AGRICULTURAL

Agricultural and Forest Meteorology 86 (1997) 273-282

Effects of ventilation on the energy balance of a greenhouse with bare soil ’

Marcel Fuchs a, * , Ehud Dayan b, David Shmuel ‘, Isaac Zipori b ’ Institute of Soils and Water, Agricultural Research Organization, The Volcani Center, PO Box 6, Bet Dagan 50250, Israel

b Institute of Soils and Water, Agricultural Research Organization, Besor Experiment Station, Negev Mobile Post OJice, Negev 85400, Israel

’ Southern Research and Development, Besor Experiment Station, Negev Mobile Post Ojke, Negev 85400, Israel

Received 10 October 1995; revised 21 October 1996; accepted 5 November 1996

Measurements of the radiation and energy balances in a polyethylene greenhouse covering a bare, dry sandy soil were used to determine the heat dissipation efficiency of four ventilation methods. The 62% of incident solar radiation transmitted by the roof provided the heat load on the system. The four ventilation treatments comprised passive ventilation obtained by rolling up opposite end walls, activating two fans mounted in one gable and opening the opposite wall, the same configuration but with a single fan, and completely closing the greenhouse. Opening opposite walls generated sufficient draught to exchange air at a rate of 44 volumes per hour. Operating one and two fans produced exchanges of 8 and 13 volumes per hour, respectively, much below the specified rating of the equipment, because of pressure losses across insect-proof nets on the vents. Closing the greenhouse limited the air exchange to three volumes per hour. For the climatic

conditions of the experiment, external wind speed and internal buoyancy forces affected passive ventilation, but had no significant effects on fan ventilation. Despite the dryness of the top layer of soil, the latent heat flux remained a large term in the energy balance. High ventilation rates diminished soil heat flux, increased sensible heat flux and slightly reduced latent heat flux. Q 1997 Elsevier Science B.V.

Keywords: Energy balance; Ventilation; Bare soil; Greenhouse

1. Introdu&oIl

The greenhouse is a cultivation environment that is partly separated from its surroundings. The roof transparency establishes a link between the internal microclimate and outdoor atmospheric conditions. To quantify the physical modifications introduced by

* Corresponding author. I Contribution from the Agricultural Research Organization, the

Volcani Center, Bet Dagnn. No. 1730-E 1995 Series.

the structure, the cladding material and the climate- control equipment, it is first necessary to identify the parameters associated with their properties. In the energy balance equations, these parameters appear as radiometric, conductive and convective transport co- efficients regulating heat absorption and dissipation. Ventilation, by modifying the heat exchange among the radiation-absorbing surfaces, the internal air space and the free atmosphere, provides a means for regu- lating the internal microclimate. The objective of the experiment was to measure the parameters affected

0168-1923/97/$17.00 Q 1997 Elsevier Science B.V. All rights reserved. PII SO168-1923(96)02418-5

274 M. Fuchs et al. /Agriculhual and Forest Meteorology 86 (1997) 273-282

by ventilation and to establish the efficacy of several ventilation methods.

2. Theory

2.1. Energy balance

Consider a greenhouse a few days after the plants have been removed. The soil surface is dry and therefore evaporation or latent heat flux density E of residual moisture in the soil is slow. This condition enables most of the solar radiation absorbed by the soil to warm the soil and the air. The steady state energy balance of the soil surface in a greenhouse is

R,=E+H+G, (1) where R, is net radiation absorbed by the soil surface, H is the sensible heat flux density from the soil to the air, E is the latent heat flux from the soil to the air, and G is the heat flux density into the soil. Fluxes are positive when the surface, the air or the soil gain energy. Air exchange between inside and outside is the ventilation rate removing sensible heat, H,, and latent heat, E,. Since the cover material is impervious to water vapor, at steady state we have E = E,. By contrast, sensible heat exchange by con- duction, H, through the cover material occurs, tbere- fore H = H, + H,. Comparing the heat losses of greenhouses with single- and double-layer poly- ethylene covers (van de Braak, 19951, we estimate that the conductive heat transport is about 2 W m-’ “C-‘. The convective heat transport for a ventilation rate of 20 volume exchanges per hour is 30 W m-* “C- ‘. Therefore, in a ventilated greenhouse, we can assume H = H,. With this approximation, the air temperature q and water vapor pressure ei inside the greenhouse, and the air temperature TO and water vapor pressure e, outside the greenhouse, allows us to derive rx, the air exchange resistance from:

PC,(? - T,) pc,(ei - e,) r, =

H = yE ’ (2)

where p is the density of air, cP is the specific heat of air and y is the psychrometric constant. Because Elq. (2) neglects sensible heat conduction through the greenhouse cladding, it is valid only for ventilated greenhouses. This approximation implies that con-

vective and radiative transport to the free atmosphere surrounding the greenhouse completely dissipates the solar radiation absorbed by the roof. Combining Eqs. (1) and (2) we obtain H and E:

H= P(Rn-‘3 1+p ’

R, - G E=-

1+p ’ (3)

where p is the Bowen ratio of the energy exchange through the vents of the greenhouse. The resulting E and H allow us to calculate r,. For engineering applications, the ventilation rate is conveniently ex- pressed as the number of air volumes exchanges per hour, X:

3600

where Z is the average height of the greenhouse. Measurements of the surface temperature of the

soil, T,, enable us to calculate the internal boundary- layer resistance ri of the bare soil:

In the presence of internal temperature gradients, this resistance depends on the location of the temperature sensor. In a greenhouse with a fully developed canopy the resistance values will differ from those deter- mined for an empty greenhouse because the large internal surface of the foliage exerts additional drag on the motion of air. The size of the greenhouse is another factor modifying the air flow because pres- sure drops across fans and vents have fixed values, whereas pressure gradients depend on greenhouse dimensions. Nevertheless, experimentation in a small, empty greenhouse can be used to compare the reia- tive efficacy of ventilation methods.

2.2. Transmission of radiation through the roof

Weathering of the cladding material and intercep tion by the structure affect the transmission of solar radiation through the roof of the greenhouse. As

M. Fuchs et al. /Agricultural and Forest Meteorology 86 (1997) 273-282 275

documented information on effective transmission of standing greenhouses is lacking, development of in situ measurement methods is necessary. The solar radiation balance of the greenhouse floor has a downward component; S,:

‘id = Trsod + srsiu9 (6)

4” = 13Ji,. (7)

In these expressions, the subscript i refers to inside the greenhouse, the subscript o to outside, d to downward and u to upward, r to the roof and s to the soil. The radiometric properties, transmissivity, 7, reflectivity, S and absorptivity, LY must satisfy the constraints:

cr+s+7=1; cy20; s20;720 (8)

Measurements of S, and Sod, the downward solar radiation inside and outside the greenhouse, respec- tively, give the value of the apparent transmissivity, 7app a quantity that depends on the reflectivity of the ground. Replacing S,, in JQ. (6) with its value from Eq. (7) yields

%I 7, 7

app = s = -- od 1 - s*s, .

Measuring S, with an inverted pyranometer enables us to obtain S, diiectly from IQ. (7). This value becomes a fixed parameter in Eq. (9).

Without independent measurements of S,, we can only obtain the values that bracket TV. For 8, = 0, 7r reaches its upper bound:

upper bound (T,) = TV. (10)

Eq. (9) yields the lower bound (7,) for (Y, = 0 (i.e. s, = 1 - 7,):

(1 - wapp lowerbound = 1 _ s 7 .

s aPP (11)

Eq. (11) is an implicit function because 78pp changes with 8,. At S, = 1, i.e. when the soil is a perfect reflector, 7.pp = 1, meaning that downward radiation inside and outside the greenhouse are the same. Thus, for a roof with low solar absorptivity, a reflec- tive ground increases, the radiation in the greenhouse (Stanhill et al., 1975:).

3. Experimental procedure

We isolated a span, 7.5 m wide and 23 m long, of a multi-span greenhouse by erecting lateral poly- ethylene side walls. The structure had a height of 5.6 m at the ridge and 4.0 m at the gutter. The ridge was oriented north-south. The roof cladding material was a polyethylene film with infrared-absorbing ad- ditives. The north and south ends had roll-up walls from 0.8 m above the floor to heights of 2.0 and 2.8 m on the south and north walls, respectively. Plastic nets, allowing the passage of spheres with 0.35 mm maximum diameter covered the openings. Two ex- haust window fans, with a rated outflow of 16000 m3 h-’ at zero static pressure, were mounted on the north gable. The soil of the greenhouse is an im- ported fine sand, with clay and silt contents less than 5%.

The experiment started on 28 June 1992, two weeks after cultivation and disinfection of the soil. At the time of the experiment the polyethylene cladding had weathered for a full year. The upper 0.05 m soil layer was air dry. The following four ventilation treatments were applied in sequence, each for 24 h on four consecutive days:

open: passive ventilation by rolling up the north and south walls; closed: closure of all vents; one-fan: forced ventilation with one fan in opera- tion, south wall closed, north wall open; two-fan: forced ventilation with two fans in oper- ation, south wall closed, north wall open.

All the measurements were taken between 11:OO and 13:00 h when solar radiation varies the least during the day. All the variables were sampled at 30 s intervals and averaged every 600 s. The solar radia- tion balance was measured with three pyranometers (model CM-5, thermopile with no temperature com- pensation, Kipp and Zonen, Delft, the Netherlands). One instrument, installed above the greenhouse, measured the outdoor downward solar radiation. A second instrument, mounted underneath the ridge 3 m above the ground, measured the inside downward radiation, and, at the same location, an inverted pyranometer measured the reflected radiation. The use of only one pyranometer to sense inside down- ward radiation did not account for the variation of transmitted radiation resulting from the interception

276 M. Fuchs et al. /Agricultural and Forest Meteorology 86 (1997) 273-282

of the sun’s rays by the structure (Stanhill et al., 1973; Heuvelink et al., 19951, but averaging the radiation reading over the midday period, when the sun’s zenith angle is changing most slowly but its azimuth motion is the fastest, diminished the effect of spatial variability. Fig. 1 shows that the downward solar radiation inside the greenhouse remained fairly constant during the measurements, except for a slight dip at around 12:30 when an element of the structure intercepted direct radiation.

A polyethylene-dome-shielded net pyrradiometer (Siemen Ersking, Frederikssund, Denmark), posi- tioned below the ridge 3 m .above the soil, measured the net short- and long-wavelength radiation. Fig. 1 indicates that the representativeness of the single fned location of this measurement was acceptable, since variation caused by the structure was small.

Air temperatures. and humidities were measured with aspirated shielded psychrometers. The wet and dry sensing elements were copper-constantan ther- mocouples. Inside the greenhouse, psychrometers were mounted 1 and 2 m above the soil. Outside, the psychrometer was in an open area 2 m above the

28 June 1992

000 oooooooooooo

0 I

A 1o:ou 11m . HOUR 4t.S.T.)

0 Solerdcwnwadwtpkle t SoterdownwadinrMe A Solar upward inside ?? Ml~venetimlide

Fig. I. Components of .the rahtion balance pf a polyeth@ene- COVdgeeghouSetith~baredrysandpsoil.

ground. A cup anemometer (Met One, model 014A, Campbell Scientific, Logan, USA) 2 m above the ground measured the wind speed.

Copper-constantan thermocouples inserted at depths 3 and 30 mm below the soil surface in four locations, measured soil temperature, and four heat flux plates (Fuchs and Hadas, 1973) buried 30 mm beneath the surface measured the soil heat flux. To obtain the heat flux at the surface of the soil, the change of heat content in the top 30 mm of soil was evaluated from the average temperature change of the soil layer during the 600 s interval between two successive data recordings. The heat capacity for dry sand is 1.13 MJ m-3 K-’ (de Vries, 1963).

All the signals were processed and stored in a CR-10 data logger connected to the sensors via an AM416 multiplexer (Campbell Scientific, Logan, USA). -

4. Results and discussion

4.1. Radiation transmission

Using the data in Fig. 1, we found that the average soil reflectivity 8, was 0.164. The measured value of 0.63 for rqP is an upper bound for the true value of the roof transmissivity and the lower bound for transmissivity, according to Eq. (11) was 0.60. The small difference between the upper and lower bounds indicates that the measurement of ~,r yielded a reasonable estimate of the roof transmissivity. However, this result is a consequence of the low value found for the reflectivity of the soil. Covering the soil with plastic mulch, which raised the ground reflectivity, S, to 0.303, increased rapt, only slightly, to 0.64, but dropped the lower bound to 0.553. This result shows that, in contrast with the finding of Stanhill et al. (1975) for a glass-covered greenhouse, a reflective coating on the floor would not apprecia- bly increase light penetration in a greenhouse of the type considered here. Combining the data collected over the bare soil with those collected over the plastic mulch allows us to solve Eq. (9) simultane- ously for TV and S,, to yield values of 0.62 and 0.11, respectively. The absorptivity of the roof, a, is thus 0.27; this is a surprisingly high value. The precision

M. Fuchs et al. /Agricultural and Forest Meteorology 86 (19971273-282 211

Table 1 Effect of ventilation m&ad on the resistvlces to heat transport and air exchange rate in a polyethylene greenhouse withabare sandysoiltoppedbya50mmairdrylayer

Treatment Internal Exchange vobJme resistance resistance exchange (5 m-l) (s m-l) (h-l)

open 113 17.0 44.1 Two-& 165 56.4 13.3 olle-fan 170 92.6 8.1 Closed 157 245.5 3.1

of this result is low because rqP in the two sets of measurements differed only by 0.01, a 1% error in the measurement of ‘rqP leading to a 20% error in the calculated a,. Yet, the very fact that the two values were so close suggests that q was high. This result merits further investigation.

4.2. Ve&ilutian

During the experiment the wind speed (a nor& westerly sea breeze) was approximately 2 m s-’ (Table 2). Opening the north and south walls of the gtcenhouse allowed this moderate wind to produce a sufficient dynamic pressure drop to provide the low- est heat transport resistance and the highest air ex- change rate (44.1 hLS1) of all treatments (Table 1). Even in the presence of an insect-proof net that impeded air movement through the vents, this method of ventilation ensured a high exchange rate. Operat-

Table 2 Bffect of ventilation regime on the average midday temperature and humidity in a polyethylene gee&mse with a bare sandy &ii topped by adtvlaver5Ommthick

ing two fans which displaced a nominal 32000 m3 h-l, should have produced an exchange tate of 38.6 h-‘. The calculated value of only 23.3 h-’ suggests that the fans could not overcome the drag caused by the nets. Whereas in the two-fan treatme* exchange resistance increased threefold over that of the open passive treatment, the internal heat transport resis- tance from soil to air grew by only 46%. Forced ventilation with a siugle fan lowered the air ex- change rate by another factor of two. This propor- tionality shows that aerodynamic resistance, proba- bly generated by the insect-proof net, prevented ef- fective operation of the fans. Better ventilation would require fans capable of working against steep pms- sure drops. The decrease of ventilation only very slightly increased the internal heat transfer resistance from the soil surface to the air. Closing the green- house considerably diminished the air exchange rate. Yet, it is necessary to recall that calculations assume convection to be much larger than conduction. For the closed greenhouse this assumption is weak and leads to an overestimate of the air exchange rate_

Similarities in outdoor temperature, humidity and wind speed provided a good experimental basis for the comparison of treatments (Table 2). In the closed greenhouse, the average air temperature was 20°C higher than that outside. Ventilation reduced this air temperature difference to 5°C (Table 21, but cooled the soil surface temperature by only 7°C.

The air temperature 2 m above the ground was

Treatment Greenhouse Outdoor Bowen ratio, /3

surface Airtempe- water vapor Air Water Wii femperahut rature (“Cl pressure &Pa) temperature vapor (“1 lm 2m lm 2m (“0 pressure &Pa)

open 54.1 34.4 36.5 267 3.00 30.4 260 2.20 3.81 1.022 Two-fan 54.4 35.0 36.1 2.29 2.81 29.0 2.00 240 1.38 0.585 ‘. One-fan 56.2 38.5 38.8 2.75 3.21 29.0 2.01 2.30 0.856 0.545 Closed 61.5 49.4 50.0 4.40 4.99 29.7 2.26 2.u) 0.614 0.4% 0

B = O&671(4 - T,)/(q - e,)]

Ti T, AT e; e, he 5 open 35.45 30.4 5.05 2.835 2.60 0.235 1.44 Two-fan 35.55 29.0 6.55 2.550 200 0.550 0.794 one-fan 38.65 29.0 9.65 2.980 2.01 0.970 0.664 Closed 49.7 29.7 20.00 4.695 2.26 2.435 0.550 I

278 M. Fuchs et al. /Agricultural and Forest Meteorology 86 (1997) 273-282

consistently higher than that at 1 m (Table 2). As the soil absorbed 52% of the outdoor solar radiation, the elevated soil temperature heated the air by convec- tion. Normally, this transport mechanism could not account for the observed temperature inversion. With a solar radiation absorptivity of 27%, it would be possible to claim that the roof served as a second source of internal sensible heat, thus explaining the inversion. The existence of an additional heat ex- change channel would invalidate the energy balance approach used in the present analysis. A roof warmer than the internal air would imply the absurd conclu- sion that transport by conduction across the ther- mally stable internal air layer below the roof would be more efficient than the convective heat exchange with the free and unstable air above the roof. The vapor pressure profile in the greenhouse provides compelling evidence against a link between energy transfer from the roof and the inversion. The vapor pressure showed the same inversion as the tempera- ture (Table 2). As the roof is in no way a water vapor source, it could not have generated the inver- sion. We attempt to explain this observation later in this paper. The stratification of air temperature and humidity raises the question of the representativeness for q and e, used for the Bowen ratio computation for the ventilated treatments. As the end walls opened from 0.8 to 2.8 m above the ground we assumed that the measurements at 1 and 2 m provided reasonable estimates of the average internal temperature and humidity.

sponding to a difference in thermal emission of 55 W m-‘, as calculated on the basis of an emissivity of 0.90 for this soil type (Fuchs and Tanner, 1968). The additional emission is slightly greater than the observed 41 W m-* decrease in net radiation, imply- ing that the incoming radiation for all the treatments was similar.

Increasing the ventilation rate reduced the temper- ature elevation in the greenhouse very rapidly. The weak air exchange of the one-fan treatment reduced the temperature excess to lO”C, but further increases in ventilation rates produced only slight additional cooling (Table 2). This result supports the prediction of a cooling model (Fuchs, 19931, that internal tem- peratures change rapidly at the low end of the venti- lation range, but that any increase in ventilation beyond 20 h-’ has practically no effect. Closing the greenhouse led to a considerable increase in the internal humidity. Although the relative humidity in the greenhouse did not rise above 40%, the vapor pressure was 2 kPa higher than the condensation vapor pressure at outdoor air temperature. A sudden drop in the heat load would induce condensation. The risk of condensation decreases with ventilation.

The closed treatment has a lower net radiation than the others (Table 31, but this difference does not indicate that the radiative load was less on that day. The surface temperatures in the open and closed treatments were 54.1 and 61.5”C, respectively, corre-

Temperature and vapor pressure elevations are linearly related to exchange resistance (Fig. 2). The humidity elevation extrapolates to 0.045 kPa (not significantly different from zero at p = 0.05) for zero resistance. According to Q. (2), this result shows that the latent heat flux density was about the same on the four days (Table 3). The same extrapola- tion for the temperature elevation yields a positive offset of 3.4”C. This offset has no specific physical meaning, because the energy balance demands that q = To at zero exchange resistance. However, it indicates that the sensible heat flux density increases

Table 3 Effect of ventilation regime on the midday energy balance of a polyethylene greenhouse over a bare sandy soil with a top dry layer 50 mm thick Each datum is the average of 12 sampling periods of 600 s duration

Treatment Net radiation soil heat flux Evaporation Sensible heat flux Bowen ratio, p (W m-‘) (W m-*) (W m-*)

(W me21 (mm h-‘)

@en 456 118 132 (0.20) 204 1.55 Two-fan 453 145 170 (0.25) 136 0.80 One-fan 457 15.5 180 (0.27) 120 0.66 Closed 415 182 144 (0.22) 86 0.60

M. Fuchs et al./Agricultural ad Forest Meteorology 86 (1997) 273-282 279

open *fan1 one-fan

AIR EXCHANGE RESISTANCE (s m-1)

Fig. 2. Temperature and humidity gains, and soil surface heating in a polyethylene-covered greenhouse with a dry bsre sandy soil, as functions of the exchange resistance.

with decreasing air exchange resistance, and the Bowen ratios in Table 3 show the same effect. As the radiative heat loads during the four measurement periods were nearly identical, the contrast between the responses of sensible and latent heat fluxes to ventilation suggests that the physical factors control- ling their transport differ.

Table 2 has already illustrated that ventilation, within the present experimental range, did not cool effectively the soil surface. Fig. 2 shows emphati- cally that a decrease of air exchange resistance (i.e. enhancing ventilation) increases the soil-to-air tem- perature difference. As higher ventilation enhances sensible heat removal from the greenhouse (Table 3), more sensible heat must come from the soil surface, imposing a steeper temperature gradient or a de- crease of the boundary-layer resistance. The trend of the soil-to-air temperature difference line in Fig. 2 suggests that the effect of air exchange on the inter- nal boundary-layer resistance is weak. Still, ventila- tion modifies the soil heat flux. The contribution of the soil heat flux to the midday energy dissipation increases from 25% of the net radiation in the well- ventilated open treatment, to 44% in the closed treatment (Table 3). The effect of atmospheric con- vective transport on the partition of heat between bare soil and air is well known in meteorology

(Lettau, 1951), but to our knowledge it has not been previously reported for greenhouses.

Whereas ventilation increases sensible heat trans- port, the effect on evaporation, shown in Table 3, is unexpected. Evaporation decreases with increased ventilation except in the closed greenhouse, for which the computation of the energy balance is inaccurate because the low ventilation weakens the assumption to neglect heat conduction. The reason for this sur- prising result is the exponential dependence of satu- ration vapor pressure on temperature. Because of its low thermal conductivity, the temperature of the dry soil layer is high. Ventilation slightly lowers the soil temperature, which sharply reduces the vapor pres- sure gradient across the dry layer. This process reduces the evaporative driving potential To meet the law of energy conservation, the sum of sensible and latent heat fluxes must equal the difference between net radiation and soil heat flux. If ventila- tion increases the sensible heat flux more than it decreases soil heat flux (Table 31, then the latent heat flux must decrease. A theoretical transpirational cooling model, when applied to a crop with high stomatal resistance, also predicts a lowering of tran- spiration when ventilation increases (Fuchs, 1993). For a stoma&l resistance below a critical value, the model predicts that transpiration increases with ven- tilation, as normally expected.

7 50

i;; -1: ._

- :.: :..._.:

2 35-

k 30-

z 25

i;;:Y$+_; .

OI :

3 :- % 2.0 2.2 2.4 2.6

WIN0 SPEED (m s-1)

Fig. 3. Correlation between wind speed and air-exchange resis- tance in a polyethylene-covered greenhouse with a dry bare sandy soil. North and south end walls are rolled up. Wind is from the northwest direction. The point in the dotted frame corresponds to data in Figs. 5 and 6 (see text).

280 M. Fuchs et al. /Agricultural and Forest Meteorology 86 (1997) 273-282

Ventilation in the open treatment relies on the dynamic pressure generated by the natural wind, and buoyancy resulting from temperature and humidity gradients. Fig. 3 illustrates the negative correlation between the wind speed and the exchange resistance for the open treatment. Each data point represents a 600 s average of the measurements. Despite the narrow range of wind speeds recorded during the midday measurement period, the data show a strong link between exchange resistance and wind (r = -0.435 at p = 0.12). Over the wind speed range of about 1.9-2.6 m s-‘, the increase in air exchange rates doubled, from 30 to 60 h-l. Fig. 4 illustrates the effect of external wind on the ventila- tion obtained when fans operated. Correlation coeffr- cients are positive, indicating that wind, which blew on the end wall through which the fans pulled the air, slightly decreased the ventilation efficiency. Yet the low statistical significance of the correlation does not allow us to generalize this result.

Movement of air should affect the value of the internal boundary-layer resistance above the soil sur- face. Fig. 5 suggests a linkage between the exchange resistance and the internal boundary-layer resistance of the air above the bare soil. Except for one point, the data show an apparently high correlation. Yet, the correlation analysis is not applicable because the

WIN0 SPEED (md)

Fig. 4. With fan forced ventilation, the correlation between wind speed and air-exchange resistance is postive, but weak. The polyethylene-covered greenhouse with a dry bare sandy soil has fans on the north gable and the opposite end wall is rolled up. Wind is from the northwest direction.

- ‘40 T E k 1 . ??

AIR EXCHANGE RESISTANCE (8 ml)

Fig. 5. Apparent correlation between air-exchange resistance and internal boundary-layer resistance above the soil in a poly- ethylene-covered greenhouse with a dry bare sandy soil. North aud south end walls are rolled up. Wiid is from the northwest direction. The point in the dotted frame corresponds to the points marked in Figs. 3 and 6 (see text).

two variables share the same value of H, the sensi- ble heat flux density. The odd point in Fig. 5 corre- sponds to the datum with the lowest recorded wind speed (dotted frame of Fig. 3) and to the data with lowest soil to air temperature difference (dotted frame of Fig. 6). This coincidence corroborates the result in Fig. 2 that the soil-to-air tempemmre gradient in- creases with ventilation. The range of variation of the internal boundary-layer resistance was only 35% as compared with lOO%, excluding, or 30096, includ- ing the odd point in Fig. 5, for the exchange resis- tance. This confirms the weakness of the coujGng between ventilation and internal boundary-layer sug- gested by the analysis of Fig. 2. Further direct investigations of the relation between intemal resis- tance and air flow would require im@ex&nt mea- surements of ventilation rates, such as those obtained with tracers and air flow visuahxation.

The data contain additional information on the factors affecting the intemal resistance. Recause of thesteadyvahleof Ndurmgthemiddayperiodas quantified by a low coefficient of variation of 10.3% the temperature elevation in the greenhouse increases linearly with air exchange resistance. The correlation coefficient was 0.872, significant at p = 0.00005~ with a zero offset of 1.7”C (Fig. not shown). Accord- ing to Eq. (5) a similar relation should hold between

M. Fuchs et al./Agticultural and Forest Meteorology 86 (1997) 273-282 281

Tsoi~ in - Tair in V.7 Fig. 6. Relation behveen soil-to-air temperature difference in a greenhouse and resistances to heat transport in a polyethylene- covered greenhouse with a dq bare sandy soil. North and south end walls are rolled up. Wmd is from the northwest direction. The point in the frame corresponds to the points tnarked in Figs. 3 and 5 (see text).

the internal boundary-layer resistance and the soil-air temperature gradient, but the correlation between these two variables was not significant (Fig. 6, solid squares). A possible explanation of this result is that the very steep temperature gradient generates inter- mittent localized heat convection cells. The experi- mental evidence of Aubinet (1991) for the existence of convective cells above heating pipes, lends credi- bility to this hypothesis. Convective cells could also explain why the temperature and the vapor pressure measured at 2 m were systematically higher than at 1 m above the ground (Table 2). Businger (1963) also postulated large convective cells, induced by the external wind, to explain the air temperature and humidity inversions, and the spatial distribution of air temperature he recorded in a large, multi-span greenhouse. As in our experiment, the ground was the major surface, with the highest temperature and was the only source of water vapor in the green- house, another mechanism for counter-gradient flow across the inversion layer is difficult to conceive. Unfortunately, neither the thermometer time con- stants nor the sampling procedures were suitable for the acquisition of direct evidence in support of the hypothesis of coherent convective cells.

The data show a strong correlation between the soil-to-air temperature difference and the resistance to air exchange (Fig. 6, hollow circles). This result suggests that buoyancy forces drive the air exchange, via the vents of the greenhouse. Yet, despite the highly significant correlation (r = - 0.749, p = 0.002), it is unlikely that buoyancy alone could account for the entire range of variation of air-ex- change resistance, because the large temperature dif- ferences are confined within a narrow range. Air exchange resistance and soil-to-air temperature dif- ference are both implicit variables, influenced by the outdoor wind, which is an independent variable. The confounding effect of the wind appears through the correlation coefficient of -0.435 with a rejection probability of 0.12, between wind speed and ex- change resistance shown in Fig. 3. The correlation coefficient between soil-to-air temperature difference and wind speed is 0.467 with a rejection probability of 0.092. As both correlations are weaker than that found in Fig. 6 (left-hand y-axis), the data provide statistical evidence of heat transport enhancement by buoyancy in the greenhouse with passive ventilation. Forced ventilation should weaken the effect of buoy- ancy, and the two-fan treatment results support this hypothesis: the correlation coefftcient between soil- to-air temperature difference and air-exchange resis- tance drops to a non-significant value of -0.282 with a rejection probability of 0.350.

5. Conclusions

The conclusion that emerges from the comparison of the ventilation methods is that rolling up opposite walls to generate a natnral flow of air is the most effective. However, the sensitivity of this method to wind speed restricts its dependable application to regions with regular winds. Forced ventilation with exhaust fans operated with low efficiency and the air exchange rates of the fans were below their specified performance, suggesting that the pressure drop across the insect-proof nets was large. External wind had a non-significant effect on the performance of the fans.

The high temperature of the dry bare soil created larger buoyancy effects than those expected over

282 hf. Fuchs et al./Agriculhu+al and Forest Meteorology 86 (1997) 273-282

vegetation where latent heat prevents the develop- ment of large temperature gradients. Therefore, the results found here may not be directly applicable to a greenhouse with a large transpiring biomass. Never- theless, the experiment represents the conditions of a newly planted greenhouse with sparse vegetation (Seginer, 1994).

The small size of the experimental greenhouse is another factor that restricts the applicability of the results to situations encountered in commercial oper- ations. The ratio between vent areas and greenhouse volume, and the effects of the fans decrease with the length of the ridge axis. Extrapolation of the results on ventilation efficacy to a greenhouse with longer ridges would require careful scaling of the drag forces.

A high ground surface temperature enhances radi- ation emission. Crops with sparse canopies intercept part of this radiation and, therefore, bear an addi- tional heat load. Increased ventilation and shading would lessen this stress. The large contribution of soil heat flux to energy dissipation is another major characteristic of the empty greenhouse. Ventilation decreases the relative soil heat flux density. The resulting change in heat partition between soil and air may ease or exacerbate the heat load on sparse vegetation, depending on ventilation and stomatal resistance (Seginer, 1994).

In spite of the soil dryness, evaporation remained a significant part of the energy balance. The high temperature of the soil elevated its vapor pressure, enabling the large vapor diffusion resistance of the dry layer of soil to be overcome. Interestingly, in- creasing the ventilation rate, because it lowered soil temperature and increased the sensible heat flux, slightly diminished evaporation. By contrast, ventila- tion enhanced the sensible heat flux, but surprisingly also increased the soil-to-air temperature difference, proving that convective air exchange had a minor effect on the internal boundary-layer.

Acknowledgements

This work was supported by the United States- Israel Binational Agricultural Research and Develop- ment Fund, under Project No. IS-1816~90R.

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