Dec 20 - McClatchy-Tribune Regional News - Tom Beal The Arizona Daily Star, Tucson

There is a shadow over the bright future of solar power in Arizona, cast by the clouds that blanket our metropolitan areas when our demand for electricity is greatest.

They call the problem "intermittency," and it could have its biggest impact midafternoon in midsummer, when we're all running our air conditioners to counter the heat.

Whether those solar panels are part of a big power plant or distributed across the rooftops of Phoenix and Tucson, they will lose their power source just when the electric grid needs it most.

If Arizona is to become "the Saudi Arabia of solar energy," it needs to find ways to keep the electrons flowing through those summer storms and during the total lack of sunshine at night.

Scientists say we simply need to expand our vision of what constitutes a storage battery to include lakes, caverns, tanks of heated liquid and fleets of parked electric cars.

In the future, we might use solar power when it's not in demand to compress air and store it underground, releasing it to spin turbines when the clouds come by.

Other solutions include fleets of privately owned electric cars whose batteries can be plugged into the electric grid or bi- level lakes where water is pumped uphill when power is plentiful and run downhill through turbines during peak demand.

Add to that the proven solar technology of solar troughs, which use mirrors to focus the sun's warming rays on liquid-filled pipes that in turn heat and vaporize gases that power turbines.

Scientists say a mix of these strategies will be needed if solar is to become a dependable solution to our urgent need to find power sources that don't give off greenhouse gases.

The solutions are a few years off, but so is the problem. Arizona's utilities aren't generating vast amounts of power from renewable sources right now because of solar's other impediment -- high cost.

Still, the intermittency of renewable power sources is already a technological problem, said Tucson Electric Power spokesman Joe Salkowski.

At its Springerville power plant, where coal is burned to produce 760 megawatts of power, TEP adds another 4.6 megawatts to the same transmission lines from a photovoltaic array. Even at that low level, said Salkowski, "we feel that hiccup" in the operation of the coal-fired generators when clouds pass by.

It's no threat to the grid yet, but TEP is currently supplying less than 1 percent of its power from intermittent, renewable sources. In the future, said Salkowski, "It's a challenge we need to overcome."

Olgierd Palusinski, of the University of Arizona's Electrical and Computer Engineering Department, is working on a cure for the hiccups, and perhaps for the entire problem.

Working with researchers from Arizona State University and the University of California-Irvine, Palusinski is electro-plating metals into the extremely small pores of a non-conductive membrane, creating a storage battery that doesn't need the wet chemistry of standard ones. It simply stores electrons.

If it works, it will be more efficient, smaller, less costly and longer lasting than a standard battery, he said. An array of the devices could store enough electrons to provide 24-hour power from solar, he said.

Ben Sternberg, a professor in the UA Department of Mining and Geological Engineering, proposes a survey of underground caverns where compressed air can be stored for days before its pressure is released to spin turbines.

Sternberg wants to employ the skills and imaging techniques he honed searching for oil and gas over the past two decades, to find appropriate places to sequester compressed air underground.

Tom Hansen, a vice president for research at TEP, said the anticipated phenomenon of power loss at peak demand during Arizona's monsoon season is one of the biggest impediments to growth in solar generation.

And, he told a group of scientists at the UA last month, it could be their biggest research opportunity in coming years. UA scientists, already researching a variety of solar topics, want to use some of the solar research money given them by the Legislature earlier this month to attack the intermittency problem.

The electric grid can't handle more than a 10 percent fluctuation in power without shutting down, said Hansen, and his company, along with other Arizona utilities, has been ordered by the Arizona Corporation Commission to generate 15 percent of its power from renewable sources by 2025.

Solar is the best bet for meeting that goal, he said, and for supplying even larger levels of power in the years to come, as oil and gas supplies dwindle, and coal falls into disfavor as an energy generator.

The Arizona Corporation Commission knew of the intermittency problem when it ordered Arizona's utilities to meet the 2025 goal, said Commissioner Bill Mundell, but dismissed the utilities' argument that it would keep them from meeting the goal.

"We said, 'Look, when you get close to the 10 percent and reliability is still a concern, some future commission will address it, but right now it's a hypothetical concern,' " said Mundell.

Mundell is betting that technology will solve the problem well before the goal is met, and he predicts that, by that time, solar will also be a cheaper source of energy than others. The price of oil and gas is going up, he said, and some sort of carbon tax on coal burning is inevitable.

The basic technology for capturing sunlight for electricity is good and increasingly reliable, Hansen said.

TEP's array of photovoltaic panels near its coal-fired plants in Springerville has been generating electricity for six years and TEP has had to replace only 150 of the 34,000 modules in that time. It costs the utility $5,000 to $10,000 a year to operate the array, Hansen said, and most of that cost is for cutting the grass.

Wind turbines are already competitive with natural gas for generating electricity, said Mundell, but the state has very few areas with sufficient, consistent wind. He said solar is the future.

"We should be the Saudi Arabia of the world for solar," he said.

On StarNet: Read more in-depth coverage of environmental issues in Southern Arizona at azstarnet.com/environment

--Contact reporter Tom Beal at 573-4158 or tbeal@azstarnet.com.