| As energy consumption continues to rise while conventional energy supplies become ever more depleted there is an increasing demand for renewable energy technologies, especially solar energy in the resource rich southwestern United States. In the 1970s, researchers proposed utilizing particle suspensions in liquids to enhance the solar absorption. More recently, nanoparticle-liquid suspensions have been proposed as a means to enhance solar collector efficiency through direct absorption of the incoming solar energy. For direct absorption systems, the base fluid plays an important role in the absorption of the effective medium therefore a method for experimentally determining the extinction coefficient of four fluids commonly used in solar thermal energy applications was developed. These fluids do not absorb solar energy well and the use of nanoparticle enhancement is necessary. Through the use of a micro-solar collector, the efficiency improvement predicted by earlier models was tested for varying nanoparticle materials, volume fractions and particle shapes. Earlier studies as well as the current work show that the size and shape of the nanoparticles as well as the scattering mode all impact the amount of energy absorbed and emitted by the nanofluid. In order to optimize the efficiency of a direct absorption solar system the optimum nanoparticle-liquid combination needs to be developed. The optimum nanofluid for a direct absorption solar thermal collector is investigated numerically through the variation of particle size, collector geometry, and scattering mode. With a demonstrated efficiency improvement due to a direct absorption solar collector utilizing nanofluids, a comparative analysis was performed comparing the environmental and economic impacts for domestic hot water systems. Results show that for current nanoparticle cost and a 3 percent improvement in efficiency the nanofluid-based solar collector at the end of its life has the same economic savings as a conventional solar collector while having a lower embodied energy and higher levels of pollution offsets than a conventional collector. In addition, if 50 percent penetration of residential nanofluid-based solar collector systems for hot water heating could be achieved in Phoenix, Arizona, it is projected that over 1 million metric tons of carbon dioxide would be offset per year. |
