Matt Ridley in his blog:

Methode%2Ftimes%2Fprod%2Fweb%2Fbin%2F17922316-2ad6-11e8-908b-95a753c47952The modern world stands on a cairn built by energy conversions in the past. Just as it took many loaves of bread and nosebags of hay to build Salisbury Cathedral, so it took many cubic metres of gas or puffs of wind to power the computer and develop the software on which I write these words. The Industrial Revolution was founded on the discovery of how to convert heat into work, initially via steam. Before that, heat (wood, coal) and work (oxen, people, wind, water) were separate worlds.

To be valuable, any conversion technology must produce reliable, just-in-time power that greatly exceeds — by a factor of seven and upwards — the amount of energy that goes into its extraction, conversion and delivery to a consumer. It is this measure of productivity, EROEI (energy return on energy invested), that limits our choice.

You could make your own electricity on an exercise bicycle, eating organic ice cream as fuel, but such a system would have wildly negative EROEI once you include the energetics of farming cattle and making ice cream. It would also produce a pathetic trickle of power: about 50 watts (joules per second). The average Briton uses about 4,000 watts, as much energy as if she had 240 slaves on exercise bicycles in the back room, pedalling eight-hour shifts. That’s roughly what “civilisation” looked like in ancient Egypt or China.

More here.