Renewable energy
Renewable energy is energy from renewable resources such as wind power, solar power, tidal power, geothermal power, hydropower, or thermal depolymerization. Biofuels are also considered renewable if their source is sustainable. Although renewable energy is used mostly to generate electricity, it is often assumed that some form of renewable energy or at least sustainable energy is used to create alternative fuels. Several alternative fuels however, such as nuclear fuel and alternative fossil fuels, are made from non-sustainable sources, and fuels for Hydrogen fuel cells and air engines can be created by non-sustainable means as well. Such non-sustainable fuels are offered as alternatives usually because they cause less pollution at the point of use.
Biomass
Biomass in the energy production industry refers to living and recently dead biological material which can be used as fuel or for industrial production. Biomass is grown from several plants, including miscanthus, switchgrass, hemp, corn, poplar, willow[5], sugarcane, oil palm (palm oil), and algae oil.
Most commonly, biomass refers to plant matter grown for use as biofuel, but it also includes plant or animal matter used for production of fibres, chemicals or heat. Biomass may also include biodegradable wastes that can be burnt as fuel. It excludes organic material which has been transformed by geological processes into substances such as coal or petroleum.
Alcohol fuels
Main Article: Ethanol fuel
Methanol and ethanol are typically not primary sources of energy; however, they are convenient fuels for storing and transporting energy. These alcohols can be used in internal combustion engines such as flexible fuel vehicles with minor modifications.
Methanol can be produced from a wide variety of sources including fossil fuels, but also agricultural products and municipal waste, wood and varied biomass. More importantly, it can also be made from chemical recycling of carbon dioxide (such as from the CO2 rich flue gases of fossil fuel burning power plants or exhaust of cement and other factories, of even atmospheric CO2).[citation needed] Ethanol can be mass-produced by fermentation of the starch or sugar in a wide variety of crops (bio-ethanol), or by hydration of ethylene from petroleum and other sources.
There has been considerable debate about how useful bio-ethanol will be in replacing fossil fuels in vehicles. Concerns relate to the large amount of arable land required for crops,[6] as well as the energy and pollution balance from the ethanol production cycle .[7][8] Recent developments with cellulosic ethanol production and commercialization may allay some of these concerns.[9]
Methanol as a fuel also has several disadvantages.
Hydrogen
Proponents of a hydrogen economy think hydrogen could hold the key to ongoing energy demands. Relatively new technologies (such as fuel cells) can be used to efficiently harness the chemical energy stored in diatomic hydrogen (H2). However, there is no accessible natural reserve of uncombined hydrogen, since what little there is resides in Earth's outer atmosphere (exosphere). Hydrogen for use as fuel must first be produced using another energy source; hydrogen would thus actually be a means to transport energy, rather than an energy source, just as common rechargeable batteries are. One existing method of hydrogen production is steam methane reformation; however, the most common source of methane is natural gas, which is in short supply. Another method of hydrogen production is through electrolysis of water which uses electricity generated from any source, or a combination of fossil fuels, nuclear, and/or renewable energy sources. Biomass or coal gasification, photoelectrolysis, and genetically modified organisms have also been proposed as means to produce hydrogen.
According to the majority of energy experts and researchers, hydrogen is currently impractical as an alternative to fossil-based liquid fuels. It is inefficient to produce, has low energy density (hydrogen gas tanks would need to be 2-3 times as large as conventional gasoline tanks), and is expensive to transport and convert back to electricity. Also, hydrogen fuel cells are still prohibitively expensive as a prime mover of transportation. However, theoretically it is more efficient to burn fossil fuels to produce hydrogen than to burn oil directly in car engines (due to efficiencies of scale). Unfortunately, this does not take into consideration the significant energy cost of having to build hundreds of millions of new hydrogen powered vehicles plus hydrogen fuel distribution infrastructure. Research on the feasibility of hydrogen as a fuel is still underway, and the outcome is uncertain.
A far more practical way to utilize hydrogen is to bond it with the nitrogen in the air to produce ammonia which can then be easily liquefied, transported and used (directly or indirectly) as a clean and renewable fuel.
Air engine
The Air engine is an emission-free piston engine using compressed air as fuel. Unlike hydrogen, compressed air is about 10x cheaper than fossil oil, making it an economically attractive alternative (hydrogen is about 10x more expensive than oil or 100x more expensive than compressed air). The air engine has also broken most barriers (storage of the energy, range, ....). Models exist which can achieve speeds over 35mph with air alone, but at least one company claims it will produce an "Air Car" hybrid by 2010 which will be able to achieve over 100mpg with a top speed of 96mph.[10]
Alternative fossil fuels
Compressed natural gas (CNG) is a cleaner burning alternative to conventional petroleum automobile fuels. The energy efficiency is generally equal to that of gasoline engines, but lower compared with modern diesel engines. CNG vehicles require a greater amount of space for fuel storage than conventional gasoline power vehicles because CNG takes up more space for each GGE (Gallon of Gas Equivalent). Almost any existing gasoline car can be turned into a bi-fuel (gasoline/CNG) car. However, natural gas is a finite resource like all fossil fuels, and production is expected to peak soon after oil does.[11]
There are large but finite coal reserves which may increasingly be used as a fuel source during oil depletion. The Fischer-Tropsch process converts carbon dioxide, carbon monoxide into heavier hydrocarbons, including synthetic oil. It is used today in South Africa to produce most of that country's diesel from coal. The Karrick process is an improved methodology for coal liquefaction, with higher efficiency. Since there are large but finite coal reserves in the world, this technology could be used as an interim transportation fuel if conventional oil were to become scarce. There are several companies developing the process to enable practical exploitation of so-called stranded gas reserves, those reserves which are impractical to exploit with conventional gas pipelines and LNG technology.
Methane hydrate is a form of natural gas. This substance consists of methane molecules trapped within the crystalline structure of water ice and is found in deposits under ocean sediments or within continental sedimentary rock formations. It is estimated that the global inventory of methane hydrate may equal as much as 10x the amount of natural gas. With current technology, most gas hydrate deposits are unlikely to be commercially exploited as an energy source. In addition, the combustion of methane results in the formation of carbon dioxide the production of carbon dioxide is no different to any conventional petrolium fuel. Methane itself is also a greenhouse gas, so if it is "spilled" or released it will contribute to global warming. In other respects methane hydrate has the same problems of fossil fuel.
Nuclear power
Nuclear power is any nuclear technology designed to extract usable energy from atomic nuclei via controlled nuclear reactions. The most common method today is through nuclear fission, though other methods include nuclear fusion and radioactive decay. All current methods involve heating a working fluid such as water, which is then converted into mechanical work for the purpose of generating electricity or propulsion. Today, more than 15% of the world's electricity comes from nuclear power, over 150 nuclear-powered naval vessels have been built, and a few radioisotope rockets have been produced.
Fission reactors use the U-235 isotope of uranium for fuel. While uranium is a fairly common element, the U-235 isotope is relatively rare. Using current reactor technology and current usage levels, and assuming an economical price of extraction, there is approximately 50 years of viable uranium available. Alternative reactor technologies exist which can use the much more common U-238 isotope, but these breeder reactors have technical issues (resulting from the higher levels of heat and radiation produced) to overcome before they can be employed economically.
Since automobiles and trucks consume a great deal of the total energy budget of developed countries, widespread electric vehicles technology would be required to convert the energy generated from nuclear power to transportation.
The long-term radioactive waste storage problems of nuclear power have not been solved, although on-site spent fuel storage in casks has allowed power plants to make room in their spent fuel pools. Today, the only industrial solution lies with storage in underground repositories. There are widespread public concerns about the health-risks, security risks and radioactive waste disposal problems of nuclear materials.
Source: Wikipedia
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