Buildings consume about 40% of the nation’s energy. In buildings, the main energy-consuming applications include heating, cooling, ventilation and so on. Applying insulation materials to building envelopes is an effective way of reducing energy consumption for heating and cooling and limiting the negative environmental impacts from the buildings sector. Department of Energy’s Emerging Technologies (ET) Program is seeking the development of low thermal conductivity insulation technologies to contribute to the goal of reducing building’s energy use by 45% by 2030 relative to 2010 high-efficiency technologies. Several institutions including Oak Ridge National Laboratory (ORNL) have been exploring multiple options for developing materials with thermal conductivity of \~ 10 mW/(m.K) or lower that are suitable for retrofitting existing buildings as well as in new construction.

Materials with low thermal conductivity can be developed by using micro- or nanoporous structures along with gases of low thermal conductivity and reduced pressure within the pores. Various types of materials have been used for building insulation, such as foams, fibers, hollow particles, and packed powders. Heat transfer mechanisms through such materials include conduction (through solid and gas) and radiation. A single model cannot be used to accurately predict the effective thermal conductivity of all types of materials. Hence, different models have been developed to study the thermal transport properties of different types of materials. The literature review shows that the current approaches use simplified models to estimate the effective thermal conductivity of the insulation materials which do not account for various important factors such as size effect, interfacial thermal resistance etc. This results in a deviation of the calculated results from the experimental values.  Further, the heat transfer mechanism becomes more complex for micro- or nanoporous structures. Therefore, suitable models are needed that considers the real structures of the materials that can help to analyze the heat transfer mechanism and to study the effects of various factors on the performance and optimization of such materials.

Existing models for calculation of effective thermal conductivity of insulation materials include various assumptions for simplification, therefore there is still some room for improvement. Such models consider simple pore geometries, regular shapes, and uniform pore sizes for representing porous materials. This can simplify the heat transfer analysis; however, the actual pore structures might be complex. This can result in thermal conductivity value different than the actual one. Models developed for a specific material by considering specific elementary shapes might need to be modified if they are used for different materials with different elementary structures. Hence, a different model is needed for different materials. It would be great if we could develop a modeling tool, such as a calculator that could capture all the important factors that have not been well captured by the existing models.

This work presents a review of the existing models used for the development of insulation materials, discusses some important factors that need to be included to improve currently used models, and presents the calculator that can more accurately predict the thermal conductivity. Models that have been widely used for predicting the thermal conductivity of insulation materials need to incorporate some fitting parameters for more accurate prediction. Therefore, we will be developing models with such fitting parameters, which will help to predict the thermal conductivity more accurately. Generally, researchers prepare a sample (time and cost consuming) and perform multiple hit and trial to obtain the preliminary results (again more time and cost). So, the calculator developed in this work will help the researchers to quickly determine the results (solid, gas and radiation contribution to total effective thermal conductivity) and then move forward for the experiment if the calculated values show expected results. More realistic results can be expected from the improved models developed from this work. Moreover, the calculator developed based on the improved models can help researchers by allowing them to evaluate various options that could achieve the desired thermal conductivity of the insulation materials.


Calculator for Development of Low\u2013thermal-Conductivity Materials

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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