RESUMEN
A global rise in HVAC-R utilization requires a deeper understanding of the industry's effect on electricity consumptions and greenhouse gas emissions. The Total Equivalent Warming Impact (TEWI) methodology was designed to analyze emissions from direct release of refrigerant and indirect emissions through electricity consumption of HVAC-R systems to increase the understanding of system design on emissions, and to guide refrigerant replacement. However, the original TEWI calculation neglects the system degradation due to corrosion. This paper studies on the impact of corrosion and highlights how the original TEWI method underrepresents the lifetime emissions due to energy efficiency decrease and refrigerant release. Corrosion impacts direct emissions by increasing refrigerant leakage rates over time and indirect emissions through heat exchanger efficiency degradation and suboptimal refrigerant level. A modified TEWI equation is proposed to capture the dynamic corrosion impacts over the lifetime of HVAC operations. Three scenarios (low corrosivity, conservative and moderate corrosivity) are examined to analyze different corrosion environments. This analysis indicates 6%-27% increase in TEWI emissions based on a typical residential air conditioner (AC), when the impacts of corrosion are included, with the greatest emissions increase from reduced electrical efficiency. The impact of several current and future corrosion protection scenarios on TEWI are also included. Appropriate corrosion mitigation can reduce total lifecycle emissions of systems by 6% ~ 10%. The proposed modified TEWI method is expected to provide a more accurate emission estimation for AC sustainability and policy making.
RESUMEN
Recent studies focusing on enhancing the thermoelectric performance of metal oxides were primarily motivated by their low cost, large availability of the component elements in the earth's crust, and their high stability. So far, these studies indicate that n-type materials, such as ZnO, have much lower thermoelectric performance than their p-type counterparts. Overcoming this limitation requires precisely tuning the thermal and electrical transport through n-type metal oxides. One way to accomplish this is through the use of optimally doped bulk assemblies of ZnO nanowires. In this study, the thermoelectric properties of n-type aluminum and gallium dually doped bulk assembles of ZnO nanowires were determined. The results indicated that a high zT of 0.6 at 1000 °C, the highest experimentally observed for any n-type oxide, is possible. The high performance is attributed to the tailoring of the ZnO phase composition, nanostructuring of the material, and Zn-III band hybridization-based resonant scattering.
RESUMEN
Gram quantities of both unfunctionalized and 1,4-benzenedithiol (BDT) functionalized zinc phosphide (Zn3P2) nanowire powders, synthesized using direct reaction of zinc and phosphorus, were hot-pressed into highly dense pellets (≥98% of the theoretical density) for the determination of their thermoelectric performance. It was deduced that mechanical flexibility of the nanowires is essential for consolidating them in randomly oriented fashion into dense pellets, without making any major changes to their morphologies. Electrical and thermal transport measurements indicated that the enhanced thermoelectric performance expected of individual Zn3P2 nanowires is still retained within large-scale nanowire assemblies. A maximum reduction of 28% in the thermal conductivity of Zn3P2 resulted from nanostructuring. Use of nanowire morphology also led to enhanced electrical conductivity in Zn3P2. Interface engineering of the nanowires in the pellets, accomplished by hot-pressing BDT functionalized nanowires, resulted in an increase on both the Seebeck coefficient and the electrical conductivity of the nanowire pellets. It is believed that filtering of low energy carriers resulting from the variation of the chemical compositions at the nanowire interfaces is responsible for this phenomenon. Overall, this study indicated that mechanical properties of the nanowires along with the chemical compositions of their surfaces play a hitherto unknown, but vital, role in realizing highly efficient bulk thermoelectric modules based on nanowires.
RESUMEN
Recent studies indicated that nanowire format of materials is ideal for enhancing the thermoelectric performance of materials. Most of these studies were performed using individual nanowires as the test elements. It is not currently clear whether bulk assemblies of nanowires replicate this enhanced thermoelectric performance of individual nanowires. Therefore, it is imperative to understand whether enhanced thermoelectric performance exhibited by individual nanowires can be extended to bulk assemblies of nanowires. It is also imperative to know whether the addition of metal nanoparticle to semiconductor nanowires can be employed for enhancing their thermoelectric performance further. Specifically, it is important to understand the effect of microstructure and composition on the thermoelectric performance on bulk compound semiconductor nanowire-metal nanoparticle composites. In this study, bulk composites composed of mixtures of copper nanoparticles with either unfunctionalized or 1,4-benzenedithiol (BDT) functionalized Zn3P2 nanowires were fabricated and analyzed for their thermoelectric performance. The results indicated that use of BDT functionalized nanowires for the fabrication of composites leads to interface-engineered composites that have uniform composition all across their cross-section. The interface engineering allows for increasing their Seebeck coefficients and electrical conductivities, relative to the Zn3P2 nanowire pellets. In contrast, the use of unfunctionalized Zn3P2 nanowires for the fabrication of composite leads to the formation of composites that are non-uniform in composition across their cross-section. Ultimately, the composites were found to have Zn3P2 nanowires interspersed with metal alloy nanoparticles. Such non-uniform composites exhibited very high electrical conductivities, but slightly lower Seebeck coefficients, relative to Zn3P2 nanowire pellets. These composites were found to show a very high zT of 0.23 at 770 K, orders of magnitude higher than either interface-engineered composites or Zn3P2 nanowire pellets. The results indicate that microstructural composition of semiconductor nanowire-metal nanoparticle composites plays a major role in determining their thermoelectric performance, and such composites exhibit enhanced thermoelectric performance.
RESUMEN
A simple method for the large-scale synthesis of gram quantities of compound semiconductor nanowires without the need for any external catalysts or templates is presented. This method is demonstrated using zinc phosphide (Zn3P2) and zinc antimonide (ß-Zn4Sb3) nanowires as example systems. Large-scale synthesis of Zn3P2 and Zn4Sb3 nanowire powders was accomplished using a hot-walled chemical vapor deposition chamber by transporting phosphorus and antimony, respectively, via the vapor phase onto heated zinc foils. The zinc foils were rolled concentrically into coils to maximize the substrate surface area, and consequently, the nanowire yield. Using this method, 250 mg of Zn3P2 nanowires were obtained on 480 cm(2) of zinc foil in a span of 45 minutes. Furthermore, a process of exposing the synthesized nanowires to a vapor of organic functional molecules immediately after their synthesis and before their removal from the vacuum chamber was developed to obtain large quantities of surface functionalized nanowire powders. This in situ vapor-phase functionalization procedure passivated the nanowire surfaces without adversely affecting their morphology or dimensions. Our studies revealed that both 4-aminothiophenol and 3-propanedithiol functionalized Zn3P2 nanowires were stable over a 120 day duration without any agglomeration or degradation. This method of mass producing nanowires can also be extended to other binary semiconductors.