When considering energy sources one of the most important issues to consider is

Question

When considering energy sources one of the most important issues to consider is the efficiency of production. ie. How much waste is produced in the process of making energy. This is a crucial but difficult measure to evaluate. Most energy sources have efficiencies which are quite low (many are well below 20% efficient, that means that 4/5 of the energy used is wasted). There are other considerations as well.

Waste is produced at all levels of energy production starting with mining and refining.

The kind of waste could be a consideration, for instance, there is a lot of CO2 involved with fossil fuels which is an important greenhouse gas. While for nuclear fission there is long-lived radioactive waste.

Solar photovoltaics are not as clean as they may seem. Some of the older manufacturing methods are quite dirty. They also don't last forever.

There are also considerations of the availability of materials, this can be a geo-political issue. For instance, the so-called rare-earth metals are only mined in certain areas. China and Afghanistan to name two, there are others, Canada has some but not all, the U.S. has none. They are crucial for many high-tech devices. Also needed for breakthrough battery, solar or wind technology. China recently announced export restrictions. Also, a ton of low-level radioactive waste is produced in the process of refining a ton of rare-earth metal from ore. The one that is important for wind turbines is Neodymium (Nd) which is used to make strong permanent magnets.

There may also be siting issues. With NIMBYism blocking new projects.

1. The future of energy use will be governed by what sources of energy are available. Write 300-400 words comparing a current energy source with a source that is not yet fully "online". What are the advantages and disadvantages of each of these sources?

No need to write that much, but just to get me started. Please and thank you!

Explanation / Answer

Electricity generation provides 18,000 terawatt-hours of energy a year, around 40% of humanity's total energy use. In doing so it produces more than 10 gigatonnes of carbon dioxide every year, the largest sectoral contribution of humanity's fossil-fuel derived emissions. Yet there is a wide range of technologies — from solar and wind to nuclear and geothermal — that can generate electricity without net carbon emissions from fuel.

The easiest way to cut the carbon released by electricity generation is to increase efficiency. But there are limits to such gains, and there is the familiar paradox that greater efficiency can lead to greater consumption. So a global response to climate change must involve a move to carbon-free sources of electricity. This requires fresh thinking about the price of carbon, and in some cases new technologies; it also means new transmission systems and smarter grids. But above all, the various sources of carbon-free generation need to be scaled up to power an increasingly demanding world. In this special feature, Nature 's News team looks at how much carbon-free energy might ultimately be available — and which sources make most sense.

Hydropower

The world has a lot of dams — 45,000 large ones, according to the World Energy Council, and many more at small scales. Its hydroelectric power plants have a generating capacity of 800 gigawatts (for a guide to power, see ‘By the numbers’), and they currently supply almost one-fifth of the electricity consumed worldwide. As a source of electricity, dams are second only to fossil fuels, and generate 10 times more power than geothermal, solar and wind power combined. With a claimed full capacity of 18 gigawatts, the Three Gorges dam in China can generate more or less twice as much power as all the world's solar cells. An additional 120 gigawatts of capacity is under development.

One reason for hydropower's success is that it is a widespread resource — 160 countries use hydropower to some extent. In several countries hydropower is the largest contributor to grid electricity — it is not uncommon in developing countries for a large dam to be the main generating source. Nevertheless, it is in large industrialized nations that have big rivers that hydroelectricity is shown in its most dramatic aspect. Brazil, Canada, China, Russia and the United States currently produce more than half of the world's hydropower.

Advantages: The fact that hydroelectric systems require no fuel means that they also require no fuel-extracting infrastructure and no fuel transport. This means that a gigawatt of hydropower saves the world not just a gigawatt's worth of coal burned at a fossil-fuel plant, but also the carbon costs of mining and transporting that coal. As turning on a tap is easy, dams can respond almost instantaneously to changing electricity demand independent of the time of day or the weather. This ease of turn-on makes them a useful back-up to less reliable renewable sources. That said, variations in use according to need and season mean that dams produce about half of their rated power capacity.

Hydroelectric systems are unique among generating systems in that they can, if correctly engineered, store the energy generated elsewhere, pumping water uphill when energy is abundant. The reservoirs they create can also provide water for irrigation, a way to control floods and create amenities for recreational use.

Disadvantages: Not all regions have large hydropower resources — the Middle East, for example, is relatively deficient. And reservoirs take up a lot of space; today the area under man-made lakes is as large as two Italys. The large dams and reservoirs that account for most of that area and for more than 90% of hydro-generated electricity worldwide require lengthy and costly planning and construction, as well as the relocation of people from the reservoir area. In the past few decades, millions of people have been relocated in India and China. Dams have ecological effects on the ecosystems upstream and downstream, and present a barrier to migrating fish. Sediment build-up can shorten their operating life, and sediment trapped by the dam is denied to those downstream. Biomass that decomposes in reservoirs releases methane and carbon dioxide, and in some cases these emissions can be of a similar order of magnitude to those avoided by not burning fossil fuels. Climate change could itself limit the capacity of dams in some areas by altering the amount and pattern of annual run-off from sources such as the glaciers of Tibet.

Because hydro is a mature technology, there is little room for improvement in the efficiency of generation. Also, the more obvious and easy locations have been used, and so the remaining potential can be expected to be harder to exploit. Small (less than 10 megawatts) 'run-of-river' schemes that produce power from the natural flow of water — as millers have been doing for four millennia — are appealing, as they have naturally lower impacts. However, they are about five times more expensive and harder to scale than larger schemes.

Nuclear fission

When reactor 4 at the Chernobyl nuclear power plant in Ukraine melted down on 26 April 1986, the fallout contaminated large parts of Europe. That disaster, and the earlier incident at Three Mile Island in Pennsylvania, blighted the nuclear industry in the West for a generation. Worldwide, though, the picture did not change quite as dramatically.

In 2007, 35 nuclear plants were under construction, almost all in Asia. The 439 reactors already in operation had an overall capacity of 370 gigawatts, and contributed around 15% of the electricity generated worldwide, according to the most recent figures from the International Atomic Energy Agency (IAEA), which serves as the world's nuclear inspectorate.

Advantages: Nuclear power has relatively low fuel costs and can run at full blast almost constantly — US plants deliver 90% of their rated capacity. This makes them well suited to providing always-on 'baseload' power to national grids. Uranium is sufficiently widespread that the world's nuclear-fuel supply is unlikely to be threatened by political factors.

Disadvantages: There is no agreed solution to the problem of how to deal with the nuclear waste that has been generated in nuclear plants over the past 50 years. Without long-term solutions, which are more demanding politically than technically, growth in nuclear power is an understandably hard sell. A further problem is that the spread of nuclear power is difficult to disentangle from the proliferation of nuclear weapons capabilities. Fuel cycles that involve recycling, and which thus necessarily produce plutonium, are particularly worrying. Even without proliferation worries, nuclear power stations may make tempting targets for terrorists or enemy forces (although in the latter case the same is true of hydroelectric plants).

A long-term commitment to greatly increased use of nuclear power would require public acceptance not just of existing technologies but of new ones, too — thorium and breeder reactors, for instance. These technologies would also have to win over investors and regulators (for nuclear fusion, see ‘Farther out’).

Nuclear power is also extremely capital intensive; power costs over the life of the plant are comparatively low only because the plants are long lived. Nuclear power is thus an expensive option in the short term. Another constraint may be a lack of skilled workers. Building and operating nuclear plants requires a great many highly trained professionals, and enlarging this pool of talent enough to double the rate at which new plants are brought online might prove very challenging. The engineering capacity for making key components would also need enlarging.

In light of these obstacles, predictions of the future role of nuclear power vary considerably. The European Commission's World Energy Technology Outlook — 2050 contains a bullish scenario that assumes that, with public acceptance and the development of new reactor technologies, nuclear power could provide about 1.7 terawatts by 2050. The IAEA's analysts are more cautious. Hans-Holger Rogner, head of the agency's planning and economic study section, sees capacity rising to not more than 1,200 gigawatts by 2050. An interdisciplinary study carried out in 2003 by the Massachusetts Institute of Technology described a concrete scenario for tripling capacity to 1,000 gigawatts by 2050, a scenario predicated on US leadership, continued commitment by Japan and renewed activity by Europe. This scenario relied only on improved versions of today's reactors rather than on any radically different or improved design.

Biomass

Biomass was humanity's first source of energy, and until the twentieth century it remained the largest; even today it comes second only to fossil fuels. Wood, crop residues and other biological sources are an important energy source for more than two billion people. Mostly, this fuel is burned in fires and cooking stoves, but over recent years biomass has become a source of fossil-fuel-free electricity. As of 2005, the World Energy Council estimates biomass generating capacity to be at least 40 gigawatts, larger than any renewable resource other than wind and hydropower. Biomass can supplement coal or in some cases gas in conventional power plants. Biomass is also used in many co-generation plants that can capture 85–90% of the available energy by making use of waste heat as well as electric power.

Advantages: Plants are by nature carbon-neutral and renewable, although agriculture does use up resources, especially if it requires large amounts of fertilizer. The technologies needed to burn biomass are mature and efficient, especially in the case of co-generation. Small systems using crop residues can minimize transportation costs.

If burned in power plants fitted with carbon-capture-and-storage hardware, biomass goes from being carbon neutral to carbon negative, effectively sucking carbon dioxide out of the atmosphere and storing it in the ground (see 'Carbon capture and storage'). This makes it the only energy technology that can actually reduce carbon dioxide levels in the atmosphere. As with coal, however, there are costs involved in carbon capture, both in terms of capital set-up and in terms of efficiency.

Disadvantages: There is only so much land in the world, and much of it will be needed to provide food for the growing global population. It is not clear whether letting market mechanisms drive the allocation of land between fuel and food is desirable or politically feasible. Changing climate could itself alter the availability of suitable land. There is likely to be opposition to increased and increasingly intense cultivation of energy crops. Use of waste and residues may remove carbon from the land that would otherwise have enriched the soil; long-term sustainability may not be achievable.

Bioenergy dependence could also open the doors to energy crises caused by drought or pestilence, and land-use changes can have climate effects of their own: clearing land for energy crops may produce emissions at a rate the crops themselves are hard put to offset.

Wind

Wind power is expanding faster than even its fiercest advocates could have wished a few years ago. The United States added 5.3 gigawatts of wind capacity in 2007 — 35% of the country's new generating capacity — and has another 225 gigawatts in the planning stages. There is more wind-generating capacity being planned in the United States than for coal and gas plants combined. Globally, capacity has risen by nearly 25% in each of the past five years, according to the Global Wind Energy Council.

Wind Power Monthly estimates that the world's installed capacity for wind as of January 2008 was 94 gigawatts. If growth continued at 21%, that figure would triple over six years.

Despite this, the numbers remain small on a global scale, especially given that wind farms have historically generated just 20% of their capacity.

Advantages: The main advantage of wind is that, like hydropower, it doesn't need fuel. The only costs therefore come from building and maintaining the turbines and power lines. Turbines are getting bigger and more reliable. The development of technologies for capturing wind at high altitudes could provide sources with small footprints capable of generating power in a much more sustained way.

Disadvantages: Wind's ultimate limitation might be its intermittency. Providing up to 20% of a grid's capacity from wind is not too difficult. Beyond that, utilities and grid operators need to take extra steps to deal with the variability. Another grid issue, and one that is definitely limiting in the near term, is that the windiest places are seldom the most populous, and so electricity from the wind needs infrastructure development — especially for offshore settings.

Geothermal

Earth's interior contains vast amounts of heat, some of it left over from the planet's original coalescence, some of it generated by the decay of radioactive elements. Because rock conducts heat poorly, the rate at which this heat flows to the surface is very slow; if it were quicker, Earth's core would have frozen and its continents ceased to drift long ago.

The slow flow of Earth's heat makes it a hard resource to use for electricity generation except in a few specific places, such as those with abundant hot springs. Only a couple of dozen countries produce geothermal electricity, and only five of those — Costa Rica, El Salvador, Iceland, Kenya and the Philippines — generate more than 15% of their electricity this way. The world's installed geothermal electricity capacity is about 10 gigawatts, and is growing only slowly — about 3% per year in the first half of this decade. A decade ago, geothermal capacity was greater than wind capacity; now it is almost a factor of ten less.

Earth's heat can also be used directly. Indeed, small geothermal heat pumps that warm houses and businesses directly may represent the greatest contribution that Earth's warmth can make to the world's energy budget.

Advantages: Geothermal resources require no fuel. They are ideally suited to supplying base-load electricity, because they are driven by a very regular energy supply. At 75%, geothermal sources boast a higher capacity factor than any other renewable. Low-grade heat left over after generation can be used for domestic heating or for industrial processes.

Surveying and drilling previously unexploited geothermal resources has become much easier thanks to mapping technology and drilling equipment designed by the oil industry. A significant technology development programme — Tester suggests $1 billion over 10 years — could greatly expand the achievable capacity as lower-grade resources are opened up.

Disadvantages: High-grade resources are quite rare, and even low-grade resources are not evenly distributed. Carbon dioxide can leak out of some geothermal fields, and there can be contamination issues; the water that brings the heat to the surface can carry compounds that shouldn't be released into aquifers. In dry regions, water availability can be a constraint. Large-scale exploitation requires technologies that, although plausible, have not been demonstrated in the form of robust, working systems.

Solar

Not to take anything away from the miracle of photosynthesis, but even under the best conditions plants can only turn about 1% of the solar radiation that hits their surfaces into energy that anyone else can use. For comparison, a standard commercial solar photovoltaic panel can convert 12–18% of the energy of sunlight into useable electricity; high-end models come in above 20% efficiency. Increasing manufacturing capacity and decreasing costs have led to remarkable growth in the industry over the past five years: in 2002, 550 MW of cells were shipped worldwide; in 2007 the figure was six times that. Total installed solar-cell capacity is estimated at 9 GW or so. The actual amount of electricity generated, though, is considerably less, as night and clouds decrease the power available. Of all renewables, solar currently has the lowest capacity factor, at about 14%.

Solar cells are not the only technology by which sunlight can be turned into electricity. Concentrated solar thermal systems use mirrors to focus the Sun's heat, typically heating up a working fluid that in turn drives a turbine. The mirrors can be set in troughs, in parabolas that track the Sun, or in arrays that focus the heat on a central tower. As yet, the installed capacity is quite small, and the technology will always remain limited to places where there are a lot of cloud-free days — it needs direct sun, whereas photovoltaics can make do with more diffuse light.

Advantages: The Sun represents an effectively unlimited supply of fuel at no cost, which is widely distributed and leaves no residue. The public accepts solar technology and in most places approves of it — it is subject to less geopolitical, environmental and aesthetic concern than nuclear, wind or hydro, although extremely large desert installations might elicit protests.

Photovoltaics can often be installed piecemeal — house by house and business by business. In these settings, the cost of generation has to compete with the retail price of electricity, rather than the cost of generating it by other means, which gives solar a considerable boost. The technology is also obviously well suited to off-grid generation and thus to areas without well developed infrastructure.

Both photovoltaic and concentrated solar thermal technologies have clear room for improvement. It is not unreasonable to imagine that in a decade or two new technologies could lower the cost per watt for photovoltaics by a factor of ten, something that is almost unimaginable for any other non-carbon electricity source.

Disadvantages: The ultimate limitation on solar power is darkness. Solar cells do not generate electricity at night, and in places with frequent and extensive cloud cover, generation fluctuates unpredictably during the day. Some concentrated solar thermal systems get around this by storing up heat during the day for use at night (molten salt is one possible storage medium), which is one of the reasons they might be preferred over photovoltaics for large installations. Another possibility is distributed storage, perhaps in the batteries of electric and hybrid cars (see page 810).

Another problem is that large installations will usually be in deserts, and so the distribution of the electricity generated will pose problems. A 2006 study by the German Aerospace Center proposed that by 2050 Europe could be importing 100 GW from an assortment of photovoltaic and solar thermal plants across the Middle East and North Africa. But the report also noted that this would require new direct-current high-voltage electricity distribution systems.

Ocean energy

The oceans offer two sorts of available kinetic energy — that of the tides and that of the waves. Neither currently makes a significant contribution to world electricity generation, but this has not stopped enthusiasts from developing schemes to make use of them. There are undoubtedly some places where, thanks to peculiarities of geography, tides offer a powerful resource. In some situations that potential would best be harnessed by a barrage that creates a reservoir not unlike that of a hydroelectric dam, except that it is refilled regularly by the pull of the Moon and the Sun, rather than being topped up slowly by the runoff of falling rain. But although there are various schemes for tidal barrages under discussion — most notably the Severn Barrage between England and Wales, which proponents claim could offer as much as 8 GW — the plant on the Rance estuary in Brittany, rated at 240 MW, remains the world's largest tidal-power plant more than 40 years after it came into use.

There are also locations well suited to tidal-stream systems — submerged turbines that spin in the flowing tide like windmills in the air. The 1.2 MW turbine installed this summer in the mouth of Strangford Lough, Northern Ireland, is the largest such system so far installed.

Most technologies for capturing wave power remain firmly in the testing phase. Individual companies are working through an array of potential designs, including machines that undulate on waves like a snake, bob up and down as water passes over them, or nestle on the coastline to be regularly overtopped by waves that power turbines as the water drains off. The European Marine Energy Center's test bed off the United Kingdom's Orkney Islands, where manufacturers can hook up prototypes to a marine electricity grid and test how well they withstand the pounding waves, is a leading center of research. Pelamis Wave Power, a company based in Edinburgh, UK, for instance, has moved from testing there to installing three machines off the coast of Portugal, which together will eventually generate 2.25 MW.

Advantages: Tides are eminently predictable, and in some places barrages really do offer the potential for large-scale generation that would be significant on a countrywide scale. Barrages also offer some built-in storage potential. Waves are not constant — but they are more reliable than winds.

Disadvantages: The available resource varies wildly with geography; not every country has a coastline, and not every coastline has strong tides or tidal streams, or particularly impressive waves. The particularly hot wave sites include Australia's west coast, South Africa, the western coast of North America and western European coastlines. Building turbines that can survive for decades at sea in violent conditions is tough. Barrages have environmental impacts, typically flooding previously intertidal wetlands, and wave systems that flank long stretches of dramatic coastline might be hard for the public to accept.

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