Models and tools for the assessment of thermal-short circuit in open-loop geothermal systems

Alessandro Casasso (a) & Rajandrea Sethi (a)
(a) Department of Land, Infrastructure and Environmental Engineering, Politecnico di Torino, corso Duca degli Abruzzi, 24, 10129, Torino, Italy. Corresponding author e-mail: alessandro.casasso@polito.it.

The use of Ground Water Heat Pumps (GWHPs) for the heating and cooling of buildings is getting more and more popular due to the sustainability and economic convenience of this technology. One of the most critical design issues of these systems is the hydraulic and thermal short-circuit between the abstraction and the injection well(s), which impairs the energy efficiency of the heat pump and may even lead to the plant failure. A number of mathematical tools has already been developed to assess thermal short-circuit in well doublets with a prescribed injection temperature (thermal feedback), but this assumption is rarely met in the reality. In order to assess thermal short-circuit for a prescribed temperature difference between injection and abstraction (thermal recycling), a numerical code called TRS (Thermal Recycling Simulator) was therefore developed and is presented in this paper. The code is based on the potential flow theory and it has been validated against coupled flow and heat transport simulations with FEFLOW. TRS is freely available at http://goo.gl/AdjCp6 and works in MATLAB environment. An empirical formula, which describes the time evolution of the extracted water temperature, has also been derived from a series of simulations with TRS. The developed mathematical tools can be used to assess the long-term sustainability of a certain plant setup, to perform sensitivity analyses and for largescale assessments of the thermal exchange capacity of aquifers.

Ground Water Heat Pump, potential flow theory, open loop, thermal recycling.