Sustainable Resource Recovery Technology (NOTE 4)

Joonhong ParkYonsei CEE Department

2014. 10. 13.

The generation of electricity from heat

FuelBoiler(Th) T

urb

ine Generat

or

Pump

River (Tc)

SmokeFly ash

Pump

Electricity

Exhaust steamlow temperatureand pressure

Condenser or heat exchange

Warm water (waste heat)

Cold water

Low Pressurewater

High pressureBottom ash,

cinders

SteamHigh temp& pressure

Inescapable Inefficiencies

The efficiency, η = E-out/E-in

The heat energy Q = m ·c · (T2-T1)

here m: mass of a body where the heat is stored.

c: specific heat capacity

T: temperature

Example: 1 kg of water at 80oC is mixed with 0.5 kg of water at 50oC. What is the final temperature of the mixture?

CMBR with NC-PollutantExample 12: Thermal Pollution of a RiverA coal-fired power plant converts one-third of the coal’s energy into electrical energy. The electrical power output of the plant is 1,000 MW. The other two-thirds of the energy content of the fuel is rejected to the environment as waste heat. About 15 percent of the waste heat goes up the smokestack, and the other 85 percent is taken away by cooling water that is drawn from a nearby river. The river has an upstream flow of 100.0 m3/s and a temperature of 20. 0 C degree.

a.If the cooling water is only allowed to rise in temperature by 10.0 C degree, what flow rate from the stream would be required?b.What would be the river temperature just after it receives the heated cooling water?

Rate of change in internal energy = 1,700 MW = Tcm

a. Find m when dT = 10oC

b. Find ∆T when m =100.0 m3/s

CEE3330 Y2012 WEEK4-5

An ideal heat engine

High TemperatureSource at T-

source

Low TemperatrueSink at T-sink

Engine

Q-source

Q-sink

ExternalWork, W

η = W/(Q-source - Q-sink)

When Q-sink => zero,

η = W/Q-source

η = 1- Q-sink/Q-source

Q-sink/Q-source = T-sink/T-source

η = 1- T-sink/T-source (Carnot Eff.) (note: unit of T is K)

Carnot Eff. = 1 at absolute zero.

The max. temperature = 2000 oCThe min. temperature = - 30oCCarnot Eff. = 0.89 [The limit of heat engine]

Carnot Efficiency

A heat pump

High Temperature

Source at T-sink

Low TemperatrueSink at T-source

Engine

Q-sink

Q-source

ExternalWork, W

Q-sink = Q-source + W

η = Q-sink/W

η= Q-sink/[Q-sink - Q-source]

η = T-sink/[T-sink - T-source] [COP] (note: unit of T is K)

T-sink = 20 oCT-source = 5oCCOP = 19.5

Coefficient of Performance, COP

Double Carnot efficiencies

Heating by electricity generation

vs. Direct heating using a boiler at home

η of electricity generation X COP > η of direct heating

As long as the product of the efficiency of the power station and the COP of the heat pump is greater than the efficiency of a typical domestic gas, oil or coal fired heating system, electrical heating would be the better solution environmentally in that fuel use would be minimized and overall emissions of carbon dioxide would be lower.

Energy Conversion Efficiencies

ChemicalEnergy(fossil fuels etc.)

Thermal

Energy(heat)

Electricalenergy

Mechanical energy

70-95 %

20-40 %100 %

85-95 %

90-95 %

Losses in the fuel burning power plant

100 GJ fuel Combustion at 2000 oC in steam boiler

90 GJ thermal energy in high pressure steam from boiler

Turbine generator

35 GJ of electrical energy

Transmission and utilization as

Thermal energy Mechanical energy

55 GJ thermal energy in low pressuresteam from turbine

Condenser

55 GJ thermal energy rejectedTo cooling water at 25-40oC

Cooling tower

55GJ rejected to temperature at 0-30 oC

10 GJ thermal energy in flue gases

Electric generators

The drift velocity: 100 times a second in Europe

1204 times a second in North America

Large power stations: 3000 rev/min

Implications for the design of generators such as wind turbines where the driving force can vary greatly second by second.

National and international electricity grids

Why does the national grid use such high voltage to transmit electricity long distance?

(Ex. 1MW using a transmission cable either at 10 V and 100,000 A or at 100,000 V and 10 A with 1.5 % electricity loss due to transmission)

For different types of power plant, what are the main considerations in choosing a suitable location?

Sustainability

• General Definition: meeting the needs of the present generation without compromising the ability of future generation to meet their own needs.

• Don’t do these: exhausting a natural resource, leaving large costs for future generations or doing irreversible harm to the planet.

• An energy technology is considered sustainable if:

1. It contributes little to manmade climate change. 2. It is capable of providing power for many generations w/o significant reduction in the size of the resource, and 3. It does not leave a burden to future generation.

☞ It is very difficult to say if an energy technology is truly sustainable or not.

13

Radiative Forcing & Global Temperature Change

14

Halocarbons

N2OCH4

CO2

Stratosphericozone

Troposphericozone

Sulfate

FossilFuel

Burning(Black C)

FossilFuel

Burning(Organic C)

Radia

tive F

orc

e (

Wm

-

2)

Coolin

gW

arm

ing

3

-2

-1

0

1

2

BiomassBurning

MineralDust

Land use(albedo)

Solar

Aerosols

Level of Scientific Understanding

High Very Low

The final change in global mean temperature: dT = Ø * ΣdFØ is the proportionality constant; dF is the change in radiative forcing(see equations at p. 115

Other Concerns

General Pollution

Acid Rains

Injuries and fatalities

Land use

Energy paybacks

External costs and sustainability

General Pollution Concerns

Source Potential causes for concern

Oil Global climate change, air pollution by vehicles, acid rain, oil spills, oil rig accidents

Natural gas Global climate change, methane leakage from pipes, methane explosions, gas rig accidents

Coal Global climate change, acid rain, environmental spoliation by open-cast pollution, mining accidents, health effects on miners

Nuclear power Radioactivity, misuse of fissile and other radioactive material by terrorists, proliferation of nuclear weapons, land pollution by mine tailings, health effects on uranium miners

Biomass Effect on landscape and biodiversity, groundwater pollution due to fertilizers, use of scarce water, competition with food producing

Hydroelectricity Displacement of populations, effect on rivers and groundwater, dams (visual intrusion and risk of accident), seismic effects, downstream effects on agriculture, methane emissions from submergend biomass

Wind power Visual intrusion in landscapes, noise, bird strikes, interference with telecommunications

Tidal power Visual intrusion and destruction of wildlife habitat, reduced dispersal of effluents (these concerns apply manly to tidal barrages, not tidal current turbines)

Geothermal energy

Release of polluting gases (SO2, H2S, etc), grounwater pollution by chemicals including heavy metals, seismic effects

Solar energy Sequestration of large land areas (in the case of centralized plant), use of toxic materials in manufacture of some PV cells, visual intrusion in rural and urban environments

Global loading from various pollutants

and human disruption

Insult NaturalBaseline(tonnes/ year)

HumanDisruption Index

CommercialEnergySupply

TraditionalEnergySupply

Agriculture

Manufacturing, other

Lead emission to air 12,000 18 0.41 negligible negligible 0.59

Oil addition to oceans 200,000 10 0.44 negligible negligible 0.56

Cadmium to air 1,400 5.4 0.13 0.05 0.12 0.70

Sulphur to air 31 mil 2.7 0.85 0.005 0.01 0.13

Methane flow to air 160 mil 2.3 0.18 0.05 0.65 0.12

Nitrogen fixation 140 mil 1.5 0.30 0.02 0.67 0.01

Mercury emission to air 2,500 1.4 0.20 0.01 0.02 0.77

N2O flows to air 33 mil 0.5 0.12 0.08 0.80 negligible

Particulate to air 3,100 mil

0.12 0.35 0.10 0.40 0.15

Non-methane hydrocarbon to air

1 billion 0.12 0.35 0.05 0.40 0.30

Carbon dioxide to air 150 billion

0.05 0.75 0.03 0.15 0.07

Acid Rain: Carbonate system

Acid Rain: SOx and NOx

SO2(g) + H2O H2SO3

2SO2(g) + O2 2SO3 (g)SO3(g) + H2O H2SO4

2NO2 (g) + H2O HNO2 + HNO3

Strong vs Weak Acids

20

SO2 and NOx Emissions of Energy Technologies

Technology SO2 t/TWh NO2 t/TWh

Hydro with reservoir 7 150

Diesel (0.25% S) 1285 310-12,000

Heavy oil (1.5% S) without scrubbing

8013 1,300-2,000

Hydro run-of-river 1 120

Coal (1%S) w/o scrubbing 5274 700-5,000

Coal with SO2 scrubbing 104 690-5,000

Nuclear 3 150

Natural gas 314 77-1,500

Fuel cell 470 -

Biomass plantation 26 1,100-2,500

Sawmill waste 26 69-1,900

Wind power 69 77-130

PV 24 150