Thermal properties of dry soils: Fundamentals and applications.

摘要

The United States of America is the largest consumer and importer of fossil fuels in the world. The increasing demand for energy costs has forced us to look for alternative sources of clean and affordable energy. Renewable energy comes from natural resources such as solar, wind, hydropower, geothermal heat and biomass, and offers the only sustainable alternative to power the world.;Geothermal energy relies on the fact that the ground temperature stays constant during the year, warmer than the atmosphere during the winter and cooler during the summer. In geothermal systems, the geothermal heat pump is used to transfer the heat stored in the earth or in ground water into a building during winter, and transferring it out of the building and back into the ground during summer. Thus, the ground is a heat source in winter and a heat sink in summer. To transfer the heat from the ground to the building, a network of underground pipes is required which can affect the efficiency and cost of the system. By improving the heat transfer between the pipes and the soil, we can increase the efficiency of the geothermal system and make it more affordable.;The first objective of this dissertation is to investigate the thermal properties of dry granular media and porous rocks. Knowledge of these thermal properties are important for the design of heat pumps in geothermal system. An experimental study was carried out to explore the physical nature of thermal conduction in dry granular media (i.e., soils) and fused granular media (i.e., porous rocks). Electrical conductors exhibit higher thermal conductivity (lambda,&Tgr;) than other solid materials. Differences can be as high as three orders of magnitude in lambda&Tgr;, and are attributed to electron heat transfer; however, thermal conduction in metallic particles decreases to values very similar to those of non-electrically conductive particles. To uncover the underlying mechanisms of heat conduction in granular media, Ottawa sand and lead shot particles were used to create specimens tested for thermal conductivity, p-wave velocity and electrical conductivity. A newly developed test chamber allowed for simultaneous measurements of the three parameters under increasing vertical stress in zero-lateral-strain loading. Results show that in granular media, whether electrically conductive or not, heat conduction is phononic in nature and the contribution from electron heat conduction is negligible. Potential changes in heat conduction mechanisms that arise from changes in the nature of inter-particle contacts (i.e., fused spoils or porous rocks) were monitored using sea salt and lead shot specimens subjected to high temperature and pressure (i.e., sintered). The next objective was to determine the effective thermal properties of granular mixtures of high and low thermal conductivity solid particles. Understanding the changes in thermal properties as a function of mass fractions should prove useful in the engineering of dry geo-materials for geothermal applications. Specimens made of lead shot and lead wires mixed with Ottawa sand were subjected to vertical loading while simultaneously monitoring the effective thermal conductivity, p-wave velocity and electrical conductivity. Results show that the relative mass fractions in the mixture play a very important role in the transfer thermal energy. The differences in particle density between lead and quartz explain the sensitivity of thermal conductivity to changes in the mass fraction. Again, the results highlight the dominant role of the stiffness of the particulate media.;Finally, a numerical study was conducted to examine the potential improvement in heat transfer and storage in backfills that incorporate phase change materials. Results indicate that using phase change materials as part of the backfill can alter the radius of influence of the borehole and the storage and release of thermal energy.