Thermal modeling of a greenhouse integrated to an aquifer coupled cavity flow heat exchanger system

Thermal modeling of a greenhouse integrated to an aquifer coupled cavity flow heat exchanger system

Solar Energy 81 (2007) 723–741 www.elsevier.com/locate/solener Thermal modeling of a greenhouse integrated to an aquifer coupled cavity flow heat exch...

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Solar Energy 81 (2007) 723–741 www.elsevier.com/locate/solener

Thermal modeling of a greenhouse integrated to an aquifer coupled cavity flow heat exchanger system V.P. Sethi a

a,*

, S.K. Sharma

b

Department of Mechanical Engineering, Punjab Agricultural University, Ludhiana 141 008, Punjab, India b Energy Research Centre, Panjab University, Chandigarh 160 017, Punjab, India Received 3 August 2005; received in revised form 10 October 2006; accepted 10 October 2006 Available online 13 November 2006 Communicated by: Associate Editor Mattaios Santamouris

Abstract A thermal model is developed for heating and cooling of an agricultural greenhouse integrated with an aquifer coupled cavity flow heat exchanger system (ACCFHES). The ACCFHES works on the principal of utilizing deep aquifer water available at the ground surface through an irrigation tube well already installed in every agricultural field at constant year-round temperature of 24 C. The analysis is based on the energy balance equations for different components of the greenhouse. Using the derived analytical expressions, a computer program is developed in C++ for computing the hourly greenhouse plant and room air temperature for various design and climatic parameters. Experimental validation of the developed model is carried out using the measured plant and room air temperature data of the greenhouse (in which capsicum is grown) for the winter and summer conditions of the year 2004–2005 at Chandigarh (31N and 78E), Punjab, India. It is observed that the predicted and measured values are in close agreement. Greenhouse room air and plant temperature is maintained 6–7 K and 5–6 K below ambient, respectively for an extreme summer day and 7–8 K and 5–6 K above ambient, respectively for an extreme winter night. Finally, parametric studies are conducted to observe the effect of various operating parameters such as mass of the plant, area of the plant, mass flow rate of the circulating air and area of the ACCFHES on the greenhouse room air and plant temperature.  2006 Elsevier Ltd. All rights reserved. Keywords: Greenhouse; Aquifer water; Thermal model; Solar energy; Cooling–heating systems

1. Introduction Thermal modeling of controlled environment greenhouse is required to optimize various parameters involved in either heating or cooling of a greenhouse. Thermal modeling is also used to optimize the greenhouse air and plant temperature for maximum production of a crop from the greenhouse for a given thermal capacity. It requires basic energy balance equations for different components of the greenhouse system for given climatic conditions (solar radiation, ambient air temperature, relative humidity, wind *

Corresponding author. Tel.: +91 161 2502643. E-mail address: [email protected] (V.P. Sethi).

0038-092X/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2006.10.002

velocity, etc.) and various design parameters such as volume, shape, height, orientation, and place. Fisher and Yanda (1974) gave the description of various designs of the greenhouses in cold regions. White and Aldrich (1977) discussed various energy conservation aspects in a greenhouse. Tiwari (1984) conducted thermal analysis of a winter greenhouse. Baille (1989) performed studies on the greenhouse microclimate and its management in mild winter climates. A number of greenhouse time dependent models were discussed by Takakura et al. (1971), Chandra et al. (1981), Cooper and Fuller (1983), Sutar and Tiwari (1995), Tiwari and Goyal (1997), etc. A number of theoretical and experimental studies for greenhouse heating systems using water storage (Grafiadellis, 1987), rock bed

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Nomenclature area of the greenhouse cover, m2 area of the greenhouse door, m2 area of the greenhouse floor, m2 total surface area of the plant, m2 area of the ACCFHES, m2 specific heat of the air, J kg1 C1 coefficient of discharge of the orifice meter, decimal Cp specific heat of the plant, J kg1 C1 d diameter of the pipe, m di diameter of the orifice plate, m ha convective heat transfer coefficient between the greenhouse floor and enclosed air, W m2 C1 haw heat transfer coefficient from the greenhouse air in the pipe to the aquifer water, W m2 C1 hb bottom heat transfer coefficient between the greenhouse floor and the ground beneath, W m2 C1 hd heat transfer coefficient from the greenhouse door to the ambient air, W m2 C1 ho outside heat transfer coefficient of the greenhouse, W m2 C1 hp convective heat transfer coefficient from the plant to the greenhouse enclosed air, W m2 C1 hpr total convective and evaporative heat transfer coefficient from the plant to the enclosed air, W m2 C1 ht overall total heat transfer coefficient from inside to ambient through walls, floor and canopy cover, W C1 hwa heat transfer coefficient from aquifer water to greenhouse air in pipe, W m2 C1 Ib beam radiation on a horizontal surface at any instant, W m2 Id diffuse radiation on a horizontal surface at any instant, W m2 Ig global radiation on a horizontal surface at any instant, W m2 Ii total incident solar radiation flux on ith inclined surface at any instant, W m2 It total incident solar radiation flux at any instant on the inclined surface, W m2 k conductivity of the ground, W m2 C1 L length of the ACCFHES pipe, m LMTD logarithmic mean temperature difference, C ma mass flow rate of the air, kg s1 Ma total mass of air in the greenhouse, kg Mp total mass of plant in the greenhouse, kg N number of air changes per hour NTU number of transfer units (UhAh/Cmin), dimensionless p partial vapor pressure at saturation, Pa Ac Ad Ag Ap Ah Ca Cd

Qp r ro R St Ta Tci Tco Td Ti Thi Tho To Tp Tpo TR TR0 Tsa T1 Tx=0 Uh Ut v va V0 V1 Vfa Vg

total thermal energy transferred to or from the ACCFHES, W reflection coefficient of the ground (taken as 0.2) radius of the pipe, m ratio of the minimum heat capacity fluid (air) to the maximum heat capacity fluid (water) total solar radiation falling on the greenhouse at each wall and roof, W ambient air temperature, C inlet temperature of the cold fluid, C outlet temperature of the cold fluid, C design air temperature of the greenhouse, C temperature of the air at the inlet of the ACCFHES pipe, C inlet temperature of the hot fluid, C outlet temperature of the hot fluid, C temperature of the air at the outlet of the ACCFHES pipe, C temperature of the plant at any instant, C initial temperature of the plant, C greenhouse room air temperature, C initial temperature of the greenhouse room air, C sol–air temperature, C temperature of the greenhouse soil at larger depth, C temperature of the greenhouse floor surface, C overall heat transfer coefficient of the ACCFHES, W m2 C1 overall heat transfer coefficient of the greenhouse, W m2 C1 wind velocity, m s1 velocity of air through ACCFHES pipe, m s1 rate of heat transfer from the greenhouse due to infiltration, W rate of heat transfer from the greenhouse due to ventilation, W volume flow rate of the circulating air, m3 s1 total volume of the greenhouse, m3

Greek letters ap absorptivity of the plant, dimensionless s transmissivity of the greenhouse cover, dimensionless ag absorptivity of the ground, dimensionless qa density of air, kg m3 c relative humidity, decimal 1 larger depth Subscripts a air c cold fluid, canopy cover d door

V.P. Sethi, S.K. Sharma / Solar Energy 81 (2007) 723–741

E g h i N NR

east ground, greenhouse hot fluid inlet north north roof

storage (Fotiades, 1987; Bouhdgar and Boulbing, 1990), phase change material storage (Abhat, 1983; Boulard et al., 1990; Ozturk, 2005), ground air collector by Barok and Aldrich (1984), Kurata and Takakura (1991) and Bargach et al. (2000), north wall storage (Santamouris, 1989), solar pond (Mohamud, 1995), etc., were performed in depth by many researchers during the last three decades. The above-mentioned studies were exclusively conducted to reduce the heating requirements of various greenhouses located around the world. However, in a composite climate where heating of the greenhouse is required in winter nights and cooling is required in summer days, no single system as mentioned above can be used for meeting the requirements of such type of a climate. At present, earth-to-air heat exchanger system (EAHES) is the only available composite system around. This system has gained an increasing acceptance for greenhouse thermal control during the last few years. A number of researchers during the last two decades have performed many experimental and theoretical studies of the greenhouses coupled with EAHES. The representative studies were conducted by Puri (1985), Kozai (1985), Mavrogiannopoulos and Kyritsis (1985), Santamouris and Lefas (1986), Immakulov (1986), Bascetincelik (1987), Bernier (1987), Yoshioka (1989), Boulard et al. (1989), Herve (1990), Agas et al. (1993), and Sawhney and Mahajan (1994). The other exclusive studies on the heating and cooling performance of EAHES were conducted by Mihalakakou et al. (1993, 1994a,b, 1995), Sodha (1994), Santamouris et al. (1994, 1995a,b), Gauthier et al. (1997), Thanu et al. (2001), Argiriou et al. (2004), Ghosal and Tiwari (2006), etc. In these studies pipes were buried underground up to the depth of 2–4 m in order to take advantage of ground heating potential. The major constraint of using EAHES is the cost of digging the soil and burying of pipe(s) up to 2–4 m depth is very difficult. Horizontal installation and monitoring of pipe network at this much depth is not easy. Moreover, for the short-term use temperature around the soil mass gradually increases due to the dissipation of heat from the outside pipe surface thereby decreasing the efficiency of the system. Therefore, in this study, an alternative composite system is developed which can be effectively used for heating the greenhouse air in winter nights and cooling the same in summer days. The system uses deep aquifer water at the ground surface through an irrigation tube well installed at every agricultural farm at almost constant year-round

o p S SR W w

725

outlet plant south south roof west water

temperature of 24 C. The system is named as aquifer coupled cavity flow heat exchanger system (ACCFHES) as aquifer water flows through a cavity (shallow trench) dug around the perimeter of the greenhouse from one direction and greenhouse air circulates through an immersed pipe placed horizontally in the trench from the other direction thus making it a counter flow heat exchanger configuration. The cost of lifting the aquifer water to the ground surface and digging of trench in the agricultural field is negligible as already existing tube well and trenches meant for irrigation purposes can be used. As per the literature review, no such system is tried anywhere in the world and it is the first working system of its kind used for year-round thermal control of an agricultural greenhouse. The major advantages of the proposed system over the EAHES are: (i) the cost of deep digging of soil mass is negligible, which significantly lowers the installation cost of the system. (ii) Aquifer water is available at a lower temperature (24 C) throughout the whole year as compared to the ground temperature (26–28 C), which improves the cooling performance of the ACCFHES. (iii) For the same area of the pipe for both the systems, ACCFHES delivers or extracts more heat to or from the circulating air due to the counter flow arrangement of air and water or for the same amount of heat transfer, area and power requirements of the ACCFHES are lesser as compared to the EAHES. (iv) For the case of EAHES, temperature of the soil mass around the pipe surface increases after some times and lowers the rate of heat flow from the pipe surface to the soil or from the soil to the pipe. This ultimately lowers the overall performance of the system. Whereas in the case of ACCFHES, continuous flow of water over the immersed pipe surface in the counter flow direction instantly carries away or delivers the heat and the heat transfer rate remains same at all times. In this study, a thermal model of a greenhouse integrated to the ACCFHES is developed and its experimental validation for cooling as well as heating purposes is presented. Finally, parametric studies are conducted to observe the effect of various operating parameters on the greenhouse room air and plant temperature. 2. Materials and methods An east-west orientation, even span greenhouse (24 m2 floor area) constructed at Chandigarh (31N latitude and 78E longitude), Punjab, India is used for the study. A

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door (1 · 1.8 m) on the west wall and a ventilator (0.8 · 0.8 m each) with screw opening and closing mechanism is provided on the center of each inclined north and south roof. The slope of both the roofs is 26.56 with the horizontal. Central and side height of the greenhouse is 3 m and 2 m, respectively. The total cover area (area of four vertical walls and two inclined roofs) of the greenhouse is 70.82 m2 and is covered with a single UV stabilized polyethylene (s = 0.7) sheet. The greenhouse is coupled with the designed ACCFHES for cooling the greenhouse

water gaining heat

aquifer water

air in summer conditions (Fig. 1) and heating the same in winter conditions (Fig. 2). In the cooling mode, greenhouse air (hot fluid) near the roof is drawn at state 1 and forced through the pipe placed in the shallow trench at state 2. Aquifer water (cold fluid) from an irrigation tube well (situated at around 3 m distance from the greenhouse sight) is supplied in the trench from the other side. The trench is dug around the perimeter (three sides) of the greenhouse, in which a 20 m long (0.1016 m diameter) plastic pipe is horizontally placed. A

6m

air losing heat in pipe

from tube well Tci ˚

air 6m

3

4

cold air delivery

Tho delivery tube

6m

˚

4m

blower & motor

top supports

1

cold air delivery Thi˚

hot air suction

immersed pipe

2 Tco

water

circulating air

˚ 8m

to field

trench 0.3 × 0.3m

Fig. 1. Top view of the greenhouse cooling process using the ACCFHES.

aquifer water

air gaining heat in the pipe

water losing heat

from tube well Thi ˚ 2 hot air delivery blower

3

Tco ˚

delivery tube 1

4 Tci ˚

Tho `

water

cold air suction air gaining heat

˚ to field Fig. 2. Top view of greenhouse heating process using the ACCFHES.

trench

V.P. Sethi, S.K. Sharma / Solar Energy 81 (2007) 723–741

plastic sheet is spread in the trench (below the pipe) to avoid any water seepage to the ground beneath. Dimensions of the trench (0.30 · 0.30 m) are kept in such a way that the pipe remains fully immersed in water and carries the minimum quantity of water required for removing or delivering the heat. As soon as the water is filled in the trench, the plastic pipe starts floating on the water surface. Hence, to keep the pipe fully immersed in water, supports at the top of the pipe are provided after every 2.5 m length. These supports are simple mild steel rods of sufficient length fixed a few centimeters below the upper water surface level (across the channel width) so that the pipe does not rise above the water surface. Air and water are in a counter flow heat exchanger configuration as this arrangement has the highest effectiveness in term of heat transfer. During the flow passage, greenhouse air loses its heat to the water and becomes cooler at state 3. This cool air is then distributed in the greenhouse through a delivery pipe placed centrally along the greenhouse length at about 30 cm height above the greenhouse soil level. Final state 4 is thus achieved when this cool air is mixed with rest of the greenhouse air. In order to provide greater comfort to the plants, two 10 mm diameter holes are drilled on opposite sides of the delivery tube (on the circumference) at about 60 cm interval throughout the whole length, which provide the cool air between each plant row. The system uses 746 W, 3 phase, NGEF induction motor running at 2820 rpm and coupled with 0.4 m diameter centrifugal blower to draw greenhouse air at the flow rate of 0.4 m3 s1 (0.47 kg s1). In the heating mode, whole process remains the same but this time the greenhouse air (cold fluid) receives heat from the aquifer water (hot fluid) moving in the trench (Fig. 2). The air is sensibly heated by receiving heat from the water along the flow length and attains state 2. The air then passes through the vanes of the blower rotating at a high velocity and is further heated up by 2–3 C (state 3). The air at state 3 is then mixed with the rest of the greenhouse air to achieve the final state 4. One important observation is made that the air temperature is raised by about 2–3 C after it passes through the blower vanes. In order to neutralize the heating effect of air in summer conditions, the blower and motor assembly is placed in the suction line so that the greenhouse air when cooled by the ACCFHES is directly fed inside the greenhouse from the delivery side. However, in the heating mode, motor and blower assembly is placed in the delivery line so that the greenhouse air heated by the ACCFHES is further heated. The maximum air temperature inside the greenhouse is measured about 50 C (Ti) in summer, without using any cooling system (8 C above ambient air). The temperature of the circulating air is lowered to about 25 C (To) at the delivery pipe outlet (25 C drop from the maximum) to achieve the final design state (Td) at 35 C (7 C below ambient air) after mixing it with rest of the greenhouse air. Similarly, the minimum air temperature inside the greenhouse during extreme winter is measured around 4 C (Ti), without using any heating system. It is raised

727

to around 24 C (To) at the delivery pipe outlet (20 C increase above ambient air) to achieve the final design state at 13 C (9 C above ambient air) after mixing it with rest of the air. Climatic and other data are measured once (for one day) in every week from the first week of November 2004 up to the last week of June 2005. This data are recorded after each hour for 24 h on the selected day. Global solar radiation on a horizontal surface is recorded outside the greenhouse with the help of SM 201, CEL solarimeter also called suryamapi (local trade name) having a measuring range of 0–1200 W m2 (least count 5 W m2). Ambient air temperature, greenhouse air temperature and relative humidity is recorded on hourly basis using four calibrated LCD display thermo-hygrometers having 0–100 C temperature measuring range (least count of 0.1 C) and 0–99% relative humidity measuring range (least count 1%). One sensor is placed outside the greenhouse in shade (at 1 m height) and remaining three sensors are placed inside the greenhouse at 0.3, 1.0 and 1.5 m height at different locations (shaded from direct sunlight). Airflow rate is calculated using Altech water manometer (least count 0.1 cm) having 100 cm high peizometric tube across a calibrated orifice meter (di/d = 0.5, Cd = 0.76) fitted along the air pipe before the delivery point. Airflow rate accuracy is also checked using a dial type vane anemometer (range 0–100,000 m) of Otakeiki Seisakusho, Japan fitted across the suction line. An error of about 5% is observed in both the readings, which is within the experimental deviation range. Plant temperature is recorded with the help of 8868 IR non contact gun type infrared thermometer having temperature measuring range of 20 C to 315 C (least count of 0.5 C and measuring accuracy of ±2%). Temperature of three selected leaves (inside and outside each) is measured by pointing the gun from 0.3 m distance and then average taken. Wind speed inside and outside the greenhouse is recorded using cup anemometers. Water flow rate is measured with the help of a peizometric tube (1 m high, least count 0.1 cm) fitted on a galvanized iron tank of 1 · 1 m cross-section along the downstream side and recording the time taken for 20 cm rise in water level. Air and water temperature is also recorded at different locations (suction, delivery and in the center) along the length of the pipe and trench using stem type mercury thermometers of 0–50 C measuring range (least count of 0.5 C). Holes of suitable size are drilled at all the state points to insert the thermometers up to the center of the pipe. The holes are then properly sealed to avoid any water entry into the pipe. 3. Thermal analysis Solar radiation, after transmission through the greenhouse cover is received inside the greenhouse at the floor and the plants as shown in Fig. 3. Radiation absorbed by the plants is convected and evaporated to the room air, whereas radiation absorbed by the floor is either convected to the room air or lost to the ground by conduction

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Solar radiation incident on all surfaces, St Ta V1

ht

Reflected part

TR

hD

Transmitted, τ St Absorbed by plant, α p τ St 2m

Air suction Radiation

Tp

Evaporation, hpr

hD 0.3 m

Convection, hp Air delivery

GL

Absorbed by floor, αg St

0.3m

Conduction, hb

T∞ = Ta Fig. 3. Working principle of the greenhouse integrated to ACCFHES.

through the floor. The enclosed room air is heated and then various thermal losses occur through the canopy cover, the door, ventilators, etc. In order to write the energy balance equations for different components of the greenhouse system, following assumptions are made: (i) Storage capacity of the walls and the roof material is neglected. (ii) Absorptivity and heat capacity of the enclosed air is neglected. (iii) Radiative heat exchange between the greenhouse walls and the roofs is neglected due to its small value. (iv) The heat flow is one-dimensional. (v) Analysis is based on the quasi-steady state condition inside the greenhouse due to the transient behavior for short time interval. (vi) Relative humidity inside the greenhouse does not vary with height due to wetted floor or watering channel. (vii) No stratification in the temperature of the plant, the greenhouse enclosure, the covers, etc., due to low operating temperature range. (viii) Ground heat loss from the greenhouse floor to the ground beneath is considered in a steady state mode. (ix) Thermal properties of the green plants are assumed to be same as those of water. (x) Depth of the pipe immersion is constant throughout the whole length of the trench. 3.1. Energy balance equations Basic energy balance equations for different components of the greenhouse as suggested by Tiwari and Goyal (1998) are as follow.

3.1.1. Greenhouse plants dT p þ hpr Ap ðT P  T R Þ dt ½Rate of energy absorbed by the plant surface ¼ ½Rate of energy used to raise the plant temperature

ap sS t ¼ M p C p

þ ½Rate of thermal energy convected and evaporated from the plant to the enclosed air ð1Þ where S t ¼ AE I E þ AW I W þ AN I N þ AS I S þ ANR I NR þ ASR I SR ð2aÞ The value of St is computed theoretically for each wall and roof of the greenhouse using Liu and Jordan formula (1960) for inclined surfaces (Eq. (2b)). Hourly data of measured global and diffuse solar radiation on a horizontal surface near the greenhouse sight with various conversion factors for beam (Rb), diffuse (Rd) and reflected radiation (Rr) are used to compute the solar radiation intensity (Ii) on each wall and roof of the greenhouse. I i ¼ I b ðRb Þ þ I d ðRd Þ þ rðRr ÞðI b þ I d Þ

ð2bÞ

where i is E, W, N, S, SR and NR, respectively. Also hpr ¼ hp þ

0:016  hp ½pðT p Þ  cpðT R Þ Tiwari and Goyal ð1998Þ Tp TR ð3Þ

where hp = 2.8 + 3 (v) as suggested by Watmuff et al. (1977).

V.P. Sethi, S.K. Sharma / Solar Energy 81 (2007) 723–741

Eq. (1) is valid for 0-t time’s interval in which heat capacity of the plant and the heat transfer coefficients are constant. For the present model, 0-t time interval is considered as 3600 s. 3.1.2. Greenhouse floor ag ð1  ap ÞsS t ¼ kAg

oT þ ha Ag ðT x¼0  T R Þ ox x¼0

ð4aÞ

The rate of thermal energy conducted in the ground is expressed in a steady state condition as oT kAg ¼ hb Ag ðT x¼0  T 1 Þ ox x¼0

ð4bÞ

Temperature inside the ground after a certain depth (T1) becomes constant and is considered equal to the underground annual temperature Ta beneath the greenhouse floor as discussed by Tiwari and Goyal (1998). Therefore, Eq. (4a) can be written as ag ð1  ap ÞsS t ¼ hb Ag ðT x¼0  T a Þ þ ha Ag ðT x¼0  T R Þ ½Rate of thermal energy received by the greenhouse floor surface ¼ ½Rate of thermal energy loss from the floor to the ground beneath þ ½Rate of thermal energy loss from the floor to the room air

ð5Þ

3.1.3. Greenhouse room air Coupling of the ACCFHES with the greenhouse enables the heating and cooling of the inside air during the winter and summer conditions, respectively. The rate of hot or cool air supplied by the system is Qp, which is negative when heat is removed from the greenhouse in summer and is positive when heat is added to the greenhouse during winter. Energy balance of the greenhouse room air coupled with the ACCFHES is shown as Ap hpr ðT p  T R Þ þ Ag ha ðT x¼0  T R Þ  Qp

þ ½Rate of energy transferred from floor to room air  ½Rate of energy added=removed by ACCFHES ¼ ½Rate of energy used to raise the air temperature þ ½Rate of total energy loss through greenhouse cover þ ½Rate of energy loss through door þ ½Rate of energy loss through ventilators ð6Þ

Eq. (5) is used to compute the expressions for Tx=0, (Tx=0  TR) and Agha(Tx=0  TR) as shown below ag ð1  ap ÞsS t þ Ag ha T R þ Ag hb T a Ag h a þ Ag hb

ð7bÞ

ð7cÞ Substituting Eq. (7c) in Eq. (6) and neglecting the heat capacity of the air along with some mathematical simplifications, an expression for TR is obtained as H G age sS t þ ZT a þ Ap hpr T p  Qp ð8Þ TR ¼ Z þ Ap hpr HG and Z are the constant terms introduced for mathematical simplifications as shown in Table 1. By substituting the expression for TR in Eq. (1), a firstorder differential equation (9) of the following form is obtained dT p þ aT p ¼ f ðtÞ ð9Þ dt U pa where a ¼ ð10aÞ M pCp H G age H p sS t þ U pa T a  Qp and f ðtÞ ¼ ð10bÞ M pCp HP is a constant introduced for mathematical simplifications and its value is shown in Table 1. Eq. (9) is solved with the following assumptions (i) (ii) (iii) (iv) (v) (vi)

Solar intensity I(t) as IðtÞ (hourly average). Integrating with limit 0  t = Dt (3600 s). Ambient temperature Ta as T a . f(t) as f ðtÞ. Initial condition Tp = Tpo. Heat transfer coefficients are constant for Dt time interval.

The general solution of Eq. (9) is given as f ðtÞ ð1  eat Þ þ T po eat a

ð11Þ

A computer program in C++ is developed to compute the room air and plant temperature for a complete cycle of 24 h using Eqs. (8) and (11), respectively.

½Rate of energy transferred from plant to room air

T x¼0 ¼

ag ð1  ap ÞsS t þ Ag hb T a  Ag hb T R Ag h a þ Ag h b

Ag ha ðT x¼0  T R Þ ¼ ðag ð1  ap ÞsS t þ Ag hb T a  Ag hb T R ÞH G

Tp ¼

dT R þ ht ðT R  T a Þ þ hd Ad ðT R  T a Þ ¼ ma C a dt þ 0:33 NVg ðT R  T a Þ þ V 0

þ ½Rate of energy loss through infiltration

ðT x¼0  T R Þ ¼

729

ð7aÞ

3.1.4. Thermal energy gain inside the greenhouse The rate of cool or hot air required inside the greenhouse depends upon the instant solar radiation, the overall heat transfer coefficient, the design air temperature and the ambient air temperature at a particular time and place along with the type of the covering material used. Sol–air temperature is therefore calculated (Eq. (14)) for generalizing the design of the ACCFHES for any time and location. Sol–air temperature is interpreted as the temperature of the surroundings that will produce the same heating effect as the incident radiation in conjunction with the actual ambient air temperature. The instant heat gain per unit area of the greenhouse is equal to the heat available inside the greenhouse through the greenhouse cover minus the heat

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loss from the greenhouse by convection, radiation, evaporation and conduction as given below q ¼ ½sI g  U t ðT d  T a Þ

ð12Þ

q ¼ U t ðT sa  T d Þ Ig where T sa ¼ s þ T a Ut

ð13Þ ð14Þ

Overall heat loss coefficient Ut for the greenhouse is calculated after neglecting the thermal resistance of the cover material as  1 1 1 1 Ut ¼ þ þ ð14aÞ ho hpr hb where ho is the outside heat transfer coefficient of the greenhouse and is computed using McAdams relation (1954) in which a combined effect of radiation and convection from the greenhouse cover to the surroundings is given ho ¼ 5:7 þ 3:8v

ð14bÞ

Therefore in summer, total heat to be removed from the greenhouse is given by Qp ¼ q  Ac

ð15Þ

(Qp) to the greenhouse in order to achieve the design air temperature. The computation of sol–air temperature on a winter day shows that there is no need to heat the greenhouse during the winters at Chandigarh (31N) as the transmitted solar radiation inside the greenhouse is sufficient to raise the greenhouse room air temperature up to the desired level for the proper plant growth. However, during the winter nights when availability of solar radiation is zero, the rate of heat required inside the greenhouse is equal to the rate of heat lost from it. 4. Numerical computations In order to solve Eqs. (8) and (11), design data and constants as given in Tables 1 and 2 are used. Using the develTable 1 Parameters used for the validation Parameters

Summer day values

Winter night values

ht = hEAE + hWAW + hNAN + hSAS + hNRANR + hSRASR

748.56

394.46

Minimum mass flow rate of the circulating air required to remove Qp from the greenhouse is given as

HG ¼

Ag ha Ag ha þ Ag hb

0.78

0.78

ma ¼ Qp =C a ðT i  T o Þ

Hp ¼

Ap hpr Ap hpr þ Z

0.81

0.71

637.58

253.16

18.78

18.78

786.36 0.18

423.25 0.18

ð16Þ

The same heat Qp is to be transferred to the ACCFHES and is given as Qp ¼ U h  Ah  LMTD

ð17Þ

After neglecting the thermal resistance of the pipe material, Uh is computed as U h ¼ haw  hwa =ðhaw þ hwa Þ Ah ¼ 2pro L ¼ Qp =U h  LMTD where LMTD ¼ ðhi  ho Þ= ln hi =ho

ð17aÞ ð17bÞ ð18Þ

and ho ¼ T ho  T ci

For the winter configuration hi ¼ T ho  T ci

1

Ag ha  Ag hb Ag ha þ Ag hb Z = ht + hdAd + Ug + 0.33 NV age = ag (1  ap) Ug ¼

Table 2 Constants used for the validation

and ho ¼ T hi  T co

During winter night, Ig term in Eq. (12) is zero and the instant heat required inside the greenhouse is equal to the instant heat loss per unit area of the greenhouse in order to maintain the design air temperature of the greenhouse. q ¼ U t ½T a  T d 

1 1 þ Z Ap hpr

Ac = 70.82 m2, AN,S,E,W,NR,SR = 12, 12, 10, 10, 13.41, 13.41 m2 Ad = 1.8 m2 Ag = 24 m2

For the summer configuration hi ¼ T hi  T co

 U pa ¼

ð19Þ

The condition Tsa > Td shows cooling of the greenhouse. It is a summer day condition in which the ACCFHES would remove the required quantity of heat (Qp) from the greenhouse in order to achieve the design air temperature. The condition Tsa < Td shows heating of the greenhouse. It is a winter day condition in which the ACCFHES would supply the required quantity of heat

Ah = 12.63 m2 Ap = 100 m2 (summer day), 50 m2 (winter day) Cp = 4190 J kg1 C1 d = 0.1016 m ha = 5.8 W m2 C1 hb = 1.0 W m2 C1 hd = 10.57 W m2 C1 hp = 10.57 W m2 C1 L = 19.82 m LMTD = 13 C ma = 0.47 kg s1

Mp = 120 kg (summer day), 75 kg (winter night) N=0 Tpo = 22 C (summer day), 6 C (winter night) TR0 = 20 C (summer day), 4 C (winter night) Ut = 10.57 W m2 C1 (summer day), 5.57 W m2 C1 (winter night) Uh = 70.52 W m2 C1 v = 2.59 m s1 (inside), 1 m s1 (outside) va = 15 m s1 V=0 Vg = 60 m3 ag = 0.30 ap = 0.40 c = 0.4–0.7 s = 0.7 qa = 1.2 kg m3

V.P. Sethi, S.K. Sharma / Solar Energy 81 (2007) 723–741

Temperature °C

Inside plant

Inside room air

roof the greenhouse for different hours on a summer and a winter day is computed as shown in Figs. 6 and 7 which further helped to compute the total solar radiation (St) received by the greenhouse (Eq. (2a)). Convective and evaporative heat transfer coefficient (hpr) for each hour of the day is computed with the help of Eq. (3). Total heat to be removed from the greenhouse at a given sol–air temperature is computed using Eq. (15). Required mass flow rate (ma) of the circulating air through the pipe is computed using Eq. (16). Required area of the ACCFHES (Ah) is

Outside air

Global

Diffuse

50

900

45

800

40

700

35

600

30

500

25

400

20

300

15 10

200

5

100

0 1

2

3 4 5 6 7

Solar radiation (W m-2)

oped computer program, results of the plant and the greenhouse room air temperature for each hour on a typical summer and a winter day are obtained for the measured climatic and other design parameters. Climatic parameters such as ambient air temperature, greenhouse room air temperature, plant temperature and solar intensity are recorded for 24 h on a typical summer day (12-05-2005) and a winter day (09-01-2005) by integrating the greenhouse with ACCFHES as shown in Figs. 4 and 5, respectively. Using Eq. (2b), solar intensity on each wall and

731

0 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Time of day (h)

Operation of the ACCFHES during sunshine hours (6am to 6pm) Fig. 4. Hourly variation of the measured climatic data and plant temperature for the greenhouse integrated to the ACCFHES during an extreme summer day (12-05-05) at 31N 78E location.

Inside room air

Outside air

Global

Diffuse

40

700

35

600

Temperature °C

30

500

25 400 20 300 15 200

10

Solar radiation (W m-2)

Inside plant

100

5 0

0 18 19 20 21 22 23 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Time of day (h)

Operation of the ACCFHES during off-shine hrs (6pm to 8am the next morning) Fig. 5. Hourly variation of the measured climatic data and plant temperature for the greenhouse integrated to the ACCFHES during an extreme winter day (09-01-05) at 31N 78E location.

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V.P. Sethi, S.K. Sharma / Solar Energy 81 (2007) 723–741 SW

NW

SR

NR

WW

EW

Solar irradiance (W m-2)

1000 900 800 700 600 500 400 300 200 100 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time of the day (h)

Fig. 6. Hourly variation of the global solar intensity on different walls and roofs of the greenhouse for a typical summer day (12-05-05) at 31N 78E location.

SW

NW

SR

NR

WW

EW

Solar irradiance (W m-2)

1000 900 800 700 600 500 400 300 200 100 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time of the day (h)

Fig. 7. Hourly variation of the global solar intensity on different walls and roofs of the greenhouse for a typical winter day (09-01-05) at 31N 78E location.

Table 3 Operating parameters of the ACCFHES and the crop Symbol

Value

Ah (m2) ma (kg s1) Ap (m2) Mp (kg)

5.0, 12.63, 30, 50 0.1, 0.47, 1.0, 1.5 50, 100, 450, 750 75, 120, 500, 1000

computed using Eq. (17b). Suitable values of the operating parameters such as area of the plant, mass of the plant, area of the ACCFHES and the air mass flow rate as shown in Table 3 are used for the parametric study. 5. Experimental validation and results Experimental validation of the developed thermal model is carried out for a typical summer day and a winter night by integrating the greenhouse with the ACCFHES. Hourly variation of the predicted and measured values of the

greenhouse room air temperature along with the ambient air temperature for a typical summer day is shown in Fig. 8. It is observed that the predicted values remain slightly lower than the measured values but are very close to each other showing mean square deviation (v2) of 3.92%, which indicates that the developed model stands validated for room air temperature. The minimum and the maximum ambient air temperatures are measured as 24 C and 43.5 C i.e. a daily swing of about 20 C is observed. The minimum and the maximum values of the predicted and measured temperature are 23.04 C and 35.55 C and 24.8 C and 37.1 C, respectively. It indicates that the daily swing of the room air temperature is reduced to about 13 C, i.e. it is lowered by 7 C which proves that the developed model is validated as it nearly achieves the design air temperature (35 C) of the greenhouse room air at the maximum ambient air temperature in summers. The difference in the ambient and predicted room air temperature is maintained between 6 and 7 K throughout the operation of ACCFHES.

V.P. Sethi, S.K. Sharma / Solar Energy 81 (2007) 723–741 Ambient

Predicted

733

Measured

50 45

Temperature °C

40 35 30 25 20 15 10 5 0 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Time of day (h) Operation of the ACCFHES during sunshine hours

Fig. 8. Hourly variation of the predicted and measured values of the greenhouse room air temperature in comparison to the ambient air temperature on a typical summer day.

When the ACCFHES in switched on (at 6 h) in the morning, the greenhouse starts cooling and inside room air temperature drops 6 C below the ambient air in about two hours as compared to the measured value and 6.53 C as compared to the predicted value (Fig. 8). This difference remains almost constant up to 11 h. However after 12 noon, the ambient air temperature increases to a very high value (43.5 C) which leads to slight decrease in the performance of the ACCFHES but the inside room air temperature still remains about 4. 5C below the ambient air as compared to the measured value and 5.56 C as compared to the predicted value up to 15 h. After 15 h, when the solar radiation intensity reduces, the greenhouse room air temperature again reaches 6.5 C below the ambient air condition as compared to the measured value and 7.63 C as compared to the predicted value. When operation of the ACCFHES is stopped in the evening (at 18 h), the greenhouse again starts warming up (both measured and predicted values) as outside temperature is higher than the greenhouse room air temperature. It is due to the effect of solar radiation falling on the greenhouse from the west and the north side, which slightly heats up the greenhouse. But in the early morning hours (1–5 a.m.), a thermal balance occurs between the greenhouse and the surrounding air. Hourly variation of the measured inside room air and inside plant temperature with respect to the ambient air temperature is shown in Fig. 4. It is observed that the plant temperature is slightly higher than the room air temperature during sunshine hours where as the trend reverses during off-shine hours. It is because plant absorbs direct sunlight during daytime, which raises its temperature above the room air temperature. However, due to the cooling effect created by the ACCFHES inside the greenhouse, its temperature is observed marginally higher (1–2 C) than the room air temperature, which in non-cooling cases is generally observed 4–5 C higher as compared to the room air temperature. However, both plant and room air tem-

peratures remain below the ambient air temperature during sunshine hours due to the cooling effect created by the ACCFHES. Hourly variations in the predicted and the measured values of the greenhouse room air temperature along with the ambient air temperature for a typical winter day (24 h) are shown in Fig. 9. It is observed that the predicted values are very close to the measured values showing mean square deviation (v2) of 3.12%, which indicates that the developed model stands validated for the room air temperature. The minimum and the maximum measured ambient air temperatures are 5 C and 23.5 C. The ACCFHES is operated from 8 h in the evening up to 8 h in the next morning. The lowest predicted and measured value is 13.22 C and 12 C, which indicates that the greenhouse room air temperature remains around 7–8 C above the ambient air. This difference is maintained throughout the whole period of operation. When the ACCFHES is switched off at 8 h in the next morning, the greenhouse room air temperature slightly drops. It is due to the heat loss from the greenhouse cover because of lower ambient air temperature. However, after 9 h when sufficient beam radiation enters the greenhouse, inside room air temperature starts rising up gradually and reaches to the maximum of 30.5 C and 28.41 C at 14 h as shown by the measured and the predicted value, respectively in Fig. 9, which is sufficient for the proper plant growth. It can thus be concluded that there is no need to heat the greenhouse during daytime in the winter months at 31N latitude. Similarly, the predicted hourly plant temperatures inside the greenhouse for the summer and the winter day (24 h) are also in close agreement with the measured values (v2 is 4.56 for the summer data and 2.24 for the winter data) as shown in Figs. 10 and 11. It is concluded that the developed model can also be used for predicting the plant temperature inside the greenhouse at any time and place. In both the cases plant temperature is maintained 5–6 K

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V.P. Sethi, S.K. Sharma / Solar Energy 81 (2007) 723–741 Ambient

Predicted

Measured

35

Temperature °C

30 25 20 15 10 5 0 18 19 20 21 22 23 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Time of day (h) Operation of the ACCFHES during off-shine hours

Fig. 9. Hourly variation of the predicted and measured values of greenhouse room air temperature in comparison to the ambient air temperature on a typical winter night.

predicted

Plant temperature °C

50

measured

outside plant

45 40 35 30 25 20 15 10 5 0 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Time of the day (h)

Operation of the ACCFHES during sunshine hours

Fig. 10. Hourly variation of the predicted and measured values of greenhouse plant temperature in comparison to the measured outside plant temperature on a typical summer day.

predicted

measured

outside plant

Plant temperature °C

35 30 25 20 15 10 5 0 18 19 20 21 22 23 24 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17

Time of the day (h) Operation of the ACCFHES during off-shine hours

Fig. 11. Hourly variation of the predicted and measured values of greenhouse plant temperature in comparison to the measured outside plant temperature on a typical winter night.

V.P. Sethi, S.K. Sharma / Solar Energy 81 (2007) 723–741

below and above the ambient air in extreme summer and winter conditions respectively during the ACCFHES operation.

mass of plant increases from 75 kg to 1000 kg during daytime. At the maximum ambient air temperature (43.5 C at 15 h), the plant temperature is observed as 39.84 C at 75 kg and 36.72 C at 1000 kg. This drop is due to the increase in the heat storage capacity of the isothermal mass. However, this trend reverses during the night hours and plant temperature remains low at the lower mass of the plant. At the minimum ambient air temperature (26 C at 5 h), the plant temperature is observed as 26.65 C at 1000 kg and 24.16 C at 75 kg. During the winter operation of the ACCFHES (Fig. 13), it is again proved that the increase in the mass of plant also increases the plant temperature during off-shine hours but decreases the same during sunshine hours. The effect of increase in the mass of plant on the room air temperature on a summer day is shown in Fig. 14. At the maximum ambient air temperature, the room air temperature decreases from 38.12 C to 36.42 C during

6. Parametric studies Effect of change in the operating parameters such as mass of the plant, area of the plant, mass flow rate of the circulating air and area of the ACCFHES as shown in Table 3 on the greenhouse room air and plant temperature is discussed in this section. 6.1. Effect of the mass of plant Effect of increase in the mass of plant on the hourly variation of plant temperature for a summer day (keeping other operating parameters constant) is shown in Fig. 12. It is observed that the plant temperature decreases as the Ambient

Mp = 75

735

Mp = 120

Mp = 500

Mp = 1000

50

Plant temperature °C

45 40 35 30 25 20 15 10 5 0 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Time of the day (h)

Operation of the ACCFHES during sunshine hours

Fig. 12. Effect of increase in the mass of plant (kg) on hourly variation of the plant temperature at constant air mass flow rate (0.47 kg s1) and ACCFHES area (12.63 m2) for a summer day.

Ambient

Mp = 75

Mp = 120

Mp = 500

Mp = 1000

40

Plant temperature °C

35 30 25 20 15 10 5 0 18 19 20 21 22 23 24 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17

Time of day (h)

Operation of the ACCFHES during off-shine hours

Fig. 13. Effect of increase in the mass of plant (kg) on hourly variation of the plant temperature at constant air mass flow rate (0.47 kg s1) and ACCFHES area (12.63 m2) for a winter night.

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V.P. Sethi, S.K. Sharma / Solar Energy 81 (2007) 723–741 Ambient

Mp = 75

Mp = 120

Mp = 500

Mp = 1000

Room air temperature °C

45 40 35 30 25 20 15 10 5 0 1

2

3 4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time of the day (h)

Operation of ACCFHES during sunshine hours

Fig. 14. Effect of increase in the mass of plant (kg) on hourly variation of the room air temperature at constant air mass flow rate (0.47 kg s1) and ACCFHES area (12.63 m2) for a summer day.

Ambient

Ap = 50

Ap = 100

Ap = 450

Ap = 750

50 Room air temperature °C

45 40 35 30 25 20 15 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time of day (h) Operation of ACCFHES during sunshine hours

Fig. 15. Effect of increase in the area of plant (m2) on hourly variation of the room air temperature at constant air mass flow rate (0.47 kg s1) and ACCFHES area (12.63 m2) for a summer day.

sunshine hours with the increase in the mass of plant from 75 kg to 1000 kg. During off-shine hours this trend also reverses. At the minimum ambient air temperature point, room air temperature is 24.16 C and 25.4 C at 75 kg and 1000 kg, respectively. An overall view of Figs. 12 and 14 reveals that increase in the mass of plant has greater effect on the plant temperature as compared to the room air temperature. 6.2. Effect of the area of plant Effect of increase in the area of the plant on hourly variation of the room air temperature for a summer day is shown in Fig. 15 and for a winter night in Fig. 16. It is observed that the room air temperature decreases with

increase in the area of plant from 50 m2 to 750 m2 during sunshine hours. This trend also reverses during off-shine hours as in Section 6.1. It is because area of the plant is directly proportional to the mass of the plant. The effect of increase in the area of plant on the hourly variation of plant temperature for a summer day is shown in Fig. 17. It is observed that the plant temperature decreases with increase in the area of plant during sunshine hours. It is because increase in the area of the plant helps in absorbing more solar radiation, which in turn increases the evaporation from the plants. This trend again reverses during offshine hours. It appears that for a 24 m2 greenhouse area, 100 m2 plant area and 120 kg of plant mass can be considered as realistic values in the summer conditions. Similarly, when the plants are just grown and small, 50 m2 plant area

V.P. Sethi, S.K. Sharma / Solar Energy 81 (2007) 723–741 Ambient

Ap =50

Ap = 100

Ap = 450

737 Ap =750

40 Room air temperature °C

35 30 25 20 15 10 5 0 18 19 20 21 22 23 24 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17

Time of day (h)

Operation of the ACCFHES during off shine hours

Fig. 16. Effect of increase in the area of plant (m2) on hourly variation of the room air temperature at constant air mass flow rate (0.47 kg s1) and ACCFHES area (12.63 m2) for a winter night.

Ambient

45

Ap = 50

Ap = 100

Ap = 450

Ap = 750

Plant temperature °C

40 35 30 25 20 15 10 5 0 1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Time of day (h)

Operation of the ACCFHES during sunshine hours

Fig. 17. Effect of increase in the area of plant (m ) on hourly variation of the plant temperature at constant air mass flow rate (0.47 kg s1) and ACCFHES area (12.63 m2) for a summer day. 2

at 75 kg plant mass gives the sufficient rise in the room air temperature in the winter conditions. An overall view of Figs. 15 and 17 reveals that increase in the area of the plant has greater effect on the room air temperature as compared to the plant temperature. 6.3. Effect of the area of ACCFHES Effect of increase in area of the ACCFHES (Ah) on the greenhouse room air temperature for different hours of ACCFHES operation during a summer day is shown in Fig. 18. It is observed that the room air temperature decreases with the increase in the area of ACCFHES. This drop is significant when the area is increased from 5 m2 to 12.63 m2. At 5 m2 area, the greenhouse room air temperature does not drop at all and remains higher than the ambi-

ent air temperature during all hours of the ACCFHES operation. However, at 12.63 m2 area, the greenhouse room air temperature drops 6–7 C below ambient air even during peak hours (11–16 h). It is because of the increase in thermal energy removal from the circulating air by the aquifer water due to greater contact area between hot (air) and cold fluids (water) as compared to 5 m2 area. Increasing the area up to 30 m2 further lowers the room air temperature by about 2.5 C and still more increase up to 50 m2 further lowers the room air temperature by about 1.5 C. This fact clearly indicates that at higher area of the ACCFHES, effectiveness of the system increases, but at a decreasing rate. It can also be seen from Fig. 19 that with the increase in the area (number of transfer units, NTU) of the ACCFHES, which acts as a counter flow heat exchanger system, effectiveness of the ACCFHES initially

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V.P. Sethi, S.K. Sharma / Solar Energy 81 (2007) 723–741 Ambient

Ah = 5

Ah = 12.63

Ah = 30

Ah = 50

50 Room air temperature °C

45 40 35 30 25 20 15 10 5 0 6

7

8

9

10

11

12

13

14

15

16

17

18

Time of ACCFHES operation (h)

Fig. 18. Effect of increase in the area of ACCFHES (m2) on hourly variation of the room air temperature at constant mass of plant (120 kg), area of plant (100 m2) and air mass flow rate (0.47 kg s1) for a summer day.

R=0.2

R=0.4

R=0.6

R=0.8

R=0.9

1

Effectiveness

0.9 0.8 0.7 0.6 0.5 0.4 0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

NTU = UA/Cmin

Fig. 19. Effect of increase in the area of ACCFHES on the effectiveness of the ACCFHES at different values of R.

increases at a rapid rate for different values of R (Cmin/ Cmax), then after a certain point it increases slowly and almost becomes constant at higher NTU values (greater than 4). Hence, there is no advantage of increasing the area of the ACCFHES after a certain point (NTU 4), which is equivalent to about 13 m2, as further increase in the area does not contribute much to the increase in the thermal energy transfer. Hence, an optimum area is selected depending upon the effectiveness and cost of the system. Effect of increase in the area of ACCFHES (Ah) on the greenhouse room air temperature for different hours of the ACCFHES operation during a winter night is shown in Fig. 20. The room air temperature also increases with increase in the area of ACCFHES, except for 5 m2 area. Increase in the room air temperature is significant when area is increased from 5 m2 to 12.63 m2. It is observed that at 5 m2 area, the room air temperature does not increase at all and remains close to the ambient air temperature. However, at 12.63 m2 area, room air temperature remains about

7–8 C above ambient air even during extremely cool hours (1–6 h). It is due to the increase in the thermal energy added by the aquifer water to the cold circulating air because of the greater contact area between the cold (air) and the hot fluids (water) as compared to 5 m2 area. However, further increase in the area up to 30 m2 raises the room air temperature by about 2.5 C and still further increase up to 50 m2 raises it by 1.5 C only, which again shows that there is no use to increase the contact area between the aquifer water and the circulating air beyond 12.63 m2. 6.4. Effect of the air mass flow rate Effect of the air mass flow rate (ma) at the constant area of ACCFHES (12.63 m2) on the room air temperature in summer is shown in Fig. 21. It is observed that at 0.1 kg s1 the room air temperature does not drop rather it remains slightly higher as compared to the ambient air for most

V.P. Sethi, S.K. Sharma / Solar Energy 81 (2007) 723–741 Ambient

Ah = 5

Ah = 12.63

739

Ah = 30

Ah = 50

Room air temperature °C

30 25 20 15 10 5 0 18

19

20

21

22

23

24

1

2

3

4

5

6

7

8

Time of ACCFHES operation (h)

Fig. 20. Effect of increase in the area of ACCFHES (m2) on hourly variation of the room air temperature at constant mass of plant (75 kg), area of plant (50 m2) and mass flow rate (0.47 kg s1) for a winter night.

Ambient

ma = 0.1

ma = 0.47

ma = 1.0

ma = 1.5

Room air temperature °C

50 45 40 35 30 25 20 15 10 5 0 6

7

8

9

10

11

12

13

14

15

16

17

18

Time of ACCFHES operation (h)

Fig. 21. Effect of increase in the air mass flow rate (kg s1) on hourly variation of the room air temperature at constant mass of plant (120 kg), area of plant (100 m2) and at constant area of ACCFHES (12.63 m2) for a summer day.

Ambient

ma = 0.1

ma = 0.47

ma = 1.0

ma = 1.5

Room air temperature °C

30 25 20 15 10 5 0 18

19

20

21

22

23

24

1

2

3

4

5

6

7

8

Time of ACCFHES operation (h)

Fig. 22. Effect of increase in air mass flow rate (kg s1) on hourly variation of the room air temperature at constant mass of plant (75 kg), area of plant (50 m2) and at constant area of ACCFHES (12.63 m2) for a winter night.

of the operational hours. However, at 0.47 kg s1, drop in the room air temperature is significant as compared to the ambient air. It is because of increase in the number of air changes per unit time, which is responsible for greater thermal energy transfer from the greenhouse to the ACCF-

HES. Number of air changes at 0.1 kg s1 is not sufficient for transferring the required thermal energy from the greenhouse to the ACCFHES. Further increase in the mass flow rate to 1 kg s1 further lowers the greenhouse room air temperature but at a decreasing rate. Still further

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V.P. Sethi, S.K. Sharma / Solar Energy 81 (2007) 723–741

increase in the mass flow rate to 1.5 kg s1 has a very marginal effect on the room air temperature drop. It is clearly indicated from the figure that there is no use of increasing the number of air changes beyond a certain value and room air temperature almost becomes independent of the mass flow rate. However, in order to have greater mass flow rate, higher power is also consumed, so an economically feasible size of blower that provides a sufficient reduction in the room air temperature at the minimum cost is selected. Hence, a mass flow rate of 0.47 kg s1 provided by a 746 W blower is sufficient for the purpose. The same is true for the winter operation as shown in Fig. 22. It is observed that at 0.1 kg s1 the greenhouse room air temperature does not increase and almost remains close to ambient air. However, at 0.47 kg s1 it increases significantly by about 7–8C as compared to ambient air. Further increase in the mass flow rate up to 1 kg s1 and 1.5 kg s1 again shows the diminishing effect. Hence, it can be concluded that the same system can be used to achieve the desired room air temperature condition inside the greenhouse in summer as well as in winter conditions. 7. Conclusions 1. Developed thermal model for a greenhouse integrated to the ACCFHES is validated as predicted values of the greenhouse room air and plant temperature are in close agreement with the experimental values. 2. Developed thermal model for the greenhouse integrated to the ACCFHES can be used to predict the plant and room air temperature for any size of greenhouse, time of year and location. 3. The plant temperature is observed slightly higher as compared to the room air temperature during sunshine hours where as the trend reverses during off-shine hours. 4. Increase in the mass of plant lowers the plant and room air temperature during sunshine hours but the trend reverses during off-shine hours. The effect of increase in the mass of plant on the room air temperature is small as compared to the plant temperature. 5. Increase in the area of plant lowers the room air and plant temperature during sunshine hours but the trend reverses during off-shine hours. The effect of increase in the area of plant on the plant temperature is small as compared to the room air temperature. 6. Increase in the area of ACCFHES beyond a certain value (12.63 m2) has a diminishing effect on the drop in the room air temperature both in summer and winter conditions. 7. Increase in the air mass flow rate beyond a certain value (0.47 kg s1) also has a diminishing effect on the drop in the room air temperature both in summer and winter conditions. 8. Finally, it can be concluded that for a greenhouse of 24 m2 area, the ACCFHES with air and water contact area of 12.63 m2 and air mass flow rate of 0.47 kg s1

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