The main problems of today’s society are; the ever increasing demand in energy, a possible resource shortage and environmental pollution. Therefore a lot of money is spent in research to find alternative ways of producing energy using Earth’s natural resources like wind, sun and the ocean. This report is a study on Ocean Thermal Energy Conversion (OTEC) which might be the answer to the energy and environmental problem our world has to face. A basic description and an introduction to the types and principles of operation of closed cycle, open cycle and hybrids of OTEC technology is given in this report. Emphasis is shown on the hybrid cycle which has advantages compared to other single purpose plants for fresh water production or power generation. The process of desalination of sea water can prove vital for countries which have water shortage. This report is focused on the technology that would be suited for a hybrid cycle OTEC plant situated in Cyprus.
Contents
Table of Figures
1. Aims and Objectives
The aim of this project is to investigate the characteristics of a dual purpose plant for simultaneous production of desalinated water and electric power. An attempt will be made to state the conditions and criteria for adjusting the ratio of desalinated water to power production and also how this ratio is affected by a variation in seawater temperature and pressure. In addition, different design considerations of the hybrid cycle particularly suitable for an OTEC plant operating in Cyprus will be discussed.
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2. Introduction
”The ocean surface absorbs more heat from the sun in one square mile than could be produced by burning 7000 barrels of oil” (Avery & Wu, 1994). Therefore the solar energy absorbed per day by the surface waters can be used to provide a green, zero-emission production of electricity. Ocean Thermal Energy Conversion or OTEC is the technology that meets the technical requirements and is economically viable for harvesting the solar energy absorbed by the ocean. OTEC plants make use of the thermal gradient that exists between the surface of the ocean and the deep cold waters to initiate a power producing cycle. The warm tropical surface waters (at a temperature of ≈22°C) and the deep ocean waters (at a depth of 1000m a temperature of ≈4°C) are used as a source of thermal energy to vaporize and condense the working fluid of a turbine-generator system. The thermodynamic cycle for this heat engine is the called the Rankine cycle. OTEC systems maybe either closed cycle, open cycle or a combination of the two also known as hybrid systems. In the closed cycle, seawater is used to vaporize and condense a working fluid, such as ammonia, which drives a turbine-generator in a closed loop, producing electricity. In the open cycle, surface water is flash-evaporated in a vacuum chamber. The resulting low-pressure steam is used to drive a turbine-generator. Cold seawater is used to condense the steam after it has passed through the turbine. The open cycle can, therefore, be configured to produce fresh water as well as electricity. The hybrid cycle consists of both the closed cycle system and the open cycle system. The system is interconnected and arranged in such a manner that the former cycle provides electricity and the latter yields desalinated water. Such a system has promising potential in countries which have issues with lack of water.
3. Literature Review
3.1 Closed Cycle OTEC System
3.1.1 Principle of Operation
The closed cycle was first proposed in 1881, by D’Arsonval in France, and was demonstrated in 1979, when a small plant mounted on a barge off Hawaii (Mini-OTEC) produced 50 kW of gross power, for several months, with a net output of 18 kW (Vega, 1992). The principle of operation of the closed OTEC system is demonstrated in figure 1 below.
Figure : Principle of operation of a closed cycle OTEC system (National Renewable Energy Laboratory)
In the closed cycle, the OTEC system utilizes the warm surface seawater to vaporize a working fluid, such as ammonia, which flows through a heat exchanger (evaporator). The vapor expands at modest pressures and drives a turbine which is coupled to a generator that produces electricity. The vapor then passes through another heat exchanger (condenser) where it is condensed back into a liquid using cold seawater from pumped from the ocean’s depths through a cold water pipe. A pressurizer or feed pump is used to pump the condensed working fluid back to the evaporator to complete the cycle, producing continuous power generation as long as the warm water and cold water continue to flow.
The closed OTEC cycle is basically the same as the conventional Rankine cycle employed in steam engines, in which the steam is condensed and returned to the boiler after driving a piston or steam turbine, OTEC differs by using a different working fluid and lower pressures and temperatures (Avery & Wu, 1994). The four processes of the ideal Rankine cycle are listed below:
1. Isentropic expansion (Turbine)
2. Isobaric heat rejection (Condenser)
3. Isentropic compression (Pump)
4. Isobaric heat addition (Evaporator)
3.2 Open Cycle OTEC System
3.2.1 Principle of Operation
The open cycle concept was first proposed in the 1920’s and demonstrated in 1930, off Cuba by its inventor, a Frenchman by the name of Georges Claude (Vega, 1992). In the open cycle, the warm seawater is the working fluid. The warm seawater is pumped into a vacuum chamber where it is ”flash”- evaporated to produce steam at an absolute pressure of about 2.4kPa. The steam passes through a low pressure turbine which in turn drives a generator to produce electricity. The steam exiting the turbine flows is condensed by cold seawater pumped from the ocean’s depths through a cold-water pipe. In this open cycle configuration a surface condenser can be used and therefore the condensed steam remains separated from the cold seawater and provides a supply of desalinated water. Figure 2 below shows the principle of operation of the open cycle OTEC system.
Figure : Principle of operation of an open cycle OTEC system (National Renewable Energy Laboratory)
3.2.2 Disadvantages of the Open Cycle Configuration
This type of configuration produces less power than the closed cycle alternative but it is attractive in places where water shortage is an issue. In addition, the very low pressure at which the system operates means that connections must be carefully sealed to prevent atmospheric air from entering the system, which could bring the operation to a halt. Another disadvantage compared to the closed cycle system is that the specific volume of the low-pressure steam is very large compared to the pressurized working fluid used in the closed cycle system. Therefore the components must have large flow areas to ensure that steam flow does not reach a high enough velocity which could damage the turbine. Also, a large turbine is required to accommodate the very high volumetric flow rates of the low-pressure steam in order to generate a sufficient amount of electrical power.
3.3 Hybrid Cycle OTEC System
3.3.1 Principle of Operation
A hybrid cycle combines the features of both the closed-cycle and open-cycle systems. In a hybrid OTEC system, warm seawater enters a vacuum chamber where it is flash-evaporated into steam, which is similar to the open-cycle evaporation process. The steam vaporizes the working fluid of a closed-cycle loop on the other side of an ammonia vaporizer. The vaporized fluid then drives a turbine that produces electricity. The steam condenses within the heat exchanger and provides desalinated water. Figure 3 is an illustration of the hybrid cycle OTEC system.
Figure : Principle of operation of a hybrid cycle OTEC system (National Renewable Energy Laboratory)
The electricity produced by the system can be delivered to a utility grid or used to manufacture methanol, hydrogen, refined metals, ammonia, and similar products.
3.4 OTEC Components
The main components of an OTEC system are described below- namely, heat exchangers, evaporators, turbines and condensers.
3.4.1 Heat Exchangers for Closed Cycle OTEC Systems
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.
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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, Larsen-Basse, & Little, 1984).
3.4.2 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). 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).
3.4.3 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.
3.4.4 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 design-a two-stage condenser (see figure 4 below) developed at Solar Energy Research Institute-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).
Figure : Illustration of a two stage condenser (National Renewable Energy Laboratory)
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, Bharathan, & Althof, 1985).
4. Methodology
In this section, pending work and project plan summary will be described. For the final project, the literature review might be expanded a bit more to include the various types of working fluids that can be used. Furthermore, a section explaining the thermodynamics behind the operation of the OTEC cycle (Carnot efficiency and Rankine cycle) will be added, followed by a more realistic calculation on the OTEC actual thermal efficiency, net power output and production of desalinated water. Once these calculations are carried out, a reasonably concise section on the application of solar heating on OTEC will be written. Then an investigation on the effect of increasing water inlet temperature on power generation and fresh water production will be carried out. In addition, the effect of an increase in pressure on power generation and fresh water production will also be included. Computational software such as ‘Wolframalpha’ or ‘Matlab’ will help in determining the relationship between variable temperatures and pressures on power generation and desalinated water production. If time allows it, simulation software such as ‘Simul8’ will be used to find the optimum conditions for the hybrid cycle OTEC plant. A section of discussion and analysis of results will follow explaining the obtained results, including suggestions for improving the design. Finally, a segment explaining by what means the proposed hybrid cycle OTEC plant is suitable for operation in Cyprus. This segment will include the economic factors involved (need for finance, government subsidies etc.), drawbacks and benefits such a plant will have on the island.
5. Gantt Chart
The Gantt Chart below represents the plan for completion of the project including important deadline dates for presentations and submission of the report.
Figure : Figure showing the Gantt Chart
6. Conclusion
Water and energy are essential for human life. A steady supply of energy and water is indispensable for improving the living standard and economic stability of any country, especially countries which face water shortage problems like Cyprus. The goal of this report is to demonstrate the characteristics of a dual purpose OTEC plant and through various design considerations find the optimum conditions for such a plant to be as productive and as efficient as possible and at the same time be economically viable to attract investors.
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