Footprint, probably. By then there will be a lot of people fairly crabby about how much space the renewable infrastructure takes up. once people realize that "an acre of solar panels" is an acre of land that we are taking all the energy that Mother Nature uses to, you know, be Mother Nature.

You can see the first few faint traces of it today with complaints about killing birds and such.

Solar power for 1,000 homes would require 32 acres of land. [1]

Food for 1,000 homes requires 7800 acres. [2,3]

That's ~240x as much for food as compared to energy. It's a rounding error in terms of "damage to Mother Nature" and could be easily compensated for by not wasting so much of our produced food.

This came to mind because filling an entire roof with solar panels would definitely meet all the power requirements of a house (except for probably electric vehicles), but I was relatively confident that it took a whole lot more area to farm food for that household.

My math could have a mistake somewhere, did this pretty quickly. Feel free to tear it apart. :)

[1] https://www.energymanagertoday.com/it-takes-2-8-acres-of-lan...

[2] http://data.worldbank.org/indicator/AG.LND.AGRI.K2?locations... -> 4,082,000 sq km / 321 million = 3.1 acres per person.

[3] https://www.statista.com/statistics/183648/average-size-of-h... -> 2.5 people per household, x 3.1 acres per person = 7.8 acres per house = 7800 acres per thousand homes.

Math seems about right to me. Photosynthesis is only 3% efficient, vs. about 30% for current-generation solar panels, so that's already 320 acres. Current agricultural crops put about 40-50% of their incoming solar energy towards the edible parts, so that's 640 (you can thank the green revolution for that - wild plants put only about 5% of their incoming energy towards fruit/seed production, but we've selectively bred them for a 10x improvement in productivity). You lose a factor of about 10 going up a trophic level to eat meat, so if half your diet is meat, that'd be 3200 + 320 = 3520 acres to support your diet. Figure on transportation losses and food waste for the additional factor of 2.

All numbers are from a sustainable agriculture course I took in college, with a few spot checks by Googling.

This, BTW, should drive home just how environmentally-damaging carnivorism is and how switching to a vegetarian diet is actually significantly more impactful than almost any household energy conservation you do. However, as a long-time meat eater, I don't care, and just accept that I'm a terrible person.

Cool, thanks for the extra info.

I justify my meat consumption because in Australia most of our beef cattle are grazed on land that's useless for anything else. But I do agree that we should reduce meat consumption. Even if just for health we're eating twice as much as we should, let alone the environment.

Red meat

Build solar where sun is abundant and space is unconstrained: in Sahara.

Quite a few technical challenges due to the sand, though.

Quite a few political challenges, as well. This is not the most stable corner of the world.

And then you end up spending far more on powerlines than it would cost to get working fusion.

Not really - ITER alone is expected to be in excess of $20 billion, that would buy you about 20 modern HVDC Transmission lines, each with a length of 1000 km and able to transmit 40 GW of power in total. That's a lot of power. :-)

http://electrical-engineering-portal.com/analysing-the-costs...

But Elon Musk says it would take only a an area of land a hundred miles square to power the whole US

https://www.inverse.com/article/34239-how-many-solar-panels-...

It's not if it would be nice to free up that land, but rather whether or not countries would be willing to spend the many trillions of dollars it would take, when they always have lots of other financial priorities.

It's desert but animals and plants and people live there.

I feel like if it were as easy as a 10x10 mile plot of solar panels to power the entire US, some enterprising disruptor would have done it already. Think of the revenue from powering the entire US...

You've also got to consider transmission and the infrastructure required, probably storage for time-shifting the energy as well, since it's solar.

"a hundred miles square" means 100 x 100 miles, 10,000 square miles. The linked article makes this clear.

At $250 per square meter, 10,000 square miles of panels would come to $6.4 trillion.

I think we should be heading in this direction anyway. But it's not so small, it's not so cheap, it will take time. No surprise; the system we need to replace is not small or cheap, and it took time, too.

>I feel like if it were as easy as a 10x10 mile plot of solar panels to power the entire US, some enterprising disruptor would have done it already. Think of the revenue from powering the entire US...

Solar is expanding rapidly, and it will do so more rapidly as it gets cheaper. And wind is advancing rapidly too.

And remember, the article is about fusion electricity not starting up for another 30 years. By then, as my original comment said, it will be mostly or all renewables, and so there will be no way to persuade countries to spend trillions on fusion.

Musk's comment were intended to demonstrate that powering the US on solar is practical - you don't need to cover the whole country in panels, you actually only need 10 square miles.

You wouldn't actually put all these panels in one place, because of transmission losses, putting all your eggs in one basket, etc...

The point being, that you can power the US with a reasonable, practical amount of panels.

You just need to cover the otherwise unused roofs to build what is essentially a distributed power generator. Maybe even use a 10 kWh battery for backup if it's cheap enough (under $1000) for each PV covered roof.

Yes. Tesla/SolarCity are doing exactly that - they're about 400,000 roofs in, so far.

Limiting those enterprising disruptors to those with 32 billion dollars to spare narrows the field a touch. :)

[1] http://innovativesolarfarms.com/solar-farm-cost-per-acre/ gives $500k per acre

That's a square a hundred miles on a side (10,000 square miles), not a square ten miles by ten miles (100 square miles).

Musk's point was that even a 10,000 square miles isn't all that big compared to the rest of the United States.

Should solar + batteries be considered truly renewable? Both panels and batteries do degrade. Solar panels last, what, 20-30 years? Big batteries probably around the same, if not less. And the production of them is not exactly clean, though probably much cleaner than gas/oil/coal for the same energy output.

The raw materials for both can be reprocessed, so yeah.

>>>...by then we will be all on renewables plus cheap storage, or at least pretty close. What would the incentive be for ripping that all out and replacing it with immensely costly fusion plants?

That's a valid concern. Probably in 37 years, we'll all be riding in self-driving electric buggies that recharge in 10 minutes from stored solar power.

Lockheed-Martin[1] is working on a compact fusion design that some day might fit on a truck yet power a city. It might keep a C-130 plane aloft for a year, or keep an aircraft carrier at sea for multiple years. It might power a spacecraft to Mars, shortening the trip from six months to one month.

There is a need for a compact, concentrated, continuously generating power plant at least for these kinds of specialized applications. Whether it will still make financial sense by the time it's actually working is another question. Perhaps the main use case will be military, though the idea of an airship that can stay aloft for months or years is rather appealing -- given enough power, you could put a city in the sky.

1. http://www.lockheedmartin.com/us/products/compact-fusion.htm...

From that LockMart page on their compact fusion reactor:

> ENERGY CREATED THROUGH FUSION IS 3-4 TIMES MORE POWERFUL THAN THE ENERGY RELEASED BY FISSION

I'm not entirely sure how to parse that...?

There's no incentive for "ripping it out", but even a prototype Fusion reactor should be, at a minimum, 4-5x more efficient than a nuclear Fission reactor and probably 10x more efficient than solar or wind. I doubt we'd ever rely on a single source of energy on Earth, but if we had to pick one, Fusion would be it.

I think fusion could probably produce energy on a much larger scale than renewables. I don't think we can get to ten times or hundred times of current energy consumption with renewables.

>I don't think we can get to ten times or hundred times of current energy consumption with renewables.

We can easily get to ten times. And we have no need for a hundred.

the incentive would be mostly the fact that fusion scales really well and can make electricity [adjusting for fixed costs] super cheap. almost too cheap to meter!

also there are other uses that can make really plentiful rocketfuel esp for exploration outside of the inner solar system.

>the incentive would be mostly the fact that fusion scales really well and can make electricity [adjusting for fixed costs] super cheap. almost too cheap to meter!

At first I was going to make a counter-argument. But then I noticed the exclamation mark at the end, and decided to defy Poe's law.

Diversity of energy sources?