A review of greenhouse engineering developments …€¦ · 1.3. Current greenhouse and tunnel...

Agricultural and Forest Meteorology 112 (2002) 1–22

A review of greenhouse engineering developmentsduring the 1990s

D.L. Critten∗,1, B.J. Bailey1

Silsoe Research Institute, Silsoe, Bedfordshire MK45 4HS, UK

Received 12 November 2001; received in revised form 25 April 2002; accepted 30 April 2002

Abstract

The paper begins with a brief review of earlier work.The 1990s saw limited extensions to the theory, computer modelling and practices relating to light transmission, apart

from an interesting, but unproven new design to improve transmission. Studies concentrated on improving the performanceof greenhouse elements (e.g. cladding), including methods of measurement.

Thermal studies show significant progress. Computer modelling was extended to include the time dimension and finiteelement techniques were used to improve resolution of detail. As with light transmission, studies intended to improve thequality of greenhouse elements were also carried out. Understanding and modelling the performance of a thermal screen hasalso been improved, both as such and as an integral part of the greenhouse. Much effort was directed towards understandingventilation. Analysis has shown that different sources exist (thermal-, wind-induced), and these have been analysed. Theexistence of internal circulating air patterns has also been established.

The optimisation of carbon dioxide concentrations in greenhouses has now reached the point where the gain to a growercan be readily predicted.

Further computer modelling of light transmission to include the effects of scattering and modelling of skylight to predictleaf irradiance are proposed. Extension of finite element techniques to further improve resolution in thermal studies isrecommended. A comprehensive investigation of the physical and engineering aspects of ventilation and internal air flowsis also needed, to support the extended thermal studies of the greenhouse. Finally, a pilot study, anticipating the design of acomputer-controlled greenhouse is envisaged.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Sky radiance; Sunlight; Heat transfer coefficient; Temperature; Wind conditions; Water uptake; CO2

∗ Corresponding author. Tel.:+44-1582-860-000x2611;fax: +44-1525-860-156.E-mail address:[email protected] (D.L. Critten).

1 Retired.

1. Introduction

1.1. Background

The greenhouse has been used in various formsfor centuries as a means of protecting plants fromextremes of weather, enabling, e.g. exotic tropical spe-cies to be grown at higher latitudes. This is achievedby creating better growing conditions, traditionally bymaintaining a higher internal ambient, compared with

0168-1923/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0168-1923(02)00057-6

2 D.L. Critten1, B.J. Bailey / Agricultural and Forest Meteorology 112 (2002) 1–22

external ambient temperature. However, the protectedenvironment can be used in other ways. For example,a constant water supply to plants can be maintainedin regions where periods of drought are common. Thegreenhouse also permits a more efficient use of avail-able water in such regions. Protection against highwinds, insects and airborne diseases is also possible.This flexibility of design has enabled the greenhouseto be used in regions as diverse as northern Europe,the Mediterranean and hot desert countries. However,a protected environment can be a ‘two-edged sword’since it also provides a comfortable environmentwithin which pests and fungi can lurk.

Since the 1950s, quantitative studies to improvethe commercial viability of greenhouse crops havebeen carried out. In the late 1970s interest in thisresearch was enhanced by a sudden increase incrude oil prices which resulted in significantly in-creased heating costs. Double glazing a greenhousewill reduce winter heat losses, but invariably causesa reduction in light transmission, thereby reducingcrop growth rate. Capital costs are also increased.Greenhouse design studies were therefore initiated toinvestigate whether grower’s income could be opti-mised by changing the shape or cladding, or generalconstruction of the house.

Reviews of these investigations were carried out byBailey (1988)and Critten (1993, 1994). The formerpaper discusses the thermal behaviour of the green-house, while the latter two consider optical, and ther-mal and optical performances, respectively togetherwith greenhouse control technology (1994 paper). Areview devoted to methods of determining the radio-metric properties of greenhouse claddings was carriedout by Nijskens et al. (1985). Transmissivities ofcommonly used materials in the photosyntheticallyactive radiation (PAR) and infra-red (IR) regions arepresented.

1.2. Factors affecting greenhouse designs

In studying greenhouse optimisation, the ultimateconcerns are the capital cost of the greenhouse andthe cost benefit of new greenhouse designs or de-sign changes to the grower. Methods of calculatingthe latter will vary with the crop. For instance, atomato grower will normally look for maximum cropproduction on a continuous basis, whereas a chrysan-

themum grower will be more concerned to time hiscrop to mature at a given date, such as MotheringSunday (UK). Broadly, however, for present pur-poses, cost benefit is usually improved by increasinglight input to the house, reducing heat losses duringcold weather, and increasing heat removal (ventila-tion) during hot weather. We also find that optimaldesign is not significantly dependent on latitude forthose regions in which greenhouses are normally em-ployed, though latitude may affect operating details.Thus, a ventilation system designed for a greenhouseunder low latitude hot daytime (desert) conditionsmay well be applicable in a house at higher latitudeduring summer time. Equally, an insulating systemused during winter at high latitudes might be usefulto keep a crop warm during cold desert nights. In thepresent work, therefore, we will not take latitude intoaccount.

1.3. Current greenhouse and tunnel structures

There are three main greenhouse types currently inuse. These are the traditional English house, whichmay be single or multi-span, each span of width 6.7 m,and supported by vertical stanchions at either side;the Dutch Venlo, multi-span only, each span 3.2 mwide, with stanchions separated by 3.4, 6.4 or 9.6 m,and the single widespan of width 22 m. Sidewalls arenormally 3.5–4.5 m high, and roof pitch is 22–26◦ tothe horizontal.

Plastic tunnels may be single or multi-span, andcompose a set of shallow generally circular arcs (steeltubes) over which is stretched a polyethylene sheet.Span width is similar to that of the traditional house.House length and number of spans are of courseoptional.

The canarian greenhouses used in Mediterraneanregions comprise a series of galvanised tubes, 3 mapart, connected by metal rods. The roof is nearlyhorizontal, and is constructed of wooden slats, 60 cmapart, over which plastic film is stretched. If insectprotection only is required, the plastic is replaced bynetting.

1.4. Object

In the present paper, we review developments oflight transmission and thermal technologies during the

D.L. Critten1, B.J. Bailey / Agricultural and Forest Meteorology 112 (2002) 1–22 3

1990s, including the effects of these on carbon dioxideoptimisation. We also discuss possible future research.First however, we present a very brief summary of theearlier work mentioned inSection 1.1. This is intendedonly to provide the background for the subsequentreview, and the reader is referred to the earlier reviews(Section 1.1) for details.

2. A summary of optical and thermalgreenhouse technology up to 1990

2.1. Light transmission

2.1.1. Transmission into full scale greenhousesEarly work consisted mainly of long-term mea-

surements of light transmission, attempting also todistinguish cladding from structural losses and diffusefrom direct light losses. New cladding materials werealso evaluated. For example, a long-term comparisonbetween double polycarbonate and double acrylic cladhouses showed that the latter transmitted about 5%more light. Measurement accuracies were discussed,and the use of a small number of ‘point’ sensors wasshown in general to give unreliable results. The gen-eral effects of surface contamination (dirt, etc.) werealso studied (Jones, 1966).

2.1.2. Transmission of single units ofcladding materials

Measurements were usually carried out withsamples mounted on an integrating sphere. Dif-fuse, parallel beam, and laser sources were all used.Transmission of sunlight as a function of incidenceangle was also studied. Typical transmissions of PARby sheets subject to normal incidence of a paral-lel beam of light are 88–90% for horticultural glass(3–4 mm thick), 85% for 16 mm twin walled acrylicand about 90% for 180�m horticultural polyethy-lene sheet. Absorption increases typically by about1–5% mm−1 for glass, but is virtually zero foracrylic.

Theoretical studies of PAR transmission were alsoperformed on dry structured plastic cladding materi-als. These comprise two or three parallel transparentsheets of plastic, contiguous with and spaced apart byparallel narrow lengths of the same material, creatinga set of consecutive rectangular cells in cross-section.

The sheets are formed by plastic extrusion processes.Losses were shown to be generally higher than insingle sheets, and were unacceptably high when thestructural cell width is less than the total claddingthickness. Absorption was also found to have anenhanced effect in structured plastic claddings.

2.1.3. Computer modelling of greenhouselight transmission

A number of complex models were producedduring or before the 1980s. These could accommo-date direct sunlight or diffuse skylight, effects ofpolarisation of incident light (shown to be small),multiple reflections (shown to be significant), vari-able roof angles, and number of spans. Effects ofstructural members were also studied. Studies didnot however include non-planar cladding surfaces,apart from the pioneering work ofKurata et al.(1991), nor scattering of light by the cladding, ex-cept as a simple extinction loss for small amounts ofscattering.

The predictions produced by these models weregenerally good, and comparison with experiment isgenerally accurate to 1 or 2%.

Some theoretical developments were also pro-duced, leading to a primitive ‘theory of greenhouselight transmission’ for multi-spans. An importantaspect of the theory was the separation of physicaland geometric aspects of the greenhouse claddingsurfaces, enabling separate treatments for each. A‘design guide’ for the greenhouse manufacturers wasalso produced, to be used in conjunction with strengthcalculations (Critten, 1987).

New designs for improving greenhouse light input,e.g. the vertical south roof, or clerestory greenhousewere also proposed and evaluated using computer andexperimental models.

2.1.4. Light enhancementThese are usually ‘bolt on’ elements. The sim-

plest form is to add a reflecting surface to the northwall of an E–W aligned greenhouse. Another methodwas to mount prisms or angled reflecting slats inthe roof, which can produce up to 35% increase inwinter light (Japan). Another scheme employed tra-ditional venetian blinds, but with slats containingprismatic or shaped reflecting elements. Using thereflecting system, an average 30% gain in a single


span house over the mid-winter period was measured(UK).

2.1.5. Light penetration into greenhouse cropsA number of detailed models were produced dur-

ing the 1980s. The subject is more complicated thantransmission into a greenhouse, and detailed infor-mation as to the intensity of incident light at anypoint above the crop as a function of elevation andazimuth is needed. We therefore require greenhousetransmission as a function of these angles. For croppenetration, we further require row crop structuraldetails, leaf angular distribution and leaf area index(LAI), all of which determine light transmission andscatter.

Models are of two types. One calculates averagedlight levels within the crop while the other retains the‘sunfleck’ nature of transmitted sunlight. The non-linear nature of the photosynthesis response to lightlevels sometimes gives rise to different answers forthese two approaches. The former method yields asingle value of irradiance as a function of depth withinthe crop, while the latter produces a correspondingprobability distribution of irradiances. Comparisonsbetween measured and predicted light levels anddistributions is generally good.

2.2. Thermal properties

2.2.1. Sources of energyNatural radiation produced by the sun is the most

prominent source of energy, and varies with season.During the winter at high latitudes, it is insufficient,and additional sources of heat are needed even duringthe day. Methods of introducing heat into the green-house interior comprise traditional hot water pipes,convectors (i.e. heat exchangers), radiant heaters, andfloor or bench heaters.

2.2.2. The thermal energy balance within agreenhouse

The three well known methods of heat exchange,conduction, convection and radiation all contribute tothe thermal properties of the greenhouse. Conductionoccurs through the cladding and greenhouse floor.Convection is generated by a temperature differencebetween the air and the floor, cladding, plants andheaters within the house, and between the air and the

cladding outside the house. Ventilation, which deli-berately transfers heat and air between the interiorand exterior is also relevant. Air leakage also occurs.Radiation exchange occurs between all tangibleelements within and outside the house, and includessun and sky radiant input, together with exchangebetween house elements and the sky. Finally, becauseplants transpire, latent heat is absorbed to producewater vapour, and is released when the water vapourcondenses on the house elements. Convection is themost difficult mechanism to quantify accurately andheat exchanges due to convection are invariably basedon empirical relationships.

At equilibrium a minimum of four non-linear equa-tions are required to link heat exchange between thegreenhouse air, plants, floor, and roof (temperaturesunknown) with heaters, sun, exterior air and sky,these latter being taken as completely specified. Thenumerical calculations require knowledge of radiativeemissivities and view factors, convective heat transfercoefficients etc. The solutions of the equations yieldsaverage temperatures for the four first mentionedelements. The approach to the problem could be des-cribed as within the finite element category in itsbroadest sense, but of the simplest kind, with minimaldivision into elements.

2.2.3. Thermal insulationDouble and triple glazing introduce considerable

reduction of heat losses, but with an accompanyinglight loss penalty.

An alternative approach is to introduce a moveablescreen into the house. This can be drawn horizontallyacross at night, to reduce losses during this period.About 40% saving in heat supply can be achieved inthis way. During the day, the screen can be withdrawn,but a 4% light loss due to the rolled-up material isproduced.

2.2.4. Greenhouse ventilationDuring summertime, the heat input to a greenhouse

may cause the internal temperature to exceed itsoptimal value, and ventilation is essential to preventdamage to the crop. An additional term is thereforeneeded in the heat balance equation (Section 2.2.2).Empirical relationships linking vent angular openingto air volume exchange, knowing wind velocity havebeen developed for some greenhouses.


3. Developments during the 1990s

3.1. Light transmission into greenhouses

3.1.1. Light transmission into full scale greenhousesOnly a limited amount of work is evident over this

period. However, one interesting and potentially use-ful development byMethy et al. (1994)is a systemfor measuring the irradiance distribution over a squaremetre of greenhouse (or other) surface area. Thesystem comprises a matrix of 8× 8 PAR detectors,together with logging facilities.

Wang and Boulard (2000)carried out detailed mea-surements in a plastic tunnel, using 32 solar cells,distributed over the house floor at two axial loca-tions (gable end and centre). A noticeable drop intransmittance (about 10%) near the tunnel side wallas compared with the central section was observed.Straightforward transmissivity measurements to com-pare different cladding materials and greenhouseswere carried out byWagner et al. (1997).

A small scale model greenhouse mounted on aturntable (Geoola et al., 2000) has been used to eval-uate different cladding materials. The model had abase, 6 m×1.2 m, and was 1 m high, and experimentscould be carried out in the open. The authors foundpolycarbonate sheets, treated to reduce droplet for-mation showed a 10% improvement in transmissionof sunlight when wetted over untreated sheets.

Adams et al. (2000)carried out an important exper-iment, comparing the performance of an experimentalgreenhouse unit with that of a full scale commercialgreenhouse. They showed that local differences in cropdepended on total radiation differences, and provi-ded this was taken into account, the small unit wasrepresentative of the commercial house.

Ricierci and Escobedo (1996)compared experi-mentally the transmission of polyethylene and poly-ethylene+ ‘sombrite’ shading material as claddingmaterials for tunnels. Transmission was generallyreduced from about 60–18%, respectively.

3.1.2. The transmission of single unitsof cladding materials

A useful development is the so-called ‘hot/cold’ box(see, e.g.Feuilloley and Issanchou (1996), or Polletand Pieters, 2000) which comprises a hollow box, oneface of which is constituted by the sample under study,

and through which light is passed. Condensation canbe generated internally, and boxes are adapted for out-door use.Pollet and Pieters (2000)evaluated the lossof light that is produced by water droplets condensedon the glass after transmission through single, doubleand low emissivity glass. For uniformly diffuse inci-dent radiation, they found additional losses of 6–7%for the ordinary glass (single or double) but only 3%extra loss with the low emissivity glass. For dry or wetstates, however, the transmittance of the low emissiv-ity glass lay between the single and double ordinaryglass values.Eykens et al. (2000)studied light scatter-ing with and without the presence of water droplets onglass and polyethylene, showing a dramatic increase inscattering for glass (4–81%), but only a small increasefor polyethylene (71–82%).Schultz and Bartnig(1996) demonstrated that there is not a simple‘characteristic’ droplet, because the droplet shape isinfluenced by both roof slope and cladding material.Pieters et al. (1992)developed a computer model topredict light loss through plastic greenhouses withthe inner surface partially covered by water droplets.Light transmission accuracies better than 5% wereobtained.

Scattering by different cladding materials underdry conditions was measured byOrden et al. (1998).They showed thermal polyethylenes generally scat-tered more light (6–11%) than glass (7.5%).

The effects of changing the spectral composition oflight incident on the greenhouse crop was studied indepth byAngus and Morrison (1998). They showed,e.g. that a reduction in the incidence of the fungaldisease Botrytis could be obtained by using a high ra-tio of blue/UV light. Murakami et al. (1996)describea similar exercise looking at the red/far red ratio andits effect on crop behaviour. They showed that thisratio can also be significant and, e.g. affects the timeof flowering of tomatoes. Comparable work has beencarried out byHe et al. (1997). Gomez et al. (1992)carried out a general study of long IR and short IRfilters, to give guidance for greenhouse design.Kittasand Baille (1998)carried out a variant on the spectralperformance of films. From monochromatic analy-ses, they computed PAR, photon flux, and energytransmissivities for different cladding materials. Theyconcluded that for existing materials, the energy trans-missivity parameter gave a sufficient figure of merit,but for new materials, it would be as well to consider


also PAR transmittance. Fluorescence was seen as ofquestionable value.

Schultz (1997a,b)considered the effects of shad-ing to reduce heat ingress during the summer monthsand developed criteria to evaluate cladding materials.Sierra et al. (1994)tested 12 different plastic ma-terials, and concluded that ‘crystal’ polycarbonateswere the most effective greenhouse cladding mate-rials available in the Argentine, being more flexiblethan glass, with a similar light transmission.

Other general studies were carried out byFeuilloleyet al. (1994), Blache (1992), Deltour et al. (1992)andDeltour (1992). Feuilloley et al. (1994)showed thatdroplets on plastic films improved the greenhouseheat balance, but observed that control was needed toprevent drops falling on the crop.Deltour et al. (1992)confirmed that low emissivity glass was generallybeneficial and gave the most economic performancefor the greenhouse.

Hunt et al. (1997)developed a model which, aftercalibration against the radiation transmission of apolyethylene tunnel can be used to predict the perfor-mance of new cladding materials. They also consid-ered methods for balancing and controlling thermaland light transmission characteristics of films.

A comprehensive survey of the properties of clad-ding materials has been carried out byPapadakis et al.(2000). The paper sets out a detailed theoretical back-ground to the subject defining relevant radiometric andthermal parameters. This is followed by a study of thephysical properties of a large variety of cladding mate-rials, together with methods of measuring the definedparameters. The scatter of results and causes thereofare also discussed. The paper concludes with an exten-sive table, setting out corresponding numerical values.

3.1.3. Detailed computer modelling of lighttransmission

There has been very little development in this field.Following Kurata et al. (1991), Wang and Boulard(2000)set-up a light transmission model of a tunnel,representing the curved cross-section by short straightplanar elements. Reflections other than the first wereneglected. The transmissivity of direct and diffuselight could be calculated, as a function of positionon the ground. Fair agreement (∼5%) between mea-sured and predicted irradiances across the house wasobtained.

3.1.4. Light enhancementStoffers (1998)describes a new form for greenhouse

cladding, comprising self-supporting corrugated sur-faces which required no opaque supporting structure.By this means, light transmission can theoretically beincreased to 90% of the incident light, even with ahigh refractive index material such as polycarbonate.

3.1.5. Light penetration into greenhouse cropsA theoretical model, broadly reflecting those des-

cribed in Section 2.1.5, has been developed byThevenard et al. (1999). Starting from direct or diffuselight incident upon the top of the crop, penetrationlosses are calculated, and resultant average direct ordiffuse light levels obtained, including a contributionfrom scattered light. Measured and predicted meanlight levels agree to within 10%.


3.2.1. Sources of energyTeitel et al. (2000)have developed a microwave

heating system, which is capable of heating plants inpreference to the surrounding air, without damagingthe plants. Experiment indicates that only 55% of theheat normally required is needed. The leaves werefound to be warmer than the surrounding air, thus,reducing the risk of moisture-induced disease.Campen and Bot (2000)have developed a localde-humidifier, to reduce the need for ventilation toremove water vapour. The recovered latent heat isre-introduced as sensible heat to the greenhouse.

3.2.2. Thermal energy balancing in a greenhouse

3.2.2.1. Whole greenhouse thermal models.TheGembloux model (see, e.g.Pieters and Deltour, 1997)represents a significant advance on the model de-scribed inSection 2.2.2. The new model is dynamic,so that variations in state parameters with time areobtained. In addition, the soil within the greenhouse issubdivided into four layers, or finite elements, insteadof just one. Further, the external subsoil is includedas a conduction source/sink. The resulting model nowcomprises eight non-linear heat and one mass balanceequations. The treatment of the effect of condensationis considerably developed, e.g. a composite thermalcapacity is used as and when liquid condenses on


to the cladding. The IR radiation transmission ofcladding is also then modified. To avoid numericalinstability problems, low levels of condensation arerepresented by a patchwork coverage of the claddingsurface rather than very thin liquid layers. The ref-erenced paper describes a comparison between fourgreenhouses, standard glass, low emissivity glass,polyethylene and thermal polyethylene. This was car-ried out over an entire year, for a tomato crop. Theeffects of condensation on heating requirements wereparticularly considered. It was found that if conden-sation were ignored, a 15% underestimate for glasscladding, and a 20% overestimate for polyethylenecladding were obtained. Effects of neglecting otherparameters, e.g. latent heat, were also studied. Thismodel was adapted byWang et al. (1997)to success-fully predict heat and mass exchanges between theinside and outside of a plastic tunnel.

A model developed byVollebregt and van de Braak(1995) is also in principle an advance on the basicmodel (Section 2.2.2). These authors tackled the prob-lem of heat loss through a Venlo greenhouse side wallwith heating pipes mounted in a vertical array near theside wall. An imaginary internal wall, 3 m from thereal wall and 3 m high was assumed to separate theregion of interest from the rest the house. The methodof solution comprised first estimating heat transfercoefficients near to the real wall and using these es-timates together with calculated radiative coefficientsas input to the heat balance equations. The output isthen used to compute more accurate convective heattransfer coefficients using a standard computationalfluid dynamics (CFD) model. These were then usediteratively to the point of convergence. The modelrequired sub-dividing the real wall into 15 elements,the entire imaginary wall constituting a sixteenthelement. The heating pipes and greenhouse surround-ings formed the final two elements. In addition, theCFD package used a fine uniform sub-division ofthe air space it was modelling. The solutions pro-vide a detailed pattern of temperatures and air flowsthroughout the simulated region of the house. The to-tal heat loss to the outer wall could then be calculated.Comparison with measured losses showed predictionswere consistent, and underestimated temperatures bya maximum of 2◦C. The use of the CFD package,thus, provides the necessary resolution and accuracyin evaluating a local hot spot in the greenhouse.

Navas et al. (1998)have constructed a dynamicmodel to represent a Mediterranean greenhouse cli-mate. The authors divide the greenhouse into ‘process’and ‘boundary’ components. These comprise thegrowing medium, soil, crop, cladding and inside airand respectively the sky, heating system, etc. Themodel is of similar complexity to that described inSection 2.2.2, except for the addition of the time di-mension. Results indicate a good prediction of overalltemperature variations with time under typical am-bient conditions during winter or spring in Madrid,though local small scale errors (∼1◦C or so) aredetectable.

Pita et al. (1998)carried out a conventional en-ergy balance analysis for a Mediterranean greenhouseobtaining good agreement with experiment for tem-peratures over the 24 h period.

3.2.2.2. Sub-elements within the greenhouse.Therehave been a number of studies looking at particularaspects of heat balance.

Yang (1995)considered the effect of air movementclose to plants (Chrysanthemums) in detail and for-mulated three resistance terms, stomatal resistance,related to physiological control, boundary layer resis-tance, controlling exchange processes close to leavesand aerodynamic resistance, which controls verticalair movement.

Shibuya et al. (1997)studied experimentally theevapotranspiration, and sensible and latent heat ratesof transfer associated with tomato plug sheets undergrowing conditions. A comparable problem was con-sidered byAl Massoum et al. (1998)who evaluated theefficiency of cooling plugs used in greenhouses oper-ating under desert conditions. Air was drawn throughthe wet pads, and the temperature on either side of thepad was noted. Water consumption was also measured.Cooling efficiency was found to increase significantlywith air and water flow rates.

Hayashi et al. (1998)measured experimentally thecooling effect of introducing fog (intermittently) intoa naturally ventilated greenhouse. They showed thistechnique was much more effective in reducing airrather than leaf temperature. Air cooling at the rate of10◦ min−1 was achieved.

Wei et al. (1998)devised and tested a sensor todetect condensation on tomato plants. Using bare elec-trodes, good agreement between detector impedance


changes and dew point measurements indicatingperiods of wetness, was obtained.

Fynn et al. (1993) established a relationshipbetween stomatal resistance and solar irradiance lev-els for Chrysanthemums, by correlating their evap-otranspiration with ambient parameters in a variableshade greenhouse.

Wu et al. (1994)set-up a thermal model of the heatflow from exchangers in the greenhouse sub-surfacein order to predict optimal design, whileRath (1994)calculated the effects of the thermal storage of agreenhouse on heat consumption.Benavente et al.(1998) compared the energy consumption predictedby a thermal model of a heated soil substrate with ex-perimental results. The model comprised basically asolution of the well known conduction equation. Fora greenhouse set-point of 18◦C, the comparison wassuccessful, with the prediction of the average monthlyenergy consumption accurate to 7%.Garcia et al.(1998)generated a time dependent model and evalu-ated the effectiveness of a concrete heated floor in agreenhouse. The model showed that savings dependedon the height of the temperature set-point. With theset-point sensor mounted at 0.5 m, 20% energy savingscould be achieved. This fell to zero with the set-pointcontrol mounted at 1.5 m. Agreement between ex-periment and predictions of heat consumption wasaround 5%.

Li et al. (1997)set-up a model of a conservatorytype greenhouse, and demonstrated the importanceof the back wall in maintaining the thermal state ofthe greenhouse.Kittas (1994)considered night-timelosses through the greenhouse side walls, calculatingradiation and convection separately.

Kostyuk et al. (1990)carried out measurements ofair and leaf temperatures of tomato plants in green-houses. They demonstrated an air/leaf temperaturedifference of only 1 or 2◦C, but found a differential of5–9◦C existed over the depth of the plants.Kempeset al. (1998)also considered the vertical temperaturedistribution in a greenhouse crop, both by modellingand experimentally. Heating was provided by pipes atdifferent heights within the crop. Comparisons weregood, but the local effects of increased pipe temper-ature were found to be very limited. The temperaturedifference over the crop depth was found to be about1.5◦C. Langton et al. (2000)carried out temperaturemeasurements on chrysanthemum and dieffenbachia

leaves growing in greenhouse compartments. Differ-ent shading treatments were employed, and forcedventilation was used to ensure uniform thermal envi-ronments within a compartment. Good agreement withpredictions of leaf temperature were obtained. Leaftemperature responded rapidly to heat input and tem-perature variations of 5◦C could occur across a singleleaf.

Boulard and Wang (2000)attempted to simulatecrop transpiration within a greenhouse in terms ofexternal ambient conditions. They found acceptableagreement when the vents were open, but correlationdeteriorated when the vents were closed.

3.2.3. Thermal insulationThe thermal screen described inSection 2.2.3has

been subjected to a number of modelling treatments.Teitel and Segal (1995)modelled the nocturnal heatradiation under a woven horizontal screen usingbroadly the approach described inSection 2.2.2, forthe thermal modelling of the greenhouse. They foundthat screen ‘solidity’, its optical properties and thescreen/ground area ratio all affected the net radiationunder the screen. Aluminisation was found to reduceradiation.van de Braak et al. (1998)modelled heatand mass transport through thermal screens as part ofa general thermal modelling of the greenhouse. Themodel was used to devise thermal screen/ventilationrate strategies in conjunction with different humidityset-points. The latter were found to strongly affectheat consumption whereas night time screen apertureshad a much weaker effect.

Miguel et al. (1998)examined experimentally thepermeability and porosity of samples of conventionalthermal screens to air flow. Permeability was foundto be of the order of 10−11 m2, while porosities wereless than 10%. They also obtained coefficients for var-ious convective heat interchanges. The location of theheating pipes had little or no effect on the convectiveheat transfer between the pipes and air.

Modelling double glazing has been very lim-ited over this period. However,Zhou et al. (1998)have studied theoretically the ‘U’ value (W m−2 ◦C−1)of aluminised honeycomb plastic sheet, finding asignificant improvement over the correspondingun-aluminised sheet (the use of wavelength selec-tive materials is reviewed under optical properties,Section 3.1.2).


Fig. 1. Influences of solar radiation and crop cover on air temperature in a ventilated greenhouse.

3.2.4. Ventilation

3.2.4.1. Ventilation requirement of a greenhouse.Insummer, the temperature and humidity in a greenhouseare limited by ventilation. The responses, shown inFigs. 1–3, were obtained using a simple energy bal-ance model (e.g.Day and Bailey, 1999). Greenhouseventilation systems are generally designed to provide amaximum air exchange rate of 0.035 m3 s−1 m−2. Theair temperature depends on the solar radiation enteringthe greenhouse and on the relative areas of evaporat-ing (plant leaves) and non-evaporating surfaces (drysoil, paths, etc.) that absorb the radiation. Plants pro-duce water vapour by transpiration and at low ven-tilation rates the relative humidity of the greenhouseair is limited by condensation on the inner surface ofthe cover. The low humidity that occurs in a green-house with few plants can result in a high transpira-tion which creates water stress in plants. The physicalprocess of transpiration is linked more closely withthe vapour pressure deficit (vpd) of the greenhouse air

than with relative humidity and this is now commonlyused as the controlled variable in relation to green-house humidity. High values of vpd indicate the possi-bility of water stress and low values indicate low planttranspiration and the possible occurrence of fungaldiseases.

3.2.4.2. The analysis and effects of external air flow.The flow of air through an open ventilator is describedby the Bernoulli equation in which a discharge co-efficient accounts for the loss in the conversion ofpotential to kinetic energy.Miguel (1998)measureddischarge coefficients for openings in a box with andwithout flaps and confirmed the earlier results ofBot(1983). In studies on a large-scale model greenhouse,Munoz et al. (1999)found that the discharge coeffi-cient depended on the location of the ventilator; lowervalues were found for ventilators in the middle of amulti-span greenhouse than near the perimeter.

Two phenomena create the pressure difference thatproduce the air flows, the internal–external tempera-


Fig. 2. Influences of solar radiation and crop cover on relative humidity in a ventilated greenhouse.

ture difference which creates a density difference, andhence, a pressure difference, and wind which producesa fluctuating pressure field over the greenhouse sur-face. The basic description of the flow resulting fromthe temperature difference was given byBot (1983),andWang (1998)gives equations to estimate ventila-tion fluxes through ventilators at various locations ina greenhouse.

Temperature driven ventilation can be significant inhouses with sidewall and roof ventilation and whenthere is a large internal/external temperature differ-ence. However, for large multi-span greenhouses withonly roof ventilation, the potential is small because thevertical dimension of ventilators is small. Tempera-ture driven ventilation only becomes significant at lowwind speeds (Bot, 1983; Papadakis et al., 1996).

Wind creates a pressure distribution over a green-house which is influenced both by the mean windspeed and turbulence. The mean pressure distributionhas been characterised by a non-dimensional pressurecoefficient that relates the local surface pressure tothe dynamic pressure of the free stream wind. Meanpressure coefficients have been studied extensively

because they are important in establishing the forcesimposed on a greenhouse by the wind (Wells andHoxey, 1980). Measurements made in a wind tunnelon a single span greenhouse model with open venti-lators (Sase et al., 1980) showed that the coefficientwas influenced by both the angle of opening and bythe wind direction relative to the ventilator.Boulardet al. (1998)showed the mean pressure coefficient fora long slot ventilation opening in the roof of a twinspan greenhouse with the wind parallel to the slot,varied in a linear way from negative at the windwardend to positive at the leeward end.

Fluctuating pressure coefficients have not been stud-ied to the same extent.Gandemer and Bietry (1989)measured both coefficients on a single span buildingin a large wind tunnel and found that the fluctuatingcoefficient was not as sensitive as the mean coefficientto wind direction nor to surface orientation.Boulardet al. (1998)reported similar values for a long slotventilator in a twin span greenhouse.Papadakis et al.(1996)found no differences between fluctuating pres-sure coefficient values measured for continuous ven-tilators in the roof or sidewall; at high wind speeds


Fig. 3. Influences of solar radiation and crop cover on the air vapour pressure deficit in a ventilated greenhouse.

the values were constant, but at low speeds there werevariations attributed to thermal forces.

Miguel (1998)measured the wind power-spectrumdensity to determine the magnitude of the turbulentcomponent and showed that it contributed between 13and 52% to the total pressure, the largest contributionwas at the highest wind speed. The contribution ofthe turbulent component to the total air flow and theeffect on heat and mass exchange during ventilationhas been studied byBoulard et al. (1997a).

An alternative approach (Boulard et al., 1996) hasbeen to consider that wind creates a time dependentpressure field over the greenhouse which results inan average pressure difference between the inside andoutside. This results in an average pressure coefficientthat combines the mean and turbulent wind effects ofthe wind.Papadakis et al. (1996)showed that, for a400 m2, single span greenhouse this coefficient was aconstant which depended on the type of ventilator (sidewall, roof or roof and side wall) provided the windspeed was greater than 2 m s−1. Bailey (2000)foundthat a coefficient of similar value could be applied to

wind driven ventilation through leeward ventilators ina 4 ha multi-span glasshouse.

Walker and Wilson (1993)reviewed methods ofcombining the estimates of airflow created by the windand temperature effects. Using an experimental dataset they showed that adding the individual pressuredifferences yielded the combined flow with a maxi-mum error of around 10%.Boulard and Baille (1995)derived an equation for the combined effects for roofventilation through continuous ventilators.

The linear relationship between wind speed and theair flow through a ventilator in a greenhouse was iden-tified by Morris and Neale (1954). To apply this toventilators of different sizes,Bot (1983)introduced adimensionless ventilation coefficient that was a func-tion of the ventilator aspect ratio and angle of opening.Values for this coefficient have been determined byBot(1983), de Jong (1990), Fernandez and Bailey (1992),Boulard and Draoui (1995), Bailey (2000), Kittas et al.(1995)andWang (1998). Bot (1983), de Jong (1990)andBailey (2000)found a non-linear dependence ofthe coefficient on angle of opening which contrasted


with the other results.Wang (1998)suggested theresults obtained by Bot and de Jong could have oc-curred because the measurements were made in smallcompartments within a large multi-span greenhouse.However,de Jong (1990)found non-linear relation-ships in greenhouses with areas between 1430 and6790 m2 which had not been subdivided, andBailey(2000)obtained a non-linear response in a 38,700 m2

Venlo greenhouse.Almost all the measurements of air exchange rates

have been made using leeward ventilation.de Jong(1990) measured windward ventilation but only forventilators opened to 12◦. The ventilation coefficientswere larger than the corresponding leeward values andthe dependence on angle of opening was related to theaspect ratio. For windward ventilator openings of upto 12◦ the total ventilation flux was found to be thesum of the leeward and windward fluxes.

3.2.4.3. Internal flows created by natural ventilation.To be effective in cooling a greenhouse the ventila-tion process must (i) provide sufficient air exchangebetween the greenhouse and the outside, (ii) providegood mixing between the incoming air and the green-house air and (iii) create an internal air movement thatinduces good heat and mass exchange between theplants and air. There have been relatively few studieson the air flows created inside greenhouses by naturalventilation or on the exchanges with plants.

The orientation of plant rows in relation to the sidewalls was shown bySase (1989)to have a considerableeffect on air flow resulting from side wall ventilators.Where the plant rows were parallel to the continuousventilators along both side walls the internal air speedwas 10–20% of the wind speed, while where the rowswere perpendicular to the side walls the air speed ratioreached 40%. In both cases, the internal flow was inthe same direction as the external wind. When onlyridge ventilators were open the average value of theair speed ratio was 12% and it was independent of roworientation and wind direction.

A number of studies have shown that in wind drivenventilation, air enters the greenhouse through venti-lators in the down wind part of the house and leavesthrough ventilators in the up wind part (Boulard andDraoui, 1995; Wang, 1998; Wang et al., 2000). In alarge multi-span glasshouse containing a fully growntomato cropWang (1998)found with leeward ventila-

tors open that the internal air flow was in the oppositedirection to the wind. With combined leeward andwindward ventilation the flow direction was reversed,with air entering at the windward side and leaving atthe leeward side of the glasshouse. When the ventila-tors were closed an air flow still occurred in the bodyof the greenhouse and was in the opposite directionto the wind. Using a model twin-span greenhouse ina boundary layer wind tunnel,Okushima et al. (1998)showed that air entering the windward ventilator inthe windward span was the dominant factor in de-termining the internal air flow. A re-circulating flowwas created which was in the direction of the wind inthe upper part of the greenhouse and in the oppositedirection at low level. The most uniform distributionof air speeds was obtained with leeward ventilation.

Re-circulating flows were found byKamarrudinet al. (2000)in studies made using a water flume witha model tunnel greenhouse with ventilator openingsin both sidewalls and on both vertical sides of a raisedlantern or jack roof. The water flow was visualisedusing white neutral density particles illuminated by athin plane of light which were video recorded and thevelocity field calculated.

Computational fluid dynamics is being used increa-singly to study the internal flows in closed (Boulardet al., 1997b) and ventilated greenhouses (Mistriotiset al., 1997a,b; Haxaire et al., 2000; Boulard et al.,1999a; Shklyar and Arbel, 2000; Reichrath et al.,2000). These studies have benefited from techniquesdeveloped for flow measurement and characterisa-tion that are based on sonic anemometry. These havebeen used to map the flow fields induced by winddriven ventilation in tunnel (Boulard et al., 1999b),twin-span (Boulard et al., 1997a; Haxaire et al., 2000)and multi-span (Wang, 1998) greenhouses.

A technique for visualising and quantifying airflows created by the stack effect based on a salt gradi-ent technique has been described byOca et al. (1999).Estimates of the temperatures were within 25% ofthose obtained from a theoretical ventilation model.The method was used byMontero and Anton (2000)to study ventilation in a tropical tunnel greenhouse.

The existence of re-circulating and secondary flowsduring ventilation means that mixing is incompletewhich will affect the uniformity of the environmentalconditions created. When greenhouses have only roofventilators, as with the multi-span greenhouses used


in commercial horticulture, coupling of the air flow inthe roof space with that in the crop zone is of majorimportance as it determines the heat and mass trans-fer from plants.Okushima et al. (2002)investigatedthe internal temperature distribution in a model fourspan Venlo glasshouse with fully open windward andleeward ventilators. Without a crop canopy the tem-perature on the windward side was higher than on theleeward side and an air circulation was considered tooccur even when the wind speed was zero. When themodel contained a simulated mature tomato crop withthe rows parallel to the ridges it was found that thehighest air temperatures occurred at low wind speeds.A large weak re-circulating flow occurred in the spaceabove the crop.

3.2.5. Insect exclusionThe protection of crops from damage by insects is of

increasing importance in warm regions. In practice thisis achieved by placing small mesh nets (insect screens)over ventilation openings, however, the resistance ofthe screens reduces the effectiveness of ventilation.The resistance of insect screens has been investigatedusing the approach of Bernoulli with experimentallydetermined discharge coefficients (Sase and Christian-son, 1990; Kosmos et al., 1993; Montero et al., 1996;Munoz et al., 1999; Teitel, 1998; Teitel et al., 1999).This has resulted in the design of systems suitable toexclude insects from greenhouses that use forced ven-tilation systems. Another approach has been based onthe flow through porous media using the Forchheimerequation (Miguel et al., 1997; Miguel, 1998; Migueland Silva, 2000). This gives similar results to the dis-charge coefficient method when the Reynolds numberexceeds 100–150, but it also enables the analysis offlows at lower Reynolds numbers. When greenhousesuse natural ventilation, the pressure loss by screens isminimised by making the area of the screens as largeas possible. However, there are still reductions in theair flow which are detrimental because in warm cli-mates effective ventilation is essential to provide nearoptimal growing conditions in greenhouses.

3.3. Optimisation of carbon dioxide concentrations

We consider only engineering aspects that affect orare affected by ventilation, light levels and thermalproperties.

Carmi (1993)looked at carbon dioxide enrichmentin conjunction with full sunlight or with 50% shading,appropriate for the winter season in a sub-tropical cli-mate. He found that carbon dioxide enhancement pro-duces earlier yields for either irradiance cases, but healso proposed that a partly shaded enriched crop mightgive a higher production of winter tomatoes than oneexposed to full sunlight, but without enrichment.

Ryu et al. (1993)has developed a CO2 enrichmentcontrol algorithm to control plant growth, whileLibikand Starzecki (1998)proposed strategies for use witha prototype CO2 controller model, EUDW-2. Theyadvised specific reduced CO2 levels when vents wereopen.

The general principles of providing optimal controlof the greenhouse CO2 concentration were outlined byChalla and Schapendonk (1986). Biological modelswere used to estimate photosynthesis and respiration,and crop yield was based on integrated net canopyphotosynthesis. A physical model was used to deter-mine the loss of CO2 by ventilation and the unit priceof CO2 used to determine the cost of CO2 enrich-ment.Chalabi (1992)developed a generalised strategyfor optimal control of CO2 enrichment.Chalabi andFernandez (1994)compared mechanistic models of netcanopy photosynthesis for tomato with measurements,using a CO2 mass balance method: discrepanciesbetween the model and experiment were explained.Ehler and Karlsen (1993)described a model based,expert system to optimise CO2 enrichment for sweetpepper. This continually adapted the set-points of theclimate controller to the greenhouse climate and thephysiological status and stage of development of thecrop. A constant economic dry matter index was usedto convert from photosynthesis to yield. The optimisedtreatment used less CO2 but gave a greater yield.Nederhoff (1988)tested the dynamic optimisation ofCO2 with cucumber, making on-line simulation cal-culations of CO2 cost and yield using real time valuesof solar radiation, wind speed and ventilator aperture.With optimisation, the CO2 was used selectively, theset-point being reduced as ventilation increased andincreased as radiation increased. Optimisation gave ahigher production and a relatively low expenditure onCO2 compared to two constant CO2 set-points treat-ments.Aikman (1996)described a procedure for CO2optimisation with a tomato crop. A Gompertz modelfor the kinetics of fruit growth was used to predict


the time distribution of photosynthate over successiveharvests. This was used with predictions of marketprices to estimate, for each day, a factor to convertCO2 assimilate to expected financial value from par-titioning to the fruit. The cost of enrichment wasestimated from a prediction of ventilation and the unitprice of CO2. Chalabi and Zhou (1997)outlined anoptimisation procedure to determine the CO2 set-pointtrajectory that maximised the instantaneous net in-come rate while ensuring that the CO2 set-point, therate of change of the set-point and the CO2 supply ratewere bounded. Using simulation models,Bailey et al.(1997)compared the economic performance of opti-mal control with control strategies used by growersand with improved, but non-optimal strategies. Com-parisons have also been made between the economicperformances of optimal and other enrichment strate-gies using pure CO2 (Bailey, 2000) and CO2 obtainedfrom the exhaust gases of natural gas fired boilers andcombined heat and power (CHP) units (Bailey, 2001).

de Zwart (1998)modelled four different strategicplans using boiler exhaust to supply carbon dioxide.The first plan couples carbon dioxide supply to heatdemand. The remainder divorce the two requirements,and cope with excess heat by ventilation or short-termstorage, thus, permitting generation of exhaust gasesfor enrichment purposes only. The use of heat storageappeared to be the best option.

Kenig and Kramer (2000)used the carbohydratebalance over a 3-day period to predict the succeed-ing 3-day optimal nocturnal temperature. This workfollowed from the observation that CO2 enrichmentfollowed by night-time heating of a musk lemon cropincreases the yield by a factor of 2.Akilli et al. (2000)have studied different crop responses to CO2 enrich-ment up to 1000 ppm. Yields were generally increased,but a melon crop showed no significant difference tothe control crop.

3.4. Overall greenhouse and crop models

We now briefly mention more extended schemeswhich use light transmission and thermal balance com-puter models as subroutines.

The greenhouse crop production model ‘HORTISM’(Gijzen et al., 1998) combines seven distinct subrou-tines which include modelling of greenhouse lighttransmission, light penetration into the crop, CO2 con-

centration and thermal models.Nederhoff and Vegter(1994a,b)adapted smaller scale earlier models to pre-dict (for example) tomato canopy net photosynthesis,with some success.Hansen and Ehler (1998)andEhler and Hansen (1998)set-up a different scheme,comprising a polycarbonate box, within which irra-diance, relative humidity CO2 concentration, and airtemperature in an associated greenhouse can be mea-sured, in order to predict and compare with actualplant photosynthesis in the mentioned greenhouse.Comparison with experimental measurements showedcrop photosynthesis could be predicted but factorssuch as nutrient induced stress in Begonias were notthereby detected.Gary et al. (1998)combined green-house climate and crop models to form an overallscheme, this time directed towards educational use,enabling students to benefit from the latest advancesin greenhouse modelling technology.

3.5. Use of combined heat and power (CHP)systems in greenhouses

Small scale CHP systems, also known as totalenergy or co-generation systems, use an internalcombustion engine or gas turbine to drive an electricgenerator. Such systems have been installed on manygreenhouse complexes particularly in The Netherlandsand the United Kingdom. In most cases, the unitswere owned and operated by power utilities in order tohave geographically dispersed electricity generation;the heat produced was supplied to the greenhousesat a discounted rate. The engine exhaust gases werefrequently passed through a catalytic converter anddistributed through perforated film plastic tubes in thegreenhouses to provide CO2 enrichment. Because ofthe greenhouse CO2 requirement, the units were gener-ally operated during the day and the heat produced thatwas in excess of the greenhouse demand was storedas hot water and used during the following night.

Using a CHP system on a nursery raises its pri-mary energy consumption, however, the electricityproduced replaces production at a large power plant.Unlike the case with CHP, the reject heat produced atlarge power plants is generally not used, so on a na-tional scale energy conservation objectives are servedby using CHP systems. Greenhouse operators havenot installed CHP systems themselves because of theirhigh capital cost and the imbalance between heat and


electricity consumption on nurseries (Bailey and Ellis,1989), except on those producing flowers which usesupplementary lighting (Huijs and Kaspers, 1985).However, the present arrangement makes the provi-sion of heat and CO2 to the greenhouse dependent onthe CHP operating policy adopted by the power utility.

4. Discussion

4.1. Light transmission

Progress during the last decade is limited mainlyto experimental studies of the behaviour of individualsheets of cladding, looking at a variety of proper-ties. Cladding materials include ordinary horticulturalglass, low emissivity glass, double glass, polyethy-lene, and other specially manufactured plastics. Prop-erties studied range from measurements of PAR andspectral transmission to investigating the effects ofchanging small regions of the spectrum on cropbehaviour. This includes the use in Mediterraneancountries of plastic films, selectively absorbing inthe near IR region, to reduce the thermal load withinthe greenhouse. The emphasis on these transmissionstudies is probably due in part to the development ofa range of new plastics during the last decade, thoughwhether the demand or the supply came first is notobvious. The effects of water on transmission, eitheras a film, or as droplets has occupied a large fractionof these studies, and is beginning to be properly un-derstood, though there are still problems in predictingsuch effects because of the variable shape of droplets.However, computer models are now available topredict light transmission in their presence.

In an attempt to introduce natural conditionsinto light transmission measurements, a number ofresearchers have used a test rig, the ‘hot/cold’ box,whereby the sample can be evaluated under outdoorconditions. The sun or sky, rather than a simulatinglight source can then be used to measure light trans-mission. Internal heaters generate water vapour andso effects of condensation can also be investigated.This is obviously a useful addition to the test facil-ities, but experiments still need to be carried overlong time spans. Any criteria for the acceptance ofa cladding system is of limited value without a timedeterioration factor.

Experiments measuring the scattering of light dur-ing transmission have been introduced, constituting auseful advance, which recognises the limitation of ear-lier measurements and modelling. The developmentof an 8× 8 PAR cell matrix to measure an area distri-bution of irradiance recognises the irregular nature oftransmitted light, especially under sunny conditionsand the problems associated with point sensors. Alsothe conclusion that compartmented test greenhouse el-ements can be considered representative of large-scalegreenhouses is of some practical importance.

Stoffers (1998)proposal for a different claddingdesign should be studied further, looking at practicalconsiderations of strength and long-term high trans-mission before any conclusions can be drawn. As ageneral point, corrugations certainly increase resis-tance to bending about an axis through them and atright angles to their length, but reduce it about an axisparallel to them.

There is no real progress in the area of computermodelling of either greenhouse light transmission orpenetration into tall rowcrops, any new work being anindependent reworking of existing ideas. The use ofthe earlier models in larger scale schemes can howeverbe seen as an advance.


There have been significant advances in the mod-elling of the thermal behaviour of the greenhouse.The Gembloux model introduces the time dimen-sion, and also uses a fairly simple but theoreticallyimportant advance, namely the use of finite elementtype sub-division of the soil substrate. Less impor-tant perhaps theoretically but necessary in practice isthe subsequent inclusion of the external subsoil. Therecognition of condensation effects is also of highsignificance, both practically and conceptually. TheVollebrecht and van de Braak model is of similarimportance, though the emphasis is now on the finiteelement aspect of the model. The motivation was toobtain high accuracy modelling of the air and heatmovement in a local ‘hot spot’, i.e. the heating pipesclose to an external wall. However the principle of sub-division into elements, even though some of them arehidden within a CFD package is an important advance.

Quite a number of smaller investigations havebeen carried out, looking, e.g. at the thermal effects


of condensation on the crop, the effects of thermalcapacity, and the value of floor heating. Several in-vestigations looked at the variation of temperatureover the depth of the crop and between the crop andits surrounding air. These results yield fine detail, andtherefore, constitute a significant advance.

New forms of heating are scarce, apart from themicrowave system, which looks promising.

A considerable advance in the theoretical and prac-tical understanding of thermal screens has occurred.Modelling work on the screen as part of the wholehouse, or as a specific item have both been carriedout. Studies range from a consideration of the effectsof floor to screen area ratio, to a study of the effectson heat consumption of the screen aperture. Detailedmeasurements on permeability and porosity of screenmaterial have also been reported.

Work on other forms of thermal insulation has beenvery limited.

4.2.1. VentilationThis subject has received a lot of attention during

the 1990s. Studies are still at a relatively early stage.They are nearly all limited to leeward ventilation, andhave shown wind-induced ventilation to be more sig-nificant than temperature induced ventilation in largemulti-spans. Attempts to combine the two are howeverevident. Wind ventilation comprises two parts whichdepend on mean wind speed and turbulence, respec-tively. The former is usually dominant except at highwind speeds, when the two are comparable. Studies ofthe variation of pressure coefficients along the lengthof the house, and as a function of wind direction havebeen carried out. Differences are evident in the rela-tionship between air speed and air flow through a ventobtained for different types and sizes of greenhouse.

Air flows within the house are highly dependent onrow orientation with side ventilation, though not withridge ventilation. Circulating flows are found to beinduced, whose direction is dependent on whether thevents are open or not, and which are open (leewardor windward). For a twinspan, there is evidence thatwindward ventilation is dominant in determining in-ternal air flows. Modelling techniques both physicaland computational are beginning to be used for thesestudies. The existence of circulating flows impliesthat temperature inhomogenities are likely to existand investigations have confirmed this to be so.

Insect exclusion by the use of nets over ventilatoropenings causes a reduction in the efficiency of nat-ural ventilation. Methods of compensating for this inwarmer climates have been studied, including the useof forced ventilation, or larger ventilating openings.

4.3. Carbon dioxide optimisation

Considerable progress has been made in develop-ing methods to provide CO2 set-point trajectories thatmaximise the margin between the financial benefit ofenrichment and the cost of the CO2 used. The mathe-matical basis of optimal control has been formulatedand the effect of system constraints encountered incommercial greenhouses included. Practical tests ofoptimal control algorithms have been made in re-search greenhouses containing tomato, cucumber andsweet pepper crops. The increases in grower incomefrom adopting optimal control over conventionalquasi-constant set-point control have been established.

The CHP system is an interesting method of en-ergy conservation, provided it is introduced on thelarge-scale. Further, CO2 can be usefully employed,instead of being discharged into the atmosphere.

5. Proposals for future work

5.1. Light transmission studies

While there is considerable progress in the evalu-ation of new cladding materials (excepting long-termtests), understanding and predicting light distributionpatterns in greenhouses and tunnels has not movedvery far. Apart from the specific case of the singlespan tunnel, there is still a need for a comprehensivecomputer model to predict irradiance distributionsin multi-span tunnels of variable roof cross-section.The creation of such a model does not require a sig-nificant advance from existing concepts (Section 2).The only novelty lies in the presence of curved ratherthan plain roof surfaces. Curved surfaces can ade-quately be represented by short straight sections, ashas already been done for the single tunnel, or wecould fit the closest ellipse to a given tunnel roof spancross-section. Either way, knowing the start point fora ray, the various intersections of transmitted and


reflected beams can be calculated as before usingthree-dimensional co-ordinate geometry.

Though small in number, the studies of light scat-ter by greenhouse cladding materials points to a needto include scattering in the light transmission mod-elling process. This is complicated, because a singleincident beam can produce scattering over the entiresolid angle of 4π , creating a new set of ‘incident’rays. However, once the scattering pattern of a givencladding is known, the problem is soluble by numer-ical means, though the stronger the scattering, thelonger and more complex any program becomes.

Irradiance measurement within a greenhouse hasalso received little attention, apart from the develop-ment of the 8× 8 cell matrix. Early studies, describedin Critten (1993), have also not progressed, despitea promising start. The earlier work showed sky-light could reasonably be analysed into lower orderspherical harmonics (Legendre polynomials). These,together with direct sunlight and taking greenhousestructure into account, can readily be used to generateinput for a greenhouse light transmission computermodel to yield local radiance distributions at anypoint above the crop. Subsequently, a light penetrationmodel can be used to predict detailed leaf irradiancethroughout the crop.

5.2. Thermal studies

The models described in the early paragraphs ofSection 3.2.2point the way towards further studiesin this area. These new models still contain a fairamount of ‘lumped’ heat sources and/or sinks, andwe may perhaps consider whether we need to refinethese further to obtain more detailed temperaturedistributions, etc. rather than using mean values.

The new experimental work described in thisreview indicates that there are temperature differ-ences between, say the top and bottom of a tall crop(Section 3.2.2). This being so it may well be worthdividing the crop into vertical elements. The need forgreater accuracy near heating pipes to predict detailedair flow and temperature patterns, also described inthe same section may equally well apply to patternsnear vents, especially on windy days.

Experimental evidence confirms that current modelpredictions of mean temperatures are good, butfine detail is not always correctly predicted. We

therefore perhaps need to treat all the state vari-ables (temperature, humidity, air velocity, latent heatchanges/condensation) as distributed variables, tocheck for local ‘hot spots’. Thermal screens shouldreasonably be included in such a study.

The finite element method is well suited to carryout these analyses. To do this, the greenhouse compo-nents may be divided into linear (e.g. heating pipes),areal (e.g. cladding surfaces) and volumetric (e.g. theair), types. These may then be divided into elements,and the corresponding model temperatures evaluatedaccording to standard techniques. This would give adetailed analytical picture of the thermal behaviour ofthe greenhouse. In principle, conductive, convectiveand radiative exchanges can all be included. However,the convective heat exchanges may require more at-tention than the other two, and it may be necessaryto develop separately an in-house CFD package, com-bining internal air convection and ventilation effects.

There is a problem with the finite element methodif it is applied to the greenhouse which stems from thenon-local nature of thermal radiation. In principle, anyelement is likely to exchange radiant heat with anyother element, resulting in a so-called ‘sparse matrix’in any numerical simulation. However, there will besome screening of the elements and moreover presentday microcomputers are very fast and contain vastamounts of RAM and so a reasonable sub-division ofthe various elements in the heat balance model shouldbe possible.

Finally, the so-called ‘hot/cold’ boxes, developedto test thermal and optical properties of claddingmaterials should be adapted to carry out long-termdeterioration studies of these materials. It would alsobe sensible to standardise any such equipment so thatresults world wide can be compared.

5.2.1. VentilationThe results ofWang (1998)and Boulard et al.

(1998)that air enters through ventilators on the downwind side of the glasshouse and leaves through venti-lators on the up wind side, is consistent with conclu-sions that can be drawn from the pressure coefficientdata of Wells and Hoxey (1980). These indicate astrong negative pressure at the windward side of thegreenhouse roof and a weak negative pressure at theleeward side which implies an inflow at the leewardside and an outflow at the windward side. There is


clearly considerable scope for further studies on theair flows created by ventilation to understand morefully the wind and stack effects individually and incombination.

The understanding of the physical processes thatdrive natural ventilation has not reached the stagewhere the rates of air exchange can be predictedfor the greenhouses used in commercial horticulture.Such information is required for the implementationof economically optimal environmental control andfor a better understanding of the way in which thesummer climate can be regulated to reduce plant stressand improve product quality. Research is required toestablish how wind driven ventilation is influenced byventilator configuration, whether greenhouse size hasan influence, to quantify the flows through windwardventilators and to determine how the flows throughleeward and windward ventilators interact and thetotal flow can be estimated.

Information is also required on the air flows createdinside greenhouses by natural ventilation. There ap-pears to be a change in the direction of air movementin a greenhouse with leeward ventilators open whenthe windward ventilators are opened. The interactionbetween the fluctuating and steady components ofwind driven ventilation is unknown. There is littleinformation on the coupling between the heat andmass exchanges of plants and the ventilation air. Theinfluence of ventilator position on the creation of sec-ondary flows and regions with low air movement hasnot been studied in depth, although it has considerableimplications on environmental uniformity. The impli-cations of the consequential incomplete mixing on themeasurement of ventilation also need to be considered.

5.3. The computer-controlled greenhouse

The development of large-scale greenhouse andcrop computer models (Section 3.4) prompts us toconsider whether we can go a stage further and set-upan automated greenhouse, for a single crop. The ma-jor obstacle to precise control is the uncontrollablenature of natural light. To completely compensate forthis by artificial means is assumed to be impractical.We cannot therefore apply techniques used in, e.g. aproduction line, where all parameters can be tightlycontrolled. However, we are in a position where theirradiance distribution on leaves throughout, say a

tomato crop can in principle be predicted (Section 5.1).If this is so, then it now should be possible to predictoptimal values for all the other variables (greenhouseair temperature, carbon dioxide concentration, nutri-ent supply composition, humidity, etc.). The energybalance equations (e.g.Section 2.2.2) can be rewrittenand solved for any parameter within them as an un-known. Energy supply, water supply for misting, etc.are therefore predictable, given a particular optimumtemperature, humidity, etc.

As already indicated (Section 1), the optimal valueswill depend on the grower’s particular crop.

In view of these conclusions, it may be appropriateto initiate a small scale pilot study, to see if fullyautomated control is possible, and if not, how near tothe ideal can we get.

Acknowledgements

The authors would like to thank Mrs Anne Jarvisand the staff of the SRI library, for their help, partic-ularly during the initial literature search.

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