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The Race for Geothermal

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Advanced Condenser Boosts Geothermal Power Plant Output

July 16th 2013

Geyser-Groto

Geothermal resources—the steam and water that lie below the earth’s surface—have the potential to supply vast amounts of clean energy. But continuing to produce geothermal power efficiently and inexpensively can require innovative adjustments to the technology used to process it.

Located in the Mayacamas Mountains of northern California, The Geysers is the world’s largest geothermal complex. Encompassing 45 square miles along the Sonoma and Lake County border, the complex harnesses natural steam reservoirs to create clean renewable energy that accounts for one-fifth of the green power produced in California.

In the late 1990s, the pressure of geothermal steam at The Geysers was falling, reducing the output of its power plants. NREL teamed with Pacific Gas and Electric (PG&E) under a cooperative research and development agreement to create a solution for boosting production efficiency at the complex.

To generate geothermal energy, power plants at The Geysers capture vapor and water from beneath the earth’s surface, directing it through steam turbines. These turbines drive generators, which produce electricity. Condensation of spent generator steam is a critical part of this power cycle, and in the 1990s, about half of The Geysers’ power plants relied on direct-contact condenser systems to process the steam. 

In NREL’s Advanced Direct-Contact Condenser (ADCC), spent steam flows downward through a cocurrent section and mixes with cool water released from above the structured packing layer. Most of the spent steam condenses on the innovative structured packing framework and drips down into a hot well to be pumped to cooling towers. Any steam that has not condensed is drawn upward through the countercurrent section, and noncondensible gases are pumped out for treatment. The ADCC achieves lower condenser pressures than conventional direct-contact condensers, boosting power plant output.

Direct-contact condensers mix cooling water with spent steam in an open chamber, typically relying on a series of perforated plates to provide surface area for condensation. The water and condensate mixture is pumped out to cooling towers to be recycled as circulating water, and noncondensible gases—including potential pollutants such as hydrogen sulfide—are removed. In Power Plant Unit 11 at The Geysers, standard direct-contact condenser technology had proven to be inefficient, consuming too much steam during the removal of noncondensible gases and creating high back pressures that decreased turbine performance.

Drawing upon previous condenser research related to ocean thermal energy conversion, NREL developed advanced direct-contact condenser (ADCC) technology to condense spent steam more effectively. ADCC systems replace traditional perforated plates with a sophisticated geometric framework resembling a three-dimensional maze. This framework—or structured packing—increases the surface area for interaction between cooling liquid and steam. Additionally, ADCC technology employs cocurrent and countercurrent flow to allow for maximum contact between the substances and to channel noncondensible gases more efficiently for removal.

After implementing ADCC technology, power production efficiency in Unit 11 improved by 5 percent—a phenomenal gain for an industry in which performance improvements are typically measured in mere fractions of a percentage. Potential generating capacity increased by nearly 17 percent, and the cost of hydrogen sulfide emission abatement was reduced by half.

In the course of developing its solution, NREL created a computer model that evaluates the thermal performance of possible packing structures for a particular condenser and power plant. In addition to helping geothermal plants determine optimal packing structures, the program models chemical interactions between cooling water and spent steam—an important development for units with high quantities of noncondensible gases.

Based on the overwhelming success of NREL’s solution at The Geysers, Ecolaire (formerly Alstom Energy Systems) licensed ADCC technology for use in its own geothermal power plants. Now a division within SPX Heat Transfer Inc., Ecolaire has deployed ADCC at numerous international locations, including sites in Mexico, Indonesia, and Turkey.

ADCC technology holds great promise for geothermal power plants seeking greater efficiency and lower operational costs. Many plants constructed after 1980 deployed surface condenser systems—in which vapor runs around sealed coolant pipes—to prevent the release of hydrogen sulfide into the atmosphere. ADCC systems control hydrogen sulfide emissions as effectively as surface condensers and expend much less energy doing so, driving down overall costs for condenser systems by half. And for new geothermal power plants, ADCC systems cost two-thirds less than traditional direct-contact condenser installations.

When combined with intermediate-plate heat exchangers, ADCC technology also offers a lower-cost alternative to the surface condenser systems used in fossil fuel power plants. NREL designed an ADCC system with modular heat exchangers that can be cleaned individually, without requiring power plant shutdowns. Sequential cleaning eliminates condenser downtime that costs the utility industry more than a billion dollars each year, according to Electric Power Research Institute estimates.

This article is adapted from an Innovation Publication from NREL.


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