Enhanced multipole method for the transient thermal response of slender geothermal boreholes*

During the design stages of a new geothermal heat exchanger special care is devoted to the peak heating and cooling demands of the building. These lead to extreme temperatures in the heat carrying liquid that may cause unwanted material damages in the installation. The short duration of these peak demands, of a few days at most, require the inclusion of the thermal inertia of the heat carrying liquid, the grout, and the ground located close to the boreholes in the theoretical model used to predict the thermal response of the geothermal heat exchanger. By expanding the grout and ground temperatures in terms of conveniently chosen multipoles, the present work develops such a model. The resulting enhanced multipole method, meant originally for peak heating and cooling demands, is also valid in the limit of slowly varying heat injection rates in which it delivers the same results as the classical multipole method developed by Bennet, Claesson, and Hellstro?m.

Automated summary

During the design of a new geothermal heat exchanger, special attention is paid to peak heating and cooling demands of the building to prevent material damages. The thermal response of the heat exchanger needs to consider thermal inertia of the heat carrying liquid, grout, and ground. By developing an enhanced multipole method, the model can accurately predict peak heating and cooling demands as well as slowly varying heat injection rates.