Design and technology for greenhouse cooling in tropical and subtropical regions- A review

ReviewDesign and technology for greenhouse cooling in tropical and subtropical regions: A reviewK.S.Kumar*,K.N.Tiwari,Madan K.JhaIndian Institute of Technology,Kharagpur721302,IndiaContents1.Introduction (1270)2.Greenhouse design and functional characteristics (1270)2.1.Design (1270)2.2.Orientation and shape (1270)2.3.Height (1271)2.4.Covering materials (1271)2.5.Ventilation area (1271)2.6.Insect-proof nets (1271)3.Greenhouse cooling technologies (1271)3.1.Natural ventilation (1272)3.2.Evaporative cooling (1272)3.2.1.Fan-pad system (1272)3.2.2.Fog/mist system (1273)3.2.3.Roof evaporative cooling (1273)4.Shading (1273)posite system (1273)5.1.Earth-to-air heat exchange system (1273)6.Greenhouse design and cooling approach in tropical and subtropical regions (1274)7.Conclusions and recommendations (1274)8.Research gaps for future research (1274)References (1274)Energy and Buildings41(2009)1269–1275A R T I C L E I N F OArticle history:Received26July2009Accepted10August2009Keywords:GreenhouseStructural designNatural ventilationEvaporative coolingNear infrared radiationFar infrared radiationInsect-proof netsEarth-to-air heat exchange systemA B S T R A C TGreenhouse technology is a viable option for sustainable crop production in the regions of adverseclimatic conditions.High summer temperature is a major setback for successful greenhouse cropproduction throughout year.The main intent of the paper is to present a comprehensive review on thedesign and technology for cooling of greenhouse during summer months.Effect of characteristic designparameters on greenhouse microclimate and the applicable cooling technologies have been discussed.Adetailed survey of literature revealed that,apart from cooling,studies on greenhouse design,evaluationof new cladding materials for greenhouse covering and natural ventilation with respect to local climateand agronomic condition is necessary to achieve desirable benefits.Analysis of the earlier studiesrevealed that a naturally ventilated greenhouse with larger ventilation areas(15–30%),provided at theridge and sides covered with insect-proof nets of20–40mesh size with covering material properties ofNIR(near infrared radiation)reflection during the day and FIR(far infrared radiation)reflection duringnight is suitable for greenhouse production throughout year in tropical and subtropical regions.Based onthe review,salient areas in need of further research are focused.ß2009Elsevier B.V.All rights reserved.*Corresponding author at:Agricultural and Food Engineering Department,Indian Institute of Technology,Kharagpur721302,India.Tel.:+913222283151/919735370213 (M);fax:+913222282243.E-mail address:kaduluru@yahoo.co.in(K.S.Kumar).Contents lists available at ScienceDirectEnergy and Buildingsj o u r n a l ho m e p a g e:w w w.e l s e v i e r.c o m/l o ca t e/e nb u i l d0378-7788/$–see front matterß2009Elsevier B.V.All rights reserved.doi:10.1016/j.enbuild.2009.08.0031.IntroductionGreenhouse technology is a breakthrough in the agriculture production technology that integrates market driven quality parameters with production system profits.Cultivation of crops in greenhouse is increasing from high altitude and temperate regions to the warmer regions of tropics and subtropics.Although, greenhouse protects crops from external bad weather,high temperature and humidity during summer months cause adverse effect on crop production in tropical region[1].Hence,in such regions,reduction of air temperature inside the greenhouse or the regulation of temperature closer to the ambient temperature during summer is necessary for successful crop production.Cooling is considered as the basic necessity for greenhouse crop production in tropical and subtropical regions to overcome the problems of high temperatures during summer months.Devel-opment of suitable cooling system that provides congenial microclimate for crop growth is a difficult task as the design is closely related to the local environmental conditions.Moreover, selection of appropriate technology for cooling depends on the choice of the crops to be grown,maintenance,ease of operation and economic viability[2].Hence,understanding the physical processes(geometrical and thermal)of the greenhouse,its quantification as a function of size,shape and external weather helps in the development of suitable cooling system(Bot,1983). Evaluation of microclimate in different designs of the greenhouse and establishing physical and physiological relationships of crops is necessary for the greenhouse designers to improve cooling system suitable for crop growth.Earlier studies on greenhouse cooling revealed that the present techniques used for cooling is not satisfactory.There is a necessity tofind suitable cooling methods for tropical climates and thus development of applicable cooling technologies is an important research endeavor[3,4].Appropriate greenhouse design(shape, dimension and roof configuration),right choice of cladding material coupled with suitable ventilation and cooling techniques address the problems of summer greenhouse production of high temperature regions[5].Unlike the studies on greenhouse climate,cooling received cursory treatment and less documented particularly in tropics and subtropics.There is a necessity to carryout in depth investigation on the design aspects of greenhouse and its functional character-istics that helps in natural cooling.In this paper,an effort has been made to critically review greenhouse design and its functional characteristics influence on microclimate.Established cooling technologies like natural ventilation,evaporative cooling,shading and composite systems and their applicable implications have been discussed with a note on research gaps in greenhouse technology.The information is useful for the researchers working on the engineering aspects of gathered greenhouse technology.2.Greenhouse design and functional characteristics2.1.DesignBrochier[6]mentioned that7mÂ54m galvanized tunnel frame work is more resistant to tornadoes(120km hÀ1wind speed)and more durable than wooden structures.He also mentioned that performance of the greenhouse with respect to major microclimatic parameters,i.e.temperature,light and humidity are satisfactory but gives little scientific evidence of such performances.Rault[7]reported that the greenhouse effect is desired in mild climates and the oasis effect is suitable for arid tropical climate and the umbrella effect is desired for humid tropical regions.He also stressed that none of the commonly used methods of shading,ventilation and cooling including cooling by evaporation are completely satisfactory to create favourable microclimate in greenhouse design for tropics.Rault[4]describes the performance of tunnel greenhouse with side and top ventilation under tropical low land climate in French Guyana. The author mentioned that there are still some improvements to be made in the design for its suitability to the respective climatic region.Von Zabeltitz[8]summarized the general design criteria for plasticfilm greenhouses under warm climates.Simple method of changing thefilm,insulation to the parts of the structure,increased ventilation,reduced leakage and prevention of dropping of condensed water from the roof were some of the important design considerations to be followed for greenhouse design in tropics.Von Zabeltitz[9]stated that the adoption of ventilation on roof and sides is suitable for humid tropic greenhouse and the best way to achieve air exchange is to keep the ratio of ventilation opening to thefloor area to be as large as possible.Campen[10]applied computationalfluid dynamics(CFD)technique to design the greenhouse suitable for tropical Indonesian conditions.Different greenhouse designs have been evaluated based on the ventilation rate and maximum temperature in the greenhouse and reported that the design without opening in the top has the highest ventilation rate and lowest maximum temperature in case of wind due to aerodynamics.Luo et al.[11]stated that adaptation of greenhouse climate management strategy to local climate condi-tions is very important for the improvement of resource use efficiency in crop production for subtropical summer conditions. Greenhouse Process model(KASPRO)was thus used for validation of the experimental data.The results showed that for a cucumber crop under the summer conditions,an average air exchange rate of 26volume changes per hour is required for optimal control of temperature.Leaf area Index(LAI)of4maximizes crop biomass production accounting for high net assimilation rate(NAR). Hemming et al.[12]have designed and developed a suitable tropical greenhouse for low lands of Indonesia using CFD technique.This design is equipped with a very high ventilation capacity and covered with a combination of nets and a special polythenefilm that blocks the near infrared radiation.Impron et al.[13]developed a simple greenhouse climate model for designing optimal tropical low land greenhouse based on the selection of greenhouse dimensions and the covering material properties. Greenhouse with large ventilation openings at ridge and sides with NIR reflecting cover is suitable for tomato cultivation under tropical Indonesian conditions.2.2.Orientation and shapeChandra[14],Chandra et al.[15]reported that the greenhouse with north south orientation contributed homogeneity of micro-climate for northern climatic conditions of India.Kurata[64] studied the effect of greenhouse orientation,number of spans,time of year and latitude on the direct solar radiation transmissivity into greenhouses.The effect of the above factors is less significant at low latitudes than at high latitudes.Tiwari and Gupta[16]evaluated the comparative performance of different configurations of the greenhouse with equalfloor area and central height for Indian climatic conditions.They reported that greenhouse with modified arch and modified IARI(Indian Agricultural Research Institute)shapes performed better where cooling is required and an uneven shape of greenhouse with north wall is suitable for cold places which require heating.Sethi[17] evaluated the performance of different shapes of greenhouse for optimal microclimate under Indian climatic conditions.He reported that,uneven span shape greenhouse with east–west orientation received maximum light transmission for year round greenhouse applications at all latitudes.The inside air temperatureK.S.Kumar et al./Energy and Buildings41(2009)1269–1275 1270rise depends upon the shape of the greenhouse and the variation from uneven-span shape to quonset shape is4.68C(maximum) and3.58C(daily average)at318N latitude.2.3.HeightThe average greenhouse height characterizes its volume.A large greenhouse volume results in slow response of the indoor environment to changes of the external weather conditions[18]. The current trend in greenhouse technology is towards higher greenhouses for high temperature regions[5].The current height of greenhouse up to gutter should be in the range of2.5–4.5m instead of traditional range of1.5–2m.The height up to ridge should be3m or above to obtain favourable climatic conditions for crop growth in high temperature regions[5].2.4.Covering materialsGarcia et al.[19]and Hoffmann and Waaijenberg[20]reported that,new cool plasticfilms having special cover properties of near infrared reflection during the day and far infrared reflection during night maintained favourable microclimate for crop growth in tropical regions.Arcidiacono et al.[21]compared the effect of different greenhouse covering materials(a polyethylenefilm,an insect-proof net and a photo-selective red coloredfilm)on the microclimate of greenhouse for tomato cultivation during sum-mer.The greenhouse covered with the insect-proof net maintained the temperatures similar to those recorded outside while the photo-selectivefilm was not effective to control temperature rise during summer.Shen and Yu[22]reported that the best cooling method for greenhouses in tropical region is ventilation,fans and with covering materials having near infrared reflection.Runkle et al.[23]reported the environmental conditions under multi-layered near infrared reflectingfilm(NIR)with minimum reduction of photo synthetically active radiation(PAR)for modulating temperature under subtropical environment.For a well ventilated greenhouse,the near infrared reflecting(NIR)film could be an attractive shading material for growers who desire to produce crops under high light conditions.Bilal et al.[24]studied the effects of different coveringfilms like ultraviolet stabilized, infrared absorbed,single layered and double layered polyethylene films on growth and productivity of aubergine.Thefinal yield of plants grown in double layer polythene cover were higher among others and where as light transmission was highest in single layer polythene,intermediate in ultraviolet stabilized and infrared absorbed and the lowest in double layered poly houses.The plants in double layered greenhouse grew faster(more leaves and flowers)than others.2.5.Ventilation areaKozai et al.[25]conducted an integrated study on the effect of ventilation area,vent opening angle,wind speed,wind angle and on air exchange rate and temperature rise in tropical climatic conditions.Air exchange rate in single span greenhouse did not vary remarkably but with continuous increase in wind velocity,air exchange rate increased linearly with wind speed.Von Zabeltitz[9] reported that side vents could be opened as much as possible but the roof vent opening should be limited to prevent entry of rain during rainy season.The volume/floor ratio of greenhouse should be as large as possible if local wind speed is not too high to maintain favourable environment for crop growth.The combined sidewall vent area should be equal to the combined ridge vent area and each should be at least15–20%of thefloor area of the greenhouse for tropical conditions[26].Connellan[27]reported that in a naturally ventilated greenhouse,the minimum ventilation opening area of20%of the greenhousefloor area be maintained so that greenhouse temperature will be nearer to the external ambient temperatures for higher temperature regions.Kamar-uddin et al.[28]reported that the arrangement of ventilation opening by more than40%of the surfacefloor area was sufficient enough to get good ventilation rate and avoid extreme tempera-ture rise in the greenhouse for tropical regions.Montero et al.[65] reported that better microclimatic conditions were achieved when size of the sidewall openings was33%of thefloor area.Albright [29]reported that inside temperatures of greenhouse do not begin to approach to ambient air temperatures until ridge and side area is more than10%of thefloor area.Campen[10]reported that the greenhouse design which has a large opening area for ventilation (40.4%of greenhouse surface area)with aerodynamic top opening maintained optimal conditions for crop growth under Indonesian conditions.Hermanto et al.[39]optimized the greenhouse ventilation area in a naturally ventilated greenhouse for tomato production in humid tropics.Ventilation area of60%of greenhouse floor area provided at ridge and sides maintained favourable greenhouse environment throughout year.2.6.Insect-proof netsSoni et al.[30]studied the effect of screen mesh size on the vertical temperature distribution of the greenhouse under natural ventilation in humid tropical climate.With decrease in porosity of the screen,increase in temperature was5–10%.Ajwang and Tantau [31]reported that an anti-thrips screen with a discharge coefficient of0.22resulted in a temperature increase up to58C above ambient conditions in a greenhouse with young plants of low transpiration rates.The increase in temperature would fall to38C in a greenhouse with a mature crop under humid tropical climatic condition.Studies on the evaluation of different greenhouse designs covered with insect-proof nets on the microclimate indicated slight variations in the ventilation rates among different designs [10].The effect of different ventilator configurations covered with nets resulted in significant variations of temperature inside the greenhouse[32].Teitel[33]reported that mounting screens on roof openings altered the air velocity near them.Peak air velocities were measured near roof openings with a17-mesh and50-mesh screens under Israel climatic conditions.Reduction of air velocities with17mesh screen is20%less than screen with50mesh size.Based on the available information on design and functional characteristics of greenhouse that influence the microclimate,it can be stated that there is no specific design suitable for tropical and subtropical regions.Optimizing the design suitable for local climate and agronomic condition is necessary to get the maximum benefit of cooling demand.Careful selection of shape,size, ventilation area and cladding material and evaluating greenhouse to the respective region and climate serves as the best cooling option in tropical and subtropical regions.3.Greenhouse cooling technologiesFontes[34]experimented with the concept of closed systems for cooling the greenhouses especially for desert climates.The results stated that for most closed system of greenhouse,a water film on the surface of the greenhouse provides the cooling effect. Mannan and Cheema[35]tested two small scale non-ventilated greenhouses in hot climate in India and obtained the measurement of inside temperatures in the summer ranging as high as55–608C. Thin layer of moving waterfilm over the roof is very effective in keeping the greenhouse cooling in summer by as much as10–128C.They did not report on the relative humidity conditions. Rault[4]considers the most commonly used cooling technologiesK.S.Kumar et al./Energy and Buildings41(2009)1269–12751271as unsatisfactory for application in the humid tropics.He stated that shading is unwanted due to limited external luminosity. Evaporative cooling systems effectiveness is reduced because of high relative humidity and cooling by refrigeration or dehumidi-fication of the atmosphere requires high investment and operating cost.From[3]and Rault[4],reported that existing techniques used for cooling the greenhouses in tropical regions are not satisfactory and represent a burden for the application of greenhouse technologies as a whole.There is a necessity tofind new,natural, cheap cooling technology for greenhouse suitable for tropical climates.Development of applicable cooling technologies is an important research endeavor.3.1.Natural ventilationNatural ventilation is the direct result of pressure differences created and maintained by wind or temperature gradients.It requires less energy and equipment and is the cheapest method of cooling a greenhouse.It depends heavily on evapo-transpirational cooling provided by the crop.In this section,research on greenhouse natural ventilation applicable to tropical and sub-tropical climates has been reviewed and presented in detail.Teitel and Tanny[36]carried out theoretical and experimental studies in a four-span greenhouse of960m2floor area in the south of Israel.Continuous vertical roof windows were used to ventilate the greenhouse.Opening the roof windows resulted in the decrease of air temperature and humidity ratio with time and approached to a steady state.Demrati et al.[66]studied the natural ventilation performance in a large scale,multi-span greenhouse for banana crop.The ventilation performance was compared with those of other types and discussed with respect to improvement in design,use of insect-proof nets and irrigation control.Teitel et al.[37]studied the effect of continuous screened side vents on the ventilation rate,temperature,humidity and air velocity distribu-tions in a naturally ventilated greenhouse.Results indicated that the ventilation rate increased linearly with wind speed.The temperature and humidity ratios were larger near the roof than near the crop while the air velocity was higher near the crop than near the roof.Ould Khaoua et al.[38]developed a two-dimensional model using computationalfluid dynamics(CFD)technique for air exchange rate measurements in a full-scale horticultural com-partmentalized glasshouse.The effect of vent arrangement on the airflow and temperature patterns was assessed.The results indicated that windward roof vent configuration generates the highest ventilation rate(11–26hÀ1).Teitel et al.[37]investigated the airflow patterns and air temperature distributions in a naturally ventilated greenhouse with vertical roof openings using computationalfluid dynamics technique.The results showed significant effect of the wind direction on theflow patterns both inside and at the roof openings.Wind direction significantly affected the ventilation rate,airflow and crop temperature distributions.Measured ventilation rates are in reasonable agreement with estimated ventilation rates predicted by a model. However,air velocities at the greenhouse openings differed from the measured values.In humid tropical climates,the effect of screen mesh size on microclimate,vertical temperature distribution and air exchange rates in naturally ventilated greenhouse was reported by Soni et al.[30]and Harmanto et al.[39].The highest temperature value was obtained at the points near to the roof which was about58C higher than the coolest point in the vertical direction.The lower60%of the height profile registered only86–92%of the maximum temperature value,while the upper40%registered92–100%.A decrease in porosity increased the vertical gradients from5to10%.A one-dimensional model to predict the microclimate inside an unheated greenhouse using external data from local meteorolo-gical station was reported by Zhang et al.[40].The predicted values are in good description with the measured values.Impron et al.[13]developed a simple model to predict the microclimate of greenhouse duly optimizing the cover properties and ventilation as main parameters for a naturally ventilated greenhouse under tropical low lands of Indonesia.The measured and predicted values are in good agreement.Patil et al.[41]investigated the variations of tropical greenhouse temperatures by using auto regressive(AR), auto regressive moving average(ARMA)and a neural network auto regressive model.(NNAR).The neural network auto regressive model performed better than other two.From the available review on natural ventilation as means of cooling,it can be concluded that wind and buoyancy effect are the main controlling agents for getting favourable microclimate and air exchange rate in tropics and subtropics.Buoyancy effect cannot be neglected completely,though its effect on natural ventilation is not prominent when the external wind speed exceeds2–2.5m sÀ1. Relatively larger ventilation openings provided at the roof and ridge can realize acceptable natural ventilation cooling in tropical and subtropical regions.3.2.Evaporative coolingEvaporative cooling is the most effective cooling method for controlling the temperature and humidity inside a greenhouse. However,its suitability is restricted to the respective region and climate as humid tropics seldom suits for its application due to high humidity levels.3.2.1.Fan-pad systemKozai and Sase[42]and Landsberg et al.[43]studied the efficiency of fan-pad system in subtropical region.They reported that for a freely transpiring crop in the glasshouse,inside air temperatures can be reduced by8–128C under high ambient temperatures and radiation intensities.Chandra et al.[44] conducted an experimental study in a24m2plastic covered greenhouse using negative pressure fan and pad system for ndsberg’s model was used to predict air temperature inside greenhouse.Decrease in inside air temperature to a range of 4–58C compared to outside condition was achieved.Evaporative cooling in a commercial greenhouse equipped with a fan and pad system during the summer period in arid countries was studied by Jamal[45].He found that the volumeflow rate of20air exchanges per hour is necessary for favourable condition in the greenhouse under dry weather conditions.Abdel-wahab[46]developed a mathematical model to estimate water evaporation rate,airflow rate and cooling effect in an evaporative cooling system for farm structures in Saudi Arabia.It was reported that covering the roof of the greenhouse with external shading would save an appreciable amount of energy and water consumption.Jain and Tiwari[47] conducted theoretical and experimental studies of fan-pad evaporative cooling system in a24m2greenhouse.They reported that the greenhouse air temperature was lowered by4–58C against ambient condition by using fan and pad system.The results of the parametric studies to optimize the fan-pad cooling parameters revealed that length of greenhouse and height of the cooling pad were found sensitive to cooling.Fuchs et al.[67] developed a procedure to evaluate the latent heat cooling by means of crop transpiration and free water evaporation from wet fan and pad system.They found that covering material property of 30%reduced solar radiation transmission at ventilation rate of30 volume exchanges per hour maintained the temperature of greenhouse with in safe limits for growing rose crop during summer.K.S.Kumar et al./Energy and Buildings41(2009)1269–1275 12723.2.2.Fog/mist systemThis system is based on spraying water as small droplets (droplet diameter of2–60m m)with high pressure nozzles.Cooling is achieved by evaporation of droplets.Fogging can also be used to increase the relative humidity apart from cooling the greenhouse.Montero et al.[68]used an air water fogging system to cool the greenhouse with shade screen of45%perforations.It was reported that maximum temperature reduction during sunny days was58C. Arbel et al.[48]tested the efficiency of the fog system with a droplet size of2–60m m in16mÂ24m greenhouse under Israel climatic conditions.The results were compared with fan and pad system.They concluded that performance of fog system was better than fan-pad system as temperature and relative humidity variations were<58C and20%,respectively.A combination of forced ventilation and fogging system for cooling greenhouses was presented by Arbel et al.[49].High pressure nozzles of more uniformity distribution coupled with fans placed at both ends were being the treatments imposed to achieve the required cooling demand.It was reported that air temperature and relative humidity of288C and80%,respectively were maintained during the summer at midday with the combination of forced ventilation and fogging.Abdel-Ghany and Kozai[50] developed a dynamic simulation model to predict the tempera-tures of air,canopy,cover andfloor surface under naturally ventilated fog cooled greenhouse.The model was validated at different fogging conditions and the results of the simulation agreed well with experimental values.3.2.3.Roof evaporative coolingRoof evaporative cooling is sprinkling of water onto a surface of the roof so as to form a thin layer of the free water surface causing increase in the evaporation rate and to fall to the wet bulb temperature of the closely surrounded air.Sodha et al.[51]stated that heatflux through the roof of greenhouse in summer period could be decreased substantially if water was evaporated on the surface of the roof by roof evaporative cooling.Cohen et al.[52]experimentally investigated the cooling efficiency of greenhouse by wetting the outer roof and inner crop soil surfaces where tomatoes were grown.They reported that wetting of roof had a smaller effect in reducing air and canopy temperatures than wetting the canopy.However,combination of both wetting treatments reduced inside air temperature by about 58C and canopy temperature by nearly78C below the ambient condition.Sutar and Tiwari[53]studied the effect offlowing water film over the roof on greenhouse air temperature in a low cost plastic greenhouse of Delhi climatic conditions,India.They concluded that inside air temperature was observed to be4–58C lower than control greenhouse.However,when a shade cloth on the roof of the greenhouse was used with waterfilm,inside air temperature drop was increased to108C.Willits and Preet[54] conducted an experiment with intermittent application of water over externally mounted shade net on greenhouse.The results revealed that rise of greenhouse air temperature was reduced by 41%under wet cloth as compared to18%under dry cloth.Ghosal et al.[69]developed a mathematical model throughflowing water film on shade cloth stretched over the roof and south wall of an even span greenhouse at Delhi,India.The model was validated with experimental data collected in three conditions,i.e.shade net with waterflow,shade net without waterflow and without shade net.The predicted results are in good description with reality.4.ShadingThe entry of unwanted radiation or light can be controlled by the use of shading.Shading does not allow important photo-synthetically active radiation(PAR)which is crucial for photo-synthesis.However,it is a viable option for crop production during peak summer.Shading paints can be applied to the outside greenhouse cover with some degree of success but paints gets washed away during the rainy season.Shade net application with different perforated mesh sizes and their evaluation with respect to local climate and region is necessary to get cooling benefits in summer.Cohen and Fuchs[55]measured the radiometric properties of various reflective shade screens used for greenhouse.These screens can be successfully used to reflect the excessive solar radiation from the greenhouse roof during extreme summer days depending upon the extent of excess radiation.Camilo et al.[56] performed a study to evaluate the use of reflective aluminized polypropylene shading nets on photosynthetic performance of citrus plants and observed reduction in photo-synthetically active radiation(PAR)levels and leaf temperatures when the reflective nets were used.Ali et al.[57]developed an efficient greenhouse design for hot climatic conditions of Kuwait.Entry of solar radiation was allowed only from the roof of the even span greenhouse of250m2floor area where tomatoes were grown.The roof was shaded with a screen mesh(55%shade).Fan-pad system was also used during peak hours to control the inside air temperature.The results showed that combination of shading and fan and pad during peak summer maintained temperature of 308C during the day and228C during night in the greenhouse. Shukla et al.[58]experimentally studied the effect of inner thermal curtain in the evaporative cooling.The results indicated that use of inner thermal curtain reduced the greenhouse temperature by 58C.posite system5.1.Earth-to-air heat exchange systemThe ground potential of the earth can also be used for cooling the greenhouses in summer due to its constant year round temperature(26–288C)by means of earth to air heat exchange system.Santamouris et al.[62]studied the performance of earth-to-air heat exchangers for energy conservation in cooling of agricultural greenhouses through a mathematical model.The results of parametric analysis illustrated that pipe length,pipe diameter and air velocity inside the pipes influenced the performance of the system considerably.Sharan et al.[63]developed earth-tube-heat-exchanger(ETHE)for controlling the hot arid environment of Kutch,India.He reported that ETHE limits the greenhouse temperature with crop up to368C with shading on top of the greenhouse.Ghosal et al.[59]developed a simplified analytical model to study round the year effectiveness of a recirculation type earth to air heat exchanger in a greenhouse located in New Delhi, India.The average temperature of greenhouse was3–48C less in summer than control greenhouse.Ghosal et al.[59]developed a thermal model to investigate the potential of using the stored thermal energy of the ground for greenhouse cooling with help of an earth to air heat exchange(EAHES).The temperatures of the greenhouse air,with the experimental parameters of EAHES were 5–68C lower in the summer than greenhouse without EAHES.Critical review on the reports of various researchers on cooling technologies applicable to tropical and subtropical regions indicated that,each technology has its own advantage and disadvantage when applied infield application.No cooling system is suitable to cater the needs of greenhouse grower.However,it can be concluded that fan-pad and fogging is effective in lowering the air temperature of the greenhouse.However,in humid tropical regions,the use of fan-pad and fogging is restricted due to high humidity levels.Roof cooling is less effective and not preferredK.S.Kumar et al./Energy and Buildings41(2009)1269–12751273。