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Sustainable energy is energy produced from renewable sources using equipment which lasts long enough to produce more energy than was used in manufacturing it. Some sources which might seem renewable are not. For example, biomass grown with the aid of fossil fuel is not very renewable.
New sources of sustainable energy can be developed if people devote enough time and effort to the problem. Considering the vast amounts of energy arriving from the sun (174 billion megawatt-hours every hour), devising clever ways of capturing a very tiny fraction of this would be highly useful. The machinery need not be large and centralized. In keeping with Permaculture, and with the fact that the sun distributes its energy evenly over the earth's surface, energy can be captured, stored, and used in small amounts using locally built and maintained equipment.
Passive solar heat collectors produce hot water. This is useful for household convenience, but also can be used to power heat engines which produce other forms of energy. The amount of energy which a heat engine can produce is directly proportional to the temperature difference of the heat source and heat sink used with the engine. At an absolute temperature of 303K (30 degrees C), a temperature difference of 30 kelvins (i.e., a difference of 30 degrees C) means at most 10% of the heat energy can be converted to work, while a difference of 60 K means at most 20% of the heat energy can be converted to work.
Thus, using solar collectors to heat hot water which is then used in heat engines to convert some of the heat to work is a much better strategy than simply operating a heat engine using a cold sink such as a river, and ambient temperature.