Benchmark Irrigated under Cover Agriculture Crops

AbstractManaging water sustainably in a ‘green’ economy means using water more efficiently in all sectors and ensuring that ecosystems have the quantity and quality of water needed to function effectively. Despite the increasing demand for water and its scarcity in some regions in Europe and the Mediterranean basin, “water use efficiency” or Water Productivity, is claimed to be unsatisfactory. In many Southern European regions up to 85% of the water is consumed by agriculture. The expected climate change will worsen the situation as it will lead to hotter summers. In this paper an initial study to benchmark agricultural irrigation practices– here, protected cultivation - with the objective of evaluating and comparing the systems through performanceindicators that can be obtained from data routinely available at the field and farm level were presented and discussed. Benchmarking, a systematic process for detecting inefficiencies based on comparisons between similar systems, is a potential tool for identifying andtargeting problem areas. The benchmarking tool was based on the results of an FP7 EU-SIRRIMED. In the present study we use this tool in order to assess the performance of two contrasted production strategies (i) hi-tech horticultural production, exemplified by soil-less greenhouse-grown tomato crops with closed, semi closed and open irrigation techniques and (ii) low-tech screenhouse production, exemplified by soil grown sweet pepper under screenhouses having different shading factors. We found that a large margin of progress in water and fertilisers use efficiency is at hand of farmers, provided they can integrate to their farming practices innovative technologies (i.e closed hydroponic systems) or structures that are well adapted to the local climatic and biotic conditions (e.g. screenhouses)

Brine utilisation for cooling and salt production in wind-driven seawater greenhouses:Design and modelling

Brine disposal is a major challenge facing the desalination industry. Discharged brines pollute the oceans and aquifers. Here is it proposed to reduce the volume of brines by means of evaporative coolers in seawater greenhouses, thus enabling the cultivation of high-value crops and production of sea salt. Unlike in typical greenhouses, only natural wind is used for ventilation, without electric fans. We present a model to predict the water evaporation, salt production, internal temperature and humidity according to ambient conditions. Predictions are presented for three case studies: (a) the Horn of Africa (Berbera) where a seawater desalination plant will be coupled to salt production; (b) Iran (Ahwaz) for management of hypersaline water from the Gotvand dam; (c) Gujarat (Ahmedabad) where natural seawater is fed to the cooling process, enhancing salt production in solar salt works. Water evaporation per face area of evaporator pad is predicted in the range 33 to 83 m3/m2·yr, and salt production up to 5.8 tonnes/m2·yr. Temperature is lowest close to the evaporator pad, increasing downwind, such that the cooling effect mostly dissipates within 15 m of the cooling pad. Depending on location, peak temperatures reduce by 8–16 °C at the hottest time of year

Greenhouse microclimate and dehumidification effectiveness under different ventilator configurations

In this paper, the efficiency of two different greenhouse ventilation opening configurations on greenhouse microclimate during dehumidification process with simultaneously heating and ventilation was analysed by means of computational fluid dynamics (CFD) using a commercial program based on the finite volume method. The numerical model was firstly validated against experimental data collected in a tunnel greenhouse identical with the one used in simulations. A good qualitative and quantitative agreement was found between the numerical results and the experimental measurements. The results of the simulations performed for an outside wind direction perpendicular to the greenhouse axis show clearly the influence of ventilation opening configurations on the velocity, temperature and humidity distributions inside the greenhouse. With the first ventilation configuration (roll-up type) maximum air velocity inside the greenhouse was reached in the greenhouse near the ground, with the lowest values observed near the greenhouse roof. As a result, temperature and humidity decreased first near the ground and afterwards in the rest of the greenhouse volume during the dehumidification process. The exactly opposite pattern was observed with the second configuration (pivoting door type). The maximum air velocities were observed near the greenhouse roof where air temperature and humidity were decreased first during the dehumidification process. Energetically the first configuration is proven to be better since the ratio of latent to sensible exchanges during the dehumidification process was higher than the first configuration. © 2006 Elsevier Ltd. All rights reserved

Influence of vent design on greenhouse microclimate during dehumidification with simultaneous heating and ventilation

The influence of greenhouse vent configuration on greenhouse microclimate and energy consumption when natural ventilation was used for greenhouse dehumidification was numerically analysed using a computational fluid dynamics model after its experimental validation. Two commonly found vent configurations (roll-up type and pivoting door type) were tested in a tunnel greenhouse with a mature tomato crop. The simulations were two dimensional and the solution was obtained in two steps. First a converged solution under steady-state conditions was obtained. Then, the flow was considered unsteady air humidity inside the greenhouse was assumed to be 90% and air temperature 20°C whereas the corresponding outside climate variables were 50% and 10°C. Ventilation openings were activated and air humidity and temperature were decreased in the greenhouse at a rate depending on the local value of the air velocity inside the greenhouse. The different vent arrangements resulted in different airflow patterns inside the greenhouse. As a result of the different airflows, the temperatures and humidities inside the greenhouse were reduced accordingly during dehumidification. The ratio of latent to sensible heat exchange during the dehumidification process was chosen as the criterion in order to evaluate the energy efficiency of each ventilation configuration. The roll-type vent configuration had the best energy performance

Effect of vent arrangement on windward ventilation of a tunnel greenhouse

The effect of ventilation configuration of a tunnel greenhouse with crop on airflow and temperature patterns was numerically investigated using a commercial computational fluid dynamics (CFD) code. The numerical model was firstly validated against experimental data collected in a tunnel greenhouse identical with the one used in simulations. The airflow patterns were measured and collected using a three-dimensional sonic anemometer and the greenhouse ventilation rate was deduced using a tracer gas technique. A good qualitative and quantitative agreement was found between the numerical results and the experimental measurements. After its validation, the CFD model was used to study the consequences of four different ventilator configurations on the natural ventilation system. The ventilation configuration affects the ventilation rate of the greenhouse and the airflow and air temperature distributions as well. For the different configurations, computed ventilation rates varied from 10 to 58 air changes per hour for an outside wind speed of 3 ms(-1) and for a wind direction perpendicular to the openings. Likewise, the simulations highlight that while the mean air temperature at the middle of the tunnels varied from 28(.)2 to 29(.)8degreesC, for an outside air temperature of 28degreesC, there are regions inside tunnels 6degreesC warmer than outside air. Average air velocity in the crop cover varied according to the arrangement of the vents from 0(.)2 to 0(.)7 ms(-1). The consequences of the marked climate heterogeneity on plant activity through the variation of crop aerodynamic resistance as well as the influence of the vent configurations on the efficiencies of ventilation on flow rate and air temperature differences between inside and outside, are also discussed. (C) 2003 Silsoe Research Institute. All rights reserved Published by Elsevier Lt

Numerical simulation of the airflow and temperature distribution in a tunnel greenhouse equipped with insect-proof screen in the openings

An analysis of the ventilation process in a tunnel greenhouse equipped with an insect-proof screen in the side openings was performed with the use of a commercial computational fluid dynamics (CFD) package (CFD2000(R)). The aim of the study was to investigate how the screen influences airflow and temperature patterns inside the greenhouse. The screens on the greenhouse inlets and outlets, as well as the crop were simulated using the porous medium approach. The first simulations were carried out with a wind direction perpendicular to the side openings. Insect screens significantly reduced airflow and increased thermal gradients inside the greenhouse. Maximum air velocity values inside the greenhouse were observed near the openings, whereas air velocity was lowest in the middle of greenhouse. Airflow rates reduced by half in the greenhouse equipped with screen, These differences were also important in the region covered by crop, thus screen affected the sensible and latent exchanges between crop and air. The effect of different wind directions was also investigated. Wind direction considerably affected climatic conditions inside the greenhouse, as contrasted air flow and temperature patterns were observed for various wind regimes, especially when the greenhouse was equipped with insect screens. (C) 2002 Elsevier Science B.V. All rights reserved

Energy needs and energy saving in Mediterranean greenhouses

A fully-automated and climate-controlled greenhouse requires heating, ventilation, cooling, dehumidification and shading systems to control the microclimate conditions and create an optimal environment for crop growth and development. Furthermore, irrigation and fertilization systems and product sorting, packing and storage systems as well as several other auxiliary systems are needed. All the above systems require energy to operate that may be an important part of the total operating cost of the greenhouse. In this paper, the energy needed to operate the above systems in a Mediterranean greenhouse is calculated and analysed and methods to reduce these energy needs are suggested

Temperature gradients in a partially shaded large greenhouse equipped with evaporative cooling pads

The main drawback of greenhouse evaporative cooling systems based on cooling pads and extracting fans is the thermal gradient developed along the direction of the airflow. High-temperature gradients of this type can markedly affect plant growth, and growers often combine cooling pads with shading. To predict the temperature gradients along a greenhouse, a simple climate model is proposed which incorporates the effect of ventilation rate, roof shading and crop transpiration. In order to calibrate the proposed model, measurements were performed in a commercial greenhouse equipped with fans and pads and shaded in the second half. Experimental data show that the cooling system was able to keep the greenhouse air temperature at rather low levels. However, due to the significant length of the greenhouse (60 m), large temperature gradients, (up to 8degreesC) were observed from pads to fans. The model was calibrated by fitting temperatures in the middle and at the end of the greenhouse. The model was validated on experimental data different from those used for the calibration and then it was used to study: (i) the influence of different ventilation rates combined with shading on air temperature profiles along the greenhouse length; and (ii) the influence of the outside air temperature and humidity on the performance of the cooling system. High ventilation rates and shading contribute to reduce thermal gradients. Despite its simplicity, the model is sufficiently accurate to improve the design and the management of the cooling pad systems. (C) 2003 Silsoe Research Institute. All rights reserved. Published by Elsevier Science Ltd