Technical Assessment of the Prevention of Micro-Fouling on OTEC Heat-Transfer Surfaces Through the Use of Ultraviolet Radiation

摘要

To reduce or eliminate biofouling by microorganisms it has been suggested that the seawater entering the heat exchanger be sterilized (or at least sanitized) by uv radiation at 253.7 nm. The feasibility of applying this technology to OTEC is examined. Trivial calculations based on the Lambert-Beer equation and reasonable assumptions about seawater quality and the intensity of irradiation obtainable from a uv lamp suggest seawater may be transparent enough to a collimated beam of uv light to deliver effective germicidal doses to nearly 150 cm under some conditions. However, the practical limit on the depth of effective radiation from commercial lamps is severely restricted by many factors including the natural divergence of light, absorption and scattering in the media, intensity of radiation from the light source and so forth. Even under very favorable conditions a common design allowing uv light to penetrate 30 cm of water would have to permit the water at that distance to be in contact with the light for 20 seconds or so to deliver the germicidal effect of high quality sanitization but not necessarily sterilization. Macro-fouling, which may be more severe than micro-fouling, will not be affected by uv radiation (presuming an absence of symbiotic relationships). Parasitic power required for uv sources may be prohibitive under unfavorable conditions (i.e., unexpectedly high absorptivities of seawater, or excessive turbidity) or the absence of an industrial effort to scale up present uv equipment appropriate to OTEC needs. This latter event is unlikely for it appears that present uv lamps can be adapted to OTEC needs without major technological advancement. Power and cost estimates for uv installation and operation vary widely depending on the number of lamps needed for the OTEC configuration and the intensity of uv radiation actually required to prevent biofouling of heat transfer systems in OTEC designs.