Averaging US East Coast Wind

There is an interesting paper in the most recent issue of PNAS by Kempton et al, Electric Power from Offshore Wind by Synoptic-Scale Interconnection. The paper is in a tradition of literature looking at how to solve the problem that renewables are intermittent by averaging over production in a wide geographical area - on the theory that the wind must be blowing (or the sun shining) somewhere. In this particular paper, the region studied is the US east coast. This map shows the study sites S1 - S11:

As you can see, the sites run pretty much the full length of the US East Coast from Florida to Maine. They then look at the power that could be generated by averaging all these sites together. This next graph will give you a flavor. Capacity factor is the fraction of the maximum power output of the turbine(s) that is currently being produced, and the graph is showing you that value for two week periods at the start of November and May respectively, in the 11 study sites, with the composite numbers at the bottom:

As you can see, the individual sites are extremely noisy, and the composite signal (the Pgrid at the bottom) is smoother and doesn't fluctuate as extremely. Thus the paper claims:

In the study region, using our meteorologically designed scale and orientation, we find that transmission affects output by reducing variance, slowing the rate of change, and, during the study period, eliminating hours of zero production. The result is that electric power from wind would become easier to manage, higher in market value, and capable of becoming a higher fraction of electric generation (thus more CO2 displacement).

From my non-specialist perspective, the overall result still doesn't look that great, however. If I zero in on the Pgrid, and rearrange the figure a bit:

You can see that there are stormy days in November when the entire hypothetical array of wind farms is producing at pretty much full capacity, and the capacity factor of the whole system is over 0.9. Then there are calm days in May when there's not a whole lot of wind and the capacity factor is down near 0.1. A 10x variation in power output doesn't seem like something to boast about too loudly.

The problem is that the US east coast is not really big enough to be above synoptic scale: the size of the big cyclones and anti-cyclones that govern weather in temperate latitudes. Another figure in the paper illustrates the kind of thing that can go wrong. Here the colors are wind speed (in m/s) and the lines show pressure isobars:

As you can see, with a big enough high off the coast there can be hardly any wind up or down the whole region.

There are several important points of context to make however. The first good news is that clearly east coast solar would tend to counter swings in east coast wind to a significant degree. Windless days of high pressure in the summer are exactly when you'd expect solar plants to be at close to maximum output - whereas the wind can pick up the slack in the winter (solar panels will not be doing a lot of good on those stormy days in November).

The other thing is, even within the US context, the east coast is not really where the wind is. This map from the National Renewable Energy Laboratory shows the distribution:

A very important point: the power in the wind is proportional to the cube of the wind speed, so those purplish regions in the high plains have about 10 times the potential per unit area of the yellowish-green regions on the coasts. That's where the wind is. And clearly, by feeding wind into the grid across the US, the performance is going to be smoother again, since a single high cannot cover the whole country.

It seems like with renewables, when feeding small amounts into a mainly fossil fuel grid, that's workable as they basically displace fuel use when they are available, but don't displace much capacity so the fossil fuel plants are still there to smooth the renewables out. An all-renewables grid would also be possible, but requires averaging over huge areas (and/or enormous amounts of storage). In between those extremes, things are going to be awkward.