Session V31: Energy Production: Combustion, Heat Engines, Solar Thermal and Thermoelectrics

Electricity from Coal Combustion: Improving the hydrophobicity of oxidized coals

To reduce pollution and improve efficiency, undesirable mineral impurities in coals are usually removed in coal preparation plants prior to combustion first by crushing and grinding coals followed by gravity separation using surfactant aided water flotation. However certain coals in the US are not amendable to this process because of their poor flotation characteristics resulting in a major loss of an energy resource. This problem has been linked to surface oxidation of mined coals which make these coals hydrophilic. In this project, we are investigating the surface and water flotation properties of the eight Argonne Premium (AP) coals using x-ray diffraction, IR spectroscopy and zeta potential measurements. The role of the surface functional groups, (phenolic -OH and carboxylic -COOH), produced as a result of chemisorptions of O$_{2}$ on coals in determining their flotation behavior is being explored. The isoelectric point (IEP) in zeta potential measurements of good vs. poor floaters is being examined in order to improved the hydrophobicity of poor floating coals (e.g. Illinois {\#}6). Results from XRD and IR will be presented along with recent findings from zeta potential measurements, and use of additives to improve hydrophobicity. Supported by USDOE/CAST, Contract {\#}DE-FC26-05NT42457.   

Electrorheology for Efficient Energy Production and Conservation

At present, most of our energy comes from liquid fuels. The viscosity plays a very important role in liquid fuel production and conservation. For example, reducing the viscosity of crude oil is the key for oil extraction and its transportation from off-shore via deep water pipelines. Currently, the dominant method to reduce viscosity is to raise oil's temperature, which does not only require much energy, but also impacts the environment. Recently, based on the basic physics of viscosity, we proposed a new theory and developed a new technology, utilizing electrorheology to reduce the viscosity of liquid fuels. The method is energy-efficient, and the results are significant. When this technology is applied to crude oil, the suspended nanoscale paraffin particle, asphalt particles, and other particles are aggregated into micrometer-size streamline aggregates, leading to significant viscosity reduction. When the temperature is below 0$^{\circ}$C and the water content inside the oil becomes ice, the viscosity reduction can be as high as 75{\%}. Our recent neutron scattering experiment has verified the physical mechanism of viscosity reduction. In comparison with heating, to reach the same level of viscosity reduction, this technology requires less than 1{\%} of the energy needed for heating. Moreover, this technology only takes several seconds to complete the viscosity reduction, while heating takes at least several minutes to complete.   

Small angle neutron (SANS) and X-ray scattering (SAXS) investigation of microstructure and porosity with fractal properties of coal, shale, and sandstone from Indiana

We have applied SAXS, SANS and adsorption isotherms to study the porosity, pore structure and interaction of confined fluids in the various Indiana rock samples. This study included a bituminous coal, a sandstone, and a grey shale from formations investigated as possible targets for CO2 sequestration. SAXS and SANS are demonstrated quantitative information about the microstructure and pore morphology of the coals and other rocks at length scale (1 nm to 0.3 micron) as well as the fractal nature of pore matrix interfaces. The different scattering cross sections of X-rays and neutrons provide information on the distribution of pore sizes in organic and inorganic components. Neutrons are relatively sensitive to the presence of either hydrocarbons or water in the pores, and always give a smaller Porod exponent that that for X-ray. Construction of LENS was supported by the NSF, the 21$^{st}$ Century Science and Technology fund of Indiana, and the DOD. LENS operation is supported by Indiana University.   

Optimizing the performance of a heat engine: A simulation study

We performed a simulation study of~a simple heat engine as it undergoes Carnot-type cyclic motion in a finite time over a wide range of piston speeds. There exists a specific piston speed at which the power delivered by the engine is maximum ($P_{max})$ and its corresponding efficiency is slightly larger than \textit{half~}of the Carnot efficiency (1/2 $\eta _{c})$. An optimization criterion leads to a trade-off between high power and high efficiency with respective values of 4/5 $P_{max}$ and 3/4 $\eta _{c}$. In addition, we found the time taken at the optimized state to be twice the time taken when operating at maximum power.   

Analysis of Binary Cycle Efficiency Using Redlich-Kwong Equation of State

Coal, natural gas and nuclear power plants operate using various forms of Rankine cycle. We present an efficiency maximization strategy of binary cycle, which has two Rankine cycles in tandem, using Redlich-Kwong equation of state for wide ranging working fluids: alkali metals, mercury, water, and ammonia. Binary cycle efficiency can approach the Carnot efficiency at a cost. The mercury/ammonia working fluid combination yields the highest efficiency for typical binary cycle conditions. We discuss practical implications given that mercury and ammonia create safety concerns, especially on finding other fluids having similar efficiency based on our simulations.   

Carbon dioxide adsorption on H$_{2}$O$_{2}$ treated single-walled carbon nanohorns

Carbon nanohorns are closed single-wall structures with a hollow interior. Unlike SWNTs, which assemble into cylindrical bundles, nanohorns form spherical aggregates. In our experiments we used dahlia-like carbon nanohorn aggregates. Our sample underwent treatment with H$_{2}$O$_{2}$ which opened access to the interior spaces of the individual nanohorns. We measured carbon dioxide adsorption at several temperatures between 167 and 195 K. We calculated the isosteric heat as a function of loading, and the binding energy values for CO$_{2}$ on the nanohorn aggregates from the isotherm data. Results on the H$_{2}$O$_{2}$-treated nanohorns will be compared with those obtained on other carbon substrates. We have also determined detailed equilibration profiles for CO$_{2}$ adsorption on the nanohorn aggregates; these results will also be presented.   

GOFs and ZIFs: Experimental Results and Analysis of Carbon Dioxide Sorption

In recent years, growing concerns about global warming and the environment have spurred an accelerated development of materials technology for carbon dioxide (CO2) capture and storage. Two recent categories of materials being investigated for their CO2 storage capabilities are graphene oxide frameworks (GOFs) [1] and zeolitic imidazolate frameworks (ZIFs). We have synthesized graphene-oxide-frameworks (GOFs) by linking the OH groups on graphene oxide with benzene-boronic acids. Our initial GOF materials exhibit isosteric heats at low coverage of 32 kJ/mol for CO2. The nitrogen BET surface area of these initial materials is around 500 m2/g. Also, ZIFs are particularly useful for CO2 capture and storage due to high selectivities, CO2 uptakes and sample robustness. Neutron scattering and spectroscopic results of GOFs and select ZIFs with in-situ gas sorption will be presented. Neutrons are able to determine locations and strengths of binding sites. We will present detailed isotherms of carbon dioxide, methane and nitrogen at different temperatures of these interesting GOF and ZIF materials. \\[0pt] [1] J. W. Burress et al., Angewandte Chemie International Edition 49, 8902 (2010).   

Eutectics and Phase Diagrams of Molten Salts from Molecular Dynamics simulations

The use of alkali nitrate salt mixtures as heat transfer fluids in solar thermal power plants is limited by their relatively high melting point. Certain compositions of quaternary and higher dimensional mixtures of alkali and alkaline earth nitrates and nitrites have low melting points. However, the high dimensionality of the search space makes it difficult to find lowest melting compositions. Molecular simulations offer an efficient way to screen for promising mixtures. A molecular dynamics scheme general enough to identify eutectics of any HTF candidate mixture will be presented. The eutectic mixture and temperature are located as the tangent point between free energies of mixing for the liquid and a linear plane connecting the pure solid-liquid free energy differences. The free energy of mixing of the liquid phase is obtained using thermodynamic integration over ``alchemical'' transmutations sampled with molecular dynamics, in which particle identities are swapped gradually. Numerical results for binary and ternary mixtures of alkali nitrates agree well with experimental measurements.   

Energy Harvesting with Stochastic, Subharmonic and Ultraharmonic Vibrations

Non-linear bi-stable systems have been shown to provide improved efficiency for harvesting energy from random and broad band vibration sources. This paper explores the distinct frequency response in the broadened spectrum of a particular non-linear energy harvester, a piezoelectric cantilever with magnetic coupling. The cantilever response evolves dynamically with frequency around the main cantilever resonance. Both stochastic and multi-frequency vibration responses are observed, and account for some of the improved efficiency. In addition, sub-harmonics and ultra-harmonics of the main resonance, along with various combinations of these appear. Taken together, the sub-harmonic and ultra-harmonic response produces an average of four-fold increase in voltage production. For energy harvesting purposes, the mixtures of the stochastic and various harmonic features together with the un-damped resonant response enhances the performance well beyond that of a standard energy harvester. An analytical model of the bi-stable dynamics produces results consistent with those observed experimentally.   

Enhanced thermoelectric properties of n-type filled skutterudite Yb0.35Co4Sb12 by substitution on both the Co and Sb sites

A dimensionless thermoelectric figure of merit (ZT) of about 1.2 was reported in Yb0.35Co4Sb12 at 550$^{\circ}$C by ball milling and hot pressing. Through alloying on both the Co and Sb sites, we expect to achieve lower thermal conductivity while maintaining the power factor. The composition tuning is aimed for reducing the electrical conductivity and increasing the Seebeck coefficient, which will lead to a lower thermal conductivity, and ultimately higher ZT. In this report, we present the thermoelectric properties of skutterudites Yb0.35FexCo4-2xNixSb12 and Yb0.35Co4Sb12-yMy (M=Si, Ge, Sn, B, Al, Ga, In, etc.).   

Optimum Working Fluid Selection For Rankine Cycle Using Redlich-Kwong Equation of State

Efficiency of Rankine cycle as a function of working fluid molecule is modeled using Redlich-Kwong equation of state. We have evaluated 12 molecules, ranging from water to ethylene glycol, and have parameterized their individual performance on several material parameters, including heat capacity and compressibility. This research aims to understand at the molecular level what drives some molecules to perform better at certain temperature and pressure range of the Rankine cycle. Immediate applications we are interested in are geothermal power, solar thermal energy conversion and waste heat recovery.   

Thermoelectric properties of correlated materials

The discovery of large Seebeck coefficients in transition metal compounds such as FeSi, FeSb2, or the iron pnictides, has stirred renewed interest in the potential merits of electronic correlation effects for thermoelectric properties. The notorious sensitivity in this class of materials to small changes in composition (doping, chemical pressure) and external stimuli (temperature, pressure), makes a reliable and, possibly, predictive description cumbersome, while at the same time providing an arena of possibilities in the search for high performance thermoelectrics. Based on state-of-the-art electronic structure methods (density functional theory with the dynamical mean field theory) we here compute the thermoelectric response for several of the above mentioned exemplary materials from first principles. With the ultimate goal to understand the origin of a large thermoelectricity in these systems, we discuss various many-body renormalizations, and identify correlation controlled ingredients that are pivotal for thermopower enhancements.   

Electron correlation effect on temperature and magnetic-field dependences of thermopower

We theoretically investigate the temperature {\it T} and the magnetic field dependences of thermopower. To focus on the strong electron correlation, the Hubbard model is solved in the dynamical mean field theory with the non-crossing approximation impurity solver. The thermopower shows a non-monotonic behavior as a function of {\it T} and asymptotes to the high-{\it T} values given by the Heikes formula, depending on the ratio of Coulomb repulsion and {\it T}. The large response to the magnetic-field, which is observed in the cobalt oxides [1], can be associated with the sharp quasiparticle peak intrinsic to the strongly correlated electron system. We discuss the effect of orbital degeneracy, which is another key factor to enhance the thermopower in the correlated system [2].\\[4pt] [1] Y. Wang {\it et al.}, Nature {\bf 423}, 425 (2003).\\[0pt] [2] W. Koshibae {\it et al.}, Phys. Rev. B {\bf 62}, 6869 (2000).   

Unusual Transport and Strongly Anisotropic Thermopower in PtCoO$_{2}$ and PdCoO$_{2}$

Thermoelectrics provide a technology for producing electrical energy from solar and other heat sources. Thermoelectric performance requires materials with high thermopower, normally found in doped semiconductors, where the thermopower is generally nearly isotropic. We discovered using first principles calculations and Boltzmann transport theory that two oxides, PtCoO$_{2}$ and PdCoO$_{2}$, which are not semiconductors, but rather good metals, have exceptionally large thermopowers in one direction, and moreover that the thermopower in these materials is highly anisotropic. This places these compounds in a highly unusual transport regime. Besides providing a new direction for thermoelectric materials research, they may be very useful in probing the fundamental limits of conventional transport theory for metals.   

Analysis of Wind Forces on Roof-Top Solar Panel

Structural loads on solar panels include forces due to high wind, gravity, thermal expansion, and earthquakes. International Building Code (IBC) and the American Society of Civil Engineers are two commonly used approaches in solar industries to address wind loads. Minimum Design Loads for Buildings and Other Structures (ASCE 7-02) can be used to calculate wind uplift loads on roof-mounted solar panels. The present study is primarily focused on 2D and 3D modeling with steady, and turbulent flow over an inclined solar panel on the flat based roof to predict the wind forces for designing wind management system. For the numerical simulation, 3-D incompressible flow with the standard k-$\varepsilon $ was adopted and commercial CFD software ANSYS FLUENT was used. Results were then validated with wind tunnel experiments with a good agreement. Solar panels with various aspect ratios for various high wind speeds and angle of attacks were modeled and simulated in order to predict the wind loads in various scenarios. The present study concluded to reduce the strong wind uplift by designing a guide plate or a deflector before the panel.