How many of the 97.4 quadrillion BTU's per year will be available in the year 2100. (And assuming that new energy sources will not come on line by that date.) We can figure out how many people can live in the USA at the the same energy use. This calculation can be made by looking at the projected deliveries of each source in the left side of the figure separately.
For example, to get cars to transport us, petroleum has to be found in the ground, drilled, pumped, transported to the refinery, refined, and transported to the gas pump. Not just fuel for the tanker trucks, but fuel for the trucks that built the roads and bridges and fuel for the steal plant, and fuel for the truck that built the steel plant, and fuel to get the steel workers to the plant and ... you get the idea that there is a lot of fuel involved in getting gas to your tank. When it takes more fuel to complete these tasks, than the fuel delivered at the pump, then we might be better off leaving the petroleum in the ground and using some other source of energy to provide transport. This means we would have to use energy from coal, gas, hydro, solar panels, wind, or GEO thermal, to transport us.
Obviously that is not the case today. We are getting, from our better wells, 18 barrels of energy for every 1 barrel of energy we use to get it. But these wells are declining in their production and new wells or surface mines (for example tar sands) have much lower energy return on energy invested. Tar sands returns 3 - 4 barrels of energy for every barrel needed to deliver it.
In the next 85 years, the 18 to 1 number is expected to fall. If it falls below 1 to 1 the energy needed to build and maintain the roads trucks, pipelines, refineries, plus discovery, drilling, pumping, etc, is more than the energy delivered. In 85 years we should stop using petroleum to transport us.
This ratio of energy delivered to energy expended for delivery, for gas, coal, and uranium may also fall below 1-1. Then, maybe this century, we won't be able to run our civilization with these energy sources either.
More striking, in the long run without these other energy sources, solar thermo, solar photo voltaic, wind, tidal, and geo thermal, require the building and maintenance of so much infrastructure, it may take more energy to get energy from these processes than they produce. They only exist today because they use infrastructures they did not build or maintain. These infrastructures were built with energy produced at EROEI's 60 -1 or higher. Some today, are being maintained with energy EROEI of 18-1 range and these ratios are falling.
To help visualize this, think of an island with a fully running self contained modern society. Education, jobs, farms, mines, manufacturing, symphonies, transport, and all the accompanying infrastructure is powered by internal combustion engines (using imported petroleum products.) and electrical power plants, (using imported coal and uranium). Think, for example, Japan.
Then imagine converting all internal combustion engines to electric motors. (Yes, no planes.) The transition would require increasing the amount of electricity production. However, instead of building new electric plants, fueled with an increased amount of coal and uranium imports, the island community erects enough wind turbines, additional power gird, and enough energy storage to run itself.
Since, there would be no need for the importation of any fossil or uranium fuel, the island could close its boarder to all imports or exports.
At first glance, the island might seem self sustaining. All the original services are in operation supported by wind turbines. But is this a sustainable system? These wind turbines wear out and need replacement and energy produced by them will be diverted to accomplish these tasks. After it is will society keep operating as it had before the transition?
Before answering, consider that all the supporting infrastructure must eventually be replaced too. The manufacturing plants; and transport systems; all the subsystems that mined, and delivered the raw materials, the produced and installed production machines to factories, and the training and life support of all the human capital involved in these many support systems. All of which were supported by the imported oil, coal and uranium.
Since the borders are now closed to imports and exports, all products and training created outside of japan using outside energy sources in these remote locations is now produced by the wind turbines associated with the Japan electrical grid.
If the wind turbines of Japan can manage all these tasks, your have a wind energy sustained civilization. If not the wind supported island civilization of Japan collapses.
And that is how we should answer the question, can wind energy support an earth-wide sustainable civilization.
A similar analysis performed on solar photo voltaic, solar thermo, geo-thermo tidal, or wave energy systems may show that each of them cannot sustain a developed-world civilization either.
The only source of energy, known today that can maintain itself in working order and produce enough net energy to sustain a developed-world civilization (not the size of the present world civilization ) is existing "in-place" "limited life" hydro electric dams.
I used the words "in-place" because these dams do not create enough energy to replace the concrete superstructure or build another similar dam at another location.
I have used the words "limited life" because the dams do not produce enough energy to dredge out the silt behind themselves making the reservoir smaller. The dams only make enough energy to do maintenance on the generators and also run everything else going on in society.
When the reservoir silts in they will not be able to store the water from snow melt in spring and use it to generate electricity in the fall when stream flow is low. Limited life implies that dams after they silt in will produce energy too erratically to support a viable civilization. (at least without storage)
So the sustainable civilization I envision will have 4 centuries to invent and implement a new energy system to support itself. For example running the generators on seasonal stream flow and finding a way to efficiently store the spring energy for use in the fall.
Figure 1
If this analysis is correct, it means (using Figure 1) we in America will have to run not on 97.4 quads but only the 2.53 quads of energy created by our hydro electric system.
