Evaluating energy dissipation during expansion in a refrigeration cycle using flue pipe acoustic resonators

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008."June 2008."Includes bibliographical references (p. 27-29).This research evaluates the feasibility of using a flue pipe acoustic resonator to dissipate energy from a refrigerant stream in order to achieve greater cooling power from a cryorefrigeration cycle. Two models of the acoustic operation of flue pipe resonant systems are examined: an electrical circuit analog that represents the linear approximation of the acoustic system and a numerical model based on empirical data. The electrical analog yields a symbolic representation for the power that can potentially be dissipated from the acoustic stream. Ongoing research into these acoustic systems, however, shows that the electrical analog, which neglects nonlinear effects, is incomplete and overestimates the operation of a pipe. However, the analogy can be used to quickly find the order of magnitude of power dissipated from the acoustic resonator. A subsequent data-based model allows for a more accurate quantitative estimation of the potential efficiency of the flue pipe in extracting work and thereby dissipating energy from a refrigerant stream. The efficiency of extracting work from a refrigerant stream using the acoustic system analyzed here ranges from 10% to 60%. The range is so large because the quality factor of the experimental flue pipe is unknown. This quality factor is imperative in determining the power dissipation. Further research should optimize the quality factor. A large quality factor causes less amplitude attenuation than a small one, but a smaller one dissipates more of the stored energy. The results of the models are compared to the efficiencies of existing technology, specifically the recently invented thermo acoustic expansion valve (TEV). It is found that the efficiency of the TEV is less than the theoretical results deduced from the numerical model. At an efficiency of approximately 10%, the technology represents a gain in cooling power, but further optimization using the results of this research can increase this gain even more.by Maria N. Luckyanova.S.B

The Sun Makes You Number One!

Representing the Solid-State Solar-Thermal Energy Conversion Center (S3TEC), this document is one of the entries in the Ten Hundred and One Word Challenge. As part of the challenge, the 46 Energy Frontier Research Centers were invited to represent their science in images, cartoons, photos, words and original paintings, but any descriptions or words could only use the 1000 most commonly used words in the English language, with the addition of one word important to each of the EFRCs and the mission of DOE energy. The mission of S3TEC is advancing fundamental science and developing materials to harness heat from the sun and convert this heat into electricity via solid-state thermoelectric and thermophotovoltaic technologies

Thermal conductivity control by oxygen defect concentration modification in reducible oxides: The case of Pr0.1Ce0.9O2−δ thin films

We demonstrate the impact on thermal conductivity of varying the concentration of oxygen vacancies and reduced cations in Pr[subscript 0.1]Ce[subscript 0.9]O[subscript 2−δ] thin films prepared by pulsed laser deposition. The oxygen vacancy concentration is controlled by varying the oxygen partial pressure between 1 × 10[superscript −4] and 1 atm at 650  °C. Corresponding changes in the oxygen non-stoichiometry (δ) are monitored by detecting the lattice parameters of the films with high-resolution X-ray diffraction, while the thermal properties are characterized by time-domain thermoreflectance measurements. The films are shown to exhibit a variation in oxygen vacancy content, and in the Pr[superscript 3+]/Pr[superscript 4+] ratio, corresponding to changes in δ from 0.0027 to 0.0364, leading to a reduction in the thermal conductivity from k = 6.62 ± 0.61 to 3.82 ± 0.51 W/m-K, respectively. These values agree well with those predicted by the Callaway and von Baeyer model for thermal conductivity in the presence of point imperfections. These results demonstrate the capability of controlling thermal conductivity via control of anion and cation defect concentrations in a given reducible oxide.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (MRSEC Program, Award No. DMR-0819762

Detecting coherent phonon wave effects in superlattices using time-domain thermoreflectance

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 79-85).Superlattices (SLs), structures consisting of periodic layers of thin films of several angstroms to tens of nanometers thick, have unique electrical and thermal properties that make them well suited for applications in optoelectronics and as fundamental learning tools in the realm of thermoelectrics. One unique characteristic of SLs is their low thermal conductivity compared to a bulk material with the same molecular composition. This property has given rise to extensive theoretical and experimental investigations regarding thermal transport through SLs. The different thermal transport characteristics have been studied in the context of various transport regimes. In this thesis, an experimental investigation of thermal transport in the coherent regime through a SL is presented. The trend in thermal conductivity that can be expected if such coherent wave effects exist is derived from the Landauer-Biittiker formalism, which treats energy transport as a transmission process. The frequency-dependent transmission probability for phonons through the SL is found via an application of the transfer matrix method (TMM). The calculations show that the integral effect of the buildup of phonon stopbands in the SL is minimal. Thus, if coherent wave effects are present, the conductance of the SL is nearly constant as the number of periods is increased, and the thermal conductivity, which is the product of the conductance and the total thickness of the SL, increases linearly with number of periods. To test the predictions, five GaAs/AlAs SLs with one, three, five, seven, and nine periods of one layer of GaAs of 12 nm thickness, and one layer of AlAs of 12 nm thickness are grown using MOCVD. The thermal conductivities of the SLs are measured using a transient thermoreflectance (TTR) technique at temperatures ranging from 30K to 300K. The results are the first-ever experimental evidence for the presence of coherent wave effects in heat transport through SLs.by Maria N. Luckyanova.S.M

Anderson Localization of Thermal Phonons Leads to a Thermal Conductivity Maximum

Our elastic model of ErAs disordered GaAs/AlAs superlattices exhibits a local thermal conductivity maximum as a function of length due to exponentially suppressed Anderson-localized phonons. By analyzing the sample-to-sample fluctuations in the dimensionless conductance, g, the transition from diffusive to localized transport is identified as the crossover from the multichannel to single-channel transport regime g ≈ 1. Single parameter scaling is shown to hold in this crossover regime through the universality of the probability distribution of g that is independent of system size and disorder strength.Solid-State Solar-Thermal Energy Conversion Center (Award DE-FG02-09ER46577

Epitaxial CrN Thin Films with High Thermoelectric Figure of Merit

A large enhancement of the thermoelectric figure of merit is reported in single-crystalline films of CrN. The mechanism of the reduction of the lattice thermal conductivity in cubic CrN is similar to the resonant bonding in IV–VI compounds. Therefore, useful ideas from classic thermo­electrics can be applied to tune functionalities in transition metal nitrides and oxides.Solid-State Solar-Thermal Energy Conversion Center (DE-SC0001299)Solid-State Solar-Thermal Energy Conversion Center (DE-FG02-09ER46577