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Maybe this will eventually result in a FAQ for this board, but I am hoping to start what I hope will be an informative discussion on what alternative energy sources can be called truely renewable and even more so, green. I'll start the ball rolling.

Some definitions first.

Renewable = an energy source that does not depend on a resource that is depleted through use.

Green = the relative impact on the environment of generating power. At the very least, this mean nonpoluting. The most strict definition of green would mean close to zero environmental impact. Everything however has some impact on the environment, at the very least, occupying land that might otherwise be green space, to at the other end of the scale, puking thousands of tonnes of CO2, NOx and SO2 into the atmosphere or generating radioactive waste. Do we consider noise polution? I rate noise fairly low in environmental impact as it is localized and unless it exceeds safety levels is fairly benign.

Wind power is obviously renewable; wind can be used but not consumed. It is also amoung the most green power generation technologies. The only negative impacts of wind generation are some noise and bird kill problems. The land use impact can be fairly minimal as wind farms can be installed on existing agricultural lands or even off shore. All told, wind energy systems probably have the lightest of all environmental impacts. Wind also has some of the most potential in terms of the it's ability to meet our growing energy demands.

Geothermal is renewable; the energy source is the hot magma beneath the earths crust. Steam fields however can be depleted if the water that is extracted for generation is not re-injected. Geothermal energy is also very green. Environmental impacts vary with plant design. Some geothermal plants dump of hot water into rivers and streams changing the local ecosystem and by not re-injecting the water into the ground, over a long period of time they can deplete a steam field. Good plant design (reinjection and cooling) can mitigate this impact to close to zero.

Solar is renewable; sunlight is a free and inexhaustable resource. Solar is generally pretty green as well however the manufacture of PV cells is still pretty energy intensive. I don't have current numbers, but I know at one point, the return on invested energy for most solar cells was measured in years. Amorphous silicone and thin film technology is improving this. Utility scale PV plants whether based on standard PV panels, concentrating PV or solar thermal systems, do require a lot of real estate that can not be effectively used for other purposes. In distributed power applications however, PV panels can be installed on existing buildings and parking structures. This approach minimizes the land use impact and is probably the most practical long term application of photovoltaics.

Hydro is renewable, but not as green as the above technologies. Hydroelectric dams have a huge land use impact - flooding in some cases hundereds or thousands of acres of land and altering whole river ecosystems. Dams interfer with estabilished spawning streams for salmon and even alter downstream agriculture and wetlands used by migratory birds. That said, hydropower that displaces fossil fuel generating plants have a positive impact on air quality and global warming. "Run of the River" small hydro systems which do not dam or restrict the flow of the river can be considered both renewable and green. They probably have some fish kill impact, but this is pretty minimal and can be further minimized with intake screens.

Fuel Cells can be renewable or not, and green or not, it depends on the fuel. If the fuel is hydrogen, the fuel cell itself is very green, but the system may still be neither green nor renewable. It depends on how the hydrogen is generated. If the hydrogen is generated by electolysis, and the electrical energy to electrolyze the water was generated from a clean renewable power source then fuel cells can be considered green and renewable. If the electrical power comes from the grid, then the fuel cell system is only as green as the mix of generation capacity on the grid. Fuel cells run on biogas are green and renewable. Fuel cells run either directly on fossil fuels or on hydrogen reformed from hydrocarbon fuels are neither green nor renewable.

Microturbines like fuel cells can be considered renewable if they are run on biogas from landfills or solid waste digesters. The CO2 is accounted for becuase it is non-anthropogenic, but because microturbines do combust the gas, they do have some NOx emissions as well.

I'm sure there are other technologies I am missing, and probably some thoughts on these technolgies as well.

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