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Performance studies of earth air tunnel cum greenhouse technology
PII:
Energy Convers. Mgmt Vol. 39, No. 14, pp. 1497±1502, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0196-8904(98)00019-3 0196-8904/98 $19.00 + 0.00
PERFORMANCE STUDIES OF EARTH AIR TUNNEL CUM GREENHOUSE TECHNOLOGY G. N. TIWARI*, R. F. SUTAR, H. N. SINGH and R. K. GOYAL Centre for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India
$
(Received 5 May 1997) AbstractÐWe present performance studies of an earth air tunnel cum greenhouse technology in terms of instantaneous thermal eciency. Analytical expressions have been obtained for heating/cooling conditions of a greenhouse. The results are in accordance with the experimental results obtained for the same system in Greece. # 1998 Elsevier Science Ltd. All rights reserved Solar energy
Greenhouse technology
Earth air tunnel
NOMENCLATURE A=Area (m2) Ap=Area of plant (m2) AD=Area of door (m2) AET=Cross-sectional area of tunnel (m2) hb=Heat transfer coecient between greenhouse ¯oor and ground beneath (W/m28C) h(t)=Overall heat transfer coecient from enclosed room to ambient through canopy cover (W/m28C) hD=Heat transfer coecient between room air and ambient air through door ( W/m28C) h6=Heat transfer coecient between ¯oor and air inside greenhouse (W/m28C) hp=Convective heat transfer coecient between plant surface and air inside greenhouse (W/m28C) hpr=Total convective and evaporative heat transfer coecient from plant to enclosure (W/m28C) Mp=Heat capacity of plant (J/8C) P = Partial pressure of water vapour at temperature T (N/m2) QL=Rate of total heat loss from inside greenhouse to outside and from ¯oor to beneath (W) ST(t)=Solar intensity (W/m2) T=Temperature inside ground at greater depth (8C) t=Time (s) Ta=Ambient air temperature (8C) Tp=Plant temperature (8C) TR=Greenhouse enclosure air temperature (8C) Tvx = 0=Greenhouse ¯oor temperature (8C) UbG=Overall heat transfer coecient from enclosed room to inside ground at far distance through ¯oor (W/m28C) Ue=Eective total heat loss coecient (W/m28C) Upa=Total heat loss coecient including external, internal, bottom and ventilation losses (W/m28C) V=Wind velocity (m/s) V0=Rate of heat transfer due to in®ltration (W/m28C) V1=Rate of heat transfer due to ventilation (W/m28C) Greek letters g=Relative humidity E=Eective fraction of energy utilised ap=Absorptivity of plant ag=Absorptivity of greenhouse cover t=Fraction of energy transmitted through cover Z=Instantaneous thermal loss eciency factor Subscripts E=East g=Floor N=North 0=Initial P=Plant R=Roof S=South W=West *To whom correspondence should be addressed. 1497
1498
TIWARI et al.: PERFORMANCE STUDIES OF EARTH AIR TUNNEL
INTRODUCTION
High temperature during the summer season is adverse to greenhouse crops. It is important to note the temperature of the plant (Tp) and the greenhouse enclosure (TR) were of the order of about 508C and 408C, respectively, during the summer with ventilation eect. Further, it is important to note that there were signi®cant changes in these temperatures with an increase of ventilation beyond the optimum ¯ow rate. It has also been observed during the experimental study at the Indian Institute of Technology, Delhi, India, that most of the cucumber plants were damaged due to high enclosed air temperature at high levels of solar intensity. In order to reduce the greenhouse enclosure temperature further, an external cooling arrangement is required. The level of cooling depends mainly on the average ambient temperature and the insolation level. There are basically three component of a greenhouse, namely, root media, environment and thermal conditions, which become the basic requirements for optimum growth of crops for maximizing yield. The design of such an ideal greenhouse will depend on the local climatic condition. In India, there are basically six climatic zones [1]. Each zone would require a variation in the design for optimum performance and eciency, and hence, there should be six types of greenhouses for Indian climatic conditions. In each type of greenhouse, either thermal heating or cooling is required to achieve optimum enclosure environmental conditions, as mentioned earlier [2]. After knowing the temperatures for optimum growth of the vegetables, ¯owers and other horticultural crops during o season production for maximum yield, a suitable greenhouse can be designed, including one of the following heating or cooling concepts. (a) Heating concepts: i, direct gain; ii, isolated gain; iii, conventional heating. (b) Cooling concepts: i, ventilation; ii, earth air tunnel; iii, evaporative cooling. In this paper, an attempt has been made to verify the validity of a mathematical model developed by Sutar and Tiwari [2] by considering the following heat losses. (a) External heat losses: i, upward heat loss from enclosed room air to ambient through canopy cover; ii, downward heat loss from enclosed room air to the ground beneath. (b) Temperature dependent internal heat losses: i, convective heat loss; ii, evaporative heat loss. It has been observed that the nature of the analytical results predicted by Sutar and Tiwari [2] is exactly the same as observed experimentally and which are reported in the present paper.
INSTANTANEOUS THERMAL EFFICIENCY
Following Sutar and Tiwari [2], the analytical expression for the plant temperature as a function of the design and climatic parameters of the system can be written as Tp
f t 1 ÿ eÿat Tp0 eÿat a
where f(t)=F(t)/Mp F(t)=[apt + h0(te2+hte1)]S(t) ÿ h0v0+QET+UpaTa Upa=[1/H + (1/Aphpr)]ÿ1 H=h(t) + HDAD+V1+UbGAG (Appendix) a=Upa/Mp UbG=hbhG/(hb+ha) h=ÿhG/(h0+ha) h0=Aphpr/(H + Aphpr) te1=ag(1 ÿ ap)t te2=(1 ÿ ag)(1 ÿ ap)t.
From equation (1), the average plant temperature can be calculated as: p 1 T t
Z Tp dt:
1
TIWARI et al.: PERFORMANCE STUDIES OF EARTH AIR TUNNEL
The above equation, after integration, becomes f t 1 ÿ eÿat 1 ÿ eÿat : 1ÿ Tp0 Tp at at a
1499
2
With the help of equation (2), an analytical expression for the thermal loss eciency factor can be written as Zi Upa Tp ÿ Ta =ST t ateff Ueff Tpa ÿ Ta =ST t
3
where _ aDTAET)/ST(t)](1 ÿ R1) (at)e=[apt + h0(te2+hte1)] ÿ [(h0V0+EmC Ue=UpaR1 R1=(1 ÿ exp(ÿat))/at. Equation (3) represents the behaviour of a straight line with gradient m = Ue and intercept C = (at)e. Further, an analytical expression for an instantaneous thermal eciency of a greenhouse can be de®ned as Zig
Mp Tp ÿ Tp0 : ST t
With the help of equation (2), the above equation can be expressed as _ a DTAET Mp 1 ÿ eÿat h0 v0 EmC Tp0 ÿ Ta ÿ Upa : ap t h0 teff2 hteff1 ÿ Zig ST t ST t Upa
4
Equation (4) represents the behaviour of a straight line with gradient m = ÿ Upa, which is similar to the characteristic curve of this condition for a ¯at plate collector. ANALYTICAL RESULTS AND DISCUSSION
It is important to observe that, for at <1, equations (3) and (4) reduce to Zi1Zig10.0. This condition refers to the case of a greenhouse which has either a large value of Mp (heat capacity) or a small time interval. The smaller time implies that the solar radiation has not been used in controlling the environment condition in summer, which is the basic requirement of a summer
Fig. 1. Cross-sectional view of the greenhouse earth air tunnel.
1500
TIWARI et al.: PERFORMANCE STUDIES OF EARTH AIR TUNNEL Table 1. Total solar radiation in MJ/m2 day for Agrinion (Greece) [3] Month January February March April May June July August September October November December
Computed
Measured
8.22 16.22 24.82 25.40 17.80 8.69 6.67 14.62 20.29 25.24 18.52 8.89
11.24 21.43 26.07 22.60 12.54 7.29 10.66 17.43 25.23 22.42 12.77 6.56
greenhouse. This supports arguments from the cooling point of view. It is achieved by providing shading over the cover. Further, for at >1, equations (3) and (4) reduce to _ a DTAET =ST t Zi ap t h0 teff2 hteff1 ÿ h0 v0 EmC and Zig
Mp Upa
ap t h0 teff2 hteff1 ÿ
_ a DTAET h0 v0 EmC Tp0 ÿ Ta ÿ Upa : ST t ST t
5
6
It refers to the case of a greenhouse which has either a smaller value of Mp or a longer period interval to use solar radiation for heating. Since in winter, there should not be any in®ltration/ventilation, hence v0=0.0, and DT = TaÿTs becomes negative because Ts>Ta. In this case, the gain term becomes positive, which again supports the case of heating the greenhouse. NUMERICAL RESULTS AND DISCUSSION
In order to validate the developed model for a greenhouse with the air tunnel attachment, climatological and design data of a greenhouse situated in Greece has been adopted from Santamouris et al. [3, 4]. The greenhouse is located in Arginion in southwestern Greece (latitude 3885'N) and is used for growing roses. The area of the greenhouse is 1000 m2. Five PVC tubes, 30 m long and 22 cm in diameter, are buried under the greenhouse at a depth of 1.5 m, as shown in Fig. 1. The measured data of the total daily solar intensity (monthly average) for each month and the measured data of the ambient and indoor air temperatures for a characteristic day of each
Fig. 2. Performance curves for a greenhouse with tunnel attachment.
January
Ð Ð 75.81 190.81 300.13 375.54 402.20 375.54 300.13 198.81 75.81 Ð Ð 2286.76 8.22
Month Time
6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 W/m2 MJ/m2
In
12.5 12.0 14.5 16.5 18.0 20.0 21.0 23.5 24.0 22.5 20.0 18.0 15.0
Time
6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00
Month
Ð 92.08 244.67 406.96 543.52 633.10 664.18 633.10 543.52 409.96 244.67 92.08 Ð 4504.84 16.22
March 48.68 188.81 364.50 535.72 675.12 765.31 796.43 765.31 675.12 535.72 364.50 188.81 48.68 5952.72 21.43
April 108.61 266.40 445.15 612.99 747.66 834.26 864.08 834.26 747.66 612.99 445.15 266.40 108.61 6894.21 24.82
May 138.81 299.71 476.08 639.73 770.40 854.26 883.11 854.26 770.40 639.73 476.08 299.71 138.81 7241.08 26.07
June 124.39 283.46 460.27 625.16 757.10 841.85 871.02 841.85 757.10 625.16 460.27 283.46 124.39 7055.51 25.40
July 69.42 216.44 392.91 562.11 698.99 787.32 817.78 787.32 698.99 562.11 392.91 216.44 69.42 6272.14 22.60
August 5.65 121.25 282.19 447.36 584.47 673.91 704.87 673.91 584.47 447.36 282.19 121.25 5.65 4934.53 17.80
September Ð 38.97 163.47 308.70 435.69 520.30 549.83 520.30 435.69 308.70 163.47 38.97 Ð 3484.08 12.54
October
January
10.0 11.5 12.5 13.0 14.5 14.5 14.0 15.0 15.0 14.0 12.5 10.5 10.0
Out 16.0 18.0 20.0 29.5 31.0 35.0 37.0 36.0 40.0 40.0 39.0 37.0 33.0
In
April
9.0 15.0 19.0 29.0 29.0 29.5 30.0 27.0 29.0 30.0 29.0 24.0 20.0
Out 19.0 19.5 28.0 30.0 35.0 40.0 44.0 48.0 44.0 40.0 41.0 42.0 40.0
In
July
19.0 19.5 22.0 27.0 30.0 34.0 35.0 38.0 36.0 35.0 36.0 38.0 34.0
Out
9.5 9.5 9.0 20.0 28.0 33.0 39.5 39.3 39.0 37.0 36.0 30.0 23.0
In
October
9.0 9.0 8.5 17.0 22.0 25.0 28.0 28.0 28.0 27.0 24.0 22.0 21.0
Out
Table 3. Hourly ambient and greenhouse temperature variation at Agrinion (Greece) for the four months under study [4]
Ð Ð 138.75 278.04 402.11 485.39 514.54 485.39 402.11 278.04 138.75 Ð Ð 3123.11 11.24
February Ð Ð 85.10 203.90 315.41 391.90 418.89 391.90 315.41 203.90 85.10 Ð Ð 2411.52 8.69
November
Table 2. Daily average solar intensity available in Agrinion (Greece) computed from total average intensity for each month over a ®ve-year period [3]
Ð Ð 58.34 164.32 287.65 339.76 365.37 339.76 287.65 164.32 58.34 Ð Ð 2025.54 7.29
December
TIWARI et al.: PERFORMANCE STUDIES OF EARTH AIR TUNNEL 1501
1502
TIWARI et al.: PERFORMANCE STUDIES OF EARTH AIR TUNNEL
month (Table 1) reported by Santamouris is considered for calculation. The value of the overall heat loss coecient Upa is taken as 6.814 W/m28C [5]. Using the method suggested by Due and Beckman [6] to calculate the hourly intensity from the total daily radiation, the values for hourly solar intensity were obtained for Arginion for each month (Table 2). The hourly variation of the inside and outside temperatures for typical months have been given in Table 3. The amount of thermal loss eciency factor for the greenhouse and the ratio of (Tp0ÿTa)/ ST(t) were calculated for each month. The variation for each month is shown in Fig. 2. It can be observed that there is a good agreement between the suggested behaviour of previous work done by Sutar and Tiwari [2], equation (3) and the nature of these curves. CONCLUSIONS
On the basis of the above study, it is inferred that the proposed model is in accordance with the experimental results observed by Santamouris [3]. Further, the proposed model can be used to optimize the design parameters for heating and cooling of a greenhouse for a given climatic condition. REFERENCES 1. Bansal, N. K. and Minke, G., Climatic zones and rural housing in India, Scienti®c Series of the International Bureau, KFA, Julich, 1988. 2. Sutar, R. F. and Tiwari, G. N., Energy, 1996, 21(1), 61±65. 3. Santamouris, M., Arginion, A. and Vallindras, M., Solar Energy, 1994, 52(5), 371±378. 4. Santamouris, M. and Katsoulis, B. D., Solar and Wind Technology, 1989, 6(1), 79±84. 5. Garzoli, K. and Blackwell, J., J. Agril. Engg. Res., 1973, 18, 205±216. 6. Due, J. A. and Beckman, W. A., in Solar Engineering of Thermal Processes. John Wiley & Sons, New York, 1991. 7. Watmu, J. H., Charters, W. W. S. and Proctor, D., Complex, 1977, 2, .
APPENDIX The h(t) is an overall heat loss coecient from an enclosed room air to ambient through transparent canopy cover and can be expressed as 1 1 ÿ1 h t h1 h2 where h1 and h2 are internal and external heat loss coecients and are expressed as: (a) Internal total heat loss coecient h1 hpc hpr : An expression for hpr is given in the text, and it can also be considered as a temperature dependent parameter, if required: (b) External total heat loss coecient The external heat transfer coecient is generally considered as [6] h2 5:7 3:0V: However, Watmu [7] pointed out that the above expression includes radiative losses. He proposed a new expression for the convective heat loss coecient as hca 2:8 3:8V:
Energy Convers. Mgmt Vol. 39, No. 14, pp. 1497±1502, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0196-8904(98)00019-3 0196-8904/98 $19.00 + 0.00
PERFORMANCE STUDIES OF EARTH AIR TUNNEL CUM GREENHOUSE TECHNOLOGY G. N. TIWARI*, R. F. SUTAR, H. N. SINGH and R. K. GOYAL Centre for Energy Studies, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India
$
(Received 5 May 1997) AbstractÐWe present performance studies of an earth air tunnel cum greenhouse technology in terms of instantaneous thermal eciency. Analytical expressions have been obtained for heating/cooling conditions of a greenhouse. The results are in accordance with the experimental results obtained for the same system in Greece. # 1998 Elsevier Science Ltd. All rights reserved Solar energy
Greenhouse technology
Earth air tunnel
NOMENCLATURE A=Area (m2) Ap=Area of plant (m2) AD=Area of door (m2) AET=Cross-sectional area of tunnel (m2) hb=Heat transfer coecient between greenhouse ¯oor and ground beneath (W/m28C) h(t)=Overall heat transfer coecient from enclosed room to ambient through canopy cover (W/m28C) hD=Heat transfer coecient between room air and ambient air through door ( W/m28C) h6=Heat transfer coecient between ¯oor and air inside greenhouse (W/m28C) hp=Convective heat transfer coecient between plant surface and air inside greenhouse (W/m28C) hpr=Total convective and evaporative heat transfer coecient from plant to enclosure (W/m28C) Mp=Heat capacity of plant (J/8C) P = Partial pressure of water vapour at temperature T (N/m2) QL=Rate of total heat loss from inside greenhouse to outside and from ¯oor to beneath (W) ST(t)=Solar intensity (W/m2) T=Temperature inside ground at greater depth (8C) t=Time (s) Ta=Ambient air temperature (8C) Tp=Plant temperature (8C) TR=Greenhouse enclosure air temperature (8C) Tvx = 0=Greenhouse ¯oor temperature (8C) UbG=Overall heat transfer coecient from enclosed room to inside ground at far distance through ¯oor (W/m28C) Ue=Eective total heat loss coecient (W/m28C) Upa=Total heat loss coecient including external, internal, bottom and ventilation losses (W/m28C) V=Wind velocity (m/s) V0=Rate of heat transfer due to in®ltration (W/m28C) V1=Rate of heat transfer due to ventilation (W/m28C) Greek letters g=Relative humidity E=Eective fraction of energy utilised ap=Absorptivity of plant ag=Absorptivity of greenhouse cover t=Fraction of energy transmitted through cover Z=Instantaneous thermal loss eciency factor Subscripts E=East g=Floor N=North 0=Initial P=Plant R=Roof S=South W=West *To whom correspondence should be addressed. 1497
1498
TIWARI et al.: PERFORMANCE STUDIES OF EARTH AIR TUNNEL
INTRODUCTION
High temperature during the summer season is adverse to greenhouse crops. It is important to note the temperature of the plant (Tp) and the greenhouse enclosure (TR) were of the order of about 508C and 408C, respectively, during the summer with ventilation eect. Further, it is important to note that there were signi®cant changes in these temperatures with an increase of ventilation beyond the optimum ¯ow rate. It has also been observed during the experimental study at the Indian Institute of Technology, Delhi, India, that most of the cucumber plants were damaged due to high enclosed air temperature at high levels of solar intensity. In order to reduce the greenhouse enclosure temperature further, an external cooling arrangement is required. The level of cooling depends mainly on the average ambient temperature and the insolation level. There are basically three component of a greenhouse, namely, root media, environment and thermal conditions, which become the basic requirements for optimum growth of crops for maximizing yield. The design of such an ideal greenhouse will depend on the local climatic condition. In India, there are basically six climatic zones [1]. Each zone would require a variation in the design for optimum performance and eciency, and hence, there should be six types of greenhouses for Indian climatic conditions. In each type of greenhouse, either thermal heating or cooling is required to achieve optimum enclosure environmental conditions, as mentioned earlier [2]. After knowing the temperatures for optimum growth of the vegetables, ¯owers and other horticultural crops during o season production for maximum yield, a suitable greenhouse can be designed, including one of the following heating or cooling concepts. (a) Heating concepts: i, direct gain; ii, isolated gain; iii, conventional heating. (b) Cooling concepts: i, ventilation; ii, earth air tunnel; iii, evaporative cooling. In this paper, an attempt has been made to verify the validity of a mathematical model developed by Sutar and Tiwari [2] by considering the following heat losses. (a) External heat losses: i, upward heat loss from enclosed room air to ambient through canopy cover; ii, downward heat loss from enclosed room air to the ground beneath. (b) Temperature dependent internal heat losses: i, convective heat loss; ii, evaporative heat loss. It has been observed that the nature of the analytical results predicted by Sutar and Tiwari [2] is exactly the same as observed experimentally and which are reported in the present paper.
INSTANTANEOUS THERMAL EFFICIENCY
Following Sutar and Tiwari [2], the analytical expression for the plant temperature as a function of the design and climatic parameters of the system can be written as Tp
f t 1 ÿ eÿat Tp0 eÿat a
where f(t)=F(t)/Mp F(t)=[apt + h0(te2+hte1)]S(t) ÿ h0v0+QET+UpaTa Upa=[1/H + (1/Aphpr)]ÿ1 H=h(t) + HDAD+V1+UbGAG (Appendix) a=Upa/Mp UbG=hbhG/(hb+ha) h=ÿhG/(h0+ha) h0=Aphpr/(H + Aphpr) te1=ag(1 ÿ ap)t te2=(1 ÿ ag)(1 ÿ ap)t.
From equation (1), the average plant temperature can be calculated as: p 1 T t
Z Tp dt:
1
TIWARI et al.: PERFORMANCE STUDIES OF EARTH AIR TUNNEL
The above equation, after integration, becomes f t 1 ÿ eÿat 1 ÿ eÿat : 1ÿ Tp0 Tp at at a
1499
2
With the help of equation (2), an analytical expression for the thermal loss eciency factor can be written as Zi Upa Tp ÿ Ta =ST t ateff Ueff Tpa ÿ Ta =ST t
3
where _ aDTAET)/ST(t)](1 ÿ R1) (at)e=[apt + h0(te2+hte1)] ÿ [(h0V0+EmC Ue=UpaR1 R1=(1 ÿ exp(ÿat))/at. Equation (3) represents the behaviour of a straight line with gradient m = Ue and intercept C = (at)e. Further, an analytical expression for an instantaneous thermal eciency of a greenhouse can be de®ned as Zig
Mp Tp ÿ Tp0 : ST t
With the help of equation (2), the above equation can be expressed as _ a DTAET Mp 1 ÿ eÿat h0 v0 EmC Tp0 ÿ Ta ÿ Upa : ap t h0 teff2 hteff1 ÿ Zig ST t ST t Upa
4
Equation (4) represents the behaviour of a straight line with gradient m = ÿ Upa, which is similar to the characteristic curve of this condition for a ¯at plate collector. ANALYTICAL RESULTS AND DISCUSSION
It is important to observe that, for at <1, equations (3) and (4) reduce to Zi1Zig10.0. This condition refers to the case of a greenhouse which has either a large value of Mp (heat capacity) or a small time interval. The smaller time implies that the solar radiation has not been used in controlling the environment condition in summer, which is the basic requirement of a summer
Fig. 1. Cross-sectional view of the greenhouse earth air tunnel.
1500
TIWARI et al.: PERFORMANCE STUDIES OF EARTH AIR TUNNEL Table 1. Total solar radiation in MJ/m2 day for Agrinion (Greece) [3] Month January February March April May June July August September October November December
Computed
Measured
8.22 16.22 24.82 25.40 17.80 8.69 6.67 14.62 20.29 25.24 18.52 8.89
11.24 21.43 26.07 22.60 12.54 7.29 10.66 17.43 25.23 22.42 12.77 6.56
greenhouse. This supports arguments from the cooling point of view. It is achieved by providing shading over the cover. Further, for at >1, equations (3) and (4) reduce to _ a DTAET =ST t Zi ap t h0 teff2 hteff1 ÿ h0 v0 EmC and Zig
Mp Upa
ap t h0 teff2 hteff1 ÿ
_ a DTAET h0 v0 EmC Tp0 ÿ Ta ÿ Upa : ST t ST t
5
6
It refers to the case of a greenhouse which has either a smaller value of Mp or a longer period interval to use solar radiation for heating. Since in winter, there should not be any in®ltration/ventilation, hence v0=0.0, and DT = TaÿTs becomes negative because Ts>Ta. In this case, the gain term becomes positive, which again supports the case of heating the greenhouse. NUMERICAL RESULTS AND DISCUSSION
In order to validate the developed model for a greenhouse with the air tunnel attachment, climatological and design data of a greenhouse situated in Greece has been adopted from Santamouris et al. [3, 4]. The greenhouse is located in Arginion in southwestern Greece (latitude 3885'N) and is used for growing roses. The area of the greenhouse is 1000 m2. Five PVC tubes, 30 m long and 22 cm in diameter, are buried under the greenhouse at a depth of 1.5 m, as shown in Fig. 1. The measured data of the total daily solar intensity (monthly average) for each month and the measured data of the ambient and indoor air temperatures for a characteristic day of each
Fig. 2. Performance curves for a greenhouse with tunnel attachment.
January
Ð Ð 75.81 190.81 300.13 375.54 402.20 375.54 300.13 198.81 75.81 Ð Ð 2286.76 8.22
Month Time
6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 W/m2 MJ/m2
In
12.5 12.0 14.5 16.5 18.0 20.0 21.0 23.5 24.0 22.5 20.0 18.0 15.0
Time
6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00
Month
Ð 92.08 244.67 406.96 543.52 633.10 664.18 633.10 543.52 409.96 244.67 92.08 Ð 4504.84 16.22
March 48.68 188.81 364.50 535.72 675.12 765.31 796.43 765.31 675.12 535.72 364.50 188.81 48.68 5952.72 21.43
April 108.61 266.40 445.15 612.99 747.66 834.26 864.08 834.26 747.66 612.99 445.15 266.40 108.61 6894.21 24.82
May 138.81 299.71 476.08 639.73 770.40 854.26 883.11 854.26 770.40 639.73 476.08 299.71 138.81 7241.08 26.07
June 124.39 283.46 460.27 625.16 757.10 841.85 871.02 841.85 757.10 625.16 460.27 283.46 124.39 7055.51 25.40
July 69.42 216.44 392.91 562.11 698.99 787.32 817.78 787.32 698.99 562.11 392.91 216.44 69.42 6272.14 22.60
August 5.65 121.25 282.19 447.36 584.47 673.91 704.87 673.91 584.47 447.36 282.19 121.25 5.65 4934.53 17.80
September Ð 38.97 163.47 308.70 435.69 520.30 549.83 520.30 435.69 308.70 163.47 38.97 Ð 3484.08 12.54
October
January
10.0 11.5 12.5 13.0 14.5 14.5 14.0 15.0 15.0 14.0 12.5 10.5 10.0
Out 16.0 18.0 20.0 29.5 31.0 35.0 37.0 36.0 40.0 40.0 39.0 37.0 33.0
In
April
9.0 15.0 19.0 29.0 29.0 29.5 30.0 27.0 29.0 30.0 29.0 24.0 20.0
Out 19.0 19.5 28.0 30.0 35.0 40.0 44.0 48.0 44.0 40.0 41.0 42.0 40.0
In
July
19.0 19.5 22.0 27.0 30.0 34.0 35.0 38.0 36.0 35.0 36.0 38.0 34.0
Out
9.5 9.5 9.0 20.0 28.0 33.0 39.5 39.3 39.0 37.0 36.0 30.0 23.0
In
October
9.0 9.0 8.5 17.0 22.0 25.0 28.0 28.0 28.0 27.0 24.0 22.0 21.0
Out
Table 3. Hourly ambient and greenhouse temperature variation at Agrinion (Greece) for the four months under study [4]
Ð Ð 138.75 278.04 402.11 485.39 514.54 485.39 402.11 278.04 138.75 Ð Ð 3123.11 11.24
February Ð Ð 85.10 203.90 315.41 391.90 418.89 391.90 315.41 203.90 85.10 Ð Ð 2411.52 8.69
November
Table 2. Daily average solar intensity available in Agrinion (Greece) computed from total average intensity for each month over a ®ve-year period [3]
Ð Ð 58.34 164.32 287.65 339.76 365.37 339.76 287.65 164.32 58.34 Ð Ð 2025.54 7.29
December
TIWARI et al.: PERFORMANCE STUDIES OF EARTH AIR TUNNEL 1501
1502
TIWARI et al.: PERFORMANCE STUDIES OF EARTH AIR TUNNEL
month (Table 1) reported by Santamouris is considered for calculation. The value of the overall heat loss coecient Upa is taken as 6.814 W/m28C [5]. Using the method suggested by Due and Beckman [6] to calculate the hourly intensity from the total daily radiation, the values for hourly solar intensity were obtained for Arginion for each month (Table 2). The hourly variation of the inside and outside temperatures for typical months have been given in Table 3. The amount of thermal loss eciency factor for the greenhouse and the ratio of (Tp0ÿTa)/ ST(t) were calculated for each month. The variation for each month is shown in Fig. 2. It can be observed that there is a good agreement between the suggested behaviour of previous work done by Sutar and Tiwari [2], equation (3) and the nature of these curves. CONCLUSIONS
On the basis of the above study, it is inferred that the proposed model is in accordance with the experimental results observed by Santamouris [3]. Further, the proposed model can be used to optimize the design parameters for heating and cooling of a greenhouse for a given climatic condition. REFERENCES 1. Bansal, N. K. and Minke, G., Climatic zones and rural housing in India, Scienti®c Series of the International Bureau, KFA, Julich, 1988. 2. Sutar, R. F. and Tiwari, G. N., Energy, 1996, 21(1), 61±65. 3. Santamouris, M., Arginion, A. and Vallindras, M., Solar Energy, 1994, 52(5), 371±378. 4. Santamouris, M. and Katsoulis, B. D., Solar and Wind Technology, 1989, 6(1), 79±84. 5. Garzoli, K. and Blackwell, J., J. Agril. Engg. Res., 1973, 18, 205±216. 6. Due, J. A. and Beckman, W. A., in Solar Engineering of Thermal Processes. John Wiley & Sons, New York, 1991. 7. Watmu, J. H., Charters, W. W. S. and Proctor, D., Complex, 1977, 2, .
APPENDIX The h(t) is an overall heat loss coecient from an enclosed room air to ambient through transparent canopy cover and can be expressed as 1 1 ÿ1 h t h1 h2 where h1 and h2 are internal and external heat loss coecients and are expressed as: (a) Internal total heat loss coecient h1 hpc hpr : An expression for hpr is given in the text, and it can also be considered as a temperature dependent parameter, if required: (b) External total heat loss coecient The external heat transfer coecient is generally considered as [6] h2 5:7 3:0V: However, Watmu [7] pointed out that the above expression includes radiative losses. He proposed a new expression for the convective heat loss coecient as hca 2:8 3:8V: