One of the most heated and interesting debates in the energy world today has to do with how far the US can get on carbon-free renewable energy alone.
One faction believes that renewables can supply 100 percent of US energy, with sufficient help from cheap energy storage and savvy management of demand.
Another faction believes that renewables will ultimately fall short and need assistance from nuclear power and natural gas or biomass with carbon capture and storage.
This war is largely being waged behind the scenes in competing academic papers, but it is highly relevant to current events as a whole host of states and cities are passing laws targeting “100 percent clean energy.” Some, like Hawaii, specifically target 100 percent renewables. Some, like Washington state, target 100 percent “clean,” allowing room for non-renewable sources.
Which target is more realistic and prudent? Just how far can renewables get?
At the heart of the debate is the simple fact that the two biggest sources of renewable energy — wind and solar power — are “variable.” They come and go with the weather and time of day. They are not “dispatchable,” which means they cannot be turned on and off, or up and down, according to the grid’s needs. They don’t adjust to the grid; the grid adjusts to them.
That means a grid with lots of renewables needs lots of flexibility, lots of different ways of smoothing and balancing out the fluctuations in wind and solar. When people predict that renewables will fall short of 100 percent, what they are predicting is that we won’t be able to find enough flexibility to accommodate them (at least not fast enough). They will require “firming” by dispatchable, nonrenewable sources.
There are many sources of grid flexibility, but the one that seems to have the most potential and is laden with the highest hopes is energy storage. To a first approximation, the question of whether renewables will be able to get to 100 percent reduces to the question of whether storage will get cheap enough. With cheap-enough storage, we can add a ton of it to the grid and absorb just about any fluctuations.
But how cheap is cheap enough?