When it comes to clean power, can you have too much of a good thing? The answer, at least in the U.S. Pacific Northwest, is yes. The Bonneville Power Authority (BPA), the federal agency that operates 15,000 miles of transmission lines and 31 hydroelectric dams in Washington, Oregon, Idaho, and parts of five other states, recently proposed a slight adjustment to its dispatch rules – the rules that determine the priority with which different power generators are deployed. (See Figure.) BPA now hopes to achieve a more harmonious coexistence between two renewable sources of electricity: wind and hydroelectricity.
Wind power is an abundant renewable resource that is generating an increasing amount of U.S. electricity each year. Nevertheless, increasing penetration of wind power can create challenges for transmission system operators. Unlike conventional fossil-fueled power plants, wind turbines cannot be turned up and down to meet electricity demand (or load). Instead, they can generate power only when wind is available. When they do produce electricity, however, they don’t require the combustion of a fuel to do it, so wind generation costs relatively little and produces no greenhouse gases.
Wind power enjoys policy support through a variety of mechanisms, including renewable portfolio standards (which mandate a certain share of a region’s electricity to be generated from clean sources) and tax credits (which lower the effective cost of generation). As a result, wind power is often at the top of the dispatch stack. System operators will prioritize wind generation over other sources because they need to meet a certain quota of kilowatt-hours from renewable generation or because tax incentives allow wind generators to operate economically at rock-bottom (and sometimes even negative) electricity prices.
Transmission system operators must use other power plants in their fleets to accommodate wind turbines’ variability, turning plants down when the wind is strong and up when it is weak. Some power plants are better able than others to fill this balancing function for wind power (nuclear plants, for example, are very inflexible, while solar plants generally resemble their wind counterparts by producing variable power).
Hydroelectric dams are generally considered to be one of the best generating technologies to have on hand for electricity systems with large amounts of variable wind generation. Assuming sufficient levels of water are available, plant operators need only to allow it to flow through water turbines to generate power, or hold it in reservoirs if power is not required. Interconnections between wind-rich Denmark and hydro-rich Sweden and Norway have allowed Denmark to generate more of its energy from wind power than any other country in the world, exporting it to Sweden and Norway when it has excess and importing hydro-powered electricity when it does not have enough.
But sometimes nature conspires to provide too much wind and water, leading to some surprising consequences. This happened last June in the Pacific Northwest, when record downpours and heavy winds arrived simultaneously. (See Chart.) When heavy rainfall fills up reservoirs, plant operators can divert it over spillways rather than through turbines. But too much spill aerates the water below, and fish can get a condition similar to the bends from high levels of dissolved air bubbles.
To avoid spilling hydropower, BPA has proposed that it use that hydropower instead of other sources of electricity when it must do so to avoid harming endangered species. Replacing thermal generation such as coal and natural gas with generation from federal dams is fairly straightforward. For thermal plants, it generally makes economic sense to accept free hydropower rather than generate their own. But, as mentioned above, tax credits and renewable energy credits can put wind plants in the unusual position of being able to make more money by generating than accepting free replacement power.
Thus, as more wind turbines are installed and as they generate ever more electricity, BPA expects that wind levels could place a growing strain on dams. Although BPA is investigating options to use excess wind generation to pump water or perform other non-time sensitive work, as a last result, it could require wind plants to reduce output, even at a loss.
System operators around the world already direct wind plants to reduce or “curtail” their output. Wind often blows hardest at times when the system simply doesn’t need it and can’t handle it. But curtailing wind output to ensure power reliability is one thing, while curtailing it to protect endangered fish species is another. BPA’s environmental redispatch proposal could set a precedent that other wind- and hydro-rich power systems follow.
At the end of the day, both hydroelectric and wind power produce clean electricity, and power systems are lucky to have them. As generation from renewable sources grows, power dispatch rules may require some tweaking, but there are certainly worse things than having to manage an abundance of renewable resources.