Direct Energy, 2018a

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to connect to the power grid. Relatedly, direct energy conversion devices
such as thermoelectric devices and fuel cells are used to power satellites,
rovers, and other aerospace systems. Many electrical engineers work in
the automotive industry. Direct energy conversion devices found in cars
include batteries, optical cameras, Hall eect sensors in tachometer used
to measure rotation speed, and pressure sensors.
Direct energy conversion is a fascinating topic because it does not t
neatly into a single discipline. Energy conversion is fundamental to the
elds of electrical engineering, but it is also fundamental to mechanical
engineering, physics, chemistry, and other branches of science and engineering.
For example, springs are energy storage devices often studied by
mechanical engineers, capacitors are energy storage devices often studied
by electrical engineers, and batteries are energy storage devices often studied
by chemists. Relatedly, energy storage and energy conversion devices,
such as springs, capacitors, and batteries, are not esoteric. They are commonplace,
cheap, and widely available. While they are found in everyday
objects, they are active subjects of contemporary research too. For example,
laptop computers are limited by the lifetime of batteries, and cell phone
reception is often limited by the quality of an antenna. Batteries, antennas,
and other direct energy conversion devices are studied by both consumer
companies trying to build better products and academic researchers trying
to understand fundamental physics.
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This book is intended to both illustrate individual energy conversion technologies
and illuminate the relationship between them. For this reason, it
is organized in two parts. The rst part is a survey of energy conversion
processes. The second part introduces calculus of variations and uses it as
a framework to relate energy conversion processes.
Due to the wide variety, it is not possible to discuss all energy conversion
devices, even all direct devices, in detail. However, by studying the example
direct energy conversion processes, we can gain an understanding of indirect
processes and other applications. The devices discussed in this book
involve energy conversion between electrical form and another form. Additionally,
devices involving magnets and coils will not be discussed. Many
useful devices, including motors, generators, wind turbines, and geothermal
power plants, convert energy electromagnetically using magnets and coils.
Approximately 90% of power supplied to the electrical grid in the United
States comes from generators that use magnets and coils [1]. Also, about
2/3 to 3/4 of energy used by manufacturing facilities goes towards motors