A Solar—Hydrogen Energy System

The first right of a reader is to learn what a book is all about. Therefore, let it be said that the term “hydrogen economy” refers to an energy system in which all energy sources are used to produce hydrogen (H₂); this hydrogen is then collected, stored, and distributed as a completely pollution-free, multipurpose fuel. In the case of a “solar-hydrogen economy,” at least some of the primary energy that produces this hydrogen comes from direct sunlight. This precise though nonspecific definition will be illustrated and discussed in great detail in the following chapters.

A rather obvious but long-ignored fact is that the supplies of raw materials and fossil fuels such as coal, natural gas, and oil on our planet are limited. For decades, it was believed that these deposits were so enormous that in the foreseeable future, we would not have to fear the exhaustion of those known deposits or of the resources not yet economically exploitable. Recently, however, since the report of the Club of Rome and in particular the diagrams published by the Meadows on the limits of growth, this viewpoint is no longer defensible.1 Because of the recent uncontrolled exponential growth of the world’s population, and the additional growth of energy demands due to increasing living standards per capita of population, exhaustion of known ore deposits and fossil fuels can be expected in the foreseeable future (see Table 2.1). The scenarios of the Meadows indicate a decade of breakdown, somewhere between 2020 and 2070, in which the per capita amount of food available for the world population will undergo sharp (>10% per year) cutbacks and the production-time curve will have passed through its maximum.

Following the discussion of the need for a transition to a hydrogen technology in Chapter 2, this chapter will present material that leads to an estimate of the time needed for the introduction and buildup of a hydrogen economy. Material concerning energy supplies that are available at present and the rate of use of these supplies, together with some prognoses about the exhaustion of fossil fuels, forms the basis for this presentation.

According to the First Law of Thermodynamics—the energy principle—one can neither destroy nor produce energy in a closed system, but only convert it from one form to another. The First Law is also sometimes called the Law of the Impossibility of Perpetual Motion. The First Law is exactly valid down to particles of the smallest known dimensions, and so universal that patent authorities always reject inventions if they claim to concern the production (rather than the conversion) of energy. Radioactive energy is not a contradiction of the First Law, because when it seems that a certain amount of radioactive energy is produced, there is in fact a small amount of mass destroyed, and the two are related by the well-known Hasenöhrl-Einstein equation: E = mc ².

Concern about the future of the world’s energy supply has sharpened awareness of the difference between nonrenewable and renewable sources of energy. Sources of water power are inexhaustible and not harmful to the environment. They are renewed by the cycle of evaporation and rain, are driven by the sun, and will soon be in use on all continents. Wind energy also originates in solar energy and is therefore inexhaustible and—if it could be efficiently utilized—sufficient to meet the needs of all mankind some 30 times over. It is all the more unfortunate, therefore, that the geographic distribution and sporadic nature of wind energy make it a difficult source to use economically. Tides and geothermal heat, even though they do not arise from solar energy, are virtually inexhaustible, but are of restricted use because of their limited distribution.

From the earlier material concerning photovoltaic cells as direct energy converters of light to electricity (Chapter 4), the physical processes that take place in them, and the electrical engineering associated with their use (Chapter 5), it can be concluded that we are dealing with simple light-to-electricity converters that have no mechanically movable parts and do not require the use of chemicals. These cells work with an efficiency as high as 20%—a relatively high degree of efficiency —and it can be seen at once that the high concentration of silicon in the earth’s crust could permit the mass production of silicon, the preferred semiconductor. The single difficulty in this picture is the (present) investment cost which is around $5000/kW_el, which is still somewhat too high as far as the terrestrial production of electricity from solar energy is concerned. The price of silicon solar cells currently in use would have to be reduced by a factor of 3–5 to allow a rapid and complete breakthrough in solar electricity. It is easy to see the direction in which development must proceed. Much of the present cost goes into the preparation of superpure perfect single crystals. In this direction, it seems that efforts in the direction of lowering cost have become asymptotic, so that future efforts to achieve a reduction in cost should be concentrated on the production of less perfect silicon crystals. It is probable that such material would have a somewhat lower efficiency of conversion, but this is acceptable so long as the reduction in cost of the starting material is greater than the reduction in efficiency.

Solar energy arrives on the earth at the rate of 170 trillion (10¹²) kilowatts. It can be converted to useful energy not only along solar thermal or solar electric paths, but also along the solar chemical pathways. From a long-term point of view, this option could be the most important. Its goal is the photochemical decomposition of water to hydrogen and oxygen, after which the hydrogen could be collected and utilized in a hydrogen economy. It may well be that hydrogen can be produced from solar energy by indirect paths—namely, by first producing heat or electricity and then later utilizing these to produce hydrogen—yet the direct decomposition (photolysis) of water is the most immediate way and therefore fundamentally the best one.

The basis of a modern hydrogen technology will be that process for the production of hydrogen which is the most favorable from both the energetic and economic points of view. A very adequate survey of the various methods of producing hydrogen has been given by Justi.¹

The vast energy sources of the future will originate far away from where they are used; because of climatic considerations, solar and OTEC (ocean thermal energy conversion) power stations will have to be located in relatively circumscribed areas of the world. The world’s remaining coal reserves are largely in Siberia and Alaska, and nuclear power plants cannot be put in cities because of the danger they present and because of their enormous requirements for cooling water.

The comparative capital and operating costs for the transmission of hydrogen over great distances and those for the transmission of the same amount of energy through high-tension power lines are so important to the objective assessment of the transition to a hydrogen economy that they should be subjected to inquiry not only by academic scientists, but also by the planning office of a uniquely qualified company. For this reason, Justi requested Messer-Griesheim GmbH, a Düsseldorf company concerned with the transmission of gases, to undertake a detailed project concerning the investment and operating costs of a hydrogen pipeline 2000 km long and having an annual throughput of 1 × 10¹⁰ Nm³(= 10 GNm³/year). This chapter, which discusses a project for transporting hydrogen over great distances and storing it in the pipeline, is not science fiction, but an account of a proposed design by the most experienced German company in this area.

The main problem in the use of solar energy is the daily and yearly variations, which are not in phase with the variations of energy demand. Therefore, for widespread application of solar energy, it is necessary to store energy collected during “on” times in the form of hydrogen.

For acceptance of the secondary energy carrier hydrogen, which in the future will be introduced on a large scale, knowledge of the safety aspects is of considerable importance. The safety and the dangers in dealing with hydrogen are determined by its physical and chemical properties. In this chapter, the characteristics of hydrogen that are relevant for safety measures are presented and compared with those of other energy carriers so that the risks in dealing with gaseous and liquid hydrogen can be estimated.

In a solar-hydrogen economy, it is likely that hydrogen will be arriving from distant sites which receive plentiful sunlight. The hydrogen will arrive either in pipelines as a gas or in tankers as a liquid, and will then be converted at the final site into mechanical or electrical energy.

A considerable part of present energy consumption occurs in the production of useful or process heat. Over 70% of energy use in the Federal Republic of Germany occurs in the production of heat that is then used by households, small-scale consumers, and industry.¹ If the energy supply were converted increasingly to gaseous fuel and consumers were over time supplied with a fuel mix in which the hydrogen content increased, the market for heat would represent a considerable potential for the introduction of a hydrogen technology. However, substitution of hydrogen for oil, natural gas, and coal would necessitate changing the types of burners to those suitable for the technical needs of hydrogen combustion—an added capital cost that would have to be faced.

Hydrogen is already in use as an important raw material in chemical industry and particularly in the petrochemical industry. Its various applications have been known and investigated for a considerable period of time.^1–3 Hydrogen for industrial use is produced almost exclusively from the fossil fuels: natural gas, oil, and coal, which will therefore be depleted sooner and become more costly as time goes on. It is as true of the fossil fuels used as raw materials in the chemical industry and the petrochemical industry as it is of those used in other applications: they cannot be replaced. In the long term, therefore, a distinction must be made between the use of fossil fuels as sources of heat energy in combustion and their use as raw materials to produce hydrogen and other chemicals. It may well be that industrial needs for very large quantities of hydrogen will give rise to the beginning of a comprehensive new hydrogen technology.

Oil and oil-related products such as gasoline and diesel oil are used to power cars and vehicles of all kinds because of their high energy density and ease of storage. Our standard of living depends in great measure on the automobile. It is further dependent on the mass transport of goods by water, rail, highway, and air; it also depends on the use of machines in agriculture and the resulting high rate of utilization of arable land. The fact that the exhaustion of the world’s supply of oil is now in sight makes it necessary to develop new fuels or modes of transportation if we are to maintain our standard of living. Even without the imminent depletion of our oil supply, however, the increasing air pollution from automobiles, particularly carbon monoxide and nitrogen oxides in the atmosphere (as well as other polluting products of the combustion of gasoline), forces us to give thought to developing other fuels for transportation.