Dispatchable Wind: Enabling Wind to Play a Mainstream Role in Power Production

The conventional wind industry has done a terrific job at reducing the cost of extracting energy out of the wind, but has not yet been able to shape wind power production to meet grid demands and secure a good price for its power.

In the Renewables Arena, Efficient Energy Storage Is the Holy Grail

For wind to take its place as a mainstream generator, wind farms will have to be able to shape the power they deliver according to the needs of their customers. Wind is an intermittent resource that current technologies convert immediately into intermittent power. This intermittent power receives a poor price in many markets, with no time-of-day energy pricing or capacity payments. In addition, because of its unpredictability, intermittent power is not an efficient user of transmission resources. The industry can forecast the output of a wind farm within 3 percent over 20 years, but it can’t predict whether, or how much, the wind will blow next Tuesday. All of these challenges – low pricing for power, limited capacity payments, inefficient transmission use, and poor forecasting abilities – collectively limit the role wind energy can play in power generation today.

This disconnect is particularly pointed for those in the power sector who are facing a trifecta of increasing power demand, imminent or present-day imposition of emissions costs, and imminent or present-day renewable portfolio standards.

Renewable Wind Energy on Demand

To help meet these challenges, the wind industry has done an excellent job of optimizing turbine efficiency, and it has come up with solutions to store wind energy on short time scales: capacitors smooth power on the scale of a few seconds; flywheels store power for a minute or two; flow batteries, though still expensive, hold some hope for storing wind energy for an hour. Still, such short-term solutions are clearly not able to make wind energy available on demand, nor can they make it competitive with other resources.

Compressed air energy storage (CAES) promises to store sufficient wind power to effect reliable daily dispatch directly comparable to fossil or nuclear sources.

In a compressed air system, air is forced into a pipe, tank, or cavern and then released through an expander to generate power during periods of peak demand. In order for this process to be efficient, there must be at least moderate wind, and to maximize capacity, there must also be suitable geologic features to provide extended storage. Fortunately, these conditions are met in many locations throughout the developed world.

Compressing Air with Wind Power

There are two basic approaches to compress air with wind power. The first is to build a conventional wind farm, utilizing the intermittent electricity to power a variable speed electric motor and have the motor spin one or more compressors on the ground. The compressors pump air into a storage vessel, and the air is expanded on demand through a gas turbine to make power.

The second approach to compress air with wind power is to remove the generators from the wind turbine altogether, replacing them with compressors. The turbine rotor drives compressors in the nacelle, converting wind’s kinetic energy to compressed air potential energy. The high-pressure air can then be run through a pipe down the tower to an underground network of steel pipes that interconnects all the wind turbines. The pipe network alone can act as a storage vessel to provide up to twelve hours of energy storage. If the storage network also incorporates a salt dome, depleted gas field, aquifer, limestone cavern, or other geologic feature, smaller diameter pipes can be used, reducing project costs, and the increased container volume raises energy storage capacity to more than a month.

Considerations for Commercial Implementation

Each method of compressing air with wind power has its own distinct advantages. With the first method of compressing air, readily available ‘off-the-shelf’ equipment can be used for the entire project. With the second method, capital costs can be reduced because there is no need for a large gearbox, generators in the turbine, or variable speed electric motors on the ground. In addition, since there is less equipment to maintain, operating costs can be reduced as well. Compressed air can be stored directly in the energy collection system, and efficiencies can be improved.

That changes wind power from an intermittent, supplemental generation resource to a scheduled, mainstream generation resource, and answers the challenge of providing clean renewable power reliably and competitively. Moreover, because wind is impervious to fuel price fluctuations and emissions costs which dog conventional generation, this method may enable fixed-price power purchase agreements to be written for 20-year terms.

Realizing the Vision of Sustainable, Profitable Renewable Energy

Traditionally, long-range forecasts for global power production have focused on coal, hydro, nuclear, and gas power, with only a small sliver of attention dedicated to the “other” category, which includes renewable energy resources. In response to mounting public pressure, new regulations, and increasing fuel costs, that view has changed dramatically in recent years, and soon appears set to be discarded altogether.

Renewables must play a central part in any comprehensive energy program, and it is clear that many options will be needed in order to meet differing circumstances and portfolio standards around the world. Still, in almost all circumstances, the twin keys to mainstream adoption of renewable energy sources are reliability and cost-competitiveness.

Wind energy from compressed air can be used to generate power for guaranteed availability and profitability. New generation options will transform wind energy from the least dispatchable resource on the grid to one of the most available energy resources around the world – providing grid-quality power sustainably, reliably, and profitably.