Energy and the New Reality, Volume 1:
Energy Efficiency and the
Demand for Energy Services
Chapter 3: Electricity from Fossil Fuels
L. D. Danny Harvey
harvey@geog.utoronto.ca
Publisher: Earthscan, UK
Homepage: www.earthscan.co.uk/?tabid=101807
This material is intended for use in lectures, presentations and as
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Recap: Primary to Secondary to End-Use Energy
Losses
P rim a ry
E n e rg y
Tra n s fo rm a tio n
Tra n s p o rta tio n
D is trib u tio n
Losses
S e c o n d a ry
E n e rg y
U tiliz a tio n
F in a l
D e v ic e o r
U s e fu l
S y s te m
E n e rg y
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 3.1 Three-phase AC Current
Voltage
1.0
0.0
-1.0
0
5
10
Time (ms)
15
20
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
G e n e ra to r
ste a m
H ig h -P ressu re B o ile r
e le ctricity o u t
fo ssil fu el in
S te a m
Tu rb in e
a ir (O 2 )
to coo lin g tow e r
o r co ld river w ate r
CO2
w a ter
co n de n sa te
a n d/o r
co g en e ra tio n
C O 2 up th e sta ck
se q ue ste red C O 2
out
C o n d e n se r
P um p
Source: Hoffert et al (2002, Science 298, 981-987)
co o ling w a te r re tu rn flo w
The upper limit to the possible efficiency of a
powerplant is given by the Carnot efficiency:
η = (Tin-Tout)/Tin
So, 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
EX H AU ST
FU EL
C O M B U S TO R
SH A FT
E L E C T R IC IT Y
G E N E R AT O R
C O M PR ES SO R
T U R B IN E
IN TA K E A IR
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 3.4 Turbine efficiency vs turbine size (power)
45
Turbine Efficiency (%)
40
35
30
25
20
15
10
5
0
0
50
100
150
Turbine Power (MW)
200
250
Figure 3.5 Efficiency and cost of a simple-cycle gas
turbine with and without water injection
50
600
45
Turbine Efficiency (%)
35
400
30
Cost
25
300
Without water injection
20
With water injection
200
15
10
100
5
0
0
0
20
40
Turbine Power (MW)
60
Cost (euros/kW)
500
40
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 generation
using natural gas
C O O L IN G T O W E R
C O ND EN S ER
EX H AU ST
E L E C T R IC IT Y
W AT E R
PUMP
S T E A M T U R B IN E
ST EA M
FU EL
H E AT R E C O V E R Y
S T E A M G E N E R AT O R
COM BUSTOR
SH A FT
E L E C T R IC IT Y
G E N E R AT O R
C O M PR ES SO R
T U R B IN E
IN TA K E A IR
Source: Williams (1989, Electricity: Efficient End-Use and New Generation Technologies and Their Planning Implications, Lund University Press)
Figure 3.6b Simple-cycle cogeneration
EX H AU ST
W AT E R
PUMP
PR O C ES S S TEA M
FU EL
H E AT R E C O V E R Y
S T E A M G E N E R AT O R
C O M B U S TO R
SH A FT
E L E C T R IC IT Y
G E N E R AT O R
C O M PR ES SO R
T U R B IN E
IN TA K E A IR
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
C O O L IN G T O W E R
PR O C ES S S TEA M
EX H AU ST
C O ND EN S ER
E L E C T R IC IT Y
W AT E R
PUMP
S T E A M T U R B IN E
ST EA M
FU EL
H E AT R E C O V E R Y
S T E A M G E N E R AT O R
COM BUSTOR
SH A FT
E L E C T R IC IT Y
G E N E R AT O R
C O M PR ES SO R
T U R B IN E
IN TA K E A IR
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
E L E C T R IC IT Y
FUEL
GAS
T U R B IN E
G E N E R ATO R
STEAM
EXHAUST GAS
H E AT
RECOVERY
STEAM
G E N E R ATO R
H E AT
EXCHANGER
EXHAUST GAS
HO T W ATER
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 hightemperature 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.
F uel (H2 )
A ir (M ostly
N 2 + O2 )
D C P ow er
E electron flow
N egative ion s
or
F uel
d istrib u tion
p late
O xid ized
F uel (H2 O )
C AT H O D E
P ositive ion s
E L E C T R O LY T E
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.
N itrogen
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
Figure 3.10 Solid Oxide Fuel Cell / Gas Turbine System
Fuel
Air
o
o
25 C
25 C
FC
AC
o
236 C
o
847 C
SOFC = Solid Oxide Fuel cell
AC,FC = Air & Fuel compressor
CB = Catalytic burner
GT = Gas turbine
HRSG = Heat recovery steam generator
HE = Heat exchanger
GT-2
o
1079 C
GT-1
o
301 C
o
HE-1
738 C
o
448 C
o
526 C
HE-2
SOFC
o
985 C CB
M
468 C
Turbine
Exhaust
o
Pump
o
440 C
HRSG
o
509 C
o
1290 C
HE-3
o
224 C
o
25 C
To heat load
From heat load
Figure 3.11a Electrical efficiency vs. load
Electrical Efficiency (%)
60
Combined
cycle
50
SOFC
40
Reciprocating engine
30
20
7.52 MW gas turbine
3.5 MW gas turbine
10
Micro-turbine
0
0.2
0.3
0.4
0.5
0.6
0.7
Relative Load
0.8
0.9
1.0
Figure 3.11b Relative electrical efficiency vs. load
1.2
SOFC
Combined cycle
Relative Efficiency
1.0
0.8
0.6
Reciprocating
engine
0.4
7.52 & 3.5 MW gas turbines
and micro-turbine
0.2
0.0
0.2
0.3
0.4
0.5
0.6
0.7
Relative Load
0.8
0.9
1.0
Summarizing the preceding slides
and other information,
• Natural gas combined-cycle has the highest fullload efficiency (55-60%) and holds its efficiency
well at part load
• Reciprocating engines have intermediate fullload efficiencies (40-45%) and load their
efficiencies well at part load
• Gas turbines and micro-turbines have low fullload 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-ofthe-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
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)
Denmark
Netherlands
Finland
Russia
Germany
Canada
China
Chile
Portugal
USA
0
10
20
30
40
50
60
Percent Decentralized Electricity Generation
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
Loss of electricity as a fraction of heat withdrawn
Figure 3.13 Example of the tradeoff between production of
useful heat and loss of electricity production
using steam turbine cogeneration
0.35
0.30
0.25
0.20
0.15
0.10
0.05
80
100
120
140
160
180
200
220
240
Steam Temperature (oC)
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
have 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 by
nmarginal = 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)
Marginal Efficiency of Electricity Generation
1.5
Boiler efficiency = 0.8
Boiler efficiency = 0.9
1.0
ηel=0.55
ηel=0.40
0.5
ηel=0.25
0.0
0
0.1
0.2
0.3
0.4
0.5
Thermal Efficiency
0.6
0.7
0.8
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
Electricity:heat ratio
• Because the marginal electricity generation
efficiency in cogeneration is generally much higher
than the efficiency of a dedicated central
powerplant, there is a substantial reduction in the
amount of fuel used to generate electricity when
cogeneration is used
• Thus, we would like to displace as much inefficient
central electricity generation as possible when
cogeneration is used to supply a given heating
requirement
• This in turn requires that the electricity-to-heat
production ratio in cogeneration be as large as
possible
• (Remember – none of the gains that we’ve talked
about occur if we can’t use the waste heat produced
by cogeneration)
Figure 3.14 Electricity:heat output ratio in cogeneration
Electricity:Useful-Heat Ratio
2.0
Reciprocating
engine/
heat recovery
steam
generator
1.5
1.0
0.5
Gas turbine/
back-pressure
steam turbine
Microturbine
0.0
Backpressure
steam
turbine
Gas turbine/
heat recovery
steam
generator
Fuel cell/
heat recovery
steam
generator
Fuel cell/
gas turbine/
back-pressure
steam turbine
Figure 3.17 Dependence of overall savings through cogeneration
on the electricity:heat ratio and on the central powerplant
efficiency, assuming a 90% overall efficiency for cogeneration and
90% efficiency for the alternative heating system
60
η=0.4
Per cent Overall Savings
50
η=0.5
40
η=0.6
30
20
10
0
0.0
0.5
1.0
Electricity:Heat Ratio
1.5
2.0
Cost of Electricity
Issues related to the cost of electricity:
• Capital cost, interest rate, lifespan
• Fuel cost (impact of depends on
efficiency)
• Fixed and variable operation &
maintenance costs
• Baseload vs peaking costs
• Transmission line costs and transmission
losses
• Amount of backup capacity
Figure 3.16 Capital cost of natural gas combined cycle
cogeneration plants
1600
Cost (US$/kW)
1200
800
400
0
0
100
200
300
Electrical Capacity (MW)
400
500
Amortization of capital cost:
CRF x Ccap / (8760 x CF) units: $/kWh
where CRF = i /(1-(1+i)-N) is the cost recovery factor
_i = interest rate
_N = financing time period
Ccap = capital cost ($/kW)
8760 is the number of hours in a year
CF= capacity factor (annual average output as a
fraction of capacity)
Fuel contribution to the final cost:
Cfuel ($/GJ) x 0.0036 (GJ/kWh) / efficiency
The cost of electricity from less efficient
powerplants will be more sensitive to the cost
of fuel than the cost of electricity from efficient
powerplants, but more efficient powerplants will
tend to have greater capital cost
Typical overnight capital costs and best efficiencies
• Pulverized coal: $1200-1400/kW,η= 0.45-0.48
• IGCC: $1400-2600/kW today, η= 0.41-0.55
$1150-1400/kW hoped for, future
• NGCC: $400-600/kW, η = 0.55-0.60
• Reciprocating engine: $600-1200/kW,η=0.40-0.46
• Micro-turbine: $1800-2600/kW, η= 0.23-0.27
• Fuel cells: $3000-5000/kW, η= 0.35-0.45
$1000-1500/kW hoped for, future
• NGCC/FC hybrid: $2000-3000/kW, η= 0.70-0.80
Figure 3.18 Cost of electricity from coal and natural gas
16
Electricity Cost (cents/kWh)
14
NG, $2000/kW, η=0.7, $10/GJ
12
NG, $500/kW, η=0.4, $10/GJ
10
8
Coal, $1500/kW, η=0.5, $4/GJ
6
4
Coal, $1000/kW, η=0.4, $4/GJ
2
0
0.2
0.4
0.6
Capacity Factor
0.8
1
Figure 3.19 Cost of heat from boilers, electricity with or
without cogeneration, and heat from cogeneration
9
Heat or Electrictiy Cost (cents/kWh)
8
7
Heat cost
Electricity cost, dedicated facility
Gross electricity cost, cogen facility
Net electricity cost, cogen facility
6
5
4
3
2
1
0
2
4
6
Fuel Cost ($/GJ)
8
10
Figure 3.20 Cost of electricity from central coal (at $2/GJ)
and from natural gas (at $10/GJ)
Central baseload
coal
Central peaking coal
Central peaking NG
Onsite peaking NG
Peaking NG cogen
0
2
4
6
8
10
Cost of Electricity (cents/kWh)
12
14
Water requirements
• Most thermal powerplants use water to cool the
condenser of a steam turbine and for other,
minor, purposes
• There are two approaches:
a once-through cooling system
a recirculating system in a cooling tower
• Water use by power generation represents the
largest or second largest use of water in most
countries (with irrigation sometimes being a
larger use)
• In once-through systems, the water is returned
to the source (but at a warmer temperature).
Large volumes of water are needed – not
available in arid regions
• In a recirculating systems, water that has
removed heat from the condenser is sprayed
through a cooling tower, where it is cooled by
evaporation, then returns to the condenser
• This consumes water, but the amount that is
withdrawn from the water source (lakes, rivers or
groundwater) is smaller than in once-through
systems
Typical water requirements
• Steam turbines (as in coal powerplants)
Once through: 80-190 litres withdrawn per kWh of
__generated electricity, ~ 1 litre/kWh consumed
Recirculating: 1-3 litres/kWh withdrawn
1-2 litres/kWh consumed
• Natural gas combined cycle
Once through: 30 litres/kWh withdrawn
~ 0.4 litres/kWh consumed
Recirculating: 0.9 litres/kWh withdrawn
0.7 litres/kWh consumed
Bottom line:
• More efficient power plants, such as
natural gas combined cycle powerplants,
use less water per kWh of generated
electricity than less efficient powerplants
• The water requirements can be a
constraining factor in arid regions
• It is possible to use air rather than water to
cool the condenser, but then the efficiency
drops