AY Honors/Renewable Energy/Answer Key

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[[pl:Odnawialne %BCr%F3d%B3a energii]] es:energía renovablenl:Duurzame energiede:Regenerative Energie Renewable energy is energy from a source which is not subject to depletion in a human timescale. Sources include the sun's rays, wind, waves, rivers, tides, biomass and the heat from radioactive decay in the Earth's core. Renewable energy does not include energy sources which are dependent upon limited resources, such as fossil fuels and nuclear fission power.

Renewable energy may be used directly (as in solar ovens, geothermal heat pumps, and windmills) or be used to generate electricity or create fuels such as ethanol. Throughout human existence, wood has been critically important as a thermal energy source. Historically, only the power of falling water in rivers (hydroelectricity) has been significantly tapped for the generation of electricity. However, recent years have seen the rapid development of wind generation farms by mainstream power companies. Solar energy's main human application has been in agriculture and forestry, via photosynthesis, but increasingly it is harnessed for heat and electricity. Geothermal power can be used to generate electricity near hot spots in the Earth's crust. Agriculturally produced biomass fuels, such as biodiesel, ethanol and bagasse (a byproduct of sugar cane cultivation) are burned in internal combustion engines or boilers.

Around 80% of energy requirements are focused around heating or cooling buildings and powering the vehicles that ensure mobility (cars, trains, airplanes). This is the core of society's energy requirements. However, most uses of renewable power focus on electricity generation.

The first US politician to focus significantly on solar energy was Jimmy Carter, in response to the long term consequences of the 1973 energy crisis.

Pros and cons

Renewable energy sources are fundamentally different from fossil fuel or nuclear power plants because of their widespread occurrence and abundance - the sun will 'power' these 'powerplants' for the next 4 billion years. Some renewable sources do not emit any additional carbon dioxide and do not introduce any new risks such as nuclear waste.

Since they are harnessing relatively low-intensity energy this new kind of power plant needs to be distributed over a large area. To put the phrases 'low-intensity' and 'large area' easier to understand one should image that in order to produce 1000 kWh of electricity - a typical per-month-per-capita consumption of electricity in Western countries - a house owner in cloudy Europe needs to cover 10 square meters of roof with solar panels.

The disadvantage of renewables is their impact on local environments and their visibility to everybody. Some people dislike the aesthetics of wind turbines or bring up nature conservation issues when it comes to large solar-electric installations outside of cities. Some people try to utilize these renewable technologies in an efficient and aesthetically pleasing way: fixed solar collectors can double as noise barriers along highways, roof-tops are available already and could even be replaced totally by solar collectors, etc.

The second difficulty is the variable and diffuse nature of renewable energies (with the exception being geothermal energy, which is however only accessible where the earth's crust is thin, such as near hot springs and natural geysers). For electricity this means that either there must be reliable overlapping sources or some means of storage on a reasonable scale (pumped-storage hydro systems, batteries, future hydrogen fuel cells, etc.). So, because of the currently expensive energy storage a stand-alone system is only economic in rare cases.

If renewable and distributed generation were to become widespread, electric power transmission and electricity distribution systems would no longer be the main distributors of electrical energy but would operate to balance the electricity needs of local communities. Those with surplus energy would sell to areas needing "top ups".

Sources of renewable energy

There are several types of renewable energy, most are mentioned below:

Of course there are some small scale applications as well.

  • Piezo electric crystals embedded in the sole of a shoe can yield a small amount of energy with each step. Vibration from engines can stimulate piezo electric crystals.
  • Some watches are already powered by movement of the arm.
  • Special antennae can collect energy from stray radiowaves or even light (EM radiation).

Most renewable energy sources can trace their roots to solar energy, with the exception of geothermal and tidal power. For example, wind is caused by the sun heating the earth unevenly. Hot air is less dense, so it rises, causing cooler air to move in to replace it. Hydroelectric power can be ultimately traced to the sun too. When the sun evaporates water in the ocean, the vapor forms clouds which later fall on mountains as rain which is routed through turbines to generate electrity. The transformation goes from solar energy to potential energy to kinetic energy to electric energy.

Some people want to put nuclear energy into the renewable category. Arguments in favour of this view include:

  • The view that nuclear energy does not contribute to global warming.
  • Fast breeder reactors can produce more fuel than they consume.
  • The view that uranium and thorium, being radioactive, are not theoretically long-term resources.
  • The view that nuclear waste, since it will eventually become less radioactive than the original ore bodies, is not theoretically a long-term problem.

However, others feel that the fact that in practice it uses a depleting resource (uranium or thorium), and that the decay of the waste to this level may take as long as three thousand years (depending on the technology used) means that it cannot be included in such a classification. Breeder reactors consume uranium or thorium to produce fissile fuel, so this particular argument is a simple misunderstanding of the basic processes involved. Similar arguments can also be applied against proposed nuclear fusion power stations using deuterium and tritium, the latter bred from lithium, as fuel.

Solar Energy

Since most renewable energy is "Solar Energy" this term is slightly confusing and used in two different ways: firstly as a synonym for "renewable energies" as a whole (like in the political slogan "Solar not nuclear") and secondly for the energy that is directly collected from solar radiation. In this section it is used in the latter category.

There are actually two separate approaches to solar energy, termed active solar and passive solar. The elements outlined below are included in active solar.

Electrical energy

However, for electricity generation ground based solar power has limited potential, as it is too diffuse and too intermittent. First, ground based solar input is interrupted by night and by cloud cover, which means that solar electric generation inevitably has a low capacity factor, typically less than 20%. Also, there is a low intensity of incoming radiation, and converting this to high grade electricity is still relatively inefficient (14% - 18%), though increased efficiency or lower production costs have been the subject of much research over several decades.

Two methods of converting the Sun's radiant energy to electricity are the focus of attention. The better known method uses sunlight acting on photovoltaic (PV) cells to produce electricity. This has many applications in satellites, small devices and lights, grid-free applications, earthbound signalling and communication equipment, such as remote area telecommunications equipment. Sales of solar PV modules are increasing strongly as their efficiency increases and price diminishes. But the high cost per unit of electricity still rules out most uses.

Several experimental PV power plants mostly of 300 - 500 kW capacity are connected to electricity grids in Europe and the USA. Japan has 150 MWe installed. A large solar PV plant was planned for Crete. In 2001 the world total for PV electricity was less than 1000 MWe with Japan as the world's leading producer. Research continues into ways to make the actual solar collecting cells less expensive and more efficient. Other major research is investigating economic ways to store the energy which is collected from the Sun's rays during the day.

The second method for utilizing solar energy is solar thermal. A solar thermal power plant has a system of mirrors to concentrate the sunlight on to an absorber, the resulting heat then being used to drive turbines. The concentrator is usually a long parabolic mirror trough oriented north-south, which tilts, tracking the Sun's path through the day. A black absorber tube is located at the focal point and converts the solar radiation to heat (about 400°C) which is transferred into a fluid such as synthetic oil. The oil can be used to heat buildings or water, or it can be used to drive a conventional turbine and generator. Several such installations in modules of 80 MW are now operating. Each module requires about 50 hectares of land and needs very precise engineering and control. These plants are supplemented by a gas-fired boiler which ensures full-time energy output. The gas generates about a quarter of the overall power output and keeps the system warm overnight. Over 800 MWe capacity worldwide has supplied about 80% of the total solar electricity to the mid-1990s.

Alternatively, many individuals have installed small-scale PV arrays for domestic consumption. Some, particularly in isolated areas, are totally disconnected from the main power grid, and rely on a surplus of generation capacity combined with batteries and/or a fossil fuel generator to cover periods when the cells are not operating. Others in more settled areas remain connected to the grid, using the grid to obtain electricity when solar cells are not providing power, and selling their surplus back to the grid. This works reasonably well in many climates, as the peak time for energy consumption is on hot, sunny days where air conditioners are running and solar cells produce their maximum power output. Many U.S. states have passed "net metering" laws, requiring electrical utilities to buy the locally-generated electricity for price comparable to that sold to the household. Photovoltaic generation is still considerably more expensive for the consumer than grid electricity unless the usage site is sufficiently isolated, in which case photovoltaics become the less expensive.

One proposal for a solar electrical plant is the solar tower, in which a large area of land would be covered by a greenhouse made of something as simple as transparent foil, with a tall lightweight tower in the centre, which could also be composed largely of foil. The heated air would rush to and up the centre tower, spinning a turbine. A system of water pipes placed throughout the greenhouse would allow the capture of excess thermal energy, to be released throughout the night and thus providing 24-hour power production. A 200 MWe tower is proposed near Mildura, Australia.

Thermal energy

The main role of solar energy in the future may be that of direct heating. Much of society's energy need is for heat below 60°C (140°F) - e.g. in hot water systems. A lot more, particularly in industry, is for heat in the range 60 - 110°C. Together these may account for a significant proportion of primary energy use in industrialised nations. The first need can readily be supplied by solar power much of the time in some places, and the second application commercially is probably not far off. Such uses will diminish to some extent both the demand for electricity and the consumption of fossil fuels, particularly if coupled with energy conservation measures such as insulation.

Domestic solar hot water systems are common in the hotter areas of Australia, and simply consist of a network of dark-coloured pipes running beneath a window of heat-trapping glass. They typically have a backup electric or gas heating unit for cloudy days. Such systems can actually be justified purely on economic grounds, particularly in some remoter areas of Australia where electricity is expensive.

With adequate insulation, heat pumps utilising the conventional refrigeration cycle can be used to warm and cool buildings, with very little energy input other than energy needed to run a compressor. Eventually, up to ten percent of the total primary energy need in industrialised countries may be supplied by direct solar thermal techniques, and to some extent this will substitute for base-load electrical energy.

Large scale solar thermal powerplants, as mentioned before, can be used to heat buildings, but on a smaller scale solar ovens can be used on sunny days. Such an oven or solar furnace uses mirrors or a large lens to foccuss the Sun's rays onto a baking tray or black pot which heats up as it would in a standard oven.

Solar towers would also use thermal energy.

Wind Energy

Wind turbines have been used for household electricity generation in conjunction with battery storage over many decades in remote areas. Generator units of more than 1 MWe are now functioning in several countries. The power output is a function of the cube of the wind speed, so such turbines require a wind in the range 3 to 25 metres/second (11 - 90 km/hr), and in practice relatively few land areas have significant prevailing winds. Like solar, wind power requires alternative power sources to cope with calmer periods.

There are now many thousands of wind turbines operating in various parts of the world, with utility companies having a total capacity of over 39,000 MWe of which Europe accounts for 75% (ultimo 2003). Additional windpower is generated by private windmills both on-grid and off-grid. Germany is the leading producer of wind generated electricity with over 14,600 MWe in 2003. In 2003 the U.S.A. produced over 6,300 Mwe of wind energy, second only to Germany. New wind farms and offshore wind parks are being planned and built all over the world. This has been the most rapidly-growing means of electricity generation at the turn of the 21st century and provides a complement to large-scale base-load power stations. Denmark generates over 10% of its electricity with windturbines, whereas windturbines account for 0.4% of the total electricity production on a global scale (ultimo 2002). The most economical and practical size of commercial wind turbines seems to be around 600 kWe to 1 MWe, grouped into large wind farms. Most turbines operate at about 25% load factor over the course of a year, but some reach 35%.

Geothermal Energy

Where hot underground steam or water can be tapped and brought to the surface it may be used to generate electricity. Such geothermal power sources have potential in certain parts of the world such as New Zealand, USA, Philippines and Italy. Iceland produced 170 MWe geothermal power and heated 86% of all houses in the year 2000. Some 8000 MWe of capacity is operating over all. There are also prospects in certain other areas for pumping water underground to very hot regions of the Earth's crust and using the steam thus produced for electricity generation. An Australian startup company, Geodynamics, proposes to build a commercial plant in the Cooper Basin region of South Australia using this technology by 2004.

Electrokinetic energy

This type of energy harnesses what happens to water when it is pumped through tiny channels. See electrokinetics (water).

Hydroelectric Energy

Hydroelectric energy produces less carbon dioxide than burning fossil fuels or gas.

Rivers

Hydroelectric power from potential energy of rivers, now supplies about 715,000 MWe or 19% of world electricity. Apart from a few countries with an abundance of it, hydro capacity is normally applied to peak-load demand, because it is so readily stopped and started. It is not a major option for the future in the developed countries because most major sites in these countries having potential for harnessing gravity in this way are either being exploited already or are unavailable for other reasons such as environmental considerations.

The chief advantage of hydrosystems is their capacity to handle seasonal (as well as daily) high peak loads. In practice the utilisation of stored water is sometimes complicated by demands for irrigation which may occur out of phase with peak electrical demands.

Tides

Harnessing the tides in a bay or estuary has been achieved in France (since 1966) and Russia, and could be achieved in certain other areas where there is a large tidal range. The trapped water can be used to turn turbines as it is released through the tidal barrage in either direction. Worldwide this technology appears to have little potential, largely due to environmental constraints.

Waves

Harnessing power from wave motion is a possibility which might yield much more energy than tides. The feasibility of this has been investigated, particularly in the UK. Generators either coupled to floating devices or turned by air displaced by waves in a hollow concrete structure would produce electricity for delivery to shore. Numerous practical problems have frustrated progress.

OTEC

Ocean Thermal Energy Conversion is a relatively unproven technology, though it was first used by the French engineer Jacques Arsene d'Arsonval in 1881. The difference in temperature between water near the surface and deeper water can be as much as 20°C. The warm water is used to make a liquid such as ammonia evaporate, causing it to expand. The expanding gas forces its way through turbines, after which it is condensed using the colder water and the cycle can begin again.

Biomatter & Biogas Energy

Biomatter or biomass can be used to produce biofuel ( bioalcohols -like methanol and ethanol- and biodiesel). Biodiesel can be used in modern diesel vehicles and can be obtained from waste and crude vegetable and animal oil and fats (lipids).

All sorts of biomatter can be burnt to heat water and to drive turbines. Plants partly use photosynthesis to store solar energy, water and CO2. Sugar cane residue, wheat chaff, corn cobs and other plant matter can be, and is, burnt quite successfully. The process releases no net CO2. Of course electricity is not the only form of practical energy. In some areas corn, sugarbeets, cane and grasses are grown specifically to produce ethanol (also known as alcohol) a liquid which can be used in internal combustion engines and fuel cells.

Animal feces release methane under the influence of anaerobic bacteria which can also be used to generate electricity. See biogas.

Hydrogen fuel cells

Hydrogen is widely seen as a possible fuel for hydrogen cars, if certain problems can be overcome economically. It may be used in conventional internal combustion engines, or in fuel cells which convert chemical energy directly to electricity without flames, in the same way the human body burns fuel. Making hydrogen requires either reforming natural gas (methane) with steam, or, for a renewable and more ecologic source, the electrolysis of water into hydrogen and oxygen. The former process has carbon dioxide as a by-product, which exacerbates (or at least does not improve) greenhouse gas emissions relative to present technology. With electrolysis, the greenhouse burden depends on the source of the power, and both intermittent renewables and nuclear energy are considered here.

With intermittent renewables such as solar and wind, matching the output to grid demand is very difficult, and beyond about 20% of the total supply, apparently impossible. But if these sources are used for electricity to make hydrogen, then they can be utilised fully whenever they are available, opportunistically. Broadly speaking it does not matter when they cut in or out, the hydrogen is simply stored and used as required.

A quite different rationale applies to using nuclear energy for hydrogen. Here the plant would be run continuously at full capacity, with perhaps all the output being supplied to the grid in peak periods and any not needed to meet civil demand being used to make hydrogen at other times. This would mean maximum efficiency for the nuclear power plants and that hydrogen was made opportunistically when it suited the grid manager.

About 50 kWh (1/144,000 J) is required to produce a kilogram of hydrogen by electrolysis, so the cost of the electricity clearly is crucial.

Renewable energy needs a reliable and efficient storage system

Sun, wind, tides and waves cannot be controlled to provide directly either continuous base-load power, or peak-load power when it is needed. In practical terms, without proper energy storage methods they are therefore limited to some 20% of the capacity of an electricity grid, and cannot directly be applied as economic substitutes for fossil fuels or nuclear power, however important they may become in particular areas with favourable conditions. If there were some way that large amounts of electricity from intermittent producers such as solar and wind could be stored efficiently, the contribution of these technologies to supplying base-load energy demand would be much greater. Already in some places pumped storage is used to even out the daily generating load by pumping water to a high storage dam during off-peak hours and weekends, using the excess base-load capacity from coal or nuclear sources. During peak hours this water can be used for hydroelectric generation.

Relatively few places have the scope for pumped storage dams close to where the power is needed, and overall efficiency is low. Means of storing large amounts of electricity as such in giant batteries or by other means have not yet been put to general use, because of the missing business case--but one possible technologies already exist: large scale flow batteries. A long term vision is hydrogen fuel cells for energy storage.

There is some scope for reversing the whole way we look at power supply, in its 24-hour, 7-day cycle, using peak load equipment simply to meet the daily peaks. Today's peak-load equipment could be used to some extent to provide infill capacity in a system relying heavily on renewables. The peak capacity would complement large-scale solar thermal and wind generation, providing power when they were unable to. Improved ability to predict the intermittent availability of wind enables better use of this resource. In Germany it is now possible to predict wind generation output with 90% certainty 24 hours ahead. This means that it is possible to deploy other plants more effectively so that the economic value of that wind contribution is greatly increased.

Countries that use renewable energy

Iceland is a world leader in renewable energy due to its abundant hydro- and geothermal energy sources. Over 99% of the country's electricity is from renewable sources and most of its urban household heating is geothermal. Israel is also notable as much of its household hot water is heated by solar means. These countries' successes are at least partly based on their geographical advantages.

Leading countries by renewable electricity production, (2000)
Hydro Geothermal Wind PV Solar
1. CanadaU.S.GermanyJapan
2. U.S.Philippines U.S.Germany
3. Brazil Italy Spain U.S.
4. China Mexico Denmark India
5. Russia Indonesia India Australia


Share of the total power consumption in EU-countries that are renewable.

1985 1990 1991 1992 1993 1994
EUR-15 5,61 5,13 4,92 5,16 5,28 5,37
Belgium 1,04 1,01 1,01 0,96 0,84 0,80
Denmark 4,48 6,32 6,38 6,80 7,03 6,49
Germany 2,09 2,06 1,61 1,73 1,75 1,79
Greece 8,77 7,14 7,63 7,13 7,33 7,16
Spain 8,83 6,70 6,56 5,73 6,49 6,50
France 7,24 6,34 6,75 7,54 7,32 7,98
Ireland 1,75 1,65 1,68 1,59 1,59 1,63
Italy 5,60 4,64 5,16 5,19 5,34 5,50
Luxembourg 1,28 1,21 1,14 1,26 1,21 1,34
The Netherlands 1,36 1,35 1,35 1,37 1,38 1,43
Austria 24,23 22,81 20,99 23,39 24,23 23,71
Portugal 25,07 17,45 17,03 13,88 15,98 16,61
Finland 18,29 16,71 17,02 18,10 18,48 18,28
Sweden 24,36 24,86 22,98 26,53 27,31 24,04
United Kingdom 0,47 0,49 0,48 0,56 0,54 0,65

Table from [1]

References

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