To most people, a book is an ordinary bundle of printed paper that may contain a few pearls of wisdom.

But to a power engineer, a book is also a thermal-storage device that can soak up a little heat. In that sense, Drexel University's Hagerty Library, with its nearly half-million volumes, is one very big heat sink.

"The books are like batteries," said Audrey Zibelman, chief executive of Viridity Energy Inc., a Conshohocken firm that is innovating ways to manage power use.

Since June, Viridity has administered power consumption at three Drexel University buildings, including the library and the law school. By shifting electrical loads and using the existing air-conditioning system, it has reduced electrical use 15 percent, much to Drexel's delight.

"If we're saving 15 to 20 percent, that puts a lot of money in our pockets that didn't exist in the past," said Chika Nwankpa, a Drexel professor of power engineering.

Using library books to store energy in the summer months - they cool off at night and help keep the building comfortable during the day - is only one component in a complicated calculus to exploit fluctuations in the wholesale electricity markets. Those markets are at the center of a quiet revolution that is reshaping the way Americans buy and use electricity.

Though the constantly shifting wholesale price is invisible to most customers who pay fixed electric rates, it represents a huge opportunity for big customers like Drexel that can curtail usage during peak hours, reducing demand - and the price - for others.

To encourage big users to draw less power when the transmission system is running near capacity, regional grid operators like PJM Interconnection L.L.C., of Valley Forge, offer incentive payments for reductions, known in the trade as "negawatts." It is those incentives that present an enticing market for companies such as Viridity.

"We identify the best hours to reduce consumption and get compensated for that," said Zibelman, who founded Viridity two years ago with another former PJM executive. Zibelman, a lawyer, was PJM's chief operating officer.

Viridity's latest market-based project involves a partnership with SEPTA to capture electricity generated by braking subway trains, much as a hybrid automobile produces power when it slows down.

The captured power will be stored in a large battery and reused when the train accelerates. The system is expected to reduce electrical-power purchases 10 percent to 20 percent at each battery location, SEPTA officials say.

The amount saved from buying less power is impressive - one battery is expected to save SEPTA 1,380 megawatt-hours a year, or about $45,450. But the real money to be made is from the incentives for selling "negawatts" to the grid.

According to Viridity, each one-megawatt battery array is expected to generate $510,000 a year in PJM incentives.

One way SEPTA can make money is a classic arbitrage - buy low and sell high. SEPTA can charge the batteries overnight at cheap prices and sell the power back to the grid during peak hours.

The transit agency can earn other incentives for providing instantaneous infusions of power to keep the grid in balance, the type of service that will become increasingly important as more variable-energy sources, such as wind and solar generators, are connected to the system.

Viridity receives a share of the incentives as its fee for managing the system using its software.

If the pilot project works, SEPTA could install more than 30 of the batteries to serve its electrified subway and trolley system - potentially putting a big dent in its $20 million annual electric bill. SEPTA says the savings would trickle down to riders.

On Thursday, Viridity received a $900,000 grant from the Pennsylvania Energy Development Authority to help fund the $1.5 million project. If successful, Zibelman said, Viridity would finance the remainder of the project with its own capital.

Pennsylvania Department of Environmental Protection Secretary John Hanger, who presented the state grant at a ceremony, called the SEPTA project "really, really entrepreneurial" and "incredibly clever and creative." He also noted that Viridity plans to sell the idea to other transit agencies.

"Viridity can take this around the world and build a huge business," Hanger said. "That's what we're talking about here."

Within the electric-power industry, there is much division about how much to pay in incentives for "negawatts" - formally known as demand-side resources.

Companies such as Viridity and providers of renewable-power systems want grid operators to pay the same rate for "negawatts" as the market rate for electricity. That would provide a substantial incentive for large power consumers to invest in their own generation equipment or storage devices.

But large electricity generators, who stand to lose business if customers curtail power purchases, say that paying customers big incentives to use electricity more efficiently is not necessary. The Federal Energy Regulatory Commission is expected to issue a ruling later this year that would standardize incentives.

For Drexel's Nwankpa, the potential market for "negawatts" is huge, and goes far beyond subway systems and library books. The three Drexel buildings now managed by Viridity represent only 6 percent of the university's building space.

Nwankpa looks at big cities filled with skyscrapers and sees towering thermal-storage systems just waiting for their power use to be optimized.

"There's a big opportunity to grab there, a lot of 'negawatts' to be traded," he said.

Contact staff writer Andrew Maykuth at 215-854-2947 or amaykuth@phillynews.com.