chapter 6. energy flows and balances
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Dr. BUNRITH SENG
Chapter 6
Energy Flows and Balances
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E-mail: [email protected]; [email protected]
Department of Civil Engineering, Zaman University
No. 8, St. 315, 12151 Phnom Penh, Cambodia
Zaman University Department of Civil Engineering
No. 8, St. 315, 12151 Phnom Penh, Cambodia
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Unit of Measure
Joule (J)
Calorie (Cal)
Kilowatt-hour (kWh)
Unit of Energy
3
Energy Balances and Conversion
CONSUMED
energy
of Rate
PRODUCED
energy
of Rate
OUT
energy
of Rate
IN
energy
of Rate
DACCUMULATE
energy
of Rate
Energy Balance Equation
At Steady State
OUT
energy of Rate
IN
energy of Rate
4
Energy Balances and Conversion (Cont.)
OUT
energy wasted
of Rate
OUT
energy useful
of Rate
IN
energy
of Rate
Energy out has two terms; energy wasted in the conversion and useful energy.
Efficiency
100INEnergy
OUTenergy Useful (%) Efficiency
5
Example: A coal-fired power plant uses 1000 Mg of coal per day. The energy value of the coal is 28,000 kJ/kg. The plant produces 2.8106 kWh of electricity each day. What is the efficiency of the power plant?
Solution:
Energy IN = (28,000 kJ/kg) (1000Mg/d) (1000 kg/Mg)
36% 100kJ/d1028
kJ/d1010.1 (%) Efficiency
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A CMF reactor would require 44% more volume than a PFR
Energy Balances and Conversion (Cont.)
= 28109 kJ/d
Useful energy output = (2.8106 kWh/d)(3.6106 J/kWh)(10-3 kJ/J)
= 10.1109 kJ/d
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Energy Balances and Conversion (Cont.)
Simplified drawing of a bomb calorimeter
Bomb Calorimeter
Results of a bomb calorimeter test
Note: 1 cal is defined as the amount of energy necessary to raise the temperature of 1g of water 1oC.
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Bomb Calorimeter at ITC
Energy Balances and Conversion (Cont.)
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Example: A calorimeter holds 4L of water. Ignition of a 10g sample of a waste-derived fuel of unknown energy value yields a temperature rise of 12.5oC. What is the energy value of this fuel? Ignore the mass of the bomb.
Solution:
Energy Balances and Conversion (Cont.)
OUTEnergy INEnergy
Tm OH2S OUTEnergy
TV .J/g.K 4.184 OUTEnergy
C)mL/L)(12.5 (4L)(10g/mL 1J/g.K 4.184 OUTEnergy o3
J 10209OUTEnergy INEnergy 3
J/g 900,20 10J/10290 fuel theof ueEnergy val 3 g
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Heat Energy
Energy Balances and Conversion (Cont.)
material theof
eTemperatur Absolute
material
of Mass
Energy
Heat
Heat Energy Balance
CONSUMED
energy
Heat
PRODUCED
energy
Heat
OUT
energy
Heat
IN
energy
Heat
DACCUMULATE
energy
Heat
At Steady state
00
OUT
energy
Heat
IN
energy
Heat
0
00TT 0 332211 outin
QQTQOr
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Energy Balances and Conversion (Cont.)
Example: A coal-fired power plant discharges 3 m3/s of cooling water at 80 oC into a river that has a flow of 15 m3/s and a temperature of 20 oC. What will be the temperature in the river immediately below the discharge?
Solution:
00TT 0 332211 outin
QQTQ
3
22113
Q
QTQTT
K 303/ 153
/m 1527320/m 3273803
33
3
sm
sKsKT
C30 be r willriver wate theof re temperatuThe o
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Energy Sources
Renewable Energy
Hydropower from rivers
Hydropower from tidal estuaries
Solar power
Refuse and other waste materials
Wind
Wood and other biomass, such as sugarcane and rice hulls
Nonrenewable Energy
Nuclear power
Coal, peat, and similar materials
Natural gas
Oil
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Energy Sources (Cont.)
Energy Flow in the United States
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Energy Equivalence
There are two important distinct energy equivalences:
Arithmetic energy equivalence
Conversion energy equivalence
Example: What are the arithmetic and conversion energy equivalents between gasoline (20,000 kJ/kg) and refuse-derived fuel (5,000 kJ/kg)?
Solution:
refuse kJ/kg 5,000
gasoline kJ/kg 20,000 eequivalencenergy Arithmetic
gasoline kg refuse/1 kg 4
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Solution:
refuse kJ/kg 2,500
gasoline kJ/kg 20,000 eequivalencenergy Conversion
gasoline kg refuse/1 kg 8
Energy Equivalence (Cont.)
But the processing of refuse to make the fuel also requires energy. This can be estimated at perhaps 50% of the refuse-derived fuel energy, so the actual net energy in the refuse is 2,500 kJ/kg. Therefore,
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Electric Power Production
Simplified drawing of a coal-fired power plant
Coal-fired Power plant
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OUT
energy wasted
of Rate
OUT
energy useful
of Rate
IN
energy
of Rate
DACCUMULATE
energy
of Rate
Electric Power Production (Cont.)
WU QQQ 00
Where, Q0 = energy flow into the black box QU = useful energy out of the black box QW = wasted energy out of the black box
Efficiency
100Q
Q(%) Efficiency
W
U
