Just how renewable is renewable?
Wikipedia says “Renewable energy is energy that is collected from renewable resources, which are naturally replenished on a human timescale.”
I no longer think this definition works for us. We have the capability to cover the globe in wind turbines, hydropower plants, or solar panels if we wish. At some point, we have a maximum amount of energy we can pull from these sources.
I remember seeing quotes from former Representative Joe Barton claiming “Wind Is a Finite Resource” and we all laughed at him. But is he wrong? If you place a wind turbine on every square meter of the world and capture 100% of the wind on earth, then there would be “no more wind” strong enough to power any of the turbines.
This isn’t the same as claiming islands will flip over if you add troops.
According to Wikipedia, our planet uses 21,000 TWh (TerraWatt Hours per Year). We will use TWh per year to compare everything.
Wind
This quote from the wind page of wikipedia bothers me: “There is no generally accepted maximum level of wind penetration.”
According to research, building wind turbines can actually cause global warming, slightly. The current wind farms are estimated to increase the temperature in the U.S.A. by 0.24 degrees Celsius. BUT our current methods of harnessing energy warm the earth far more.
So, how many turbines before we “run out of wind”?
According to Tom Murphy at University of California, we could get about 21 TW (183,960 TWh per year) out of wind farming from a purely physics point of view.. This is nearly twice our current global energy need.
Carlos de Castro at the University of Valladolid calculated about 1 TW (8,760 TWh per year) sustainably from his research paper.
Mark Jacobson at Stanford calculates 253 TW (2,216,280 TWh per year)
Assuming 2 MW turbines at 30% efficiency means each turbine gives out 0.005256 TWh.
Carlos de Castro: 8,760 TWh / 0.005256 TWh = 1.67 million Wind Turbines
Tom Murphy: 183,960 TWh / 0.005256 TWh = 35 million Wind Turbines
Mark Jacobson: 2,216,280 TWh and actually did the math to find out 575 million Wind Turbines
1.67 Million turbines generating 9,000 TWh per year sustainably
and
Up to 575 million wind turbines generating 2.2 Million TWh per year maximum
At that level, it would cause massive global warming beyond what our fossil fuels cause as well.
If we built say 600 million wind turbines, our wind will be so slow we actually get less energy out of the system. WE WOULD BE RUNNING OUT OF WIND!
There is currently only ~350,000 Wind turbines in the world so we have a ways to go before hitting the capacity.
David Keith at Harvard had to say this: “No easy answers. Wind is certainly renewable, but renewable things can still have environmental impacts.”
Wind is still a great resource we could generate far more energy from without less effects than fossil fuels.
Solar
Solar is currently just wasted. So it doesn’t really affect the source whether we cover the planet in solar cells or even just cover the sun in a crazy sci-fi Dyson Sphere. At Dyson Sphere levels, we could get 300 million million million million watts at any time.
Most solar energy is just thrown away into space before it gets anywhere close to Earth’s surface. By that time it is down a whopping 1,000 W/m2. The sun lights about half the Earth’s surface at any time which is about 510,000,000,000,000 square meters.
1,000 W/m2 * 510,000,000,000,000 m2 = 510,000,000,000,000,000 Watts
510,000,000,000,000,000 Watts = 510,000 TW
510,000 TW * 8,760 hours per year = 4,467,600,000 TWh of sun hitting earth’s surface
4.5 Million TWh per year hits the earth while over 1 billion TWh per year comes off the sun. That’s only 0.45%!
Scientists think 30% may be the peak efficiency for solar cells in which case we have
4.5 Million TWh * 30% =
1.35 Million TWh per year from solar maximum
until it runs out in 50 billion years of course.
Hydro
I always wondered how water randomly evaporates, it turns out the sun causes that. The energy from hydroelectricity comes from the potential energy in that evaporated water causes movements in the water cycle.
Tom Murphy once again did all the math for me. I found it interesting, that 23% of solar energy goes into evaporating water. The trick with Hydro is area of water vs. land on Earth matters when estimating how much energy it can produce. His calculation estimated about 11 TWh.
11 TWh * 8,760 hours per year =
96,360 TWh per year from Hydro power maximum
Plenty to cover our entire usage.
Geothermal
Geothermal energy is harnessed using heat from the Earth’s core to produce steam to run a generator. Usually we just inject water deep into rocks and then pump them back up to the surface when they are nice and hot.
The initial heat from Earth’s formation (also known as primordial heat) and radioactive decay from isotopes together create the heat within our planet. It is important to note this heat is not directly from the Earth’s core as I initially assumed and so these heat sources will provide no matter what we do for billions of years to come. William McDonough at University of Maryland described it to me like this:
You can think of the Earth’s interior and its thermal properties in a kitchen cooking sense. The metallic core is like a hot cast iron skillet and the surrounding mantle and crust like a enveloping ceramic trivet. The skillet is a great conductor of heat (flame to food), whereas the trivet is a poor thermal conductor, good for protecting your counters.
Because we are not (and perhaps cannot!) tap the heat directly from the Earth’s core, the ~47 TW that rise to the surface will be there for billions of years to come.
Geotherm Power Plant Efficiency ~10%.
47 TW * 8,760 hours * 10% =
41,172 TWh per year Maximum from Geothermal Energy
Bioenergy
Bioenergy comes from burning or using living organisms for energy. Kinda crazy stuff.
Helmut Haberl at the Institute of Social Ecology Vienna calculates 100 ej per year could be potentially produced per year.
That is 277 TWh per year maximum for Bioenergy
I can’t even begin to understand how to estimate the potential for bioenergy so I’ll just leave the link to his research paper. It involves crop yields and the different sources of bioenergy. I will have to learn more about it in the future.
Summary
Whenever I read up on any of these sources, they kept talking about energy storage. It took me awhile to realize why that is important. You lose energy when you try to store or convert it. I always thought everything just goes to the electrical grid and is held there for however long. It makes sense why coal and gasoline and oil are so popular, they can be stored easily.
One important thing I learned is that all renewable energy is either solar energy, or a derivative of solar energy. Solar causes evaporation which makes the water cycle move. Solar directly causes biomass to grow through photosynthesis. Solar causes wind because it unevenly heats the planet. The one exception is geothermal (meaning heat from the planet’s core) which was already there.
Just because I was curious in the upper bound of renewable energy, does not mean I am against it. It is still far greater than any fossil fuel. We should be transparent about the effects though and not claim it is unlimited.