The main components of an OTEC system are described below- namely, heat exchangers, evaporators, turbines and condensers. The design of heat exchangers to meet industrial requirements for efficiency, durability, ease of manufacture, packaging, system integration reliability and cost has led to an extensive technology devoted just to this subject. The special requirements of OTEC can be met by heat exchangers with different operating characteristics than conventional designs. Also, research has been done to increase the overall heat transfer coefficients in ways that will reduce the heat exchanger costs per kilowatt of net power generated. This has led to the investigation of various potential types of heat exchangers with features designed to be optimal for OTEC applications. Some of these are briefly described below.
Shell and Tube Heat Exchangers: This is the most widely used type of heat exchanger for industrial evaporator and condenser applications. As the name implies, this type consists of a shell and a bundle of tubes inside it. Specifically for OTEC applications, water flows through the tubes and the working fluid flows across the tube bank in the middle section. In conventional ones, seawater flows through the tubes, and the working fluid evaporates or condenses in a shell around them. This design can be enhanced by using fluted tubes: the working fluid flows into the grooves and over the crests, producing a thin film that evaporates more effectively.
Plate Heat Exchangers: Another type of heat exchanger that would offer advantages in performance and cost is the plate heat exchanger. The plate type heat exchanger is more compact than the shell and tube configuration. In this type, the seawater and the working fluid flow in alternate channels separated by parallel plates. Suitable manifolds are used to guide the fluid into the proper channels. With this type of heat exchanger the gains in heat transfer coefficient can be up to 100-200%, compared with the conventional shell and tube designs.
The material which heat exchangers are made of is very important in terms of cost and performance. Titanium was the original material chosen for closed-cycle heat exchangers because it resists corrosion. However, it is an expensive option for plants that use large heat exchangers. Therefore other cheaper materials such as corrosion-resistant copper-nickel alloys can be used to protect platform and cold-water pipes, but are not compatible with ammonia, the most common working fluid. A suitable alternative is aluminium which performs well under marine conditions and results indicate that selected aluminium alloys may last 20 years in seawater (Thomas & Hills, 1989). Marine organisms and slime can quickly grow on surfaces exposed to warm seawater- a buildup known as biofouling- and this reduces the heat transfer efficiency. Laboratory experiments indicate that the addition of chlorine in the pipes can prevent biofouling (Panchal, et al., 1984).
3.5.2 Evaporators for Open Cycle OTEC Systems
Open-cycle flash-evaporators include those with open-channel flow, falling films, and falling jets. These conventional evaporators typically perform to within 70% to 80% of the maximum thermodynamic performance at acceptable hydraulic losses. The technological development led to a vertical-spout evaporator that can perform to within 90% of the thermodynamic limit (National Renewable Energy Laboratory, n.d.). In this evaporator, water is drawn upward through a vertical pipe (a spout) and violently sprayed outward by escaping steam (Bharathan & Penney, 1984). To enhance performance, the spray may fall on screens that further break up the droplets and increase the evaporation rate. To avoid pressure loss, the evaporator has simple intake and exit systems that separate the steam from the discharge. Steam continues through the system, and the remaining seawater is discharged from the bottom of the evaporator. Violent flashing in a spout evaporator causes seawater droplets to be entrained by the steam. If they are not removed, these droplets can cause erosion and stress-corrosion cracking in turbine blades and contaminate the desalinated water discharge as well. Passing the steam through the commercially available mist eliminators used in the process industry removes a sufficient quantity of these seawater droplets (Bharathan & Penney, 1984).
3.5.3 Turbines
In the open cycle process, after the droplets are removed, steam flows through large, low-pressure turbines, entering at a pressure of about 2.4 kPa. These turbines must be able to handle the large steam flows necessary to produce a significant amount of electric power. Multistage turbines used in nuclear or coal-fired power plants are already available. The low-pressure stages of these turbines typically operate at conditions close to those needed in an open-cycle OTEC plant. In close cycle OTEC systems the turbine needs not be so large because it works with vapor at elevated pressures.
3.5.4 Condensers for Open Cycle OTEC Systems
Once the steam passes through the turbines, it can be condensed in direct-contact condensers or surface condensers. A surface condenser consists of an intermediate solid wall, which is absent in direct-contact condensers and therefore the latter provides more effective condensation (Bharathan, Parsons, & Althof 1988). In one designâ€"a two-stage condenser (see figure 4 below) developed at Solar Energy Research Instituteâ€"cold seawater is distributed through two open-ended vessels filled with a commercially available structured packing material. About 80% of the steam is condensed as it flows through the first vessel in the same direction as the cold seawater. The remaining steam is routed into the bottom of the second vessel and flows through it in the opposite direction to the seawater. At the top of the second vessel, a vacuum system pumps out the non-condensable (inert) gases along with any uncondensed steam (National Renewable Energy Laboratory, n.d.).
Figure : Illustration of a two stage condenser (National Renewable Energy Laboratory, n.d.)
Surface condensers keep the cooling seawater separate from the spent steam during condensation. By using indirect contact, the condensers produce desalinated water that is relatively free of seawater impurities. The surface condensers considered for use in OTEC systems are similar to those used in conventional power plants; however, these surface condensers must operate under lower pressures and with higher amounts of non-condensable gases in the steam. These non-condensable gases which are present in the open cycle system are released from the seawater when it is exposed to low pressures under vacuum and are namely oxygen, nitrogen and carbon dioxide. Air can also enter the open cycle vacuum chamber through leaks therefore decent construction techniques can reduce the rate of air leakage to very low levels. These gases, if are not removed from the vacuum vessel, they can build up enough pressure to stop evaporation. An exhaust compressor is usually used to remove these non-condensable gases. The compressor however requires about 10% of the total power generated by the system (Parsons, et al., 1985).
