Thermodynamics

Carlos SilvaNovember 11th 2009

The power of heat

From the greek therme (heat) and dynamis (power,force)

• The capacity of hot bodies to produce work

Sadi Carnot(1796-1832)

Réflexions sur la puissance motrice du feu et sur les machines propres à développer cette puissance

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

1inputHeat

outputWork 0

inputHeat

outputHeat 1

High

Low

T

T1

Coefficient of Performance

Some devices use work to move heat from one place to other

• inverse process of thermal machines

Heat Pumps

Air conditioners

CH

Hheating TT

TCOP

inputWork

outputHeat

CH

Ccooling TT

TCOP

inputWork

outputHeat

ZEROTH LAWLaws of thermodynamics

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

FIRST LAWLaws of thermodynamics

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)

SECOND LAWLaws of thermodynamics

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;

THIRD LAWLaws of thermodynamics

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

nRTPV

Different Processes

Isobaric Isometric

Adiabatic ΔT ≠ 0 but Q = 0

IsothermalΔT = 0 but Q ≠ 0

CyclicIf 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