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Environmental Sustainability of Greenhouse Covering Materials

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Environmental Sustainability of Greenhouse Covering Materials sustainability Review Environmental Sustainability of Greenhouse Covering Materials Chrysanthos Maraveas Department of Civil Engineering, University of Patras, 26500 Patra, Greece; [email protected] Received: 15 October 2019; Accepted: 1 November 2019; Published: 3 November 2019 Abstract: The fundamental objective of the review article was to explore the ecological sustainability of greenhouse covering material based on the following themes; considerations for greenhouse materials, properties of polymers and glass, additives, fillers, stabilizers and reinforcements, performance, Ultraviolet (UV) transmittance, phase change materials (PCMs), and environmental sustainability. A comparison of various polymers (polyvinyl chloride (PVC), acrylic, D-polymer, Linear low-density polyethylene (LLDPE), polyolefins), and silica glasses illustrated that each type of greenhouse cladding material has its unique merits and limitations. The performance of silica glasses, PVC, polyolefins was influenced by weather, greenhouse design, plant under cultivation, percentage UV transmittance, incorporation of additives and stabilizers, reinforcements, and integration of photovoltaic panels into the greenhouse roof among other factors. Polymers can be customized to achieve 0%UV transmittance, slow-insecticide release, and anti-microbial properties. In contrast, glass materials are preferred based on suitable photosynthetically active radiation (PAR) transmittance and near-infrared (NIR) reflection and less risk of photo-oxidation. From an ecological perspective, polymers can be recycled via mechanical and chemical recycling, closed-loop cycling, and polymerization of bio-based feedstock. However, post-consumer plastic films do not possess the same optical and energy properties as virgin polymers. The combined benefits of different polymers suggest that these materials could be adopted on a large scale over the long-term. Keywords: cladding materials; greenhouse; environmental sustainability; plastics; glass; semi-transparent solar panels; ultra-violet radiation 1. Introduction Greenhouse structures are central to the sustainability of modern civilizations considering the unreliability of traditional methods of farming, global population increase, and a projected increase in food and energy demand. Hassanien and co-researchers estimate that global food demand would increase exponentially by 2050 [1]. Industrialization and post-industrial growth have induced anthropogenic degradation of the environment [2], leading to global warming and climate change and unpredictable weather patterns, which diminish the reliability of rain-fed agriculture, especially in arid and semi-arid areas. These factors underscore the need for commercial farming in controlled environments such as greenhouses [3]. The ecological sustainability of greenhouse covering materials is considered in the context of energy sustainability [4,5], global production of PET [6], energy-efficient materials [7–11] and customized optical properties [12], and synthesis techniques [13]. This research article explores the ecological sustainability of the materials utilized in the cladding/covering of greenhouses. The primary emphasis is placed on radiometric and physical properties such as the heat transfer coefficient, absorptivity, UV and IR reflectivity, and transmissivity [14]. The properties determine photo selectivity and ability of the material to filter ultraviolet radiation (UV) and infra-red (IR) radiation and achieve the desired cooling effect [14]. The protection against UV-B is critical, given radiation levels between 280 nm and 315 nm damage plants Sustainability 2019, 11, 6129; doi:10.3390/su11216129 www.mdpi.com/journal/sustainability Sustainability 2019, 11, x FOR PEER REVIEW 2 of 24 Sustainability 2019, 11, 6129 2 of 24 and inhibit photosynthesis by triggering stress and photomorphogenic responses [15]. In contrast, UV-A radiation does notand have inhibit significant photosynthesis adverse effects by triggering on plants stress and andis of photomorphogenicminimal importance responses in [15]. In contrast, the selection of claddingUV-A materials radiation for greenhouses does not have [15]. significantThe selection adverse of greenhouse effects on materials plantsand is also is of minimal importance informed by seasons; inthis the is selection because of seasons cladding determine materials forthe greenhousesvariations in [15 energy]. The selectionand heat of greenhouse materials requirements in summeris (in also the informed south) and by winter seasons; (in thisthe north) is because [16]. seasonsIn particular, determine materials the variationswith a in energy and heat high transmittance coefficientrequirements are suitable in summer for constructing (in the south) greenhouse and winter (inmaterials the north) during [16]. winter In particular, materials with a because they do not filterhigh out transmittance photosynthetically coefficient active are suitableradiation for (PA constructingR) [17]. In greenhousecontrast, materials materials during winter because possessing high PAR coefficientsthey do not are filter unsuitable out photosynthetically during summer active due to radiation the risk (PAR)of overheating [17]. In contrast, and materials possessing the need for alternative coolinghigh PAR such coe asffi cientsevaporation are unsuitable cooling. during summer due to the risk of overheating and the need for Standard greenhousealternative covering coolingmaterials such are as glass evaporation [18], plastic cooling. sheets, and films, double or single glazing, and the material characteristicsStandard greenhouse of interest covering for greenhouse materials cladding are glass are [18 ],thermal plastic sheets,efficiency and films, double or single and optical properties becauseglazing, they and determine the material radiation characteristics control, ofheat interest transfer, for UV, greenhouse soil [19], cladding and IR are thermal efficiency absorbance or transmittanceand optical[20,21]. propertiesThe values because listed inthey Table determine 1 confirm radiationthe variations control, in energy heat transfer, and UV, soil [19], and IR optical properties dependingabsorbance on the or composition transmittance and [20 cross,21].-linking The values of the listed polymers; in Table the1 confirm least energy the variations in energy and transmittance was reportedoptical in VPVC properties-cladded depending greenhouse on the (τPAR composition = 30%). The and highest cross-linking was glass of the (GL) polymers; the least energy at 89% [22]. The symbolstransmittance τPAR, ρPAR, was Q reporteddenote energy in VPVC-cladded transmission, greenhouse reflectance, (τ PARand absorption,= 30%). The highest was glass (GL) and quantum transmission.at 89% Q [22is ].a Thepredictor symbols forτ PAR,plant ρyieldPAR, Qefficiency denote energy(ɳ) and transmission, other plant- reflectance,related and absorption, and coefficients. In particular,quantum there is transmission. a positive and Q is direct a predictor correlation for plant between yield effi Qciency and (ɳ ) and. Another other plant-related coefficients. practical observation is Inthe particular, impact of there layers is a[23] positive—multilayer and direct plastic correlation sheets betweenhave better Q and mechanical. Another practical observation properties, but thicknessis could the impact impact of PAR layers transmittance. [23]—multilayer plastic sheets have better mechanical properties, but thickness could impact PAR transmittance. Table 1. Optical and energy properties of greenhouse covering materials [22]. Type of Material Table 1. OpticalτPAR andρPAR energy αPAR properties τNIR of greenhouse ρNIR coveringαNIR materialsQ [22]. Glass Type of Material84 6 τPAR10 ρ73PAR α7PAR 20τNIR 83ρ NIR αNIR Q Low-density polyethylene film (LDPE) 83 8 9 87 8 5 85 Glass 84 6 10 73 7 20 83 Thermal polyethylene film (TPE) Low-density polyethylene83 film (LDPE)8 839 85 8 9 96 8784 8 5 85 Bubbled polyethylene plastic filmThermal (BPE) polyethylene film63 (TPE) 14 23 83 68 8 149 18 8563 9 6 84 Ethylene-vinyl acetate (EVA) Bubbled polyethylene plastic89 film (BPE)8 633 89 14 7 234 6889 14 18 63 Three-layer (3L) coextruded film comprisingEthylene-vinyl ofacetate (EVA) 89 8 3 89 7 4 89 Three-layer (3L) coextruded86 film 8 6 88 7 5 86 EVA and TPE 86 8 6 88 7 5 86 comprising of EVA and TPE Violet colored polyvinylchloride-Violetbased coloredfilm polyvinylchloride-based 39 8 5239 74 8 10 52 16 7439 10 16 39 (VPVC) film (VPVC) Rose-colored polyvinylchloride-basedRose-colored ßuorescent polyvinylchloride-based 59 59 76 7662 62 material (FPVC). ßuorescent material (FPVC). Beyond the material properties, the selection of the greenhouse materials is influenced by the Beyond the material properties, the selection of the greenhouse materials is influenced by the energy requirements of the plant [4,15], budget, photo-oxidation resistance, weather, design of the energy requirements of the plant [4,15], budget, photo-oxidation resistance, weather, design of greenhouse (Quonset, arch-tunnel, even-span, and uneven span) [3,24,25] and environmental the greenhouse (Quonset, arch-tunnel, even-span, and uneven span) [3,24,25] and environmental sustainability (recyclability) [6,7]. The shape of the greenhouse structure is critical because it sustainability (recyclability) [6,7]. The shape of the greenhouse structure is critical because it determines determines aperture efficiency and radiant energy capture [26]. Optimal aperture efficiency has been aperture efficiency and radiant energy
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