A TUNNEL GREENHOUSE ADAPTED TO THE TROPICAL LOWLAND CLIMATE P.A, RAULT Institut de Recherches Agronomiques Tropicales I.R.A.T. / C.I.R.A.D. BP 427 97204 PORT DE FRANCE MARTINIQUE - FRENCH WEST INDIES

Abstract In 1986 when the "Soilless culture" program started in the

I.R.A.T. research station of Martinique," the previous problem to re-solve was the choice of an adequate shelter. Until then, the few shelters in production where "classic tunnels" : (plastic cover burried on the sides, without gable ends). It rappidly made no doubt that it was inadequate. The inside diurnal temperatures raised too high for most vegetables. Through the world none satisfying cotnnercial and technical solution appeared to our exigencies : - a simple shelter structure to protect the plants from the heavy di-verted rains. - a particularly performant aeration, specially for the humid lowland tropics. -a low cost (investment and functioning) with a view towards the deve-lopment and the extension of the protected crops in these areas.

Since 1965 in french Guyana, successful 1 vegetables productions under shelters are conducted. I.R.A.T. had developped a very perfor-mant woodden framed structure two span plastic greenhouse. Because of the lack of wood in Martinique and the frequency of intense cyclones, this technology could not be spread. In a first phase we decided to corrpare the performances in regard to the temperatures of : - a classic "umbrella tunnel". - a two-span steel framed greenhouse. - a completely closed tunnel with dynamic ventilation. Then on a second phase, we oriented our studies towards the adaptation of the tunnel for an optimal natural ventilation through two commer-cial prototypes. At the issue of this work, we are new in position to propose a sirrple and efficace solution to the thermic exigencies under plastic green-house in the lowland tropics, this "tropical tunnel" is a chimney ef-fect tunnel with a natural and static aeration favoured by continued low and top openings. Under the ridge opening, a transparent plastic gutter is disposed to collect the ram. 1.Introduction 1.1. Characteristics of the tropical climate

The plain of "le Lamentin" is quite representative of a lowland tropical area with the characteristics of a maritine climate : tempe-ratures rarely excessive, but constantly high'; abundant rainfall ; intense solar radiation ; presence of trade winds.

Acta Horticulturae 281, 1990 Greenhouse Construction, Design 95

At IRAT station, situed close by the level of the sea, (altitude : 35 meters), the rains are the most remarkable element : inequally reparted all year long, it usually rains 2 times more during the rainy season (220 rati per month from June to November) than during the dry season. The annual average rainfall height is 2 meters. . The average temperature is quite regular : annual average 26,1* C (mean min s 22,7 ; mean max : 29,6) ; only 3* C between the hottest (27,1 in June, July and August) and the coolest months (24,7 in Janua-ry and February). i The relative hvmidity rate is always high : 81 % from June to November. . The global radiation intensity is 1921 joules/cm2 per day equival-lent to 168 kcal/cm2/year.

It is distributed through 2883 hours of solar heat. The insolation is a constant of the Caribbean climate : an ave-

rage of 8 hours in "le Lamentin" (7,3 in October ; 8,3 in April). In fact, globally there is no excess of solar energy but it is

very concentrated 58 cal/cm2/solar heat hour), comparatively for exam-ple to Perpignan (south of France), where it is about 40 cal/cm2/solar heat hour. . The trade winds regularly blow and come from the coast (33 % parti-cularly from East and 28 % from East-North-East). 1.2. Climatic contraints under plastic shelter

The lack of locally outdoor grown fresh vegetables during the rainy season justify the sheltered culture.

Clouded over sky, high temperatures (night and day), high rela-tive humidity rate : all characteristics of a confined atmosphere li-miting for most vegetables, and that worsen in the greenhouse if it is not correctly thought ; Thought in terms of geographic implantation area in terms of site, of orientation and above all in terms of design.

Under a polyethylene film cover the greenhouse effect is significant. The resulting increasing temperatures have to be modulate by a constant and rapid exhaust.

During the day and especially from 10 a.m to 3 p.m, the objecti-ve is to keep the temperature as close as possible inside and outside the plastic cover.

Indeed with heat tolerant cultivars it is difficult to obtain a first quality vegetable production (tomato fruit for exanple) with long expositions to highest temperatures than 30* C.

This is the main reason why since 3 years we have looked for the ideal shelter in a tropical atmosphere. What is the ccnmercial struc-ture allowing the best compromise between protection, aeration, ro-bustness and cost ? 2. Materials and method 2.1. Hie different greenhouses types observed

They are illustrated and characterized in table 1. All greenhouses are oriented East-South-East.

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2.1.1. Experiments inr 1987 A. Umbrella tunnel - the aeration is natural and static : continued low lateral aeration - there is no cover on gable ends and the polyethylene 180 microns (y) is stopped on each lateral side at 1 meter from the soil. - the opening surface is 100 m2. B. Ventilated tunnel - the areation is forced, dynamic and automatic : dynamic ventilation - the system is constitued by :

2 ventilators with an extraction capacity of 41 000 m3/h each ; situed at the middle of each side.

. 4 air intakes (1,5 m x 1,5 m) (2 on each gable end) - theoretically the inside air volime is renewed 80 times per hour in the best case. - the plastic cover is a polyethylene film 180 y

C. Two span greenhouse - the aeration is natural and static. Integral 4 sides aeration - only the 4 pitches of the roof are covered with a polyethylene film 200 y - moreover there is a nanous opening 5 cm width on each side, at full length, of the ridge. - the opening surface is 243 m2

2.1.2. Experiments in 1989 A. Modified two span greenhouse - the aeration is natural and static. 3 sides complete aeration. - in this case only the gabble ends and a lateral side are uncovered. A polyethylene cover (200 y) has been set on the fourth side to pro-tect the plants from the strong winds and the heavy diverted rains. - narrow openings are kept at the bottom (5 cm) and at the top (10 cm) on full length of this covered side. - the opening surface is 160 m2

B. Chimney effect tunnel - the aeration is natural and static : continued low and top aeration. - to the continued lew lateral aeration of the "tmbrella tunnel is added a continued top opening of 0,35 m on both sides of the ridge. A rigid transparent P.V.C. gutter is disposed 0,5 m under the opening to collect the rain. - the opening surface is 157 m2. C. Tunnel with continuous opening window - the aeration is natural and static : and composed : - on the windy lateral side : of a continued lew aeration : the j5oly-thylene film (200 y) is stepped at 1,30 m from the soil. - on the out of wind side albove the level of the crops support bars : of a continuous moveable window (0,5 m of real opening). - the opening surface is 112 m2.

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2.2. Observations on the temperature To characterize the climate inside the greenhouses, only the

diurnal air temperatures is measured. The equipment for the climatic data acquisition is identical

with the CTIFL Device (conception and installation) (TOOOTIN H. 1987) - Data acquisition control unit 3421 (Hewlett Pacquard) - Thermocouple T placed in a ventilated shelter for air temperature, is situed in the center of the greenhouse at 1,50 m from the soil.

All our measures where executed in presence of a soilless tomato crop, (indeterminate type).

The data acquisition frequency is 10 minute between 6 a.m and 6 p.m 3. Results 3.1. Experiments in 1987

In a first phase, in 1987, the umbrelle tunnel, the ventileted tunnel and the two span greenhouse where compared.

The objective was, with regard to the umbrella tunnel and the outside surroundings, to establish the inside thermic data of - a dynamic ventilation in a tunnel structure. - a natural aeration in a commercial structure approaching the woodden framed shelter of Guyana. . Figure 1 situates the level of temperatures inside the umbrella tun-nel according to the type of weather. It underlines the fast rising temperature with a clear sunny sky. . The greenhouse effect is sensible from 7.30 a.m to 5.30 p.m and maxinun from 10 a.m to 3 p.m in touch with the solar radiation level. . Continuous recordings indicate that despite an equipment allowing 80 renewals of the inside air volume (900 m3), per hour, an electric ven-tilated is not significantly colder than a natural ventilated umbrella tunnel, (figure 2) Moreover both are for the mean maximum 3* C hotter (33,3* C) than out-side (30,1* C) over these 13 days of measure.

- on the other hand the temperatures inside and outside the "two span greenhouse" are quite similar respectively for the mean maximun : 30,7 and 30,1* C.

. Conclusion For 1 square meter of protected soil the multispan greenhouse

offers 0,72 m2 of opening and only 1,02 m2 of plastic surface : two explanations for its good performances.

At the opposite : 153 % of plastic cover surface for the dynamic ventilated tunnel worsen the greenhouse effect.

A bell effect without high exutory keeps the hot air captive in the case of the umbrella shelter.

The disadvantage of the two span greenhouse is the inadequacy of protection from the heavy diverted rains.

Because of the investment and working costs, the dynamic venti-lation is not justified in a tropical low altitude area.

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The most promising direction of investigation for a better mana-gement of microclimate under cover is an adaptation of both the mul-tispan greenhouse and the umbrella tunnel. 3.2. Experiments in 1989

In a second phase, in 1989, the two span greenhouse after modi-fication was ccrrpared to two prototypes conceived by two french firms : - Serres BN for the tunnel with the chimney effect - Serres de FRANCE RICHEL for the tunnel with a continuous opening window.

the good results obtained with the two span greenhouse in 1987 are lost when in 1989 the complete side exposed to the wind is closed by a plastic film.

This shelter is on average one degree hotter than outside on the diurnal period (6 a.m to 6 p.m) (see table 2).

The extension of the plastic film surface (25 % more) correspon-ding to a diminution of the ratio of ventilation explain the differen-ce of performance.

The greenhouse effect is increased and the circulation of air is less effective.

The two prototypes allow more confortable thermic conditions. The tunnel with the continuous opening window, situed above the level of the crop support bars, on the out of wind side is still perfectible with regard to the results of the chimney effect tunnel (figure 3). Only 0,4 *C separate the average diurnal temperature inside (28,4 *C) and outside (28 *C) the chimney effect tunnel.

A continued top ventilation added to a continued low lateral ae-ration on the 2 sides is the most effective solution (figure 4). The mean max is 1,3 *C higher than outside.

. Conclusion In the case of the two span greenhouse, a good compromise bet-

ween our two models observed would be the extension of the wind expo-sed span in a penthouse to permit both aeration and protection f rem the diverted rains.

In the case of a top aeration, the choice of protecting the ope-ning from the rain by a moveable window or with a fixed transparent gutter is a question of cost and also resistance to the winds.

With the BN chimney effect tunnel, it seems difficult to go fur-ther in the notions of simplicity, resistance and low cost. For those reasons, it deserves the denomination "Tropical tunnel". 4. Discussion

The observations effected show obviously that the tunnel green-house with sides and top ventilation, also named "tropical tunnel", offers good thermic performances in our conditions.

This superiority in comparison with the others shelters tested (except for the two span greenhouse) goes with other advantages s - Simplicity : the process can be easily put into operation by rapid modifications on any kind of tunnel structure.

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- Robustness The winds have no hold on the top gutter because of its inside position. - Good efficacy for a tolerable investment. Although, actually in Martinique, specially because of the transport costs and import taxes prices stay high, the "tropical tunnel" is a Relative low cost greenhouse.

. classic tmbrella tunnel : 80 FF/m2

. tunnel with opening window : 120 FF/m2

. "tropical tunnel" : 90 FF/m2

The investment cost for a moveable side wall or roof window has no justification in our conditions. The ventilation openings must be left open throughout the year both day and night.

With the "tropical tunnel" a great progress is now possible for the inprovment of quality and quantity production of vegetables under shelter. The first owners of this tunnel express their satisfaction for the very better working conditions.

If some improvements can still be done, concerning - a cheaper gutter material - an optimal calculation of the ratios of opening for the fastest air renewal, the most important result is that greenhouse growers in the lowland humid tropics can now reckon with a tunnel greenhouse adaptated to their climate and exigencies (figure 8).

REFERENCES ERUN,R et IA3IER,J., 1984. Etude d'un nouveau type d'abri mieux adapté au climat méditerranéen. Revue Horticole. 245. pp 25-32. COUEROY,A., 1987. Caractérisation du climat spontané sous trois types d'abris en zona tropicale humide. Mémoire E.N.I.T.A.H. Angers : 38 p. MEKIKDJIAN,C et PAPAIODANNIDIS,I., 1987. Résultats des travaux sur les structures de serres âdâptées au climat méditerranéen. Extraits de ocnEte-rendu des séances du comité des .plastiques en Agriculture. Agen 1986 - Chanfcourcy 1987 - CEM&31EF - Montpellier 86 p. RADLT,P.A., 1988. LA MARTINIQDE : Situation des cultures protégées. Revue Horticole, 284 : pp 45-47, 49, 50 ; 285 : 11, 13-16. RADLT,P.A., 1988. Protected crops in Humid tropical regions. How could we avoid or reduce excessive temperatures ? How could we select the cladding materials and the greenhouse design ? Acta Horticulture, 230, 1988. High technology in Protected cultivation : pp 565-572. TROTTIN,H., 1987. Logiciel d'acquisition de données et de contrôle de processus sur centrale de mesure mobile HP 82479 A - CTIFL - Nîmes : 30 p.

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Table 1 - Characteristics of the different greenhouses types observed

Temperatures

from 6 am Mean

Difference with

outside

) ) ) Mean max )

Difference with

outside

to 6 |li •1987 * 1989 1987 1989 ) 1987 1989 1987 1989

Outside 28 3 28,1 30,1 30,0

Umbrella tunnel 29 5 +1,2 33,3 +3,2

Ventilated tunnel 30 0 +1,7 33,3 •3,2

Two span greenhouse 28 5 +0,2 30,7 • 0,6

Modified two span greenhouse 29,1 • 1,0 33,2 +3.2

Chimney effect tunnel 28,5 • 0,4 31,3 +1,3

Tunnel with opening window 28,9 +0.8 32,3 +2,3

* 1987 : 13 days of measure during the period 28 july - 10 august

* 1989 : 12 days of measure during the period 8 july - 27 july

Table 2 - Oiurnal air temperature.for each greenhouse tested, Mean and Mean max temperatures and their differences with outside during the periods of measure in 1987 and 1989

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Type days : 1st may, 11 may, 31 may 38 -, -

06:00 07:30 09:00 10:30 12:00 13:30 15:00 16:30 18:00

TIME

• Changeable sky : 1st may + Cloudy sky : 11 may 0 Clear sunny sky : 31 may

F i g u r e 1 - D i u r n a l a i r t e m p e r a t u r e s under t h e " u m b r e l l a t u n n e l "

J u l y Augus t

o Dynamic v e n t i l a t i o n + u m b r e l l a t u n n e l a i r t u n n e l a i r

0 two span g reenhouse û o u t s i d e a i r a i r

F i g u r e 2 - Max and min d i u r n a l a i r t e . u p e r a t u r e d u r i n g 13 days o f measure i n 1987

A 3 7 -

36_

3 5 "

34-

33

32

3 1 -

30-

2 9 -

2 8 -

27"

2 6 -

25"

24-23

o u t s i d e chimney e f f e c t t u n n e l t u n n e l w i t h open ing window m o d i f i e d two spen g r e e n h o u s e

8 / 0 7 9/07 14/07 15/07 16/07 17/07 18/07 22/07 23/07 24/07 25/07 26/07 Oat.

Figure 3 - Hex end «in diurnel a i r teaiperoture during 12 days of measure in 1989

34-

33-

O 32-

I 31-

i 30.

s 1. 29-• 28 .

10:00 — * —

11:00 12:00 13:00 14:00 15:00

global radiation maan modified two span greenhouse tunnel with opening window chimney effect tunnel outside

.1000

- 900

800

r 600 Z 500 o

775 W/a* 32,2 'C 31,« *C 30,7 -C 29,7 -C

Figure 4 - Evolution of the air temperatures insids snd outside the different greenhouses on a tunny day

Figure 5 : Tropical tunnel greenhouse I.R.A.T Martinique 1989

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