Essentials of Energy Technology

Sources, Transport, Storage, Conservation

Jochen Fricke,Walter L. Borst

• Publisher: Wiley-VCH
• Year: 2013
• ISBN:
  9783527673605
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Werkstoffwissenschaft

An in-depth understanding of energy technology, sources, conversion, storage, transport and conservation is crucial for developing a sustainable and economically viable energy infrastructure. This need, for example, is addressed in university courses with a special focus on the energy mix of renewable and depletable energy resources.

Energy makes our lives comfortable, and the existence of amenities such as heaters, cars, warm water, household appliances and electrical light is characteristic for a developed economy. Supplying the industrial or individual energy consumer with energy 24 hours a day is a non-trivial challenge, especially in times where the energy is coming from very diverse resources such as oil, gas, nuclear fuels, wind, sun, or waves.

This book gives physics, chemistry, engineering, and materials science students insights in the basics of energy and energy technology. It was developed along a successful course for advanced bachelor or graduate students and is written in a didactic style. The problems and solutions at the end of each chapter are ideal for exams and make self-study easy. Topics covered include energy from fossil and nuclear fuels, renewable sources, energy transport, storage, and conservation.

Chapter 1

Introduction

1.1 Global Energy Flow

The global demand for primary energy has grown enormously during the past decades. It is now about 5.0 · 10²⁰ J per year or 16 TW (Figure 1.1). Most of this energy is dissipated as waste heat. As the solar power reaching the Earth (insolation) is 170 000 TW, we recognize that, on a global scale, the heat dissipation caused by human activities is about 10 000 times smaller than the solar input. However, inside cities, the anthropogenic heat dissipation and the solar input can become comparable. This leads to a warmer microclimate.

Figure 1.1 Present global energy flow in Watt. The numbers in parentheses are relative to the solar input. About 80% of our primary energy is provided by fossil fuels, about 10% by biomass, and 6% by nuclear reactors. The contributions from photovoltaics, solar thermal, wind, geothermal, and tides are not shown, as each of them still amount to <1% of the primary energy demand.

(Source: Adapted from [1].)

1.2 Natural and Anthropogenic Greenhouse Effect

A much more severe and global problem associated with the flow of energy is the anthropogenic emission of greenhouse gases. Most important among these is carbon dioxide (CO₂) released by burning of fossil carbon (Table 1.1). The average dwell time of CO₂ in the atmosphere is about 120 years. CO₂ is a natural constituent of the atmosphere together with water vapor, the latter being the dominant greenhouse gas. These gases interact with a thermal radiation of 1.1 · 10¹⁷ W or about 220 W/m² from the Earth (Figures 1.1 and 1.2). Their molecules either have a permanent electric dipole moment, as with H₂O, or are vibrationally excited, as in the case of CO₂ and CH₄, another greenhouse gas.

Table 1.1 The amount of CO₂ emitted per thermal kilowatt hour depends strongly on the atomic carbon/hydrogen ratio of the fossil fuel (1 kg of C is oxidized into 3.7 kg CO₂).

Figure 1.2 Hypothetical atmosphere of the Earth without infrared-active trace gases assumed in the left half of the figure. About two-thirds of the incoming solar radiation is absorbed at the surface of the Earth (with an albedo or reflectivity of 0.35), reemitted as thermal radiation, and completely given off into space. The resulting temperature would be about 18 °C below zero, preventing life as we know it. Greenhouse gases present in the real atmosphere are added in the right half of the figure. They absorb part of the outgoing thermal radiation and send it back to Earth. This greenhouse effect provides life-supporting temperatures of +15 °C. The most important greenhouse gas is H₂O with typically 1–2% by weight, followed by CO₂, CH₄, NO_x, and so on.

These gases thus reduce the radiative heat transfer from the Earth into space, raising the global mean temperature from −18 to +15 °C, a precondition for a habitable Earth. A stable mean temperature requires a balance between solar input and thermal output (Figure 1.3).

Figure 1.3 Normalized solar radiation input and thermal radiation at 300 K as a function of wavelength. The solar blackbody spectrum at 6000 K is modified by absorption in the Earth's atmosphere.

It is important to answer the question why the concentration of CO₂ is of any consequence. After all, the concentration of water vapor is about 100 times larger. Figure 1.4 shows that some of the absorption bands of CO₂ coincide with “windows” in the H₂O spectrum. Thus, a relatively small amount of CO₂ can reduce the thermal flow, that would otherwise escape into space through these windows. The effect of the other greenhouse gases on the thermal flow into space is characterized by the global warming potential (GWP). For example, CH₄ has a GWP ≈ 25, indicating that one molecule of CH₄ is 25 times more effective than one molecule of CO₂.

Figure 1.4 Relative spectral absorption of water vapor and carbon dioxide in the atmosphere. A value of 1 means a saturated absorption or complete opaqueness, 0 indicates a “window” for radiative escape. One sees, for example, that CO₂ drastically reduces the escape of thermal radiation in the H₂O-window of 4–5 µm. Note that the three CO₂-absorption bands shown are saturated only in their center but not in the flanks. Therefore, a further increase in CO₂ in the atmosphere can definitely enhance the greenhouse effect.

(Source: Adapted from [2].)

CO₂ and other noncondensing greenhouse gases together account for about 25% of the terrestrial greenhouse effect. Atmospheric modeling [3] shows that these gases via feedback processes provide the necessary infrared absorption to sustain the present levels of water vapor and clouds, which make up the remaining 75% of the terrestrial greenhouse effect. (Without CO₂ and the other noncondensing greenhouse gases, the atmospheric water vapor would condense. The terrestrial greenhouse would collapse within a few decades, sending Earth into an ice-bound state.)

In summary, the natural greenhouse effect determined the climate on the Earth in the past and supported the development of life. About 150 years of anthropogenic activities, however, accompanied by the burning of coal, oil, and natural gas, have led to a drastic increase in the concentration of greenhouse gases in the atmosphere. This is causing an additional, human-related reduction in the thermal radiation transfer to space. The imbalance, also called radiative forcing, is about 1 W/m² today [4]. This is only a 0.5% contribution to the total radiative heat transfer from the Earth. Furthermore, the large thermal mass of the oceans has stored large amounts of heat. Nonetheless, a global warming of about 0.8 K since 1870 and 0.6 K since about 1960 is observed.

The main culprit for the warming of the Earth is anthropogenic CO₂. Its concentration in the atmosphere rose from a preindustrial value of 280 to about 390 ppm in the year 2010 (Figure 1.5).

Figure 1.5 The concentration of CO₂ in the atmosphere at present is increasing by nearly two parts per million by volume per year (ppmv) and was about 390 ppmv in 2010. The oscillations on the continuous rise are about 6.5 ppmv peak-to-peak and are caused by annual variations in bioactivity and oxidation of biomass. Photosynthesis in summer causes a relative minimum in September/October, while oxidation of biomass in winter leads to a relative maximum in May. The preindustrial value was 280 ppmv.

(Source: Adapted from Mauna Loa, Hawaii.)

In order to put the anthropogenic influence on our climate in perspective, we have to look at the history of CO₂. The CO₂ concentration during the past 400 000 years fluctuated between about 180 and 280 ppm, and never exceeded 300 ppm. A higher CO₂ concentration was always accompanied by warmer temperatures and vice versa. The increase to 390 ppm thus is rather dramatic. The retreat of mountain glaciers and the north polar ice sheet appear to be manifestations of the problem.

Problem 1.1

A warmer atmosphere can hold more moisture [5] and thus more torrential rains can be expected. Calculate the relative increase in water vapor pressure for an atmosphere at 20 °C, assuming a temperature increase of 1 K. The exponential dependence of vapor pressure on temperature is p(T) = p₀ · exp[−ΔE/(k_B · T)]. (In order to find ΔE, start with the mass specific heat of vaporization, then find the molar mass of water and the number of water molecules per mole.)

We should mention here that aerosols in the atmosphere are responsible for a negative radiative forcing. Combustion caused by humans has increased the amount of atmospheric aerosols substantially. The interaction between these aerosols and solar radiation leads to a direct cooling of the atmosphere. In addition, aerosols enhance the condensation of moisture and modify the optical properties of clouds. The sign of this indirect aerosol effect – whether positive or negative – is still uncertain. A third indirect aerosol effect involves the change of biochemical cycles [6]. All three effects may have reduced global warming substantially. Anticipating future worldwide installations of scrubbing devices, much higher CO₂ mitigation costs could result than previously thought.

1.3 Limit to Atmospheric CO₂ Concentration

In order to prevent catastrophic climate changes in the future, causing, for example, a rise in sea level of several meters, the CO₂ concentration in the atmosphere will have to be limited. The actual limit is the subject of much discussion at present.

As an example, let us consider a maximum tolerable CO₂ concentration of 560 ppm, that is, twice the preindustrial value. From the known global annual us...