Energy and the Environment Second Edition Chapter 3: Heat Engines Copyright © 2006 by John Wiley & Sons, Inc. Robert A. Ristinen Jack J. Kraushaar

The Mechanical Equivalent of Heat The energy equivalent of the burning of 1 match is equal to 1 BTU (778 foot-pounds). The possibility of easing human labor by utilizing heat sources has been the driving force behind a long history of development of what we now call heat engines.

The Energy Content of Fuels Burning of hydrocarbon compounds (fossil fuels) is the reverse of photosynthesis and follows these basic chemical reactions: C + O 2 → CO 2 + heat energy H 2 + O → H 2 O + heat energy (Burning of fossil fuels returns to the atmosphere the carbon dioxide and water vapor that was once present in the ancient Earth and was removed through photosynthesis.)

The Thermodynamics of Heat Engines A heat engine is defined as any device that can take energy from a warm source and convert a fraction of this heat energy to mechanical energy. There is a fundamental law of physics that states that some of the heat energy taken from the source must always be rejected, at a temperature cooler than that of the source to the environment. The Carnot efficiency is a number always less than 100% that represents the percentage of the energy taken from the source that is converted to useful mechanical work, under the assumption of an ideal engine.

The Thermodynamics of Heat Engines Efficiency = (work done) / (energy put into the system), or Efficiency = (Q hot – Q cold ) / (Q hot ), or Rearranging this becomes (1 – Q cold /Q hot ) x 100% Efficiency (Carnot) = (1 – T cold /T hot ) x 100% To make the Carnot efficiency as high as possible, it would be desirable to increase T hot and decrease T cold.

Generation of Electricity Michael Faraday (1831) – discovered electromagnetic induction. The movement of a magnet near a loop of wire induces a magnetic current in the wire. The energy required to rotate the wire within a magnetic field may come in the form of heat from a boiler, burning of fossil fuels, steam, etc. Standard large modern power plants may provide 1000 MW e of electricity and release 2000 MW t of thermal power as waste heat to the environment.

Electric Power Transmission Thomas Edison was responsible for the first electric power system in NYC in It initially used direct current (DC). It was later realized that alternating current (AC) offered greater flexibility in changing the voltage at different points in the system with transformers.(DC)(AC) Energy is lost in power lines proportional to the line resistance (in ohms) multiplied by the square of current (in amperes). By increasing the voltage delivered through the lines, the resistive loss is lowered. Step-up transformers perform the task of raising the voltage before connecting to long-distance transmission lines. Transformers reduce the voltage prior to entering our homes.

Practical Heat Engines Practical heat engines began to appear about 300 years ago with the use of steam engines. The steam turbine was introduced by Parsons in the 1880s and is now the basis for most electricity generation. Steam engines – (Efficiencies of steam engines have increased from 1% early on to nearly 30% nowadays.) Steam Gasoline engines – Most early vehicles were powered by external combustion engines. Most modern vehicles use internal combustion. Fuel (gasoline) is vaporized and mixed with air to form a combustible mixture in a closed chamber then ignited with an electric spark. (About 25% of the chemical energy is converted to mechanical energy.)

Practical Heat Engines Diesel engines – similar to gasoline engines but no electric spark ignition is required. The fuel and air is not mixed prior to being admitted to the combustion chamber. Diesel engines are more efficient at converting its fuel energy into mechanical energy (greater than 30%). Carbon monoxide emissions are also very low (less than 10% that of gasoline). However, it has greater noisiness, harder starting in cold weather, odor, and greater emissions of smoke, particulates, and nitrogen oxides. Gas turbines – (jet aircrafts) Air is brought in at the front of the engine and mixed with fuel and ignited. The expanding gases enter through a turbine and exit the back at much higher velocities.

Heat Pumps Heat pumps work in the opposite direction of that of heat engines. (Removes energy from a cold place and delivers it to a warm one.) Heat pumps are the basis for space heating, refrigeration, and air conditioning. The efficiency of a heat pump can be measured using the ratio of the coefficient of performance (C.O.P): C.O.P. = Q h / W = (Q h ) / (Q h – Q c ) = (T h ) / (T h – T c )

Cogeneration Heat engines always rejects heat energy as waste. This lost energy is a concern from a monetary and environmental standpoint. The rejected heat is of lower quality but can be useful in many applications. This is known as cogeneration. The University of Colorado in Boulder uses an electric- generating plant to provide energy to the campus and uses its waste heat for space heating and cooling. The excess is put into the public utility lines. This facility has an energy consumption efficiency of ~70%.