The Potential & Challenges Of Developing Blue Energy

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Earlier this month, I heard a speaker from the local university talk about blue energy. As someone who’s been researching power sources that move us away from fossil fuels, the topic piqued my interest. I’m also a Florida resident, so I’m fascinated about the vast ocean I see before me.

How can we harness the ocean to create renewable energy?

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The lecture I heard about blue energy, titled “Power from the Gulf Stream: The Potential and Challenges of Developing Blue Energy,” was delivered by Bill Baxley, who is chief engineer at Florida Atlantic University’s (FAU) Harbor Branch. Bill works with the Southeast National Marine Renewable Energy Center (SNMREC) at FAU, and he has conducted many studies to advance the science and technology of recovering energy from the oceans’ renewable resources.

Ocean tides, currents, and waves represent marine hydrokinetic energy — the energy of moving seawater. Ocean energy, while renewable, clean, and plentiful, must be converted to electricity before it can replace more traditional forms of energy. To do this requires technology — machines of one kind or another.

Like many blue energy engineers, Bill takes down obstacles, one by one, using cutting-edge technology with the hopes of drawing accessible energy out of the ocean. Sometimes a technology seems “intuitive,” he says, “but you need to prove it, too” in order to make new technologies applicable to multiple applications. Patterns of data are necessary for funding “to put resources to it.”

What is Blue Energy, Anyways?

Blue energy, sometimes called ocean energy, refers to technologies that harvest renewable energy from the oceans, excluding winds. Ocean energy can be harvested in many forms:

• Wave energy: Depends on wave height, speed, length, and the density of the water
• Tidal stream energy: Generated from the flow of water in narrow channels
• Tidal range technologies (or tidal barrages): Exploit the difference in surface height in a dammed estuary or bay
• Ocean thermal energy conversion: Uses temperature differences between surface and subsurface water to generate energy
• Salinity gradient power: Relies on the difference in salinity between salt and freshwater

There are many considerations when developing blue energy. For example, the farther you are from the equator, the higher the tides are: 3 feet in Florida, 30 feet in Maine. Additionally, to be a viable energy source, renewable energy needs to be harvested fairly close to where it will be used by a human population.

The SNMREC places special emphasis on ocean currents and offshore thermal resources available to the southeastern US.

Large-scale observations of the structure of the Florida Current reveal a “core” of relatively high-speed (~2 m/s) flow near the surface about 20 km offshore of the southeast coast of Florida. Although, on average, all of the water in the Florida Straits flows northward, it is this core of the Florida Current that is of the most interest to energy developers, because the power that can be obtained from a moving fluid is proportional to the cube of the fluid’s speed.

Ocean thermal energy is conceptually quite simple, because it works just like traditional electrical power plants.

• A heat source (such as burning coal) is used to boil a working fluid (water), creating high-pressure steam.
• The high-pressure steam is used to turn a turbine and a generator, and electricity is produced.
• Once past the turbine, the steam is cooled back to liquid water using a “cold” source — generally air, in the case of traditional power plants.

This process is called a Rankine Cycle. Ocean thermal energy conversion commonly uses the temperature difference between warm surface seawater and the cold water near the ocean bottom to drive a Rankine cycle, in which a working fluid evaporates at the higher temperature and re-condenses at the lower temperature. The resulting “steam” (whether water or other substance) can drive a turbine and generator or other mechanical conversion device.

So the temperature difference between the ocean surface and the deep water becomes a source of blue energy — ocean thermal energy.

Current in the Florida Straits, Possibilities for Blue Energy

What is less well understood is the variability of the speed and position of the Florida high-speed core. Because such variability is of great interest to the ocean energy community, SNMREC has undertaken an observational program using long-term deployments of acoustic current profilers. These systems use underwater sound waves, much in the same fashion that radar uses radio waves in the atmosphere.

By positioning an upward-looking acoustic current profiler near the bottom, it is possible to obtain the current speed and direction throughout the water column. Such current profiles are measured every half hour; by using several of these profiling systems, variations over both time and space can be inferred, analyzed, and assessed for their implications for marine renewable energy recovery.

SNMREC has also deployed shore-based radar systems that use backscattering from the sea surface to infer the surface current over a large offshore area, one that includes the positions of the acoustic profiling systems. The combination of these two approaches provides a more detailed assessment of the Florida Current and its small-scale variations than has been previously available.

At oceanic temperatures, ammonia/water mixtures can be used as the working fluid, provided a surface-water/deep-water temperature difference of ~20°C is available. Because the Florida Current provides a steady source of warm, tropical water into the Florida Straits, and because the bottom water in the Straits remains much colder, there is ocean thermal energy conversion (OTEC) potential offshore of southeast Florida.

The question is where and how much?

To address this question, SNMREC has undertaken a program of temperature measurements using a standard conductivity-temperature-depth (CTD) instrument deployed from a small research vessel. East-west cross sections that measure temperature as a function of depth — that is, the temperature stratification — are repeated from Miami, Fort Lauderdale, Lake Worth, and Stuart on a monthly schedule.

Early results have revealed that the cold water at the bottom of the Florida Straits is also present on the bathymetric feature known as the Miami Terrace, which means that from about North Miami to Boca Raton there is a cold-water reservoir close to shore and about 200 meters deep.

Devices are ideally placed in the center of the Florida Straits due to consistency and lack of impact from Florida or Bahamas.

Multibeam mapping uses sand to measure a “swath” of ocean bottom. This is followed by underwater robots who reproduce the same data with the base map. Then a habitat map is drawn to see if organisms or the seafloor would be damaged.

Open Ocean Current Generating Systems

Perhaps nowhere is the notion of interactions embodied more than in the case of open-ocean current generating systems and the physical environment, especially when commercial-scale deployments are considered.

It seems obvious that removing a significant fraction of the Florida Current’s kinetic energy to generate electricity will have some effect on the flow. While it can be argued that the large-scale processes responsible for the Florida Current will not be changed, and therefore that the total amount of water transported northward through the Florida Straits will not change, the same cannot be said for the details of the flow and its variations.

Conversely, very small-scale alterations of the details of the flow (that is, an individual turbine system’s wake) will be an important consideration for the design of arrays of systems and even for the design of individual components such as rotors.

Challenges to blue energy research in the Florida Straits include the deep water, the distance from shore, continual high flows, prime flow near surface, and tropical storms.

Given the prohibitive cost of true experiments, often the most efficient approach to these problems lies in computer simulation. To this end SNMREC and the Center for Ocean Atmosphere Prediction Studies (COAPS) at the Florida State University have teamed up to use state-of-the-art ocean circulation models to investigate these interactions. In the process, useful and interesting relationships between the power available in the Florida Current and the total mass transport through the Florida Straits are being discovered, information that will assist developers’ strategies for the future.

Source: Southeast National Marine Renewable Energy Center