A Rankine cycle with a working fluid such as ammonia is a first choice for extraction energy from the temperature difference. The essential components of the cycle are an evaporator, turbine-Generator, Condenser and a Pump for circulating the working fluid. Other components required for the OTEC plant include pumps for supply of warm water (from the surface of the ocean) to the evaporator and cold water (from a depth of 1000 m) to the condenser. This is shown in <xref ref-type="fig" rid="fig001">Figure 1</xref>. Also Shown in the T-S (Temperature –Entropy) diagram for the Rankine cycle.The bathymetry of the coast around India, where cold water at a depth of 1000m is available at about 40 km from the shore necessitates the use of a floating platform to house the OTEC plant. NIOT proposes to build a 1 MW floating OTEC plant working on a closed cycle off the coast of Tamilnadu.</sec><sec id="sec002"><title>Objective of 1 MW Otec Plant

The aim of the demonstration plant is the design, manufacture, and erection commissioning and performance evaluation of a floating OTEC plant rating 1 MW gross.

In addition to the power module operating on closed loop ammonia and associated turbine generator, control system technologies, this will imply the need to develop and assimilate the following technologies:

Design, Manufacture and Performance evaluation of high quality, compact heat exchangers with phase change under marine conditions.

Measurements and development of methods for control of bio fouling in heat exchangers/

Structural analysis under marine environment of 1.0 diameter, 1000 m long HDPE pipe coupled to a floating barge.

Offshore structures design.

Deepwater moorings

Offshore operations.

Development of instrumentation and control strategies for optimization from an OTEC plant

Development of Governor control strategies for variation in the load and load variables

Power transmission through under water cables

Establishment of base line environmental effects due to an OTEC plants

Power Modules

The closed cycle OTEC system utilizes a low boiling liquid like ammonia as the working fluid. The fluid is evaporated in an evaporator using the warm seawater from the surface. After the vapor drives the turbine it is condensed by cold seawater. The condensate is pumped back to the evaporator. The baseline design of the power cycle using Titanium plate heat exchangers. Warm water at 29 degree centigrade is drawn from the surface of the ocean with a flow rate of 2.1 tons/sec while cold water at 7 degree centigrade from a depth of 1000 m is drawn at 1.5 tons/sec. The flow rate of ammonia is 31.6 kg/s to produce 1 MW across an axial turbine.

Heat Exchangers

The heat exchangers in an OTEC power plant is significant as they exchange large quantity (nearly 40–50 times of the gross power) of heat energy due to the low thermal efficiency of the power cycle. Nearly 30 – 40% of the total cost of the plant should be allotted to this single component. For the present 1 MW OTEC plant plate heat exchangers are used for both evaporator and condenser. The heat energy is exchanged between seawater and ammonia with phase change through corrugated titanium plates of 0.7 mm thickness with CR gaskets. The evaporator and condenser consist of 4 modules arranged in parallel. The ammonia side of the evaporator is coated with stainless steel powder to enhance the heat transfer co-efficient. It is the first time this technology is experimented on OTEC heat exchangers where small D T exists. The coating or thermal spraying is expected to increase the heat transfer co-efficient by 30 – 50%. The evaporator makes use of low angle plates and condenser medium angle plates. The heat transfer co-efficient and pressure drop in the plate heat exchanger is related to the plate angles.

Turbine Generator

The OTEC turbine is characterized by low-pressure ratios and relatively high mass flow of working fluid. The output of the turbine may vary more than? 20% due to the variation in temperature and hence the turbine is to be designed to have a good isentropic expansion efficiency over a considerable range of pressure ratio. The sea water flow rates and hence the system lead loss is very much dependent on the isentropic efficiency of the turbine.

For the 1 MW OTEC plant, a 4-stage axial flow reaction turbine coupled to a synchronous generator through 2: 1 speed reduction gear box is chosen for power conversion. For a considerable range of pressure ratios the turbine efficiency remains above 85.

Platform

The platform consists of a non-self propelled barge. The sea water system comprises a cold water pipe (CWP) a warm water pipe (WWP), a mixed discharge pipe and ducting on the barge. The CWP is of outer diameter of 1.0 m and is of High-Density polyethylene (HPDE) and forms an integral of the mooring of the barge. Other components of the single point mooring are a surface buoy, hanger, anchor lines and the anchor.

The figure in the following page shows the mooring configuration such that the CWP acts as a part of the mooring.

Utility of Otec Plants

For an energy starved nation like India, the main use of an OTEC plant would be the electricity generated. However there are other benefits which can be simultaneously derived from an OTEC plant. These are desalinated water and mariculture. Smaller rating plants in the range of 1–5 MW can be directly used as base power for islands like the Andamans, though they may not be as economical as fossil fuelled ones in such low capacity ranges. Once the 1 MW plant is commissioned, a hands-on experience would have been achieved on various aspects of an OTEC plant. This expertise may then be utilized for an island community where an OTEC plant would satisfy three different objectives simultaneously i.e base power, desalination and mariculture.

Otec plant for islands

As is well known, supplying power to small island communities from the mainland is a difficult proposion. Islands especially in the tropics would be best beneficiaries of electricity from an OTEC plant. The plant could be land based avoiding the need for a floating platform.

Energy

A net power of 1 MW could meet the major requirement for islands like the Andamans. Thus, the rating of the OTEC plant need be only incrementally higher than the current 1 MW plant. The technologies established in the implementation if the 1 MW plant can therefore be utilized immediately.

Desalination

One way to produce large quantities of desalinated water is with a hybrid OTEC system in which desalinated water is produced by vacuum flash distillation and power is produced by a closed cycle loop. For example, it has been shown that a 2 MW net plant could produce about 4320 cubic meters for desalinated waters a day. This water would be welcome on an island where fresh water supply is limited for agriculture and drinking.

Deep water supported mariculture

The cold seawater drawn from large depths is rich in nutrients and relatively free of pathogens. It is a very good medium for growing phyto-plankton and microalgae, which in turn can be used for growing commercially valuable fish. Large volumes of cold water are involved and mariculture is in its infancy in India. Thus, the OTEC plant could promote developments in these areas as well.

Ocean thermal energy conversion