Introduction

When engineers design geothermal energy systems, they are essentially figuring out how to move heat from underground reservoirs of hot rock, soil, or water up to the surface, where it can be used to generate electricity or provide direct heating. The thermal conductivity of underground materials determines how effectively this heat can move through the Earth to reach wells, pipes, or heat exchangers.

• High thermal conductivity materials (like granite or water-saturated rock) allow heat to flow more quickly and evenly. This means geothermal wells drilled into such regions can deliver a steady, reliable supply of heat.
  
• Low thermal conductivity materials (like dry clay, sand, or porous rock) act as insulators, slowing down the transfer of heat. In these conditions, the rate of heat extraction must be carefully managed to avoid cooling the reservoir too quickly.

High-conductivity sites are better for electricity generation (where high temperatures and steady heat flow are required). Moderate or low-conductivity sites may be more suitable for direct-use systems, like heating buildings or greenhouses, where lower but steady temperatures are sufficient.
Thermal conductivity is a key factor in determining how much heat can be harvested, how quickly it can be replenished, and how long a geothermal resource will remain viable. Without this knowledge, engineers risk designing systems that either underperform or deplete the geothermal resource too soon.

Materials

• Tray or beaker that can be heated to hold test material (e.g., stainless steel tray or glass beaker)
• One test material (examples below); 5 cm thick layer in designated tray or beaker
  • Dry sand
  • Moist sand
  • Dry soil
  • Moist soil
  • Clay
  • Small stone gravel
• Hot plate (low setting)
• Insulating mat (cork mat or silicone pad)
• 2 thermometers (digital probes preferred)
• Safety glasses, heat-resistant gloves and/or beaker tongs
• Ruler

Write a Hypothesis

In today’s lab, your group will be testing the thermal conductivity of a material provided by your teacher by measuring the temperature over time as it is exposed to a heat source. Look at the materials list and identify the variables.

In comparison to the other materials being tested by your classmates, will your material have high or low thermal conductivity? Write a hypothesis and explain your reasoning.

Safety Guidelines

Before starting the lab experiment, review these safety guidelines.

1. Hot plates and trays/beakers get HOT. Wear safety glasses and heat-resistant gloves when handling equipment. 
2. Keep liquids away from electrical cords and hot plate controls. 
3. DO NOT set thermometer probes directly on the hot plate surface.

Set Up

1. Add and level a 5 cm layer of your material in your tray or beaker, and gently compress to a consistent packing. 
2. Place your container on the hot plate, but do not turn it on yet.
3. Use a ruler to mark probe depth 1 cm (near the bottom). Insert Probe A 1 cm above the bottom (close to the heat source). Insert Probe B 0.5 cm below the surface. Keep both probes centered and not touching the container walls or bottom. 
4. Record the initial temperatures of both probes before heating in the data table below. 
5. Unplug the hot plate and let materials cool before disposal and clean up.

Procedure

1. Place the container on the hot plate set to LOW. 
2. Start the timer. 
3. Record both probe temperatures every two minutes for 20 minutes. 
4. Do not stir or move the probes during the run. 

Data Table

Graphing

• Using graphing paper or a digital graphing tool, create a line graph with time (minutes) on the x-axis and temperature (℃) on the y-axis.
• Plot two separate lines on the same graph: Probe A (near the bottom, near the heat source) and Probe B (near the top, farther from the heat source). Be sure to label each line clearly with different colors or symbols, and include a legend.
• To calculate the rate of heat transfer, determine the slope of the Probe A line during the initial heating period (the first 6-8 minutes). In this section, the line should be steep and fairly linear. You will use this portion to calculate slope because it best represents the material’s ability to conduct heat from the source into the sample.
• The units for the slope will be ℃/time, which is the rate of heat transfer.

Analysis and Conclusion Questions

1. Share the rate of heat transfer for your material according to teacher instructions. Your teacher will direct you to where the rate of heat transfer data from all groups is displayed. Using that data, rank all the materials tested in order from fastest to slowest heat transfer.

2. What properties of the materials (e.g., density, moisture, composition) may influence thermal conductivity? 

3. Based on class data, does your material have high, moderate, or low thermal conductivity? 

4. Was your hypothesis supported by the data? Why or why not? 

5. What specific sources of measurement error may have affected your results (for example: timing, temperature readings, or material thickness)? How might repeating the experiment with more trials help reduce the impact of these errors and increase confidence in your conclusions?

6. Why would engineers want to know the thermal conductivity of underground materials before designing a geothermal system? 

7. In a geothermal power plant, what problems might occur if the reservoir material has low thermal conductivity? 

8. Imagine the material that you tested represented the subsurface beneath a geothermal well. How might the heat extraction strategy need to change for long-term efficiency? 

9. If two geothermal sites have the same temperature, but one has high-conductivity granite and the other has low-conductivity clay, which would be better for electricity generation? Why?

10. How does this experiment help explain in part why geothermal energy is more successful in some regions (like Iceland) than in others?

Assessment Rubric