It was unrealistic, it is now fashion. Lack of realism came from the higher cost of electricity generated by renewables. But now you can find studies concluding that it is no longer the case. For the “fire reinventer” Amory Lovins (read his book “Reinventing fire” or watch his TED conference)[i] it is possible for the US to have a 2.6× bigger economy in 2050 with no oil, coal, or nuclear energy, one-third less natural gas, 82–86% lower carbon emissions, tripled end-use efficiency, and 74% renewable supplies.
More recently the French Agency ADEME[ii] released a report that is an atomic bomb for France where brand new nuclear power plants are supposed to be built. Actually it is not officially released but you can find it on Mediapart[iii] or summarized in Le Monde[iv] . The main idea is that renewables are not more expensive than nuclear. It is only valid under some scenarios (in particular on demand management) and requires a careful mix but it would not be impossible. The cost would notably be a convex function of the percentage of renewables implying for instance that electricity cost is lower for 95% of renewables than for 40%.
What about intermittency? Well, Amory Lovins would correct you by stating that renewables are variable not intermittent since predictable and therefore easy to tackle. Even if there is some predictability in wind and sun I do not think weather is perfectly known in advance and even variability is not that easy to compensate for. This is a problem that Germany is already facing and I had a chance to attend a seminar given at CityU HK in March 2015 by researchers from the Chair of Energy Economics (EE2)[v] at the University of Dresden and from that of Karlsruhe Institute of Technology (KIT)[vi]. Not only are they concerned with not having enough wind or sun when there are demand peaks but they also study what to do to with excess supply off-peak. Surprisingly they recommend export and storage but curtailing is relevant option as well, due to its lower cost in terms of grid extension and storage. Also note that exports put some stress on the electricity markets of importing countries (how could Switzerland then use its hydro to exploit demand peaks?). Researchers at EE2 also pay attention on the way uncertainty on renewable energy sources has affected different electricity markets and how forecast errors on the renewable energy sources can be tackled. Electricity balancing is the process through which the transmission system operators ensure that they are able to access a sufficient amount of energy to balance the differences between supply and demand that occur in every electricity transmission system. They show that wind and solar should be integrated in balancing markets to add flexibility and allow for forecast errors compensation. This could even strengthen renewable energy integration.
Quite convincingly, the ADEME report has successfully checked the robustness of a 100% electricity generation using renewable, to extreme weather events. And of course a mix of storage capacities (including pumped hydroelectric storage Mr Lovins does not like –hydro is for baseload in his mix- and Switzerland could think of to recycle its capacity) are needed and should be carefully designed for the costs not to be inflated too much. In his TED conference, Amory Lovins keeps saying sun and wind have the lowest marginal cost of energy generation (well, everybody would agree that once solar panels and wind power plants are installed wind and sun are not really expensive). The ADEME report focuses a lot on electricity generation capacity using renewable sources but, at least in the final report, the cost computations (including those of storage capacities) are not a lot detailed. And Germany is thinking about reducing feed-in tariff that helped so much solar energy development.
Anyway, considering the problem in the reverse way has never been so relevant: why not designing the optimal fully green electricity generation (whatever the remoteness of the future) and then design how to optimally get there rather than groping period by period to introduce a little bit of renewable with no idea of the big picture? The ADEME report obtains a ratio of 1 for sun to 4 for wind when there is 80% renewables (also obtained for Germany by Fraunhofer Institute)[vii]. Any guess for Hong Kong?
[vii] EnergieWirtschaftliche Bedeutung der Offshore WindEnergie fûr die EnergieWende, Fraunhofer IWES, 2013