How can we harness the ocean to create renewable energy?
The lecture I heard on blue energy, entitled “Power from the Gulf Stream: the potential and challenges of blue energy development”Was delivered by Bill Baxley, who is chief engineer at the Harbor Branch of Florida Atlantic University (FAU). Bill works with the Southeast National Marine Renewable Energy Center (SNMREC) at FAU and has conducted many studies to advance science and technology to recover energy from the ocean’s renewable resources.
Tides, currents and ocean waves represent marine hydrokinetic energy, the energy of the movement of sea water. Ocean energy, while renewable, clean and abundant, must be converted into electricity before it can replace more traditional forms of energy. To do this, you need technology: machines of one kind or another.
Like many blue energy engineers, Bill breaks down obstacles, one by one, using cutting-edge technology in hopes of extracting accessible energy from the ocean. Sometimes a technology seems “intuitive,” he says, “but you also have to prove it” to make new technologies applicable to more applications. Data models are needed for funding “to put in resources”.
What is blue energy, anyway?
Blue energy, sometimes called ocean energy, refers to technologies that harvest renewable energy from the oceans, excluding wind. Ocean energy can be harvested in many forms:
There are many considerations when developing blue energy. For example, the farther you are from the equator, the higher the tides will be: 3 feet in Florida, 30 feet in Maine. Also, to be a viable source of energy, renewable energy must be collected close enough to where it will be used by the human population.
SNMREC places special emphasis on ocean currents and offshore thermal resources available to the southeastern United States.
Large-scale observations of the structure of the Florida Stream reveal a “core” of high velocity (~ 2 m / s) flow near the surface about 20 km off the southeastern coast of Florida. Although, on average, all the water in the Florida Strait flows north, it is this core of the Florida Current that is of most interest to energy developers, because the power that can be obtained from a moving fluid is proportional to the cube. fluid velocity.
Ocean thermal energy is conceptually quite simple, because it works just like traditional power plants.
- A heat source (such as coal burning) is used to boil a working fluid (water), creating high-pressure steam.
- High-pressure steam is used to spin a turbine and generator, and electricity is produced.
- Once past the turbine, the steam is cooled again in liquid water using a “cold” source, generally air, in the case of traditional power plants.
This process is called the Rankine Cycle. Ocean thermal energy conversion commonly uses the temperature difference between warm surface seawater and cold water near the ocean floor to drive a Rankine cycle, in which a working fluid evaporates at the highest temperature and it condenses again at the lowest temperature. The resulting “steam” (whether water or other substance) can drive a turbine and generator or other mechanical conversion device.
Then the difference in temperature between the ocean surface and deep water becomes a source of blue energy: the thermal energy of the ocean.
Stream in the Florida Strait, Chance for Blue Energy
What is less well understood is the variability of the velocity and position of the Florida high-speed core. As such variability is of great interest to the ocean energy community, SNMREC has embarked on an observation program using long-term implementations of acoustic current profilers. These systems use underwater sound waves, in much the same way that radar uses radio waves in the atmosphere.
By placing an acoustic current profiler facing up near the bottom, it is possible to obtain the speed and direction of the current throughout the water column. These current profiles are measured every half hour; using many of these profiling systems, variations in time and space can be deduced, analyzed and evaluated for their implications for the recovery of marine renewable energy.
SNMREC has also implemented ground-based radar systems that use backscatter from the sea surface to infer surface current over a large offshore area, which includes the locations of acoustic profiling systems. The combination of these two approaches provides a more detailed assessment of the Florida Stream and its small-scale variations than was previously available.
At ocean temperatures, ammonia / water mixtures can be used as a working fluid, provided there is a temperature difference between surface water and deep water of ~ 20 ° C. Because the Florida Current provides a constant source of tropical warm water in the Florida Strait and because the bottom water in the Strait remains much colder, there is ocean thermal energy conversion potential (OTEC) off Southeast Florida.
The question is where and how much?
To answer this question, SNMREC embarked on a temperature measurement program using a standard conductivity-temperature-depth (CTD) instrument deployed by a small research vessel. The east-west cross-sections that measure temperature as a function of depth – the temperature layering – are repeated from Miami, Fort Lauderdale, Lake Worth and Stuart on a monthly basis.
Early results revealed that cold water at the bottom of the Florida Strait is also present on the bathymetric feature known as the Miami Terrace, meaning that from around North Miami to Boca Raton there is a cold water reservoir near the shore and about 200 meters deep.
The devices are ideally placed in the center of the Florida Strait due to the consistency and lack of impact from Florida or the Bahamas.
Multibeam mapping uses sand to measure a “belt” of the ocean floor. This is followed by underwater robots reproducing the same data with the basemap. Then a map of the habitat is drawn to see if the organisms or the seabed would be damaged.
Open ocean current generation systems
Perhaps nowhere is the notion of interaction more embodied than in the case of offshore power generation systems and the physical environment, especially when considering commercial scale implementations.
It seems obvious that removing a significant fraction of the Florida Current kinetic energy to generate electricity will have some effect on flow. While it can be argued that the large-scale processes responsible for the Florida Current will not change, and thus that the total amount of water transported north through the Florida Strait will not change, the same cannot be said for the details of the flow and its variations.
Conversely, very small-scale alterations in flow details (i.e. the wake of a single turbine system) will be an important consideration for system array design and even for the design of individual components such as rotors.
Challenges to blue energy research in the Florida Strait include deep water, distance from the coast, continuous high flows, near-surface main flow, and tropical storms.
Given the prohibitive cost of real experiments, often the most efficient approach to these problems lies in computer simulation. To this end, SNMREC and Florida State University’s Center for Ocean Atmosphere Prediction Studies (COAPS) have collaborated to use state-of-the-art ocean circulation models to study these interactions. In the process, useful and interesting relationships are discovered between the power available in the Florida Stream and total mass transportation across the Florida Strait, information that will aid the developers’ strategies for the future.
Source: National Center for Marine Renewable Energy of the Southeast
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