Luis A. Vega, Ph.D., Project Manager and Donald E. Evans, P.E., Chief Engineer
Pacific International Center for High Technology Research 2800 Woodlawn Drive, Honolulu, Hawaii (USA)
A small but important step in the development of OTEC power and desalinated water plants has been taken in Hawaii. A land-based open cycle experimental plant has been designed and built 60 years after Georges Claude's pioneering work. The power block is built around a single, vertical axis, mixed-flow turbine, rated at 210 kW-gross, supported by a concrete vacuum vessel, 7.6m in diameter and 9.5m high. Steam is produced in an annular flash evaporator at the periphery of the vacuum vessel. The steam flows up from the evaporator and enters the turbine radically inward. The steam exits the turbine axially in the center of the vessel. A conical exhaust diffuser is used for pressure recovery. A direct-contact condenser composed of two coaxial stages is utilized. The non-condensable gases liberated from the seawater streams, at pressures of 27 to 1200 Pa, and a small amount of uncondensed steam are compressed and exhausted using a multiple-stage vacuum compression system.
All subsystems are instrumented to measure and record input and output temperatures and pressures as well as power output. These time history records are used to estimate the control (input) parameters and the resulting performance (output) for the system and its components. The facility has been operational since December 1992 and has produced a record 255 kWe at the generator terminals, utilizing 152 kWe to operate the seawater pumps, vacuum compression pumps and auxiliaries. The corresponding net power is 103 kWe. This was accomplished with 575 kg/s (8895 gpm) of 27.52¢XC surface seawater and 407 kg/s (6289 gpm) of 6.1¢XC seawater from a depth of 700m with a rpessure of 1314 Pa (0.19 psi) at the inlet to the vacuum compression system.
Two major problems have been encountered while operating the experimental apparatus. The large relative inertia of the open cycle turbine-generator resulted in relatively fast and large oscillations in power output while connected to the grid. Engineering analysis showed that for this particular apparatus using a fluid coupler between the turbine and generator shafts eliminates the problem. For larger OC-OTEC systems, a rectifier-inverted (ac/dc/ac static power converter) would be required. The high speed vacuum compression centrifugal pumps designed for this process operate at high efficiencies but experience frequent bearing failures that appear to result from the inability to properly cool the bearings. Analysis and field testing indicated that the compressor design was deficient by not accounting for the reduction in the internal-diametrical-clearance of the bearing races induced by the relatively high temperature differential (50¢XC to 100¢XC) between the inner and outer bearing races. The factors which compound to cause this problem are the use of grease lubricated ball bearings at speeds as high as 48,000 rpm. The solution appears to be to exchange the ceramic balls with smaller balls to accommodate the differential thermal expansion of the bearing cases.