Long-Term Thermal Performance Modelling and Simulations of a Borehole
2005 (English)In: Proceedings of the 7th Symposium on Building Physics in the Nordic Countries, Reykjavik, Iceland., 2005, 1-8 p.Conference paper (Refereed)
This paper presents the long-term thermal performance modelling and simulation of a single heat extraction borehole with a U-shaped pipe, by applying the Macro Element Modelling (MEM) method, developed by Schmidt (2004). The dynamic thermal performance of the ground around the borehole is analysed in the frequency domain. Thereafter, the ground properties are transformed into a network of discrete resistances and capacitances (RC’s) that together with the pipe, models the thermal performance of the borehole. The method allows the parameters of the RC- network components to be estimated and optimised for time domain simulations. The advantage of this modelling method is that it establishes a simplified yet accurate thermal borehole model, which requires less computing time and power compared to a traditional finite difference/element model. This makes it easier to perform several decades of long-term thermal performance simulations. In this paper the U-pipe was modelled by applying a star resistance network that calculates the weighted heat fluxes between the solid borehole and the U-pipe. The presented work shows that at small temperature increases and at relative large fluid velocities the U-pipe fluid temperature can be seen as rising linearly. This provides the borehole modelling with the possibility to model the entire borehole construction by using only one macro element. To compensate for any errors when using the linearly increasing fluid temperature for calculating the energy extraction from the borehole, the total U-pipe star resistance network is compensated. The U-pipe star network was also compared with a FEM (Finite Element Method) simulation, showing that the correspondence between the two models is good. The half 5-node network that was used by Schmidt (2004) for modelling the solid construction of the borehole was modified in this paper, with an additional resistance to ensure the stability of the network when simulation long-term heat extraction periods. The borehole RC-network was optimised for a time period of 100 years. The results from the long-term performance simulation made with the derived borehole model, were also compared to other presented borehole simulations. The model proved to have, at a first stage of comparison, a good correspondence with the other presented results.
Place, publisher, year, edition, pages
2005. 1-8 p.
Borehole, long-term thermal performance, Macro Element Modelling (MEM), simulations.
Building Technologies Materials Engineering
IdentifiersURN: urn:nbn:se:kth:diva-19456OAI: oai:DiVA.org:kth-19456DiVA: diva2:338081
QC 201008102010-08-102010-08-102010-11-17Bibliographically approved