Sustainable hydrogen energy

Hydrogen is the simplest and most abundant chemical element in our universe— it is the power source that fuels the Sun and its oxide forms the oceans that cover three quarters of our planet. This ubiquitous element could be part of our urgent quest for a cleaner, greener future. Hydrogen, in association with fuel cells, is widely considered to be pivotal to our world’s energy requirements for the twenty-first century and it could potentially redefine the future global energy economy by replacing a carbon-based fossil fuel energy economy. The principal drivers behind the sustainable hydrogen energy vision are therefore: • the urgent need for a reduction in global carbon dioxide emissions; • the improvement of urban (local) air quality; • the abiding concerns about the long-term viability of fossil fuel resources and the security of our energy supply; • the creation of a new industrial and technological energy base—a base for innovation in the science and technology of a hydrogen/fuel cell energy landscape. The ultimate realization of a hydrogen-based economy could confer enormous environmental and economic benefits, together with enhanced security of energy supply. However, the transition from a carbon-based(fossil fuel) energy system to a hydrogen-based economy involves significant scientific, technological, and socio-economic barriers. These include: • low-carbon hydrogen production from clean or renewable sources; • low-cost hydrogen storage; • low-cost fuel cells; • large-scale supporting infrastructure, and • perceived safety problems. In the present chapter we outline the basis of the growing worldwide interest in hydrogen energy and examine some of the important issues relating to the future development of hydrogen as an energy vector. As a ‘snapshot’ of international activity, we note, for example, that Japan regards the development and dissemination of fuel cells and hydrogen technologies as essential: the Ministry of Economy and Industry (METI) has set numerical targets of 5 million fuel cell vehicles and10 million kW for the total power generation by stationary fuel cells by 2020. To meet these targets, METI has allocated an annual budget of some £150 million over four years.

Nuclear fission

This chapter will cover the nuclear fission option as a future energy supply, and will essentially address the question: can nuclear fission plug the gap until the potential of nuclear fusion is actually realized? (The potential for fusion is considered in detail chapter 7.) To put this question into context, let us first look at some of the key issues associated with nuclear fission, which currently supplies around one fifth of the UK’s electricity. Most large scale power stations produce electricity by generating steam, which is used to power a turbine. In a nuclear power station, the principle is the same, but instead of burning coal, oil, or gas to turn water into steam, the heat energy comes from a nuclear reactor. A reactor contains nuclear fuel, which remains in place for several months at a time, but over that time it generates a huge amount of energy. The fuel is usually made of uranium, often in the form of small pellets of uranium dioxide, a ceramic, stacked inside hollow metal tubes or fuel rods, which can be anything from a metre to four metres in length, depending on the reactor design. Each rod is about the diameter of a pencil, and the rods are assembled into carefully designed bundles, which in turn are fixed in place securely within the reactor. There are two isotopes (or different types) of uranium, and only one of these is a material which is ‘fissionable’—that is to say, if an atom of this uranium isotope is hit by a neutron, then it can split into two smaller atoms, giving off energy in the process and also emitting more neutrons. This, and other pathways, are illustrated in Fig. 6.1 (Source: CEA). Controlling the reaction, so that the energy from the fission of uranium atoms is given out slowly over a period of years, requires two aspects of the process to be carefully balanced. 1. First, there must be enough fissile atoms in the fuel so that—on average— each fission leads to exactly one other. Any fewer, and the reaction will die away.

Wave and tidal power

This chapter reviews how electricity can be generated from waves and tides. The UK is an excellent example, as the British Isles have rich wave and tidal resources. The technologies for converting wave power into electricity are easily categorized by location type. 1. Shoreline schemes. Shoreline Wave Energy Converters (WECs) are installed permanently on shorelines, from where the electricity is easily transmitted and may even meet local demands. They operate most continuously in locations with a low tidal range. A disadvantage is that less power is available compared to nearshore resources because energy is lost as waves reach the shore. 2. Nearshore schemes. Nearshore WECs are normally floating structures needing seafloor anchoring or inertial reaction points. The advantages over shoreline WECs are that the energy resource is much larger because nearshore WECs can access long-wavelength waves with greater swell, and the tidal range can be much larger. However, the electricity must be transmitted to the shore, thus raising costs. 3. Offshore schemes. Offshore WECs are typically floating structures that usually rely on inertial reaction points. Tidal range effects are insignificant and there is full access to the incident wave energy resource. However, electricity transmission is even more costly. Tidal power technologies fall into two fundamental categories:1. Barrage schemes. In locations with high tidal range a dam is constructed that creates a basin to impound large volumes of water. Water flows in and out of the basin on flood and ebb tides respectively, passing though high efficiency turbines or sluices or both. The power derives from the potential energy difference in water levels either side of the dam. 2. Tidal current turbines. Tidal current turbines (also known as free flow turbines) harness the kinetic energy of water flowing in rivers, estuaries, and oceans. The physical principles are analogous to wind turbines, allowing for the very different density, viscosity, compressibility, and chemistry of water compared to air. Waves are caused by winds, which in the open ocean are often of gale force (speed >14 m/s).

Governing the transition to a new energy economy

Over the next two or three decades a new energy economy should begin to take shape in the developed industrial countries. This will not be a post-fossil fuel economy. But it could be an economy in which non-fossil sources play a more important role; where efficiency in the production, distribution, and use of energy is significantly enhanced; where new storage and carrier technologies are being adopted; and where the fossil sector is being transformed by the imperative of carbon sequestration. Such an energy economy would represent a critical staging post in a much longer transition towards a carbon neutral, low-environmental impact, energy system. The extent to which a new energy economy actually materializes will depend on many factors including the pace and orientation of international economic development, the rate and direction of technological innovation and diffusion, as well as patterns of geo-strategic cooperation and conflict. But there is no doubt the trajectory will be significantly influenced by political decisions and government action on the energy file. This is the issue with which this chapter is concerned. At the moment there are two main political drivers for the move to look beyond oil. First, there are supply concerns. Increasing global demand, production bottlenecks, and political instability have pushed oil prices towards historic highs. Although the oil intensity (oil consumption per unit of GDP) of the OECD economies is less than during the oil crises of the 1970s (IMF, 2005), there is no doubt that the long term economic impact of high oil prices would be considerable. There are also critical issues associated with the geographic distribution of reserves. Production from areas opened up following the turbulence of the early 1970s (such as the North Sea) is peaking. In coming years the United States will be more heavily dependent on imported oil, with an increasing percentage of these imports destined to come from politically volatile areas in the Middle East and Asia. And this presents a serious risk of supply disruption.

Photovoltaic and photoelectrochemical conversion of solar energy

The Sun provides about 100,000 Terawatts (TW) to the Earth, which is approximately ten thousand times greater than the world’s present rate of energy consumption (14 TW). Photovoltaic (PV) cells are being used increasingly to tap into this huge resource and will play a key role in future sustainable energy systems. Indeed, our present needs could be met by covering 0.5% of the Earth’s surface with PV installations that achieve a conversion efficiency of 10%. Fig. 8.1 shows a simple diagram of how a conventional photovoltaic device works. The top and bottom layers are made of an n-doped and p-doped silicon, where the charge of the mobile carriers is negative (electrons) or positive (holes), respectively. The p-doped silicon is made by ‘doping’ traces of an electron-poor element such as gallium into pure silicon, whereas n-doped silicon is made by doping with an electron-rich element such as phosphorus. When the two materials contact each other spontaneous electron and hole transfer across the junction produces an excess positive charge on the side of the n-doped silicon (A) and an excess negative charge on the opposite p-doped (B) side. The resulting electric field plays a vital role in the photovoltaic energy conversion process. Absorption of sunlight generates electron-hole pairs by promoting electrons from the valence band to the conduction band of the silicon. Electrons are minority carriers in the p-type silicon while holes are minority carriers in the n-type material. Their lifetime is very short as they recombine within microseconds with the oppositely charged majority carriers. The electric field helps to collect the photo-induced carriers because it attracts the minority carriers across the junction as indicated by the arrows in Fig. 8.1, generating a net photocurrent. As there is no photocurrent flowing in the absence of a field, the maximum photo-voltage that can be attained by the device equals the potential difference that is set up in the dark at the p-n junction. For silicon this is about 0.7V. So far, solid-state junction devices based on crystalline or amorphous silicon (Si) have dominated photovoltaic solar energy converters, with 94% of the market share.

Energy efficiency in the design of buildings

The buildings sector accounts for 40% of European energy requirements. Two thirds of the energy used in European buildings is consumed by private households, and their consumption is growing every year as rising living standards lead to an increased use of air conditioning and heating systems. Research shows that more than one-fifth of the present energy consumption and up to 30–45 million tonnes of CO2 per year could be saved by 2010 by applying more ambitious standards both to new and refurbished buildings–these savings would represent a considerable contribution to meeting the European Kyoto targets (European Council, 2002). Without comprehensive measures, energy consumption and CO2 emissions from the building sector will continue to grow. Sustainable energy strategies for buildings will therefore increase in importance. Even today, so-called ‘zero emission buildings’ can be realized with existing planning approaches and technologies. Such buildings do not need an external energy input (for example from oil, gas or supplied electricity) other than solar energy. This is achieved by a combination of a high-level of energy efficiency and renewable energy technologies. This chapter focuses on buildings in the housing and service sectors, presents new building design strategies, technologies, and building components as well as the new legal framework set by the European Buildings Directive. It also discusses the question of raising awareness, and presents some thoughts on how changing life patterns may impact the buildings of the future. Residential buildings mainly need energy for space heating; with present building standards, space heating represents about 70% of the overall energy demand of existing buildings. In many European countries there are substantial efforts to increase energy efficiency—nevertheless, not all the potential for energy savings has been realized by far, and oil is still a major energy source for heating. In recent years, heat demand for new buildings was reduced significantly by technical measures. However, the number of low energy or passive buildings in Europe is still very limited, despite the fact that they can be constructed at acceptable costs.

Biological solar energy

Oil, gas, and coal provide us with most of the energy needed to power our technologies, heat our homes, and produce the wide range of chemicals and materials that support everyday life. Ultimately the quantities of fossil fuels available to us today will dwindle, and then what? Even before that we are faced with the problem of increasing levels of carbon dioxide in the atmosphere and the consequences of global warming (Climate Change, 2001). To address these issues it is appropriate to remind ourselves that fossil fuel reserves are derived from the process of photosynthesis. Plants, algae, and certain types of bacteria have learnt how to capture sunlight efficiently and convert it into organic molecules, the building blocks of all living organisms. It is estimated that photosynthesis produces more than 100 billion tons of dry biomass annually, which would be equivalent to a hundred times the weight of the total human population on our planet at the present time, and equal to about 100 TJ of stored energy. In this chapter we emphasize the enormity of the energy/carbon dioxide problem that we face within the coming decades and discuss the contributions that could be made by biofuels and developing new technologies based on the successful principles of photosynthesis. We will particularly emphasize the possibility of exploiting the vast amounts of solar energy available to extract hydrogen directly from water. The success of this energy generating and storage system stems from the fact that the raw materials and power needed to synthesise biomass are available in almost unlimited amounts: sunlight, water and carbon dioxide. At the heart of the reaction is the splitting of water by sunlight into oxygen and hydrogen. The oxygen (a ‘waste product’ of the synthesis) is released into the atmosphere where it is available for us to breathe and to use for burning our fuels. The ‘hydrogen’ is not normally released into the atmosphere as H2, but instead is combined with carbon dioxide to make organic molecules of various types.

Summary

Energy . . . Beyond Oil is important and timely and should be understood within the wider context of global climate change and future energy demands. In the 1780s John Watts developed his steam engine and so began the Industrial Revolution. At this time, ice-core records show that levels of CO2 in the atmosphere were around 288 parts per million (ppm). Give or take 10 ppm, this had been their level for the past 6,000 years, since the dawn of the first cities. As industrialization drove up the burning of fossil fuels in the developed world, CO2 levels rose. At first the rise was slow. It took about a century and a half to reach 315 ppm. The rise accelerated during the twentieth century: 330 ppm by the mid-1970s; 360 ppm by the 1990s; 380 ppm today. This change of 20 ppm over the past decade is equal to that last seen when the most recent ice age ended, ushering in the dawn of the Holocene epoch, 10,000 years ago. If current trends continue, then by about 2050 atmospheric CO2 levels will have reachedaround500 ppm, nearly double pre-industrial levels. The last time our planet experienced such high levels was some 50 million years ago, during the Eocene epoch, when sea levels were around100 m higher than today. The Dutch Nobelist, Paul Crutzen, has, indeed, suggested that we should recognize that we are now living in a new geological epoch, the Anthropocene. He sees this epoch as beginning around 1780, when industrialization began to change the geochemical history of our planet. Even today, there continues to exist a ‘denial lobby’, funded to the tune of tens of millions of dollars by sectors of the petrochemical industry, and highly influential in some countries. This lobby has understandable similarities, in tactics and attitudes, to the tobacco lobby that continues to deny smoking causes lung cancer, or the curious lobby denying that HIV causes AIDS. This denial lobby is currently very influential in the USA.

Geothermal energy

The term ‘geothermal energy’ describes all forms of heat stored within the Earth. The energy is emitted from the core, mantle, and crust, with a large proportion coming from nuclear reactions in the mantle and crust. It is estimated that the total heat content of the Earth, above an assumed average surface temperature of 15◦C, is of the order of 12.6×1024 MJ, with the crust storing 5.4×1021 MJ (Armstead, 1983). Based on the simple principle that the ‘deeper you go the hotter it gets’, geothermal energy is continuously available anywhere on the planet. The average geothermal gradient is about 2.5–3◦C per 100 metres but this figure varies considerably; it is greatest at the edges of the tectonic plates and over hot spots–where much higher temperature gradients are present and where electricity generation from geothermal energy has been developed since 1904. Geothermal energy is traditionally divided into high, medium, and low temperature resources. Typically, temperatures in excess of 150◦C can be used for electricity generation and process applications. Medium temperature resources in the range 40◦C to 150◦ C form the basis for ‘direct use’ i.e. heating only, applications such as space heating, absorption cooling, bathing (balneology), process industry, horticulture, and aquaculture. The low-temperature resources obtainable at shallow depth, up to 100–300 metres below ground surface, are tapped with heat pumps to deliver heating, cooling, and hot water to buildings. The principles of extracting geothermal energy, in applications ranging from large scale electrical power plants to smallscale domestic heating, are illustrated in Fig. 3.1. Geothermal energy can be utilized over a temperature range from a few degrees to several hundred degrees, even at super critical temperatures. The high temperature resources, at depth, are typically ‘mined’ and are depleted over a localized area by extracting the in situ groundwaters and, possibly, re-injecting more water to replenish the fluids and extract more heat. Although natural thermal recovery occurs, this does not happen on an economically useful timescale.

Energy…beyond oil: a global perspective

Coal and oil, which are the buried products of several hundred million years of solar energy, photosynthesis, and geological pressure, have fuelled our industries and transport systems since the Industrial Revolution, a period of only 200 years. Although opinions differ as to when the peak in oil production will occur (perhaps in 2010, perhaps in 2030), it is hard to avoid the conclusion that oil is being consumed about one million times faster than it was made and, further than this, the twenty-first century, will be the century when societies have to learn to live without gas and oil (coal will outlast oil and gas by a few hundred years). But, there is an entirely separate motivation for living without fossil fuel: obtaining energy from oil, coal, and gas will continue to put carbon dioxide (CO2) into the atmosphere at levels which it is widely acknowledged are elevating the average temperature on the planet. Carbon dioxide is a good heat absorber and acts like a blanket: this is because CO2 molecules resonate strongly with infrared radiation causing it to be trapped as heat instead of all being transmitted into space. Global warming is already causing the polar ice caps to melt and it is inevitable that there will be higher sea levels resulting in less land for an increasing population, along with changes in climate. These changes are not easy to predict and may be difficult to reverse. Either of these two motivations, be it the depletion of oil reserves or the need to arrest global warming caused by the combustion of fossil fuels, mandates new thinking from all those with concern for the future. How will future generations view our policies and our decision making today? Unless we change course now, these people will be left in a world where energy is a scarce resource and the mobility we have taken for granted in the late twentieth and early twenty-first century will be long gone. Our generation—rightly—would be blamed for knowingly squandering the planet’s resources.