Solar Hydrogen

Fuel of the Future

By Mario Pagliaro, Athanasios G. Konstandopoulos

The Royal Society of Chemistry

Copyright © 2012 Mario Pagliaro and Athanasios G. Konstandopoulos
All rights reserved.
ISBN: 978-1-84973-195-9

Contents

About the Authors, xvii,
Chapter 1 Hydrogen and Solar Hydrogen, 1,
Chapter 2 Water Electrolysis with Solar Electricity, 40,
Chapter 3 Thermochemical Water Splitting, 82,
Chapter 4 Solar Hydrogen Utilization, 119,
Subject Index, 155,

CHAPTER 1

Hydrogen and Solar Hydrogen

1.1 Hydrogen: Structure and Properties

First produced by Robert Boyle in 1671 by reacting mineral acids with iron, hydrogen was recognized as a discrete substance by Henry Cavendish in 1766. Cavendish named the gas “flammable air” and further reported in 1781 that it produced water when burned. In 1783, Lavoisier reproduced Cavendish’s findings, formulating the mass conservation law, and named the element “hydrogen” from the Greek for “water-creator”.

At standard temperature and pressure, hydrogen is a colorless, odorless and non-toxic diatomic gas with the molecular formula H2. It is the lightest (average atomic weight 1.007825 u for 1H) and most abundant chemical element (75% of the Universe’s elemental mass). Being highly reactive, the H2 molecule on Earth is available only in chemical compounds and its extraction from said substances requires consumption of energy.

Hydrogen gas, furthermore, has an enormous volume (Table 1.1): 1 kg of H2 at ambient temperature and atmospheric pressure has a volume of 11 m3, therefore hydrogen storage for practical energy or chemical applications basically implies a reduction in volume. This is generally accomplished by increased pressure (in gas cylinders with a maximum pressure up to 80 MPa) or by liquefaction (achieved by lowering the temperature).

We recall here that the boiling point of a pure substance increases with the applied pressure, up to a certain point. Propane, with a boiling point of -42 °C, for example, can be stored as a liquid at 21 °C under a moderate pressure of 7.7 bar. The boiling point of hydrogen can only be increased to a maximum of -240 °C, through the application of approximately 13 bar, beyond which additional pressure has no beneficial effect. At standard conditions, hydrogen has a density of about 0.09 g L-1 while in liquid state its density increases to 70.8 g L-1 and its boiling point is only 20.3 K (-252.77 °C). Even as a liquid, therefore, hydrogen is not very dense. For comparison, every liter of water contains 111 kg of hydrogen, whereas a liter (cubic meter) of liquid hydrogen contains only 70.8 kg of hydrogen. Thus, water packs more mass of hydrogen per unit volume, because of its tight molecular structure, than hydrogen itself.

The phase diagram in Figure 1.1 indicates that liquid hydrogen exists only in a small region between the solid line and the line from the triple point at 21.2 K and the critical point at 32 K. This implies that once hydrogen is evaporated from liquid it is not possible to re-liquefy it by applying higher pressure, a method that works for many other gases. As shown in Table 1.1, hydrogen burns in air at a concentration in the range 4–77% by volume. The highest burning temperature of hydrogen, 2318 K, is reached at 29% concentration by volume. As little as 0.02 mJ is the minimum energy (thermal activation energy) required to ignite a stoichiometric hydrogen : oxygen mixture, which is one-tenth of the energy required to ignite a methane : oxygen mixture, for which the value is 0.29 mJ.

On a mass basis, the amount of energy produced during hydrogen combustion is higher than that released by any other fuel, with a low heating value (LHV, also known as net calorific value) 2.4, 2.8 and 4 times higher than that of methane, gasoline and coal, respectively. Hydrogen indeed reacts easily with oxygen in a highly exothermic reaction (Equation 1.1) whose huge enthalpy is –286 kJ mol-1.

2H2(g) + O2(g) -> 2H2O(l) + 572 kJ (286 kJ mol-1) (1.1)

Table 1.2 shows that hydrogen has the highest energy-to-weight ratio of any fuel because hydrogen is the lightest element and has no heavy carbon atoms.

Therefore, for a given load duty, the mass of hydrogen required is only about one-third of the mass of hydrocarbon fuel needed. It is for this reason that hydrogen has been used extensively in the space pro- gram, where weight is crucial. For decades, for example, NASA has used liquid hydrogen to power space vehicles such as the Space Shuttle (Figure 1.2), chilling H2 to near absolute zero (-252.87 °C), when hydrogen gas turns into a high-energy liquid. Compared with hydrocarbons, hydrogen gas has a good energy density by weight (33.3 kWh kg-1) but a poor energy density by volume (2.5 kWh L-1), which is about 3.5 times lower than the energy density by volume of gasoline (Table 1.3). Higher gas pressure values improve the energy density by volume, allowing the tank to be smaller, but not lighter (at least currently).

Alternatively, to increase its volumetric energy density, liquid hydrogen may be used. However liquefaction imposes large energy expenditure and the storage tanks must be well insulated to prevent boil-off.

The temperature of spontaneous ignition of hydrogen in air is high, ca. 500 °C. However, molecular hydrogen is highly flammable and will burn in air at a very wide range of concentrations, between 4% and 74% by volume, when the gas forms explosive mixtures with air. The mixtures spontaneously explode when triggered by spark, heat or sunlight. Pure hydrogen–oxygen flames emit ultraviolet light and are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle main engine (see Figure 1.2) compared with the highly visible plume of the rocket boosters of the same shuttle.

The liquefaction process, involving pressurizing and cooling steps, is energy intensive. However, Equation 1.1 shows that pure, liquid water is the unique exhaust by-product of the reaction, and this fact alone – the absence of CO2 emissions – immediately shows the enormous, benign environmental potential of the combustion of hydrogen when compared with combustion reactions employing hydrocarbons or coal.

Hydrogen indeed is an excellent fuel that can replace hydrocarbons with numerous advantages. For example, in 2009 the Italian power company Enel started operating a 12 MW H2-powered electricity plant, in the industrial zone of Porto Marghera in Venice, which is fueled uniquely by hydrogen by-products from local petrochemical industries (Figure 1.3). The turbines were specially designed to resist embrittlement caused by hydrogen, but in any case the main emission of hydrogen combustion in air is water (the formation of trace amounts of nitrogen oxides is minimized by choosing the best combustion conditions).

Moreover, hydrogen burns much more e?ciently and rapidly than gasoline, and replacement of gasoline with hydrogen fuel in automobiles is simple and fast, requiring substantial changes to the electronic and ignition systems only (Figure 1.4). Hydrogen tanks for storage can be installed in virtually any available space within the vehicle, and gaseous H2 is precisely metered into the air intake of the engine.

Protium (1H) is the most common hydrogen isotope, with an abundance of more than 99.98%. First prepared in 1932 by Urey, deuterium (2H or D, with an atomic weight of 2.0140, containing one proton and one neutron in its nucleus) is the other stable hydrogen isotope.

Given its relatively simple atomic structure, consisting only of a proton and an electron (Figure 1.5), the hydrogen atom, together with the spectrum of light produced from it or absorbed by it, has played a central role in the development of the theory of atomic structure culminating, in 1926, in the Schrödinger equation for the hydrogen atom. Similarly, the corresponding simplicity of the hydrogen molecule was instrumental in allowing a fuller understanding of the nature of the chemical bond, which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.

Differing by the relative spin of their nuclei, there are two different spin isomers of hydrogen diatomic molecules: ortho– and parahydrogen. In the orthohydrogen form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1 (½ + ½); in the parahydrogen form the spins are antiparallel and form a singlet with a molecular spin quantum number of 0 (½ – ½).

The ortho form is an excited state and has a higher energy than the para form. At standard temperature and pressure, the normal form of hydrogen gas contains about 25% of the para form and 75% of the ortho form. The ortho: para ratio in condensed H2 is an important consideration in the preparation and storage of liquid hydrogen; the con- version from ortho to para is exothermic and produces enough heat to evaporate some of the hydrogen liquid, leading to loss of liquefied material. In general, rapidly condensed H2 contains large quantities of the high-energy ortho form that converts to the para form very slowly.

Hence, during hydrogen cooling, chemical catalysts (such as ferric oxide, activated carbon, platinized asbestos, rare earth metals, uranium compounds, chromic oxide, or some nickel compounds) are employed for the ortho–para interconversion.

Molecular hydrogen is commonly used in power stations, as a coolant in generators, because its specific heat capacity is considerably higher than that of any other gas. Half a century before quantum mechanical theory was developed, Maxwell observed that the specific heat capacity of H2 unaccountably departs from that of a diatomic gas below room temperature and begins increasingly to resemble that of a monatomic gas at cryogenic temperatures. This finding is ascribed by quantum theory to the wide spacing of the (quantized) rotational energy levels that results from the low mass of the molecule. These widely spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gases composed of heavier atoms, such as N2, do not have such widely spaced levels and do not exhibit the same effect.

Hydrogen can also react with oxygen to generate electricity in a “fuel cell”, a concept that was first proposed in 1838 by Schönbein. Soon afterwards, in 1842, Grove introduced the first “gas voltaic battery”, using a design (Figure 1.6) that is remarkably similar to today’s fuel cell designs. In contrast to batteries, which store and release chemical energy as electricity until complete discharge, an H2 fuel cell produces energy by combining hydrogen with oxygen, continuing to function and produce power as long as fuel and oxygen are supplied.

Liquid water is the only reaction product so that, for instance, the toxic and polluting emissions produced by a boat’s traditional internal combustion engine, are replaced in a boat powered by a hydrogen fuel cell by the emission of a benign flow of highly pure water (Figure 1.7).

1.2 Hydrogen Production and Utilization

Hydrogen is widely employed in the chemical and petrochemical industries. The total world production of hydrogen as a chemical constituent and as an energy source was valued at US$120 billion in 2010 (Figure 1.8). The two main applications are the production of ammonia (NH3), via the Haber process, which is used directly or indirectly as fertilizer, and hydrocracking, namely converting heavy petroleum sources into lighter fractions suitable for use as fuels (and in the de-sulfurization of middle distillate diesel fuel). Furthermore, H2 is used in the manufacture of methanol and hydrochloric acid, and as a hydrogenating agent, used particularly to increase the level of saturation of unsaturated fats and oils in the oleochemicals industry.

Finally, other important hydrogen users are the space flight business, the electronics industry, and metallurgical companies that require the reduction of metallic ores. In the latter cases, smaller quantities of “merchant” hydrogen are also manufactured and delivered to end users. The economies of scale inherent to large-scale oil refining and fertilizer manufacture make possible the on-site production and “captive” use of hydrogen.

Given that both the world’s human population and the intensive agriculture used to support it are growing, the demand for ammonia is growing. The use of hydrocracking grows at an even faster rate, because rising oil prices encourage oil companies to extract poorer source material, such as tar sands and oil shale. Accordingly, the hydrogen market is expected to increase at a compound annual growth rate (CAGR) of 6.3%, to reach a value of US$163 billion in 2015 (see Figure 1.8).

Overall, around 96% of hydrogen is derived from fossil fuels and a minor fraction is produced in large electrolyzers during the electrolysis of brine. Most (49%) of the world’s hydrogen is currently produced from the steam reforming of natural gas (Figure 1.9), followed by partial oxidation of oil (29%) and coal (18%). The graphs in Figure 1.10 show that the industrial manufacturing technology (Table 1.4) is rapidly changing, because in 2000 crude oil was the dominant source (55%).

Given the huge and increasing volumes of H2 produced in the world, it is perhaps no surprise that hydrogen plants are major energy-demanding processes and important releasers of, CO2 emitting some 100 million tonnes of CO2 equivalent per year.

The current best electrolytic processes, on the other hand, have an efficiency of 50% to 80%, so that 1 kg of hydrogen (which has a specific energy of about 40 kWh kg-1) requires 50 to 79 kWh of electricity. At 8 cents per kWh, this translates into $4.00 per kg, which is, 3 to 10 times the price of hydrogen from steam reformation of natural gas produced using traditional methods.

On the other hand, low cost renewable electricity is increasingly used to manufacture H2 electrolytically, for example in the German Hydrogen Challenger ship (Figure 1.11). This vessel is equipped with vertical axis wind turbines, which harness the strong winds of the outer seas and use them to create the electricity used to electrolyze the water beneath the ship.

Unlike electricity, hydrogen can be stored in large quantities and for long periods of time. Storing energy in the form of hydrogen allows the generator to sell power only when and where the price is highest. This means that every kWh of hydrogen energy is much more valuable than a kWh of electricity.

The use of hydrogen as a fuel is still a niche market. To date the largest demand for hydrogen as a fuel has come from the United States space program. Now, however, fuel cells that use hydrogen as a unique fuel are opening up new markets for hydrogen suppliers, with potentially high demand if some key applications take off (Figure 1.12).

Fuel cell vehicles (FCVs), for instance, involve technology under development with five car makers (Daimler AG, Honda, General Motors, Hyundai and Toyota) that are currently operating the largest fleets of FCVs. According to a recent market report, commercial sales of FCVs will reach the key milestone of 1 million vehicles by 2020, with a cumulative 1.2 million vehicles sold by the end of that year.

For the same US market analysts, the entire growth of the FCV market is balancing on two key items: the growth of H2 refueling stations and the improved durability and efficiency of the fuel cells. The growth of H2 refueling stations will be boosted, we argue, by distributed solar hydrogen generation using photovoltaic electricity.

1.3 Solar Hydrogen

Hydrogen generated by water splitting induced by solar energy is the fuel of the future, which can replace fossil fuels and ultimately cease our dependence (or “addiction”, to quote a former US president) on fossil hydrocarbons and coal, thus ending the emission of CO2 into the atmosphere that causes global warming and climate change.

In other words, clean solar-based H2 technologies not only produce hydrogen but also employ entirely renewable and abundant energy sources and raw materials: solar energy and water, respectively, which produce no CO2 emissions (Figure 1.13).

In the words of Winter, a long-time advocate of hydrogen energy:

The solar water-to-hydrogen-to-water cycle is the only closed material cycle of any human energy scheme. All the others are open systems: they take something irrecoverable from the earth’s crust, convert it chemically or nuclearly, and return it to the biosphere, sometimes toxic, sometimes radioactive, and sometimes of negative environmental or climatic influence.

In the last decade, much hype has been associated with the topic of hydrogen. For example, in a widely read book, Romm has questioned the idea that hydrogen is an economically viable fuel for transportation because of its cost and the greenhouse gases generated during production, the low energy content per volume and weight of the container, the cost of the fuel cells, and the cost of the infrastructure.

In contrast to fossil fuel deposits, which are a concentrated source of high-quality energy, commonly extracted with power densities (the rate of energy production per unit of Earth’s area) of 102 or 103 W m-2 for coal or hydrocarbon fields, biomass energy production has densities well below 1 W m-2, while the density of electricity produced by photovoltaic generation is around 20 W m-2 of peak power.

(Continues…)Excerpted from Solar Hydrogen by Mario Pagliaro, Athanasios G. Konstandopoulos. Copyright © 2012 Mario Pagliaro and Athanasios G. Konstandopoulos. Excerpted by permission of The Royal Society of Chemistry.
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