Renewable energy

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Renewable energy sources are energy sources which theoretically are constantly being renewed. The name may be a bit of a misnomer and perhaps ambient energy sources might be a better term.

Contents

[edit] Solar energy

Solar energy is the energy of the Sun shining on the Earth, originating from entropic fusion reactions at the heart of the star. It can be tapped in many different ways, directly and indirectly.

Direct methods usually refer to either solar panels or passive heat absorption.

Indirect methods would include hydroelectric power, wind power and energy crops. With hydroelectric systems, the sun provides the energy to make the water evaporate, so it can begin its travel anew by raining down on a high ground. With wind power, solar radiation is the ultimate energy source that drives the Earth's weather and climate, including the winds harnessed by turbines. Plants also utilize solar energy in order to perform many energy costly chemical reactions, which later in the food chain are broken down by life for energy.

[edit] Direct solar

Solar power rises above the goo burning mediocracy.

There are two main ways of directly getting useful energy from sunlight: either directly converting it to electricity with photovoltaic cells, or using it to heat stuff in various ways.

For now, both kinds of methods are limited by the vagaries of a planet's surface, including cloud cover (curse you, water vapour!), varying amounts of sunlight (curse you, axial tilt!) and the day-night cycle (curse you, planetary rotation!). Space-based solar collection systems that don't suffer from these limitations have been proposed, but these are unlikely to be built in the foreseeable future.[1]

Photovoltaic cells, a.k.a. "solar panels", convert solar radiation into electricity through the use of the photovoltaic effect. Although the Earth's surface receives daily about 4.2 kilowatt-hours per square meter, most solar panels are not perfectly efficient (and they can never be for thermodynamics reasons) and in reality most of them struggle to get over 20% efficiency (8%-15% being more common, even for commercial uses).

The PS10 Solar Tower in Spain, rated at 11MW.

Heat collection cells take many forms. The one encountered most often is used for heating water for domestic use,[2] which might look like Solar Panels, but in reality at no points is a conversion to electricity taking place. Other forms of heat collection are greenhouse structures, as well as passive heating architectural schemes making use of either controlled glazed surfaces or a material's native thermal capacity. Since such systems only try to convert into heat (so any "heat losses" are actually aiding them), for their specific applications they are actually far more efficient than solar panels with conversion rates of over 60%. (The reason for this being that they don't work equally well in the entire spectrum).

Concentrations towers are essentially a specialized heat capturing system where a lot of mirrors aim thermal energy into a central point, essentially utilizing the same mechanism Archimedes did when he (allegedly) burned a Roman fleet. In a solar tower's focus point there is usually some kind of heat absorbing liquid, which is heated and used to turn turbines, similar to the way conventional power plants generate electricity. The advantages of that approach is that, while solar panels are expensive objects, each and everyone one requiring electrical installations and maintenance, mirrors are ... just dumb mirrors which you could clean with a hose. In addition, because the important energy conversation takes place en masse at the central focus points, such facilities only have to improve on that one, whereas photovoltaic ones which would have to do so over their entire area. Heat storage strategies can also allow such a plant to maintain generation during brief interruptions (ie, clouds) or even for a significant part of the night.

Experimental installations like this are already in place in the Southwestern desert in the United States, which are in reasonably close proximity to large cities in California and Nevada. Were the US to switch into a centralized solar generation from sunlit states, it would require the overhauling of the current, decades old energy distribution systems. A first step towards that would be a nationwide direct current backbone.

Project Desertec[3] is a similar scheme involving huge solar arrays in the Sahara desert which would then transmit low-loss DC power to Europe.

[edit] Advantages of direct solar

  • If the technology works well, it can be scaled up to huge production levels quite well.
  • Relatively little maintenance is required.
  • Daily variations in power output are mostly predictable.
  • It is the technology with the most appeal to the Greens.
  • Works really well for powering communications satellites.
  • Also works pretty well for low powered devices where a grid connection is impractical.
  • Direct use of heat for low temperature purposes such as hot water is very cost effective in the right climate

[edit] Disadvantages of direct solar

  • Photovoltaic cells degrade over time, making "break even" calculations not so obvious.
  • Photovoltaic cells require the use of toxic chemicals for their manufacture.
  • Solar power plants fail mysteriously for many hours every day. Recent research suggests that an astronomical phenomenon called "night" might be the culprit. Although, coincidentally demand does also tend to be at its lowest during these periods.[4]
  • Clouds also reduce solar power by a significant amount.
  • Many highly populated parts of the world don't get consistent high levels of sunlight. This is a problem especially in countries where peak power use is during the winter (which it usually is). But this could be solved by using HVDC lines to transport power over long distances efficiently[citation needed].
  • Low power density (a lot of land is needed per unit of power), but higher than wind or biofuels.
  • Solar collectors (whether PV or thermal) require cleaning if built in a dusty environment (such as say, the middle of a desert where there is the most sunlight) which can often mean high water requirements (not a good thing in the middle of a desert).
  • Space-based solar which could get around most of the problems of solar requires infrastructure we just don't have and is unproven (so we can't depend on it). (It also isn't direct since it converts sunlight into microwaves so maybe shouldn't be mentioned here, but that's not going to stop us).

[edit] Hydroelectric energy

Hydroelectric power, or water power in general, involves taking advantage of water that has been raised due to evaporation by the sun, and deposited on high ground. As it then flows back towards the ocean, its potential energy can be tapped by turbines or water wheels, which are usually built into dams to increase the drop.

Many hydroelectric schemes are also designed to store energy, as water is pumped into the dam when energy is cheap at night and then allowed to run out when demand is higher during the day. Although not particularly efficient, it is perhaps the only commonly used method of "storing" significant amounts of electrical energy. Sometimes an artificial upper reservoir is constructed - the facility generates no net power and is dedicated to energy storage.

[edit] Advantages of water power

  • It is relatively clean, once the equipment has been built and installed
  • If you need a flood control dam it is very cheap and low impact to add electricity generators to it (at least compared with leaving the dam without turbines)
  • Creation of artificial lakes upstream of the dams, to be used for recreation, wildlife preserves, or potable water supplies
  • Supplies power on demand (actually about the most responsive power source on any grid)
  • Can be started with little energy need using only muscle power which is useful for black starts (i.e., when a power grid needs to be started from nothing)
  • By installing pumps that return water upstream, the dam can act like a gigantic battery
  • The only renewable proven to be able to take significant market share away from fossil fuels

[edit] Disadvantages of water power

  • Flooding of landscapes, causing a disruption of ecosystems.
  • Displacement of residents and burial underwater of important cultural landmarks.
  • Limited by geography. In most developed countries, all the good spots are already taken.
  • The flooded vegetation and soil decomposes anaerobically to methane, a far worse greenhouse gas than carbon dioxide. which causes further global warming.[5]
  • The failure of a dam can be disastrous. Large dam failures rank among the most deadly industrial accidents in history.
  • Hydroelectric dams pose barriers to migrating fish. This can be remedied to some extent by constructing fish ladders.

[edit] Wind energy

This is one of the more developed forms of renewable energy and major projects are underway worldwide.

Although only 1% of the world's power comes from the wind, some European countries produce a lot more. The heaviest user is Denmark at 20%, followed by Spain's 11.5% in 2009 (and 40% on a good day)[6]. European targets are also very ambitious, with Spain planning to obtain 15% of its power from wind by 2010. In general, annual wind power growth in Europe is a surprising 32%.[7] However, it should be noted that the wind-heavy countries have among the highest electricity prices in the EU.[8]

[edit] Advantages of wind power

  • Once the infrastructure is in place, it is very clean.
  • The technology is well-established.
  • Land between the wind turbines can be used for farming or pastures.
  • Can realistically be built on water, reducing land usage and, with it, a number of the below-listed disadvantages.

[edit] Disadvantages of wind power

  • Completely unpredictable output, due to the power in the wind varying as the cube of the wind speed; many turbines are required over a large area to compensate for this (even with many turbines over a large area, the total power will sometimes drop near zero) [9]
  • Less intuitively, wind turbines must be shut down and their blades feathered if there is too much wind, since this can damage their gearboxes or in extreme cases cause them to catch fire.
  • Due to the unpredictable output, other sources of energy are needed to back up wind power. In the obvious case, you need extra power to come online when the wind is not high enough to meet demand; but in the less obvious case, when more wind power is available than is demanded, you need extra power to go offline. Coal and nuclear plants are not flexible enough for this, so in practice wind power displaces hydroelectric and natural gas. Furthermore, dams generally are expected to provide or withhold water for many conflicting needs, such as flood control, irrigation, and salmon migration as well as baseload power production, and balancing wind production is another conflicting need. [10]
  • Depending on where the wind farm is built and site management practices, land use change might release more CO2 than the wind farm will save, even ignoring the carbon cost of construction (peat bogs are an especially bad location) [11]
  • Low power density: 2 W/m2, meaning large amounts of land are required [12]
  • Land between the wind turbines is less desirable for human settlement due to risk of turbine blades breaking away, ice throw, and noise

[edit] Fake disadvantages of wind power

Most "anti wind power" groups, however, tend to focus on the following, which aren't particular good reasons against wind power (or wouldn't be if wind power lived up to its proponents' hype), but they're easy to understand.

  • Visual impact; wind turbines have to be built in highly exposed locations, while conventional power plants can be placed anywhere with decent road access, including being integrated into existing industrial developments. This one comes closest to being a legitimate concern since it is very rare even the most perfunctory efforts are made to made the wind turbines look like they actually belong where they are, they are generally painted high-gloss white and look like they just escaped from an airport.
  • Wind Turbine Syndrome.
  • Wind turbines rising up to crush humanity

[edit] Plant-based energy (biofuels)

Plant-based energy consists of allowing plants to fix carbon in sugars and cellulose via sunlight, and then to either produce liquid fuels (such as alcohol) from them or burn them directly.

[edit] Advantages of plant energy

  • It's carbon neutral in the long run
  • It can produce convenient, high energy density liquid fuel, which is very useful in transportation
  • Can use waste products that would otherwise have to be thrown out

[edit] Disadvantages of plant energy

  • It competes with food production and has led to significant price increases and food shortages
  • Some systems, such as making ethanol from corn, are very inefficient and might even have an energy return below 1% and may release more CO2 than just burning petroleum directly. To meet the energy demands for the U.S, an area 5 times the size of the land area of this planet would be needed.
  • Often, plant matter is harvested without proper concern for replenishment
  • In some cases tropical rainforest is being cut down to grow biofuels [13]
  • Where wood is burnt directly for heat it can present a fire risk
  • Very low power density
  • Heavy fertiliser use to grow crops may increase nitrogen-associated pollution (algal blooms, NOx emissions etc.)[14]


[edit] Waste-based energy

Human civilisation is constantly producing waste of one type or another, providing a renewable resource for many things including energy. Much like plant-based energy, waste-based energy relies on producing a combustible material. With waste the composition of said material may be adjusted via sorting mechanisms to optimise the energy density of the fuel. Prominent examples of this are biogas from bacterial degradation of waste and Energy-from-waste incineration. This is part of the energy recovery option in the waste hierarchy and thus is most sustainably benefited from where the waste cannot be prevented or recycled.

[edit] Biogas

Biogas, a fuel consisting of mostly methane, is typically produced during anaerobic digestion (microbial conversion in the absence of oxygen) of waste that is high in organic content in waste processing facilities or less controlled anaerobic degradation of such waste in landfills. While landfills produce biogas we can use, they are also a source for releasing the methane into the environment under degradation of organic waste, which is dangerously combustible and constitutes a severe greenhouse gas. Generally the more controlled facilities are preferred for management of organic waste.

[edit] Advantages of biogas

  • Convenient, high energy density liquid fuel. Very useful in transportation
  • A way to recycle some types of waste and thus get more value out of our products
  • Burns relatively cleanly, and with a very small carbon footprint
  • Waste is a pretty reliable resource
  • Sludge from wastewater can also be anaerobically digested
  • The digestate product of anaerobic digestion is useful as fertilizer in soil

[edit] Disadvantages of biogas

  • Requires a certain amount of organic content in waste (at least on the level of municipal solid waste) and sometimes it is not feasible to produce
  • In the case of solid organic waste, composting is usually cheaper and deals much more efficiently with lignocellulosic material (like wood)
  • In the case of solid organic waste, it is difficult to separate plastic contamination out which reduces the quality of the digestate product

[edit] EfW incineration

Energy-from-waste incineration is the most common method of direct energy recovery from waste and involves directly burning the waste. It is used, or should be used, to recover some value from different kinds of waste where other resource recovery methods have already had their share. In the past this method used to produce by-products irresponsibly and puff them into the surrounding area, like dust or heavy metals; these also included extremely hazardous substances like dioxins and furans. With current engineering standards and properly managed by-products, however, this is no longer a significant problem.

[edit] Advantages of EfW incineration

  • Incineration close to where waste is generated/collected
  • No long-term liabilities
  • EfW now has a track record in many countries
  • Produces biologically sterile ash with a tenth of the volume and a third of the weight of original waste
  • Emissions are controlled
  • A way to reclaim value from some types of waste
  • Bottom ash can be reused as aggregate in construction
  • BPEO (Best Practicable Environmental Option) for some hazardous wastes
  • Waste is a pretty reliable resource

[edit] Disadvantages of EfW incineration

  • Generates carbon dioxide
  • High costs and long pay back periods
  • Needs long-term waste disposal contracts
  • Needs high calorific value wastes
  • Needs constant emissions monitoring against dioxins and furans
  • Production of ash residues requiring disposal

[edit] Fake disadvantages of EfW incineration

Groups that are anti-incineration tend to be of the "act first, think later" variety, like Greenpeace or Friends of the Earth, and some of their arguments reflect that:

  • Resources are being lost by incinerating waste. This argument is ignorant of the fact that EfW incineration is so low on the common waste priority pyramid that only the waste from which nothing more can be derived using other methods is supposed to go through it. This also ignores the fact that resources are being recovered by converting waste to energy.
  • Incineration is incompatible with recycling. Again, it is more complementary to recycling than incompatible.
  • It unsustainably produces toxic substances that we have to live with. As mentioned above, times have moved on and nowadays problematic by-products are prevented, minimised or controlled by modern engineering standards.

[edit] Geothermal energy

Geothermal technologies tap the temperature difference between the surface of the earth and shallow or deep underground regions. Alternatively they may make use of high-temperature hot springs in geologically active areas. The primary source of the temperature difference is the decay of radioactive elements, so in some sense it is a form of nuclear power.

A ground source heat pump is often confused with geothermal energy but is not actually geothermal but a way of making heat pumps more efficient than air source heat pumps since most of the heat is actually from the sun or from the heat pump have been operated in the other direction in the other season (in summer pump heat from the house into the ground, then in winter pump heat from the ground into the house).

Also, it is possible to tap the much higher temperatures hundreds or thousands of meters below the earth's surface via boreholes.

[edit] Advantages of geothermal energy

  • Very high energy densities may be naturally available
  • In practical terms the environmental impact is low
  • Minimal land usage

[edit] Disadvantages of geothermal energy

  • Drilling can cause man-made earthquakes [15]
  • Very few good resources, with only Iceland being able to use geothermal for a significant amount of power (and even then hydro dominates there)
  • If energy extraction is too fast, it is no longer renewable
  • Uses lots of water
  • Deep geothermal is effectively fracking for power and is likely to be subject to the same objections if ever adopted on any large scale.

[edit] Tidal and wave energy

This relatively new technology is designed to obtain energy from tidal movements or from waves. Although the concept seems simple few, if any, commercial installations are in place.

If efficient systems could be built they could provide significant predictable power (at least from tides).

There are essentially two ways to "trap" tidal energy - to build generation plants that are run by large amounts of water flowing in and out of large estuaries, and to build open ocean devices that somehow tap the energy by letting the tides (and perhaps waves, too) force a floating object up and down relative to an anchored one. The Aguçadoura Wave Farm[wp] is an example of the latter technology.

Note that, while tides are driven by the moon's gravity (and to a lesser extent by the sun's gravity), waves are driven primarily by the wind. This means wave power is a kind of wind power, and is thus ultimately a form of solar power.

[edit] Advantages of tidal and wave energy

  • Whatever we build will provide energy until the moon "runs down"
  • Power generation systems can be built into, or merged with, flood control systems that protect large cities located on major estuaries, such as London and the Thames; this generates the energy close to a large need for it

[edit] Disadvantages of tidal and wave energy

  • They would need to be shown not to interfere with fish stocks or fisheries
  • The local energy output will wax and wane from maximum to virtually zero four times a day, and this time slowly changes from day to day. This can, however, be mitigated by dammed mill pond type systems.

[edit] Miscellaneous

[edit] Piezoelectric generators

Piezoelectric materials generate voltage when deformed (the opposite is also true - when voltage is applied, they deform). This can allow them to act as potential "free" and renewable power sources in certain applications. For example there are proposals for piezoelectric floors in dance clubs[16], or piezoelectric sections in roads to power nearby street lights regardless of the grid's status. Usually the amounts of electricity generated that way / cost ratio are pretty underwhelming. However things are better when it comes to small scale applications. For example, piezoelectric fibers woven in someone's clothing could allow them some day to trickle charge their gadgets via their daily activity.

[edit] Thermoelectric generators

Thermoelectric generators use materials which generate voltage from heat differences using the Seebeck effect[wp] (the opposite is also true - applying a voltage will generate a heat differential, known as the Peltier effect). Because of their low efficiencies, they are rarely encountered in anything but small scale uses, such as temperature sensors and Soviet kerosene-lamp-powered radios.[17] A notable exception are radioisotope thermoelectric generators, the likes of which have been used in deep space exploration probes, making them independent from the fading sunlight in the far reaches of the Solar System. As the name suggests, in RTGs the heat is provided by the decay of radioactive materials,[18] which automatically makes them a non-renewable energy source.

[edit] Earth batteries

Earth batteries[19] create voltage by placing two rods of dissimilar metal into the soil. Functionally, they are no different than sticking two electrodes in a potato, and like any other galvanic battery, they are not eternal, although due to the changing soil conditions they might end up "recharging" themselves. The voltages generated this way are usually too small for any practical use. A notable exception was their use as an energy source for signal amplifiers in early telegraph installations.[20] Larger installations might also tap into Earth's natural telluric currents.[21]

[edit] Footnotes

  1. New Scientist Space energy
  2. http://en.wikipedia.org/wiki/Solar_water_heater
  3. http://www.desertec.org/
  4. http://www.gridwatch.templar.co.uk/
  5. http://www.newscientist.com/article/dn7046-hydroelectric-powers-dirty-secret-revealed.html
  6. gales provide 40% of Spain's energy
  7. http://www.cleanpowernow.org/index.php?name=News&file=article&sid=202
  8. http://www.finfacts.ie/irelandbusinessnews/publish/article_10006575.shtml
  9. The Capacity Factor: Uptime & downtime
  10. [1], OregonLive
  11. See this report, section 7.6.14. for relevant charts.
  12. Sustainable Energy - without the hot air, I.4: Wind, p. 32
  13. http://www.newscientist.com/article.ns?id=mg18825265.400
  14. http://e360.yale.edu/feature/the_case_against_biofuels_probing_ethanols_hidden_costs/2251/
  15. Popsci: Does Geothermal Power Cause Earthquakes?
  16. http://inhabitat.com/green-a-go-go-at-londons-first-eco-disco/
  17. http://blog.modernmechanix.com/kerosene-lamp-powers-radio/
  18. See the Wikipedia article on Radioisotope thermoelectric generator.
  19. They are really cells, it takes more than one to make a battery
  20. See the Wikipedia article on Earth battery.
  21. See the Wikipedia article on Telluric current.
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