Investigation of the Potential of Long Wave Radiation Heating to Reduce Energy Consumption for Greenhouse Heating

Transcription

1 Investigation of the Potential of Long Wave Radiation Heating to Redue Energy Consumtion for Greenhouse Heating A. Kavga S. Pantelakis and Th. Panidis Deartment of Mehanial Engineering & Aeronautis University of Patras 65 Patras Greee V. Bontozoglou Deartment of Mehanial & Industrial Engineering University of Thessaly Volos Greee Keywords: greenhouse thermal erformane water ies heating long wave radiation energy balane heating effiieny Abstrat A arametri study has been arried out to quantify the ontribution of eah of the omonents of a greenhouse to the energy onsumtion during heating. The greenhouse loated in Western Greee is heated by water ies. Steady-state thermal balanes are develoed under general heating onditions and an aroximate exression to estimate heating effiieny is introdued. The results onfirm the dominant ontribution of onvetive and radiative heat losses through the greenhouse over. For the resent ase study the estimated heating effiieny oeffiient does not exeed 7%. Thermal erformane of the same greenhouse by assuming diret heating of the lants by long wave radiation is ahieved by exloiting the general formulations of the thermal roblem develoed. A signifiant otential for inreasing heating effiieny by involving long wave radiation heating is demonstrated. Imrovement starts from 45% and may areiably inrease deending on the heating time assumed. INTRODUCTION Energy onsumtion for greenhouse heating reresents a serious onern for greenhouse oerators throughout the world (Bot ). Conventionally heating in a greenhouse ours either by means of a iing system or by air heaters (Teitel et al. 999; Bartzanas et al. 5). Thus the interior of the greenhouse is heated to the same or even slightly higher temerature than the value targeted for the lants. 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 arameters inluding the strutural features of the greenhouse (i.e. shae dimensions materials used) heating and ventilation systems orientation latitude and loation tye of lantation as well as lantation and bare soil surfae area et. To state the general thermal roblem a set of non-linear equations are required to link heat exhanges between greenhouse air lants overs and floor with heaters sun exterior air and sky. To redue energy onsumtion some straightforward measures an be alied suh as double glazing (Guta et al. ) thermal sreens (Ghosal et al. 4) et. The above measures ontribute to overome the basi ause for energy onsumtion in a greenhouse whih boil down to the unavoidable thermal losses. An alternative for reduing energy onsumtion in greenhouse heating ould emerge by the use of long wave radiation. By ativating a long wave radiation heating soure lants and soil may reeive heat diretly. As air and over temeratures remain relatively low heat losses are signifiantly redued. However the use of long wave radiation for greenhouse heating has been so far sarely investigated. In (Blom and Ingratta 98) energy savings of 33-4% are reorted by using a long wave radiation heating system as omared to the onventional heating method. It is worth noting that long wave radiation heating has reently ome to be onsidered as a welome substitute for onventional heating in ertain food roessing aliations (Galindo et al. 5; Tanaka et al. 7) beause of its sueriority in terms of redued osts and inreased rodut quality. Pro. IS on Greensys7 Eds.:S. De Pasale et al. Ata Hort. 8 ISHS 8 74

2 In the resent work the thermal erformane of a greenhouse heated by water ies is investigated theoretially and exerimentally and the exeted imrovements by onsidering long wave radiation for diret heating of lants and soil are assessed. CHARACTERISTICS OF THE CASE-STUDY GREENHOUSE The greenhouse onsidered for the investigation is laed in the Tehnologial Eduational Institute (T.E.I) of Messologi in Western Greee and it onsists of three strutural units. It is equied with a onventional heating system whih onsists of a entral boiler and a hot water ie system. The struture is a metalli framework with glass over. The total area of the greenhouse A s is equal to 5 m the area of the over is A = 85 m and the volume of the greenhouse is V = 7 m 3. The number of air hanges er hour N is equal to.5 h ; it refers to a new greenhouse onstrution with good maintenane. The ultivation onsists of lants of lettue and the greenhouse s floor an be onsidered as being omletely overed by lants thus the area of lant anoy A = 5 m. The temerature data are olleted at a meteorologial station for a time eriod of 3 years whih is loated next to the greenhouse and omlies with the seifiations of World Meteorologial Organization (WMO). Proessing of the meteorologial data was erformed using the statistial rogram SPSS 3. For the energy alulations of the greenhouse the average night temerature of the statistially most unfavorable month of the eriod examined was taken; it gives for the outside temerature T o to the value of 7 C. The desirable temerature for the ultivation is taken to be 4 C. THERMAL MODELING OF THE GREENHOUSE A thermal analysis is erformed in order to quantify the ontribution of eah omonent of the greenhouse to the required energy onsumtion under general heating onditions. The heat transfer roblem is formulated and solved under steady-state onditions for satially uniform temeratures of overt inside air T a and lantst. The lantation temerature T is assumed fixed at the target value and the unknowns are the temeratures of the over T and of the inside airt a. The equations develoed below are aliable to both heating alternatives whih are onsidered in the resent study namely: (a) heating of the greenhouse interior by a water ieline system and (b) diret heating of the lantation and soil by long wave radiation. Three additive loss terms are onsidered: Term Q is due to inevitable onstrution defets of the greenhouse that ause air leakage as well as to the required ventilation through ventilation oenings. C αραnv Q = ( Ta Tο ) ( W) () 36 where C α the seifi heat of air (J/kg C) ρ α the density of air (kgr/m 3 ) and Tα Tο the temerature differene between inside and outside air ( C). Losses Q refer to ombined onvetive and radiative losses from the greenhouse over. The over 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 h o the onvetive heat transfer oeffiient between over and outside air (W/m K) ε the emissivity of over (-) σ the Stefan - Boltzmann onstant (W/m K 4 ) and T To the temerature differene between over and outside air ( C). The sky temerature Tsky is always set equal to the mean monthly outside air temerature To. The unknown temerature of the over T is alulated by writing an energy balane using the over itself as the ontrol volume. The over exhanges heat by onvetion with the inside and outside air and at the same time it gains heat by radiation from the lants and loses heat by radiation towards the sky. A balane of these terms at steady-state leads to the following equation T a 74

3 4 4 σε A( T T ) Ah a( Tα T) εaσ( T Tο ) hοa( T Tο ) = (3) + ( ε) ( Aε / Aε) where a the onvetive heat transfer oeffiient between inside ambient air and over (W/m h K) ε the emissivity of lants (-) and T T ο the temerature differene between lantation and outside air ( C). The radiation exhange between lants and over takes into aount the grey nature of the surfaes and the geometri onstraint. Losses Q 3 refer to losses from the greenhouse floor and the floor temerature is taken equal to the temerature of the lantation T. Q3 = KsAs( T Tο ) ( W) (4) where K s the total heat transfer oeffiient through the soil (W/m K). To evaluate the quality of different heating shemes a heating effiieny oeffiient n may be formulated: Qt n = (5) Qtotal In eq. (5) Qt stands for the amount of energy absorbed by the lants of an initially old greenhouse in order to reah the desired temerature and Q total is the total amount of energy rovided by the heating system during the entire transient heating eriod. Rigorous evaluation of Q total would neessitate integration of a system of ordinary differential equations in time. However a onservative estimate is rovided by aroximating thermal losses by their maximum value whih is attained under steadystate onditions. Thus Q total may be alulated from the following exression Qtotal = ( Q+ Q + Q3 ) Δ t+ Qa t + Q t (6) where Q to Q 3 are the steady values of the various thermal losses and Q at Q t the energy needed to heat the inside air and lants. The energy stored in the greenhouse omonents during heating an be evaluated simly by onsidering the mass m (Kg) and seifi heat of eah omonent C (J/kg C). The major ontributions ome from the lant anoy and the interior air and are given by the exressions Qt = mc( T T ο )(7a) Qα t = mac α( Tα T ο) (7b) Finally Δt stands for the duration of heating eriod whih in this study is taken equal to Δ t = h. It aroximates well the time interval observed exerimentally from the atual oeration of the greenhouse under onsideration. Furthermore this hoie is justified by the omuted harateristi time of thermal resonse of the lantation. COMPUTATION OF THE ENERGY NEEDS OF THE GREENHOUSE In order to rodue indiative figures for the ase-study greenhouse the following temeratures are set at the resetive values: (i) The desirable temerature for the growth of the onsidered ultivation is taken as T = 4 C. (ii) The temerature of the environment outside the greenhouse is taken from the statistially most unfavorable weather onditions for this geograhial 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 rodued is exloited to oerate a water ies or a long wave radiation heating system. Finally the thermal roerties of lant mass are equal to those of water due to the high ontent of water in the lant. It should be notied that the resent model may be readily alied also to other ase studies by adoting different sets of data. Estimation of the Energy Needs Using a Water Pies Heating System In the ase of heating by means of water ies the greenhouse interior is uniformly heated and the lants reeive energy mainly by onvetion from the inside air. As a result T α = T. Substituting in eq. (3) it remains the over temerature T as the single unknown. Instead of solving this equation numerially and then returning to eq. () 743

4 to alulate the thermal losses Q one may in this ase derive an analytial aroximation by linearizing the radiative ontributions. Thus eqn () may be written as: Q T To A h + ελ = ( ) o where λ = σ( T + To )( T + To) (α) Adding eqs () and (3) it gives: 4 4 ε Aσ ( T T ) Q = hα A( T T) + + ( ε ) ( Aε / Aε (8) ) In eqn (8) it has been also made use of the substitution T α =T. Equation (8) may be similarly linearized to give Q = T T A hα + ε ( A / A) λ (8α) where λ = σ( T + T )( T + T)/ + ( ε) ( Aε / Aε) The temerature summation terms λ λ are aroximated by the onstant λ=5.78 W/m K. This numerial value is equal to the algebrai mean of λ and λ when the unknown is set equal to the mean value between the desirable internal temerature in the greenhouse T = 4 T C and the external ambient temerature T o = 7 C. Adding eqs (a) and (8a) the unknown T is eliminated and the following exression for the heat losses from the over in terms of the total temerature differene ( T To) is derived: Q = A( T To) = KA( T To) ( 9) + ha + ε( A / A) λ ho + ελ In eqn (9) the total heat transfer oeffiient through the over K has been defined as K = () + ha + ε ( A / A ) λ ho + ελ it inororates both the onvetive and radiative ontributions. Using equations () and (4) and the above equation (9) the ontribution of eah greenhouse omonent to the required energy onsumtion during heating has been alulated. All values used in the alulations are summarized in Table. The results are dislayed (Fig. ) where the values give the ontribution of eah omonent in terms of absolute energy amounts. The same values are given as erentage of the required energy onsumtion during heating. As exeted losses through the over are the most signifiant. It reresents 77.3% of the total energy onsumtion required during heating. However ventilation and soil losses are also not negligible. Using the ratio h ο / ( h ο + ε λ ) aearing in eq. (a) the ontribution of onvetion and radiation to the total over losses are determined to 8.6 % for the onvetive ontribution and 8.4 % for the radiative ontribution resetively. The energy amounts Qatand Qt needed to heat inside air and lants are alulated from eqs (7a) and (7b) to 5.5 MJ and 55 MJ resetively. By using eqn (6) the total amount of energy required to heat the initially old greenhouse is derived to 744

5 Qtotal = 34.7 MJ and the thermal effiieny oeffiient for heating eriod of the greenhouse is alulated from eq. (5) as 7. %. It is worth noting that although ase seifi this value is not exeted to differ areiable for other ultivations as well as other loations with omarable limati onditions. Estimation of the Exeted Imrovements by Assuming Infrared Radiation System An alternative to inrease the thermal effiieny of the greenhouse ould emerge by the diret heating of lants and soil using long wave radiation. With this mode of heat transfer the air temerature T a is exeted to be signifiantly different than that of the lants 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 lants anoy and the inside surfae of the over and also exhanges mass with the exterior (air renewal by leakages and ventilation).the balane of these ontributions at steady-state is exressed by the equation Ah a( T Tα) Ah a( Tα T) nv.36( Tα Tο) = () Equations (3) and () need to be solved simultaneously to determine the two unknown temeratures T α and T. This task may be aomlished either numerially or analytially by a symboli mathematis software. For the resent ase study the Mathematia software tool has been used. By taking again T = 4 C and T o = 7 C the values derived for air and over are T α =. C and T = 8.3 C resetively. It is worth noting that the air equilibrates at a temerature signifiantly lower than that of the lants and that the over remains even older. These hanges are exeted to influene favorably the heat losses of the greenhouse. Indeed eah ontribution is re-alulated from eqs () () and (4) and the results are omared to those orresonding to onventional heating (Fig. ). With the exetion of the ondutive losses through the soil (whih remain by definition idential) all other thermal losses are areiably redued in the ase of infrared heating. The total result is a 45-5 % eonomy in energy needs during steady-state oeration. It is worth mentioning that heat losses from over although areiably redued in absolute values as omared to water ies heating are still reresenting the most signifiant ontribution to the total energy losses and reah 73.7% of it (Fig. ). Hene searhing for alternative over material solutions would remain an urgent need also by using long wave radiation heating systems. Using the eqs (5) and (6) and taking again a heating time equal to h the values alulated for total thermal load Q total and thermal effiieny oeffiient n are Qtotal =.4 MJ and n = Qt Qtotal = 4.7% resetively. These values evidently underline the signifiant otential of the long wave radiation heating systems for ahieving areiable redution of the energy needs to heat the greenhouse. It should be noted that the assumtion of same heating time by the use of diret long wave radiation as for heating with water ies learly reresents an unfavourable heating time senario for the ase of long wave radiation heating as an essential benefit of long wave radiation is the otential for dramati redution in heating time. This redution is ahieved by inreasing the intensity of radiation soures and is robably only limited by the reeiving ability of the lants. Exerimental results resented in (Teitel et al. ) indiated that diret heating of tomato and eer 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 aears feasible. Thus the resent analysis onludes with an estimate of ossible gains in effiieny stemming from the redution of heating time and results are shown (Fig. 3) for times ranging from 5 min u to 6 min. Comarison of these results with the effiieny n=7. % ahieved by water ies heating indiates imrovements starting from 46% and reahing u to 8% for the shortest heating time assumed. 745

6 CONCLUSIONS A arametri study has been undertaken to quantify the ontribution of eah of the omonents of a greenhouse loated in Western Greee and heated by water ies system to the energy onsumtion during heating. For the alulations ondutive onvetive and radiative resistanes have been identified and aroriate energy balanes have been formulated for temorally steady and satially uniform (but not neessarily equal with eah other) temeratures of over inside air and lants. Also a reliminary onservative estimate of thermal effiieny during heating has been develoed. Using the thermal model develoed thermal erformane of the ase-study greenhouse has been examined; it gives for the thermal effiieny oeffiient the unaetably low value of 7%. As the results of the alulation reresent atual figures of a thermal model formulated under general onditions they an be onsidered as reresentative also for other ultivations and other loations with similar limati onditions. Combined onvetive and radiative losses from the over reresent nearly 8% of the overall thermal losses of the greenhouse during heating and are redominantly resonsible for the unaetably low thermal effiieny of the greenhouse and thus their redution is of rimary onern. A signifiant imrovement of the heating effiieny ould emerge by the diret heating of lants and soil by the means of long wave radiation. By assuming for diret long wave radiation heating the same heating time as by using water ies whih learly reresents the most unfavourable senario by the use of long wave radiation it results to a heating effiieny inrease of about 45% as omared to the derived heating effiieny when using water ies heating. Literature Cited Bartzanas T. Thamithian M. and Kittas C. 5. Influene of the Heating Method on Greenhouse Mirolimate and Energy Consumtion. Biosystems Eng. 9: Bot G.. Develoments in indoor sustainable lant rodution with emhasis on energy saving. Comuters and Eletronis in Agriulture 3:5-65. Critten D.L. and Bailey B.J.. A review of greenhouse engineering develoments during the 99s Agriultural and forest Meteorology :-. Galindo F.G. Toledo R. and Sjoholm I. Tissue damage in heated arrot slies. Comaring mild hot water blanhing and infrared radiation. J. Food Eng. 67: Ghosal M.K. and Tiwari G.N. 4. Mathematial modeling for greenhouse heating by using thermal urtain and geothermal energy. Solar Energy 76: Guta M.J. and Chandra P.. Effet of greenhouse design arameters on onservation of energy for greenhouse environmental ontrol. Energy 7: SinghG. 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 3(): 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. Segal L. Shklyar A. and Barak M A omarison between ie and air heating methods for greenhouses. J. Agri. Eng. Researh 7: Teitel M. Shklyar A. Elad Y. Dikhtyar V. and Jerby E.. Develoment of a mirowave system for greenhouse heating. Ata Hort. 534:

7 Tables Table. Inut arameters used for the omutations. m = 875 kg A 85 m m = α k g A ha ha h = ο = 8.5W m K = 8.5W m K V T = 87.6 K = C = 49 J / kg C 3 = 5 m To = 8.6K ρ a =.3 kg / m N W m K n h =.85 3 = 7 m =.5 h ε =.9 ε =.9 Ca = 4 / J kg C 8 σ = 5.66 W / m K λ = 5.78 / Ks 4 W m K =.85 W / m K Figures Q [MJ] 8 6 Q [%] ventilation over radiation ventilation over onvetion over radiation soil ondution Fig.. Contribution of eah greenhouse omonent to the energy balane in MJ and in erentage of the total energy onsumtion. 747

8 Q [MJ] Q [%] ventilation over radiation ventilation over onvetion over radiation soil ondution Fig.. Comarison between onventional and in MJ and long wave radiation heating in erent of the total energy onsumtion. 8 Thermal effiieny time (min) Fig. 3. Relation between thermal effiieny oeffiient and heating time for the ase of long wave radiation heating. 748