Transient Thermoelectric And Electrokinetic Energy Conversion In Nanoporous Materials

A central goal of modern energy researches relies on developing more efficient and need-satisfied way of energy conversion. Nano-materials provide a big opportunity for realizing this goal. Among the different types of nano-materials, nanoporous materials have the most advantages both in manufacture and application. Although many researches foucusing on the energy conversion properties of nanoporous materials have been reported, there are still special fields not been involved and new ideas can be explored.Thermoelectric and fluid-electric phenomena in nanoporous materials have attracted great interests in recent years. Owing to the extremely low thermal conductivity, nanoporous materials are ideal in thermoelectric application. But most of the researches reported only focus on the steady-state properties, while transient properties are rarely concerned. The double layer effect and so-caused electrokinetic generation phenomenon in nanoporous material can convert the kinetic power of fluids into electricity, which have great potential in energy applications. However most of the electrokinetic generators reported in literatures are driven by external pressure, which will cost large numbers of high-grade energy due to the inherit low efficiency. In considering of the problems above, the main effort and conclusion of this work are as follows:(1) The transient thermal properties of non-homogeneous nanoporous materials are investigated by the transient hot-probe method, and gaint thermal relaxation time is observed. The measured relaxation time (RT) of CNT beds at room temperature are on the order of100s, which is about one magnitude higher than the largest value reported in non-homogeneous porous materials before. The RT value is also found to remain nearly unchanged with reduction of air pressure. Both the large RT value and variation trend with air pressure are contrary to the generally accepted two phase nonequilibrium theory.(2) Under the revelation of the thermal phase lagging phenomenon, a transient thermoelectric method utilizing the lagging behavior is proposed for the first time. The performance of such a transient thermoelectric generator is theoretically investigated based on CV model. It is found that the lagging behavior can effectively reduce the heat leak through conduction during the generating, thus promote the energy conversion efficiency. While non-dimensional relaxtion time is larger than1, The efficiency of the transient TE generator is found to exceed that of normal steady-state operation. By applying the transient method, we build a TE waste heat recovery systerm in the CPU transistor level. The efficiency is also found to be much higher than that of traditional TE system in CPU waste heat recovery, due to the high temperature heat source and reduction of heat leak.(3) A novel capillary driven electrokinetic generation method is proposed. Unlike traditional electrokinetic generator, the fluidical flow in nanochannels here is driven by the capillary force, coupling with the liquid transpiration. Thus the generator can harvest environmental energies ranging from wind power, waste heat to solar energy. The performance of the generator is both theoretically and experimentally exanalyzed. According to theoretical calculations, the maximum power of such a generator with transpiration surface diameter of1.5mm can be as large as0.39mW, which would stimulate a new way for power supply in self-driven micro electronics.(4) Under the revelation of the capillary driven electrokinetic generation, a live tree driven electrokinetic generation method is proposed. The performance of the generator is also theoretically and experimentally analyzed. The generators in experiments exhibit excellent solar energy and waste heat collecting property, hence can be utilized to harvest different kinds of energy in environment by coupling with plant-growing. It is also found in experiments that the open-circuit voltage of the generator is closely related to the transpiration rate of the leaves, thus can be used as plant sensor to investigate the physiological properties of plants.