Just curious - is that much silica needed to power the whole planet? We've got entire deserts full of silica sand and most rocks are composed mostly of silicon. I feel like refining enough silicon to power the world via solar wouldn't be hugely destructive.

Most of the raw Si is used in the steel industry. Only a tiny fraction is used in electronics applications. So, Si would not be the bottleneck (refining it to the high purity needed for electronics applications is very energy-intensive, though).

It's the rare-earth doping elements that are the issue, not the silicon. We have silicon in extreme abundance.

Typical dopants for solar cells are elements like B and K, not rare earths.

Industry average for solar PV is currently about 5 grams of silicon consumed per nameplate watt of module capacity. That includes processing losses (wafer sawing, etching, etc.)

I'm going to assume a capacity factor of 20% for solar PV. That's significantly lower than the 28.6% capacity factor recorded for utility scale PV in the US last year, much higher than the ~10% you might expect in Germany. Choosing a high-ish solar capacity factor I'm going to note that most of the world's population lives in countries with better solar resources than Germany and that more solar PV is being installed on the utility scale than on e.g. household rooftops. Utility scale projects are sited for better sun resources and can use mechanical trackers that aren't suited for rooftops; both of those contribute to higher capacity factors than you can get out of opportunistic installations on existing rooftops, whichever country you're building in.

I'm also going to assume that modules have 25 years of factory-spec-power-equivalent output. Data from real-world solar installations show that the median module lasts about 30 years before it needs replacement (and more rigorous pre-sale testing may be pushing that number up in recent years), but maximum power output also declines over time due to a variety of degradation mechanisms, so I'm fudging these two factors together for 25 years' equivalent of "like new" power output.

Multiplying the capacity factor of 0.2 by the expected 25 years of like-new-power-output, we see that each nameplate watt of solar power installed is expected to provide 5 watt-years of energy output. The steady-state production rate you need for the world to have X watts of solar-generated power over the course of the year is then X/5.

Concrete numbers time: world electricity consumption in 2013 averaged about 2.28 terawatts. The world would need to produce about 457 gigawatts-peak of solar modules per year to produce this much electricity over the course of a year, consuming about 2,285,000 tonnes of silicon. World primary energy consumption in 2015 averaged about 17.3 terawatts. To match that on an annualized basis you'd need to produce 3.46 terawatts-peak of solar modules per year for a silicon consumption of about 17,340,000 tonnes.

Silica is about 46% silicon, so you'd need to mine ~5,000,000 to 38,000,000 tonnes of silica per year in order to produce enough silicon for solar PV to take over world electricity production or all world energy consumption respectively. For comparison, world sand consumption in 2014 was estimated at 15 billion tonnes, roughly 3 orders of magnitude greater. (Silicon producers usually don't start with sand anyway; they use coarsely crushed lumps of silica-rich materials like quartzite, quartz, or chert, which are even more abundant than clean sand.)

Note that I am NOT claiming that you could actually replace all the world's diverse energy sources with electricity from silicon based solar PV. There are of course no oceangoing ships that can be powered by electricity, no electrical passenger aircraft, etc. And even if we were just trying to replace fossils for current electricity generation, storage remains a hard open problem. Without cheap, abundant electricity storage it will be impossible for solar PV to take even a 40% share of world electricity consumption. But I hope that the numbers make it clear that silica availability is not a material constraint on solar PV deployment scale.

You could probably cover the Sahara in solar panels and still have a lot of sand there

Silica is not the bottleneck

> Rather, the point is to say "here's a huge vein of untapped energy, some of which could be tapped without damaging the environment and without releasing carbon."

Did we say that about trees? Fossil fuels? Have we said that about silica, lithium, or rivers (we're having plenty of issues from Georgia to Pennsylvania along the Appalachians because of just that)?

I agree with what you're saying, I just don't think your last sentence is right. Kinda seems like a contradiction if everyone overuses resources (keeps burning oil) and it's causing problems (and we know that is a class of problem that we've experienced with other substances like rubber, trees, fish, dodo birds, lithium - hell, anything we can get our hands on), shouldn't we ensure other problems don't pop up from overusing resources (what if everybody else did this)?

I think the exact point is to not make the same mistake we've made with pretty much every other resource we've ever found...

>I think the exact point is to not make the same mistake we've made with pretty much every other resource we've ever found...

So... keep burning oil instead of experimenting with this?

So... don't overuse resources. And when presented with the question of "how does this scale?" don't ignore how fabulously efficiently we damaged the environment every other time we've scaled production up.

Not sure how you got "keep burning oil" from that...

We're not even near the scaling part. It's silly to argue about how something will have issues if the entire world did it. There are tons of things we do today that wouldn't scale environmentallly if the entire world wanted to do the same thing (e.g. flying).

We have more trees in the USA now than we had in Colonial times.

The forest area seems to have decreased significantly between 1760 and 1910.

http://forestry.about.com/od/forestresources/tp/forest_fact_...

Hum, no. Solar scales very well.

Silica already covers most of the places were we would like to place panels, we will have much worse problems way before we run out of land, and there is no required rare resource for it.

There are shorter bottlenecks on energy storage. But even then Na-S will get us a long fraction of the way solar can go.