Ocean Thermal Energy





Land based ocean thermal energy in Hawaii.

Land based ocean thermal energy in Hawaii.

Earth’s largest solar panel is the ocean. Around 70% of the Earth is covered in water and the tropical oceans alone collect the equivalent of 250 billion barrels of oil in solar radiation daily.[1] That is enough energy to power the United States at its current consumption rate for over five thousand years. The potential of this ocean thermal energy is seemingly limitless.

Unlike fossil fuels thermal energy stored in the top layers of the ocean is renewable as long as the sun keeps shining. How can we convert this heat into a usable form of electricity? Ocean Thermal Energy Conversion, or OTEC, uses the drastic temperature difference between the surface layers and deeper waters of the ocean to convert the ocean’s stored supply of thermal energy into a practical form[5]. While there is room for much improvement on OTEC, the potential benefits of this new technology are strong enough to seriously consider the system as another renewable, sustainable, alternative fuel.

Tropical areas are ideal for OTEC operations. The cold, dense sea water sinks south from the Arctic passing underneath the warm equatorial waters[4]. The high temperature difference is optimal for OTEC as the difference between the upper and lower layers must be at least 20ºC (36ºF) for efficient operation[2]. To gather the energy from the hot and cold sea water, three conversion systems can be assembled: closed-cycle, open-cycle, and hybrid systems.

Closed Cycle Ocean Thermal Energy

In a closed-cycle system, the thermal energy stored in the upper layers of the ocean causes a liquid with a low boiling point, such as ammonia or ethanol, to evaporate. This liquid is known as the working fluid. Evaporation of the liquid propels a turbine which powers a generator producing usable electricity. The cold water from the deeper layers of the ocean—which is barely above freezing temperature—is pumped through a condenser, and the vapour condenses back into a liquid to be recycled within the system [2, 3].

Diagram of a closed circuit ocean thermal energy converter.

Diagram of a closed circuit ocean thermal energy converter. Image by RobbyBer. License: CC BY-SA 1.0

Open Cycle Ocean Thermal Energy

Rather than heating a liquid with a low boiling point, open-cycle systems actually use the warm upper layer sea water as the working fluid. When fed into a near vacuum, the warm ocean water boils in the extreme low pressure. The water vapor acts as a propellant for the turbine and, as in a closed-cycle system, powers a generator. The vapor is then condensed into a liquid by the cooler deep sea water[2, 3].

Diagram of a closed loop ocean thermal energy converter.

Diagram of a closed loop ocean thermal energy converter. Image by RobbyBer. License: CC BY-SA 1.0

Conveniently, an open-cycle OTEC system also provides a new method of sea water desalination. The evaporated warm water, separated from the salt, can be isolated and used for drinking water or irrigation. Unfortunately, this process weakens the overall efficiency of an open-cycle system. Desalination is optional; the evaporated water can easily be recycled within the system, similar to the closed-system process [2].

Hybrid Ocean Thermal Energy

As the name suggests, hybrid systems are a blend of open- and closed-cycle systems designed to optimise the efficiency of the OTEC process. First, the warm sea water evaporates within a vacuum. The vapor from this process then heats a working liquid with a low boiling point, and the vapor from said working liquid powers the turbines [2, 3].

Locations for Ocean Thermal Energy

While the technology itself seems straightforward, there are very few stable locations at which such technology would properly and safely function. The sea is far from docile. Harsh weather conditions, strong currents, large waves, and overexposure to the elements can significantly hinder the construction and overall functionality of OTEC plants. There are three categories of locations at which an OTEC system could possibly be built: on the continental shelf, mid-sea, or near/on the shore. Each location has its own drawbacks.

A modern off-shore oil rig is an example of a shelf-based facility. When OTEC technology is positioned on the continental shelf, the transfer of supplies and power is significantly more difficult and expensive than a land-based operation. The construction of the system itself is also far more costly than the other options, as complex structural support is necessary to stabilize the facility.[1]

It is aslo possible to construct a floating OTEC facility mid-sea. A floating facility would be tricky to stabilize as mooring becomes increasingly difficult as the water deepens. Modern mooring systems can function at a maximum 2000 meters depth and are extremely costly. In addition, water pipes and underwater machinery must be carefully designed to withstand high pressures, strong currents, and other forms of deep sea elemental abuse.[1]

Land- or shore-based operations seem to be the least effective but simplest option. Near the shore, extensive wiring, cabling, and mooring are not necessary. Security and stability of the operation is more easily maintained. Furthermore, the potential desalinated water from on-shore open-cycle systems could be transported as drinking or irrigation water without difficulty. The surf near the shore, however, is subject to violent and turbulent currents and waves from strong storms, and access to the colder deep sea water is significantly limited on shore.[1]

This image shows ideal locations for ocean thermal energy.

This image shows ideal locations for ocean thermal energy.

Benefits and Obstacles of Ocean Thermal Energy

Despite the technological complications of building an OTEC plant, the basic technology has certain benefits over other types of renewable energy sources. Solar and wind power both depend on a several inconsistent environmental conditions. OTEC systems, if stabilized, can generate power almost continuously.[6] In addition, the potential desalination of the sea water and collection of the nutrient-rich deep sea waters could even provide new resources in aquaculture (fish farming) as well as general irrigation.[6]

Unfortunately, OTEC may also negatively impact the surrounding environment. Pumping deep sea water to the surface not only releases stored molecules of carbon dioxide, but it may also cause an imbalance of nutrients and concentrations of other dissolved gases.[6] The Ocean Energy Council claims that these effects can be minimised by releasing the cold water at specific depths, and experts are working to develop more sophisticated, waste-free OTEC technology.

The tropical ocean: our planet’s 23 million square mile solar panel. The sea around our equator soaks up the beaming radiation of the sun, and we have discovered a way to harness this energy. “The technology is there, and the science is there,” says Stephen Oney of Honolulu’s Ocean Engineering and Energy Systems, commenting on the great potential of future OTEC systems. “It just needs to be improved”. With the progression of mooring and stabilizing technologies in the near future, perhaps we will soon find a way to efficiently access this extremely valuable renewable resource.

Sources:

1. NREL; Ocean Thermal Energy Conversion
2. Energy Savers: Ocean Thermal Energy Conversion
3. Ocean Thermal Energy – Department of Business, Economic Development, & Tourism
4. Ocean Thermal Energy – YouTube
5. How Ocean Thermal Energy Works
6. Wired; The Mad Genius from the Bottom of the Sea

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