Transcript 924 Lecture, Energy

Family Homecoming Special Event
"Can Climate Engineering Serve as a Complementary Step to Aggressive
Mitigation?"
¨Dr. Michael MacCracken, The Climate Institute, Washington, DC
¨Friday, Sept. 25 at 4:00 pm in Olin 1, with cookies
Hydrologic Cycle
Annual
Precipitation,
Washington
State
The Atmosphere’s Energy
Read Anthes chapter 3
Energy is the ability to do work
Units are mass x distance2 / time2
Potential energy: E = mgh
Kinetic energy: E = 1/2 mv2
Heat energy: sensible and latent
Radiant energy: visible and infrared
Laws of Thermodynamics
1. Conservation of energy: Energy is neither created
nor destroyed; it is transformed.
you can't take out of a system more than you put in.
you can't win
2. The entropy of the universe is continually
increasing.
perpetual motion and a heat engine with 100%
efficiency are both impossible.
you can't break even
3. It is impossible to attain absolute zero or absolute
0 entropy.
you can't even get out of the game
Energy transformation example:
Hydroelectric power plant
More complete picture:
Solar power (drives hydrologic cycle)
Potential energy (water stored in reservoir)
Kinetic energy (spillway)
Mechanical energy (spinning turbines)
Electrical energy (transmitted over wires)
Lightbulbs (converts energy to light)
Waste heat (IR) is lost to space
Transfer of Energy
Conduction
Convection
Advection
Latent heat
Radiation
-- Molecular motion
-- Mass transfer vertical
-- Mass transfer horizontal
-- Ice and liquid phases
-- SW and LW photons
Conduction (molecular motion)
Thermal conductivity is the ability of a substance to
transfer heat via molecular motion.
Measured in units of cal/sec/cm/oC
Conductivity of solids > liquids > gases.
Silver (good conductor)
Water (1000 times worse)
Ice
Air (good insulator)
= 1.0
= 1.4 x 10-3
= 5.3 x 10-3
= 6.1 x 10-5
Convection and Advection
(mass transfer)
Rising air currents (thermals) carry sensible
heat and latent heat from the surface into
the upper air.
Winds (advection) carry sensible heat and
latent heat (moisture) into northern
latitudes.
Ocean currents transfer warmer waters to
northern latitudes and vice-versa.
The Electromagnetic Radiation
Every object in the universe emits radiation.
From 1012 cm radio waves to 10-12 cm gamma rays
Stefan-Boltzmann Law
Hotter bodies emit more total energy than colder
bodies.
The total energy of a blackbody is proportional to the
fourth power of temperature.
Etot = T4
Compare energy emitted by Sun and Earth
Energy emitted per unit of surface area:
E / E = T4 / T4 = (6000 / 300)4 = 204 = 1.6 x 105
Energy emitted by the entire surface
Multiply by R2/ R2 = (100/1)2 = 104
So Sun emits 1.6 x 109 more energy than Earth
Power in watts
Sun
3.6 × 1026
Total human consumption, global
1.3 × 1013
Total human consumption, US
3.2 × 1012
Large commercial power plant
109 to 1010
human, daily average from diet
100 (one light bulb)
per capita world
2 x 103 (20 lightbulbs)
per capita US
104 (100 lightbulbs)
Planck energy distribution curve
(energy density per unit time per unit wavelength)
Wein’s Law
The wavelength of maximum emission depends
inversely on a body’s Kelvin temperature.
max = 2897/T (microns)
Emission from hotter bodies peaks at shorter
wavelengths.
What is max for the Sun?
max = C/T = 2897/ 6000 = 0.48 microns = yellow visible light
What is max for the Earth?
max = C/T = 2897/ 300 = 10,1 microns = infrared
Trace gases absorb radiation at selected wavelenghts.
Atmosphere is transparent to sunlight at 0.5mm and to IR at 10mm
Net result
Make a heat budget at the top and bottom of the atmosphere
Top of atmosphere: Gains = Losses
100 SW - 31.3 SW - 68.7 LW = 0
\
Surface: 7.6 SW + 43.2 SW + 98 LW - 7.6 SW - 4.4 C - 22.8 E - 114 LW = 0
This is the average balance sheet -- Dynamic balance is never achieved!