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8.2 thermal energy transfer


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global energy issues

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8.2 thermal energy transfer

  1. 1. 8.2 Thermal Energy transfer
  2. 2. There are three forms of heat transfer: Conduction Convection Radiation 2Forms of energy transfer
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  4. 4. Explaining Conduction 4 What happens if we heat part of a metal rod? The particles in that part of the rod vibrate more. The particles pass on the vibration to other particles along the rod. The colder area warms up.
  5. 5. Convection The transfer of heat in a liquid or gas, by the movement of particles, when less dense liquids rise and more dense liquids flow in to take its place. This movement is called convection current 5
  6. 6. What happens to the particles in a liquid or a gas when you heat them? The particles spread out and become less dense. This effects fluid movement. A liquid or gas. Heating Fluids 6
  7. 7. Why is the freezer compartment in a fridge at the top? Freezer compartment It is put at the top, because cool air sinks, so it cools the food on the way down. It is warmer at the bottom, so this warmer air rises and a convection current is set up. 7
  8. 8. Radiation A transfer of heat from a hot object through space or air to a cold object. This process does not depend on particles to transmit heat. Heat energy is transferred as radiation even when no particles are present. Emissivity is the property that allows us to compare radiators – find out about the emissivity of snow and coal. 8
  9. 9. Apply 1. Why is there a sea breeze? 2. How do air conditioning units work? 3. Compare ceramic and metal pans 4. How did the mid-atlantic ridge form? 5. Are black cans better heaters?
  10. 10. Solar Radiation The Sun, with a surface temperature of nearly 6,000 degrees K, emits radiation in the ultra- violet(8%), visible(43%) and infra-red (49%) regions of the electro-magnetic spectrum. The Earth intercepts about 0.5 billionth of the Sun’s total radiation About half of the solar radiation reaching the lower atmosphere actually warms the Earth’s surface (15 degrees C average). The remainder of the radiation is either reflected by clouds and oceans, or absorbed by gases in the atmosphere.
  11. 11. Solar Radiation The solar constant is defined as the amount of solar energy per second that falls on an area of 1m2 of the upper atmosphere perpendicular to the suns’ rays Another name for the solar constant is the sun’s energy flux The solar constant has a value of 1.35 kWm-2 The solar constant varies due to the elliptical orbit of the Earth and age of Sun. The total solar radiation reaching the top of the atmosphere is about 1.7 x 10 17 W which is 170 Wm-2 averaged over a day and night
  12. 12. Solar Radiation  The total solar radiation is 170 Wm-2 averaged over a day and night how much energy does 1km2 of the Earth receive in a day?
  13. 13. Black body radiation A black body is an object that absorbs all electromagnetic radiation that falls on it. No electromagnetic radiation passes through it and none is reflected. Because no light (visible electromagnetic radiation) is reflected or transmitted, the object appears black when it is cold. If the black body is hot, these properties make it an ideal source of thermal radiation.
  14. 14. Black body radiation From the measured values for the Sun, Ts=5778K Rs=6.96 * 108m D=1.496*1011 albedo=0.367 we'll find the effective temperature of the Earth to be Te=248.53K This is the black body temperature that would cause the same amount of energy emission, as measured from space, while the surface temperature is higher due to the greenhouse effect.
  15. 15. Black body radiation  Graph of light emission at different Temperatures
  16. 16. Black body radiation  Stefan Boltzmann The energy radiated by a blackbody radiator per second per unit area is proportional to the fourth power of the absolute temperature and is given by
  17. 17. Black body radiation  Emissivity The emissivity of a material (usually written ε or e) is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature. It is a measure of a material's ability to radiate absorbed energy. A true black body would have an while any real object would have . Emissivity does not have units. In general, the duller and blacker a material is, the closer its emissivity is to 1. The more reflective a material is, the lower its emissivity
  18. 18. Black body radiation  Surface heat capacity Also known simply as specific heat, is the measure of the heat energy required to increase the temperature of a unit quantity of a substance by a certain temperature interval. More heat energy is required to increase the temperature of a substance with high specific heat capacity than one with low specific heat capacity. Molecules have internal structure because they are composed of atoms that have different ways of moving within molecules. Kinetic energy stored in these internal degrees of freedom contributes to a substance’s specific heat capacity and not to its temperature.
  19. 19. Albedo Albedo at a surface is the ratio between the incoming radiation and the amount reflected …expressed as a % or coefficient Recall solar radiation is mainly in the visible and infrared regions (we call the incoming short wave infrared insolation)
  20. 20. Albedo
  21. 21. Albedo Incoming radiation will be INSOLATED, REFLECTED and RETRANSMITTED in various ways.  30% reflected  51% absorbed at surface (23% of which is used in the water cycle)  19% absorbed in atmosphere Albedo is affected by seasons, day, latitude. Global annual mean albedo is 0.3
  22. 22. Albedo  The proportion of absorbed, emitted, and reflected incoming solar radiation steers the Earth's climate system causing fluctuations in temperature, winds, ocean currents, and precipitation.  The climate system remains in equilibrium as long as the amount of absorbed solar radiation is in balance with the amount of terrestrial radiation emitted back to space.
  23. 23. Carbon dioxide emissions  Carbon dioxide is one of the main contributors to greenhouse effect. It contributes 9-26% to the greenhouse effect.  The image shows the increasing density of carbon dioxide in the atmosphere contributed by human activity
  24. 24. Carbon dioxide emissions The seven sources of CO2 from fossil fuel combustion are (with percentage contributions for 2000–2004):  Solid fuels (e.g. coal): 35%  Liquid fuels (e.g. gasoline): 36%  Gaseous fuels (e.g. natural gas): 20%  Flaring gas industrially and at wells: <1%  Cement production: 3%  Non-fuel hydrocarbons: <1%  The "international bunkers" of shipping and air transport not included in national inventories: 4%
  25. 25. IR spectra of greenhouse gases
  26. 26. Greenhouse gases absorb infra-red radiation strongly in the bands with wavelengths between 12,500 – 17,000 nm and 4,500 – 7,000 nm. In between these two bands more than 70% of the radiation is emitted from the Earth’s surface escapes through the atmosphere. When infra-red radiation is absorbed the bonds in the molecules to vibrate, where the distances between bonded atoms increases and decreases in a rhythmical manner as if connected by springs. It is possible for resonance to occur if the vibration frequency matches the molecules natural frequency (for greenhouse gases their natural f is in the infrared region)
  27. 27. IR absorption of greenhouse gases  Polyatomic molecules have more numerous vibrations, at least some of which are ‘infrared-active’. The main infrared absorbers in the atmosphere are water and carbon dioxide. At the long wavelengths of terrestrials radiation, the bending vibrations of these molecules are chiefly responsible for absorption.
  28. 28. What is the Greenhouse Effect? The greenhouse effect is a natural warming process. Put simply natural greenhouse gases absorb outgoing long wave radiation and re-radiate some of it back to Earth The absorbed energy warms the earth's surface which then emits heat energy back toward space as longwave (infra-red) radiation. This outgoing infra- red radiation is partially absorbed by greenhouse gases which then radiate the energy in all directions, warming the earth's surface and lower atmosphere. Without these greenhouse gases the earth's average surface temperature would be about 33 degrees Celsius cooler, that is the same temperature as the Moon.
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  30. 30. Greenhouse Gases The main natural greenhouse gases are water vapour, carbon dioxide, methane, and nitrous oxide. All except water vapour have increased in concentrations in the atmosphere since the 1950s. Other greenhouse gases include ozone and halocarbons. Most of the greenhouse effect is due to water, with carbon dioxide contributing about 1 degree to the warming effect. Only polar molecules can absorb infra-red radiation so the most common gases in the atmosphere, oxygen (O2) and nitrogen (N2), do not contribute to the Greenhouse Effect.
  31. 31. Source of Gases Greenhouse Gas Main Source Human Activity Carbon dioxide Combustion of fossil fuels Transport, power and energy production, burning of rainforests. Methane Anaerobic breakdown of plant matter Rice growing, cattle and sheep farming Nitrous Oxide Denitrificatio n of nitrates by microbes Use of nitrogenous fertilizers in agriculture
  32. 32. Energy balance models There are four main categories of model:  EBM – use spreadsheets to study incoming/outgoing global radiation, using different latitudes from equator to pole  RCM – simulate atmospheric environments, only radiation balance and convection heat transfer  STM – combination of energy balance and radiative-convective models  GCM – 3D general circulation model simulating global climate
  33. 33. Global Warming?  The natural greenhouse effect has been significantly intensified by human activities that increase the concentrations of greenhouse gases (mostly carbon dioxide, methane and nitrous oxide) in the atmosphere.  These greenhouse gases increase the retention of heat in the troposphere and contribute toward (cause) global warming or, more accurately, climate change. (Note: Not all parts of the Earth may experience increases in temperatures e.g. UK temperatures may decrease.)
  34. 34. Enhanced Greenhouse Effect
  35. 35. Major Impacts  A Rise in Temperature varying with latitude & season, with longer summers and shorter winters.  Change in Climate with small temperature changes resulting in large impacts on climate.  Higher Evaporation Rates and increased, but uneven, global rainfall (drier South Australia, wetter Northern Territory).  Change in Natural Ecosystems with increasing extinction of native flora and fauna.  Increase in Photosynthesis with domination of weed species.  Rises in Sea Level due to warming and expansion of oceans and melting of land-based ice.  Weather Extremes during the transition to an enhanced greenhouse Earth.
  36. 36. Major Impacts What evidence do we have for global warming?  Ice core data  Tree ring data  Sedimentary records  Glacial melting