environmental physics chapter 9: global warming and waste heat copyright © 2012 by dbs

Environmental Physics

Chapter 9:Global Warming and Waste Heat

Copyright © 2012 by DBS

Introduction

• 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


• 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


“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 EffectMean surface T = 15 ºC

Global Warming and the Greenhouse

Effect

How do we take the Earth’s temperature?

Proxy (indirect)records

Direct records

Global Warming and the Greenhouse Effect• 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

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.

Sensitivity - 80 ppm in CO2 produces a 10 ºC change at Vostok

Lead lag issue, CO2 first then T, or other way around?


Direct temperature measurements over past 350 yrs

Inirect temperature measurements over past 2000 yrs


Evidence for Warming• 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


• 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.


• 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 %


• 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 EffectGreenhouse 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

Global Warming and the Greenhouse EffectFigure 9.5: Distribution of carbon dioxide emissions from fossil fuels, 2002.


• A

Table 9.2: Annual per capita carbon dioxide (CO2) releases for countries with the highest total emissions, 2009. (1 metric ton = 1000 kg)


1958: Keeling began measuring CO2 at Mauna Loa, HI


• 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

http://www.cmdl.noaa.gov/ccgg/index.html


Evidence for Warming• 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 (1 ºF) in past 100 years• Increased frequency of hurricanes• Methane levels have risen 145 %• Since industrial revolution CO2 concentrations have risen 34 %, highest in 650,000 yrs


• 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


• 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


Evidence for Warming• 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 (1 ºF) in past 100 years• Increased frequency of hurricanes• Methane levels have risen 145 %• Since industrial revolution CO2 concentrations have risen 34 %, highest in 650,000 yrs• Rising sea temperatures• Last century, the world’s sea level rose by 10-20 cm


• 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.


Evidence for Warming• 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 (1 ºF) in past 100 years• Increased frequency of hurricanes• Methane levels have risen 145 %• Since industrial revolution CO2 concentrations have risen 34 %, highest in 650,000 yrs• Rising sea temperatures• Last century, the world’s sea level rose by 10-20 cm• Disappearing glaciers• Melting Arctic sea ice• Melting Antarctic sea ice


• 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


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


• The key question is how much and how fast the temperature will rise

• Without human influence C-cycle is essentially in balance

• Fossil fuel combustion adds about 5 x 109 tons C/yr

• Approximately half of this is absorbed by the ocean and plants

Climate Change Feedbacks and Impacts

Emissions 6.8 x 109 tons C/yr – simple for CO2, more difficult for others

Atmos. concentration – historical records

½ of anth. CO2 emissions enter sinks

Change in absorption spectrum described by

Radiative forcing

Carbon Cycle

Flows and sinks of carbon

Determines how much is in the atmosphere

Largest active sink

Question

Convert tonnes C to metric (Pg C, Petagrams 1015) and re-draw the diagram. (NB: does not include deforestation)

Ocean

39000 Pg C

Fossil Fuels

4000 Pg C

1 billion tonnes = 1 x 109 ton x 1000 kg/ton x 1000 g/kg

= 1 x 1015 g = 1 Pg

Land

(plants and soil)

2000 Pg C

Atmosphere

720 Pg C

5 Pg C yr-1120 Pg C yr-1

100 Pg C yr-1

Question

How much more CO2 does the ocean store than the atmosphere?

39000 / 720 = x 50


• 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

Higher surface temperatures

Positive forcing

(heating)

Enhanced GH effect

Water vapor content

Strong Positive Feedback

Low Clouds High Clouds

Cloud Feedback

Net effect depends on cloud type

Positive forcing

(heating)

Negative forcing

(cooling)

Enhanced GH effect

Increased albedo

Increased Cloud

High Uncertainty

Ice-Albedo Feedbacks

Higher surface temperatures

Positive forcing

(heating)

Lower albedo

Melting ice sheets

Small Positive Feedback More evaporation

→ cloud

(cooling)


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.


• Great deal of uncertainty about climate change impacts• Two opposing viewpoints:

1. We don’t know enough to take action2. We should accept climate change as inevitable, and we should act now to prepare ourselves

• 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

• Review

Thermal Pollution

• 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

Figure 3.3: Block diagram of a fossil-fueled electric generating station.

Condensing unit takes cold water from a body of water

Thermal Pollution

• Quantity of water passing through the condenser is large

Q = mcΔT

• 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

• Higher water flows reduce the temperature change

• Limits ΔT to 8 °C (15 ° F)

• 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

• 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

• Nuclear plants emit about 40 % more waste heat than a similarly sized FF plant

• 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

• 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

• Temperature is one of the most important factors governing the occurrence and behavior of life


By how many mg/L does the O2 level drop when water is released from a power plant?


• 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.


• 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• 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)


Lake Processes: Eutrophication

• Summer – lake is naturally stratified

– Warm water on top – epilimnion

– Cold water at depth – hypolimnion

– Middle – thermocline

• Winter – lake is well mixed

– cold water sinks

– Sets up convection current

– Mixing brings up nutrients and supplied oxygen at depth

• 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

• 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.


• Mechanical-draft Wet Cooling– 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– Larger, more expensive– Similar to car radiatior

Figure 9.16/17: Mechanical-draft wet-cooling tower and natural-draft dry-cooling tower.


• Cooling Ponds

– Artificial lake

– Shallow to allow a maximum surface are to volume ratio for heat los via evaporation

– Can be used for recreation, fishing, swimming, boating etc.

Using Waste Heat

• 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

• 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

• 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

• 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

• 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

environmental physics chapter 9: global warming and waste heat copyright © 2012 by dbs

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