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HEAT ENGINES. Introduction. Lifting an object that weighs 1 lb into the air through a height of 778 ft would require an energy of 778 ft·lb. By comparison, lighting only one match releases about the same amount of energy. If we could find a way to capture the heat energy of the match and convert it to mechanical energy to lift the object, we would ease human labor significantly. This has been the driving force behind the long history of the development of Heat Engines.
Mechanical Equivalent of Heat. • Before mid-18th century, the distinction between temperature and heat was not clear and the two were often confused. Heat was thought to be some type of fluid, called caloric, which could be added to or taken away from a substance making the substance hot or cold. • We now know that heat is a form of energy transfer that occurs when there is a temperature difference between different objects or different parts of the same object.
Mechanical Equivalent of Heat 2. • Benjamin Thompson, count Rumford (1753-1814) was the first to investigate the connection between heat and energy transfer. While supervising the boring of a canon, he became curious about the tremendous amount of heat generated during the boring process. His interest led to some detailed experiments on the nature of heat and heat capacities of materials. He concluded that the increase in temperature was due to the work done during the boring process. Despite his findings, popular notion of heat as a fluid caloric still persisted, since that theory explained many of the results in which people were interested.
Mechanical Equivalent of Heat 3. • Next person to investigate the connection between heat and energy was Julius Mayer who, in 1842, suggested that heat and mechanical work were equivalent and that one could be transformed into the other. He went as far as showing that the temperature of water could be raised by 1oC by mechanical agitation alone; however, he failed to determine the amount of work needed for such a change in temperature. • The quantitative connection between heat and mechanical work was conclusively demonstrated by James Prescott Joule in1843. Joule devised an experiment in which the potential energy of falling weights was used to churn water in an insulated container. He showed that 1Btu = 772ft·lb. More precise measurements yield 1Btu = 778ft·lb.
Diagram above shows the general pathways by which we utilize energy from fossil fuels. Heat is derived from fossil fuels by ‘burning’ the fuel. Burning of hydrocarbon fuels involve the combining of the carbon and the hydrogen from the fuel with oxygen from the air. Energy Content of Fossil Fuels.
Energy Content of Fossil Fuels 2. A more scientific term for ‘burning’ (the addition of oxygen to) a substance is oxidation. The oxidation of hydrocarbons proceed according to the following basic chemical reactions: • C + O2→ CO2 + heat • H2 + O → H2O + heat The two reactions above are included in the overall reaction formula for an actual fuel. For example, the oxidation reaction for heptane, C7H16 is: C7H16 + 11O2 → 7CO2 + 8H2O + 1.15x106cal per 100g of C7H16 CO2 and H20 are the only products of this reaction.
Energy Content of Fossil Fuels 2. • Burning or oxidation of a hydrocarbon compound is the reverse of photosynthesis. By burning a hydrocarbon, we put back into air and water what was once taken from them billions of years ago. • The 1.15x106 cal per 100g shown in the oxidation reaction for heptane is known as the heat of combustion for heptane. Every fuel has a tabulated value for this quantity. The heat of combustion is the definite maximum amount of energy available from the fuel.
Thermodynamics of Heat Engines. • A heat engine is any device that can take energy from a warm source and convert a fraction of the heat energy to mechanical energy. Diagrammatically, a heat engine is represented as follows: Heat Source, Thot Heat Engine Qhot Work Output W = Qhot - Qcold Heat Engine Qcold Heat Sink, Tcold
Thermodynamics of Heat Engines. • The thermal efficiency of any system is defined to be the ratio of the work done to the heat input: • For any heat engine, • A Carnot engine is an ideal heat engine. For the Carnot Engine, Qcold/Qhot = Tcold/Thot. Therefore,
Generation of Electricity In 1831, Michael Faraday discovered the phenomenon now called electromagnetic induction. Faraday observed that by moving a magnet near a loop of conductor wire an electric current is induced in the wire. This discovery is often considered the greatest invention of all time; it made the generation and transmission of electricity possible. Electricity generators and motors were quickly invented as a result of this discovery; communication and computer systems were also developed as a result of this discovery. A simple form of an electricity generator is shown in fig. 3.3 of textbook.
Generation of Electricity 2 • In electric power plants the coils of the generator are mechanically connected to steam turbines or water driven hydro-electric turbines at large dams. Fig 3.4 shows the main components of a fuel burning electric power plant. • Efficiencies of fuel burning electric power plants have increased steadily over the years. Typical efficiencies are about 30%; i.e. about two-thirds of the fuel energy is rejected as waste heat. For a plant generating 1000MWe, 2000MWt is released into the environment as waste heat. The energy from the fuel into the steam boiler is 3000MWt. In general, three units of fuel is burned to output one unit of electric energy. The best new plants now have efficiencies close to 40%.
Practical Heat Engines • The first practical heat engine was developed by Thomas Savery in 1698 and was called the Miner’s Friend because it was meant to save coal miners from accidental drowning by pumping water out of the mines. The next steam engine, called the Newcomen engine, was designed and built by Thomas Newcomen in collaboration with Savery. Major advances were made by James Watt and others; and before long steam engines appeared everywhere (in Europe in the 18th century) to ease human labor. • The steam turbine was developed by Parsons in 1880. The steam turbine is now the basis for most of our electricity generation.
Steam Engines • When water is boiled to steam at atmospheric pressure its volume expands a thousand times; this is the basis of how steam engines operate. If the steam is confined, pressure builds up and the steam tries to expand with great force. The force can be exerted against a piston or the blades of a turbine. • Earliest steam engines had efficiencies of less that 1%; they burned prodigious amounts of fuel. Improvements in technology resulted in efficiencies above 10% by 1900 and now above 30%. • Steam engines belong to a broad class of engines called External Combustion Engines. In this type of engine, the fuel is burned outside the pressurized part of the engine at atmospheric pressure and relatively low temperature in the presence of plenty of air. As a result, relatively low amounts of carbon monoxide and nitrous oxide are emitted.
Gasoline Engines Gasoline engines belong to the class of heat engines called Internal Combustion Engines. In this type of engine a fuel, such as gasoline, is vaporized and mixed with air to form a combustible mixture inside a closed chamber (fig 3.9). The mixture is compressed to about 6 to 10 times atmospheric pressure and then ignited with an electric spark timed to fire at the right instant. The fuel burns suddenly (explodes) upon ignition, forming hot gases ( mainly CO2 and H2O for hydrocarbon fuels). The resulting hot gases expand with great force against a piston causing a crankshaft attached to the piston to rotate. About 20% of the chemical energy stored in the fuel can be converted into mechanical energy in a modern gasoline engine.
Diesel Engines The diesel engine is also an internal combustion engine, in many ways similar to the gasoline engine. The main differences between diesel and gasoline engines are: • The diesel engine does not need an electric spark to ignite the fuel and air mixture. • The diesel engine does not mix the fuel and air before admitting them into the combustion chamber. • The diesel engine is generally heavier and bulkier than the gasoline engine. • The diesel engine usually runs at a lower speed and it may be slower to respond to the need for more power. • Diesel engines are significantly more efficient than gasoline engines (30% for diesel; 20% for gasoline).
Diesel Engines 2 • Diesel engine completely replaced steam engines on railroad locomotives by the 1960s. The efficiency of about 6% for locomotives with reciprocating steam engines was no match for the higher efficiency of the diesel engine. • For the diesel engine (fig 3.10) the combustion chamber contains only air during the compression stroke. The compression ration is high, about 15 atms, causing the temperature to increase to the ignition point for fuel-air mixture. At the peak of the compression stroke, a short burst of fuel is injected by spraying unto the combustion chamber and instantly ignites as it mixes with the compressed air in the chamber. Due to the higher combustion temperature than the gasoline engine, a higher thermodynamic efficiency is achieved.
Diesel Engines 3 Common diesel fuel is middle-grade fuel heavier than gasoline. It has about 10% more BTU per gallon than gasoline. This and the higher efficiency adds to the operating cost advantage of diesel over gasoline engines on a cost per gallon basis. CO emission by diesel engines is very low, less than 10% of gasoline engine emissions, due to the fact that diesel combustion takes place in an excess of air (oxygen) in the combustion chamber relative to the amount of fuel admitted; any CO formed is quickly oxidized to CO2. • Disadvantages of diesel include: (1) greater noisiness; (2) greater initial cost; (3) greater weight; (4) harder to start in cold weather; (5) Characteristic odor; (6) Occasional emissions of visible (black) smoke and particulates.
Gas Turbines. One of the newer types of internal combustion heat engines that powers aircraft and a few smaller electric power plants. In the gas turbine, air is drawn in from the front of the engine and compressed by a fanlike compressor. After the compression the air is mixed with finely dispersed fuel and the mixture is ignited, causing the heated gases to expand. The expanding gases move through the turbine as they exit through the exhaust at the rear of the engine. Due to the expansion, the exhaust gases move out at higher velocities than the air entering the compressor. This is what gives the jet engine its thrust. The gas turbine can be viewed as an internal combustion engine operating smoothly and continuously, in contrast to the sequential pulses of piston engines.
Gas Turbines 2. • Current gas turbines have efficiencies of about 20% - 30% for converting thermal energy into mechanical energy. • Turbines have the advantage of being light weight relative to their power applications (e.g. helicopter applications). They respond well to sudden power demands (as needed by helicopters and electric utility peaking power); and relatively inexpensive. • Gas turbine power plants are finding their way into cogeneration facilities where they are a good match to the need.
Heat Pumps. Heat pumps are heat engines run backwards (as shown in diagram on the right above); i.e. heat is taken from the cold reservoir and delivered to the hot reservoir.
Heat Pumps 2. • Heat pumps are highly attractive for use in space heating applications. They are also the basis for the functioning of refrigerators and air conditioners. Heat pumps take heat from outside (which is the cold source) and deposit it inside an enclosure (e.g. a room, which is the hot source) . • The Carnot efficiency of an ideal heat pump is called the Coefficient of Performance, C.O.P. C.O.P(hp) = Qh/W = Qh/(Qh-Qc) = Th/(Th-Tc). • For a refrigerator or ac, the heat pump takes heat from inside the enclosure (e.g. inside a fridge or a room; the cold source) and deposits it outside, (the hot source). C.O.P(ref. or ac) = Qc/W = Qc/(Qh-Qc) = Tc/(Th-Tc).
Cogeneration. The operation of a heat engine is accompanied by the rejection of large amounts of waste heat energy. For large plants, this waste heat is dissipated into the atmosphere through cooling towers, or large bodies of water such as cooling ponds, rivers, lakes, even the ocean. In addition to the expensive equipment involved in disposing of the waste heat, there are concerns about the environmental effects of the waste heat added to the atmosphere or the body of water. The rejected heat is of lower quality than that which powered the heat engine; however, it can still be used in a beneficial way instead of just throwing it away. In some applications, the waste heat can be used for space heating and cooling of homes, factories , as well as industrial process heat used in manufacturing.
Cogeneration 2. The beneficial use of the waste heat as discussed on the previous slide is termed cogeneration. An example of cogeneration is provided by the heater in our automobiles. Waste heat from the engine, which otherwise would be thrown away, is used to provide a comfortable passenger compartment. An excellent example of cogeneration is provided by the University of Colorado in Boulder, where a newly installed electricity generating plant powered by gas turbines provides electricity to the entire campus. The rejected heat from the plant is used for space heating (and even cooling). Sometimes the plant generates more electricity than the University needs and the excess is sold to the public utility and put into the grid. The result is a significant reduction in the University’s energy costs. (Fig.3.13 shows a small cogeneration plan).
