UC San Diego

Municipal solid waste (MSW) in landfills can reach temperatures greater than 50°C that may be sustained for several decades due to methanogenic bacterial activity. The generated heat is an alternative energy source that can be exploited for direct heating of nearby infrastructure or for augmenting industrial processes. However, the proper design of heat extraction systems for landfills requires in-situ measurements of MSW thermal properties and knowledge of the process of heat generation within the waste. Accordingly, the main objective of this study is to understand the processes governing heat generation and heat extraction in MSW and to characterize the thermal properties of the waste. To reach this objective a new MSW landfill cell at the Sycamore Landfill in Santee, California was instrumented with temperature sensors and geothermal heat exchangers, installed at three different elevations in the cell during waste placement. The spatial and temporal evolution of the waste temperature was monitored during 13 months of a self-heating period, followed by a 17-day heat extraction thermal response test, and lastly, a 3-month temperature recovery period. The data collected during the monitoring was analyzed using three different methods of increasing complexity: an infinite line source analysis, a two-dimensional inverse analysis using a finite difference model with simplified heat transfer boundary conditions, and a three-dimensional inverse analysis using a finite element model for heat transfer and nonisothermal pipe flow. The estimated values of in-situ thermal conductivity and thermal diffusivity of the MSW were consistent with values on the higher range of those obtained from laboratory tests on MSW reported in the literature. The data also suggests a dependence of the heat generation rate on the temperature of the surrounding waste, similar to that observed for methanogenic bacterial activity. The experimental and simulation results constitute the basis for the design of geothermal heat exchange systems in MSW landfills. The successful implementation of the heat exchange system in MSW landfill indicates that similar systems can be used for long-term heat extraction, or for maintenance of waste temperatures to enhanced methane generation or waste settlement, or to protect landfill liner or cover systems.