By George Crabtree, University of Illinois at Chicago and Argonne National Laboratory & Jim Misewich, Brookhaven National Laboratory
Nothing highlights the modern challenges facing the US electricity grid more than renewable electricity from wind and solar sources. The grid inherits its challenges from an earlier day when its primary function was supplying electricity reliably to every customer who wanted it. The grid did this remarkably well, by bringing power from local generating plants to local customers no more than a few tens of or at most a hundred miles away. There was no concern for carbon emissions in those days, digital quality and uninterruptable power were unknown concepts, and the primary metric was price. With inexpensive coal as fuel and generating plants strategically placed near demand centers, power was delivered at continuously decreasing price from 1890 to about 1970, a singular technological and economic achievement.
Renewable electricity changes the game. Renewable electricity cannot always be used locally, especially at midcontinent where wind resources are high or in the southwest US where sunlight is strong. In both cases, population density is low, requiring long distance transmission of renewable electricity over hundreds of miles to find needy customers. Wind and solar sources do not produce power reliably and often not even predictably, averaging only 10-20% of the nameplate capacity for solar and 20-30% for wind energy. This variability in renewable electricity creates reliability issues that must be addressed, especially in the digital age when even one cycle of reduced voltage can trigger expensive shutdowns of robotic semiconductor fab lines or internet data centers.
These issues are compounded by the historically local structure of ownership and regulatory policy for the grid. In the days when generation and customers were only tens of miles apart, dividing the grid into local or regional units for performance and oversight allowed maximum flexibility for solving local challenges. As a result, we now have 3200 utilities providing power and 130 balancing areas where generation and demand are matched. With renewable electricity, the challenges and solutions are manifestly national. We need to transmit electricity over as much as half a continent, and greater variability of supply requires much larger balancing areas to find enough ready customers.
Storage of electricity is another issue with long distance implications. Without storage, conventional backup generation for renewable sources must be built to insure reliability, offsetting the benefit of zero carbon emissions and requiring inefficient up and down ramping to follow renewable fluctuations. Storage captures excess generation (the wind often blows harder at night than during the day) and releases it when renewable generation subsides, reducing the need for long distance transmission to balance the supply. Storage not only improves reliability, it also lowers cost through arbitrage, capturing inexpensive off-peak power and selling it at high on-peak prices. The grid is built to provide worst-case peak power, often 50% higher than the average power, even without the variation of renewable sources. If networked storage were in operation, the grid could run reliably with much less capacity, avoiding significant capital improvement costs over the next two decades. This "saving grace" is often overlooked in the cost-benefit analysis of storage projects.
The local heritage of the grid in terms of ownership and regulatory structure erects artificial barriers to capturing the full value of renewable projects. Often the business case is made from the perspective of a single or a few cooperating utilities serving a local area. The source or market of renewable electricity may be far away, in another utility's jurisdiction, so the distant value to other utilities is not counted. The value of balancing supply with demand over long distances is likewise often lost in the local perspective. Storage that meters out electricity to precisely match the maximum safe transmission capability of a line raises transmission efficiency and delays or eliminates expensive upgrades, a long distance benefit that is often overlooked. Capturing these benefits requires a holistic "big picture" view of the grid as an integrated network that is far greater than the sum of its parts. The historical patchwork structure of the grid in ownership and regulatory policy ignores the grid's long-distance synergies and needs serious re-examination if renewables are to see wide use.
Solving the technological challenges of storage and long distance transmission will require breakthroughs in materials research. This research will also be useful for materials researchers working on renewable energy related topics, such as solar and wind, as it is useful to keep a high level perspective on the grid since it is so critical for the ultimate success and large-scale adoption of renewable energy. Semiconductor-based power electronics accomplishes AC-DC conversion needed at each end of a long distance DC transmission line, but the small band gap of silicon requires stacking hundreds or thousands of devices to deal with the high voltage of DC lines. Wide band gap semiconductors such as silicon carbide and diamond are much better choices, but much more research and development is needed for these materials to be used in power electronics. Superconducting DC transmission is an attractive option because it can operate at much lower voltages than conventional DC lines and deliver more power. Higher operating currents are needed to lower the cost of the wire. Superconducting Magnetic Energy Storage (SMES) has high round trip efficiency, fast response, and lifetimes up to 30 years, attractive features for smoothing the output fluctuations of solar farms. Many other storage technologies for larger scale grid applications need development, such as molten salt thermal storage for solar driven steam generators, cathodes, anodes and electrolytes for flow batteries, sodium sulfur batteries, and longer-lived advanced lead acid batteries. These storage technologies are just emerging for large-scale grid applications.
The Panel on Public Affairs of the American Physical Society recently released a study outlining the challenges of integrating renewables on the grid, with recommendations for addressing each challenge. The full report deals with the issues discussed above and can be found at http://www.aps.org/about/pressreleases/integratingelec.cfm
