One of the greatest challenges of integrating renewable power into the US grid is its intermittent nature. This is especially true for wind power, which is prone to rapid fluctuations that can leave utilities scrambling to either add or dump power. But the temptation of wind is large—the US has wind resources to cover 23 times its current electric use—and that has led to many ideas about how best to deal with the erratic supply. A study that will appear in PNAS later this week suggests a radical solution: connect offshore wind up the entire Eastern Seaboard of the US into a single, huge, baseline generating system.The authors of the new study note that, currently at least, the fluctuations in wind power are handled by redundant generating and transmission equipment, which generally involves the burning of fossil fuels when the wind slacks off. One of the two major alternatives currently under consideration involves the use of energy storage, either large, on-grid facilities, or ad-hoc aggregation of the excess capacity in items like electric vehicles. The final option, and the one they consider, is the potential to aggregate geographically diverse collections of wind farms.
They're hardly the first to consider this prospect, and a variety of other studies have examined the potential of distributed wind in specific locations. So, for example, a study of wind potential in the UK found a tendency for the entire geographic area to experience similar wind conditions, meaning that even a dispersed generating system might not work there.
The new study builds on the earlier work by considering why this is the case. The UK is about 1,100km along its north-south axis, and the high pressure systems that bring it low wind tend to be roughly 1,000km in size. In contrast, the US East coast is roughly 2,500km in length, and has a tendency to spawn storms that move up the coast in a roughly northeasterly direction. Many states along the coast are already in the planning or permitting stages for large offshore wind facilities (New Jersey alone is considering at least three) that will total over a TeraWatt in capacity, assuming they're all built.
So, for the new analysis, the authors considered a total of 11 sites on the continental shelf, ranging from the Florida Keys to the Gulf of Maine. Wind speed data was available over a five-year period for each of the sites, available with one-hour resolution between readings.
As expected, sites close to each other showed a fair degree of correlation in wind speeds—if the winds had died at one, they were likely to be dead at a site that was relatively nearby. By the time sites 750km apart were considered, however, the correlation had dropped below 0.2, and it dropped below 0.1 for sites over 1,300km apart. As expected, this means that the large geographic spread of the sites means that they're unlikely to be hit by a single weather system that causes a synchronized rise or fall in production.
The authors didn't seem much in the way of negative correlations, however, where a lack of wind in one site would typically mean high winds in a different one. Still, the aggregated wind power was very stable. Although production from individual sites would rise and fall by as much as 50 percent within an hour, the aggregate as a whole rarely saw changes greater than 10 percent. Throughout the entire period, the ensemble was above 5 percent of its rated capacity except for a grand total of 20 days, and never dropped to zero. For the most part, output was typically near the middle of the capacity range.
The authors also analyzed individual weather events, including one where a large anticyclonic system was parked over North Carolina. Although the center of the seaboard was largely quiet, stations in Florida saw strong westward winds, while those in New England had a strong eastward wind, exactly as the authors had predicted. All 20 days of low production were analyzed, but the authors conclude there's no pattern involved; each of the instances was the product of unique circumstances.
The authors seem rather interested in the idea of actually physically connecting all the sites with high-capacity undersea cables into what they term the Atlantic Transmission Grid. At roughly $4 million a mile to install, this would still account for less than 15 percent of the total price of the full system of wind farms, a figure that's in line with building redundant generating capacity onshore. Still, there would seem to be advantages to building the interconnect on land, where it would be easier to service and could integrate other intermittent power sources, like solar.
There's also a certain irony to the fact that the authors suggest that planning and licensing the Atlantic Transmissions Grid would be simpler because it involves a single nation. While that's true to an extent, the grid would have to service a patchwork of local utilities, and incorporate sites in states that have very different perspectives on (and legislated requirements for) renewable power.
Source: Arstechnica.com
