Renewable energy notes for me power system engg

1. OCEAN THERMAL ENERGY
Ocean thermal energy conversion (OTEC) is a method which consists of extracting
energy from the difference in temperature between shallow and deep waters by way of a heat
engine. The biggest difference in temperature (around 20 degrees Celsius, generally located
near the equator or tropics), between a heat and cold source provides the greatest amount of
potential energy. Thus, the main technical challenge is to generate the most amount of power
from a small temperature ratio.
Compared with technologies such as wave energy, the energy available from OTEC is one
or two orders of magnitude higher. But the thermal efficiency is very low; the theoretical
maximum efficiency is 6 or 7%. Furthermore, the extraction of energy is difficult and thus
expensive (pumping material). This technology could be economically competitive as compared
with conventional power technologies. Electrolysis could produce hydrogen to enhance energy
output from the process of OTEC.
Suitable Site for OTEC
 Coastal Zone land
 Sea Floor must be descend from the Shore based Plant locations
 Significant Temperature difference
- at least 20ºC between Surface & Deep water
 Seasonal availability of warm & cold water – Not much variation due to season changes.
Types of OTEC technology
Several options exist for OTEC technology depending of the location and the cycle used.
Plants may be based on land, ocean shelf or floating facilities. The first two have the most
advantages. Three kinds of cycles are used:
 Open cycle - Claude Cycle
 Closed cycle - Anderson Cycle
 Hybrid cycle.

2. The main concept is the same for all systems. There is a heat engine placed between a
high temperature and low temperature reservoir. As in a steam turbine, the engine converts
heat energy into kinetic energy. To operate, the cold sea water is either pumped directly (can be
more than one kilometer) or desalinated near the sea bed to then float up through a pipe to the
surface.
Open-cycle OTEC
The open-cycle process does not use an intermediate fluid like the closed-cycle but
directly uses the sea water. It uses the warm surface in tropical seas. The warm water is placed
in a low-pressure container which carries out the evaporation. Like in others cycles, the steam
is driven into the turbine attached to a generator. The second effect is the production of
desalinated water (fresh water) through evaporation. The steam is condensed into liquid by the
deep cold water. Contrary to the first cycle, the water is captured because it is now clean
drinking water.
Open Cycle OTEC System
Fig: OTEC open-cycle block diagram

3. Open Cycle OTEC Operation
It uses Surface sea water as the working fluid. The deaerator removes gases with the
help of Vacuum pump. Sea Water is flash evaporated under partial vacuum which produce
vacuum. Low Pressure Steam is passed through a turbine which extracts energy. The spent LP
Steam/Vapour is cooled in a condenser. Spray condenser is used to condensate the spent LP
steam. Condensate is mixed with cooling water & the mixture is discharged back into sea. Warm
surface water at 270C is admitted into an evaporator in which pressure is maintained at a value
slightly below saturation pressure corresponding to water temperature. Water from the deep
ocean, which is at about 100C,on reaching the condenser.
Due to heat transfer cold water temp. rise is 150C. In case of Open Cycle OTEC, very large
amount of water mass & volume flow rates are used and very Low Pressure Turbine is used.
Specific Volume is 2000 times that of modern fossil fuel power plant. More deaerators must be
used to remove gases dissolved in the sea water. Cost of Open Cycle OTEC system is very high.
Turbine cost is half of the power plant cost. Bio-fouling is less in case of Open OTEC. Water from
the bottom of the ocean, which is very cold, is brought up to the surface. The water vaporizes
due to the much higher temperature at the surface. The vaporization of the water creates an
expansion, an expansion that spins a turbine.
Closed-cycle OTEC system
Fig: OTEC closed-cycle block diagram

4. This cycle uses a working fluid (with a low boiling point) which is cooled down and
heated up in a full cycle. The warm water located at the surface is injected into a heat
exchanger; the working fluid gets the heat to change its state, and is vaporized. The vapor is
directed through the turbine and generator where the electricity is produced. Then the vapor is
condensed by the deep cold water. The liquid goes back to the heat exchanger and so on. India
has built a closed-cycle, floating plant of 1 MW rated power.
Closed cycle OTEC operation
It requires a special working fluid that receives & rejects heat to the source to sink.
Working fluid may be ammonia, propane or Freon. Operating pressure of this fluid is
10Kg/Cm2 (10Bar). Specific volume is much lower to steam in conventional power plant. This
leads to smaller size turbine, when compared to Open cycle OTEC turbine. Closed cycle system
is to transfer heat efficiently across the heat exchanger surfaces and uses very large heat
exchangers having the typical efficiency of 2%.In ocean environment, it is likely that a layer of
slime known as bio fouling will accumulate on the water side of heat exchanger. Slime is due to
microorganism and can be removed by periodically cleaning the heat exchanger.
Evaporators / Heat Exchangers
Conversion Efficiency of heat into mechanical work in a turbine depends on the drop in
temp. of working fluid. Inlet temp. is 200C & Outlet Temp. is 100C. Drop in temp. is 100C and
Max. thermal Efficiency is (10/293) - 3.4%. Heat Exchanger constructional materials must have
good heat conductivity and free from corrosion & erosion.
Materials used for Heat Exchanger
 Titanium
- Good Mechanical Strength
- Resistant to corrosion & erosion
- Rare Element in nature & High Cost
 Aluminum
- More susceptible for corrosion by ocean water
- Aluminum alloys found suitable for salt water uses
 Copper Nickel (90:10) Alloy
- Extensively used for shipboard PP condenser

5. Hybrid cycle OTEC
The last cycle proposes to combine both open and closed cycles. The warm water is
vaporized in a vacuum chamber. The steam transforms working liquid into vapor (heat
exchanger) which is directed in a turbine. As in the open-cycle the condensed water provides
fresh water.
Fig: Block diagram of hybrid cycle OTEC technology
Hybrid Cycle OTEC Operation
There is an attempt to combine the best futures of Open & Closed Cycle OTEC. Sea Water
is Flash evaporated to steam. Heat resulting steam is then transferred to ammonia.
Applications for OTEC
The main goal of this system is to output electric power and, secondly, desalinated water
(2-megawatt electric plant could produce about 4300 cubic meters of desalinated water each
day) for some cycles. But OTEC technology offers others possibilities, like support for deep
water mariculture (deep waters rich in nutrients) and air conditioning. It also can be used to
produce ammonia (the working fluid), hydrogen, aluminum, methanol and others chemicals.

6. Advantages
 Reduction in the dependence on fossil fuels.
 Source of energy is free, renewable and clean.
 Clean electricity is produced with no production of greenhouse gas or pollution (Liquid
or solid).
 Energy produced is free once the initial costs are recovered.
 These technologies are renewable sources of energy. OTEC systems can produce fresh
water as well as electricity. This is a significant advantage for an island, such as the
Virgin Islands for example, where fresh water is limited.
 There is enough solar energy received and stored in the warm tropical ocean surface
layer to provide most, if not all, of present human energy needs.
Disadvantages
 At present, electricity produced would cost more than electricity generated from fossil
fuels at their current costs.
 OTEC is very expensive, and cost-effective turbines and heat exchangers need to be
developed.
 Construction of OTEC plants and laying pipes in coastal waters may cause localized
damage to reefs and near-shore marine ecosystems.
 It leads to the displacement of wild life habitats.
 Technologies are not fully developed.
 Problems exist with the transport of electricity to onshore loads.
 A 38 degree difference is necessary for OTEC to work.
Prepared by
R. RamaRaj, KCET
J. S. Sakthi suriya raj, KCET
K. Selva Narayanan, KCET