Abstract
Ocean thermal energy conversion (OTEC) uses large temperature differences between the surface and deep waters in the ocean for dependable renewable energy production. Compared to traditional approaches which use a working fluid, thermoelectric (TE) OTEC utilizes TE materials and has fewer moving parts, therefore promising greater corrosion resistance, biofouling resistance, and more widespread applicability. Current research on TE OTEC has shown that it is economically competitive. Our research aims to further optimize commercial-scale TE OTEC by minimizing TE material usage and water flow. Our TE OTEC plant brings up cold water from 600 meters deep and pumps it through a series of tubes suspended in the warm surface ocean. Each tube consists of TE material between two 70/30 copper-nickel alloy walls. After passing through the tubes, the cold water is returned 120 meters deep. To find the optimal TE material thickness, cross-sectional area, and shape, we used MATLAB to model the heat flow through each TE module. We found that cross-sectional area does not impact material usage or the amount of water needed and that the hourglass is the most efficient shape. We considered the performance of the TE material within the context of each pipe by additionally modeling heat flow within the pipe and accounting for the energy spent by pumping. This model showed the coupled effects of pipe radius, pipe length, TE material thickness, and cold water velocity on material usage and water flow required. The results of our model suggest that pipes and material thicknesses on the micro- and millimeter scales would be the most efficient. With manufacturing feasibility in mind, we determined that a pipe radius of 2.1 centimeters, pipe length of 4.85 meters, TE material thickness of 1.1 millimeters, and a water velocity of 0.1 meters per second minimize water flow and material usage. With this design, a 500 MW plant requires 220.5 cubic meters of bismuth telluride and a water flow rate of 131 cubic meters per second. To determine appropriate pipe spacing, FEATool Multiphysics was used to simulate how surface ocean currents would interact with the pipes. To maintain adequate surface current velocity through the plant, pipes should be spaced at least 30 centimeters apart perpendicular to current flow and 2 meters apart parallel to current flow. This results in a final plant dimension of 10 meters x 54 meters x 1 kilometer. To analyze environmental disturbances, water outflow was simulated in FEATool Multiphysics. Results indicated that a larger pipe radius and lower flow speed reduce eddy formation, thereby water column disturbance and internal mixing. Given these results, we concluded that a TE OTEC plant has minimal environmental impacts and can become a competitive renewable energy option in the world today.