POTENSI OTEC DI INDONESIA
4. Isentropic compression of the gas at entropy S A (from D to A)
FIGURE 6 Temperature–entropy diagram for an ideal Carnot cycle.
As shown in Figure 6, the typical Carnot cycle acting as a heat engine consists of four steps:
1. Isothermal expansion of the gas at
The amount of thermal energy transferred between the cold reservoir and the system is: Q C = T C (S B − S A ).
The maximum possible efficiency would be
Therefore
Or
Ocean Thermal EnergyHarvesting 311
or
TH −TC TC
TH TH
η= =1− . (5.7)
efficiency is η =0.060.
Equation 5.6 defines the principles of the Carnot efficiency. Based on this equation, it can be concluded that ideally the maximum thermal efficiency (or the thermal efficiency of a reversible heat engine) depends only on the temperature of the two thermal-energy reser- voirs involved. Therefore, seasonal sea temperature variations may influence the overall OTEC power generation rates.
For warm seawater at 77◦F (298.15◦K) and cold seawater at 45◦F (280.37◦K), the Carnot A generalized thermodynamic cycle is shown in Figure 5.7.
Based on this cycle
(5.8) and
(5.9)
(5.10) QH = ∫
H THdS
QC = ∫
C TC dS.
Therefore, the net thermodynamic cycle work becomes W = ∫
H TH dS − ∫
CTC dS or
W = Tds. (5.11)
Consequently, the efficiency of a nonideal OTEC system would be
QH
QH−QC TC TH
η= <1− , (5.12)
T TH
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W QH QC TC
QC
SA SB S
FIGURE 5.7 Temperature–entropy diagram for a generalized thermodynamic cycle.
312 Energy Harvesting: Solar, Wind, and Ocean Energy Conversion Systems
S 1
3
4 T
TH
TC
W QH QC 2
FIGURE 5.8Temperature–entropy diagram for a Rankine cycle.
whereηusually is less than 6%. Another reason for this low efficiency is the operation of pumps in transfer processes. They consume about 20–30% of the power generated by the turbine generator [11]. For other various practical reasons, 3% is the typical efficiency that can be reached.
For an OTEC system, the typical cycle used is the Rankine cycle, as shown in Figure 5.8.
In a Rankine cycle, there are four processes:
1. The working fluid is pumped from low to high pressure. Since the working fluid is liquid at this process, the pump requires a small amount of input energy (from 1 to 2).
2. The high-pressure liquid enters a boiler, where it is heated at constant pressure by the warm surface seawater to become dry saturated vapor (from 2 to 3).
3. The dry saturated vapor expands through a turbine, generating power. This process decreases the temperature and the pressure of the vapor. Approximately, it is an isentropic expansion (from 3 to 4).
4. The vapor then passes through a condenser, where it is condensed at a constant pressure by cold water to turn into liquid form (from 4 to 1).
3. Technical Obstacles of Closed-Cycle OTEC Systems 1. Working Fluids and Its Potential Leakage
The working fluid should have a low boiling point, high density, and high pressure.
Ammonia and fluorinated carbons are popular choices [15]. Ammonia has superior transportation properties, easy availability, and low cost; however, it is toxic and flammable. On the other hand, fluorinated carbons are potential threats to the ozone layer.
Hydrocarbons like ethane, propane, or butane are other appropriate options, but they are also flammable.
Amixture of working fluid, composed of 1% ethane, 98% propane, and 1% normal butane, on a mole percentage basis is proposed as an alternative working fluid [16]. The mixtures can vaporize and condense at varying temperatures in constant-pressure processes, while pure fluids vaporize and condense at constant temperature in constant-pressure processes,
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Ocean Thermal EnergyHarvesting 313
Working fluid Pure Hot seawater
Mixture
Temperature
Cold seawater
Entropy FIGURE 5.9Comparison of mixture and pure fluid.
as shown in Figure 5.9. The dashed lines represent pure working fluid, whereas the solid lines represent mixture working fluid.
With the assumption of 10◦F inlet temperature difference and 3◦F exit temperature differ- ence for both the evaporator and condenser, the log mean temperature differences (LMTDs) of the pure fluid cycle is 90% of that of the mixture cycle [16].
Another major advantage of mixtures over pure fluids is that under progressive fouling, the mixture cycle possibly can be maintained operative for a longer time when compared to pure fluids, as shown in Figure 5.10, since the driving force for heat transfer is uniform throughout the heat exchanger for the mixture cycle [16].
In Figure 5.10,ΔTis the temperature difference across the water-side fouling. It increases during the fouling progress.
The evaporator, turbine, and condenser operate in partial vacuum ranging from 1% to 3% atmospheric pressure. Therefore, the system must be carefully sealed to prevent the potential leakage of working fluid and potential in-leakage of atmospheric air.
Cold seawater Pure working fluid
Temperature
Cold seawater Mixture working
fluid
Entropy
Temperature
ΔT
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ΔT
(a) (b)
Entropy
FIGURE 5.10 Comparison of mixture and pure fluid with biofouling.
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314 Energy Harvesting: Solar, Wind, and Ocean Energy Conversion Systems
5.3.1.1 Degradation of Heat Exchanger Performance by Microbial Fouling
Degradation of surfaces by biological entities and deposition of living matter on intake pipes is collectively termed as biofouling. In a closed-cycle OTEC system, since seawater must pass through the heat exchanger, the microbial fouling could degrade its thermal conduc- tivity and affect its performance. It depends on several factors such as water temperature, construction material of the heat exchanger, and water nutrient level [17].
In a study in 1977 [18], some mock heat exchangers were exposed to seawater for 10 months and researchers discovered that although the level of microbial fouling was low, the thermal conductivity of the system was significantly impaired. It is concluded that the apparent discrepancy between the level of fouling and the heat transfer impairment is the result of a thin layer of water trapped by the microbial growth on the surface of the heat exchanger.
Another study, conducted in 1985 at Hawaii, concluded that microbial fouling layers as thin as 25–50μm can degrade heat exchanger performance by as much as 40–50% [19].
The conclusion is that although the physical cleaning, even simple brushing or passing sponge rubber balls through the cubes, can decrease the rate at which fouling occurs, it is not enough to completely halt microbial growth [19]. Furthermore, it is found that the microbes began to grow more quickly after cleaning under the selection pressure [20].
Nickel or zinc plating can be used for biofouling reduction. However, nickel-based designs cause thicker channel walls, which may reduce the efficiency of the heat exchanger [21]. In addition, instead of taking the actual sea surface water, the HWP intake could be placed at some point below the actual sea surface, for example, 30 m depth, to solve the biofouling problem, because there are less microorganisms there than on the sea surface.
However, deeper sea layers have cooler water resulting in OTEC power reduction, due to the lower temperature difference.
Chlorination is another alternative. Berger’s study examined this approach and con- cluded that chlorination levels of 0.1 mg/L treated for 1 h per day slowed microbial growth appreciably and may show effective results in the long-term operation of a plant [19].
However, there is another consideration associated with the environmental impact of dis- charging chlorination into the ocean. Fortunately, experiments conducted at the Natural Energy Laboratory of Hawaii have demonstrated that very small, environmentally benign, levels of chlorine can successfully control the microfouling [22].
5.3.2 Thermal Energy Conversion for OTEC Systems
The main basis for OTEC is the mechanical powerPmavailable from the process, which is expressed as [4]
Pm=Pfrom − Pfrom.
evaporator CWP
(5.13)
The temperature relationships of the intake temperature of the HWP and CWP can be explained as
Tai = Tω − dTω −dTa, Taf = Tc − dTc −dTa,
(5.14) (5.15) whereTωis the intake temperature of the HWP,Tcis the intake temperature of the CWP, Tai and Taf are the initial and final temperatures of the working fluid, respectively. In
Ocean Thermal EnergyHarvesting 315
Equations 5.14 through 5.16, d is the temperature loss factor associated with the HWP and CWP intake temperatures and varies from 0 to 1.
The difference between the final and initial temperatures of the working fluid can be calculated as
(5.16)
Si=
Taf − Tai = (Tω − Tc)(1 −d).
The entropy intake and discharge rates, Si and Sf, can be expressed as Pfrom evaporator
Tai (5.17)
and
Pfrom CWP
Sf= . (5.18)
Taf
Using a lumped loss coefficientL, the actual power that can be extracted can be expressed as Poutput = LSi (Tai − Taf). (5.19) Generally,Lvaries around 0.75–0.85. The electrical conversion efficiency is usually in the range of 90% ofPoutput. In order to achieve the cited cold seawater temperature, 600–1000 m depth ranges are required for typical CWPs.
3. Open-Cycle OTEC Systems
1. Structure and Principles of Open-Cycle Systems
In an open-cycle OTEC process, which is shown in Figure 5.11, seawater functions as the working fluid. The boiling temperature of water is a function of pressure [13]. It drops as the pressure decreases. The first step is boiling the warm shallow water by placing it in a low-pressure container of about 2% atmospheric pressure at sea level. Then the expanding steam drives a low-pressure turbine coupled to an electrical generator. The
Vacuum pump
Power
Desalinated water
Cold water in Warm
water in Discharge
water to sea Discharged
warm water Discharged cold water Vacuum chamber
flash evaporator
Condenser Steam
Vacuum chamber flash evaporator FIGURE 5.11 Block diagram of opencycle.
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316 Energy Harvesting: Solar, Wind, and Ocean Energy Conversion Systems
steam, which leaves its salt and contaminants behind in the low-pressure container, is desalinized. Afterwards, it is chilled and condensed back into a liquid by exposure to cold temperatures from deep ocean water. The by-product, desalinized fresh water, which is suitable for human consumption or irrigation, is valuable especially in local communities, where natural freshwater supplies are limited.
In contrary to the closed-cycle OTEC systems, seawater is used as the effective working fluid in the open-cycle OTEC systems. Some of the warm seawater intake is flashed and boiled by bringing it to a low-pressure chamber (vacuum). The resultant expansion of the steam drives a very low-pressure turbine. The cold seawater intake from the ocean deep is used for condensation. The vacuum vessel is coupled to the low-pressure steam turbine within the low-pressure environment. The steam produced in the vacuum vessel is desalinated; therefore, the condensed discharge water is also desalinated.
The heat exchangers are partially eliminated in the open-cycle design, which results in eliminating the losses associated with the heat exchanger, which is the main advantage of the open-cycle method. However, direct use of the seawater as working fluid and the need for a special design for the vacuum vessel are the disadvantages of the open-cycle system [4].
In 1993, the largest open-cycle OTEC plant was designed by Pacific International Center for High Technology Research. The plant was constructed and operated at Keahole Point, Hawaii, which is a 210 kW plant [23]. Considering the seawater pumps and vacuum sys- tems’ electricity consumption of about 170 W, the nominal net output of this experimental plant was about 40 kW. Following the successful completion of experiments, this open- cycle OTEC plant was shut down in January 1999 [24], since it was established for these experiments only.
5.3.3.2 Technical Difficulties of Open-Cycle OTEC Systems
Most of the drawbacks of open-cycle OTEC systems are due to the great turbine sizes. Since the turbine operates at a very low-pressure condition, ranging from 1% to 3% of atmo- spheric pressure, open-cycle systems require very large turbines to capture relatively small amounts of energy. Georges Claude, the inventor of the open-cycle process, calculated that a 6 MW turbine would need to be about 10 m in diameter. Recent re-evaluation of Claude’s work [25] indicates that modern technology cannot improve his design, signifi- cantly. Therefore, it seems that the open-cycle turbines are limited to 6 MW, unless some new specialized turbines are developed, which may utilize fiber-reinforced plastic blades in rotors with diameters bigger than 100 m. With current technology, increasing the gross power-generating capacities of a Claude cycle plant above 2.5 MW will incur significant increase in its complexity and cost, and reduce its efficiency [15].
4. Hybrid Cycle OTEC Systems
1. Structure and Principles of Hybrid OTEC Systems
Another option is to combine the two processes together into an open-cycle/closed-cycle hybrid, which combines the features of both the systems, as shown inFigure 5.12. In a hybrid OTEC design, both seawater and other fluids such as ammonia are used as working fluids [26,27].
In a hybrid OTEC system, warm seawater first enters a vacuum chamber, where it is partly flash-evaporated into steam, similar to the open-cycle evaporation process. Then the steam
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Desalinized Power water
Ammonia turbine
Warm seawater
Liquid ammonia pump Steam
Ammonia condenser Spouts
Steam condenser/
ammonia vaporizer
Vacuum pump
Noncondensable gases Steam turns to
desalinized water
Cold seawater
FIGURE 5.12General structure of the hybrid OTEC process. (Redrawn from Natural Energy Laboratory of Hawaii Authority, USA, available at:http://www.nelha.org,March 2008.)
goes to another vaporizer to release its heat to vaporize the working fluids, like ammonia, in a method similar to the closed-cycle evaporation process. An effervescent two-phase, two-substance mixture is obtained by physically mixing the second fluid with the warm seawater. The evaporated second working fluid is separated from the steam/seawater, which is recondensed similar to the closed cycle. The low-pressure turbine is easily driven by the phase change of the seawater/ammonia mixture. The vaporized fluid then drives a turbine to produce electricity, and the steam condenses within the heat exchanger and turns into desalinated water.
The close coupling of seawater and second working fluid is the main advantage of the hybrid OTEC design. A condenser heat exchanger is needed, even though an evaporator heat exchanger is not required. The separation of two fluids also needs some special design [28,29]. The ammonia vapor/liquid mixture needs to be rendered completely to the liquid phase using a compressor.
Compared to the closed-cycle system, the hybrid system avoids the problem of the degradation of the heat exchanger, caused by microbial fouling, since the cold water heat exchanger has little or even no microbial fouling [19]. In the second vaporizer of this system, desalinized water can be produced as the by-product. When compared to the open-cycle system, power-generating capacity can be improved.
5.4 Components of an OTEC System
Heat exchangers (evaporators and condensers) are critical components utilized in OTEC systems. In this section, the basic structures of these components and vacuum flash evaporators are described and analyzed.
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318 Energy Harvesting: Solar, Wind, and Ocean Energy Conversion Systems
Cold stream inlet
Hot stream inlet Hot stream outlet
Cold stream outlet
Exchange Hot stream
Cold stream
FIGURE 5.13Countercurrentexchange.
5.4.1 Heat Exchanger
A heat exchanger is used within the closed-cycle OTEC systems to evaporate the working fluid. Heat exchanger can work at two mechanisms: countercurrent exchange and parallel exchange. In the countercurrent exchange, hot and cold streams flow in opposite directions, as shown in Figure 5.13.
In the parallel exchange, hot and cold streams flow in the same direction, as shown in Figure 5.14.
To determine the temperature driving force for heat transfer in a heat exchanger, the LMTD is used. It is the logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger. For countercurrent flow, it is expressed as
ln((T1 − t2)/(T2 −t1)) (T1 − t2) − (T2 −t1)
LMTD= . (5.20)
For parallel flow, it is expressed as
ln((T1 − t1)/(T2 −t2)) (T1 − t1) − (T2 −t2)
LMTD= , (5.21)
where T1 is the hot stream inlet temperature, T2 is the hot stream outlet temperature, t1 is the cold stream inlet temperature, and t2 is the cold stream outlet temperature.
5.4.2 Evaporator
An evaporator is a heat exchanger, which vaporizes a substance from its liquid state to its gaseous state, as shown inFigure 5.15. The liquid working fluid (low temperature) is fed
Hot stream inlet Hot stream outlet
Cold stream inlet Cold stream outlet
Hot stream
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Exchange Cold stream
FIGURE 5.14 Parallel heat exchangeroperation.
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Liquid working fluid
Gaseousworking fluid
Warm seawater
FIGURE 5.15Schematic diagram of an evaporator.
into the U-type tubes, absorbing heat from the outside heat source (passing warm seawater), which will turn it to gas.
In an evaporator, the main heat transfer passage isconduction, not theradiationorconvec- tion. The conduction heat transfer can be defined as the transfer of thermal energy through direct molecular interaction within a medium or between mediums in direct physical contact without the flow of the medium material.
The rate of heat transfer can be expressed by Newton’s cooling law [30,31]:
dQ
dt = hA(T0−Tenv), (5.22)
whereQis the transferred thermal energy,tis the time,his the heat transfer coefficient, A is the heat transfer surface area,T0is the temperature of the object’s surface, andTenvis the temperature of the environment.
It can be stated that the rate of heat loss of a body is not only proportional to the tempera- ture difference between the body and its surroundings environment, but also proportional to the heat exchanging surface area. The tubes usually are designed in U shape to increase the heat exchanging surface area.
If a plate-fin exchanger is used, the heat transfer coefficient can be calculated as [32]
⎡
h =⎣ A 1−AApf 1− hp
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. Σ . ΣΣ Σ−1
tan hl(2h /pk )δ aA
p
A l,
(2hp/kδ) + kAW + hSW1⎤⎦−1
, (5.23)
whereais the wall thickness (m),Ais the total outside area of the plate-fin panel (m2), Af
is the total fin area of the contact with propane (m2),Apis the total area of the contact surface with propane,Awis the average wall area (m2),δis the fin thickness (m),hpis the film conductance of propane, and hswis the film conductance of seawater. The film conductance is a parameter that describes the heat transfer capability of fluids.hpandhsw
are functions of the physical and chemical fluid properties such as temperature and salinity.
The conductance also depends on the flow rates of the fluids. In Equation 5.23, the other parameters such askis the thermal conductivity of evaporator material [J/(ms,◦K)] andl is one-half of the fin length (m).
A schematic diagram of an evaporator is presented in Figure 5.16, and a dynamic model and its controller are presented inFigure 5.17[33]. In this model, the temperature of the
320 Energy Harvesting: Solar, Wind, and Ocean Energy Conversion Systems
Evaporator Hot watercycle Thi
phi Tho pho Twopwo
Twipwi Working fluid FIGURE 5.16 Evaporator schematicdiagram.
hot water is given, and the manipulated variable is the hot water mass flow rate [33]. By changing the rate of the hot water mass flow through the hot water pump, the heat quantity of the outlet working fluid can be controlled.
In Figure 5.16,Thiis the temperature of inlet hot water to the evaporator,Thois the temperature of outlet hot water from the evaporator,phois the pressure of outlet hot water, phiis the pressure of inlet hot water, wherephi=pho=ph, andmhis the mass flow rate of hot water.Qhiis the heat quantity of inlet hot water,Qhois the heat quantity of outlet hot water,Twiis the temperature of inlet working fluid,Twois the temperature of outlet working fluid,pwiis the pressure of inlet working fluid,pwois the pressure of outlet working fluid, andmwis the mass flow rate of working fluid.
This model is based on the relationship between the heat exchange value of hot water in the evaporator,ΔQh, and heat exchange value of the working fluid,ΔQw. The heat quantity can be selected as the state variable [34].
In the evaporator, the heat exchange value of the hot water is
ΔQh =(hho−hhi)mh (5.24)
and the heat exchange value for the working fluid is
ΔQw=(hwo−hwi)mw, (5.25)
wherehdenotes the water enthalpy. In Equations 5.24 and 5.25, the evaporator inlet enthalpy of hot water (hhi)is a function of the evaporator temperature of inlet hot water (Thi)and the pressure of hot water (ph):
(5.26)
G–1 F–1 F G
+– +
Qrwo ΔQh w ΔQw Qwo
ΔQm ΔQrw
Controller
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hhi = h(Thi, ph).
Model ΔQ¢
w
Qwi
FIGURE 5.17 System model and itscontroller.