Towards the maximization of energy performance of an energy-saving Chinese solar greenhouse: A systematic analysis of common greenhouse shapes

Highlights

• Shape interpolations between four common front shape extrema of the Energy-saving Chinese Solar Greenhouse (ECSG) were studied.

• Two kinds of optimal ECSG shapes were identified in this study.

• PCA analysis were conducted to determine the optimal greenhouse construction parameters.

• The proposed model framework allows the investigation of the effects of different shape variations of solar greenhouses for any arbitrary latitude.

Abstract

Determining the optimal shape for greenhouses that is suitable for cold northern regions is essential for non-seasonal fruit production. In the present study, a structural model was developed for energy-saving Chinese solar greenhouses (ECSG), which combines a greenhouse energy balance model with a detailed shape analysis. All possible greenhouse shape interpolations within four common front shape extrema were systematically analysed to determine an optimal ECSG shape with a maximal energy performance for use during winter. The analysis revealed a direct relationship between the interception of solar radiation in the greenhouse and the height of the ridge. Our results indicated that the flatter the curve of the front cover, the more radiation is intercepted by the ground and north wall of the greenhouse. As a result, compared to the commonly used greenhouse type, two types of optimal ECSG shapes were identified, each attaining an increase of 2°C in the minimum night temperature. The general model framework developed in this study allows the investigation of the effects of different variations in the many small detailed interpolation shapes of ECSGs for any arbitrary latitude which can be used directly to provide guidance for the construction of a new generation of energy-efficient solar greenhouses. Also, with the interpolation method proposed in this paper, large-scale shape statistical analysis now can be performed to help qualified decision-making during the process of greenhouse construction.

Introduction

As the dominant free, sustainable, and clean energy of the natural environment, solar energy is an unmatched driving energy source for planet Earth (Ezzaeri et al., 2020). Solar energy is also the primary substitute for fossil fuels, and will help reduce the negative impacts of greenhouse gas emissions (Obama, 2017) and lead to the adoption of the ‘solar energy harvesting’ (SEH) concept (Khaligh and Onar, 2017). Solar greenhouses, a typical form of SEH structure, have developed rapidly in recent years and have achieved the goal of year-round production of fresh vegetables and fruits worldwide (Iddio et al., 2020, Perez et al., 2019). Greenhouse crop production relies mainly on the thermal performance of greenhouses, which depends heavily on the energy-harvesting performance of solar energy (Al-Helal and Abdel-Ghany, 2011). Energy-saving Chinese solar greenhouses (ECSGs) are specially designed to capture and accumulate solar energy during the day and release it uniformly at night to provide a desirable microclimate for plants without auxiliary heating to increase the production period and to simultaneously reduce the carbon footprint and costs of sustainable food production. Greenhouses of the ECSG type (Choab et al., 2019) are the most commonly used type of greenhouse in northern China, and are built with a thin plastic film covering the south roof as the solar radiation collecting component (Tong et al., 2013). In addition to load-bearing function, the primary function of the massive north wall is its ability to absorb and store solar radiation during the day and release the equivalent energy to the greenhouse during the night (Yang, 2012). The north roof, however, is a non-transparent roof made of light thermal insulation materials that can maintain the absorbed diurnal solar energy inside the greenhouse. With the introduction of the slanting north roof in the early 20th century, greenhouses could be constructed with a strikingly increased span while having only a minor influence on the thermal performance (Yanfei et al., 2017) and simultaneously reduced the building cost. However, the width of the north roof has a negative shading effect when the solar radiation angle is very high. Because the ECSG has the characteristic of zero auxiliary heating during winter, greenhouses require a large amount of solar energy to maintain warmth during cold winter days. The most dominant factor influencing SEH performance in ECSG configurations is the shape and, therefore, the slope of the south roof (G. Tong et al., 2018). The light transmission of the curved south roof is relatively challenging to simulate and is often considered to be constant or divided into small sections (Ahamed et al., 2018). In addition to the concrete shape of the front cover, the size, i.e., the span and height, has a significant effect on the achieved SEH performance. The height of the ECSG varies from 2 to 6 m, whereas the span ranges from 5 to 14 m (Tong et al., 2013). However, the optimal span and height solution of the ECSG has not yet been determined, aggravated by the fact that the effect of each shape depends on the specific location (latitude) of the greenhouse, resulting in a different ecliptic at each spot.

Scholars have done a lot of comparative optimization research on the greenhouse shapes. Çakir and Şahin (2015) compared the total solar radiation capture of five common greenhouse shapes (even-span, uneven-span, semi-circular, vinery, and elliptic) and found that the elliptic type is the optimum among all analysed shapes. The solar energy capture performance of a sliding cover greenhouse and a Liaoshen-type greenhouse (LSG) were compared mathematically by (X. Tong et al., 2018); the sliding cover greenhouse was found to demonstrate a better solar energy capture capability during winter. Using a mathematical approach, Chen et al. (2020) found that a sawtooth-shaped greenhouse has the ability to capture the maximum amount of global solar radiation among six typical greenhouse shapes (even-span, uneven-span, ellipse, arch, sawtooth, and vinery) in southern China. Savytskiy et al. (2021) used five greenhouse shapes for comparison. Their results show that the shape with single-gable roofing and an opaque wall on one side is the most profitable form, which is directly consistent with the ECSG greenhouse configurations. With respect to the ECSG, the primary concern is the angle of the south roof. If the angle is too small, the solar reflection percentage may increase, and the amount of radiation transmitted into the greenhouse is reduced. Therefore, regions at different latitudes should accordingly design greenhouses with different south roof angles (Tong et al., 2013). Studies have also been conducted to simulate solar radiation effects on photovoltaic greenhouses (Gao et al., 2019). Colantoni et al. (2018) described the shading effects of photovoltaic roof panels on tomatoes. Cossu et al. (2014) assessed the solar radiation distribution pattern of a south-oriented photovoltaic roof greenhouse. However, considering only the cover transmittance as a constant value (Berroug et al., 2011, Joudi and Farhan, 2015, Mobtaker et al., 2019, Tong et al., 2009) can introduce significant errors in the estimation of greenhouse solar radiation absorption, which will significantly influence the results of thermal performance calculations.

In addition, most of the simulation results show low accuracy and rough resolution owing to the limitations of the methods used (Tong et al., 2009, Xu et al., 2017). To overcome this problem, G. Zhang et al. (2020) considered the beam, diffuse, and ground-reflected radiation of a glass greenhouse cover using a mathematical model. Tanaka et al. (2015) analysed the performance of an elliptic curved-shape solar collector mathematically and found that an elliptically curved collector can capture more solar radiation than conventional flat-plate collectors. El-Maghlany et al. (2015) investigated the optimum elliptic curved-surface aspect ratios and orientation for maximum solar energy capture. Wu et al. (2020) compared four front cover shapes of Chinese solar greenhouses (circular-parabolic, circular, elliptic, and parabolic) by taking the incidence angle of the sun into consideration and found that the circular-parabolic is the best shape for accumulating direct solar radiation. The studies mentioned above were only conducted under a 2D scenario, which increases the need to find a more accurate and efficient approach with changeable parameters, particularly for 3D geometries. Gupta et al. (2012) simulated the solar fraction ratio of each greenhouse surface using 3D-shadow analysis in Auto-CAD® (Autodesk Inc., California, USA). The approaches mentioned earlier analysed only the solar radiation interception aspect, but how these shapes affect the thermal microclimate inside the greenhouse remains unknown. This is mainly because the thermal environment of the greenhouse is affected not only by the variable of solar radiation harvesting but also by the heat preservation performance of greenhouse structural materials. Therefore, considering only the shape as a single variable is not sufficient to properly analyse the thermal performance of a greenhouse, because all variables are highly related to each other. To overcome this difficulty, researchers have used the computational fluid dynamics method to assess the solar radiation harvesting and thermal performance of greenhouses using a discrete ordinates model (Modest, 2013) to model the radiative heat transfers of the greenhouse cover to the plants inside (Boulard et al., 2017). Saberian and Sajadiye (2019) studied the effect of fan-assisted ventilation systems on common greenhouses in the Islamic Republic of Iran under excessive solar heat load in summer day scenarios. However, the computational fluid dynamics approach is computationally intensive. Even with up-to-date hardware, substantial simplifications need to be made (Nebbali et al., 2012), which dramatically limits the scale and number of simulated scenarios. Extensive 3D greenhouse shape simulation and analysis of real-world dimensions remain challenging.

Borrowed from computer graphics, modern 3D ray-tracing methods provide extremely sophisticated means of light simulation to visualise how the sunlight travelling through the cover is partially absorbed, reflected, and transmitted inside the greenhouse, and have the capability of actually simulating the path of each single light ray (Hemmerling et al., 2008b). Therefore, this technique offers a considerably accurate and effective means to quantify the concrete light distribution of sunlight for both the greenhouse structure and the specific crop (Henke and Buck-Sorlin, 2017). Combined with the functional-structural plant modelling (FSPM) approach, one has a powerful tool to model the influence of the structure on light interception by the plants inside the solar greenhouse (Buck-Sorlin et al., 2010). Hitz et al. (2019) successfully simulated the full visible light spectrum and intensity inside an LED growth chamber using the GPUFlux model in GroIMP. Perez et al. (2019) analysed the architecture of maize hybrids on light interception inside a glasshouse using OpenAlea. Buck-Sorlin et al., 2011, Zhang et al., 2019 simulated the light environment of a glass greenhouse and the light absorption and cut-rose plant photosynthesis, respectively, using FSPM. Sarlikioti et al. (2011) explored the light interception of tomato canopy photosynthesis distribution in a commercial greenhouse. de Visser et al. (2014) analysed the optimum LED light use strategy in a tomato greenhouse in the Netherlands. Zhang et al. (2020b) successfully simulated the high-resolution thermal performance of a tomato greenhouse based on a simulated micro-light climate. All these studies demonstrated impressive capabilities of light simulations using ray-tracing techniques and their application in FSPM with or without additional light sources, thus this technique can be used on analysing the daylighting in buildings such as the greenhouse.

The north-eastern China has bitterly long cold harsh winters. Shenyang generally experiences temperature of −20℃. The month with the fewest daily hours of sunshine in Shenyang is near the end of December to early January with an average of 6.9 h of sunshine per day. Therefore, it is crucial to design and build greenhouses based on these cold critical time periods, such as winter solstice. The ECSG is a passive solar collector without auxiliary heating which should be built using the most advantageous shape to ensure maximum penetration of solar radiation and heat during the day to reach the maximum energy performance. To the best of our knowledge, the ECSG with an optimal energy performance has not yet been addressed, and it would be very time- and cost-consuming to build and test all possible greenhouse shapes under real-world conditions.

The objective of this study was to explore the optimal ECSG shape configuration at latitudinal region of 41°N in north-eastern China (night temperature can be down to −20℃ on winter solstice day). To identify the most prominent factors that contribute to the SEH performance of the ECSG, the FSPM method was first used to build a 3D LSG model and a tomato crop FSP model. Furthermore, an energy balance model, as described in (Y. Zhang et al., 2020) was integrated. Secondly, the radiation interception of 121 greenhouse front portion shape scenarios and 121 back portion shape scenarios in a total of 14,641 scenarios were simulated. Thirdly, the daily indoor temperature of each scenario was simulated to determine the optimal ECSG shape. Finally, principal component analysis was performed to estimate the quantitative relationships between the greenhouse shape variables and the similarity between the simulated shape scenarios.