Effect of covering materials on energy consumption and greenhouse microclimate

AGRICULTURAL AND FOREST METEOROLOGY Agricultural and Forest Meteorology 82 (19%) 227-244

Effect of covering materials on energy consumption and greenhouse microclimate Y. Zhang a, L. Gauthier b**, D. de Halleux b, B. Dansereau a, A. Gosselin a a Dept. de phytologie, Centre de Recherche en Horticulture, Universitk Laval, St-Fey, Que’. GIK 7P4, Canada b Dept. de ghnie rural, Centre de Recherche en Horticulture, Universite’ Laval, St-Fey, Que’. GIK 7P4, Canada

Received 16 May 1995; accepted 5 December 1995

Abstract The objective of the study was to conduct an extensive energy and microclimatic assessment of different greenhouse covering materials. Single glass (CL) and three types of double polyethylene (PE) claddings were compared. The double polyethylene cladding consisting of an anti-fog thermal film for the inner layer and a standard PE film for the outer layer was the most energy efficient. It had an average measured heat transfer coefficient (U value) of 2.9 W me2 K- ‘. The average U value for the other PE cladding was 3.4 W m ~’ K- ’. The use of thermal screens in the PE houses during the night reduced heat loss rates by 23-24%. The differences in climates under different claddings is presented in terms of PAR transmission and humidity levels. The measured average PAR transmission during the winter months (Novembe-March) were 0.68, 0.62, 0.65 and 0.60 for glass, anti-fog 1-year, anti-fog 3-year and anti-fog thermal claddings, respectively. In the summer months (April-October) the values were higher. The average vapour pressure deficit in the double PE houses was found to be 0.2 kPa lower than under single glass during the winter season, but no significant difference was observed between various anti-fog films. The use of a thermal screen in a double PE houses caused only a slight increase in greenhouse humidity. The contribution of supplementary lighting to greenhouse heating demand is also presented and discussed.

1. Introduction Greenhouse heating costs in the central part of Canada represent 30% to 40% of total production costs (Lavoie and Dufour, 1994). For this reason, double cladding, infrared

* Corresponding author. 0168.1923/96/$15.00

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PII SO168-1923(96)02332-5

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absorbing polyethylene coverings and removable thermal screens are used as means to decrease energy consumption. Substantial energy savings (2540%) have been obtained by these methods (O’Flaherty and Maher, 1978, 1980; Weimann, 1989; Ferare and Goldsberry, 1984; Magnani, 1987; Lagier, 1991). However, decreased light (10% to 30%), increased humidity conditions (Nisen, 1979; Nisen and Coutisse, 1981) and loss of production and quality (Steinbuch and de Vooren, 1984) were also reported. A PE film that is covered with a continuous water layer is more transparent to solar radiation than a film that induces the formation of droplets (Jaffrin and Urban, 1990). Therefore, the light transmission of double cladded greenhouses is improved by using anti-fog film for the inner layer (Lagier, 1991). It is only during the last ten years that films with satisfactory anti-fog characteristics have been commercially available. Hence, studies on these films are very limited. The PAR transmissivity of some anti-fog films was investigated under conditions that are different from these encountered during winter in central Canada or in double cladded greenhouses (Weimann, 1985; Jaffrin and Urban, 1990; Jaffrin and Morisot, 1994). Long-term studies on the thermal properties of anti-fog films, on the other hand, are more limited. Furthermore, most of the studies related to the energy saving aspects of anti-fog properties were carried out without a plant population (e.g. Nijskens et al., 1984). For this reason, a three-year greenhouse study was undertaken to determine the thermal properties and PAR transmission coefficients of three double anti-fog PE coverings and their relative values to a single glass covering, as well as humidity conditions under each cladding during several plant cycles. The effect of ageing on the thermal and transparency properties of films and the effect of thermal screens and supplementary lighting on greenhouse energy consumption are also discussed.

2. Material

and methods

2.1. Experimental

design

The experiment was carried out from January 1991 to July 1993 in a multi-span greenhouse located at Lava1 University, Quebec, Canada (46”47’ N, 71”6’ W>. The greenhouse, oriented Northeast-Southwest, was covered with double polycarbonate walls (8 mm thick) and double air-inflated polyethylene roofs (200-250 mm apart at centre). Experiments were conducted in four separate and contiguous compartments (6.0 X 6.4 m) situated in the Southern-most span of the greenhouse (Fig. 1). The Southeast wall of each compartment was exposed directly to the outside. The Northwest side was adjacent to an area in which tomatoes and cucumbers were grown (18°C at night and 21°C in the day). To eliminate the difference in energy consumption among compartments due to differing surrounding environments, 3.0 m buffer zones were kept at both ends of the span. Double polycarbonate plus an opaque plastic film was used to separate the different compartments. Compartments 1 and 4 were insulated from the buffer zone by a 50 mm Styrofoam panel. With this additional insulation, the I/ value decreased from 4 W rnb2 K- ’ for d ou bl e polycarbonate to 0.6 W mm2 K- ‘. Compartment 3 (GL) was cladded with single glass (4 mm>. The three other compartments were

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film

229

d

N

I

~1

0

vents HID lamps

I

Fig. I. Layout of the four experimental compartments and the location of instruments in the compartments. GL = single glass, AFL = PE + anti-fog 1-year PE, AF3 = PE + anti-fog S-year PE, AlT = PE + anti-fog thermal PE, HID = high intensity discharge (high pressure sodium vapour) lamps.

covered with double polyethylene. The outside layer was standard polyethylene (AT D3 40, AT Plastics Inc., Ont., Canada). For the inside layers, compartment 1 (AF3) had anti-fog 3-year PE (AT D3 AF80); compartment 2 (AFT) had anti-fog thermal PE (CIL DT 59); and compartment 4 (AFl) had anti-fog 1-year PE (CIL Dl AF40). When necessary the air space between the two covers was deflated to increase the melting rate of the snow. In each compartment, the air temperature was maintained between 18-20°C during the day and between 13-14°C during the night using a heating and ventilation system consisting of two electric unit heaters (7.5 kW each) and an extracting fan. Heating and ventilation were controlled by programmable digital thermostats (White-Rodgers, Emerson Electric Co., Ont., Canada). The fan jet insured constant air mixing and cooling by injecting outside air when necessary. A 50% aluminised thermal screen (Svensson LS-15, Inveka, Sweden) was automatically deployed above the plants to reduce heat loss from 17:OOh to 08:OOh in the winter and to reduce radiation from 11:OOh to 13:OOh in the summer. The heat saving effect of thermal screens was determined using a method proposed by Amsen and Nielsen (1988). Heat consumption was measured with thermal screens deployed at night two weeks out of four weeks. This method ensures that the site, house, heat control, heating system, and culture etc. are identical. The thermal screen was always deployed at night in GL. Several cut flower crops were grown in each compartment during the three year study. A supplementary lighting system consisting of four high intensity discharge (HID) high pressure sodium vapour lamps (400 W) ensured a photoperiod of 16 h (00:0&16:OOh) and a PAR level of 60 pmol m-’ s- ’ for one block of each species. The other block of each species was exposed only to ambient light conditions. 2.2. Instrumentation In each compartment, air temperature and relative humidity levels were measured using HMD 20YB (Vaisala, Finland) humidity and temperature sensors. The sensors

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were housed in an aerated box 1.5 m above the ground near the centre of each compartment. A Li-190SB (Li-Cor Inc., Lincoln, NE, USA) quantum sensor was mounted horizontally directly underneath the roof but above the thermal screen to measure photosynthetically active radiation (PAR) due to solar radiation. Four thermocouples (copper-constantan) were fixed to the covering material surface. Two were placed on the outside layer and two on the inside layer of the northern and southern sides. Humidity sensors were calibrated and the quantum sensors were cleaned regularly. Outside temperature and humidity, wind speed and wind direction, PAR, global radiation and infrared radiation were measured by devices made respectively by Campbell Scientific (Model 107, Logan, UT), Met One (Model 013 and 023, Grants Pass., OR), Li-Cor (190SB, Lincoln, NE) and Eppley (PIR, Newport, RI) and located on a weather station on the roof of an adjacent building. 2.3. Data collection

and analysis

Air temperature, relative humidity and PAR in the greenhouse and at the weather station were measured every minute and their average was recorded at 15min intervals. Data was collected using a CR 10 (Campbell Scientific, Logan, UT) in the greenhouse and a CR 21 in the weather station and transferred to a desktop computer every third day. Identical air temperature set points were maintained in all four compartments. Under the same temperature conditions, the energy used for heating in AFl, AF3 and AFT was compared with that used in GL, which served as a control. Since all compartments were subjected to the same climatic conditions, the relative heat retention ability of the three anti-fog polyethylene films could be evaluated. Data collected during three seasons (1991-1993) under the local winter climate (November-March) were used in the evaluation. In 1991, the temperature in the buffer zone beside AFl was not controlled. Hence the data in AFl for 1991 were not used. Since no thermal screen was used during the day, daytime energy consumption represents an overall average for the whole experimental period while the night-time heating was divided into two parts according to whether or not a thermal screen was deployed in the double PE houses.

3. Results and discussion 3.1. Energy consumption Air temperatures were maintained at similar levels in the four compartments (Table 1). The differences in average air temperature between compartments did not exceed 1°C during the night and 2°C during the day. Average relative humidities in GL was slightly lower than in the PE houses and there was no significant difference in relative humidity levels between PE houses. Therefore, the effect of humidity on energy consumption was not considered in this study. Due to the effect of solar radiation on heat balance during the day, day and night heat consumptions are discussed separately. According to previous studies, the heat supplied by supplementary light cannot be neglected (Brault, 1988; Thimijan and Heins, 1983).

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Table I Average and standard deviation of air temperature CT,) and humidity (VPD) (November-March) in the four greenhouses with different cladding materials

the winter

season

VPD (kPa)

T, (“C) GLa

during

231

AFl

AF3

AFT

GL

AFI

AF3

AFT

18.9+1.2 19.5fl.l

18.7kO.4 19.3k1.3 20.3k1.6

18.7kO.6 19.2+1.2 20.1k1.6

0.87*0.15 0.9.5+0.16 1.2OkO.14

0.82kO.18 1.07kO.14

0.77f0.15 0.82f0.19 1.13rtO.18

0.73+0.13 0.82+0.18 1.05+0.16

14.5+0.6

14.7kO.6

0.62f0.14

-

0.60*0.12

0.60+0.12

14.8zkO.6 l4.3kO.6

14.8+0.6 14.1kO.9

0.72kO.16 0.87f0.16

0.68+0.18 0.871tO.19

0.62kO.19 0.82f0.14

0.68+0.18 0.82+0.18

Day (08:OO - 17:OO)

1991 1992 1993

18.5*0.6b 19.25 1.0 20.2k 1.6

Night (17:OO - 08:OOh) 1991 14.6kO.7 1992 14.8*0.5 14.8+0.5 1993 14.6+0.7 14.OfO.8

a CL = single glass; AFl = PE + anti-fog thermal PE; PE = polyethylene. b Mean value f standard deviation.

l-year

PE; AF3 = PE + anti-fog

3.year

PE, AFT = PE + anti-fog

To eliminate this effect, the energy consumptions from 17:OOh to 24:OOh (when no supplementary lighting was employed in the greenhouse) are used to compare night-time energy consumption for four cladding materials. Due to yearly variations in outside climate, it is not possible to make comparisons between years. It is possible, however, to compare covering materials when they are subjected to the same conditions. In this study, the glasshouse (GL) was used as a reference treatment. Our comparative study showed that during the day, when no thermal screens were used in any of the compartments, the relative heat consumption of AF3 and AFT over GL was in the range of 52-56%. For AFl, the ratio was 68-73% (Table 2). Therefore, compared to a single glass cladded roof, energy savings of 44 to 48% were observed when anti-fog 3-year or anti-fog thermal films were used as the

Table 2 Average energy consumption (MJ) during cladding materials

the day (08:00-17:OOh)

in the greenhouses

covered

GL’

AFl

AF3

AFT

with four

1991 (January,

February,

March, December)

117.8 (lOO%)b

-

66.8 (56%)

66.6 (56%)

1992 (January,

February,

March, December)

85.7 (lOQ%o)

58 (68%)

:4;,

44.9 (52%)

1993 (January,

February,

March)

94.1 (100%)

68.8 (73%)

52.3 (56%)

52.5 (56%)

’ GL = single glass; AF1 = PE f anti-fog thermal PE; PE = polyethylene. b Percentage relative to single glass.

l-year

PE; AF3 = PE + anti-fog

3.year

PE; AFT = PE f anti-fog

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Table 3 Average energy consumption (MJ) during the night (17:00-24:OO) when a thermal screen was used in the GL compartment only (A) and in all the compartments (BJ in the greenhouses covered with four cladding materials A. Thermal screen in GL only GL+TS 1991 (March 7-29; December 1- 15) 1992 (January 15-29; February l-16; March 27-29)

AFl

AF3

78.1 (100%) b 89.6 (lOO%c)

a

(124%) 122.3 (136%)

96.8 (109%ro) II 1.6 (125%)

AFT

96.1 (107%)

GL+TS

AFI +TS

AF3 + TS

AFT+TS

@9%/o) 84.6 (94%)

125.0 (73%) 80.0 (89%)

85.0

B. Thermal screen in all the compartments

1991 (January 13-30; February 2-24; December 16-30) 1992 (January l-14, February 17-28; March I-14; November 2- 15; December 16-30) a GL = single glass; AFl = PE k anti-fog thermal PE; PE = polyethylene. b percentage relative to single glass.

140.1 (100%) 90.4 (lOO%J

1-year PE; AF3 = PE + anti-fo g 3-year

101.8 68.9 (76%)

PE; AFT = PE + anti-fog

inner layer of the roof in a PE house, while a saving of 27 to 32% was obtained when an anti-fog l-year film was used. During the night, when there was no thermal screen in the PE houses, 36-64% more energy was consumed in AFl and 24-40% in AF3 (Table 3(A)). In AFT, heat consumption was 7-28% higher than in GL depending on the experimental period. When thermal screens were also used in double PE houses (Table 3(B)), energy consumption in AFl and AF3 were 89-106% that of GL. In AFT, the percentage was reduced to 73-88%. The greatest energy saving effect from double PE houses was obtained in the first two years. During this period, energy consumption in AFT was the closest to GL combined with thermal screen (107- 109% of GL). In the third year, the relative energy consumption in the PE houses increased significantly. This can be explained by the fact that the anti-fog property of the film degrades with time. It has been shown (Nijskens et al., 1984) that the presence of a film of water (due to condensation) on a polyethylene film acts as an IR barrier and thus has significant impact on the thermal characteristics of claddings. It has also been demonstrated (Jaffrin and Morisot, 1994) that the anti-fog property of films degrades with time due to the leaching effect of condensation. Hence, a degraded film will not benefit as much from the presence of condensation, since the later will be in the form of droplets rather than in the form of a film of water. The greater energy consumption in AFl compared with AF3 may be due to the poorer anti-fog property of this film. The anti-fog property of AFl is expected by the manufacturer to last for only one year, while the anti-fog property of AF3 and AFT is expected to last for two years. Since the greatest energy saving was obtained in AFT, it is reasonable to assume that the energy saving effect of AFT comes from both the

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Table 4 Average 24 hours energy consumption (MJ) during the period when a thermal comoartment only and (B) in all the comuartments

233

screen was used (A) in GL

A. Thermal screen in GL only GL+TS 1991 (March 7-29; 1992 (January

December

1- 1.5)

15-29; February

1993 (February

16-28;

I-16;

March 27-29)

March l-15)

a

AFl

AF3

195.9 (100%) b 175.3 (100%) 175.4 (100%)

(84%) 180.3 (103%) 202.3 (115%)

GL+TS

AF1 +TS

163.6 (77%) 158.0

AFT 151.6

(90%) 166.3 (95%)

141.0 (80%) 156.4 (89%)

AF3 + TS

AFT+TS

B. Thermal screen in all compartments

1991 (January 13-30; February 2-24; December 16-30) 1992 (January l-14; February 17-28; March 1-14; November 2- 15; December 16-30) 1993 (January l-30; March 16-28) a GL = single glass; AFl = PE f anti-fog thermal PE; PE = polyethylene. b percentage relative to single glass.

l-year

257.9 (100%) 176.1 (100%)

(74%) 142.6 (94%)

191.0 (65%) 126.4 (72%)

113.8 (65%)

177.9 (lOO%o)

147.5 (94%)

141.1 (79%)

125.9 (71%)

PE; AF3 = PE + anti-fog

168.4

3-year PE, AFT = PE f anti-fog

thermal and anti-fog properties of the film in the first two years and mostly from the thermal property in the third year. APT performed well for two years with a slight degradation in the third year. Whole day energy consumptions in AF3 and AFT (Table 4(A)) were lower than in GL (5- 16% reduction for AF3 and 1 l-23% reduction for AFT) even though a thermal screen was deployed in GL during the night. When a thermal screen was also deployed in the PE houses during the night (Table 4(B)), whole day energy consumptions in all the PE houses were always lower than in the glasshouse (reductions of 6% for API, 21-28% for AP3 and 29-35% for AFT). The difference in whole day energy consumption between 1991 and 1993 was not as significant as that between night energy consumptions which means that whole day energy consumption was less affected by the degradation of the anti-fog property. This is probably due to the fact that in the daytime there was much less (if any) water condensate on the film surface. 3.2. Experimental

determination

of U values

The thermal property of greenhouse glazing materials is usually quantified transfer coefficient, which is calculated with the following equation: UC-

Q AJT

by a heat

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where U is in W rnp2 K- ’ and represents the rate of heat loss through a surface due to one degree of temperature difference between inside and outside air. Q is the total heat loss rate in W, A, the area of cladding surface in m*, and AT the temperature difference in “C between inside air and outside air. To compare the thermal properties of single glass and double anti-fog films obtained in this study with related studies, CT values were calculated from energy consumption measurements according to the following equation: u =

r

Q- &,4v(AT+A~i) A,AT

where Q is energy consumption rate measured between 22:00-24:OOh (no HID lighting) on climatically stable nights. A stable night was defined as a period during which the outside temperature fluctuation is less than 1°C and the heating demand is greater than zero. lJ, and U, are the U values of roof and wall materials, A, and A,,, are the area of the roof and wall exposed to the outside or the adjacent span, ATi is the temperature difference between the inside air of experimental compartments and the adjacent span. This equation is based on the following assumptions: 1. there was no heat exchange between compartments, 2. there was no heat storage, and 3. heat loss through infiltration was negligible. In all greenhouses, natural air exchange rates can be very low during subfreezing conditions since the leaks become sealed with frozen condensate (ASAE, 1993). A value of 3.5 W m-* K- ’ for a double polycarbonate wall (Nijskens et al., 1984) was used for CJ,,, in this study. The advantage of this method, compared to laboratory experiments, is that U values could be determined in conditions where condensation was present and thereby the effect of the anti-fog property on heat retention could be evaluated. A study by Chaibi (1991) has shown that neglecting condensation in the evaluation of U values resulted in a poor correlation between the theoretical and measured values. Average U values of AP3 and AFT were 3.4 and 2.9 Wmw2 K-‘, respectively (Table 5). The U value for AF3 was decreased, on the average, by 0.6 Wmm2 K- ’ compared to standard double PE (ASAE, 1993) indicating the importance of anti-fog properties of the materials on heat retention for double PE claddings. The U value for AFT, on the other hand, was close to the ASAE value for double IR PE. Therefore, the use of thermal film resulted in a decrease in U values in the order of 0.4-0.5 W rnp2 K- ’. This reduction due to thermal film is relatively less important than that for

Table 5 U values of the different Greenhouse

covering

greenhouse

covering

material

AF3 AFIAF3 + TS AFT+TS a ASAE (1993). TS: thermal screen. db: double.

materials

determined

CI values(Wm-2

from energy consumption

K-‘)

Measured

ASAE a

3.4 2.9 2.6 2.2

4.0 (db PE) 3.0 (db PE IR) 2.5 (db PE + TS)

f * + *

0.87 0.58 1.35 0.76

measurements

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m

235

GL+TS

----AFT

y = 0.16x + 2.61

r2 = 0.36

0.20x + I .58 0.20x + 1.97

AFT

? = 0.85 1.2= 0.84

GL+TS AFT

01234567012345678

Fig. 2. Effects of outside wind speed on U values (Wm-’ skies (B) for the different covering materials.

K- ‘) measured

under clear skies (A) and cloudy

double standard PE cladded greenhouses (decrease of 1.O W m-* K- ’ from the use of thermal film) suggested by ASAE. It can be concluded that an IR barrier in an anti-fog film is relatively less effective than when it is added to a standard PE. Eq. (1) assumes that U values are not affected by weather conditions. Our results (Fig. 2) indicate that, as expected, wind speed and sky conditions (clear or cloudy sky) do affect U values. Fig. 2 shows that under clear skies (Fig. 2(A)) heat loss rates are generally higher than under cloudy skies (Fig. 2(B)). It also shows that sky conditions have the greatest effect on AF3 and the least effect on GL + TS. 3.3. Effect of thermal screen on energy consumption To study the effect of thermal screens, Amsen and Nielsen (1988) suggested that 20 observations are sufficient and that year round or at least winter experiments are required. In this study, data for 140 nights covering three winter seasons were used. With a thermal screen, U values decreased to 2.6 and 2.2 Wm-* K-’ for AF3 and AFT, respectively (Table 5). It was found that the use of a thermal screen in AF3 and AIT resulted in a decrease of 0.7-0.8 Wm-’ K-’ in U values, indicating a reduction of 23-24% in the heat transfer coefficient. If the U values suggested by ASAE are used, a thermal screen in a standard double PE house causes a reduction of 37% in the heat transfer coefficient. Previous studies have shown that under a single glass cladding, reductions of 58% in heat loss was achieved with aluminised plastic film (Bailey, 198 1). Our own observations with aluminised thermal screens indicate that the energy saving effect of thermal screens in double anti-fog PE houses are lower than the value given for single glasshouses and double standard PE houses. These results suggest that, as expected, the greenhouse with the lower heat retention property benefits the most from a thermal screen.

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The difference between relative energy consumption (Table 3) in 1991 and 1993 was at the same level (15-19%) regardless of thermal screen. This indicates that the ageing effect of the materials and the effect of outside climate on heat consumption of the greenhouse were not affected when a thermal screen was deployed. 3.4. Effect of supplementary

lighting on energy consumption

The contribution of supplementary lighting to greenhouse heating is important in predicting total energy consumption costs. We define the fraction of useful heat generated by the lamps (Fr,$,> and the proportion of heating load contributed by the lamps (Pr,,J as:

QT-QH QT

pruse =

(4)

where Qr is the total heating load (kW) measured from 22:00-24:OOh, Q,, is the power absorbed by the electric heater (kW) measured from 01 :OO-03:00h, and Q,_ is the power absorbed by the lamps (kW) calculated from Ol:OO-03:OOh. Since supplementary light was applied from 0O:OOh to 16:OOh in this experiment, the average heating loads (kW) between 22:OOh and 24:OOh (when no HID lighting was used) and between 01:OOh and 03:OOh (when HID lighting was applied) were calculated and compared for each of the selected days. The period from 0O:OOh to 01:OOh was dismissed to avoid crossover effects. Also, nights on which the amplitude of the variations in outside temperature and wind velocity between the two periods were respectively less than 1°C and 4 kmh-’ were chosen for the comparison. In this analysis, the underlying assumption is that the heating load did not vary during the selected nights and that Qr represents the heating load that prevailed during the whole period. Thus, in Eq. (31, (Qr - Q,> g’tves the difference between the heat load with and without HID lighting applied, Fruse gives the fraction of the power absorbed by the lamps that serves as useful heat and Pruse gives the proportion of the total heating demand supplied by the HID lamps. When AT was around 10 to 12°C Fruse was zero regardless of covering materials (Fig. 3), indicating that no heat generated from the lamps was used in heating the greenhouse. This can be explained, in part, by the fact that heat was supplied by the adjacent span and by the greenhouse floor. Fruse increased to values ranging from 80% to 98% when the AT was greater than 20°C. At a AT of 15°C FruEe was equal to 50%. With a light level of 20 Wm-* PAR, Lawand (1983) reported an Fruse of 54% from September to May for double PE plus thermal screen cladding, which is close to the values at AT of 15°C with the light level of 12 Wm-* PAR used in our experiment. Pr,,, is also strongly temperature dependant (Fig. 4). When the temperature difference between inside and outside (AT) was between lo-12°C Pruse reached lOO%, which means that no heating from electric heaters was required beyond the heat supplied by supplementary lighting (23 W m-*1. Pruse decreased to 50% at a AT of 15°C and

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237

25

0 0

5

10

15

20

25

30

:

AT (‘C) Fig. 3. The fraction of the energy supplied by the HID lamps that is used for heating (Fr,,,) temperature difference between inside and outside air (AT) for different cladding materials.

further decreased to less than 30% when AT was greater than 25°C. difference was found for different covering materials. This result confirms of many commercial growers that the contribution of the HID lighting load is 25 and 30% (Mpelkas, 1981). With HID light levels of 30 W m-*

versus the

No significant the estimation to the heating (PAR) Brault

100 AF3+TS r2=0.50 - - - - AFT+TS r2=0.72

75

25

0 0

5

10

15

20

25

30

5

AT (“C) Fig. 4. The proportion of total heat demand that is supplied by the HID lamps (Pr,,,) difference between inside and outside air (AT) for different cladding materials.

versus the temperature

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(1988) presented Pruse values for various months of the year that vary between 17% and 57%. Lower Pruse was measured in the cold months. The dependence of Pruse on outside climate is more clearly demonstrated in this study when it is plotted versus temperature differences between inside and outside. It is interesting to note that when the temperature difference between inside and outside reached 10 to 12°C Fruse was zero and Pruse was 100%. During the night, three heat sources contributed to the heating of the greenhouse compartments: the electric heaters, the HID lamps, and the heat transfer from the adjacent span and from the greenhouse floor. Our measurements indicate that when AT is less than or equal to lo-12°C no heating from the electric unit heaters is necessary (PrUse = 100%) and no heat from the lamps is used (Fruse = 0%). Therefore the only factor that keeps greenhouse air lo-12°C higher than outside air is the heat flux from the adjacent span (which was kept 6°C higher than the experimental units) and from the greenhouse floor. In fact, White and Sherry (1981) have suggested that when the lamps (HID, 15 W m-* PAR) are on and the temperature difference is less than 12”C, some means of recycling excess heat was necessary. This AT threshold, however, may vary with the type of greenhouse structures (single or multispan) and the temperature set point of surrounding spans. 3.5. Transnlissivity

to PAR

The transmissivity to PAR measured during the cold season (November-March) was analysed separately from the warm season (April-October), since the transmissivity factor can be affected by incidence angles, frost and condensation. In fact, cladding transmissivity is of less importance during the warm period since most of the time, light is too intense and has to be reduced through the use of shading screens. Generally, single glass was the most transparent material followed by AF3 and AFl. The lowest transmissivity to PAR was measured in AFT. During the cold months, the transmissivities of GL, AFl, AF3 and AFT were around 0.68, 0.62, 0.65 and 0.60, respectively (Table 6). During the warm season, transmissivities of GL, AFl, AF3, and AFT were 0.80, 0.69, 0.71, and 0.66, respectively (Table 7). The difference in transmission coefficients between the cold and warm seasons can be explained by the difference in incidence angles (Magnani, 1987) and by the presence of frost and condensation. During

Table 6 Average PAR transmissivity Covering materials

GLa AFl AF3

of four cladding

materials during cold season (November-March)

PAR transmissivity

AVG versus GL (%)

1991

1992

1993

AVG

GL

0.69 + 0.05 b 0.66 * 0.01 0.68 f 0.01

0.67 k 0.03 0.61* 0.02 0.65 k 0.01

0.67 + 0.07 0.59 k 0.06 0.62 5 0.06

0.68 0.62 0.65

100 88-96 92-98

a GL: single glass, AFl: PE * anti-fog PE. b Standard deviation.

l-year PE, AF3: PE + anti-fog

3.year FE, Am:

PE+ anti-fog

thermal

Y. Zhang et al./Agricultural Table 7 Average PAR transmissivity

of four cladding

Covering materials

PAR transmissivity

GLa AFl AF3 AFT

0.80 f 0.70 + 0.7 1 f 0.65 +

1991

and Forest Meteorology 82 (1996) 227-244

materials

during the warm season (April-October) AVG versus GL (%o)

1992 0.03 b 0.02 0.01 0.0 1

a GL: single glass, AFl: PE f anti-fog PE. b Standard deviation.

239

0.79 0.69 0.72 0.67

1993 f f f +

0.03 0.02 0.02 0.02

0.8 1 + 0.67 f 0.70 + 0.65 f

I-year PE, AF3: PE f anti-fog

0.04 0.04 0.04 0.04

AVG

GL

0.80 0.69 0.71 0.66

100 84-88 86-91 SO-86

S-year PE, AFI? PE + anti-fog

thermal

the warm months, the incidence angle is closer to the normal angle and hence more favourable to light transmission. The difference in transmissivity to PAR between single glass and double PE coverings is smaller during the cold season (2-12%) than during the warm season (9-20%). This is due to the fact that a layer of frost (frozen condensate) is often present on the glass surface during the very cold periods. Fig. 5 presents an example of this phenomenon. It shows that for the day of 23 December 1992, when it was relatively - O.l”C>, the quantity of light measured in the warm (average daytime temperature: glass compartment was generally higher than that measured in the double PE houses. In contrast, for the day of 25 December 1992, when it was very cold (average daytime temperature: - 19.3”C) a reverse phenomenon was observed due to the presence of frost

0 GL H AFl I AF3

EAFT

z 3

3.0

3 % a

2.5

23 Dec. 92

24 Dec. 92

25 Dec. 92

Fig. 5. PAR (molday- ‘ ) in the four compartments between 23 December and 25 December 1992. GL = single glass; AFl = PE+ anti-fog l-year PE; AF3 = PE+ anti-fog 3-year PE, AFf = PE + anti-fog thermal PE; PE = polyethylene.

240

Y. Zhung

ef al./Apkulrurul

und Forest Meteorology 82 (1996) 227-244

and ice on the glass surface. On a double PE cladding, no frost accumulates on the inner layer since its temperature is much higher. Ferare and Goldsberry (1984) measured the transmissivity to PAR of double PE claddings between October and April in Colorado, USA and found an average value of 0.65. Their measurements were done with dry films (no condensation) and for PAR measured one meter above the ground surface. With the presence of condensation, our measurements in AFl for the first winter season and in AF3 for the first two winter seasons were equal to or slightly greater than their values. This situation could be explained by two possibilities: firstly, in our study, PAR was measured just beneath the roof rather than one meter above the ground surface; secondly, condensation on the surface of the anti-fog films in our study formed a continuous water layer rather than droplets, and according to Jaffrin and Urban (1990) a perfectly wettable film covered with a continuous water layer is equally or even more transparent than a dry film. van den Kieboom (1981) measured the transmissivity to PAR of a double standard PE cladded greenhouse. He obtained values in the range of 0.50 to 0.55 with an average of 0.53, which was 82% of the values they obtained for single glass cladding (0.65). In our study, the ratios PE versus GL were 96% for AFl the first year, 97-98% for AF3 during the first two years and 90-91% for AFT during the first two years (Table 5). The relative values are higher than those found by van den Kieboom (15-16%). According to Gilby (1989) water droplet condensate causes a 15% reduction in transmitted light. Hence, compared to double standard PE cladding our measurement with anti-fog claddings suggests again that during the first winter season for AFl and during the first two winter seasons for AF3, condensation on the surface of the films formed a continuous water layer rather than droplets. During the winter, the transmissivity to PAR decreased by 1 l%, 9% and 6% in three years for AFl, AF3 and AFT, respectively. The decreases in transmissivity observed in the second year for AFl and in the third year for AFT were relatively large when compared to other yearly variations in transmissivities. Transmissivity in AF3 declined by 4% per year during this three year experiment. During the warm season, the transmissivities remained at the same levels for the three years. Therefore, ageing of the materials in terms of PAR transmission was only observed in winter. This phenomenon suggested that it is the anti-fog or anti-droplet ability rather than the light transmissivity of the materials that decreased with time. The anti-fog characteristics for AF3 and AFT lasted for two years while it lasted for only one year for AFl. The transmissivities during the warm season for AFl and AF3 were similar to the observations of Weimann (1985) on inflated double antifog PE during June, July and August. The lower PAR transmissivity in AFT compared to other anti-fog films studied in this experiment supports the observation by Blom and Ingratta (1985) and by Simpkins et al. (1984) that infrared (IR) absorbing compounds in thermal films lead to a slight reduction in transmissivity in the visible spectrum. 3.6. Greenhouse

humidity

The comparison of vapour pressure deficits (VPD) under different cladding materials in the winter season is presented in Fig. 6. In this figure, the VPD values in GL are

Y. Zhang et al./Agriculnual

and Forest Meteorology 82 (1996) 227-244

241

1.6

PE

y=l.lx-0.25

12=0.85

0.6

0.2

0.4

0.8

0.6

1.2

1.0

1.4

1.6

1.6 1.4 --o_PPE+TS

y= 1.01x-0.06 y=l.O9x-0.12

;L=O.82 ?=0.97

1.2 1.0 k E

0.8

> 0.6

0.2 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Fig. 6. Comparison of vapor pressures deficit (VPD) in double PE greenhouses with that in GL during (A) day and (B) night when thermal screen CT.51 was applied in all compartments and thermal screen was applied in glasshouse only. On the x axis is the everyday VPD values in compartment CL during 1 December 1991 to 28 February 1992. On the y axis are the VPD values under PE houses during same period. Each point represents the mean of one day. The same symbols are used for the different PE films since the difference between them is negligeable. GL = single glass; PE = polyethylene.

taken on the X axis and VPDs in PE houses are taken on the Y axis. Due to the insignificant difference between VPD values under different PE claddings the symbols used to represent VPDs in AFl, AF3 and AFT are identical. Therefore, the line Y = X was considered as a standard to show the difference in VPD between PE houses and GL.

242

Y. Zhang et d/Agricultural

and Forest Meteorology

82 (1996) 227-244

During the day (Fig. 6(A)), the VPD varied between 0.4 and 1.6 kPa in all the compartments depending on outside temperatures. The VPD in PE houses was 0.2 kPa lower than in GL when VPD values were low. The difference decreased when VPD values increased. This is as expected since high VPD values are often due to important air exchange rates (as they occur in the summer). During the night (Fig. 6(B)), VPDs varied between 0.3 and 1.2 kPa. The VPD in the PE houses was about 0.1 kPa lower than in GL. These results agree with previous observations that humidity was higher (10% in RH) in insulated houses (double glass) than in single glass on sunny days during the winter months and during the summer months there were no differences in humidity levels (van Winden and van Uffelen, 1984; Steinbuch and de Vooren, 1984). When VPDs are between 0.3 and 1 kPa, no effect on the physiology and development of high value vegetable and ornamental crops grown under protected cultivation is observed (Hand, 1988). However, the growth of many crops is adversely affected when VPD is above 1 kPa (Hoffman, 1979; Bakker, 1990). VPDs above 1 kPa, which are rarely encountered in energy saving greenhouses during the winter, were observed on half winter days in this study. That is due to the very low outside temperatures resulting in high condensation rates. For insulated greenhouses in Quebec, inside low humidity rather than high humidity conditions should be monitored when ornamental plants are grown during the winter months. 3.7. EfSect of thermal screen on humidity levels When thermal screens were deployed in double anti-fog PE houses on winter nights, the VPD was decreased by 0.02 to 0.03 kPa, compared to the VPD values when thermal screens were not used (Fig. 6(B)). Thermal screens are expected to increase humidity in the greenhouse, since the condensation rate on the screen is lower due to an increase in the apparent temperature (Stanghellini, 1987). This effect, however, was not significant for double cladded greenhouses in this experiment.

4. Conclusions From this three year study, we can conclude that the anti-fog thermal film was the most effective and anti-fog l-year film was the poorest in terms of whole day energy savings. Double claddings with anti-fog 3-year or anti-fog thermal polyethylene as the inner layer can be used as an economical substitute for single glass combined with a thermal screen. However, since anti-fog 3-year polyethylene has the highest and anti-fog thermal polyethylene has the lowest transparency to incident PAR, anti-fog 3-year film can be considered the best polyethylene material when transmissivity to PAR is the most important factor. This research has shown that the anti-fog property of the covering materials is important not only for light transmission but also for heat retention in greenhouses. Also, the degradation of the anti-fog property results in a decreased transparency and an increased heat loss rate. This study also shows that the energy saving effect of thermal

Y. Zhang et d/Agricultural

and Forest Meteorology

82 (1996) 227-244

243

screens in double anti-fog PE houses is lower than its effect in other types of claddings and that the presence of a thermal screen causes no significant change in humidity levels in PE cladded greenhouses.

Acknowledgements The authors thank Mr. Andre Boisvert, Serge Gagnon, Fabien LabbC, Jean-Francois Goulet, RenC Pouliot and Rachel Daigle for their professional and technical assistance. The financial support for this work given by AT Plastics Inc., Industries Harnois, Ciba Geigy Canada, Ministere de 1’Energie et des Ressources du Quebec and L’Institut Quebecois de Developpement de 1’Horticulture Omementale, is gratefully acknowledged.

Appendix A. Equations

used for the best fit curves shown in Fig. 3 and Fig. 4

A.I. Fig. 3

GL + TS:

y=

-0.3x2

+ 18.0x-

145.7

AFT+TS:

y=

-0.3x2

+ 15.5x-

123.7

AFT+TS:

y=

-0.3x2+

15.9x-

129.4

10
GL+TS:

y= 4161.1~-‘~~*

AFT+TS:

y = 3049.0x-

AFT+TS:

y = 3082.1 x- ’.47

‘.49

lO
References Amsen, M.G. and Nielsen, O.F., 1988. Thermal screen in greenhouses: methods for estimation of heat saving effect. Tidsskr. Planteavl, 92: 363-366. ASAE, 1993. Heating, Ventilating and Cooling Greenhouses. ASAE Engineering Practice, ASAEStandards EP406.1, pp. 547-550. Bailey, B.J., 1981. The reduction of thermal radiation in glasshouses by thermal screens. J. Agric. Eng. Res., 26: 215-224. Bakker, J.C. 1990. Effect of day and night humidity on yield and fruit quality of glasshouse eggplant (Solanum melongerm L.). J. Hart. Sci., 65: 747-753.

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Blom, Th.J. and Ingratta, F.J. 1985. The use of polyethylene film as greenhouse glazing in North America. Acta Hortic., 170: 69-80. Brault, D.P., 1988. Contribution de I’eclairage HPS au chauffage d’une serre. M.Sc. Thesis. University of Laval, 99 pp. Chaibi, M.T., 1991. Coefficient of heat transfer from greenhouses heated by geothermal energy in South Tunisia. Plastic., 92: 41-48. Ferare, J. and Goldsberry, K.L., 1984. Environmental conditions created by plastic greenhouse covers. Acta Hortic.. 148: 675-682. Gilby, G.W. 1989. Greenhouse cladding film developments. Plastic., 81: 19-28. Hand, D.W. 1988. Effect of atmospheric humidity on greenhouse crops. Acta Hortic., 229: 143-155. Hoffman, G.J. 1979. Humidity. In: T.W. Tibbits and T.T. Kozlowski (Editors), Controlled Environment Guidelines for Plant Research. Academic Press, London, 413 pp. Jaffrin, A. and Urban, L., 1990. Optimization of light transmission in modem greenhouses. Acta Hortic., 281: 25-33. Jaffrin, A. and Morisot, A., 1994. Role of structure, dirt and condensation on the light transmission of greenhouse covers. Plastic., 101: 33-44. van den Kieboom, A., 1981. Light transmittance of ‘alternative’ greenhouse. Acta Hortic., 115: 105-l 12. Lagier, J., 1991. Choice of flexible cladding materials in relation to greenhouse and plants growth. Plastic., 90: 33-44. Lavoie, E. and Dufour, J.C., 1994. Les Clectrotechnologies pour le chauffage des serres. Option Serre, Mai 94: 22-23. Lawand, T.A. 1983. Electric illumination and heating for low energy greenhouse system, Final report. Engineering and Statistical Research Report on Contract File Number 23SU.01843-l-EC20. Magnani, G., 1987. Performance of different plastics films in double-clad greenhouse: four year research. Plastic., 73: 9-22. Mpelkas, C.C., 198 1. Greenhouse supplemental lightin g for roses with high-pressure sodium lamps. Acta Hortic., 128: 85-97. Nijskens, J., Deltour, J., Coutisse, S. and Nisen, A., 1984. Heat transfer through covering materials of greenhouses. Agric. For. Meteorol., 33: 193-214. Nisen, A., 1979. Transparence aux rayonnements des mat&iaux de couverture des serres et abris: constquences horticoles. P. H. M. Rev. Hortic., 147: 15-24. Nisen, A. and Coutisse, S., 1981. Photometric properties of double wall plastics used as covering for greenhouses. Acta Hortic., 115: 85-97. O’Flaherty, T. and Maher, M.J., 1978. Evaluation of double-clad polyethylene greenhouses for energy saving in early tomato production. Acta Hortic., 76: 335-339. O’Flaherty, T. and Maher, M.J.. 1980. Fuel consumption and crop performance in double-covered polyethylene greenhouses. Acta Hortic., 107: 81-85. Simpkins, J.C., Mears, D.R., Roberts, W.J. and Janes, H. 1984. Evaluation of an experimental greenhouse film with improved energy performance, Paper No. 84-4033. Am. Sot. Agric. Eng., Knoxville, TN. Stanghellini, C. 1987. Transpiration of Greenhouse Crops: an Aid to Climate Management. JMAG Press, Wageningen, The Netherlands, 150 pp. Steinbuch, F. and de Vooren, V., 1984. Production and quality of cutflowers and potplants grown in greenhouses covered with energy saving double layer materials. Acta Hottic., 148: 555-559. Thimijan, R.W. and Heins, R.D., 1983. Photometric, radiometric, and quantum light units of measure: a review of procedures for interconversion. HortScience, 18(6): 818-822. Weimann, G. 1985. Light transmissivity of different filmcoverings on greenhouses. Acta Hortic., 170: 143-146. Weimann, G., 1989. The characteristics of light transmissivity, heat consumption and condensation processes in different film greenhouses. Acta Hortic., 245: 133- 140. White, J.W. and Sherry, W.J., 1981. High intensity lighting and reflective thermal blanket combination for economy of energy. Acta Hortic., 128: 119-129. van Winden, C.M.M. and van Uffelen, J.A.M., 1984. Comparison of the effect of single and double glass greenhouses on environmental factors and production of vegetables. Acta Hortic., 148: 567-574.