AltaRock will soon be exploring Weyerhaeuser-owned forests.

AltaRock, a Sausalito, Calif.-based startup, announced in September 2008 that it had reached an agreement with forest products giant Weyerhaeuser (NYSE: WY), granting the company geothermal exploration rights on portions of Weyerhaeuser's West Coast properties.

The deal gives AltaRock exclusive rights to assess the geothermal potential of 667,000 acres in northern California, western Washington and western and south-central Oregon. It also allows AltaRock to transfer up to 40 percent of the exploration acreage rights into geothermal development leases within two years.

Weyerhaeuser spokesman Greg French says the agreement is a "potentially environmentally-friendly way to release the value of land holdings," working toward the company's stated sustainability goals while tapping the land's latent resources.

The deal follows AltaRock's wrap-up of its second round of financing, which brought in more than $26 million from investors, including $6.25 million from Google.org, the philanthropic arm of Google (Nasdaq: GOOG).  The funds would go toward the development and demonstration of AltaRock's proprietary geothermal technology. Aiming to provide utilities with a renewable baseload power supply, AltaRock has an eye toward states with renewable portfolio standards, such as California, Oregon and Washington, according to AltaRock.

Unlike conventional geothermal resources, which depend on naturally occurring caches of underground hot water, AltaRock's engineered geothermal system (EGS) can harness the Earth's heat from a broad range of sites, AltaRock claims. EGS systems inject water though engineered pathways in hot rock, bringing the heated water to the surface and using the steam to create electricity before circulating the water back underground.

Geothermal sources are especially appealing to renewable energy developers because they typically operate 24 hours a day, rather than intermittently like wind and solar, and don't require large energy storage systems. A 2007 MIT study found that with increased investment and technological improvements, geothermal sources could meet 10 percent of the United States' energy demands by 2050.