5:15 PM - 6:45 PM
[MZZ43-P01] Development of a method for estimating apparent thermal conductivity based on hydrogeological information
Keywords:Shallow geothermal utilization, Apparent thermal conductivity, Hydrogeology, estimation equation, Thermal response test
1. Introduction
Ground-source heat pump (GSHP) systems utilize shallow ground approximately 200 meters underground as a heat source for heating, hot water supply, snow melting, etc., positioning it as one of the renewable energy heat utilization methods. This system includes closed-loop systems and open-loop systems, both boasting high energy efficiency. Additionally, they are expected to contribute to CO2 emission reduction and mitigation of heat island effects. However, the adoption of GSHP systems domestically is not widespread due to factors such as high installation costs and lack of awareness.
2. NEDO Project "Development and Standardization of Apparent Thermal Conductivity Estimation Method"
New Energy and Industrial Technology Development Organization (NEDO) has been advancing various technology development and standardization projects related to renewable energy heat utilization. Currently, they are implementing the "Research and Development Total Cost Reduction of Heat Utilization as Renewable Energy " project (2019-2023). The National Institute of Advanced Industrial Science and Technology (AIST), through a consortium with Hokkaido University and Akita University, is working on this project under the theme of "Development and standardization of a method for estimating apparent thermal conductivity, simple heat response test method, and integrated design tools." In this presentation, we will report on "Development and standardization of a method for estimating apparent thermal conductivity” led by AIST.
Apparent thermal conductivity (λa value) represents the effective thermal conductivity of rock or sediment, including the influence of groundwater flow, and is currently estimated only through thermal response tests (TRTs). Accurate estimation of λa value at system installation points based on existing geological and groundwater information would enable appropriate system design and significantly contribute to cost optimization for both installation and operation. Therefore, AIST has developed an estimation method for λa value based on hydrogeological background as part of the above NEDO project. The estimated λa value represents an average value from the surface to a depth of 100 meters, with a target estimation accuracy within 0.5 W/(m･K) error.
3. Development and Validation of apparent thermal conductivity estimation equation based on hydrogeological methods
To estimate λa value, it is necessary to quantify the influence of darcy flow velocity. Therefore, in this study, based on the results of numerical TRTs under various geological structures and groundwater flow conditions, we developed an estimation equation for λa value with effective thermal conductivity and darcy flow velocity as variables. Numerical TRTs were conducted using methods employing exact solutions and numerical models to quantify the relationship between effective thermal conductivity, darcy flow velocity, and λa value. Effective thermal conductivity data used as variables input into the estimation equation were compiled from the constructed geological models and the 1:200,000 Seamless Geological Map of Japan. Darcy flow velocity data were obtained from a wide-area 3D groundwater flow analysis using geological models. In addition, hydraulic gradient data was compiled from an AI model and hydraulic conductivity data from the Seamless Geological Map, from which darcy flow velocity was determined based on hydraulic gradient and hydraulic conductivity.
To validate the estimation equation, the estimated λa value was compared with that obtained from TRTs conducted at eight locations within study areas. As a result, the error in λa value between the two methods was within 0.5 W/(m･K) at six locations, confirming generally good reproducibility. Verification of the accuracy of the estimation equation and measured values will be conducted in the future.
Ground-source heat pump (GSHP) systems utilize shallow ground approximately 200 meters underground as a heat source for heating, hot water supply, snow melting, etc., positioning it as one of the renewable energy heat utilization methods. This system includes closed-loop systems and open-loop systems, both boasting high energy efficiency. Additionally, they are expected to contribute to CO2 emission reduction and mitigation of heat island effects. However, the adoption of GSHP systems domestically is not widespread due to factors such as high installation costs and lack of awareness.
2. NEDO Project "Development and Standardization of Apparent Thermal Conductivity Estimation Method"
New Energy and Industrial Technology Development Organization (NEDO) has been advancing various technology development and standardization projects related to renewable energy heat utilization. Currently, they are implementing the "Research and Development Total Cost Reduction of Heat Utilization as Renewable Energy " project (2019-2023). The National Institute of Advanced Industrial Science and Technology (AIST), through a consortium with Hokkaido University and Akita University, is working on this project under the theme of "Development and standardization of a method for estimating apparent thermal conductivity, simple heat response test method, and integrated design tools." In this presentation, we will report on "Development and standardization of a method for estimating apparent thermal conductivity” led by AIST.
Apparent thermal conductivity (λa value) represents the effective thermal conductivity of rock or sediment, including the influence of groundwater flow, and is currently estimated only through thermal response tests (TRTs). Accurate estimation of λa value at system installation points based on existing geological and groundwater information would enable appropriate system design and significantly contribute to cost optimization for both installation and operation. Therefore, AIST has developed an estimation method for λa value based on hydrogeological background as part of the above NEDO project. The estimated λa value represents an average value from the surface to a depth of 100 meters, with a target estimation accuracy within 0.5 W/(m･K) error.
3. Development and Validation of apparent thermal conductivity estimation equation based on hydrogeological methods
To estimate λa value, it is necessary to quantify the influence of darcy flow velocity. Therefore, in this study, based on the results of numerical TRTs under various geological structures and groundwater flow conditions, we developed an estimation equation for λa value with effective thermal conductivity and darcy flow velocity as variables. Numerical TRTs were conducted using methods employing exact solutions and numerical models to quantify the relationship between effective thermal conductivity, darcy flow velocity, and λa value. Effective thermal conductivity data used as variables input into the estimation equation were compiled from the constructed geological models and the 1:200,000 Seamless Geological Map of Japan. Darcy flow velocity data were obtained from a wide-area 3D groundwater flow analysis using geological models. In addition, hydraulic gradient data was compiled from an AI model and hydraulic conductivity data from the Seamless Geological Map, from which darcy flow velocity was determined based on hydraulic gradient and hydraulic conductivity.
To validate the estimation equation, the estimated λa value was compared with that obtained from TRTs conducted at eight locations within study areas. As a result, the error in λa value between the two methods was within 0.5 W/(m･K) at six locations, confirming generally good reproducibility. Verification of the accuracy of the estimation equation and measured values will be conducted in the future.