Abstract
Ocean thermal energy conversion (OTEC) is a renewable energy source that uses warm, surface seawater to evaporate a working fluid that runs a turbine before being condensed by cold, deep seawater in a simple Rankine cycle. There are two types of OTEC cycles, the closed cycle (CC-OTEC) uses a working fluid that circulates through the system continuously and the open cycle system (OC-OTEC) that flash evaporates seawater and uses steam to run the turbine before it is condensed and either ejected from the system or harvested as fresh water. MATLAB software is used to model open, closed and hybrid OTEC cycles. The hybrid cycle uses the open cycle stage to harvest freshwater and to power a superheater installed before the turbine inlet in the closed cycle stage. The OC-OTEC and CC-OTEC models were validated individually by comparing the results to similar research and then combined into the hybrid cycle. The focus of this research is to evaluate the viability of closed, open and hybrid cycle OTEC systems at various locations around the United States coast and to explore replacement working fluids for ammonia, which is toxic. The working fluids tested are ammonia, butane, R-134a, R-22 and R-410a. Each system is assessed at 11 different seawater temperature differences ranging from 16oC to 26oC. The open cycle system has the lowest system efficiency, ranging from 1.3-1.6%, however freshwater production is significant at 83,160 L/s. For the closed cycle system, R-22 is shown to have the best performance with efficiencies ranging from 1.6-3% while R-410a is the worst at 1.4-2.4%. The hybrid system shows a percent increase in net power production for all working fluids where ammonia is the highest at 36-39% and the remaining working fluids increase between 26-29%. Ammonia also had the largest increase in efficiency for the hybrid system ranging from 36-39% while R-410a is the lowest at 26-28%. R-22 out-performed R-134a, butane and R-410a, however R-22 is a hydrochlorofluorocarbon (HCFC) and is banned by the Montreal Protocol. R-410a is the most benign of the working fluids but had the worst performance. Further investigation on replacement refrigerants for ammonia is necessary for closed cycle systems. The coast off Key West, Florida is shown to be the most suitable location for OTEC technology in the United States with a seawater temperature difference of 22.41oC and net power outputs of 52.04MW, 102.55MW and 142.71MW for the open, closed and hybrid cycles, respectively. Kailua-Kona, Hawaii has the second best performance and is the best location for freshwater production with a site location only 1.8 miles from shore.