New condensers for geothermal power

by H Brown, CADDET US National Team

Using ADCC technology, developed by the US Department of Energy's National Renewable Energy Laboratory (NREL), the Pacific Gas and Electric Company (PG&E) greatly improved the operation of one of its geothermal power plants at The Geysers, in California - the world's largest geothermal power complex. Installation of the technology in Unit No.11 at The Geysers improved power production efficiency by 5%, increased potential plant capacity by 17%, and cut chemical requirements for abatement of hydrogen sulphide emissions in half. Benefits such as these mean that refurbishing similar geothermal plants with ADCC will pay for itself within about two years.

New condenser technology improves performance for geothermal plants.

Background

Geothermal power is one of the largest sources of renewable energy today. It now provides around 7% of California's electricity. Worldwide, there are already more than 200 geothermal power plants, and major new geothermal developments are planned for Indonesia and the Philippines. With proper management - which ADCC helps provide - this is a renewable resource with minimal emission of pollutants and greenhouse gases. In addition, geothermal electricity generation is very efficient. Each unit of geothermal electricity displaces about three units of fossil-fuel energy combustion with its associated pollution, greenhouse gas emissions, and resource depletion.

Flowchart of the unit with ADCC technology.

ADCC technology

Condensation of spent steam is a key part of the power cycle in electricity generating plants. Direct-contact condensers mix cooling water with spent steam in an open chamber to condense the steam from an electric power plant rather than running the steam around sealed coolant pipes ("shell and tube" surface condensers). Most existing direct-contact condensers have only perforated plates inside the chamber to provide surface area for condensation. NREL's ADCC technology uses sophisticated, geometric shapes to provide optimal surface area for condensing spent steam. These packing structures channel the steam and water for maximum contact with each other, as well as providing surface area for condensation. A computer model evaluates the thermal performance of possible packing structures, allowing rapid identification of the optimal packing for a particular condenser and power plant. Importantly for geothermal plants, which have steam with a high content of reactive gases which cannot be condensed, the computer program also models chemical reactions in the spent steam and cooling water.

Condenser research was initiated during the 1980s as part of a programme for development of ocean thermal energy conversion (OTEC). Because OTEC is based on the smallest temperature differential of any thermal energy generation technology, it was critical to get the maximum performance from the condensation process. The researchers conducted extensive experiments and built analytical models to understand the key features of the condensation process and build the best direct-contact condenser possible.

Advanced direct contact condenser packing material installed in Unit 11 at The Geysers.

The market

The Geysers has a total of 8 units with direct-contact condensers that can be readily renovated with ADCC plus 6 units with surface condensers that can be retrofitted. Because of electric utility restructuring in California, PG&E is currently seeking to sell The Geysers complex, but any new owners can also benefit from ADCC, both for maintaining economic viability in the face of declining resources and for fully utilising regained capacity from planned wastewater injection. ADCC has been clearly demonstrated to be a practical, viable technology with economic benefits for geothermal power plants. It should apply equally well for many of the 200 existing geothermal plants around the world and for new ones such as those planned for the Philippines and Indonesia.

ADCC can also be used for fossil-fuel power plants and for food processing or any other industrial process that needs to condense steam. For the closed-cycle cooling systems of fossil-fuel plants, it is also necessary to use intermediate heat exchangers. This eliminates the need to shut plants down for condenser cleaning ­ a process that now costs US fossil steam power plants about $1 billion per year (where $ is the US dollar).

NREL has granted the Ecolaire Corporation of Easton, Pennsylvania, USA, an exclusive licence for geothermal power plants and is looking for licensees for other process applications.

For more information contact the CADDET US National Team in Golden, Colorado.

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