Solar Heat Transfer in Soil

• Introduction to Solar Heat Transfer

  • Use of dry solar heat for heating applications

  • Common practice to use 100 grams as the mass of dry samples for calculations

  • Assignment reference to fractions in heat calculations

Heat Transfer Properties of Soil

• Overview of Thermodynamic Concepts

  • Discussion on solar radiation, specific heat, and water content in soil

• Fourier's Law of Heat Conduction

  • Examining how heat flows through soil:

  • Use of Fourier equation for heat transfer

  • Mathematical representation in similar form to Darcy's Law for fluid flow

  • Fourier equation components:

  • Heat flux (denoted as $q$)

  • Thermal conductivity denoted as $k$

  • Temperature gradient expressed as $ rac{ ext{d}T}{ ext{d}x}$

  • Units:

    • Heat transfer: Watts per square meter ($ ext{W/m}^2$)

    • Thermal conductivity: Watts per meter Kelvin ($ ext{W/(m K)}$)

Factors Influencing Thermal Conductivity

• Materials and Structures

  • Comparison of thermal conductivity across different materials

  • Air (lowest thermal conductivity)

  • Water (moderate thermal conductivity)

  • Mineral soils (highest thermal conductivity)

  • Influence of soil structure:

  • Loose structures with larger pores exhibit low thermal conductivity

  • Dense structures with less pore space are generally better conductors of heat

• Building Materials Implications

  • Common use of porous rocks for construction to insulate from external temperatures

  • Rock types:

  • Igneous rocks (intrusive and extrusive) hold air pockets formed during rapid cooling

  • Historical and geographical context of material selection for building purposes

Thermal Conductivity Variability in Soil

• Variability Due to Moisture Content

  • Wet soils generally have higher thermal conductivity compared to dry soils

  • Aggregated soils have lower thermal conductivities due to larger pore spaces

  • Introduction of water in soil influences thermal dynamics:

  • Movement of water through soil can either cool it or heat it depending on the temperature differential

Temperature Changes with Soil Depth

• Soil Temperature Profiles

  • General behavior of soil temperature with depth:

  • Temperature often decreases with depth due to water presence

  • Discussion surrounding stabilization of temperatures at a certain depth

  • Concept of Temperature Gradients

  • Temperature in soil varies daily and seasonally

  • Peak temperatures occur signifying daily fluctuations in temperature

• Seasonal Changes in Soil Temperature

  • Observed differences in soil temperature across seasons, influenced by depth:

  • Cold months show increasing temperature with depth, while warm months show the opposite trend

  • Presentation of graphical data representing soil temperature profiles

Soil Temperature Management Techniques

• Techniques to Manipulate Soil Temperature

  • Use of mulches as an insulating layer:

  • Dry plant residues with low thermal flux reduce temperature variations

  • Allow for stabilization of soil temperature across fluctuations in air temperature

  • Black plastic mulches used for soil solarization decrease pathogen presence

  • Challenges associated with synthetic mulches:

  • Breakdown into microplastics poses environmental risks

• Effects of Mulch in Various Seasons

  • Observed trends between air temperature and soil temperature with mulches versus without:

  • Mulched soil reflects lower surface temperatures during warmer months

  • Discussion of potential impacts during winter months on soil conditions

Soil and Climate Interaction

• Impact of Soil Moisture and Temperature on Climate

  • Relationship between soil moisture depletion and the atmospheric vapor pressure deficit

  • Thermal radiation emissions from soil contribute to climate change dynamics

  • Role of permafrost in carbon release and greenhouse gas effects

• Conclusion

  • Summary of class topics leading into winter break

  • Encouragement and final thoughts for students regarding the learning material and upcoming breaks.