Reducing human reliance on energy-inefficient cooling methods such as air conditioning would have a large impact on the global energy landscape. By a process of complete delignification and densification of wood, we developed a structural material with a mechanical strength of 404.3 megapascals, more than eight times that of natural wood. The cellulose nanofibers in our engineered material backscatter solar radiation and emit strongly in mid-infrared wavelengths, resulting in continuous subambient cooling during both day and night. We model the potential impact of our cooling wood and find energy savings between 20 and 60%, which is most pronounced in hot and dry climates.

/pdf/a-radiative-cooling-structural-material-18m9j9z5y8.pdf

A radiative cooling structural material.

The energy used for regulating building temperatures accounts for 14% of the primary energy consumed in the U.S. One-quarter of this energy is leaked through ineffcient glass windows in cold weather. The development of transparent composites could potentially provide affordable window materials with enhanced energy effciency. Transparent wood as a promising material has presented desirable performances in thermal and light management. In this work, the performance of transparent wood is optimized toward an energy effcient window material that possesses the following attributes: 1) high optical transmittance (≈91%), comparable to that of glass; 2) high clarity with low haze (≈15%); 3) high toughness (3.03 MJ m−3) that is 3 orders of magnitude higher than standard glass (0.003 MJ m−3); 4) low thermal conductivity (0.19 W m−1 K−1) that is more than 5 times lower than that of glass. Additionally, the transparent wood is a sustainable material, with low carbon emissions and scaling capabilities due to its compatibility with industryadopted rotary cutting methods. The scalable, high clarity, transparent wood demonstrated in current work can potentially be employed as energy effcient and sustainable windows for signifcant environmental and economic benefts.

https://www.fpl.fs.fed.us/documnts/pdf2020/fpl_2020_mi001.pdf

A Clear, Strong, and Thermally Insulated Transparent Wood for Energy Efficient Windows

Thermal conductivity of hybrid nanofluids: A critical review

Solar steam generation is regarded as one of the most sustainable techniques for desalination and wastewater treatment. However, there has been a lack of scalable material systems with high efficiency under 1 Sun. A solar steam generation device is designed utilizing crossplane water transport in wood via nanoscale channels and the preferred thermal transport direction is decoupled to reduce the conductive heat loss. A high steam generation efficiency of 80% under 1 Sun and 89% under 10 Suns is achieved. Surprisingly, the crossplanes perpendicular to the mesoporous wood can provide rapid water transport via the pits and spirals. The cellulose nanofibers are circularly oriented around the pits and highly aligned along spirals to draw water across lumens. Meanwhile, the anisotropic thermal conduction of mesoporous wood is utilized, which can provide better insulation than widely used superthermal insulator Styrofoam (≈0.03 W m−1 K−1). The crossplane direction of wood exhibits a thermal conductivity of 0.11 W m−1 K−1. The anisotropic thermal conduction redirects the absorbed heat along the in-plane direction while impeding the conductive heat loss to the water. The solar steam generation device is promising for cost-effective and large-scale application under ambient solar irradiance.

Scalable and Highly Efficient Mesoporous Wood-Based Solar Steam Generation Device: Localized Heat, Rapid Water Transport

This paper presents a critical review of heat transfer applications of nanofluids. The effects of nanoparticle concentration, size, shape, and nanofluid flow rate on Nusselt number, heat transfer coefficient, thermal conductivity, thermal resistance, friction factor and pressure drop from numerous studies reported recently are presented. Effects of various geometric parameters on heat transfer enhancement of system using nanofluids have also been reviewed. Heat transfer devices covered in this paper include radiators, circular tube heat exchangers, plate heat exchangers, shell and tube heat exchangers and heat sinks. Various correlations used for experimental validation or developed in reviewed studies are also compiled, compared and analyzed. The pros and cons associated to the applications of nanofluids in heat transfer devices are presented in details to determine the future direction of research in this arena.

Recent advances in application of nanofluids in heat transfer devices: A critical review

Thermal conductivity and interfacial thermal conductance play crucial roles in the design of engineering systems where temperature and thermal stress are of concerns. To date, a variety of measurement techniques are available for both bulk and thin film solid-state materials with a broad temperature range. For thermal characterization of bulk material, the steady-state absolute method, laser flash diffusivity method, and transient plane source method are most used. For thin film measurement, the 3{\omega} method and transient thermoreflectance technique including both frequency-domain and time-domain analysis are employed widely. This work reviews several most commonly used measurement techniques. In general, it is a very challenging task to determine thermal conductivity and interface contact resistance with less than 5% error. Selecting a specific measurement technique to characterize thermal properties need to be based on: 1) knowledge on the sample whose thermophysical properties is to be determined, including the sample geometry and size, and preparation method; 2) understanding of fundamentals and procedures of the testing technique and equipment, for example, some techniques are limited to samples with specific geometrics and some are limited to specific range of thermophysical properties; 3) understanding of the potential error sources which might affect the final results, for example, the convection and radiation heat losses.

Measurement Techniques for Thermal Conductivity and Interfacial Thermal Conductance of Bulk and Thin Film Materials

Water cooled minichannel heat sinks for microprocessor cooling: Effect of fin spacing

Minichannel heat sink geometries with varying fin spacing were tested with de-ionized water and MWCNT (1 wt %) nanofluid to evaluate their performance with flow components of a liquid cooling kit. Four heat sinks with fin spacing of 0.2 mm, 0.5 mm, 1.0 mm, and 1.5 mm were used in this investigation. Heat sink base temperature was analogous to processor operating temperature which was the prime parameter of interest in this investigation. The base temperature decreased by reducing the fin spacing and using multiwalled carbon nanotube (MWCNT) nanofluid. The lowest value of heat sink base temperature recorded was 49.7 °C at a heater power of 255 W by using a heat sink of 0.2 mm fin spacing and MWCNT nanofluid as a coolant. Moreover, as a result of reduced fin spacing and using MWCNT nanofluid as a coolant the value of overall heat transfer coefficient increased from 1200 W/m2K to 1498 W/m2K, translating to about a 15% increase. The value of thermal resistance also dropped by reducing the fin spacing and using MWCNT nanofluid. The most important aspect of the study is that the heat sinks and MWCNT nanofluid proved to be compatible with the pump and radiator of the commercial CPU liquid cooling kit. The pump was capable to handle the pressure drop which resulted by reducing the heat sink fin spacing and by using MWCNT nanofluid. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(7): 653–666, 2014; Published online 11 November 2013 in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21107

Saad A. Jajja

Papers

Measurement Techniques for Thermal Conductivity and Interfacial Thermal Conductance of Bulk and Thin Film Materials

Measurement Techniques for Thermal Conductivity and Interfacial Thermal Conductance of Bulk and Thin Film Materials

Water cooled minichannel heat sinks for microprocessor cooling: Effect of fin spacing

Multiwalled Carbon Nanotube Nanofluid for Thermal Management of High Heat Generating Computer Processor

Experimental investigation of supercritical carbon dioxide in horizontal microchannels with non-uniform heat flux boundary conditions