Earth tube cooling systems, also known as earth-air heat exchangers, use the earth’s stable underground temperature to cool or heat air before it enters a building. This system leverages the consistent temperatures found underground to moderate the temperature of air being funneled into a structure. By circulating air through buried tubes, the air either gains or loses heat to the surrounding soil.
Purpose and Functionality
The primary purpose of an earth tube cooling calculator is to estimate the cooling or heating effect provided by an earth tube system. This helps in designing efficient systems that can reduce the need for additional energy-consuming air conditioning or heating.
How It Works
Inputs Required
- Ambient Air Temperature (T_air): The outdoor air temperature.
- Soil Temperature (T_soil): The average temperature of the soil at the depth of the tubes.
- Airflow Rate (Q_air): The rate at which air flows through the tubes, usually measured in cubic meters per hour (m³/h).
- Tube Material and Characteristics: Including the thermal conductivity of the material, diameter, and length of the tubes.
- Tube Surface Area (A): Calculated based on the diameter and length of the tube.
- Temperature Difference (ΔT): Difference between the soil temperature and the ambient air temperature.
Basic Calculations
Heat Transfer Calculation
The heat transfer QQQ through the earth tube can be estimated using the following equation:
cssCopy codeQ = h ⋅ A ⋅ ΔT
Where:
- hhh is the heat transfer coefficient, depending on the material and flow characteristics.
- AAA is the surface area of the tube.
- ΔTΔTΔT is the temperature difference between the soil and the incoming air.
Determining the Exit Temperature of Air (T_exit)
To find the exit temperature of the air after it has passed through the earth tubes, you can use the following equation:
cssCopy codeT_exit = T_air - \frac{Q}{\rho ⋅ c ⋅ Q_air}
Where:
- ρ\rhoρ (rho) is the density of air (approximately 1.2 kg/m³ at standard conditions).
- ccc is the specific heat capacity of air (approximately 1005 J/kg°C).
- QairQ_airQair is the airflow rate in kg/s, which can be calculated from the volumetric flow rate divided by the density of air.
Example Calculation
Suppose you have the following parameters:
- Ambient Air Temperature (T_air): 35°C
- Soil Temperature (T_soil): 15°C
- Temperature Difference (ΔT): 35°C−15°C=20°C35°C – 15°C = 20°C35°C−15°C=20°C
- Tube Length: 50 m
- Tube Diameter: 0.2 m
- Tube Surface Area (A):
cssCopy codeA = π ⋅ d ⋅ L = π ⋅ 0.2 ⋅ 50 ≈ 31.4 m²
- Heat Transfer Coefficient (h): 10 W/m²°C (typical value)
cssCopy codeQ = h ⋅ A ⋅ ΔT = 10 ⋅ 31.4 ⋅ 20 = 6280 W
- Airflow Rate (Q_air): 0.5 m³/s (≈ 0.6 kg/s, assuming air density as 1.2 kg/m³)
mathematicaCopy codeT_exit = 35°C - \frac{6280}{0.6 ⋅ 1005} ≈ 25°C
This indicates the air exiting the earth tubes would be cooled from 35°C to approximately 25°C, thus providing significant cooling without the use of additional energy for air conditioning.
Relevant Information Table
| Parameter | Value |
|---|---|
| Ambient Air Temperature (T_air) | 35°C |
| Soil Temperature (T_soil) | 15°C |
| Temperature Difference (ΔT) | 20°C |
| Tube Length | 50 m |
| Tube Diameter | 0.2 m |
| Tube Surface Area (A) | 31.4 m² |
| Heat Transfer Coefficient (h) | 10 W/m²°C |
| Heat Transfer (Q) | 6280 W |
| Airflow Rate (Q_air) | 0.5 m³/s (0.6 kg/s) |
| Exit Air Temperature (T_exit) | 25°C |
Conclusion
Earth tube cooling systems offer a sustainable way to moderate indoor temperatures using the stable temperatures of the earth. By using an earth tube cooling calculator, you can design systems that effectively reduce or even eliminate the need for traditional air conditioning. This not only saves energy but also provides a comfortable indoor environment using natural resources.