Summary: Fossil fuels (i.e., petroleum, natural gas and coal), which meet most of the world’s energy demand today, are being depleted rapidly. Also, their combustion products are causing global problems, such as the greenhouse effect, ozone layer depletion, acid rains and pollution, which are posing great danger for our environment, and eventually, for the total life on our planet. Many engineers and scientists agree that the solution to all of these global problems would be to replace the existing fossil fuel system with the Hydrogen Energy System. Hydrogen is a very efficient and clean fuel. Its combustion will produce no greenhouse gases, no ozone layer depleting chemicals, and little or no acid rain ingredients and pollution. Hydrogen, produced from renewable energy (solar) sources, would result in a permanent energy system which we would never have to change. However, there are other energy systems proposed for the post-petroleum era, such as a synthetic fossil fuel system. In this system, synthetic gasoline and synthetic natural gas will be produced using abundant deposits of coal. In a way, this will ensure the continuation of the present fossil fuel system. The two possible energy systems for the post-fossil fuel era (i.e., solar hydrogen energy system and synthetic fossil fuel system) are compared with the present fossil fuel system by taking into consideration production costs, environmental damages and utilization efficiencies. The results indicate that the solar hydrogen energy system is the best energy system to ascertain a sustainable future. The paper is presented in a question/answer format. Question 1: What are the energy related global problems: Answer 1: Soon after the invention of the steam engine in the 1860’s, when the Industrial revolution started to replace human and beast toil with nature’s energy sources, a bright future seemed to be certain for humankind. More and more of nature’s energy, initially in the form of wood and coal, and later as oil and natural gas, were being harnessed for the benefit of humans. This resulted in the mass production of goods, with a corresponding reduction in prices and rising living standards. Communities asked for factories, railroad, highways, seaports and airports. These meant more jobs, income, goods and services. The world’s standard of living was rising. When the Industrial revolution started, the annual gross world product per capita was in tens of dollars; today, it is $6,600 and rising exponentially. Fossil fuels, which fed this amazing economic growth, were the medicine to cure deprivation. But it was an untested medicine at that. As planet Earth consumed more and more fossil fuels, two important predicaments started to emerge: (1) the fossil fuels would be depleted in a foreseeable future, and (2) the fossil fuels and their combustion products were causing global environmental problems. Depletion of Fossil Fuels The demand for energy continues to rise because of two main reasons: (a) the continuing increase in world population, and (b) the growing demand by the developing countries in order to improve their living standards. At the present time, a large portion (about 70%) of the world energy demand is met by the fluid fossil fuels (i.e., petroleum and natural gas) because of their availability and convenient use. However it is expected that the world fluid fossil fuel production will soon peak, and thereafter begin to decrease [1-4]. Figure 1 shows estimates of the production rates of the fossil fuels and the world demand. It can be seen that the fluid fossil fuel (petroleum and natural gas) production worldwide will continue to rise for the next 15 years, and then will start to decrease. The coal production, du to environmental reason, is expected to remain nearly constant for the next decade, and then start to decrease. In the meantime, as a result of the growing world population and the desire of the people to better their living standards, the world demand for fluid fuels is rising (Fig. 1). It is expected that the world population growth (which is 5.88 billion at the moment and rising at a rate of 1.55 per year) will slow down and reach about ten to twelve billion by the end of the next century [5]. Consequently, the world demand for fluid fuels will slow down and reach around 1.6 x 1012 GJ per year. There will be a growing gap, starting within the next ten years, between the demand and production of fluid fuels. Numbers in square brackets refer to references listed at the end of the paper. "1 GJ of energy is approximately equal to the energy contained in 3.4 gallons of petroleum Figure 1. Estimates of World Fossil Fuel Production. Environmental Damage The second predicament involving the fossil fuels is the environmental damage being caused by the fossil fuels and their combustion products. Technologies for fossil fuel extraction, transportation, processing and particularly their end use (combustion), have harmful impact on the environment, which cause direct and indirect negative effects on the economy. Excavation of coal devastates the land, which has to be reclaimed and is out of use for several years. During the extraction, transportation and storage of oil and gas, spills and leakages occur, which cause water and air pollution. Refining processes also have an environmental impact. However, most of the fossil fuel environmental impact occurs during the end use. The end use of all fossil fuels is combustion, irrespective of the final purpose (i.e., heating, electricity production or motive power for transportation). The main constituents of fossil fuels are carbon and hydrogen, but also some other ingredients, which are originally in the fuel (e.g., sulfur), or are added during refining (e.g., lead, alcohols). Combustion of the fossil fuels produces various gases which are all released into the atmosphere and cause air pollution. Air pollution may be defined as the presence of some gases and particulates which are not a natural constituent of the atmosphere, or even the presence of the natural constituents in an abnormal concentration. Air pollution causes damage to human health, animals, crops, structures, reduces visibility, etc. Once in the atmosphere, triggered by sunlight or by mixing with water and other atmospheric compounds, these primary pollutants may undergo chemical reactions, change their form and become secondary pollutants, like ozone, aerosols, peroxyacyl nitrates, various acids, etc. Precipitation of sulfur and nitrogen oxides, which have dissolved in clouds and in rain droplets to form sulfuric and nitric acids is called acid rain; but also acid dew, acid fog and acid snow have been recorded. Carbon dioxide in equilibrium with water produces weak carbonic acid. Acid deposition (wet or dry) causes soil and water acidification, resulting in damages to the aquatic and terrestrial ecosystems, affecting humans, animals vegetation and structures. The remaining products of combustion in the atmosphere, mainly carbon dioxide, together with other so-called greenhouse gases (methane, nitrogen oxides and chlorofluorocarbons), result in thermal changes by absorbing the infrared energy the earth radiates into the atmosphere, and by re-radiating some back to earth, causing global temperatures to increase. The effects of the temperature increase are melting of the ice caps, sea level rise and climate changes, which includes heat waves, droughts, floods, stronger storms, more wildfires, etc. Using the studies of scores of environmental scientists, the above stated damages have been calculated for each of the fossil fuels[6]. Table 1 presents the results for each type of damage, in 1998, U. S. dollars. It can be seen that the environmental damage for coal in 1998 is $14.51 per GJ of coal consumed; for petroleum in 1998, $12.52 per GJ of petroleum consumed; for natural gas in 1998, $8.26 per GJ of natural gas consumed, and the weighted mean damage in the world is 1998, $12.05 per GJ of fossil fuel consumption. These damage costs are not included in the prices of fossil fuels, but are paid for by the people, directly or indirectly, through taxes, health expenditures, insurance premiums, and through a reduced quality of living. In other words, today fossil fuels are heavily subsidized. If the respective environmental damages were included in the fossil fuel prices, it would force earlier introduction of cleaner fuels, such as hydrogen, with many benefits to the economy and the environment. In order to see the worldwide dimensions of the fossil fuel environmental damage, Table II has been prepared. It can be seen that 37% of the total damage is caused by coal while the coal consumption is 31% of the total fossil fuel consumption. On the other hand, only 20% of the damage is caused by natural gas, which has a market share of 29%. It is clear that increasing the natural gas consumption, at the expense of coal and petroleum, would be environmentally beneficial. This would also prepare the way for greater public acceptance of gaseous fuels, which would result in a smoother changeover to hydrogen, also a gaseous fuel. Question 2: What is the Hydrogen Energy System? Answer 2: Because of the foregoing, energy researchers are looking at the possible sources of energy to replace the fossil fuels. There are quite a number of primary energy sources available, such as thermonuclear energy, nuclear breeders, solar energy, wind energy, hydropower, geothermal energy, ocean currents, tides and waves. At the consumer, end, about one-quarter of the primary energy is used as electricity and three-quarters as fuel. The above mentioned primary energy sources must, therefore, be converted to these energy carriers needed by the consumer. In contrast with the fossil fuels, none of the new primary energy sources can be used directly as a fuel, e.g., for air transportation, land transportation. Consequently, they must be used to manufacture a fuel or fuels, as well as to generate electricity. Since we need to manufacture a fuel for the post fossil fuel era, we are in a position to select the best possible fuel. There are many candidates, such as synthetic gasoline, synthetic natural gas (methane), methanol, ethanol and hydrogen. The fuel of choice must satisfy the following conditions [7]: It must be convenient fuel for transportation. It must be versatile or convert with ease to other energy forms at the user end. It must have high utilization efficiency. It must be safe to use. In addition, the resulting energy system must be environmentally compatible and economical. Transportation Fuel Surface vehicles and airplanes must carry their fuel for a certain distance before replenishing their fuel supply. In the case of space transportation, the space vehicles must carry their fuel, as well as the oxidant necessary for their scheduled range. Therefore, ti is important that the transportation fuel be as light as possible and also take as little space as possible. We can combine these requirements in a dimensionless number, termed the motivity factor [8]: (1) where E is the energy generated by the fuel, M the mass of the fuel, V the volume of the fuel, and subscript h refers to hydrogen. The higher the motivity factor, the better the fuel for transportation. Table III lists the pertinent properties of some fuels, as well as the motivity factors calculated using Equation (1). It can be seen that among the liquid fuels, LH2 has the best motivity factor, while methanol has the lowest motivity factor. Among the gaseous fuels, GH2 has the best motivity factor. Consideration of the utilization efficiency advantage of hydrogen further improves hydrogen’s standing as the best transportation fuel. Of course, this is one of the reasons why hydrogen is the fuel of choice for the space programs around the world, even though presently, it is more expensive than fossil fuels. Versatility At the user end, all fuels must be converted through a process (such as combustion) to other forms of energy, e.g., thermal energy, mechanical energy and electrical energy. If a fuel can be converted through more than one process to various forms of energy at the user end, it becomes more versatile and more convenient to utilize. Table IV lists various fuels and processes by which they can be converted to other forms of energy at the user end. It can be seen that all the fuels, except hydrogen, can be converted through one process only; that of combustion. Hydrogen, however, can be converted to other forms of energy in five different ways; i.e., in addition to flame combustion, it can be converted directly to steam, converted to heat through catalytic combustion, act as a heat source and/or heat sink through chemical reactions, and converted directly to electricity through electrochemical processes [9]. In other words, hydrogen is the most versatile fuel. by, HYDROGEN ENERGY SYSTEM: A PERMANENT SOLUTION TO GLOBAL PROBLEMS T. Nejat Veziroglu Clean Energy Research Institute University of Miami, Coral Gables, FL 33124, USA http://www.iahe.org/