Autonomous greenhouse microclimate through hydroponic design and refurbished thermal energy by phase change material

Accepted Manuscript Autonomous greenhouse microclimate through hydroponic design and refurbished thermal energy by phase change material

Sara Baddadi, Salwa Bouadila, Wahid Ghorbel, AmenAllah Guizani PII:

S0959-6526(18)33595-9

DOI:

10.1016/j.jclepro.2018.11.192

Reference:

JCLP 14951

To appear in:

Journal of Cleaner Production

Received Date:

14 May 2018

Accepted Date:

21 November 2018

Please cite this article as: Sara Baddadi, Salwa Bouadila, Wahid Ghorbel, AmenAllah Guizani, Autonomous greenhouse microclimate through hydroponic design and refurbished thermal energy by phase change material, Journal of Cleaner Production (2018), doi: 10.1016/j.jclepro. 2018.11.192

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ACCEPTED MANUSCRIPT Autonomous greenhouse microclimate through hydroponic design and refurbished thermal energy by phase change material Sara BADDADIa*, Salwa BOUADILAa, Wahid GHORBELb, AmenAllah GUIZANIa a The

Thermal Processes Laboratory - Research and Technology Center of Energy, Tunisia b The

Center of Biotechnology of Borj Cedria, Tunisia *[email protected]

Abstract The rapidly increasing demand for agricultural food needs coupled with rising energy costs marked challenges for ensuring food bigger than any previous period. Therefore, greenhouses agriculture is a main focus for farmers, engineers and greenhouses designers in view of ways of improvement that provides. Compared to the conventional greenhouses, hydroponics warrant better quality, greater nutrient content, higher yield, efficient water and fertilizer use, but also more energy requirements. A new Hydroponic Greenhouse was designed and installed in the Research and Technology Center of Energy in Tunisia. A Solar Air Heater with Latent thermal storage using Phase Change Material was also realized for the hydroponic greenhouse heating taking into account the thermal heat loads of the specific application. In this work, the microclimate of the Hydroponic Greenhouse without heating was pursued to evaluate the performance of the hydroponic design. Hydroponic greenhouse allowed better environment than conventional greenhouses. During daytime, the new greenhouse temperature exceeded 18 °C and the difference between the inside and the outside reached mostly 6 °C. The relative humidity ranged between 20-35 % the day and 70-85 % at night. Several measurements were also carried out after the heating to pursue the Solar Air Heater contribution. The temperature of the hydroponic greenhouse during nighttime after the heating raised by 6 °C and the nocturnal temperature was mainly over 15 °C. The diurnal temperature of the Heated Hydroponic Greenhouse was generally higher than 32 °C. Compared to conventional solar heating, the two packed beds of latent storage energy improved the indoor greenhouse environment especially during harsh and nocturnal periods. Keywords: hydroponic greenhouse, solar air heater with latent thermal storage, microclimate, greenhouse heating.

1

ACCEPTED MANUSCRIPT Nomenclature A

surface (m2)

C

orientation and inclination coefficient

λ

thermal conductivity (W/m2 K)

e

thickness (m)

𝛕

temperature reduction coefficient

e‘

exposure to wind coefficient

f

correction factor

abs

absorber

Fred

solar energy reduction factor

amb

ambient

Ftr

solar transmission factor

G

gain (W)

d-w

door and windows

h

global exchange coefficient (W/m2 K)

ex

external

H

thermal loss coefficient (W/°C)

eq

equipment

Heatreq

heating requirements

h

holes

HHG

Heated Hydroponic Greenhouse

in

inlet

Hydroponic Greenhouse

Inf

infiltration

K

global losses coefficient (W/m2 K)

int

internal

L

thermal losses (W)

glass

glass

N

number of occupants

max

maximum

P

permeability

min

minimum

heat outputs per occupant (W)

out

outlet

Phase Change Material

p

person

air flow rate (m3/h)

r

radiation

relative humidity (%)

R

air renewal

Solar Air Heater with Latent storage

s

solar

HG

Pocc PCM Q RH SAHL

Greek symbols

Subscripts

c

convection

T

temperature (C )

tot

total

V

wind velocity (m/s)

Tr

transmission

w

coefficient of exposure to wind

Un

unheated place

z

height (m)

v

2

volumetric

ACCEPTED MANUSCRIPT 1.

Introduction The food security is a topic of much interest. As world population is rapidly increasing,

there is a growing demand for food (Sims et al., 2016). In fact, the world population is expected to raise, during only 150 years, by 8.7 billion people (Gerhard K. Heilig, 2015), and according to the State of Food Security and Nutrition in the world, the global hunger increased in 2016 and affects 815 million people (FAO IFAD UNICEF, 2017) and the global demand for food is estimated to increase by 50 % until 2030 (FAO, 2011). One of the main challenges of our time is to feed the growing and more demanding world population (Pittelkow et al., 2014). This task demands an ever-increasing development of new technologies for higher food production (Allardyce et al., 2017; Vadiee and Martin, 2012). Indeed, agricultural productivity must rise by 70 % in 2050 (Stevens and Gallagher, 2015). To ensure the food security, greenhouse food production is a potential alternative for feeding continuous food demand year around because of its ability to intensify productivity (Vadiee and Martin, 2014). In fact, greenhouse cultivation has evolved to offer the optimum environment to cultivate the desirable crop in an enclosed structure where all plant growth factors can be controlled and maintained at optimum level to create a favorable microclimate (Esen and Yuksel, 2013; Jain and Tiwari, 2002). Actually, greenhouses are becoming more and more popular for food crop production, and around 405 000 ha of greenhouses spread over all the continents (Savvas et al., 2013). Agriculture is the major consumer of freshwater (Unesco and United, 2009) and around 80% of water resources is presently devoted to agriculture sector (Martinez-Mate et al., 2018). In addition of the large quantities of water needed for irrigation (Hardin et al., 2008), cultivation in soil presents numerous disadvantages. Conventional greenhouses require large areas which in turn need high concentrations of nutrients and pesticides (Barbosa et al., 2015; Killebrew and Wolff, 2000). Moreover, pollutants and chemical waste released during the cultivation may have dangerous effects such as soil degradation, erosion, and contamination (Stanghellini et al., 2003). Thus, technics for food production must evolve. Hydroponic culture seems to be the solution to many problems associated with the conventional greenhouse. Actually, hydroponic crop production has significantly increased worldwide (Hickman, 2017) and various plant species have been successfully grown hydroponically. The cultivated crops include the most popular vegetables as cucumber (Grewal et al., 2011), lettuce (Cometti et al., 2013; Ioslovich, 2009), pepper (Lopez-Galvez et al., 2016), tomato (Lopez-Galvez et al., 2014; Ntinas et al., 2015). 3

ACCEPTED MANUSCRIPT Investigations into hydroponic alternatives could be beneficial to sustainably feed fresh crops all the year irrespective of seasons with (i) greater yields using minimal land area, (ii) efficient use of water, fertilizers and chemicals for disease and pest control and (iii) safer culture with inferior environmental impacts and specific greenhouse gas emissions. Concerning the improvement of hydroponic yields (i), Bradley and Marulanda described a simplified hydroponic technology for reducing hunger. They mentioned that simplified hydroponic requires approximately 25% of the area used for soil cultivation (Bradley and Marulanda, 2001). Barbosa et al. demonstrated that land requirements of the hydroponic technology can even be ten time less. A quantitative comparison of the lettuce production showed that this technique of culture offered over eleven times greater yields compared to conventional agriculture (Barbosa et al., 2015). Crops grow hydroponically into stacked trays as vertical farming for more growing spaces to multiply the quantity of crops for higher yields (Link, 2017). The second benefit (ii) of hydroponics consists in offering an efficient fertilizer consumption and reducing the use of chemicals for pest and disease control (Hussain et al., 2014). As well, hydroponic farming has a significant potential to save water. Hydroponic systems are highly efficient in recycling water as the drainage can be easily captured for reuse (Carmassi et al., 2005). A study of a hydroponic system delivering water and nutrients for vegetable crops found that cucumber and tomatoes can be produced by reusing 33 % of drainage water which reduce the potable water consumption for irrigation (Grewal et al., 2011). For safer culture with less environmental impacts, hydroponic culture is considered as the beneficial technology (iii). Hence, hydroponic farming can substantially reduce pollution caused by the drainage water discharge (Grewal et al., 2011). As crop nutrients are contained and recycled, there is no residual salts lost to environment (Bradley and Marulanda, 2001). Moreover, this agricultural system presents a low risk of contamination (Lopez-Galvez et al., 2016). In fact, hydroponic greenhouses offer physical barriers against some causes of enteric bacterial and reduce the contamination factors which makes the hydroponic technology safer than open field (Orozco et al., 2008). Also, and as there is no soil, crops remain clean and do not need washing (Coolong, 2012). Furthermore, compared to soil cultivation (0.23 kg CO2 equivalent), gas emissions are notably inferior for the hydroponic systems ( 0.11 kg CO2 equivalent) (Martinez-Mate et al., 2018). Despite of the numerous advantages promoting the use of the hydroponic culture, the specific energy consumption of this agriculture form is 17 % higher than land cultivation 4

ACCEPTED MANUSCRIPT (Martinez-Mate et al., 2018). Among the greenhouse annual energy needs, the demand of heat corresponds to 95.3 % of the total energy needs while the electricity consumption corresponds to only 4.7 % (Vourdoubas, 2015). The intensive greenhouse energy requirements make the heating a major issue for producers around the world. Indeed, an auxiliary heating system is necessary to maintain the optimum temperature inside the greenhouse in cold climate especially during the night when the temperature drops below the optimal range for crops (Benli, 2013). Renewable energy sources come along as the most prospective energy resource for reducing the energy load for greenhouses acclimatization and covering the heating needs. Thus, the scientific community all over the world have tested several systems. Esen and Yuksel designed and set up biogas, solar and a ground source heat pump greenhouse heating system with horizontal slinky ground heat exchanger to heat a greenhouse and found that the different energy sources can be successfully tested for greenhouse heating (Esen and Yuksel, 2013). Joudi and Farhan investigated a greenhouse formed mainly with a conventional greenhouse and six solar air heaters connected in parallel and covering 45 % of the greenhouse roof. The solar air heater provided the daily heating load of the greenhouse. The stored energy from the heater and the solar heat inside the greenhouse covered all the daily heating requirements with an surplus around 46 % (Joudi and Farhan, 2014b). Hussain et al. compared linear and spot Fresnel lens solar collectors with similar storage capacities and Fresnel lens surface areas for the heating of identical greenhouses under the same weather and operating conditions. Both collectors provided heating load exceeding the greenhouse heating demand. The excessive heat was stored in the storage tank for the nighttime heating. The thermal efficiency for the spot Fresnel lens collector was higher than for the linear Fresnel lens collector and the spot Fresnel lens collector performance was around 7-12 % higher than that of linear Fresnel lens collector (Imtiaz Hussain et al., 2015). Attar and Farhat have proved experimentally the efficiency of a solar water system based on capillary heat exchangers integrated in the greenhouse for the heating and the ground heat storage. For a greenhouse of 10 m3, the heating system efficiency was around 64.9 % in December and about 133.6 % in April (Attar and Farhat, 2015). As some renewable energy forms (solar, wind,…) are intermittent in nature and not available all time, searching for suitable devices of thermal energy storage as new sources of energy is a main challenge for reducing the mismatch between the energy supply and demand and improving the energy systems performance (Sharma et al., 2009). Thermal energy storage as sensible and latent heat has attracted a lot of attention since it plays an important role in 5

ACCEPTED MANUSCRIPT conserving energy and can be applied to several different energy systems for reducing the wastage of energy. The latent storage by Phase Change Material (PCM) presents a greater advantage compared to sensible storage, such as rock and water storage. In fact, PCM has an important potential to store larger amounts of heat during the phase change with smaller temperature difference between storing and releasing heat (Sethi and Sharma, 2008). The high energy storage densities over narrow temperature range make PCM attractive for greenhouse heating (Öztürk, 2005). Several researches have studies different systems of PCM for the heat requirements of greenhouses. Benli and Durmuş tested an experimental device constructed with ten solar air collectors with latent heat storage system. The proposed size of collectors integrated PCM provided 6 to 9 °C higher temperature inside the greenhouse compared to outside. This heating system allowed until 23 % of total daily thermal energy requirements of the greenhouse for 3-4 h compared to the conventional heating systems (Benli and Durmuş, 2009a). Benli have designed a ground source heat pump heating system with a latent heat thermal storage tank and investigated the use of the phase change materials for energy saving in the greenhouse. He found that this system is appropriate for the greenhouse heating and important savings were realized (Benli, 2011). Bouadila et al. inserted a solar air heater with latent storage collector inside a small chapelshaped greenhouse during the winter. The results showed that the internal temperature was until 5 °C higher than the ambient temperature. The amount of heat stored with the collector presented about 56 % of the day total excess heat inside the greenhouse and ensured 30 % of the total heating requirements at night (Bouadila et al., 2014). Barley is one of the most important cereal crops in the Mediterranean region and is widely cultivated in semi-arid and arid regions especially in the North of Africa. However, the scarcity of fresh water, the climate change and the bad land cultivation practices promote the interest of using hydroponic greenhouses for barley culture. In order to save surface and groundwater consumption, some action should be required to ensure pasture management, livestock and future food availability. A national project funded by Ministry of Higher Education and Scientific Research (Tunisia) focused on the implementation of a technically feasible, economically viable and socially acceptable strategy for a sustainable management of barley productivity in Tunisia. The 3-year project is based on a collaboration between two partners: the Research and Technology Center of Energy (CRTEn) and Centre of Biotechnology of Borj Cedria (CBBC) (2014 - 2017).

6

ACCEPTED MANUSCRIPT As hydroponics are highly sophisticated systems, their microclimate management is seldom, nay absent, in the greenhouse acclimatization researches. The novelty of this work is to ensure an improved hydroponic greenhouse indoor climate for the barley growth availing the hydroponic greenhouse conception and benefiting from the solar heating using a solar collector with phase change material during cold periods. To compensate for the drops of temperature at night which may severely damage plants, it is necessary to use the additional heating: the SAHL, a pilot unit that uses exclusively solar energy for greater acclimatization and better culture conditions. Considering these issues, this paper will present an eco-friendly hydroponic greenhouse and a new solar air heater using two packed beds of latent storage energy designed and realized in the Thermal Processes Laboratory of CRTEn. This study seeks then to determine the feasibility of an innovative Hydroponic Greenhouse (HG) and a new Solar Air Heater with Latent storage energy (SAHL) for its nocturnal heating. In sections (2) and (3), the experimental set-up and the measuring equipment will be described. In section (4), we will present the thermal analysis of the hydroponic greenhouse. The HG microclimate evaluation, the SAHL comportment and the PCM behavior will be illustrated in section (5). In the conclusion, the main remarks will be reported. 2.

Experimental set-up The experimental prototype is a new Heated Hydroponic Greenhouse (HHG) composed of

two components: (1) an experimental Hydroponic Greenhouse (HG), and (2) a new Solar Air Heater with Latent storage energy on two beds (SAHL). The (Fig. 1) shows the real device of the experimental system.

HG

SAHL

Fig. 1. The experimental Heated Hydroponic Greenhouse

7

ACCEPTED MANUSCRIPT 2.1

The experimental Hydroponic Greenhouse description The experimental Hydroponic Greenhouse (HG) occupies 24 m2 of area and 3 m of height

with Southern-Eastern orientation. The hydroponic greenhouse is constructed with a galvanized steel structure (Z300) covered by a sandwich panel with polyurethane as insulator. It is characterized by a light weight, a good compressive strength, a low toxicity and a thermal conductivity around 0.028 W/m K. It is composed of two isothermal boxes; the principal compartment is destined for the hydroponic culture and the small one is used for the germination and the control systems. The principal compartment has 54 m3 of volume and destined for the hydroponic culture (Fig.2). A metallic support is installed to arrange the culture trays. The dimensions of the tray are: 4 mm of thickness, 72 mm of width and 350 mm of length. The trays are placed in two rows, each row is composed of five vertical levels spaced by 0.5 m. The metallic support is constructed in such way that the trays of each level are placed with a low inclination. The lower side of all trays are perforated in extremities for the water recuperation.

Solenoid valve Culture tray Support Extractor

Ventiltor

Shading screen Lamp Manual valve Drip tubing Recovery pipe

Fig. 2. The real view of the experimental HG

8

ACCEPTED MANUSCRIPT The HG is equipped with a double-glazed window with an area of 4.73 m2 and a thermal transmittance U < 1.3 W/m2 K placed in the Southern wall and represents 30 % of the total area of this frontage. This window ensures the transmission of the solar radiation for the crops growth. The Northern wall contains two sliding windows (simple glazing) with the same dimensions. Two removable opaque shading screens with 2 mm of thickness covered the windows to reduce the radiation intensity, to prevent the overheating of plants during the summer and to mitigate thermal losses during the winter. The supplemental lighting of the HG is ensured by four lamps used at night and when the solar radiation is not available or not sufficient. In fact, the optimum lighting is essential to ensure the photosynthesis process of plants. 2.2

The control panel of irrigation and ventilation systems

The second compartment occupies a total volume of 18 m3 and is fitted out with the germination equipment and a control panel of irrigation and ventilation systems as shown in (Fig. 3). The panel is composed of electric cabinet, Hunter X-core controller, cartridges water filter, dosing pump, DAB water pump, AquaOxy aerator, two Hunter solenoid valves and chint inverter. -

The electric cabinet contains push button, main breaker, (3) circuit-breakers, (3) electric switches, (8) manual switches On/Off, power transformer and (2) temperature controllers.

-

The Hunter X-core controller regulates current time, start times, run time and water days for the automatic watering program.

-

The cartridges water filter (XS Type SENIOR 20-27) treats the water flowing in the irrigation system.

-

The dosing pump Green Line model Dosatron D25 doses the nutriment solution to enrich the water.

-

The DAB 0.5HP Peripheral Centrifugal water pump model KPS 30/16 M actuates the water circulation.

-

The AquaOxy 4800 aerator with a power consumption equal to 60 W and a maximum flow rate of 4800 l/h allows the oxygen during the germination.

-

The Hunter PGV 24 V scheduling solenoid valve used for the flow adjustment.

-

The CHINT NVF2-1.5/TS4 inverter modifies the velocity of the air flow exchanged with the fans.

9

ACCEPTED MANUSCRIPT

(a) Control panel

(a) (e) Inverter

(b) X-core Controller

(e)

(b)

(c) (d)

(c)

Dosing pump

(f)

(g)

(f) Cartridges water filter

(d) Aerator

(g) Pump

Fig. 3. The different components of the control system

10

ACCEPTED MANUSCRIPT 

Irrigation control system The irrigation technique chosen for the HG is the drip irrigation method for a suitable,

economical and effective water use. The automatic program of irrigation assured by the Hunter X-core controller (Fig. 3 (b)) which schedules the solenoid valves to allow the water circulation. Water circulates from the water tank to crops in a polyethylene drip tubing. The drip tubes branch out into ten horizontal drilled pipes equipped with drippers, each pipe contains a manual valve that permits the water flow. The drippers deliver uniformly a measured quantity of water in regular intervals to ensure that the plants do not suffer from stress of less or over watering. The trays holes allow the flow water from a level to another until reaching the recovery pipe and thereafter to the tank (Fig. 4).

(Upper view)

(Side view)

Drip tubing Line of emitters Recovery pipe Culture tray

Solenoid valve Manual valve Tank

Fig. 4. The irrigation system of the HG



Ventilation control system Generally, the natural ventilation is not sufficient to obtain optimum climate inside the

greenhouse. A ventilation is then required to maintain favorable levels of relative humidity and to reduce excess heat. To homogenize the temperature and the humidity within the greenhouse, three THERMIVENT helical fans are distributed into greenhouse. A fan is placed at 2.5 m of 11

ACCEPTED MANUSCRIPT high in the Northern wall of the germination room and two fans are placed in the Northern and the Southern walls respectively at 0.5 m and 2 m from the floor. The fans are actuated automatically if the optimum temperature is not available. The inverter (Fig. 3 (e)) adjusts the revolution speed from 0.5 m/s to 5 m/s. Table 1 summarizes the fans characteristics. Table 1. The fan Characteristics Voltage

380V

Amperage

0.6 A

Frequency

50 Hz

Electrical power

150/250 W

RPM (revolutions per minute)

1360 r/min

2.3

The solar air heater with latent storage energy description

The heating system used to control the interior environment of the HG during the nighttime, is a new solar air heater with latent storage energy (Fig. 5). The SAHL is designed and realized in the Research and Technology Center of Energy, it is placed in the exterior near the Southern wall of HG. The solar collector is composed of spherical capsules confining phase change material. The capsule coat is molded with a black blend polyolefin with 0.002 m of thickness. The nodules (312 nodules) are placed on two superposed beds arranged on two metal wire racks fixed in a galvanized parallelepiped box. The number of nodules must necessarily satisfy the maximal greenhouse heat requirements during the coldest climate period, it was determined by the thermal analysis assessed hereafter. The energy needs during the heating period quantify the required amount of phase change material necessary to maintain the desired environment inside the greenhouse and then determine the satisfactory number of PCM capsules. The solar air heater with latent storage energy sizing was determined as follows; the mass of phase change material was obtained through dividing the required amount of energy by the heat fusion of the PCM. Subsequently, the number of nodules was measured considering the amount of PCM per nodule. And consequently, the sizing was accomplished by choosing the appropriate design of the absorber and then the solar air heater. The packed beds absorber is the most important component ensuring the solar thermal energy storage. A 0.035 m thick transparent glass cover (emissivity of 94 % and a transmission of 81 %) is placed 0.04 m apart from the absorber. The SAHL is isolated by a layer polyurethane with 0.05 m of thickness and mounted on a metallic support with a Southern orientation and a slope of 37° to the horizontal. Indeed, the intensity of incident solar radiation on the solar collector is affected by the azimuth and tilt angles, which influence the incident angle of sunlight on it. The 12

ACCEPTED MANUSCRIPT optimal angles of a panel vary from one geographic latitude to another and depend also on the atmospheric composition, the climate, utilization period. The most reasonable values of the azimuth and tilt angles are based on several studies having worked on tilt angles in solar energy applications as (Tlijani et al., 2017) and (Hafez et al., 2017).

Fig. 5. The schematic view of the SAHL

The SAHL operating on two processes (charging and discharging). During the day, the charging process, the air inlet and outlet openings are closed and the PCM is heated and melted at 27 °C (melting temperature). The phase change material stores the energy as sensible and latent heat. At night, the discharging process begins, the air inlet and outlet orifices are opened and a helical fan is actuated to blow the hot air with a fixed speed of 1 m/s. The SAHL is properly sized to allow suitable heating and the fan running speed was chosen low in order to achieve a long term restoration of heat. Moreover, a Pt100 sensor in associated to the solar collector to detect the internal temperature and actuate the SAHL fan; once the temperature reaches its limit and exceeds the appropriate temperature range, the fan will be immediately stopped. 13

ACCEPTED MANUSCRIPT The new SAHL with latent storage energy on two beds is connected to the hydroponic greenhouse by a funnel to transfer the stored thermal energy (Fig. 1). The length of the connection conduit is equal to 0.7 m and insulted with 0.05 m of glass wool to reduce the thermal losses. 

Phase Change Material

The phase change material used is the calcium chloride hexahydrate, CaCl2.6H2O with additive. The CaCl2.6H2O is a promising heat storage material utilizing latent heat of fusion. It is a proper PCM with big latent heat, good thermal conductivity and inflammability. The thermophysical properties of the calcium chloride hexahydrate are tabulated in Table 2. The additives were used to solve the problem of supercooling and minimalize changes in the latent heat of transition phase during thermal cycling processes of the Calcium chloride hexahydrate. The PCM used is characterized by a melting in the desired operating temperature range and a small volume change during phase transition. It melts in a stable range of temperature and has shown minor variations in the latent heat of fusion during the thermal cycling process; only about 1– 1.5 °C and 4 % average variation respectively during the 1000 thermal cycles (Tyagi and Buddi, 2008). Figure 6 illustrates the DSC measurement curve of latent heat of fusion and melting temperature of CaCl2.6H2O at different cycles. The CaCl2.6H2O nodules have a remarkable life span of more than 10 000 cycles (more than 27 years of normal use) (Cristopia, 2013). Table 2. The PCM thermo-physical characteristics PCM

CaCl2.6H2O

Physical state

Liquid

Solid

Heat of fusion (kJ/kg)

192.6

192.6

Specific heat (kJ/kg K)

2.22

1.42

Thermal conductivity (W/m K)

0.58

1.05

1710

1530

Density

(kg/m3)

14

ACCEPTED MANUSCRIPT

(a)

(b)

(c)

(d)

Fig. 6. DSC measurement curve of latent heat of fusion and melting temperature of CaCl2.6H2O at different cycles (Tyagi and Buddhi, 2008)

3.

Measurement methodology An experimental study is performed to evaluate the climatic parameters of the HHG heated

by the solar air heater with thermal storage energy (SAHL). Measurements have been done during the month of February 2017. To accomplish the experimental measuring diverse instrumentations were used. 3.1

Measuring equipment in the HHG

The measurement device was equipped with the following instruments and sensors noted as: - (2) HMP155A sensors, the first one was placed in the center at 1.2 m above the floor to measure the temperature and the relative humidity ( THMP in and

RH HMP in ) inside the

HHG and the second HMP155A sensor was placed outside to register the external temperature and relative humidity ( THMP ex and

15

RH HMP ex ).

ACCEPTED MANUSCRIPT - (2) pyranometers KIPP & ZONEN measured the horizontal global solar radiation: one was placed at 1.2 m inside the greenhouse and the second pyranometer was fixed in horizontal plane at 3 m outside the greenhouse. - (5) thermocouples to measure temperature in the center of the greenhouse at different vertical levels ( T 1 , T 2 , T 3 , T 4 and T 5 ), - (4) thermocouples ( T 6 , T 7 , T 8 and T 9 ) placed in the horizontal plane at 1.2 m above the floor to measure the air temperature at North, South, Est and West sides, - (4) thermocouples placed on different windows glass sides ( T 10 , T 11 , T 12 and T 13 ), - (1) thermocouple to measure the temperature of the floor ( T 14 ). -

The schematic layout illustrated in (Fig. 7) describes (14) localizations of K-type thermocouples (accuracy ± 0.01°C) distributed inside the greenhouse at different triple positions (X, Y, Z). The reference center of the localizations parameters corresponds to the middle of the greenhouse at 1.2 m above the ground, it corresponds to the thermocouple T 3 position.

Every thermocouple position is defined by (X, Y, Z) detailed as follow: 

X = {-2, -1, 0, 1 or 2} presented the values that define the position of the thermocouple -from the North to the South.



Y = {-1, 0, or 1} presented the values that define the position on a scale from the East to the West.



Z = {0, -2, -1, 0, 1 or 2} presented the values that define the vertical position from the ground.

3.2

Measuring equipment in the SAHL

To measure the temperatures of the SAHL components (10) K-type thermocouples were used (Fig. 5). -

(2) thermocouples to measure the inlet ( Tin ) and outlet ( Tout ) air temperatures of the SAHL,

-

(2) thermocouples to measure the surface of the nodules temperature in the center of the first bed ( Tabs ) and the glass temperature ( Tglass ),

-

(6) thermocouples were integrated in the first, the seventh and the last nodules of each bed bed 1

named respectively ( T1

bed 1

, T7

bed 1

bed 2

, T13 ) and ( T1

16

bed 2

, T7

bed 2

, T13

).

ACCEPTED MANUSCRIPT -3 X Y Z

-2

-1

0

0.5 1.35 2.2 1.5

0

0.2 0.7

0

3 1.2

1

2 3

4.5 1.7 2.2

Fig. 7. The thermocouples distribution in the HG

3.3

The meteorological station The NRG weather station of the Research and Technology Center of Energy of Borj

Cedria is equipped with an acquisition system recording every 10 min the average, the maximum, the minimum and the standard deviation values for the ambient temperature, the global sun flux on a horizontal plan and the wind velocity and direction at 20 m. The wind velocity at other high can be obtained from the wind speed measurement at 20 m using the correcting equation U1 U 2    z1 z2  where U is the wind speed, z is the height, β is the 

power law exponent and the subscripts 1 and 2 indicate the different heights (Nightingale et al., 2013). All climatic and measured parameters of HHG system are recorded using a Campbell Scientific CR5000 measurement and control Datalogger connected to a microcomputer with LoggerNet 17

ACCEPTED MANUSCRIPT acquisition program. Due to the shortcut interface and the CRbasic commands, a program measuring the different parameters every 2 seconds and providing the average values every 10 minutes was realized. The data-acquisition system recorded the measurements during all the experimental period. 3.4

Uncertainty analysis

Due to repeated measurements, experimental results may have uncertainty. An uncertainty analysis is then needed to estimate the experiment measurements accuracies and to increase the precision of the performed experiments. Experimental errors came mainly from temperature, relative humidity, solar irradiation measurements and the sensitiveness and measurement error of data acquisition system (type CR 5000). Table 3 itemizes the details of instrumentation and uncertainties. The sensitiveness were obtained from a catalog of the instruments. These uncertainties were likely to be related to instrumentation issues as measurement inaccuracy, sensitivity and calculated uncertainties. The calculated uncertainties of the dependent parameters were estimated by Eq. (1). 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 of 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):

wR  [(

R R R w1 ) 2  ( w2 ) 2      ( wn ) 2 ]1/2 x1 x2 xn

18

(1)

Table 3. Instrumentation and uncertainties details

Parameter Internal relative humidity External relative humidity Internal global solar radiation External global solar radiation Ambient temperature Internal temperature Internal temperature Internal temperature Internal temperature Internal temperature Internal temperature Internal temperature Internal temperature Internal temperature Internal temperature Internal temperature Internal temperature Internal temperature Internal temperature Glass temperature Outlet heater temperature PCM temperature Acquisition

Symbole RHHMP-in RHHMP-ext Tamb T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 T13 T14 Tglass Tout Tjbed i -

Unit % % W/m2 W/m2 °C °C °C °C °C °C °C °C °C °C °C °C °C °C °C °C °C °C -

Sensor HMP155A HMP155A KIPP & ZONEN pyranometer KIPP & ZONEN pyranometer K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple K-type thermocouple CR 5000 data logger

19

Position in the center of the greenhouse at 1.2 m above the floor outside near the collector (Fig. 5) in horizontal surface at 1.2 m inside the greenhouse in inclined surface near the collector (Fig. 5) outside at 3 m above the floor in the center of the greenhouse at 2.2 m (Fig. 6) in the center of the greenhouse at 1.7 m (Fig. 6) in the center of the greenhouse at 1.2 m (Fig. 6) in the center of the greenhouse at 0.7 m (Fig. 6) in the center of the greenhouse at 0.2 m (Fig. 6) at 1.2 m in the northern side of the greenhouse (Fig. 6) at 1.2 m in the southern side of the greenhouse (Fig. 6) at 1.2 m in the eastern side of the greenhouse (Fig. 6) at 1.2 m in the western side of the greenhouse (Fig. 6) on the internal southern glass (Fig. 6) on the external southern glass (Fig. 6) on the internal northern glass (Fig. 6) on the external northern glass (Fig. 6) on the floor in the center of the greenhouse (Fig. 6) the glass of the solar heater (Fig. 5) (Fig. 5) the j position of the bed i , i={1,2}, j={1,7,13} inside the hydroponic greenhouse

Uncertainty ±2% ±2% ± 3 W/m2 ± 3 W/m2 ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.01°C ± 0.1°C

ACCEPTED MANUSCRIPT

4. 4.1

Thermal analysis and efficiencies Thermal analysis The thermal analysis corresponds to the hydroponic greenhouse before the cultivation

(without crops). The isothermal greenhouse as described above can be assimilated to a building application. In this section, the thermal analysis is developed to evaluate the internal thermal gains and losses inside the HG and then to quantify the heating requirements of the greenhouse during cold climate period (Fig. 8).

Fig. 8. The thermal balance of the HHG

The amount of energy provided via the heating equipment chosen to maintain the desired environment can be determined by the greenhouse losses and gains. Otherwise, it is equal to the total thermal losses minus the sum of useful gains by solar irradiation and internal energy sources (Cassagne and Gicquel, 1989). As mentioned, energy requirements are generally given by:

Heating requirements   Losses   Gains

 Heat Req  Ltot  Gtot

(2)

The total thermal losses are assumed as the following form:

Ltot  H (Te  Ti ) 20

(3)

ACCEPTED MANUSCRIPT where

(Te  Ti ) is the difference between interior and exterior temperatures (°C) and H is the

total thermal loss coefficient of the hydroponic greenhouse expressed in ( W / m 2 ). H includes the different forms of thermal losses, it can be written as:

H  H R  H Inf  HUn  H Tr 

(4)

H R : represents the coefficient of losses by specific air renewal. It takes the form of: H R  0.34 QV

(5)

where 0.34 ( Wh / m3 K ) is the value of the volumetric heat capacity of the air at 20 °C,

Qmin and

Qmax are respectively the minimum and maximum values of the total extracted air flow and QV is the ventilation volumetric flow rate ( m3 / h ) and expressed as

Qv   5 Qmin  Qmax  6

(6)

 H Inf : is the coefficient of losses by infiltration. These losses can be written as:

H Inf  0.32 ( P . w) with P is the permeability of each façade and written as: P  0.25 Ah 

(7)

 (mA

d w

)

(8)

Ah and Ad  w are respectively the ventilation holes and the opening (door and windows) surfaces,

m is a 

permeability coefficient (m / h) and

is the coefficient of exposure to wind.

HUn : is the coefficient of losses by transmission through unheated spaces.

In this case, we consider HUn 

w

:0

H T : denotes the transmission losses coefficient. They are given as follow: HT    K A

(9)

A is the surface of the transmission area ( m2 ) and K is the global losses coefficient ( W / m2 K ) and it is equal to:

1 1 e 1    K hi  he

e

(10)

is the thickness of each material constituting the wall ( m ) and  is its thermal conductivity

( W / m K ).

21

ACCEPTED MANUSCRIPT -

the global external exchange coefficient,

he , includes the heat transfer by convection

and radiation, and expressed as:

he  hec  her -

the global internal exchange coefficient,

(11)

hi , can be written as

hi  hic  hir

(12)

the values of the convective heat transfer coefficient, hic, are provided in accordance

-

with the heat flow direction (Morel and Gnansounou, 2009). hic equals to 5 W/m2 K if the heat flow is directed upwards, 2.5 W/m2 K if it is horizontal and 0.7 W/m2 K once the flow is directed downwards The external convective heat transfer coefficient is estimated to take the form as (Duffie et al., 2003):

hec  5.2  3.8 Vamb where

(13)

Vamb is the ambient wind velocity ( m / s ).

The useful gains include all source providing an amount of energy to the greenhouse. These gains contain the solar energy inputs and all outputs heat inside the greenhouse (occupation, equipment, lighting…). The useful gains are determined as follow:

Gtot  GS  GP  Geq 

(14)

GS : is the solar inputs calculated as follows (CASSAGNE and GICQUEL): Gs   ( A Ftr Fred C ) I

where A is surface of each wall( m 2 ),

(15)

Ftr is the solar transmission factor of the wall, Fred is the

solar energy reduction factor, C is the coefficient of orientation and inclination of the wall and

I is the solar irradiation intensity ( W / m 2 ). The internal gains include Geq and 

GP .

Geq : is the internal gain of equipment: Geq  Pel f

Pel is the heat outputs supplied by the devices

(W ) and f is a correction factor.

22

(16)

ACCEPTED MANUSCRIPT 

GP : is the internal gain of occupation determined as :

GP  n Pocc (17) where n represents the number of occupants, Pocc defines the heat outputs per occupant during its presence ( W ). In this case, 4.2

GP

: 0.

The HG thermal performance The greenhouse microclimate is directly dependent on weather conditions. Then, to

define the heating requirement needed to cover all the heat desired, the thermal performance of the hydroponic greenhouse was evaluated during the coldest period. To predict the nocturnal heating requirement necessary for maintaining the nighttime temperature over 16 °C (for barley) inside the greenhouse, the previous equations were used to determine the useful gains and the thermal losses of the HG cultivation room in a testing day (Fig. 9). 700

Solar radiation

Ambient temperature Temperature inside the HG

40 35

500

30

2

25

400

20

300

15

200

Temperature (°C)

Solar radiation (W/m )

600

10

100 5 0 00:00

04:00

08:00

12:00

16:00

20:00

0 24:00

Time (hh:mm) 8/12/2016 (Reference day)

Fig. 9. Weather parameters for reference

Thermal losses are determined by Eq. (3) to Eq. (13) and summarized in the Table 4 for a typical day. The obtained results showed that windows were the responsible of the big part of thermal losses with around 38 % and the walls together shared 29 % of the total losses. The door and the floor contributed respectively to 7 and 5 % the least value of thermal losses and the rest was distributed practically equally to the roof and air renewal and infiltration. The repartition of the energetic gains showed the important role of the southern window in tapping the solar radiation. 23

ACCEPTED MANUSCRIPT Contrarily to the thermal losses, the double glazing brought almost the half of the useful gains to the HG. The roof participated with 14 % as the internal gains. For the walls, they contributed to more than 15 % of solar inputs with a bigger part for the sunny walls than not sunny facades. Table 5 recapitulates the useful gains determined by Eq. (14) to Eq. (17). Table 4. The thermal losses through the HG Loss sources

Thermal loss (W)

Walls

110

Windows

144

Roof

40

Floor

19

Door

26

Air renewal and infiltration

40

Total

379

Table 5. The useful gains of the HG Gain sources

Useful gains (W)

Sunny walls

100

Not sunny walls

35

Southern window

375

Northern window

80

Roof

110

Gains by equipment

110

Total

810

The distribution of the thermal losses and the energetic gains shows the big importance of the greenhouse orientation choice and the insulation for increasing the energetic gains and decreasing the thermal losses in cold periods. According to Eq. (2), to maintain the optimal temperature at night, the heating system must insure around 430 W at nighttime as heat for the HG. Then, the solar air heater was designed to satisfy all the heating requirement during the nocturnal period (12 hours). Based on energy losses and benefits calculated, the SAHL was sized to warrant the required heating even during the least favorable climatic conditions. In fact, the amount of heat necessary to compensate for the greenhouse requirements for heating defined the quantity of encapsulated phase change material, around 95 kg, considering the amount of heat stored by every nodule. The 310 nodules

24

ACCEPTED MANUSCRIPT necessary to satisfy the heat required defined then the packed beds absorber configuration and thereafter the SAHL sizing. 5.

Economic analysis The basic aim of every agricultural production is to increase products and decrease costs.

Thus, the financial analysis is crucial to assess the production efficiency. Optimum energy use in agriculture can be reflected in two ways; increasing the productivity with the same level of energy inputs or reducing energy without affecting the productivity. In this context, the energy use and the cost production in the hydroponic greenhouse heating was evaluated by comparing solar heating (SAHL + fuel boiler) and conventional heating (fuel boiler). The payback period PP of the heating system can be calculated by dividing the amount of the investment cost ( Cinvest ) by the energy gain cost ( Cenergy gain ) as follow:

PP 

Cinvest Cenergy gain

(18)

Wherein the investment cost Cinvest includes the system cost Csys, the operation cost Cop and the maintenance cost Cmaint. The system cost Csys in turn is defined as the costs of equipment, installation and realization. The operation cost Cop corresponds to the boiler circulating pump for the conventional system (fuel boiler), and the pump and fan costs for the solar heating (SAHL + backup system). The maintenance cost Cmaint corresponds to the annual cost of maintaining each component of the systems. Capital costs of the heating systems were based on the price quotations obtained from providers. The heat demand required is the amount necessary to keep the greenhouse within the appropriate temperature range, between 16°C and 27°C, during five months (November to March). In Tunisia, it is entirely possible to remove the need for heating during daytime periods. Then the heating duration is about 1800 hours. The amount of energy that must be supplied by the heating system is around 775 kWh. Table summarizes the main results.

25

ACCEPTED MANUSCRIPT Table 6. Summary cost analysis

Solar heating (SAHL+ boiler)

Conventional heating (boiler)

System cost (€)

900

600

Operating cost (€)

35

130

Maintenance cost (€)

30

20

Energy gain cost (€)

165

-

PP (year)

5.8

The reduction of the annual energy production due to the solar heating was over 4600 kWh. The average mean life of the SAHL was estimated to be 20 years and the payback period of using the SAHL system was 6 years and this period of time could be shortened by choosing the most suitable methods to increase the annual yield production. Findings indicate that important energy saving benefits were realized from the reduction in consumption of the fuel used conventionally to provide energy. Due to the solar air heater with thermal latent storage, the total energy inputs were reduced and the heating demands were satisfied freely. The solar heating led to significant improvements in the energy use efficiency. On top of the saving in heating costs of the SAHL, the environmental benefits of using this system are considerable due to the reduction of CO2 emission. Therefore, the use of the SAHL in the greenhouse heating process would be an appropriate system. 6.

Results and discussion The experimental measurements of this study were taken in February 2017. They

concerned the climatic conditions, the solar air heater performance, the phase change materials states and the thermal behavior of the HG (without heating) and the HHG (with heating). The external global solar radiation, the ambient temperature and the wind velocity at 2 m above the hydroponic greenhouse during one week in February 2017 are presented in (Fig. 10). The climatic weather prevailing in the experimental site was characterized by significant fluctuations. The solar radiation intensity during this period attained mainly 800 W/m2 at midday. The 22nd and the 23rd of February marked perturbed values of solar intensity were characterized by the lower value which didn’t reach 600 W/m2. Concerning the ambient temperature during this period, the maximal diurnal values ranged between 18 and 24 °C. In accordance with the solar intensity variation, the highest value of ambient temperature was

26

ACCEPTED MANUSCRIPT recorded in 20 February the sunniest day. At night, the outdoor temperature varied between 4.5 and 15 °C but it was mainly lower than 10°C. The wind velocity in the experiment’s site had an average in the standards which equals 4 m/s. However, the wind velocity was characterized by unstable rates and varied widely between 0.4m/s and 11.6 m/s. The least values of wind velocity were recorded between the 19th and 20th of February when the weather was very sunny and the temperature exceeded 23 °C. The highest speed was recorded in 22 February at 16h. 45

Ambient temperature

Wind speed Solar irradiation

40

18 16

1000 900 800

35

14 12

25

10

500

20

8

400

15

6

10

4

600

300 200

Solar irradiation (W/m2)

30

Wind speed (m/s)

Temperature (°C)

700

100 5

2

0 0 00:00 00:00 00:00 00:00 00:00 00:00 00:00 00:00 18 Feb 20 Feb 22 Feb 24 Feb 23 Feb 19 Feb 21 Feb

0 -100

Date and time

Fig. 10. The climatic parameters evolution

6.1

Performance of the hydroponic greenhouse without heating To evaluate the hydroponic greenhouse performance before the heating, measurements

have been taken during a week (from 4 to 10 February 2017) with fluctuated weather conditions to pursue the temperature evolution inside the hydroponic greenhouse. The air temperature inside the HG and the external climatic conditions are illustrated in (Fig. 11).

27

ACCEPTED MANUSCRIPT 45

Solar irradiation

800 700 600

30

500

25

400

20

300

15

200

10

100

5

2

35

Solar irradiation (W/m )

Temperature (°C)

40

Ambient temperature Temperature inside the HG

0

0 00:00

00:00 04 Feb

00:00 05 Feb

-100 00:00 00:00 00:00 00:00 00:00 09 Feb 10 Feb 06 Feb 07 Feb 08 Feb

Date and time

Fig. 11. The air temperature evolution inside the HG

Figure 11 shows that the internal air temperature had the same trend as the ambient temperature and it also depended on the global solar radiation. For a diurnal ambient temperature varying among 14 and 27 °C, the HG air temperature ranged between 18 and 34°C. During this testing week, the diurnal temperature inside the HG was always higher than the outdoor climate. In 8 February, the ambient temperature varied among 20 and 23 °C while the HG temperature was over 34 °C at 13h. The 9th of February, when the solar irradiation didn’t attain 270 W/m2 and the ambient temperature was about 16 °C, the HG temperature was around 19 °C. The temperature inside the HG was permanently higher than the ambient temperature and the difference was around 11 °C and even more. However, during the early hours of the day the difference decreased and the temperature of the HG was even colder than outside but it still mostly over 12 °C. At night, the ambient temperature varied between 10 and 13 °C and the HG temperature was approximately between 12 and 16 °C. Figure 12 illustrates the temperature of three positions (T1, T2 and T4) at different high levels (respectively 2.2, 1.7 and 0.7 m) inside the HG (Fig. 6 section 2.1). The stratification was more observed from midday till the sunset and almost absent from 17h to 9h. The higher air temperature corresponded to the upper position. However, from 10h to 13h, temperatures in lower positions were greater than other positions which can be explained by the sun position and the irradiation direction during these hours. 28

ACCEPTED MANUSCRIPT 45

Solar irradiation

800 700 600

30

500

25

400

20

300

15

200

10

100

2

35

Solar irradiation (W/m )

Temperature (°C)

40

Ambient temperature T1 T2 T4

5 0 04/02/2017 00:00

0

05/02/2017 00:00

06/02/2017 00:00

07/02/2017 00:00

-100 08/02/2017 00:00

Date and time

Fig. 12. The vertical stratification inside the HG

As acknowledged, it is crucial to control the change in relative humidity. In fact, it is a critical factor affecting the spread of epidemics of certain crop diseases and inappropriate values of relative humidity threaten growth plants and damage vegetation. Figure 13 exhibits the shape of the external, internal relative humidity and ambient temperature curves during four days under the previous conditions. Temperature and humidity are inversely proportional. Low humidity values accompanied the maximal temperatures during the sunny periods.

29

ACCEPTED MANUSCRIPT 40

Ambient temperature

Internal humidity External humidity

35

110 100 90 80 70

25

60 20 50 15

40 30

10

Relative humidity (%)

Ambient temperature (°C)

30

20 5 0 13/02/2017 00:00

10

14/02/2017 00:00

15/02/2017 00:00

16/02/2017 00:00

0 17/02/2017 00:00

Date and time

Fig. 13. The relative humidity inside and outside the HG

The pace of the relative humidity inside the HG is visibly more stable than the external humidity, this fact explains the importance role of HG insulation in reducing the weather fluctuation effect and stabilizing the interior humidity. The internal humidity was mainly lower than the relative humidity outside. The higher difference between the internal and external humidity attained was recorded at 14h and it was over 30 %. The lower internal humidity was recorded at 14h and it was around 23% while the temperature exceeded 17 °C. Then, the humidity started to increase and attained greater values from midnight to the sunshine until reaching over 80 % at 9h. The emplacement of the greenhouse (to close to the sea) justifies the high values of relative humidity. 

The shading screen effect on the HG (without heating) Figure 14 exposes the HG temperature evolution with the ambient temperature before and

after the use of the Southern shading screen in practically comparable weather. The 13th and 14th February, when the hydroponic greenhouse was totally exposed to the sun through the Southern double glazing, the HG temperature was greater than ambient temperature by more than 18 °C and it reached 34 °C in 13 February at 14h when the outdoor temperature was about 15.6 °C. After using the movable Southern screen continuously during 15 and 16 February, the 30

ACCEPTED MANUSCRIPT diurnal temperature inside the greenhouse didn’t attain 28 °C even when the solar irradiation intensity exceeded 1000 W/m2. Using the shading screen decreased the temperature in the HG the day by reducing the amount of solar radiation intensity incoming to the greenhouse. The movable screen of the HG moderated the sun intensity incoming inside the greenhouse by varying the position of the screen covering the double glazing. Shading screen allowed a good solution when it is sunny but there is no need for high solar radiation intensity. 45 40

1200 Ambient temperature Temperature inside the HG

Solar irradiation

Without shading

With shading

1100

40

1000 35

900 800

30

700 25

600

20

500 400 300

2

15

Solar irradiation (W/m )

Temperature (°C)

35

45

200

10

30 25 20 15 10

100 5

0

0 13/02/2017 00:00

14/02/2017 00:00

15/02/2017 00:00

16/02/2017 00:00

-100 17/02/2017 00:00

Date and time

Fig. 14. The temperature evolution inside the HG without and with the shading

6.2

Performance of the hydroponic greenhouse with heating As mentioned above, the greenhouse conception and design warranted daily internal

temperatures permanently higher than external temperature. However, temperature may suddenly drop at night time which makes the heating necessary. To confront the severe weather and improve the nocturnal climate under the greenhouse, a solar air heater with latent thermal storage was connected to the HG from the 18th of February. Figure 15 shows the air temperature evolution inside the heated hydroponic greenhouse with the ambient temperature and the solar irradiation intensity from 22 to 28 February. The integration of the SAHL to the HG increased significantly the temperature inside the HHG. At daytime, the temperature was mostly higher than 32 °C and reached 37 °C at 13:50. The higher diurnal temperature during this week was recorded in 24 February. The lowest temperature was 31

5 0

ACCEPTED MANUSCRIPT recorded in 28 February when the solar radiation intensity didn’t attain 400 W/m2 and it was over 21 °C. During the period of heating, the SAHL was activated overnight to compensate the drop of temperature. The nocturnal temperature inside the HHG varied between 17 and 20 °C. The heating allowed an increase of temperature by more than 6 °C the nighttime. 50

1000

Ambient T Temperature inside the HG

45

Solar irradiation

800

40

700

35

600

30

500 25 400 20

300

15 10

2

200

Solar irradiation (W/m )

Temperature (°C)

900

100

5

0

0 00:00

22 Feb

00:00

-100 00:00 00:00 00:00 00:00 00:00 00:00 26 Feb 23 Feb 27 Feb 28 Feb 25 Feb 24 Feb

Date and time Fig. 15. The air temperature evolution inside the HHG



Thermal Load Leveling (TLL)

As the air temperature is a function of time, the fluctuation of the temperature inside the greenhouse plays a crucial role. Hence, it is important to determine the thermal load leveling (TLL) to quantify the swings in temperature. The TLL is a relative index giving an idea about the fluctuation of temperatures inside a greenhouse, it is an important factor for optimizing heating in greenhouses and is defined by equation Eq. (19). For a greenhouse thermal heating with minimum fluctuations, the thermal load leveling factor should have low values due to the increase of

Ti ,max  Ti ,min as well as the decrease of Ti ,max  Ti ,min (Sutar and Tiwari, 1994). TLL 

Ti ,max  Ti ,min Ti ,max  Ti ,min

32

(19)

ACCEPTED MANUSCRIPT The performance of the hydroponic greenhouse was evaluated in terms of daily variation of thermal load leveling as shown in Fig.16 and 17. The TLL of the HG was mainly around 0.4 and exceeded 0.5 in 8 February (Fig.16). The lowest value was recorded in the 9th of February. After using the solar air heater, the TLL of the HHG didn’t attain 0.4 and varied mainly between 0.2 and 0.36 (Fig.17). Compared to the TLL without heating system, the thermal load leveling was significantly lower which signifies that the fluctuations of greenhouse air were reduced and the internal environment was improved after the heating. Hence, the SAHL was effective for reducing the daily swings of temperatures of air in the HG and then enhancing the greenhouse microclimate.

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 4/2

5/2

6/2

7/2

8/2

9/2

10/2

Fig. 16. Variation of thermal load leveling of the HG 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

22/2

23/2

24/2

25/2

26/2

27/2

Fig. 17. Variation of thermal load leveling of the HHG

33

28/2

ACCEPTED MANUSCRIPT 6.3

The SAHL performance To pursue the behavior of the solar air heater, the evolution of the SAHL inlet, outlet

and absorber temperatures from 18 to 22 February are presented below (Fig. 18). The daytime, the air inlet and outlet openings were closed the SAHL cover trapped the solar irradiation and the absorber stored the excess of the solar energy as latent heat to be restored when the sun is not available. At evening, the air inlet and outlet openings were opened and the heater worked continually overnight. The thermal energy heat stored due to the phase change materials the day was after that recovered the nocturnal period to heat the greenhouse. Figure 18 shows that the SAHL temperatures were closely related to weather condition. The daily average thermal efficiency of the SAHL during these days is also given. 60 55 50

SAHL intel temperature SAHL outlet temperature Absorber temperature

45

60

900

55

800

50

40

600

35

500

30 400

25

300

20

10 5

18 Feb

00:00

19 Feb

00:00

20 Feb

00:00

Date and time

21 Feb

00:00

22 Feb

2

200

15

0 00:00

45

700

Solar irradition (W/m )

Temperature (°C)

1000 Solar irradiation

35 30 25 20 15

100

10

0

5

-100 00:00

0

Fig. 18. The SAHL air temperature evolution

During sunny days, the absorber temperature exceeded 30 °C and the outlet temperature was over 40 °C. The 20th of February was characterized by the important values of temperatures. For an outdoor temperature of 24 °C and a solar radiation intensity over 810 W/m2, the absorber temperature reached 35 °C and the SAHL outlet temperature- was above 43 °C at 13h. The lowest temperatures were in 22 February when the ambient temperature was about 18 °C and the solar intensity didn’t attain 600 W/m2. The solar air heater allowed mainly 20 °C higher output temperature than the ambient temperature. The absorber temperature evolution clearly 34

40

ACCEPTED MANUSCRIPT marked distinct pace which can be explained by the performance phase change materials encapsulated in the spherical capsules of the absorber. Figure 19 depicts the phase change materials temperatures evolution in nodules 13 and 7 of the bed _1

bed_1 ( T13

bed _1

and T7

bed _ 2

) and the nodules 13 and 1 of the bed_2 ( T13

air heater during the charging and discharging processes (Fig. 5 section 1.3) .

35

bed _ 2

and T1

) of the solar

70

70 bed1

Charging process

60

PCM temperature (°C)

PCM temperature (°C)

T13

bed2

40

30

20

40

40

30

25

20

10

Discharging process

0 Time (hh:mm)

30

Discharging process

0 18:00

Charging process

15

10

Discharging process

06:00

06:00

T13

35

20

10

bed2

T1

Charging process

50

50

06:00

45

bed1

T7

T13

PCM temperature (°C)

60

bed1

bed2

T13

18:00

Time (hh:mm)

(a)

(b) Fig. 19. The PCM temperature evolution

36

06:00

5 06:00

18:00 Time (hh:mm)

(c)

06:00

ACCEPTED MANUSCRIPT The PCM temperature variations of the different nodules show a slight difference concerning the charging and discharging duration as well as the fusion and solidification peaks. Concerning the nodules temperatures placed in a same position in both beds (Fig. 19 (a)), the PCM temperature of the first bed is more important than the PCM temperature of the second bed. The phase change material constituting the first bed received more solar intensity than the bed _1

second bed which explains the fact that T13

bed _ 2

exceeded 58 °C while T13

didn’t attain 37 °C.

More the nodule is near the transparent cover of the collector more its PCM temperature is greater. Comparing two nodules temperatures in different positions in a same bed, PCM temperatures in upper location are greater than PCM temperatures in lower position. The thermo-siphon bed _1

effect was developed in the upper part of the collector which explains the fact that T13 bed _1

more important than T7 bed _ 2

than T1

bed _ 2

(56.6 °C against 38.7 °C at 17:50) (Fig. 19 (b)) and T13

is

is higher

(36 °C against 30 °C at 16:40) (Fig. 19 (c)).

During the charging process, the PCM temperature started raising at 7h until reaching around 27 °C at 10:30. Then, temperature remained constant about two hours when the PCM stored an amount of energy as latent heat. This phenomenon is more perceived in nodules of higher positions by the thermo-siphon effect. For the discharging process, more the nodule is near the bed _ 2

air inlet more it discharges faster. The PCM temperature T1

discharged more rapidly than

other nodules and drop less than 10 °C (Fig. 19 (c)). The discharging duration of nodules near the cold air inlet is quicker than nodules of upper locations which discharged over a longer period (after 7h). (Fig. 20) clarifies the phenomena of the hysteresis and the phenomena of the supercooling presented during the charging-discharging processes in the nodule 13 of the second bed (Fig. 5 section 1.3).

37

ACCEPTED MANUSCRIPT 40 bed2

T13

PCM temperature (°C)

35

30

Tmelting

25

Supercooling

Hysteresis

Tcrystallization

20

15

10

5 19/02/2017 06:00

19/02/2017 18:00

20/02/2017 06:00

20/02/2017 18:00

21/02/2017 06:00

Date and time Fig. 20. The PCM phenomena

During the sunshine when the PCM stored the thermal energy from the solar irradiation, its temperature started to increase until reaching the melting temperature, approximately 27 °C, when the first drops of liquid appeared. During the changing phase of the PCM, the melting temperature was slightly greater than the crystallization temperature; and then, the temperature continued to raise until reaching about 37 °C. This particularity corresponds to the phenomenon of hysteresis. The hysteresis is characterized by a lag of the solidification temperature compared to that of fusion. It is due to some thermo-physical properties of PCM such as the slow formation of the crystalline lattice, the modification of the crystalline organization of the solid during the phase change. During the discharging process, the temperature of the PCM decreased below the solidification temperature and the PCM was in a metastable state. This state was maintained over a period of time and then the temperature evolved slowly and remained constant around a lower temperature than the temperature of the melting representing the total crystallization. The PCM temperature decreased afterward till dropping less to 15 °C. This state corresponded to the supercooling which is a delay in crystallization; the liquid dropped to a temperature below the theoretical phase transition temperature (sensible heat of the liquid phase) because the first

38

ACCEPTED MANUSCRIPT crystal did not appear. Supercooling is generally characterized by a difference between the melting temperature and the most likely temperature of appearance of the first crystal. 

The thermal efficiency of SAHL

The daily average thermal efficiency of the SAHL  is defined by the quotient of the amount of the accumulated heat extracted from the collector during the discharging process and the amount of the accumulated absorbed heat during the charging process. It takes the form as:

=



 Extracted heat amount  Absorbed heat amount

=



QExtracted

Disch arg ing process



(20)

QAbsorbed

Ch arg in gprocess

Figure 21 illustrates the daily thermal efficiency with the accumulated useful and absorbed heat. The daily average thermal efficiency of the solar air heater with latent thermal storage varied

160

0.7

140

0.6

120

0.5

100

0.4

80 0.3

60

0.2

40

0.1

20 0

0 18 Feb

19 Feb

20 Feb

21 Feb

22 Feb

Date Date Accumulated useful heat

Accumulated absorbed heat

Fig. 21. Thermal efficiency of the SAHL

39

Daily thermal efficiency

Daily thermal efficiency (%)

Accumulated heat (kJ)

between 0.29 % and 0.38 %.

7.

Survey of greenhouse heating applications

Table 7. Various greenhouse heating systems studies

Reference

Heating system description

(Öztürk and Başçetinçelik, 2003) (Ghosal and Tiwari, 2004) (Ozgener and Hepbasli, 2005)

Sensible heat technique by storing solar energy using the volcanic material Inner thermal curtain and natural flow of geothermal warm water through the floor Solar-assisted ground-source (geothermal) heat pump system Aquifer coupled cavity flow heat exchanger (Sethi and Sharma, 2007) system (Benli and Durmuş, 2009b)

Ground-source heat pump-phase change material latent heat storage system

(Benli and Durmuş, 2009a) (Benli, 2013)

Ten solar collectors with latent heat storage

(Joudi and Farhan, 2014a) (Zhang et al., 2015)

Horizontal and vertical ground- source heat pumps Six solar air heaters with a single glass cover on the roof Seasonal solar soil heat storage system

(Imtiaz Hussain et al., 2015)

Linear Fresnel lens (LFL) and Spot Fresnel Lens (SFL) solar collectors

Findings - the average daily rates of the thermal energy and exergy recovered from the heat storage unit were 601.3 and 20.9 W during the charging period - the heat storage unit assured 18.9 % of the total heating requirement. - the air temperature surrounding plants under thermal blanket was maintained between 14 – 23 °C during winter night and early morning - the greenhouse air had a maximum diurnal temperature of 31.05 °C, nocturnal temperature of 14.54 °C and a relative humidity of 40.35 %. - the average greenhouse room air temperature was maintained at 7 – 9 °C above the outside air during winter nights - the average relative humidity inside the greenhouse decreased by 10 – 12 % - the heat pump increased the temperature by 5 – 10.8 °C and the chemical material increased it by 1 – 3.8 °C - compared with the conventional heating systems, the collectors provided about 18–23 % of total daily thermal energy requirements of the greenhouse for 3 – 4 hours - the overall system efficiency ranged between 2.7 - 3.3 for horizontal groundsource heat pump and 2.9 - 3.5 for vertical ground-source heat pump system - the greenhouse inside air temperature was kept at 18 °C - the daily heating load of the greenhouse was provided - the energy saving realized by using the seasonal solar soil heat storage system compared to conventional solar heating system was 27.8 kW h/m2 - SFL collector performance was about 7 – 12 % higher than that of LFL collector. 40

(Llorach-Massana et al., 2016)

PCM as a root zone temperature control system

(Semple et al., 2017)

Solar collector system for low and high temperature applications

(Anifantis et al., 2017)

Ground source geothermal heat pump with to photovoltaic panels

(Hassanien et al., 2018)

Evacuated tube solar collector as a solar water heater assisted an electric heat pump

The present study

New solar air heater with two packed beds of latent storage energy

- the thermal efficiency was higher for the SFL than for the LFL collector. - significant environmental and economic benefits were obtained - an increase of 18 % of farmers net benefit was obtained - crop productivity increased of 20 % - the system was able to cover up to 65 % of the annual greenhouse heating requirement - solar collector and thermal energy storage efficiency for the low-temperature system were significantly higher than that of the high-temperature system - the overall system efficiency obtained, starting from the amount of solar energy available during daylight hours was 11 % - the internal air temperature in the greenhouse raised by 2 - 3 °C, whereas, the relative humidity decreased by 10 % - the system provided more than 35% of the annual required heat. - the thermal efficiency of the solar collector was 0.49. - the solar air heater increased the temperature by over 6 °C all the nighttime - the nocturnal temperature inside the greenhouse varied between 17 and 20 °C - the thermal efficiency of the SAHL reached 0.38 %

41

ACCEPTED MANUSCRIPT 8.

Conclusion The feasibility of an innovative Hydroponic Greenhouse (HG) and a new Solar Air

Heater with Latent storage energy (SAHL) has been evaluated. The novelty of this work is to warrant an enhanced hydroponic greenhouse microclimate due to an hydroponic concept and a pilot unit with latent thermal storage using exclusively solar energy. The hydroponic greenhouse guaranteed a suitable microclimate during the whole daytime with internal temperatures exceeding the ambient temperature by around 6 °C. The diurnal temperature inside the greenhouse ranged between 18 and 34 °C. To compensate for nocturnal temperature drops, a solar air collector with latent storage energy was connected to the greenhouse. The packed beds absorbed the sun radiations and stored the excess of the thermal energy as latent heat. The PCM used proved to be a promising solution for this energy storage application and melted with minor variations in the appropriate operating temperature range. At night, the SAHL recovered the stored energy and obviously improved the indoor climate. The temperature was increased by over 6 °C all the nighttime and the nocturnal temperature varied between 17 and 20 °C. The daily average thermal efficiency of the solar air heater with latent thermal storage varied between 0.29 % and 0.38 %. The SAHL appeared to be cost effective compared to the conventional heating systems and good improvement has been occurred in the greenhouse. Annual energy production due to the solar heating was reduced by over 4600 kWh. The payback period of using the SAHL system was 6 years. By using this kind of solar air heater, economic terms show considerable good manners. Subsequently, the use of the SAHL and the HG could be a beneficial option for having optimal microclimate, improved energy saving benefits and reduced CO2 emission.

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ACCEPTED MANUSCRIPT

Highlights 

Hydroponic greenhouse required less additional heating needs compared to conventional greenhouses (glass, plastic …).



The solar heater with latent thermal storage mitigated obviously the nocturnal temperature drops inside the greenhouse.



The system hydroponic greenhouse-latent thermal storage heater warranted an enhanced microclimate even during harsh climates.