Green Energy Course- Renewable Energy Systems Biên sọan: Nguyễn Hữu Phúc Khoa Điện- Điện Tử- Đại Học Bách Khoa TPHCM CHAPTER 2: The Electric Power Industry •Little more than a century ago there were no lightbulbs, refrigerators, air conditioners, or any of the other electrical marvels that we think of as being so essential today. •Indeed, nearly 2 billion people around the globe still live without the benefits of such basic energy services. •The electric power industry has since grown to be one of the largest enterprises on the planet, with annual sales of over $300 billion in the United States alone. •It is also one of the most polluting of all industries, responsible for three-fourths of U.
sulfur oxides (SOX) emissions, one-third of our carbon dioxide (CO2) and nitrogen oxides (NOX) emissions, and one- fourth of particulate matter and toxic heavy metals emissions. Major Electricity Milestones THE ELECTRIC UTILITY INDUSTRY TODAY Conventional power generation, transmission, and distribution system. Electric utilities, monopoly franchises, large central power stations, and long transmission lines have been the principal components of the prevailing electric power paradigm since the days of Insull. Utilities and Nonutilities Entities that provide electric power can be categorized as utilities or nonutilities depending on now their business is organized and regulated Nonutility generators (NUGs) are privately owned entities that generate power for their own use and/or for sale to utilities and others.
Nonutility generators have become a significant portion of total electricity generated in the United States. From EIA Annual Energy Review 2001 (EIA, 2003). primary energy: The energy going into power plants end-use energy, which is the energy content of electricity that is actually delivered to customers. • The numerical difference between primary and end-use energy is made up of losses during the conversion of fuel to electricity, losses in the transmission and distribution system (T&D), and energy used at the power plant itself for its own needs.
• Less than one-third of primary energy actually ends up in the form of electricity delivered to customers. •For rough approximations, it is reasonable to estimate that for every 3 units of fuel into power plants, 2 units are wasted and 1 unit is delivered to end-users. Electricity flows as a percentage of primary energy. Based on EIA Annual Energy Review 2001 (EIA, 2003).
Distribution of retail sales of electricity by end use. Residential and commercial buildings account for over two-thirds of sales. Total amounts in billions of kWh (TWh) are 2001 data. The load profile for the a peak summer day in California (1999) shows maximum demand occurs between 2 P.
Lighting and air conditioning accounts for over 40% of the peak. End uses are ordered the same in the graph and legend. From Brown and Koomey (2002). Average retail prices of electricity, by sector (constant $1996).
From EIA Annual Energy Review 2001 (EIA, 2003). CARNOT EFFICIENCY FOR HEAT ENGINES Over 90% of world electricity is generated in power plants that convert heat into mechanical work. The heat may be the result of nuclear reactions, fossil-fuel combustion, or even concentrated sunlight focused onto a boiler. Almost all of this 90% is based on a heat source boiling water to make steam that spins a turbine and generator, but there is a rapidly growing fraction that is generated using gasturbines.
The best new fossil-fuel power plants use a combination of both steam turbines and gas turbines to generate electricity with very high efficiency. Steam engines, gas turbines, and internal-combustion engines are examples of machines that convert heat into useful work. What we are interested in here is, How efficiently can they do so? This same question will be asked when we describe fuel cells, photovoltaics, and wind turbines, and in each case we will encounter quite interesting, fundamental limits to their maximum possible energy-conversion efficicencies. Heat Engines a heat engine extracts heat QH from a high-temperature source, such as a boiler, converts part of that heat into work W, usually in the form of a rotating shaft, and rejects the remaining heat QC into a low- temperature sink such as the atmosphere or a local body of water.
A heat engine converts some of the heat extracted from a high- temperature reservoir into work, rejecting the rest into a low- temperature sink. Entropy and the Carnot Heat Engine The definition of Entropy (extremely important quantity ) is not very intuitive. It can be described as a measure of molecular disorder, or molecular randomness. At one end of the entropy scale is a pure crystalline substance at absolute zero temperature.
Since every atom is locked into a predictable place, in perfect order, its entropy is defined to be zero. In general, substances in the solid phase have more ordered molecules and hence lower entropy than liquid or gaseous substances. When we burn some coal, there is more entropy in the gaseous end products than in the solid lumps we burned. That is, unlike energy, entropy is not conserved in a process.
In fact, for every real process that occurs, disorder increases and the total entropy of the universe increases. if an amount of heat Q is removed from a “large” thermal reservoir at temperature T (large enough that the temperature of the reservoir doesn’t change as a result of this heat loss), the loss of entropy S from the reservoir is defined as (*) where T is an absolute temperature measured using either the Kelvin or Rankine scale. Equation * suggests that entropy goes down as temperature goes up. •The following constraint on the efficiency of a heat engine •The maximum possible efficiency of a heat engine is given by STEAM-CYCLE POWER PLANTS Conventional thermal power plants can be categorized by the thermodynamic cycles they utilize when converting heat into work.
Utility-scale thermal power plants are based on either (a) the Rankine cycle, in which a working fluid is alternately vaporized and condensed, or (b) the Brayton cycle, in which the working fluid remains a gas throughout the cycle. (c) Most baseload thermal power plants, which operate more or less continuously, are Rankine cycle plants in which steam is the working fluid. (d) Most peaking plants, which are brought on line as needed to cover the daily rise and fall of demand, are gas turbines based on the Brayton cycle. (e) The newest generation of thermal power plants use both cycles and are called combined-cycle plants.
Basic Steam Power Plants A fuel-fired, steam-electric power plant. Let us use the Carnot limit to estimate the maximum efficiency that a power plant such as that shown in Fig. can possibly have. A reasonable estimate of TH , the source temperature, might be the temperature of the steam from the boiler, which is typically around 600◦C.
For TC we might use a typical condenser operating temperature of about 30◦C. the average efficiency of power plants is only about half this amount. Coal-Fired Steam Power Plants Mass flows to generate 1 kWh of electricity in a 33.3% efficient, coal-fired power plant burning bituminous coal. Typical modern coal-fired power plant using an electrostatic precipitator for particulate control and a limestone-based SO2 scrubber.
A cooling tower is shown for thermal pollution control. the average new steam plant is about 34% efficient and has a heat rate of approximately 10,000 Btu/kWh. The best steam plants have efficiencies near 40% COMBUSTION GAS TURBINES Basic Gas Turbine A basic gas turbine driving a generator is shown in Fig. In it, fresh air is drawn into a compressor where spinning rotor blades compress the air, elevating its temperature and pressure.
This hot, compressed air is mixed with fuel, usually natural gas, though LPG, kerosene, landfill gas, or oil are sometimes used, and subsequently burned in the combustion chamber. The hot exhaust gases are expanded in a turbine and released to the atmosphere. The compressor and turbine share a connecting shaft, so that a portion, typically more than half, of the rotational energy created by the spinning turbine is used to power the compressor. Basic gas turbine and generator.
Temperatures and efficiencies are typical. Basic Gas Turbine Fuel Combustion AC 100% chamber 1150 C o Power 33% Generator Compressor Turbine Fresh 550 oC Exhaust air gases 67% Brayton Cycle: Working fluid is always a gas Maximum Efficiency 550 273 Most common fuel is natural gas 1 42% 1150 273 Typical efficiency is around 30 to 35% Gas Turbine Source: Masters One way to increase the efficiency of gas Steam-Injected Gas Turbines (STIG) turbines is to add a heat exchanger, called a heat recovery steam generator (HRSG), to capture some of the waste heat from the turbine. As shown in Fig. , water pumped through the HRSG turns to steam, which is injected back into the airstream coming from the compressor.
The injected steam displaces a portion of the fuel heat that would otherwise be needed in the combustion chamber. These units, called steam injected gas turbines (STIG), can have efficiencies approaching 45%. Moreover, the injected steam reduces combustion temperatures, which helps control Steam-injected gas turbine (STIG) for NOx emissions. increased efficiency and reduced NOx emissions.
Efficiencies may approach They are considerably more expensive than 45%. simple gas turbines due to the extra cost of the HRSG, and the care that must be taken to purify incoming feedwater. COMBINED-CYCLE POWER PLANTS Combined-cycle power system with representative energy flows providing a total efficiency of 49%. Efficiencies of up to 60% can be achieved, with even higher values when the steam is used for heating GAS TURBINES AND COMBINED-CYCLE COGENERATION Simple-cycle gas turbine with a steam generator for cogeneration showing typical conversion efficiencies.
Representative energy flows for a combined-cycle, cogeneration plant with back-pressure steam turbine, delivering thermal energy to a district heating system. Combined Heat and Power (CHP) Fuel Combustion 100% Exhaust gases AC chamber Power 33% Compressor Turbine Generator Fresh air Steam 53% Feedwater Heat recovery steam Process heat Water pump generator (HRSG) Absorption cooling Space & water heating Exhaust 14% Overall Thermal Efficiency = 33% (Electricity) + 53% (Heat) = 86% BASELOAD, INTERMEDIATE AND The fluctuations in demand suggest that during the PEAKING POWER PLANTS peak demand, most of a utility’s power plants will be operating, while in the valleys, many are likely to be idling or shut off entirely. In other words, many power plants don’t operate with a schedule anything like full output all of the time. It has also been mentioned that some power plants, especially large coal-fired plants as well as hydroelectric plants, are expensive to build but relatively cheap to operate, so they should be run more or less continuously as baseload plants; others, such as simple-cycle gas turbines, are relatively inexpensive to build but expensive to operate.
They are most appropriately used as peaking power plants, turned on only during periods of highest demand. Other plants have characteristics that are somewhere in between; these intermediate load Example of weekly load fluctuations and plants are often run for most of the daytime and then roughly how power plants can be cycled as necessary to follow the evening load. categorized as baseload, intermediate, or peaking plants. Figure suggests these designations of baseload, intermediate, and peaking power plants applied to a weeklong demand curve.
Screening Curves screening curves that show annual revenues required to pay fixed and variable costs as a function of hours per year that the plant is operated. capacity factor as the ratio of average power to rated power The average cost of electricity is the slope of the line drawn from the origin to point on the revenue curve that corresponds to the capacity factor. Screening curves for coal-steam, combustion turbine, and combined-cycle plants based on data in Table 3. For plants operated less than 1675 h/yr, combustion turbines are least expensive; for plants operated more than 6565 h/yr, a coal-steam plant is cheapest; otherwise, a combined-cycle plant is least expensive.
Load–Duration Curves A load–duration curve is simply the hour-by-hour load curve rearranged from chronological order into an order based on magnitude.