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can you estimate how many people can be supported by growing plants?


Yes Jheri, but the details are complex. You have to worry about the crop mix, climate, water for irrigation and so on. For a vegetarian world the number is usually between 11 and 14 billion people. If you add animal based food it drops. If everyone ate like Americans it would be under 3 billion.

There is a lot to do with the efficiency of farming and distribution. Currently about a billion people are hungry and a larger number are malnourished.


Thanks great blog post.

That graph by David MacKay is a great way of thinking about energy production and consumption. Is the difference between "before conversion" and "Solar PV parks" mostly a relationship between efficiency of turning photons into electrons, number of hours of sun a day and W/m2?

Do you think the "steady" progress of PV technology since say 1970 will continue for decades to come?


Hi David. The 1350 W/m^2 is the power density of sunlight just outside our atmosphere. It is projected onto a sphere the radius of our orbit, so the trick is distribute it over the Earth's surface area. The difference is a factor of 4, so the average spread over a day is about 335 W/m^2. The 170 number is because the atmosphere and clouds attenuate roughly 50% of that on average. In a high desert directly under the Sun the numbers go up to about 250 W/m^2, but only when the Sun is directly overhead. These numbers are before the conversation to some other form of energy. Thermal for a concentrating solar plant, electricity for PVs...

The charts author is picking some average figures for locations in his home country (England). They assume English weather, seasons and day and night except where noted (very few deserts in England:-)

Inexpensive PVs are about 15% efficient. More efficiency can be had at a cost, but this is close to a sweet spot. In the US PV cell costs are lower than mounting and permitting costs - you could easily drop the installed cost by a factor of two with streamlined laws and more clever construction techniques - the Germans have shown that and it balances for their relatively cloudy weather.

Long term projections on cost is tricky. It is clear there will be reductions - how close to the current curve (which has seen the impact of overproduction recently) is not as clear. There are also some important infrastructure technologies that will impact solar adoption - notably storage.

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