Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.
-
date post
21-Dec-2015 -
Category
Documents
-
view
219 -
download
1
Transcript of Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.
![Page 1: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/1.jpg)
Energy, Society,and
the Environment
Unit IV: How Do Power Plants Work? Combustion and Heat Engines
![Page 2: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/2.jpg)
Outline
• Combustion: Energy Generation and Pollutants
• 1st Law of Thermodynamics
• 2nd Law of Thermodynamics
• Efficiency
![Page 3: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/3.jpg)
Combustion
• What do we do with fossil fuels: burn them – Combustion: impacts
• Fuels
• Balancing combustion chemical equations
• Combustion products
![Page 4: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/4.jpg)
What happens in combustion?
• Fuel + oxidizer -> Products + light + heat • Combustion, in its simplest form, e.g: methane
CH4 + 2O2 CO2 + 2H20
A clean reaction, except for the issue of carbon dioxide and the global climate
This idealized reaction takes place in an ‘atmosphere’ (oxygen) free of impurities
![Page 5: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/5.jpg)
How much CO2 is produced when 1 ton of cellulose (C6H12O6) is burned?
(Cellulose is the primary component of plant matter, wood etc)
Write the equation:
C6H12O6 + O2 CO2 + H2O
Balance first the carbon:
C6H12O6 + O2 6CO2 + H2O
Then the hydrogen:
C6H12O6 + O2 6CO2 + 6H2O
Last, the oxygen (because you can change the oxygen without altering other elements):
C6H12O6 + O2 6CO2 + 6H2O,
So, 6 + x = 18 x = 12, or 6 O2
The balanced equation is:
C6H12O6 + 6O2 6CO2 + 6H2O
![Page 6: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/6.jpg)
How much CO2 is produced when 1 metric ton of wood is burned,continued ...
How much do these weigh?
1 molecule of cellulose produces 6 molecules of CO2
![Page 7: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/7.jpg)
Remember your atoms
C = 6 p + 6 n = 12 O = 8 p + 8 n = 16
(atomic mass unit, OR1 mole of C is 12 grams)
![Page 8: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/8.jpg)
How much CO2 is produced when 1 metric ton of wood is burned,continued ...
So, from C6H12O6 6CO2 + 6H2O, we see that:
€
=264 10g x 6g
180g=1.47x 106 g =1.47 tons of CO2
How much do these weigh?
C6H12O6 = (6 x 12) + (12 x 1) + (6 x16) = 180 grams (in 1 mole)
6 CO2 = 6 x [(1x12) + (2 x16)] = 264 grams (from 1 mole of cellulose)
1 molecule of cellulose produces 6 molecules of CO2
180 grams cellulose 264 grams CO2
106 grams cellulose ?? grams CO2
??
![Page 9: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/9.jpg)
Simple combustion equation,but put it in air
• Example: methane reacts with airCH4 + (O2 + 3. 76N2) -> CO2 + H2O + N2
(this is termed the ‘unbalanced’ version)
CH4 + 2(O2 + 3. 76N2) -> CO2 + 2H2O + 7.52N2
(balanced: exactly the correct amount of oxidizer to convert all C to CO2, and all H to H2O)
Note: Air is 78% nitrogen, 21% oxygen, + other stuff
![Page 10: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/10.jpg)
Real Combustion
• If combustion occurs without complete oxidation, we get instead:
CH4 + O2 + N2 mostly (CO2 + 2H20 +N2)
+ traces (CO + HC + NO...)
• This can occur when:– temperature too low
– insufficient O
– combustion too rapid
– poor mixing of fuel and air, etc. ...
![Page 11: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/11.jpg)
Real, Real Combustion
• At higher temperatures, N reacts with O:
air(N2 +O2) + heat NOx (nitrogen oxides, x can be 1 or 2)
So much for pure fuels, now add impurities:
enter N, S, metals and ash (non-combustibles)
What we really get:
• Fuel (C, H, N, S, ash) + air (N2 +O2)
(CO2, H2O, CO, NOx, SOx, VOCS, particulates) + ash– Volatile Organic Compounds: VOCs
![Page 12: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/12.jpg)
Real, Real Combustion and Emissions
NOx + VOCs = SMOG
CO2, NOx, SOx + oxidants + water = ACID RAIN
Particulates (especially ultrafine):
• Create inflammatory response • Affect heart rate variability• Inducing cellular damage• May be associated with premature death
![Page 13: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/13.jpg)
Emissions Controls
• Clean Air Act requires EPA to set National Ambient Air Quality Standards (NAAQS) for seven pollutants (referred to as criteria pollutants): carbon monoxide (CO), lead (Pb), ozone (ground level), nitrogen oxides (NOx), particulate matter (PM2.5 and PM10), sulfur oxides (SO2). Last amended in 1990.
![Page 14: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/14.jpg)
National Ambient Air Quality Standards
Pollutant Metric Fed. Std. CA Std. Effects
CO 8 hr 10 mg/m39 ppm Angina, pre-natal
asthmaNO2 Ann. mean 40 mg/m3 20 ppm Respiratory dis.
PM10 Ann. Mean 50 μ / 3g m 30 / 3mg m . , Resp disease
2.5PM . Ann Mean 15 / 3mg m in review . , Resp diseaseSO2 . Ann Mean 80 μ / 3g m in review , .wheez plant dam
![Page 15: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/15.jpg)
Criteria Pollutants & the Clean Air Act
POLLUTANT Emissions Air Concentration ‘83 - ‘02 ‘93 - ‘02 ‘83 - ‘02 ‘93 - ‘02
CO - 41% - 21% - 65% - 42%Pb - 93 % - 5 % - 94% - 57%O3 [1 hr] - 40 % - 25% - 22% - 2%PM10 - 34% - 22% - - 10%PM2.5 - - 17% - - 8%SO2 - 33% - 31% -34% - 39%NOx - 15% - 9 % - 2% +.5%
![Page 16: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/16.jpg)
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
![Page 17: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/17.jpg)
Announcements 2/16
• No Class THIS Friday (note the change)
• Homework #3 posted today on D2L, due
next Monday
![Page 18: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/18.jpg)
POWER PLANTS areHeat Engines
Using Heat to Do Work(a bit of Thermodynamics)
![Page 19: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/19.jpg)
What use is thermodynamics?
• For different energy sources compare:– Efficiency
– Amount of fuel needed
– Pollution produced
• Use as a tool for improving energy systems– Analyze each part of a power plant (pumps, turbine,
heat exchangers, etc.)
• Analyze alternative energy scenarios– Ethanol, biodiesel, hydrogen
![Page 20: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/20.jpg)
• How many fewer power plants would need to be built in Arizona if we increased our efficiency by 10% by 2020?
• How much could SO2, CO2, and other
pollutants be reduced by that efficiency increase?
Some Questions That Can Be Answered
![Page 21: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/21.jpg)
Coal Fired Steam Power Plant
![Page 22: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/22.jpg)
1st Law of Thermodynamics
Water in
Water out
![Page 23: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/23.jpg)
1st Law of Thermodynamics
• Conservation of Energy Principle
Esystem = Ein - Eout
EinEsystem Eout
![Page 24: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/24.jpg)
Earth Energy Balance
EARTHEnergyStored
Energy from Sun
Energy Radiated to Space
![Page 25: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/25.jpg)
Earth Energy Balance
EARTHEnergyStored
Energy from Sun
Energy Radiated to Space
Estored = Ewind + Eplants + Eheat + etc. = Esun - Elost
![Page 26: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/26.jpg)
Steady-State Condition
• Under steady-state conditions, there is no change in the stored energy of the system.
Esystem = 0 = Ein – Eout
or Ein = Eout
![Page 27: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/27.jpg)
Energy Balance for a Power Plant
PowerPlant
1000 MJ
Energy from Fuel
Efuel = 1000 MJ
![Page 28: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/28.jpg)
Energy Balance for a Power Plant
PowerPlant
1000MJ 350MJ
Energy from FuelUseful Energy (Work)
Efuel = 1000 MJ
Euseful = 350 MJ
Efuel = Euseful + Ewaste
![Page 29: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/29.jpg)
Energy Balance for a Power Plant
Useful Energy (Work)
PowerPlant
1000MJ 350MJ
650MJ
Energy from Fuel
Wasted Energy
Efuel = Euseful + Ewaste
1000 MJ = 350 MJ + 650 MJ
Efuel = 1000 MJ
Euseful = 350 MJ
Ewaste = 650 MJ
![Page 30: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/30.jpg)
Power Balance for a Power Plant
PowerPlant
1000MW 350MW
650MW
Pin = Pout
Pout = Pout 1 + P out 2
Pfuel = Puseful + Pwaste
1 W = 1 J/s
![Page 31: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/31.jpg)
Power Balance for a Power Plant
PowerPlant
1000MW 350MW
650MW
Pin = 1000 MW
Pout = 350 MW + 650MW
Pfuel = Puseful + Pwaste
Pin = Pout
![Page 32: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/32.jpg)
Power Plant
![Page 33: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/33.jpg)
Parts of a power plant• Energy Source
– Combustion: Coal, natural gas, oil, biomass, garbage
• Heat Generated– Boiler: Coal burned, heats water, creates steam– Combustion Chamber: CH4 burned, hot gases
• Work Produced– High pressure steam or hot gases turn turbine blades
• Wasted Heat Removed/Exhaust Treatment– hot water dumped into a body of water or cooling tower– exhaust gases dumped into atmosphere
• Some waste heat can be recovered
![Page 34: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/34.jpg)
Connection to Environment
• Fuel Side: Mining, drilling, transporting• Waste Heat Side:
– Increase temperature of body of water• Affect fish, algae blooms, etc.
– Pollutants in waste heat stream• Air pollutants
• Pollutants in waste water stream
• Environmental justice– Location, impact & management of power plants
![Page 35: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/35.jpg)
Heat Engine
• A device that converts heat into mechanical energy
• Used to approximate thermal systems
![Page 36: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/36.jpg)
Heat Engine
HeatEngine
Wnet
Qcold
Qhot
High Temp. Source of Heat
Low Temp. Sink of Waste Heat
Energy Balance: Qhot = Wnet + Qcold
![Page 37: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/37.jpg)
DefinitionsHigh Temp Source of Heat: This is the source of energy that drives the power
plant (heat of combustion, geothermal heat source, nuclear reactor, etc.).
Qhot: This is the heat transferred from the hot source.
Heat Engine: This includes the working parts of the power plant
(including pumps, turbines, heat exchangers, condensers, etc.).
Wnet: This is the net amount of work that exits the power plant. A turbine generates energy, but the pumps and compressors use energy.
Qcold: This is the rejected or waste heat, which is dumped to a cold source (i.e. river, atmospheric air, lake, etc.).
Low Temp Sink of Waste Heat: This is the reservoir (river, air, lake, etc.) that the waste heat is dumped into (often goes through a cooling tower).
![Page 38: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/38.jpg)
Efficiency
(Several names:
I =1st Law, Actual, or Thermal Efficiency)
Efficiency = what you want
what you pay for
work done
energy put into the system=
Qhot
=Wnet
Qhot
=Qhot-Qcold
![Page 39: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/39.jpg)
Ein=1000 MJ from fuel
Eout=350 MJ useful energy
+650 MJ wasted energy
= Wnet/Qhot
= 350 MJ / 1000 MJ = 0.35 = 35%
Energy Balance for a Power Plant
PowerPlant
1000MJ 350MJ
650MJ
Energy from Fuel Useful Energy (Work)
Wasted Energy
Note: Qhot = Qin = Qfuel
![Page 40: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/40.jpg)
Second Law of Thermodynamics
• Order tends to disorder, concentration tends to chaos
• No process can occur that only transfers energy from a cold body to a hot body (heat must flow from hot to cold)
• No process can occur that converts a given quantity of thermal energy into an equal quantity of mechanical work (always some degradation-always some wasted energy)
![Page 41: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/41.jpg)
2nd Law of Thermodynamics,alternate statements:
• states in which direction a process can take place– heat does not flow spontaneously from a cold to
a hot body– heat cannot be transformed completely into
mechanical work– it is impossible to construct an operational
perpetual motion machine
![Page 42: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/42.jpg)
2nd Law of Thermodynamics and Carnot Efficiency
• 2nd Law: Heat cannot be converted to work without creating some waste heat.
• What does this mean for a heat engine?
Wnet < Qhot ALWAYS
In other words, efficiency is always less than 100%.
Well, how much less?
![Page 43: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/43.jpg)
2nd Law of Thermodynamics and Carnot Efficiency
Carnot Efficiency: The net work produced and the heat into the system only depend on temperatures. No thermal system can be more efficient than the Carnot efficiency.
€
Qcold
Qhot
=Tcold
Thot
c =Qhot - Qcold
Qhot
= 1 - Qcold
Qhot
= 1 - Tcold
Thot
This is the best you can achieve (under ideal conditions)
![Page 44: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/44.jpg)
2nd Law of Thermodynamics and Carnot Efficiency
Important: Temperatures must be in units of Kelvin.
K = ºC + 273.15
1 ºC = 1.8 F
Example: In the power plant we considered earlier, Thot = 900 K (very hot steam) and Tcold = 300 K (room temperature). What is its Carnot efficiency?
c = 1 - Tcold / Thot = 1 - (300/900) = 0.67 = 67%
If this plant were an ideal heat engine, 33% of the energy wouldbe lost (to nearby water or to atmosphere via cooling tower)
![Page 45: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/45.jpg)
Ein=1000 MJ from fuel
Eout=350 MJ useful energy
+650 MJ wasted energy
Efficiencies of the Power Plant
PowerPlant
1000MJ 350MJ
650MJ
Energy from Fuel Useful Energy (Work)
Wasted Energy
Thigh=900 K
Tlow =300 K
I= Wnet/Qhigh=350 MJ / 1000 MJ = 0.35 = 35% Reality
c = 1 - Tcold / Thot = 1 - (300/900) = 0.67 = 67% Ideal
![Page 46: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/46.jpg)
Comparison of Efficiencies
Type Th (K) Tc (K) Carnot
Eff.
Actual
Eff.
Coal 800 300 62.5% 35%
Nuclear 1200 300 75% 35%
Geo-
Thermal
525 350 33% 16%
![Page 47: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/47.jpg)
Waste Energy
Note: When the working fluid reaches Tlow no more energy can be used – although the working fluid still contains energy
(typically > 60% of Efuel)
![Page 48: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/48.jpg)
Heat PollutionThe circulation rate of cooling water in a typical 700 MW coal-fired power plant with a cooling tower amounts to about 71,600 cubic metres an hour (315,000 U.S. gallons per minute) and the circulating water requires a supply water make-up rate of about 5 percent (i.e., 3,600 cubic metres an hour).
If that same plant had no cooling tower and used once-through cooling water, it would require about 100,000 cubic metres an hour and that amount of water would have to be continuously returned to the ocean, lake or river from which it was obtained and continuously re-supplied to the plant. Discharging such large amounts of hot water may raise the temperature of the receiving river or lake to an unacceptable level for the local ecosystem.
![Page 49: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/49.jpg)
Cogeneration• Use waste energy in another application
• e.g., Heaters in cars use waste energy from the engine
• Easy uses: Space and water heating, especially if small power plants can be built near urban areas: increase total efficiency from ~ 35% to ~ 70%
• Take the Qcold through another cycle with Qcolder
• Great potential in reducing fuel use and environmental impacts (by bringing Qcolder as close to atmospheric temperature as possible)
![Page 50: Energy, Society, and the Environment Unit IV: How Do Power Plants Work? Combustion and Heat Engines.](https://reader035.fdocuments.us/reader035/viewer/2022062313/56649d555503460f94a31f06/html5/thumbnails/50.jpg)
Gasoline and Diesel EnginesGasoline engines in our cars are also heat engines called internal combustion engines Fuel combined with gas in a closed chamber and ignited: combustion proceeds at a very high temperature and rateTypically 25% efficiency (mechanical energy/combustion energy), lots of nasty pollutants
Diesel engine also an internal combustion engineFuel and air mixed differently than gasoline engineAir compressed for heating, no electric sparkCombustion temperature even higher than gasoline engineEfficiency > 30%Less CO emission, more NOx and particulate emissions