Green Wombat

In Wednesday’s New York Times, I wrote about two experimental projects in California to store solar energy produced by photovoltaic rooftop arrays:

In the garage of Peter Rive’s San Francisco home is a battery pack. It is not connected to Mr. Rive’s electric Tesla Roadster sports car, but to the power grid.

The California Public Utilities Commission has awarded $1.8 million to Mr. Rive’s company, SolarCity, a residential photovoltaic panel installer, to research the feasibility of storing electricity generated by rooftop solar arrays in batteries.

As rooftop solar systems provide a growing percentage of electricity to California’s grid, regulators and utilities are increasingly concerned about how to balance the intermittent nature of that power with demand.

One possible solution is to store energy generated by solar arrays in batteries and other systems and then feed that electricity to the grid when, say, a cloudy day results in a drop in power production. And when demand peaks, electricity generated from renewable sources could be dispatched from batteries rather than fossil-fuel burning power plants.

“As soon as distributed solar starts providing 5 to 10 percent of demand, its intermittent nature will need to be addressed,” said Mr. Rive, who is SolarCity’s co-founder and chief operating officer.

SolarCity is teaming with Tesla Motors, the Silicon Valley electric car company run by Mr. Rive’s cousin, Elon Musk, and the University of California, Berkeley, to study how to integrate solar arrays and off-the-shelf Tesla lithium-ion battery backs into the grid. SolarCity plans to put such systems in six homes.

“We think in the years ahead this will be the default way that solar is installed,” Mr. Rive said. “Getting the costs down, though, is not going to be an easy task.”

Homeowners could potentially benefit by tapping batteries at hours when electricity rates are high or using them to provide backup power if the grid goes down.

The research has just begun, and at the moment SolarCity is testing the impact of charging and discharging electricity from the Tesla battery pack in Mr. Rive’s garage. His roof sports a three-kilowatt solar array.

“We’re at the point now where we can direct the battery to charge and discharge at specific times by sending a signal over the Internet,” Mr. Rive said.

Included in the $14.6 million awarded for solar energy storage research by the utilities commission was $1.9 million to SunPower for a project that will store in ice and batteries electricity generated by solar arrays at Target stores.

SunPower, a Silicon Valley solar panel manufacturer and power plant developer, will work with Ice Energy, a Colorado company that makes systems that use electricity when rates are low to form ice. When rates are high, air conditioning refrigerant is cooled by the melting ice rather than by an electricity-hogging compressor.

The Ice Bear system and a solar array will be installed at one Target store while battery packs will be used at two other stores in California.

You can read the rest of the story here.