Until a few years back, one of the biggest challenges facing renewable energy sources was their cost of generation.
Now, however, with the cost of solar power dropping fast, and the cost of fossil based power generation increasing in many countries, the cost of the renewable energy is perhaps no longer the toughest challenge facing the sector.
The real challenge now for renewables is their intermittent nature and as a consequence, their infirmness and lack of reliability.
The two main renewable energy sources (solar and wind) are intermittent in nature and cannot be fully relied upon as standalone resources for electricity. As a result, storing these is the only known way now to make them a reliable source of energy on their own.
(One might question about biomass and also about renewable energy sources such as geothermal power which can actually work 24×7. These sources however have a limited capacity to scale and cannot contribute anywhere close to the quantum of power that the world requires today)
Storage of power, mainly done through batteries, is however a very expensive proposition even today, even after the cost of battery storage itself had dropped significantly in the past few years.
Thus, if one were to state the one key bottleneck that stops the world go completely renewable, it would be the economics of large scale energy storage.
I have been in cleantech since 2007, and during this period, I have heard enough bull about battery storage having become really economical.
That bull lasts only until you actually ask the proponent to put down hard numbers on the paper and do the actual math.
I have been surprised how even some bright guys in the energy business simply believe the hype that battery storage is now affordable to all. This hype appears to have percolated to the top in countries like India where there have been suggestions of implementing 100+ MW of ground mounted solar power plants with battery storage.
Good luck with that!