Will adding inner glazing films improve Greenhouse performance? A Thermal Analysis

Introduction:

Accurately quantifying the actual R-value of components with variable characteristics, such as air gaps, is critical in sustainable greenhouse design. This is particularly important when assessing the potential economic value of changes intended to improve greenhouse efficiency.

Obtaining effective standard measures can be difficult; R-values are not always published, and the R-value of some components like air gaps can vary depending on installation method. The lack of a reliable field expedient method to measure and convert the results into standard measures like R-Value that can be used to compute savings in heating costs can make it difficult to justify the investment needed to implement innovative technologies in greenhouses.

I developed a easily applied methodology capable of determining  actual R-values for these variable components. This approach is vital for evaluating the thermal performance of existing installations and estimating the potential heat savings from proposed enhancements to greenhouse glazing.    

Accurately quantifying these R-values provides a solid foundation for making informed decisions in sustainable greenhouse design and operation.  This capability is vital for decision-makers, as it provides a clear indication of the potential return on investment when considering installation of energy-efficient technologies in greenhouses on a larger scale.

Proposed methodology

The methodology utilizes temperature gradients across two or more insulating layers such as air gaps to estimate U-factors and R-factors in greenhouse environments.  It employs temperature differentials across insulating layers, with at least one known U-Factor as a reference point.  It uses an assumption of serial transmission of heat through multiple layers and uses change in temperatures across those layers to develop a ratio based method to derive new U-Factor, R-Value and facilitates the estimation of proportional heating cost savings based on simple measures from experimental approaches.

By streamlining the thermal analysis process, this approach significantly enhances the efficiency of micro-experiments, allowing for precise conversion to standard R-values and U-factors.

Advantages of the Methodology:

  • Practicality: Simplifies data collection by using easily accessible temperature measurements.
  • Flexibility: Adaptable to a wide range of glazing systems, suitable for both standard and customized greenhouse designs.
  • Comparative Analysis: Uses known U-factors or R-factors of one component to estimate values for other components.
  • Efficiency: Offers a straightforward and quick calculation process.

Technical Procedure:

For this example we used 3 sheets a plastic membrane stretched over a wood frame that forms two well sealed air gaps.  A 3rd air gap is formed between the outer most plastic membrane and the existing twin-wall polycarbonate glazing.     The twin-wall polycarbonate is the material we know has a R-value of 1.43.  We use a data logger with DS1820B sensors to measure the air temperature in each air gap plus the interior air temperature and exterior air temperature.

  1. Temperature Measurements: Simple temperature readings form the basis of the thermal analysis. Example: Indoor air at 39.54°F, after the first air gap at 37.4°F, after the second air gap (by polycarbonate) at 34.12°F, and outdoor air at 32.11°F.
  2. Calculate Temperature Differences: Analyze the temperature changes across the system. Across air gaps: a difference of 5.42°F (39.54°F – 34.12°F). Across the polycarbonate layer: a difference of 2.01°F (34.12°F – 32.11°F).
  3. Known R-factor of Polycarbonate Layer: Utilize the vendor-supplied R-factor of 1.43 ft²·°F·hr/Btu.
  4. Calculate U-factor of Polycarbonate Layer: U-factor is approximately 0.699 Btu/(hr·ft²·°F), derived from 1/1.43.
  5. Comparative Ratio Method for Air Gaps U-factor Estimation: Temperature difference ratio is approximately 2.696 (5.42°F/2.01°F). Estimated U-factor for air gaps is approximately 0.259 Btu/(hr·ft²·°F), calculated from 0.699/2.696.
  6. Calculate R-factors: R-factor for air gaps is approximately 3.86 ft²·°F·hr/Btu, calculated from 1/0.259.
  7. Combined R-factor: Total R-factor is a combined value of 5.29 ft²·°F·hr/Btu (1.43 + 3.86).
  8. Combined U-factor: Total U-factor is approximately 0.189 Btu/(hr·ft²·°F), calculated from 1/5.29.

Conclusion:

This methodology offers a streamlined and efficient solution for determining the thermal resistance of advanced multi-layer glazing systems in eco-friendly greenhouses. By demystifying the complex thermal dynamics within greenhouse structures, it greatly aids in the decision-making process for sustainable greenhouse design and energy management.

This technique is especially effective in scenarios where conventional data collection methods pose challenges, such as in innovative greenhouse projects focusing on sustainability. It’s important to note that measurements with this system yield the most accurate results during nighttime, eliminating the variable of inbound solar radiation, which is a key consideration in passive solar greenhouse design.”

Also published as:

  • A Novel Approach to Greenhouse Thermal Analysis on Academia.edu

Joseph Ellsworth
https://RainAmp.com
info@rainamp.com

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