Evaluation of Low-Grade Geothermal Energy Recovery From a Cold Climate Municipal Solid Waste Landfill

Abstract

With an increased focus on climate change and environmental footprint in recent years, methods to mitigate the use of non-renewable energy resources have received increased attention. Over the last two decades, research has been performed exploring the processes associated with gas and heat generation within municipal solid waste (MSW) landfills. Discussions have since commenced about the potential to utilize landfills as a low-grade geothermal energy source. Field tests investigating this topic have taken place globally, but they have not taken place in a colder and semi-arid climate where the initial waste temperatures are known to be lower. In addition, the analysis of the previous field tests (including the MSW thermal properties, radius of influence, and heat extraction rate) have either not been possible due to the testing methods, and/or have been simplified to one-dimensional analytical solutions. The research presented investigates the feasibility of low-grade geothermal heat extraction from medium-sized MSW landfills in cold and semi-arid climates, while also exploring different methods of analysis.

The Northern Landfill, operated by Loraas Disposal Ltd., is a medium-sized landfill located 10 km north of Saskatoon, Saskatchewan. As of 2018, approximately 2.5 megatonnes of MSW over three million cubic metres have been placed. A vertical borehole heat exchanger (BHE) and surrounding temperature-measuring thermistors were installed into the landfill using sonic drilling methods. A thermostatic system was also constructed to provide a constant fluid temperature as it entered the BHE. Two active thermal response tests (TRTs) with recovery periods (one heat injection test and one heat extraction test) were performed with the temperature of the circulating fluid and the surrounding MSW being measured along the landfill depth. The results from the tests were compared to each other, as well as to passive thermal responses previously measured at the Northern Landfill, and to previous active TRTs in MSW performed globally. In addition, the tests were analyzed with one-dimensional analytical models and a two-dimensional finite element model (with and without the backfilled materials), to determine the thermal properties of the MSW with depth. The heat transfer rate was also estimated with a two-dimensional model.

The maximum thermal response of the waste from the heat extraction test was similar to a previous test performed in a warmer California climate (Yeşiller et al., 2016) in terms of normalized depth location and radial trend. The radius of influence for the heat extraction test was found to be between 1.8 m and 5.1 m using the observational method and 4.0 m when evaluated with the two-dimensional finite element model. It was determined that the analytical infinite line source (ILS) method was suitable for providing initial MSW thermal properties with depth for a more complex two-dimensional finite element model. For the heat injection and heat extraction test, the two methods estimated the MSW thermal diffusivity to range from 1.9 x 10-7 m2/s to 3.9 x 10-7 m2/s for the sections of MSW known to not be dependent on atmospheric temperatures. The analytical Cooper Jacob distance-drawdown method was determined to be an unsuitable method for estimating initial MSW thermal properties with depth, overestimating the MSW thermal diffusivity by an order of magnitude with a thermal diffusivity of 4.0 x 10-6 m2/s. However, it did verify that at a constant depth, the MSW could be treated as a homogenous material within the spatial testing limits.

It was determined that the MSW thermal diffusivity was greatest at the central depths. This finding did not follow the theory of MSW thermal diffusivity increasing with depth. It did follow the trend of the measured volumetric water content of in-situ MSW cores from a borehole that was retrieved without additional drilling fluid and within proximity of the test. The results and analysis of the field testing found that low-grade geothermal heat extraction is not a practical in-situ energy recovery method for medium-sized landfills in cold and semi-arid regions. Twenty-six vertical BHEs would be required to extract 11.5 kW of power, the capacity of an average closed loop ground heat available for Canadian residents (Government of Canada, 2021). While also not practical, the number of required vertical BHEs can be reduced to 14 if the circulation fluid temperature is reduced to -4.0°C.

In terms of in-situ energy recovery at MSW landfills in cold and semi-arid regions, it is recommended to evaluate the feasibility of optimizing the quantity of methane generated in a mesophilic bacteria landfill with heat exchangers and extract the methane as a source of energy. Future active TRTs performed in MSW landfills are recommended to account for the mechanical, hydraulic, and pneumatic processes inside landfills that are known to influence heat transport, as well as additional external factors such as radiation, wind, and snow to predict the shallower waste depth more accurately.