Solar-Hydrogen Energy Systems: An Authoritative Review of Water-Splitting Systems by Solar Beam and Solar Heat: Hydrogen Production, Storage and Utilisation

By Tokio Ohta

Solar-Hydrogen Energy Systems is a collection of papers that discusses the advancements in the research of alternative energy technologies that utilizes solar-hydrogen energy systems. The text first introduces the concept of solar-hydrogen energy system, and then proceeds to covering the technical topics in the subsequent chapters. The next chapters talks about the thermodynamics of water-splitting and water electrolysis. Next, the selection details direct thermal decomposition of water. The selection also discusses different processes to produce hydrogen, such as thermochemical, photochemical, and biochemical. The ninth chapter talks about solar energy storage by metal hydride, and the last chapter deals with direct solar energy conversion at sea. The book will be of great interest to scientists, engineers, and technicians involved in the research, development, and implementation of alternative energy technology.

• Language: English
• Publisher: Elsevier Science
• Release date: Oct 22, 2013
• ISBN: 9781483188423

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PREFACE

Photosynthesis is the process fundamental to all life on the Earth. Plant-roots absorb water from the soil and the chlorophyll splits it into hydrogen and oxygen using solar energy. The oxygen is released to the atmosphere while the hydrogen is combined with carbon dioxide extracted by the plant from the atmosphere producing carbohydrates, the basic substance of plants. And, of course, plants provide the food source directly or indirectly for all living organisms. When plant material is combusted for energy purposes, or metabolized otherwise, water and carbon dioxide are created and usually released to the atmosphere, thus completing a natural cycle.

But with more than 4 billion people living on the Earth and demanding a reasonable or high quality of living, we have seen a dramatic scale-up of supporting industrial developments. Often this scale-up proceeds in almost a run-away manner. Various adverse effects have accrued which interrupt or counter this natural cycle, e.g., environmental pollution.

More significantly, the consumption of valued resources of traditional kinds, those that are maldistributed on the Earth produces an unstable supply system. The resulting stresses on the techno-economic operation of many nations, especially highly developed internationally-tied countries has been dramatic. The resulting impacts on internal affairs and international relationships have escalated markedly in recent years.

It is apparent that the ultimate remedy to the basic problems we now face in this regard is to develop those technologies which, in essence, accelerate the natural cycles. For example, we must learn to produce hydrogen and oxygen from water using solar energy processes. We might also fix atmospheric carbon, but at the present stage of consideration, hydrogen production seems more fundamental.

To be more specific, the strong sunlight falling upon the tropical zone (ocean and desert) must be used to split seawater or underground water to produce hydrogen and oxygen. Hydrogen is a clean, efficient fuel which can be used, for example, to power aircraft. It is also an important, even basic, chemical intermediary for the production of fertilizers and commodities of high market value. Thus the energy, the water and the food can all be provided by this system.

–– From my thesis Technologies Today and Tomorrow published in International Journal of Hydrogen Energy, Vol. 1, p. 241, (1976) ––

The feasibility study on such hydrogen energy systems has been undertaken since the autumn of 1972 in Japan. As one of the working groups, the small round-table conferences had been frequently held by the Japan Society for the Promotion of Science in 1973 and 1974. Since then, the main participated scientists have been organized as Academic Association for Hydrogen Energy (A.A.H.E.) and which has continued to study this issue to date.

This book contains selected papers presented at the A.A.H.E.-conferences. Most of them are concerned with How to split water by sunlight. Along with them, the long period storage of solar energy using metal hydrides and a system-level concept paper on direct solar energy conversion at sea are included. These two chapters address the potentially important subjects of using desert- and ocean-based solar energy conversion facilities.

I believe that this book does contain an up-to-date and top level contents. It is my sincere hope that the readers looking for new areas to enter professionally may find some in this book and join this interdisciplinary field.

March 1979

Tokio Ohta,     Yokohama, JAPAN

CHAPTER 1

INTRODUCTION - A REVIEW OF THE SCOPE

Publisher Summary

This chapter reviews the scope of hydrogen. Hydrogen is produced in a closed cycle at a proper temperature range and then, electricity is generated at a different temperature range. Electric power system and hydrogen energy system are the two subsystems with organized links. Hydrogen is used as an important chemical raw material for various substances as well as it is an excellent energy medium. Air is composed of 79% of nitrogen, 21% of oxygen, 0.034% of carbon dioxide, and small amounts of other gases. If hydrogen is released into atmosphere and it reacts with nitrogen, this will form ammonia, which provides a raw material for fertilizer to grow food plants. This is the famous Haber-Bosch process. If hydrogen reacts with oxygen, intense energy is generated and water is produced. Plants absorb water through their roots and decompose it into oxygen and hydrogen with the aid of solar rays. The oxygen is emitted into the atmosphere, whereas the hydrogen combines with carbon dioxide gas from the surrounding air to synthesize starch under the catalytic action of chlorophyll.

1-1 SIGNIFICANCE OF SOLAR HYDROGEN ENERGY SYSTEMS

Fossil fuel can be denoted as CnHm, where n carbon atoms C are combined with m hydrogen atoms H. Coal, for instance, has a formula in which n is 1 and m is zero. In the case of hydrogen, n is zero and m is 2. A natural trend in fossil hydrogen carbon utilization is shown in Fig. 1.1. On the vertical axis is the number of carbons and on the horizontal axis the ratio of the number of hydrogen atoms to the number of carbon atoms (m/n). It is clearly noted that the trend is toward utilization of heavy oil, light oil, kerosene, naphtha, gasoline, propane, and methane in this order. On the vertical axis of the right hand side are indicated the boiling points measured in the absolute temperature of these fractions. As is known, the boiling point drops as the number of hydrogen atoms rises in generous tendency. Propane and methane, which are gases at room temperature, can be transported and stored as liquefied. This means that cryogenics has an important role to play in advanced energy systems. It should be also noted that the order of boiling points corresponds to that of per-mole heating value.

Fig. 1.1 Utilization-trends in fossil hydrocarbon as plotted n vs. m/n in CnHm and boiling temperature TB(K) [1].

There are grounds for this trend in hydrocarbon utilization. Firstly, fossil fuel with high carbon content almost always contains a high level of impurities such as sulfur and emits polluting gases upon burning. Secondly, high-carbon hydrocarbons are heavy, highly viscous and therefore difficult to handle. The thought that m/n will reach an infinitely large value at C0H2 as it keeps increasing agrees with the concept of the energy-economy taking the environmental problems into consideration.

To stabilize and optimize energy economy, at least the following two measures are basically necessary

(1) Diversification of energy sources. Besides coal, natural gas, tar sand, oil shale, and nuclear power, as much natural energy sources as possible, such as hydraulic, solar, geothermal, and oceanic power should be utilized. It is preferable to overcome the exclusive dependence upon petroleum energy by many kinds of competitive energies.

(2) Organized multiple use of secondary energy. One of the forms of clean secondary energy being supplied to users is electricity. However, losses are inherent to electric power transport. High-voltage transmission is subject to corona discharge loss and large current transmission to Joulian loss. These losses average 4∼9%. Besides, highly urbanized society in limited land space has made it impossible to erect higher voltage and larger scale electric transport facilities. Storage of large electric power is presently depending upon pumping-up power generation, which is said to have a 70% efficiency. The pumping-up system involves siting difficulties, so that the power station, consumers, and reservoirs are often located far apart; hence the cost if erecting towers in the transmission network will be enormous.

Considering these difficulties along with the fact that about 65% of end usage of energy in advanced industrial nations today is in the form of heat, one would readily realize the significance of introducing hydrogen – which is a fuel as clean as electricity and which, on combustion, produces almost only water – as another secondary energy. Diversified secondary energies must not be used independently as they are now. They should be organically linked. Figure 1.2 is a conceptual diagram of a post petroleum energy system.

Fig. 1.2 Post petroleum energy system [1].

Water is dissociated by means of natural or nuclear energy. Hydrogen is produced in a closed cycle at a proper temperature range, then electricity is generated at a different temperature range. Electric power system and hydrogen energy system are the two subsystems with organized links. There are two main links with few moving parts, quiet, labor-saving and automatic in operation. These are realized when hydrogen/air fuel cells are used to convert hydrogen to electricity. Another electric storage method that meets the above conditions is to store hydrogen produced by water electrolysis. From the above, the reader should be able to understand the importance of a solar-hydrogen energy system. This is the ideal and ultimate technological innovation in this field. It will be the most preferable form of energy utilization by human beings.

It is well-known that hydrogen is used as an important chemical raw material for various substances as well as an excellent energy medium. The basic functions of hydrogen in the atmosphere will be described below. Air is composed of 79% of nitrogen (N2), 21% of oxygen (O2), 0.034% of carbon dioxide (CO2), and small amounts of other gases. If hydrogen is released into atmosphere and reacts with nitrogen, this will form ammonia (NH3) which provides a raw material for fertilizer to grow food plants. This is the famous Haber-Bosch process. If hydrogen reacts with oxygen, intense energy is generated and water (H2O) is produced. Plants absorb water through their roots and decompose it into oxygen and hydrogen with the aid of solar rays. The oxygen is emitted into the atmosphere whereas the hydrogen combines with carbon dioxide gas from the surrounding air to synthesize starch (C6H10O5)n under the catalytic action of chlorophyll. We should know that the production of hydrogen from water by solar rays is a try imitative of plant’s organization. In view of this, there should be some microorganism which, if given hydrogen, will produce starch and protein in the dark by absorbing carbon dioxide from the air. In fact, single cell microbes of this type do exist. These are called hydrogen protein, actually a mixture of protein and starch, and could provide a future food for mankind.

What is produced from hydrogen reacted in air is shown in Fig. 1.3. The diagram indicates how important a role hydrogen plays in producing food, energy, and water, which are essential to human existence. If the hydrogen used in the system is manufactured by solar energy, the great natural system of the sun, water and air will revolve around hydrogen to support us. The science and technology of producing hydrogen from water by solar energy will become more and more important from now on.

Fig. 1.3 Products of reaction of hydrogen in air

1-2 SOLAR COLLECTORS

Catalytic action of plant’s chlorophyll does not need concentrated solar ray since it utilizes the beams with shorter wave length having higher quality of photon energy rather than heat. Solar beams with a wave length shorter than 500 nm (1 nm = 10−9m), the green light, are absorbed in plant’s organization.

However, as Fig. 1.4 shows, most of the solar beams arriving at the earth’s surface have a rather longer wave length. An outline of the energy-ratio of each color-range is shown in Table 1.1. One should notice that the energy-ratio of infra to far infra red range is more than half. This is the reason why we need solar collectors ineffective solar energy systems where solar heat is inevitably introduced. Such systems have been treated in detail [2].

Table 1.1

Energy-ratio of each color-ranges in solar beam

Fig. 1.4 Solar energy density at each wave length. Enveloped area by the curve and the horizontal line means the solar constant.

1-2-1 REFLECTIVE COLLECTION

High temperature solar furnace utilizes reflective collection method on a large scale. There are two reasons. The first is that one can collect the sunshine falling upon a wide area by heliostats. The second is that the reflective collection can be done more economically than refractive method because the structure of large lens-systems is expensive. It is a defect of the reflective system that collected energy flux density is sensitive to the reflective index, nevertheless the reflector surface is apt to get dirty.

An ideal concentrator is a parabolic mirror which is defined as a surface made by the revolution of parabola. Diameter of solar image at the focus of an ideal parabolic mirror whose focal distance is f is given by

(1.1)

Fig. 1.6 A gutter-collector

where δ=16’ is the solid angle for the sun. Maximum solar energy flux density on the circular area with diameter d is

(1.2)

where a is the absorption coefficient of the atmosphere, R is the reflective index of the mirror surface, S=1.35kW/m² is the solar constant, and θ is the rim angle as shown in Fig. 1.5.

Fig. 1.5 Solar beam concentration by an ideal parabolic mirror.

To design a parabolic mirror, it is noted that the ratio of the mirror diameter D to the forcal distance f, D/f, should be less than 4 because of technical difficulties in manufacture of the mirror. All the existing large solar reflectors have values of the ratio from 2.2 to 3.1.

Utilization of the parabolic mirror as defined previously needs a delicate 2-axis tracking mechanism. Simpler tracking can be realized by using a two dimensional para-boloid mirror – a gutter mirror. Concentration ratio of such gutter-collector is defined as the ratio of aperture to absorber area and is given by

(1.3)

The concentration ratio reaches a maximum when the relationship

(1.4)

is satisfied.

Figure 1.7 [3] shows the relationship between the temperature and the concentration ratio. The lower limit and the region for practical design represent the collector efficiency of 40–60%.

Fig. 1.7 Temperature and concentration ratio [2].

1-2-2 REFRACTIVE COLLECTION

The Fresnel lens is mostly used in a solar collector systems of small and medium sizes. It has merit especially in hybrid systems, an example of which is a combination with optical fiber-tube. The hybrid system is pictured in Fig. 1.8. Multiple use of this type of hybrid system will generate a temperature high enough to be used in the direct decomposition of water vapor into hydrogen and oxygen molecules. It will yield also a light source strong enough to promote the chemical reactions in dense solutions. A typical example of application of the Fresnel lens is Yokohama Mark 5 where photochemical and thermoelectric generation are combined. Details will be described in Chap. 6. Another merit of the Fresnel lens is that the collection efficiency is not so sensitive as in the case of parabolic mirror. Fresnel lenses are made of plastics and are not expensive compared to glass and certain mirror materials. One can easily get Fresnel lens as thin as 3mm and surface area of lm² which can generate a temperature higher than 1, 000K.

Fig. 1.8 A combination of Fresnel lens and tube of optical fibers.

1-2-3 FLAT PLATE COLLECTORS

Parabolic mirror and Fresnel lens systems have a spherical or a cylindrical surface which absorbs collected solar beams at the focal site. The surface of this absorber needs a special layer coated with chemical materials which is so-called selective absorbing layer. Popular flat solar collectors have a black surface with the same layer as the cylinders. In the case of flat collector, temperature of the collected heat is about 50–150°C, and it can be as high as 1, 000°C in the case of parabolic mirror and Fresnel lens

There are three kinds of selective absorbing layers.

(1) Multiple thin layers formed on a metal surface. These are metal oxides or semiconductor oxides and have been found first by Haas in 1956 [4]. Since then, many examples have been invented, a few of which are listed in Table 1.2.

Table 1.2

Selective absorbing layers of multiple layer structure. a and εegr; are absorptivity and emissivity, (st) and (sub) mean semitransparent and substratum, respectively.

(2) Absorptive and reflective double layers formed on a metal surface. Absorptive layer is coated thick enough on the substratum metal surface. The layer has a high absorptivity in solar beam region, but is transparent in the thermic rays region. On the other hand, the reflectivity of the metal surface is very high to the thermic rays. Then the complex has a selective absorbing property. Some examples are shown in Fig. 1.3.

(3) Colored surface films of metal colloid could be a third structural material but no actual application has been found yet.

Another type of plane collector is the photochemical cell, in which a photochemical reaction takes place. Photochemical cells must satisfy at least the two following condititons.

(1) Medium of reactants, the solution of some chemical substances, should not be transparent to the light from visible to ultra violet ranges. Absorption coefficient of the solution α(λ) is important.

(2) In order to design the most efficient photochemical cell, one must choose the relationship between the thickness δ of the cell and the initial activity (or concentration) of key chemical substance C as given by [14]

(1.5)

is an average of the absorption coefficient over the solar spectrum given by

(1.6)

f(λ) in Eq. (1.6) is the solar energy density distribution curve as shown in Fig. 1.4, λo and λc are the shortest wave of the solar beam arriving on the earth’s surface and λc is the longest wave length of the solar light available to operate the photochemical reaction, respectively.

Photochemical cells are constructed by glass plates, so the optical property of the glass becomes important. If almost all of the ultra violet rays incident to the cell are unable to pass through the cover glass plate, the function of the cell is greatly lowered. Ordinary commercial sheet glass with thickness of 2mm passes more than 82% of solar ray whose wave lengths are shorter than 350nm, which is 46% of those shorter than 330nm. Special quartz glass is transparent to light with wave length shorter than 200nm. One must also pay attention to the reflection loss of about 4%/mm about the thickness. An example of photochemical cells applied to a water-splitting system will be introduced in Chap. 6.

Table 1.3

Selective absorbing layers of double layer structure

1-3 HYDROGEN PRODUCTION BY SOLAR ENERGY

Although the present book will treats those subjects concerned mainly with how to split water by solar energy, to be emphasized are these selected five methods, electrolysis, direct thermal, thermochemical, and photoelectrochemical. All of these are believed to be more promising and are interesting from the scientific aspects. A brief review has already been published by Ohta and Veziroglu[15], in which these selected subjects are given initial treatment. However, there are other promising ways of solar production of hydrogen by indirect methods which mean that solar energy generates electric power then it electrolyzes water to evolve hydrogen. Here, a review is given about every way of solar production of hydrogen, avoiding repetition.

1-3-1 DIRECT

, an energy input equal to the enthalpy change given by

(1.7)

is required. In Eq.(1.7), ΔG and ΔQ (= T ΔS, the absolute temperature times the entropy change) are the change in Gibbs’ free energy and the change in thermal energy, respectively. The detail of Eq.(1.7) will be discussed in Chap. 2. The numerical value in Eq.(1.7) is valid when water is in the liquid state of 25°C and 1 atm and the produced hydrogen and oxygen are in the gaseous state under the same conditions.

Direct methods are defined in such manner as no Gibbs’ free energy is needed or no thermal energy is necessary. The former case is called the direct thermal method and will be described in Chap. 3. There are several different methods in the latter case, an example of which is electrolysis (ΔG is provided by electrical energy) introduced in Chap. 3. Besides electrolysis, radiolysis (ΔG is given by radial rays) and photolysis (ΔG is given by photon energy, the photoelectrochemical method described in Chap. 6 belongs to this category) are