The challenge of "green" is aggregating small amounts of energy – ultimately days of sunlight per surface – into amounts useable in quantity and continuity. Plants convert some of that energy continuously in daylight hours, but aggregating is the challenge. Currently we rely almost entirely upon a fossil fuel process that takes eons and is not sustainable – even if the amounts of recoverable fuels remains large, the environmental side effects are rising, not falling. Global economic growth has almost immeasurable benefits – hundreds of millions of Chinese no longer face hunger daily. Only recently has the government sufficiently overcome the fear of famine to eliminate the mandate that farmers grow grain. In China point- and regional-source pollution is now sufficiently bad to generate local political action, as it was first in California and then in the US as a whole in the 1960s. But no local government, and most national governments, are uninterested in denying access to electricity (air conditioning, refrigeration, lighting) or mobility (cars). Desirable or not, I don't think it's realistic to expect that governments will do much to repress energy demand. Supply-side developments are thus crucial. That means improving the feasibility of solar, wind, hydro and biomass.
One challenge is operational size. To what extent are economies of scale so intrinsic in the physics (and their engineering implementation) that only large facilities are feasible? Let me speculate on alternatives for wind power to frame this question.
Currently the trend is towards very large turbines. Winds blow stronger above ground; if you're building a tall tower, you then want to generate a lot of power per tower to cover costs. That may work, with better engineering of blades and generators and mechanical connections. Scale on the manufacturing side can help, as standardized designs lead to economies in production, from poles to turbine blades.
What would a small system look like, something found in every backyard? First, the turbines would have to be short and spin on a vertical rather than a horizontal axis; they couldn't look like windmills, but rather spinning windpoles that would face different wind sheer and so might be cheaper structurally – the pole would be the turbine access, with lower stresses cheap bearings or even bushings would do. Now close to the ground they'd "enjoy" far less wind, so would have to be really cheap. Windpoles might be relative to windmills on a watt-hour basis.
Then there's the aggregation issue. Such windpoles probably couldn't each turn a generator, that would be too high in cost per unit of energy. They might however be able to turn a small scroll compressor that would feed through standard lines to a centrally located turbine. Scroll compressors are pretty well understood, there are lots of refrigerators and air conditioners out there. Storing compressed air is also a mature technology, providing a means to enhance continuity. Small air tools – small turbines – have also been around a long time. So the pieces could be assembled quite readily.
I'm not enough of an engineer to cost any of this out. There may be simply too little wind energy at ground level. But versions of this – systems whose cheapness and small size make up for conversion efficiency – seem worth exploring. Perhaps they already have been, and have been found wanting. But in some parts of the world small rooftop solar water panels are pervasive – highly inefficient in the amount of energy they convert but so cheap as to make sense.
...[we'll see] a multiplicity of energy systems … [as in] vehicle drivetrains
In any case, any attempt to move away from fossil fuels is likely to lead to a multiplicity of energy systems – just as we are currently seeing a growing variety of vehicle drivetrains, depending on local fuel options and driving patterns.
Note 1. Nuclear – including geothermal – and tidal sources are exceptions. While in principle fusion is possible only uranium-based fission is commercially available, but that suffers from both political and economic pressures that make it a small slice of currently harnessed energy. Geothermal and tidal energy are at present unimportant.