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Continuing its streak of recent renewable deals, California utility PG&E announced yesterday that it would purchase 106.8 megawatts of solar thermal-biofuel hybrid power from San Joaquin Solar, a subsidiary of Portugal’s Martifer Renewables.

The two 53.4 megawatt plants combine the use of conventional solar thermal technology with biofuels derived from agricultural waste, livestock manure and grass clippings produced in surrounding Fresno County to allow for 24/7 electricity generation. Although the plants are smaller than other solar thermal deals cinched by PG&E, they mark the first time it has tried combining solar with other generation schemes, and help cement the utility’s role as a leader in renewables.

The plants, which will begin operations in 2011, will be located near Coalinga, in the center of the state, and provide enough energy to supply nearly 75,000 homes. Martifer’s solar thermal technology works by harnessing the heat produced by concentrating sunlight on liquid-filled tubes with curved mirrors, called solar troughs, to run steam turbines which, in turn, generate electricity. Its major downside, of course, is that it can only operate when the sun is shining.

This is where Martifer’s innovative hybrid technology comes in handy: By switching over to biofuels at nighttime or on overcast days, the company will be able to keep its plants running continuously. The 250,000 pounds of biomass each plant will require to operate on a yearly basis will also produce heat that will be used to run the turbines. Getting that amount of biomass shouldn’t be hard: Andrew Byrnes, one of the project’s developers at San Diego-based Spinnaker Energy, told Fortune that Fresno County alone produces close to 2 million tons annually.

The company said the electricity generated from this hybrid technology would be competitive with energy generated from natural gas and coal plants. In recent months, PG&E has also inked contracts with BrightSource Energy, which will provide it with 900 megawatts of solar thermal energy, and Calpine, which will supply geothermal energy from The Geysers field north Calistoga.

Southern California Edison, another large utility, recently said it would invest $875 million in a 250 megawatt project to blanket unused commercial rooftops in three L.A. counties with solar panels. The project, which will power 162,000 homes, will take around 5 years to complete.

Earth2Tech reported that CEO Peter Darbee has expressed interest in using PG&E’s considerable assets, around $36 billion, to purchase solar thermal plants outright in the future. For now, however, its ability to do so is constrained by the lack of an investment tax credit (ITC), a 30 percent tax credit for solar investments that doesn’t extend to the utility industry.

According to the San Jose Mercury News, the utility has signed contracts for 2,500 megawatts of renewable energy since 2002 in an effort to meet California’s tough energy mandate — 20 percent derived from renewable sources by 2010. PG&E put out a request for offers (RFO) in April to ask for an additional 800 - 1,200 megawatts in renewable energy contracts by 2015. The company will announce the winning bids next month. Earth2Tech also reported that CEO Peter Darbee has expressed interest in using PG&E’s considerable assets, around $36 billion, to purchase solar thermal plants outright.

Google.org, Google’s philanthropic arm, has taken a number of stakes in solar and wind startups over the past year, most recently joining a $115 million investment in solar thermal firm BrightSource Energy. It now seems to be focusing its attention on the bustling geothermal energy sector, with Google co-founder Sergey Brin recently expressing a strong interest in Ormat, a geothermal startup headquartered in Reno, Nevada.

During an interview with the Israeli newspaper, The Marker, Brin confirmed that his company was in discussions with Ormat to collaborate on several clean energy projects, calling the startup a “great company” and praising it for its potential to turn geothermal energy “into a big business.” Though he wouldn’t say whether Google was in talks to purchase any Israeli cleantech companies, he did say that the conditions were good for his firm to buy companies in 2009. He said there were a lot of interesting companies that worked in renewable energy and electric cars — perhaps a nod to Shai Agassi’s Project Better Place.

According to Haaretz, senior executives at Google have already met with their counterparts from Ormat twice, and Larry Page recently visited one of the company’s plants in Steamboat Hills, Nevada. Ormat chairman Lucien Bronicki said he and Google officials were pushing legislation in the U.S. advocating more R&D for advanced geothermal technology. Ormat announced in February that it would work with the DOE and several geothermal companies — GeothermEx and Pinnacle Technologies — to test Enhanced Geothermal Systems (EGS) technology at its 11 megawatt Desert Peak facility.

The DOE has committed $1.6 million to support the project, which could eventually yield over 50 MW of power. The partnership will test hot fractured rock (HFR) technology to attempt to increase the output of its geothermal wells. Sydney, Australia-based Geodynamics, which I wrote about a few weeks ago, has been on the forefront with this technology and is nearing the completion of a 50 MW demonstration plant to supply up to 75,000 people by 2012.

Ormat has several existing projects in Guatemala, Kenya and Nicaragua is considered the world leader in geothermal energy.

In addition to making a series of high-profile investments in eSolar, BrightSource and Makani Power as part of its RE<C initiative, Google has also donated over $1 million in grants to support plug-in vehicle adoption. The foundation’s RechargeIT initiative recently gave $200,000 to CalCars.org. Page said Google.org’s goal is to produce 1 gigawatt of renewable energy capacity from wind, geothermal and solar thermal sources cheaper than coal, an objective he and Brin are optimistic will be met in years, rather than decades.

BrightSource Energy, an Oakland, Calif., solar thermal startup, has landed a hefty $115 million funding round from investors including Google to develop its solar power tower technology.

Solar thermal technology is one the leading hopes for alternative energy. It uses like mirrors and lenses to boil water, the steam of which is harnessed to generate electricity.

This third round was led by Google.org, VantagePoint Venture Partners, BP Alternative Energy, Statoil Hydro Venture and Black River; returning investors included DBL Investors, Draper Fischer Jurvetson and Chevron Technology Ventures. The company has now raised $160 million.

The company recently signed a massive contract with PG&E to supply it with up to 900 megawatts from its plants, whose construction will begin in 2009. Its Distributed Power Tower (DPT) technology is basically an array consisting of thousands of small mirrors, called heliostats, which concentrate sunlight on a single point — in this case, a boiler chamber mounted on top of the tower. Because its heliostats are able to follow the sun in two dimensions, BrightSource claims they are much more efficient than rival solar thermal technologies.

Each field of heliostats, dubbed a Solar Power Cluster (SPC), can produce 20 MW of solar power; a typical BrightSource power plant, made up of five SPCs, is therefore capable of generating up to 100 MW. To reach the 900 MW mark, the company plans on having one 100 MW plant up and running by 2011 and four 200 MW plants up by 2016.

Rivals Ausra, Solel and eSolar use similar technologies to produce electricity, though their specific designs differ. The latter, for example, uses flat mirrors in smaller groupings to produce up to 33 MW at a time –a practical strategy that allows the firm to plug directly into the existing grid and to eschew the burdensome process of obtaining permits. Costs will likely remain the biggest obstacle for these companies, but BrightSource, which is chaired by Arnold Goldman, a man with an extensive background in solar thermal, will be well positioned to handle them.

[See also: CNET Green Tech Blog