Modeling Of Evaporative Cooling System For Naturally-Ventilated Tropical Greenhouses

Natural ventilation in tropical greenhouse is a common method for ventilation which gives higher inside temperatures compared to the outside temperatures. This type of ventilation is not enough to reduce high temperature inside the structure in low land areas. Thus the requirement of cooling is increased. Use of fossil fuel to run the cooling fans are not economically viable due to increasing fuel cost and greenhouses are not always located near the electrical grid. The objectives of this research was to study the inside microclimate of the greenhouse, the natural ventilation of the greenhouse, the evaporative cooling system by means of misting fans and to study the use of photovoltaic (PV) as an alternative energy source to cool down the greenhouse. This study presents the theoretical and experimental results of the inhouse microclimate, ventilation rate induced by stack effect, wind effect and combination of both stack and wind effects for naturally ventilated single and multispan tropical greenhouses, effect of height and plants on inside temperature and ventilation rate. Ventilation rate induced by the stack effect was found to increase with increasing temperature difference between inside and outside of greenhouse structures according to power law, with an index of 0.5. The wind effect ventilation rate was found to increase linearly with increasing outside wind speed. Ventilation rate inside single span structure (smaller floor area) was higher than in multi-span structure. This was due to that the fast movement and air exchanges in single span structure. However, the bigger floor area gives higher in-house temperatures. Inside temperature and outside wind speed were calculated to verify the mathematical models which were developed. This method was used because of the difficulty to use tracer gas method in porous large scale greenhouse structures. The comparison between calculated and measured inside temperature showed there was no significant difference between them. The effect of height on ventilation rate is also crucial in greenhouse design. The ventilation rate increases with the increasing height (distance between the middle of the side opening and the middle of the roof opening) of the structure. Ventilation rate inside the house with plant was found lower than inside the house without plant. This was due to the temperature difference between inside and outside structure. Evaporative cooling by means of misting fans in single span was also presented in this study. Four misting fans were used to cool the greenhouse. They were installed two meters above ground with two fans near the southern sidewall and two fans in the middle of the greenhouse operating from 10:00 am to 16:00 pm daily. Data was collected from three rows with total 57 points inside the greenhouse. The distance between points was 2.5 m along the length of the greenhouse. The data was processed using GIS (Geographic Information System) to model the inside temperature and wind speed. The temperature inside the greenhouse with fans was found to be lower than that without fans, while the inside relative humidity of the air was found to be higher in the greenhouse with fans than that without and the outside. However these values of relative humidity lie at the optimum value that is less than 90% which do not give negative effect on the plants inside the structure. Based on the results, the contours and 3D maps of the in-house temperature and wind speed distribution in the single span greenhouse were developed. The efficiency of misting fans was lower than the previous studies. This was due to the usage of misting fans in porous naturally ventilated greenhouse, while the previous studies were carried out in closed greenhouses and glasshouses. Photovoltaic (PV) hybrid system design and simulation was discussed in this study. The results show that PV system would be suitable to supply electricity to cover the load requirement without purchasing energy from grid, and the battery state of charge was found to be in the range of 75-100%.

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