Int. J. Hydrogen Energy Vol. 3, pp. 203-208 036~3199/78/0601-2003 $02.00/0 Pergamon Press Ltd. Printed in Great Britain ~) International Association for Hydrogen Energy

SYSTEM EFFICIENCY OF A WATER-SPLITTING SYSTEM SYNTHESIZED BY PHOTOCHEMICAL A N D THERMOELECTRIC

CONVERSION OF SOLAR ENERGY T. OHTA, N. KAMIYA, M. YAMAGucm, N. GOTOH, T. OTAGAWA and S. ASAKURA

Yokohama National University, 156 Tokiwadai, Hodogaya, Yokohama 240, Japan

(Received for publication 28 November 1977)

Abstract--Valuable hybrid systems for hydrogen production by solar energy have been developed and the system efficiency has been estimated and discussed from various points of view. In order to decompose water into hydrogen and oxygen without the consumption of additional reactants, a steady stream of the reacting materials must be maintained in consecutive reaction processes and if the system has a rate determining step extra energy should be supplied to promote the reaction. In the Yokohama Mark 5 Process, the efficiency of the thermoelectric device is as low as 5 ~; however, the overall efficiency of hydrogen production can be raised to 20 9/o by addition of extra electric power.

The energetics of a hybrid system combining photochemical, thermochemical and electrochemical reactions have also been discussed.

INTRODUCTION

EFFECTIVE utilization of the cleanest and cheapest energy source (i.e. solar energy) has been considered but up to now it has not been practically demonstrated. Although green plants can convert light energy into chemical energy with high efficiency, the light is absorbed in photosynthesis only at certain wavelengths and the remainder of the solar energy is wasted.

To utilize low density solar flux as effectively as possible, all wavelengths of light should be used and the efficiency of each step oftbe energy conversion steps should be improved. For this reason, we have investigated several cycles and have concluded that the best means of hydrogen production by solar energy without consumption of additional reactants is a hybrid system. A hybrid system which combines photochemical, thermocbemicai and electrochemical reactions is considered.

At the U.S.-Japan joint seminar [1] and 10th IECEC [2], we presented a hybrid system using photo- chemical cells. A redox system containing iodine absorbs visible to ultraviolet light and any unabsorbed light is transmitted through the cell and concentrated with a Fresnel lens to provide a heat source for other steps. Products of the photolysis are treated in thermochemical and/or electrochemical steps to produce hydrogen.

In the consecutive reactions of the hybrid system the mass balance must be constant and a single step with a low reaction rate will lower the overall efficiency. In this report, the overall efficiency of the system is estimated using the efficiency of each step and the optimum conditions are also discussed.

YOKOHAMA MARK 5

One of the hybrid systems previously proposed [1, 2], is:

Yokohama Mark 5 :

2FeSO 4 + 12 + H2SO,t --. Fe2(SO4) a + 2HI (1)

2HI - , H 2 + 12 (2)

Fe2(SO,) 3 + H20 -* 2FeSO, + H2SO , + ½0 2. (3)

The key reaction (1) is photochemical and the substantial part can be rewritten as:

2Fe 2+ + 13 + light - , 2Fe 3+ + 3I-, (4)

where the Gibbs' free energy change, AG298, is 10.8 kcal. As the colour of iodine indicates, the reactants absorb light of the blue to ultraviolet re#on and the absorbance of the solution varies with the

203

204 T. OHTA et al.

concentration of iodine. The maximum wavelength of the light absorbed, 2 o is dependent on the iodine concentration, C. At an iodine concentration C = 2.5 × 10- 3 mole l, 2 c = 5800A while for C = 2.5 × 10 -4 mol/l, 2c = 4300 A. At 4000 A the solution transmittance is 50%. The efficiency of the photo- chemical system, qPc, in (A - kcal/mole) is given by:

3.¢ AG~o 8 IXo~b[l - exp{ -K(3,) CI}] 2g(2)d2

r/Pc = 2.86 × 105 ~]og(2)d 2 , (5)

where 20 is the shortest wavelength of the sunlight arriving at the cell surface, K is the absorption coefficient, l is the thickness of the photochemical cell and 0(2) is the solar distribution function. Substituting the values: AG29 a = 10.8 kcal/mole, ~b = 1, exp{ - K(2) Cl} ~, 0.2, and using the results of solar parameters known for 20, 0(2) and 2 c = 6000 A, we obtain qec = 0.4 as an optimum value. A detailed discussion of qec will be presented in a paper which will soon be published in this Journal. Hydrogen iodide in reaction (2) may be decomposed by photochemical, thermoehemical or electro- chemical methods. Since reaction (1) is conducted in aqueous solution, the hydrogen iodide solution may be electrolysed directly.

The third step is also conducted electrochemically. Since the separation of Fe 3 + from other species in the first step is rather difficult, an electrolyser with special ion exchange membranes, which provide the anodic, the cathodic and the intermediate compartments, is introduced to accomplish the third reaction and the separation of hydrogen iodide. The necessary voltage including overvoltage for reactions (2) and (3) are about 0.74 V and 0.66 V, respectively. The electricity for the second and third steps is generated by a thermoelectric device, constructed with p- and n-semiconductors, using the long wavelength light transmitted through the photochemical cell. The materials used are Bi-Te-Sb for p-type and Bi-Te-Se for n-type semiconductors.

In the preliminary field test carried out on 24 December 1974 at Yokohama, the following data for the thermoelectric power generator were obtained. The absorber and ambient temperatures were 453 K and 292 K, respectively. The efficiency of the collector was 0.81. The figure of merit of the thermoelement is 1.89 x 10 -3 1/deg and the real efficiency of the device was about 5 %. The thermoelectric module composed of 32 thermocouples gave an open circuit voltage of 2.01 V of AT = 150°C. The overall efficiency of the total system, qo, can be expressed by:

1 tlo = Woo {Wedlec + ( W e + We)q,eqec}, (6)

where W 0 and We~ are the incident solar energy and the fraction of W o used for the photochemical reaction and qfe and r/e~ are the effieiencies of the thermoelectric device and the electrolysis, respectively.

Efficiency o f the hybrid cycle

For the entire system, the minimum amount of total energy input, regardless of the number of steps, is the enthalpy change given by

A H = AG + T A S , (7)

which, for water decomposition, is 68.32 kcal/mole. In a reversible system, T A S is the energy input as heat and will be replaced by AQ. The theoretical work requirement is given by the Gibbs' function which is 56.69 kcal/mole for water decomposition. The Gibbs energy can be shared with many kinds of ener- gies, e.g. light energy, AGL, electric energy, AG~, mechanical energy, A G u , and so on depending upon the reaction conditions.

AG = AG L + A G e + AG u + . . . . (8)

For the multiple step process, each component of eq. (7) is rewritten as the sum of the contributions in each step:

AH = YAH

AG = Y..AG L + Y, AG e + Y.,AG u + . . . . (9)

AQ = EAQ.

SYSTEM EFFICIENCY OF A WATER-SPLITTING SYSTEM 205

As a whole, the overall efficiency is defined as

AH = X W (10)

qo

after Funk [3], where ZW in this case is the total energy consumed. Conservation of mass must also be considered. All reacting materials must be carried on to the next step without loss. For that reason, the following matching condition

AG L = AG E AQ WLrtec ~ . . . . . ~ = 1, (11)

should be maintained. The total efficiency, ~o, is expressed as

1 ?]0 = ~-w(WL~lp¢ + WE~ec -~-"" "Jr- WT?ltc) (12)

Z W = W L = W~ + . . . + W r. (13)

This theory is applied to Yokohama Mark 5 which has one photochemical step and two electrochemical steps. Inserting the values for AGLx and AGE2 plus AGE3 of 10.8 kcal/mole and 45.9 kcal/mole, respectively, into eq. (11):

10.8 45.9 Wecqe¢ = WE2rlec2 ..[_ WE3~ec 3 1. (14)

Values of W r and W E are given by

wT = Wo - w,c (15)

and

W E = Wrrlt, = ( W o - Wpc ) the. (16)

Assuming qecl = qe~3 = qec, the matching condition (14) is then replaced by

10.8 45.9 Wectle ~ - ( W ° _ Wec)qtj l ,~ = 1. (17)

The overall efficiency is then

1 7o =-Wo {Weeqec + ( W - We~)qteqe¢}. (18)

In general, the efficiency of the thermoelectric conversion is too small (5-10 % at most in the present system) to satisfy the matching condition (17). A more economical method is to supply an additional auxiliary electric power, E*, from another source so that eq. (18) becomes

1 q~ = W o + E* [Wecqe~ + {(w° - WP~)qt" + E*} ~/,~]. (19)

A numerical analysis [4] and the exlSerimental results of our real system Mark 5 shows an overall efficiency of more than 20 %.

YOKOHAMA MARK 6

The bottleneck of Mark 5 is due to the deficiency of electric power. In order to improve the process, the decomposition of hydrogen iodide by a thermochemical process instead of the electrolysis was considered. We propose a new cycle named Yokohama Mark 6 in which reaction (2) is carried out thermochemically. Estimating the following data,

AG L = 10.8 kcal/mol, AQ = 37.8 kcal/mole, AG E = 19.7 kcal/mole, (20)

we get the matching condition

10.8 37.8 19.7 = l, (21) We¢qe~ - {(Wo - Wee)(1 - Tire) -}- Q*} q,¢ = {(Wo - We~) Ylte -~ E*} ~Vec

206 T. OHTA et al.

where Q* expresses the auxiliary heat supply. The overall efficiency under the matching condition is

1 tl* = W o + Q* + E * [We?e" + {(W° - We)(1 - t/'e) + Q*} r/,, + {(W o - Wp) ~,, + E*}q,c]. (22)

Detailed analysis of Yokohama Mark 6 shows an overall efficiency of about 30 %.

DISCUSSION

The efficiency of the photochemical reaction, qec is expressed by eq. (5) which depends on the maxi- mum wavelength, 2c, absorbed through the photochemical cell. The value ofqe c is estimated to be 30, 35 and 40 ~ at '~c values of 40OO, 5000 and 6000 A. For the Mark 5 process, the relationships between q~ and qte, and also E* and qte, are shown in Fig. 1. In this case, qec is taken to be 40~o with 2 = 6OO0 A.

60 ~ 6 0 E"

50 50

~o ~" - ~

"B

.u 50 30

20 20 ",'/p~" 4 0 %

~0 - - "gec = 80 %

I I o 0 lO 20

"9)~ °/o

FIG. I. Overall efficiency and auxiliary electric power supply in Mark 5.

It is clearly impossible to get a proper matching condition unless E* is added from an auxiliary electri- city source. Low efficiency oftbe electrolysis process, i.e., qJ/ec determines the overall rate of hydrogen production and so the supply of an auxiliary electric power E* has a large effect for Mark 5. For the Mark 6 process, Q* and E* can be determined from

(Wo - WPc) (1 - qte) + Q* = AQ/q,c (23)

and

( W o - Wpc ) l'~te Jr" E* = .t~GE/?~ec, (24)

which are derived from eqs (9) and (11). In Figs 2, 3 and 4, the relationships between the overall effi- ciency and the auxiliary electric power or heat, assuming the efficiency of conversion of (W o - Wpc ) into effective heat by 80 ~o are shown.

From Fig. 3, it is clear that with the small value of the ~tc the overall efficiency cannot be improved even if large amounts of heat are supplied from an auxiliarly source. The efficiency of Mark 6 will be 34-36 ~o from Figs 2, 3 and 4. If no auxiliary power supply is available (i.e. E* = 0, Q* = 0) a maximum overall efficiency of 20 ~o is obtained at qPc = 32.6 ~o, q,c = 17.2 ~o and qte = 10 ~o from eqs (22), (23) and (24).

SYSTEM EFFICIENCY OF A WATER-SPLITTING-SYSTEM 207

4 0 % ~ "q= = 4 0 % /

"qtc" 30 % 300 ~ "the - IO %

- . " \ -~ 80 Y, A O r /~ - 8 0 % 50 ~ so

ioo 4,o ~ ,+ 20o 4o

. 5 0 - - 2 0 • o - I 0 0 2 0

• o ~- "o

E* I I I o I I I I

~o 20 o ~o 20 30 40 50

,rh,, % ,r/e %

FIG. 2. Auxiliary supplies of electric- and heat- FIG. 3. Auxiliary supplies of electric- and heat- energy, and overall efficiency vs thermoelectric energy, and overall efficiency vs thermochemical

efficiency in Mark 6. efficiency in Mark 6.

~t,: " 3 0 *

"q~e " I 0 %

• r/,~ • 8 0 % (3,

I00 40

o 3 0

~o

0

30 40 50

~70/o

FIG. 4. Auxiliary supplies of electric- and heat-energy, and overall efficiency vs photochemical efficiency in Mark 6.

C O N C L U S I O N

The best method of hydrogen product ion from solar energy is a combinat ion of photochemical, thermochemical and electrochemical reactions. Nevertheless, auxiliary energy should be added to low efficiency steps in order to obtain better overall efficiency. In the Mark 6 process, an opt imum overall efficiency of 35 % is obtained with an auxiliary power supply and about 20 % without any additional energy.

208 T. OHTA et al.

Nomenclature

2 solar wavelength (A) 2 o, 2~ the shortest wave length of the incident solar radiation and the longest o/~e absorbed by the

solution 0(2) solar distribution function (W/cm 2. A) K absorption coefficient (1/mole. cm) l thickness of the photochemical cell (cm) C concentration of iodine (mole/l) • quantum yield r/pc, t/t~, qec efficiency of the photochemical process, the thermoelectric device and the electrochemical process ~/~, )7 o overall efficiency with and without auxiliary power supply r/ec2 the subscript 2 indicates that the second reaction is electrochemical AG~9 a Gibbs" free energy change of the photochemical reaction (kcal/mole) AGL, AG~, AG u Gibbs' free energy shared by light, electric and mechanical energy (kcal/mole) Q*, E* auxiliary supply of heat and electric energy (kcal) W o incident solar energy Wec , W~. the part of W o used for the photochemical process Wr, W~ the part of W o supplied for the thermoelectric device and the electricity generated by the device.

R E F E R E N C E S

1. T. OHTA et al., Water-Splitting System Synthesized by Photochemical and Thermoelectric Utilization of Solar Energy, Proe. U.S.-Japan Joint Seminar on Key Technolooyfor the Hydrooen Energy System, Tokyo, 21-23 July, 1975.

2. T. OHTA et al., Water-Splitting System Synthesized by Photochemical and Thermoelectric Utilization of Solar Energy, Proc. lOth IECEC, Newark, Delaware, 19-22 August, 1975.

3. J. E. FUNK, The Generation of Hydrogen by the Thermal Decomposition of Water, Proc. 9th IECEC, San Francisco, CA, 26-28 August, 1974.

4. J. A. DEAN (Ed.), Lanae's Handbook of Chemistry (12th Edition) McGraw-Hill, New York (1973).