Moss Landing Power Plant Post-modernization Thermal Plume Evaluation
- Jeff Paduan
Navy Postgraduate School
End Date: January 01, 2004
A thermal plume evaluation study, which was designed to measure the distribution of heated waters discharged by the modernized Moss Landing Power Plant (MLPP), was conducted in compliance with Central Coast Regional Water Quality Control Board (RWQCB) permit and California Energy Commission (CEC) certification requirements.
Duke Energy Moss Landing, LLC recently completed re-powering and modernizing the Moss Landing Power Plant (MLPP). The modernized facility is located within the existing coastal site, 12 miles northwest of Salinas, CA in Monterey County near the Moss Landing Harbor. The surrounding area includes industrial facilities, agricultural lands, sparse residences, beaches, and tidal wetlands. Within the facility, two combined-cycle combustion turbine generators, each with a capacity of 530-megawatts (MW), have been installed. In addition, the project included steam turbine rotor upgrades of two existing units (6 and 7), which produce an additional 15 MW per unit. The total generating capacity of the upgraded MLPP is 2,538 MW.
Cooling water for the modernized facility is, as it always has been, drawn from intake structures within the harbor area and discharged to Monterey Bay. The cooling water intake for the new combined cycle generators utilizes the previously existing seawater intake structure for retired units 1-5. The new units share the ocean discharge structures for existing units 6 and 7. The maximum cooling water flow rate is approximately 250,000 gallons per minute (gpm) for the two new combined-cycle units and 600,000 gpm for Units 6 and 7. Under peak power production, the project results in the discharge of an estimated 850,000 gpm, which can be compared with recent pre-modernization discharge rates around 532,000 gpm for discharge to Monterey Bay.
The maximum heat loading for the modernized facility increased about 41% over present conditions to 182 million BTU/min, although the addition of the new units' cooling water to the existing discharge lowered the maximum temperature by about 2.4° F. A detailed listing of historic and post-modernization flow rates, heat loads, cooling water temperature increases, and generation capacities can be found in the Duke Energy report entitled “Moss Landing Power Plant Modernization Project: Evaluation of the Proposed Discharge System with Respect to the Thermal Plan” (April 28, 2000). This information is also part of the extensive materials submitted to the California Energy Commission (CEC) and the Central Coast Regional Water Quality Control Board (RWQCB) as part of the permit applications for the modernization
Summary to DateThe study found that the thermal discharge resulting from operation of MLPP is manifest as a surface-intensified plume over the discharge point in Monterey Bay. Strong mixing with ambient ocean waters reduces the maximum temperature elevations to less than several degrees Fahrenheit above ambient within a few hundred feet of the discharge location. Observations throughout the study period (June-October 2002) show the thermal plume distribution to be biased toward the inshore side of the discharge with a strong thermal signature from the flow moving out of Elkhorn Slough typically found offshore of the discharge. Throughout the study period, both thermal plumes were observed to elongate preferentially toward the south.
The tidally driven flow into and out of Elkhorn Slough dominates the fluctuations of ambient temperatures in the immediate vicinity of the MLPP thermal discharge. Temperature fluctuations are predominantly semidiurnal (i.e. twice daily) in response to the tidal forcing. Because the inshore waters in the shallow areas of Elkhorn Slough are typically several degrees warmer than the ocean waters further offshore, this tidal pumping creates natural temperature variations in the region that are comparable to temperature elevations created within the MLPP thermal plume. Despite the dominance of natural tidal exchange processes in this area, thermal output from the MLPP discharge does affect the waters around the discharge, particularly those waters between the discharge and the shoreline. Subsurface temperature sampling conducted during boat surveys indicates that the MLPP thermal plume exceeds 8 feet in thickness on many occasions, although sensors placed 2 feet above the bottom indicate that actual contact with the benthic habitat away from the surf zone is rare. Images and maps from boat and aircraft surveys strongly suggest that the MLPP thermal plume was contained primarily inshore of the discharge during the study period. Measurements made offshore and north of the discharge were dominated by water flowing out of Elkhorn Slough.
Continuous estimates of the horizontal temperature gradients were made using observations from a buoy array, which made it possible to estimate the surface temperatures at shoreline contact points surrounding the MLPP thermal discharge. Temperature anomalies created at the shoreline by the presence of the MLPP thermal discharge were estimated relative to one of the buoy observing locations and relative to adjusted MLPP intake temperatures. When the distributions of temperature anomalies are compared to normal distributions with the same variance, evidence of the positive bias imparted by the presence of the MLPP thermal plume can be seen. Coastal sites beginning with the south breakwater and continuing southward along the coast to the location of the former Sandholdt Pier all show clearly skewed temperature anomaly distributions with a preponderance of temperature anomalies in the range of 1–4 degrees Fahrenheit under full, or close to full, MLPP discharge heat loading conditions. There is no statistical evidence, however, for MLPP-generated temperature anomalies in excess of 4 degrees Fahrenheit at these coastal sites.
In summary, the MLPP thermal plume is clearly detectable along the shoreline just inshore of the MLPP thermal discharge. At the surface, it most commonly occupies the region between the discharge and the edge of the surf zone and often extends more than 2,000 feet southward along the coast as seen in IR images of the region. Below the surface, a more limited-area core plume is detectable between the surface and 10 feet below the surface. In all cases, including full heat loading conditions, the detectable edges of the MLPP thermal plume represent positive temperature anomalies above ambient levels of 1–4 degrees Fahrenheit, with anomalies greater than 4 degrees Fahrenheit at the shoreline or at 1,000 feet from the discharge being rare, certainly less than 50 percent of the duration of any complete tidal cycle.