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Ocean Thermal Energy
Essay Contents:
- Essay on the Introduction to Ocean Thermal Energy
- Essay on the Availability of Ocean Thermal Energy
- Essay on the Ocean Temperature Differences
- Essay on the Conversion Technologies
- Essay on the Basic Designs for OTEC System
Essay # 1. Introduction to Ocean Thermal Energy:
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The oceans and seas constitute some 70 percent of the earth’s surface area. These absorb solar radiation which causes moderate temperature gradients from the water surface downwards, especially in tropical waters. This ocean thermal energy is immense. The temperature gradient can be utilized in a heat engine to generate power.
This is called ocean temperature energy conversion (OTEC). But because the temperature difference is small, even in the tropics, OTEC systems have very low efficiencies and consequently have very high capital cost.
Essay # 2. Availability of Ocean Thermal Energy:
The availability of ocean thermal energy can be estimated as follows:
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Total extraterrestrial solar energy received by the earth.
= 1.516 × 1018 kWh/year or 5.457 × 1018 MJ/year
Average clearness index = 0.5
Fraction of area of oceans = 0.7
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... Total terrestrial ocean thermal energy = 1.516 × 1018 × 0.5 × 0.7
= 0.53 × 1018 kWh/year or 1.9 × 1018 MJ/year
Average terrestrial ocean thermal energy is nearly equal to 50% of solar constant
= 1353 W/m2 × 0.5 = 676 W/m2
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This total terrestrial incident energy is not absorbed by water because some of it is reflected back to the sky .The energy absorbed by the ocean water can be estimated from the annual evaporation of water. Although the same amount is replenished, back by rainfall & runoff from the land.
Annual evaporation = 120 cm
or 120 cm3/cm2
or 1.20 m3/m2
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Average water surface temperature = 20°C
The latent heat of vaporization = 2454 kJ/kg
Sea Water density = 1000 kg/m3
... The annual energy absorbed = 1.20 × 1000 × 2454
= 3 × 106 kJ/m2 per year
This is equivalent to 95 W/m2
Absorbed energy as percentage of incident energy
= 95/676 × 100 = 14%
This figure may be higher in the tropics and lower in arctic waters.
Essay # 3. Ocean Temperature Differences:
Solar energy absorption by the water takes place according to Lambert’s Law of absorption which states that each layer of equal thickness absorbs the same fraction of light that goes through it.
If I is the intensity of radiation and y is the depth of water,
dI (y)/dy = µI
or I(y) = Io e-µy
where
Io = Intensity of radiation at the surface where y = 0.
µ = Extinction coefficient or absorption coefficient
For very fresh clear water, µ = 0.05 m-1
For turbid fresh water, µ = 0.27 m-1
For very salty water, µ = 0.50 m-1
The intensity falls exponentially with the depth (y) and depending upon the value of almost all of the absorption occurs very close to the surface of deep waters. Therefore, maximum temperature occurs just below the surface due to heat and mass transfer at the surface itself.
Water density decreases with an increase in temperature. However, pure water at 3.98°C has the maximum density.
There will be no thermal convective currents between the warmer, lighter water at top and coolar, heavier water at a depth. Similarly, heat transfer by thermal conduction between the water layers at the surface and a depth is too low to alter the picture and mixing is retarded. The warm water stays at the top and the cool water stays at the bottom.
In tropical waters there are essentially two infinite heat reservoirs a heat source at the surface at about 27°C and a heat sink at about 4°C at some depth of one kilometer. Both the reservoirs are maintained annually by solar incidence. These temperatures vary with latitude and season.
Essay # 4. Conversion Technologies:
The maximum possible efficiency of a heat engine working between two temperature limits cannot be more than that of a Carnot cycle operating between the same temperature limits.
The Carnot efficiency,
Ƞc = T1 = T2/T1
where,
T1 = source temperature
T2 = sink temperature
If t1 = 27°C and t2 = 7°C
Ƞc = 27 – 7/(27 + 273) = 6.67%
For a 300K (27°C) surface temperature an OTEC system cannot have efficiency greater than ΔT/3 where ΔT is the temperature difference between surface and deep waters. Taking into consideration the temperature and pressure drops in the plant equipment such as turbine, condenser, heat exchangers and pumps, the efficiency of a real OTEC power plant is less than two percent.
Although there are no fuel costs, the capital costs are extremely high due to large size of power plant.
Essay # 5. Basic Designs for OTEC System:
There are two basic designs for OTEC systems:
I. Open cycle also called Claude Cycle.
II. Closed cycle also called Anderson Cycle.
I. The Open or Claude Cycle:
The seawater plays the role of heat source, working fluid, coolant and heat sink. The flow diagram and T-s diagram are shown in Fig. 9.1 and 9.2 respectively.
Warm water at 27°C is admitted into an evaporator where pressure is maintained slightly below the saturation pressure corresponding to warm water temperature. The steam from evaporator is superheated at pressure of 0.0317 bars. The water undergoes volume boiling; water is partly flashed into steam. Process 1-2 is a constant enthalpy throttling process.
The low pressure in the evaporator is maintained by a vacuum pump which also removes the dissolved non-condensable gases from the evaporator. The steam expands in a specially designed steam turbine. The steam at turbine exit is condensed in a direct contact condenser where cold deep water at 13°C is mixed with this steam and discharged into the ocean.
A temperature difference of 10°C (25° – 15°) is available for the cycle work. The plant has to handle very large quantities of warm and cold water which requires large pumping power and large heavy cold water pipes.
Example 1:
A Claude cycle of 100 kW gross capacities operates on the parameters given in the table. The turbine has a poly tropic efficiency of 0.80 and the turbine- generator has a combined mechanical-electrical efficiency of 0.90. Calculate the surface and deep-water flow rates and the gross cycle and plant efficiencies.
Solution:
1. Evaporator:
2. Turbine:
3. Condenser:
4. The Cycle:
Note:
The gross power, 100 kW, and the gross plant efficiency 2.5%, do not take into account pumping and other auxiliary power inputs to the plant.
Modification of Open OTEC Cycle:
Certain changes are proposed in the open OTEC cycle to make it economically viable.
1. Controlled Flash Evaporator.
2. OTEC plant for cogeneration of electricity and fresh water.
1. Controlled Flash Evaporation (CFE):
A CFE chamber is a vertical vessel of 2.5m height which is filled with a large number of vertical parallel chutes. The water flows on both sides of the chutes vaporises without violent process of bubble formation and bursting. Pure vapour free of entrained solids flows downwards. Both brine and vapour cool down as they travel down the chutes.
The pure low pressure steam enters the steam turbine and expands to the condenser. The de-aeration required is minimal in CFE. The high quality of steam going to the turbine is well suited to electrical and fresh-water cogeneration. The corrosion-problem in steam turbine is alleviated.
2. Cogeneration of Fresh Water and Electricity:
A surface condenser either shell and tube or plate type is used. The deep cold water is pumped through the condenser and discharged back to the ocean. The turbine fresh water condensate is used for various uses. When combined with a controlled flash evaporator, the quality of condensate is potable water with salt content 1 to 5 ppm. There is slight reduction in efficiency due to slightly higher pressure and temperature in the condenser as compared to direct contact condenser.
II. The Closed OTEC Cycle:
This is also called Anderson Cycle and uses Ammonia, Propane or Freon as the working fluid for the Rankine Cycle.
High pressure of working fluid results in low specific volumes and compact plant and less costs. The problems of evaporator are also avoided. However, a large size heat exchanger has to be used as boiler to keep the temperature differences small.
An Anderson OTEC power plant uses propane with 20°C temperature difference between warm surface and cool deep water. The propane is vaporized in the boiler at 10 bars and exhausted in the condenser at 5 bars. It employs thin plate-type heat exchangers to minimize the amount of material and cost. The heat exchangers are placed at depths in the ocean where static pressure of water is roughly equal to the pressure of the working fluid.
Limitations of OTEC Power Plants:
Problems facing commercial development of OTEC systems are enormous:
1. Low temperature differences result is very low plant efficiencies, very large plant size and huge capital cost.
2. For open systems, the problems associated with the evaporator design, operation and maintenance are many.
3. Steam turbines capable of generating 10 MW or more using low pressure steam have yet to be developed.
4. For closed systems, heat exchangers of very large size have to be designed and built.
5. Pumps handling large amounts of water have to be developed.
6. A 100 MW OTEC power plant may require 1 km long and 30m diameter pipe.
7. The whole plant has to be stationed and moored at large depth.
8. The deployment of the electrical cables required to carry the power to shore must be carefully done because it is subjected to severe stresses from its own weight and ocean currents & eddies.
9. An OTEC power plant will have to be capable of withstanding sever ocean storms over its lifetime, corrosion by seawater salts, erosion due to large volume flows, bio fouling due to algae growth and encruotation by various marine life such as barnacles.
Special Applications:
(i) OTEC Plant Ship:
The commercial-size OTEC systems are very large and suffer from various problems when placed in a stationary position. OTEC Plant ship concept can be used. The OTEC plant can be built as a specially designed ship. The grazing plant ship would move at low speed (about 0.5 knots) in search of warmest surface water.
The electricity generated can be used on board to manufacture energy-intensive products like aluminium, magnesium, and nickel, various alloys, semiconductors, etc. deep sea bed mining, chemical processing of products like ammonia.
(ii) Island OTEC Plants:
The case for OTEC is more promising for islands that have low population densities, that are dependent upon imported fossil fuels for generating electricity, and that have large warm-water resource in relation to their energy needs.
(iii) Hydrogen Energy:
The power generated in the OTEC plants can be used for hydrogen production by means of water electrolysis. In this case, the investment costs will be lower compared to electrolysis powered by electricity generated in conventional thermal and nuclear power plants. The energy transmission to the consumer locations shall be by means of hydrogen transport.
India is planning 1 MW OTEC plant off Chennai coast.