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Comparison of calculated and measured performance of a fluid-roof and a standard greenhouse
Energy in Agriculture, 2 (1983) 75--89 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
75
COMPARISON OF CALCULATED AND MEASURED PERFORMANCE OF A FLUID-ROOF AND A STANDARD GREENHOUSE
J.-P. CHIAPALE 1 , C.H.M. VAN BAVEL 2 and E.J. SADLER 2
Station de Bioclimatologie, Institut National de Recherche Agronomique, Montfavet (France) 2 Texas Agricultural Experiment Station, Texas A & M University, College Station, T X 77843 (U.S.A.) (Accepted 8 March 1983)
ABSTRACT Chiapale, J.-P., Van Bavel, C.H.M. and Sadler, E.J., 1983. Comparison of calculated and measured performance of a fluid-roof and a standard greenhouse. Energy Agric., 2: 75--89. A side-by-side comparison was made in Avignon, France, during 1980, of two greenhouse types: a standard greenhouse with forced air heating and forced ventilation, and a fluid-roof house in which the temperature was controlled by water flowing over a roof made of infrared absorbing glass. Heat exchange with groundwater at 14°C kept the circulating water between 12 and 23°C. Both systems worked essentially as predicted by a simulation model on a cold day in April and a warm day in June. The model used measured meteorological data to calculate the control action and the actual conditions prevailing in both greenhouses. The predictions were adequate, except for a tendency to over-estimate interior humidities. The model was useful in suggesting that, in spite of essentially equal air temperatures, the conditions for plant growth were substantially different in the two greenhouses. In the fluid-roof house, there would be less light, a higher humidity, a much reduced transpiration, and, during the day, a somewhat lower CO 2 level and lower leaf temperature, all as compared to the standard glasshouse.
INTRODUCTION I t is e s s e n t i a l t h a t g r e e n h o u s e s p r o v i d e a p r o d u c t i v e e n v i r o n m e n t a t the least e x p e n d i t u r e of energy, b o t h for h e a t i n g a n d for cooling. Tradit i o n a l l y , e n e r g y r e q u i r e m e n t s h a v e b e e n e s t i m a t e d b y m e t h o d s t h a t correlate energy consumption to standard climate data and to horticultural requirements, with due regard to method of greenhouse construction and o p e r a t i o n . S u c h m e t h o d s are b a s e d u p o n m e a s u r e d p e r f o r m a n c e d a t a , and therefore, have direct application only in a restricted physiographic area. Also, t h e y c a n n o t be used to p r e d i c t the effect of n e w m a n a g e m e n t or construction techniques.
0167-5826/83/$03.00
© 1983 Elsevier Science Publishers B.V.
76 A more general approach is to consider the problem as a physical one: that is, to analyze the energy and mass flows in a greenhouse-crop-environm e n t system in detail and to calculate greenhouse performance without collecting data on an existing system, or prior to construction of an experimental facility. Particularly for the case of unusual designs or unusual environments, a theoretical m e t h o d can be of great help, even though it remains necessary to do experiments to verify predicted results before application in a practical situation. Several investigators have proposed such methods to analyze and predict greenhouse performance. There are a number of reports in which such mechanistic models have been verified, generally with satisfactory results. An early example is the work of Walker (1965), who used an energy balance, steady-state m e t h o d to predict temperatures in a greenhouse, as well as heating requirements. A more detailed computer model was developed by Selcuk (1971), who predicted air and plant temperatures with an accuracy of 1--2°C. The work by Takakura et al. (1971) is an example of a dynamic simulation model. It was verified by measurement of the relation between leaf and air temperatures, an important element in greenhouse engineering that is often overlooked. A dynamic simulation model was proposed by Soribe and Curry (1973), in which plant growth was calculated, as well as the conditions in the greenhouse. Tests with lettuce proved the potential of this approach. Other instances of agreement between conditions calculated from computer models and found in experiments are cited in work by Kimball (1973), Takami and Uchijima (1977), and Von Elsner (1982). We present here a test of a greenhouse modeling m e t h o d that is aimed to determine the potential advantage of a so-called fluid-roof greenhouse over a standard greenhouse. In a fluid-roof greenhouse, at least one half of the solar radiation, primarily the infrared fraction, is absorbed by the translucent roof and is stored or rejected, as the case may require, using a fluid that runs over or through the roof. For earlier reports on this approach to energy conservation in greenhouses, reference is made to Van Bavel et al. (1981). There, a more extensive review of the problem is given and the merits of the fluid-roof concept are analyzed with a numerical model, described in detail by Van Bavel and Sadler (1979), and given here, in outline, in the Appendix. Calculations and preliminary experiments did suggest that, given an adequate storage volume of the working fluid, such as 100 1/m 2 of greenhouse floor area, considerable savings in both heating and ventilation could be achieved. In that case, the average temperature of the storage system is close to the average daily outside air temperature, but the diurnal amplitude of the former is several times smaller than that of the latter. Later, it was proposed by Damagnez et al. (1981) that the fluid-roof principle would be particularly effective if the heat rejection or absorption by the roof were assisted by a heat exchanger that uses groundwater as a heat reservoir.
77 Parallel to the analytical and modeling research, an experimental program was started in Avignon, France, where a set of experimental greenhouses was constructed in early 1980, as described by Chiapale (1981). In this report, we present the actual performance of the fluid-roof greenhouse and t h a t of its conventional, glass counterpart, and the calculated performance of each, as obtained from the simulation model. METHODOLOGY A detailed description of the experimental installation in Montfavet near Avignon, France, has been given by Chiapale (1981). In brief, in a 1000-m 2 greenhouse, two compartments of 200 m 2 each were used for the present study. One had a standard glass roof, was heated with hot water air handlers situated above the crop, and was cross-ventilated with electric fans. The other was in every other way identical to the standard c o m p a r t m e n t , but it had a roof made of double-glazed panels, the lower glazing being infrared absorbing glass, 4.5 mm thick (KOA, Boussois S.A.) and the upper, clear glass, 4 mm thick. A layer of demineralized water was made to flow down-slope over the heat absorbing glass, when the inside air temperature, as sensed by an air thermostat, was above a m a x i m u m or below a m i n i m u m setpoint value. After this water drained off the glass, it was collected in a tank of 5000-1 capacity and thence pumped back to the roof ridge at a flow rate of 225 l/min, or 1.17 1/min per m 2 of greenhouse space. On its way back to the roof, the water passed through a heat exchanger with a capacity of 12 kW/°C. Groundwater from a shallow well was pumped through the other side of the heat exchanger, and returned to the aquifer. The groundwater had a constant temperature of about 14°C, close to the local mean annual air temperature. The tank temperature was held between 12 and 23°C by a dual t h e r m o s t a t that controlled the groundwater pump. By controlling the roof circulation pump from another dual thermostat, which sensed the greenhouse air temperature, the system prevented the latter temperature to exceed the limits judged suitable for crop production, in this case 13 and 23°C, respectively. Hence, excess solar radiation, primarily in the infrared part of the spectrum, was rejected into the groundwater. Undue heat loss to the environment, on the other hand, was made up by extracting heat from the aquifer. The storage tank acted as a buffer in this system, but its heat storage capacity was n o t significant, compared to typical diurnal gains and losses. The thermostats controlling the heating and ventilation in both greenhouses were set slightly outside the range of the roof pump thermostats, at 11 and 25°C, respectively. The purpose of the fluid-roof system is to reduce both the normal heating and ventilation requirements, if n o t to eliminate them. In a preliminary design simulation for the Avignon site, Damagnez et al. (1981) reported
78 that, indeed, the normal heating and ventilation action needed in the standard glasshouse would be essentially eliminated by the radiant heating and cooling provided by the fluid-roof design, support ed by the use of the aquifer as a source or sink for heat, and by the selective shading action o f the infrared absorbing glass. Here, we examine the quantitative agreement between calculation and measurement, rather than the qualitative adequacy of the c o n c e p t as already report ed by Chiapale {1981}. During the experimental period, 1 March--1 August 1980, a t o m a t o crop was grown in bot h greenhouses. Planted in full soil on 5 March, the crop yielded by 11 July, the final harvest data, 9.1 kg/m 2 in the normal glasshouse and 6.2 kg/m 2 in the fluid-roof house. F r o m the available record, 2 days of data were selected to make a detailed comparison of calculated and measured conditions for plant growth and climate control action. The first was 13 April 1980, a cold, clear day with outside temperatures below freezing during the early morning, followed by a slight warming trend during the afternoon. The second was 19 June 1980, a warm and humid day with clear skies, typical for the su mmer climate of the area. On bot h days, all greenhouse systems functioned as required, and a complete, error-free set of data was available every 30 min for the 24-h period from midnight to midnight. The following data were collected every 30 min by an automatic datalogger, as the 30-min average or the accumulated values, based on a data scan every 30 s. Solar radiation outside was measured with a p y r a n o m e t e r (Kipp en Zonen), windspeed with a cup a n e m o m e t e r , and outside dry and wet bulb t e m p e r a t u r e with an aspirated and shielded p s y c h r o m e t e r . Two additional psychrometers were used to measure the t e m p e r a t u r e and the h u m i d i t y inside each of the two test greenhouses. In addition, the t e m p e r a t u r e of the water stored in the tank of the circulation system in the fluid-roof solar greenhouse was measured. Finally, a record was obtained o f the a m o u n t of time the ventilation fans, the two pumps, and the heating system were running during each 30-min period and in each greenhouse. A simplified diagram of the m e t h o d of operation of each of the two systems in the cooling and in the heating m ode is shown as Fig. 1. Actually, there were two sets of ventilation fans, the first actuated at 25°C and the second at 28°C. The heating system was activated at 12°C. A more detailed diagram showing the water circulation system in the fluid-roof system is given as Fig. 2. The greenhouse p e r f o r m a n c e was calculated with the continuous simulation model described by Van Bavel et al. (1981), using as inputs the actually measured weather data and the initial conditions at the beginning of 13 April and of 19 June. The simulation model was used with the following significant modifications. Sky long-wave radiation was c o m p u t e d with the formula proposed by Idso (1981), and the infiltration rate was made d e p e n d e n t upon wind-
79 GLASS ROOF
DAY
R
! NIGHT
FLUID ROOF
t
Fig. 1. Simplified operating systems for the glass-covered and the fluid-roof greenhouse. In the first, daytime energy gain from solar radiation (R) is rejected by ventilation (V), while nighttime radiation loss (R) is made up by heating (H), to maintain air temperatures between 12 and 25°C. In the fluid-roof system, R is balanced by rejection or withdrawal of heat from the groundwater (UW), using a heat exchanger (HE) and pump (P2) to keep the temperature in the tank (T) between 12 and 23°C, and pump (P1) to circulate the water through the roof. speed, using a f o r m u l a derived b y Gac et al. ( 1 9 6 9 ) . T h e l a t t e r e x p r e s s i o n states: i n f i l t r a t i o n r a t e = 0 . 0 0 0 4 2 + 0 . 0 0 0 28 U0
(m3/(m2s))
in w h i c h U0 is the w i n d s p e e d ( m / s ) . A t 1 m / s , the i n f i l t r a t i o n rate is equiva l e n t to 1.0 air c h a n g e p e r h. A n u m b e r o f o t h e r f o r m u l a s have b e e n p r o p o s e d , b u t a p p e a r t o d i f f e r little. T h e d a t a base u s e d b y Gac et al. ( 1 9 6 9 ) was large a n d d e t a i l e d , t h o u g h t h e e x p r e s s i o n t h a t r e s u l t e d has a considerable r a n d o m u n c e r t a i n t y . T h e p r i n c i p a l p a r a m e t e r s describing t h e g r e e n h o u s e in t h e s i m u l a t i o n m o d e l w e r e t h e following: 0.75 t r a n s m i s s i o n b y glass r o o f ( 4 0 0 - - 7 0 0 n m ) ; 0.75 t r a n s m i s s i o n b y glass r o o f (700---2000 n m ) ; 0.40 t r a n s m i s s i o n b y f l u i d - r o o f ( 4 0 0 - - 7 0 0 rim); 0.05 t r a n s m i s s i o n b y f l u i d - r o o f ( 7 0 0 - - 2 0 0 0 n m ) ; 140 W / m 2 m a x i m u m p o w e r o f air h e a t i n g s y s t e m ; 32 air c h a n g e s p e r h c a p a c i t y o f f o u r v e n t i l a t o r s ;
80
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Fig. 2. D e t a i l e d d i a g r a m o f w a t e r c i r c u l a t i o n in t h e f l u i d - r o o f g r e e n h o u s e . A n air t h e r m o s t a t ( T 1 ) c o n t r o l s p u m p (P1), a n d a w a t e r t h e r m o s t a t ( T 2 ) c o n t r o l s p u m p (P2), in s u c h a way t h a t P2 can r u n o n l y if P1 does. H e a t e x c h a n g e r ( H E ) a n d b o t h p u m p s were a c t u a l l y l o c a t e d in t h e g r e e n h o u s e , as is t h e filter (F).
13°C controlled air temperature in heating mode; 24 or 27°C controlled air temperature in cooling-by-ventilation mode (two or four ventilators, respectively); 23°C controlled air temperature in cooling mode with fluid-roof circulation; 1.17 1/(m: min) capacity of fluid-roof circulation pump; 64 W/(m: °C) capacity of heat exchanger with groundwater. The values for transmission are based on actual measurements in the t w o structures, and include the effect of supporting structure and tubing suspended below the glazing. It may be noted that the light transmission by the fluid r o o f in the 400--700onm band was quite low. Replacement with a blue-green glass (V66, Boussois S.A.) in 1981 resulted in a higher light transmission, and in t o m a t o yields entirely comparable to those under normal glass, whereas the 1980 yields were significantly lower than in the glasshouse. In addition, however, it was also found in 1981 that both the irrigation frequency and the mineral nutrition need to be adapted to the climate in the fluid-roof greenhouse to obtain optimum results. RESULTS AND DISCUSSION
A comparison of calculated and measured performance of the climate control functions (heating, ventilation and fluid pumping) is given, as a
81
comparison of the calculated and measured greenhouse environment for plant production. These comparisons were made for two contrasting weather patterns. The first, on 13 April 1980, is shown in Fig. 3. The night temperatures approached freezing, and the atmosphere was generally dry. The pattern for the second day, 19 June 1980, is shown in Fig. 4. The actual performance of the system on 13 April is shown in Fig. 5, as well as its predicted behavior. The standard greenhouse required heating during the night until about 7 am to maintain an air temperature above 12°C. The hours during which heating occurred were accurately predicted, but not the duration, given as the number of minutes of heating during each hour. The discrepancy is likely due to the fact that the power-in-use of the heating system was not accurately known, as no wattmeters had been installed. The solar greenhouse, on the other hand, did not require any heating, since the air temperature was maintained by pumping the tank fluid through the roof. This action was accurately calculated, as shown in Fig. 5. 13 RPRIL 1980 30
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Fig. 3 (left). Hourly averages of solar radiation (W/m2), outside air and dewpoint temperatures (°C), and windspeed (m/s) at the Avignon greenhouse site on 13 April 1980. Fig. 4 (right). Hourly averages of solar radiation (W/m2), outside air and dewpoint temperatures (°C), and windspeed (m/s) at the Avignon greenhouse site on 19 June 1980.
82 13
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Fig. 5. M e a s u r e d (M) and calculated (C) values o f d u r a t i o n o f h e a t i n g a n d v e n t i l a t i o n in t h e glass r o o f g r e e n h o u s e , and o f r o o f w a t e r and g r o u n d w a t e r c i r c u l a t i o n in the fluidr o o f g r e e n h o u s e , all in m i n / h for every h o u r , o n 13 April 1980.
During the daytime, a small a m o u n t of ventilation was required to keep t h e glasshouse air t e m p e r a t u r e below 25°C. The actual ventilation occurred in two pulses, whereas the calculated action was m ore even, with the total ventilation volume being nearly equal. It should be poi nt ed o u t t hat the mo d el idealizes and smoothes the external and internal conditions, hence no p er f ect agreement in on- of f cont r ol can be expected. In the solar greenhouse, the ventilation was supplanted by fluid circulation, the action again being accurately calculated. On the warm day in June, shown in Fig. 6, no heating was required. During the day, a large a m o u n t of ventilation was measured as well as calculated f o r the glasshouse, and the quantitative agreement was reasonably good. In the fluid-roof greenhouse, the t e m p e r a t u r e was entirely controlled by pumping: no ventilation was measured. This action was almost exactly as calculated. In bot h greenhouses, ventilators or pumps were running c o n t i n u o u s l y during the afternoon, and air temperatures slightly exceeded the set points. We turn n o w t o the measured and calculated air temperatures and dewp o i n t temp er a t ur e s in the glasshouse, given in Fig. 7. The form er were in substantial agreement, but the d a y t i m e d e w p o i n t values were over-estim a t e d b y as m u c h as 10°C. In the fluid-roof house, the calculated and measured tem pe r at ur e s and dew-points, shown in Fig. 8, related in the same way as in t he glasshouse, but the overestimation of the d e w p o i n t was 5°C at the most.
83 19
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Fig. 6. Measured (M) and calculated (C) values of the duration of heating and ventilation in the glass roof greenhouse, and of roof water and groundwater circulation in the fluid-roof greenhouse, all in min/h for every hour, on 19 June 1980. Ventilation is maximally 120 min/h, as there are two separately controlled sets of ventilators.
I n e v a l u a t i n g b o t h sets o f d e w p o i n t d a t a , it m u s t be b o r n e in m i n d t h a t t h e r e was o n l y o n e p s y c h r o m e t e r in e a c h g r e e n h o u s e , l o c a t e d a t a b o u t 1 m a b o v e t h e g r o u n d , in t h e c e n t e r . H e n c e , a bias e r r o r c o u l d h a v e occ u r r e d , w h e r e a s in t h e m o d e l c a l c u l a t i o n , a h o m o g e n e o u s g r e e n h o u s e space was a s s u m e d . T h e cause o f a real d i s c r e p a n c y b e t w e e n t h e t w o sets o f d e w p o i n t values is a m a t t e r o f c o n j e c t u r e , b u t it is p r o b a b l e t h a t t h e vent i l a t i o n b y l e a k a g e was g r e a t e r t h a n a s s u m e d . N o d i r e c t m e a s u r e m e n t s o f t h e leakage o f t h e a c t u a l s t r u c t u r e w e r e m a d e , a n d a ' t y p i c a l ' value was u s e d in t h e calculations. A n o t h e r possible c a u s e f o r the f a c t t h a t m e a s u r e d h u m i d i t i e s w e r e l o w e r t h a n t h e c a l c u l a t e d o n e s is t h a t t h e soil surface b e t w e e n the p l a n t s was d r y , since a drip irrigation s y s t e m was used. In c o n t r a s t , t h e m o d e l a s s u m e d a c o n t i n o u s c r o p c o v e r a n d a c o n s t a n t l y w e t soil surface. F r o m a h o r t i c u l t u r a l v i e w p o i n t , we call a t t e n t i o n t o t h e f a c t t h a t t h e relative h u m i d i t y , as m e a s u r e d , was higher in t h e f l u i d - r o o f s t r u c t u r e t h a n in t h e glasshouse, p a r t i c u l a r l y in the early m o r n i n g hours. D e t a i l e d m o d e l analysis has s h o w n t h a t , d u r i n g t h e n i g h t and at e q u a l air t e m p e r a t u r e s , p l a n t t e m p e r a t u r e s are higher in the f l u i d - r o o f h o u s e t h a n in t h e glasshouse. S u c h a d i f f e r e n c e c o u l d e x p l a i n higher d e w p o i n t s a n d relative h u m i d i t y values. N o m e a s u r e m e n t s o f p l a n t or leaf t e m p e r a t u r e s w e r e m a d e to supp o r t this s u p p o s i t i o n .
84 13
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Fig. 7 (left). Hourly averages of measured and calculated values of the air and dewpoint temperature in the glass greenhouse on 13 April 1980. Fig. 8 (right). Hourly averages of measured and calculated values of the air and dewpoint temperature in the fluid-roof greenhouse on 13 April 1980.
The corresponding results for 19 J une are shown in Fig. 9 for the glasshouse, and in Fig. 10 for the fluid-roof house. In the glasshouse, good agreement existed between calculation and measurement. This result is n o t surprising, as the ventilation was almost continous for most of the day. The fluid-roof house was n o t ventilated at all, but the r o o f circulation was practically cont i nuous during the day, indicating t h a t the air t e m p e r a t u r e set poi nt o f 23°C was exceeded, as the recorded data indeed show. The calculated air temperatures were slightly higher than measured, and the calculated d e w p o i n t values were significantly higher. Again, we are led to assume that the infiltration, or leakage, o f the structure was higher than believed. As a final model verification, we show in Fig. 11 the measured and calculated t e m p e r a t u r e of the water in the storage tank of the fluid-roof house, b o th for the cold and the warm day. This comparison integrates, as it were, the ability o f the model to predict bot h the climate control and the absolute disposition of the available solar energy. It can be seen that the predictions are close to the measured values, w h e t h e r the groundwater is used as a source or as a sink for heat.
85 19 JUNE
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Fig. 9 (left). Hourly averages of measured and calculated values of the air and d e w p o i n t t e m p e r a t u r e in the glass greenhouse on 19 J u n e 1980. Fig. 10 (right). Hourly averages of measured and calculated values of the air and d e w p o i n t t e m p e r a t u r e in the fluid-roof greenhouse on 19 J u n e 1980.
CONCLUSIONS AND APPLICATIONS
T h e m a i n o b j e c t i v e o f t h e e x p e r i m e n t a n d its analysis was t o see if t h e g r e e n h o u s e m o d e l was r e a s o n a b l y a c c u r a t e . On t h e w h o l e , t h e a n s w e r is a f f i r m a t i v e , b u t an e x c e p t i o n m u s t be m a d e in regard t o the e s t i m a t e s o f i n t e r i o r h u m i d i t i e s . We m u s t n o t e t h a t t w o i n p u t p a r a m e t e r s t h a t b e a r d i r e c t l y on this m a t t e r w e r e n o t m e a s u r e d d i r e c t l y : the i n f i l t r a t i o n rate, a n d t h e e f f e c t o f w i n d s p e e d on b o t h t h e i n f i l t r a t i o n and t h e f o r c e d vent i l a t i o n rate. Also, t h e m o d e l c a l c u l a t i o n s s i m p l i f y the n a t u r e o f the c r o p surface, w h i c h is a s s u m e d to be h o m o g e n e o u s and a single-layer i n t e r f a c e . H e n c e , t h e c a l c u l a t i o n o f t h e e v a p o t r a n s p i r a t i o n c o u l d be in e r r o r as well. T h e m e a s u r e m e n t o f actual w a t e r use a n d o f actual e n e r g y use f o r heating w o u l d be useful a d d i t i o n s t o g r e e n h o u s e e x p e r i m e n t s o f this n a t u r e . Also, it w o u l d s e e m t h a t m o r e t h a n a single p o i n t m e a s u r e m e n t o f air t e m p e r a t u r e and h u m i d i t y o u g h t t o be m a d e - - p a r t i c u l a r l y w i t h a tall c r o p such as t o m a t o e s , g r o w n in r o w s w i t h w a l k i n g s p a c e in b e t w e e n .
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Fig. 11 (left)• Hourly averages o f the measured and calculated water t e m p e r a t u r e in the storage t a n k of t h e fluid-roof greenhouse on 13 April and 19 J u n e 1980. Fig. 12 (right)• Hourly averages of the calculated leaf t e m p e r a t u r e in t h e glass and in the fluid-roof greenhouse o n 13 April and 19 J u n e 1980.
Nevertheless, the climate control actions are well enough predicted so that one can estimate the energy requirements of the two different designs that were compared, for different times of the year and for different locations. Also, but with less certainty, one can predict in which respect the plant climate differs between the two designs. As a m a t t e r of horticultural interest, we have calculated the leaf t e m p e r a t u r e , the carbon dioxide level of the greenhouse air, and the plant water use, for each of the two test days, although corresponding measurements were n o t made. Calculated leaf temperatures are shown in Fig. 12, for bot h 13 April and 19 June, in the two types of greenhouse. Calculated levels of carbon dioxide in the greenhouse air are likewise shown in Fig. 13, and the calculated plant water use, or evapotranspiration, is given in Fig. 14. It is obvious from Figs. 12, 13 and 14 that the fluid-roof design, as compared to a standard glasshouse, is expected to result in a rather different plant climate, and should be managed differently for optimal results. First, Fig. 12 shows that, if air thermostats are used to control plant temperature, t h e y should be set lower at night and higher in the daytime to achieve
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Fig. 13 (left). Hourly averages of the calculated carbon dioxide level in the glass and in the fluid-roof greenhouse on 13 April and 19 June 1980. Fig. 14 (right). Average hourly calculated water use in mm in the glass and in the fluidroof greenhouse on 13 April and 19 June 1980.
comparable conditions for plant growth. Second, the virtual absence of ventilation in the fluid-roof design is likely to produce a significant depletion of CO2. Not only should this be counteracted, but in fact, enrichm e n t above normal outside levels is indicated, since a minimal a m o u n t of CO2 would be wasted, provided the structure has a low infiltration rate. In the absence of ventilation, both the glass and the fluid-roof house show, in Fig. 13, a depletion to less than 200 ppm. Incidental measurements, reported by Chiapale (1981), gave CO2 levels around 250 ppm, suggesting, again, that the leakage of the system was underestimated for the model calculations. Thirdly, Fig. 14 shows that water use in a fluidroof structure should be drastically reduced, as compared to a standard glasshouse. This has implications for plant nutrition, and suggests, also, that plant water potentials would be higher t h r o u g h o u t the day, with possible effects on cell elongation rates• Hence, a horticultural comparison of the two types of greenhouse cannot be made by providing identical treatments in the two environments, which is the usual approach. A greenhouse model, such as the one examined here, makes it possible, in spite of its shortcomings, to rationally plan
88 experiments effectively.
in w h i c h
d i f f e r e n t g r e e n h o u s e designs are t o be c o m p a r e d
ACKNOWLEDGEMENTS This w o r k was s u p p o r t e d b y the C E A ( C o m m i s s i o n ~ l'Energie A t o m i q u e ) o f F r a n c e , the U n i t e d States D e p a r t m e n t o f E n e r g y t h r o u g h t h e U n i t e d States D e p a r t m e n t o f Agriculture, and by the C e n t e r for E n e r g y a n d Mineral R e s o u r c e s o f Texas A&M University. The help o f the f o l l o w i n g persons is gratefully a c k n o w l e d g e d : J. D a m a g n e z o f R e n a u l t , F r a n c e a n d G.C. H e a t h m a n , f o r m e r g r a d u a t e research assistant at Texas A&M University. REFERENCES Chiapale, J.-P., 1981. La serre solaire INRA-CEA: R~sultats physiques. Acta Hortic., 115:387--393 (In French with English summary). Damagnez, J., Van Bavel, C.H.M., Sadler, E.J. and Chouani~re, M.P., 1981. Simulation of the effect of storage characteristics upon the dynamic response of the fluid-roof solar greenhouse. Acta Hortic., 106: 27--38. Gac, A., Jacquot, M., Chomont, J., B~thery, J., G6rard, A. and Foucault, J., 1969. Etude experimentale concernante le renouvellement de l'atmosph~re et le bilan 4nergetique des serres maraich~res rev6tues de plastique et de verre. Bull. Tech. G4nie Rural 95, R~publique Fran~aise, Min. Agriculture, CERAFER, Paris, 57 pp. Idso, S.B., 1981. A set of equations for full spectrum and 8--14 um and 10.5--12.5 um thermal radiation from cloudless skies. Water Resour. Res., 17: 295--307. Kimball, B.A., 1973. Simulation of the energy balance of a greenhouse. Agric. Meteorol., 11: 243--260. Selcuk, M.K., 1971. Analysis, design and performance evaluation of controlled-environment greenhouses. Am. Soc. Heating Refrig. Aircond. Eng. Trans., 77: 72--78. Soribe, F.L. and Curry, R.B., 1973. Simulation of lettuce growth in an air-supported plastic greenhouse. J. Agric. Eng. Res., 18: 133--140. Takami, S. and Uchijima, Z., 1977. A model of the greenhouse with a storage-type heat exchanger and its verification. J. Agric. Meteorol. Japan, 33: 155--166. Takakura, T., Jordan, K.A. and Boyd, L.L., 1971. Dynamic simulation of plant growth and environment in the greenhouse. Trans. ASAE, 14: 964--971. Van Bavel, C.H.M., Damagnez, J. and Sadler, E.J., 1981. The fluid-roof solar greenhouse: Energy budget analysis by simulation. Agric. Meteorol., 23: 61--76. Van Bavel, C.H.M. and Sadler, E.J., 1979. SG79: A computer simulation program for analyzing energy transformation in a solar greenhouse. Mimeo, Texas A&M University, College Station, TX, 75 pp. Von Elsner, B., 1982. Das Kleinklima und der W~rmeverbrauch yon geschlossenen Gew~/chsh~iusern. Gartenbautech. Inf. 12. Institute for Agricultural Engineering, Hannover University. Walker, J.N., 1965. Predicting temperatures in ventilated greenhouses. Trans. ASAE, 8: 445--448. APPENDIX Here a s u m m a r y is given o f the s i m u l a t i o n m o d e l , as described in detail b y Van Bavel et al. ( 1 9 8 1 ) .
89 The system is divided into physical compartments consisting of the r o o f (with or without a fluid running through it), the greenhouse space, the crop, an appropriate n u m b e r of soil layers, and a storage tank (omitted for the glasshouse). At its outer boundary the roof interacts with the atmosphere by absorption and emission of longwave radiation, by absorption and reflection of shortwave (solar) radiation, and by convective heat exchange. The greenhouse space (air) exchanges sensible and latent heat with the outside by infiltration and forced ventilation, by convection with the roof, the crop, and the soil. Roof, crop, and soil exchange energy by longwave radiation. The crop itself is simplified to a plane without heat capacity, b u t e n d o w e d with a capability to exchange carbon dioxide in photosynthesis and respiration, and water by transpiration, all as regulated by a stomatal mechanism that responds to light and plant water stress. Calculation of the flow of heat into the soil, and between its several layers below the surface, allows a dynamic analysis of the diurnal heat storage and release in the floor. A pump connects the roof and the tank in a closed loop, when applicable. Starting from known or assumed initial conditions, all fluxes of energy, water and carbon dioxide are c o m p u t e d from the state of the system and the ambient weather conditions, the latter interpolated from a data set. The state of each c o m p a r t m e n t is then computed by adding the net flux to the known content of energy and mass, and the system condition is updated. The algorithms are simple except for the ones needed to find the crop temperature and the soil temperature. For these an iterative method is required because of the implicit functions of temperature in both cases. The updating takes place every 30 s and appropriate lists of variables are printed every hour, with daily summaries, if desired. The general method is one of forward, rectangular integration using a finite difference approach. The program is written in CSMP III, a user-oriented simulation language. It has been successfully translated into F O R T R A N by others. The program can and has been readily adapted to simulate different types of r o o f construction, different heating methods, evaporative cooling, and various thermostatic and flow controls.
75
COMPARISON OF CALCULATED AND MEASURED PERFORMANCE OF A FLUID-ROOF AND A STANDARD GREENHOUSE
J.-P. CHIAPALE 1 , C.H.M. VAN BAVEL 2 and E.J. SADLER 2
Station de Bioclimatologie, Institut National de Recherche Agronomique, Montfavet (France) 2 Texas Agricultural Experiment Station, Texas A & M University, College Station, T X 77843 (U.S.A.) (Accepted 8 March 1983)
ABSTRACT Chiapale, J.-P., Van Bavel, C.H.M. and Sadler, E.J., 1983. Comparison of calculated and measured performance of a fluid-roof and a standard greenhouse. Energy Agric., 2: 75--89. A side-by-side comparison was made in Avignon, France, during 1980, of two greenhouse types: a standard greenhouse with forced air heating and forced ventilation, and a fluid-roof house in which the temperature was controlled by water flowing over a roof made of infrared absorbing glass. Heat exchange with groundwater at 14°C kept the circulating water between 12 and 23°C. Both systems worked essentially as predicted by a simulation model on a cold day in April and a warm day in June. The model used measured meteorological data to calculate the control action and the actual conditions prevailing in both greenhouses. The predictions were adequate, except for a tendency to over-estimate interior humidities. The model was useful in suggesting that, in spite of essentially equal air temperatures, the conditions for plant growth were substantially different in the two greenhouses. In the fluid-roof house, there would be less light, a higher humidity, a much reduced transpiration, and, during the day, a somewhat lower CO 2 level and lower leaf temperature, all as compared to the standard glasshouse.
INTRODUCTION I t is e s s e n t i a l t h a t g r e e n h o u s e s p r o v i d e a p r o d u c t i v e e n v i r o n m e n t a t the least e x p e n d i t u r e of energy, b o t h for h e a t i n g a n d for cooling. Tradit i o n a l l y , e n e r g y r e q u i r e m e n t s h a v e b e e n e s t i m a t e d b y m e t h o d s t h a t correlate energy consumption to standard climate data and to horticultural requirements, with due regard to method of greenhouse construction and o p e r a t i o n . S u c h m e t h o d s are b a s e d u p o n m e a s u r e d p e r f o r m a n c e d a t a , and therefore, have direct application only in a restricted physiographic area. Also, t h e y c a n n o t be used to p r e d i c t the effect of n e w m a n a g e m e n t or construction techniques.
0167-5826/83/$03.00
© 1983 Elsevier Science Publishers B.V.
76 A more general approach is to consider the problem as a physical one: that is, to analyze the energy and mass flows in a greenhouse-crop-environm e n t system in detail and to calculate greenhouse performance without collecting data on an existing system, or prior to construction of an experimental facility. Particularly for the case of unusual designs or unusual environments, a theoretical m e t h o d can be of great help, even though it remains necessary to do experiments to verify predicted results before application in a practical situation. Several investigators have proposed such methods to analyze and predict greenhouse performance. There are a number of reports in which such mechanistic models have been verified, generally with satisfactory results. An early example is the work of Walker (1965), who used an energy balance, steady-state m e t h o d to predict temperatures in a greenhouse, as well as heating requirements. A more detailed computer model was developed by Selcuk (1971), who predicted air and plant temperatures with an accuracy of 1--2°C. The work by Takakura et al. (1971) is an example of a dynamic simulation model. It was verified by measurement of the relation between leaf and air temperatures, an important element in greenhouse engineering that is often overlooked. A dynamic simulation model was proposed by Soribe and Curry (1973), in which plant growth was calculated, as well as the conditions in the greenhouse. Tests with lettuce proved the potential of this approach. Other instances of agreement between conditions calculated from computer models and found in experiments are cited in work by Kimball (1973), Takami and Uchijima (1977), and Von Elsner (1982). We present here a test of a greenhouse modeling m e t h o d that is aimed to determine the potential advantage of a so-called fluid-roof greenhouse over a standard greenhouse. In a fluid-roof greenhouse, at least one half of the solar radiation, primarily the infrared fraction, is absorbed by the translucent roof and is stored or rejected, as the case may require, using a fluid that runs over or through the roof. For earlier reports on this approach to energy conservation in greenhouses, reference is made to Van Bavel et al. (1981). There, a more extensive review of the problem is given and the merits of the fluid-roof concept are analyzed with a numerical model, described in detail by Van Bavel and Sadler (1979), and given here, in outline, in the Appendix. Calculations and preliminary experiments did suggest that, given an adequate storage volume of the working fluid, such as 100 1/m 2 of greenhouse floor area, considerable savings in both heating and ventilation could be achieved. In that case, the average temperature of the storage system is close to the average daily outside air temperature, but the diurnal amplitude of the former is several times smaller than that of the latter. Later, it was proposed by Damagnez et al. (1981) that the fluid-roof principle would be particularly effective if the heat rejection or absorption by the roof were assisted by a heat exchanger that uses groundwater as a heat reservoir.
77 Parallel to the analytical and modeling research, an experimental program was started in Avignon, France, where a set of experimental greenhouses was constructed in early 1980, as described by Chiapale (1981). In this report, we present the actual performance of the fluid-roof greenhouse and t h a t of its conventional, glass counterpart, and the calculated performance of each, as obtained from the simulation model. METHODOLOGY A detailed description of the experimental installation in Montfavet near Avignon, France, has been given by Chiapale (1981). In brief, in a 1000-m 2 greenhouse, two compartments of 200 m 2 each were used for the present study. One had a standard glass roof, was heated with hot water air handlers situated above the crop, and was cross-ventilated with electric fans. The other was in every other way identical to the standard c o m p a r t m e n t , but it had a roof made of double-glazed panels, the lower glazing being infrared absorbing glass, 4.5 mm thick (KOA, Boussois S.A.) and the upper, clear glass, 4 mm thick. A layer of demineralized water was made to flow down-slope over the heat absorbing glass, when the inside air temperature, as sensed by an air thermostat, was above a m a x i m u m or below a m i n i m u m setpoint value. After this water drained off the glass, it was collected in a tank of 5000-1 capacity and thence pumped back to the roof ridge at a flow rate of 225 l/min, or 1.17 1/min per m 2 of greenhouse space. On its way back to the roof, the water passed through a heat exchanger with a capacity of 12 kW/°C. Groundwater from a shallow well was pumped through the other side of the heat exchanger, and returned to the aquifer. The groundwater had a constant temperature of about 14°C, close to the local mean annual air temperature. The tank temperature was held between 12 and 23°C by a dual t h e r m o s t a t that controlled the groundwater pump. By controlling the roof circulation pump from another dual thermostat, which sensed the greenhouse air temperature, the system prevented the latter temperature to exceed the limits judged suitable for crop production, in this case 13 and 23°C, respectively. Hence, excess solar radiation, primarily in the infrared part of the spectrum, was rejected into the groundwater. Undue heat loss to the environment, on the other hand, was made up by extracting heat from the aquifer. The storage tank acted as a buffer in this system, but its heat storage capacity was n o t significant, compared to typical diurnal gains and losses. The thermostats controlling the heating and ventilation in both greenhouses were set slightly outside the range of the roof pump thermostats, at 11 and 25°C, respectively. The purpose of the fluid-roof system is to reduce both the normal heating and ventilation requirements, if n o t to eliminate them. In a preliminary design simulation for the Avignon site, Damagnez et al. (1981) reported
78 that, indeed, the normal heating and ventilation action needed in the standard glasshouse would be essentially eliminated by the radiant heating and cooling provided by the fluid-roof design, support ed by the use of the aquifer as a source or sink for heat, and by the selective shading action o f the infrared absorbing glass. Here, we examine the quantitative agreement between calculation and measurement, rather than the qualitative adequacy of the c o n c e p t as already report ed by Chiapale {1981}. During the experimental period, 1 March--1 August 1980, a t o m a t o crop was grown in bot h greenhouses. Planted in full soil on 5 March, the crop yielded by 11 July, the final harvest data, 9.1 kg/m 2 in the normal glasshouse and 6.2 kg/m 2 in the fluid-roof house. F r o m the available record, 2 days of data were selected to make a detailed comparison of calculated and measured conditions for plant growth and climate control action. The first was 13 April 1980, a cold, clear day with outside temperatures below freezing during the early morning, followed by a slight warming trend during the afternoon. The second was 19 June 1980, a warm and humid day with clear skies, typical for the su mmer climate of the area. On bot h days, all greenhouse systems functioned as required, and a complete, error-free set of data was available every 30 min for the 24-h period from midnight to midnight. The following data were collected every 30 min by an automatic datalogger, as the 30-min average or the accumulated values, based on a data scan every 30 s. Solar radiation outside was measured with a p y r a n o m e t e r (Kipp en Zonen), windspeed with a cup a n e m o m e t e r , and outside dry and wet bulb t e m p e r a t u r e with an aspirated and shielded p s y c h r o m e t e r . Two additional psychrometers were used to measure the t e m p e r a t u r e and the h u m i d i t y inside each of the two test greenhouses. In addition, the t e m p e r a t u r e of the water stored in the tank of the circulation system in the fluid-roof solar greenhouse was measured. Finally, a record was obtained o f the a m o u n t of time the ventilation fans, the two pumps, and the heating system were running during each 30-min period and in each greenhouse. A simplified diagram of the m e t h o d of operation of each of the two systems in the cooling and in the heating m ode is shown as Fig. 1. Actually, there were two sets of ventilation fans, the first actuated at 25°C and the second at 28°C. The heating system was activated at 12°C. A more detailed diagram showing the water circulation system in the fluid-roof system is given as Fig. 2. The greenhouse p e r f o r m a n c e was calculated with the continuous simulation model described by Van Bavel et al. (1981), using as inputs the actually measured weather data and the initial conditions at the beginning of 13 April and of 19 June. The simulation model was used with the following significant modifications. Sky long-wave radiation was c o m p u t e d with the formula proposed by Idso (1981), and the infiltration rate was made d e p e n d e n t upon wind-
79 GLASS ROOF
DAY
R
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FLUID ROOF
t
Fig. 1. Simplified operating systems for the glass-covered and the fluid-roof greenhouse. In the first, daytime energy gain from solar radiation (R) is rejected by ventilation (V), while nighttime radiation loss (R) is made up by heating (H), to maintain air temperatures between 12 and 25°C. In the fluid-roof system, R is balanced by rejection or withdrawal of heat from the groundwater (UW), using a heat exchanger (HE) and pump (P2) to keep the temperature in the tank (T) between 12 and 23°C, and pump (P1) to circulate the water through the roof. speed, using a f o r m u l a derived b y Gac et al. ( 1 9 6 9 ) . T h e l a t t e r e x p r e s s i o n states: i n f i l t r a t i o n r a t e = 0 . 0 0 0 4 2 + 0 . 0 0 0 28 U0
(m3/(m2s))
in w h i c h U0 is the w i n d s p e e d ( m / s ) . A t 1 m / s , the i n f i l t r a t i o n rate is equiva l e n t to 1.0 air c h a n g e p e r h. A n u m b e r o f o t h e r f o r m u l a s have b e e n p r o p o s e d , b u t a p p e a r t o d i f f e r little. T h e d a t a base u s e d b y Gac et al. ( 1 9 6 9 ) was large a n d d e t a i l e d , t h o u g h t h e e x p r e s s i o n t h a t r e s u l t e d has a considerable r a n d o m u n c e r t a i n t y . T h e p r i n c i p a l p a r a m e t e r s describing t h e g r e e n h o u s e in t h e s i m u l a t i o n m o d e l w e r e t h e following: 0.75 t r a n s m i s s i o n b y glass r o o f ( 4 0 0 - - 7 0 0 n m ) ; 0.75 t r a n s m i s s i o n b y glass r o o f (700---2000 n m ) ; 0.40 t r a n s m i s s i o n b y f l u i d - r o o f ( 4 0 0 - - 7 0 0 rim); 0.05 t r a n s m i s s i o n b y f l u i d - r o o f ( 7 0 0 - - 2 0 0 0 n m ) ; 140 W / m 2 m a x i m u m p o w e r o f air h e a t i n g s y s t e m ; 32 air c h a n g e s p e r h c a p a c i t y o f f o u r v e n t i l a t o r s ;
80
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Fig. 2. D e t a i l e d d i a g r a m o f w a t e r c i r c u l a t i o n in t h e f l u i d - r o o f g r e e n h o u s e . A n air t h e r m o s t a t ( T 1 ) c o n t r o l s p u m p (P1), a n d a w a t e r t h e r m o s t a t ( T 2 ) c o n t r o l s p u m p (P2), in s u c h a way t h a t P2 can r u n o n l y if P1 does. H e a t e x c h a n g e r ( H E ) a n d b o t h p u m p s were a c t u a l l y l o c a t e d in t h e g r e e n h o u s e , as is t h e filter (F).
13°C controlled air temperature in heating mode; 24 or 27°C controlled air temperature in cooling-by-ventilation mode (two or four ventilators, respectively); 23°C controlled air temperature in cooling mode with fluid-roof circulation; 1.17 1/(m: min) capacity of fluid-roof circulation pump; 64 W/(m: °C) capacity of heat exchanger with groundwater. The values for transmission are based on actual measurements in the t w o structures, and include the effect of supporting structure and tubing suspended below the glazing. It may be noted that the light transmission by the fluid r o o f in the 400--700onm band was quite low. Replacement with a blue-green glass (V66, Boussois S.A.) in 1981 resulted in a higher light transmission, and in t o m a t o yields entirely comparable to those under normal glass, whereas the 1980 yields were significantly lower than in the glasshouse. In addition, however, it was also found in 1981 that both the irrigation frequency and the mineral nutrition need to be adapted to the climate in the fluid-roof greenhouse to obtain optimum results. RESULTS AND DISCUSSION
A comparison of calculated and measured performance of the climate control functions (heating, ventilation and fluid pumping) is given, as a
81
comparison of the calculated and measured greenhouse environment for plant production. These comparisons were made for two contrasting weather patterns. The first, on 13 April 1980, is shown in Fig. 3. The night temperatures approached freezing, and the atmosphere was generally dry. The pattern for the second day, 19 June 1980, is shown in Fig. 4. The actual performance of the system on 13 April is shown in Fig. 5, as well as its predicted behavior. The standard greenhouse required heating during the night until about 7 am to maintain an air temperature above 12°C. The hours during which heating occurred were accurately predicted, but not the duration, given as the number of minutes of heating during each hour. The discrepancy is likely due to the fact that the power-in-use of the heating system was not accurately known, as no wattmeters had been installed. The solar greenhouse, on the other hand, did not require any heating, since the air temperature was maintained by pumping the tank fluid through the roof. This action was accurately calculated, as shown in Fig. 5. 13 RPRIL 1980 30
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Fig. 3 (left). Hourly averages of solar radiation (W/m2), outside air and dewpoint temperatures (°C), and windspeed (m/s) at the Avignon greenhouse site on 13 April 1980. Fig. 4 (right). Hourly averages of solar radiation (W/m2), outside air and dewpoint temperatures (°C), and windspeed (m/s) at the Avignon greenhouse site on 19 June 1980.
82 13
RPRIL
GLR$$ ROOF
1980 FLUID-ROOF
60 z 2.E
~
60
c!iiiiiiiil iiiJii M
3o
3O
o
o
u.
c
M
E,N
30 o
30 ::
!
i
0 30
~c
0
o
3O o
ao N
:.i:.!:.~i~i!iii:,. o
~ g
!
12 TIME OF OAT (h)
o
12
o 2Lt
TIME OF OAT (h)
Fig. 5. M e a s u r e d (M) and calculated (C) values o f d u r a t i o n o f h e a t i n g a n d v e n t i l a t i o n in t h e glass r o o f g r e e n h o u s e , and o f r o o f w a t e r and g r o u n d w a t e r c i r c u l a t i o n in the fluidr o o f g r e e n h o u s e , all in m i n / h for every h o u r , o n 13 April 1980.
During the daytime, a small a m o u n t of ventilation was required to keep t h e glasshouse air t e m p e r a t u r e below 25°C. The actual ventilation occurred in two pulses, whereas the calculated action was m ore even, with the total ventilation volume being nearly equal. It should be poi nt ed o u t t hat the mo d el idealizes and smoothes the external and internal conditions, hence no p er f ect agreement in on- of f cont r ol can be expected. In the solar greenhouse, the ventilation was supplanted by fluid circulation, the action again being accurately calculated. On the warm day in June, shown in Fig. 6, no heating was required. During the day, a large a m o u n t of ventilation was measured as well as calculated f o r the glasshouse, and the quantitative agreement was reasonably good. In the fluid-roof greenhouse, the t e m p e r a t u r e was entirely controlled by pumping: no ventilation was measured. This action was almost exactly as calculated. In bot h greenhouses, ventilators or pumps were running c o n t i n u o u s l y during the afternoon, and air temperatures slightly exceeded the set points. We turn n o w t o the measured and calculated air temperatures and dewp o i n t temp er a t ur e s in the glasshouse, given in Fig. 7. The form er were in substantial agreement, but the d a y t i m e d e w p o i n t values were over-estim a t e d b y as m u c h as 10°C. In the fluid-roof house, the calculated and measured tem pe r at ur e s and dew-points, shown in Fig. 8, related in the same way as in t he glasshouse, but the overestimation of the d e w p o i n t was 5°C at the most.
83 19
JUNE 1980
GLASS ROOF 120
i:i:!:i:
g~
60
.:.:.:.:,.:.:.:.:.:.:.
C
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90
3O
60
0
30
~-~o ~
FLUID-ROOF
.:.:.:.:
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30
0
~
o-~,
iiiiiiiii Iii i!i i i!i i il 3O
.:.:.:~.:.:.:.:.:
.:.:.:.:,.:.:.:.:.:.:.
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0
t iiiiil Iiiiiii iiiill
0¢12
f Iiiiiii!iii il t iiiiiiiiiiiiii'o 0
12
TIME OF DRY ( h )
0
12
2q
TIME OF DAY ( h )
Fig. 6. Measured (M) and calculated (C) values of the duration of heating and ventilation in the glass roof greenhouse, and of roof water and groundwater circulation in the fluid-roof greenhouse, all in min/h for every hour, on 19 June 1980. Ventilation is maximally 120 min/h, as there are two separately controlled sets of ventilators.
I n e v a l u a t i n g b o t h sets o f d e w p o i n t d a t a , it m u s t be b o r n e in m i n d t h a t t h e r e was o n l y o n e p s y c h r o m e t e r in e a c h g r e e n h o u s e , l o c a t e d a t a b o u t 1 m a b o v e t h e g r o u n d , in t h e c e n t e r . H e n c e , a bias e r r o r c o u l d h a v e occ u r r e d , w h e r e a s in t h e m o d e l c a l c u l a t i o n , a h o m o g e n e o u s g r e e n h o u s e space was a s s u m e d . T h e cause o f a real d i s c r e p a n c y b e t w e e n t h e t w o sets o f d e w p o i n t values is a m a t t e r o f c o n j e c t u r e , b u t it is p r o b a b l e t h a t t h e vent i l a t i o n b y l e a k a g e was g r e a t e r t h a n a s s u m e d . N o d i r e c t m e a s u r e m e n t s o f t h e leakage o f t h e a c t u a l s t r u c t u r e w e r e m a d e , a n d a ' t y p i c a l ' value was u s e d in t h e calculations. A n o t h e r possible c a u s e f o r the f a c t t h a t m e a s u r e d h u m i d i t i e s w e r e l o w e r t h a n t h e c a l c u l a t e d o n e s is t h a t t h e soil surface b e t w e e n the p l a n t s was d r y , since a drip irrigation s y s t e m was used. In c o n t r a s t , t h e m o d e l a s s u m e d a c o n t i n o u s c r o p c o v e r a n d a c o n s t a n t l y w e t soil surface. F r o m a h o r t i c u l t u r a l v i e w p o i n t , we call a t t e n t i o n t o t h e f a c t t h a t t h e relative h u m i d i t y , as m e a s u r e d , was higher in t h e f l u i d - r o o f s t r u c t u r e t h a n in t h e glasshouse, p a r t i c u l a r l y in the early m o r n i n g hours. D e t a i l e d m o d e l analysis has s h o w n t h a t , d u r i n g t h e n i g h t and at e q u a l air t e m p e r a t u r e s , p l a n t t e m p e r a t u r e s are higher in the f l u i d - r o o f h o u s e t h a n in t h e glasshouse. S u c h a d i f f e r e n c e c o u l d e x p l a i n higher d e w p o i n t s a n d relative h u m i d i t y values. N o m e a s u r e m e n t s o f p l a n t or leaf t e m p e r a t u r e s w e r e m a d e to supp o r t this s u p p o s i t i o n .
84 13
30
=
APRIL
13 APRIL
1980
|
30
u
o._.2O
o2O
=2 Z
.2 IE
|
1980
!
|
a:1o MEASUREO
MEASURED
..... C A L C U L A T E D
..... C A L C U L A T E D
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I
I
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I
I
- ........ s°'°',
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I
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iI
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X
MEA5URED
. . . . . CALCULf:ITEO |
!
6
12
I
18
TIME OF DAY ( h )
2q
.....
CALCULATEO
!
|
!
6
12
18
TIME
OF
DAY
2q
(h)
Fig. 7 (left). Hourly averages of measured and calculated values of the air and dewpoint temperature in the glass greenhouse on 13 April 1980. Fig. 8 (right). Hourly averages of measured and calculated values of the air and dewpoint temperature in the fluid-roof greenhouse on 13 April 1980.
The corresponding results for 19 J une are shown in Fig. 9 for the glasshouse, and in Fig. 10 for the fluid-roof house. In the glasshouse, good agreement existed between calculation and measurement. This result is n o t surprising, as the ventilation was almost continous for most of the day. The fluid-roof house was n o t ventilated at all, but the r o o f circulation was practically cont i nuous during the day, indicating t h a t the air t e m p e r a t u r e set poi nt o f 23°C was exceeded, as the recorded data indeed show. The calculated air temperatures were slightly higher than measured, and the calculated d e w p o i n t values were significantly higher. Again, we are led to assume that the infiltration, or leakage, o f the structure was higher than believed. As a final model verification, we show in Fig. 11 the measured and calculated t e m p e r a t u r e of the water in the storage tank of the fluid-roof house, b o th for the cold and the warm day. This comparison integrates, as it were, the ability o f the model to predict bot h the climate control and the absolute disposition of the available solar energy. It can be seen that the predictions are close to the measured values, w h e t h e r the groundwater is used as a source or as a sink for heat.
85 19 JUNE
30
i
19 JUNE
1980
i
1980
30
i
0 ~20
~20
E Ig
.2 a:]o
I
- -
MEASURED
.....
CALCULATED
I
I
I
I
I
0o
~2o
,,'20 Z:
W
X
I
I
6 TIHE
--REASURED
--MEASUREO
.....
.....
CALCULATED I
12 18 OF OAT ( h )
O
2q
CALCULATED
I
!
12
18
TIME OF DAY (h)
Fig. 9 (left). Hourly averages of measured and calculated values of the air and d e w p o i n t t e m p e r a t u r e in the glass greenhouse on 19 J u n e 1980. Fig. 10 (right). Hourly averages of measured and calculated values of the air and d e w p o i n t t e m p e r a t u r e in the fluid-roof greenhouse on 19 J u n e 1980.
CONCLUSIONS AND APPLICATIONS
T h e m a i n o b j e c t i v e o f t h e e x p e r i m e n t a n d its analysis was t o see if t h e g r e e n h o u s e m o d e l was r e a s o n a b l y a c c u r a t e . On t h e w h o l e , t h e a n s w e r is a f f i r m a t i v e , b u t an e x c e p t i o n m u s t be m a d e in regard t o the e s t i m a t e s o f i n t e r i o r h u m i d i t i e s . We m u s t n o t e t h a t t w o i n p u t p a r a m e t e r s t h a t b e a r d i r e c t l y on this m a t t e r w e r e n o t m e a s u r e d d i r e c t l y : the i n f i l t r a t i o n rate, a n d t h e e f f e c t o f w i n d s p e e d on b o t h t h e i n f i l t r a t i o n and t h e f o r c e d vent i l a t i o n rate. Also, t h e m o d e l c a l c u l a t i o n s s i m p l i f y the n a t u r e o f the c r o p surface, w h i c h is a s s u m e d to be h o m o g e n e o u s and a single-layer i n t e r f a c e . H e n c e , t h e c a l c u l a t i o n o f t h e e v a p o t r a n s p i r a t i o n c o u l d be in e r r o r as well. T h e m e a s u r e m e n t o f actual w a t e r use a n d o f actual e n e r g y use f o r heating w o u l d be useful a d d i t i o n s t o g r e e n h o u s e e x p e r i m e n t s o f this n a t u r e . Also, it w o u l d s e e m t h a t m o r e t h a n a single p o i n t m e a s u r e m e n t o f air t e m p e r a t u r e and h u m i d i t y o u g h t t o be m a d e - - p a r t i c u l a r l y w i t h a tall c r o p such as t o m a t o e s , g r o w n in r o w s w i t h w a l k i n g s p a c e in b e t w e e n .
2~
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0 O
i 6
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CALCULATED
J 12
TIME OF DAY (h)
l 18
.....
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FLUID-ROOF
I
I
I
6
12
18
2~
TIME OF OQT (h)
Fig. 11 (left)• Hourly averages o f the measured and calculated water t e m p e r a t u r e in the storage t a n k of t h e fluid-roof greenhouse on 13 April and 19 J u n e 1980. Fig. 12 (right)• Hourly averages of the calculated leaf t e m p e r a t u r e in t h e glass and in the fluid-roof greenhouse o n 13 April and 19 J u n e 1980.
Nevertheless, the climate control actions are well enough predicted so that one can estimate the energy requirements of the two different designs that were compared, for different times of the year and for different locations. Also, but with less certainty, one can predict in which respect the plant climate differs between the two designs. As a m a t t e r of horticultural interest, we have calculated the leaf t e m p e r a t u r e , the carbon dioxide level of the greenhouse air, and the plant water use, for each of the two test days, although corresponding measurements were n o t made. Calculated leaf temperatures are shown in Fig. 12, for bot h 13 April and 19 June, in the two types of greenhouse. Calculated levels of carbon dioxide in the greenhouse air are likewise shown in Fig. 13, and the calculated plant water use, or evapotranspiration, is given in Fig. 14. It is obvious from Figs. 12, 13 and 14 that the fluid-roof design, as compared to a standard glasshouse, is expected to result in a rather different plant climate, and should be managed differently for optimal results. First, Fig. 12 shows that, if air thermostats are used to control plant temperature, t h e y should be set lower at night and higher in the daytime to achieve
87 0.6
i
|
|
600 13 APRIL 13 APRIL
GLASS ROOF
--GLASS ROOF . . . . . FLUID-ROOF
..... FLUID-ROOF
~O.tt E~O0 Q.
~:0.2
0
u 200
:X
LJ
I
I
--GLASS
19 JUNE
0.0
I
.
.
.
.
ROOF
19 JUNE
ROOF
. . . . . FLUID-ROOF
FLUID-ROOF
.
--GLASS
~0.~
~L&O0 0.
.
.
,,'5,
0
~:o.2
~200
Z
I
I
I
6
12
18
TIME OF OAT (h)
0.0 2~
O
6
12 18 TIME OF OAT (h)
2~
Fig. 13 (left). Hourly averages of the calculated carbon dioxide level in the glass and in the fluid-roof greenhouse on 13 April and 19 June 1980. Fig. 14 (right). Average hourly calculated water use in mm in the glass and in the fluidroof greenhouse on 13 April and 19 June 1980.
comparable conditions for plant growth. Second, the virtual absence of ventilation in the fluid-roof design is likely to produce a significant depletion of CO2. Not only should this be counteracted, but in fact, enrichm e n t above normal outside levels is indicated, since a minimal a m o u n t of CO2 would be wasted, provided the structure has a low infiltration rate. In the absence of ventilation, both the glass and the fluid-roof house show, in Fig. 13, a depletion to less than 200 ppm. Incidental measurements, reported by Chiapale (1981), gave CO2 levels around 250 ppm, suggesting, again, that the leakage of the system was underestimated for the model calculations. Thirdly, Fig. 14 shows that water use in a fluidroof structure should be drastically reduced, as compared to a standard glasshouse. This has implications for plant nutrition, and suggests, also, that plant water potentials would be higher t h r o u g h o u t the day, with possible effects on cell elongation rates• Hence, a horticultural comparison of the two types of greenhouse cannot be made by providing identical treatments in the two environments, which is the usual approach. A greenhouse model, such as the one examined here, makes it possible, in spite of its shortcomings, to rationally plan
88 experiments effectively.
in w h i c h
d i f f e r e n t g r e e n h o u s e designs are t o be c o m p a r e d
ACKNOWLEDGEMENTS This w o r k was s u p p o r t e d b y the C E A ( C o m m i s s i o n ~ l'Energie A t o m i q u e ) o f F r a n c e , the U n i t e d States D e p a r t m e n t o f E n e r g y t h r o u g h t h e U n i t e d States D e p a r t m e n t o f Agriculture, and by the C e n t e r for E n e r g y a n d Mineral R e s o u r c e s o f Texas A&M University. The help o f the f o l l o w i n g persons is gratefully a c k n o w l e d g e d : J. D a m a g n e z o f R e n a u l t , F r a n c e a n d G.C. H e a t h m a n , f o r m e r g r a d u a t e research assistant at Texas A&M University. REFERENCES Chiapale, J.-P., 1981. La serre solaire INRA-CEA: R~sultats physiques. Acta Hortic., 115:387--393 (In French with English summary). Damagnez, J., Van Bavel, C.H.M., Sadler, E.J. and Chouani~re, M.P., 1981. Simulation of the effect of storage characteristics upon the dynamic response of the fluid-roof solar greenhouse. Acta Hortic., 106: 27--38. Gac, A., Jacquot, M., Chomont, J., B~thery, J., G6rard, A. and Foucault, J., 1969. Etude experimentale concernante le renouvellement de l'atmosph~re et le bilan 4nergetique des serres maraich~res rev6tues de plastique et de verre. Bull. Tech. G4nie Rural 95, R~publique Fran~aise, Min. Agriculture, CERAFER, Paris, 57 pp. Idso, S.B., 1981. A set of equations for full spectrum and 8--14 um and 10.5--12.5 um thermal radiation from cloudless skies. Water Resour. Res., 17: 295--307. Kimball, B.A., 1973. Simulation of the energy balance of a greenhouse. Agric. Meteorol., 11: 243--260. Selcuk, M.K., 1971. Analysis, design and performance evaluation of controlled-environment greenhouses. Am. Soc. Heating Refrig. Aircond. Eng. Trans., 77: 72--78. Soribe, F.L. and Curry, R.B., 1973. Simulation of lettuce growth in an air-supported plastic greenhouse. J. Agric. Eng. Res., 18: 133--140. Takami, S. and Uchijima, Z., 1977. A model of the greenhouse with a storage-type heat exchanger and its verification. J. Agric. Meteorol. Japan, 33: 155--166. Takakura, T., Jordan, K.A. and Boyd, L.L., 1971. Dynamic simulation of plant growth and environment in the greenhouse. Trans. ASAE, 14: 964--971. Van Bavel, C.H.M., Damagnez, J. and Sadler, E.J., 1981. The fluid-roof solar greenhouse: Energy budget analysis by simulation. Agric. Meteorol., 23: 61--76. Van Bavel, C.H.M. and Sadler, E.J., 1979. SG79: A computer simulation program for analyzing energy transformation in a solar greenhouse. Mimeo, Texas A&M University, College Station, TX, 75 pp. Von Elsner, B., 1982. Das Kleinklima und der W~rmeverbrauch yon geschlossenen Gew~/chsh~iusern. Gartenbautech. Inf. 12. Institute for Agricultural Engineering, Hannover University. Walker, J.N., 1965. Predicting temperatures in ventilated greenhouses. Trans. ASAE, 8: 445--448. APPENDIX Here a s u m m a r y is given o f the s i m u l a t i o n m o d e l , as described in detail b y Van Bavel et al. ( 1 9 8 1 ) .
89 The system is divided into physical compartments consisting of the r o o f (with or without a fluid running through it), the greenhouse space, the crop, an appropriate n u m b e r of soil layers, and a storage tank (omitted for the glasshouse). At its outer boundary the roof interacts with the atmosphere by absorption and emission of longwave radiation, by absorption and reflection of shortwave (solar) radiation, and by convective heat exchange. The greenhouse space (air) exchanges sensible and latent heat with the outside by infiltration and forced ventilation, by convection with the roof, the crop, and the soil. Roof, crop, and soil exchange energy by longwave radiation. The crop itself is simplified to a plane without heat capacity, b u t e n d o w e d with a capability to exchange carbon dioxide in photosynthesis and respiration, and water by transpiration, all as regulated by a stomatal mechanism that responds to light and plant water stress. Calculation of the flow of heat into the soil, and between its several layers below the surface, allows a dynamic analysis of the diurnal heat storage and release in the floor. A pump connects the roof and the tank in a closed loop, when applicable. Starting from known or assumed initial conditions, all fluxes of energy, water and carbon dioxide are c o m p u t e d from the state of the system and the ambient weather conditions, the latter interpolated from a data set. The state of each c o m p a r t m e n t is then computed by adding the net flux to the known content of energy and mass, and the system condition is updated. The algorithms are simple except for the ones needed to find the crop temperature and the soil temperature. For these an iterative method is required because of the implicit functions of temperature in both cases. The updating takes place every 30 s and appropriate lists of variables are printed every hour, with daily summaries, if desired. The general method is one of forward, rectangular integration using a finite difference approach. The program is written in CSMP III, a user-oriented simulation language. It has been successfully translated into F O R T R A N by others. The program can and has been readily adapted to simulate different types of r o o f construction, different heating methods, evaporative cooling, and various thermostatic and flow controls.