Analytical Approaches to Modelling Temperature Variations for a Vertical Ground-Coupled Heat Pump System

English Abstract:

When designing a vertical ground-coupled heat pump (GCHP) system it is important to optimize

the system by considering the thermal potential of the subsurface and various economic factors.

This paper reviews the analytical models used to size the ground heat exchanger (GHX)

component of the system; the models under consideration include the infinite-line source (ILS)

model, the infinite-cylindrical source (ICS) model, and the composite cylindrical

source (CCS) model. For comparison purposes, each model is applied to an example building utilizing a

single borehole system. The ILS model has been proven to be acceptable when investigating

long-term trends whereas the ICS and CMC models are more appropriate

for short-term trends. For a cycling system, attention should be placed on the

transient behavior of a GCHP during the first minutes of operation. Using data from building

loads and time of operation for the heat pump, the temperature variations of the heat transfer

fluid and the borehole wall are predicted for the system. It is discussed that when a system is

sized according to the larger of its peak heating or cooling demand, in situations where one is

larger than the other, annual ground temperature imbalances may result and degrade the

performance of the system over time. The possible benefits of properly cycling a geothermal heat

pump include the reduction of annual ground temperature imbalances and economic savings.

This paper investigates the appropriateness of using the CCS model for illustrating the short-

term ground and fluid temperature responses when cycling a GCHP.