The extraction of power and fresh water from the ocean off the coast of KZN utilising ocean thermal energy conversion (OTEC) techniques

Abstract

Ocean thermal energy conversion (OTEC) is an electric power generation system which uses the temperature difference between warm water at the surface (26 oC) and cold water from the depths (5 oC) of the ocean. Generating electricity is not the only function of OTEC as it can also produce significant amounts of fresh water. This can be very important, for example on islands and in some regions, such as Port Edward, where fresh water is limited. This thesis sets out to harness this fluidic energy, thus generating significant amounts of useful electric power for insertion into the national grid, as well as fresh water in Port Edward on the KwaZulu-Natal (KZN), South Coast. The site of Port Edward is naturally suited to the establishment of alternate energy collection sources such as OTEC; the geographical location of this region is additionally suited to the development of Open Cycle - Ocean Thermal Energy Conversion (OC- OTEC). Port Edward lies just beneath the tropic of cancer and on the shore of the Indian Ocean thus two important elements needed for OTEC namely constant sunlight and large coastal areas can easily be found in this region. More importantly, the steep drop in water depth down to 3000 meters makes this an ideal research site for ocean thermal energy conversion in KwaZulu-Natal (KZN). If the proposed theories are correct, this can possibly be used for base generated energy capacity and fresh water. The results are presented with reference to the temperature difference between the sea surface and the sea bottom because it is an important parameter in choosing an actual plant site and system design of OC-OTEC. This research is mainly laboratory based concentrating on design, calculations, modelling and simulation of OC-OTEC. The thermodynamic fluid calculations were undertaken with a view to design the main mechanical components of an OC-OTEC system, i.e. flash evaporator, condenser and steam turbine. SOLID EDGE software was utilized to design OC-OTEC plant and ASPEN PLUS V8.6 software was used to simulate and model the experiment. An OC-OTEC demonstration plant was designed and constructed in an Electrical Power Laboratory at Durban University of Technology (DUT). The experimental study was carried out on the demonstration plant with consideration given to water temperature, mass flow rate of fluid, and pressure. The measurements were taken before and after each component. The selection of a good process modelling and simulation tool was of extreme importance for the success of this work. Throughout the measurements, we found that the thermal efficiency (%) and the power output increased with increasing temperature difference Δt = tw - tc. The power output was produced when the total temperature difference was sufficient to allow heat transfer within the evaporator and provide a pressure drop across the turbine. There was more heat transfer (steam produced) in the flash evaporator at a constant flow rate because the warm water continuously supplied heat energy to the evaporator without losing much energy through the process, therefore continuous feed to the turbine improved constant power output. The thermal efficiencies were increased with increasing pressure across the turbine. The increase of pressure drops across the steam turbine caused the output power to increase. The larger flow rates of the warm water lead to higher amounts fresh water produced from the condenser. The final step in this process was the design of the main components of a practical plant to be used as a pilot plant at a selected location on the KwaZulu-Natal South coast. This will address the problem of lack of water in the region.