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Greenhouses shelter the crop from unfavourable environmental conditions and the covering largely contributes to creating beneficial growing conditions inside. There is no perfect greenhouse cover for all combinations of crop and climatic regions. Usually a greenhouse cover has permanent optical properties determining the amount of solar radiation entering the greenhouse. Consequently during crop growth, the amount and quality (spectrum, direct/diffuse ratio) of the solar radiation is not ideal for the crop. Growers try to compensate for this by using different additional techniques such as temporary coatings, screens (mobile or fixed, etc.) and heating or cooling. New materials are currently being developed, whose optical properties can be (almost) instantaneously changed (materials with switchable properties). This will allow growers to gain real-time control on the quantity and quality of the light entering the greenhouse to match crop requirement. The present study uses advanced simulation models to predict the potential of covers with switchable properties to improve tomato yield and use of resources in different climatic regions (mild winter and tropical) and with different greenhouse types (artisan and industrial type). Results indicate that covers with switchable properties have advantages over permanent properties for most combinations of filter type/location. Only in very extreme tropical climates will covering materials with permanent filter properties have advantages. Furthermore, simulations models can play a major role in optimising the switchable filter design. • No single type of greenhouse cover has optimum performance in every climate & location. • Switching radiometric properties looks promising for high light demand crops. • Switchable materials have no advantage over permanent filters in tropical climates. • Simulation could be a powerful tool to determine optimum radiometric properties. • Simulation should include specific combination of crop, greenhouse location & type. [ABSTRACT FROM AUTHOR]

Research on closed plant production systems, such as artificially illuminated and highly insulated plant factories, has offered perspectives for urban food production but more insight is needed into their resource use efficiency. This paper assesses the potential of this ‘novel’ system for production in harsh climates with either low or high temperatures and solar radiation levels. The performance of plant factories is compared with cultivation in traditional greenhouses by analysing the use of resources in the production of lettuce. We applied advanced climate models for greenhouses and buildings, coupled with a lettuce model that relates growth to microclimate. This analysis was performed for three different climate zones and latitudes (24–68°N). In terms of energy efficiency, plant factories (1411 MJ kg −1 dry weight) outperform even the most efficient greenhouse (Sweden with artificial illumination; 1699 MJ kg −1 dry weight). Additionally, plant factories achieve higher productivity for all other resources (water, CO 2 and land area). With respect to purchased energy, however, greenhouses excel as they use freely available solar energy for photosynthesis. The production of 1 kg dry weight of lettuce requires an input of 247 kWhe in a plant factory, compared to 70, 111, 182 and 211 kWhe in greenhouses in respectively the Netherlands, United Arab Emirates and Sweden (with and without additional artificial illumination). The local scarcity of resources determines the suitability of production systems. Our quantitative analysis provides insight into the effect of external climate on resource productivity in plant factories and greenhouses. By elucidating the impact of the absence of solar energy, this provides a starting point for determining the economic viability of plant factories. [ABSTRACT FROM AUTHOR]

Computational fluid dynamics (CFD) was used to study the effect of greenhouse sidewall vents on buoyancy-driven natural ventilation, which is the most difficult situation for greenhouse cooling. A CFD model was validated and then used to compare roof ventilation with combined roof and sidewall vents. The effects of the distance between opposing sidewall vents and the presence of anti-insect screens were investigated and quantified. In a 20 span greenhouse with a distance of 152m between the sidewalls, combining roof and sidewall vents gave a ventilation rate per unit ground area that was twice that with only roof vents. In a 3 span greenhouse with 22.8m between sidewalls but with the same roof vent area per unit ground area, seven times more ventilation was obtained with combined ventilation compared to only roof ventilation. In the 7 and 20 span greenhouses with roof ventilation, 79% and 48.3%, respectively, of the cross sectional area had a temperature difference between inside and outside the greenhouse greater than 4°C, whereas with combined ventilation, these areas were 23.4% and 36.1%. In the latter case, these hot areas were located in the centre of the houses. These results show that, with buoyancy-driven ventilation, the contribution of the sidewall vents is important even for quite large greenhouses but is more critical for greenhouses with a small number of spans. Numerical simulations with an anti-insect screen having a porosity of 25% showed that the air exchange rate with combined ventilation was reduced by 77–87%, depending on greenhouse size. These reductions were much larger than those obtained for wind-driven ventilation. It was concluded that, to maximise ventilation when wind speeds are low, combined roof and sidewall ventilation should be used. Also, large greenhouses should be relatively narrow to make better use of the sidewall vents. [Copyright &y& Elsevier]

The increasing demand for efficiently growing vegetation in greenhouses requires continual improvement of the control of the growth environment experienced by the plants. The single most important factor in maximizing the crop's growth is the quantity, quality, and geometrical distribution of radiation intercepted at every moment. Few available greenhouse coverings are capable of responding to changes in sunlight conditions by themselves, limiting the grower's control: they must use additional technologies, including screens, artificial lighting, heating, and cooling. This review considers existing efforts in providing adaptable greenhouse covering systems and advanced optical materials for controlling the color, intensity, and/or distribution of sunlight transmitted into greenhouse‐like structures by describing existing static materials and their responsive equivalents. This work also offers speculation on potential applications of other light‐control elements, mostly designed for use in the urban environment that can be adapted for greenhouse use in the future. [ABSTRACT FROM AUTHOR]

The aim of this study was to analyse air exchange and temperature distribution in a greenhouse with combined mechanical and natural ventilation and to design more efficient mechanical ventilation systems. For this purpose, a computational fluid dynamics (CFD) model of the greenhouse was used. Three configurations were considered: Configuration 1 (mechanical ventilation and closed roof ventilators), Configurations 2 and 3 (mechanical ventilation and roof ventilators open 30% and 100%, respectively). After validation, the CFD model was used to improve the design of the greenhouse mechanical ventilation system in each of the three configurations analyzed. Four greenhouse lengths, 28 m, 50 m, 75 m and 100 m, were used in the simulations. Compared to fan ventilation only, roof ventilation improved the climate of fan-ventilated greenhouses in terms of the air exchange rate (22%) and climate uniformity because the internal air was mixed better than with mechanical ventilation only. As the greenhouse length increased, more advantages were achieved with natural ventilation compared to mechanical ventilation. For most configurations, there was a strong linear correlation between temperature gradient and greenhouse length. The greenhouse whose regression line had the steepest slope was the one with closed roof ventilators. Increasing the fan capacity produced a general reduction in temperature, but the effect was less intense for the greenhouses with open roof ventilators. Compared to box inlet ventilators, an enlarged continuous inlet in the wall opposite the fans increased overall system performance because it eliminated backflow recirculation zones, which are prone to produce high temperatures. [ABSTRACT FROM AUTHOR]

A computational fluid dynamics (CFD) model for simulating greenhouse night-time climate and condensation is presented. The model was applied to a four-span plastic covered greenhouse. Film condensation was simulated by applying a user defined function (UDF) added to the commercial CFD package. The CFD model was verified by comparing CFD predictions with experimental measurements. The effect of cover temperature on greenhouse humidity was then determined by the CFD model and compared to experimental results. Root mean square values for the differences between the CFD and experimental values of internal temperature and humidity showed there was good agreement between. The results showed the importance of heat transfer losses by radiation, particularly for low values of soil heat flux (SHF). They also showed the roof was the coolest surface in the greenhouse, and therefore the sink for the water vapour produced by the crop. For each configuration (SHF 10, 25, 50 and 100Wm−2 and equivalent sky temperature 263K, 273K and 276K), the condensation rate curves and relative humidity (RH) evolution are presented. It was observed that all the condensation rate curves had the same characteristic shape and could be represented by a single logistic function. The response of the CFD model to a step-change in the water vapour source (night-time transpiration from the crop) was then analysed. It was observed that the model predicted the same steady-state temperature, RH and condensation rate independent of the time when the water vapour source was enabled. The CFD condensation model is intended to be used for the design of strategies for humidity control, particularly in unheated greenhouses. [ABSTRACT FROM AUTHOR]

The present study was conducted to study the effect of pre-harvest factors such as salinity and greenhouse air CO 2 enrichment on the quality of a high commercial value, tomato cultivar. The experiment was carried out in two identical, adjacent multispan greenhouses, with polyethylene plastic covering. Hybrid Raf (cv. Delizia), tomato plants, were exposed to two treatments of salinity (5 and 7 dS m −1 ) and two ambient CO 2 concentrations (350 and 800 μmol mol −1 ) in which CO 2 enrichment was applied during the daytime according to a strategy linked to ventilation. The high salinity treatment (7 dS m −1 ) increased firmness (N), total soluble solids content (SSC), titratable acidity (TA) and the percentage of dry matter of the fruit (% DM), however, the pH was reduced. In return, CO 2 enrichment did not affect firmness, TA and pH, whereas SSC and % DM were increased. On the other hand, the climatic variations as temperature, radiation and CO 2 concentration, during the period of fruit development (PFD) affected its duration. The effective degree days (EDDs) model that combines temperature and radiation was used to study the relationship with the PFD and quality parameters. A good correlation was found between PFD and EDDs ( r 2 = 0.95). Thus the thermal time model by incorporating radiation can be recommended for our region to predict the period of fruit development in a hybrid Raf tomato crop. With respect to quality parameters, an increasing trend was observed between fruit firmness in high salinity conditions and the EDDs though the correlation was weak ( r 2 = 0.50). We did not find a relationship on SSC, TA, pH and % DM. CO 2 enrichment shortened two days of the PFD and salinity did not affect this parameter. According to the results, CO 2 enrichment allows increasing the production of a high value commercial crop grown under saline conditions by reducing the time needed for complete fruit development without compromising organoleptic quality. [ABSTRACT FROM AUTHOR]

An economic model was developed as a key component of a model-based method to design greenhouses for a broad range of climatic and economic conditions. This economic model was linked to an existing greenhouse climate-crop yield model to calculate the annual Net Financial Result (NFR) of a greenhouse. The aim of this study was to identify – among ten predefined design alternatives – the greenhouse with the highest annual NFR for tomato production under southern Spanish conditions. The basic designs were either the parral greenhouse, or a multi-tunnel, possibly fitted with any combination of heating, fogging and CO2 enrichment. Results demonstrated that the multi-tunnel, fitted with only a fogging system was most profitable, followed by the multi-tunnel with heating, CO2 enrichment and fogging. However, the difference in NFR between such a design and a simple parral was small with respect to the difference in investment. A sensitivity analysis of the NFR of the two technology extremes shows that tomato price, the fraction of marketable yield and the Photosynthetically Active Radiation (PAR) transmission of the cover had the largest bearing on NFR. With increasing technology level, the NFR depended less on outdoor climate and more on tomato price. This indicates that a low-tech greenhouse diminishes the risk of variations among price paths in different years, whereas a high-tech greenhouse covers better the “weather risk”. The best design was also affected by climate management and the joint impact of climate modification techniques. These results demonstrated that a model-based design approach can cope with multi-factorial design aspects. [ABSTRACT FROM AUTHOR]