How do we fly without fossil fuels? With great difficulty. My latest column over at Energy Collective is on the issue, and anyone inclined to read it can do so here. In it I compare the power density of Boeing 747s, which is many orders of magnitude greater than what can be achieved with renewables such as wind and solar, along with comparisons of the energy density of batteries and fossil fuels. Low power density of renewables is as much of a problem as intermittency, yet we rarely discuss it. Power density is something I hope to explore more in the next month or two.