Classification of OTEC Power Plants

OTEC power plants may be classified on the basis of- (i) Location and (ii) Cycle.

Classification of OTEC Power Plants Depending Upon Location:

i. Land-Based Power Plant:

Land-based and near-shore facilities offer three main advantages over those located in deep water. Power plants constructed on or near land do not require sophisticated mooring, lengthy power cables, or the more extensive maintenance associated with open-ocean environments.

They can be installed in sheltered areas so that they are relatively safe from storms and heavy seas. Electricity, desalinated water, and cold, nutrient-rich sea water could be transmitted from near-shore facilities via trestle bridges or causeways. In addition, land-based, or near-shore sites allow OTEC power plants to operate with related industries such as mariculture or those that need desalinated water.

Favoured locations include those with narrow shelves (volcanic islands) steep (15-20 degrees) offshore slopes, and relatively smooth sea floors. These sites reduce the length of the cold-water in take pipe. A land-based power plant could be built well inland from the shore, offering more protection from storms, or on the beach, where the pipes would be shorter. In either case, easy access for construction and op­eration helps lower the costs.

Land-based or near-shore sites can also support mariculture. Mariculture tanks or lagoons built on shore allow workers to monitor and control miniature marine environments. Mariculture products can be delivered to market with relative ease via standard transport (rail roads or high­ways).

One disadvantage of land-based facilities arises from the turbulent wave action in the surf zone. Unless the OTEC power plants water supply and discharge pipes are buried in protective trenches, they will be subject to extreme stress during storms and prolonged periods of heavy seas. Also, the mixed discharge of cold and warm seawater may need to be carried several hundred meters offshore to reach the proper depth before it is released. This arrangement needs addi­tional expense in construction and maintenance.

OTEC systems can avoid some of the problems and expenses of operating in a surf zone if they are built just offshore if waters ranging from 10 to 30 meters deep (Ocean Thermal Corporation 1984). This type of plant would use shorter (and therefore less costly) intake and discharge pipes, which would avoid the dangers of turbulent surf. The plant itself, however, would require protection from the marine environment, such as breakwaters and erosion-resistant foun­dations and the plant output would need to be transmitted to shore.

ii. Shelf Based Power Plant:

To avoid turbulent surf zone as well as to have closer access to the cold water resource, OTEC power plants can be mounted to the continental shelf at depths up to 100 m. A shelf-mounted plant could be built in a shipyard, towed to the site, and fixed to the bottom of the sea.

Such construction is already used for offshore oil rigs. The additional problems of operating an OTEC plant in deep water, however, may make shelf-mounted facilities less desirable and more expensive than land-based counterparts. Problems with shelf-mounted plants include the stress of open- sea conditions and more difficult product delivery.

Having to consider strong ocean currents and large waves requires additional engineering and construction expense. Platforms need extensive pilings to maintain a stable base for OTEC operation. Power delivery could also become costly due to requirement of long under water cables for reaching land. These reasons make the shelf-mounted plants less attractive.

iii. Floating Power Plant:

Floating OTEC facilities could be designed to operate off-shore. Although potentially preferred for systems with a large power capacity, floating facilities present several difficulties. This type of power plant is more difficult to stabilize, and the difficulty of mooring it in very deep water may cause problems with power delivery.

Cables attached to floating platforms are more susceptible to damage, especially during storms cables at depth exceeding 1,000 m are difficult to maintain and repair. Riser cables, which span the distance between the sea bed and the plant, need to be constructed to resist entanglement.

As with shelf-mounted plants, floating plants require a stable base for continuous OTEC operation. Major storms and heavy seas can break the vertically suspended cold-water pipe and interrupt the intake of warm water as well. To reduce such problems pipes can be made of relatively flexible polyethylene attached to the bottom of the platform and gimbaled with joints or collars.

Pipes may need to be uncoupled from the plant to prevent storm damage. As an alternative to a warm-water pipe, surface water can be drawn directly into the platform; however, it is necessary to prevent the intake flow from being interrupted during violent motions caused by heavy seas.

If a floating power plant is to be connected to power delivery cables, it requires the plant to remain relatively stationary. Mooring is an acceptable method, but current mooring technology is limited to depths of about 2,000 metres. Even at shallower depths, the cost of mooring may prohibit commercial OTEC ventures.

Classification of OTEC Power Plants Depending on the Cycle Used:

The cold seawater is an integral part of each of the three types of OTEC systems:

(i) Closed-Cycle OTEC Power Plant,

(ii) Pen-Cycle OTEC Power Plant, and

(iii) Hybrid OTEC Power Plant.

For operation, the cold seawater is to be brought to the surface. This can be done through direct pumping. A second method is of desalination of the seawater near the sea floor; this lowers its density, which will cause it to “float” up through a pipe to the surface.

The alternative to costly and massive, cold water pipes bring condensing cold water to the surface is to pump the much smaller volume of vapourised low boiling point fluid into the depths to be condensed thus overcoming a massive technical and environmental problem and lowering the cost of OTEC.

(i) Closed-Cycle OTEC Power Plant:

In a closed-cycle system, fluid with a low boiling point, such as ammonia, is used to rotate a turbine coupled to an electric generator. Warm surface seawater is pumped through a heat exchanger where the low- boiling-point fluid is vapourized. The expanding vapour rotates the turbo-generator. Then, cold, deep water pumped through a second heat exchanger condenses the vapour back into a liquid, which is then recycled through the system.

(ii) Open-Cycle OTEC Power Plant:

In open-cycle OTEC power plant, the tropical ocean’s warm surface is used to produce electricity. When warm seawater is placed in a low-pressure container, it boils. The expanding steam drives a low-pressure turbine coupled to an electric generator. The steam, which has left its salt and contaminants behind the low-pressure container, is pure fresh water. It is condensed back into a liquid by exposure to cold temperatures from deep-ocean water. This method has the advantage of producing desalinized fresh water, suitable for drinking water or irrigation.

(iii) Hybrid OTEC Power Plant:

A hybrid cycle combines the features of both the closed-cycle and open-cycle systems. In a hybrid OTEC system, warm seawater enters a vacuum chamber where it is flash-evaporated into steam, similar to the open- cycle process. The steam vapourises the ammonia working fluid of a closed-cycle loop on the other side of an ammonia vapourizer. The vapourised fluid then drives a turbine coupled to an electric generator producing electricity. The steam condenses within the heat exchanger and provides desalinated water.

The electrical energy generated by the system can be supplied to a utility grid or used to produce methanol, hydrogen, refined metals, ammonia, and similar products.