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 i...
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| Format: | Thesis |
| Language: | English English |
| Published: |
2007
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| Online Access: | http://psasir.upm.edu.my/id/eprint/5178/1/FK_2007_14.pdf http://psasir.upm.edu.my/id/eprint/5178/ |
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| Summary: | 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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