by Luis Yega and Donald Evans PICHTR U.S.A.
A small but important step in the development of OTEC power and desalinated water plants has been taken in Hawaii. An open cycle 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 evaprator and enters the turbine radially 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 noncondensable gases liberated from the seawater streams, at pressures of 2700 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 input and output temperatures and pressures as well as power output.
The land-based experimental 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 compressor system and auxiliaries. The corresponding net power is 103 kWe. This was accomplished with 574 kg/s (8894 gpm) of 27.5¢XC surface seawater and 407 kg/s (6366 gpm) of 6.1¢XC seawater from a depth of 700m with a pressure of 1300 Pa (0.19 psi) at the inlet to the vacuum compression system.
Several 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 systems a rectifier-inverter (ac/dc/ac static power converter) would be required. The high speed (26,000 to 48,000 rpm) vacuum compression centrifugal pumps designed for this process operate at high efficiencies but experience frequent bearing failures. This problem is currently under investigation. The data records obtained will be presented along with some of the questions related to the operation of OTEC plants and plans for the future.