Artificial Upwelling Driven by Salinity Differences in the Ocean

A concept for an artificial upwelling driven by salinity differences in the ocean to supply nutrients to a mariculture farm is described and analyzed. A long shell-and-tube counterflow heat exchanger built of inexpensive plastic and concrete is suspended vertically in the ocean. Cold, nutrient rich, but relatively fresh water from deep in the ocean flows up the shell side of the heat exchanger,and warm but relatively saline water from the surface flows down the tube side. The two flows exchange heat across the thin plastic walls of the tubes, maintaining a constant temperature difference along the heat exchanger. The plastic tubes are protected by the concrete outer shell of the heat exchanger. The flow is maintained by the difference in density between the deep and surface water dueto their difference in salinity. This phenomenon was first recognized by the oceanographer Stommel, who termed it 'The Perpetual Salt Fountain.' The heat transfer and flow rate as a function of tube number and diameter is analyzed and the size of the heat exchanger optimized for cost is determined fer a given flow of nutrients for various locations. Reasonable sizes (outer diameter on the orderof 5 m) are obtained. The incremental capital cost of the salinity-driven artificial upwelling is compared to the incremental capital cost and present value of the operating cost of an artificial upwell fueled by liquid hydrocarbons. The salinity-driven upwelling is generally cheaper.