Reduction of lattice thermal conductivity from planar faults in the layered Zintl compound SrZnSb_2

The layered Zintl compound SrZnSb_2 is investigated using transmission electron microscopy (TEM) to understand the low lattice thermal conductivity. The material displays out-of-phase boundaries with a spacing from 100 down to 2 nm. Density functional theory calculations confirm that the TEM-derived defect structure is energetically reasonable. The impact of these defects on phonon scattering is analyzed within the Debye–Callaway model, which reveals a significant reduction in the acoustic phonon mean free path. This enhancement in phonon scattering leads to an ~30% reduction in lattice thermal conductivity at 300 K

Ca_3AlSb_3: an inexpensive, non-toxic thermoelectric material for waste heat recovery

Thermoelectric materials directly convert thermal energy into electrical energy, offering a promising solid-state solution for waste heat recovery. For thermoelectric devices to make a significant impact on energy and the environment the major impediments are the efficiency, availability and toxicity of current thermoelectric materials. Typically, efficient thermoelectric materials contain heavy elements such as lead and tellurium that are toxic and not earth abundant. Many materials with unusual structures containing abundant and benign elements are known, but remain unexplored for thermoelectric applications. In this paper we demonstrate, with the discovery of high thermoelectric efficiency in Ca_3AlSb_3, the use of elementary solid-state chemistry and physics to guide the search and optimization of such materials

Valence band study of thermoelectric Zintl-phase SrZn_2Sb_2 and YbZn_2Sb_2: X-ray photoelectron spectroscopy and density functional theory

The electronic structure of SrZn_2Sb_2 and YbZn_2Sb_2 is investigated using density functional theory and high-resolution x-ray photoemission spectroscopy. Both traditional Perdew-Burke-Ernzerhof and state-of-the-art hybrid Heyd-Scuseria-Ernzerhof functionals have been employed to highlight the importance of proper treatment of exchange-dependent Zn  3d states, Yb 4f states, and band gaps. The role of spin-orbit corrections in light of first-principles transport calculations are discussed and previous claims of Yb^(3+) valence are investigated with the assistance of photoelectron as well as scanning and transmission electron microscopy

Enhancement of thermoelectric properties by energy filtering: Theoretical potential and experimental reality in nanostructured ZnSb

Energy filtering has been suggested by many authors as a means to improve thermoelectric properties. The idea is to filter away low-energy charge carriers in order to increase Seebeck coefficient without compromising electronic conductivity. This concept was investigated in the present paper for a specific material (ZnSb) by a combination of first-principles atomic-scale calculations, Boltzmann transport theory, and experimental studies of the same system. The potential of filtering in this material was first quantified, and it was as an example found that the power factor could be enhanced by an order of magnitude when the filter barrier height was 0.5~eV. Measured values of the Hall carrier concentration in bulk ZnSb were then used to calibrate the transport calculations, and nanostructured ZnSb with average grain size around 70~nm was processed to achieve filtering as suggested previously in the literature. Various scattering mechanisms were employed in the transport calculations and compared with the measured transport properties in nanostructured ZnSb as a function of temperature. Reasonable correspondence between theory and experiment could be achieved when a combination of constant lifetime scattering and energy filtering with a 0.25~eV barrier was employed. However, the difference between bulk and nanostructured samples was not sufficient to justify the introduction of an energy filtering mechanism. The reasons for this and possibilities to achieve filtering were discussed in the paper

Forestry Club-The Year in Review

Looking back at the past year for the Forestry Club, I can be truly pleased with the level of activity and participation by the members. We were able to bring back some old traditions as well as try some new things. All in all, it was an enjoyable year

Development of Control Strategies for Demand Controlled Ventilation using IoT

Som et resultat av utbredelsen av energieffektive bygg, er lufttette konstruksjoner blitt vanlige i nye bygg for å redusere energibruk til oppvarming. Dette påvirker innendørs luftkvalitetsparametere og kan gi utslag på folks helse og velvære. Behovsstyrt ventilasjon (DCV) er et vanlig tiltak for å redusere energibruk, hvor sensorer brukes for å måle forskjellige parametere innendørs som brukes til kontroll av ventilasjonssystemet. Utviklingen av sensorteknologi gjør at lavkostnadssensorer nå kan konkurrere mot kjente, pålitelige detektorer. Derfor er det fordelaktig å undersøke kontrollstrategier for ventilasjon for å optimalisere både energieffektivitet og inneklima. En full-skala kontormodell er konstruert og blir brukt til eksperimenter for å undersøke forskjellige kontrollstrategier for det tilhørende DCV-systemet. Arduino-baserte lavkostnadssensorer brukes for å måle CO2, PM2.5, TVOC, formaldehyd, temperatur og relativ fuktighet. Hver kontrollstrategi er satt sammen av en eller flere kontrollparametere, som måles av sensorene, for kontroll av luftmengde inn til rommene. I tillegg er forskjellig logikk for omluft implementert i kontrollstrategiene. Dataen fra eksperimentene brukes for å validere en bygningsmodell som brukes i samsimularing mellom EnergyPlus og CONTAM. Simuleringer av forskjellige kontrollstrategier for DCV-systemer setter dermed grunnlaget for analysen av de forskjellige kontrollstrategiene. Analysen vil basere seg på å evaluere både energibruk og inneklima. Simuleringene indikerer at ved å endre kontrollparameterne for ventilasjonskontroll kan både energibruk og inneklima forbedres. Ved å introdusere omluft til systemet, blir innendørs konsentrasjon av utendørsgenererte forurensninger, som eksempelvis PM2.5, redusert. Omluft resulterer også i større energibesparelser. Relatert til romluftskontroll vil hvilke parametere som passer best avhenge av klima og bygningskarakteristikk. Simuleringene indikerte at formaldehyd bør vurderes for bruk til kontroll av luftmengder i DCV systemer, mens utendørsgenererte forurensninger er mindre egnet.With the prevalence of energy-efficient buildings, airtight constructions to reduce energy for space heating have become common in new buildings. This impacts parameters in the indoor environment which may affect occupants' health and well-being. Demand-controlled ventilation (DCV) is a common energy-saving measure, where sensors measure indoor air quality (IAQ) parameters for control of the ventilation system. The recent development in sensing technologies allows for low-cost sensors (LCSs) to be able to compete with known, reliable detectors. Hence, investigating control strategies to optimize both energy efficiency and IAQ is advantageous. A full-scale office model is constructed and is used to conduct experiments investigating control strategies for the associated DCV system. Arduino-based low-cost sensors are used to measure CO2, PM2.5, TVOC, formaldehyde, temperature, and RH concentrations. Each control strategy is constructed by one or several control parameters, measured by the LCSs, for control of supply airflow to the rooms. Additionally, control of recirculation of air is implemented in the control strategies. The obtained data is used to validate a building model for co-simulation between EnergyPlus and CONTAM. Thus, simulations with various control strategies for DCV systems are performed and set the base for the analysis of control strategies. The analysis aims to evaluate both IAQ and energy use related to each control strategy. The simulations indicate that changing the control parameters for ventilation control can improve both energy consumption and IAQ. By introducing recirculation to the system, indoor concentrations of outdoor generated pollutants, such as PM2.5, are improved. Recirculation of air also results in larger energy savings. With regard to room airflow control, applying more control parameters to the ventilation logic showed great results. The simulations indicated that formaldehyde should be considered used for airflow control in DCV systems, while outdoor-generated pollutants may be less suited. However, what parameters are most efficient may depend on climate and building characteristics