Estimations of undisturbed ground temperatures using numerical and analytical modeling

Abstract

The interaction of buildings and ground source heat pump systems with the surrounding ground is quite important for design and energy calculation procedures. Building design load calculations, building energy calculations, ground heat exchangers design and design and energy analyses of district heating and cooling systems often require as inputs the undisturbed ground temperatures.

Currently, the available undisturbed ground temperatures are rather limited. In the U.S., the ground temperatures are usually represented with a three-parameter one-harmonic model. The model parameters for the continental US or North America are presented in maps in the ASHRAE handbooks. The results presentation in small maps can be quite difficult to read for a specific location. Furthermore, the sources of some results are unknown, and where the source is known, the results were published more than half a century ago. ASHRAE district heating manual also published a world-wide data set presented in one-harmonic model with model parameters presented in tables. However, the data are computed based on a simplified approximation that the ground surface temperature is equal to the air temperature; this approximation can lead to significant error in the cold climates and arid climates.

Therefore, the main objective of this research is to provide a new set of ground temperature estimates for use by engineers. A numerical model and a simplified design model have been developed for the estimations of the typical year ground temperature and maximum/minimum ground temperatures of multiple years. Both models have been validated against the experimental results. The validated numerical model will be run with 1020 TMY3 weather files in the U.S., 80 CWEC weather files in Canada and 3012 IWEC-2 weather files around the world. The simplified design model relies on empirical parameters to estimate the ground temperatures. Therefore, the numerical model results will be used to generate parameters for the design model. Two sets of ground temperature estimates approximated for with two different earth surface conditions will be developed; these two earth surface conditions are short grass, tall grass. These ground temperatures are presented in a two-harmonic form using parameters estimated from the numerical model results.