What does a world without fossil fuels look like? There are lots of different options, but none of them look much like the rich developed nations of the world today. David MacKay’s approach in Sustainable Energy Without the Hot Air is to hold our rate of energy consumption constant, and explore the kinds of carbon-free energy systems that could satisfy that demand. The uncomfortable conclusion he comes to is that if we want to run our world on renewables, the energy farms have to be comparable in scale to nations. Comparable in scale to our agricultural systems. This is because all renewable energy is very diffuse, and we use a whole lot of energy.
Just as an example, of all the renewable power sources solar is the most concentrated, and PV farms like the ones cropping up in Bavaria because of Germany’s generous feed-in tariff average about 5W/m2. With better siting (the Sahara, Arizona) you can do a bit better, and there’s a little more efficiency to be eked out of the panels, but for large scale deployments, you’re not going to get above 10W/m2. If you’re an average citizen of the EU or Japan, your 5kW of power thus demands 500m2 of land. Multiply that by 700 million people in the EU, and you get the total area of Germany. An average North American’s 10kW requires 1000m2. Multiply that by 300 million people, and you get an the entire area of Arizona.