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One obvious possibility is conservation. The United States uses twice the energy per person as is used in Europe or Japan (Fig. 9.11). In part this is because the human terrain of the United States developed in a more automotive way, whereas public transit and trains are better developed in Europe and Japan. In addition, Americans drive larger cars and live in larger houses. I personally enjoy life in Europe. I would far prefer to take a train to work, in which I can read or watch people, rather than curse at other drivers in a private car. Certainly it makes more sense, in a global sense, for Americans to learn to live like Europeans rather than Europeans and other earthlings to learn to live like Americans.

On the other hand, it seems unlikely that conservation alone will solve the world's energy problems. Most of the world lives at a standard far below that of Europe or the United States. If the citizens of the developing world ultimately consume as much as the European model, the footprint of humanity on the Earth would increase by about a factor of five. The Kaya projection of energy use in the future already assumes increases in energy and carbon efficiency, and it still predicts a thirst for energy that results in higher CO2 concentrations than the stabilization scenarios.

None of our traditional sources of carbon-free energy seems capable of making up the shortfall for the coming century by itself (Hoffert et al. 1998; Pacala and Socolow 2004). The first option that seems to come to people's minds is nuclear energy because it generates no carbon, and we know it works. The French are using nuclear for more than a third of their energy needs. However if we want to generate 17 TW of nuclear energy, using present-day standard 1000 MW reactors, it would require 17,000 new reactors within 100 years, for an average reactor construction rate of one new reactor every second day, continuously, for 100 years. This seems like a lot of nuclear reactors, or else development of reactors much larger than today's.

It is a close call whether or not there would be enough minable uranium to support this level of nuclear energy production. The global inventory of minable uranium today would be sufficient to generate 10 TW of energy at our current efficiency for a few decades before it would be exhausted. However, uranium today is rather inexpensive, and we only bother to mine the purest uranium deposits because production of nuclear energy is limited by supply of reactors, not by supply of uranium. We could move to lower-purity uranium deposits. Ultimately it may be possible to extract uranium from seawater, where it is present in huge amounts but at very low concentrations. In addition, it is theoretically possible to stretch the energy yield of natural uranium by converting it to plutonium in what is known as a breeder nuclear reactor. This increases the energy yield from the uranium by a factor of 50, but the downside is that it is very easy to produce nuclear weapons from plutonium. For this reason, breeder nuclear reactors are not used today.

Opposition to nuclear energy arises because of the potential for catastrophic accidents. The explosion at the Chernobyl power plant in the former USSR could have been much worse if the core had melted down to the water table, contaminating the groundwater supply to Ukrainian agriculture and the city of Kiev. Nuclear energy has the potential to poison large stretches of land to human use essentially forever. The other objection to nuclear energy is the problem of waste storage. Breeder reactors have the advantage of incinerating the long-lived radioactive wastes.

Windmills are becoming economically competitive with new traditional power plants. (The trick word in that sentence was "new." An already existing coal power plant can be run very inexpensively; nothing can compete with that.) Windmills supply 7% of the energy needs of Denmark with no adverse impact on the beauty of the landscape of Denmark in my opinion. Wind energy currently accounts for 0.3% of energy globally and is growing at 30% per year. At this rate wind could supply 10% of the world's energy within the next couple of decades. Scaling up current wind energy production by a factor of 50 would generate about 2 TW of energy.

It may be possible to extract energy from winds at high elevations in the atmosphere. Winds get faster with altitude in the troposphere, peaking in the jet stream winds in mid-latitudes of both hemispheres. The air up there is at lower pressure than down here, and therefore has less density to drive wind blades, but still the power density is much higher at high altitude than it is at the Earth's surface (Fig. 13.4). Wind energy collectors could function like giant kites tethered to the ground by electrically conducting cable, remaining aloft passively by riding the wind. It has been proposed that high-altitude windmills could scale up to tens of terawatts of energy (http://www.skywindpower.com).

Solar cells, or photovoltaics, generate electricity from sunlight. Photovoltaics are currently rather expensive to produce, but if solar energy generation were scaled up by a factor of 700 from present-day rates, they could generate several terawatts of energy. Similarly to surface windmills, photovoltaics seem unlikely to solve the entire energy problem but they can certainly help. One new idea is to build solar cells on the moon (Hoffert et al. 2002), beaming energy back to the Earth as microwaves. The moon would be an ideal location for solar cells, in that it never rains there, there are no clouds, and no birds. The cells could be produced from lunar soil, reducing the amount of mass that would have to be lifted out of Earth's gravity by rockets, a huge savings

Latitude

Fig. 13.4 Power density of winds in the atmosphere as a function of latitude and altitude. From Caldeira, via windskypower.com.

Latitude

Fig. 13.4 Power density of winds in the atmosphere as a function of latitude and altitude. From Caldeira, via windskypower.com.

in energy and cost. The energy from the solar cells could be transmitted to Earth via microwaves; this is apparently feasible, safe, and relatively efficient. An array of solar cells in orbit around the earth would also share the benefits of a clean environment and solar flux that is unimpeded by the atmosphere. Solar cells in orbit however have to be produced on Earth and lifted to orbit.

Both surface wind and solar power share a technical difficulty that energy must be stored because periods of maximum power demand may not coincide with times of maximum windiness or sunshine. To some extent, each technology benefits from the other, in that, when the sun ain't shining, the wind often be blowing. Another way to deal with this problem is to globalize the power grid, so that electricity could be efficiently transported around the world. Electrical energy can also be used to generate hydrogen. You have probably heard the hype about the hydrogen economy. Hydrogen itself is not a primary energy source. There are no minable sources of energy as hydrogen like the energy in fossil fuel deposits. Hydrogen can be produced from other sources of energy, for example, electricity can be used to break water into components hydrogen and oxygen gas, a process called hydrolysis, or produced chemically from coal in a process called gasification.

Hydrogen could be used to store energy for use in transportation. Because hydrogen is a gas, it must be contained under pressure, as opposed to gasoline, which is liquid at room temperatures, or propane, which can be liquefied at moderate pressure. Propane is already being used to power some buses, as well as things like forklifts. Hydrogen is a very flammable gas, and burns invisibly unless the flames happen to hit some other material that emits light. Hydrogen is more explosive than propane, but in crash tests, the added danger is offset by the tendency of hydrogen to rise up, like helium would, escaping from a crashed vehicle.

Another option for carbon-free power is to use fossil fuels and then dispose of the carbon, called CO2 sequestration. CO2 could be captured from the exhaust stream of a power plant, or coal could be treated with steam to release hydrogen gas and CO2. The CO2 could be injected into the Earth into geological formations that must be porous enough for CO2 to flow away from the injection site, but isolated from the surface so that the CO2 doesn't escape back to the surface. The largest type of geological formation that would fit the bill is called saline aquifers. These deposits contain water in their pore spaces, but the water has salt dissolved in it, so the assumption is that this water is no good for anybody and we might as well inject CO2 into it. Methane gas has remained stable in deep Earth reservoirs for hundreds of millions of years, so the idea is in principle possible. These aquifers are thought to have the capacity to store 10,000 Gton of C as CO2.

Scientists are also discussing the possibility of sequestering CO2 in the deep ocean. CO2 released into the atmosphere will ultimately mostly dissolve in the ocean anyway; 75% of the carbon is in the ocean when the air and water reach equilibrium after hundreds ofyears. One could envision direct injection into the ocean as simply bypassing the transit of CO2 through the atmosphere. CO2 released in the ocean will equilibrate with the atmosphere toward the same equilibrium point; 25% of ocean-released CO2 would escape to the atmosphere after hundreds of years. The immediate difficulty with ocean injection is that the CO2 is very concentrated right near the injection site, acidifying the water (Chapter 10) and killing marine life.

You may have read about the idea of fertilizing the plankton in the ocean to take up CO2 as a means of carbon sequestration. The idea is superficially attractive in that plankton in many parts of the ocean, in particular in the cold surface waters around Antarctica, are starving for tiny amounts of the element iron. One atom of iron would allow plankton to take up 100,000 atoms of carbon (more or less). Iron is typically supplied to the surface ocean through dust blown through the atmosphere, but the Southern Ocean is remote enough that not much dust blows down there. When oceanographers add iron to surface water, the plankton bloom. The idea is that the plankton would grow, die, then sink to the deep ocean, thus taking charge of the jobs of extracting CO2 from the surface ocean, ultimately from the atmosphere, and transporting it to the deep ocean for us. The problem is that models of the carbon cycle in the ocean and atmosphere predict that even if the entire Southern Ocean could be successfully fertilized, the effect on atmospheric CO2 in the coming century would be discouragingly small. The reason is that it takes hundreds of years for the atmosphere and ocean to reach equilibrium. Ocean fertilization could have a larger impact on atmospheric CO2 if we were willing to wait 500 years. The impact in the next few decades is too small to be worth it.

The last possibility to mention is the idea of deliberately altering the climate in such a way so as to counteract the warming effects of rising CO2 concentrations. One possibility is to deliberately inject sulfate aerosols into the stratosphere. Sulfate aerosols cool by scattering light. Particles in the stratosphere remain there for several years, as opposed to aerosols in the troposphere where rain scrubs them out in a few weeks. The cooling effect of volcanic eruptions such as Mt. Pinatubo has been clearly documented. The aerosols could be shot up to the upper atmosphere in 1-ton shells fired by large cannons. Relative to most of the alternatives, cooling the Earth by this method seems inexpensive and benign. My own personal objection to this idea is that the warming effects from CO2 will last for centuries, even millennia, committing mankind to taking an active role in climate maintenance essentially forever for a few decades of carelessness. Other proposals for climate engineering include the placement of a large reflective object in space, in orbit around the Earth or at the stable "Lagrange" point between the earth and the Sun where objects can sit indefinitely. The space-based option would be more costly and would require active removal if its effects were eventually judged to be detrimental, but it may not require ongoing participation in order to continue working.

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Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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