Download Evidence for Warming

Environmental Physics
Chapter 9:
Global Warming and Waste Heat
Copyright © 2012 by DBS
Introduction
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Two major environmental problems of 20th century:
– “Global warming” from increased man-made CO2 emissions
– Depletion of stratospheric ozone layer
Global Warming and the Greenhouse Effect
With the combustion of fossil fuels, our atmosphere has become one large experimental
laboratory, leading to consequences that might case disastrous alterations in our climate
Global Warming and the Greenhouse Effect
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Consequences of global warming:
– Rise in temperature
– Weather extremes
– Farmland becomes dust bowls – drought, water stress
– Rising sea levels around coastal areas – flooding, loss of small islands
– Loss of biodiversity / extinction – shift of habitats, mass migration
Global Warming and the Greenhouse Effect
“there is a discernable human influence on global climate from the buildup of greenhouse gases”
United Nations’
Intergovernmental panel
on Climate Change (IPCC)
Global Warming and the Greenhouse Effect
Mean surface
T = 15 ºC
Global Warming and the Greenhouse Effect
Global Warming and
the Greenhouse
Effect
Direct records
How do we take the Earth’s temperature?
Proxy (indirect)
records
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Vostok, Antarctica - pretty grim existence, largest graveyard by far
Furthest point from coastline, coldest place on earth -126 ºF
4 cycles most of the time either in an ice age or getting to one, warm conditions are rare
(5% of time), abrupt changes
Global Warming and the Greenhouse Effect
Lead lag issue, CO2 first then T,
or other way around?
Sensitivity - 80 ppm in CO2 produces
a 10 ºC change at Vostok
Figure 9.2: The correlation between carbon dioxide concentrations and the earth’s temperature over the past
400,000 years. Data was obtained from measurements of the Vostok, Antarctica ice cores.
Global Warming and the Greenhouse Effect
Direct temperature measurements
over past 350 yrs
Inirect temperature measurements
over past 2000 yrs
Global Warming and the Greenhouse Effect
Evidence for Warming
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The 20th century was the warmest century in the past 1000 years.
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2005 was the warmest year on record
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Mean global temperature rose about ½ º C (1º F) in past 100 years
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Increased frequency of hurricanes
Global Warming and the Greenhouse Effect
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Must not forget other greenhouse gases!
Figure 9.4: Global concentration of methane gas over the past 1000 years indicates a dramatic increase
beginning about 100 years ago. The data was obtained from air bubbles trapped in ice in Greenland.
Global Warming and the Greenhouse Effect
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The 20th century was the warmest century in the past 1000 years.
2005 was the warmest year on record
Mean global temperature rose about ½ º C (1º F) in past 100 years
Increased frequency of hurricanes
Methane levels have risen 145 %
Global Warming and the Greenhouse Effect
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Amount absorbed is described by a gases absorption spectrum
– GH gases absorb in the IR region
– Atmosphere is transparent to visible radiation
Radiation windows
Global Warming and the Greenhouse Effect
Greenhouse Gases and GWP’s
CO2 most important
(why?)
Absorbs between
13 - 100 μm
Naturally present, little
radiation in this range
escapes
Other GH gases absorb
in IR region
Effect of adding these
gases is much larger
N2O GWP same due
to long life
Figure 9.5: Distribution of carbon dioxide emissions from fossil
Global Warming and
the Greenhouse Effect
fuels, 2002.
Global Warming and the Greenhouse Effect
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A
Table 9.2: Annual per capita carbon dioxide (CO2) releases for countries with the highest total
emissions, 2009. (1 metric ton = 1000 kg)
Global Warming and the Greenhouse Effect
1958: Keeling began measuring CO2
at Mauna Loa, HI
Movie
Global Warming and the Greenhouse Effect
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What is the significance of the Keeling curve?
What could be
responsible for this
seasonal up-down
fluctuation?
Since 1958 atmospheric
carbon dioxide has risen
by more than 15%
http://www.cmdl.noaa.gov/ccgg/index.html
Global Warming and the Greenhouse Effect
Evidence for Warming
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The 20th century was the warmest century in the past 1000 years.
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2005 was the warmest year on record
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Mean global temperature rose about ½ º C (1 ºF) in past 100 years
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Increased frequency of hurricanes
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Methane levels have risen 145 %
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Since industrial revolution CO2 concentrations have risen 34 %, highest in 650,000 yrs
Global Warming and the Greenhouse Effect
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At present rate of fossil fuel use, doubling of CO2 expected by middle of this century
Average global temperature rise of 1.5 to 4.5 °C is expected
Earth would be warmer than at any point in last 2 million years
Temperature has only risen 0.3 – 0.6 degrees in last 100 years
Global Warming and the Greenhouse Effect
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2005 report of a 5-year study of ocean temperatures indicated rising sea temperatures
Rise in sea levels of 3.2 cm / decade since 1993
Global Warming and the Greenhouse Effect
Evidence for Warming
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The 20th century was the warmest century in the past 1000 years.
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2005 was the warmest year on record
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Mean global temperature rose about ½ º C (1 ºF) in past 100 years
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Increased frequency of hurricanes
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Methane levels have risen 145 %
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Since industrial revolution CO2 concentrations have risen 34 %, highest in 650,000 yrs
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Rising sea temperatures
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Last century, the world’s sea level rose by 10-20 cm
Global Warming and the Greenhouse Effect
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2005 study reported nearly 90 % of the glaciers of the Antarctic peninsula are losing mass
Arctic glacier and permafrost melt
Recession of glaciers outside polar areas and decrease in snow cover
The Ross Ice Shelf. This is the southernmost navigable point on the planet and the point where
Roald Amundsen started the first successful trek to the South Pole in 1911.
Global Warming and the Greenhouse Effect
Evidence for Warming
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The 20th century was the warmest century in the past 1000 years.
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2005 was the warmest year on record
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Mean global temperature rose about ½ º C (1 ºF) in past 100 years
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Increased frequency of hurricanes
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Methane levels have risen 145 %
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Since industrial revolution CO2 concentrations have risen 34 %, highest in 650,000 yrs
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Rising sea temperatures
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Last century, the world’s sea level rose by 10-20 cm
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Disappearing glaciers
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Melting Arctic sea ice
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Melting Antarctic sea ice
Global Warming and the Greenhouse Effect
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Determining the impacts of global warming is difficult
Large computer models are used to do the simulations and predictions of future scenarios
Numerical representations of complex physical processes
Global Warming and the Greenhouse Effect
Potential implications of the warming trend:
1. Increased global temperatures – larger at the poles – ice caps melt – sea level rises anywhere
from 0.3 to 7 m (1 to 23 ft)
2. Changes in precipitation and weather patterns lead to shifts in productive areas and growing
patterns
3. Unbearable summer high temperatures and number of extreme temperature days
4. Changes in ocean currents leading to a cooler European climate
Global Warming and the Greenhouse Effect
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The key question is how much and how fast the temperature will rise
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Without human influence C-cycle is essentially in balance
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Fossil fuel combustion adds about 5 x 109 tons C/yr
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Approximately half of this is absorbed by the ocean and plants
Climate Change
Feedbacks and Impacts
Atmos. concentration
– historical records
½ of anth. CO2
emissions enter sinks
Emissions
6.8 x 109 tons
C/yr
– simple for CO2,
more difficult for
others
Change in absorption spectrum
described by
Radiative forcing
Flows and sinks of carbon
Carbon Cycle
Largest
active sink
Determines how much is in
the atmosphere
Question
Convert tonnes C to metric (Pg C, Petagrams 1015) and re-draw the diagram.
(NB: does not include deforestation)
120 Pg C yr-1
Atmosphere
5 Pg C yr-1
720 Pg C
100 Pg C yr-1
Land
(plants and soil)
Ocean
Fossil Fuels
39000 Pg C
4000 Pg C
2000 Pg C
1 billion tonnes = 1 x 109 ton x 1000 kg/ton x 1000 g/kg
= 1 x 1015 g = 1 Pg
Question
How much more CO2 does the ocean store than the atmosphere?
39000 / 720 = x 50
Global Warming and the Greenhouse Effect
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Much of the uncertainty associated with climate model predictions has to do with understanding the
sizes of feedback mechanisms
Negative feedback = cooling effect
Positive feedback = warming effect
Water Vapor
Positive
forcing
(heating)
Higher surface
temperatures
Enhanced GH
effect
Strong Positive Feedback
Water vapor
content
Low Clouds
High Clouds
Cloud Feedback
Increased
Cloud
Increased
albedo
Enhanced GH
effect
High Uncertainty
Negative
forcing
(cooling)
Positive
forcing
(heating)
Net effect depends on cloud type
Ice-Albedo Feedbacks
Positive
forcing
(heating)
Higher surface
temperatures
Lower
albedo
Melting ice sheets
Small Positive Feedback
More
evaporation
→ cloud
(cooling)
Global Warming and the Greenhouse Effect
Figure 9.10: Potential feedbacks to global warming. Positive feedbacks are expected to increase the
warming, while negative feedbacks will probably have a cooling effect.
Global Warming and the Greenhouse Effect
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Great deal of uncertainty about climate change impacts
Two opposing viewpoints:
1. We don’t know enough to take action
2. We should accept climate change as inevitable, and we should act now to prepare ourselves
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Ways to act:
– Energy policy
– Emphasis on conservation
– Increased energy efficiencies
– Economic incentives
– Invest in renewable energy technologies
Q17
17. How much CO2 is emitted into the atmosphere at a coal-fired power plant for every 1 kWh of
electrical energy used in our homes?
Use the energy value of coal (assumed to be pure carbon) and an efficiency of 40 %. 1 kWh =
3413 Btu.
1 kWh x (3413 Btu/kWh) / (40/100) = 8532 Btu
Energy value of coal from Table 3.4 (fuel relationships, 1 ton = 2200 lb = 25 x 106Btu)
8532 Btu x (2200 lb/25 x 106 Btu) = 0.75 lb carbon
0.75 lb C x (44 g/mol CO2 / 12 g/mol C) = 2.7 lb CO2
End
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Thermal Pollution
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Thermal pollution is defined as the addition of unwanted heat to the environment
“Pollution” in this case is not the visible “dirtying” of water but the modification of a lake or rivers
environment
The greatest source of heated water is from steam electric generating stations
Condensing unit
takes cold water from
a body of water
Figure 3.3: Block diagram of a fossil-fueled electric generating station.
Thermal Pollution
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Quantity of water passing through the condenser is large
Q = mcΔT
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The amount ΔT by which the temperature increases is inversely proportional to the mass, m, of
water and proportional to the amount of heat added
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Higher water flows reduce the temperature change
Limits ΔT to 8 °C (15 ° F)
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Require 2 gallons for every kWh produced
Question
For a standard 1000 MWe plant flow is ~ 10,000 gallons per second (1200 cubic feet / s)
How many gallons is this per year?
315 x 109 gallons of water per year
This volume is equivalent to ¼ of the daily needs of NY City
Thermal Pollution
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Water demands by power plants
account for 50 % of usage today
Thermal pollution from plants rose
significantly
To meet this problem plants built
after 1977 were required to have
closed cooling systems
Figure 9.8: Present and historical water uses in the United States.
Thermal Pollution
Fossil vs. Nuclear Plants
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Nuclear plants emit about 40 % more waste heat than a similarly sized FF plant
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Reason: higher efficiency and loss of heat through smoke stack
This 1988 thermal image of the Hudson River highlights
temperature changes caused by discharge of 2.5 billion
gallons of water each day from the Indian Point power
plant. The plant sits in the upper right of the photo —
hot water in the discharge canal is visible in yellow and
red, spreading and cooling across the entire width of
the river.
Two additional outflows from the Lovett coal-fired power
plant are also clearly visible against the natural
temperature of the water, in green and blue.
Ecological Effects of Thermal Pollution
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Aquatic Life
– Decreased ability of water to hold oxygen
– Increased rate of chemical reactions
– Changes in reproduction, behavior and growth throughout the food chain
– Long-term damage to natural waters
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Temperature is one of the most important factors governing the occurrence and behavior of life
Ecological Effects of Thermal Pollution
Ecological Effects of Thermal Pollution
By how many mg/L does the
O2 level drop when water is
released from a power plant?
Ecological Effects of Thermal Pollution
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Gradual changes are more tolerated than sudden changes
A temperature of 34 °C (93 °F) is usually take as an upper limit for aquatic life
Figure 9.12: Sensitivity of fish to temperature. Preferred temperature ranges for some species, as
determined in the field and laboratory, are shown as blocks. The solid dot • indicates the upper lethal
limit. The open dot ◦ is the temperature found to be best for spawning.
Ecological Effects of Thermal Pollution
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Growth an reproduction as a function of water temperature
Fish grow faster with increased temperatures
e.g. shrimp growth is increased by 80 % when water is 27 ° C vs. 21 °C
Figure 9.13: Effect of temperature on growth and production of food animals.
Ecological Effects of Thermal Pollution
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One famous case is that of Sockeye Salmon - Columbia River
A series of hydroelectric dams changed it from cool/fast-flowing
to warmer/slower moving lakes
Bacterial diseases drastically reduced the population
Total commercial landings of chinook and sockeye
salmon in the Columbia River, 1866-1990
(from NPPC 1986, ODFW and WDF, 1991)
Ecological Effects of Thermal Pollution
Lake Processes: Eutrophication
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Summer – lake is naturally stratified
– Warm water on top – epilimnion
– Cold water at depth – hypolimnion
– Middle – thermocline
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Winter – lake is well mixed
– cold water sinks
– Sets up convection current
– Mixing brings up nutrients and supplied oxygen
at depth
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Power plant disturbs natural process
– Cold water taken from depths
– Hot water discharged at surface
– Leads to longer stratification, shorter mixing time, lower O2 and higher nutrient levels
Figure 9.14: Stratification, or layering, of a lake during the summer. The average temperature of each
layer is shown.
Cooling Towers and Ponds
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Recent laws dictate methods other than direct dumping of coolant water into the aquatic
environment
Natural draft cooling towers at coal-fired power station, Nottingham, England.
Cooling Towers and Ponds
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Mechanical-draft Wet Cooling
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Hot water from the condenser enters the top and is sprayed downward
Small droplets are cooled by evaporation as a stream of air is drawn from the outside and circulates upwards
Natural-draft Dry Cooling
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Larger, more expensive
Similar to car radiatior
Figure 9.16/17: Mechanical-draft wet-cooling tower and natural-draft dry-cooling tower.
Cooling Towers and Ponds
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Cooling Ponds
– Artificial lake
– Shallow to allow a maximum surface are to volume ratio for heat los via evaporation
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Can be used for recreation, fishing, swimming, boating etc.
Using Waste Heat
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Hot water for industrial use – cogeneration
Aquaculture, increased fish growth through warm water cultivation
Greenhouse heating
Desalination of seawater
Increased crop growth and frost protection
Air preheating
Using Waste Heat
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Large amounts of waste heat are lost from buildings – vented air, steam, hot water
Energy recovered using heat exchangers
Heat is transferred to a liquid
Figure 9.18: Air-to-liquid heat exchanger.
Using Waste Heat
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Exhaust gases can also be used to preheat combustion air for boilers and furnaces through a
‘recuperator’, recovering half of the waste energy that normally goes up the stack
Figure 9.X: Industrial furnace recuperator to extract waste heat from exhaust gases.
Summary
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Evidence of human impact on climate change is becoming clearer
Emissions of greenhouse gases are changing the composition of the atmosphere
Predicted warming of 2-6 °F in the next 100 years may increase sea levels
Warming may also affect global weather patterns and modify agricultural productivity
National plans to reduce greenhouse gas emissions involve increasing energy efficiency and
switching to cleaner fuels
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Steam electric generating plants discharge large quantities of waste heat into the environment
Increased water temperatures damages the health of aquatic ecosystems
Waste heat can accelerate eutrophication
Cooling towers can be used to reduce the effects of waste heat