Keywords: Greenhouse; Thermal performance; Water pipes heating; Infrared radiation; Energy balance; Heating efficiency

Transcription

1 International Symposium Greensys 7 "High Tehnology for Greenhouse system Management" Naples Italy, 4-6 Otober 7 Investigation of the Potential of Infrared-radiation (IR) to Redue Energy Consumption in Greenhouse Heating A. Kavga, S. Pantelakis and Th. Panidis Department of Mehanial Engineering & Aeronautis, University of Patras, 65 Patras, Greee V. Bontozoglou Department of Mehanial & Industrial Engineering, University of Thessaly, Volos, Greee Keywords: Greenhouse; Thermal performane; Water pipes heating; Infrared radiation; Energy balane; Heating effiieny Abstrat Energy absorbed by the plants of an initially old greenhouse in order to reah the desired temperature represents only a small perentage of the total amount of energy onsumed during the entire heating period of a greenhouse. A parametri study has been arried out to quantify the ontribution of eah of the omponents of a greenhouse loated in Western Greee to the energy onsumption during heating. The greenhouse is heated by water pipes. To this end, first steadystate thermal balanes are developed under general heating onditions and an approximate expression to estimate heating effiieny is introdued. Then, for the greenhouse under onsideration, atual figures are alulated for one representative outdoor temperature derived by exploiting meteorogial data measured over a period of three years. The results onfirm the dominant ontribution of onvetive and radiative heat losses through the greenhouse. For the present ase study the estimated heating effiieny oeffiient does not exeed 17%. Thermal performane of the same greenhouse by assuming diret heating of the plants by infrared radiation is made by exploiting the general formulations of the thermal problem developed. A signifiant potential for inreasing heating effiieny by involving IR heating is demonstrated. Improvement starts from 45% and may appreiably inrease depending on the heating time assumed. INTRODUCTION Energy onsumption for greenhouse heating represents a serious onern for greenhouse operators throughout the world (Bot, 1). Conventionally, heating in a greenhouse ours either by means of a piping system or by air heaters (Teitel et al, 1999; Bartzanas et al, 5). Thus, the interior of the greenhouse is heated to the same or even slightly higher temperature than the value targeted for the plants. Several efforts have been undertaken to formulate the thermal behaviors of a greenhouse (Critten et al, ; Singh et al, 6). The thermal behavior of a greenhouse is influened by a variety of parameters inluding the strutural features of the greenhouse (i.e. shape, dimensions, materials used), heating and ventilation systems, orientation, latitude and loation, type of plantation as well as plantation and bare soil surfae area, et. To state the general thermal problem a set of non-linear equations are required to link heat exhanges between greenhouse air, plants, s and floor, with heaters, sun, exterior air and sky. To redue energy onsumption, some straightforward measures an be applied suh as double glazing (Gupta et al, ), thermal sreens (Ghosal et al, 4), et. The above measures ontribute to overome the basi ause for energy onsumption in a greenhouse, whih boil down to the unavoidable thermal losses.

2 An alternative for reduing energy onsumption in greenhouse heating ould emerge by the use of infrared radiation (IR). By ativating an IR soure, plants and soil may reeive heat diretly. It overomes the onventional onept to inrease the inside air temperature in a greenhouse in order to deliver to the plants the neessary heat by onvetion. Furthermore, as air and temperatures remain relatively low, heat losses are signifiantly redued. However the use of IR for greenhouse heating has been so far sarely investigated. Few works published in the early 8 s (Blom and Ingratta, 1981), investigate the suitability of low intensity IR for greenhouse heating. For example, in Blom (1981), energy savings of 33-41% are reported by using an IR system, as ompared to the onventional heating method. It is worth noting that IR has reently ome to be onsidered as a welome substitute for onventional heating in ertain food proessing appliations (Galindo et al, 5; Tanaka et al, 7), beause of its superiority in terms of redued osts and inreased produt quality. In the present work, the thermal performane of a greenhouse heated by water pipes is investigated theoretially and experimentally and the expeted improvements by onsidering infrared radiation for diret heating of plants and soil are assessed. CHARACTERISTICS OF THE CASE-STUDY GREENHOUSE The gable greenhouse onsidered for the investigation is plaed in the Tehnologial Eduational Institute (T.E.I) of Messologi in Western Creee and it onsists of three strutural units. It is equipped with a onventional heating system, whih onsists of a entral boiler and a hot water pipe system. The struture is a metalli framework with glass. The total area of the greenhouse As is equal to 5 m, the area of the is A = 815 m and the volume of the greenhouse is V = 17 m 3. The 1 number of air hanges per hour, N, is equal to 1.5 h ; it refers to a new greenhouse onstrution with good maintenane. The ultivation onsists of plants of lettue and the greenhouse s floor an be onsidered as being ompletely ed by plants. The temperature data are olleted at a meteorologial station for a time period of 3 years, whih is loated next to the greenhouse and omplies the speifiations of World Meteorologial Organization (WMO). Proessing of the meteorologial data was performed using the statistial program SPSS 13. For the energy alulations of the greenhouse the average night temperature of the statistially most unfavorable month of the period examined was taken; it gives for the outside temperature T o to the value of 7 C. The desirable temperature for the ultivation T a is taken to be 14 C. THERMAL MODELING OF THE GREENHOUSE A thermal analysis is performed in order to quantify the ontribution of eah omponent of the greenhouse to the required energy onsumption, under general heating onditions. The heat transfer problem is formulated and solved under steady-state onditions for spatially uniform temperatures of, inside air and plants. The plantation temperature is assumed fixed at the target value and the unknowns are the temperatures of the and of the inside air. The equations developed below are appliable to both heating alternatives whih are onsidered in the present study, namely: (a) heating of the greenhouse interior by a water pipeline system, and, (b) diret heating of the plantation and soil by IR. Three additive loss terms are onsidered: Term Q 1 is due to inevitable onstrution defets of

3 the greenhouse that ause air leakage, as well as to the required ventilation through ventilation openings. Cp αραnv Q1 = ( Ta Tο ) ( W) (1) 36 Losses, Q, refer to ombined onvetive and radiative losses from the greenhouse. The loses heat by onvetion to the outside air and by thermal radiation towards the sky. 4 4 Q = ho A ( T To ) + ε Aσ( T To ) () where, in the present paper the sky temperature is always set equal to the mean monthly outside air temperature. The unknown temperature of the is alulated by writing an energy balane, using the itself as the ontrol volume. The exhanges heat by onvetion with the inside and outside air, and at the same time, it gains heat by radiation from the plants and loses heat by radiation towards the sky. A balane of these terms at steady-state leads to the following equation, 4 4 σε pap( Tp T ) Ah a ( Tα T ) ε Aσ( T Tο ) h οa ( T Tο) = (3) 1 + (1 ε) ( Apε p / Aε) where the radiation exhange between plants and takes into aount the grey nature of the surfaes and the geometri onstraint (the surrounds the floor), and the sky temperature has again been set equal to the outside air temperature. Losses Q 3 refer to losses from the greenhouse floor and the floor temperature is taken equal to the temperature of the plantation. Q3 = KA s s( Tp Tο ) ( W) (4) To evaluate the quality of different heating shemes a heating effiieny oeffiient may be formulated: Qpt, n = (5) Qtotal In eq. (5), Qpt, stands for the amount of energy absorbed by the plants of an initially old greenhouse in order to reah the desired temperature and Q total is the total amount of energy provided by the heating system during the entire transient heating period. Rigorous evaluation of Q total would neessitate integration of a system of ordinary differential equations in time. However, a onservative estimate is provided by approximating thermal losses by their maximum value, whih is attained under steadystate onditions. Thus, Q total, may be alulated from the following expression, Qtotal = ( Q1+ Q + Q3 ) + t Qa, t + Qp, t (6) where Q 1 to Q 3 are the steady values of the various thermal losses and Q a,t, Q p,t the energy needed to heat the inside air and plants. The energy stored in the greenhouse omponents during heating an be evaluated simply by onsidering the mass and speifi heat of eah omponent. The major ontributions ome from the plant anopy and the interior air, and are given by the expressions, Qp, t = mc p pp ( Tp T ο ) (7a) Qα, t = mc a pα( Tα Tο) (7b) Finally, t, stands for the duration of heating period, whih in this study is taken equal to t = 1h. It approximates well the time interval observed experimentally from the

4 atual operation of the greenhouse under onsideration. Furthermore, this hoie is justified by the omputed harateristi time of thermal response of the plantation. COMPUTATION OF THE ENERGY NEEDS OF THE GREENHOUSE In order to produe indiative figures for the ase-study greenhouse, the following temperatures are set at the respetive values: (i) The desirable temperature for the growth of the onsidered ultivation is taken as Tp = 14 C. (ii) The temperature of the night environment outside the greenhouse is taken from the statistially most unfavorable weather onditions for this geographial area as To = 7 C. The onversion effiieny oeffiient n h for the heater is taken to be.85 equal, if the amount of energy produed is exploited to operate a water pipes or an infrared radiation heating system. Finally the thermal properties of plant mass are equal to those of water, due to the high ontent of water in the plant. It should be notied that the present model may be readily applied also to other ase studies by adopting different sets of data. Estimation of the Energy Needs using a Water Pipes Heating System In the ase of heating by means of water pipes, the greenhouse interior is uniformly heated and the plants reeive energy mainly by onvetion from the inside air. As a result, T = α T. Substituting in eq. (3), it remains the temperature, T p, as the single unknown. Instead of solving this equation numerially and then returning to eq. () to alulate the thermal losses, Q, one may in this ase derive an analytial approximation by linearizing the radiative ontributions. Thus, eqn () may be written as: Q T To A ( ho + ελ 1) = where λ1 = σ( T + To )( T + To) (α) Adding eqs () and (3), it gives: 4 4 ε papσ( Tp T ) Q = hα A( Tp T) (1 (8) ε) ( Apε p / Aε) In eqn (8) it has been also made use of the substitution T α =T p. Equation (8) may be similarly linearized to give Q = Tp T (8α) A hα + ε p( Ap / A) λ where λ = σ( Tp + T )( Tp + T) / 1 + (1 ε) ( Apε p / Aε) The temperature summation terms λ 1, λ are approximated by the onstant λ= W/m K. This numerial value is equal to the algebrai mean of λ 1 and λ, when the unknown T is set equal to the mean value between the desirable internal temperature in the greenhouse, Tp = 14 C, and the external ambient temperature, To = 7 C. Adding eqs (a) and (8a), the unknown T is eliminated and the following expression for the heat losses from the in terms of the total temperature differene (T p -T o ) is derived:

5 1 Q = A( Tp To) = KA( Tp To) (9) ha + εp( Ap / A) λ ho + ελ In eqn (9) the total heat transfer oeffiient through the K, has been defined as 1 K = (1) ha + ε p( Ap / A) λ ho + ελ it inorporates both, the onvetive and radiative ontributions. Using equations (1) and (4) and the above equation (9), the ontribution of eah greenhouse omponent to the required energy onsumption during heating has been alulated. All values used in the alulations are summarized in table 1. The results are displayed in Figure 1 where the values give the ontribution of eah omponent in terms of absolute energy amounts. The same values are given as perentage of the required energy onsumption during heating. As expeted, losses through the are the most signifiant. It represents 77.3% of the total energy onsumption required during heating. However, ventilation and soil losses are also not negligible. Using the ratio h ο / ( h ο + ελ ) appearing in eq. (a), the ontribution of onvetion and radiation to the total losses are determined to 81.6 % for the onvetive ontribution and 18.4 % for the radiative ontribution respetively. The energy amounts Q at, and Qpt, needed to heat inside air and plants are alulated from eqs (7a) and (7b) to 15.5 MJ and 55 MJ, respetively. By using eqn (6) the total amount of energy required to heat the initially old greenhouse is derived to Qtotal = MJ and the thermal effiieny oeffiient for heating period of the greenhouse is alulated from eq. (5) as 17. %. It is worth noting that although ase speifi, this value is not expeted to differ appreiable for other ultivations as well as other loations with omparable limati onditions. Estimation of the Expeted Improvements by assuming Infrared Radiation System An alternative to inrease the thermal effiieny of the greenhouse ould emerge by the diret heating of plants and soil using infrared radiation. With this mode of heat transfer, the air temperature, T a, is expeted to be signifiantly different than that of the plants, and its alulation at steady-state requires another equation. As suh, the energy balane around a ontrol volume that ontains only the inside air will be taken. The air exhanges heat by onvetion with the plants anopy and the inside surfae of the and also exhanges mass with the exterior (air renewal by leakages and ventilation).the balane of these ontributions at steady-state is expressed by the equation Aphap( Tp Tα) Ah a( Tα T ) n.36( V Tα Tο) = (11) Equations (3) and (11) need to be solved simultaneously to determine the two unknown temperatures, T α and T. This task may be aomplished either numerially or analytially by a symboli mathematis software. For the present ase study the Mathematia software tool has been used. By taking again T = 14 C and T = 7 C the values derived for air and are T = α 1. C and T = 8.3 C respetively. It is worth noting that the air equilibrates at a temperature signifiantly lower than that of the plants, and that the remains even older. These hanges are expeted to influene p o

6 favorably the heat losses of the greenhouse. Indeed, eah ontribution is re-alulated from eqs (1), () and (4), and the results are ompared to those orresponding to onventional heating in figure. With the exeption of the ondutive losses through the soil (whih remain by definition idential) all other thermal losses are appreiably redued in the ase of infrared heating. The total result is a 45-5 % eonomy in energy needs during steady-state operation. It is worth mentioning that heat losses from, although appreiably redued in absolute values as ompared to water pipes heating, are still representing the most signifiant ontribution to the total energy losses and reah 73.7% of it (Figure ). Hene, searhing for alternative material solutions would remain an urgent need also by using infrared radiation heating systems. Using the eqs (5) and (6) and taking again a heating time equal to 1h, the values alulated for total thermal load Q total and thermal effiieny oeffiient n are Qtotal =.4 MJ and n= Qp, t Qtotal = 4.7%, respetively. These values evidently underline the signifiant potential of the infrared radiation heating systems for ahieving appreiable redution of the energy needs to heat the greenhouse. It should be noted that the assumption of same heating time by the use of diret IR heating as for heating with water pipes learly represents an unfavourable heating time senario for the ase of IR heating as an essential benefit of infrared radiation is the potential for dramati redution in heating time. This redution is ahieved by inreasing the intensity of radiation soures and is probably only limited by the reeiving ability of the plants. Experimental results presented in (Teitel et al, ) indiated that diret heating of tomato and pepper within only a few seonds by using a mirowave system, did not ause visible injury to leaves, flowers and fruits. At any ase, a tenfold derease appears feasible. Thus, the present analysis onludes with an estimate of possible gains in effiieny stemming from the redution of heating time, and results are shown in figure 3 for times ranging from 5 min up to 6 min. Comparison of these results with the effiieny n=17. % ahieved by water pipes heating indiates improvements starting from 46% and reahing up to 8% for the shortest heating time assumed. CONCLUSIONS A parametri study has been undertaken to quantify the ontribution of eah of the omponents of a greenhouse loated in Western Greee and heated by water pipes system to the energy onsumption during heating. For the alulations, ondutive, onvetive and radiative resistanes have been identified and appropriate energy balanes have been formulated for temporally steady and spatially uniform (but not neessarily equal with eah other) temperatures of, inside air and plants. Also, a preliminary, onservative estimate of thermal effiieny during heating has been developed. Using the thermal model developed, thermal performane of the ase-study greenhouse has been examined; it gives for the thermal effiieny oeffiient the unaeptably low value of 17%. As the results of the alulation represent atual figures of a thermal model formulated under general onditions they an be onsidered as representative also for other ultivations and other loations with similar limati onditions. Combined onvetive and radiative losses from the represent nearly 8% of the overall thermal losses of the greenhouse during heating and are predominantly responsible for the unaeptably low thermal effiieny of the greenhouse, and thus their redution is of primary onern. The materials whih are nowadays used as materials do not provide the potential for signifiant improvements of the heating

7 effiieny. Researh to provide alternative material solutions for the s is urgently needed. A signifiant improvement of the heating effiieny ould emerge by the diret heating of plants and soil by the means of infrared radiation. By assuming for diret IR heating the same heating time as by using water pipes, whih learly represents the most unfavourable senario by the use of IR, it results to a heating effiieny inrease of about 45% as ompared to the derived heating effiieny when using water pipes heating. Literature Cited Bot, G. 1. Developments in indoor sustainable plant prodution with emphasis on energy saving. Computers and Eletronis in Agriulture. 3: Teitel, M., Segal, L., Shklyar, A., and Barak, M A omparison between pipe and air heating methods for greenhouses. J. Agri. Eng. Researh 7: Bartzanas, T., Thamithian, M., Kittas, C. 5. Influene of the Heating Method on Greenhouse Mirolimate and Energy Consumption. Biosystems Eng. 91: Critten, D.L., and Bailey, B.J.. A review of greenhouse engineering developments during the 199s, Agriultural and forest Meteorology 11:1- Singh,,G., Singh, P.P., Lubana, P.P.S., and Singh, K.G. 6. Formulation and validation of a mathematial model of the mirolimate of a greenhouse. Renewable Energy 31(1): Gupta, M.J., and Chandra, P.. Effet of greenhouse design parameters on onservation of energy for greenhouse environmental ontrol. Energy 7: Ghosal, M.K., and Tiwari, G.N. 4. Mathematial modeling for greenhouse heating by using thermal urtain and geothermal energy. Solar Energy 76: Galindo, F.G., Toledo, R., Sjoholm I. Tissue damage in heated arrot slies. Comparing mild hot water blanhing and infrared radiation. J. Food Eng. 67: Tanaka, F., Verboven, P., Sheerlink, N., Morita, K., Iwasaki, K., and Niolai, B. 7. Investigation of far infrared radiation heating as an alternative tehnique for surfae deontamination of strawberry. J. Food Eng. 79(): Teitel M., Shklyar A., Elad Y., Dikhtyar V., and Jerby, E.. Development of a mirowave system for greenhouse heating, Ata Hortiulturae 534: Tables Table 1. Input parameters used for the omputations T = K 419 / mp = 1875 kg A = 815 m Cp p p = J kgc m = 3 α 1kg Ap = 5 m To = 8.16K ρ a = 1.3 kg / m hap = 8.5W m K V = 17 m ε p =.9 σ = W / m K 1 ha = 8.5W m K N = 1.5 h ε =.9 λ = W/ mk h ο = W m K n h =.85 Cpa = 14 J / kgc Ks = 1.85 W/ mk

8 Figures 14 19, , Q [MJ] 8 6 Q [%] ,16 9,1 3, ,3 14, 11,4 ventilation onvetion radiation soil ondution ventilation onvetion radiation soil ondution Fig 1: Contribution of eah greenhouse omponent to the energy balane in MJ and in perentage of the total energy onsumption , ,1 6,1 1 5 Q [MJ] ,8 Q [%] ,16 9,78 9,1 17,11 3,31 3, ,3 7,8 14, 13,6 11,4 18,5 ventilation onvetion radiation soil ondution ventilation onvetion radiation soil ondution Fig : Comparison between onventional and infrared heating in MJ and in perent of the total energy onsumption 8% 7% 6% 5% 4% 3% % 1% % Thermal effiieny time (min) Fig 3: Relation between thermal effiieny oeffiient and heating time