Thermodynamics and Heat Engines

  • Heat engines and refrigerators are important in energy use
    • A heat engine extracts energy for movement from heat as it flows from hot to cold
    • A refrigerator inputs energy to move heat from cold to hot
  • The efficiency of these is dictated by thermodynamics

Fundamental Concepts

  • Temperature
  • Fahrenheit, Celsius, Kelvin Scale
  • First Law of Thermodynamics
  • Second Law of Thermodynamics

Applied Concepts

  • Carnot cycle heat engine
  • Carnot refrigerator
  • Carnot efficiency
  • Coefficient of performance for refrigeration
  • Coefficient of performance for heating

Temperature

  • Measure of the internal energy in a system or material
  • This energy is the motion, vibration, or rotation of atoms and molecules

Zeroth Law of Thermodynamics

  • If two systems are each in thermal equilibrium with a third system, they are also in thermal equilibrium with each other.
  • Real world example: Coffee gets cold, ice cream melts

First Law of Thermodynamics

  • Energy is conserved
  • Energy cannot be created or destroyed
  • “You can’t get something for nothing”

First Law Efficiency

  • Most commonly used measure of efficiency
  • Useful energy out divided by total energy in

Second Law of Thermodynamics

  • The amount of entropy (disorder) in a closed system always increases
  • Heat flows spontaneously from hot to cold
  • “You can’t break even”

Second Law Thermodynamic limit to heat engine

  • Carnot derived the upper limit of efficiency for a heat engine \eta = 1 - \frac{T_C}{T_H} \eta = \frac{T_H - T_C}{T_H}
  • This law dictates the maximum possible efficiency for power plants
  • Some of the heat must be released into the environment

Second Law Efficiency

  • Compares the efficiency of a system to the maximum possible efficiency given by the temperatures and the second law of thermodynamics

Exploring the Carnot Equation

What is the efficiency of a carnot heat engine when

  • T_C is near zero?
  • T_H is very high?
  • T_C and T_H are close to each other?

Heat Engine Definition

  • Heat engines convert thermal energy to mechanical kinetic energy

Heat Engine Examples

  • Coal power plant turbines
  • Internal combustion engines

Heat Engines

  • The heat engine is a mathematical model
  • Takes the heat (flow) between two thermal reservoirs and converts some of that heat to work
  • Heat can come from combustion or natural sources of heat
  • This conversion can never be 100 percent efficient because of the second law

Heat Engines

  • A heat engine is more efficient when it uses a wider temperature range between the hot and cold sides

Heat Engine

Quality

Quality

Carnot Heat Engine

  • The most efficient heat engine possible uses a Carnot cycle
  • Heat is used to expand a gas and do work and heat is removed during the compression of the gas.
  • There are many other cycles used in steam generators and internal combustion engines

Power plant

Power plant