Transcript Thermodynamics

Thermodynamics
Carlos Silva
November 11th 2009
The power of heat
From the greek therme (heat) and dynamis (power,force)
• The capacity of hot bodies to produce work
Réflexions sur la puissance motrice du feu et
sur les machines propres à développer cette puissance
Sadi Carnot
(1796-1832)
Laws of thermodynamics
0th
• Definition of temperature
• Systems at different temperatures exchange energy until reaching
a thermal equilibrium
1st
• Conservation of energy
• heat is a form of energy
2nd
• Entropy of an isolated system never decreases
• perpetual motions of machines is impossible
3rd
• Entropy at absolute zero temperature (0 K)
• it is impossible to cool a system until zero
BASIC DEFINITIONS
Closed Systems and Control Volume
System
• a set of interacting or interdependent entities, real or
abstract, forming an integrated whole
• Closed System
• System that is isolated from its surroundings
• In thermodynamics
• a closed system can exchange heat and work (energy),
but not matter, with its surroundings
• Isolated system cannot exchange anything
Control Volume
• Region of space through which mass flows
Work, Power, Energy
Work (J)
• Measure of motion accomplishment of a system due to the action of a
force over a distance and time (Dynamics)
• (…) work expresses the useful effect that a motor is capable of producing. This
effect can always be linked to the elevation of a weight to a certain height(…) the
product of the weight multiplied by the height to which it is raised” (Sadi Carnot)
Power (W=J/s)
• The rate at which work is done
Energy (J)
• Amount of work that can be accomplished by a force
• Is the capacity of a system to perform work
Tonne of oil equivalent
Energy released by burning one tonne of crude oil (Toe)
• Approximately 42 GJ (oil properties can vary)
• International Energy Agency:
• 10 Gcal
• 41,868 GJ
• 11,630 MWh
• 7,4 barrels of oil
Energy: Primary and Final
Primary
• Energy contained in raw fuels
Final
• Energy available after conversion and transportation systems
Útil
• Energy after utilization
Sankey diagram for S.Miguel 2007
Property, State and Process
Property – macroscopic characteristic of a system
• Extensive properties
• The value for the overall system is the sum of the values for its parts
(mass, volume, energy)
• Intensive properties
• The values are not additive, may vary from one place to the other at
any time (pressure, temperature, specific volume)
State – a condition of a system, described by the properties
• usually a snapshot in time x(t)=(P,T)
Process – change of properties and therefore state of the system
• brings the system from x(t) to x(t+1)
Specific volume, Density
Specific volume
• volume occupied by a unit of mass (mass / volume)
• water 4º - 1dm3/kg
• Iron -128,2 cm3/kg
Density
• mass by unit of volume (volume/mass)
• Water at 4º - 1000kg/m3 / water at 20º - 998kg/m3
• Iron - 7800kg/m3
Temperature and Pressure
Pressure (Pa=N/m2)
• Effect of a force in a surface
• Caused by the collision of molecules to the boundaries of a
system
Temperature (K)
• At the microscopic scale, is a measure of
the energy of the particles
• solid state (vibration of molecules)
• liquid (translation movement)
• gas (vibration and rotation movements
• Thermal equilibrium – system does not change temperature
Heat, Specific Heat
Heat (J)
• is the process of energy transfer from one body or system due to thermal
contact
• can be defined as thermal energy
• energy of a body that increases with temperature
Specific heat
• energy required to increase 1 degree of a 1unit (kg or mol) of a substance
• Can be measured at constant pressure (Cp)
• Water - 4,186 J/(g·K) (25 º C) / 2,080 J/(g·K) (100º C)
• Can be measured at constant volume (Cv)
Efficiency
Thermal Efficiency
Heat Engines
Carnot Efficiency
Work output
0
1
Heat input
Heat output
  1
Heat input
TLow
  1
THigh
Coefficient of Performance
Some devices use work to move heat from one place to other
• inverse process of thermal machines
Heat Pumps
Heat output
TH
COPheating 

Work input TH  TC
Air conditioners
TC
Heat output
COPcooling 

Work input TH  TC
Laws of thermodynamics
ZEROTH LAW
Systems thermodynamic equilibrium
When two systems are put in contact with each other, there will
be a net exchange of energy between them unless or until
they are in thermal equilibrium, that is, they are at the same
temperature
• "If A and C are each in thermal equilibrium with B, A is also in
thermal equilibrium with C.“
• single temperature and pressure can be attributed to the whole
system
Laws of thermodynamics
FIRST LAW
Enthalpy (H)
Measure of internal energy of a closed system
• sum of internal energy plus the product of pressure and volume
• For constant pressure, the enthalpy increases with heat
Specific enthalpy (J/kg)
• Energy per unit of mass (PCI)
• Low (hidrocarbonets)
• Fuel 42MJ/kg
• Propane 46 MJ/kg
• High
Conservation of Energy
The total amount of energy in a closed system remains constant
over time (are said to be conserved over time)
• The increase in the internal energy of a system is equal to the
amount of energy added by heating the system minus the amount
lost as a result of the work done by the system on its surroundings.
• Energy cannot be created nor destroyed
• Energy can change form (for example chemical to thermal)
Laws of thermodynamics
SECOND LAW
Entropy (S)
Thermodynamics
• Measure of uniformity of the distribution (quality) of energy
Information
• For a system whose exact description is unknown, its entropy is defined
as the amount of information needed to exactly specify the state of the
system
Entropy increases in nature
Temperature differences between systems in contact with each
other tend to even out and that work can be obtained from
these non-equilibrium differences, but that loss of heat
occurs, in the form of entropy, when work is done
• In a system, a process that occurs will tend to increase the total
entropy of the universe
• Heat generally cannot flow spontaneously from a material at lower
temperature to a material at higher temperature (Clausius)
• It is impossible to convert heat completely into work in a cyclic
process (Kelvin)
Reversible and Irreversible Processes
Reversible (ideal)
• system and surroundings can be restored to the initial state from the
final state without producing any changes in the thermodynamics
properties
• it should occur infinitely slowly due to infinitesimal gradient
• all the changes in state occurred in the system are in thermodynamic
equilibrium with each other
Irreversible (natural)
• All processes in nature are irreversible
• Finite gradient between the two states of the system
• heat flow between two bodies occurs due to temperature gradient
between the two bodies;
Laws of thermodynamics
THIRD LAW
Entropy at absolute zero (0 K)
As a system approaches absolute zero, all processes cease and
the entropy of the system approaches a minimum value
• decreasing entropy of a system requires increasing the entropy of
surroundings
THERMODYNAMIC
PROCESSES
Boyle’s Law
The absolute pressure and volume of a gas (ideal) are inversely
proportional, if the temperature is kept constant within a
closed system
Ideal Gas law
• k - Boltzman constant (8.314 J·K−1mol-1)
• n – number of moles
PV  nRT
Different Processes
Isobaric
Isometric
Isothermal
ΔT = 0 but Q ≠ 0
Adiabatic
Cyclic
ΔT ≠ 0 but Q = 0
If clockwise – heat engine
If counterclockwise – heat pump
Ideal (Carnot) Cycle
Carnot Theorem
• No engine operating between two heat reservoirs can be more
efficient than a Carnot engine operating between those same
reservoirs
Pressure-Volume
Temperature-Entropy
Real Cycles
There are no ideal cycles
• Irreversible systems, losses of heat