1. The Energy Content of Fuels

Transcription

1 Heat Engines

2 1. The Energy Content of Fuels How heat is derived from fuels? For example, we may consider the burning process for heptane, C 7 H 16, colorless liquid constituent of gasoline. C 7 H O 2 7CO 2 + 8H 2 O x10 6 calories per 100g C 7 H 16 Carbon dioxide and water are the only material products of the reaction, and the energy liberated is in the form of heat. The number at the right in the formula is 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 a fuel, which cannot be exceeded.

3 1. The Energy Content of Fuels Two basic purposes to obtain fossil fuels: to provide direct heating and lighting, and to power heat engines Figure 3.1 The general pathways by which we utilize energy from fossil fuels

4 2. The Mechanical Equivalent of Heat 1 Btu = 778 foot-pound Unit for heat energy: 1Btu (raise the temperature of one pound of water by one degree Fahrenheit) Unit for mechanical energy: 1 foot-pound (raise one pound of water one foot higher) Which one is larger? You can lifting a one-pound weight 778 feet into the air with the energy released by the burning of only one match. Capture the heat energy of the fuel and turn it into mechanical energy. 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.

5 3. The Thermodynamic of Heat Engines A heat engine is any device that can take energy from a warm source and convert a fraction of this heat energy to mechanical energy. Figure 3.2 A thermodynamic diagram of a heat engine operating between a heat source and heat sink at a lower temperature. The work output must equal the difference between the heat energy extracted from the source and that rejected to the sink the Principle of Energy Conservation.

6 Not all of the heat energy taken from the source is being used to performed useful work. Some fraction of the heat energy must always be rejected, at a temperature cooler than that of the warm source, to the environment. The second law of thermodynamics Kelvin statement: It is impossible to convert heat completely into works in a cyclic process. Clausius statement: Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature. In a system, a process that occurs will tend to increase the total entropy of the universe. Efficiency = work done energy put into the system < 100%

7 Efficiency = work done energy put into the system Efficiency = Q hot - Q cold Q = (1 - cold ) x 100% Q hot Q hot For an ideal engine, the ratio of two energy terms is identical to the ratio of two temperature terms: Q cold / Q hot = T cold / T Carnot hot where the temperatures are given on the absolute (Kelvin) scale. T cold Efficiency = (1 - ) x 100% T hot It is remarkable that this efficiency (Carnot) depends only on the temperatures of the two reservoirs between which the heat engine operates.

8 A generalized thermodynamic cycle A Carnot cycle taking place between a hot reservoir at temperature T H and a cold reservoir at temperature T C. T cold Efficiency = (1 - ) x 100% T hot

9 Example: For a coal-fire electric power plant, T hot (the boiler temperature) would be 825 K, and T cold (the cooling tower) would be about 300 K. This leads to Efficiency = (1 300 / 825) x 100% = (1 0.36) x 100% = 64% In this case, 36% of the heat energy from the energy of the fuel must be wasted by rejecting it through the cooling tower to the surrounding atmosphere. To make the efficiency as high as possible, it would be desirable to increase T hot and decrease T cold. The limit on T hot is imposed by the materials from which the boilers can be constructed and the limit on T cold is imposed by the availability in nature of large sinks at sufficiently low temperature.

10 4. Generation of Electricity In 1831, in London, Michael Faraday ( ) discovered electromagnetic induction one of the greatest discoveries of all time. Electromagnetic induction is the production of voltage across a conductor situated in a changing magnetic field or a conductor moving through a stationary magnetic field. The discovery made the generation and transmission of electricity possible, and quickly lead to the invention of electric generators. "This is all very interesting, but of what possible use are these toys?" "I cannot say what use they may be, but I can confidently predict that one day you will be able to tax them." Of what use is a newborn baby?

11 Figure 3.4 An elementary alternating current generator. A loop of wire is forced to rotate in a magnetic field. The induced alternating current enters the external circuit through contacts (carbon brushes) that rub against rotating metal rings, called slip rings, attached to the coil. The current generated, I, reverses in direction as the coil rotates. right hand rule & left hand rule The current induced in the coils according to the Faraday law interacts with the magnetic field to resist the motion of the coils through the field. Therefore it takes energy from some external source to force the rotation.

12 Most electric power plants have the rotating coils of the generator mechanically connected to steam turbines or to water-driven hydroelectric turbines at large dams. The main components of a typical electric power plant are shown in the figure below. Figure 3.3 A diagram of a fuel-burning electric power plant. Here a river provides cooling water to the condenser, but lake water or a cooling tower could serve the same purpose.

13 Figure 3.5 Typical efficiency of an electric power plant for converting chemical energy in the fuel into electric energy. The best new plants now achieve nearly 40%.

14 5. Electric Power Transmission In order for electric energy to be useful to society, it must be transported in some way from the power plants to factories or residences. The first electric power system was developed by Thomas Edison in New York in 1882, using direct current (DC). Alternating current (AC) offered greater flexibility in changing the voltage at different points in the system with transformers. Raising the voltage continues to be the main way to reduce transmission losses. (The loss is proportional to I 2 R.)

15 Diagram of an electrical system Transmission and distribution losses in the USA were estimated at 7.2% in The system of generating stations, substations and transmission lines is called the electric power grid. Most of the electric power companies in North America are integrated into a single power grid (3000 power plants, 200,000 miles of high-voltage transmission lines) for reasons of economy, availability of backup power for emergencies, and ability to trade energy. Wireless Transmission of Electricity?!

16 6. Practical Heat Engines Heat engines have steadily improved since they were first invented 300 years ago. Early Watt pumping engine (James Watt, 1770s) First compound Steam Turbine, built by Charles Parsons in 1887 basis for most of our electricity generation now. The 1698 Savery Engine the first commercially-useful steam engine: built by Thomas Savery.

17 5.1 Steam Engines A steam engine is a heat engine that performs mechanical work using steam as its working fluid. Principle of operation for a steam engine: When water is boiled to steam at atmosphere pressure, its volume expands about a thousand times. If the steam is confined, pressure builds up and the steam tries to expand with great force. This force can exerted against a piston or it can work against the blades of a turbine. A locomotive powered by the force of steam against pistons. The motion of the pistons is coupled by connecting rods directly to the drive wheels.

18 Working Mechanism for Steam Turbine When steam at high pressure is admitted to one side of a turbine, it will force the blade assembly to rotate. This rotation is achieved most effectively if the exhaust side of the turbine is maintained at low pressure. The low pressure condition is assured by the presence of a condenser, typically a chamber into which the exhaust steam is admitted and kept cool by a flow of water from a river, lake, or other source. A rotor of a modern steam turbine, used in a power plant The mechanical energy of the rotating turbine can be coupled to other machinery, often to an electric generator.

19 Steam engines belong to the broad class of external combustion heat engine. In engine of this type, the fuel is burned outside of the pressurized part of the engine, at a relatively low temperature, at atmospheric pressure, and in the presence of an abundance of air. Low emission of carbon monoxide and nitrogen oxides Emission of sulfur oxides and particulates depends on the fuel being burned.

20 5.2 Gasoline Engines The heat engine we now have in almost all of our motor vehicles are internal combustion engines. Gasoline is vaporized and mixed with air to form a combustible mixture inside a closed chamber. The mixture is compressed to about 6 to 10 times atmospheric pressure, then ignited with an electric spark. On ignition, the fuel explodes, forming hot gases (CO 2, H 2 O, N 2 ). The resulting hot gases (1000 o C) expand with great force against the piston, causing the crankshaft to rotate. About 25% of the chemical energy in the fuel can be converted to mechanical energy in a modern gasoline engine. Figure 3.9 The four strokes of a four spark-ignited internal combustion engine: (a) compression, (b) combustion, (c) exhaust, and (d) fuel-air intake.

21 5.3 Diesel Engines The diesel engine is an internal combustion engine similar to the gasoline engine. Difference: Diesel engine does not use electric spark ignition and it does not mix the fuel and air before admitting them to the combustion chamber. During the compression stroke, the diesel combustion chamber contains only air. The pressure can reach as high as 15 atm and the temperature of the compressed air has been increased to the ignition point for a fuel-air mix. A short burst of fuel is injected into the chamber. The fuel mixes with the hot air and immediately ignites without the help of an electric spark. Figure 3.10 Cutaway drawing of a diesel engine. Ignition is accomplished by the high temperature produced by the compressed of air.

22 Advantage: The combustion temperature of diesel engine is higher than in a spark-ignited gasoline engine, producing a higher thermodynamic efficiency (> 30%). Diesel fuel has about 10% more Btu per gallon than gasoline. Low CO emission due to an excess of air (and oxygen) in the combustion chamber Disadvantage: Greater noisiness, initial cost, and weight Harder starting in cold weather Characteristic odor and visible smoke Greater emission of nitrogen oxides

23 5.4 Gas Turbines Diagram of a gas turbine engine A continuously running gasoline engine A gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Energy is added to the gas stream in the combustor, where air is mixed with fuel and ignited. Combustion increases the temperature and volume of the gas flow. This is directed through a nozzle over the turbine's blades, spinning the turbine and powering the compressor. Energy is extracted in the form of shaft power, compressed air and thrust, in any combination, and used to power aircraft, trains, ships, generators, and even tanks.

24 Diagram of a gas turbine engine different design for jet plane and electricity generator For electricity generator: the turbine is designed to give larger fraction of its power to the rotating shaft which is connected to the shaft of the generator. For aircraft jet engine: the shaft needs only enough power for the compressor fan, the rest of the energy going into kinetic energy of the exhaust gases. Advantage: light, inexpensive, respond well to sudden power demand Disadvantage: relative low efficiency (20 to 30%)

25 7. Heat Pumps The second law of thermodynamics (Clausius statement) Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature. Can we remove energy from a cold place and deliver it to a warmer place? Can we run the heat engine backward? Heat pump: a device using energy input in the form of work to cause the transfer of heat energy from the low T to the another reservoir with higher T. Figure 3.11 A thermodynamic diagram of a heat pump. A work input, W, is required to transfer an amount of energy Q cold, out of a cold reservoir and a larger amount, Q hot, into a hot reservoir. Because energy is conserved, Q hot must equal W + Q cold.

26 The Coefficient of Performance (COP) is used to measure the effectiveness of the heat pump. COP = Q h /W = Q h /(Q h Q c ) = T h /(T h T c ) Example: Calculate the ideal COP for an air-to-air heat pump used to maintain the temperature of a house at 21 o C when the outside temperature is -1 o C. Solution T h = 21 o C = 294 K T c = -1 o C = 272 K COP = T h / (T h T c ) = 294K/(294K 272K) = 13.3 Thus, for each watt of power used to drive this heat pump, 13.3 watts are delivered to the hot reservoir (the interior of the house), and 12.3 watts are extracted from the cold reservoir (the outside the house). In practice, the COP for such a situation would be much less favorable (in the range of 2-6).

27 COP = Q h /W = Q h /(Q h Q c ) = T h /(T h T c ) The COP will diminish if the outside temperature drops. So electrically driven air-to-air heat pumps are most useful in moderate climate (> -10 o C). In addition to the air-to-air heat pumps, it is common to use the ground at several feet of depth, or surface water such as a river for the cold reservoir.

28 On passage through an expansion valve adiabatic into a region of expansion lower pressure, the liquid expands into a gas, becoming very much colder. The warm gas flows through a heat exchanger where it is cooled by a flow of room T air and condensed to a liquid, thus giving up heat to the room. The extremely cold gas flows through a second heat exchanger where it is warmed to the outside air T, thus extracting heat from the outside air. Freon gas is compressed by a compressor to raise its T and P. Figure 3.12 An electrically driven heat pump using Freon as a working fluid.

29 8. Cogeneration ( 废热发电, 热电联产 ) Cogeneration (also combined heat and power, CHP) is the use of a heat engine to simultaneously generate both electricity and useful heat. The operation of a heat engine for any purpose is necessarily accompanied by the rejection of heat energy, often in large amounts. The waste heat is generally dissipated into the atmosphere or an adjacent body of water with possible negative environmental effects. A simple example of beneficial use of waste heat energy is using the heater in an automobile. A cogeneration plant uses the rejected heat energy from electricity generator for space heating in cities. A cogeneration plant is usually a small, decentralized electric generating plants near the point of use, even right in the middle of a university campus.

30 Efficiency = work done + Q cold energy put into the system

31 Figure 3.13 A small cogeneration plant that uses the combustion of natural gas to drive a gas turbine coupled to an electric generator. The hot exhaust gases boil water to steam for use in space heating. Efficiency of a new coal-fired electric power plant 38% Overall efficiency of a cogeneration can reach 70%.