The potential and challenges of blue energy development

Earlier this month, I heard a lecturer from the local university talk about blue energy. As someone who has researched energy sources that take us away from fossil fuels, the topic piqued my interest. I’m also a resident of Florida, so I’m fascinated by the vast ocean I see in front of me.

How to exploit the ocean to create renewable energy?

The lecture I heard about blue energy, titled “Gulf Stream Energy: The Potential and Challenges of Blue Energy Developmentwas presented by Bill Baxley, who is a chief engineer in the port branch of Florida Atlantic University (FAU). Bill works with the Southeast National Marine Renewable Energy Center (SNMREC) at the FAU, and he has conducted numerous studies to advance the science and technology of energy harvesting from renewable ocean resources.

Ocean tides, currents and waves represent marine hydrokinetic energy – the energy of moving sea water. Ocean energy, although renewable, clean and plentiful, must be converted into 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 is breaking down barriers, one by one, using advanced technology in hopes of tapping accessible energy from the ocean. Sometimes a technology seems “intuitive,” he says, “but you also have to prove it” in order to make new technologies applicable to multiple applications. Data models are needed for funding to “dedicate resources to it”.

What is blue energy, anyway?

Blue energy, sometimes referred to as ocean energy, refers to technologies that harvest renewable energy from the oceans, excluding the winds. Energy from the oceans can be harvested in several forms:

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

SNMREC places particular emphasis on ocean currents and offshore thermal resources available in the southeastern United States.

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

Ocean thermal energy is conceptually quite simple, as it works just like traditional 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 spin a turbine and generator, and electricity is generated.
  • After passing the turbine, the steam is cooled to liquid water using a “cold” source – usually air, in the case of traditional power plants.

This process is called a Rankine cycle. Ocean thermal energy conversion typically 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 s evaporates at the higher temperature and recondenses at the lower temperature. The resulting “steam” (whether water or some other substance) can drive a turbine and generator or other mechanical conversion device.

Thus, the temperature difference between the surface of the ocean and the deep waters becomes a source of blue energy – the thermal energy of the oceans.

Current in the Florida Strait, possibilities for blue energy

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

By positioning an upward-pointing acoustic current profiler 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 several of these profiling systems, variations over time and space can be inferred, analyzed and evaluated for their implications for marine renewable energy harvesting.

SNMREC has also deployed land-based radar systems that use backscatter from the sea surface to infer surface current over a wide offshore area, which includes the positions of acoustic profiling systems. The combination of these two approaches provides a more detailed assessment of the Florida Current and its small-scale variations than before.

At ocean temperatures, ammonia/water mixtures can be used as the working fluid, provided a surface water/deep water temperature difference of approximately 20°C is available. Since the Florida Current provides a constant source of warm tropical water in the Florida Strait and the water at the bottom of the Strait remains much cooler, there is potential for ocean thermal energy conversion (OTEC ) off southeast Florida.

The question is where and how much?

To answer this question, SNMREC undertook 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 – i.e. 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 Strait is also present on the bathymetric feature known as Miami Terrace, meaning that from approximately North Miami to Boca Raton there is a reservoir of cold water 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 “strip” of the ocean floor. Then come underwater robots that reproduce the same data with the base map. Then a habitat map is drawn to see if the organisms or the seabed would be damaged.

Offshore power generation systems

Perhaps nowhere is the notion of interactions more embodied than in the case of deep sea power generation systems and the physical environment, particularly when commercial scale deployments are contemplated.

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

Conversely, very small-scale changes in flow detail (i.e. the wake of an individual turbine system) will be an important consideration for the design of system networks and even for the design of individual components such as rotors.

Challenges for blue energy research in the Florida Straits include deep water, distance from shore, continuous high flows, near-surface main flow, and tropical storms.

Given the prohibitive cost of real experiments, the most effective approach to solving these problems is often computer simulation. To that end, SNMREC and Florida State University’s Center for Ocean Atmosphere Prediction Studies (COAPS) have partnered to use state-of-the-art ocean circulation models to study these interactions. In the process, useful and interesting relationships between the power available in the Florida Current and the total mass transport across the Florida Strait are discovered, information that will help developers’ strategies for the future.

Source: Southeast National Marine Renewable Energy Center


 

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Valerie J. Wallis