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.
 Solar energy
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 from by raining down on a high ground anew. With wind power, solar radiation is often the prime cause behind the kinetic energy of air molecules, driving the wind, which we harness with propellers driving 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.
 Direct solar
"Direct" solar power means capturing the energy in sunlight and converting it to a usable form via various technologies.
There are three main ways of doing this:
- Photovoltaic cells
- Heat capture cells
- Concentration towers
Sometimes space-based solar collection systems have been proposed, but these will probably not be built in the foreseeable future.
Photovoltaic cells 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).
Heat collection cells take many forms. The one encountered most often is used for water heaters, 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.
Spain has more available sunshine than any other European country. Its government is committed to achieving a target of 12 percent of primary energy from renewable energy by 2010 with an installed solar generating capacity of 3000 megawatts.
 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
 Disadvantages of direct solar
- Really expensive.
- 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.
- 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).
- 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).
 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.
 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
 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.
- 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.
 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). 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%. However, it should be noted that the wind-heavy countries have among the highest electricity prices in the EU. Denmark has the highest energy prices in the world, and must rely heavily on Scandinavian pumped hydroelectricity.
 Advantages of wind power
- Once the infrastructure is in place, it is very clean.
- The technology is now well established.
- Land between the wind turbines can be used for farming or pastures
- Can realistically be built on water, reducing land usage
 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.
- 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. 
- 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.
- Low power density: 2 W/m2, meaning large amounts of land are required.
- Land between the wind turbines cannot be used for human settlement due to risk of turbine blades breaking away, ice throw and noise.
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.
- Bird kills. In reality they are negligible compared to the number killed by cars, cats and windows.
- Visual impact.
- Wind Turbine Syndrome
 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.
 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.
 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.
- Often, plant matter is harvested without proper concern for replenishment.
- In some cases tropical rainforest is being cut down to grow biofuels.
- Where wood is burnt directly for heat it can present a fire risk.
- Very low power density.
 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.
 Advantages of geothermal energy
- Very high energy densities may be naturally available.
- In practical terms the environmental impact is low.
- Minimal land usage.
 Disadvantages of geothermal energy
- Drilling can cause man-made earthquakes.
- 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.
 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.
 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.
 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.
 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, 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.
 Thermoelectric generators
Thermoelectric generators are materials which generate voltage from heat differences using the Seebeck Effect (the opposite is also true - applying a voltage will generate a heat differential, known as the Peltier effect). They are rarely encountered in anything but small scale uses (i.e., temperature sensors, or non-moving parts coolers), however a notable exception are radioisotope thermoelectric generators, the likes of which have been used in deep space exploration probes. They use thermal energy generated by the decay of radioactive materials, capturing it and converting it into electricity with a blanket of such materials.
 Earth batteries
Earth batteries 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. Larger installations might also tap into Earth's natural telluric currents.
A notable case of practical use was as an energy source for signal amplifiers in early telegraph installations.
- ↑ New Scientist Space energy
- ↑ http://en.wikipedia.org/wiki/Solar_panel
- ↑ http://en.wikipedia.org/wiki/Solar_water_heater
- ↑ http://www.desertec.org/
- ↑ Spain expects 3,000 MW in Solar Plants by 2010
- ↑ http://www.newscientist.com/article/dn7046-hydroelectric-powers-dirty-secret-revealed.html
- ↑ gales provide 40% of Spain's energy
- ↑ http://www.cleanpowernow.org/index.php?name=News&file=article&sid=202
- ↑ http://www.finfacts.ie/irelandbusinessnews/publish/article_10006575.shtml
- ↑ http://www.world-nuclear.org/info/inf99.html
- ↑ The Capacity Factor: Uptime & downtime
- ↑ , OregonLive
- ↑ See this report, section 7.6.14. for relevant charts.
- ↑ Sustainable Energy - without the hot air, I.4: Wind, p. 32
- ↑ Sustainable Energy - without the hot air, I.10: Offshore wind, p. 64
- ↑ http://www.newscientist.com/article.ns?id=mg18825265.400
- ↑ Popsci: Does Geothermal Power Cause Earthquakes?
- ↑ http://inhabitat.com/green-a-go-go-at-londons-first-eco-disco/
- ↑ See http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator
- ↑ They are really cells, it takes more than one to make a battery
- ↑ http://en.wikipedia.org/wiki/Telluric_current
- ↑ http://en.wikipedia.org/wiki/Earth_battery