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Geothermal contribution to greenhouse heating
Applied Energy 64 (1999) 241±249
www.elsevier.com/locate/apenergy
Geothermal contribution to greenhouse heating Jorge A. Adaro, Pablo D. Galimberti, Alba I. Lema, AmõÂlcar Fasulo, Jorge R. Barral* Facultad de IngenierõÂa, Universidad Nacional de RõÂo Cuarto, Ruta Nacional 36 km 601, 5800 RõÂo Cuarto, Argentina
Abstract Plant freezing and plant-growth inhibition are among the major problems in greenhouse cultivation in the central part of Argentina. The possibility of using a constant temperature underground geothermal water source, which ¯ows naturally, has been studied as an economic option to solve these problems. A system of heating by means of geothermal energy, with energy-conservation measures, was designed and evaluated for typical production greenhouses in the southern part of CoÂrdoba, Argentina. The results of tests carried out during 3 years are presented. These results are really promising, taking into account the high bene®t/cost relation of the design and the availability of similar geothermal resources in many farms of this region. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Economics; Geothermal; Greenhouse; Heating; Horticulture; Out-of-season production; Thermal curtain
1. Introduction Good plant-growth conditions can be achieved by means of using greenhouses. A greenhouse can be managed to protect the plants by creating a favourable environment, allowing intensive use of soil, and helping sanitary plant control. From an economic point of view, the main objective of horticultural greenhouses is to advance the normal season production or to obtain a completely out-of-season production, which corresponds with higher crop prices.
* Corresponding author. Tel.: +54-358-467-6246; fax: +54-358-676-6246. E-mail address: [email protected] (J.R. Barral). 0306-2619/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0306-2619(99)00049-5
242
J.A. Adaro et al. / Applied Energy 64 (1999) 241±249
Many variables must be controlled in order to provide the good environmental conditions previously mentioned. The most important parameters to be controlled inside a greenhouse are temperature, humidity and light [1]. Most winter days in the south of CoÂrdoba are dry and sunny, which implies a low relative humidity and a moderate temperature during daytime. But, at night, the temperature descends usually to only a few degrees above 0 C and sometimes to below 0 C. These low temperatures can cause the freezing of greenhouse plants. Low ambient humidity, together with moderate daytime temperatures, allow one to minimise humidity-control problems. Generally the lateral walls of greenhouses are partially opened during the daytime to provide ventilation. This is possible because of the moderate ambient temperature. Therefore, before closing these walls, when preparing the greenhouse for a cold night, the air in the greenhouse has almost the same relative humidity as the external ambient air has, which means it is dry, and, although this air is con®ned inside the greenhouse during the night, it takes time for the air to reach a high water-vapour content; and, if this happens, ventilation takes place the next day. Moreover, the photo-period for the horticultural products used, matches, or is indierent to the light duration in winter days at this latitude. Consequently, only the temperature at night appears as an important critical variable to be controlled. Although dierent species are cultivated, the requirement is always the same: the avoidance of low temperatures at night. The conventional solution for this problem is the burning of some fossil fuel inside the greenhouse during dangerous freezing nights. Since fuel prices are high, this option is very expensive, but, it is necessary, considering the possibility of the complete destruction of plants. Although total destruction can be avoided, the low night temperatures delay or interrupt the plant growth [2], which results in low production and inferior quality production during the cold season, when the price of the product reaches its highest value. A system of polyethylene tubes placed on the ground and ®lled with warm water from a geothermal source was used experimentally on a farm to provide night heating to greenhouses. In addition, permanent thermal curtains were used as a ceiling to decrease the night heat-losses. Water as the heat-storage medium has been widely used in passive solar greenhouse applications [3]. In the present experiences, although the principle used is the same, the source of energy is now the warm water of a geothermal source instead of the Sun. Then, due to the continuous water circulation, the relation between the volume of water required per square metre of covered ground surface is substantially less than the average recommended values for passive-solar applications. Another desired eect is the heating of the ground. Many systems usually tend to provide heating only to the greenhouse air, which is justi®ed because the temperature of the aerial part of the plant aects plant growth and development with higher intensity. But, some physiological problems in winter cultivation of horticultural plants are caused by inadequate thermal conditions of the ground, which limit the nutrients absorption processes. Moreover, the ground can work as a radiant surface emitting heat to the greenhouse ambient during night.
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Internal thermal curtains have proved to be ecient for decreasing night heat losses [4] and dierent materials have been tested and used. Although it is recommended to have the curtains opened during daytime and closed at night [5], a thin polyethylene ®lm was used for these experiments and, in order to avoid ®rst costs and operational costs, the thermal curtains were static. In addition to the results of 3 years of experimentation, greenhouse behaviour estimations have been made using a 15-year weather database [6,7] corresponding to the southern and central part of CoÂrdoba province. The described two elements proved to be highly convenient, which was shown by temperature measurements, agronomic analysis and economic results. It is worth noting that the farm's owner adopted the system in all his greenhouses during the 1997 campaign. Other bene®cial consequences of this type of system are the environment preservation and the saving of fossil fuels. 2. System description The experiences described in this paper were developed on a farm called SIQUEM, which works with greenhouses. SIQUEM and the National University of RõÂo Cuarto (UNRC) have signed an agreement by means of which they commit themselves to work jointly in this type of project, where the farm basically provides some materials and the standard greenhouse labour and the University does research work using its measurement equipment and the materials to be tested. The farm is situated 10 km away from the UNRC, 33.2 S latitude and 64.3 W longitude, and has a low-temperature geothermal source [8], which provides underground water at 28 C. The water ¯ows freely and no pump is necessary for the farm consumption requirements. The ``chapel'' greenhouses used in the tests are typical constructions in the central part of Argentina: they are made of wood beams and polyethylene walls and roof. The tests were undertaken for one of the main production greenhouses (1000 m2) and on a prototype built by the UNRC (105 m2). Cheap common materials were selected for the heating system and a simple construction methodology was used to build it, in order to minimise raw material costs and labour hours. The tubes, made of 200 mm thick black low-density polyethylene, have a diameter of 10 cm. They were placed on the greenhouse ground, between furrows, trying not to obstruct the employees' pathway, as shown in Fig. 1. The tubes were connected in series with only one inlet and one outlet for each greenhouse. The position of the tubes allows heat transmission to the air and the ground simultaneously. In normal operation, the water ¯ow is controlled manually, opening the valves before sunset, allowing the water to ¯ow through the greenhouse tubes, and closing the valves in the middle of the next morning, when the Sun begins to warm the greenhouse. The greenhouse exit water is then used for watering in other farm activities. Data acquisition systems were used to measure and record dierent variables, mainly temperatures, every 15 min, during the winter season. Other variables, like
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Fig. 1. (a) Section of the greenhouse prototype; (b) top view of the greenhouse prototype.
relative humidity and some temperatures, were measured by means of a portable weather station placed inside the greenhouse, whereas water ¯ow rate and water pressure were determined manually. 3. Experimental methodology and results 3.1. First experiments The ®rst test was made in the 1995 winter period [9], in order to analyse the possibility of heating by means of this system and to check the behaviour of some local cheap materials. In order to estimate the possible heat transfer from the plastic tubes to the air and the ground, it was necessary to determine the heat-transfer coecient of the tube in normal operating conditions. The calculation of this heat transfer coecient was made in a laboratory in the National University of San Luis (UNSL). In one of the main greenhouses, a tube was placed between furrows to check its behaviour. The water ¯ow was ®xed at 0.0045 m3/s, and heat ¯uxes were calculated using temperature data of dierent days for the greenhouse inner air, external ambient air, greenhouse ground and water inlet and outlet. Table 1 shows some results for one typical winter day. After this ®rst winter season, the physical condition of the tubes was checked. The most important conclusions of this ®rst experience were: (a) When the inner temperatures were low, the tubes transferred a lot of energy to the greenhouse ambient air and ground, which indicated it was possible to warm the greenhouses during winter nights in order to prevent freezing and so improve productivity. (b) The tubes material did not suer deterioration, which allowed it to be used for several winter seasons
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Table 1 First test results for a typical winter's day Total available energy (MJ) Energy transferred to the air (MJ) Energy transferred to the ground (MJ) Unused energy (MJ) Greenhouse mean temperature ( C) Greenhouse maximum temperature ( C) Greenhouse minimum temperature ( C) Water mean temperature ( C) External ambient mean temperature ( C) Max. external ambient temperature ( C) Min. external ambient temperature ( C) Global eciency of the system (%)
461.0 53.9 9.7 397.3 19 31 12 27 16 28 10 14
3.2. Evaluation of a prototype The next step was to prepare a prototype [10], which was furnished with a system of tubes, as shown in Fig. 1. For obvious economic reasons, this prototype (105 m2) was smaller than a conventional greenhouse (1000 m2). The tubes were connected in series and a total length of 60 m of tubes was used in this greenhouse prototype. The water ¯owed through the prototype from 5:00 p.m. until 9:00 a.m. of the next day. A thermal curtain was also used to improve the prototype night energy-conservation. Since its main function was to decrease heat losses by convection and in®ltration, it was made of 50 mm thick transparent polyethylene, with no special optical properties. The thermal behaviour of this prototype was compared with the thermal behaviour of a typical greenhouse in normal production without a system of tubes or thermal curtain. This second test series began in February 1996. The same variety of green pepper was planted both in the prototype and the typical greenhouse. The data were collected using a set of sensors distributed in the prototype and in the conventional greenhouse, and connected to a personal computer, which was placed beside the prototype, protected by a masonry construction. The power delivered from the geothermal system was calculated and the energy given to the air, the ground, and unused energy, were considered separately. Temperatures and powers delivered in the prototype from the tubes for a cold winter day are shown in Table 2. A comparison of the ambient temperature, conventional greenhouse air-temperature, and prototype air-temperature, for the same day previously mentioned, is shown in Fig. 2. Fig. 3 represents the inlet and outlet water temperatures and the prototype air-temperature. The most salient event occurred in May 1996, when the ®rst freezing days took place. In the conventional greenhouse, 70% of the total plantation was destroyed, whereas there was no damage to any plant in the prototype. The minimal ambient temperature registered during this winter season was ÿ6 C, the air temperature in
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Table 2 Results for the prototype greenhouse during one cold-winter's day Water mass ¯ow rate Inlet water mean temperature ( C) Outlet water mean temperature ( C) Mean power provided (kW) Max. Power provided (kW) Mean heat power available (kW) System's eciency (%) Energy transferred to the prototype (MJ)
0.25 20 15 5.2 6.1 13.6 37.6 240
Fig. 2. Comparison among air temperatures of prototype, conventional greenhouse, and external ambient environmental for a cold-winter's day.
Fig. 3. Water temperatures at tube inlet and outlet and air prototype temperature for a cold-winter's day.
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the typical greenhouse reached ÿ3 C, but the minimum temperature registered in the prototype greenhouse was 1 C. Moreover, not only plant freezing was prevented, but also out-of-season production was obtained. This caused the prototype production to be sold at a price ®ve times higher than for the normal season's production. 3.3. Comparison among dierent systems In order to determine the best economic solution for freezing prevention, an experimental greenhouse (315 m2 ¯oor area) was divided into three equal-sized sectors and three dierent systems were analysed during the 1997 campaign [11]: . Sector 1: greenhouse only with tubes. . Sector 2: greenhouse only with thermal curtain. . Sector 3: greenhouse with tubes and thermal curtain. Following the same procedure of the previous tests, the temperatures in dierent parts and water ¯ows were measured for the corresponding calculations. Tomatoes were planted in the three sectors and, in addition to the temperature measurements and thermal calculations, the responses of the cultivation were analysed. The sectors' temperatures for one cold winter day are shown in Fig. 4, and the plant situation in 14 August 1997 is shown in Table 3. 4. Analysis and discussion Although the opening and closing operations of the valves for the water-¯ow control were manually performed, it did not imply extra costs since farmers normally
Fig. 4. Comparison among air temperatures in sectors 1, 2 and 3, and the external ambient environment for a cold-winter's day.
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Table 3 Plant developments by 14 August 1997
Number of replaced plants Average height of plants (cm) Average number of leaves per plant
Sector 1
Sector 2
Sector 3
48 23.9 6.9
42 29 6.65
36 32.7 6.7
worked in the greenhouses during that time, performing other typical farm tasks. This operation demanded just a few minutes per day. Since the water ¯ows clean through the tubes system and at approximately 26 C, the system is not damaged by fouling or high temperatures; so, it does not require maintenance or incur interruptions. Some minor problems of small perforations occurred due to the normal work of the farmers near the tubes, which were immediately solved by sealing the holes. The exit water from the tubes was used for watering purposes in other sectors of the farm, which was necessary because of winter droughts. The second test shows a higher daytime temperature for the conventional greenhouse compared with the prototype's temperature, as shown in Fig. 2. This happened because the parameter ``external cover surface/covered terrain'' was greater for the conventional greenhouse. But, in spite of this circumstance, the results clearly show that night temperatures determine the plant growth. Then, if tube systems and thermal curtains were applied to a typical greenhouse, the same daytime temperatures and better ®nal results in plant protection and net out-of-season production, would be achieved. On sunny days, the air temperature inside the greenhouse prototype was higher than the tube's water temperature, and during daytime the water left the prototype at a higher temperature than that at its entrance. Then, water circulation could be combined with ventilation to prevent overheating. Daytime temperature was almost the same all the days in each sector of the third experiment, when the greenhouses were partially opened for ventilation, and the dierences among the sectors show that the night temperature is a decisive factor in the plant growth acceleration. Since the thermal curtain was transparent and very thin, it did not cause a signi®cant decrease of the solar-radiation absorption in the greenhouses. 5. Conclusion Thermal analysis and agronomic results showed that, for this particular climate, the combination of tubes with water from a geothermal source and internal thermal curtains provides excellent conditions for preventing freezing and producing out-ofseason crops. The application of only one of the previous improvements is not advisable in this region, because freezing prevention cannot be guaranteed. The availability of a geothermal source of heat is the only requirement for installing this
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type of system. Fortunately, many farms in the region have similar, and sometimes much better, geothermal sources. To conclude, the following advantages apply: (a) It is not necessary to make a perforation to look for the geothermal source, because the farm must always have a well for consumption and irrigation and the same water can be used for the heating system. (b) Although the eciency is low, due to the high exit temperature, the water is at no cost because it ¯ows freely and no pumps are required. (c) The greenhouse's exit-water represents neither an elimination problem nor a waste, because during winter periods watering is necessary in other open sectors of the farm. (d) Materials for tubes, accessories, polyethylene, etc., are cheap common market materials. (d) It is really easy to assemble the complete system. The connections of the tubes and valves require only a few working hours. (f) The operation and maintenance of the system are simple and low time demanding. References [1] Albright LD. 1991. Production solar greenhouses. In: Parker BF, editor. Solar energy in agriculture. New York: Elsevier, 1991. [2] ASHRAE. Ashrae handbook, fundamentals. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineering, 1993. [3] Santamouris M, Balaras CA, Dascalaki E, Vallindras M. Passive-solar agricultural greenhouses: a worldwide classi®cation and evaluation of technologies and systems used for heating purposes. Solar Energy 1994;53(5):414±26. [4] Chandra P, Albright LD. Analytical determination of the eect on greenhouse heating requirements of using night curtains. Transactions of the ASAE 1980;23(4):994±1000. [5] Seginer I, Albright LD. Rational operation of greenhouse thermal-curtains. Transactions of the ASAE 1980;23(5):1240±5. [6] Fasulo A, Barral J, Adaro A, Lema A. Variables ClimaÂticas de la RegioÂn Centro Sur de CoÂrdoba, Estado de Avance, AsociacioÂn Argentina de EnergõÂa Solar. Proceedings of ASADES 1994;94(2):503± 7. [7] Galimberti P, Adaro J, Barral J, Lema A, Fasulo A. 1995. Variables ClimaÂticas de la RegioÂn Centro Sur de CoÂrdoba, AsociacioÂn Argentina de EnergõÂa Solar. Proceedings of ASADES 95(1): 04.67± 04.72 [8] ASHRAE. Ashrae handbook, HVAC applications. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineering, 1995. [9] Adaro J, Lema A, Galimberti P, Schneider M, Fasulo A, Iriarte A. Aporte GeoteÂrmico para CalefaccioÂn de Invernaderos: Ensayos Preliminares, AsociacioÂn Argentina de EnergõÂa Solar. Proceedings of ASADES 1995;95(1):01.45±1.50. [10] Adaro J, Galimberti P, Lema A, Barone A, Fasulo A, Iriarte A. Aprovechamiento GeoteÂrmico en la CalefaccioÂn de Invernaderos, AsociacioÂn Argentina de EnergõÂa Solar. Proceedings of ASADES 1996;96(1):01.49±1.52. [11] Galimberti P, Adaro J, Lema A, Barone A, Grosso L, Fasulo A. Estudio Comparativo de Diferentes Mejoras en Invernaderos. Avances en EnergõÂas Renovables y Medio Ambiente 1997;1(1):21±4.
www.elsevier.com/locate/apenergy
Geothermal contribution to greenhouse heating Jorge A. Adaro, Pablo D. Galimberti, Alba I. Lema, AmõÂlcar Fasulo, Jorge R. Barral* Facultad de IngenierõÂa, Universidad Nacional de RõÂo Cuarto, Ruta Nacional 36 km 601, 5800 RõÂo Cuarto, Argentina
Abstract Plant freezing and plant-growth inhibition are among the major problems in greenhouse cultivation in the central part of Argentina. The possibility of using a constant temperature underground geothermal water source, which ¯ows naturally, has been studied as an economic option to solve these problems. A system of heating by means of geothermal energy, with energy-conservation measures, was designed and evaluated for typical production greenhouses in the southern part of CoÂrdoba, Argentina. The results of tests carried out during 3 years are presented. These results are really promising, taking into account the high bene®t/cost relation of the design and the availability of similar geothermal resources in many farms of this region. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Economics; Geothermal; Greenhouse; Heating; Horticulture; Out-of-season production; Thermal curtain
1. Introduction Good plant-growth conditions can be achieved by means of using greenhouses. A greenhouse can be managed to protect the plants by creating a favourable environment, allowing intensive use of soil, and helping sanitary plant control. From an economic point of view, the main objective of horticultural greenhouses is to advance the normal season production or to obtain a completely out-of-season production, which corresponds with higher crop prices.
* Corresponding author. Tel.: +54-358-467-6246; fax: +54-358-676-6246. E-mail address: [email protected] (J.R. Barral). 0306-2619/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0306-2619(99)00049-5
242
J.A. Adaro et al. / Applied Energy 64 (1999) 241±249
Many variables must be controlled in order to provide the good environmental conditions previously mentioned. The most important parameters to be controlled inside a greenhouse are temperature, humidity and light [1]. Most winter days in the south of CoÂrdoba are dry and sunny, which implies a low relative humidity and a moderate temperature during daytime. But, at night, the temperature descends usually to only a few degrees above 0 C and sometimes to below 0 C. These low temperatures can cause the freezing of greenhouse plants. Low ambient humidity, together with moderate daytime temperatures, allow one to minimise humidity-control problems. Generally the lateral walls of greenhouses are partially opened during the daytime to provide ventilation. This is possible because of the moderate ambient temperature. Therefore, before closing these walls, when preparing the greenhouse for a cold night, the air in the greenhouse has almost the same relative humidity as the external ambient air has, which means it is dry, and, although this air is con®ned inside the greenhouse during the night, it takes time for the air to reach a high water-vapour content; and, if this happens, ventilation takes place the next day. Moreover, the photo-period for the horticultural products used, matches, or is indierent to the light duration in winter days at this latitude. Consequently, only the temperature at night appears as an important critical variable to be controlled. Although dierent species are cultivated, the requirement is always the same: the avoidance of low temperatures at night. The conventional solution for this problem is the burning of some fossil fuel inside the greenhouse during dangerous freezing nights. Since fuel prices are high, this option is very expensive, but, it is necessary, considering the possibility of the complete destruction of plants. Although total destruction can be avoided, the low night temperatures delay or interrupt the plant growth [2], which results in low production and inferior quality production during the cold season, when the price of the product reaches its highest value. A system of polyethylene tubes placed on the ground and ®lled with warm water from a geothermal source was used experimentally on a farm to provide night heating to greenhouses. In addition, permanent thermal curtains were used as a ceiling to decrease the night heat-losses. Water as the heat-storage medium has been widely used in passive solar greenhouse applications [3]. In the present experiences, although the principle used is the same, the source of energy is now the warm water of a geothermal source instead of the Sun. Then, due to the continuous water circulation, the relation between the volume of water required per square metre of covered ground surface is substantially less than the average recommended values for passive-solar applications. Another desired eect is the heating of the ground. Many systems usually tend to provide heating only to the greenhouse air, which is justi®ed because the temperature of the aerial part of the plant aects plant growth and development with higher intensity. But, some physiological problems in winter cultivation of horticultural plants are caused by inadequate thermal conditions of the ground, which limit the nutrients absorption processes. Moreover, the ground can work as a radiant surface emitting heat to the greenhouse ambient during night.
J.A. Adaro et al. / Applied Energy 64 (1999) 241±249
243
Internal thermal curtains have proved to be ecient for decreasing night heat losses [4] and dierent materials have been tested and used. Although it is recommended to have the curtains opened during daytime and closed at night [5], a thin polyethylene ®lm was used for these experiments and, in order to avoid ®rst costs and operational costs, the thermal curtains were static. In addition to the results of 3 years of experimentation, greenhouse behaviour estimations have been made using a 15-year weather database [6,7] corresponding to the southern and central part of CoÂrdoba province. The described two elements proved to be highly convenient, which was shown by temperature measurements, agronomic analysis and economic results. It is worth noting that the farm's owner adopted the system in all his greenhouses during the 1997 campaign. Other bene®cial consequences of this type of system are the environment preservation and the saving of fossil fuels. 2. System description The experiences described in this paper were developed on a farm called SIQUEM, which works with greenhouses. SIQUEM and the National University of RõÂo Cuarto (UNRC) have signed an agreement by means of which they commit themselves to work jointly in this type of project, where the farm basically provides some materials and the standard greenhouse labour and the University does research work using its measurement equipment and the materials to be tested. The farm is situated 10 km away from the UNRC, 33.2 S latitude and 64.3 W longitude, and has a low-temperature geothermal source [8], which provides underground water at 28 C. The water ¯ows freely and no pump is necessary for the farm consumption requirements. The ``chapel'' greenhouses used in the tests are typical constructions in the central part of Argentina: they are made of wood beams and polyethylene walls and roof. The tests were undertaken for one of the main production greenhouses (1000 m2) and on a prototype built by the UNRC (105 m2). Cheap common materials were selected for the heating system and a simple construction methodology was used to build it, in order to minimise raw material costs and labour hours. The tubes, made of 200 mm thick black low-density polyethylene, have a diameter of 10 cm. They were placed on the greenhouse ground, between furrows, trying not to obstruct the employees' pathway, as shown in Fig. 1. The tubes were connected in series with only one inlet and one outlet for each greenhouse. The position of the tubes allows heat transmission to the air and the ground simultaneously. In normal operation, the water ¯ow is controlled manually, opening the valves before sunset, allowing the water to ¯ow through the greenhouse tubes, and closing the valves in the middle of the next morning, when the Sun begins to warm the greenhouse. The greenhouse exit water is then used for watering in other farm activities. Data acquisition systems were used to measure and record dierent variables, mainly temperatures, every 15 min, during the winter season. Other variables, like
244
J.A. Adaro et al. / Applied Energy 64 (1999) 241±249
Fig. 1. (a) Section of the greenhouse prototype; (b) top view of the greenhouse prototype.
relative humidity and some temperatures, were measured by means of a portable weather station placed inside the greenhouse, whereas water ¯ow rate and water pressure were determined manually. 3. Experimental methodology and results 3.1. First experiments The ®rst test was made in the 1995 winter period [9], in order to analyse the possibility of heating by means of this system and to check the behaviour of some local cheap materials. In order to estimate the possible heat transfer from the plastic tubes to the air and the ground, it was necessary to determine the heat-transfer coecient of the tube in normal operating conditions. The calculation of this heat transfer coecient was made in a laboratory in the National University of San Luis (UNSL). In one of the main greenhouses, a tube was placed between furrows to check its behaviour. The water ¯ow was ®xed at 0.0045 m3/s, and heat ¯uxes were calculated using temperature data of dierent days for the greenhouse inner air, external ambient air, greenhouse ground and water inlet and outlet. Table 1 shows some results for one typical winter day. After this ®rst winter season, the physical condition of the tubes was checked. The most important conclusions of this ®rst experience were: (a) When the inner temperatures were low, the tubes transferred a lot of energy to the greenhouse ambient air and ground, which indicated it was possible to warm the greenhouses during winter nights in order to prevent freezing and so improve productivity. (b) The tubes material did not suer deterioration, which allowed it to be used for several winter seasons
J.A. Adaro et al. / Applied Energy 64 (1999) 241±249
245
Table 1 First test results for a typical winter's day Total available energy (MJ) Energy transferred to the air (MJ) Energy transferred to the ground (MJ) Unused energy (MJ) Greenhouse mean temperature ( C) Greenhouse maximum temperature ( C) Greenhouse minimum temperature ( C) Water mean temperature ( C) External ambient mean temperature ( C) Max. external ambient temperature ( C) Min. external ambient temperature ( C) Global eciency of the system (%)
461.0 53.9 9.7 397.3 19 31 12 27 16 28 10 14
3.2. Evaluation of a prototype The next step was to prepare a prototype [10], which was furnished with a system of tubes, as shown in Fig. 1. For obvious economic reasons, this prototype (105 m2) was smaller than a conventional greenhouse (1000 m2). The tubes were connected in series and a total length of 60 m of tubes was used in this greenhouse prototype. The water ¯owed through the prototype from 5:00 p.m. until 9:00 a.m. of the next day. A thermal curtain was also used to improve the prototype night energy-conservation. Since its main function was to decrease heat losses by convection and in®ltration, it was made of 50 mm thick transparent polyethylene, with no special optical properties. The thermal behaviour of this prototype was compared with the thermal behaviour of a typical greenhouse in normal production without a system of tubes or thermal curtain. This second test series began in February 1996. The same variety of green pepper was planted both in the prototype and the typical greenhouse. The data were collected using a set of sensors distributed in the prototype and in the conventional greenhouse, and connected to a personal computer, which was placed beside the prototype, protected by a masonry construction. The power delivered from the geothermal system was calculated and the energy given to the air, the ground, and unused energy, were considered separately. Temperatures and powers delivered in the prototype from the tubes for a cold winter day are shown in Table 2. A comparison of the ambient temperature, conventional greenhouse air-temperature, and prototype air-temperature, for the same day previously mentioned, is shown in Fig. 2. Fig. 3 represents the inlet and outlet water temperatures and the prototype air-temperature. The most salient event occurred in May 1996, when the ®rst freezing days took place. In the conventional greenhouse, 70% of the total plantation was destroyed, whereas there was no damage to any plant in the prototype. The minimal ambient temperature registered during this winter season was ÿ6 C, the air temperature in
246
J.A. Adaro et al. / Applied Energy 64 (1999) 241±249
Table 2 Results for the prototype greenhouse during one cold-winter's day Water mass ¯ow rate Inlet water mean temperature ( C) Outlet water mean temperature ( C) Mean power provided (kW) Max. Power provided (kW) Mean heat power available (kW) System's eciency (%) Energy transferred to the prototype (MJ)
0.25 20 15 5.2 6.1 13.6 37.6 240
Fig. 2. Comparison among air temperatures of prototype, conventional greenhouse, and external ambient environmental for a cold-winter's day.
Fig. 3. Water temperatures at tube inlet and outlet and air prototype temperature for a cold-winter's day.
J.A. Adaro et al. / Applied Energy 64 (1999) 241±249
247
the typical greenhouse reached ÿ3 C, but the minimum temperature registered in the prototype greenhouse was 1 C. Moreover, not only plant freezing was prevented, but also out-of-season production was obtained. This caused the prototype production to be sold at a price ®ve times higher than for the normal season's production. 3.3. Comparison among dierent systems In order to determine the best economic solution for freezing prevention, an experimental greenhouse (315 m2 ¯oor area) was divided into three equal-sized sectors and three dierent systems were analysed during the 1997 campaign [11]: . Sector 1: greenhouse only with tubes. . Sector 2: greenhouse only with thermal curtain. . Sector 3: greenhouse with tubes and thermal curtain. Following the same procedure of the previous tests, the temperatures in dierent parts and water ¯ows were measured for the corresponding calculations. Tomatoes were planted in the three sectors and, in addition to the temperature measurements and thermal calculations, the responses of the cultivation were analysed. The sectors' temperatures for one cold winter day are shown in Fig. 4, and the plant situation in 14 August 1997 is shown in Table 3. 4. Analysis and discussion Although the opening and closing operations of the valves for the water-¯ow control were manually performed, it did not imply extra costs since farmers normally
Fig. 4. Comparison among air temperatures in sectors 1, 2 and 3, and the external ambient environment for a cold-winter's day.
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J.A. Adaro et al. / Applied Energy 64 (1999) 241±249
Table 3 Plant developments by 14 August 1997
Number of replaced plants Average height of plants (cm) Average number of leaves per plant
Sector 1
Sector 2
Sector 3
48 23.9 6.9
42 29 6.65
36 32.7 6.7
worked in the greenhouses during that time, performing other typical farm tasks. This operation demanded just a few minutes per day. Since the water ¯ows clean through the tubes system and at approximately 26 C, the system is not damaged by fouling or high temperatures; so, it does not require maintenance or incur interruptions. Some minor problems of small perforations occurred due to the normal work of the farmers near the tubes, which were immediately solved by sealing the holes. The exit water from the tubes was used for watering purposes in other sectors of the farm, which was necessary because of winter droughts. The second test shows a higher daytime temperature for the conventional greenhouse compared with the prototype's temperature, as shown in Fig. 2. This happened because the parameter ``external cover surface/covered terrain'' was greater for the conventional greenhouse. But, in spite of this circumstance, the results clearly show that night temperatures determine the plant growth. Then, if tube systems and thermal curtains were applied to a typical greenhouse, the same daytime temperatures and better ®nal results in plant protection and net out-of-season production, would be achieved. On sunny days, the air temperature inside the greenhouse prototype was higher than the tube's water temperature, and during daytime the water left the prototype at a higher temperature than that at its entrance. Then, water circulation could be combined with ventilation to prevent overheating. Daytime temperature was almost the same all the days in each sector of the third experiment, when the greenhouses were partially opened for ventilation, and the dierences among the sectors show that the night temperature is a decisive factor in the plant growth acceleration. Since the thermal curtain was transparent and very thin, it did not cause a signi®cant decrease of the solar-radiation absorption in the greenhouses. 5. Conclusion Thermal analysis and agronomic results showed that, for this particular climate, the combination of tubes with water from a geothermal source and internal thermal curtains provides excellent conditions for preventing freezing and producing out-ofseason crops. The application of only one of the previous improvements is not advisable in this region, because freezing prevention cannot be guaranteed. The availability of a geothermal source of heat is the only requirement for installing this
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type of system. Fortunately, many farms in the region have similar, and sometimes much better, geothermal sources. To conclude, the following advantages apply: (a) It is not necessary to make a perforation to look for the geothermal source, because the farm must always have a well for consumption and irrigation and the same water can be used for the heating system. (b) Although the eciency is low, due to the high exit temperature, the water is at no cost because it ¯ows freely and no pumps are required. (c) The greenhouse's exit-water represents neither an elimination problem nor a waste, because during winter periods watering is necessary in other open sectors of the farm. (d) Materials for tubes, accessories, polyethylene, etc., are cheap common market materials. (d) It is really easy to assemble the complete system. The connections of the tubes and valves require only a few working hours. (f) The operation and maintenance of the system are simple and low time demanding. References [1] Albright LD. 1991. Production solar greenhouses. In: Parker BF, editor. Solar energy in agriculture. New York: Elsevier, 1991. [2] ASHRAE. Ashrae handbook, fundamentals. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineering, 1993. [3] Santamouris M, Balaras CA, Dascalaki E, Vallindras M. Passive-solar agricultural greenhouses: a worldwide classi®cation and evaluation of technologies and systems used for heating purposes. Solar Energy 1994;53(5):414±26. [4] Chandra P, Albright LD. Analytical determination of the eect on greenhouse heating requirements of using night curtains. Transactions of the ASAE 1980;23(4):994±1000. [5] Seginer I, Albright LD. Rational operation of greenhouse thermal-curtains. Transactions of the ASAE 1980;23(5):1240±5. [6] Fasulo A, Barral J, Adaro A, Lema A. Variables ClimaÂticas de la RegioÂn Centro Sur de CoÂrdoba, Estado de Avance, AsociacioÂn Argentina de EnergõÂa Solar. Proceedings of ASADES 1994;94(2):503± 7. [7] Galimberti P, Adaro J, Barral J, Lema A, Fasulo A. 1995. Variables ClimaÂticas de la RegioÂn Centro Sur de CoÂrdoba, AsociacioÂn Argentina de EnergõÂa Solar. Proceedings of ASADES 95(1): 04.67± 04.72 [8] ASHRAE. Ashrae handbook, HVAC applications. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineering, 1995. [9] Adaro J, Lema A, Galimberti P, Schneider M, Fasulo A, Iriarte A. Aporte GeoteÂrmico para CalefaccioÂn de Invernaderos: Ensayos Preliminares, AsociacioÂn Argentina de EnergõÂa Solar. Proceedings of ASADES 1995;95(1):01.45±1.50. [10] Adaro J, Galimberti P, Lema A, Barone A, Fasulo A, Iriarte A. Aprovechamiento GeoteÂrmico en la CalefaccioÂn de Invernaderos, AsociacioÂn Argentina de EnergõÂa Solar. Proceedings of ASADES 1996;96(1):01.49±1.52. [11] Galimberti P, Adaro J, Lema A, Barone A, Grosso L, Fasulo A. Estudio Comparativo de Diferentes Mejoras en Invernaderos. Avances en EnergõÂas Renovables y Medio Ambiente 1997;1(1):21±4.