Download Introduction in energy systems - Faculty of Mechanical Engineering

University of Ljubljana
Faculty of Mechanical Engineering
Department of Energy Engineering
Laboratory for Heat and Power
ENERGY CONVERSION SYSTEMS
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Authors: dr. Boštjan Drobnič
Rok Stropnik
dr. Mitja Mori
Ljubljana, march 2016
Energy Conversation Systems
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Energy conversation systems
The concept of 'energy conversation systems’ covers a wide range of processes,
machines, devices and systems that are running energy conversion, to change the energy
from its naturally available primary forms in contemporary society the most useful
forms, especially in electricity. Conversions can be roughly divided into 'hot' and 'cold'.
In the first exploit internal, chemical or nuclear energy primary sources and through
heat pretend firstly into mechanical work and continue in electrical power.
A Basic laws of thermodynamics
Processes for energy conversion may be very different, but all are limited to the basic
laws of nature. Those that relate specifically to energy conversion are included in the
basic laws of thermodynamics:
Zeroth law of thermodynamics
 If two systems (A and B) are in thermal equilibrium independently with a third system
they must be in thermal equilibrium with each other (A and C, B and C). This law helps
define the notion of temperature.
First law of thermodynamics (energy law)
 The increase in internal energy of a closed system is equal to the heat and work
supplied to the system.
Second law of thermodynamics (entropy law)
 Heat can never pass from a colder to a warmer body without some other change,
connected therewith, occurring at the same time (Clausius statement).
 In every neighborhood of any state S of an adiabatically enclosed system there are
states inaccessible from S (Principle of Carathéodory).
 It is impossible, by means of inanimate material agency, to derive mechanical effect
from any portion of matter by cooling it below the temperature of the coldest of the
surrounding objects (Kelvin statement).
Third law of thermodynamics
 The system is not possible cooled, in a finite number of steps, to absolute zero.
B Working substance, state, transformation, process
For thermodynamic process it’s necessary to have the working substance (water, air …).
The working substance brings the energy different forms in the machines and devices, where
the energy transformation taking place, and carried out. In doing so, the working substance
change its thermodynamic properties. That can be adequately described by
thermodynamic state variables such as pressure, temperature and density or volume,
which the substance occupies. In addition to these, for the analysis of the energy
transformation in the energy system are important two more state variables - enthalpy
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Energy Conversation Systems
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(the amount of energy carried by the working substance) and entropy (allowing the
assessment of the reversibility of the processes ...). State variables, which will be used
for the analysis of the thermodynamic processes related to producing (useful forms of)
the energy are:
variable
symbol
unit
pressure
p
Pa, bar, mmHg, ...
temperature
T
K, °C, F, ...
specific volume
v
m3/kg, ...
specific enthalpy
h
kJ/kg, ...
specific entropy
s
kJ/kgK, ...
Pressure (symbol p) is the force (F) applied perpendicular to the surface (S) of an object
per unit area over which that force is distributed. The pressure at any given point of a
non-moving (static) fluid is always perpendicular to the surface and a sign of pressure is
chosen so that the direction is always positive in the fluid.
Temperature is a physical property of matter that quantitatively express general
concepts such as 'hot' and 'cold'. Subjects which have low temperatures are cool, different
degrees of higher temperatures it is described as warm or hot. It can be measured with a
thermometer or a calorimeter. It is a Means of determining the internal energy contained
within the system. Formally the temperature is equal to the derivative of the entropy
with respect to the internal energy.
The specific volume of a substance is the ratio of the substance's volume to its mass. It is
the reciprocal of density and an intrinsic property of matter as well. Specific volume is
defined as the number of cubic meters occupied by one kilogram of a particular
substance.
Enthalpy is a measure of energy in a thermodynamic system. It includes the internal
energy, which is the energy required to create a system, and the amount of energy
required to make room for it by displacing its environment and establishing its volume
and pressure. Enthalpy is defined as a state function that depends only on the prevailing
equilibrium state identified by the variables internal energy, pressure, and volume. It is
an extensive quantity. The enthalpy is the preferred expression of system energy changes
in many chemical, biological, and physical measurements at constant pressure, because it
simplifies the description of energy transfer.
Entropy is a thermodynamic property and it’s a measure of the energy not
available to do work in the thermodynamic processes. Since the second law of
thermodynamics entropy of an isolated system always increases or remains
constant, the entropy is a measure of the tendency of processes that
spontaneously flow in a specific direction (heat always flows from a higher to a
lower temperature). Such processes reduce the order in the systems, so the
entropy is also a measure of disorderly or randomness in a system.
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Entropija je termodinamična lastnost, ki je med drugim merilo energije, ki ni na voljo za
opravljanje dela v termodinamičnih procesih, kjer potekajo pretvorbe energije. Ker se po
drugem zakonu termodinamike entropija izoliranega sistema vedno povečuje ali ostaja
konstantna, je entropija tudi merilo težnje procesov, da spontano tečejo v točno določeni
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smeri (toplota teče vedno od višje temperature proti nižji). Taki procesi zmanjšujejo
urejenost sistemov, zato je entropija tudi merilo neurejenosti ali naključnosti v sistemih.
Energy Conversation Systems
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In the energy processes the state of the working fluid and thermodynamic state
variables changes as a result of feeding or extraction of energy in the form of heat or
mechanical work. Changing the state of the working substance is called thermodynamic
process and is usually described with the initial state (point) and the final state, and
additionally we can also describes the changes between the start and end points. In
simplified cases, the thermodynamic process taking place in four typical ways in which
one of the thermodynamic state variable remains unchanged:
process
Constant
example
variable
Isobaric
isochoric
isothermal
isentropic
p
v
T
S
heating/cooling of substance at constant pressure
heating/cooling of substance at constant volume
compression/expansion of substance at constant temperature
fast compression/expansion (temperature is amended)
The
sequence
of
the
various
thermodynamic processes, which brings
the state of the working substance back to
the initial state, called a thermodynamic
cycle and is the basis for most technical
processes, to convert different forms of
energy through the heat into mechanical
work. Examples include the production of
mechanical work (which is often further
converted into electricity) power plant,
nuclear power plants, internal combustion
engines, jet engines, ...
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The primary purpose of the thermodynamic cycle is the conversion of heat, which is
relatively easy to obtain from the primary sources of energy into mechanical work or
electricity, which is in practice more useful form of energy. In general, in the cycle
process is supplied the energy in the form of heat (high temperature), and the
mechanical part, then the heat is drawn off from the process (low temperature), and the
mechanical work. In the cycle is more discharged mechanical work than supplied on the
other hand we get less output heat than input. For a full thermodynamic cycle, as well as
for individual parts of the process, subject to the second law of thermodynamics, or the
law of conservation of energy.
Two basic and most simple cycle processes, which are in practice the most widely used
in the 'hot' energy transformations, are the gas (Joule or Brayton) and steam (Rankine)
cycle process. On the previous figure shows two typical installations (systems of
machines and devices), in which previous mentioned process takes place. On the
simplified scheme of these systems are shown only four basic elements (input of
mechanical work, heat input, output mechanical work (useful work), heat sink (unused
heat). In engineering sciences we use agreed symbols to show the basic elements of the
system. The individual elements of the gas and the steam thermodynamic process are
labeled the same as the corresponding pictures above.
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Energy Conversation Systems
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To see the changing state properties of the working substance in the process we use the
relevant diagrams. Typically used diagrams are T-s, h-s, h-s. T-s diagram is used in
thermodynamics to visualize changes to temperature and specific entropy during a
thermodynamic process or cycle. It is a useful and common tool, particularly because it
helps to visualize the heat transfer during a process. For reversible (ideal) processes, the
area under the T-s curve of a process is the heat transferred to the system during that
process. H-s chart or Mollier diagram plots the total heat against entropy, describing the
enthalpy of a thermodynamic system. A p-v diagram plots the change in the pressure
with respect to the volume for some process or processes. Typically in thermodynamics,
the set of processes forms a cycle, so that upon the completion of the cycle there has
been no net change and the state of the system.
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