http://en.wikipedia.org/wiki/Renewable_energy_in_Brazil http://en.wikipedia.org/wiki/Renewable_energy_in_Norway
Most hydroelectric power in Norway are generated with the help of a reservoir, usually a natural lake that's been regulated with the help of a relatively small dam, but there are some wholly artificial ones as well, the biggest one being 84 square km.
That said, yes - the environmental impact is small. It also helps that the Norwegian population is very small.
There are breeder reactor designs which convert far more abundant non-fissile isotopes to fuel-grade fissible isotopes, usually starting with U-238 or Th-232. While not technically "renewable", these do off a theoretically sustainable energy supply, with projections suggesting over 5 billion years' supply at forseable energy consumption rates (that's long enough for solar stability to be the principle limiting factor).
As far as new breeder designs, it's a bit too late. Nuclear promised us "energy too cheap to meter" and construction and maintenance costs have kept going down. Meanwhile solar and wind have been regularly going down in price with each doubling of production.
The case against nuclear isn't environmental; it's economic.
No, it's mostly just fear.
Personally, I consider the current generation of fission reactors technologically unsound: Neither the problem of radiactive waste nor shutdown in case of catastrophic failures are adequately solved.
Research in Thorium reactors is worthwhile considering the growing energy needs of China and India, but smarter transnational grids with more efficient load balancing can probably make renewable energy production viable in Europe.
Nuclear represents high risks at multiple areas of operation:
- The fuel, waste, and plants are all radioactive, in some cases for geological time periods, presenting unprecedented disposal issues. Perhaps not insurmountable, but very, very significant.
- Scale of operations of nuclear plants means that the plants themselves, their operations, and their revenues are immense. A large nuclear plant generates over $2 billion worth of electricity (retail) annually. As we've learned with other large financial systems, profit maximization can skew risk management objectives.
- Risks of scale include management, financial, terrorist, natural disaster, labor action, and other factors. Any of these could put, at the least, power reliability at risk, and at worst, health and safety for millions of people. One consequence of the 2011 Tōhoku earthquake and tsunami in Japan was TEPCO's loss of 27% of its generating capacity (http://www.pcworld.idg.com.au/article/379610/japan_facing_ma...). Exacerbated by Japan's dual-cycle grids (eastern and western Japan operate on 50 and 60 Hz respectively: http://www.wired.com/gadgetlab/2011/03/tech-legacy-tokyo/).
- The timescale and emergent phenomena associated with nuclear power mean that risks aren't known, appreciated, and/or understood at construction time. Chernobyl, TMI, and Fukushima all exhibited fundamental engineering and/or operational flaws. Siting of the Fukushima plant turned out to be problematic. Several California plants are now known to be on or near active seismic zones. We still don't have a permanent waste disposal solution in the US (and much of the rest of the world).
Most alternate forms of renewable / sustainable power are much more diverse and distributed. Though this means higher capital and maintenance costs per MWh of generation, it also means vastly smaller risks of any individual plant failure, and far lower likelihood of cascade failures.
Interesting times.
But my major problem with most forms of nuclear is that from a geopolitical standpoint, if you are proposing a technology for clean global power, it has to by definition be something you would be able to share freely with a potential enemy. Most current nuclear power technologies, including the pebble bed, conspicuously fail at this.
Solar, wind, and hydro are renewable in the sense that we consume them as they are produced.
Agriculturally-based power sources (wood, ethanol, biomass, algae oil generation) are renewable in the sense that the biological source grows back after it's harvested (assuming sustainable harvesting).
A sustainable consumptive resource is one which, at projected rates of consumption, would be sufficient for forseable demand until the end of time on Earth. In 4-5 billion years, the sun will expand to the point that it encompasses Earth's orbit. Somewhat sooner than that, increased solar radiation will boil off the oceans and atmosphere. We'll have bigger things to worry about than sources of electricity.
Over what period of time (or is the rate of consumption fixed?) and what region of space?
"at projected rates of consumption, would be sufficient for forseable demand until the end of time on Earth"
Not sure I believe 4B-year forward demand projections...
Isn't the amount of energy expended (and/or mass of raw materials mixed) per TWh produced what we really care about?
If you look at the Timeline of the Far Future, somewhere between 100m and 800m years from now life as we know it on Earth becomes untenable (asteroid impacts, solar flux), if not before (other effects).
There are some practical upper long-term limits to human population on Earth. I suspect it's anywhere between a few hundred millions to low billions (I don't believe present populations are long-term sustainable).
There are other resource constraints which may serve to set an upper limit on energy consumption per capita. From this we can establish long-term energy demands, available supplies, and feasibility of these supplies.
4 billion year forecasting is rather more fantasy than science, so far as human endeavors go.
The machines we use to harness energy don't last forever either. If every 30 years I have to replace either 2,600 tons of equipment for energy source A or 200 tons for energy source B, don't I want B?
Yes, we all believe in the Third Law of Thermdynamics, and understand that the arrow of time is defined by entropy.
"Renewable" means that the specifically tapped resource will renew, by its own access to its prime driver, faster than we can deplete it. In your geothermal instance, it's the geothermal reservoir that's considered renewable. It's also possible to tap geothermal as a nonrenewable resource (generally in open-cycle systems in which groundwater is vented, release, or otherwise not returned to the reservoir, resulting in eventual depletion).
Even fossil fuels are renewable if the extraction rate is kept below the creation rate. Given that FF were created over roughly 200 million years, and will likely be practically exhausted in fewer than 200, that rate is on the order of 1 million times less than we're utilizing them now (probably more if you consider that extraction rates have accelerated the past 100 years).
Your harnessing plant isn't considered part of the fuel resource, and isn't considered in the sustainability equation, generally, unless you're doing something silly like open-pit nuclear fission to capture waste heat for generation, in which you'll likely exhaust your available habitable planetary surface.
Most likely you're not physically transforming your plant in a way that excludes, say, recycling of raw materials, and/or your plant is comprised of very abundant materials (iron, silicates, etc.) relative to your fuel source. It's not the absolute magnitude of plant material involved in generating energy, it's the portion of same relative to the available resources for replenishing these. Generally, structural materials are much more abundant than energy-containing materials. Starting with the matter surrounding and below you at this moment.
Sources of useful energy are not hugely abundant. And, in the case of nonrenewable energy sources, are consumed in the process of utilizing them.
If you're looking at the Earth's crust, it's 60% silica, with most of the remaining 40% being various highly oxidized compounds, including certain quantities of dihydrogen oxide.