Going through David Holmgren's version of the Permaculture Design Principles, here's #2.
Catch and Store Energy
There are only a few actual sources of energy around us: solar is the primary driver for the energy I'd expect to capture in a permaculture system, but there is also the geological energy in the planet's core/crust (subduction, tectonics, vulcanism), the gravitational effects of the sun and moon, as well as radioactive decay of the higher elements. (Interesting question, is there a sustainable way to use nuclear energy? The decay is going to happen whether or not we capture it...) Yes, I recognize that none of these are actual energy creation processes, but other than nuclear the relative size of source vs sink is such that any energy we extract from the process has no effect on the process itself.
This principle is all about fighting the increase of entropy. As the universe tends toward general disorder, we biological systems create local order, and the more order we create, the more resources we have available for our success. Catching energy for immediate needs and reorganizing it to make energy available for future needs provides for both growth and flexibility.
So how do we catch energy?
- Grow stuff
- Heat stuff
- Move stuff
Plants do a great job of turning sunlight, air and water into stored energy. We can consume the plants directly, feed them to animals to create higher density energy storage, use them as combustibles for heating, and about a billion other things :-)
Solar can be as simple as a black curtain behind your window, or as complex as photovoltaic panels - a greenhouse is a great way of capturing some energy. If you live in Iceland then volcanic heat capture is a real option, though that has its risks. So does standing next to a pile of pitchblende.
Wind, hydro, tidal flow capture - all of these convert "free" kinetic energy into more usable forms.
Once we've captured the energy, storing it for future use is its own challenge. Plant seeds can last for a long time, and our animals are energy on the hoof/claw. We can use rocks as a heat sink and let them radiate, we can move water uphill so that we can capture its energy again when we decide to let it down, we can compress gasses and liquids for pressure systems, and we can ferment some of our biomass into combustibles. Imagination is the only limiting factor. (Okay, the laws of physics probably come into it, too).
There is an implicit assumption here that a system isn't viable if it requires external inputs, e.g. conventional agriculture's energy equation of 10 calories of fossil fuel burned to create 1 calorie of edible food. We have to be careful about where we draw the boundary for "external" but a system that captures as much of the energy available to it as possible is a key design goal.
Pretty picture of the principle at permacultureprinciples.com