Investigation into the Condensation Effect on the Greenhouse Operational Energy Performance and Energy Saving Strategies.
Open Access
- Author:
- Zhang, Enhe
- Graduate Program:
- Architectural Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 06, 2024
- Committee Members:
- James Freihaut, Program Head/Chair
Rahman Azari, Outside Unit & Field Member
Robert Berghage, Major Field Member
Julian Wang, Chair & Dissertation Advisor
Greg Pavlak, Major Field Member - Keywords:
- Condensation Effect
Solar Irradiance
Building Envelope
Photothermal Effect
Energy Efficiency
Operational Energy - Abstract:
- What is Condensation? Condensation is the process by which a substance transitions from a gaseous to a liquid state, typically when it encounters a cold surface and leads to a loss of molecular energy. This phenomenon has diverse applications, including water collection, air conditioning, heat exchange systems, and architectural plumbing networks. However, condensation in greenhouse environments can have unintended consequences which would affect energy consumption, the durability of covering materials, and the transmission of solar radiation essential for crop growth (as shown in Figure 1). The use of low emissivity (Low-E) and infrared (IR) reflective coatings, designed to enhance energy efficiency by reflecting excess solar energy and reducing cooling needs in hot climates, can inadvertently lower the surface temperature of greenhouse coverings. This reduced temperature increases the likelihood of condensation forming on these surfaces. Thus, greenhouses maintain relative humidity levels between 60% and 80%, experience more significant condensation compared to typical residential settings with humidity levels ranging from 40% to 60%. This increased propensity for condensation in greenhouses requires careful consideration and scientific analysis. It can negate potential energy savings. In hot and humid climates (Zone 2), condensation can reduce energy savings by up to 12%. While in cold climates (Zone 7), it can lead up to a 36% increase in energy costs. How does Condensation affect greenhouse covering materials insulation? When water condenses on these surfaces, it noticeably diminishes their ability to insulate against heat loss. This outcome can be primarily attributed to two key factors. Firstly, the buildup of condensed water increases the emissivity of the inner surface. This is due to water having a higher emissivity compared to most coated surfaces of the glazing. Consequently, there is an increased efficiency in radiative heat transfer. Secondly, the moisture trapped within the surface layer amplifies convective heat transfer between the greenhouse's internal conditions and the nearby inner surface. This development arises from creating a new condensation film, composed of liquid, vapor, and dry air components. This new film replaces the previous surface layer governed solely by dry air-related dynamics. The study underscores the substantial impact that an internal surface condensation can exert on the overall heat transfer properties of the greenhouse coverings. This effect becomes more pronounced when the surface has characteristics of Low-E or IR, leading to a nearly threefold increase in the U-factor—a parameter used to assess thermal insulation. The study suggests potential limitations despite the widespread adoption of these products and retrofitting approaches for energy conservation in greenhouses. These strategies could inadvertently result in higher operational energy usage, particularly when considering the influence of condensation dynamics. Notably, the study has developed a prediction model using the contact angle (which is measurable) to estimate the condensation heat transfer coefficient (namely condensate U-factor). These insights and methodologies are necessary for selecting appropriate covering materials and additives, thus enhancing the effectiveness of energy conservation in greenhouse facilities. How to evaluate Greenhouse Covering Materials’ performance: The measuring and evaluation methodologies are listed as follows: (1) Optical and radiometric data about greenhouse covering materials are being collected and then used to determine their solar heat gain coefficient (SHGC), U-factor, and visual transmittance. Selecting the right materials for greenhouse systems is vital because they significantly impact the transmission of photosynthetically active radiation (PAR), which is part of solar radiation and essential to plant growth, managing heat retention, and overall energy efficiency. The study covers common greenhouse coverings from the current market: clear glass (CG), CG with UV protection coatings, CG with Low-E coatings, polyethylene (PE), polycarbonate (PC), PE with infrared-reflective and anti-condensation (IRAC) properties, and PE with low density and anti-condensation (LDAC) properties. The investigation includes properties like spectral transmittance, absorptance, reflectance, and the SHGC, which all regulate the amount of solar radiation and PAR that enters the greenhouse. The SHGC and U-factor stand out as key parameters for assessing how energy-efficient these materials are. (2) A simplified theoretical framework rooted in double-film theory has been developed to aid in numerical analyses. This framework's accuracy was confirmed through empirical validation using a scaled physical experiment conducted within a controlled environmental chamber using a hotbox testing methodology shown in Figure 2. Both experimental results and computational simulations highlight the pivotal role of the contact angle in determining how much condensation forms as a film on a surface. This film significantly affects the condensation heat transfer coefficient. It's important to note that surfaces with higher contact angles—often found in materials treated with IR additives and Low-E coated glass—tend to show reduced insulation when water condenses, especially in winter conditions. (3) Using a three-dimensional (3D) model of a typical greenhouse setup, an extensive energy performance simulation was carried out using EnergyPlus software. This model serves two main purposes. First, it comprehensively overviews overall energy metrics and heating requirements. It also evaluates interior lighting consumption across various greenhouse cover materials. Second, it explores the potential of parametric energy simulations, allowing for an analysis of how changing the U-factor and SHGC attributes of covering materials can impact energy efficiency. Special case studies 1: Utilizing Solar Radiance using a photothermal effect-based spectrally selective solar (S3) film to enhance Condensation Resistance: As shown in figure 3, thin films made of metallic nanoparticles can generate strong photothermal effects when exposed to near-infrared light sources. This offers possibilities for designing innovative spectrally selective glazing that can modulate solar infrared radiation without affecting the transmission of photosynthetically active radiation (PAR). The conversion of light to heat through surface plasmons leads to localized heating, which minimally affects the substrate. This study pursued a dual objective by incorporating these nanoscale photothermal effects into the design of greenhouse coverings. Initially, a comprehensive analysis was conducted to understand the heating mechanism induced by photothermal effects. This analysis covered various heat exchange modes, including conduction, convection, and radiation. Subsequently, a numerical analytical approach was developed that combined spectral characteristics, solar radiance profiles, and the nanoscale photothermal effect. The primary aim was to enhance the resistance of greenhouse coverings to condensation. In practice and simulations alike, the temperature on the side with the film increased significantly by 5 ℃ with up to 16% more solar heat gains than Low-E coatings, effectively preventing the occurrence of condensation. Special case studies 2: Enhancement of Energy Efficiency in Greenhouses with Solar-Selective Plastic Incorporating ATO Nanoparticles: This study focused on the integration of plasmonic nanoparticle coatings into greenhouse coverings, specifically polyethylene (PE) and polycarbonate (PC) plastic films as a strategy to address these challenges. The primary objective is to improve solar control properties while ensuring adequate PAR for optimal plant growth. The experiment findings reveal that the Antimony Tin Oxide (ATO) nanoparticle coatings significantly reduce summer heat gain -- 41.5% for PE and 42.4% for PC, thus leading to a reduction in cooling energy demands in greenhouses. Spectral analysis and energy estimation demonstrate a slight decrease in PAR transmission due to the coatings, which can be effectively compensated with energy-efficient LED lighting. The study underscores that the reversible application of ATO nano coatings on PE and PC films (as shown in figure 4) strikes a balance between solar radiation mitigation and maintaining necessary PAR levels, resulting in substantial energy savings. These advancements align with the principles of clean energy and environmental policy, offering a sustainable approach to greenhouse management. By significantly reducing the operational energy footprint of greenhouses by 49% in zone 7, this research contributes to the broader goals of energy efficiency and environmental stewardship in agricultural practices.