Guest Post by John Morgan. John is Chief Scientist at a Sydney startup developing smart grid and grid scale energy storage technologies. You can follow John on twitter at @JohnDPMorgan.

The fastest path to decarbonization would seem to be combining every kind of low carbon energy available – the so-called “all of the above” camp of clean energy advocacy. The argument runs that different kinds of clean energy are complementary and we should build as much of each as we can manage. This is not in fact the case, and I’ll show that a mix of wind and solar significantly decreases the total share of energy that all renewables can capture. The “all of the above” approach to emissions reduction needs to be reconsidered.

In a recent essay Breakthrough Institute writers Jesse Jenkins and Alex Trembath have described a simple limit on the maximum contribution of wind and solar energy: it is increasingly difficult for the market share of variable renewable energy [VRE] sources to exceed their capacity factor. For instance, if wind has a capacity factor of 35%, this says it is very difficult to increase wind to more than 35% of electrical energy. Lets look at why this is so, and extend the principle to a mix of renewables.

The capacity factor (CF) is the fraction of ‘nameplate capacity’ (maximum output) a wind turbine or solar generator produces over time, due to variation in wind, or sunlight. Wind might typically have a CF of 35%, solar a CF of 15% (and I’ll use these nominal values throughout).

Jesse and Alex’s “CF% = market share” rule arises because it marks the point in the build out of variable renewables at which the occasional full output of wind and solar generators exceeds the total demand on the grid.

At this point it gets very hard to add additional wind or solar. If output exceeds demand, production must be curtailed, energy stored, or consumers incentivized to use the excess energy. Curtailment is a direct economic loss to the generators. So is raising demand by lowering prices. Energy storage is very expensive and for practical purposes technically unachievable at the scale required. It also degrades the EROEI of these generators to unworkable levels.

Jesse and Alex make this argument in detail, backed up with real world data for fully connected grids (i.e. not limited by State boundaries), with necessary qualifications, and I urge you to read their essay.

The “CF% = market share” boundary is a real limit on growth of wind and solar. Its not impossible to exceed it, just very difficult and expensive. Its an inflexion point; bit like peak oil, its where the easy growth ends. And the difficulties are felt well before the threshold is crossed. I’ve referred to this limit elsewhere as the “event horizon” of renewable energy.

So if wind is limited to say 35% of energy, and solar to 15%, can we add them together and achieve 50% share? The Breakthrough authors seem to think so, writing that “this threshold indicates that wind and solar may be able to supply anywhere from a third to a half of all electricity needs”. That would be a very considerable addition of low carbon energy. But unfortunately this is not the case.

Here’s the problem with adding solar: it produces about half as much energy as wind for the same capacity. And the capacity factor rule sets a limit on total variable renewable capacity. So at the limit solar capacity is not additive to wind, it displaces wind, while producing less energy. Any amount of solar lowers the share of energy that wind and solar together can acquire, and the optimal mix for decarbonization is all wind and no solar.

This is a general corollary to the capacity factor rule – adding lower capacity factor generation to the mix reduces the potential share of variable renewable energy. It is the energy equivalent of Gresham’s Law – “Bad energy drives out good”. Far from targeting a “mix of renewables”, we are better off targeting just the one with the highest capacity factor. We should build wind and not solar.

You can see this dynamic in the following figure, which plots the limiting share of wind and solar energy (VRE) in the grid as a function of solar’s share of wind and solar capacity. Adding solar capacity cannibalizes wind capacity, and reduces the total amount of low carbon energy that these sources can ultimately provide. Solar is not additive to wind; its subtractive.

The situation becomes even clearer if we shift focus from installed capacity to energy delivered. In the plot below, the x-axis now shows the fraction of wind and solar energy that is produced by solar.

Introducing solar energy into the mix causes a rapid drop in the maximum grid penetration of all variable renewable energy. Wind alone could potentially achieve 35% of grid energy share. But with 50% solar, the maximum share that wind and solar together can achieve is just 21%.

In other words, building out solar effectively robs us of a whole climate stabilization “wedge”.

It should be remarked that this capacity factor rule sets too optimistic a limit. The Breakthrough writers cite estimates that only 55%-60% of grid energy could be replaced by variable sources, due to stability requirements. This means VRE share will struggle to exceed 60% of capacity factor, and the limits described above will be reduced by that factor. So while wind alone could achieve up to about 21% of all electricity, a 50-50 mix of solar and wind is practically limited to only 12%.

This is a lot to give away.

So long as we only have a small amount of solar and wind we can build as much of either as we like. The limit only becomes apparent at higher penetration. But this happens much more quickly if there’s a lot of solar in the mix.

There may be good reasons to build solar in the early stages of a clean energy expansion. The rate of emissions reduction matters, and while supply chains are developing, building both solar and wind might help. But if this trajectory is to continue we will need to shift resources to wind fairly early on, and allow solar capacity to decline.

This should prompt a rethink of the simplistic “all of the above” response to emissions reduction, and the popular notion that there should be a mix of renewables. If it doesn’t even work for wind and solar, does it work anywhere at all? Its time to pick some winners, and support for renewable energy at scale should increasingly favour wind over solar.

And we should also think about how to decarbonise the remaining eighty percent of the grid that variable renewables can’t touch.