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This chapter explores the basics of electricity, generation from fossil fuels, and technical considerations and efficiencies.
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Energy and the New Reality, Volume 1:Energy Efficiency and the Demand for Energy ServicesChapter 3: Electricity from Fossil FuelsL. D. Danny Harveyharvey@geog.utoronto.ca This material is intended for use in lectures, presentations and as handouts to students, and is provided in Powerpoint format so as to allow customization for the individual needs of course instructors. Permission of the author and publisher is required for any other usage. Please see www.earthscan.co.uk for contact details. Publisher: Earthscan, UKHomepage: www.earthscan.co.uk/?tabid=101807
Outline • Electricity Basics • Electricity from Fossil Fuels • Cogeneration and Trigeneration • Economics
Electricity Basics • Electricity can be either direct current (DC) or alternating current (AC) • In AC current, the voltage and current fluctuate up and down 60 times per second in North America and 50 times per second in the rest of the world • The power (W) in a DC current is equal to current (amps) x voltage (volts): P=VI • The power in an AC current is equal to the product of the root mean square (RMS) of the fluctuating current and voltage if the current and voltage are exactly in phase (exactly tracking each other): P=Vrms x Irms • The standard electricity distribution system consists of 3 wires with the current in each wire offset by 1/3 of a cycle from the others, as shown in the next figure
Figure 3.2 Two Pole Synchronous Generator Source: EWEA
Electricity demand continuously varies, and power utilities have to match this variation as closely as they can by varying their power production. The following distinctions are made: • Baseload powerplants: these are plants that run steadily at full load, with output equal to the typical minimum electricity demand during the year. Plants (such as coal or nuclear) that cost a lot to build but are cheap to operate (having low fuel costs) are good choices • Peaking powerplants: these are plants that can go from an off state to full power within an hour or so, and which can be scheduled based on anticipated variation in demand (natural gas turbines or diesel engines would be a common choice) • Spinning reserve: these are plants that are on but running at part load – this permits them to rapidly (within a minute) vary their output, but at the cost of lower efficiency (and so requires greater fuel use in the case of fossil fuel power plants).
Electricity from Fossil Fuels • Pulverized coal • Integrated Gasification/Combined Cycle (IGCC) • Natural gas turbines and combined cycle • Diesel and natural gas reciprocating engines • Fuel cells
Technical issues related to electricity from fossil fuels • Full load efficiency • Part-load efficiency • Rates of increase of output • Impact of temperature on output • Auxiliary energy use
Figure 3.3 Generation of electricity from a conventional, pulverized-coal powerplant Source: Hoffert et al (2002, Science 298, 981-987)
The upper limit to the possible efficiency of a powerplant is given by the Carnot efficiency:η = (Tin-Tout)/TinSo, the hotter the steam supplied to the steam turbine, the greater the efficiency. Hotter steam requires greater pressure, which requires stronger steel and thicker walls – so there is a practical limit to the achievable Carnot efficiency (and actual efficiencies are even lower)
Coal powerplant operating temperatures and efficiencies • Typical: 590ºC, 35% efficiency • Best today: > 600ºC, 42-44% efficiency • Projected by 2020: 720ºC, 48-50% efficiency
Integrated Gasification Combined Cycle (IGCC) • This is an alternative advanced coal powerplant concept • Rather than burning pulverized solid coal, the coal is heated to 1000ºC or so at high pressure in (ideally) pure oxygen • This turns the coal into a gas that is then used in a gas turbine, with heat in the turbine exhaust used to make steam that is then used in a steam turbine • Efficiencies of ~ 50% are expected, but are much lower at present
Generation of electricity with natural gas • Simple-cycle power generation • Combined-cycle power generation • Simple-cycle cogeneration • Combined-cycle cogeneration
Simple-cycle turbine • Has a compressor, combustor, and turbine proper • Because hot gases rather than steam are produced, it is not restricted in temperature by the rapid increase in steam pressure with temperature • Thus, the operating temperature is around 1200ºC
Figure 3.6a Simple-cycle gas turbine and electric generator Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)
Efficiency of generating electricity using natural gas • One might expect a high efficiency from the gas turbine, due to the high input temperature (and the resulting looser Carnot limit) • However, about half the output from the turbine has to be used to compress the air that is fed into it • Thus, the overall efficiency is only about 35% in modern gas turbines
Figure 3.5 Efficiency and cost of a simple-cycle gas turbine with and without water injection
Due to the afore-mentioned high operating temperature of the gas turbine, the temperature of the exhaust gases is sufficiently hot that it can be used to either • Make steam and generate more electricity in a steam turbine (this gives combined cycle power generation), or • provide steam for some industrial process that can use the heat, or to supply steam for district heating (this gives simple cycle cogeneration)
Figure 3.6c Combined-cycle power generationusing natural gas Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)
Figure 3.6b Simple-cycle cogeneration Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)
The energy can be cascaded even further, as follows: • Gas turbine → steam turbine → useful heat as steam from the steam turbine (combined cycle cogeneration), or • Gas turbine → steam turbine → steam → hot water (also combined cycle cogeneration), or • Gas turbine → steam → hot water
Figure 3.6d Combined-cycle cogeneration Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)
Figure 3.7 Cogeneration system with production of steam and hot water Source: Malik (1997, M. Eng Thesis, U of Toronto)
State-of-the-art natural gas combined-cycle (NGCC) systems have electricity generation efficiencies of 55-60%, compared to a typical efficiency of 35% for single-cycle turbines • However, NGCC systems are economical only in sizes of 25-30 MW or greater, so for smaller applications, only the less efficient simple-cycle systems are used • Thus, a number of techniques are being developed to boost the electrical efficiency of simple gas turbines to 42-43%, with one technique maybe reaching 54-57%
In cogeneration applications, the overall efficiency (counting both electricity and useful heat) depends on how much of the waste heat can be put to use. However, overall efficiencies of 90% or better have been achieved
Reciprocating engines • These have pistons that go back and forth (reciprocate) • Normally they use diesel fuel – so these are the diesel generators normally used for backup or emergency purposes • However, they can also be fuelled with natural gas, with efficiencies as high as 45%
Fuel cells • These are electrochemical devices – they generate electricity through chemical reactions at two metal plates – an anode and a cathode • Thus, they are not limited to the Carnot efficiency • Operating temperatures range from 120ºC to 1000ºC, depending on the type of fuel cell • All fuel cells require a hydrogen-rich gas as input, which can be made by processing natural gas or (in the case of high-temperature fuel cells) coal inside the fuel cells
Fuel cells (continued) • Electricity generation efficiencies using natural gas of 40-50% are possible, and 90% overall efficiency can be obtained if there is a use for waste heat • In the high-T fuel cells, the exhaust is hot enough that it can be used to make steam that can be used in a steam turbine to make more electricity • An electrical efficiency of 70% should be possible in this way – about twice that of a typical coal-fired powerplant.
Figure 3.8 Cross section of a single fuel cell. Several such cells would be placed next to each other to form a fuel cell stack.
Figure 3.9 United Technologies Company 200-kW phosphoric acid fuel cell that uses natural gas as a fuel. 1=fuel processor, 2=fuel cell stack, 3=power conditioner, 4=electronics and controls Source: www.utcfuelcells.com
Summarizing the preceding slides and other information, • Natural gas combined-cycle has the highest full-load efficiency (55-60%) and holds its efficiency well at part load • Reciprocating engines have intermediate full-load efficiencies (40-45%) and load their efficiencies well at part load • Gas turbines and micro-turbines have low full-load efficiencies (typically 25-35%, but ranging from 16% to 43%) and experience a substantial drop at part load • Fuel cells using natural gas have intermediate full-load efficiency (40-45%) but this efficiency increases at part load
Capital Costs Today • Pulverized coal powerplant with state-of-the-art pollution controls: $1200-1400/kW • Natural gas combined cycle: $400-600/kW in mature markets, $600-900/kW in most developing countries • Reciprocating engines: $600-1200/kW • Fuel cells: $3000-5000/kW
Cogeneration is the simultaneous production of electricity and useful heat – basically, take the waste heat from electricity generation and put it to some useful purpose. Two possible uses are to feed the heat into a district heating system, and to supply it to an industrial process
Figure 3.12 Proportion of electricity produced decentrally (overwhelmingly as cogeneration)
Technical issues • Impact of withdrawing useful heat on the production of electricity • Ratio of electricity to heat production • Temperature at which heat is supplied • Electrical, thermal and overall efficiencies • Marginal efficiency of electricity generation
Four efficiencies for cogeneration: • The electrical efficiency – the amount of electricity produced divided by the fuel use (later I’ll need to call this the direct electrical efficiency) • The thermal efficiency – the amount of useful heat provided divided __by the fuel use • The overall efficiency – the sum of the of two • The effective or marginal efficiency of electricity generation – explained later
Impact of withdrawing heat • In simple-cycle cogeneration, capturing some of the heat in the hot gas exhaust does not reduce the production of electricity, but the electrical production is already low • In cogeneration with steam turbines, the withdrawal of steam from the turbine at a higher temperature than would otherwise be the case reduces the electricity production • The higher the temperature at which we want to take heat, the more that electricity production is reduced
Figure 3.13 Example of the tradeoff between production of useful heat and loss of electricity production using steam turbine cogeneration Source: Bolland and Undrum (1999, Greenhouse Gas Control Technologies, 125-130, Elsevier Science, New York)
Thus, to maximize the electricity production, we want to be able to make use of heat at the lowest possible temperature.If the heat is to be provided to buildings, that means having well insulated buildings that can be kept warm with radiators that are not very hot
The alternative to cogeneration is the separate production of heat and electricity. The effective efficiency in generating electricity is the amount of electrical energy produced divided by the extra fuel used to produce electricity along with heat compared to the amount of fuel that would be used in producing heat alone. The extra amount of fuel required in turn depends on the efficiency with which we would have otherwise produced heat with a boiler or furnace.
For example, suppose that we have a cogeneration system with an electrical efficiency of 25% and an overall efficiency of 80%. Then, the thermal efficiency is 80%-25%=55% - we get 55 units of useful heat from the 100 units of fuel. If the alternative for heating is a furnace at 80% efficiency, we would have required 68.75 units of fuel to produce the 55 units of heat. Thus, the extra fuel use in cogeneration is 100-68.75=31.25 units, and the effective electricity generation efficiency is 25/31.25=80%. I call this the marginal efficiency, because it is based on looking at things on the margin (this is a concept from economics).
With a little algebra, it can be shown that the marginal efficiency is given bynmarginal = nel/(1-nth/nb)where nel and nth are the electrical and thermal efficiencies of the cogeneration system, and nb is the efficiency of the boiler or furnace that would otherwise be used for heating
Figure 3.15 Marginal efficiency of electricity generation in cogeneration (ηel = efficiency of the alternative, central powerplant for electricity generation)
Key points • For a given thermal efficiency, the effective electrical efficiency is higher the higher the direct electrical efficiency • However, very high effective electrical efficiencies can be achieved even with low direct electrical efficiencies if the thermal efficiency is high – that is, if we can make use of most of the waste heat • To get a high thermal efficiency requires being able to make use of low-temperature heat (at 50-60ºC), as well as making use of higher temperature heat
