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ScienceDirect Solar Energy 115 (2015) 217–228 www.elsevier.com/locate/solener

The effect of nocturnal shutter on insulated greenhouse using a solar air heater with latent storage energy Sami Kooli ⇑, Salwa Bouadila, Mariem Lazaar, Abdelhamid Farhat Research and Technology Center of Energy, Thermal Processes Laboratory, Hammam Lif, B.P. 95, 2050 Tunis, Tunisia Received 5 January 2014; received in revised form 25 July 2014; accepted 25 February 2015

Communicated by: Associate Editor G.N. Tiwari

Abstract In order to reduce the energy consumption in agricultural greenhouses at night, two similar greenhouses with a nocturnal shutter are constructed and installed in the CRTEn (Research and Technologies Centre of Energy) in Tunisia. The first is equipped with a heating system. The solar heating system is a solar air heater collector with latent heat storage. At daytime, thermal solar energy is stored, however, at night it can be restored. Moreover, the shutter is used only at night. The analysis of the thermal energy is used to examine the repartition of the absorbed, the useful, the stored and the losses of energy in the greenhouse; with or without nocturnal shutter. The balances of the various components of the greenhouse are used to study the portions of the energy recovered, absorbed, stored and lost. The experimentally obtained results show that: the nocturnal variations of temperature inside the two greenhouses exceed 2 °C between the first (with shutter) and the second one (without). Also, the nocturnal temperature inside the greenhouse equipped with solar heating system was maintained to 15 °C while the outside temperature decreases to 8 °C. Ó 2015 Elsevier Ltd. All rights reserved.

Keywords: Latent heat storage; Greenhouse climate control; Packed bed solar air heater

1. Introduction From 2005 to 2008, the greenhouse areas of Tunisia increased from 4600 ha to 8683 ha (APIA). One of the greatest problems encountered in greenhouses is the control of the internal climate. The lack of heating has unfavorable effects on the precocity of production. The basic strategy of greenhouse passive heating system is to reduce the heat losses and at the same time to transfer excess heat from inside the greenhouse during the day to heat storage. The use of thermal screens to reduce the heat losses in the greenhouse are cited in literature. Bailey (1981) ⇑ Corresponding author. Tel.: +216 52 362 357; fax: +216 71 430 934.

E-mail address: [email protected] (S. Kooli). http://dx.doi.org/10.1016/j.solener.2015.02.041 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.

verified that the use of the thermal screens are commonly drawn over the crop at sunset and removed at sunrise; can reduce the overnight heat loss by 35–60%. Thermal radiation can become the dominant mechanism of night-time heat loss from a greenhouse, particularly when there is a clear sky (Silva and Rosa, 1987). In addition to reducing thermal radiation, screens that are impermeable to air decrease the volume of the greenhouse air that needs to be heated and form an extra air gap between ¨ ztu¨rk and the crop and the greenhouse roof (O Basc¸etinc¸elik, 1997), thereby reducing the heat transfer to the surroundings. The use of energy-saving screens allows for an increase in night temperatures but when they are fixed screens they decrease radiation so their use is of no interest (Lo´pez et al., 2003). However, some growers do use them, in order

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Nomenclature A C Cp da F h I dir I dif IT kv LAI l P ðT Þ pv Q RE ra rs T T t U Vo y z

surface area (m2) thermal capacity specific heat of air at constant pressure (J/kg K) density of air (kg/m3) view factor heat transfer coefficient (W/m2 K) diffuse solar radiation (W/m2) direct solar radiation (W/m2) total solar radiation (W/m2) canopy attenuation coefficient leaf area index characteristic length of the leaf canopy (m) saturated water vapor pressure at temperature T (kPa) fraction of area heat rate (W) rate of air infiltration (m3/s) aerodynamic resistance (s/m) stomatal resistance (s/m) temperature (K) average temperature (K) time (s) loss coefficient (W/m2 K) wind speed (m/s) psychometric constant (0:0667 kPa=K) depth of soil (m)

Greek symbols a absorptivity for solar radiations at absorptivity for thermal radiations e emissivity

to limit the fall of water droplets (from condensed water vapor or from the rain in artisan low-cost greenhouses) over the crop. Furthermore, because screens reduce thermal radiation, heat loss by radiation from the crop is reduced and crop temperature is expected to be raised. Kittas et al. (2003) considered the influence of an aluminized thermal screen on greenhouse microclimate and canopy energy balance, and reported that with a thermal screen the microclimate at crop level was more homogeneous and the average air and canopy temperatures were higher than without a screen. However, the energy saving with a 65% aluminized thermal screen in their experiments was only about 15%. Mobile thermal screens improve the yield and adapt well to multi-tunnel greenhouses, being of more interest in heated greenhouses than in unheated ones (Meca et al., 2003). Thus, they keep the internal air temperature higher than it would be without a screen (Montero et al., 2005). Baille et al. (2006) analyzed the night energy balance of an air-heated greenhouse in mildwinter climatic conditions. They enhanced that the energy efficiency would be: to improve the air tightness of the

c k h q r s

fraction of the solar radiation thermal conductivity (W/m K) incident angle reflectivity Stefan–Boltzmann constant ð5:670  108 W= m2 K4 Þ transmissivity

Subscripts c cover of greenhouse col collected g greenhouse i inside greenhouse inf infiltration los loss net net o outsider greenhouse s soil sto stored sky sky sun sun v canopy Exposants C convective heat Cd conductive heat inf infiltration L latent heat R radiation heat S solar radiations

greenhouse, to reduce radiative losses by means of thermal screens, and to increase the soil efficiency in storing solar energy and releasing it during the night. Teitel et al. (2009) reported that the thermal screen did not reduce the heat loss from the greenhouse because it was relatively small in area and only 20% of its area was covered by reflective aluminized material. The solar energy is an attractive substitute for conventional fuels for passive and active heating applications. However, the intermittent characteristic of the solar radiations has led to the improvement of suitable collection and storage technologies. In a greenhouse, the use of solar thermal energy covers one part of the crop heating demand needed during the day; its handling during nighttime requires an efficient storage system, so that the excess of heat is stored for later use during the night. The use of PCMs (Phase Change Materials) is one of the most efficient ways of storing thermal energy for heating and cooling applications (Jurinak and Adbel-Khalik, 1978; Morrison and Abdel-khalil, 1978; Ghoneim and

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Klein, 1989; Sokhansan and Schoenau, 1991; Esen et al., 1998; Hasnain, 1998; Barba and Spiga, 2003; Farid et al., 2004). Studies about the application and utilization of PCM for space heating greenhouses dates back to 1980, when Kern and Aldrich (1979) used 1650 kg of CaCl26H2O in aerosol cans each weighing 0.74 kg were used to investigate energy storage possibilities both inside and outside a 36 m2 ground area greenhouse. Results of this study indicated that, while the stored energy by the outside unit was between 105.5 and 158.25 MJ with a solar energy fraction of 38–43%, these were respectively 21.1 and 31.65 MJ for the internal unit with a somewhat much smaller solar fraction of 6–8%. Furthermore, it was calculated that while the external unit released about 80–90% of the energy absorbed, this was 60–80% for the internal energy storage unit. Paris (1981) had used an eutectic mixture of sodium hydroxide (NaOH) solution and chromium nitride (Cr2N melting point 15.6 °C) in a 445 m2 experimental glasshouse. The material was stored in 21 containers. The heat storage produced a minimum air temperature of 8 °C inside the greenhouse under extreme winter conditions and saved about 5000 l of oil. Jaffrin et al. (1982) studied the performances of a solar greenhouse equipped with a storage system using 13.5 tons of CaCl26H2O phase change materials encapsulated into flat plastic pouches; in a glass-covered multi-span greenhouse with 500 m2 ground area. Thermal and crop performance of this greenhouse was compared with an air-bubble double plastic-covered and a conventional greenhouse. The containers were placed in concrete shelves with five layers in semi-circular tunnels and air was used to transfer heat. This system saved 80% and 60% of propane gas in comparison with the conventional and double-covered greenhouses, respectively. Electricity consumption of the fans was determined to be less than 10% of the heating load of the greenhouse. Nishina and Takakura (1984) conducted a research applying Na2SO410H2O with some additives to prevent phase separation and degradation for heating a greenhouse in Japan. They achieved that only 40–60% of the latent heat potential of the PCM was realized which indicated that almost half of the PCM was not used efficiently during the energy exchange processes. Huang et al. (1986) has designed and constructed a storage system with two different stacking configurations and air baffling integrated with greenhouse solar system. Cylindrical storage rods were used as the primary storage elements. The result showed that the designed latent storage systems demonstrated significantly higher compact storage capacity than the rock or water storage. Levav and Zamir (1987) tested two different storage arrangements containing CaCl26H2O. In the first, two stores containing PCM tubes were placed in the North side of the greenhouse and in the second one similar PCM tubes were contained in an underground store. The required air temperature in the greenhouse was achieved without any increase in the relative humidity. They indicated that the 24 °C melting temperature of the PCM could

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still be high for some crops and the pipe diameter seemed larger than necessary together with the undesired pipe thickness which reduced heat conduction to the material inside. Boulard et al. (1990) used a latent heat storage system in a double-skin polycarbonate greenhouse equipped with a forced ventilation system. They found that the inside of the greenhouse temperature is 10 °C higher than the outside temperature during typical nights of March and April. ¨ ztu¨rk (2005) presented a seasonal thermal energy storage O using 6000 kg of paraffin wax as a PCM with the latent heat storage technique and attempted to heat the greenhouse of 180 m2 floor area. During the experimental period, it was found that the average net energy and exergy efficiencies were 40.4% and 4.2%, respectively. Benli and Durmusß (2009) conducted a study on the performance analysis of a latent heat storage system. In their study, a 10-piece flat plate air solar collector was used to heat up a greenhouse located in Turkey. A corrugated plate, reverse corrugated plate, trapeze plate, reverse trapeze plate and a flat absorption plate in combination with a phase change material were used for this work. They reported that such system is capable to provide 18–23% of the total daily thermal energy of a greenhouse for about 3-4 h in comparison to conventional heating devices. And the difference in temperatures between indoors and outdoors was around 6–9 °C. The aim of this work is to determine the effect of nocturnal shutter and the heat provided by a SAHLSC (Solar Air Heater with Latent Storage Collector) inside an Insulated Greenhouse (IG). An analytical study of the greenhouse (with and without shutter) parameters are used to determine the fraction of the solar radiation absorbed by the greenhouse, the overall heat transfer coefficient, the energy loss coefficient and the stored heat. Experiments were carried out in two greenhouses designed and realized in the CRTEn (Research and Technology Center of Energy) in Tunisia. We will present in Section 2, the description of the system and the site. In Section 3, thermal analyses of the greenhouses climate are carried out. Finally, we will report the experimental results in Section 4. The main remarks of this work will be reported in the conclusion. 2. Description of the system and site In this study, experiments were carried out in two greenhouses oriented to East–West shown in Fig. 1(a) and (b). The first is an Insulated Greenhouse (IG); the second used a Solar Air heater with Latent Storage Collector (SAHLSC) and named Insulated Greenhouse with Latent Heat System (IGLHS). The experiments were performed during December 2012 in the CRTEn in Borj Ce´dria. The experiment’s site is a sunny region, situated on the Mediterranean coast of north Africa, near the city of Tunis in Tunisia, with the following coordinates: Latitude 36°430 N and Longitude 10°250 E.

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Fig. 1. (a) External view of IG, (b) external view of IGLHS and (c) covered greenhouses at night.

Fig. 3. Photograph of the experimental SAHLSC inside greenhouse.

Fig. 2. Schematic view of SAHLSC.

2.1. The latent heat storage system The SAHLSC is used as a means to heat the interior environment of the greenhouses during the nighttime. A schematic arrangement of a new solar air heater with phase change energy storage using spherical capsules is given in Fig. 2 and a photograph of the SAHLSC inside the

greenhouses is shown in Fig. 3 (Bouadila et al., 2013). The experimental device consists of a packed bed absorber formed of spherical capsules; with a black coating and fixed with a steel matrix; with the PCM confined inside those capsules. The packed bed absorbs the sun radiations and stores the solar thermal energy as sensible and latent heat. The length, the width and the total volume of the collector are 2 m, 1 m and 0.28 m3, respectively. A 0.004 m thick transparent glass cover was placed 0.015 m apart from the absorber. A 0.05 m thick polyurethane insulation, with

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Table 1 Thermal properties of PCM. Material

Melting point (°C)

Heat of fusion (kJ kg1)

Specific heat (kJ/kg °C)

Density (kg/m3)

Thermal conductivity (W/m °C)

Capsule (AC27)

27

192.6

Liquid

Solid

Liquid

Solid

Liquid

Solid

2.22

1.42

1710

1530

0.58

1.05

heat conductivity 0.028 W/m K, is placed in the bottom of the collector. The thermo-physical properties of the capsule are given in Table 1. 2.2. Experimental greenhouses In this study, experiments were carried out in two small chapel-shaped greenhouses with same dimensions. The experimental greenhouses occupy a floor area equal to 14.8 m2, 3.7 m width, 4 m long and 3 m high at the center. The greenhouse wall and roof oriented to the south are made by plexiglass with 3 mm of thickness. Sidewalls and the northern roof are built by sandwich panels with 0.4 m and 0.6 m of thickness, respectively. At night, an aluminum roller shutter with small blades automatically spreads over the southern face of the IGLHS (Fig. 1(c)). The greenhouses were equipped with a centrifugal fan controlled by a differential thermostat. The fan operates when the temperature inside the greenhouse exceeds the optimal growth temperature of the plant. A tomato crop was planted in both greenhouses. The reflectivity and transmissivity spectra of the canopy and cover layer recorded in the wavelength range (380– 1400 nm) were listed in Table 2. The reflectivity and transmissivity spectra were realized using LAMBDA950 UV– vis–NIR spectrometer equipped with an integrated sphere. 2.3. Measuring equipment inside and outside the greenhouse K-type thermocouples are used and placed in the IG to measure the horizontal distributions of temperature; the vertical temperature distributions, for measuring the top and the bottom temperatures of the plexiglass and sandwich panels. The average temperature of the canopy was measured using an infrared temperature sensor IR120. The IR-SS Solar Shield is used to protect the IR120 sensor from direct solar radiation (Campbell scientific). Temperatures under the soil inside the IG at 0, 0.25 and 0.5 m depth were measured using three PT-107 sensors. The inside greenhouse temperature and relative humidity sensor Table 2 Cover, canopy and soil characteristic.

Reflectivity for solar radiations Transmissivity for solar radiations Reflectivity for thermal radiations Transmissivity for thermal radiations Emissivity

Cover

Canopy

Soil

0.10 0.85 0.10 0.88 0.02

0.1 0 0.55 0.4 0.05

0.2 0 0.16 0 0.84

(HMP155A, Campbell Scientific) was inserted in the center at 1.5 m of level from the ground. The HMP155A sensor was put into the probe shelter; composed of 14-plate radiation shield houses which protect the HMP155A temperature and RH probe. Its louvered construction allows air to pass freely through the shield, thereby keeping the probe at or near ambient temperature. The probe shelter is white, allowing it to reflect solar radiation (Campbell scientific). The global solar irradiation in the horizontal plane was measured with a Kipp and Zonen pyranometer (CM11). It’s situated at 1.5 m above the ground in the center of the IG. Ambient temperature and relative humidity are measured by a HMP155A sensor situated at a height of 1.5 m above the greenhouse. Temperatures in the SAHLSC were measured using Ktype thermocouples. It is placed to measure the temperature inside capsules, the absorber surface of one capsule, the glass, the inlet and the outlet air temperatures of the collector. All climatic and measured parameters are sample recorded every 10 min using a CR5000 data logger (Campbell Scientific Inc). 3. Thermal analysis of the greenhouse climate The energy analysis presented in this section is mainly based on the first law of thermodynamics. Our interest has been focused on the absorbed, useful, stored and lost energy of the greenhouse and the solar air heater. Fig. 4 described the heat flux network chart showing the energy relationship in the studied greenhouse. The theoretical analysis employed for the study of the IGLHS consists of using static thermal energy balance and involves the following heat fluxes: – The fluxes entering the greenhouse: net needs of energy delivered by the heating system (Qnet ) and total solar energy collected inside the greenhouse (Qcol ). – The fluxes leaving the greenhouse: total thermal losses (Qlos ). – The fluxes stored inside the greenhouse: stored energy in the greenhouse (Qsto ). Qnet þ Qcol  Qlos  Qsto ¼ 0

ð1Þ

The expressions for each term of the above equation are formulated as: ½Collected energy ¼ Qcol ¼ cI T Ag

ð2Þ

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Fig. 4. The heat flux network chart of the IGLHS.

The solar efficiency c is the fraction of the solar radiation absorbed by the greenhouse components.   ð3Þ ½Loss energy ¼ Qlos ¼ Ug Ag T IGLHS  To i The energy loss coefficient, Ug , includes only the thermal losses by the plexiglass cover. Ug was calculated from experiments. The stored energy inside the greenhouse suggested by Boulard and Baille (1987) is expressed as follows: ½Stored energy ¼ Qsto ¼ Ag Cg

dT g dt

ð4Þ

where Cg is the effective heat capacitance of the greenhouse. Then Eq. (1) becomes: Qnet ¼ cI T Ag þ



Ug Ag T IGLHS i

 To



dT g þ Ag Cg dt

The energy balance of the IG can be expressed: dT g   dt 0 ¼ cI T Ag  Ug Ag T IG i  T o  Ag C g

ð5Þ

the cover QSv!c , solar radiations reflected by the soil surface and absorbed by the cover QSs!c , solar radiations absorbed by the soil after transmission through the cladding QSsun!s , solar radiations reflected by the vegetation and absorbed by the soil QSv!s , solar radiations absorbed by vegetation after transmission through the cladding QSsun!v , and solar radiations reflected by soil and absorbed by vegetation QSs!v . According to Eq. (2) the fraction of the solar radiation absorbed by the greenhouse is written as follows: c¼

ðQSsun!c þ QSv!c þ QSs!c Þ þ ðQSsun!s þ QSv!s Þ þ ðQSsun!v þ QSs!v Þ Ag I T ð8Þ



QSc þ QSs þ QSv Ag I T

ð9Þ

where ð6Þ

The solar collector is integrated inside the greenhouse then the delivered heat becomes to the useful and loses heat of the SAHLSC. This amount of energy rise the air temto T IGLHS . The perature inside the greenhouse from T IG i i recovered energy is obtained by adding the energy balance of the IG (without collector), Eq. (5), to the energy balance of the IGLHS (without collector), Eq. (6).   Qnet ¼ Ug Ag T IGLHS  T IG ð7Þ i i 3.1. Collected solar energy The collected solar energy includes: solar radiations directly absorbed by the plexiglass cover QSsun!c , solar radiations reflected by the vegetation and absorbed by

 The solar radiation absorbed by the cover is expressed by: QSc ¼ ðaSc Ac þ pv qSv aSc sSc Ac þ ð1  pv ÞqSs aSc sSc Ac ÞðI dir cos h þ I dif Þ ð10Þ where h is the incident angle of the solar radiations.  The solar radiation absorbed by the soil is given by: S S QSs ¼ ðð1  pv ÞaSs sSc As þ pv sS v as sc As ÞðI dir cos h þ I dif Þ

ð11Þ

The determination of the fraction of the solar radiation flux that is absorbed by the vegetation can be described as a special case of Beer’s law (Monteith and Unsworth, 1990).   kv LAI S aS ð12Þ v ¼ 1  qv e S S sS v ¼ 1  qv  av

ð13Þ

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With k v is the canopy attenuation coefficient and LAI is leaf area index.  The solar radiation absorbed by the canopy is given by: QSv

¼

S ðpv aS v As s c

þ

S S S pv sS v qs av Ac sc ÞðI dir

cos h þ I dif Þ ð14Þ

3.2. Greenhouse thermal losses

R QCco þ Qinf io  Qgo ðT i  T o ÞAg  The convection heat, QCco , is given by:

QCco ¼ hCco Ac ðT c  T o Þ

ð15Þ

ð16Þ

Different relations have been used to calculate the external heat loss coefficient at the outside surface of greenhouse covers (Mac Adams, 1954; Seginer and Livne, 1978; Kindelan, 1980; Papadakis et al., 1992). It depends to the greenhouse geometry under any environmental conditions. Mac Adams (1954) suggested a relation, which has been used in this study: hCco ¼ 5:7 þ 3:8V o

ð17Þ

where V o is the wind speed outside the greenhouse.  The infiltration heat, Qinf io , is given by: Qinf io ¼ d a C p REðT i  T o Þ

ð18Þ

ð19Þ

where 1. The thermal radiation through the soil is given by: QRssky ¼ es QRsky!s  QRs!sky ð20Þ   QRssky ¼ es As ð1  pv ÞF sc sRc rT 4sky þ As pv F sc sRc sRv rT 4sky    sRc As ð1  pv ÞF sc res T 4s þ As pv F sc rsRv es T 4s ð21Þ The appropriate approximation to sky temperature is given by Swinbak (Duffie and Beckman, 1991): T sky ¼ 0:0552T 1:5 o

QRvsky ¼ ev QRsky!v  QRv!sky

ð23Þ

QRvsky ¼ Av F vc ev sRc rT 4sky þ sRv qRs Av F vc ev sRc rT 4sky  sRc As pv F sc rev T 4v

QRcsky ¼ ec QRsky!c  QRc!sky   QRcsky ¼ Ac rec T 4sky 1 þ sRc F sc ðð1  pv Þqs þ pv qv Þ  Ac rec T 4c

ð24Þ

ð22Þ

ð25Þ

ð26Þ

3.3. The greenhouse stored heat The stored heat inside the greenhouse is the sum of three thermal exchanges: conductive heat transfer into the soil surface QCd s , latent heat transfer between the canopy and the air inside the greenhouse QLv!i and thermal radiation transfer inside the greenhouse, QRgi . According to Eq. (4) the stored heat is: L R QCd s  Qvi þ Qgi ð27Þ Ag  The conductive heat transfer into soil QCd s , is given by:

Qsto ¼

QCd s ¼

 ks  Ag T sjz¼0  T sjz¼50 z

ð28Þ

 The latent heat, QLv!i , is evaluated as follows: QLvi ¼

 The radiation outwards QRgo is made up of the radiation passing directly through the cover from the soil to the sky QRssky , from the vegetation QRvsky , and that emitted outwards from the cover itself, QRcsky . QRgo ¼ QRssky þ QRvsky þ QRcsky

2. The thermal radiation through the vegetation is given by:

3. The thermal radiation through the cover surface is given by:

Greenhouse thermal losses are the sum of three thermal exchanges: between outside plexiglass cover and outside air by convection QCco , between inside and outside air by air infiltration Qinf io and between greenhouse components (cover, soil and vegetation) and sky by thermal radiation QRgo . According to Eq. (3) the energy loss coefficient is written: Ug ¼

223

d a C p :LAI ðP  ðT v Þ  P ðT i ÞÞ Av y ra þ rs

ð29Þ

The saturated water vapor pressure P  ðT v Þ at the temperature of the canopy is calculated by Tetens equation Eq. (30), the following empirical formula relating accurately saturated water vapor pressure to temperature for the temperature range between 0 and 60 °C (Boulard and Wang, 2000; Tetens, 1930):   17:27T v  P ðT v Þ ¼ 0:6108 exp ð30Þ T v þ 237:3 The aerodynamic resistance, ra (s m1) of the canopy, mainly depends on the aerodynamic regime that prevails in the greenhouse and the leaf length. It is expressed as (Boulard and Wang, 2000): ra ¼ 220

l0:2 V 0:8 i

ð31Þ

where l is the characteristic length of the leaf and V i is the mean interior air speeds. The stomatal resistance rs (s m1) of the canopy is derived from a simple empirical relationship, Eq. (32), with global radiation (thermal and humidity dependences of

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greenhouse plants transpiration were neglected) (Kindelan, 1980; Kittas et al., 2003):   1 ð32Þ rs ¼ 200 1 þ expð0:05ðsc I T  50Þ

 The internal thermal radiation, QRgi , includes: thermal radiation emitted by the soil surface toward the cover QRs!c , thermal radiation emitted by the cover surface toward the soil QRc!s , thermal radiation emitted by the cover toward the canopy QRc!v , thermal radiations emitted by the canopy toward the cover QRv!c . QRgi ¼ QRci þ QRsi þ QRvi

ð33Þ

where QRci ¼ ec ðAs ð1  pv ÞF sc res T 4s þ As pv F sc rsRv es T 4s Þ  As ð1  pv ÞF sc rec T 4c þ ec sRc As pv F sc rev T 4v  As pv F vc rec T 4c     QRsi ¼ es QRc!s þ sRv QRc!v þ qRc QRs!c  1  sRc QRs!c   QRvi ¼ ev QRc!v  1  sRc QRv!c

ð34Þ

ð35Þ ð36Þ

4. Uncertainty analysis The calculated uncertainties of the dependent parameters were estimated by Eq. (37). The result R is a given function in terms of the independent variables. Let wR be the uncertainty in the result and w1 ; w2 ; . . . ; wn be the uncertainties in the independent variables. The result R is a given function of the independent variables x1 ; x2 ; . . . ; xn . If the uncertainties in the independent variables are all given with the same odds, then the uncertainty in the result having these odds is calculated by Holman (1994): " 2  2  2 #1=2 @R @R @R wR ¼ w1 þ w2 þ    þ wn @x1 @x2 @xn ð37Þ Uncertainty analysis is needed to prove the accuracy of the experiments. The independent parameters measured in the experiments reported here are temperature, air velocity, and solar radiations. To carry out these experiments, the sensitiveness of data acquisition system is about ±0.001 °C, the measurement error is ±0.002 °C, the sensitiveness of the thermocouple is ±0.01 °C, the sensitiveness of the IR120 is ±0.0004 °C and sensitiveness of the PT107 is ±0.01 °C. The HMP155A errors are ±0.02 °C of temperature and ±2% of humidity. An anemometer with ±0.01 ms1 accuracy, and Kipp and Zonen pyranometer with ±3% measurements uncertainties are used. The sensitiveness was obtained from catalogs of the instruments.

5. Results and discussion 5.1. Influence of the shutter on air temperature inside greenhouses In order to evaluate the effect of the shutter on inside the greenhouses air temperature, experimental studies in the month of December 2012 for two cases (without and with shutter) are done. The first set of experiments with shutter at night was done from 18th to 20th December, 2012; the second sets of measurements without the shutter were done from 21th to 22th December, 2012. The external climate conditions for the 5-days experimental period are shown in Fig. 5. The maximum of the transmitted global solar radiation intensity inside the greenhouse in the horizontal plane ranging between 200 and 450 W m2, and outdoor temperature varied from 6 to 19 °C. The wind velocity in the region of the greenhouses varied between 2 and 8 ms1 and the mean value of relative ambient humidity is around 70%. The daily variations of the ambient temperature, air temperature inside IG and air temperature inside IGLHS with and without shutter at night are presented in Fig. 6. The 18th and 21th of December, 2012 are a comparative climatic conditions days; the diurnal temperature attained 42 °C inside the two greenhouses and an average nocturnal temperature equal to 15 °C. At night, the air temperature inside the IG, without shutter was equal to the outside temperature, whereas for the greenhouse with shutter the difference between the outside and the inside increased up to 2.5 °C, in the first half of night, and decreased gradually until become equal to the outside temperature. For the IGLHS a similar air temperature trend was observed, with higher values due to the solar air heater collector with the latent heat storage system integrated inside this greenhouse. The solar collector is used to store the excess of the transmitted solar irradiation in the packed bed absorber and to provide it at night. The discharging process of the SAHLSC starts at 16:00 (local time). A fan used to blow the air at a fixed speed equal to 1 ms1. For both cases (greenhouse with or without shutter), we found that the nocturnal variation of temperature inside the IGLHS exceeds the one inside IG with a difference of 2 °C. On the 19th of December, 2012 characterized by better climatic conditions, the SAHLSC has maintained air temperature inside IGLHS with shutter constant, around 17 °C, along the night. The temperature inside IG was also maintained to 15 °C with nocturnal outside temperature decreasing to 8 °C. We deduced that the shutter reduced the nocturnal heat loss by reducing the convective heat between the glazing material and the exterior and the radiation loss to the sky. 5.2. The insulated greenhouse thermal interaction Greenhouses show a thermal interaction with their components and the surrounding (cover, canopy, soil, inside

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Fig. 5. Global solar radiation and ambient temperature in function of days (18th–22th December, 2012).

Fig. 6. Ambient temperature, air temperature inside IG and air temperature inside IGLHS function of days (18th–22th December, 2012).

and outside air). The change in energy within a greenhouse can be expressed by the energy balance equation Eq. (1). The energy gain is due to solar radiation, Eq. (2), the energy loss is due to radiation, convection and infiltration heat, Eq. (3) and the energy stored inside their components, Eq. (4). Fig. 7. present the heat gain repartition during the sunshine period in 19th of December, 2012. The heat gain to the IG was assessed with solar radiation. The calculation of the solar radiation absorbed by the cover is determined by Eq. (10), the solar radiation absorbed by the soil is given by Eq. (11) and the radiation absorbed by the canopy is determined by Eq. (14). In this study, the collected heat inside IG provided: 8% was collected by the cover, 10%

Fig. 7. The collected heat inside IG (19th December, 2012).

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by the canopy and 82% by the soil. Then, the soil is the largest absorber of the solar radiation inside the greenhouse; this value depends on soil quality, humidity and day and night temperatures. The heat transfer modes convection, Eq. (16), infiltration, Eq. (18), and radiation, Eq. (19), contribute to greenhouse energy loss. Based on these equations, the greenhouse energy losses calculation may be carried out as follows Figs. 8a and 8b during respectively the 19th of December, 2012 (with shutter) and 21th of December, 2012 (without shutter). The radiative flux loss is mainly determined by the transmittance, emittance and reflectance of the glazing material. The radiation heat loss rate is estimated by 24% of the total heat loss of the IG with shutter and 61% of the IG without shutter at night. Thermal radiation loss becomes the dominant mechanism of heat loss inside IG without shutter at night. It turns out that the thermal radiation exchange is a very important factor in determining the thermal environment inside a greenhouse. The value of the total heat loss inside the greenhouse with shutter on 19th of December, 2012 is 2.8 kW smaller than losses inside the IG without shutter where the value is about 7.9 kW. It should be noted that the shutter reduced 35% of the loss resulting between glazing material and the exterior. As described by Eq. (15), the global heat loss coefficient can be expressed as function of temperatures inside air, outside air, sky and the outside surface of the cover. The

calculated nocturnal coefficient for the IG with or without shutter at night, are respectively 4.39 W m2 K1 and 6.28 W m2 K1. These coefficients are considered smaller than the conventional greenhouse where the average value is about 10 W m2 K1 (Jolliet et al., 1991). The design of the IG (northern wall covered by sandwich panels) and shutter maximizes solar radiation input and reduces heat losses. Data on climate, the greenhouse and the canopy characteristics and its conditions of use are assembled to estimate the stored heat inside the greenhouse. Figs. 9a and 9b illustrated the stored heat inside the IG greenhouse with shutter at night on 19th of December 2012 and without shutter at night on the 21th of December, 2012. The stored energy is evaluated by the following equations: the conduction heat absorbed by soil, Eq. (28), latent heat, Eq. (29), the radiation heat absorbed by cover, Eq. (34), the radiation heat absorbed by soil, Eq. (35) and the radiation heat absorbed by canopy, Eq. (36). The heat stored inside the greenhouse with shutter at night by conduction heat absorbed into soil is about 49% of total stored heat. The form of latent heat as well as sensible heat is taken into account, the former representing an average of 36% of the overall stored heat. Energy stored in the form of radiation heat represent an

Fig. 9a. The stored heat inside IG with shutter at night (19th December, 2012). Fig. 8a. The loss heat inside IG with shutter at night (19th December, 2012).

Fig. 8b. The loss heat inside IG without shutter at night (21th December, 2012).

Fig. 9b. The stored heat inside IG without shutter at night (21th December, 2012).

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average of 11% absorbed by soil, 3% absorbed by cover and 1% by canopy of total stored heat inside the IG with shutter at night. The thermal interactions inside the IG without shutter at night changes and depend to the transmissivity, emissivity and reflectivity of the greenhouse glazing materials, ground and canopy. Fig. 9b examines the contributions to maximizing or minimizing stored heat inside the components of the IG without shutter. Then, the conduction heat absorbed by soil becomes 41.5%, latent heat is 30%, the radiation heat absorbed by cover is 7%, the radiation heat absorbed by soil is 21% and the radiation heat absorbed by canopy becomes 0.5%. 5.3. The recovered heat from the SAHLSC Figs. 10a and 10b presented the variations of the diurnal stored heat in the PCM and the night time recovered heat for different times during two days, the 19th and 21th of December, 2012 respectively with and without shutter. Potential recoverable heat of the SAHLSC inside the IG at night was determined by using Eq. (7). The charging process tests are performed from 9:00 to 16:00 (local time). It is observed that during the initial period of charging, the instantaneous heat stored increases with insolation and towards a maximum value of 475 W at 13:00 (Fig. 10a). During the charging process, the instantaneous stored heat fluctuates at the same time as insolation. As the discharging process proceeds at 16:00 (local time), the PCM starts solidifying and the used heat is uniform for a longer period (Fig. 10a). The uniform value of the used heat is about 123 W all the night with shutter and around 50 W without shutter. It can also show that the recovered heat was not affected by the diurnal solar radiations fluctuation. Therefore, the uniform amount all the night with shutter of heat which can be recovered from the solar air heater with latent heat storage is the major advantage of the heating greenhouse application.

Fig. 10b. The recovered heat from SAHLSC inside the IGLHS without shutter at night (21th December, 2012).

6. Conclusion In order to maximize plant production, it is important to maintain greenhouse’s temperature within an optimal range. In this study, two identical insulate greenhouses are used to evaluate the effect of the nocturnal shutter at night to maintain nighttime temperature. The experimental results show that the nocturnal variation of temperature inside the greenhouse with shutter at night exceeds the one without shutter, with a difference of 2 °C. A thermal energy analysis was also applied to calculate the absorbed, useful, stored and losses energy inside the greenhouse with or without nocturnal shutter. The radiation heat loss rate is estimated by 24% of the total losses heat of the IG with shutter and 61% of the IG without shutter at night. Thermal radiation exchange is a very important factor in determining the thermal environment inside a greenhouse. A solar air heater collector using a packed bed of spherical capsules with latent heat system was operated and installed inside the first greenhouse. This collector stores solar energy during daytime and supplies it for heating at night. To sum up, on December, cold season in Tunisia, the amount of nighttime recovered heat of the solar system is about 440 W (per volume of collector); that is 62 W (per area of collector), during all the night inside the greenhouse with shutter. References

Fig. 10a. The recovered heat from SAHLSC inside the IGLHS with shutter at night (19th December, 2012).

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