OTEC Power Plant:

Introduction – OTEC Power Plant or Ocean Thermal Energy Conversion is an energy technology, which uses the ocean’s natural temperature gradient to drive a turbine, which is connected to a generator. It is desirable that the temperature difference between the warm surface water and the cold deep water in sea should be at least 20°C (68°F).

Basic principle:

OTEC Power Plant systems rely on the basic relationship between pressure (P), temperature (T) and volume (V) of a fluid, which can be expressed by the following equation:

where pressure, temperature and the volume of a fluid can be closely controlled by manipulating the other two variables.

Hence the differential in temperature of the fluid can be used to create an increase in pressure in another. The increase in pressure is utilized to generate mechanical work.

Most of the sun’s radiation is absorbed by sea water. Thus warm water on the ocean’s surface flow from tropics towards poles. Cold water circulates at the ocean bottom from poles to the tropics. Hence, in the tropical regions the water temperature is around 5°C at a depth of 1000 m, where as at the surface, it remains almost constant at 25°C.

Thus, we can employ a closed Rankine cycle to generate power between these two steady temperatures. Such plants are called Ocean Thermal Energy Conversion Plants (OTEC).

These systems would be located offshore on large floating platforms or inside floating bulls. The warm surface water is used for supplying the heat input in the boiler, while the cold water brought up from the ocean depths is used for extracting the heat in the condenser.

Fig.4.52 Shows a schematic diagram of an Ocean Thermal Energy Conversion Plant – OTEC.

OTEC Power Plant works on a closed cycle Rankine cycle and use lower boiling point working fluid like propane, ammonia, R-12, etc., This system can be located off shore i.e on large floating platforms. To supply heat input to boiler, warm surface water is used and for extracting heat in condenser, cold water is brought up from depths of the ocean.

The efficiency of the rankine cycle will be low due to small temperature difference between hot and cold streams. This system allows only small temperature drop of 4 to 5°C across the boiler and condenser. OTEC system is attractive because both the collection and storage of energy is done automatically by nature.

Types of OTEC Power Plant:

There are basically three types of OTEC systems developed that can utilise sea water temperature differentials.

• Closed cycle
• Open cycle
• Hybrid cycle

Closed-cycle OTEC System:

The closed-cycle system uses a working fluid, like ammonia, pumped around a closed loop, which has three components: a pump, turbine and heat exchangers (evaporator and condenser). Warm seawater passing through the evaporator converting the ammonia liquid (4) into high-pressure ammonia vapour at (5).

The high-pressure vapour at (1) is then fed into an expander where it passes through and rotates a turbine connected to a generator. Low-pressure ammonia vapour leaving the turbine (2) is passed through a condenser, where the cold seawater cools the ammonia, returning the ammonia back into a liquid (3).

Open-Cycle OTEC system:

The open-cycle system is generally similar to the closed-cycle system and uses the same basic components. The open-cycle system uses the warm seawater as the working fluid. The warm seawater passing through the evaporator (2) is converted into steam (3), which drives the turbine/generator. After leaving the turbine (5), the steam is cooled by the cold seawater to form desalinated water. The desalinated water is pure fresh water for domestic and commercial use.

Hybrid OTEC System:

The hybrid system uses parts of both open-cycle and closed-cycle systems to produce electricity and desalinated water. In this arrangement, electricity is generated in the closed-cycle system and the warm and cold seawater discharges are passed through the flash evaporator and condenser of the open-cycle system to produce fresh water.

Advantages of OTEC Power Plant:

• It is clean form of energy conversion.
• It does not occupy land areas.
• No payment for the energy required.
• It can be a steady source of energy since the temperature are almost steady.

Disadvantages:

• About 30% of power produced will be used to pump water.
• The system should to withstand strong harmful effects of sea water. (hurricane, debris, fishes)
• The material used should withstand corrosive atmosphere.
• Very high investment is required.
• Plant size is limited to 100 MW due to large size of components.