Ground Temperature

Thermistor string

Temperature strings like this one were buried in the ground behind Weller to measure the temperature of the soil.

The ground source heat pump at Weller School takes heat from the soil behind Weller School and moves it to a heat pump in the fan room, which transfers the heat to incoming ventilation air. GSHPs are not very common in Alaska because the ground here is not as warm as in other climates. Also, Alaska has a very long heating season; we have to heat our homes and buildings for more months of the year than we have to cool them. This raises a lot of questions about GSHPs in Alaska:

1. Can a GSHP provide enough heat from the cold soil to help heat a building?

 2. As the GSHP takes heat from the soil, will the sun be able to replenish it, or will the soil become colder?

 3. If the soil becomes colder, will the GSHP still work as well?

4. Will solar panels help to restore heat to the ground?

The scientists and engineers at CCHRC and the Fairbanks North Star Borough School District are trying to answer these questions. They want to learn more about GSHPs and their effect on the ground in a cold climate. So they decided to monitor the temperature of the ground where the ground loop is taking heat from the soil at Weller.

To monitor the temperature, the researchers buried four temperature strings in the ground outside Weller School. Each temperature string has sensors to measure soil temperature at different depths.  The sensors are strung together in a line and protected by a pipe.  They record the soil temperature every 15 minutes and send that  data through a wire back to a computer. The computer stores the data so that we can look at it later.The picture below shows the area without grass where workers buried the ground loop for the heat pump in Semptember 2010. One temperature string is located on the western edge of this area, one temperature string is 8 feet east of the edge, and a third one is in the center of the ground loop field.  These strings will show how the ground loop changes the temperature of the ground. The  fourth string is located away from the ground loop.  This string is a control, which means that it will show what the soil temperatures are in an area undisturbed by the ground loop.

Thermistor Locations

There are 4 temperature strings buried in the ground behind Weller School. Each string can measure the soil temperature down to a depth of 22 feet.

The temperature strings each have 9 sensors so that we know the temperature at different depths where each string is. The sensors are placed at 1 foot under the ground surface, 4 feet deep, 6 feet deep, 8 feet deep, 10 feet deep, 12 feet deep, 14 feet deep, 18 feet deep and 22 feet deep. During the year, the temperature of the ground changes a lot near the surface, because it receives heat from the sun.  Deeper down, the ground temperature doesn’t change very much, because it is insulated  from the cold of winter and heat of summer by the soil above it.

This type of graph is called a "trumpet curve" because the outside lines form a cone, just like at the end of the instrument.

Monitoring the ground temperatures will help researchers know how the heat pump is doing.  If the heat pump takes too much heat away from the ground, then it won’t work very well.  This could mean that the ground loop is too small, or that heat pumps might need more help from solar panels. The temperatures can also show when the heat pump is performing well, and if heat from the solar panels is helping to keep the ground warm. Every day the temperature strings record data so that we can tell how the ground is doing.  We make a graph of the ground temperatures every week.  The latest graph is below:

Soil Temperatures

If you would like to see more soil temperature data from the ground loop field, or past temperature graphs, you can visit our Graph Archive.

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