Thermal Numerical Analysis of Vertical Heat Extraction Systems in Landfills

Author

Michael Thomas Onnen, California Polytechnic State University, San Luis ObispoFollow

DOI: https://doi.org/10.15368/theses.2014.69
Available at: https://digitalcommons.calpoly.edu/theses/1223

Date of Award

6-2014

Degree Name

MS in Civil and Environmental Engineering

Department/Program

Civil and Environmental Engineering

Advisor

James Hanson

Abstract

An investigation was conducted to determine the response of landfills to the operation of a vertical ground source heat pump (i.e., heat extraction system, HES). Elevated landfill temperatures, reported various researchers, impact the engineering performance of landfill systems. A numerical model was developed to analyze the influence of vertical HES operation on landfills as a function of climate and operational conditions.

A 1-D model of the vertical profile of a landfill was developed to approximate fluid temperatures in the HES. A 2-D model was then analyzed over a 40 year time period using the approximate fluid temperatures to determine the heat flux applied by the HES and resulting landfill temperatures. Vertical HES configurations simulations consisted of 15 simulations varying 5 fluid velocities and 3 pipe sizes. Operational simulations consisted of 26 parametric evaluations of waste placement, waste height, waste filling rate, vertical landfill expansions, HES placement time, climate, and waste heating.

Vertical HES operation in a landfill environment was determined to have 3 phases: heat extraction phase, transitional phase, and ground source heat pump phase. During the heat extraction phase, the heat extraction rate ranged from 0 to 2550, 310 to 3080, and 0 to 530 W for the first year, peak year, and last year of HES operation, respectively. The maximum total heat energy extracted during the heat extraction phase ranged from 163,000 to 1,400,000 MJ. The maximum difference in baseline landfill temperatures and temperatures 0 m away from the HES ranged from 5.2 to 43.2°C. Climate was determined to be the most significant factor impacting the vertical HES.

Trends pertaining to performance of numerous variables (fluid velocity, pipe size, waste placement, waste height, waste filling rate, vertical landfill expansions, HES placement time, climate, and waste heating) were determined during this investigation. Increasing fluid velocity until turbulent flow was reached increased the heat extraction rate by the system. Once turbulent flow was reached, the increase in heat extraction rate with increasing fluid velocity was negligible. An increase in the heat extraction rate was caused by increasing pipe diameter. Wastes placed in warmer months caused an increase in the total heat energy extracted. Increasing waste height caused an increase in the peak heat extraction rate by 43 W/m waste height. Optimum heat extraction per 1 m of HES occurred for a 30 m waste height. Increasing the waste filling rate increased the total heat energy extracted. Heat extraction rates decreased as time between vertical landfill expansions increase. Total heat energy extracted over a 35 year period decreased by approximately 21,500 MJ/year for every year after the final cover was placed until HES operation began. For seasonal HES operation, the total heat energy obtained each year differs and the fourth year of operation yielded the most energy. Wet Climates with higher heat generating capacities yielded increased heat extraction rates. Maximum temperature differences in the landfill due to the HES increased by 16.6°C for every 1 W/m³ increase in peak heat generation rate. When a vertical HES was used for waste heating, up to a 13.7% increase in methane production was predicted.

Engineering considerations (spacing, financial impact, and effect on gas production) for implementing a vertical HES in a landfill were investigated. Spacing requirements between the wells were dependent on maximum temperature differences in the landfill. Spacing requirements of 12, 12, 16, and 22 m are recommended for waste heating, winter-only HES operation, maximum temperature differences in the landfill less than 17°C, and maximum temperature differences in the landfill greater than 17°C, respectively. A financial analysis was conducted on the cost of implementing a single vertical HES well. The energy extracted per cost ranged from 0.227 to 0.150 $/MJ for a 50.8 mm pipe with a 1.0 m/s fluid velocity and a 50.8 mm pipe with a 0.3 m/s fluid velocity, respectively. A vertical HES could potentially increase revenue from a typical landfill gas energy project by $577,000 per year.