Title:

Coupling Subsurface and Above-surface Models for Optimizing the Design of Borefields and District Heating and Cooling Systems in the Presence of Moving Groundwater

Authors:

Jianjun HU, Christine DOUGHTY, Patrick DOBSON, Peter S. NICO, Michael WETTER

Key Words:

Modelica Buildings Library, TOUGH, Borefields, Reservoir, District heating and cooling, code coupling

Conference:

Stanford Geothermal Workshop

Year:

2022

Session:

Low Temperature

Language:

English

Paper Number:

Hu1

File Size:

1200 KB

View File:

Abstract:

Dynamic energy simulation is important for the design and sizing of district heating and cooling systems with geothermal heat exchange for seasonal energy storage. Current modeling approaches in building and district energy simulation tools typically consider heat conduction through the ground between boreholes without flowing groundwater. While detailed simulation tools for subsurface heat and mass transfer exist, these fall short in simulating above-surface energy systems. To support the design and operation of such systems, we have developed a coupled model including a software package for building and district energy simulation, and software for detailed heat and mass transfer in the subsurface. For the first, we use the open-source Modelica Buildings Library, which includes dynamic simulation models for building and district energy and control systems. For the heat and mass transfer in the soil, we use the TOUGH simulator. TOUGH can model heat and multi-phase, multi-component mass transport for a variety of fluid systems, as well as chemical reactions, in fractured porous media. In previous work, we described the coupling of these software packages, including how time-dependent boundary conditions for the borehole walls are synchronized for use in Modelica and TOUGH. We verified that the coupled Modelica/TOUGH code produced consistent results with the original Modelica code for an idealized problem in which heat transfer was purely by conduction in a uniform geologic medium. Here, we examine less idealized problems for which TOUGH’s advanced capabilities for modeling fluid flow are required. The first example includes a thick vadose zone with a water-table depth that varies in time, which requires a fine vertical grid discretization for the 2D TOUGH model. The second example considers strong regional groundwater flow, which requires a 3D TOUGH model. Both examples require a new, more sophisticated coupling between the TOUGH and Modelica grids, which is described. Such models can be used to optimize borehole design and systems operations.


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