Charakterisierung der transienten Wärmeübertragung koaxialer Erdwärmesonden

  • Characterization of the Transient Heat Transfer Processes in Coaxial Borehole Heat Exchangers

Steins, Christopher; Kneer, Reinhold (Thesis advisor); Spindler, Klaus (Thesis advisor)

Aachen (2017, 2018)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

The transformation towards a renewable energy based energy supply is one of the key challenges of this time. Ground-coupled heat pump systems can serve the demand of heating and cooling and, in particular, can replace heating systems fired by fossil fuels. The use of borehole heat exchangers is considered to be very efficient because temperatures in deeper earth layers are independed from atmospheric conditions. However, the high development costs result particularly from drilling the wellbores, so that the extensive expansion of these systems is still limited. In order to overcome these limitations, it is not only necessary to reduce the development costs by reducing the absolut drilling depth, but also to optimize the ground-coupled heat pump systems in relation to the fluctuating energy requirements.Within this dissertation, the phenomenological transient heat transfer processes are investigated in detail by means of a locally and time-resolved coaxial borehole heat exchanger model and their effects on the system efficiency are characterized. Through a comprehensive parameter study, the temporal changes in the temperatures and heat flows around the borehole heat exchanger are investigated particularly with respect to the internal volume. With the used short-time modell, it can be shown, that coaxial borehole heat exchanger with an increased internal volume can significantly contribute to the flexibility of the system operation and a higher efficiency. Finally, transient indicators are introduced as an alternative evaluation approach for borehole heat exchangers to quantify the influence of the different site and operating parameters. Thereby, the evaluation involves the assessment of the entropy production in transient operation.