what"s the most efficient renewable energy sources?

Answer 1

In one sense, there are no renewable energy sources. All rely on energy in chemical bonds (already existing) or nuclear bonds (also already existing). In neither case is there any known process replenishing the chemical or nuclear bonds storing the energy.

However, if you mean renewable in the sense that the ultimate source of the energy is so large we won't notice, in practice, that it's not really renewable, there are several choices. IN no particular order then...

Hydro
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Hydroelectric is popular, but is only practical in some circumstances (close by mountains, for instance) and many of those sites are already in use. In any case, they are not renewable in a larger sense in that they have had (so far) large and damaging ecological effects. Interference with migration patterns (eg for salmon), altered and usually greatly decreased water flow downstream from the dams...

Tidal
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Tidal power is also tempting, but even more constrained by location -- at least for large plants. Small generation, from tethered equipment perhaps, is not yet possible. There are lots of possible designs, but it's a hard engineering problem. And, in any case, the capital investment (and maintenance/repair) seems high compared to the payoff. And there will be ecological effects, especially in the most tempting sites such as the Bay of Fundy.

Geothermal
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Geothermal power is very tempting, available in only a few places, requires large capital investments in comparison to energy produced, is a very hard engineering problem, but has been shown to be practical in some situations, eg, Iceland.

biological sources
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All biological sources of energy (eg, ethanol or bio diesel) are ultimately from sunlight. Some require a considerable input of energy (usually in the form of diesel for machinery) and considerable capital investment. Some of these require more energy in than energy out with present techniques. The best are not huge net energy producers. Bio diesel is perhaps most attractive as they can use waste that would have been discarded in any case. French fry perfumed trucks, anyone?

Wind
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Wind power is relatively low density energy (at least as compared to hydroelectric) but requires reduced capital investment (except for storage problems), have many sites on which generators can be deployed, but run up against considerable opposition (NIMBY, they're ugly) or not a chance this utility will buy power from your generators and distribute it for you. We might buy the whole thing, and once we control it...

Efficiency in existing operations
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Increased efficiency from current operations is the highest immediate payoff. Many industrial plants vent a lot of heat into the air or local waters. Lots of that heat, though no longer in a form usable by industrial processes, can be used for heating purposes. It's called co-generation by some, and is one of those uses for waste (heat in this case) which can payoff in a big way. In addition, the example of the sawmill which uses sawdust and scraps to heat the plant and generate some electrical power is another example. Altogether, this approach, if employed widely, will result in very large savings. Overall, not high capital investment, either.
Another increased efficiency measure would be a way to use superconduction in electrical power distribution. Not yet possible, but much research is being devoted to it. This would have the effect of adding many very large power plants to national power grids after installation.
Increased efficiency in insulation, reduced power requirements (eg, efficient appliances, more efficient light bulbs, etc) would again give the effect of more large power plants for large countries with populations using lower efficiency products.

nuclear
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Nuclear power (as in current fission plants or proposed fusion plants, if we can ever figure out how to make it work) is tempting. There are some problems however. We've not solved the radioactive waste problem, we've not solved the proliferation of weapons problem, and in an increasingly terroristic world filled with fanatics for whom the price anyone else pays to support their goals is a matter of indifference. Huge capital investment, much shorter lifetime than had been expected just after WWII, very large regulatory supervision which the current business climate regards as without any value to anyone, ... Accidents are increasingly likely (eg, Chernobyl, Three Mile Island, assorted accidents in Japan, ...) and can have horrendous results.

solar
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Solar power is relatively expensive, though high production will bring costs down, but is intermittent (weather, storm damage, etc). As well, storing electrical energy from solar is hard and unless the local electrical utility is willing to purchase excess power, the economic case for small power installations. Lower grade solar (small cooking stoves, direct hot water generation, etc) have more promise if widely used. Reduced investment, reduced maintenance, etc.

the winner
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The big payoff in solar is orbital solar. This has been fully analyzed from an engineering perspective over the past 35 years. The capital investment would be large, but the power produced would be 24/7/365 and could be increased merely by adding more panels or the equivalent. There would be large resistance from existing interests (oil, existing utilities, existing power plant generation owners, ...), and it would be 'expressed' politically. The engineering problems are simply not much of an issue, having been essentially worked out. It's the others that will be hardest.

Answer 2

Water by far. Sunlight is good only for clear days, and for only 6-8 hrs at the most. Plus it is too expensive to buy the solar panels for electricity. Under glass tubing is less expensive but still too short a day light time source.
Wind is cheaper mechanically, and probably more hours of production overall. But still depends on sufficient air movement.
Water from river flow is the most constant 24/7 production. Dams, are expensive but are multipurpose. Electricity, irrigation, drinking, recreation, flood control.
Paddle wheels anchored along rivers or streams also produce continuous electricity and are cheap. They can be smaller or larger.
Ocean waves and tides are being looked at as a continuous source also.
All water sources of renewable energy are by far and away the most efficient. Water is heavy at 8+ lbs per gal. it does not take very much flow on a lever arm to multiply its effect dramatically.

Answer 3

Sunlight.

In the long run, practically all energy sources can be traced back to sunlight.

Photosynthisis is a process that uses solar energy. So when you use ethanol... you re using converted solar energy. Even fossile fuels are just stored solar energy. Wind is a result of uneven heating by sunlight.

Tidal forces and nuclear powerare about the only energy sources on earth that does not eventually trace back to sunlight. (and in a way, nuclear power is harnessing a controlled miniature sun... since the sun is a "fission-fusion engine")

Answer 4

i guess sunlight and water. its just that we don't have enough technology to use them full scale to meet our needs.

Answer 5

sunlight,water and wind