Loop Pipe Length
Sizing the ground heat exchanger length requires a knowledge of the earth temperatures and thermal conductivity at planned design depths.
The sub-steps are:
- Defining the buried heat exchanger location - This includes pipe depth and spacing for horizontal loops or bore-hole depth, the number of U-bend loops, and spacing of bore holes.
- Determining the underground soil temperatures - Annual air temperatures, moisture content, soil type, and vegetative cover all have an effect. Several sources should be used, including local knowledge based on the loop contractor's experience and the measured data.
- Determining the maximum and minimum earth temperatures - This step uses information developed in the preceding step, along with analytical equations.
- Determining the heat pump maximum and minimum entering fluid temperatures - Do this using the manufacturer's published data and estimates based on the site location.
- Calculating the earth-to-loop temperature difference - This is the difference between the earth temperature minimum and the heat pump minimum entering fluid temperature for heating, and conversely the maximum temperature for the cooling cycle.
- Determining the pipe resistance to heat flow - This calculation uses thermal conductivity data for the pipe selected and the loop configuration.
- Determining the soil/field resistance to heat flow - This can be a time consuming process, so using one of the personal computer software programs available for this task is recommended.
- Calculating the heating and cooling run fractions - These are based on the building's heating and cooling loads, balance temperatures where no heating or cooling is required, heat pump capacity and hours in each temperature bin. Again, using a personal computer software program is recommended.
- Calculating the ground heat exchanger size based on:
a. The length of pipe for a horizontal ground heat exchanger,
b. The feet of bore hole for a vertical system, or
c. The defining parameter for other heat exchanger designs.
There are a number of design approaches that yield similar results. The IGSHPA Installation Guide outlines one straightforward method that is generally accepted, and used in some of the software programs.
Usually designers examine several entering water temperatures to get a range of choices. Shorter lengths may be the lowest first cost, but a longer length increases the heat pump capacity and efficiency, and lowers the operating cost. The final size selection will be a compromise.