Wind varies widely in energy and availability. If you have the misfortune of living in a windy environment, you might console yourself with the use of this source. On the plus side, the amount of energy in wind increases with the cube of the velocity, so it can be a valuable resource.

Solar, although cyclic, is far more predictable. Solar cells can give you about 10% of the sun's energy in the form of electrical power. If it has to be converted to another voltage, it will lose a bit – likely at least 25% (of the 10% collected). If stored in batteries for later use, it will lose more in charging and discharging the batteries.

Panels that collect heat can deliver 70% and more, which can be efficiently stored for later use. The efficiency of focusing collectors varies widely with design and application.

Fire -- Wood is a versatile source that can be efficiently stored and produce energy on demand. It goes without saying that reliance upon this source requires intentionally planting trees for an ongoing energy supply. For most applications methane is overrated. If you run a dairy or a sewer plant, you might have enough biomass to do some good, but for the rest of us, it is a poor use of effort. For a broadened picture of the opportunities available through this source, check out Energy Tree

The Perpetual Motion of Fools   I am often amazed at how otherwise intelligent people tout energy sources they can neither prove nor produce.

Keeping warm: There's more to this than heat.

Alternative engines can be powered by heat from any source.

Electrical power can be derived from mechanical, radiation, combustion, or chemical sources of energy.

Lighting is available from electrical power or flames

Refrigeration can be produced in a number of ways

The World We Face (and what to do about it) Provides an over-all perspective of the energy picture.

  A sustainable lifestyle should depend upon as little energy as possible. Structures should be well insulated, and conservative in size. Lighting should be focused, and strategically placed. An efficient community design would minimize transportation needs.

  Energy should be produced as close to its point of use possible. Small distributed systems should be optimized. Timing is another factor; plan the use of energy so that it will achieve as much as possible within the time it is being used.

  Whenever energy is changed from one form to another there are losses. Electrical heating is convenient, but if thermal energy has been converted to mechanical energy to produce electrical energy, most of the original thermal energy is lost. It is obviously much better to use thermal energy directly whenever possible.

  Sometimes thermal energy can be scavenged from high-intensity applications – for instance: Exhaust from a cooking fire or motor may still have enough heat left over to heat water for washing dishes or bathing. As Ben Franklin so aptly put it “A B.T.U saved is a B.T.U. earned.”

  Energy from solar collectors that heat air has commonly been stored in insulated bins of rocks. Fluid-based solar heating systems commonly rely on tanks of water. There is also a category of thermal storage that involves the use of a “eutectic”.

  A eutectic refers to anything that can be melted and frozen. The advantage of a eutectic is that it stores and yields a lot of energy when changing from one phase to another. The most commonly used material in this category is water. Ice keeps your drink at a constant low temperature until the last of it has melted (or frozen).

  During a period of inadequate employment I worked as a security guard (I’ll never get around to writing my book titled “The Adventures of Rent-a-Pig”), My beat sometimes involved sitting still in a pickup truck for hours at a time in Colorado winter nights. Paraffin melts at about 140 degrees F, and remains at about that temperature until it re-freezes. I filled a gallon can with paraffin, melted it by putting the can in boiling water (houses have burned down because people were melting containers of paraffin directly over the fire), wrapped it in an old blanket, and used it as a foot-warmer during my watches. One tricky thing about paraffin however it that it expands and contracts a lot as it melts and freezes. So the cap should not be on the can while heating, and needs to be loosened periodically while cooling.

  There are also chemical reactions that can store heat. Calcium chloride for instance has a powerful attraction for water molecules. When it is heated enough to drive out all the water and then allowed to cool, it will produce significant heat when re-exposed to moisture. I know of at least one commercial thermal storage system based upon this principle. There are commercially available hand-warmers that are activated by allowing two chemicals to mix.

  Batteries have been here longer than any of us, but there is a price to pay. A lead-acid battery that is about 70% charged may store over 80% of the charging current. By the time it’s 90% charged however, the charging efficiency will drop off to about 60%. Varying efficiency losses are unavoidable with any battery technology. Another problem with batteries is that they typically last only a few years. Replacing a massive bank of batteries is an ecological and financial disaster.

  Compressed air stores energy proportional to the square of the pressure. Years – decades – ago I read of a go-cart powered by a modified air-hammer. The claim was that with a tank of nitrogen pressurized to the obscene extreme of twenty thousand PSI (I have never heard of such pressures, and frankly remain skeptical), it had a theoretical range of 600 miles.

  The problem with compressing air or any other gas is that the temperature increases with the pressure. So right off the bat you lose energy in the form of heat when you allow it to return to ambient temperature. The interesting side of this is that as you allow the pressure to escape while yielding mechanical energy, it also cools. Those of us who have used jackhammers or other air-powered equipment are familiar with the frost build-up that can occur; if you happen to want a refrigerator however, you’re in luck.

  While speaking of compressed air (though not directly related to storing heat), there is an incredible fire-starting device discovered in occasional use in Southeast Asia called a fire-piston. A close tolerance stick sealed by greased winding of thread was fitted into a precision hole about three inches deep (the stick and hole were about 3/8” diameter). There was a shallow hole in the inside end of the stick that held tinder. When the piston was forcibly plunged down the cylinder and immediately withdrawn, the air (heated to about 800F) ignited the tinder. A traveler from Europe observed a Filipino using one to light a cigarette and traded him a Zippo lighter and some bubble-gum for it. Some believe that this was the original inspiration for the diesel engine. I have not yet successfully built one of these – a little smoke, but so far no fire.