Electricity Generation

Transcription

1 Electricity Generation Page 1 Outline Combustion Generation Based on - Thermodynamic Cycles, Chapter 4 of Energy Resources and Systems by T.K. Ghosh and M.A. Prelas, Springer Structure Operation and Management of the Electric Supply Chain, Chapter 2 of Electricity Markets by C. Harris. Published by Wiley Electric Power Systems by S.W. Blume IEEE Press 2007.

2 Methods of Generation Page 2 Electricity can be generated by converting kinetic or solar energy to electrical energy. Kinetic energy Electricity: Kinetic energy comes from the flow of steam, water or air: Hot Air/Steam flow By burning natural gas in gas-powered plants gas oil in oil-powered plants gas coal in coal-powered plants By performing nuclear reactions in nuclear plants By capturing thermal solar energy Water flow at hydroelectric plants Air flow at wind farms Wave and tidal movements can also be used Photovoltaic Solar energy Electricity Energy resource (oil & gas, coal, nuclear, thermal solar) Combustion Air/Steam flow by heat Air flow by wind Water flow by elevation Kinetic energy Generation Electrical energy

3 Combustion Page 3

4 Combustion Cycles Page 4 When fuel is burnt, its heat energy can be passed to turbines in several ways Open Cycle: Heat warms up air and hence increases pressure. Hot air flow to turbines to rotate them. Low efficiency: 25%-40%. They can be turned on quickly. Conventional Thermal Cycle (Generation): Heat boils the water into steam and increases the steam s pressure. Steam temperature is about 1000 o F and pressure about 2000 pounds per square inch. Efficiency 35% for plants of the 1970 s and increasing at the rate of 0.25 per year (roughly speaking). Efficiency is obtained at high temperatures, turning on and off lowers the efficiency. Thick metals are required to contain high-temperature steam and water. Frequent heating and cooling of the metals leads to cracks, leaks and failures. Combined Cycle Turbine: 1st cycle is identical to open cycle. Its output hot exhaust gas/air is used to boil water in the 2nd cycle. The resulting steam is passed to a steam turbine. Efficiency 45% for plants of the 1980 s and increasing at the rate of 0.25 per year. Combined Heat and Power: After the power is produced with the high temperature gas or steam, the exhaust is relatively colder. That exhaust can still be used to heat offices/homes. Nuclear Cycles: Boiling Water Reactor Pressurized Water Reactor 1 st Cycle 2 nd Cycle

5 Some Thermodynamics, Ideally Speaking Page 5 Conventional thermal cycle = Brayton cycle. Single-phase medium: Air Pressure P Air in the Combuster pipes Kinetic & Heat Energy Compressor Cooler Turbine Kinetic & Heat Energy Heat at constant P Compress P and V Turbine P and V Cool at constant P Volume V Rankine Cycle = Two-phase medium: Water + Steam Kinetic & Heat Energy Water in the pipes Compressor Combuster Cooler Turbine Kinetic & Heat Energy Steam in the pipes Pressure P Heat at constant P Compress P and V Turbine P and V Cool at constant P Volume V Combined Cycle Turbine=Brayton cycle + Rankine cycle

6 Combined Cycle Turbines Page 6 Kinetic & Heat Energy Compressor Combuster Turbine 1 Air in the pipes Rotating shaft Kinetic energy Generator 1 Cooler Heat Energy Compressor Heat Exchanger Turbine 2 Steam in the pipes Rotating shaft Kinetic energy Generator 2 Water in the pipes Cooler Heat Energy Combined Cycle Turbines Combined Heat and Power Turbine

7 Coal Power Plants Page 7 Coal provides 1 MMBtu at $4-5 or less. It can be cheaper if you buy bulk at the coal mine. The price is relatively low. Except for the recently low priced gas price, coal is the cheapest energy resource among fossil fuels. Coal is abundant: US reserves is 267,000 MM Ton and produces 1,131 MM Ton, so reserves will last at least 236 years. BUT Coal combustion creates: More carbon dioxide emissions than oil and natural gas per unit of heat. Natural Gas Combined Cycle (NGCC) plants (45-50%) are more heat efficient than (36-40%) Supercritical Pulverized Coal (SCPC) plants. Coal has more carbon than oil and gas. Sulfur dioxide, Nitrogen oxides, Mercury

8 Pulverized Coal Power Plants Pulverized Coal: Finely ground coal particles. When coal is in small particles, it burns easily, like a gas. Page ton/hr 149 o C Lime slurry 22.6 ton/hr lime CaO 145 ton/hr water 18.2 ton/hr Stack Gas 2,770 ton/hr 55 o C; 1 atm N % H % CO 2 11% O 2 4.9% Ar 0.8% SO 2 22ppm NO x 38ppm Hg <1ppb Hg: Mercury ppm 1 in million ppb 1 in billion 2,445 ton/hr 163 atm 538 o C Wet FGD Solids 41 ton/hr Flue Gas Desulfurization to remove Sulfur Dioxide Bottom Ash 4.6 ton/hr 500 MW Subcritical 500 MW Plant without CO 2 Capture Source: p. 116 of The Future of Coal by MITEI

9 Heat Efficiency & Emissions Supercritical Pulverized Coal Plants have higher temperature and pressure in the boiler. Page ton/hr 2,200 ton/hr 164 ton/hr 14.4 ton/hr Source: p. 116 of The Future of Coal by MITEI 316 atm 610 o C 3.64 ton/hr 500 MW 32.5 ton/hr Post-Combustion CO2 capture: Bind/capture the CO2 with fluids. Cost = $50-100/ton of CO2. Chemical scrubbing liquids: Mono-ethanol-amine (common), Chilled ammonia, Potassium carbonate. Physical absorption. Membrane separation Use Carbondioxide in enhanced recovery Carbon Sequestration: Storing carbon in depleted reservoirs/caves Heat efficiency is energy content in the output (500 MW) divided by content in the input coal. Subcritical Pulverized Coal Plant: 36.8%. Supercritical Pulverized Coal Plant: 39.1%.

10 Subcritical vs. Supercritical PC Power Plants Page 10 Heat Rate = input in Btu divided by output in kwh per hour. Input per hour = 208,000 kg/h in subcritical plants = 5,200 MM Btu/h = 5,200,000,000 Btu/h Recall 1 kg coal gives about MM Btu Output per hour = 550 MW = 550,000 kw Heat Rate = Input Output = 5,200,000,000 = 9,454 Btu 550,000 kwh Physical meaning: It takes 9,454 Btu to generate 1 kwh. $4.5 per million Btu 9454 Btu costs $ $0.045 = cost of coal fuel in 1 kwh. Heat rate in financial context is [electricity output price]/[gas input price]. A ratio with no physical meaning. Burn up rate in a nuclear reactor is [energy output] / [weight of fuel input]. In addition to fuel costs, Capital cost and Operating costs: Department of Energy uses 30 years lifetime to amortize costs below. Capacity Efficiency Plant cost Electricity cost Retail price Subcritical 550 MW 36.8% $852 M 6.40 cents/kwh 10 cents/kwh Supercritical 550 MW 39.1% $866 M 6.33 cents/kwh 10 cents/kwh Source:

11 Electric Energy kwh = kw h Page 11 Power is height of a bar; Energy is area under the bar. power in kw P(3) P(4) P(1) P(2) E(s) P(s) s t P(t) time in h E(s) s t P(t) E s = 1 P P P s = P P(s) s E s = P(t) t=1 E s = 0 s P t dt

12 Voltage utdallas Electric Energy from Kinetic Energy Single-phase Generator Faraday s law: A changing magnetic field creates a voltage on a conductor. Charge moves: If a charge q moves in a magnetic field B with speed v, it experiences force with magnitude F = qvb and with direction perpendicular to both B and v. Moving charge Force. Force Magnetic field changes Moving charge. Magnetic field changes: It is sufficient to change either the location of the conductor or the magnetic field. Rather than moving the conductor (charge), move the magnet (magnetic field) as shown below: Conductor 1,1 Page 12 Time S N Magnet 1,2 1,3 1,4 2,1 A voltage cycle S N Figures above are based on Figure 2-1 of Electric Power System Basics by Steven W. Blume, Published in 2007 by IEEE Press.

13 Frequency of Rotor vs. Frequency of Voltage Page 13 Rotor: Magnet and the shaft. Rotating the rotor twice faster increases frequency by a factor of 2. Frequency of rotor: revolutions per second for steam turbines. E.g., Voltage has 60 cycles per second in the USA, i.e., 60 hertz. 50 hertz in Europe. 1,1 S N 1,2 1,3 1,4 2,1 S N Blue is twice as frequent as gray.

14 Frequency of Rotor < Frequency of Voltage Multi-polar Rotors USA has 60 hertz voltage frequency; This does not necessarily mean that rotor completes 60 revolution per second. Water is dense and water turbines rotate at a low frequency: Hoover Dam rotors have 3 revolutions per second. 60 hertz voltage can be obtained with quadrupolar rotor rotating at 30 revolutions per second. 1 revolution of rotor yields 2 voltage cycles with a quadrupolar rotor. 1,1 Page 14 N N 1,2 1,3 1,4 Half-revolution of rotor 2,1 completes 1 voltage cycle Hoover Dam rotors have 3 revolutions per second, how many poles should be on the rotor to obtain 60 hertz voltage? 2 poles (N & S) yield 3 hertz; 4 poles (2N & 2S) yield 6 hertz (figures above); 6 poles yield 9 hertz; 8 poles yield 12 hertz; 2n poles yield 3n hertz, solving for n, we obtain n=20. Hoover Dam rotors have 40-poles.

15 Three-phase, Bipolar Generator Page 15 If there are more loops on the conducting coil, the magnitude of voltage is higher. Rather than putting more loops in a coil, put more coils. Three phase generator has 3 coils around the rotor. 120 degree angle S N Stator: Cylinder that covers the rotor and includes coils. Stator is static; rotor rotates. Stator Polyphase generators have multiple coils. A 6-phase, 8 polar generator has 6 coils and a magnet with 8 poles.

16 Summary Page 16 Combustion Generation

17 Which is correct? Electric vs. Electrical Page 17 Electric must be used when referring to a device that uses electrical power to operate: Electric car sales are going up. Electric guitar gives metalic sound that irritates some people. Electric blankets can cause electric shocks. Electric can be used in a metaphoric sense: The stockholder meeting had an electric atmosphere. Electric is used in specific contexts while electrical is more appropriate for general contexts: GE produces electric bulbs, microwaves, ovens, dryers and makes a substantial amount of its revenue from selling electrical goods. When the TXU technician arrived at my house, he asked for the location of electric cables. When the ATT technician asked for the location of electrical cables in my house, I showed him the electric cables, phone cables and TV cables. High definition TV is an electric machine that is designed by electrical engineers. Your suggestions for appropriate use?