Ground Heat Exchanger (GHE) is the key to the efficiency of a Ground Source Heat Pump (GSHP). However, GHE is not standardized as other parts of the GSHP. It is basically a bunch of tubes that need to be properly sized and burred in the ground. Improperly sized GHE can lead to choked flow and higher pump cost or stagnate flow, and less heat transfer.
Sizing Ground Heat Exchanger methods and data of GHE are mostly scattered in installation manuals and other documentation, such as the International Ground Source Heat Pump Association Installation Guide and others listed in the references. There are also heat exchanger manufacturers’ manuals and several software programs you can use to facilitate the design calculations.
Below are the general steps needed to size a Ground Heat Exchanger.
The indoor ground fluid temperature. This number can be found in the heat pump unit. The heat pump should specify what this ground fluid temperature is supposed to be inside the unit when the heat pump is used for cooling and heating. Yes, there are two different values, one for cooling, one for heating.
Earth temperature where the tubes to be burred. You may choose to use the yearly average, the coldest temperature to ensure enough heating in winter, or the hottest temperature to ensure enough cooling in summer. Estimation may be OK, or you can dig a hole to the depth where you are going to install the tubes. Drop a thermometer to the hole, let it stay there for 5 minutes and pull it out to read the temperature immediately.
Evaluate the soil properties. Wet and fine soil will transfer heat better than dry and sandy soil. Using fine mud outside tubes in the installation will help. Calculate the thermal resistances of the soil and the tube wall. The smaller the sum of the resistance, the closer the fluid tube temperature will be to the ground temperature.
Calculate the temperature differences between the indoor fluid and the in-ground fluid. Again there are two values for this calculation one is for cooling, and the other is for heating.
Compare these two values to see which is smaller. Hereafter, we will only use the smaller value as the fluid temperature difference and ignore the bigger value since it will be automatically satisfied if the smaller value is covered in the design.
Choose the fluid and find its properties. It should be mostly water, but some anti-freezing agent if you cannot avoid freezing temperature. Some anti-freezer may love the heat capacity and heat transfer coefficient.
Find the BTU (or thermal) capacity of the heat pump unit. Divide the thermal capacity by the fluid temperature difference, and the fluid heat capacity will yield the fluid flow rate needed for the ground heat exchanger.
Find available tube sizes and calculate the cross-section inside the tubes. Divide the fluid flow rate obtained above by the cross-section will produce the fluid velocity. Choose the tube size that will give you the recommended fluid velocity inside the ground tubes. This velocity should be preferred with 2 to 8 ft/sec. That is the tube you are going to use for the ground heat exchanger.
With the fluid velocity, the soil thermal resistance, the thermal capacity derived above, you can calculate the total heat exchange surfaces needed. Divide this total surface by the circumference of the tube will give you the total length of the tube needed.
Understanding these basic factors in sizing a ground heat exchanger will help you to perform the calculation and get a better result. Some of the steps are involved, and assumptions may have to be made. Once the assumed value is found, repeat the calculation to get a better Ground Source Heat Pump installation.