Ocean Thermal Energy Conversion (OTEC)

Energy Research and Development Administration research progress in Ocean Thermal Energy Conversion (OTEC) is outlined. The development program is being focused on cost effective heat exchangers; ammonia is generally used as the heat exchange fluid. Projected costs for energy production by OTEC vary between $1000 to$1700 per kW

NSF presentation

Wind energy conversion research is considered in the framework of the national energy problem. Research and development efforts for the practical application of solar energy -- including wind energy -- as alternative energy supplies are assessed in: (1) Heating and cooling of buildings; (2) photovoltaic energy conversion; (3) solar thermal energy conversion; (4) wind energy conversion; (5) ocean thermal energy conversion; (6) photosynthetic production of organic matter; and (7) conversion of organic matter into fuels

Solar-Thermal Energy Conversion

The fact that our conventional fuel resources are finite and will be exhausted some time in the future gives impetus to a consideration of solar radiation for conversion into heat or electric power. The characteristics of the solar radiation - that the enrgy flux density is small and that it arrives intermittently on the ground of our glove has to be considered in any system utilizing this energy source. For home heating parts of the roof offer a sufficiently large area, for electric power production areas of some km2 are required to collect a sufficient amount of energy. Optical concentration, thermal collection by a fluid and electric collection are generally used in series. A crucial element in any scheme is the design of the solar collector. The conditions Imposed by the specific application and the possibilities to obtain high collection efficiencies are Investigated. Recent developments in thin film technology have provided means for improvement of the absorber and the glass envelope of the collector, and have brought solar thermal plants closer to a condition where it is competitive with other energy sources

Thermal Energy Conversion in Nanofluids

abstract: A relatively simple subset of nanotechnology - nanofluids - can be obtained by adding nanoparticles to conventional base fluids. The promise of these fluids stems from the fact that relatively low particle loadings (typically <1% volume fractions) can significantly change the properties of the base fluid. This research explores how low volume fraction nanofluids, composed of common base-fluids, interact with light energy. Comparative experimentation and modeling reveals that absorbing light volumetrically (i.e. in the depth of the fluid) is fundamentally different from surface-based absorption. Depending on the particle material, size, shape, and volume fraction, a fluid can be changed from being mostly transparent to sunlight (in the case of water, alcohols, oils, and glycols) to being a very efficient volumetric absorber of sunlight. This research also visualizes, under high levels of irradiation, how nanofluids undergo interesting, localized phase change phenomena. For this, images were taken of bubble formation and boiling in aqueous nanofluids heated by a hot wire and by a laser. Infrared thermography was also used to quantify this phenomenon. Overall, though, this research reveals the possibility for novel solar collectors in which the working fluid directly absorbs light energy and undergoes phase change in a single step. Modeling results indicate that these improvements can increase a solar thermal receiver's efficiency by up to 10%.Dissertation/ThesisPh.D. Mechanical Engineering 201

Rectennas for Thermal-Energy Conversion

We demonstrated the conversion of thermal energy in the form of far- and mid-infrared using a micro rectenna, consisting of a spiral antenna coupled to an ultra-fast nanodiode, the so-called self-switching device (SSD). A maximum efficiency of 0.02 % was measured at a 973 K (700 °C) using a calibrated black-body radiator illuminating the rectenna. The relatively low efficiency was due to the impedance mismatch between the diode and the antenna, and can be reduced by designing a suitable matching structure. The fabrication of larger rectenna array could be exploited, for example, to harvest wasted thermal energy from exhaust pipes and industrial machinery

Graphene-based photovoltaic cells for near-field thermal energy conversion

Thermophotovoltaic devices are energy-conversion systems generating an electric current from the thermal photons radiated by a hot body. In far field, the efficiency of these systems is limited by the thermodynamic Schockley-Queisser limit corresponding to the case where the source is a black body. On the other hand, in near field, the heat flux which can be transferred to a photovoltaic cell can be several orders of magnitude larger because of the contribution of evanescent photons. This is particularly true when the source supports surface polaritons. Unfortunately, in the infrared where these systems operate, the mismatch between the surface-mode frequency and the semiconductor gap reduces drastically the potential of this technology. Here we show that graphene-based hybrid photovoltaic cells can significantly enhance the generated power paving the way to a promising technology for an intensive production of electricity from waste heat.Comment: 5 pages, 4 figure

Electron emission thermal energy conversion

Title from PDF of title page (University of Missouri--Columbia, viewed on Aug. 18, 2010).The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file.Thesis advisor: Dr. Gary Solbrekken.M.S. University of Missouri--Columbia 2010.Electron emission from a surface can be achieved via two mechanisms: tunneling and thermionics. Converting thermal energy to electrical power using these mechanisms is achieved by generation of an electron current from the emitter to the collector, and production of a voltage potential between electrodes due to the potential energy difference between the electrodes. Efficient low temperature energy conversion is investigated in this thesis utilizing these two emission mechanisms. Two device concepts were developed based on thermal field (a form of tunneling) and thermionic emission that incorporate nontraditional design elements and novel implementations of existing technologies. These device concepts were developed with the intent to help mitigate some of the common downfalls of this solid state energy conversion. In addition to the novel implementation and device concepts, a unique system level modeling approach is taken that combines a more detailed thermal network with the emission modeling. Advantages of this method include a better estimate for boundary conditions and emission temperatures. Typically emission models assume constant temperature boundary conditions which can over estimate device performance. Modeling of a magnetically enhanced thermionic diode illustrated significant reductions in thermal radiation exchange between emitter and collector. This reduction is attributed to the ability to spatially reorient the electrodes due to the magnetically altered electron trajectories, and was shown to have a substantial effect on the energy conversion efficiency. Efficient low temperature thermionic energy conversion is currently not viable due to the high temperatures required to excite electrons above the material work function. With lower material work functions low temperature thermionic energy conversion would be achievable. The second design concept investigated in this thesis utilizes the transition region between field emission and thermionic emission known as thermal-field emission. This type of emission uses a high electric field produced by a gate electrode to increase the probability of electron tunneling. High electric fields at relatively low gate voltages are achieved by concentrating the field around nanowire tip emission sites. Unlike field emission the electrode is heated by a heat source which further increases the probability of electron emission. Unlike thermionic devices which suffer poor emission rates at low temperature, the thermal-field nanowire converter can produce appreciable emission at low temperatures. Modeling showed promising conversion efficiencies for this device at low temperature. However, the model does not account for gate leakage currents which will likely be the primary obstacle of this technology. Initial steps towards fabrication of this device have been taken including the growth of Si nanowires.Includes bibliographical reference

Impact of alternative energy forms on public utilities

The investigation of alternative energy sources by the electric utility industry is discussed. Research projects are reviewed in each of the following areas; solar energy, wind energy conversion, photosynthesis of biomass, ocean thermal energy conversion, geothermal energy, fusion, and the environmental impact of alternative energy sources