24 June 2014

A NOVEL NUMERICAL MODEL FOR THE THERMAL RESPONSE OF BOREHOLE HEAT EXCHANGER FIELDS


The design of a borehole field should be based on a long-term simulation of its thermal response for specified energy loads. This response can be calculated from a precalculated
dimensionless function, the g-function. This paper is focused on a new approach to the calculation of g-functions; in particular with a precise representation of the boundary
conditions at the borehole wall. First, the almost constant temperature along the borehole heat exchanger requires a boundary condition of essentially isothermal boreholes along the
depth. In a common case the borehole heat exchangers are connected in parallel, thus all boreholes should have the same temperature. Also, the total heat flow to the borehole field
should be constant over time. A numerical model in which the boreholes are filled with a highly conductive material has been built, reproducing the isothermal condition. By thermally
interconnecting the boreholes, the equal temperature condition is satisfied. The g-functions
of some simple borehole field configurations are presented in this paper. The results show, in
general, a good agreement with the existing solutions for a similar boundary condition.