Optimizing Energy Efficiency and Light Transmission in Greenhouses Using Rotating Low-Emissivity-Coated Envelopes

Abstract

Growing demand for sustainable agricultural solutions has driven innovations in greenhouse design, particularly in urban areas. This study evaluated the relationship between transparent envelope thermal properties and greenhouse energy loads through regression analysis using DesignBuilder simulations. The thermal performance of the envelope was designated as independent variables to quantify its impact on heating and cooling loads. Based on this analysis, a rotatable low-emissivity (low-E) coating envelope system optimized for temperate climate zones was proposed. This system allows seasonal adjustment of coating orientation to enhance energy efficiency. Compared to traditional materials, this approach achieved up to 16% energy savings without compromising visible light transmittance, essential for crop growth. While double-glazed low-E glass demonstrated the highest energy reduction (22%), it reduced visible light transmittance by 20%, potentially affecting crop productivity. In contrast, the proposed system maintained high visible light transmittance while achieving significant energy efficiency, balancing energy performance and light environment requirements. Additionally, integrating the greenhouse with building structures resulted in a 31.91% reduction in building energy consumption through improved insulation. These findings highlight the potential of adaptable greenhouse envelopes to improve energy performance and support urban sustainability.

1. Introduction

Climate change has caused severe global effects, significantly impacting the agricultural sector, including reduced crop yields [1]. The frequency and intensity of extreme weather events, droughts, floods, and pest outbreaks are increasing [2], further disrupting agricultural productivity and threatening global food security, especially in densely populated areas.

In response, the global market for controlled environment greenhouses is rapidly expanding, at an annual rate of 11.9% [3]. This market now includes a diverse range of structures, from large-scale indoor farming facilities in suburban areas to urban greenhouses and plant factories, which are highly controlled agricultural systems that utilize artificial lighting, hydroponic or aeroponic cultivation methods, and automated climate control technologies to optimize plant growth regardless of external environmental conditions. The integration of information technology has led to the development of smart greenhouses, designed to optimize and control environmental conditions. In particular, urban greenhouses are gaining attention because they efficiently utilize vacant urban spaces while addressing the energy demands associated with agricultural product transportation. The adaptation of vacant rooftops for urban agriculture has become a key strategy in sustainability initiatives [4]. Rooftops comprise approximately 21–26% of total building area, offering significant potential for development [5]. These areas support urban agriculture by enabling the use of photovoltaic cells and rainwater harvesting, thus promoting sustainability, energy savings, and reduced environmental impact [6].

Rooftop greenhouses (RTGs) have been successfully implemented in cities across Canada and the United States. Notable examples include Lufa Farms in Montreal (31,000 m² RTG), the Vinegar Factory in Manhattan (830 m²), and Gotham Greens in New York (15,000 m²). Other examples in New York include a 743 m² rooftop “Sky Vegetables” and a 130 m² greenhouse at a public school. The Arbor House in New York features a 1000 m² greenhouse that harnesses waste heat to warm the building, collecting approximately 225 MWh of waste heat annually (averaging 26 kWh). Globally, research and social benefits continue to drive RTG adoption. In Japan, particularly Tokyo, the Pasona HQ Tokyo Urban Farm is a 4000 m² RTG. In Europe, Germany hosts projects such as the Fraunhofer UMSICHT inFARMING building, while Newcastle University in the UK is constructing an urban science building with an RTG. Other examples include Urban Farmers in Switzerland (250 m² installation) and UF002 De Schilde in The Hague, The Netherlands (1900 m² greenhouse), as well as the Institute of Environmental Science and Technology at the Autonomous University of Barcelona, which houses the first building designed with an integrated RTG [7].

However, one of the significant challenges faced by these greenhouses is their substantial energy demand for environmental control [8]. In some cases, most of the energy consumed is allocated to heating, cooling, and supplementary LED lighting. These energy costs can account for up to 40% of the total operational expenses, contributing to the financial difficulties faced by many startups in the sector [9]. Some companies have faced bankruptcy due to these challenges [10].

To address these issues, research on passive and active technologies for solving greenhouse energy challenges has been conducted. To reduce energy consumption in greenhouses, not only can the application of insulated glazing, commonly used in buildings, help minimize heating energy demand, but also recent advancements in micro- and nanostructured polymer technologies can be considered for implementing passive cooling techniques [11]. In particular, for urban greenhouses, solutions include utilizing waste heat from existing buildings or power plants and incorporating renewable energy sources such as photovoltaic modules. This study focuses on passive techniques, such as optimizing transparent envelopes, to reduce energy load while improving crop production outcomes. Additionally, it assessed the impact of RTG installations on the energy consumption of existing buildings.

2. Literature Review

Greenhouses are essential for sustainable agriculture and meeting the increasing global food demand. However, their lightweight structures and inefficient operations, characterized by inadequate insulation and outdated environment control systems, often result in higher fossil fuel energy consumption compared to similarly sized buildings, making the greenhouse sector one of the most energy-intensive in agriculture. Reducing energy consumption while increasing crop yields remains a key objective for sustainable greenhouse operations [12,13].

Interest in RTGs has grown due to their potential to address urban food security while improving building energy performance. RTGs create an energy symbiosis with host buildings by capturing waste heat from HVAC systems, reducing overall heating loads [14]. Studies have indicated that integrating RTGs with renewable energy sources, such as photovoltaic panels and thermal storage, enhances energy self-sufficiency [15]. RTGs achieve additional carbon emission reduction and economic benefits compared to stand-alone greenhouses through energy and environmental synergies with existing buildings, thereby presenting a new building typology where energy efficiency and food production are interconnected or symbiotic [7].

Research on energy-saving techniques in greenhouses has focused on envelope optimization, heating and cooling systems, and recent advancements in renewable energy integration. Passive approaches include analyzing energy consumption based on greenhouse shapes [16], optimizing greenhouse envelopes and ventilation, while active methods incorporate thermal energy storage (TES) to stabilize temperature fluctuations and enhance energy conservation. TES technologies, including phase change materials (PCMs) and underground heat storage, have demonstrated significant potential in reducing heating demands by capturing and releasing heat as needed [15,16,17]. Additionally, energy transfer between greenhouses has been studied as a means to improve overall energy efficiency by redistributing surplus heat from one structure to another. Hybrid heating systems, which integrate multiple heat sources, including geothermal and biomass, offer a promising solution for achieving net-zero energy consumption [15].

Active approaches have evolved beyond conventional heating and cooling systems to incorporate renewable energy sources. Researchers have examined the integration of photovoltaic (PV) systems, photovoltaic–thermal (PVT) hybrids, and solar thermal collectors to meet greenhouse heating and cooling needs while minimizing dependence on fossil fuels [15]. However, challenges such as efficiency losses in cold climates and PV shading effects on plant growth remain critical concerns [18].

Various simulation tools are employed to analyze and optimize greenhouse energy performance. EnergyPlus and TRNSYS are commonly used for heating and cooling load assessments, while computational fluid dynamics (CFD) modeling analyzes airflow and humidity control within greenhouses [19,20,21,22]. MATLAB/Simulink has been utilized for AI-driven climate control optimization, further reducing operational energy consumption [23]. Life cycle analysis (LCA) methods are also increasingly used to assess the environmental impact and sustainability of different greenhouse configurations [24]. Another study conducted real-world experiments on greenhouses across different climatic conditions, demonstrating that integrating geothermal heating with TES systems can improve energy efficiency by over 40% [25]. Several notable studies have contributed to understanding RTG and greenhouse energy efficiency.

Despite these advancements, challenges remain in balancing greenhouse energy use with crop productivity. Optimizing the greenhouse envelope is one of the most effective methods to achieve energy efficiency while maintaining suitable growing conditions for plants [12]. Improvements in insulation materials, adaptive shading systems, and hybrid glazing technologies significantly impact thermal performance and light penetration. Studies on solar greenhouses have demonstrated that strategic modifications to the greenhouse shape and structural components can increase energy efficiency while supporting year-round crop production [18]. Additionally, passive strategies, such as greenhouse orientation and thermal buffering walls, further enhance energy conservation.

Optimizing greenhouse envelopes remains a priority, as passive design strategies can significantly reduce energy demand and improve long-term sustainability. However, the limited selection of greenhouse envelope materials, compared to conventional building materials, highlights the need for further research. As greenhouse demand rises, future studies should focus on developing advanced envelope materials that minimize heating and cooling loads while maintaining optimal growing conditions. Greenhouses have become a critical solution for sustainable agriculture and meeting the growing global demand for food.

3. Method and Target Model

3.1. Methodology

Various factors influence greenhouse energy performance, including shape, orientation, thermal properties of the envelope, ventilation methods, and equipment choices. The transparent envelope plays a crucial role in blocking or absorbing heat while providing the necessary light to support crop growth. These characteristics directly impact energy consumption and crop productivity.

Polyethylene is the most commonly used material for greenhouse envelopes due to its economic advantages In the Republic of Korea, more than 98% of greenhouses are constructed using plastic film, primarily polyethylene (PE). However, this material has limitations in thermal insulation, which leads to excessive energy consumption for heating in winter and cooling in summer. To address these inefficiencies, recent developments in greenhouse envelope materials have introduced alternatives such as polymethyl methacrylate (PMMA), polycarbonate panels, and glass. These materials provide enhanced thermal performance, but they also pose challenges in terms of installation cost and light transmittance. In response to the growing demand for energy-efficient greenhouses, research into low-emissivity (low-E) coated glass has gained attention, particularly in the Netherlands. Low-E coatings selectively control infrared radiation while allowing visible light to pass through, making them an effective solution for reducing heat loss in winter and minimizing solar heat gain in summer. This study evaluates the energy-saving potential of different transparent envelope materials and proposes an innovative rotatable low-E-coated glass system to optimize greenhouse energy performance.

To achieve this objective, a comprehensive methodology was adopted, encompassing data collection, energy simulation, regression analysis, and system development. First, a dataset was compiled containing 206 different glass configurations, including single-pane, double-pane, and triple-pane glass, with and without low-E coatings. The key thermal and optical properties analyzed in this study include the U-value (thermal transmittance), Solar Heat Gain Coefficient (SHGC), and visible light transmittance. These parameters serve as independent variables influencing the heating and cooling loads of the greenhouse.

The energy performance of different envelope configurations was assessed using DesignBuilder 7.3, a simulation tool based on the EnergyPlus engine. DesignBuilder enables the calculation of heating and cooling loads under various environmental conditions by considering factors such as external climate, greenhouse envelope properties, and ventilation rates. Additionally, the WINDOW 8.0 software was utilized to analyze the thermal and optical properties of glass and to calculate surface temperatures of different glazing materials.

A multiple regression analysis was conducted to quantify the relationship between glazing properties and greenhouse heating/cooling loads. The U-value and SHGC were designated as independent variables, while heating and cooling loads served as dependent variables.

Based on the regression analysis and simulation results, an innovative rotatable low-E-coated glass system was developed. The proposed system allows seasonal adjustment of the low-E coating orientation, optimizing greenhouse energy efficiency throughout the year. In winter, the low-E coating faces inward, reflecting infrared radiation back into the greenhouse and minimizing heat loss. In summer, the low-E coating faces outward, reducing solar heat accumulation and improving cooling efficiency. This system maintains high visible light transmittance while enhancing seasonal adaptability. To verify the feasibility of the proposed system, a patent was filed in the Republic of Korea under the title “Greenhouse with Rotary Panels” (Patent Number: 10-2024-0092864).

This methodological approach provides a comprehensive evaluation of greenhouse envelope performance and introduces a scalable, energy-efficient solution for sustainable agriculture. The findings contribute to the advancement of energy-efficient greenhouse designs by integrating simulation-based analysis, statistical modeling, and practical system development. The overall research process is outlined in Figure 1, illustrating the step-by-step approach from data analysis to the development of an energy-efficient greenhouse envelope system.

3.2. Simulation Model

In this study, a rooftop greenhouse (RTG) was modeled on a university building in Busan, the Republic of Korea, to evaluate the energy load of the greenhouse and its impact on the experimental building (see Figure 2). The university building was selected due to its potential for immediate use of the crops produced, efficient demand management, and the ease of future validation with empirical data.

The modeled building was assumed to have been constructed in 2014, and the 2013 national building insulation standards were applied. The building has a single-story structure with a total floor area of 873 m² and a window-to-wall ratio of 50%. Additional details on environmental control settings and envelope conditions are provided in Figure 2. The RTG modeled in this study was based on standard glass greenhouse designs distributed by the Rural Development Administration and the Korean Rural Community Corporation in the Republic of Korea. The design follows the standard single-span, double-pitched-roof glass greenhouse model commonly used in agricultural facilities. The greenhouse layout was optimized to fit the rooftop configuration while ensuring structural compatibility.

3.3. Heating and Cooling Load Based on the Thermal Performance of the Envelope

Generally, for buildings, superior insulation performance of the envelope, that is, a lower U-value, enhances both heating and cooling efficiency. However, in transparent envelopes, the U-value and shading coefficient complicate the heat exchange. Therefore, optimizing the greenhouse envelope requires investigating trends in cooling and heating loads based on the U-value and solar heat gain performance.

Because greenhouses typically have five surfaces composed of transparent materials, careful consideration of the thermal properties of the envelope during summer and winter is essential. Unlike opaque materials, the transparent ones must account for solar radiation, making the U-value and solar heat gain coefficient (SHGC) particularly important. In some cases, transparent materials with high insulation performance (low U-values) may fail to dissipate absorbed solar heat near the glass surface through conduction or radiation, leading to heat accumulation. Consequently, high-insulation glass may inadvertently increase the cooling load inside the greenhouse.

Table 1. Surface temperature simulation cases.

Case Classification	Composition of Glass	U-Value [w/m2K]	SHGC [-]
Case 1	5 Clear	5.849	0.833
Case 2	5 Clear + 12 Air + 5 Clear	2.716	0.725
Case 3	5 Clear + 12 Air + 5 Low-E	1.797	0.644
Case 4	5 Clear + 12 Air + 5 Clear + 5 Low-E	1.289	0.571
Case 5	5 Clear + 12 Air+ 5 Low-E + 5 Low-E	1.020	0.544

Simulation conditions (NFRC * 100-2010); outside temperature in winter: 21 °C; outside temperature in summer: 24 °C; * National Fenestration Rating Council.

, Figure 3 shows the thermal performance results of the glass systems analyzed using the WINDOW 8.0 simulation program. During winter, the indoor surface temperature increased as insulation improved. Specifically, the application of low-E glass resulted in an indoor surface temperature increase of at least 4 °C. However, this enhancement led to an unintended rise in the indoor surface temperature of the glass. Although the SHGC decreased by 11% with low-E glass (as seen in cases 2 and 3 in Figure 3), the improved insulation increased the indoor surface temperature by 2.4 °C. Therefore, while lower U-values were advantageous for heating during winter, they had a detrimental effect in summer by increasing indoor surface temperature.

To identify clearer relationships, simulations and statistical analyses were conducted to examine cooling and heating load consumption trends based on the greenhouse envelope. Two independent variables, the U-value and shading coefficient, were considered to determine the thermal performance of the transparent envelope, whereas the dependent variables were the cooling and heating loads. The building model used was the same as described in Section 3.2; however, to minimize the influence of other variables, the model assumed ground installation.

The simulation used the DesignBuilder library, analyzing 206 different glass types across six categories, including single-pane, double-pane, and triple-pane configurations, with and without low-E coatings. Figure 4 illustrates the distribution of thermal transmittance (U-value) and Solar Heat Gain Coefficient (SHGC) for these six categories. The U-values range from 0.5 to 5.0 W/m²K, while the SHGC values are distributed between 0.1 and 1.0.

The thermal performance characteristics demonstrate distinct patterns based on glass configuration. A clear trend of improving insulation performance is observed in the progression from single-pane to double-pane to triple-pane, each exhibiting clearly defined U-value ranges. The SHGC values, however, show broad distribution patterns across all glass types, indicating that these values are not primarily dependent on the number of panes. Rather, the analysis reveals that SHGC values are more significantly influenced by factors such as glass color and the iron content. Nevertheless, a trend of decreasing maximum SHGC values is observed as the number of glass panes increases.

The heating and cooling load patterns were analyzed for 206 different glass configurations using DesignBuilder simulations. Figure 5 and Figure 6 present bubble chart analyses of heating and cooling loads in relation to thermal transmittance (U-value) and the Solar Heat Gain Coefficient (SHGC).

For heating loads, a distinct inverse correlation is observed with improved insulation performance (decreasing U-values). While the SHGC also influences heating loads, its impact leads to only marginal increases as values decrease.

By contrast, the cooling load patterns demonstrate inverse trends. Cooling loads rise significantly with higher SHGC values due to increased solar heat gain. Additionally, improved insulation (lower U-values) leads to a moderate increase in cooling loads, as absorbed solar radiation cannot dissipate effectively through conduction. In the Republic of Korea’s temperate climate, heating loads are approximately three times higher than cooling loads. Consequently, the annual heating and cooling load patterns predominantly follow heating load trends related to the U-value and SHGC. This relationship is further explored in subsequent sections through detailed statistical analysis.

Multiple regression analysis was conducted to examine the relationship between glass thermal properties and heating/cooling loads. Thermal transmittance (U-value) and the Solar Heat Gain Coefficient (SHGC) were designated as independent variables, while heating and cooling loads served as dependent variables. Prior to the regression analysis, potential multicollinearity between the independent variables was evaluated. The Variance Inflation Factor (VIF) was 3.69 for both variables, confirming their suitability for inclusion in the analysis.

The regression analysis for heating loads demonstrated high model reliability, with an R-squared value of 0.958 and an adjusted R-squared value of 0.957, indicating that the U-value and SHGC account for approximately 95.7% of the variance in heating loads. This high correlation is attributed to the greenhouse’s exclusively transparent envelope, which eliminates interference from opaque elements. Equation (1) represents the regression equation for heating load per unit area as a function of the U-value and SHGC. Consistent with the bubble chart analysis, the heating load is positively correlated with the U-value and negatively with the SHGC. The regression coefficients are −60.466 for the U-value and 16.893 for the SHGC. However, given that U-values range from 0.5 to 6.0 and SHGCs range from 0.1 to 1.0, the coefficients alone do not determine their relative impact on heating loads. A t-test was conducted to evaluate the significance of each independent variable. In the heating load model, the U-value exhibited a t-value of 63.41, indicating strong significance, with a p-value of 9.14 × 10⁻¹³⁶ (p < 0.05). Similarly, the SHGC demonstrated statistical significance, with a t-value of −38.21 and a p-value of 1.16 × 10⁻⁹⁴ (p < 0.05) (

Table 2. Statistical results of regression coefficients for the heating load model.

Variable	Coefficient	t-Value	p-Value
U-Value	16.893	63.409	9.14 × 10⁻136
SHGC	−60.446	−38.210	1.16 × 10⁻94

). While both variables contribute significantly to the model, the t-test results indicate that the U-value is a more robust predictor of heating loads than the SHGC.

Heating Load (kWh/m²) = 290.188 + 16.893⋅U-Value − 60.446⋅SHGC

(1)

Figure 7 presents the multiple regression analysis results of the heating load in relation to the U-value and SHGC, along with residual distribution plots for reliability assessment.

Residual analysis demonstrated that all data points were normally distributed around zero, with no systematic bias. The residuals exhibit symmetrical patterns with no discernible skewness, indicating independence from predictor variables. This randomness in distribution validates the model’s predictive reliability.

The cooling load regression model yielded an R-squared value of 0.761 and an adjusted R-squared value of 0.759, explaining approximately 76.1% of the variance. Although lower than that of the heating load model, these values indicate reliable explanatory power (R-squared > 0.7). The weaker correlation suggests that factors beyond envelope thermal performance, such as greenhouse orientation and external climate conditions, exert greater influence on cooling loads.

Unlike heating loads, summer solar radiation intake and output mechanisms exhibit inverse correlations, with the cooling load negatively correlated with the U-value and positively correlated with the SHGC. The regression coefficients are −2.215 for the U-value and 48.763 for the SHGC (Equation (2)). However, as with heating loads, direct comparisons based solely on coefficient magnitudes are limited due to differences in the distribution ranges of the U-value and SHGC. Notably, the U-value has a substantially weaker effect on cooling loads than on heating loads (heating load U-value coefficient: 16.893; cooling load coefficient: −2.215). While the SHGC for cooling loads is somewhat lower than for heating loads, it remains influential (heating load SHGC: −60.446; cooling load coefficient: 48.763). T-test results for the cooling load model show a t-value of −6.856 (p = 8.35 × 10⁻¹¹, p < 0.05) for the U-value and a t-value of 12.97 (p = 5.30 × 10⁻⁶⁵, p < 0.001) for the SHGC (

Table 3. Statistical results of regression coefficients for the cooling load model.

Variable	Coefficient	t-Value	p-Value
U-Value	−2.215	−6.856	8.35 × 10−11
SHGC	48.763	25.420	5.31 × 10−65

). While both variables demonstrate statistical significance in explaining cooling loads, contrary to the heating load model, the SHGC emerges as the dominant predictor.

Cooling Load (kWh/m²) = 113.734 − 2.215⋅U-Value + 48.763⋅SHGC

(2)

Figure 8 presents the multiple regression analysis results for the cooling load in relation to the U-value and SHGC, along with residual distribution plots for model reliability assessment. Residual analysis confirms that the data meet normality assumptions, exhibiting predominantly symmetrical patterns with residuals centered around zero, further validating model reliability.

Analysis of annual heating and cooling loads confirmed the regression model’s reliability, with R² = 0.918 and Adjusted R² = 0.918. The overall trends reflect the characteristic pattern of heating loads being approximately three times higher than cooling loads annually. The results demonstrate that total energy loads decrease with improved insulation performance (lower U-values) and increased solar heat gain (higher SHGC values). T-tests further validate the significance of both independent variables in explaining the regression model (Equation (3),

Table 4. Statistical results of regression coefficients for the heating and cooling load models.

Variable	Coefficient	t-Value	p-Value
U-Value	14.678	47.523	9.87 × 10⁻110
SHGC	−11.683	−6.370	3.97 × 10⁻8

).

Total Load (kWh/M²) = 403.922 + 14.678⋅U-Value − 11.683⋅SHGC

(3)

The distribution plots of predicted values and residuals exhibit symmetrical patterns consistent with heating and cooling load analyses. The absence of skewness or systematic patterns confirms the independence of residuals from predictor variables (Figure 9).

The thermal characteristics of greenhouse envelopes present opposing requirements for cooling and heating loads. Heating load reduction benefits from low U-values and high SHGC values, with U-value reduction proving more effective. Conversely, cooling load reduction favors high U-values and low SHGC values, with SHGC control showing substantially greater effectiveness than U-value adjustments.

In temperate climates requiring both heating and cooling, adaptable transparent envelopes that respond to seasonal changes represent the optimal solution. However, where such adaptability is not feasible, envelope selection should prioritize controlling the predominant load type. In the Republic of Korea, where heating loads are approximately three times higher than cooling loads, optimizing the envelope for heating load reduction is the most effective approach.

3.4. Development of a Low-E-Coated Greenhouse

As demonstrated in Section 3.3, temperate climates with distinct seasons require significantly different greenhouse envelope performance in summer and winter. To address this issue, thermochromic or electrochromic glass can be applied to vary insulation performance seasonally. However, these technologies only regulate the SHGC, rendering them insufficient as stand-alone solutions.

Typically, seasonal demands are managed through shading devices or by applying powder-type shading agents to the ceiling in summer and removing them in the fall. While this method controls solar radiation, it does not regulate insulation. Additionally, shading screens can obstruct visible light necessary for plant growth, reducing productivity.

Therefore, this study optimizes the thermal and optical properties of low-E coatings through physically controlling their thermal characteristics. Low-E-coated glass, featuring a nanoscale low-E coating, maintains high transmittance in the visible spectrum while selectively controlling long-wave infrared radiation.

In architecture, silver is commonly used as a low-E coating for insulation, typically applied in multilayers to prevent oxidation. This type of coating is well-suited for greenhouses, as it transmits visible light required for plant growth while regulating long-wave radiation for insulation. However, silver-coated low-E glass in double glazing presents challenges in cost-effectiveness and requires thicker structural frames to support the additional weight, limiting its greenhouse applications.

To address these limitations, this study explored tin oxide (SnO₂) as a coating material. While tin oxide offers lower thermal performance than silver, it provides superior durability, remaining stable even when exposed to air. This durability allows tin oxide-coated glass to function as a single-pane glazing system, eliminating the need for double glazing. When used as a single pane, the U-value and SHGC of low-E glass change depending on the orientation of the coating. If the coating faces inward, it reflects long-wave infrared radiation back inside, enhancing insulation. If oriented outward, this effect diminishes, yielding insulation performance (U-value) comparable to uncoated glass.

By utilizing the long-wave infrared radiation properties of low-E glass, greenhouse loads can be effectively controlled. In winter, an inward-facing coating reflects infrared radiation into the interior, minimizing heat loss and maintaining insulation. In summer, an outward-facing coating allows accumulated infrared radiation near the glass surface to dissipate outside, reducing cooling loads.

These characteristics enable optimal seasonal envelope control by adjusting the orientation of the coating. Greenhouses offer various openable designs, such as pivot-style rotating systems. Figure 10 illustrates a rotating frame design that adjusts the low-E coating’s direction according to the season, along with thermal performance results analyzed using the WINDOW 8.0 program. Depending on the orientation of the low-E coating, the U-value varied by 37%, the SHGC fluctuated by 3%, while visible light transmittance remained unchanged.

These findings provide valuable insights into indoor environmental conditions based on glass system thermal performance, offering critical data for greenhouse envelope design. With greenhouses having five transparent surfaces excluding the floor, heating and cooling loads are highly sensitive to the thermal and optical properties of the envelope. Therefore, selecting appropriate materials is crucial to addressing distinct summer and winter requirements.

4. Results and Discussion

This study assessed the effects of repositioning a low-emissivity (low-E) coating on a transparent envelope during summer and winter using a pivot window system. The proposed system was compared with commonly used materials for greenhouse envelopes and architectural glazing.

Table 5. Eenrgy loads Simulation cases.

	Case 1	Case 2	Case 3	Case 44	Case 5	Case 6
Composition	PE * Film	PMMA ** Double	Single Glazing	Double Glazing	Double Glazing/Low-E	Single Glazing/Low-E
Overall Thickness [mm]	0.1	10.0	4.0	20.0	20.0	4.0
U-value [W/m2·K]	5.886	3.163	5.879	2.730	1.880	Summer: 5.758 Winter: 3.640
SHGC [-]	0.902	0.803	0.848	0.745	0.694	Summer: 0.797 Winter: 0.774
Tvis [-]	0.884	0.847	0.868	0.760	0.717	0.819

Note: * PE: polyethylene; ** PMMA: polymethyl methacrylate.

summarizes the six representative cases considered for simulation.

Load calculations were performed using DesignBuilder, and the results are presented in Figure 11. Regarding PMMA (Case 2), a material increasingly applied in greenhouse construction, it demonstrated an energy-saving effect of 14% compared to PE film, the most commonly used greenhouse material. However, the material with the highest energy reduction rate was low-E double-glazed glass, which is primarily used in architectural applications. Nevertheless, due to its two glass panes, low-E double glazing presents challenges in structural load support.

As observed in previous analyses, materials with superior insulation performance significantly reduce heating loads but simultaneously increase cooling loads, partially offsetting the overall energy savings. By contrast, the rotating single-pane low-E-coated glass system proposed in this study reduced heating loads by 28% without increasing cooling loads. The implementation of the proposed rotating single-pane low-E-coated glass system in approximately 54,000 ha of domestic greenhouse facilities [26] is projected to yield a potential energy load reduction of 49.8 TWh. Additionally, its single-pane composition results in a lighter structural requirement compared to double-glazed glass.

Furthermore, evaluation of building heating and cooling load patterns demonstrated that, compared to the base case (before greenhouse installation), the insulation effect of the greenhouse resulted in an average 31.91% energy savings. The results showed a decrease in heating energy consumption while cooling energy demand increased.

The properties of the greenhouse envelope not only influence energy consumption, but also affect the amount of visible light transmission, which directly impacts crop productivity. While optimizing the greenhouse envelope can achieve energy savings, a reduction in crop yields due to such optimization would undermine the fundamental purpose of a greenhouse. Previous research indicates a significant correlation between crop yields and pre-flowering temperature; however, solar radiation has been found to exert an even greater influence on yields [27].

Previous studies calculated heating and cooling loads under the assumption that greenhouse temperatures were maintained at 20–25 °C, the optimal growth temperature for tomatoes. Additionally, illuminance distribution was analyzed for each case. By integrating these findings, this study aims to propose an optimized greenhouse envelope that effectively balances energy consumption and crop productivity.

Figure 12 and Figure 13 show a comparison of light transmission across the six cases.

In Case 5, while energy performance was optimal, multiple layers, including the low-E coating, reduced visible light transmittance and indoor illumination by approximately 20% compared to traditional materials (Case 1). Conversely, traditional materials (Cases 1–3) performed better in light transmission but were less energy-efficient. The proposed rotating frame system with low-E glass demonstrated a favorable balance between energy efficiency and light transmission, with only a 7–9% reduction in transmittance compared to PE film, PMMA. While polymer-based materials deteriorate due to UV exposure, resulting in decreased transmittance (1–2% per year) and yellowing over time [28], glass maintains consistent performance throughout its lifespan.

Therefore, considering both energy savings and optimal growing conditions, Case 6 is identified as the most balanced greenhouse system among the evaluated configurations.

5. Conclusions and Implications

The findings of this study underscore the critical role of selecting appropriate transparent envelopes in designing energy-efficient greenhouses. While enhancing the thermal insulation performance of transparent envelopes effectively reduces heating loads, it can also lead to increased cooling loads, necessitating careful seasonal optimization to maintain energy efficiency. In contrast, the proposed rotatable-frame greenhouse envelope system demonstrates adaptability to external environmental conditions, enabling seasonal optimization for both summer and winter climates.

A multiple regression analysis was conducted to quantify the relationship between glazing thermal properties (U-value and SHGC) and greenhouse heating and cooling loads to establish a predictive energy efficiency model. Based on the analysis, an optimized system was developed for temperate climate regions. The proposed rotatable low-E-coating system effectively reduces heating loads in winter while preventing an increase in cooling loads during summer, achieving up to 22% energy savings compared to conventional greenhouse materials. Although multi-layered low-E glass, commonly used in architectural applications, exhibited the highest energy-saving potential, it resulted in a 19% reduction in visible light transmittance, which may adversely impact plant growth. In contrast, the proposed rotatable system maintained higher visible light transmittance while maximizing energy performance and balancing thermal insulation and crop productivity. Additionally, integrating greenhouses with buildings resulted in an average 31.91% reduction in total building energy consumption, confirming the potential of energy-efficient greenhouse operations through building-integrated agricultural solutions. These findings suggest that the proposed rotatable low-E-coating system has significant potential to enhance urban agricultural sustainability and support energy-efficient food production.

Despite these promising outcomes, several limitations should be acknowledged. First, this study primarily focused on the thermal properties of greenhouse envelopes; however, future research should incorporate additional environmental factors, such as regional climate conditions, crop-specific temperature requirements, and ventilation strategies, to develop a more comprehensive performance model. Second, as the findings are based on simulation analysis, empirical validation through real-world experimental studies is necessary to confirm the practical feasibility of the proposed system. Third, the mechanical complexity of the rotating low-E-coating system may introduce structural and economic challenges in real-world applications. Future research should evaluate the system’s long-term durability, maintenance costs, and economic feasibility. Furthermore, the impact of glazing optical properties on crop yields should be assessed to ensure that both energy efficiency and agricultural productivity are optimized.

To further refine the predictive greenhouse load model, future research will incorporate a broader range of variables beyond envelope thermal performance and conduct experimental validation of the proposed system. A comparative economic evaluation will also be performed, considering construction costs, maintenance expenses, and crop yield outcomes from greenhouses utilizing the proposed system.

This study confirms that a seasonal control approach using a rotatable low-E-coating system effectively reduces greenhouse energy consumption while maintaining crop productivity. These findings provide valuable insights for architects, urban planners, and policymakers seeking to enhance the sustainability of urban agricultural systems. As urban expansion continues, integrating greenhouse envelope technologies with buildings presents a viable strategy for reducing the energy footprint of food production while improving environmental sustainability. The proposed intelligent greenhouse envelope system offers a practical solution for energy conservation, food security, and the advancement of sustainable building technologies. Future research will focus on expanding environmental variables, conducting empirical validation, and performing economic feasibility assessments further to validate the potential of next-generation urban greenhouse technologies.

6. Patents

Patent Number: 10-2024-0092864

Title: Greenhouse with Rotary Panels

Inventors: SuBin Song, SungHwan Yoon

Filing Date: 15 July 2024

Jurisdiction: The Republic of Korea