Dossier Océan et énergie - Énergie Thermique des Mers

by
L. A. Vega. Ph. D., Hawaii, USA

3) OTEC and the Environment

OTEC offers one of the most benign power production technologies, since the handling of hazardous substances is limited to the working fluid (e.g., ammonia), and no noxious by-products are generated. OTEC requires drawing seawater from the mixed layer and the deep ocean and returning it to the mixed layer, close to the thermocline, which could be accomplished with minimal environmental impact. The carbon dioxide out-gassing from the seawater used for the operation of an OC-OTEC plant is less than 1 percent of the approximately 700 grams per kWh amount released by fuel oil plants. The value is even lower in the case of a CC-OTEC plant.

A sustained flow of cold, nutrient-rich, bacteria-free deep ocean water could cause sea surface temperature anomalies and biostimulation if resident times in the mixed layer and the euphotic zone respectively are long enough (i.e., upwelling). The euphotic zone is the upper layer of the ocean in which there is sufficient light for photosynthesis. This has been taken to mean the 1 percent-light-penetration depth (e.g., 120 m in Hawaiian waters). This is unduly conservative, because most biological activity requires radiation levels of at least 10 percent of the sea surface value. Since light intensity decreases exponentially with depth, the critical 10 percent-light-penetration depth corresponds to, for example, 60 m in Hawaiian waters. The analyses of specific OTEC designs indicate that mixed seawater returned at depths of 60 m results in a dilution coefficient of 4 (i.e., 1 part OTEC effluent is mixed with 3 parts of the ambient seawater) and equilibrium (neutral buoyancy) depths below the mixed layer throughout the year. This water return depth also provides the vertical separation, from the warm water intake at about 20 m, required to avoid reingestion into the plant. This value will vary as a function of ocean current conditions. It follows that the marine food web should be minimally affected and that persistent sea surface temperature anomalies should not be induced. These conclusions need to be confirmed with actual field measurements that could be performed with the pre-commercial plant described in Section 9.

To have effective heat transfer it is necessary to protect the heat exchangers from biofouling. It has been determined that biofouling only occurs in OTEC heat exchangers exposed to surface seawater. Therefore, it is only necessary to protect the CC-OTEC evaporators. Chlorine (Cl₂) has been proposed along with several mechanical means. Depending upon the type of evaporator, both chemical and mechanical means could be used. To protect marine life, the Environmental Protection Agency (EPA) in the USA allows a maximum Cl₂ discharge of 0.5 mg l^-1 and an average of 0.1 mg l^-1. CC-OTEC plants need to use Cl₂ at levels of less than 10 percent of the EPA limits. The power plant components will release small quantities of working fluid during operations. Marine discharges will depend on the working fluid, the biocides, the depth of intake and the discharge configuration chosen.

Other potentially significant concerns are related to the construction phase. These are similar to those associated with the construction of any power plant, shipbuilding and the construction of offshore platforms. What is unique to OTEC is the movement of seawater streams with flow rates comparable to those of rivers and the effect of passing such streams through the OTEC components before returning them to the ocean. The use of biocides and ammonia are similar to other human activities. If occupational health and safety regulations like those in effect in the USA are followed, working fluid and biocide (most probably anhydrous ammonia and chlorine) emissions from a plant should be too low to detect outside the plant sites. A major release of working fluid or biocide would be hazardous to plant workers, and potentially hazardous to the populace in surrounding areas, depending on their proximity. Both ammonia and chlorine can damage the eyes, skin, and mucous membranes, and can inhibit respiration. Should an accident occur with either system, the risks are similar to those for other industrial applications involving these chemicals. Ammonia is used as a fertilizer and in ice skating rink refrigeration systems. Chlorine is used in municipal water treatment plants and in steam power plants. Chlorine can be generated in situ; therefore storage of large quantities of chlorine is not recommended.

Organisms impinged by an OTEC plant are caught on the screens protecting the intakes. Impingement is fatal to the organism. An entrained organism is drawn into and passes through the plant. Entrained organisms may be exposed to biocides, and temperature and pressure shock. Entrained organisms may also be exposed to working fluid and trace constituents (trace metals and oil or grease). Intakes should be designed to limit the inlet flow velocity to minimize entrainment and impingement. The inlets need to be tailored hydrodynamically so that withdrawal does not result in turbulence or recirculation zones in the immediate vicinity of the plant. Many, if not all, organisms impinged or entrained by the intake waters may be damaged or killed. Although experiments suggest that mortality rates for phytoplankton and zooplankton entrained by the warm-water intake may be less than 100 percent, in fact only a fraction of the phytoplankton crops from the surface may be killed by entrainment. Prudence suggests that for the purpose of assessment, 100 percent capture and 100 percent mortality upon capture should be assumed unless further evidence exists to the contrary. Metallic structural elements (e.g., heat exchangers, pump impellers, metallic piping) corroded or eroded by seawater will add trace elements to the effluent. It is difficult to predict whether metals released from a plant will affect local biota. Trace elements differ in their toxicity and resistance to corrosion. Few studies have been conducted of tropical and subtropical species. Furthermore, trace metals released by OTEC plants will be quickly diluted with great volumes of water passing through the plant. However, the sheer size of an OTEC plant circulation system suggests that the aggregate of trace constituents released from the plant or redistributed from natural sources could have long-term significance for some organisms.

OTEC plant construction and operation may affect commercial and recreational fishing. Fish will be attracted to the plant, potentially increasing fishing in the area. Enhanced productivity due to redistribution of nutrients may improve fishing. However, the losses of inshore fish eggs and larvae, as well as juvenile fish, due to impingement and entrainment and to the discharge of biocides may reduce fish populations. The net effect of OTEC operation on aquatic life will depend on the balance achieved between these two effects. Through adequate planning and coordination with the local community, recreational assets near an OTEC site may be enhanced.

Other risks associated with the OTEC power system are the safety issues associated with steam electric power generation plants: electrical hazards, rotating machinery, use of compressed gases, heavy material-handling equipment, and shop and maintenance hazards. Because the CC-OTEC power plant operates as a low-temperature, low pressure Rankine cycle, it poses less hazard to operating personnel and the local population than conventional fossil-fuel plants. It is essential that all potentially significant concerns be examined and assessed for each site and design to assure that OTEC is an environmentally benign and safe alternative to conventional power generation. The consensus among researchers is that the potentially detrimental effects of OTEC plants on the environment can be avoided or mitigated by proper design.

4) Engineering Challenges

The design and installation of a cost-effective pipe to transport large quantities of cold water to the surface (i.e., cold water pipe, CWP) presented an engineering challenge of significant magnitude complicated by a lack of evolutionary experience. This challenge was meet in the USA with a program relying on computer-aided analytical studies integrated with laboratory and at-sea tests. The greatest outcome achieved has been the design, fabrication, transportation, deployment and test at-sea of an instrumented 2.4 m diameter, 120 m long, fiberglass reinforced plastic (FRP) sandwich construction pipe attached to a barge. The data obtained was used to validate the design technology developed for pipes suspended from floating OTEC plants. This type of pipe is recommended for floating OTEC plants. For land-based plants there is a validated design for high-density polyethylene pipes of diameter less than 1.6 ¡@oped for pipes suspended from floating OTEC plants. This type of pipe is recommended for floating OTEC plants. For land-based plants there is a validated design for high-density polyethylene pipes of diameter less than 1.6 ¡@m. In the case of larger ameter pipes offshore techniques used to deploy large segmented pipes made of steel, concrete or FRP are applicable. Pressurized pipes made of reinforced elastomeric fabrics (e.g., soft pipes), with pumps located at the cold water intake, seem to offer the most innovative alternative to conventional concepts. However, the operability of pumps in 800 m to 1000 m water depths over extended periods must be verified and the inspection, maintenance and repair (IM&R) constraints established before soft pipes can be used in practical designs.

Other components for OTEC floating plants that present engineering challenges are the position keeping system and the attachment of the submarine power cable to the floating plant. Deep ocean-mooring systems, designed for water depths of more than 1000 m, or dynamic positioning thrusters developed by the offshore industry can be used for position keeping. The warm water intake and the mixed return water also provide the momentum necessary to position the surface vessel. The offshore industry also provides the engineering and technological backgrounds required to design and install the riser for the submarine power cable.

The design of OTEC CWPs, mooring systems and the submarine power cable must take into consideration survivability loads as well as fatigue induced loads. The first kind is based on extreme environmental phenomena, with a relatively long return period, that might result in ultimate strength failure while the second kind might result in fatigue-induced failure through normal operations.

5) Open Cycle OTEC

The open cycle consists of the following steps: (i) flash evaporation of a fraction of the warm seawater by reduction of pressure below the saturation value corresponding to its temperature (ii) expansion of the vapor through a turbine to generate power; (iii) heat transfer to the cold seawater thermal sink resulting in condensation of the working fluid; and (iv) compression of the non-condensable gases (air released from the seawater streams at the low operating pressure) to pressures required to discharge them from the system. These steps are depicted in Figure 1. In the case of a surface condenser the condensate (desalinated water) must be compressed to pressures required to discharge it from the power generating system.

Figure 1. Open-Cycle OTEC Flow Diagram.

The evaporator, turbine, and condenser operate in partial vacuum ranging from 3 percent to 1 percent atmospheric pressure. This poses a number of practical concerns that must be addressed. First, the system must be carefully sealed to prevent in-leakage of atmospheric air that can severely degrade or shut down operation. Second, the specific volume of the low-pressure steam is very large compared to that of the pressurized working fluid used in closed cycle OTEC. This means that components must have large flow areas to ensure that steam velocities do not attain excessively high values. Finally, gases such as oxygen, nitrogen and carbon dioxide that are dissolved in seawater (essentially air) come out of solution in a vacuum. These gases are uncondensable and must be exhausted from the system. In spite of the aforementioned complications, the Claude cycle enjoys certain benefits from the selection of water as the working fluid. Water, unlike ammonia, is non-toxic and environmentally benign. Moreover, since the evaporator produces desalinated steam, the condenser can be designed to yield fresh water. In many potential sites in the tropics, potable water is a highly desired commodity that can be marketed to offset the price of OTEC-generated electricity.

Flash evaporation is a distinguishing feature of open cycle OTEC. Flash evaporation involves complex heat and mass transfer processes. In the configuration tested by a team lead by the author, warm seawater was pumped into a chamber through spouts designed to maximize the heat-and-mass-transfer surface area by producing a spray of the liquid. The pressure in the chamber (2.6 percent of atmospheric) was less than the saturation pressure of the warm seawater. Exposed to this low-pressure environment, water in the spray began to boil. As in thermal desalination plants, the vapor produced was relatively pure steam. As steam is generated, it carries away with it its heat of vaporization. This energy comes from the liquid phase and results in a lowering of the liquid temperature and the cessation of boiling. Thus, as mentioned above, flash evaporation may be seen as a transfer of thermal energy from the bulk of the warm seawater to the small fraction of mass that is vaporized to become the working fluid. Approximately 0.5 percent of the mass of warm seawater entering the evaporator is converted into steam.

A large turbine is required to accommodate the huge volumetric flow rates of low-pressure steam needed to generate any practical amount of electrical power. Although the last stages of turbines used in conventional steam power plants can be adapted to OC- OTEC operating conditions, existing technology limits the power that can be generated by a single turbine module, comprising a pair of rotors, to about 2.5 MW. Unless significant effort is invested to develop new, specialized turbines (which may employ fiber-reinforced plastic blades in rotors having diameters in excess of 100 m), increasing the gross power generating capacity of a Claude cycle plant above 2.5 MW will require multiple modules and incur an associated equipment cost penalty. Condensation of the low-pressure working fluid leaving the turbine occurs by heat transfer to the cold seawater. This heat transfer may occur in a DCC, in which the seawater is sprayed directly over the vapor, or in a surface condenser that does not allow contact between the coolant and the condensate. DCCs are relatively inexpensive and have good heat transfer characteristics due to the lack of a solid thermal boundary between the warm and cool fluids. Although surface condensers for OTEC applications are relatively expensive to fabricate they permit the production of desalinated water. Desalinated water production with a DCC requires the use of fresh water as the coolant. In such an arrangement, the cold seawater sink is used to chill the fresh water coolant supply using a liquid-to-liquid heat exchanger.

Effluent from the low-pressure condenser must be returned to the environment. Liquid can be pressurized to ambient conditions at the point of discharge by means of a pump or, if the elevation of the condenser is suitably high, it can be compressed hydrostatically. Non-condensable gases, which include any residual water vapor, dissolved gases that have come out of solution, and air that may have leaked into the system, must be pressurized with a compressor. Although the primary role of the compressor is to discharge exhaust gases, it usually is perceived as the means to reduce pressure in the system below atmospheric. For a system that includes both the OC-OTEC heat engine and its environment, the cycle is closed and parallels the Rankine cycle. Here, the condensate discharge pump and the non-condensable gas compressor assume the role of the Rankine cycle pump.

The analysis of the cycle yields (Figure 1):

Heat (added) absorbed from seawater (J/s) qw = m. _ww Cp (T_wwi - T_wwo)

Steam generation rate (kg/s) m . s = q_w/h_fg

Turbine work (J/s) w_T = m . _s (h₃ - h₅) = m . _s h _T (h₃ - h _5s)

Heat (rejected) into seawater (J/s) q_c = m. _cwCp (T_cwo - T_cwi)

where,

m._ww is the mass flow rate of warm water; Cp the specific heat; T_wwi and T_wwo the seawater temperature at the inlet and outlet of the heat exchanger; hfg the heat of evaporation; and the enthalpies at the indicated points are given by h, with the subscript s referring to constant entropy. The turbine isentropic efficiency is given by h _T. The subscript cw refers to the cold water.

6) The 210 kW OC-OTEC Experimental Apparatus

As stated above, a 210 kW OC-OTEC Experimental Apparatus was conceived as the next step towards answering questions related to operation of OTEC plants. The apparatus was operated for six years (1993-1998) providing valuable data and pointing the way for future modifications and improvements in the OC-OTEC process. The turbine-generator was designed for an output is 210 kW for 26 ¢XC warm surface water and a deep water temperature 6 ¢XC. A small fraction (10 percent) of the steam produced was diverted to a surface condenser for the production of desalinated water. The highest production rates achieved were 255 kW (gross) with a corresponding net power of 103 kW and 0.4 l s^-1 of desalinated water. These are world records for OTEC. It must be noted that the net power was not optimized because pumping losses were relatively high due to the use of a seawater system that was already available. It is expected that for a commercial size plant the ratio of net to gross power will be approximately 0.7.

The data and experience obtained demonstrated that the OC-OTEC process is technically feasible for the production of base load electricity and desalinated water. It was also demonstrated that frequency control in the island mode is achieved with either a load-diversion-governor (LDG) or with the vacuum compression system. The most significant and exciting lessons learned were those that were fundamental new insights into the OC-OTEC process. The two main discoveries were the unstable synchronous generator output, and the violent outgassing of seawater in the heat exchangers. The most annoying problem was the frequent failures of the grease lubricated bearings of the centrifugal pumps used for the vacuum and exhaust system. In retrospect, this was due to a major design oversight. Equipment operating at speeds higher than approximately 27 000 revolutions-per-minute should, in general, not use grease lubricated bearings. It was concluded that high speed centrifugal pumps with, for example, magnetic bearings can be used in future OC-OTEC systems to achieve extended life cycles, relatively low power consumption and, therefore, optimum net power.

7) Design of a Small Land-Based OC-OTEC Plant

To understand the details of the design and operation of a Claude Cycle plant, it is useful to consider a specific example given by the design of a small land-based plant. The design philosophy reflects an emphasis on feasibility: a state-of-the-art cold water supply pipeline, a 1.6 m high-density-polyethylene (HDPE) conduit, was selected as the design starting point, rather than a prescribed net power output. Consistent with this choice, some critical hardware components were preferred over alternative configurations because they are presently available off-the-shelf. The major conclusion is that cost-effective electricity and desalinated water could be supplied to small South Pacific Island communities if appropriate financing is available. Depending on the importance of desalinated water for these communities, two global design options were identified:

	OC-OTEC power plant proper, with a net power production capability of about 1.2 MW and desalinated water production of 2200 m3 per day;
	OC-OTEC power plant fitted with a second-stage water flashing unit, with a net power production capability of about 1.1 MW and desalinated water production of 5150 m3 per day.

In addition, a 300-room air conditioning system can be included for resort development.

These configurations are cost competitive with conventional power plants if investment loans with interest rates of less than 5 percent are available and if credit for desalinated water and air-conditioning byproducts are included. However, the transfer of OTEC to small Pacific Islands should not be exclusively assessed from the perspective of present-day cost effectiveness since it offers these isolated communities some degree of energy independence while preserving their environment. In this regard the most important finding is not related to technical matters but rather to financial considerations: the announcement by industrialized countries, after the Kyoto Conference, that low interest loans would be made available to developing countries for renewable energy projects with minimal environmental impact.

Figure 2. Heat and Mass Balance of a 1.8MW-gross OC-OTEC Plant with Additional 2^nd Stage Seawater Desalination.

The heat and mass balance is given in Figure 2. A 6156 kg s^-1 flow rate of warm seawater at 26 ¢XC is supplied via a 2.5 m ID FRP pipe. The pipe has an intake depth of 25 m and is 120 m long. Five inline dry motor vertical turbine propeller pumps (three operational, two standby) supply the flow to an intake pool below the first stage evaporator. The intake pool has a nominal operating level of 2.78 m from mean sea level (MSL). This level is selected to provide enough head (level) in the mixed flow discharge pool for gravity discharge into the ocean. Three inline submersible propeller-type pumps (two operational, one standby) bring 3175 kg s ^-1 of cold seawater through a 1.6 m OD pipe from a depth of 1000 m. The pipe length is 2590 m. A 3085 kg s^-1 flow rate of 4 ¢XC cold seawater is available for the OTEC system, whereas 90 kg s^-1 is reserved for air-conditioning applications. An upriser takes the warm water into the evaporator. A pre-deaeration nozzle removes a portion of the non-condensables from the warm water accumulated below the spout plate. The evaporator spout plate has 122 spouts and the warm water flashes through the spouts into the evaporation chamber at a pressure of 2.76 kPa. A small fraction (26.1 kg s^-1) of supply water is changed into steam and the rest is discharged into the first stage discharge pool at a temperature of 23.4 ¢XC. The discharge pool at a level of 1.76 m MSL also acts as the supply pool for the second stage evaporator. The evaporation pressure in the second stage is 2.22 kPa. No pre-deaeration is provided in the second stage as the water has been deaerated in the first stage. The steam flow from the second stage evaporator is 34 kg s^-1. The effluent water from the second stage evaporator at 20 ¢XC goes into the mixed water discharge pool.

Steam from the first stage evaporator enters the turbine at 2.74 kPa and leaves the turbine diffuser system at 1.29 kPa. The turbine generator system gives a gross output of 1838 kW. Steam exhausted from the turbine-diffuser system enters the first stage main surface condenser. The main condenser receives 2702 kg s^-1 of cold seawater at 4 ¢XC and condenses 92 percent of the incoming steam. The vent condenser gets 281 kg s^-1 of 4 ¢XC cold seawater supply and condenses 90 percent of the steam leaving the main condenser. The remaining steam and the non-condensables are evacuated by the vacuum compressor system. Steam generated by the second stage evaporator enters the second stage main condenser at 2.18 kPa and 19.2 ¢XC. This steam is expected to be virtually free of non-condensables. The condenser receives 3085 kg s^-1 of cold seawater at 9.4 ¢XC and discharges it at 16.2 ¢XC. The minimal amount of uncondensed steam, along with any non-condensables, goes to a vent condenser. A hook-up is provided to let the vapor compressor system remove any non-condensables and water vapor from the vent condenser.

Non-condensables and steam vapor from the first and second stage vent condenser enters the vacuum compressor system through a counter-current direct contact precooler. The precooler receives 4 ¢XC cold seawater and ensures that the mixture temperature at the first stage inlet of the compressor system is not more than 5 ¢XC and that the entire vapor is condensed until its partial pressure becomes equal to the seawater saturation pressure at 5 ¢XC. The basic compressor system has four stages with direct contact coolers in-between. The fourth stage compressor takes the non-condensables from the third stage. The discharge from the fourth stage is re-injected at 30 kPa into the warm water effluent

returning from the second stage evaporator. A fifth stage compressor is also provided to alternately bypass the re-injection scheme and discharge into the atmosphere. The first four stages are centrifugal, whereas the fifth stage is a positive displacement type.

The cold and warm water effluents from the second stage combine into a mixed discharge pool with a nominal level of 0.75m MSL. A 3 m ID, 190 m long and 60 m deep pipe provides gravity discharge recourse for the mixed water system. The net power from the system is 1126 kW obtained after subtracting 334 kW for cold water supply pumping; 284 kW for warm water supply pumping; 80 kW for compressor system; and 14 kW for desalinated water pumping from the gross power produced. Without second stage water production the combined pumping losses will be reduced by approximately 100 kW (net increase) and the desalinated water production would be 25.8 kg s^-1.

~to be continued