The renewable sources I have the most confidence in as serious (though not complete) alternatives are the ones that can be implemented at the consumer level. For small-scale consumers (households to commercial buildings), that's currently limited to solar and (to some extent) wind. The near future might bring some kind of integrated turn-key biomass digestor/generator. All of those are more suitable for suburban and rural settings than for cities.
(Suburbs are an interesting case. On one hand, they're the most obvious example of the energy-intensive lifestyle by design that arose in the mid 20th century. [The suburb I grew up in has roads through commercial areas with no sidewalks, among other abominations. Such as, intersections with "no pedestrian crossing allowed" signs -- so you have to literally break the law to go from place to place on foot.] On the other hand, the sprawl will help to accommodate local and individual solar power collection and limited local food production, so suburbs might persist instead of reverting to farm land.)
The reason consumer solutions have an advantage is the speed and flexibility inherent in parallel action by numerous individual households and businesses. Imagine if around 1975, the worldwide consumption of an exaFlop of computing power in the year 1995 was foreseen. The leaders of the computer industry probably would have said something like, "To meet that need, we would have to build new large mainframe computer facilities at a rate of thirty per day." Sounds scary and impossible. But consumers and manufacturers made it happen in a different way.
(The analogy is imperfect, of course, because power generation technology cannot possibly multiply in effect per unit cost the way computers did. On the other hand, the increase in demand isn't as extreme either.)
Demand per capita can be decreased a lot more with design, technology, and minor social changes. So much of the "normal" way we do things was designed to exploit cheap energy. Like, heating whole houses instead of just the room(s) we're using, or just ourselves. When home heating came from fireplaces, houses used to be designed with the expectation that specific rooms or floors would be closed for the winter (that is, not heated; you could still go there to get something). Today, having two or three separate HVAC zones is considered a luxury feature, and plumbing is still designed in a way that makes it difficult or dangerous to allow any part of the building to freeze. That has to, and will, change.
Fleece and thermal clothing has made it as comfortable for me to set my thermostat to 55 or 60 degrees F in winter time, for normal activities, as it used to be at 70 to 75, without feeling "bundled up." If someone starts selling thin insulated gloves with fingertip pads with the right friction for keyboarding, I could lower that another 5 degrees. The next 5 degrees after that would require some additional innovations, like a warmed dining table top that keeps your cooked food from getting cold before you can eat it. Beyond that, mains powered (rather than battery powered) thermally active indoor clothing would become useful. You're not going to be able to make money selling any of those things while heating your whole house or office is still affordable to most, but see what happens after another doubling or tripling of the price.
The point is, if we did need to go back to year 1900 levels of per capita energy consumption, that does not mean going back to a 1900 level of comfort and productivity, because we will still have 2010 (and forward) technology to work with. What's holding us back is all the technology and social conventions that are based on mind-1900's solutions using cheap energy.
Energy consumption by industry, commerce, and agriculture is subject, to varying degrees, to similar changes. The processes used to manufacture and distribute a given item are the ones that are cheapest overall, not the ones that use least energy. If I ran a widget factory in the 20th century, and someone invented a widget manufacturing process that used ten times more energy per unit (say, $10.00 worth instead of $1.00 worth), and saved $11.00 in materials cost or overhead (e.g. by speeding up the process), then I'd have been managing poorly if I don't implement that method. If the energy price goes up more than 10% so that that method no longer pays, the price I have to charge for widgets doesn't increase by $11.00 + profit margin, it increases by $1.00 + the increase in cost for the $1.00 worth of energy needed for the other method + profit margin. That's an extreme example, but the point is that energy demand is often very sensitive to energy price, especially over the long term where capital tied up in energy-intensive production methods can be moved into alternatives.
When I first made an effort many years ago to look into the energy future of the western world, I found the doom-and-gloom predictions (such as those of the Peak Oil crowd) almost convincing. But then I started asking, "how comfortably could people live at about 1 kilowatt per capita?" and some of the possibilities for solutions started looking reasonable. The most troublesome issue is food production and transport. If a kcal of the food I eat requires more than 10 kcals of energy to plant, grow, harvest, process, and transport (not counting the solar energy used by the plants) -- which it currently does, by a large margin -- then staying under the 24 kwh/day "limit" is going to be very difficult.
However, the total usage "ration" of 1 kw (for all consumption, not just household) is pretty strict. It's what I could collect by covering my roof with solar panels and filling part of my basement with batteries, at about the cost of a new luxury car. A share in a co-op wind/solar facility in the plains a few hundred miles west would probably be more economical. This is also a worst-case scenario in a different way, in that it assumes alternatives alone have to do the whole job.
Respectfully,
Myriad
sorry but clean coal is an oxymoron