Offshore Wind Energy: A Viable Option for California?

In the future, offshore wind energy could provide 174–224% of California’s (CA) current electricity needs (Dvorak et al. 2010).  This estimate is based on the development of floating and other turbine tower support technologies that will enable the placement of turbines in deep water (50–200 m).  The advancement of these technologies is critical to the viability of offshore wind energy in CA since approximately 90% of wind resources are located in deep water.  Utilizing only existing technologies, for depths up to 50 m, the estimate decreases to wind energy providing 17–31% of the state’s electricity needs.  Compared to Southern (SCA) and Central CA (CCA), the Northern coast (NCA) has the greatest potential for immediate development.  NCA’s potential annual delivered energy by turbines at depths of 0–50 m, utilizing winds speeds ≥ 7.0 ms-1 is 63.1 terawatt hours (TWh).  This potential delivered energy would offset approximately 36% of CA’s current carbon electricity sources.  Although NCA has the most shallow water wind resources, it has limited transmission capacity compared to the other regions.  At the time of writing, the potential for offshore wind energy has not yet been developed in CA or elsewhere in the United States.—Juliet Archer

          Dvorak, M., Archer, C., Jacobson, M., 2010. California offshore wind energy potential. Renewable Energy 35, 1244–1254.

          M. J. Dvorak and his colleagues quantified CA’s potential for offshore wind energy by locating potential turbine sites using bathymetry data, modeling multiple years of mesoscale weather data and then calculating the potential energy and power provided by offshore turbines.  To give context, the CA coast was divided into three areas, NCA, CCA, and SCA.  Within these regions, potential sites were classified by depth using high-resolution bathymetry data.  To determine average offshore wind speed, a mesoscale model version 5 (MM5) weather model was run for all of 2007 and for the months of January, April, July, and October of 2005 and 2006.  This modeling allowed the calculation of annual and seasonal average wind speeds at turbine hub height (80 m) as well as the average power density of the wind resource.  The modeling data were validated using offshore weather buoy data from the National Oceanic and Atmospheric Administration (NOAA) National Data Buoy Center (NDBC) for years 1998–2008.  The MM5 data very closely matched the NDBC buoy data. 
          To estimate the energy production potential, the number of turbines that could be built and the potential production capacity of each site was calculated.  The REpower 5M, 5 MW wind turbine, requiring 0.442 km of area, was used for all calculations.  In calculating turbine density, the authors accounted for surface area that could not be utilized due to shipping lanes, wildlife areas, viewshed considerations, etc., by including a conservative 33% exclusionary factor.  The turbine capacity factor (CF) is defined as the ratio of actual output over a period of time and maximum output at nameplate capacity over that time.  It was calculated for each site using the relationship between average wind speed, rated power and rotor diameter of the REpower 5M turbine.  This calculation allowed annual energy and average power output to be calculated for each site.  In all calculations, winds were assumed to follow a Rayleigh probability distribution over time. 
          The results show that the potential for wind energy in CA is significant, but not currently feasible, in all regions.  The relatively shallow waters of NCA have the most potential using current turbine foundation technology.  The development of sites in CCA is limited because most resources exist far from San Francisco and in deep waters.  The Farallon Islands is one such site whose development is dependent on lengthy undersea transmission cables and a study of the environmental effects of wind turbines on nearby bird, marine mammal, and fish populations.  SCA has similar problems since the CA Bight shields the Los Angeles coast and sends most winds to sites 50 km or farther from shore.  These distant sites include Point Conception, San Miguel Island and Santa Rosa Island.  If technologies for deepwater resources are developed, then the combination of SCA’s high demand and many grid interconnection points will make it an ideal region for offshore wind energy development.
          A hypothetical, but currently feasible, wind farm near Cape Mendocino in NCA is proposed by the authors.  The farm would be located in water that is less than 50 m deep and therefore could utilize current monopole or multi-leg turbine foundations.  It would occupy about 138 km2 in area and contain 300 REpower 5M turbines.  The farm could be connected to the local electrical grid via an existing power plant in Humboldt Bay.  The authors predicted that it would be most productive in summer months and that its hourly activity would be consistent throughout daytime hours.  This represents a significant advantage over onshore wind farms which peak at night and thus do not match the high daytime summer demand.  Using the aforementioned exclusionary factor, the proposed farm could replace 4% of CA’s current carbon electricity generation.  This great potential to offset carbon energy sources suggests that offshore wind energy sites should be seriously considered in CA.   

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