Acta Horticulturae

Technical tour: explanations about cucumbers on a commercial nursery.
Technical tour: a shaded pepper crop at Las Palmerillas.
A significant aspect of the symposium was the
number of new ideas presented on technologies
that could contribute to greenhouse cooling.
These included transparent photovoltaic
cells, linear Fresnel lenses, the collection, removal
and reuse of solar energy, and the use of the
very salt water from Salinas together with seawater
in evaporative cooling systems. The continued
development by plastics companies of
PAR transmitting films that reflect solar infrared
radiation, not utilised by plants, but contributing
half of the solar input to greenhouses,
offers the potential for a very significant advance
in greenhouse cooling.
Many presentations used the relatively new
modelling technique of computational fluid
dynamics (CFD) to show how the natural ventilation
of greenhouses is affected by the size of
a greenhouse and the shape of its roof, and by
the position, size and type of the ventilators.
This development indicates that it should soon
be possible to design a natural ventilation system
for a greenhouse that will give a specified
air exchange rate. CFD was also used to investigate
the movement of air inside greenhouses
showing the influences of crop row orientation
and external wind direction, and also how pesticide
sprays move inside greenhouses and are
removed by natural ventilation. Good internal
air movement is important to ensure climate
uniformity and effective heat, water vapour and
carbon dioxide exchange between plant leaves
and greenhouse air.
The ways in which many important greenhouse
crops respond to different methods of cooling
were reported. Information was presented on
the effects of temperature and vapour pressure
deficit on plant temperature and plant transpiration,
which are significant factors in ensuring
good plant growth and development. New
ways of designing and controlling fog evaporative
cooling systems were presented, which
offered independent control of temperature
and humidity while reducing water usage.
Good control of greenhouse temperature and
humidity was shown to be especially important
immediately following transplanting when
plants have a low leaf area index.
The function of greenhouses is to enable farmers
to grow crops that can be sold at a profit.
Therefore, the economics of the different cooling
methods is the factor of greatest interest to
farmers. Many presenters reported the effects
of cooling on the yield and quality of greenhouse
crops. Of special interest was a comparison
of three methods of cooling: fans, natural
ventilation combined with fogging, and natural
ventilation combined with shading applied to
the greenhouse cover as a whitewash. An economic
analysis of the results showed, for pepper
production, that natural ventilation with
whitewashing gave the highest financial margin
between crop value and the costs of production
- this information is of direct value to the farmer.
Bernard Bailey
CONTACT
Dr. Jerónimo Pérez Parra, Estación Experimental
de Cajamar, Autovia del Mediterráneo, km.
416.7, 04710 El Ejido, Almería, Spain, Phone:
+34 950 580 548, Fax: +34 950 580 450,
email: jpparra@cajamar.es
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