12.4.3 Solar Radiation and Temperature
Some studies have used solar radiation and transpiration models based on the heat
and water balances of the crop, to investigate the distributions of air temperature
and humidity and also the interactions between the crop and the air, in addition to
the airflow distribution (Sase 2006). According to Tablada et al. (2005), the factor
of solar protection plays a crucial role in maintaining stable thermal conditions
indoors, even if the outside air temperature is higher. The slightly higher air speed
on the top floor is insignificant in view of reducing the negative effect of the solar
radiation over the roof and facade. The temperature of the greenhouse cover is an
essential parameter needed for any analysis of energy transfer in the greenhouse. A
sub-model developed by Impron et al. (2007) calculated the transmission of
radiation through the greenhouse, including the reduction of NIR transmission
through the roof. Tong et al. (2009) developed a numerical model to determine
time-dependent temperature distributions based on hourly measured data for solar
radiation, indoor air, soil, and outside temperature, taking into account variable
solar radiation and natural convection inside the greenhouse during the winter in
northern China.
12.4.4 Temperature and Air Exchange
The effect of solar and thermal radiation is often taken into account by setting
specific wall or heat fluxes at the physical boundaries of the greenhouse. Radiation
transfer within the crop itself is still the major concern since it determines the two
main physiological crop processes: transpiration and photosynthesis. This challenge is now launched and will probably receive more attention within the next
few years (Bournet and Boulard 2010). Pontikakos et al. (2006) analyzed data
obtained from a CFD model, showing that the external boundary temperature is a
critical parameter in the pattern of internal greenhouse temperatures and that for
specific external temperatures and wind directions, airspeed becomes the crucial
parameter. According to Molina-Aiz et al. (2006), opening vents affect the airflow,
the ventilation rate, and the air temperature distribution in a greenhouse, where the
mean air temperature at the middle varied from 28.2 to 32.9 °C with an outside air
temperature of 26 °C, there were regions inside the greenhouse that were 13 °C
warmer than the outside air. Nebbali et al. (2006) used a semi-analytical method to
determine the ground temperature profile from weather parameters and other
characteristics, to help in evaluating heat flux exchange between the surface and
the air. Rico-García et al. (2008) showed that ventilation in greenhouses due to the
temperature effect produces high air exchange rates; however, those air patterns
occur near the openings, causing almost no air exchange in the central zone of the
greenhouse due to a stagnant effect that reduces the wind effect throughout the
greenhouse. In agreement with the results of Majdoubi et al. (2009), convection
350
G. De la Torre-Gea et al.
Some studies have used solar radiation and transpiration models based on the heat
and water balances of the crop, to investigate the distributions of air temperature
and humidity and also the interactions between the crop and the air, in addition to
the airflow distribution (Sase 2006). According to Tablada et al. (2005), the factor
of solar protection plays a crucial role in maintaining stable thermal conditions
indoors, even if the outside air temperature is higher. The slightly higher air speed
on the top floor is insignificant in view of reducing the negative effect of the solar
radiation over the roof and facade. The temperature of the greenhouse cover is an
essential parameter needed for any analysis of energy transfer in the greenhouse. A
sub-model developed by Impron et al. (2007) calculated the transmission of
radiation through the greenhouse, including the reduction of NIR transmission
through the roof. Tong et al. (2009) developed a numerical model to determine
time-dependent temperature distributions based on hourly measured data for solar
radiation, indoor air, soil, and outside temperature, taking into account variable
solar radiation and natural convection inside the greenhouse during the winter in
northern China.
12.4.4 Temperature and Air Exchange
The effect of solar and thermal radiation is often taken into account by setting
specific wall or heat fluxes at the physical boundaries of the greenhouse. Radiation
transfer within the crop itself is still the major concern since it determines the two
main physiological crop processes: transpiration and photosynthesis. This challenge is now launched and will probably receive more attention within the next
few years (Bournet and Boulard 2010). Pontikakos et al. (2006) analyzed data
obtained from a CFD model, showing that the external boundary temperature is a
critical parameter in the pattern of internal greenhouse temperatures and that for
specific external temperatures and wind directions, airspeed becomes the crucial
parameter. According to Molina-Aiz et al. (2006), opening vents affect the airflow,
the ventilation rate, and the air temperature distribution in a greenhouse, where the
mean air temperature at the middle varied from 28.2 to 32.9 °C with an outside air
temperature of 26 °C, there were regions inside the greenhouse that were 13 °C
warmer than the outside air. Nebbali et al. (2006) used a semi-analytical method to
determine the ground temperature profile from weather parameters and other
characteristics, to help in evaluating heat flux exchange between the surface and
the air. Rico-García et al. (2008) showed that ventilation in greenhouses due to the
temperature effect produces high air exchange rates; however, those air patterns
occur near the openings, causing almost no air exchange in the central zone of the
greenhouse due to a stagnant effect that reduces the wind effect throughout the
greenhouse. In agreement with the results of Majdoubi et al. (2009), convection
350
G. De la Torre-Gea et al.