Energy, Power and Climate Change

Transcription

1 Energy, Power and Climate Change Thermal energy can be completely converted to work in a single process. A continuous conversion of thermal energy into work requires a cyclical process. Example: isothermal expansion Q = U + W U = 0 so Q = W Power Generation in a typical electrical power plant 1. Some fuel is used to release thermal energy coal, natural gas, oil, uranium. Thermal energy is used to boil water to make steam. 3. Steam turns turbines attached to coils of wire.. Coils of wire turning a magnetic field induce alternating potential difference. Second Law of Thermodynamics: 1) The total entropy of the universe is increasing. ) No cyclical process (engine) is ever 100% efficient. Some energy is transferred out of the system (lost to the surroundings) as unusable energy (degraded energy). 5. Potential difference is stepped up for transmission in order to reduce I R loss of power in lines. Fossil fuels: coal, oil, natural gas Origins of fossil fuels: organic matter decomposed under conditions of high temperature and pressure over millions of years Degraded energy: energy transferred to the surroundings that is no longer available to do work can t be converted into other forms Examples: thermal energy due to friction, sound, energy of deformation Sankey diagrams (energy flow diagrams) are used to keep track of energy transfers and transformations in any process. 1) Thickness of arrow is proportional to amount of energy. ) Degraded energy points away from main flow of energy. 3) Total energy in = total energy out. Non-renewable fuels: fuels that cannot be replaced as fast as they are used as so will be used up will run out eventually limited supply of resource Renewable fuels: resource that cannot be used up (permanent) or is replaced at same rate as being used World Energy Sources Type of fuel Renewable? CO emissions? Fossil fuels No Yes Sankey diagram for a flashlight: Sankey diagram for a car engine: Sankey diagram for an electric motor: Nuclear No No Hydroelectric Yes No See Hamper p. 171 Wind Yes No Solar Yes No Wave Yes No Other energy sources: biofuels, tidal, geothermal NOTE: In most instances... the Sun is the prime energy source for world energy. 1

2 Energy density of a fuel: the ratio of the energy released from the fuel to the mass of the fuel consumed Use: to compare different types of fuels How is choice of fuel influenced by energy density? Fuels with higher energy density cost less to transport and store Other considerations: cost of production, political, social and environmental factors Units: J/kg Fuel Energy Density (MJ/kg) Fusion fuel 300,000,000 Uranium-35 90,000,000 Gasoline (Petrol) 6.9 Diesel 5.8 Biodiesel. Crude oil 1.9 Coal 3.5 Sugar 17.0 Wood 17.0 Cow dung 15.5 Household waste 10 The energy released by one atom of carbon-1 during combustion is approximately ev. The energy released by one atom of uranium-35 during fission is approximately 180 MeV. a) Based on this information, determine the ratio of the energy density of uranium-35 to that of carbon-1..3 x 10 6 Fossil Fuel Power Production Historical and geographical reasons for the widespread use of fossil fuels: 1. industrialization led to a higher rate of energy usage (Industrial Revolution). industries developed near large deposits of fossil fuels (coal towns) Transportation and storage considerations: 1. Natural gas is usually transported and stored in pipelines. Advantages: cost effective Disadvantages: unsightly, susceptible to leaks, explosions, terrorist activities, political instability (withholding use of pipelines or terminals based for political reasons). Many oil refineries are located near the sea close to large cities. Oil is transported via ships, trucks, and pipelines. Advantages: workforce and infrastructure in place, easy access to shipping Disadvantages: oil spills and leakage, hurricanes, terrorist activities 3. Power stations using coal and steel mills are usually located near coal mines. Advantages: minimizes shipping costs Disadvantages: environmental impact (strip mining), mine cave-ins Advantages: Use of fossil fuels for generating electricity Disadvantages: b) Based on your answer above, suggest one advantage of uranium-35 compared with fossil fuels. Higher energy density implies that uranium will produce more energy per kilogram less fuel needed to produce the same amount of energy 1. high energy density. relatively easy to transport 3. cheap compared to other sources. can be built anywhere 5. can be used in the home 1. combustion produces pollution, especially SO (acid rain). combustion produces greenhouse gases (CO ) 3. extraction (mining, drilling) damages environment. nonrenewable 5. coal-fired plants need large amounts of fuel 3

3 1. State typical efficiencies for fossil fuel power generating plants:. State the energy transformations in using fossil fuels to generate electrical energy: Chemical energy in fuel.thermal energy in steam rotational mechanical energy/kinetic energy electrical energy in turbines At each stage energy is dissipated in thermal/sound energy Fossil Fuel Typical Efficiency Coal 35% Natural Gas 5% Oil 38% 5. A 50 MW coal-fired power plant burns coal with an energy density of 35 MJ/kg. Water enters the cooling tower at a temperature of 93 K and leaves at a temperature of 350 K and the water flows through the cooling tower at a rate of 00 kg/s. a) Calculate the energy removed by the water each second MW 3. Sketch a Sankey diagram for a typical fossil fuel power plant. b) Calculate the energy produced by the combustion of coal each second. 150 MW c) Calculate the overall efficiency of the power station. 0%. An oil-fired power station produces 1000 MW of power. a) How many joules of energy will be produced in one day? d) Calculate the mass of coal burned each second kg b) How much energy is released from the fuel per day? Use efficiency c) What is the rate of oil consumption by the plant per day? Use energy density 5 6

4 World Fuel Consumption The histogram shows the relative proportions of world use of the different types of energy sources that are available, though it will vary from country to country. This shows their use for all purposes, such as manufacture, transportation, electric power generation, heating, etc. Most oil and natural gas is used for manufacture of plastics, pharmaceuticals and other synthetic materials. They are also used for transportation fuels. The next biggest use is for production of electricity. The pie chart below shows the relative use of each type of fuel to generate electricity. Solar heating panel (active solar heater): converts light energy from Sun into thermal energy in water run through it Use: heating and hot water Solar Power Photovoltaic cell (solar cell): converts light energy from Sun into electrical energy Use: electricity Nuclear Energy Advantages of solar heating panel over solar cell: requires less (storage) area, less cost, more efficient Source: fissioning of uranium-35 and conversion of mass into energy State the energy transformations in using nuclear fuels to generate electrical energy: Nuclear energy in fuel.thermal energy in coolant.. thermal energy in heat exchanger/steam rotational mechanical energy/kinetic energy electrical energy in turbines The amount (intensity) of sunlight varies with: a) time of day b) season (angle of incidence of sunlight altitude of Sun in sky Earth s distance from Sun) Sketch a Sankey diagram for a typical nuclear power plant. c) length of day d) latitude (thickness of atmosphere) Which way should a solar panel or cell be facing in the Northern hemisphere? Why? South to receive Maximum radiation from the sun to provide maximum energy for whole day Advantages: 1. No global warming effect no CO emissions Comparing Nuclear Fuel to Fossil Fuel. Waste quantity is small compared with fossils fuels 3. Higher energy density 1 kg of uranium releases more energy than 1 kg of coal Disadvantages: 1. Storage of radioactive wastes. Increased cost over fossil fuel plants 3. Greater risks in an accident (due to radioactive contamination) Advantages: 1. Renewable source of energy. Source of energy is free 3. No global warming effect no CO emissions. No harmful waste products Disadvantages: 1. Large area needed to collect energy. Only provides energy during daylight 3. Amount of energy varies with season, location and time of day. High initial costs to construct/install. Larger reserves of uranium than oil 7 8

5 1. An active solar heater whose efficiency is 3% is used to heat 100 kg of water from 0 0 C to 50 0 C. The average power received from the Sun in that location is 0.90 kw/m. a) How much energy will the solar heater need to provide to heat the water? b) How much energy will be needed from the Sun to heat the water? Basic features of a horizontal axis wind turbine: a) Tower to support rotating blades. Wind Power b) Blades that can be rotated to face into the wind. c) Generator. d) Storage system or connection to a distribution grid. Energy transformations: Solar energy heating Earth... Kinetic energy of air... kinetic energy of turbine.. electrical energy c) Calculate the area of the solar heater necessary to heat the water in.0 hours. 1. Determine the power delivered by a wind generator: length of cylinder of air = d = vt. A photovoltaic cell with an area of 0.0 m is placed in a position where the intensity of the Sun is 1.0 kw/m. a) If the cell is 15% efficient, how much power does it produce? b) If the potential difference across the cell is 0.50 V, how much current does it produce? volume of air V Ad through turbines per second = = = Av t t mass of air through m ρv turbines per second = = = ρav t t power delivered to KE 1/ mv 1 turbines by air = = = Aρv t t 3 3. Why is it impossible to extract all power from air? a) Speed of air cannot drop to zero after impact with blades b) Frictional losses in generator and turbulence around blades c) Compare placing 10 of these solar cells in series and in parallel.. Reasons why power formula is an estimate: a) Not all KE of wind is transformed into mechanical energy b) Wind speed varies over course of year c) Density of air varies with temperature d) Wind not always directed at 90 0 to blades. Why are turbines not placed near one another? a) Less KE available for next turbine b) Turbulence reduces efficiency of next turbine 9 10

6 1. A wind turbine has blades 0 m long and the speed of the wind is 5 m/s on a day when the air density is 1.3 kg/m 3. Calculate the power that could be produced if the turbine is 30% efficient. Hydroelectric Power There are many schemes for using water to generate electrical energy. But all hydroelectric power schemes have a few things in common. Hydroelectric energy is produced by the force of falling water. The gravitational potential energy of the water is transformed into mechanical energy when the water rushes down the sluice and strikes the rotary blades of turbine. The turbine's rotation spins electromagnets which generate current in stationary coils of wire. Finally, the current is put through a transformer where the voltage is increased for long distance transmission over power lines. By far, the most common scheme for harnessing the original gravitational potential energy is by means of storing water in lakes, either natural or artificial, behind a dam, as illustrated in the top picture at right.. A wind generator is being used to power a solar heater pump. If the power of the solar heater pump is 0.50 kw, the average local wind speed is 8.0 m/s and the average density pf air is 1.1 kg/m 3, deduce whether it would be possible to power the pump using the wind generator. A second scheme, called tidal water storage, takes advantage of big differences between high and low tide levels in bodies of eater such as rivers. A barrage can be built across a river and gates, called sluices, are open to let the high-tide water in and then closed. The water is released at low tide and, as always, the gravitational potential energy is used to drive turbines to produce electrical energy. A third scheme is called pumped storage. Water is pumped to a high reservoir during the night when the demand, and price, for electricity is low. During hours of peak demand, when the price of electricity is high, the stored water is released to produce electric power. A pumped storage hydroelectric power plant is a net consumer of energy but decreases the price of electricity. Energy transformations: Gravitational PE of water... Kinetic energy of water... kinetic energy of turbine.. electrical energy Advantages: Disadvantages: Advantages: Disadvantages: 1. Renewable source of energy. Source of energy is free 3. No global warming effect no CO emissions. No harmful waste products 1. Large land area needed to collect energy since many turbines are needed. Unreliable since output depends on wind speed 3. Site is noisy and may be considered unsightly. Expensive to construct Renewable source of energy. Source of energy is free 3. No global warming effect no CO emissions. No harmful waste products 1. Can only be utilized in particular areas. Construction of dam may involve land being buried under water 3. Expensive to construct 1

7 1. A barrage is placed across the mouth of a river at a tidal power station. If the barrage height is 15 meters and water flows through 5 turbines at a rate of 100 kg/s in each turbine, calculate the power that could be produced of the power plant is 70% efficient..6 x 10 W Use average height for E P = mgh Wave Power Energy can be extracted from water waves in many ways. One such scheme is shown here. Oscillating Water Column (OWC) ocean wave energy converter: 1. Wave capture chamber is set into rock face on land where waves hit the shore.. Tidal power forces water into a partially filled chamber that has air at the top. 3. This air is alternately compressed and decompressed by the oscillating water column.. A reservoir that is 1.0 km wide,.0 km long and 5 meters deep is held behind a dam. The top of this artificial lake is 100 meters above the river where the water is let out at the base of the dam. Assume the density of water is 1000 kg/m 3. a) Calculate the energy stored in the reservoir..3 x J. These rushes of air drive a turbine which generates electrical energy. Energy transformations: Kinetic energy of water... Kinetic energy of air... kinetic energy of turbine.. electrical energy Determining the power per unit length of a wavefront λ volume of shaded region = A L λ mass of shaded region = ρv= ρa L b) Calculate the power generated by the water if it flows at a rate of 1.0 m 3 per second through the turbine. 875 kw λ potential energy of shaded region = mgh= ρa LgA λ ρa LgA EP 1 1 power per wave = λ = = ρ = T T T ρ A Lg A Lgv 13 power per unit length for a wave = 1 A ρ gv 1

8 1. If a wave is 3 meters high and has a 100 meter wavelength and 0.1 Hz frequency, estimate the power of each meter of the wavefront. 11 kw per meter Black-Body Radiation When heated, a low-pressure gas will...emit a discrete spectrum When heated, a solid will... emit a continuous spectrum Black-body radiation: radiation emitted by a perfect emitter This means that an object that acts as a black body will... perfectly absorb all incoming radiation and not reflect any then perfectly radiate it. Emission Spectra for Black-Bodies. Waves of amplitude 1.5 meter roll onto a beach at a rate of one every 1 seconds with a speed of 10 m/s. Calculate: a) how much power they carry per meter of shoreline 1. Not all wavelengths of light will be emitted with equal intensity.. Wavelength with highest intensity is related to... temperature. 3. As body heats up, wavelength at which most power is radiated... decreases.. Area under curve is proportional to... total power radiated by body Your Turn b) the power along a km stretch of beach. Use the axes at right to sketch the emission spectra for a black-body radiating at low and high temperature. Be sure to label the axes and indicate which curve represents which temperature. Stefan-Boltzmann Law: relates intensity of radiation to temperature of body Advantages: 1. Renewable source of energy. Source of energy is free Disadvantages: 1. Can only be utilized in particular areas. High maintenance due to pounding of waves I = σt P = σt A P= σ AT where σ =Stefan-boltzmann constant σ =5.67 x 10 8 W m K 3. No global warming effect no CO emissions. No harmful waste products 3. High initial construction costs 15 The total power radiated by a black-body per unit area is proportional to the fourth power of the temperature of the body. 16

9 1. The average temperature of the surface of the Sun is approximately 5800 K. Calculate the total intensity of the radiation emitted by the Sun.. The average radius of the Sun is 7.0 x 10 8 m. Calculate the total energy radiated by the Sun each second. Solar Radiation Calculate the intensity of the Sun s radiation incident on Earth. P I = A σ AST I = A σ ( π R ) T I = π r S See pg. 3 of data booklet for 1.5 x m 3. How much energy does the Sun radiate each hour? State some assumptions made in the above calculation. 1. Orbit is a perfect sphere.. Sun and Earth are perfect spheres. 3. No radiation is absorbed by atmosphere.. Sun and Earth are perfect black bodies. Emissivity (e) ratio of power emitted by an object to the power emitted by a black-body at the same temperature. P e= P BB P= ep BB P= eσ AT. Calculate the power emitted by a square kilometer of ocean surface at 10 0 C. Solar Constant: the amount of solar energy that falls per second on an area of 1 m above the earth s atmosphere at right angles to the Sun s rays Accepted value: 1380 W/m Average incoming solar radiation: Power incident on a disc of area πr and averaged over a sphere of area πr Accepted value: = ¼ solar constant =30 W/m 17 18

10 Albedo ratio of total solar radiation power scattered by a planet to total solar radiation received by a planet Meaning: fraction of the total incoming solar radiation received by a planet that is reflected back out into space Global annual mean albedo on Earth: 0.30 = 30% Greenhouse Gases: each has natural and man-made origins 1. Carbon Dioxide (CO ): product of photosynthesis in plants, product of fossil fuel combustion. Methane (CH ): product of decay and fermentation and from livestock, component of natural gas 3. Water Vapor (H O): evaporation. Nitrous Oxide (N O): product of livestock, produced in some manufacturing processes The Earth s albedo varies daily and is dependent on: Absorption of infrared radiation by greenhouse gases 1. season. cloud formations 3. latitude Resonance a transfer of energy in which a system is subject to an oscillating force that matches the natural frequency of the system resulting in a large amplitude of vibration Application to the greenhouse effect: The natural frequency of oscillation of the molecules of the greenhouse gases is in the infrared region Incoming solar radiation: 30 W/m Radiation reflected (by atmosphere, clouds, ground): 0.30 (30) =100 W/m Radiation absorbed (by atmosphere, clouds, ground): 0.70 (30) = 0 W/m Transmittance/Absorbance Graphs for Greenhouse Gases Total energy reflected and re-emitted by Earth: 30 W/m since in thermal equilibrium Greenhouse Effect a) Short wavelength radiation (visible and short-wave infrared) received from the Sun causes the Earth s surface to warm up. b) Earth will then emit longer wavelength radiation (long-wave infrared) which is absorbed by some gases in the atmosphere. c) This energy is re-radiated in all directions (scattering). Some is sent out into space and some is sent back down to the ground and atmosphere. Enhanced (Anthropogenic) Greenhouse Effect Human activities have released extra carbon dioxide into the atmosphere, thereby enhancing or amplifying the greenhouse effect. Major cause: the burning/combustion of fossil fuels d) The extra energy re-radiated causes additional warming of the Earth s atmosphere and is known as the Greenhouse Effect. 19 Possible effect: rise in mean sea-level Outcome: climate change and global warming 0

11 Using a simple energy balance model to predict the heating of a planet Surface Heat Capacity (C S) energy required to raise the temperature of a unit area of a planet s surface by 1 K. Surface heat capacity of Earth: C S =.0 x 10 8 J m - K -1 Temperature change formula: Q CS = A T P t CS = A T ( Iin Iout ) t CS = T ( Iin Iout ) t T = C S C Formula: S Q = A T Units: J m K 1. Explain, using a simple energy balance model, that the intensity of the energy re-radiated by the Earth is about 30 W/m. If the Earth is at thermal equilibrium, then it is emitting energy at the same rate that it is absorbing it. Average incoming solar radiation intensity is 30 W/m so radiated intensity is also 30 W/m. 5. Use the measured equilibrium temperature of Earth to determine the emissivity of Earth. solar radiation absorbed = radiation emitted W (.70)30 = e σ T m 0 = e(5.67 x 10 )(88) 8 e= Suppose a change causes the intensity of the radiation emitted by Earth to decrease 10%. a) Suggest a mechanism by which this might happen. Increased amounts of greenhouse gases cause more solar radiation to be trapped in atmosphere b) Calculate the new intensity of radiation emitted by Earth. 0.90(30) = 306 W/m. Calculate the expected equilibrium temperature of the Earth. solar radiation absorbed = radiation emitted W (.70)30 =σt m T = 55K 3. State two assumptions made in the model used above. a) assumed Earth is a black-body radiator - not true Emissivity is <1 c) Calculate the amount by which earth s temperature would rise over the course of a year as a result. ( Iin Iout ) t T = C S (30 306)(365)()(3600) T = 8.0 x 10 T =.7K b) neglected the greenhouse effect. The measured equilibrium temperature of the Earth is 88 K. Comment on why this is greater than the temperature calculated above. significant amounts of energy is trapped in atmosphere to warm up Earth to 88 K by GHE d) State some limitations of the model used in the calculation above. 1. treats the planet as a whole and ignores interactions between parts (eg. Between oceans and atmosphere). ignores feedback mechanisms (eg. Increase in temperature may melt ice caps which would reduce albedo of earth) 1

12 Global Warming. increased solar flare activity may increase solar radiation 3. cyclical changes in the Earth s orbit may increase solar radiation. volcanic activity may increases amount of solar radiation trapped in Earth s atmosphere Global Warming: records show that the mean temperature of Earth has been increasing in recent years. In specific terms, an increase of 1 or more Celsius degrees in a period of one hundred to two hundred years would be considered global warming. Over the course of a single century, an increase of even 0. degrees Celsius would be significant. The Intergovernmental Panel on Climate Change (IPCC), a group of over,500 scientists from countries across the world, convened in Paris in February, 007 to compare and advance climate research. The scientists determined that the Earth has warmed.6 degrees Celsius between 1901 and 000. When the timeframe is advanced by five years, from 1906 to 006, the scientists found that the temperature increase was 0.7 degrees Celsius. Possible models suggested to explain global warming: Global mean surface temperature anomaly relative to changes in the composition of greenhouse gases may increases amount of solar radiation trapped in Earth s atmosphere In 007, the IPCC report stated that: Most of the observed increase in globally averaged temperature since the mid-0 th century is very likely due to the increase in anthropogenic [human-caused] greenhouse gas concentrations. (the enhanced greenhouse effect) Major likely cause of the enhanced greenhouse effect: increased combustion of fossil fuels Evidence linking global warming to increased levels of greenhouse gases International Ice Core Research: Between 1987 and 1998, several ice cores were drilled at the Russian Antarctic base at Vostok, the deepest being more than 3600 meters below the surface. Ice core data are unique: every year the ice thaws and then freezes again, forming a new layer. Each layer traps a small quantity of the ambient air, and radioactive isotopic analysis of this trapped air can determine mean temperature variations from the current mean value and carbon dioxide concentrations. The depths of the cores obtained at Vostok means that a data record going back more than 0,000 years has been built up through painstaking analysis. Mechanisms that may increase the rate of global warming 1. Global warming reduces ice and snow cover, which in turn reduces the albedo. This will result in an increase in the overall rate of heat absorption.. Temperature increase reduces the solubility of CO in the sea and increases atmospheric concentrations. 3. Deforestation results in the release of more CO into the atmosphere due to slashand-burn clearing techniques, as well as reduces the number of trees available to provide carbon fixation.. Continued global warming will increase both evaporation and the atmosphere s ability to hold water vapor. Water vapor is a greenhouse gas. 5. The vast stretch of permanently frozen subsoil (permafrost) that stretches across the extreme northern latitudes of North America, Europe, and Asia, also known as tundra, are thawing. This releases a significant amount of trapped CO. Rise in sea-levels Coefficient of Volume Expansion (β) fractional change in volume per degree change in temperature V β = V T o As the temperature of a liquid rises, it expands. If this is applied to water, then as the average temperature of the oceans increases, they will expand and the mean sea-level will rise. This has already been happening over the last 100 years as the sea level has risen by 0 cm. This has had an effect on island nations and low-lying coastal areas that have become flooded. Formula: Precise predictions regarding the rise in sea-levels are hard to make for such reasons as: 1. Anomalous expansion of water: Unlike many liquids, water does not expand uniformly. From 0 0 C to 0 C, it actually contracts and then from 0 C upwards it expands. Trying to calculate what happens as different bodies of water expand and contract is very difficult, but most models predict some rise in sea level.. Melting of ice: Floating ice, such as the Arctic ice at the North Pole, displaces its own mass of water so when it melts it makes no difference. But melting of the ice caps and glaciers that cover land, such as in Greenland and mountainous regions throughout the world, causes water to run off into the sea and this makes the sea level rise. Units: 1 K Inspect the graphical representation of the ice core data and draw a conclusion. There is a correlation between Antarctic temperature and atmospheric concentrations of CO 3

13 Possible solutions for reducing the enhanced greenhouse effect 1. Greater efficiency of power production. To produce the same amount of power would require less fuel, resulting in reduced CO emissions.. Replacing the use of coal and oil with natural gas. Gas-fired power stations are more efficient that oil and coal and produce less CO. 3. Use of combined heating and power systems (CHP). Using the excess heat from a power station to heat homes would result in more efficient use of fuel.. Increased use of renewable energy sources and nuclear power. Replacing fossil fuel burning power stations with alternative forms such as wave power, solar power, and wind power would reduce CO emissions. 5. Carbon dioxide capture and storage A different way of reducing greenhouse gases is to remove CO from waste gases of power stations and store it underground. 6. Use of hybrid vehicles Cars that run on electricity or a combination of electricity and gasoline will reduce CO emissions. International efforts to reduce the enhanced greenhouse effect 1. Intergovernmental Panel on Climate Change (IPCC): Established in 1988 by the World Meteorological Organization and the United Nations Environment Programme, its mission is not to carry out scientific research. Hundreds of governmental scientific representatives from more than 100 countries regularly assess the up-to-date evidence from international research into global warming and human induced climate change.. Kyoto Protocol: This is an amendment to the United Nations Framework Convention on Climate Change. In 1997, the Kyoto Protocol was open for signature. Countries ratifying the treaty committed to reduce their greenhouse gases by given percentages. Although over 177 countries have ratified the protocol by 007, some significant industrialized nations have not signed, including the United States and Australia. Some other countries such as India and China, which have ratified the protocol, are not currently required to reduce their carbon emissions. 3. Asia-Pacific Partnership of Clean Development and Climate (APPCDC): This is a non-treaty agreement between 6 nations that account for 50% of the greenhouse emissions (Australia, China, India, Japan, Republic of Korea, and the United States.) The countries involved agreed to cooperate on the development and transfer of technology with the aim of reducing greenhouse emissions. 5