International lournal of Hydrogen Energy, Vol 1, pp. 113-i16. Pergamon Press, 1976. Printed in Northern Ireland

PHOTOCHEMICAL AND THERMOELECTRIC UTILIZATION OF SOLAR ENERGY IN A HYBRID WATER-SPLITTING

SYSTEM?

T. OHTA, S. ASAKURA, M. YAMAGUCHI, N. KAMIYA, N. GOTOH and T. OTAGAWA

Hydrogen Energy Research Laboratories Yokohama National University, Yokohama, Japan

Abstract-A hybrid thermochemical water-splitting cycle using solar energy is proposed and experimental results are presented. The cycle consists of a photochemical reaction conducted in a flat cell with a Fresnel lens and concentrating the remaining solar energy on a thermoelectric generator which produces electric power for the electrolysis steps. The photochemical reaction is:

2Fe2+ + 13- +light+ 2Fe3+ + 31

The overall efficiency is estimated to be as high as 15-25%.

INTRODUCTION

CONVERSION of solar radiation consisting of ultraviolet to infrared wavelengths into a secondary energy form has been an aim of scientists for some time [l]. The secondary forms of electrical, thermal, and chemical energy have been widely investigated without effective success. The concept of the proposed hydrogen energy system [2] coupled with the production of hydrogen from water via solar energy is believed to be the ultimate energy system. The most important part of such a system is the production of hydrogen and oxygen from water in a closed cycle. Most closed water-splitting cycles require heat input at very high temperatures and therefore assume a nuclear reactor as the heat source.

CYCLES

This research involves the following two hybrid cycles which consist of a photochemical and electrochemical reactions at ambient temperatures.

Yokohama Mark 2A

2FeS04 + IZ + 2H20+ 2Fe(OH)SOd + 2HI (1.1)

2HI+Hz+I2 (1.2)

2Fe(OH)S04-+2FeS04 + H20 + $02. (1.3)

Yokohama Mark 5

2FeS04 + 12 + H&04 + Fez(S04)1+ 2HI (2.1)

ZHI*Hz+Iz (2.2)

Fez(S04)3 + H20+ 2FeS04 + HzSO.+ + $02. (2.3)

The two key reactions, (1.1) and (2.1), are photochemical and can be written as [4]:

2Fe2++K+light+2Fe3’+ 31-. (3)

The change in Helmholtz free energy for this reaction is: A68 = +10.8 kcal/mol. Reactions (1.2) and (2.2) can be conducted using either photon energy at a wavelength shorter

than 3500 A or thermal energy at a temperature of 427°C. However, electrolysis is found to be the most effective method for this reaction. Reaction (1.3) can theoretically be accomplished

i- Presented at the U.S.-Japan Seminar on Key Technologies for the Hydrogen Energy System, Tokyo, Japan, July 1975.

113

I 1.4 PHOTOCHEMICAL AND THERMOELECTRIC UTILIZATION

using thermal energy at 250°C but is difficult in reality. Reaction (2.3) is best accomplished electrolytically.

EXPERIMENTAL RESULTS The water-splitting system used in this experiment and based on the above cycles is shown in

Fig. 1. The apparatus is composed of three subsystems, the first being the photochemical reactor in which reaction (3) takes place. This reactor faces the solar beam and is a flat cell 50 x 65 x 1 cm through which the liquid reactants flow at a space velocity of 1 .O l/hr. As indicated by the iodine color, the reactants absorb the light in the blue to ultraviolet region with a quantum yield of 2 with respect to Fe3’. If light of wavelength shorter than A, is almost completely absorbed by the reactant solution of iodine concentration C, A, = 5800 A for C= 2.5 x 10e3 mol/l. and A, = 4300 A for C = 2.5 x 10m4 mol/l. The transmittance of Fe’.‘-K solution is 50% even at 4000 A.

The efficiency of the photochemical system is exactly formulated to give, in kcal/mol,

I AC

AFm @[l -exp {-K(i)cl}]g(h)A dh

ho VP’ = 2.86 x 10’

c *< (4)

g(A) dA

where AO is the shortest wavelength of the sun beam arriving at the cell surface, K is the absorption coefficient, and g(A) is the distribution function of the solar beam from the intensity vs wavelength plot.

By this formula, we have 77p= = 0.4 as its best value by substituting the numerical values into each parameter.

The second subsystem consists of a solar collector, thermoelectric generator and cooling fins which also serve as the electrodes in the electrolysis subsystem A Fresnel lens, with surface area A,, is attached to the back surface of the photochemical cell. This lens focuses the unabsorbed light from the photochemical cell into a heat collector of area A,. The efficiency of the collector is given by:

EU[ T; - Ta4] wo ’ (5)

(Y and E are the absorptivity and emissivity of the absorber surface respectively. WO is the solar constant, u is the Stefan-Boltzmann constant and Th and T, are the surface and ambient temperatures respectively. In a preliminary field test carried out on 24 December 1974 in

/ Solar Beam

Fresnel Lens

T. OHTA et al. 115

Yokohama, the values of the above parameters were:

Th =453K

T,=292K

a = 0.82

E = 0.08

AC/An = 20.

Using these values, Eq. (5) yields: qsc = 0.81. The absorbed heat is converted into electrical energy by the thermoelectric device. The

efficiency of this conversion under a constant heat flux has been discussed by Castro et al. [6]. The maximum efficiency of the device is obtained when m, the ratio of the electrical resistance of the thermoelectrical system, r, to that of the electrolysis system is the solution of the equation:

,m-n-1][2m’+(l-2n)m+n+1]-2ZWo*=0. K

Where: n=ZT,

z = s/Kr

Wo* = A, Wo

S = the thermoelectric power of the thermocouple

K = the thermal conductance

T, = the temperature of the electrodes immersed in the electrolytic solution.

Using m,, as the optimum value of m from the solution of Eq. (6), the maximum efficiency is given by:

2mo(mo-n- 1) ~rh=2mo~+(1-2n)mo+n+1~ (7)

Equation (7) is valid for one P-N junction and will be somewhat different for a real system of multiconnected thermoelements. The thermoelement used in this work was a Bi-Te-Sb alloy with a figure of merit (equal to Z if the size of the element is properly designed) equal to 1.89~ IO-‘“C-‘. The real efficiency was about 5%. This value is considered to be very low However, unconverted heat will heat the electrodes as well as the solution to reduce the overvoltage of the electrolysis to a reasonable level. A tentatively constructed thermoelectric module composed of 32 thermocouples gives an open circuit voltage of 2.0 V for a AT of 150°C.

The third subsystem is composed of two electrolyzers. The products of the photochemical reaction flow into the first electrolyzer which is divided into three compartments, the electrodes being in the first and third compartments. The reactions occurring in each chamber are given in Eq. (8.1)-(8.3).

2Fe3’+61- +2e-+2FeZ’+61- (8.1) 2H’ + 21. -+ 2HI (8.2)

HzO+2H’+:Oz+2e. (8.3)

The products 2FeZ + + 41- of the reaction in the first chamber are returned to the photochemical system and the species 21- migrates into the second compartment through an anion selective membrane. Oxygen is evolved from the third compartment and the 2H’ species migrates through a cation selective membrane into compartment two where it combines with the 21- to form 2HI. The hydrogen iodide is carried into the second electrolyzer for decomposition.

116 PHOTOCHEMICAL AND THERMOELECTRIC UTILIZATION

The voltage necessary for this first electrolyzer is given by:

(0.459-0.059 pH + E, + Eo} V (9)

Eo is the overvoltage [7], and EC is a function of the concentration of each species as given by the Nernst’s equation.

In the second electrolyzer, the reaction:

2HI-+Hz+ 12 (10)

is carried out by an applied voltage of:

(0.536-0.059 pH+E,‘+Eo’} V (11)

For this relationship, the overvoltage, Eo’ is found to be very small. Reduction of the overvoltage is critical to improvement of the efficiency of the system. In this experiment, EO + EO is as high as 0.6 V using Pt electrodes. The overall efficiency of our system can be expressed by

?u=~{wpnpc+(wu-- W,,)q,=}, (12)

where W, is the part of W, used by the photochemical reaction, and v,~ is equal to nth multiplied by the efficiency of the electrolysis system. n0 is estimated to be 15-25% theoretically. However, a production rate of only one liter per hour of hydrogen was achieved in the 24 December 1974 field test. The causes of this low production rate are considered to be: (1) The incident solar energy was small (lower than 0.6 kW/m’, the exact value was not measured). (2) Power loss by the contact voltage in the thermoelectric system. (3) Heat leak via convection in solar collector system. (4) Mismatching of resistance between thermoelectric and electrolysis systems. (5) The design of the electrolyzers was in error and gave low efficiency for the electrolysis system.

In our new equipment under construction, we expect an improved efficiency up to the ideal value.

Acknowledgemen&--The present work has been performed under the auspices of a Scientific Research Grant of the Ministry of Education and the Sunshine Project of the Ministry of International Trade and Industry. Gratitudes are due to these authorities and to Messrs. Y. Takagi, T. Sakaki, Y. Hiramatsu, K. Kimura, H. Honma, and Dr. K. Uemura of Komatsu Electronics Co., Ltd. for their assistance.

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REFERENCES A. M. ZAREM & D. D. ERWAY (Editors) Introduction to the Utilization of Solar Energy, McGraw-Hill, New York (1963). .I. O’M. BOCKRIS, Science 176, 1323 (1972); W. E. WINSCHE, K. C. HOFFMAN & F. J. SALZANO, ibid. 180, 1325 (1973); T. N. VEZIROGLU (Editor) Hydrogen Energy (Part A, B), Plenum Press, New York (1975). R. H. WENTARF, Jr. & R. E. HANNEMAN, Science 185, 311 (1974); Relevant works are cited in this literature. E. J. RABINOWITCH, J. Chem. Phys. 8, 551, 560 (1940); T. OHTA & N. KAMIYA, Proc. 9th ZECEC, p. 317 (1974). J. A. DEAN (Editor) Lange’s Handbook of Chemistry, 7th Edn., Section 9. McGraw-Hill, New York (1973). R. R. HEIKS & R. W. URE (Editor) Thermoelectricity. Interscience, New York (1961). T. OHTA & N. KAMIYA, Proc. f&h ZECEC (1973, p. 772. Japanese patents pending.