To what extent is hydroelectric power really renewable?

Question

One of the more widespread "renewable" ways of generating power is by harnessing hydropower, i.e. by exploiting the movement of falling or flowing water. Some of the most commonly seen implementations are river dams and tidal power stations.

All hydroelectric power stations obviously remove some kinetic energy from the water, usually by requiring it to turn a turbine. What I'm curious about is whether the removal of this energy is actually sustainable, i.e. whether energy is introduced back into these bodies of water at such a rate that we don't need to worry about depleting our bodies of water of kinetic energy.

Some might say that removing kinetic energy from the oceans on that scale is an obvious impossibility, but this strikes me as a foolish assumption in light of the dilution hypothesis, which clearly turned out false. Two-hundred years ago, people assumed that the oceans were so incredibly vast that man could never have an environmental impact on them. Today, here we stand, with oceanic acidification, warming, and overfishing being prime environmental concerns.

The exact same question and argument can be repeated for wind energy.

Do we have any idea exactly what the patterns and rates of energy transfer into our planet's water (and wind) systems are?

What are the potential environmental impacts of the large-scale removal of kinetic energy from our bodies of water?

(Note: among these environmental impacts, please don't focus on well-known impacts like damage to local ecosystems, loss of land, noise pollution, etc. I am chiefly interested in how this way of generating power affects our natural systems of energy storage and circulation, like oceanic and wind currents.)

The following question is off-topic, but it's a good follow-up: how is energy even introduced into our oceanic and wind current-systems? For example, I assume the moon's push-pull is responsible for the tidal movement of the oceans. What impact does the sun play? How fast is this entire system of energy transfer?

Comment: I strongly endorse the use of wind and hydropower as sources of energy over the further use of fossil fuels. However, I still think it is important to do research into the actual renewability of presumed-renewable energy sources, as we don't want to end up with another fossil fuel-type situation, in which we become aware of dependency on these energy sources and their malignant environmental side-effects long after widespread enthusiastic adoption.

Answer 1

Electricity from waves, from hydro (both run-of-river and storage) and from wind, are all indirect forms of solar power. Electricity from tides is different, and we can deal with that in a separate question. Global tidal electricity generation is not yet at the scale of gigawatts, so it's tiny for now.

Winds come about from the sun heating different parts of the planet at different rates, due to insolation angles, varying cloud cover, varying surface reflectivity, and varying specific heat of surface materials. Temperature differentials create wind currents.

Waves come about from wind, so they're a twice-indirect form of solar power.

Sunlight on water speeds up evaporation, lifting the water vapour into clouds, giving them lots of gravitational potential. That rain then falls, sometimes onto high land, from where it can be gathered into storage reservoirs that are tapped for electricity, or where it flows into rivers that are then harnessed in run-of-river hydro.

How much power is there? Well, the insolation from the sun is, at the outer boundary of the Earth's atmosphere, at an intensity of about 1400 Watts per square metre. The Earth's albedo is roughly about 30% - i.e. on average about 400 Watts are reflected back into space, giving an average irradiation into the Earth of about 1000 Watts per square metre. Picture the Earth's surface as seen from the Sun: wherever the Earth is in its orbit on its own axis, and around the Sun, the Sun sees a disc that has the Earth's diameter, so the surface area exposed to the Sun is just $\pi$ times the square of Earth's radius, which is about 6 300 kilometres.

So the incoming solar radiation is $1000 \times 6,300,000^2 \times \pi \approx 125 \times 10^{15} \rm \ W$

Current human consumption of non-food energy is about 17 TW, i.e. $17 \times 10^{12} \rm \ W$, which is about four orders of magnitude smaller than insolation.

There is an interesting open academic discussion (see "modelling methodology smackdown" section) about how much energy we can physically extract from wind: some models say it's only a few times as much as all the energy we currently use; others say it's another order of magnitude more again. Either way, it's way more than we need, so while it's an interesting modelling discussion, the answer wouldn't change anything right now.

There are also open academic discussions about the impact of removing terawatts of wind energy, on regional climates. However, regional climates are already one of the more uncertain parts of long-term climate forecasts; add to that the high uncertainties around where the energy extraction would be, and what the specific effects of extraction are, and we end up with a lot of speculation and very little usable information. Not that this has prevented people from trying to model it, and reporting their results: but the uncertainties dwarf the results.

And harnessing renewables doesn't change anything significant in the Earth's total heat budget either, because almost all of that $1000\ \mathrm{W/m^2}$ from the sun already ends up as low-grade heat anyway. If harnessed for electricity, that electricity will then get used, and almost all of it will then end up as low-grade heat. The bit that will change when we get close to 100% renewables is that we will no longer be injecting about 15 TW of energy from fossil and nuclear into the biosphere (2 TW is renewables). But that's only about 0.01% of the insolation anyway.