Plasmonic Solar Collection for Ocean Thermal Energy Conversion

Ocean Thermal Energy Conversion (OTEC) is a renewable energy technology that makes use of the temperature difference between the surface and the depth of the ocean to run a low-pressure turbine and produce the electricity. As shown in Fig. 1(a), a closed-cycle OTEC uses a refrigerant, such as ammonia or R-134a, as a working fluid to allow its evaporation and condensation using warm and cold seawater, respectively. OTEC has the potential to be adopted as a sustainable, baseload energy source that requires no fuel or radioactive materials, which also makes much less environmental impacts than conventional methods of power generation. However, the main technical challenge of OTEC is the low efficiency resulting from small temperature differences. Even in the tropical area, the temperature difference between surface and deep water is only 20-25 deg. C. The thermodynamic efficiency of OTEC is thus of order of 5 to 6 % at best (Goto et al., Elec. Eng. Jap. 176, p.272, 2009), requiring large seawater flow rates for power generation, e.g., approximately 3 ton/s of deep cold seawater and as much warm seawater to generate one megawatt of electrical power. Increasing the temperature difference between warm and cold seawater is thus critically important to improve the thermodynamic efficiency of OTEC and make the OTEC technology more attractive besides tropical regions. Since the deep seawater temperature is fairly constant across the ocean in 3−5 deg C, the objective of this research is to seek for an effective way to preheat surface seawater with solar power by implementing plasmonic solar thermal absorption. The plasmonic solar thermal absorption aims to enhance the light-absorption characteristics by adopting a plasmonic nanofluid as a working fluid of a solar thermal collector: see the inset of Fig. 7(b). However, there are two challenges in realizing the plasmonic solar thermal absorption. First, the LSP enhances the light absorption only around at its excitation frequency, which is not desirable for broad-band solar absorption. Another challenge can be found at the LSP frequencies of typical metallic nanoparticles in the ultraviolet to visible spectra, which too short to be beneficially used for the solar thermal absorption. To address these challenges, the proposed research will synthesize and characterize different shapes and configurations of plasmonic nanoparticles and realize a solar thermal collection system that has a strong light absorption over a broad spectrum from the visible to the near-infrared range. Figure 1(b) shows the absorption coefficient when four spherical gold-nanoshell (GNS) structures (20/10, 30/5, 45/5, and 55/3 for silica core size/gold layer thickness in nm) are blended in water as an illustrative working fluid. When compared with the nanofluid that has Al nanoparticles (r = 2.5 nm) with the same volumetric concentration (0.07%), the GNS-blended nanofluid has a broad range absorption spectrum from ~400 nm to 2 μm that covers most of the useful solar spectrum. As shown in Fig. 7(c), the resultant solar collectorefficiency of the GNS-blended nanofluid reaches around 70% when the GNS concentration is only ~0.02%, which is one order of magnitude lower than the concentration of the Al nanofluid required to achieve as good efficiency (Lee et al., J. Solar Energy Eng., accepted). The aforementioned theoretical research will be a stepping stone for designing a solar thermal collector of OTEC and optimizing the plasmonic working fluid therein.   This research is a part of research collaborations with the Deep Ocean Water Application Research Center at the Korea Ocean Research and Development Institute.

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