Thermochemical Water Splitting

3.1 Concentrating Solar Power for Heavy Energy Demand

Solar energy technologies have the flexibility to address global power needs. Earth receives a vast amount of solar energy that is estimated to be approximately 120 000 TW (1 TW = 1012W), which vastly exceeds the current annual worldwide energy consumption rate of B15TW.1 The latter figure includes all available forms of energy from electricity to gasoline combustion and is proportional with the population growth.

For example, energy consumption in 2010 increased by 5.6% com­pared to 2009.2 Most of this power is currently produced by burning fossil fuels, namely coal, oil and natural gas (Table 3.1), which affords a clearly unsustainable situation given the limited reserves of these pri­mary sources and the rapidly growing economies of large countries such as China, India and Brazil that undergo rapid industrialization. More­over, the figures in Table 3.1 show a worrying growth in the con­sumption of coal, which is taking place not only in China and in India but also in the USA, countries where large reserves of coal exist and whose exploitation is increasingly pursued owing to the consistently high price of oil at > $90 per barrel.

Therefore, when planning the transition towards a sustainable energy future we need a good understanding of the available, scalable and long­term solutions that can be applied globally.1

In other words, we must focus on solutions that meet not only our current energy demands but will have the potential to sufficiently cover

Solar Hydrogen: Fuel of the Future Mario Pagliaro and Athanasios G. Konstandopoulos © Mario Pagliaro and Athanasios G. Konstandopoulos 2012 Published by the Royal Society of Chemistry, www. rsc. org

Table 3.1 Fossil fuels and renewable sources: share of global energy con­sumption. (Adapted from Ref. 2, with kind permission.)

Oil’s share of global energy consumption; rose by 3.1% over the year before 34% Coal’s share of global energy consumption; up by 7.6%, 29.6%

the highest since 1970

Gas’s share of global energy consumption; up by 7.4%, 24%

the biggest annual growth since 1984

Share of renewables in global energy consumption 1.8%

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Figure 3.1 Trend of platinum load in PEM fuel cells, 2006-2015, according to the United States Department of Energy.

(Reproduced from Ref. 3, with kind permission.)

the world future demand. For example, current hydrogen fuel cells and lithium-based electric batteries for cars are not sustainable because the world reserves of both lithium (for batteries) and platinum3 (for fuel cells) would be rapidly exhausted.

Assuming, with the US Department of Energy (DOE; Figure 3.1), that in 2015 the stack in fuel cells will use 0.2 g of platinum per kW, a fleet of 50 000 fuel cell vehicles (FCVs) with 80 kW stacks will demand 800 kg of platinum. Assuming that the current yearly output of General Motors, 2 980 000 cars, will be replaced with state-of-the-art FCVs, it would require 48 tonnes of platinum per year just for an automaker owning 4% of the world market in car production.

Given that the sun delivers 8000 times the present global power needs, it is rather safe to conclude that solar power is the only truly sustainable energy source. Sunlight of course is diluted: the yearly (average) solar power that reaches Earth’s surface is about 170 Wm~2. Hence, when it comes to generating enough power to cover the escalating energy demands worldwide, we must necessarily focus on simple, low-tech

image109

Chapter 3

Figure 3.2 Comparison of the useful transport energy requirements for a vehicle powered by hydrogen (left) vs. clean electricity (right).

(Reproduced from Ref. 5, with kind permission.)

solutions such as concentrating solar power (CSP) plants coupled to energy storage in energy carriers, such as H2, that can be made available anywhere and at any time thanks to effective long-term storage of solar energy.

Now, arguing that a hydrogen economy does not make sense, Bossel showed in 2006 that if we assume standard production techniques for hydrogen, the inefficiencies simply render the costs of storage and transportation too high (Figure 3.2).4 Emphasizing that hydrogen will compete with its own source of energy, i. e. with electricity from the grid, Bossel insists that we have to solve an energy problem, not an energy carrier problem.5 Low-tech CSP technology does indeed solve the energy problem, while solar hydrogen addresses the inevitable demand for effective storage of clean electricity. The CSP plants usually involve solar thermal collection via parabolic mirrors, where focused sunlight heats steam to about 600 °C to drive a turbine and generate electricity (Figure 3.3).6

When we generate energy, the arrangement of matter becomes dis­ordered, for example when steam is unavoidably heated to a high state of disorder. Ordered structures such as nanostructures or crystalline materials are not able to survive the unavoidable by-product of disorder

85

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Figure 3.3 The PS-10 solar tower plant near Seville, Spain (courtesy of Abengoa Solar). Solar energy is concentrated with heliostats to generate heat for electricity generation. A similar concept can be applied to a plant for solar hydrogen production.

(Reproduced from Wikipedia. org, with kind permission.)

when generating large quantities of energy. In other words, for heavy energy demand a high-tech solution will never give both optimal relia­bility and efficiency.1

Подпись: C
Подпись: IA Подпись: (3.1)

The CSP technology is both low-cost and low-tech and is capable of producing large amounts of energy at a tiny fraction of the surface area needed for photovoltaics (PV). Collection systems to concentrate solar energy traditionally use parabolic reflectors (with trough, tower, dish and, more recently, Fresnel’s planar optical configurations) whose flux concentration ratio C over a targeted area A at the focal plane, nor­malized with respect to the incident normal beam insolation I, is given by Equation (3.1):7

Higher concentration ratios imply lower heat losses from smaller areas and, consequently, higher attainable temperatures at the receiver. When normalized to I = 1000 W m~2 (the highest solar flux on a typical sunny day), C is often expressed in units of ‘‘suns’’. The solar flux concentration ratio typically obtained is at the level of 100, 1000, and 10 000 suns for trough, tower, and dish systems, respectively.

The first commercial CSP plants were erected in the 1980s in the Californian Mojave Desert by the Israeli company Luz Industries (Figure 3.4). These plants have a combined capacity of 354MW and today they generate enough electricity to meet the power needs of approximately 500 000 people.8

Chapter 3

image114

Figure 3.4 Nine separate trough power plants, called Solar Energy Generating Sys­tems (SEGS), were built in the 1980s in the Mojave Desert by the Israeli company Luz Industries. Synthetic oil captures this heat as the oil circu­lates through the pipe, reaching temperatures as high as 390 °C. (Reproduced from Wikipedia. org, with kind permission.)

Since then the technology has evolved considerably. For example, in modular CSP plants relatively small heliostats (movable mirrors) that use cheap linear Fresnel technology (costing between 50 and 60% of the costs of a parabolic collector per square meter)9 track the sun and focus its energy onto tower-mounted receivers using non-toxic and readily available water as the unique thermal fluid (Figure 3.5).

The focused heat converts the fed water into superheated steam that drives a turbine generator to produce electricity. The steam passes through a steam condenser, reverts back to water through cooling, and the process repeats. Europe’s first commercial solar concentrating power plant has operated smoothly since 2007, close to the Spanish city of Seville (see Figure 3.3). Overall, this CSP plant produces 11 MW of electricity, enough to power 6000 households. The plant currently operates with 624 heliostats, which concentrate the solar radiation on a thermal receiver located on a tower at a height of 115 m. The receiver converts the thermal energy into steam, which drives turbines that produce electricity.

image115

The best location for solar thermal power plants is the Earth’s Sun Belt (Figure 3.6) because this is where the sun shines most frequently and where radiation is most intense. The CSP plants located within regions

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Figure 3.7 Interconnected to the Californian grid, the 5 MW Sierra Sun Tower plant, built by eSolar in 2009, is the only commercial CSP tower facility in North America.

(Reproduced from esolar. com, with kind permission.)

of the Sun Belt have higher potential to store solar energy more effi­ciently, either as thermal power or by converting it into chemical fuels (solar fuels).10

These systems may use a variety of different field designs (heliostat allocation), depending on the location of the solar plant, the geo­graphical characteristics of the land, the size of the heliostats, etc., to control the concentration of solar radiation on a relatively small area, the face of the absorber (Figure 3.7), which in the case of solar tower facilities for power generation may develop temperatures ranging from 200 to 1000 °C.

Future projections for industrial CSP facilities generating electricity estimate that they will have the same cost as coal-, gas-, and oil-fired power plants in less than 15 years for ‘‘midload’’ electricity, i. e. in the middle ranges of cost and demand.11 By the end of 2011, 850 MW of solar-thermal capacity will be installed in Spain alone, and ground­breaking for the construction of 2500 GW of CSP plants will have occurred in the USA. In total, project pipelines could represent 7000 MW of generating capacity worldwide. An overview of state-of – the art of technologies for solar thermal power production and fuel production has been published recently.12

Using some conservative assumptions and simple calculations, Abbott has shown that a total desert surface area of 500 km x 500 km can supply the whole world’s energy needs.1 If the world’s power

requirement is P = 15 TW, then the solar farm footprint area is (Equation 3.2):

A = (3.2)

1ZaZgZeZl Zb

Abbott assumed the employment of a solar Stirling dish of the type shown in Figure 3.8 but, of course, similar conclusions could be reached assuming the use of any other recent CSP technology.

The average insolation of a desert is conservatively set at I = 300Wm~2; za = 0.54 is the area fill factor of 10m diameter dishes each occupying a plot of 12 m x 12 m to allow room for maintenance vehicles and cleaning equipment; zg = 0.3 is the efficiency of the electricity pro­duction from a Stirling engine driven generator; ze = 0.5 is the efficiency for electrolytically generating hydrogen; zi = 0.7 is the efficiency to liquefy all the hydrogen; and zb = 0.5 the efficiency of storage and transportation of hydrogen.

With all this taken into account, Equation (3.2) leads to an area of 1.76 x 1012m2, which is equivalent to a plot of size of 1330 km x 1330 km. In reality, with less pessimistic assumptions, the total area required equates to 500 km x 500 km only.1

As shown by Figure 3.6, many parts of the world’s Sun Belt have hot desert regions ideal for hosting efficient solar CSP plants. Australia,

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Figure 3.8 A Stirling solar dish, manufactured until 2011 by Stirling Energy Systems in Arizona.

(Reproduced from Wikipedia. org, with kind permission.)

China and the USA, for example, all have expansive stable dry deserts and could potentially supply power exceeding the whole world’s energy needs. However, it will be far more economical in terms of energy dis­tribution to have these solar farms widely distributed throughout the world. Solar dish farms around 4 x 4 km2 in size are ideal for both economy of scale and wide distribution. This will also avoid the known geopolitical stresses caused by uneven distribution of oil in the world even if, given the higher sunlight requirements of CSP (compared with PV), the vast potential for energy generation by CSP will remain geographically unequally distributed relative to the main electricity consumers (located in Europe and in the USA).

Updated: August 16, 2015 — 6:31 pm