Hydro: Easier to use and more reliable than wind, hydropower is the use of naturally occurring running water, big dams alter the environment to increase this factor, to spin something, either a water-wheel or a turbine to generate mechanical/electrical energy or in the roman case to supply water to aqueducts. It has the advantage of being reliable but most of the planets capacity for hydro, especially big dams, is already in use and climate change could severely disrupt the use of hydropower.
Comes in three scales.
- Big dams being what most people think of, these disrupt local environments and offer little room for expansion. The main versions of this in Australia are the snowy mountain scheme and hydropower in Tasmania
- Macro being the powering of towns or villages by hydropower
- Micro for single houses/buildings
The last two offer a good source of power for individuals or communities if the locality supports it and would be especially beneficial as the national grid declines. Currently the majority is used for electricity production but as the power grid fails declines and the energy available for industry declines a return to direct mechanical energy should occur.
Wind: Similar to hydropower in that naturally occurring air currents (wind) is used to spin something. Together with hydropower the foundations of the industrial revolution was built and they could easily sustain a low (relative to ours) consumption industrial complex (the evidence is that we have already done so once). While small scale is possible and it would certainly help (especially during salvage industrialism, see Arch druid report), wind power is best done n at least a community basis (think windmills). While current use is mainly electricity, like hydropower direct mechanical energy use should return, luckily advances in this mode have been made since the industrial revolution.
Both of these technologies will be utilised for electricity production, most likely for communication (radios, telegraph etc), lights and sensor (soil, air monitors) tech.
Sun: The energy source that the above ultimately descend from, the hydrosphere and atmosphere are powered by sunlight. Sunlight is also the main ingredient of agriculture and biofuels but they are separate topics so here I’ll look at direct solar use (think solar panels, solar hot water or solar furnaces). As experience has shown, which admittedly compared to wind or hydro is miniscule for solar but low-tech magazine has an article on heating/cooling cities with it. its main use is the production of thermal energy, something that wind and hydro can only provide indirectly and would otherwise have to be provided by biomass. It can produce electricity , however the current method of using solar panels is unlikely to survive, alternatives such as dye based solar panels or solar thermoelectric are possible. Solar furnaces, first used as scientific tools (it doesn’t interfere with the reactions), offer a way to replace fossil fuels in the important techs that require high heat (think metallurgy) while others other forms can provide low to medium heat quite well. This means a return to charcoal (which causes deforestation), in full scale, might not be necessary. In daily life it could easily combine with biomass to form the chief means of thermal energy for operations such as cooking, space heating etc while human muscle power provides the majority of the mechanical energy needs.
These three energy sources could easily support a low-energy industrial complex and agricultural sector. But a large amount of power will come from human muscles and any tech that can amplify this (bicycles are one) would also provide a significant energy resource.