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Tuesday, 08 April 2014 00:00

The cooling system of buildings is applied to greenhouses in order to increase their energy efficiency

Diego Valera Diego Valera Fundación Descubre

Researchers from the Universities of Almeria and Seville have found that installing evaporator water boxes, similar to those installed in the cooling towers of buildings, reduces water and energy consumption in unsealed greenhouses

Researchers from the Agricultural Engineering Group of the University of Almeria, belonging to the Agrifood Campus of International Excellence CeiA3, have implemented a system of evaporator water boxes, typical of building cooling, to greenhouses in order to check its effect on water and energy consumption. Experts have found that these evaporation devices are a good choice to cool unsealed greenhouses, as those which are more frequently installed in the Mediterranean basin, since they increase energy efficiency.

In their article 'Energy Efficiency in Greenhouse Evaporative Cooling Techniques: Cellulose Cooling versus Pads Boxes’, recently published in the journal Energies, the scientists compare the evaporator boxes system with those currently most used in technically advanced greenhouses: nebulization and evaporator panels. The first technique uses a network of pipes in which water under pressure is injected and released through tiny holes, creating micro-size drops (one millionth of a meter). Nebulization systems do not require that greenhouses be sealed, since the moisture generated inside often requires external air currents. ‘The problem is that, during summer, due to the high temperatures often combined with strong winds, the farmer must close the windows to prevent the structure from breaking. This implies that the system may not work, because the inside is saturated with water vapor and no cooling occurs’, explains Diego Luis Valera, from the University of Almeria, researcher responsible for the study, to Fundación Descubre.

Cellulose panels

Meanwhile, the system using evaporator panels consists of placing a cellulose material covering the windows on one side of the greenhouse. This cellulose support is permanently wetted with an upper pipe which is soaking it. On the opposite side of the greenhouse, fans are placed. This way all the air passing inside is made through the cellulose, thus it is loaded with droplets which cool the environment. In contrast to nebulization, this method is not altered by high temperatures or wind. However, it requires highly sealed structures.

In order to overcome the drawbacks of nebulization and panels, the system used by the engineers from Almeria is based on cooling boxes. In this case, the fan and the cellulose panel are together on the same side of the greenhouse, so the moist air is drawn into the inside. ‘This mechanism is widely used in buildings, but this is the first time its application is scientifically analyzed in traditional greenhouses’ he says. After the tests undertaken in the wind tunnel at the University of Almeria, experts have concluded that the evaporator boxes are more efficient than other cooling systems since they require less energy and water consumption in greenhouses, even in those that were unsealed. ‘The results obtained show that the boxes produce a pressure drop between 51.27% and 94.87% less than that produced by cellulose panels. This pressure drop entails a lower specific consumption of water and energy’, he summarizes.

Experts are already testing the system in the experimental greenhouses at the University of Almeria, where the ventilation systems are being tested in Almeria-type standard greenhouses (more traditional) and in multi-tunnel types (the most sophisticated).

Wind tunnel tests

Before applying it to the field, researchers undertook theoretical designs of the system using energy balances to adjust the design of cooling systems. Then the model was transferred to the wind tunnel developed at the University of Almeria. ‘We generated different wind speeds and measured the pressure drops that each system generated on the air stream. We also monitored the temperature, moisture and water consumption of each item for each airspeed ‘ he says.

The results of this study are framed within the project of excellence known as ‘Ahorro y eficiencia energética en invernaderos tipo Almería’, funded by the Andalusian Regional Ministry of Economy, Innovation, Science and Employment (Junta de Andalucía).

Research in progress

Currently, the research group led by Diego Valera is also immersed in improving the natural ventilation systems through modifications that require low investments for farmers, but which imply great improvements in the microclimate of their greenhouses. ‘We are using techniques at the forefront of knowledge for agricultural applications such as implementation of systems for measuring turbulent fluxes of water vapor and CO2, triaxial sonic anemometry and infrared thermography’, he specifies. They also have support from the Centre for Research in Agro Biotechnology (BITAL) of the University of Almeria and from a network of public and private partners.

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