprocesses and the basic properties of the equipment involved arc specilicd. Then, the pertinent cost data are compiled from the published reports and from the manufacturers and the equipment costs are estimated. The cost data arc updated, when neccssary, employing the Marshall and Swift all-industry cost index. Electrolyser. The cost of clcetrolyser is cstintated using the following expression, the paramcters of which are listed in Table 2 [5]: ('e-
777(APROD..3()0).',[f,.i4-(I
L).i, lx,,
•t O . 5 f . [ l l i + l . . i , Jx,,,',
(33)
where x,, = t,i,&:100 and the coefficient has the dimension of (GJ..day). (mA.cm ? ).
1)~
5. I. I'r,,,c.sA d(',S(Til)llOlt A self-sufficient solar water thernmlysis planl module pov..ered by a 230 kW capacity parabolic dish type reflector is studied thermodynamically from the engineering point of vie,a and economically. In the process shown, Fig. 2. v, atcr to be decomposed is introduced into the sectioned reactor cavity R as steam at 453 K, superheated tip to 1700 K and tinally undergoes thermolysis reaching the central section of the cavity where the temperature goes up to 2500 K The cavity is sized with respect to the radiant flux and temperature distribution available at the reflector, to produce 172 GJ..year equivalent of thcrmol}tic hydrogcn. The product gases are quenched with watcr m quencher Q, and very rapidly cooled to 473 K. The resulting gas is a mixture of molecular hydrogen, oxygen and steam. Water vapor is removed by passing the gas through condenser ('1 where the rtuect heat is utilized to produce steam at 453 K and 10 atm.. part of which is fed to the reactor, remaining part expanded to 0.1 arm. in steam turbine T, to produce 21.2 kW clectricit_,, which is enough to cater I\)r the total electricit,, requirement of the process and power a Stuart ('ell t(" produce 152 GJ.year equivalent of electrol,,..tic hydro-. gen. Thus the total plant capacity is increased b\ near[,, 90%. Hydrogen resulting from thermolysis is separated from oxygen diffusing through a Pd membrane and is compressed to atmospheric pressure. The condensed steam in ('1 is pumped back to quencher Q through pump PI. The water from condenser ("2 is mixed with make-up ,.,,:tier and pumped through P2 to condenser CI. Process characteristics arc given in f a b l e 3. 5.2. Process evaluation and re.su/t.~ The process of Fig. 2 with the characteristics given in Table 3 is evaluated using a computer code
Solar hydrogen production Table 3. Process characteristics Item
187
20 --
Value
',00
Sotor hydrogen cost. ÷
80 --
Reflector (parabolic dish) Diameter Focal plane diameter ~/(concentrator efficiency)
~. . 6o
17.00 m 0.60 m 0.95
+ ~
.~
-
$ 5 3 4 5 / m e concentrator 40
20 O0 2o
Recei~er (concentric cavities) Central section (reactor) Outer sections (steam superheater) Efficiency
0.90
Chemical plant Operating time Thermolysis H2 production rate Reactor temperature Reactor pressure Mole quenching H20/mole feed water Conversion rate Electrolysis H, production rate
3000 h 172 G J/year 2500 K I arm. 2 37.4 mole H20/H2 152 G J/year
I 005
L Ci
I 0,5
i 02
I 025
i 03
Cap,tat charge rote
Fig. 3. Parametric study and cost results of solar hydrogen production by the hybrid process.
developed earlier [1]. The thermodynamic and engineering results are reduced to obtain energy loads in various equipment and are presented in Table 4. The installed equipment costs for the module described in Section 4 are obtained from [6] and various publications [7]. They are updated when necessary to 1987 $ as described earlier in Section 4.3 and listed also in Table 4. Using the economical assumptions listed in Table 1 and costs in Tables 3 and 4, the overall solar hydrogen cost is calculated using the methodology described in Section 4 and the above code [1]. The capital charge rate and concentrator Table 4. Energy types and loads in the process and installed equipment costs updated to 1987 $ Equipment
Energy type
Energy load (M J/h)
Cost ($1987)
Concentrator Receiver Reactor Steam superheater Condenser CI Separator Compressor Turbine Condenser C2 Pump PI Pump P2 Pump P3 Electrolyser
Thermal Thermal Thermal Thermal Thermal Thermal Mechanical Mechanical Thermal Mechanical Mechanical Mechanical Electrical
828.00 828.00 439.20 388.80 (1044.00) 1.30 7.20 (76.32) (648.00) 0.43 0.01 1.80 64.80
14368 222 --21571 11300 14500 6540 13700 1178 1280 710 4370
Total
÷
--
89739
Values in parentheses indicate energy yield from equipment.
costs are taken as variable. The results are presented in Fig. 3. The total energy supplied to the system is 828 MJ/h which is used in the decomposition process to superheat steam and decompose water. The available thermal energy shown in parentheses is used to heat water and generate steam. The mechanical energy produced from the available thermal energy amounts to 76.32 MJ/h a small portion of which is used to power the compressor and the pumps. The major part is supplied to the electrolyser. The module overall thermal efficiency calculated using eq. (13) is found as 13%. 6. DISCUSSION AND CONCLUSIONS The hybrid module produces 200 mole H2/h by thermolysis and 177 mole H2/h by electrolysis. Therefore, the overall thermal efficiency can also be estimated from Table 4 as (200+177)(286 kJ/mole H2)/(828,000 k J/h) = 0.13. The earlier studies on the thermolysis process [I, 2] indicate a thermal efficiency of about 8-10%. It can be seen that the thermal energy is better utilized in the hybrid process and for the same amount of high temperature solar energy, the hydrogen production capacity is increased by about (152 GJ/year)/(172 G J/year) = 90%. For the base case (53.45 $/m 2 concentrator) the total module cost at start up is $115,344. This becomes $107,242 for 30.00 $/m 2 concentrator and $103,356 for 20.00 $/m 2 concentrator. The effect of this improvement on the hydrogen cost is negligible as can be seen in Fig. 3. The other big cost items such as Pd membrane, condensers and compressor do not lend for further improvements in cost, however they constitute study areas for alternative solutions. For example Pd membrane can be replaced by cheaper, however less efficient materials such as vycor glass and polymers. The economical calculations are carried out for 1987 constant dollar. The results in Fig. 3 indicate that for a typical capital charge rate of 10%, the solar
18g
S Z. II¢YK.'~RA and [ . IJll(;t:x
hydrogen cost is about 39 $..'(iJ. This can bc compared to the solar hydrogen costs obtained by other systems : P.V. electrolytic hydrogen of 60 115 $..GJ [8], hybrid thermochemical solar hydrogen ol'15 70 $(i.! [9]. It can be seen that the solar hydrogcn I'rom the hybrid thermolysis electrolysis systenl is quite ect~llolllicul and competitive with other solar schemes. The systenl will
bc m o s t
suitable
ltn
nlodtliar
hydrogen
d u c t i o n in s u n n y r e m o t e a r e a s ~ here the d e m a n d
pronla\
be ['ton1 300 GJ..year to se,,eral Jlundred nll_llliples o1" a module. In c o n c l u s i o n it m a y he s l a t e d t h a i the p r c s e n l p l a n l module is technologically possible, rcllcclors of 20 ill diameler being comrnerciallx available. Thermolytic hydrogen is also getting less cxpensive ,,'iih rcccnl breakthrough in materials and manufacturing technologies. If the determined research programs aiming at $20..m ~ succeed, then the present gas cost of S3q.(LI can be reduced to $35..(7J. which would render hydrogen production by water thermolysis cvcn more attractive. It is possible to expand plant capacity hundreds of times simply bv increasing the number ~ i modules.
"['he linancial supporl o1" thc Natural Sciences ~tnd l-ngineering Research ('ouricil of CInada and .IrkmJlrled.qeme#lt.~'
lilt: N A N )
('ollaboratl~.C Research (il'aills !~rogralllnlc . arc
acknowledged. R I ' } l " l l I.'N('I.Ts
I S. /.. |'l~l)'k~.ml, I { y d l o g c n pr~duclj~;I b,. v..aler thermol,.sis Ph.D. Thesis. [ c o l e I)olytcchniquc. Montreal. (':In,ida 2. [!. liilgct!. Solar h).dr~gcv] protlklctlon b. dirc~.l x~,~ll~..r dc'u'Olllpl~sition proc'css : .,~. prciilninar) cn,ginccrilliZ assu's,,, n]ent. Int..L II.rdr~.¢'n /:)~r,'q.v 9. 53 flc)144) 3 I';. Ililgen and ('. Itilgen. Solar synthetic I'tlcl plodllciloii /Ill .Z Hrdroqen Fn,,'r¢' 6, 349 3(2 (l<,lgl) -1 .I. ( l a l i n d o lind I! Bilgen, I"Iux tilld Icnlpcr/.ilurc disi r i b u l i o n in the roe'eider of" pclrabo]lC ~,~ltlr l'urnaccs .'~'t>/,, t:',rr.q.r 3,t. 125 I.t5 l l¢441 $ I4 1.. I.cRo) ~lnd ..~, K ~lu;.irl. I.mpt>]~il clcdroi),,Cl Icchnology. A compctili~c Icct]nolt~2) I'r,, _*rid If'Ill.'('. pi t J59 375(1~)7~) (,. ~'l S. Peters zlnd K. 1) ]llnnlcril~tu:-;. t~lattl .Or.w!l. ~t##~! I:rottotnirs / . r ('/trmir, l/ t:'#l~llll~'d', \It(|raw-Hill. Nov, Y o r k (198(I) 7 DI':VI It brochure for I 7 in dianlcici COllC¢lllrat~,r s~,Icil! I<1647: K. rzihonl:.llli allcl 13. i]utlcr. ,\d\ant_'tkt conlp<~sitcn b r qrt.'s,'-;t'd-lllelllbrilnu, hclioslals t.'lld'¢tl I~. ['45 7ql) (1<)~7). ~..,\. I|~inlmachc and IL Bii~2cn../tis:gcsnnlcnl tl[ mlar h)ell (~ gL'n production by pkoto~oltaic cleciroly/cr ,;).s;Ic'nis Solar ~7 />mr Ig~,'" |.S'li3; ..tnm~a! ..ll(,eti#~q. I(1~ 1I"
(19b17) <). li Ililgen. Solar h)'drl>gell prodliclloil bv hbbrid Ih¢l nlochc'nllcal processes .'~',,l<," I:'tt(>r:ll 41. I~)tj ( 19XX I