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The common definition for renewable energy sources includes at least:

  • Wind
  • Solar
  • Biofuels
  • Hydro
  • Geo-thermal

However, the common definition of renewable makes this grouping a bit awkward. Webster says:

renewable, adjective: capable of being replaced by natural ecological cycles or sound management practices.

This easily describes biofuels and hydro - some resource (biological material, flowing water) is used, and then through a natural cycle (photosynthesis, rain), is replenished.

However, in the cases of wind, solar, and geothermal, this isn't so obvious -- nothing is being used up (at least not in a measurable quantity) which would need to be renewed.

Furthermore, there's no need to manage wind, solar, and geothermal energy -- it isn't possible, on human scales, to overuse these.

But obviously it is possible to overuse hydro and biofuel resources.

So, should there be subcategories?

  • It certainly is possible to overuse geothermal energy, not because of limited existence of it, but because of limited access to it. Older geothermal plants also commonly extracted steam or water from below the ground without replacing it, which slowly lowered the pressure of the water and in some cases also caused surface subsidence. Newer plants pump water through hot rock to heat it, reducing the subsidence and pressure problems, but they also have a limit to how much heating can be gained from a given size of facility. – Highly Irregular Dec 3 '16 at 7:28
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There are lots of ways to subcategorise renewables, for example:

  • Indirect + direct solar (wind, wave, hydro, biomass, biogas, PV, CSP, solar thermal, - and what's used up here in all cases is incoming solar energy), versus others (tidal & geothermal)
  • Combustion (biomass, biogas) versus others
  • Heat engines (geothermal, biomass, biogas) versus others
  • portable (biofuel, wind) versus others

And so on.

Which subcategories we use, depend on what the context is. It is theoretically possible to over-exploit any of them, leading to significant harm. That's most conspicuously true of biomass and hydro, but is true of the others too.

You say it isn't possible on human scales to over-exploit them, but the science isn't unambiguous on this. For example, hot-dry-rock geothermal can cause earth tremors, which may cause damage to urban areas.

And conceivably, we could extract so much energy from the wind that it starts to disrupt climatic patterns. It's not clear how much energy we can extract before such disruption occurs, but it might be as low as 10 terawatts [TW]. Now, given human energy consumption is already ~18 TW, and there's a lot of development work still to do in emerging economies, that will increase further, so it is conceivable that we might get to 10 TW: and before we do, we'll need better modelling and monitoring to ensure that that's safe.

So we (globally) need to be mindful that there may be limits on exploitation of any of the renewable resources.

  • Still, I would argue that the difference between solar/wind/geothermal and the others is one of where and how they are used - the quantity itself isn't the issue. Do you have a source on the 10TW figure? Given the fact that most wind systems have about a 30% capacity factor, you'd need a tremendous amount to get to 10TW output at any given moment. – LShaver Dec 5 '16 at 0:11
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However, in the cases of wind, solar, and geothermal, this isn't so obvious -- nothing is being used up (at least not in a measurable quantity) which would need to be renewed.

Furthermore, there's no need to manage wind, solar, and geothermal energy -- it isn't possible, on human scales, to overuse these.

For wind and solar there isn't a "store" that is being used up and replenished - instead the energy flux from the sun is being diverted. Wind power removes some of the kinetic energy from the atmosphere. This energy comes from the sun as heat, and after being converted to electricity and "used" will end up as heat, so the net result is the same, with some of the conversion being done in human appliances rather than atmospheric turbulence. The scale of what we divert is tiny in comparison with the global energy flux, but as others have noted the human-induced changes can be seen in local areas, and if we were to scale wind up enough might be noticeable globally.

Solar is more direct: There is a certain amount of sunlight falling on a given area of the planet. Normally it will be converted to heat, but with a photovoltaic panel in the way a fraction of it will be converted to electricity instead, and will then later be converted to heat (by an electrical appliance).

Geothermal is different, and at human scales may or may not be renewable, depending on the rate at which heat is extracted compared to the rate at which it is replaced from underground heat sources. These heat sources may simply be adjacent hot rocks (in which case only a low rate of extraction can be replaced) or it may be that you're Iceland and you're extracting from a volcano, in which case you're not going to make a dent in the thermal energy just beneath ;-) Being strictly correct, geothermal power is not renewable at a planetary, long-term, scale, because the Earth is cooling and geothermal energy extraction hastens that cooling by a tiny fraction, but that hardly matters for human purposes.

Note that ground source heat pumps and the like, commonly advertised as "geothermal", are not usually operating off the internal heat of the planet, but instead are working in the top few metres that has been warmed by the sun - so they are arguably a form of solar energy. In this case there is a "reservoir", which is the heat energy in the soil, which is replenished by the sun, and it's conceivable that in densely populated areas an overuse of heat pumps could temporarily deplete this reservoir.

  • Do you know the proper terminology to differentiate between ground-source heat systems (which work any where) and power plants which depend on geothermal activity? – LShaver Dec 5 '16 at 0:13
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Actually with wind power it may be possible to overuse them.

From the wiki article: "Environmental Impact of Wind Power"

Weather and climate change Wind farms may affect weather in their immediate vicinity. This turbulence from spinning wind turbine rotors increases vertical mixing of heat and water vapor that affects the meteorological conditions downwind, including rainfall.[107] Overall, wind farms lead to a slight warming at night and a slight cooling during the day time. This effect can be reduced by using more efficient rotors or placing wind farms in regions with high natural turbulence. Warming at night could "benefit agriculture by decreasing frost damage and extending the growing season. Many farmers already do this with air circulators".[108][109][110]

A number of studies have used climate models to study the effect of extremely large wind farms. One study reports simulations that show detectable changes in global climate for very high wind farm usage, on the order of 10% of the world's land area. Wind power has a negligible effect on global mean surface temperature, and it would deliver "enormous global benefits by reducing emissions of CO2 and air pollutants".[111] Another peer-reviewed study suggested that using wind turbines to meet 10 percent of global energy demand in 2100 could actually have a warming effect, causing temperatures to rise by 1 °C (1.8 °F) in the regions on land where the wind farms are installed, including a smaller increase in areas beyond those regions. This is due to the effect of wind turbines on both horizontal and vertical atmospheric circulation. Whilst turbines installed in water would have a cooling effect, the net impact on global surface temperatures would be an increase of 0.15 °C (0.27 °F). Author Ron Prinn cautioned against interpreting the study "as an argument against wind power, urging that it be used to guide future research". "We’re not pessimistic about wind," he said. "We haven’t absolutely proven this effect, and we’d rather see that people do further research".[112]

This article points out that there is a limit to how much wind energy we can sustainably harvest. With high altitude turbines it's about 1800 terawatts. Not something to worry about before Tuesday.

Solar power too. A solar panel is much darker than the land around it. Suppose you put down a 20 mile square of panels covering 80% of the available surface. Now you have a large chunk that has an albedo of .1 instead of the more typical .35. The solar cells get hot. Hot air rises. And you end up with a semipermanent cloud directly over your solar array.

Geothermal is NOT a renewable resource. But it is far more benign than fossil fuel. As you use it, you cool off the rocks where the steam is created. It doesn't decay all at once, but you have to wait longer and longer as a larger and larger volume of rock is cooled. I recall that in iceland it becomes uneconomic after a decade or so, and they move the power plant.

  • Do you have sources on the solar power and geothermal data? "A decade or so" sounds too short to me - it'd be hard to recoup costs in that time frame. And on solar power, it seems this would be very dependent on where you put panels - installation on rooftops probably wouldn't affect global albedo much, since rooftops tend to be more darkly colored anyway. – LShaver Dec 5 '16 at 0:16
  • Sorry, can't find the source. Rooftop installations won't be enough.. You can cover single floor residents that way, but residential isn't a big part of the picture. In Canada statcan.gc.ca/tables-tableaux/sum-som/l01/cst01/envi41a-eng.htm claims around 23% for household use. – Sherwood Botsford Dec 10 '16 at 3:18
  • I was only giving rooftops as an example, to argue that the effect of solar PV on albedo would depend on where panels are installed. In some places they could lower albedo. – LShaver Dec 13 '16 at 1:56

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