Exploring the Reliability and Potential Climatic Impacts of Large-Scale Deployments of Offshore Wind Turbines

Wind power is widely available renewable energy source, but a large-scale deployment of several million wind turbines would be required to meet the estimated global energy demand in 2100 of 44 TW. Wang and Prinn sought to examine the environmental impacts and reliability of such an extensive use of offshore wind power (2011). The researchers had previously conducted a study to examine the effects of a similar large-scale deployment of wind turbines over land to meet about 10% of the predicted global energy needs in 2100. Their model suggested that such a significant use of wind turbines over land could lead to a significant temperature increase in the lower atmosphere over the installed regions. This model also predicted a significant disturbance in air circulation patterns as well as cloud and precipitation distribution. Unlike the previously modeled land-based wind turbine installations, the offshore wind turbine installations were found to cause a surface cooling over the installed regions. The disturbance to the global climate caused by offshore wind installations was calculated to be relatively small when compared to land-based installations. Yet there are significant concerns about the intermittent nature of power generation from offshore wind turbines caused by seasonal wind variations, so the operation of such a substantial offshore wind would demand significant measures taken on the part of system operators to cope with this variability. —Meredith Reisfield
Wang, C., Prinn, R. G., 2011. Potential climatic impacts and reliability of large-scale offshore wind farms. Environmental Research Letters. doi: 10.1088/1748-9326/6/2/025101.

Wang and Prinn, working at the Center for Global Change Science and the Joint Program of the Science and Policy of Global Change at the Massachusetts Institute of Technology, sought to compare the effects of deploying wind turbines over semi-arid grasslands to the effects of large-scale offshore wind installations. The land-based model found that using wind turbines on this scale could cause global surface warming exceeding 1 K over designated surface areas and alter global distributions of rainfall and clouds. To model the effects of large-scale deployment of offshore wind turbines, the researchers used the Community Atmospheric Model of the Community Climate System Model developed by the US National Center for Atmospheric Research. This model was coupled with a slab ocean model and the Community Land Model to simulate the long-term climate responses to large-scale offshore wind turbine use.
This three-dimensional climate model presented several advantages including a high spatial resolution of 2° by 2.5° along the latitudinal and longitudinal directions respectively, 26 vertical layers of atmospherics modeling, coverage of a large range of geographic areas, and consideration of multiple assumed strengths of wind turbine effects. To simulate the climate effect of offshore wind farms, Wang and Prinn modified the surface drag coefficient to represent the turbine-induced change to sea surface roughness.
The researchers conducted six simulations of offshore wind turbine effects. The wind turbines in each simulation were installed over regions between 60°S and 74°N in latitude, at depths of 200, 400, or 600 m and an assumed sea surface drag coefficient of either 0.007 or 0.001. The turbines were simulated to be in five regions free from sea ice that are likely to become actual offshore turbine installation sites, including the Southeast and East Asian coasts, the North American coast, the West European coast, the South American coast and Oceania. The higher drag coefficient was based on reported measurements over mesoscale wind farms, while the lower coefficient approximately doubles the average sea surface drag coeffient in areas without wind turbines. Each model was set up to run 60 years and take about 40 years to reach an annually repeating climatic steady state. These simulations utilized present day greenhouse gas levels to isolate the climatic effects of wind turbines from effects due to greenhouse gasses. The results compared the mean parameters of the last 20 years of each model used in analysis. Power gain from mean flow kinetic energy due to wind turbines was calculated by using the models with and without wind turbines, then subtracting. Raw wind power consumption increases proportionately with installation area and the sea surface drag coefficient assumed in the model. The researchers assumed a 25% conversion rate of raw wind power converted to electric power by wind turbines. The estimated output of electric power ranged from 6.8 to 11.9 TW with the higher drag coefficient, and from 1.7 to 3.1 TW with the lower drag coefficient. At most, these numbers would account for 25% of the predicted 44 TW of future global energy needs.
The climatic impacts of the simulated installations were significant. The surface air temperature over tropical and mid-latitude sites were reduced by nearly 1 K, with even greater cooling observed in the Arctic region and a slight warming in Antarctica. The cooling was due principally to enhanced latent heat flux from the sea surface to the lower atmosphere, driven by an increased turbulent mixing caused by the wind turbines, and extended vertically into the lower and middle troposphere through mixing. The annual averaged surface air temperature ranged from 0.4 to 0.6 K in models with a high surface drag coefficient and was about 0.2 K in the low drag coefficient cases. Cooling was also shown to be greater as a smaller fraction of the turbines were installed in tropical latitudes. Due to changes in the patterns of geographical locations of installed regions as turbines were modeled to be in depths of 200, 400, and 600 m. Though these turbines also create impacts on clouds, temperatures, precipitation and air circulation beyond the installation sites, the impacts were less significant then those observed with a similar deployment of turbines over land.
The large deployment of offshore wind turbines also presents concerns about intermittency and reliability, as well as the need to lower the high current unit wind power costs. Intermittency is especially serious over European coastal sites, where the potentially harvested with power could vary by a factor of 3 seasonally, and was generally found to be greater than a factor of 2 . The inconsistency of offshore wind as a power source would require solutions such as on-site energy storage, backup generation and long-distance power transmission for an electrical system dominated by offshore wind power.

Diversifying Variable Renewable Energy Sources to Reduce Utility Reserve Requirements

Sources of renewable energy, including solar, offshore wind, and ocean wave technologies, offer significant advantages including no fuel costs and no emissions from generation. However, the renewable and nondispatchable nature of these technologies severely impacts grid reserve requirements. Like many areas in the U.S., the Pacific Northwest is rapidly expanding its wind power resources. An additional 5000 MW of offshore wind power is expected to come online in this area in the next five years. This trend in renewable energy resource development presents significant problems for system operators. The variability of wind resources can create a need for greater ramp-up rates, interhour variability, and scheduling errors in conventional power plants. These factors combine to increase the amount of energy generation capacity the system operators must hold in reserve to prevent rolling blackouts and energy shortages. Halamay and colleagues (2011) analyzed the interaction of variations in utility load, wind power generation, solar power generation, and ocean wave power generation. Their research suggests that a diversified portfolio of energy resources can reduce the effects of variability and decrease utility reserve requirements.  —Meredith Reisfield
Halamay, D.A., Brekken, T. K. A., Simmons, A., McArthur, S, 2011. Reserve requirement impacts of large-scale integration of wind, solar and ocean wave power generation. IEEE Transaction on Sustainable Energy 2, 321–328.

Halamay and his colleagues analyzed the effects of offshore wind, solar and ocean wave renewable energy sources on reserve requirements for the Pacific Northwest. The output of each of these renewable power sources varies over time. While the variation is typically small, the output of a large plant can occasionally go from full output to low production or vice versa over the course of several hours. System operators also have limited control over renewable power generation, so in this analysis the researchers chose to subtract the contribution of renewable energy sources from the total load. The researchers hypothesized that a diversified renewable energy portfolio would enable a greater penetration rate than just one predominant renewable energy source. Penetration is the ratio of the peak load within the year to the peak generation within the year. In each of these scenarios is greater than or equal to penetration by solar and wave energy.
The researchers calculated the energy reserve requirements for six scenarios; no renewable energy; 15% wind power penetration; 10% wind and 5% solar penetration; 10% wind and 5% wave penetration; 10% wind, 2.5% solar, and 2.5% wave penetration; and 5% wind, 5% solar, and 5% wave penetration. The second scenario, 15% wind generation, most closely reflects the current energy portfolio in the area studies, which has 14% wind penetration. The researchers studied the area within the Bonneville Power Administration (BPA) Balancing Authority Area (BAA). Wind generation and load data were freely available from the BPA. Wind power data were collected from approximately 1600 MW of wind under the BPA BAA, which was scaled as necessary to model the desired penetration rate. Irradiance data were gathered from 10 different locations in the Pacific Northwest to calculate potential for solar power generation, with the assumption that each location hosted 50% photovoltaic and 50% concentrating solar sources. The data were combined, weighting each site equally, and scaled to model the desired power generated for each particular scenario. Buoys measuring wave height at three locations were used to calculate theoretical ocean wave power outputs. The data were also combined and scaled as necessary. All data and analysis focused on the 2008 calendar year and used 10-minute sample times.
The researchers also used three different time scales to describe power reserve requirements. The first, regulation, examined the difference in small changes in power that can be readily met through Automatic Generation Control (AGC) via spinning reserves. The following time scale, defined as the difference between hourly power generation and 10-minute average power load describes larger changes in power demand and supply. The imbalance time scale describes the accuracy of forecasted power generation by comparing hourly forecasted power generation with hourly average power generation. Imbalance components of reserve requirements have grown significantly with increased use of wind power and are predicted to continue to grow rapidly. The 2008 load was forecasted using historical data from 2007 as a baseline, with additional correction terms added to account for load-growth from one year to the next and smooth transitions between monthly averages to prevent discontinuities. The scenarios with diversified renewable energy sources showed improvement over use of wind alone.
Halamay and his colleagues demonstrated the adverse affect of wind power on reserve requirements. These results suggest the need for an integrated approach to develop renewable energy sources. This analysis did not include tidal energy conversion, harvesting energy from tidal flows to generate power, despite the strong tidal resource in populous areas such as Puget Sound. 

Oceanographic Parameters to Explore the Environmental Impacts of OTEC Installations

Before field trials of ocean thermal technology (OTEC) become operational, researchers need a solid understanding of the environmental impacts of these installations. Due to the large thermal gradient and irregular bathymetry off the coast of Hawaii, the archipelago has several potential OTEC sites in the works. A pilot plant is under construction south of Barber’s Point, Oahu, and a commercial plant may be constructed off of Kahe Point, Oahu. A Final Environmental Impact Statement was conducted in 1981 for the Barber’s Point site, but this report needs to be brought up to current oceanographic and engineering standards. Comfort and Vega (2011) suggest a protocol for environmental baseline monitoring, which focuses on ten chemical oceanographic parameters, and addresses existing gaps in knowledge of ecology and oceanography near the two OTEC sites. In the operation of an OTEC plant, seawater intake pipes draw warm water from a depth of 20 m and cold water from a depth of approximately 1000 m. The water masses are mixed and discharged at 60 m or deeper. An environmental impact analysis can help to determine the optimal mixed seawater discharge level. —Meredith Reisfield
Comfort CM, Vega L. 2011. Environmental Assessment of Ocean Thermal Energy Conversion in Hawaii. Hawaii National Marine Renewable Energy Center, Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, HI, p 1-8.

During the operation of an OTEC plant, large water masses are redistributed. A 5 MW OTEC plant requires 25 m3/s of both cold and warm water flow. The daily flow tops 2 million cubic meters of water. The redistribution of large volumes of water could significantly impact an ecosystem, affecting primary production, nutrient concentrations, and densities of larval fish and other plankton. Comfort and Vega propose using existing data sets as baseline environmental information for OTEC. They recommend an additional year-long, directed, baseline monitoring program to address gaps in existing knowledge. Fortunately, as researchers at the Hawaii Natural Energy Institute acknowledge, significant data are already available to describe current circulation, and oceanographic parameters such as temperature, salinity, nutrients, and primary production off the coast of Oahu. The OTEC plume’s trajectory has also been modeled. OTEC operation raises several concerns about biological impacts. The redistribution of water on a large scale will affect the temperature stratification, salinity, oxygen and nutrient levels near the site. A primary concern in OTEC installations is the potential for upwelled nutrients to fertilize surface waters and prompt phytoplankton blooms. To avoid altering the primary productivity in surrounding waters, it is crucial that the plumes discharged from OTEC facilities settle at a sufficiently low depth so that the potential for functional biomass increase is reduced. Small organisms, including plankton and fish larva, can easily be trapped in the intake pipes with high mortality due to sudden and significant temperature and pressure changes. Many organisms migrate vertically throughout the water column on a daily basis, so understanding which organisms may be entrapped requires further knowledge of the ecosystem. Since floating objects in the ocean tend to accumulate large groups of fish and seabirds, larger organisms are likely to interact with the OTEC installations.  These organisms are less likely to be entrained in the system due to their larger size and swimming abilities which will allow them to easily manage the current flow. Vibration of the deep water pipe will create a signal that could be detected by marine mammals and fish, creating a risk of disruptions in marine mammal communication and navigation. The researchers propose additional monitoring of seasonal oceanographic parameters at relevant locations, further plankton sampling across multiple depths and time periods, and acoustical monitoring at the installation site to quantify baseline noise levels both before and after installation of the OTEC facility. Comfort and Vega note that given the wide availability of current data, gaps in knowledge could be quickly and efficiently addressed with one year of directed baseline monitoring. Studying the effects of an OTEC facility in operation with sufficient baseline data, rather than simply modeling these outcomes, could ensure that commercial OTEC plants have a minimal environmental impact. 

Mapping the Global Potential of Ocean Thermal Energy Conversion

A global transition to sustainable energy infrastructure will require long-lead-time research and development of myriad technologies such as ocean thermal energy conversion (OTEC). Although OTEC plants are capital-intensive installations, new technology is reducing the cost of investing in this technology. OTEC exploits the temperature difference across the thermocline to produce electricity. To explore the potential of the oceanic thermocline as a renewable energy resource on a global scale, researchers used a Geographic Information System (GIS) to map a detailed model of global climatology of oceanic stratification (Nagurny et al.2011). Nagurny and colleagues employed this information, coupled with statistics for an OTEC plant model, to estimate OTEC potential at locations worldwide, to understand the distribution of these resources, and to pinpoint locations with high potential for utilizing OTEC. This visual system enabled the researchers to estimate the potential renewable energy available in the ocean’s thermocline at any given location using a model of an OTEC installation. This model is an improvement over past thermocline studies; it yielded a worldwide distribution of power on a 1/12th degree grid across multiple time periods. It was obtained from a baseline single stage Rankine cycle design and can be simply adjusted to employ different plant designs. —Meredith Reisfield
Nagurny, J., Martel, L., Jansen, E., Plump, A., Gray-Hann, P., Heimiller, D., Rauchenstien, L.T., Hanson, H.P., 2011. Modeling global ocean thermal energy resources. Oceans, 2011, 1-7.

Nagurny and colleagues at Lockheed Martin and the National Renewable Energy Lab mapped global climatology model of ocean thermoclines based on open-source data from the Naval Research Laboratory’s (NRL’s) Hybrid Coordinate Ocean Model (HYCOM). The data for this model were gridded at 1/12th degree latitude and longitude using GIS, which improved spatial resolution over previous models. A closed cycle OTEC plant model developed by Lockheed Martin was applied to this GIS map. The nominal OTEC plant design produces 150 MW gross (100 MW net power) of electricity. The researchers chose this design because sufficient data about the plant was available, the size of the plant is feasible with existing technology, and this design produces enough power to have the potential to be economically reasonable in the near future. To calculate net power, gross power, and fixed and variable losses the researchers examined aspects accounting for major contributing and loss factors to OTEC power production. They focused on three groups of factors: gross power, cold water pipe pumping cost, all other pumping and transmission power costs. Gross power was calculated using established thermodynamic equations of a Rankine cycle. The model was formulated so that the gross power and variable loss factors were the variables that depended on oceanic conditions. Nagurny and colleagues added an algorithm that optimized the depth of the cold water source, relaxing the assumption that successfully installing an OTEC power plant would require 1-km-deep cold water. Previous global assessment of potential resources were limited by the assumption that sufficiently cold water lay 1000 m below the ocean’s surface and left out shallower regions with potential OTEC resources. The researchers chose the most shallow depth out of the bottom water, the water at a level where the temperature gradient of 3°C per km balances production and loss, and water at 1000 m (for consistency with previous studies).
The net power potential worldwide varied from 0 to 197 MWe. Large regions offer potential for net power production. Large areas of the Pacific, Atlantic, and Indian Oceans can supply 100 MWe or greater from the nominal design. Areas in the Philippines and off New Guinea where found to have the greatest potential for OTEC, with upwards of 190 MWe obtained from the 100MWe plant conditions. The higher resolution of this model and the inclusion of shallower cold water availability allowed the mapping of the Gulf Stream’s July thermocline off the U.S. east coast, the western Equatorial Atlantic, and the Central Pacific near and to the northeast of Hawaii. The study was limited by a lack of data concerning ocean currents at depth, which are needed to replace the cold water in OTEC for the power production to be sustainable, and climate change data affecting the solar heating of surface water. 

Simulating Marine-Hydro-Kinetic Energy Generation Using SNL-EFDC

The world’s growing power usage, coupled with the rapid depletion of fossil fuels and increased atmospheric CO2concentrations increase the demand for renewable energy solutions. Marine hydrokinetic (MHK) energy has potential as an effective and efficient source of renewable power, but a full evaluation of ecological and economic considerations should be undertaken before installing MHK technology. Researchers at Sandia National Laboratories recognized the need for knowledge of near and far field hydrodynamics in exploring the optimal placement of MHK technologies (James et al. 2011). They simulated an experimental water flume, which was calibrated against the results of a previous flume experiment at the University of Southampton. They studied the impacts of MHK energy generation on factors such as water flow, sediment dynamics, and water quality using SNL-EFDC. Their model successfully imitated the original flume studied at Southampton, suggesting is can be used to perform MHK-array site optimization studies. —Meredith Reisfield
James, S.C., Barco, J.Johnson, E.Roberts, J.D.Lefantzi, S., 2011. Verifying marine-hydro-kinetic energy generation simulations using SNL-EFDC. Oceans, 1-9.

Sources of marine hydrokinetic power, such as water turbines and wave energy converters, can be implemented at a reasonable cost, with higher predictability and decreased intrusion on the environment than many traditional power systems. MHK technology has not been implemented on a wide scale, so there is a high availability and low exploitation of available sites. Many of these potential sites are located near population centers, which can help to keep transmission costs at a minimum. James and his colleagues stressed the importance of economic and environmental cost-benefit analyses before installing MHK technologies, and designed their model in the hopes of enabling these types of examinations. Further research will be needed to understand and quantify the environmental impacts of MHK devices. MHK devices remove energy from marine systems, so they can impact volumetric flows, tidal ranges and sediment dynamics. Changes in water circulation patterns can change, altering flushing rates and the concentration of nutrients and dissolved gasses in a marine ecosystem, possibly altering algae growth. Changes in sediment dynamics can alter patterns of deposition and erosion, as well as the size and composition of eroded particles. Additionally, acoustic energy and electromagnetic waves generated by MHK devices could disturb wildlife. Taken together, these factors may interfere with biological activities such as migration, life cycles, communication, and resource availability. Conversely, an MHK installation could benefit an ecosystem by creating a reef-like presence that could provide shelter and food resources, or by causing an area to be designated a marine sanctuary, thereby reducing other intrusive effects. The authors also discuss the importance of evaluating economies of scale associated with the diminishing levels of return of installing additional turbine at an MHK installation.
            James and his colleagues applied SNL-EFDC, an upgraded version of the US Environmental Protection Agency’s environmental flow and transport code, Environmental Fluid Dynamics Code (EFDC) enhanced with the US Army Corp of Engineers’ water quality code, and a sediment dynamics code developed by researchers at the University of California, Santa Barbara. EFDC is an open-source 3D flow and transport code that directly combines sediment transport and water quality calculations. The various components of the model had also been used successfully at numerous sites. The module considered energy removal by MHK devices with changes in turbulent kinetic energy and the turbulent kinetic energy dissipation rate, in addition to being able to map levels of dissolved CO2 and O2, algae, and components of carbon, nitrogen, phosphorous and silica. The original University of Southampton study the authors reference measured the flow field in the wake of a tidal current turbine in the circulating water channel at Boulogne-sur-Mer in France. An SNL-EFDC was built to simulate this experiment. An MHK device and support structure was built to represent the turbine used in this experiment. James and his colleagues ensured that the difference in total energy, or kinetic and potential energy, upstream and downstream from the device was equal to the energy converted by the MHK plus frictional losses to the MHK support structure, so that the flume sidewalls could be specified as frictionless. They compared their model output to the wake data from the original flume in France, and found they had successfully simulated the original experiment. This model could perform simulations with the ultimate goal of optimizing design and minimizing environmental damage. Their model does have some limitations. SNL-EFDC is designed for macro-scale systems and cannot simulate a fine level of detail. However, the model could be very appropriate for systems with large amounts of turbulence. 

Hybrid Wind-Tidal System Holds Potential to Guarantee Continuous Availability of Grid Power

Offshore wind power is subject to short-term fluctuations, limiting the potential for this technology to serve as a source of continuously available grid power. Scientists at the Graduate School of Energy Science in Kyoto have suggested a hybrid system that combines an offshore wind turbine with a corresponding tidal turbine to make offshore power available to the grid at a constant level (Rahman et al.2011). In the proposed system, tidal power is used to balance the variations in the load of offshore wind power by operating a flywheel motor/generator system. When wind power exceeds a specified level, the tidal system functions as a motor to store surplus power as rotational energy. When wind power falls below a certain level, the tidal system works as a generator to complement the wind power and counter large fluctuations in wind power that can affect the frequency and voltage of output. The hybrid system could enable the development and utilization of offshore renewable energy sources by proposing new load fluctuation control strategies. A laboratory performance analysis favorably evaluated the feasibility of this system. —Meredith Reisfield
Rahman, M., Shunsuke, O., Shirai, Y., 2011. Hybrid offshore wind and tidal turbine power system to compensate for fluctuation (HOTCF). Green Energy and Technology. doi: 10.1007/978-4-431-53910-0_24.

The proposed system utilizes two types of power generation, the tidal motor/generator and the offshore wind turbine generator. While the tidal generator creates smooth output power, the output power of a wind turbine is directly dependent on wind velocity. Rahman and his colleagues built a laboratory scale prototype model of the hybrid system.
The offshore wind turbine generator component of this hybrid system consists of a coreless synchronous generator and a servo-motor. Servo-motors can be combined with encoders to provide an information feedback about position and speed and continuously correct performance. The servo-motor, controlled by a computer, simulated the rotation of an offshore wind turbine. The rpm (rotations per minute) of the motor determined the generation of electrical energy. A 6-pulse diode rectifier converts the AC power generated by the wind turbine to DC power. The tidal turbine component of the systems seeks to apply and control a two way energy flow scheme, so that energy can either be injected into the offshore wind turbine or stored as kinetic energy as wind power fluctuations demand. The tidal component combines a servo-motor with a generator/motor. The servo-motor serves as an input of tidal energy to the generator, which converts the mechanical energy from tides into electrical energy. The tidal turbine induction output is connected to a DC capacitator through a dual way converter. The researchers also placed several small controllers at both ends of the system to monitor operating conditions.
Tests of the system found that the tidal system turned to generator mode was successfully able to compensate for variations in wind generator output. Conversely, the tidal system could store rotational energy as a flywheel with small losses. The main challenge facing this model is to reduce the delay in recovering grid power to initial value after a drop in wind power generation, which suggests the control flexibility and overall stability of the system can be improved. This framework can produce a relatively stable power output when connected to a commercial grid, avoiding the inherent power fluctuations of traditional offshore wind technologies. The hybrid design makes the system stable, adaptable and easily scalable. Wind and tidal resources can complement each other to general large amounts of power in an economically feasible manner. Successful evaluation of load demands and resource forecasting could make the hybrid system method a successful technique for converting tidal energy and wind energy into electricity. 

The Seaweed Solution: A New Source of Ethanol

Biofuels represent a significant potential sustainable energy source, yet land-based biofuel crops pose land use challenges, using large stretches of arable land that could otherwise be utilized to produce food. Scientists are now turning to an ocean-based alternative: seaweed. Unlike other raw plant materials, brown seaweed does not contain a lignin structure. Biodegradation of lignin is a requirement for processing biofuel from raw plant materials. The sugars in brown seaweed can be released by simply milling or crushing the macroalgae, eliminating the need for a pretreatment and hydrolysis process before fermentation and lowering the potnential cost of bioconversion of the raw plant material into ethanol. Brown seaweed contains four types of sugars—alginate, laminarin, mannitol, and cellulose. Of these, alginate is most abundant. Previous attempts to convert macroalgae into biofuels were limited by the availability of microorganisms that could metabolize alginate polysaccharides. Wargacki et al. (2012) have engineered a microbial platform for direct ethanol synthesis of brown seaweed. The newly designed microbial platform can simultaneously degrade, uptake, and metabolize alginate to produce ethanol. The strain could potentially be engineered to produce a wide range of chemical and fuels. This project is a significant step in realizing production of renewable fuels from sustainable and scalable biomass sources. —Meredith Reisfield

Wargacki, A., Leonard, E., Win, M., Regitsky, D., Santos, C., Kim, P., Cooper, S., Raisner, R., Herman, A., Sivitz, A., Lakshmanaswamy, A., Kashiyama, Y., Baker, D., Yoshikuni, Y., 2012. An Engineered Microbial Platform for Direct Biofuel Production from Brown Macroalgae. Science 20, 308–313.

Wargacki et al. Engineered the bacterium Escherichia coli to digest brown seaweed and ferment ethanol. They isolated the genes of a marine microbe, Vibrio splendidus, that were responsible for breaking down alginate into simple sugars like pyruvate, and inserted the genes into an E. coli strain. This modification of E. coli allowed the bacterium to take up alginate oligomers which carried a stretch of DNA bearing genes for alginate-degrading enzymes. The strain was further engineered to convert sugars into ethanol and enable a single-step manufacture of ethanol from brown seaweed. The researchers added a chemical pathway borrowed from Zymomonas mobilis, a bacterium isolated from cane juice, to turn the pyruvate into ethanol.

When the new strain was fed crushed kombu (a common brown seaweed), the cells fermented a concentration of 5% ethanol, which is comparable to the benchmark for bioconversion of woody biomass. The U.S. Department of Energy reported a macroalgae productivity of 59 dry metric tons/ha/year, and an ideal ethanol yield from macroalgae of 0.254 weight ethanol / weight dry macroalgae. These numbers estimate an peak bio-ethanol productivity of 19,000 liters/ha/year, approximately twice the ethanol productivity from sugarcane and 5 times the ethanol productivity from corn. Initial evaluations of the E. coli platform in ethanol production achieved over 80% of the maximum theoretical yield of ethanol from sugars in seaweed. The engineered strain successfully processed alginate and glucose in addition to increasing the rate of mannitol fermentation. These results support the team’s hypothesis that alginate pathways play a significant role in balancing intracellular redox reactions by consuming excess-reducing equivalents generation in the fermentation of mannitol.

Brown seaweed seems to be an ideal source for the production of biofuels since it avoids the economic concerns associated with land management and the adverse effect on global food supplies, eliminating the “food versus fuel” concerns presented by the production of ethanol using sugar cane and corn. A few barriers remain to be conquered before we can take advantage of biofuels from beneath the waves. Seaweeds are currently harvested for use in human consumption, animal feeds, and agricultural fertilizers and are not currently grown at a sufficient scale for wide use as fuel. Although brown seaweed holds several advantages over currently existing methods to produce ethanol via microbial fermentation of biomass, the question remains whether seaweed can be produced at a scale that would have a significant impact on the global energy economy. The platform engineered by Wargacki, et al. is a significant step toward scalable and diverse feedstocks that would enable sustainable use of biomass technologies.

Key Requirements for Installing an Ocean Thermal Energy Conversion (OTEC) Plant

Ocean Thermal Energy Conversion (OTEC) is recognized as a significant potential renewable energy resource for coastal communities, yet OTEC plants can only be constructed on sites that meet key logistical and technical requirements. Finding a suitable candidate site requires extensive study of the atmosphere, ocean and seabed environment surrounding the location where the plant will be installed. McCallister et al. (2010) studied the specific aspects of a candidate site for installation of an OTEC pilot plant off Pearl Harbor, O’ahu, Hawai’i. Using geophysical information and oceanographic data, the authors were able to conduct a preliminary survey of the installation area and identify potential risks and hazards at the installation site. The Pearl Harbor site was found to meet the key characteristics of a preliminary study, including a sufficient temperature differential (more than 20° C difference from deep water to cool water) and a favorable seafloor environment for anchoring the physical system. The site also presents low seismic and tsunami risk, moderate risk of mass sediment movement, and a significant long-term risk presented by tropical cyclones. Additionally, the authors explored the data collection and physical characteristics required to undertake installation of an OTEC system. –Meredith Reisfield

McCallister, M., Switzer, T., Arnold, F., Ericksen, T., 2010 .Geophysical and oceanographic site survey

requirements for ocean thermal energy conversion (OTEC) installations. Oceans 2010, 1–10, 20–23.

OTEC works by utilizing the difference in temperature between deep water and surface water to drive turbines and generate electricity. A fluid with a low boiling point, typically ammonia, is circulated through a network of pipes. The fluid is vaporized by a heat exchanger warmed by surface water, and the resulting gas is used to spin a turbine and generate electricity. The gas is then cooled by a second heat exchanger and cooled by seawater pumped up from lower depths. It condenses into a liquid and the process is repeated. Pearl Harbor is the site of a ten-megawatt OTEC pilot plant that should be operational by 2015.

McCallister et al. obtained the geophysical data required to install an OTEC system through high-resolution sonar and video imaging. The O’ahu site was found to share common characteristics of favorable OTEC locations, including a steep slope, irregular bathymetry (depth of ocean floors), and uncertain sediment thickness. Sediment sampling revealed significant sediment thickness, suggesting that the O’ahu site could support the multiple anchoring locations required to secure the physical system in place. Additionally, an examination of the primary risks to facility found the highest potential risks to be from major caldera collapse slides, which would cause the anchors to destabilize. There is also the prospect of tropical cyclones damaging equipment.

McCallister et al. also discuss the need for extensive knowledge of physical and chemical seawater properties, recorded in situ over time to observe seasonal and local variations in temperature and current structure. Nutrient levels must be monitored at a variety of depths. The introduction of nutrient-rich deep waters into the nutrient-poor surface waters by the operation of the plant could stimulate plankton blooms and adversely affect the local ecological balance. Additional ecological challenges arise from the potential presence of endangered of threatened species, biological nursery areas, and other sensitive habitat. Since OTEC facilities must be located near islands or within several miles of coastal zones due to cabling constraints, near-shore circulation patterns play a crucial role in determining the thermal structure of the water column. Upwelling, bringing deep water closer to the surface, can benefit an OTEC facility by reducing the required length of a cold-water intake pipe. Downwelling, by contrast, can push colder waters out of range of the cold-water pipe. Thus, the construction of an OTEC plant requires a detailed model of the water column’s temperature structure across myriad conditions.