Physics of Energy Conversion

Katharina Krischer, Konrad Schönleber

• Publisher: De Gruyter
• Year: 2015
• eBook ISBN: 9781501502682
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Energy

A profound understanding of the physical laws underlying energy converters is a prerequisite for a sustainable use of our energy resources.
The aim of this textbook is to provide a unified view on the different energy conversion processes ranging from power plants to solar cells. It offers an interdisciplinary introduction to energy sciences for senior undergraduate and graduate students from natural sciences and engineering. The central theme is the treatment of energy converters as open thermodynamical systems and the performance of efficiency analyses, based on the concept of exergy.

• Presents the physics behind the most important energy converters in a unified framework.
• Evaluates the performance of ideal and realistic energy converters in terms of energy and exergy efficiencies
• Provides basic concepts needed for a discussion of energy converters, such as chemical and applied thermodynamics, electrochemistry and solid state physics.

About the Authors

Katharina Krischer

is a professor of physics at the Technische Universität München, Germany. She has taught lectures on energy sciences for undergraduate and graduate students for more than 10 years. Her research topics include the photo-electrochemical production of solar fuels.

Konrad Schönleber

is a researcher in the group of Prof. Krischer which he joined after graduating in physics from the Technische Universität München. His research interest focuses on light-driven semiconductor electrochemistry and its application for renewable energies.

1 Introduction

It is a central challenge for humanity to convert its energy infrastructure into a sustainable one. Sustainable energy supply means that the energy needs of the present are met without restricting the ability of future generations to satisfy their needs, as defined in the ‘Agenda 21’ by the United Nations. Only such an energy infrastructure is able to sustain a high standard of living in the future. To achieve this goal the sources of energy used should at least satisfy the following conditions:

1. they should not be consumed;
2. they should not lead to the emission of environmental pollutants;
3. they should not lead to health risks and inherent social injustices.

All three conditions are violated by the most common energy sources used today, e.g. fossil fuels. While the first condition can be strictly met by the renewable energy sources such as water, wind, and solar power, the other two can only be approximated even in these cases. For example, water power requires huge dams with possible severe consequences for the people and animals living in that area. In order to minimize the potential harm while maximizing the benefit, it is hence necessary to make a careful evaluation of the specific energy needs and the possible mix of sources. This evaluation is a complex process and requires a detailed understanding of the energy supply technologies and the processes involved. It is the goal of this book to provide such a detailed understanding on the physical level for the important processes involved in the generation of electrical energy.

1.1 Terms and definitions

In the discussion of energy consumption, three terms with different meanings are frequently used: primary energy, final energy, and net energy. These terms are also synonymously applied to powers instead of energies, i.e. primary power, final power, and net power.

Primary energy

The total amount of energy stored in the different natural energy sources, such as fossil fuels, is called primary energy. Primary energy cannot be used directly but has to be refined in a process that takes away a part of the energy. The end product of the refinery process is the final energy. Refinery first means the refinery process necessary to produce energy in the form of directly usable fuels, such as diesel or gasoline, from natural sources such as crude oil. More importantly, another process that can also be seen as a refining of energy is the conversion of primary energy carriers, for example coal, gas, or uranium, into electrical energy as final energy. It is the latter process that accounts for most of the difference between primary and final energy.

For fossil fuels the primary energy is the heating value (old: lower heating value), i.e. the total amount of heat released if the products of the combustion of coal, oil, or natural gas are all in the gaseous state. This value differs from the upper heating value, which is the heat of combustion obtained when all combustion reactants and products are at a standard temperature of T = 25 °C. In the latter case, water is in the liquid state, the difference between heating value and upper heating value being the heat of vaporization or latent heat of water. The upper heating value is therefore always larger than the heating value.

The other sources of primary energy such as nuclear fuels and renewable energies are treated according to the following standard method:

The electricity produced is divided by an assumed efficiency for the conversion of the natural energy source into electrical energy. This assumed efficiency is 33% for nuclear power and 100% for renewable energies. The optimal conversion efficiency for renewable energies is justified by the idea that the actual amount of energy from sunlight or wind is infinite and cannot be consumed, making the produced electrical energy the energy source. With this method the amount of primary energy stemming from the conventional sources fossil fuels and nuclear power is overestimated compared to renewable energies.

Another no longer official method for calculating the primary energy of these nonfossil energy sources is the substitution method, where the primary energy is associated with the amount of fossil fuels saved by the use of nonfossil fuel energy sources. With this method the share of renewable energies has a higher value; for example, 1 kWh of renewable electricity can replace ca. 2.5 kWh of coal bound primary energy and is thus counted as 2.5 kWh primary energy.

Final energy

Final energy is the energy that is readily delivered to the consumers. Examples are electrical energy or the heating values of refined organic fuels. The share of renewable energies in the final energy is higher than the share in the primary energy, as the conversion efficiency of conventional power plants and refinery processes is already factored in. For example, 1 kWh of coal bound primary energy will produce no more than 0.4 kWh of final energy in the form of electricity, whereas primary and final energies are the same for renewable energy sources. Typically the overall final energy for all energy sectors is about 2/3 of the overall primary energy as long as efficient energy converting systems are used.

Net energy

The net energy is the energy in its final desired form, i.e. the energy output of the consuming device. The conversion efficiency associated with the consumer is responsible for the difference between final and net energy. The overall net energy output is about 1/2 of the final energy consumption, leading to a rough estimate of the ratios of primary to final to net energy of 3 : 2 : 1. Already these very basic considerations show, that the most efficient way to reduce the consumption of primary energy is to reduce the amount of net energy consumption. This can be achieved for example by minimizing transport routes, heating rooms to lower temperatures in winter, or decreasing consumption of nonessential goods and services. Any unit net energy saved roughly translates into three units of primary energy saved.

Two examples will illustrate the differences between the different energy terms:

1. Room illumination:
In this process the purpose of the lamp is to provide a certain number of photons in the visible range per unit of time. The energy of these photons is hence the net energy. The lamp itself is driven by electrical energy, which is the final energy in this case. Hence, the efficiency of the lamp gives the conversion factor between final and net energy. The electricity is provided by a power plant or a renewable energy source. In the former case the primary energy needed is either the heating value of the required fossil fuel (e.g. coal) plus the energy taken out during the refinery of this fuel, or three times the final energy if a nuclear power plant is providing the energy. If a renewable energy source is used, the final and primary energies are identical.

2. Driving a car:
Here the net power is the mechanical power of the engine. The final energy in this case is the heating value of the fuel, e.g. gasoline, which differs from the primary energy only by the energy taken away by the refinery process. In the case of an electric car the final energy is electrical energy and thus identical to the final energy in the first example, and everything with regards to primary energy can be taken from there.

All three quantities specifying energy consumption are important and are unfortunately often confused in public discussions. Care has to be taken especially looking at the share of renewable energies in the overall energy mix, as this might have political implications. As an example, in Figure 1.1 the respective shares of different sources of primary energy are shown together with the share of renewable energies in the final energy for both the European Union (EU 28) and the USA in 2012. The sources are public data given out by the relevant statistics agencies EUROSTAT and the Energy Information Administration of the Department of Energy for the EU and the USA, respectively.

Fig. 1.1: Left: Respective shares of the different sources of primary energy for both the EU (top) and the USA (bottom) in the year 2012. Right: contribution of renewable energies to the mix of final energy of the EU (top) and the USA (bottom). The total areas of the circles are proportional to the total amounts of energy consumed (left: EU 1.68 Gtoe, USA 2.44 Gtoe; right: EU 1.10 Gtoe, USA 1.74 Gtoe). Data are taken from EUROSTAT (http://epp.eurostat.ec.europa.eu/portal/page/portal/energy/data/main_tables) and the DOE (U.S. Energy Information Administration, Monthly Energy Review, August 2014) for the EU and the USA, respectively.

As evident from Figure 1.1 the primary energy consumed in both the EU and the USA stems mostly from the fossil fuels coal, oil, and natural gas. In the EU the share of these energy carriers is clos...

Table of contents

• Cover
• Title Page
• Also of Interest
• Copyright Page
• Preface
• Contents
• 1 Introduction
• 2 Exergy in closed systems
• 3 Exergy in open systems
• 4 Thermal power plants
• 5 Electrical exergy
• 6 Chemical exergy
• 7 Electrochemical energy conversion
• 8 Solar energy
• 9 Solarthermal energy conversion
• 10 Photovoltaic energy conversion
• 11 Exergy storage
• A Basics: Thermodynamics
• B Basics: Solid state physics
• C Further Reading
• Index