No one likes overhead transmission lines. Yet, connecting geographically separate wind or solar regions will be key to increasing the percentage of electricity generated from renewables. This is because if wind farms are far enough apart, their output is uncorrelated, which means that even if the wind dies down at one farm, it will still be blowing at the other. In statistical terms, the average of the two farms together has a lower variance than either farm separately. The lower the correlation the more this is true. In practical terms, this means that the combined output of several geographically diverse wind farms is more stable than the output from any single farm. The same principle applies to the correlation between output from wind and solar farms, even in the same region. Wind is at worst uncorrelated with solar, and at best negatively correlated (the wind blows stronger when the sun isn't shining). A mixture of wind and solar is less variable than either by itself.
Long distance power lines will also be necessary for carrying electricity from places where the wind blows strongly or where the sun shines a lot to where demand for electricity is actually located. For example in the US, the mid-western "wind corridor" is a fecund source of cheap electricity, but the big consumption centres are thousands of kilometres away in the east.
So if we could build long distance high voltage power lines and put them underground that would reduce community hostility. If they could be reduced in cost that would be even better.
It’s become clear in recent years that expanded transmission from the windy Great Plains to the east is a prerequisite to developing more of the wind potential in the Midwest. If his project comes to fruition, Ward said, “We will pull some of the cheapest, most robust wind from the upper Midwest and bring it to the East Coast.”
|Wind resources in the US (Source)|
|US solar irradiance (Source)|
As a result of recently completed multi-value transmission projects across the region, he said, his project would be able to tap into a wide swath of the windiest land this side of Canada.
However, Ward’s decision to pursue this project now is a function of technology, not policy. Moving high voltages of electricity generally has required copious amounts of space, he said, meaning that transmission developers would hoist their lines high overhead. But over the past five years or so, “The technology of high-voltage cables has changed dramatically. I think everybody understands that solar and wind and batteries have changed a lot, but nobody is thinking about transmission.”
The German manufacturing conglomerate Siemens, looking for a way to unobtrusively move wind power from the North Sea to southern Germany, has been “leading the charge,” Ward said. The company has used a new rubber-based cable that is “very easy to handle in the field, easy to splice,” Ward said.
High voltages of power moved underground now require “a relatively small footprint,” about two-and-half feet across, Ward said. Two transmission cables are installed about three to five feet underground. It seems that the price has a smaller footprint as well.
“Compared to five years ago,” Ward said, “we can transmit much more power at a much lower price.”
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The 100% renewables grid of the future will have a mixture of wind and solar (because of their low correlation), spread across different regions (ditto), connected by underground HVDC lines, with battery storage for grid stability and time shifting. We might keep legacy gas peaking plants for backup, and might even use power to gas to fuel these gas plants. But the core of our new 100% renewable grid will be interconnectors and storage, and both are falling in cost.