| Abstract |
The Northern California Power Agency (NCPA), headquartered in Roseville, California, is a joint power agency formed in 1968 to provide its members with electrical energy purchasing aggregation, scheduling and management. The NCPA owns and operates a number of power plants in a 96% emission-free generation portfolio, including two geothermal plants at The Geysers, in the Mayacamas Mountains of California. NCPA’s geothermal plants went into operation in the mid-1980’s with nominal rating of 110 megawatts each produced by two steam turbine generator trains of 55 MW (nominal) each. The steam turbine generators are fed by 70 original deep production wells, along with 8 deep injection wells that began operation 12 years ago. Electricity output at The Geysers (from all concessions) reached a peak in the late 1980’s and, since then, the steam fields have been in gradual decline as the natural underground steam source decreases. The deep injection wells are used to re-supply the geothermal resource production wells with condensate from the turbines as well as waste water obtained through the Southeast Geysers Effluent Pipeline (SEGEP) of which NCPA is a co-owner. For more than two decades, the NCPA Geothermal Project has been a source of clean, renewable energy that has consistently provided the participating NCPA Members with a reliable supply of electricity at very competitive costs. However, in a similar manner to all of the geothermal assets operating at The Geysers, the NCPA Plants have experienced a significant decline in output over their operating life. Despite the benefits of the reinjection wells, which helped mitigate the naturally occurring decline in steam flow (stabilized via the SEGEP project noted above), the plant output declined by nearly fifty percent, to approximately 56 MW gross output (or 28 MW per turbine generator) due primarily to a significant decline in resource pressure. In 2009, NCPA, in conjunction with turbine designer and manufacturer, TurboCare, Inc., undertook a major development and redesign to retrofit one of the existing geothermal turbines (Unit 4). This retrofit addressed the significantly diminished steam conditions inherent in these ageing geothermal fields to restore output to near original design (an increase in output of nearly 100% above the unit’s last actual output). The retrofit consisted of replacing the original steam path components in Unit 4 with new technology and materials optimized for the lower steam resource conditions. The retrofit turbine, which achieved increased generating capacity at much reduced operating pressures and temperatures, used TurboCare’s MagmaTM geothermal steam-path turbine design. In addition to taking advantage of the higher efficiencies available from modern turbine blade and vane designs, the steam path “swallowing capacity” was increased to accept the entire steam flow available for both Unit 3 and 4. Two main steam inlets were added during the implementation outage to accommodate the increased steam volumetric flow while keeping inlet steam velocities within acceptable boundaries. Project implementation was completed in May 2010 with performance testing conducted in the ensuing weeks. The test results demonstrated that the retrofit achieved the new guaranteed capacity, while enabling Unit 3 to be permanently shutdown, eliminating the Unit 3 plant parasitic load. This not only optimized the net plant output available for dispatch to the grid, but eliminated all Unit 3 Operation and Maintenance (O&M) costs going forward. Of importance is the fact that the turbine casing and main steam valves could be adapted and retained for the application, ensuring that the existing civil works did not need to be modified, thus reducing the time required for the retrofit outage to a few weeks instead of months. The project has demonstrated that new and modern geothermal steam turbine technology can be successfully and economically adapted for use in older geothermal units to extend life, recover lost power generation and enhance ageing asset values. |