thermal modeling of a ridge-ventilated greenhouse - ijetae

International Journal of Emerging Technology and Advanced Engineering

Volume 3, Special Issue: ICERTSD 2013, Feb 2013, pages 348-355

An ISO 9001:2008 certified Int. Journal, ISSN 2250-2459, available online at www.ijetae.com

Presented at International Conference on Energy Resources and Technologies for Sustainable Development, 07-09 February 2013, Howrah, India.

ICERTSD2013-12-254 © IJETAE2013

THERMAL MODELING OF A RIDGE-VENTILATED GREENHOUSE EQUIPPED WITH LONGITUDINALLY DISTRIBUTED EVAPORATIVE

COOLING PADS

D.Misra 1, S.Ghosh 2* 1 Department of Mechanical Engineering,

Saroj Mohan Institute of Technology,Guptipara, Hooghly712512,West Bengal,India; Ph-+91-3213-224041, fax-+91-3213-224137

2* Department of Mechanical Engineering, Bengal Engineering & Science University, Shibpur, Howrah-711103, West Bengal, India;

Ph-+91-33-26684561-63, fax-+91-33-26682916

* Corresponding author email: [email protected]

ABSTRACT In the present paper, a thermal model of a fan-pad ventilated greenhouse with distributed evaporative cooling has been presented to minimize the temperature gradient along the length. An uneven-span ridge type greenhouse is considered, the fans being aligned along the ridge of the greenhouse and the cooling pads being aligned along the side wall segments. The analysis is based on energy balance equations for various elements like plants, floor and inside air of the greenhouse. A computer program in C has been developed to solve the energy equations and compute the performance parameters for a given set of input climatic and geometric data. The model has been compared with an earlier model available in the literature which was based on a single-element analysis considering the whole greenhouse as a single entity. The analysis suggests that during extreme summer days, the greenhouse air temperature can be maintained about 5-7oC below the ambient temperature for a given ventilation rate of 1.2 ACM and canopy shading of 50%. Keywords: Greenhouse; Solar radiation; Evaporative cooling; Ventilation 1. INTRODUCTION A greenhouse is a framed structure covered with a transparent material, which enable the cultivation of crops, vegetables, flower etc. in qualities, quantities when it's impossible to achieve cultivating in an open environment. The structures protect the plants from wind and natural hazards, provide cover from rain in order to manage the irrigation and fertilization schedule, and contribute to preventing outbreaks of disease and pest infestation. Greenhouses should provide a controlled environment for plant production with sufficient sunlight, temperature and humidity. Nowadays, greenhouse technology has been in use in hot and humid countries like India, by using appropriate cooling and ventilation method. A lot of research work has been carried out on greenhouse technology for the last few decades but very few of them are relevant to Indian climatic condition with fan pad evaporative cooling. Ganguly and Ghosh [1] have presented a thermal model of a fan-pad ventilated floriculture greenhouse to predict the inside greenhouse temperature. They have also shown the effects of shading and ventilation rate on greenhouse temperature. Shukla et al. [2] have carried out an experimental study to see the effect of an inner thermal curtain in an evaporative cooling system of a cascade greenhouse. A

thermal model has also been developed to predict the air temperature. Kittas et al. [3] have shown that the evaporative cooling system is able to keep the greenhouse air temperature at rather low levels. Impron et al. [4] demonstrated that air temperature was affected more by variations of ventilation and leaf area index than by the applied cover properties. The leaf area index had the highest impact on greenhouse air temperature, implying that a large proportion of the cooling is achieved by the crop itself. Sethi and Sharma [5] developed a thermal model for heating and cooling of an agricultural greenhouse integrated with an Aquifer Coupled Cavity Flow Heat Exchanger System (ACCFHES). Kittas et al. [6] reported experimental investigation of the climatic variables of greenhouse such as air temperature, solar radiation, outside wind speed and direction and their interactions affecting the air temperature in a fan-ventilated multi-span greenhouse with rose crop. Ghosal et al. [7] presented a mathematical model considering heat transfer through flowing water film on shade cloth, stretched over the roofs and south wall of an even span greenhouse to study the effectiveness of cooling in greenhouse. In the conventional fan-fad evaporative cooling and ventilation system greenhouse fans and the cooling pads are installed on the opposite walls of the house. When

© IJETAE2013 ICERTSD2013-12-254

2

Int. J Emerging Technology and Advanced Engineering

ISSN 2250-2459, Volume 3, Special Issue: ICERTSD 2013, Feb 2013, pages 348-355

outside air is drawn by the induced draught fan in the greenhouse through the wet pad, it gets cooled as latent heat of evaporation of water is taken from the air. This cold air picks up heat while flowing from the pad end to the fan end, causing a temperature gradient along the length of the greenhouse. This puts a restriction on the construction of longer greenhouse. In this paper, a thermal model of a greenhouse with distributed evaporative cooling is presented. An uneven-span ridge type greenhouse is considered, the fans being aligned along the ridge of the greenhouse and the cooling pads being aligned along the side wall segments (Fig.1). During the operation, air is drawn by the fans through the wet pads and ventilated out of the greenhouse through the roof. This avoids temperature gradient along the length but a small temperature gradient is set up along the width and height of the greenhouse. Therefore, temperature gradient does not put any

restriction on the length of the greenhouse. The study is based on the climate data for the city of Kolkata (22.83oN, 88.82oE), which bears the mixed climatic conditions of the plains and coastal areas of India. In the present model an east-west oriented, un-even span, single ridge greenhouse has been considered, with a floor area of 180-m2. Fig.1 represents the general arrangement of the proposed system. Central and side wall heights of greenhouse are 4 m and 2 m, respectively. The greenhouse is covered with Fiber Reinforced Plastic (FRP). The cooling pads are on the north and south walls, one on each side of each of the six bays or segments. A door is on the west wall and fans are aligned along the ridge of the greenhouse as shown in Fig.1. Shade nets are provided along both of the canopy in inclined manner.

Fig.1: General arrangement of proposed greenhouse

N

Fig.1(a): Side elevation (North) of the system

0.7 m

2 m

2 m

30 m

F

F

F

F

F

F

Y

Y

P P P P P P

CWP


3



2. THERMAL MODELING The following assumptions were made in the development of the thermal model:

(i) Analysis is considering steady-state condition. (ii) Heat flow is considered to be one dimensional. (iii) No energy is absorbed by the structural

elements. (iv) Radiative heat exchange between canopy and

side walls has been neglected. Following input parameters are used for the thermal model of the greenhouse: Transmissivity of the covering material for normal beam radiation [1]: 0.8 Transmissivity for diffused and reflected radiation [1]: 0.76. Saturation efficiency of cooling pad [1]: 0.88. Heat absorptivity of Ground [5]: 0.30. 2.1 Total incident solar energy on greenhouse cover The total intensity of solar radiation falling on different inclined and vertical surfaces of greenhouse is calculated using solar radiation geometry for the relevant surfaces and solar radiation. The total incident heat load of greenhouse is given by:

tiit IA)SF1(S (1)

Where Ai is the area of the surface ‘i’ and Iti is the intensity of transmitted solar radiation into the greenhouse through that surface. The intensity of transmitted solar radiation (It) into the greenhouse can be written as:

rrdbdddbbbt R)II(RIRII (2)

where Ib and Id are intensity of beam and diffuse radiation respectively. Rb, Rd, and Rr are tilt factors for beam, diffuse and reflected radiation respectively. ζb is the transmisivity of the beam radiation which varies

with hour angle and the maximum value is assumed as 0.8 for the material fiber reinforced plastic (FRP). ζr and ζd are the transmisivity of the global and diffuse radiation which are assumed to be constant and value chosen is 0.76 respectively. To calculate the total heat load into the greenhouse it is considered that the vertical surfaces receive only diffused radiation because during the peak radiation hours the contribution of beam radiation on the vertical walls is insignificant compared to the total radiation [1]. 2.2. Energy balance equations It is considered that the transmitted solar radiation into the greenhouse is fully absorbed by the plants, inside-air and floor of the greenhouse. The temperature inside the greenhouse air can be calculated by applying the law of conservation of energy for the different elements (plant, floor, inside air) of the greenhouse[14]. In the following section energy balance equations for each of the greenhouse components has been presented. 2.2.1. Greenhouse plants

)TT(Ahdt

dTCMS Gpppr

ppptp (3)

[Rate of energy absorbed by the vegetation] =[ Rate of energy used to increase plant temperature] + [Rate of energy convected and evaporated to the surrounding through leaves] Where, (αp) is absorptivity of plant suggested by Sameshima [12], is given by

)1)(r1( fp (4)

In equation (4) ‘rf’ is the reflectivity of leaves which depends on the variety of plants; ‘τ’ is transmisivity of leaves; ‘hpr’ is convective-evaporative heat transfer coefficient and ‘PT’ is saturation pressure, suggested by Tewari [7].

Fig.1(b): Sectional elevation of Y-Y

2 m

0.7 m Shade net

2 m

Air out

Air in

6m


4



2.2.2. Greenhouse floor

)TT(hA|dx

dTKAS)1( Go|xg

oxgtpg

(5)

[Rate of energy absorbed by the floor] = [Rate of energy conducted through the floor] +[Rate of energy convected into the greenhouse air] The rate of thermal energy conducted in the ground is expressed in a steady state condition as

)TT(Ah|dx

dTKA oo|xgb

oxg

(6)

Where, Tx=o is the surface temperature of the ground, °C. Temperature in the ground after a certain depth (Tα) becomes constant and is considered equal to the underground annual temperature, which is assumed to be constant (To) beneath the greenhouse floor as discussed by Tiwari and Goyal [14]. Thus, eqn. (5) can be written as

)TT(Ah)TT(AhS)1( Go|xgaoo|xgbtpg (7)

[Rate of energy absorbed by the floor] = [Rate of energy convected from floor to underground] +[Rate of energy convected into the greenhouse air] 2.2.3. Greenhouse Air The total heat accumulation in the greenhouse air is the summation of transmited heat after absorption by the plant and floor, convected-evaporated heat from the plants and convected heat from the floor. It is also considered that the heat exchange occurs across the greenhouse coverings due to temperature difference between inside and the ambient. If ambient temperature is more than the inside temperature then heat transfer takes place from outside to inside. This total heat increases the inside air temperature and is required to be ventilated out of the greenhouse by ID fans. Thus energy balance equation for the air becomes

)8()TT(VC)TT(hA

)TT(Ah)TT(AhS)1)(1(

padfanaaGgc

GoxgaGppprtpg

[Rate of energy gained by the greenhouse air after absorption by plants and floor] + [Rate of energy convected and evaporated in the greenhouse air through plant leaves] + [Rate of energy convected from the floor to the greenhouse air] = [Rate of heat transfer between the greenhouse air and the ambient] + [Rate of sensible heat gain of greenhouse air which is to be ventilated out under steady state condition] Where V is the volume of air handled by ID fan and expressed as

60/)HBL(ACMV (9)

Where, H is the effective height considering the entire shape of the greenhouse as a parallelepiped. Thus the total volume of air inside the greenhouse is equivalent to the volume of the parallelepiped . Temperature of pads can be expressed as

)TT(TT wbaapad (10)

Where ε is the saturation efficiency of cooling pads. In equation (8) ‘h’ is the overall heat transfer coefficient, given by Sethi [5]. From Eq. (7) the surface temperature of the floor of the greenhouse (Tx=0) can be written as

gagb

Ggaogbtpgox AhAh

TAhTAhS)1(T

(11)

The average greenhouse air temperature (TG) can be considered to be the arithmetic average of temperature at pad and fan end and Thus

2

TTT

padfanG

(12)

Combining equations (8), (11) and (12) greenhouse air temperature (TG) can be obtained.

gbGgc

padgctGptG AhHRQhA

QTTahA)NMS(HRTPST

(13) Considering

ba

aG hh

hH

)1(M pg

ogb TAhN )1)(1(P pg

aa VC2Q ppr AhR

In simplified form TG can be written as

pG BTAT (14)

Where

gbGgc

padgctGt

AhHRQhA

QTTahA)NMS(HPSA

gbGgc AhHRQhA

RB

Combining equation (14) and equation (3) and after simplification it can found as


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1p1p

BTAdt

dT (15)

Where pp

1 CM

)B1(RA

pp

tp1 CM

ARSB

Solving Eqn. (15) temperature of the plant (TP) can be obtained as

tApo

1

1

1

1p

1e)TA

B(

A

BT (16)

Where Tpo= initial plant temperature of the greenhouse and‘t’ is the time in seconds. By the value of plant temperature(Tp), greenhouse air average temperature (TG) can be obtained from eqn.(14) and fan (Tfan) temperature can be found out from the eqn.(12).

Accordingly, temperature gradient along the flow path

c

padfanLc L

TTd

(17)

Considering Lc (length Lc that can be geometrically

evaluated by the fig.2) is the linear distance from the centre of pad to fan which can be calculated greenhouse geometry. It has seen that the average temperature of greenhouse air (calculated by eqn. 14) located above gutter level of shed net (point D in fig.2). Though greenhouse vegetation below the gutter level therefore it may be considered average air temperature of the effective plantation zone occurs in between the gutter level and pad (shown at point B in fig.2). From the geometry average greenhouse temperature around plantation zone

)6/LdT(TT cLpadGp c (18)

Fig.2: Air flow path and temperature profile

3 m

Approximate Air flow path

Temperature profile

Tfan

TGutter

2 m

2 m

Lc

A

E

TG

TGP

D

C

B

T

l


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3. RESULTS AND DISCUSSION The model performance has been carried out using radiation and temperature data published for Kolkata [13], assuming a relative humidity. The results have been compared with the results predicted by Ganguly and Ghosh [1]. Fig. 3 shows the hourly variation of greenhouse air temperature for a given value of ventilation rate, shading and LAI on 15th May (a representative hot and dry day in summer). At 15 hours of the day when ambient temperature is the maximum it is seen that using 50% shading, 1.2 ACM ventilation rate and assuming 50% relative humidity a temperature difference of about 7.52°C between ambient and greenhouse air is obtained by using longitudinally pads and fan ventilated distributed evaporative cooling system when LAI is one. Maximum greenhouse air temperature is limited by 28.28°C when the ambient temperature is 35.8°C. Fig. 4 shows the variation of greenhouse air temperature on16th August (a representative hot and humid day in the monsoon). The model considers the average hourly radiation intensities for clear sky as the input values. The relative humidity is assumed 75% in this month. It is seen that during hot and humid condition greenhouse inside temperature is well manageable though high relative humidity restricts the temperature reduction by evaporative cooling occurring in the cooling pad. The result arises when greenhouse provides 50% shading, 1.2 ACM ventilation and one LAI. Fig. 5 shows the variation of greenhouse air temperature on 17th January (a representative day in winter). As the thermal load in the month of January is low, ventilation rate of 0.6 ACM is used instead of 1 ACM with 50% shading and assuming 50% relative humidity. It is seen that the model predicts the greenhouse inside air temperature below 16°C during the off peak radiation hours in the morning up to 10 a.m. Fig. 6 shows the variation of greenhouse air temperature on 17th January with dry pads. It is seen that greenhouse temperature becomes more than the ambient temperature using the dry run during the winter season. The greenhouse temperature can be maintained between 14.01-24.12°C, a favourable temperature for the plant growth, using 1 ACM ventilation rate with dry pad and 50% shading when LAI is one. Relative humidity assumed to be 50% throughout the day. In all the cases stated above it is seen that the results predicted by the model are lower than the results predicted by Ganguly and Ghosh [1]. There is a favourable difference of about 1.5-2°C between the outputs of the two models at peak radiation hours. The model of Ganguly and Ghosh predicts higher value. This is due to the fact that they have not considered the convective heat transfer from floor surface to underground, plant absorptivity which is considered in the present model.

6 8 10 12 14 16 18

8

12

16

20

24

28

32

36

January, ACM=0.6, SF=0.5, RH=0.5

Tem

pera

ture

(o C

)

Time (hr)

Ta T

Ganguly

TModel

Tp

6 8 10 12 14 16 18

8

12

16

20

24

28

32

36

August, ACM=1.2, SF=0.5, RH=0.75

Tem

pera

ture

(o C

)

Time (hr)

Ta T

Ganguly

TModel

Tp

6 8 10 12 14 16 18

8

12

16

20

24

28

32

36 May, ACM=1.2, SF=0.5, RH=0.5

Tem

pera

ture

(o C

)

Time (hr)

Ta T

Ganguly

TModel

Tp

Fig.3: Variation of greenhouse plant and air temperature for a representative hot and dry summer day of 15th May.

Fig.4: Variation of greenhouse plant and air temperature for a representative hot and humid day of 16th August.

Fig.5: Variation of greenhouse plant and air temperature for a representative winter day of 17th January.


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5. CONCLUSIONS It can be concluded that the length of the greenhouse can not put any restriction for moderating room air temperature. From the analysis it has been shown that the greenhouse and plant temperature can be maintained satisfactorily throughout the season in a place like Kolkata. In the month of summer the model it is most effective. In winter the model shows very low temperature during the non peak radiation hours. Thus during those hours in winter natural ventilation with dry pads can be employed. Appendix

Table 1: Parameter values used Parameters Values Ag (m

2) 180 Ac (m

2) 102.74 Agc (m

2) 372.3

Ap (m2) 144

Mp (Kg) 120 Cp (JKg-1 °C-1) 4190 Ca (JKg-1 °C-1) 1005 ρa (Kgm-3) 1.2 T0 (

0C) 17

LAI 1 rf 0.22

k 0.57 v (m/s) 1 ρ 0.2 ε 0.88

REFERENCES

1. Ganguly. A., Ghosh. S., 2007, “Modeling and analysis of a fan–pad ventilated floricultural greenhouse”, Energy and Buildings, 39 (10): 1092–1097.

2. Shukla. A., Tiwari. G..N., Sodha. M.S., 2008, “Experimental study of effect of an inner thermal curtain in evaporative cooling system of a cascade greenhouse”, Solar Energy, 82

(1): 61-72. 3. Kittas. C., Bartzanas. T., Jaffrin. A., 2003,

“Temperature Gradients in a Partially Shaded Large Greenhouse equipped with Evaporative Cooling Pads”, Biosystems Engineering, 85 (1): 87-94.

4. Impron. I., Hemming. S., Bot. G.P.A., 2008, “Effects of cover properties, ventilation rate, and crop leaf area on tropical greenhouse climate”, Biosystems Engineering, 99 (4): 553– 564.

5. Sethi. V. P., Sharma. S. K., 2007, “Thermal modeling of a greenhouse integrated to an aquifer coupled cavity flow heat exchanger system”, Solar Energy, 81 (6) : 723-741.

6. Kittas. C., Karamanis. M., Katsoulas. N., “Air temperature regime in a forced ventilated greenhouse with rose crop”, Energy and Buildings, 37 (8) : 807-812.

7. Tiwari. G. N., Gupta. A., 2002, “Performance evaluation of greenhouse for different climatic zones of India”, Solar Energy Society of India (SESI), 12 : 45–57.

8. Ghosal. M. K., Tiwari. G. N., Srivastava. N. S. L., 2003, “ Modeling and experimental validation of a greenhouse with evaporative cooling by moving water film over external shade cloth”, Energy and Building, 35 (8): 843–850.

9. Chen. W., Liu. W., Liu. B., 2006., “ Numerical and experimental analysis of heat and moisture content transfer in a lean-to greenhouse”, Energy and Buildings, 32 (2): 99-104

10. Fuchs. M., Dayan. E., Presnov. E., 2006, “Evaporative cooling of a ventilated greenhouse rose crop”, Agricultural and Forest Meteorology, 138 (1-4): 203-215.

11. Marina. H., Dan. G. Blumerg., Stanley. R. Rotman., 2010, “Estimation of vegetation parameters from polarimetric sar data EGU General Assembly”, available on line at ieeexplore.ieee.org.

12. Sameshima. R, 1995, “Estemating the Absorptivity of Solar Radiation in Soybean Canopies for Use in Crop Models”, Agricultural and Forest Meteorology 51(1) : 37-45.

13. Tiwari, G. N., 2002, “Solar Energy- Fundamentals, Design, Modelling and application”. Narosa Publishing House.

14. Tiwari, G. N., Goyal, R.K., 1998. Greenhouse Technology. Narosa Publishing House, New Delhi, India

15. Sukhatme. S. P., 2004,”Solar Energy-Principle of Thermal Collection and Storage”, Tata Mc Graw Hill.

16. Willits. D. H., 2003, “Cooling Fan-ventilated Greenhouses, a Modeling Study”, Biosystems Engineering, 84 (3): 315-329.

6 8 10 12 14 16 18

8

12

16

20

24

28

32

36

Dry pads, ACM=1, SF=0.5, RH=0.5

Tem

pera

ture

(o C

)

Time (hr)

Ta Tp T

Model

Fig.6: Variation of greenhouse plant and air temperature for a representative winter day of 17th January.


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NOMENCLATURE Symbol I Intensity of radiation (W/m2) R Tilt factor (dimensionless) ζ Transmissivity (dimensionless) T Temperature (°C) P Pressure (Pa) A Area (m2) C Specific heat (JKg-1 °C-1) ρa Density of air (Kgm-3) M Mass (Kg) ho Outside heat transfer coefficient of

greenhouse (Wm-2°C-1 ) ha Heat transfer coefficient between greenhouse

floor and room air (Wm-2°C-1 ) hb Heat transfer coefficient between greenhouse

floor and ground beneath (Wm-2°C-1 ) hpr Convective-evaporative heat transfer

coefficient between plants and air (Wm-2°C-1)

h Overall heat transfer coefficient (Wm-2°C-1 ) hp Convective heat transfer coefficient between

plant and greenhouse air (Wm-2°C-1 ) K Thermal conductivity (Wm-1°C-1)

LAI Leaf area index (dimensionless) ACM Air change per minute

(dimensionless) RH Relative humidity (dimensionless) V Volume of air handled, (m3/s) v Air velocity inside the greenhouse (m/s) SF Shading factor (dimensionless) τ Transmisivity of solar radiation through

leaves (dimensionless) rf Reflectivity of solar radiation on leaves

(dimensionless) α Absorptivity (dimensionless) ε Efficiency of cooling pad (dimensionless) St Solar energy transmitted into the greenhouse

(W) L Length (m) B Width (m) H Effective height (m) P Cooling pad CWP Circulating water pump Subscrits a Ambient b Beam c Canopy d Diffuse g Ground/Floor gc Cover of greenhouse canopy G Greenhouse Gp Greenhouse plantation p Plant r Reflected wb wetbulb

AUTHOR BIOGRAPHY

Debajit Misra was born on 1st January, 1979 in India. He obtained his bachelor equivalent degree in Mechanical Engineering from The Institution of Engineers (India) in 2006 and Master of Engineering from Bengal Engineering and Science University, India in 2007. His

major field of study is Renewable Energy Systems like Cooling Techniques for Greenhouse cultivation. Presently he is working as an Assistant Professor in the Department of Mechanical Engineering, Saroj Mohan Institute of Technology, Guptipara, Hooghly (India). He is teaching the subjects like Power Plant Engineering, Internal Combustiion Engine, Heat Transfer.

Dr. Sudip Ghosh is an Associate Professor of Mechanical Engineering Department, Bengal Engineering & Science University, Shibpur, INDIA. He received his Bachelor of Engineering (Mechanical) degree from Jalpaiguri Government Engineering College in 1991,

Master of Technology degree from Bengal Engineering & Science University in 1993, Shibpur and completed Ph.D. from Jadavpur University, Kolkata in 2005. He is teaching the subjects like Power Plant Engineering, Thermodynamics and Heat Power for UG and Graduate students. His areas of research interest include Clean Coal Technology, Renewable Energy Technologies like Fuel Cell and Biomass Gasification Systems and Greenhouse Technology. He has several publications in various national and international journals

thermal modeling of a ridge-ventilated greenhouse - ijetae

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