AES Solar, a joint venture owned by utility AES and private equity firm Riverston Holdings LLC, said Wednesday that it acquired the 709-megawatt solar-thermal power project from Tessera Solar.

Sempra Energy’s (SRE) San Diego Gas & Electric Co. utility said Friday it plans to proceed with a contract for the output from a solar-thermal power project in California’s Imperial Valley, after a developer co-owned by AES Corp. (AES) took over the project.

AES Solar, a joint venture owned by utility AES and private equity firm Riverston Holdings LLC, said Wednesday that it acquired the 709-megawatt solar-thermal power project from Tessera Solar, a unit of Ireland-based NTR Plc. Terms of the deal were not disclosed.

Tessera signed a long-term power purchase agreement with SDG&E for 300 megawatts of power from the plant, and obtained federal and state permits last year to build it. SDG&E plans to work with AES to execute the contract, said SDG&E spokeswoman Jennifer Ramp. The current contract requires the project to be operational by December 2012. It was unclear what type of technology AES Solar would use for the project. NTR suggested that the SunCatcher Stirling engine technology originally envisioned for the project would not be available.

NTR said in a statement Thursday that its Stirling Energy Systems unit, which designed the SunCatcher engines, would "delay the original deployment schedule" of the devices, "ensuring that the (Imperial Valley) project will deliver its full potential of solar power in the near-term." Unlike traditional solar panels, which convert sunlight into electricity, SunCatcher Stirling engines convert solar heat into electricity. AES Solar has built several smaller solar-panel facilities in Europe.

In December, NTR said that it had written down the value of the SunCatcher engine technology, which is the primary component of Tessera’s solar-thermal power plant design. NTR didn’t reveal the size of the write-down, and said it didn’t affect the California projects. However, the move suggested that Tessera might have had difficulty raising financing for its projects and they might be reconfigured to use another technology.

Ramp of SDG&E said she didn’t know whether AES Solar planned to use the SunCatcher technology or a different technology. "If there are development changes down the road, then we would look at that agreement again and work those new developments into the agreement," Ramp said.

In late December, Tessera sold a second, 663-megawatt solar-thermal power project it was developing near Barstow, Calif., to privately held K Road Power. Days earlier, Edison International’s (EIX) Southern California Edison utility terminated its contract with Tessera to purchase the power from that facility, called Calico Solar.

K Road said it planned to convert most of that project to a solar-panel facility, and use the SunCatcher engine technology for a smaller, second phase of the project. The California Energy Commission, which issued permits last year for both projects, has said that K Road will have to apply for permission to modify the Calico Solar project.

Both the Imperial Valley and Calico Solar projects have been planned for large parcels of federal land in the California desert and both are the subject of lawsuits that aim to block construction, citing environmental concerns. Local Native American and resident groups have sued the federal government over its approval of the Imperial Valley project, saying the project violates environmental laws.

The Sierra Club has sued to block construction of the Calico Solar project, arguing that it was poorly sited in a vital migration corridor for several threatened species, including the desert tortoise. SDG&E has high hopes for the Imperial Valley solar project and other renewable energy facilities under development in the area that will generate power that will be shipped to customers in San Diego over a new transmission line the utility is building, Ramp said.

In December, SDG&E started construction on the $1.9 billion, 117-mile Sunrise Powerlink transmission line. When completed in 2012, the transmission line will be able to ship 1,000 megawatts of wind, solar and other power from the Imperial Valley to San Diego, enough to serve about 650,000 homes, SDG&E said.

California regulations require utilities to use solar, wind or other renewable power generation for one-third of the electricity they sell by 2020. The rules are part of the state’s 2006 plan to combat climate change.