In regions characterized by extreme solar irradiance and high ambient temperatures, greenhouse operations must overcome significant thermal challenges. This paper presents an in‐depth investigation of a novel multilayer greenhouse system engineered to minimize cooling loads and optimize indoor climate stability. The study systematically examines a progression from a conventional single-layer enclosure to a dual-layer configuration with an air gap, further augmented by active ventilation and evaporative cooling. Analytical derivations of thermal transmittance (U-value) and solar heat gain coefficient (SHGC) for each configuration are coupled with dynamic simulations using Carrier’s Hourly Analysis Program (HAP). The outcomes demonstrate up to a 45% reduction in cooling load relative to the baseline, thereby offering compelling evidence for the system’s potential to contribute to sustainable energy practices.
Controlled-environment agriculture increasingly demands innovations that mitigate extreme heat gains while reducing energy consumption. In hot and arid climates such as Thatta, Pakistan, traditional greenhouse designs often suffer from excessive internal temperatures due to high solar irradiance. To address these challenges, a multilayer system is proposed in which an additional interior glazing layer is added to a conventional greenhouse, creating an air gap. Subsequent integration of active ventilation and evaporative cooling further enhances thermal regulation. This paper systematically examines the impact of these modifications by:
Calculating baseline U-values and SHGC for a single-layer greenhouse.
Determining the effective thermal and solar properties for a dual-layer system with an air gap.
Incorporating ventilation effects using empirical and ASHRAE-based methods.
Assessing the additional cooling benefits of evaporative cooling in the ventilated cavity.
The overall thermal resistance Rtotalof a composite wall is given by:
+Rair-gap+
whereU1 and U2 are the U-values of the outer and inner glazing, and Rair-gapis derived from ISO 6946 standards. The effective U-value is then:
Ucombined
The corresponding SHGC is adjusted to account for the dual-layer transmission.
Ventilation introduces convective heat removal, thereby lowering the effective U-value. This is expressed as:
Umodified =Upolycarbonate×(1−η)
whereη (Ventilation Effectiveness) is estimated using multiple methods:
For an assumed 5 air changes per hour (ACH) and a constant CCC (typically 0.2–0.5), η is estimated at approximately
This method typically indicates a smaller reduction in U-value if ventilation is low.
Observations indicate that effective ventilation can reduce the temperature difference by 60–70%, corresponding to η≈0.6–0.7.
The addition of evaporative cooling is modelled by introducing an efficiency factor that quantifies the latent cooling effect:
Umodified =Upolycarbonate×(1−(η+ϵ))
Empirical data from psychrometric analyses suggest that, under optimal conditions, evaporative cooling can contribute an additional 10–15°F temperature reduction
Using Carrier HAP:
The table below presents the effective thermal parameters for each configuration, along with the estimated cooling tonnage and relative energy savings versus the baseline (single-layer greenhouse).
Configuration | Effective U-value | Modified SHGC | Cooling Tonnage (approx.) | Energy |
Single-Layer Greenhouse | 0.577 | 0.887 | 60 tons (baseline) | 0% |
Dual-Layer (Static Air Gap) | ~0.412 | ~0.753 | ~49 tons | ~18% |
Ventilated Air Gap η ≈ 0.50) | ~0.289 | ~0.650 | ~42 tons | ~30% |
Ventilated | ~0.173 (with η + ε ≈ 0.70) | ~0.450 | ~33 tons | ~45% |
Notes:
The sequential implementation of system enhancements illustrates a clear pathway toward reduced thermal loads:
These findings are supported by analytical calculations, empirical methods, and HAP simulations, aligning with established literature
The integration of an interior glazing layer, ventilated air gap, and evaporative cooling within a greenhouse structure offers substantial improvements in thermal management and energy efficiency. The system reduces the effective U-value and SHGC, leading to significant cooling load reductions (up to 45%) and improved microclimate stability. These innovations are particularly beneficial for regions such as Thatta, Pakistan, where high solar radiation and ambient temperatures challenge conventional greenhouse designs. Future research should focus on dynamic simulation studies and field validation to further optimize these strategies.
