GEOTHERMAL-SOLAR HYBRID IN HAWAII

Geothermal and Solar Power to Get Married in Hawaii; How do you make solar thermal technology economical at a small scale?
Michael Kanellos, April 7, 2010 (Greentech Media)

"In Hawaii…Pacific Light & Power will build a 10-megawatt solar thermal plant that will combine a trough solar collector from Spain's Albiasa with a turbine traditionally used in geothermal systems.

"…Ten megawatts is unusually small for a solar thermal field. BrightSource Energy, by contrast, wants to build [a 396-megawatt] one in California…Most solar thermal systems, however, collect heat from the sun to turn water into steam [to drive turbines so]…solar thermal parks can only be built economically in places like North Africa or Arizona where the sun shines almost every day of the year, lots of empty land exists, and humidity remains almost nonexistent…[A] few clouds can depress the power output."

Schematic of the hybrid process. (click to enlarge)

"Geothermal turbines swap water and steam for organic fluids like butane, which turn to vapor at lower temperatures. Thus, geothermal turbines require less heat, which in turn allows for smaller solar fields in a wider range of climates and geographies. Like traditional solar thermal systems, excess heat can be stored and run through the system in the evening or when cloud cover descends…[The technology] may not be as efficient but it can generate energy in a wider variety of circumstances.

"When completed in 2011, the plant -- located on the island of Kauai -- will provide close to seven percent of the power needed on the island."

Schematic of the hybrid process when there is cloud cover. (click to enlarge)

"Alibasa and PLP describe it as a hybrid plant, but it's more of an unusual concatenation. Generally, hybrid plants are power plants that combine renewable energy generation -- like solar thermal systems or biogas burners -- with gas turbines to provide more baseline-like power. Florida Power and Light and Abengoa are currently building hybrid plants.

"Power from the [solar-geothermal] plant will [cost nearly the same as Hawaii’s costly grid power]…Electric power in Hawaii costs around 25.78 cents a kilowatt hour, the highest rate in the U.S…Hawaii generates most of its power from diesel generators. But Albiasa will study ways to bring the cost down to make these systems feasible elsewhere."