Quick Answer
Yes, plastic bottles can be repurposed as thermal mass in greenhouses, but they are not the most efficient option due to their lower thermal conductivity and specific heat capacity compared to more traditional materials.
Using Plastic Bottles as Thermal Mass
Plastic bottles can be an affordable and creative way to incorporate thermal mass into a greenhouse design. By burying plastic bottles filled with water or sand, you can create a thermal mass that absorbs and releases heat, helping to regulate the greenhouse temperature. For example, a 2-liter plastic bottle filled with 1.5 liters of water can absorb approximately 30-40% of the sun’s radiation, releasing it slowly at night.
Design Considerations
To effectively utilize plastic bottles as thermal mass, consider the following design principles:
- Dig the plastic bottles 18-24 inches deep to maximize heat transfer.
- Bury multiple bottles in a pattern to create a consistent temperature gradient throughout the greenhouse.
- Use a layer of insulation, such as straw or shredded bark, to minimize heat loss and maximize the thermal mass effect.
- Combine plastic bottles with other thermal mass materials, like brick or concrete, to achieve optimal temperature stability.
Calculating Thermal Mass Effectiveness
To calculate the effectiveness of plastic bottles as thermal mass, consider the following factors:
- Specific heat capacity (SHC): 1.9-2.1 J/g°C for water and 0.88 J/g°C for plastic.
- Thermal conductivity (k): 0.6 W/m-K for plastic and 0.5 W/m-K for water.
- Mass of the thermal mass material (in kg).
- Temperature difference between day and night (in °C).
- Duration of the temperature difference (in hours).
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