- Email: [email protected]
Greenhouse heating and cooling using aquifer water
ARTICLE IN PRESS
Energy 32 (2007) 1414–1421 www.elsevier.com/locate/energy
Greenhouse heating and cooling using aquifer water V.P. Sethia,, S.K. Sharmab a
Department of Mechanical Engineering, Punjab Agricultural University, Ludhiana 141008, Punjab, India b Energy Research Centre, Panjab University, Chandigarh 160017, Punjab, India Received 18 January 2006
Abstract An aquifer coupled cavity flow heat exchanger system (ACCFHES) was designed using underground aquifer water for the heating as well as cooling of a composite climatic greenhouse. The performance of ACCFHES was experimentally evaluated for a full winter and a summer season. The ACCFHES makes use of constant temperature aquifer water (24 1C) available at an agricultural field through an irrigation tube well for heating in winter nights and cooling in summer days. The results showed that the average greenhouse room air temperature was maintained 7–9 1C above the outside air during extreme winter nights and 6–7 1C below the outside air in extreme summer days, and temperature fluctuations inside the greenhouse also decreased daily. The average relative humidity (RH) inside the greenhouse also decreased by 10–12% in the winter and increased by more than double in the extreme summer conditions as compared to the outside conditions. A comparison of economic feasibility of the ACCFHES coupled greenhouse was also conducted with conventional greenhouse and open field condition based on the yield of Capsicum annum. The ACCFHES was also compared economically with other existing heating/cooling technologies such as earth-to-air heat exchanger system (EAHES), ground air collector, evaporative cooling using foggers and fan & pad system in terms of net present worth (NPW) and pay back period. It was observed that the NPW of the ACCFHES coupled greenhouse was much higher as compared to the conventional greenhouse and open field condition. The payback period of the ACCFHES coupled greenhouse was the lowest among all other existing heating/cooling systems. r 2006 Elsevier Ltd. All rights reserved. Keywords: Greenhouse; Solar energy; Cooling/heating systems; Thermal control; Aquifer water
1. Introduction Agricultural greenhouses are a viable solution to the worldwide increased demand for expanding production, facilitating out of season cultivation and allowing the growth of certain varieties in areas where it was not possible earlier. In order to have out of season agricultural production inside a greenhouse, a heating or a cooling system is required to maintain the inside micro-climate. Currently there are many cooling and heating systems being used for greenhouses depending upon the climatic conditions of an area. In the cooling systems, natural and forced ventilation as discussed by Bot [1] and Oca [2] is useful only in the mild hot weather conditions and becomes ineffective during extreme summer conditions. Use of shading screens by Bailey [3] and reflector sheets by Sethi Corresponding author. Tel.: +91 161 2553367; fax: +91 161 2401794.
E-mail address: [email protected] (V.P. Sethi). 0360-5442/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2006.10.022
and Gupta [4] showed that these restrict the entry of excessive solar radiation inside the greenhouse but not much drop in the inside air temperature could be achieved during extreme summer conditions. Evaporative cooling systems using fan & pad as studied by Landsberg et al. [5], Chandra et al. [6] and Jain and Tiwari [7] and foggers used by Press [8] and Arbel et al. [9] proved that these systems are highly effective during extreme dry summer conditions and can cover upto 80% cooling needs of the greenhouse. However, higher levels of moisture inside the greenhouse promote the growth of micro-organisms thus making the crop more susceptible to the diseases. In heating systems, water storage using plastic pipes described by Sorensen [10] and drums filled with water as discussed by Gupta and Tiwari [11] can meet 30–70% heating needs of the greenhouse. However, about 25–30% heating space becomes unavailable due to placing of these heat storage media inside the greenhouse. Studies conducted on rock bed heat storage systems by Garzoli [12], Sagara and
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421
Nakahara [13] and Choudhury and Garg [14] revealed that these systems can maintain 2–10 1C higher temperature inside the greenhouse as compared to outside air temperature and can cover 30–75% heating needs of the greenhouse. However, these systems consume very high energy due to greater air pressure differences across the bed. Studies conducted by Abhat [15], Hamada et al. [16] and Farid et al. [17] about the use of phase change materials for greenhouse latent heat storage systems described that these systems can increase the greenhouse air temperature by about 2–8 1C but due to higher cost and incongruent melting these systems become less trustworthy for long duration experiments. In a composite climate, where heating of greenhouse is required in winter nights and cooling is required in summer days, earth-to-air heat exchanger system (EAHES) can be used for creating favorable environmental conditions inside the greenhouse as earth temperature below its surface at a depth of about 3–4 m remains almost stagnant at about 26–28 1C throughout the year as studied by Santamouris et al. [18]. Other researchers like Kozai [19], Immakulov [20], Sodha [21] and Mihalakakaou et al. [22] have also discussed the heating and cooling potential of EAHES. However, the limitation of using EAHES is the higher cost of deep digging and horizontal layout of pipe(s) up to 3–4 m depth is not easy. Moreover, for the short-term use temperature around the soil mass gradually increases due to dissipation of heat from the outside pipe surface that decreases the efficiency of the system. In this study, a composite system called aquifer coupled cavity flow heat exchanger system (ACCFHES) using aquifer water on the ground surface is proposed. The system utilizes underground aquifer water (at around 24 1C, year round) at the outlet of an irrigation tube well in an agricultural field for effectively heating a greenhouse
1415
in winter and cooling the same in summer. The advantages of the proposed system over EAHES are that the cost of deep digging of soil is not required at all, which saves a lot of labor cost. Aquifer water is available at a lower temperature (around 24 1C) throughout the whole year as compared to the ground temperature (28 1C) that improves the cooling performance of the system. Moreover, for the same pipe length, higher heat transfer takes place due to counter flow arrangement of greenhouse air and aquifer water in the shallow trench. The cost of lifting the aquifer water upto the ground level is zero as this water is already available on the ground surface for irrigation purposes. 2. Methods and materials A 6 4 m size greenhouse was constructed near Chandigarh (311N latitude), Punjab, India. The east–west oriented even span greenhouse has central and side height of 3 and 2 m, respectively. It has a door of 1 1.9 m size on the west wall and has a ventilator of 0.8 0.8 m size each on the center of the inclined north and south roof. Greenhouse was covered with a single sheet of UV stabilized polyethylene of 0.7 transmissivity in which Capsicum annum was grown. A shallow cavity (also called trench) of 0.3 0.3 m size was dug around the perimeter of the greenhouse in which aquifer water from irrigation tube well (near the greenhouse sight) was passed before sending it to the field for irrigation purposes as shown in Fig. 1. A 20 m long plastic pipe (0.1016 m diameter) was placed in the trench. A thin plastic sheet was spread below the pipe (in the trench) to avoid any water seepage to the ground. The trench was dug at a distance of 1 m away from the greenhouse walls in order to avoid any seepage of water to the greenhouse foundation. One end of the pipe was connected with the suction side and the other end with the
Aquifer water from tube well Water inlet 5
Air 6m
Water tank
Holes for water mixing 6
Cold air delivery
7
4m Blower
2
Top supports Outside air mixing
3
1
Hot air suction
7 Immersed pipe
4 Air in pipe losing heat ` Water outlet to f ield
Trench 0.3 × 0.3m
Fig. 1. Top view of the greenhouse integrated with the ACCFHES in cooling mode.
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421
1416
sensible heating process. This air was further heated to state 3 by the vanes of blower and was distributed inside the greenhouse. Thus a final state 4 was achieved after the heated air was mixed with the rest of the greenhouse air. One important observation was made that the air temperature after passing through the blower was raised by about 3–4 1C in winter and 5–6 1C in summer. In order to nullify the heating effect of air in summer (state points 3 and 4, Fig. 1), blower and motor assembly was placed on the suction side so that hot air from the greenhouse once cooled was directly fed to the greenhouse from the delivery side. However, in winters, the motor blower assembly was placed on the delivery side so that air after heating through the channel was further heated (state points 2 and 3, Fig. 2) by the blower in order to improve the heating effect.
delivery side of the blower. Greenhouse air was drawn near the roof by a 746 W blower (giving a flow rate of 1800 m3/h) and was circulated through the pipe placed in the trench in the opposite direction of the water flow in order to make it a counter flow configuration. To keep the pipe fully immersed in water, supports at the top of the trench were provided after every 2.5 m length. These supports were simple iron rods fixed across the channel at the water level so that the plastic pipe could not rise above this level and remained fully immersed. In summer, hot circulating air loses its heat to the aquifer water flowing in the trench and in winter it gains heat from the same water. The cool or hot circulating air was uniformly distributed through a delivery pipe placed centrally inside the greenhouse at a height of 30 cm in order to provide the maximum cooling effect near the plants. Circulating air was distributed inside the greenhouse by means of two 10 mm holes drilled on either side in upper half of the delivery pipe at 60 cm interval so that conditioned air could reach in each row. In the cooling mode (Fig. 1), hot air near the roof was drawn (state 1) and pushed through the fully immersed pipe placed in the trench by means of a blower and motor assembly. Aquifer water was allowed to enter the trench from the opposite direction in order to make it a counter flow configuration. Hot air loses its heat to the aquifer water (sensible cooling) along the flow length. Cool air at state point 5 was further cooled to state point 6 by mixing water with the circulating air (evaporative cooling process). This cool air was distributed inside the greenhouse to achieve state 7 after mixing it with rest of the greenhouse air. In the heating mode (Fig. 2), cool air was drawn near the roof (state 1) and forced through the pipe placed in the trench. The circulating air gained heat along the flow length and attained state 2 after the completion of the
2.1. Evaporative cooling process In the extreme summer conditions, addition of a simple evaporative cooling process further maximized the cooling effect of ACCFHES. Measured quantity of water at around 25 1C, i.e. at the wet bulb temperature was sprayed inside the air pipe along the circulating air just after the completion of sensible cooling process (between state points 5 and 6, Fig. 1). The Quantity of water required for evaporation purposes was calculated about 12 l/h for the required airflow rate and relative humidity (RH) (65%). The required water was supplied through a drum of 0.1 m3 capacity placed at about 4 m height from the delivery pipe. The drum had three outlets with flow control valves to regulate the water inside the pipe. Three small holes of about 10 mm size were drilled at 15 cm interval in the pipe. Three 8 mm (inner diameter) flexible rubber pipes were inserted inside each hole for mixing water with air and Trench
Aquifer water inlet from the tube well Hot air in pipe
2
Blower
Hot air delivery
4
3
4 1
Cold air suction
Air in pipe gaining heat
Water outlet to field
Trench (0.3×0.3m)
Fig. 2. Top view of the greenhouse integrated with the ACCFHES in heating mode.
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421
for providing sufficient contact of water with the air through 30 cm distance. The principle of operation is that the high velocity moving air through the pipe breaks these water droplets into smaller particles. These particles evaporate when air at high velocity comes in contact with it and takes away the latent heat of vaporization from the air that lowers the air temperature inside the pipe to around wet bulb temperature and the overall temperature of the greenhouse is significantly lowered as compared to the outside air temperature conditions. It was observed that most of the sprayed water was evaporated and the small left over water that came out of the delivery pipe along with the cool air was allowed to flow in the central path inside the greenhouse. It was also observed that the average RH inside the greenhouse dropped very low (o25%) during these hot and dry days. Hence, the addition of evaporative cooling process helped in increasing the RH inside the greenhouse and decreasing water stress on the plants besides significantly lowering the inside air temperature. It was however observed that due to the addition of water vapor, inside the greenhouse, a continuous increase in the inside RH was observed that reduced the efficiency of the evaporative cooling process. Thus a small percentage of outside dry air (20% by volume) was mixed with the recirculating greenhouse air (state point 2, Fig. 1) to prevent too much increase in the inside RH. This value was selected on the basis of maintaining the minimum number of air changes of the greenhouse air without affecting the cooling effect. Further increase in the percentage of the outside air mixing with the greenhouse air lowered the volume flow rate of the greenhouse circulating air, which eventually lowered the rate of heat transfer of the greenhouse air to the ACCFHES. Other advantages of mixing the outside air are that it maintains the carbon dioxide levels inside the greenhouse, which is also one of the important parameters for optimum plant growth. Most importantly, addition of outside air in the greenhouse creates positive pressure inside the greenhouse (greater than outside), which prevents any infiltration or leakage of air from outside the greenhouse. 3. Measurement of parameters Air temperature was measured at all the state points 1–6 shown in Fig. 1 (cooling mode) and state points 1–3 shown in Fig. 2 (heating mode) by inserting calibrated mercury thermometers in the pipe. Holes were drilled in the pipe to insert the thermometers. These holes were then properly sealed to avoid any water entry into the pipe. Greenhouse room air temperature was also recorded by hanging three mercury thermometers at 1 m height from the ground at different places inside the greenhouse (protecting their bulbs from the direct sunlight) and then the average was taken. As the thermometers were not aspirated, an error of about 1–1.5 1C was induced as compared to the aspirated
1417
thermometers. Temperature readings were recorded after every hour from 6 a.m. to 6 p.m. in summer and 8 p.m. to 8 a.m. in winter once in a week. Airflow rate was calculated by using a water manometer across an orifice meter fitted inside the pipe just after the state point 6 in Fig. 1 and state point 3 in Fig. 2. Flow rate accuracy was also checked using a dial-type vane anemometer (range 0–100,000 m) of Otakeiki Seisakusho, Japan fitted across the suction line. Root mean square deviation of 4.78% between both the readings was observed that is within the permissible deviation range. Water flow rate was measured by passing it through a peizometer fitted tank of known cross-section along the downstream end and recording the time taken for 20 cm rise in water level. Water temperature was also measured at trench inlet and outlet along the length of the pipe using mercury thermometers at a depth of 10 cm from the water surface. Only 2–3 1C increase in the water temperature was observed during the flow length. RH was measured using two thermo-hygrometers hung at 1 m from the ground at different locations inside the greenhouse. 4. Results and discussion Experimental evaluation of the ACCFHES was conducted from November to February (winter months, Fig. 3) and March to June (summer months, Fig. 4) in the year 2004–05. These figures show the hourly average of inside air temperature, outside air temperature, delivery air temperature from the pipe outlet, inside RH and outside RH for different weeks of each month. 4.1. The heating potential of ACCFHES The heating potential of the ACCFHES was tested during the winter months of November–February. It is observed from Fig. 3 that during different weeks of November (N1–N4), the average night air temperature inside the greenhouse remained about 8 1C above the outside air conditions. During the first week of December (D1), average night outside air temperature dropped below 11 1C and reached up to 6.5 1C in the last week (D4) as compared to 19.3 and 13.4 1C during the same period which again shows an increase of about 7–8 1C above outside air conditions. Similarly, in the extreme cold month of January, the average night outside air temperature further decreased to 4.4 1C in the mid January (J2) and then increased to 8.5 1C in the last week (J4) as compared to 10.8 and 16.1 1C for inside the greenhouse still showing an increase of about 8 1C above the outside air conditions. In the month of February, the effect of severe cold decreased and the average outside air temperature increased up to 11.4 1C in the second week (F2). During the same period, greenhouse air temperature also increased up to 20.3 1C showing an increase of about 9 1C above the outside conditions. The gradual decrease in the delivery air temperature from 25.5 to 23.4 1C during November to
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421
1418
Inside air
Outside air
Delivery air
Inside RH
Outside RH
100
30
90 80 70
20
60 50
15
40 10
30
Relative humidity, %
Temperature, deg C
25
20
5
10 0
0 N1
N2
N3
N4
D1
D2
D3
D4
J1
J2
J3
J4
F1
F2
Month, week Fig. 3. Heating potential of the ACCFHES in winter months.
Inside air
Outside air
Delivery air
Inside RH
Outside RH
45
70
40
60 50
30 25
40
20
30
15 20
Relative humidity,%
Temperature, deg C
35
10 10
5
0
0 M3
M4
A1
A2
A3
A4
M1 M2 M3 Month, week
without mixing of water
M4
J1
J2
J3
J4
with mixing of water
Fig. 4. Cooling potential of the ACCFHES in summer months.
mid-January was also observed. It was due to the extreme winter conditions prevailing outside the greenhouse, which increased the convective heat transfer to outside air from the moving water surface. However, this variation in the delivery air temperature did not affect the performance of the ACCFHES and the system was still able to maintain the greenhouse air temperature higher by about 9–7 1C during winter months. The RH inside the greenhouse was 10–12% lower as compared to the outside RH during the whole of the heating period (Fig. 3). It was due to the sensible heating of the inside air that reduced the RH at the same moisture level.
4.2. The cooling potential of ACCFHES Sensible cooling performance of the ACCFHES (without mixing of water) is discussed from the third week of March (M3) up to the last week of April in Fig. 4. During this period, the maximum outside air temperature remained between 33 and 38 1C. It is known that under no cooling situation (only natural ventilation), greenhouse air temperature in summers increases by about 6–7 1C above the outside air conditions. It is observed from Fig. 4 that the average greenhouse air temperature remained 1–2 1C below the outside air for different weeks and was about
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421
7–8 1C lower as compared to no cooling situation. The average greenhouse air temperature remained comfortably below 35 1C up to the first week of April (A1), which was 1–2 1C below the outside air conditions and was well within the desirable range. During A2–A4, when the average daytime outside air temperature increased from 34.2 to 37.8 1C, the inside greenhouse air temperature was observed between 33.7 and 38.6 1C. However, in the last week of April (A4), the inside greenhouse air temperature increased above the outside air conditions showing that the sensible cooling alone is not effective in extreme summer conditions. Hence, the addition of evaporative cooling process became necessary when the greenhouse air temperature increased above 38 1C. During this period, the inside RH was about 3–10% higher as compared to the outside conditions but both the values dropped to about 25% in the last week of April, which was quite low for the plants. Temperature of the delivery air also gradually increased from 25 to 28 1C during M3–A4. This increase was due to the rise in the outside air temperature during this period, which slightly raised the temperature of moving water in the trench. The combined sensible and evaporative cooling performance of the ACCFHES (with mixing of water) is discussed from the first week of May (M1) up to the last week of June (J4) in Fig. 4. During this period, the maximum outside air temperature remained above 40 1C. The RH inside the greenhouse also dropped very low (25%) in A4. It was observed that by mixing the water with the circulating air after the completion of the sensible cooling process (state point 5 in Fig. 1), the average daytime outside and inside air temperature during the different weeks of May was 38.6 & 34.9 1C (M1), 41.2 & 35.6 1C (M2), 41.5 & 35.9 1C (M3) and 42.0 & 35.5 1C (M4). This difference was about 5–7 1C below the outside air conditions. Similarly, in the month of June, the average daytime outside and inside air temperature was 43.5 & 36.5 1C in J1, 44.2 & 36.9 1C in J2, 44.2 & 37.6 1C in J3 and 44.5 & 37.8 1C in J4. This difference was again 6–7 1C below the outside air conditions, which showed that the ACCFHES could significantly cool the greenhouse in the extreme summer conditions. It was observed that by mixing the water, average daytime RH inside the greenhouse appreciably increased from 28.3% in A4 to 58.5% in M1. The outside RH further dropped from 25.7% in A4 to 14.2% in J4 but the inside RH was maintained around 58–62% during the same period. 5. Economic analysis Economic analysis of the ACCFHES coupled greenhouse was conducted on the basis of the yield of C. annum crop. Total life of the project was assumed to be 20 years. Life of the greenhouse cover (polyethylene, UV stabilized) was considered to be 5 years. The ACCFHES was operated with a 746 W motor for 12 h/day at the cost of Rs. 2.90/
1419
kWh. Discounting rate was assumed to be 8% as compared to the bank-lending rate. The salvage value of the greenhouse and the ACCFHES was considered as 40% of the initial investment. Off-season selling price of capsicum per kg for different months was considered as Rs. 40 in February, Rs. 32 in March and Rs. 20 in April. Total revenue generated was calculated as per the yield obtained in each month. Average yield of capsicum under a conventional greenhouse was considered as 0.600 kg per plant (standard). While conducting the comparison of ACCFHES with the other existing heating/cooling technologies, it was assumed that the yield obtained was same for all the systems. This assumption was made on the fact that each system would maintain the same inside air temperature of the greenhouse. 5.1. Cost break up of different components of the selected greenhouse and other systems Cost break up of different components of the selected greenhouse shows that a total length of 80 m GI pipe (12.7 mm diameter) was used for the greenhouse construction incurring a cost of Rs. 2656 (1 $ ¼ 50 Rs., based on year 2004 prices). A total length of 6.3 m GI pipe (19.05 mm diameter) at the cost of Rs. 336 was used for the greenhouse foundation. The total weight of angle iron (19.05 mm size) and MS flat (19.05 6.35 mm size) used for the construction of ventilators and door was 18 and 30 kg, respectively, which incurred a total cost of Rs. 954. 13.52m long sieving net (32 mesh) at the total cost of Rs. 600 was fixed along the walls of the greenhouse for creating natural ventilation inside the greenhouse. The total cost of cement, bricks and screws was Rs. 300. The total labor cost paid for the greenhouse construction was Rs. 1000. Adding the above-mentioned costs, total cost for the greenhouse construction excluding the polyethylene sheet was computed as Rs. 5846. The total weight and the cost of PE sheet utilized for the greenhouse cover was 15 kg and Rs. 1875, respectively. By adding the cost of the sheet in the cost of greenhouse construction, total cost of the greenhouse was Rs. 7721. Similarly, cost break up of the components of the ACCFHES showed that the cost of 20m long PVC pipe was Rs. 1188. The cost of a 746 W motor and blower assembly was Rs. 5500. The cost of digging the trench was Rs. 300. The cost of bends, adhesive tape and other adhesives was Rs. 500. Thus, total cost of the ACCFHES excluding the operation and maintenance (O & M) cost was Rs. 7488. Annual O & M cost of the ACCFHES operation was Rs. 2366. Similarly, the cost of other selected systems (on the basis of year 2004 prices) such as EAHES, fan & pad, fogging system and ground air collector was Rs. 8500, Rs. 14 000, Rs. 20 000 and Rs. 12 000, respectively. Details of the income and the expenditure of the project under different environmental conditions are given in Table 1. The net present worth (NPW) was computed by subtracting the total discounted present worth of the cost
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421
1420
Table 1 Details of the income and the expenditure under different selected conditions Sl. no
Particulars
Greenhouse with ACCFHES
Conventional greenhouse
Open field
1. 2. 3. 3. 4. 5. 6. 7. 8.
Yield of capsicum (kg) Total revenue (Rs.) Initial investment ¼ greenhouse construction+cost of ACCFHES Cost of greenhouse construction Cost of the ACCFHES Cost of sheet after every 5th year Cost of cultivation, fertilizers, pesticides, etc. (O & M cost) Cost of electricity (O & M cost) Total O & M every year (6+7) Every 5th year (5+6+7) Total initial investment (3+4+6+7)
249.35 8165.00 15 209.00 ¼ 7721.00+7488.00 7721.00 7488.00 1875.00 800.00 1566.00 2366.00 4241.00 17 575.00
113.30 2720.00 7721.00+ni 7721.00 Nil 1875.00 800.00 Nil 800.00 2675.00 8521.00
62.90 1258.00 Nil+nil Nil Nil Nil 800.00 Nil 800.00 800.00 800.00
9.
stream from that of the benefit stream. Computations showed that the NPW of ACCFHES coupled greenhouse was Rs. 38 434.90 as compared to Rs. 3868.13 for open field condition. At the same time, NPW for the conventional greenhouse condition was Rs. 6223.90. The cumulative present worth of the cash inflow in seventh year was (Rs. 43 506.18) greater than the present worth of the total cash outflow (Rs. 43 028.91) during the whole period of the project. Computations showed that the payback period was 7 years for the ACCFHES coupled greenhouse whereas for the conventional greenhouse the payback period was 13 years. 5.2. Economic comparison with other existing systems In order to find the total initial investment for other existing systems, cost of each system was added in the total cost of the greenhouse construction including the annual O & M cost. Thus the total initial investment was Rs. 17 587 for the ACCFHES, Rs. 18 587 for the EAHES, Rs. 23 636 for the fan & pad system, Rs. 29 636 for the fogging system and Rs. 22 087 for the ground air collector. Calculations showed that the ACCFHES is the cheapest of all the existing heating/cooling systems. The NPW and the payback period was computed as Rs. 36 114.89 and 7.6 years for the EAHES, Rs. 35 396 and 8.3 years for the fan & pad system, Rs. 29 396 and 10.2 years for the fogging system and Rs. 32 614 and 9.5 years for the ground air collector. It was observed that the ACCFHES coupled greenhouse had the highest NPW and the lowest payback period. 6. Conclusions Based on the experimental results the following conclusions have been drawn: 1. The ACCFHES is capable of maintaining the greenhouse room air temperature 7–9 1C above outside air in winter months and 6–7 1C below outside air in summer months. 2. RH inside the ACCFHES coupled greenhouse was maintained 10–12% lower as compared to outside conditions in winter months and around 60% during
the extreme summer months of May and June as compared to about 25% for outside conditions during the same period. 3. NPW for the ACCFHES coupled greenhouse was much higher as compared to the conventional greenhouse and for the open field condition. NPW for the ACCFHES was also the highest among the existing thermal control systems. 4. Payback period for the ACCFHES coupled greenhouse was the lowest among the existing thermal control systems. 5. Finally, it can be concluded that the capsicum crop is worth growing under the greenhouse conditions (as an off-season crop) using ACCFHES as a year round thermal control system due to substantial increase in the revenue of the farmer under composite climatic conditions.
References [1] Bot GPA. Greenhouse climate from the physical processes to a dynamic model. PhD dissertation, University of Wageningen, Wageningen, The Netherlands, 1983. [2] Oca J. A method for studying natural ventilation by thermal effects in a tunnel greenhouse using laboratory scale models. J Agric Eng Res 1999;72:93–104. [3] Bailey BJ. Reduction of thermal radiation in glass houses by thermal screen. J Agric Eng Res 1981;26:215–22. [4] Sethi VP, Gupta YP. Reduction of greenhouse temperature using reflector sheet. Agricultural Mechanization in Asia, Africa and Latin America (AMA). 2004;35(2):51–4. [5] Landsberg JJ, White B, Thorpe MR. Computer analysis of the efficacy of evaporative cooling for glasshouses in high energy environments. J Agric Eng Res 1979;24:29–39. [6] Chandra P, Singh JK, Majumdar G. Some results of evaporative cooling of a plastic greenhouse. J Agric Eng ISAE 1989;26(3): 274–80. [7] Jain D, Tiwari GN. Modeling and optimal design of evaporative cooling system in controlled environment greenhouse. Energy Conversion Manage 2002;43:2235–50. [8] Press TF. Propagation: fog not mist. In: Combined Proceedings, International Plant Propagators Society, 1984. p. 100–9. [9] Arbel A, Yekutieli O, Barak M. Performance of a fog system for cooling of greenhouses. J Agric Eng Res 1999;72(2):129–36.
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421 [10] Sorensen B. Experiments with energy storage in a high-latitude greenhouse. Sol Energy 1989;42(4):293–301. [11] Gupta A, Tiwari GN. Computer model and its validation for prediction of storage effect of water mass in a greenhouse: a transient analysis. Energy Conversion Manage 2002;43:95–101. [12] Garzoli KV. Design of rock piles for greenhouse energy storage. Acta Hortic 1989;257:21–8. [13] Sagara K, Nakahara N. Thermal performance and pressure drop of rock beds with large storage materials. Sol Energy 1991;47(3): 157–63. [14] Choudhury C, Garg HP. Performance of air-heating collectors with packed airflow passage. Sol Energy 1993;50(3):205–21. [15] Abhat A. Low temperature latent heat thermal energy storage: heat storage materials. Sol Energy 1983;30(4):313–32. [16] Hamada Y, Ohtsu W, Fukai J. Thermal response in thermal energy storage material around heat transfer tubes: effect of additives on heat transfer rates. Sol Energy 2003;75:317–28.
1421
[17] Farid MM, Khudhair AM, Razack AKS, Al-Hallaj S. A review on phase change energy storage: materials and applications. Energy Conversion Manage 2004;45:1597–615. [18] Santamouris M, Mihalakakou G, Balaras CA, Agririou A, Asimakopoulos D, Vallindras M. Use of buried pipes for energy conservation in cooling of agricultural greenhouses. Sol Energy 1995;55(2):111–24. [19] Kozai T. Thermal performance of a solar greenhouse with an underground heat storage system. In: Proceedings of the symposium on thermal applications of solar energy, Japan, ISES, 1985. p. 503. [20] Immakulov AI. Storage of solar energy by forced circulations in shed greenhouses. Geliotechnika 1986;22:44. [21] Sodha MS. Thermal performance of parallel earth air-pipes system. Int J Energy Res 1994;18:437–47. [22] Mihalakakaou G, Santamouris M, Asimakopoulos D, Tselepidaki I. Parametric prediction of the buried pipes cooling potential for passive cooling applications. Sol Energy 1995;55(3):163–73.
Energy 32 (2007) 1414–1421 www.elsevier.com/locate/energy
Greenhouse heating and cooling using aquifer water V.P. Sethia,, S.K. Sharmab a
Department of Mechanical Engineering, Punjab Agricultural University, Ludhiana 141008, Punjab, India b Energy Research Centre, Panjab University, Chandigarh 160017, Punjab, India Received 18 January 2006
Abstract An aquifer coupled cavity flow heat exchanger system (ACCFHES) was designed using underground aquifer water for the heating as well as cooling of a composite climatic greenhouse. The performance of ACCFHES was experimentally evaluated for a full winter and a summer season. The ACCFHES makes use of constant temperature aquifer water (24 1C) available at an agricultural field through an irrigation tube well for heating in winter nights and cooling in summer days. The results showed that the average greenhouse room air temperature was maintained 7–9 1C above the outside air during extreme winter nights and 6–7 1C below the outside air in extreme summer days, and temperature fluctuations inside the greenhouse also decreased daily. The average relative humidity (RH) inside the greenhouse also decreased by 10–12% in the winter and increased by more than double in the extreme summer conditions as compared to the outside conditions. A comparison of economic feasibility of the ACCFHES coupled greenhouse was also conducted with conventional greenhouse and open field condition based on the yield of Capsicum annum. The ACCFHES was also compared economically with other existing heating/cooling technologies such as earth-to-air heat exchanger system (EAHES), ground air collector, evaporative cooling using foggers and fan & pad system in terms of net present worth (NPW) and pay back period. It was observed that the NPW of the ACCFHES coupled greenhouse was much higher as compared to the conventional greenhouse and open field condition. The payback period of the ACCFHES coupled greenhouse was the lowest among all other existing heating/cooling systems. r 2006 Elsevier Ltd. All rights reserved. Keywords: Greenhouse; Solar energy; Cooling/heating systems; Thermal control; Aquifer water
1. Introduction Agricultural greenhouses are a viable solution to the worldwide increased demand for expanding production, facilitating out of season cultivation and allowing the growth of certain varieties in areas where it was not possible earlier. In order to have out of season agricultural production inside a greenhouse, a heating or a cooling system is required to maintain the inside micro-climate. Currently there are many cooling and heating systems being used for greenhouses depending upon the climatic conditions of an area. In the cooling systems, natural and forced ventilation as discussed by Bot [1] and Oca [2] is useful only in the mild hot weather conditions and becomes ineffective during extreme summer conditions. Use of shading screens by Bailey [3] and reflector sheets by Sethi Corresponding author. Tel.: +91 161 2553367; fax: +91 161 2401794.
E-mail address: [email protected] (V.P. Sethi). 0360-5442/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2006.10.022
and Gupta [4] showed that these restrict the entry of excessive solar radiation inside the greenhouse but not much drop in the inside air temperature could be achieved during extreme summer conditions. Evaporative cooling systems using fan & pad as studied by Landsberg et al. [5], Chandra et al. [6] and Jain and Tiwari [7] and foggers used by Press [8] and Arbel et al. [9] proved that these systems are highly effective during extreme dry summer conditions and can cover upto 80% cooling needs of the greenhouse. However, higher levels of moisture inside the greenhouse promote the growth of micro-organisms thus making the crop more susceptible to the diseases. In heating systems, water storage using plastic pipes described by Sorensen [10] and drums filled with water as discussed by Gupta and Tiwari [11] can meet 30–70% heating needs of the greenhouse. However, about 25–30% heating space becomes unavailable due to placing of these heat storage media inside the greenhouse. Studies conducted on rock bed heat storage systems by Garzoli [12], Sagara and
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421
Nakahara [13] and Choudhury and Garg [14] revealed that these systems can maintain 2–10 1C higher temperature inside the greenhouse as compared to outside air temperature and can cover 30–75% heating needs of the greenhouse. However, these systems consume very high energy due to greater air pressure differences across the bed. Studies conducted by Abhat [15], Hamada et al. [16] and Farid et al. [17] about the use of phase change materials for greenhouse latent heat storage systems described that these systems can increase the greenhouse air temperature by about 2–8 1C but due to higher cost and incongruent melting these systems become less trustworthy for long duration experiments. In a composite climate, where heating of greenhouse is required in winter nights and cooling is required in summer days, earth-to-air heat exchanger system (EAHES) can be used for creating favorable environmental conditions inside the greenhouse as earth temperature below its surface at a depth of about 3–4 m remains almost stagnant at about 26–28 1C throughout the year as studied by Santamouris et al. [18]. Other researchers like Kozai [19], Immakulov [20], Sodha [21] and Mihalakakaou et al. [22] have also discussed the heating and cooling potential of EAHES. However, the limitation of using EAHES is the higher cost of deep digging and horizontal layout of pipe(s) up to 3–4 m depth is not easy. Moreover, for the short-term use temperature around the soil mass gradually increases due to dissipation of heat from the outside pipe surface that decreases the efficiency of the system. In this study, a composite system called aquifer coupled cavity flow heat exchanger system (ACCFHES) using aquifer water on the ground surface is proposed. The system utilizes underground aquifer water (at around 24 1C, year round) at the outlet of an irrigation tube well in an agricultural field for effectively heating a greenhouse
1415
in winter and cooling the same in summer. The advantages of the proposed system over EAHES are that the cost of deep digging of soil is not required at all, which saves a lot of labor cost. Aquifer water is available at a lower temperature (around 24 1C) throughout the whole year as compared to the ground temperature (28 1C) that improves the cooling performance of the system. Moreover, for the same pipe length, higher heat transfer takes place due to counter flow arrangement of greenhouse air and aquifer water in the shallow trench. The cost of lifting the aquifer water upto the ground level is zero as this water is already available on the ground surface for irrigation purposes. 2. Methods and materials A 6 4 m size greenhouse was constructed near Chandigarh (311N latitude), Punjab, India. The east–west oriented even span greenhouse has central and side height of 3 and 2 m, respectively. It has a door of 1 1.9 m size on the west wall and has a ventilator of 0.8 0.8 m size each on the center of the inclined north and south roof. Greenhouse was covered with a single sheet of UV stabilized polyethylene of 0.7 transmissivity in which Capsicum annum was grown. A shallow cavity (also called trench) of 0.3 0.3 m size was dug around the perimeter of the greenhouse in which aquifer water from irrigation tube well (near the greenhouse sight) was passed before sending it to the field for irrigation purposes as shown in Fig. 1. A 20 m long plastic pipe (0.1016 m diameter) was placed in the trench. A thin plastic sheet was spread below the pipe (in the trench) to avoid any water seepage to the ground. The trench was dug at a distance of 1 m away from the greenhouse walls in order to avoid any seepage of water to the greenhouse foundation. One end of the pipe was connected with the suction side and the other end with the
Aquifer water from tube well Water inlet 5
Air 6m
Water tank
Holes for water mixing 6
Cold air delivery
7
4m Blower
2
Top supports Outside air mixing
3
1
Hot air suction
7 Immersed pipe
4 Air in pipe losing heat ` Water outlet to f ield
Trench 0.3 × 0.3m
Fig. 1. Top view of the greenhouse integrated with the ACCFHES in cooling mode.
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421
1416
sensible heating process. This air was further heated to state 3 by the vanes of blower and was distributed inside the greenhouse. Thus a final state 4 was achieved after the heated air was mixed with the rest of the greenhouse air. One important observation was made that the air temperature after passing through the blower was raised by about 3–4 1C in winter and 5–6 1C in summer. In order to nullify the heating effect of air in summer (state points 3 and 4, Fig. 1), blower and motor assembly was placed on the suction side so that hot air from the greenhouse once cooled was directly fed to the greenhouse from the delivery side. However, in winters, the motor blower assembly was placed on the delivery side so that air after heating through the channel was further heated (state points 2 and 3, Fig. 2) by the blower in order to improve the heating effect.
delivery side of the blower. Greenhouse air was drawn near the roof by a 746 W blower (giving a flow rate of 1800 m3/h) and was circulated through the pipe placed in the trench in the opposite direction of the water flow in order to make it a counter flow configuration. To keep the pipe fully immersed in water, supports at the top of the trench were provided after every 2.5 m length. These supports were simple iron rods fixed across the channel at the water level so that the plastic pipe could not rise above this level and remained fully immersed. In summer, hot circulating air loses its heat to the aquifer water flowing in the trench and in winter it gains heat from the same water. The cool or hot circulating air was uniformly distributed through a delivery pipe placed centrally inside the greenhouse at a height of 30 cm in order to provide the maximum cooling effect near the plants. Circulating air was distributed inside the greenhouse by means of two 10 mm holes drilled on either side in upper half of the delivery pipe at 60 cm interval so that conditioned air could reach in each row. In the cooling mode (Fig. 1), hot air near the roof was drawn (state 1) and pushed through the fully immersed pipe placed in the trench by means of a blower and motor assembly. Aquifer water was allowed to enter the trench from the opposite direction in order to make it a counter flow configuration. Hot air loses its heat to the aquifer water (sensible cooling) along the flow length. Cool air at state point 5 was further cooled to state point 6 by mixing water with the circulating air (evaporative cooling process). This cool air was distributed inside the greenhouse to achieve state 7 after mixing it with rest of the greenhouse air. In the heating mode (Fig. 2), cool air was drawn near the roof (state 1) and forced through the pipe placed in the trench. The circulating air gained heat along the flow length and attained state 2 after the completion of the
2.1. Evaporative cooling process In the extreme summer conditions, addition of a simple evaporative cooling process further maximized the cooling effect of ACCFHES. Measured quantity of water at around 25 1C, i.e. at the wet bulb temperature was sprayed inside the air pipe along the circulating air just after the completion of sensible cooling process (between state points 5 and 6, Fig. 1). The Quantity of water required for evaporation purposes was calculated about 12 l/h for the required airflow rate and relative humidity (RH) (65%). The required water was supplied through a drum of 0.1 m3 capacity placed at about 4 m height from the delivery pipe. The drum had three outlets with flow control valves to regulate the water inside the pipe. Three small holes of about 10 mm size were drilled at 15 cm interval in the pipe. Three 8 mm (inner diameter) flexible rubber pipes were inserted inside each hole for mixing water with air and Trench
Aquifer water inlet from the tube well Hot air in pipe
2
Blower
Hot air delivery
4
3
4 1
Cold air suction
Air in pipe gaining heat
Water outlet to field
Trench (0.3×0.3m)
Fig. 2. Top view of the greenhouse integrated with the ACCFHES in heating mode.
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421
for providing sufficient contact of water with the air through 30 cm distance. The principle of operation is that the high velocity moving air through the pipe breaks these water droplets into smaller particles. These particles evaporate when air at high velocity comes in contact with it and takes away the latent heat of vaporization from the air that lowers the air temperature inside the pipe to around wet bulb temperature and the overall temperature of the greenhouse is significantly lowered as compared to the outside air temperature conditions. It was observed that most of the sprayed water was evaporated and the small left over water that came out of the delivery pipe along with the cool air was allowed to flow in the central path inside the greenhouse. It was also observed that the average RH inside the greenhouse dropped very low (o25%) during these hot and dry days. Hence, the addition of evaporative cooling process helped in increasing the RH inside the greenhouse and decreasing water stress on the plants besides significantly lowering the inside air temperature. It was however observed that due to the addition of water vapor, inside the greenhouse, a continuous increase in the inside RH was observed that reduced the efficiency of the evaporative cooling process. Thus a small percentage of outside dry air (20% by volume) was mixed with the recirculating greenhouse air (state point 2, Fig. 1) to prevent too much increase in the inside RH. This value was selected on the basis of maintaining the minimum number of air changes of the greenhouse air without affecting the cooling effect. Further increase in the percentage of the outside air mixing with the greenhouse air lowered the volume flow rate of the greenhouse circulating air, which eventually lowered the rate of heat transfer of the greenhouse air to the ACCFHES. Other advantages of mixing the outside air are that it maintains the carbon dioxide levels inside the greenhouse, which is also one of the important parameters for optimum plant growth. Most importantly, addition of outside air in the greenhouse creates positive pressure inside the greenhouse (greater than outside), which prevents any infiltration or leakage of air from outside the greenhouse. 3. Measurement of parameters Air temperature was measured at all the state points 1–6 shown in Fig. 1 (cooling mode) and state points 1–3 shown in Fig. 2 (heating mode) by inserting calibrated mercury thermometers in the pipe. Holes were drilled in the pipe to insert the thermometers. These holes were then properly sealed to avoid any water entry into the pipe. Greenhouse room air temperature was also recorded by hanging three mercury thermometers at 1 m height from the ground at different places inside the greenhouse (protecting their bulbs from the direct sunlight) and then the average was taken. As the thermometers were not aspirated, an error of about 1–1.5 1C was induced as compared to the aspirated
1417
thermometers. Temperature readings were recorded after every hour from 6 a.m. to 6 p.m. in summer and 8 p.m. to 8 a.m. in winter once in a week. Airflow rate was calculated by using a water manometer across an orifice meter fitted inside the pipe just after the state point 6 in Fig. 1 and state point 3 in Fig. 2. Flow rate accuracy was also checked using a dial-type vane anemometer (range 0–100,000 m) of Otakeiki Seisakusho, Japan fitted across the suction line. Root mean square deviation of 4.78% between both the readings was observed that is within the permissible deviation range. Water flow rate was measured by passing it through a peizometer fitted tank of known cross-section along the downstream end and recording the time taken for 20 cm rise in water level. Water temperature was also measured at trench inlet and outlet along the length of the pipe using mercury thermometers at a depth of 10 cm from the water surface. Only 2–3 1C increase in the water temperature was observed during the flow length. RH was measured using two thermo-hygrometers hung at 1 m from the ground at different locations inside the greenhouse. 4. Results and discussion Experimental evaluation of the ACCFHES was conducted from November to February (winter months, Fig. 3) and March to June (summer months, Fig. 4) in the year 2004–05. These figures show the hourly average of inside air temperature, outside air temperature, delivery air temperature from the pipe outlet, inside RH and outside RH for different weeks of each month. 4.1. The heating potential of ACCFHES The heating potential of the ACCFHES was tested during the winter months of November–February. It is observed from Fig. 3 that during different weeks of November (N1–N4), the average night air temperature inside the greenhouse remained about 8 1C above the outside air conditions. During the first week of December (D1), average night outside air temperature dropped below 11 1C and reached up to 6.5 1C in the last week (D4) as compared to 19.3 and 13.4 1C during the same period which again shows an increase of about 7–8 1C above outside air conditions. Similarly, in the extreme cold month of January, the average night outside air temperature further decreased to 4.4 1C in the mid January (J2) and then increased to 8.5 1C in the last week (J4) as compared to 10.8 and 16.1 1C for inside the greenhouse still showing an increase of about 8 1C above the outside air conditions. In the month of February, the effect of severe cold decreased and the average outside air temperature increased up to 11.4 1C in the second week (F2). During the same period, greenhouse air temperature also increased up to 20.3 1C showing an increase of about 9 1C above the outside conditions. The gradual decrease in the delivery air temperature from 25.5 to 23.4 1C during November to
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421
1418
Inside air
Outside air
Delivery air
Inside RH
Outside RH
100
30
90 80 70
20
60 50
15
40 10
30
Relative humidity, %
Temperature, deg C
25
20
5
10 0
0 N1
N2
N3
N4
D1
D2
D3
D4
J1
J2
J3
J4
F1
F2
Month, week Fig. 3. Heating potential of the ACCFHES in winter months.
Inside air
Outside air
Delivery air
Inside RH
Outside RH
45
70
40
60 50
30 25
40
20
30
15 20
Relative humidity,%
Temperature, deg C
35
10 10
5
0
0 M3
M4
A1
A2
A3
A4
M1 M2 M3 Month, week
without mixing of water
M4
J1
J2
J3
J4
with mixing of water
Fig. 4. Cooling potential of the ACCFHES in summer months.
mid-January was also observed. It was due to the extreme winter conditions prevailing outside the greenhouse, which increased the convective heat transfer to outside air from the moving water surface. However, this variation in the delivery air temperature did not affect the performance of the ACCFHES and the system was still able to maintain the greenhouse air temperature higher by about 9–7 1C during winter months. The RH inside the greenhouse was 10–12% lower as compared to the outside RH during the whole of the heating period (Fig. 3). It was due to the sensible heating of the inside air that reduced the RH at the same moisture level.
4.2. The cooling potential of ACCFHES Sensible cooling performance of the ACCFHES (without mixing of water) is discussed from the third week of March (M3) up to the last week of April in Fig. 4. During this period, the maximum outside air temperature remained between 33 and 38 1C. It is known that under no cooling situation (only natural ventilation), greenhouse air temperature in summers increases by about 6–7 1C above the outside air conditions. It is observed from Fig. 4 that the average greenhouse air temperature remained 1–2 1C below the outside air for different weeks and was about
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421
7–8 1C lower as compared to no cooling situation. The average greenhouse air temperature remained comfortably below 35 1C up to the first week of April (A1), which was 1–2 1C below the outside air conditions and was well within the desirable range. During A2–A4, when the average daytime outside air temperature increased from 34.2 to 37.8 1C, the inside greenhouse air temperature was observed between 33.7 and 38.6 1C. However, in the last week of April (A4), the inside greenhouse air temperature increased above the outside air conditions showing that the sensible cooling alone is not effective in extreme summer conditions. Hence, the addition of evaporative cooling process became necessary when the greenhouse air temperature increased above 38 1C. During this period, the inside RH was about 3–10% higher as compared to the outside conditions but both the values dropped to about 25% in the last week of April, which was quite low for the plants. Temperature of the delivery air also gradually increased from 25 to 28 1C during M3–A4. This increase was due to the rise in the outside air temperature during this period, which slightly raised the temperature of moving water in the trench. The combined sensible and evaporative cooling performance of the ACCFHES (with mixing of water) is discussed from the first week of May (M1) up to the last week of June (J4) in Fig. 4. During this period, the maximum outside air temperature remained above 40 1C. The RH inside the greenhouse also dropped very low (25%) in A4. It was observed that by mixing the water with the circulating air after the completion of the sensible cooling process (state point 5 in Fig. 1), the average daytime outside and inside air temperature during the different weeks of May was 38.6 & 34.9 1C (M1), 41.2 & 35.6 1C (M2), 41.5 & 35.9 1C (M3) and 42.0 & 35.5 1C (M4). This difference was about 5–7 1C below the outside air conditions. Similarly, in the month of June, the average daytime outside and inside air temperature was 43.5 & 36.5 1C in J1, 44.2 & 36.9 1C in J2, 44.2 & 37.6 1C in J3 and 44.5 & 37.8 1C in J4. This difference was again 6–7 1C below the outside air conditions, which showed that the ACCFHES could significantly cool the greenhouse in the extreme summer conditions. It was observed that by mixing the water, average daytime RH inside the greenhouse appreciably increased from 28.3% in A4 to 58.5% in M1. The outside RH further dropped from 25.7% in A4 to 14.2% in J4 but the inside RH was maintained around 58–62% during the same period. 5. Economic analysis Economic analysis of the ACCFHES coupled greenhouse was conducted on the basis of the yield of C. annum crop. Total life of the project was assumed to be 20 years. Life of the greenhouse cover (polyethylene, UV stabilized) was considered to be 5 years. The ACCFHES was operated with a 746 W motor for 12 h/day at the cost of Rs. 2.90/
1419
kWh. Discounting rate was assumed to be 8% as compared to the bank-lending rate. The salvage value of the greenhouse and the ACCFHES was considered as 40% of the initial investment. Off-season selling price of capsicum per kg for different months was considered as Rs. 40 in February, Rs. 32 in March and Rs. 20 in April. Total revenue generated was calculated as per the yield obtained in each month. Average yield of capsicum under a conventional greenhouse was considered as 0.600 kg per plant (standard). While conducting the comparison of ACCFHES with the other existing heating/cooling technologies, it was assumed that the yield obtained was same for all the systems. This assumption was made on the fact that each system would maintain the same inside air temperature of the greenhouse. 5.1. Cost break up of different components of the selected greenhouse and other systems Cost break up of different components of the selected greenhouse shows that a total length of 80 m GI pipe (12.7 mm diameter) was used for the greenhouse construction incurring a cost of Rs. 2656 (1 $ ¼ 50 Rs., based on year 2004 prices). A total length of 6.3 m GI pipe (19.05 mm diameter) at the cost of Rs. 336 was used for the greenhouse foundation. The total weight of angle iron (19.05 mm size) and MS flat (19.05 6.35 mm size) used for the construction of ventilators and door was 18 and 30 kg, respectively, which incurred a total cost of Rs. 954. 13.52m long sieving net (32 mesh) at the total cost of Rs. 600 was fixed along the walls of the greenhouse for creating natural ventilation inside the greenhouse. The total cost of cement, bricks and screws was Rs. 300. The total labor cost paid for the greenhouse construction was Rs. 1000. Adding the above-mentioned costs, total cost for the greenhouse construction excluding the polyethylene sheet was computed as Rs. 5846. The total weight and the cost of PE sheet utilized for the greenhouse cover was 15 kg and Rs. 1875, respectively. By adding the cost of the sheet in the cost of greenhouse construction, total cost of the greenhouse was Rs. 7721. Similarly, cost break up of the components of the ACCFHES showed that the cost of 20m long PVC pipe was Rs. 1188. The cost of a 746 W motor and blower assembly was Rs. 5500. The cost of digging the trench was Rs. 300. The cost of bends, adhesive tape and other adhesives was Rs. 500. Thus, total cost of the ACCFHES excluding the operation and maintenance (O & M) cost was Rs. 7488. Annual O & M cost of the ACCFHES operation was Rs. 2366. Similarly, the cost of other selected systems (on the basis of year 2004 prices) such as EAHES, fan & pad, fogging system and ground air collector was Rs. 8500, Rs. 14 000, Rs. 20 000 and Rs. 12 000, respectively. Details of the income and the expenditure of the project under different environmental conditions are given in Table 1. The net present worth (NPW) was computed by subtracting the total discounted present worth of the cost
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421
1420
Table 1 Details of the income and the expenditure under different selected conditions Sl. no
Particulars
Greenhouse with ACCFHES
Conventional greenhouse
Open field
1. 2. 3. 3. 4. 5. 6. 7. 8.
Yield of capsicum (kg) Total revenue (Rs.) Initial investment ¼ greenhouse construction+cost of ACCFHES Cost of greenhouse construction Cost of the ACCFHES Cost of sheet after every 5th year Cost of cultivation, fertilizers, pesticides, etc. (O & M cost) Cost of electricity (O & M cost) Total O & M every year (6+7) Every 5th year (5+6+7) Total initial investment (3+4+6+7)
249.35 8165.00 15 209.00 ¼ 7721.00+7488.00 7721.00 7488.00 1875.00 800.00 1566.00 2366.00 4241.00 17 575.00
113.30 2720.00 7721.00+ni 7721.00 Nil 1875.00 800.00 Nil 800.00 2675.00 8521.00
62.90 1258.00 Nil+nil Nil Nil Nil 800.00 Nil 800.00 800.00 800.00
9.
stream from that of the benefit stream. Computations showed that the NPW of ACCFHES coupled greenhouse was Rs. 38 434.90 as compared to Rs. 3868.13 for open field condition. At the same time, NPW for the conventional greenhouse condition was Rs. 6223.90. The cumulative present worth of the cash inflow in seventh year was (Rs. 43 506.18) greater than the present worth of the total cash outflow (Rs. 43 028.91) during the whole period of the project. Computations showed that the payback period was 7 years for the ACCFHES coupled greenhouse whereas for the conventional greenhouse the payback period was 13 years. 5.2. Economic comparison with other existing systems In order to find the total initial investment for other existing systems, cost of each system was added in the total cost of the greenhouse construction including the annual O & M cost. Thus the total initial investment was Rs. 17 587 for the ACCFHES, Rs. 18 587 for the EAHES, Rs. 23 636 for the fan & pad system, Rs. 29 636 for the fogging system and Rs. 22 087 for the ground air collector. Calculations showed that the ACCFHES is the cheapest of all the existing heating/cooling systems. The NPW and the payback period was computed as Rs. 36 114.89 and 7.6 years for the EAHES, Rs. 35 396 and 8.3 years for the fan & pad system, Rs. 29 396 and 10.2 years for the fogging system and Rs. 32 614 and 9.5 years for the ground air collector. It was observed that the ACCFHES coupled greenhouse had the highest NPW and the lowest payback period. 6. Conclusions Based on the experimental results the following conclusions have been drawn: 1. The ACCFHES is capable of maintaining the greenhouse room air temperature 7–9 1C above outside air in winter months and 6–7 1C below outside air in summer months. 2. RH inside the ACCFHES coupled greenhouse was maintained 10–12% lower as compared to outside conditions in winter months and around 60% during
the extreme summer months of May and June as compared to about 25% for outside conditions during the same period. 3. NPW for the ACCFHES coupled greenhouse was much higher as compared to the conventional greenhouse and for the open field condition. NPW for the ACCFHES was also the highest among the existing thermal control systems. 4. Payback period for the ACCFHES coupled greenhouse was the lowest among the existing thermal control systems. 5. Finally, it can be concluded that the capsicum crop is worth growing under the greenhouse conditions (as an off-season crop) using ACCFHES as a year round thermal control system due to substantial increase in the revenue of the farmer under composite climatic conditions.
References [1] Bot GPA. Greenhouse climate from the physical processes to a dynamic model. PhD dissertation, University of Wageningen, Wageningen, The Netherlands, 1983. [2] Oca J. A method for studying natural ventilation by thermal effects in a tunnel greenhouse using laboratory scale models. J Agric Eng Res 1999;72:93–104. [3] Bailey BJ. Reduction of thermal radiation in glass houses by thermal screen. J Agric Eng Res 1981;26:215–22. [4] Sethi VP, Gupta YP. Reduction of greenhouse temperature using reflector sheet. Agricultural Mechanization in Asia, Africa and Latin America (AMA). 2004;35(2):51–4. [5] Landsberg JJ, White B, Thorpe MR. Computer analysis of the efficacy of evaporative cooling for glasshouses in high energy environments. J Agric Eng Res 1979;24:29–39. [6] Chandra P, Singh JK, Majumdar G. Some results of evaporative cooling of a plastic greenhouse. J Agric Eng ISAE 1989;26(3): 274–80. [7] Jain D, Tiwari GN. Modeling and optimal design of evaporative cooling system in controlled environment greenhouse. Energy Conversion Manage 2002;43:2235–50. [8] Press TF. Propagation: fog not mist. In: Combined Proceedings, International Plant Propagators Society, 1984. p. 100–9. [9] Arbel A, Yekutieli O, Barak M. Performance of a fog system for cooling of greenhouses. J Agric Eng Res 1999;72(2):129–36.
ARTICLE IN PRESS V.P. Sethi, S.K. Sharma / Energy 32 (2007) 1414–1421 [10] Sorensen B. Experiments with energy storage in a high-latitude greenhouse. Sol Energy 1989;42(4):293–301. [11] Gupta A, Tiwari GN. Computer model and its validation for prediction of storage effect of water mass in a greenhouse: a transient analysis. Energy Conversion Manage 2002;43:95–101. [12] Garzoli KV. Design of rock piles for greenhouse energy storage. Acta Hortic 1989;257:21–8. [13] Sagara K, Nakahara N. Thermal performance and pressure drop of rock beds with large storage materials. Sol Energy 1991;47(3): 157–63. [14] Choudhury C, Garg HP. Performance of air-heating collectors with packed airflow passage. Sol Energy 1993;50(3):205–21. [15] Abhat A. Low temperature latent heat thermal energy storage: heat storage materials. Sol Energy 1983;30(4):313–32. [16] Hamada Y, Ohtsu W, Fukai J. Thermal response in thermal energy storage material around heat transfer tubes: effect of additives on heat transfer rates. Sol Energy 2003;75:317–28.
1421
[17] Farid MM, Khudhair AM, Razack AKS, Al-Hallaj S. A review on phase change energy storage: materials and applications. Energy Conversion Manage 2004;45:1597–615. [18] Santamouris M, Mihalakakou G, Balaras CA, Agririou A, Asimakopoulos D, Vallindras M. Use of buried pipes for energy conservation in cooling of agricultural greenhouses. Sol Energy 1995;55(2):111–24. [19] Kozai T. Thermal performance of a solar greenhouse with an underground heat storage system. In: Proceedings of the symposium on thermal applications of solar energy, Japan, ISES, 1985. p. 503. [20] Immakulov AI. Storage of solar energy by forced circulations in shed greenhouses. Geliotechnika 1986;22:44. [21] Sodha MS. Thermal performance of parallel earth air-pipes system. Int J Energy Res 1994;18:437–47. [22] Mihalakakaou G, Santamouris M, Asimakopoulos D, Tselepidaki I. Parametric prediction of the buried pipes cooling potential for passive cooling applications. Sol Energy 1995;55(3):163–73.