WORTHINGTON, Minn. (AP) — On the hottest day of the summer, with thousands of air conditioners operating, the power grid needs every pulse of electricity it can find to meet demand.
To keep everyone cool, the grid harnesses electricity from a wide variety of sources, including coal, nuclear and natural gas plants. It also taps renewable sources like wind, solar and one of the oldest power sources known, water.
"Hydroelectric is the quiet renewable," said Doug Spaulding, president of Twin Cities-based Nelson Energy. "It doesn't get a lot of press or anything but it has a lot of potential."
Spaulding's company has several water projects in the works, one of them a $2 billion large-scale power storage facility in southwest Minnesota. Sill in development, the Granite Falls project, is a 1,000-megawatts pumped storage project also would tap into the region's growing wind energy industry.
A typical pumped storage system has two reservoirs, at greatly different elevations. When electricity is in high demand, water is released from the upper basin to the lower reservoir, spinning turbines and generating electricity as it runs.
Later, when demand has receded, electric pumps send the water from the lower basin back to the upper reservoir.
The economics work because the plant can sell electricity generated at peak times for more than it costs to pump the water back up during off-peak hours.
The first U.S. pumped storage systems were built around 1930. The most recent went on line in Georgia about 15 years ago. Spaulding said it's time to revisit the technology.
"Pumped storage is the only technology available to store large quantities of electrical energy," he said.
The system doesn't literally store an electric charge like a battery. Instead, it uses a combination of water and gravity to capture energy that can be used to generate electricity.
Spaulding said the upper reservoir at Granite Falls would be on farmland. The lower unit would be bored into bedrock a couple of thousand feet down.
He said the concept is especially useful now because of the large amounts of wind energy being produced. Unlike other forms of electricity, there's very little human control over when wind turbines produce electricity. It happens when the wind blows. As a practical matter that often means the highest output is when the electricity is needed the least.
The Granite Falls system could capture that wind energy and release it during peak demand periods.
"The energy that comes to pump the water up the hill, or so to speak, would come from off-peak resources such as wind," Spaulding said.
As it will take at least four years to obtain the necessary permits for the project, the earliest the project could be on-line would be 2018, he said.
Environmental groups are studying the project, but so far no one has formally objected to the concept. One concern is whether drawing water from the Minnesota River to fill the project's reservoirs will damage the river.
The Granite Falls idea is one of several energy storage proposals in the works. Xcel Energy is using a large battery to store wind energy in southwest Minnesota.
In central Iowa, a group of utility companies plan to build what's called a compressed air energy storage system. That project plans to use wind energy to pump compressed air into deep underground caverns, said Steve Thompson, deputy CEO for the Central Minnesota Municipal Power Agency, one of several companies involved.
"The basic concept of compressed air energy storage is that you compress air off-peak and use to generate power on-peak," Thompson said.
When released, compressed air would flow through turbines to generate electricity. Like the water storage project near Granite Falls, the Iowa facility is years away from operation.