Plant Factory Reviewpdf

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Review

Resource use efficiency of closed plant production system with arti1047297cial

light Concept estimation and application to plant factory

By Toyoki KOZAI1dagger

(Communicated by Satohiko SASAKI MJA)

Abstract Extensive research has recently been conducted on plant factory with arti1047297cial

light which is one type of closed plant production system (CPPS) consisting of a thermallyinsulated and airtight structure a multi-tier system with lighting devices air conditioners and fansa CO2 supply unit a nutrient solution supply unit and an environment control unit One of the

research outcomes is the concept of resource use efficiency (RUE) of CPPS

This paper reviews the characteristics of the CPPS compared with those of the greenhousemainly from the viewpoint of RUE which is de1047297ned as the ratio of the amount of the resource 1047297xedor held in plants to the amount of the resource supplied to the CPPS

It is shown that the use efficiencies of water CO2 and light energy are considerably higher in theCPPS than those in the greenhouse On the other hand there is much more room for improving thelight and electric energy use efficiencies of CPPS Challenging issues for CPPS and RUE are alsodiscussed

Keywords closed plant production system plant factory resource use efficiency

Introduction

The world is increasingly faced with globalproblems including unusual weather environmentalpollution and shortages of water fossil fuel and plantbiomass Accordingly the stable and safe supply of plant-derived food and other products will beendangered1) When leaf vegetables are grown inthe open 1047297eld their quality and productivity tend tovary with the local climate weather conditions andsoil fertility2) On the other hand when plants aregrown in the greenhouse their quality and produc-tivity are generally improved3) In this case howeveroveruse of agrochemicals for plant production and

fossil fuel for heating in winter andor cooling in

summer results in excessive emission of environ-mental pollutants including CO2 gas and watercontaining fertilizer4)

Leaf vegetables are increasingly produced underthe controlled environment of greenhouses that arelocated in or near a large city This is mainly becauseof the signi1047297cant savings in resource consumptionin terms of the cost and time involved in transportingfresh leaf vegetables from the production site to theconsumer5) In addition the short transport distancesigni1047297cantly reduces the loss of quality and quantityduring the transport of fresh vegetables with about90 water content6) These characteristics haverecently been discussed with respect to lsquoverticalfarmingrsquo and urban agriculture7)

Thus there is a need to develop a resource-efficient system for producing high-quality leaf vegetables at high yield in a limited land area in alarge city where most consumers live to minimizethe distance between the production and consump-tion sites of fresh vegetables

At moderate temperatures indispensable re-sources for growing photoautotrophic plants are lightenergy or photosynthetically active radiation (PAR

1 Japan Plant Factory Association Chiba Japandagger Correspondence should be addressed T Kozai Japan

Plant Factory Association co Center for Environment Healthand Field Sciences Chiba University Kashiwa Chiba 277-0882Japan (e-mail kozaifacultychiba-ujp)

Abbreviations COP Coefficient of performance forcooling CPPS Closed plant production system EC Electricconductivity of nutrient solution FL Fluorescent lamp LAILeaf area index LCA Life cycle assessment LED Light emittingdiode PAR Photosynthetically active radiation PFAL Plantfactory with arti1047297cial light RUE Resource use efficiency SPPSSustainable plant production system VPD Water vapor partialpressure de1047297cit

Proc Jpn Acad Ser B 89 (2013)No 10] 447

doi 102183pjab89447copy2013 The Japan Academy



hereafter wavelength 400ndash700 nm) water CO2 andinorganic nutrients8) In order to evaluate andimprove the resource use efficiency (ratio of the

amount of the resource 1047297

xed or held in plants to theamount of the resource supplied (RUE hereafter) inplant production the concept of lsquoclosed plant pro-duction system with arti1047297cial lightrsquo (CPPS hereafter)has been proposed9)ndash15)

The purpose of using CPPS is to maximize theplant growth with the minimum inputs of lightenergy water CO2 and inorganic fertilizer in otherwords to maximize plant growth with the maximumRUE resulting in the minimum emission of environ-mental pollutants and minimum costs for theessential resources It should be noticed that CPPSis not for production of staple food plants such as

rice wheat maize and potatoesIn this paper the de1047297nition and characteristics

of the CPPS with respect to RUE are reviewedmainly in terms of the use efficiencies of water CO2

and light energy Then it will be shown that their useefficiencies are considerably higher in the CPPS thanin the greenhouse Electric energy use efficiency of CPPS is also discussed in terms of the light energyconversion ratio of the light source

In Japan the number of plant factories witharti1047297cial light (PFAL hereafter) for commercialproduction of leaf vegetables such as lettuce and

spinach plants increased from 35 in December 2009 to106 in December 201116) It is estimated that thenumber increased to over 130 by the end of 2012 andis estimated to increase further in 2013 Researchdevelopment and commercial operation of PFALhave recently been extensively conducted in KoreaChina and Taiwan as well15) This movement has alsobeen initiated by the private sector in the US and theNetherlands according to related websites althoughscienti1047297c papers on this matter are scarce It is notedhowever that most PFAL are not designed based onthe concept of CPPS and are not operated on thebasis of the monitoring of RUE

On the other hand most facilities with arti1047297ciallight for producing only transplants (seedlings andplantlets from cuttings or bulblets) are designedand operated based on the concept of CPPS13)14)

The transplants include those of fruit vegetables(tomato cucumber eggplant etc) leaf vegetables(lettuce and spinach plants etc) for hydroponicsand 1047298owering bedding plants such as pansy (Viola

wittrockiana Gams) In 2011 CPPS was usedcommercially at more than 130 locations in Japan14)

In this paper the RUE of CPPS for production of

transplants and leaf vegetables found in the literatureis compared with that of the greenhouse andmethods for improving RUE are discussed

The 1047297

rst commercial PFAL was developed byGeneral Electric (GE) of the US in the early 1970sbut it closed at the beginning of the 1980s In Japanthe 1047297rst commercial PFAL Miura Farm was estab-lished in 1983 This was followed in 1985 by a PFALat a vegetable sales area in a shopping center17) Untilaround 1995 high-pressure sodium lamps were usedas the light source After that straight-tube 1047298uo-rescent lamps were the preferred choice mainly due toimprovements with respect to PAR output per wattThen a multi-layer system with a vertical distance of about 40 cm could be installed in the PFAL whichsigni1047297cantly improved the annual productivity per

land area PFAL using LEDs (light-emitting diodes)as the light source began commercial operation in2005 in Japan although the initial investment costof LEDs was still several times higher than that of 1047298uorescent lamps as of 201317) LEDs will become animportant light source in PFAL in the near futuresince the cost performance has more than doubledevery year since 201017)18)

The spectral energy distribution and electric-to-light energy conversion factor of the light source arethe important factors for assessing the efficiency of the lighting system17)18) The spectral energy distri-

bution of the light source is important in two aspectsphotosynthesis and photomorphogenesis Photosyn-thesis is basically aff ected by photosynthetic photon1047298ux density or photosynthetically active radiation1047298ux with wave band of 400ndash700nm On the otherhand photomorphgenesis and morphology are af-fected by the light spectrum ranging from around300ndash800 nm Currently the conversion factors of typical high-pressure sodium and 1047298uorescent lampsare respectively around 038 and 026 Whereas theconversion factor of recent LED is around 040 whichwill be further improved the forthcoming years17)18)

Con1047297guration and function of closed plantproduction system (CPPS)

The CPPS consists of six principal structuralelements (Fig 1)13)14) 1) thermally well-insulatedand nearly airtight warehouse-like structure coveredwith opaque walls 2) multi-tier system (mostly 4ndash16tiers or layers about 40 cm vertically between tiers)equipped with lighting devices such as 1047298uorescentlamps and LEDs over the culture beds 3) airconditioners (also known as heat pumps) principallyused for cooling and dehumidi1047297cation to eliminate

T KOZAI [Vol 89448



heat generated by lamps and water vapor transpiredfrom plants in the culture room and fans forcirculating the room air to enhance photosynthesisand transpiration and to achieve uniform spatial airdistribution 4) a CO2 delivery unit to maintain CO2

concentration in the room at 1000ndash2000 micromol mol1

(or ppm) under light for enhanced plant photosyn-thesis 5) a nutrient solution delivery unit and 6) anenvironmental control unit including EC (electricconductivity) and pH controllers for the nutrient

solutionThe CPPS must be designed and operated to

maximize the following functions13)14) 1) the materi-al and energy balance is controlled to produce themaximum possible amount of usable or salable partsof plants with the highest values using the minimumamount of resources 2) the percentages of allresources added as energy or raw materials areconverted into plants or products (parts of plants)meaning that the RUE of the CPPS is the highestpossible and as a result 3) there is a minimumrelease of pollutants into the environment 4)minimum resource consumption with the highestRUE results in the lowest costs for resources and forrecovering environmental pollution

Among the resources a considerable amount of electric energy is consumed in the CPPS mainly forlighting and air conditioning Thus electric andlight energy are the most important resources forimproving the use efficiencies in the CPPS

De1047297nition of resource use efficiency (RUE)

The concept of RUE is schematically shown inFig 2 With respect to essential resources for growing

plants in CPPS namely water CO2 light energyelectric energy and inorganic fertilizer the useefficiencies per unit time interval (lsquohourrsquo is used forunit time in this paper) are de1047297ned in Eqs [1]ndash[10]and illustrated schematically in Fig 319)ndash24) Theseare WUE (water use efficiency) CUE (CO2 useefficiency) and LUEL (light energy use efficiency withrespect to PARL) LUEP (light energy use efficiencywith respect to PARP) EUEL (electric energy useefficiency) and FUEI (inorganic fertilizer use effi-

ciency) The unit for each variable on the right-hand side of Eqs [1] and [3] is kgm2 (1047298oor area) h1These use efficiencies are de1047297ned with respect toCPPS containing plants It should be noted thatin the 1047297elds of plant ecology and agronomy WUE isde1047297ned with respect to plants or plant community8)

CUE in Eq [3] is de1047297ned only when CO2 is suppliedor enriched in the CPPS Methods for measuringor estimating the values of variables given on theright-hand side of Eqs [1]ndash[7] are described in Liet al 23)ndash25)

Water use efficiency (WUE)

WUE frac14 ethWC thorn WPTHORN=WS frac14 ethWS WLTHORN=WS frac121WL frac14 N VA ethXin XoutTHORN frac122

where WC is the mass (or weight) of water collectedat the cooling panel of the air conditioner forrecycling use WP is the change in the mass of waterheld in plants and substrates WS is the mass of waterirrigated (or supplied) to the CPPS WL is the massof water vapor lost to the outside by air in1047297ltrationthrough small gaps in the entranceexit and wallsIn Eq [2] N is the number of air changes per hour inthe culture room (h1) VA is the air volume in the

3 Air conditioners and fans

2 Multi-tier system

with lightingdevices

5 Nutrientsolutionsupply unit

4 CO2 supply

unit

1 Thermally well-

insulated nearly

airtight walls

6 Environmentcontrol unit

Fig 1 Con1047297guration of the closed plant production system (CPPS) consisting of six principal components Electricity is needed for

operating Nos 2ndash

6

Resource use efficiency of closed plant production systemNo 10] 449



ResourcesLight water CO2

fertilizer seeds etc

PlantsProduce

ValueOpportunities

Environment

CPPS

Environmental pollutants includingCO2 and heat energy

A B

C

D

Concept of Resource Use Efficiency (RUE)RUE BA where A = B + C + D

RUE is estimated for each component of resources

Heat energy

Fig 2 Scheme showing the concept of resource use efficiency (RUE) In the CPPS each resource is converted into produce at itsmaximum level so that resource consumption and emission of environmental pollutants are minimized resulting in the maximumresource use efficiency and lowest cost for resource and pollution processing

WL = N bull VA bull (Xin ndash Xout)CL = kC bull Nbull VA bull (Cin ndash Cout)

Cs

Ws

CPXin Xout

Cin CoutWT

WC

WU

Wout

VA

IoutIin

Win

CR

Culture beds

Plant community WP D

VW

Lamps h Air conditioners

PARL

AL AA

Pumps Fans AM

IU

WUE = (WC+WP)WS

CUE = CP (CS+CR)

Workers

PARP

LUEL = f bull DPARL [5]LUEP = f bull D PARP [6]

EUEL = h bull LUEL [7]AL = PARL h

COP = Hh AA [9]

FUEI = IU IS [10]

AT = AL+ AA+ AM [11]AA= (AL + AM+ HV) COP

Plants

Hh

HV

[12]

[2]

[4]

[1]

[3]

[8]

Fig 3 Schematic diagram showing the rate and state variables in the CPPS Solid line represents 1047298ow of material and dotted linerepresents 1047298ow of energy Numbers in brackets represent equation numbers in the text For the meanings of symbols see List of symbols etc Relationships among the variables are given in Eqs [1]ndash[16] in the text

T KOZAI [Vol 89450



culture room (m3) and Xin and Xout are respectivelythe mass of water vapor per volume of air containingwater vapor inside and outside the culture room

(kgm3

) Generally no liquid wastewater is drainedfrom the culture room to the outside If there is anyit must be added as a variable on the right-hand sideof Eq [1]

CO2 use efficiency (CUE)

CUE frac14 CP=ethCS thorn CRTHORN frac14 ethCS CLTHORN=ethCR thorn CSTHORN frac123

CL frac14 kC N VA ethCin CoutTHORN frac124

where CP is the net photosynthetic rate CS is theCO2 supply rate CR is the respiration rate of workersin the culture room if any CL is the rate of CO2

lost to the outside due to air in1047297ltration CR can beestimated using the data on the number of workers

working hours per person and hourly amount of CO2 release by human respiration per person (about005kgh1)24) In Eq [4] kC is the conversion factorfrom volume to mass of CO2 (180kgm3 at 25degC)N and VA are the same as in Eq [2] and Cin andCout are respectively CO2 concentration inside andoutside the culture room (mol mol1)

It is assumed in Eqs [2] and [4] that X in and Cin

at time t are the same as at time (t D ) where is theestimation time interval If not the terms (X in(t) Xin(t D )VA) t and (Cin(t) Cin(t D )VA) t arerespectively added on the right-hand side of Eq [2]

and Eq [4] the value of which is negligibly small inmost cases In a case where Xout andor Cout changewith time during the estimation time interval theaverage value is used in Eq [2] andor Eq [4]

Light energy use efficiency of lamps andplant community (LUEL and LUEP)

LUEL frac14 f D=PARL frac125

LUEP frac14 f D=PARP frac126

where f is the conversion factor from dry massto chemical energy 1047297xed in dry mass (about20MJkg1) D is the dry mass increase of wholeplants or salable parts of plants in the CPPS(kgm2 h1) PARL and PARP are respectivelyphotosynthetically active radiation (PAR) emittedfrom lamps and received at the plant communitysurface in the CPPS (MJ m2 h1)

LUEL and LUEP can also be de1047297ned respec-tively as b CPPARL and b CPPARP where bis the minimum PAR energy to 1047297x one mole of CO2

in plants (0475 MJ mol1) and CP is the net photo-synthetic rate of plants (mol m2 h1) The ratio of PARP to PARL is often referred to as the lsquoutilizationfactorrsquo in illumination engineering

Electric energy use efficiency of lighting(EUE)

EUEL frac14 h LUEL frac14 h f D=PARL frac127

where h is the conversion coefficient from electricenergy to PARL energy which is around 025 forwhite 1047298uorescent lamps and 03ndash04 for recentlydeveloped LEDs17)18) The electric energy consumedby lamps AL can be expressed by

AL frac14 PARL=h frac128

Electric energy use efficiency of heat pumpsfor cooling (COP)

COP frac14 Hh=AA frac129

where Hh is the heat energy removed from the culture

room by heat pumps (air conditioners) (MJ m2 h1)and AA is the electricity consumption of the heatpumps (MJ m2 h1) This efficiency is often referredto as the coefficient of performance for cooling of heatpumps (COP hereafter)

Inorganic fertilizer use efficiency (FUEI)

FUEI frac14 IU=IS frac1210

where IU is the absorption rate of inorganic fertilizerof ion element lsquoIrsquo by plants and IS is the supply rateof lsquoIrsquo to the CPPS lsquoIrsquo includes nitrogen (NO3

NO4

D) phosphorous (PO4) potassium (KD) etc N

is dissolved in water as NH4D

or NO3

but is countedas N in Eq [10] Then N use efficiency P useefficiency K use efficiency etc can be de1047297ned foreach nutrient element

Results and discussion

WUE CUE LUE and EUE for CPPS foundin the literature are summarized in Table 1 incomparison with those of a greenhouse with ventila-tors closed andor open together with their theo-retical maximum values In Table 1 LUEL andEUEL are de1047297ned only for CPPS using lamps NA(not available) in Table 1 means that the efficienciesof WUE CUE and LUEP can be obtained exper-imentally but the data were not found in theliterature

It is shown in Table 1 that WUE and CUE of CPPS are considerably higher compared with thoseof the greenhouse with ventilators open LUEP of CPPS ranges between 0032ndash0043 while LUEP of the greenhouse with ventilators open varies from0003 to 0032 As for CPPS LUEP of 0032ndash0043 isabout 60ndash70 of LUEL of 006 which means thatPARP is about 60ndash70 of PARL




EUEL and LUEL in Table 1 are respectively0027 and 0007 which means that 07 of electricenergy and 27 of PAR energy were converted intochemical energy contained in dry matter of plantsChemical energy contained in dry matter is 200MJkg1 (Eqs [5] and [6]) Thus electricity con-sumption to produce one kg of dry matter are 2857(F 2000007) MJkg or 794 (F 285736) kWhSimilarly PAR energy consumed to produce one kgof dry matter is 740 (F 2000027) MJkg or 205

(F 74036) kWhWater use efficiency (WUE) WUE is 095ndash

098 in the CPPS and 002ndash003 in the greenhousemeaning that WUE of the CPPS is approximately30 to 50 (nearly equal to 095003ndash098002) timesgreater than that of the greenhouse20) NamelyCPPS is a highly water-saving plant productionsystem compared with the greenhouse

CPPS with thermal insulation walls and a highlevel of airtightness (N is 001ndash002h1) must becooled by air conditioning while the lamps are turnedon even on cold winter nights in order to maintain asuitable internal temperature by eliminating the heatgenerated from the lamps With cooling a largeportion of the evaporated water WC in Eq [1] canbe collected as condensation on the cooling panels of the heat pump and recycled for irrigation Only asmall percentage of WS is lost to the outside becauseof the high airtightness of CPPS The N value mustbe lower than about 002h1 to minimize CO2 loss tothe outside and prevent entry of insects pathogensand dust into the culture room

On the other hand water vapor evapotranspiredin the greenhouse cannot be collected for recycling

use because most of the evapotranspired watervapor is lost to the outside with the ventilated airThe rest of the water vapor is mostly condensed onthe inner surfaces of the greenhouse walls whichcannot be collected The amount of water vapor lostto the outside WL in Eq [2] increases with in-creasing (Xin Xout) and N of the greenhouse withventilators open N varies in a range between 05ndash100h1 depending on the number of ventilatorsand degree of opening as well as the wind velocity

outside26)

If all the lamps are turned off the relativehumidity of the room air in the CPPS approaches100 resulting in little transpiration which maycause physiological disorders of the plants To avoidthis the tiers in the CPPS are often divided into twoor three groups and the lamps in each group areturned on for 12ndash16 hours per day in rotation togenerate heat from the lamps at any time of the dayso that the heat pumps are activated all day fordehumidi1047297cation and cooling of room air

CO2 use efficiency (CUE) CUE is 087ndash089in CPPS with N of 001ndash002h1 and CO2 concen-tration of 1000 micromol mol1 whereas CUE is around05 in the greenhouse with ventilators closed havingN of about 01h1 and enriched CO2 concentrationof 700 micromol mol119)21) Thus CUE is roughly 18(F 088050) times higher in CPPS than in thegreenhouse with all ventilators closed and CO2

enrichment21) This is because the amount of CO2

released to the outside CL in Eq [4] increases withincreasing N and (Cin Cout) It is thus natural thatthe set point of CO2 concentration for CO2 enrich-ment is generally higher (1000ndash2000 micromol mol1)

Table 1 Estimated representative values of use efficiencies of water (WUE) CO2 (CUE) light energy (LUEL) and electric energy(EUEL) for CPPS (closed plant production system) and WUE CUE and LUEP for greenhouses with ventilators closed andor openMaximum value for CPPS based on theoretical consideration of each use efficiency is also given in the column lsquoTheoretical maximumvaluersquo For the de1047297nitions of WUE CUE LUEL EUEL and EUEP see Eqs [1]ndash[7] in the text The numerical data in Table 1 is cited

from or based on the literature in references Nos 18ndash

25 and 26ndash

31

Use efficiency CPPS

Greenhouse with

ventilators closed

and enriched CO2

Greenhouse

with ventilators

open

Theoretical

maximum value

for CPPS

WUE (water) 095ndash098 NA 002ndash003 100

CUE (CO2) 087ndash089 04ndash06 NA 100

LUEL (lamps PARL) 0027 - - About 010

LUEP (plant community) 0032ndash0043 NA 0017 About 010

005 0003ndash0032

EUEL (electricity) 0007 - - About 004

Note LUEL and EUEL are de1047297ned only for CPPS using arti1047297cial lamps NA (not available) means that the efficiencies of WUE CUEand LUEP can be obtained experimentally but the data were not found in the literature

T KOZAI [Vol 89452



in the CPPS than in the greenhouse (700ndash1000micromol mol1)

When N and (Cin Cout) of the CPPS are

constant with time CUE increases with increasingLAI (leaf area index or ratio of leaf area to cultivationarea) in a range between 0 and 319) This is becausein Eq [2] CP increases with increasing LAI but CL isnot aff ected by LAI In order to maintain a high CUEregardless of LAI the set point of the CO 2 concen-tration for CO2 enrichment should be increased withincreasing LAI

Light (PARL) energy use efficiencies (LUEL

and LUEP) The average LUEP over tomatoseedling production in the greenhouse was 001727)

On the other hand the average LUEL over tomatoseedling production in the CPPS was 002722) It

should be noted however that LUEL of the CPPS inEq [5] is estimated based on PARL emitted from thelamps17)22) whereas LUEP in Eq [6] of the green-house is estimated based on PARP received at thecommunity surface The ratio PARPPARL is esti-mated to be 063ndash07122) Thus LUEP in the CPPSwould be approximately in a range between 0038(F 0027071) and 0043 (F 0027063) which is 19(F 00320017) to 25 (F 00430017) times higherthan that in the greenhouse LUEP in the CPPSincreases almost linearly with increasing LAI at LAIof 0ndash322) The maximum LUEP which is strongly

related to the inverse of lsquoquantum yieldrsquo is estimatedto be of the order of 018)28) The LUEP of variouscrops in space research by the NASA group in theUS is around 00529)ndash32) Thus there should be moreroom for improving the LUEL and LUEP of CPPS

Improving LUEL and EUEL

Various methods can be considered to improveLUEL in Eq [5] and thus EUEP in Eq [6]33)

However as research on improving LUEL is limitedsome of the methods described below are not basedon experimental data but rather theoretical consid-eration and practical experience in commercial plantproduction using PFAL

Interplant lighting LEDs can be installed justabove the culture panels within the plant communityfor sideward and upward lighting This interplantlighting provides more light energy to the lowerleaves compared with downward lighting onlyThen the net photosynthetic rate of a whole plantcommunity with LAI of about 3 would be increasedThis is because the net photosynthetic rate of thelower leaves which is often negative or nearly zerowill turn to be positive by interplant lighting34)

The net photosynthetic rate of the upper leavesof a plant community under downward lighting isreduced when a portion of the light energy emitted

from the lamps is used for sideward and upwardlighting Net photosynthetic rate of the whole plantcommunity is considered to be maximized when thelight energy emitted from the lamps is distributedevenly over all the leaf surfaces

Improving the ratio of light energy receivedby leaves The LUEL increases with the increasingratio of light energy received by leaves to lightenergy emitted by lamps This ratio can be im-proved by well-designed light re1047298ectors reductionin vertical distance between lamps and plantsincrease in distance between plants (or plantingdensity) as plants grow etc32) The ratio increases

linearly from zero up to unity with increasing LAIof 0ndash322) Thus LUEL can be signi1047297cantly improvedby maintaining LAI at around 3 throughout theculture period by automatic or manual spacing of plants

Using LEDs A direct method for improvingEUEL is to use a light source with a high h valuein Eq [7] The h value of recently developed LEDsis around 0435) whereas that of 1047298uorescent lamps(FL) is around 02533) Although the price of LEDswith h of about 04 is several times higher com-pared with that of FL as of 2013 the price has

decreased considerably every year and this trend willcontinue in the forthcoming several years Spectraldistributions or light quality of LED lamps aff ectplant growth and development and consequentlyLUEL18)32)35)ndash37)

Controlling environmental factors otherthan light LUEL is largely aff ected by the plantenvironment and ecophysiological status of plantsas well as by the genetic characteristics of plantsAn optimal combination of temperature CO2 con-centration air current speed water vapor pressurede1047297cit and the composition pH and EC (electricconductivity) of the nutrient solution must beoptimized to improve LUEL28)

ndash31)33)

ndash36)38)

ndash41)

Air current speed Horizontal air currentspeed of 03ndash05 m s1 within the plant communityenhances diff usion of CO2 and water vapor fromroom air into the stomatal cavity in leaves andconsequently photosynthesis transpiration and thusplant growth if VPD (vapor pressure de1047297cit) iscontrolled at an optimum level39) Control of aircurrent speeds in accordance with plant growthstage by changing the rotation speed of fans is anadvantage of CPPS for improving plant growth




Increasing salable portion of plants Elec-tricity is consumed to produce whole plants consist-ing of leaves with petioles roots stems and some-

times 1047298

ower stalks and buds Thus in order toimprove EUEL and LUEL of salable parts of plants itis important to minimize the dry mass of the non-salable portion of plants Leaf vegetables such aslettuce plants should be grown with a minimumpercentage of root mass typically lower than 10 of the total mass This is not so difficult because waterstress of plants can be minimized by controlling VPDof the room air and water potential of the nutrientsolution in the culture beds of CPPS In the case of root crops such as turnip plants the salable portioncan be signi1047297cantly increased by harvesting themearlier than usual so that the aerial part will be

edible This is widely practiced in commercial PFAL

Electricity consumption Its percentageby components and COP

Commercial production from CPPS or PFAL islimited to value-added plants because the electricityconsumption for lighting to increase the dry mass of plants is signi1047297cant and in general its cost accountfor around 25 of the total production cost15)

Accordingly plants suitable for production in CPPSare those that can be grown at high planting density(50ndash1000 plantsm2) to the harvestable stage under

relatively low light intensity (100ndash

300micromolm2

s1

)and within a short cultivation period (30ndash60 days)Also the plant height needs to be lower than around30 cm because the multi-tier system must be used toincrease annual productivity per land area

Electricity consumption in the culture roomof PFAL per 1047298oor area (MJm2 h1) AT and itscomponents are given by

AT frac14 AL thorn AA thorn AM frac1211

AA frac14 ethAL thorn AM thorn HVTHORN=COP frac14 Hh=COP frac1212

where AL which is expressed as (PARLh) in Eq [8]is the electricity consumption for lighting AA for airconditioning and AM for other equipment such as aircirculation fans and nutrient solution pumps HV isthe cooling load due to air in1047297ltration and heatpenetration through walls both of which account foronly a small percentage of AA Hh is the heat energyremoved from CPPS using heat pumps COP is thecoefficient of performance of heat pumps as de1047297nedin Eq [9] which increases with decreasing outsidetemperature at a 1047297xed room air temperature

On the annual average AL AA and AM of CPPSaccount for respectively 85 10 and 5 of AT in

Tokyo44) The annual average COP for cooling in aCPPS with room air temperature of 25degC was 5ndash6in Tokyo where the annual average temperature

is about 15degC approximately 4 in summer and 10in winter42) Thus in order to reduce the totalelectricity consumption the reduction in AL byimproving LUEL is most efficient followed by theimprovement of COP

Increasing annual production capacityand sales volume per land area

Relative annual production capacity per landarea of CPPS with 10 tiers can be nearly 100-foldcompared with that of open 1047297eld by improving thefollowing 1047297ve factors 1) 10-fold by use of 10 tiers 2)2-fold by shortening the culture period from trans-

planting to harvest by optimal environmental control(germination and seedling production are conductedelsewhere in PFAL) 3) 2-fold by extending theannual duration of cultivation by all the year roundproduction with virtually no time loss betweenharvest and next transplanting 4) 2-fold by increas-ing planting density per cultivation area mainly bycontrolling the air current speed and VPD within theplant community and increased ratio of cultivationarea of each tier to 1047298oor area and 5) 15-fold percropping because there is no damage due to abnormalweather such as strong wind heavy rain and drought

and outbreak of pest insects In total 96-fold (F 10 2 2 2 12 F 96) Sales price of PFAL-grownlettuce is generally 12-foldndash15-fold due to improvedquality compared with that of 1047297eld-grown plantsresulting in relative annual sales volume of about115-fold (96 12 F 115)

Monitoring and controlling RUEand rate variables

Interactions between plants and their environ-ments in a CPPS equipped with a hydroponic culturesystem are fairly simple compared with those in thegreenhouse and open 1047297eld This is because the CPPSis thermally well-insulated and nearly airtight sothat the environment is not aff ected by the weatherespecially the 1047298uctuations in solar radiation withtime Therefore the energy and mass balance of theCPPS and related rate variables (namely 1047298ows thatinclude the time dimension in units) such as thosegiven in Eqs [1] and [2] can be measured evaluatedand controlled relatively easily23)ndash25)

The quality of environmental control will besigni1047297cantly improved by monitoring visualizingunderstanding and controlling the RUE (WUE

T KOZAI [Vol 89454



CUE LUEL EUEL and FUE in Eqs [1]ndash[10]) andrate variables The rate variables include the rates of net photosynthesis (gross photosynthesis minus dark

and photorespirations) dark respiration transpira-tion water uptake and nutrient uptake whichrepresent the ecophysiological status of the plantcommunity Another group of rate variables includesthe supply rates of CO2 irrigation water fertilizerconstituents and electricity consumption by itscomponents in the CPPS

The set points of environmental factors can bedetermined for example for maximizing LUEL withthe minimum resource consumption or at the lowestcost33) In order to maximize the cost performance(ratio of economic bene1047297t to production cost)instead of maximizing the LUEL the concept and

method of integrative environmental control must beintroduced43)44) Continuous monitoring visualizingand controlling of the rate variables and RUE willbecome important research subjects in the integratedenvironmental control for plant production usingCPPS43)44)

Net photosynthetic rate (CP) CP at time tduring the time interval of t can be estimated bymodifying Eqs [3] and [4] as follows

CP frac14 CS thorn CR ethkCNVA=FTHORNethCin CoutTHORN

thorn ethethkC VA=FTHORN ethCinetht thorn t=2THORN

Cinetht t=2THORNTHORN= tTHORN frac1213where the fourth term expresses the change in CO2

mass in the air of the culture room during the timeinterval of t between (t t2) and (t D t2) Forpractical application t would be 05ndash10h CP canbe estimated fairly accurately using Eq [13] since1) kC and VA are constant 2) CS Cin and Cout canbe accurately measured and 3) N can be estimatedusing Eq [13] as described later

Transpiration rate (WT) In a CPPS thetranspiration rate of plants grown in the hydroponicculture beds in the CPPS at time t during the timeinterval of t WT can be estimated by Eq [14] Inthe CPPS no evaporation from the substrate in theculture can be assumed because the culture bedsare covered with plastic panels with small holes forgrowing plants In the CPPS neither evaporation norcondensation fromto the walls and 1047298oor can also beassumed because the walls and 1047298oors are thermallywell insulated

WT frac14 WC thorn ethNVA=FTHORN ethXin XoutTHORN

thorn ethVA=FTHORN ethXinetht thorn t=2THORN

Xin etht t=2THORNTHORN= t frac1214

where the third term expresses the change in watervapor mass in the air of the culture room during thetime interval of t between (t t2) and (t D t2)

Since VA and F are constants and WC Xin and Xoutcan be measured relatively accurately N can beestimated for the time interval of (t t2) and(t D t2) using Eq [14B]23)43)ndash45)

N frac14 ethWT WC ethVA=FTHORN ethXinetht thorn t=2THORN

Xin etht t=2THORNTHORN= tTHORN=ethethVA=FTHORN ethXin XoutTHORNTHORN

[14B]

Water uptake rate by plants (WU) WU canbe expressed by modifying Eq [14] as

WU frac14 WP thorn WT frac1215

where WP is the change in water mass of plants

during time interval which is difficult to estimateaccurately On the other hand WU can be estimatedrelatively easily by the equation

WU frac14 Win Wout ethkLW=FTHORNethVLW etht thorn t=2THORN

VLW etht t=2THORNTHORN= t frac1216

where Win and Wout are respectively the 1047298ow rateof nutrient solution into and from the culture beds(VLW(t D t2) VLW(t t2)) is the change innutrient solution volume in the culture beds andorthe nutrient solution tank during the time interval of t WU in Eq [16] can be estimated for each culture

bed or for all the culture beds by changing themeasuring points of Win and Wout

Ion uptake rate by plants (IU) The ionuptake rate by plants in the culture bed IU can beestimated by

IU frac14 IinWin IoutWout

thorn ethethethIin thorn IoutTHORN=2THORN=FTHORNethVLW etht thorn t=2THORN


where Iin and Iout are respectively the concentrationof ion lsquoIrsquo at the inlet and outlet of the culture bedWin and Wout are respectively the 1047298ow rate of nutrient solution at the inlet and outlet of the culturebed The third term shows the change in amount of ion lsquoIrsquo in the culture beds during the time interval of IU can be measured for each culture bed more thanone culture bed or all the culture beds

Inorganic fertilizer use efficiency (FUEI)The culture beds in the CPPS are isolated from thesoil and the nutrient solution drained from theculture beds is returned to the nutrient solutiontank for recirculation Nutrient solution is rarelydischarged to the outside usually about once ortwice a year when certain ions such as NaD and Cl




which are not well absorbed by plants haveexcessively accumulated in the nutrient solution orwhen certain pathogens are spread in the culture

beds by accident In these cases the supply of fertilizer is stopped for several days before removingthe plants from the culture beds so that mostnutrient elements in the culture beds and nutrienttank are absorbed by the plants Then the dis-charged water from the CPPS to the outside isrelatively clean Therefore FUEI of the CPPS shouldbe fairly high even though no literature is foundregarding this subject

On the other hand FUEI of the greenhouse andopen 1047297eld is relatively low which occasionally causessalt accumulation on the soil surface46) In the open1047297eld excess supply of nitrates and phosphates

occasionally causes nutrient run-off and leachingresulting in eutrophication in river and lakes46)47)

Applications The estimation methods of ratevariables given above can be applied to small CPPS-type plant growth chambers used for researcheducation self-learning and hobby purposes48) aswell as PFAL for commercial plant production

Since all the major rate and state variables in theCPPS can be measured and graphically displayedthe quantitative relationships among these variablescan be analyzed and understood relatively easilyThis bene1047297t could be enhanced when the relation-

ships are displayed together with camera images of the plants Major rate and state variables of theCPPS are schematically shown in Fig 3

Toward sustainable plant productionsystem (SPPS)

Any plant production system needs to bedeveloped or improved to be as sustainable aspossible48)ndash50) The CPPS should relatively satisfythe necessary (but not sufficient) conditions of asystem with high sustainability for production of functional plants such as leaf vegetables medicinalplants and transplants compared with the green-house because

1) The CPPS is operated to obtain themaximum yield or value with the minimum amountof resources and minimum emission of environmentalpollutants including CO2 gas resulting in high useefficiencies of water CO2 light energyelectricityand inorganic fertilizer

2) The CPPS consists of six principal structuralcomponents as shown in Fig 1 which are estab-lished in society to recycle the structural componentsafter they have reached the end of their service lives

3) The CPPS heightens the stability of plantproduction in the face of abnormal weather and thepresence of pathogens and other contaminants and

achieves planned high quality and high yields allyear4) The CPPS saves considerable land area per

plant productivity resulting in saving labor hoursper production In addition the CPPS can be built ina shaded area an unused space or the basement of abuilding etc

5) The CPPS is safe and pleasant for theoperators under unfavorable weather conditionsproviding greater potential to bring out the creativityof the operators and contributing to creating anindustry in which environmental safety and welfarecan coexist High airtightness of the CPPS prevents

the entry of insects andor pathogens Thus nopesticides are necessary Also the rigid structureprevents theft and physical damage

6) Thermally well-insulated CPPS needs noheating even on cold winter nights by turning on acertain number of lamps which generate heat energyto keep the room air temperature sufficiently highAlso since heat penetration and air in1047297ltration areminimal with no penetration of solar radiationenergy the cooling load of the CPPS is almost equalto the heat generated by the lamps and other electricequipment

7) The CPPS has high potential to contribute toexpanding employment opportunities throughout theyear under any climate conditions and giving mean-ing to life for a broad range of people including theaged and disabled since the working environment ismostly maintained at an air temperature of about25degC and relative humidity of 70ndash80 The workerscan also enjoy watching the plants grow at a relativelyhigh speed It is estimated that a PFAL with 10 tiersand total 1047298oor area of 1 ha creates job opportunitiesfor approximately 300 persons for plant productionalone At a commercial PFAL with culture room 1047298oorarea of 338 m2 in Kashiwa city in Chiba Japan has anannual production capacity of one million leaf lettuceheads by one manager and 10 part-time workers(7 working hours per day) for production Thus it isroughly estimated that nearly 300 (F 10 (10000338) part-time workers would be necessary to operatea PFAL with 1047298oor area of 1 ha

8) The CPPS as a whole including its operatorscan aptly evolve with changes in the naturalenvironment and diverse social environments Inthe CPPS with a well-controlled environment therelationships among the environment plant growth

T KOZAI [Vol 89456



and the plantrsquos functional components are relativelystraightforward It is then easy to devise and imple-ment eff ective methods of controlling the environ-

ment for the optimal growth of plants and toimprove the cultivation systems The environmentaleff ects on plant growth in CPPS become muchsimpler and more understandable compared withthose in the greenhouse

9) The CPPS facilitates international technol-ogy transfer through the development of stand-ardized systems to be shared globally After allLCA of CPPS including its construction operationand termination can be evaluated more easilyregardless of its physical size which may make theCPPS a relatively universal plant production systemIt should be noted however that application of

CPPS is limited to the production of leaf vegetablestransplants (seedlings etc) and some other high-value or functional plants usually grown in thegreenhouses Thus CPPS will never absorb thetraditional outdoor farming systems On the otherhand CPPS has been commercially used at around150 diff erent locations in Japan and the number of CPPS has been increasing steadily year by year

Challenges

On the other hand there are several challengingissues to improving the CPPS as sustainable as

possible For examplesUsing the information of rate variables

together with state variables Measurement andcontrol of the CPPS and greenhouse environmentsare conventionally conducted based on the physicalstate variables (without time dimension in units)such as air temperature relative humidity CO2

concentration EC and pH of nutrient solution withthe exception of PAR (MJ m2 h1) Research oncontinuous intact measurement estimation or visual-ization of state variables of plants such as stomatalconductance chlorophyll 1047298uorescence and watercontent in plant leaves has also been conducted49)

The information on state variables will becomemore useful when used for control purposes togetherwith the information on rate variables such as ratesof net photosynthesis transpiration and ion uptakeMeasurement and control of the plant environmentbased on both the rate and state variables will opena new 1047297eld of technology and science in plantproduction24)

LCA food mileage and virtual water Lifecycle assessment (LCA) is required for a quantitativediscussion on RUE of a whole food supply chain50)

including the resources consumed in establishing aplant production system human and machineryresources for cultivation and harvesting processing

of harvests transportation and packaging of produceand processing of plant residue at the consumersrsquo sideLCA and analyses of food mileage and virtual

water of agricultural and horticultural productionin the open 1047297elds and in the greenhouse have beenconducted50)ndash54) Similar studies need to be conductedin the near future for comparisons among open 1047297eldgreenhouse and CPPS55) For the LCA of PFAL thefollowing points need to be considered 1) the plantsgrown in CPPS are clean and free of insects andagrochemicals so that washing and inspection foreliminating insects and agrochemicals after produc-tion are unnecessary 2) the CPPS can be built close

to consumers or manufacturer sites so that resourceconsumption for transportation and packing and lossof produce due to damage during transportation canbe signi1047297cantly reduced

Networks of real and virtual PFALs Anetwork linking PFALs via the Internet needs to beopened up which brings about the following possi-bilities56)57) The latest information on cultivationmethods and plant varieties can be downloaded fromcloud servers Growers can exchange ideas aboutcultivation and food preparation using social networkservices (SNS) and they can remotely check in real

time how their plants are doing by using built-incameras They can ascertain environmental data andadjust these settings Growers can upload their owncultivation instructions onto cloud servers makingthem available to others Social media tools such asTwitter and Facebook can also be used by the growermembers for exchanges of information and opinions

A trial project along these lines using smallhousehold PFAL was launched in 2012 in KashiwaCity Japan55) The project is examining (1) howpeople can grow vegetables in household spaces(2) the eff ectiveness of providing cultivation adviceand the operability of factory equipment and (3) thevalue added from creating a network It alsoexamines the usefulness and commercial feasibilityof an exclusively web-based service where growerscan ask experts questions on cultivation manage-ment share information on their own circumstancesand experiences and arrange to swap the vegetablesthey have grown The experiences and resultsobtained from this project can be applied to networksthat link schools local communities hobby growergroups restaurants hospitals and hotels By beingconnected to household and local community energy




management systems these networks can also form apart of those systems

With a virtual PFAL system incorporating an e-

learning function and a real PFAL system involvingthe actual cultivation of plants these networks havea dual aspect as an extended form of both In thevirtual system growers can operate a PFAL using asimulator that incorporates a plant growth model anenvironment model and a business model and afterattaining a certain level of pro1047297ciency there they canmove on to trying their hand at operating a realPFAL or they can engage in both in parallel

As people become familiar with this model theirability to achieve the greatest possible production of food and quality of life will become second natureas they minimize their environmental impact by

reducing their electricity and water consumptioneliminating the production of wastewater absorbingcarbon dioxide and producing more oxygen for theatmosphere

Residentsusers living in urban areas and havinglittle chance to grow plants in the open 1047297eld mayenjoy using a household PFAL It is suggested thatsuch a PFAL and its network have a potential tocontribute to a better life of some people in urbanareas56)

Conclusion

Community interest in PFAL has grown alongwith the unusual weather uncontrollable 1047298uctua-tions in the oil price and greater community concernfor health safety and security On the other handPFAL has evoked criticism and concern regardingthe involvement of electricity- and other resource-consuming characteristics for plant production

This paper addresses the criticism that PFALsystems make heavy use of electricity and that theyhave high production costs and are not 1047297nanciallyviable This paper shows that PFAL if properlydesigned and operated can contribute to theproduction of value-added plants at high yield withefficient resource consumption

mdashespecially electric-

ity consumptionmdash and without environmental pol-lution On the other hand there are many possibil-ities and challenges for the further improvement anddiverse application of CPPS

Acknowledgement

The author would like to express his sincerethanks to Dr Michihiro Hara Professor Emeritus of Iwate University for his skillful examination of Equations [1]ndash[17] in the text

References

1) Rosenzweig C and Liverman D (1992) Predictedeff ects of climate change on agriculture A compar-

ison of temperate and tropical regions In GlobalClimate Change Implication Challenges andMitigation Measures Philadelphia (ed MajumdarSK) Pennsylvania Academy of Sciencespp 342ndash361

2) Wilson JP Gerhart ES Nielsen GA and RyanCM (1992) Climate soil and crop yield relation-ships in Cascade County Montana Appl Geogr12 261ndash279

3) Castilla N and Hernandez J (2007) Greenhousetechnological packages for high quality cropproduction Acta Hortic 761 285ndash297

4) Ozkan B Kurklu A and Akcaoz H (2004) Aninput-output energy analysis in greenhouse vege-table production A case study for Antalya regionof Turkey Biomass and Energy 26 89

ndash95

5) Ohyama K Takagaki M and Kurasaka H (2008)Urban horticulture its signi1047297cance to environ-mental conservation Sustainability Science 3241ndash247

6) Atanda SA Pessu PO Agoda S Isong IUand Ikotun I (2011) The concepts and problemsof post-harvest food losses in perishable cropsAfrican J Food Science 5 603ndash613

7) Despommier D (2010) The Vertical FarmmdashFeeding the world in the 21st century StMartinrsquos Press New York

8) Salisbury FB and Ross CW (1991) Plantphysiology Wadsworth Publishing CompanyBelmont Calif

9) Kozai T (1999) Development and Application of Closed Transplant Production System YokendoPub Co Tokyo

10) Kozai T Kubota C Chun C Afreen F andOhyama K (2000) Necessity and concept of the closed transplant production system In

Transplant Production in the 21st Century (edsKubota C and Chun C) Kluwer AcademicPublishers The Netherlands pp 3ndash19

11) Kozai T (2005) Closed systems for high qualitytransplants using minimum resources In PlantTissue Culture Engineering (eds Gupta S andIbaraki Y) Springer Berlin pp 275ndash312

12) Kozai T Afreen F and Zobayed SMA (eds)(2005) Photoautotrophic (Sugar-Free Medium)Micropropagation as a New Micropropagationand Transplant Production System SpringerDordrecht The Netherlands

13) Kozai T Ohyama K and Chun C (2006)Commercialized closed systems with arti1047297ciallighting for plant production Acta Hortic 71161ndash70

14) Kozai T (2007) Propagation grafting and trans-plant production in closed systems with arti1047297ciallighting for commercialization in Japan JOrnamental Plants 7 (3) 145ndash149

15) Kozai T (2012) Plant Factory with Arti1047297cial LightOhm Publishing Company (in Japanese)

T KOZAI [Vol 89458



16) SuperHort Project Association (2012) Report onAdvance Horticulture 2012 (in Japanese)

17) Takatsuji M and Mori Y (2011) LED Plantfactory Nikkan Kogyo Co Tokyo p 4 (in

Japanese)18) Mitchell CA Both A-J Bourget CM BurrJF Kubota C Lopez RG Morrow RC andRunkle ES (2012) LEDs The future of green-house lighting Chron Horticult 52 (1) 6ndash11

19) Yoshinaga K Ohyama K and Kozai T (2000)Energy and mass balance of a closed-type trans-plant production system (Part 3)mdashCarbon di-oxide balance J SHITA 13 225ndash231 (in Japanesewith English abstract and captions)

20) Ohyama K Yoshinaga K and Kozai T (2000)Energy and mass balance of a closed-type trans-plant production system (Part 2)mdashWater bal-ance J SHITA 12 (4) 217ndash224 (in Japanese withEnglish abstract and captions)

21) Yokoi S Kozai T Hasegawa T Chun C andKubota C (2005) CO2 and water utilizationefficiencies of a closed transplant productionsystem as aff ected by leaf area index of tomatoseedling populations and the number of airexchanges J SHITA 18 182ndash186 (in Japanesewith English abstract and captions)

22) Yokoi S Kozai T Ohyama K Hasegwa TChun C and Kubota C (2003) Eff ects of leaf areaindex of tomato seedling population on energyutilization efficiencies in a closed transplantproduction system J SHITA 15 231ndash238 (inJapanese with English abstract and captions)

23) Li M Kozai T Niu G and Takagaki M (2012)Estimating the air exchange rate using water

vapour as a tracer gas in a semi-closed growthchamber Biosystems Eng 113 94ndash101

24) Li M Kozai T Ohyama K Shimamura SGonda K and Sekiyama S (2012) Estimation of hourly CO2 assimilation rate of lettuce plants in aclosed system with arti1047297cial lighting for commer-cial production Eco-engineering 24 (3) 77ndash83

25) Li M Kozai T Ohyama K Shimamura DGonda K and Sekiyama T (2012) CO2 balance of a commercial closed system with arti1047297cial lightingfor producing lettuce plants HortScience 47 (9)1257ndash1260

26) Chiapale JP de Villele O and Kittas C (1984)Estimation of ventilation requirements of a plasticgreenhouse Acta Hortic 154 257ndash264 (in French

with English abstract)27) Shibuya T and Kozai T (2001) Light-use and

water-use efficiencies of tomato plug sheets in thegreenhouse Environment Control in Biology 3935ndash42 (in Japanese with English abstract andcaptions)

28) Bugbee BG and Salisbury FB (1988) Exploringthe limits of crop productivity I Photosyntheticefficiency of wheat in high irradiance environ-ments Plant Physiol 88 869ndash878

29) Sager JC Edwards JL and Klein WH (2011)Light energy utilization efficiency for photosyn-thesis Trans ASAE 25 (6) 1737ndash1746

30) Wheeler RM Mackowiac CL Stutte GWYorio NC Ruff fe LM Sager JC PrinceRP and Knott WM (2006) Crop productivitiesand radiation use efficiencies for bioregenerative

life support Adv Space Res 41 706ndash

71331) Wheeler RM (2006) Potato and human explorationof space Some observations from NASA-sponsoredcontrolled environment studies Potato Res 4967ndash90

32) Massa GD Kim H-H and Wheeler RM (2008)Plant productivity in response to LED lightingHortScience 43 (7) 1951ndash1956

33) Kozai T (2011) Improving light energy utilizationefficiency for a sustainable plant factory witharti1047297cial light Proc of Green Lighting ShanghaiForum 2011 375ndash383

34) Dueck TA Grashoff C Broekhuijsen G andMarcelis LFM (2006) Efficiency of light energyused by leaves situated in diff erent levels of a sweet

pepper canopy Acta Hortic 711 201ndash

20535) Liu WK Yang Q and Wei LL (2012) Light

Emitting Diodes (LEDs) and Their Applications inProtected Horticulture as a Light Source ChinaAgric Sci Tech Pub Beijing (in Chinese)

36) Morrow RC (2008) LED lighting in horticultureHortScience 43 (7) 1947ndash1950

37) Jokinen K Sakka LE and Nakkila J (2012)Improving sweet pepper productivity by LEDinterlighting Acta Hortic 956 59ndash66

38) Thongbai P Koai T and Ohyama K (2011)Promoting net photosynthesis and CO2 utilizationefficiency by moderately increased CO2 concen-tration and air current speed in a growth chamberand a ventilated greenhouse J of ISSAAS 17

121ndash

13439) Yabuki K (2004) Photosynthetic Rate and Dynam-

ic Environment Kluwer Academic PublishersDordrecht

40) Nederhoff EM and Vegter JG (1994) Photosyn-thesis of stands of tomato cucumber and sweetpepper measured in greenhouses under variousCO2-concentrations Ann Bot (Lond) 73 353ndash361

41) Evans JR and Poorter H (2001) Photosyntheticacclimation of plants to growth irradiance therelative importance of speci1047297c leaf area and nitro-gen partitioning in maximizing carbon gain PlantCell Environ 24 755ndash767

42) Ohyama K Kozai T Kubota C and Chun C

(2002) Coefficient of performance for cooling of ahome-use air conditioner installed in a closed-typetransplant production system J SHITA 14141ndash146 (in Japanese with English abstract andcaptions)

43) Kozai T Ohyama K Tong Y Tongbai P andNishioka N (2011) Integrative environmentalcontrol using heat pumps for reductions in energyconsumption and CO2 gas emission humiditycontrol and air circulation Acta Hortic 893445ndash449

44) Dayan E Presnov E Dayan J and Shavit A(2004) A system for measurement of transpiration




air movement and photosynthesis in the green-house Acta Hortic 654 123ndash130

45) Kozai T Li M and Tong Y (2011) Plantenvironment control by integrating resource use

efficiencies and rate variables with state variablesof plants Proc of Osaka Forum 2011 JapaneseSociety of Agricultural Biological and Environ-mental Engineers and Scientists Osaka pp 37ndash54(in Japanese)

46) Nishio M (2005) Agriculture and environmentalpollutionmdash soil environment policy and technol-ogy in Japan and the world Nohbunkyo Tokyo(in Japanese)

47) Sharpley ANT Daniel T Sims J LemunyonR Stevens R and Parry R (2003) Agriculturalphosphorous and eutrophication Second ed USDepartment of Agriculture Agricultural ResearchService ARS-149 Maryland

48) Kozai T (2013) Innovation in agriculture plant

factories with arti1047297cial light APO News January-February Issue 2ndash3

49) Ohmasa K and Takayama K (2003) Simultaneousmeasurement of stomatal conductance non-photo-chemical quenching and photochemical yield of photosystem II in intact leaves by thermal andchlorophyll 1047298uorescence imaging Plant Cell Phys-iol 44 1290ndash1300

50) Brentrup F Kusters J Kuhlmann H andLammel J (2001) Application of the Life CycleAssessment methodology to agricultural produc-tion Eur J Agron 14 221ndash233

51) Ozkan B Kurklu A and Akcaoz H (2004) Aninput-output energy analysis in greenhouse vege-table production A case study for Antalya region

in Turkey Biomass and Energy 26 89ndash9552) Christopher LW and Matthews HS (2008) Food-

miles and the relative climate impact of foodchoices in the United States Environ Sci Technol

42 3508ndash

351353) Roy P Nei D Okadome H Nakamura NOrikasa T and Shiina O (2008) Life cycleinventory analysis of fresh tomato distributionsystems in Japan considering the quality aspect JFood Eng 86 225ndash233

54) Oki T Sato M Kawamura M Miyake MKanae S and Musiake M (2003) Virtual watertrade to Japan and in the world In Virtual WaterTrade (ed Hoekstra AY) Proc of the Interna-tional Expert Meeting on Virtual Water TradeDelft The Netherlands Value of Water ResearchReport Series No 12 pp 221ndash235

55) Ohyama K Manabe K Ohmura Y Kubota Cand Kozai T (2003) A comparison between

closed- and open-type transplant production sys-tems with respect to quality of tomato plugtransplants and resource consumption duringsummer Environment Control in Biology 41 57ndash61 (in Japanese with English abstract and cap-tions)

56) Kozai T (2013) Plant factory in Japanmdash currentsituation and perspectives Chron Horticult 53(2) 8ndash11

57) Yang QC Wei LL Liu WK and Cheng RF(2012) Plant Factories systems and practiceChemical Industry Pub Co Beijing (in Chinese)

(Received Mar 10 2013 accepted Sep 19 2013)

Pro1047297le

Toyoki Kozai was born in 1943 After establishing his early work on greenhouse

solar light transmission ventilation and energy saving he performed pioneering work on

photoautotrophic culture of in vitro plants This study led to a development of closed

plant production system with arti1047297cial light and its commercialization He was promoted

to Professor of Chiba University in 1990 and became Dean of Faculty of Horticulture in

1999 Director of Center for Environment Health and Field Sciences in 2003 and

President of Chiba University in 2005 He was back in research in 2008 as a professor

at the Center mentioned above with a focus on medicinal plant production under

controlled environments He was awarded Japan Prize of Agricultural Science from

Association of Japanese Agricultural Scienti1047297c Societies Purple Ribbon Award from

Japanese Ministry of Education Culture Sports Science and Technology and Lifetime Achievement Award from

The Society for In Vitro Biology Since 2010 he has been working as the Chief-director of Japan Plant Factory

Association (non-pro1047297t organization) and is leading research development and extension of plant factories

T KOZAI [Vol 89460



List of symbols variables and coefficient names units and equation numbers

Symbol Variable name Unit Eq No

AA Electricity consumption of air conditioners (heat pumps) MJ m2 9 11 12

AL Electricity consumption of lamps MJ m2 8 11 12

AM Electricity consumption of water pumps air fans etc MJ m2 11 12

AT Total electricity consumption (AA D AL D AM) MJ m2 11

CUE CO2 use efficiency - 3

COP Coefficient of performance of heat pumps for cooling - 9 12

Cin CO2 concentration of room air mol mol1 4 13

Cout CO2 concentration of outside air mol mol1 4 13

CL CO2 loss to the outside kg m2 3 4

CP CO2 1047297xed by plants kg m2 3 13

CR CO2 released in room air by human respiration kg m2 3 13

CS CO2 supplied to room air from CO2 cylinder kg m2 3 13

D Dry mass increase of plants kg m2 5 6 7

EUEL Electric energy use efficiency - 7F Floor area of culture room m2 13 14 16 17

f Conversion factor from plant dry mass to chemical energy 20MJkg1 MJkg1 5 6 7

FUEI Inorganic fertilizer use efficiency mol m2 10

h Conversion factor from electric energy to PARL - 7 8

Hh Heat energy removed from culture room by heat pumps MJ m2 h1 9 12

HV Heat energy exchange by air in1047297ltration and penetration through walls MJ m2 h1 12

Iin Ion concentration of lsquoIrsquo in nutrient solution at the inlet of culture beds molmol1 17

Iout Ion concentration of lsquoIrsquo in nutrient solution at the outlet of culture beds molmol1 17

IS Supply rate of inorganic fertilizer ion element lsquoIrsquo supplied to the CPPS mol m2 h1 10

IU Absorption rate of inorganic fertilizer ion element lsquoIrsquo by plants mol m2 h1 10 17

kC Conversion factor from volume to mass of CO2 180kgm3 at 25degC and 1013k PA kgm3 4 13

kLW Conversion factor from volume to mass of liquid water kg m3 16

(997kgm3 at 25degC and 1013kPa)

LUEL Light energy use efficiency with respect to PARL - 5 7

LUEP Light energy use efficiency with respect to PARP - 6

N Number of air exchanges h1 2 4 13 14

PARL Photosynthetically active radiation emitted from lamps MJ m2 h1 5 7 8

PARP Photosynthetically active radiation received at plant community surface MJm2 h1 6

VA Volume of room air m3 2 4 13 14

VLW Volume of nutrient solution in culture beds m3 16 17

Xin Water vapor density of room air kg m3 2 14

Xout Water vapor density of outside air kg m3 2 14

WC Liquid water collected for recycling use in the CPPS kg m2 h1 1 14

WL Water vapor loss rate from the CPPS to the outside kg m2 h1 1 2

WP Water held in plants in the CPPS kg m

2 h

1 1 15WS Liquid water supply rate into the CPPS kg m2 h1 1

WT Tanspiration rate of plants in the CPPS kg m2 h1 14 15

WU Water uptake rate of plants in culture beds in the CPPS kg m2 h1 15 16

Win Water in1047298ow rate to hydroponic culture beds in the CPPS kg m2 h1 16 17

Wout Water out1047298ow rate from hydroponic culture beds in the CPPS kg m2 h1 16 17

WUE Water use efficiency - 1




hereafter wavelength 400ndash700 nm) water CO2 andinorganic nutrients8) In order to evaluate andimprove the resource use efficiency (ratio of the

amount of the resource 1047297

xed or held in plants to theamount of the resource supplied (RUE hereafter) inplant production the concept of lsquoclosed plant pro-duction system with arti1047297cial lightrsquo (CPPS hereafter)has been proposed9)ndash15)

The purpose of using CPPS is to maximize theplant growth with the minimum inputs of lightenergy water CO2 and inorganic fertilizer in otherwords to maximize plant growth with the maximumRUE resulting in the minimum emission of environ-mental pollutants and minimum costs for theessential resources It should be noticed that CPPSis not for production of staple food plants such as

rice wheat maize and potatoesIn this paper the de1047297nition and characteristics

of the CPPS with respect to RUE are reviewedmainly in terms of the use efficiencies of water CO2

and light energy Then it will be shown that their useefficiencies are considerably higher in the CPPS thanin the greenhouse Electric energy use efficiency of CPPS is also discussed in terms of the light energyconversion ratio of the light source

In Japan the number of plant factories witharti1047297cial light (PFAL hereafter) for commercialproduction of leaf vegetables such as lettuce and

spinach plants increased from 35 in December 2009 to106 in December 201116) It is estimated that thenumber increased to over 130 by the end of 2012 andis estimated to increase further in 2013 Researchdevelopment and commercial operation of PFALhave recently been extensively conducted in KoreaChina and Taiwan as well15) This movement has alsobeen initiated by the private sector in the US and theNetherlands according to related websites althoughscienti1047297c papers on this matter are scarce It is notedhowever that most PFAL are not designed based onthe concept of CPPS and are not operated on thebasis of the monitoring of RUE

On the other hand most facilities with arti1047297ciallight for producing only transplants (seedlings andplantlets from cuttings or bulblets) are designedand operated based on the concept of CPPS13)14)

The transplants include those of fruit vegetables(tomato cucumber eggplant etc) leaf vegetables(lettuce and spinach plants etc) for hydroponicsand 1047298owering bedding plants such as pansy (Viola

wittrockiana Gams) In 2011 CPPS was usedcommercially at more than 130 locations in Japan14)

In this paper the RUE of CPPS for production of

transplants and leaf vegetables found in the literatureis compared with that of the greenhouse andmethods for improving RUE are discussed

The 1047297

rst commercial PFAL was developed byGeneral Electric (GE) of the US in the early 1970sbut it closed at the beginning of the 1980s In Japanthe 1047297rst commercial PFAL Miura Farm was estab-lished in 1983 This was followed in 1985 by a PFALat a vegetable sales area in a shopping center17) Untilaround 1995 high-pressure sodium lamps were usedas the light source After that straight-tube 1047298uo-rescent lamps were the preferred choice mainly due toimprovements with respect to PAR output per wattThen a multi-layer system with a vertical distance of about 40 cm could be installed in the PFAL whichsigni1047297cantly improved the annual productivity per

land area PFAL using LEDs (light-emitting diodes)as the light source began commercial operation in2005 in Japan although the initial investment costof LEDs was still several times higher than that of 1047298uorescent lamps as of 201317) LEDs will become animportant light source in PFAL in the near futuresince the cost performance has more than doubledevery year since 201017)18)

The spectral energy distribution and electric-to-light energy conversion factor of the light source arethe important factors for assessing the efficiency of the lighting system17)18) The spectral energy distri-

bution of the light source is important in two aspectsphotosynthesis and photomorphogenesis Photosyn-thesis is basically aff ected by photosynthetic photon1047298ux density or photosynthetically active radiation1047298ux with wave band of 400ndash700nm On the otherhand photomorphgenesis and morphology are af-fected by the light spectrum ranging from around300ndash800 nm Currently the conversion factors of typical high-pressure sodium and 1047298uorescent lampsare respectively around 038 and 026 Whereas theconversion factor of recent LED is around 040 whichwill be further improved the forthcoming years17)18)

Con1047297guration and function of closed plantproduction system (CPPS)

The CPPS consists of six principal structuralelements (Fig 1)13)14) 1) thermally well-insulatedand nearly airtight warehouse-like structure coveredwith opaque walls 2) multi-tier system (mostly 4ndash16tiers or layers about 40 cm vertically between tiers)equipped with lighting devices such as 1047298uorescentlamps and LEDs over the culture beds 3) airconditioners (also known as heat pumps) principallyused for cooling and dehumidi1047297cation to eliminate

T KOZAI [Vol 89448


















2 Multi-tier system



4 CO2 supply

unit

1 Thermally well-

insulated nearly

airtight walls




6






PlantsProduce

ValueOpportunities

Environment

CPPS


A B

C

D



Heat energy



Cs

Ws

CPXin Xout

Cin CoutWT

WC

WU

Wout

VA

IoutIin

Win

CR

Culture beds


VW


PARL

AL AA

Pumps Fans AM

IU

WUE = (WC+WP)WS

CUE = CP (CS+CR)

Workers

PARP



COP = Hh AA [9]

FUEI = IU IS [10]


Plants

Hh

HV

[12]

[2]

[4]

[1]

[3]

[8]


T KOZAI [Vol 89450




(kgm3




























NO4



or NO3














outside26)







25 and 26ndash

31

Use efficiency CPPS

Greenhouse with

ventilators closed

and enriched CO2

Greenhouse

with ventilators

open

Theoretical

maximum value

for CPPS





005 0003ndash0032



T KOZAI [Vol 89452






















ndash31)33)

ndash36)38)

ndash41)






times 1047298







300micromolm2

s1
















T KOZAI [Vol 89454




















[14B]





































T KOZAI [Vol 89456







Challenges























Conclusion





Acknowledgement


References






ndash95













T KOZAI [Vol 89458































121ndash
























42 3508ndash








Pro1047297le














T KOZAI [Vol 89460











































2 h

























2 Multi-tier system



4 CO2 supply

unit

1 Thermally well-

insulated nearly

airtight walls




6






PlantsProduce

ValueOpportunities

Environment

CPPS


A B

C

D



Heat energy



Cs

Ws

CPXin Xout

Cin CoutWT

WC

WU

Wout

VA

IoutIin

Win

CR

Culture beds


VW


PARL

AL AA

Pumps Fans AM

IU

WUE = (WC+WP)WS

CUE = CP (CS+CR)

Workers

PARP



COP = Hh AA [9]

FUEI = IU IS [10]


Plants

Hh

HV

[12]

[2]

[4]

[1]

[3]

[8]


T KOZAI [Vol 89450




(kgm3




























NO4



or NO3














outside26)







25 and 26ndash

31

Use efficiency CPPS

Greenhouse with

ventilators closed

and enriched CO2

Greenhouse

with ventilators

open

Theoretical

maximum value

for CPPS





005 0003ndash0032



T KOZAI [Vol 89452






















ndash31)33)

ndash36)38)

ndash41)






times 1047298







300micromolm2

s1
















T KOZAI [Vol 89454




















[14B]





































T KOZAI [Vol 89456







Challenges























Conclusion





Acknowledgement


References






ndash95













T KOZAI [Vol 89458































121ndash
























42 3508ndash








Pro1047297le














T KOZAI [Vol 89460











































2 h












PlantsProduce

ValueOpportunities

Environment

CPPS


A B

C

D



Heat energy



Cs

Ws

CPXin Xout

Cin CoutWT

WC

WU

Wout

VA

IoutIin

Win

CR

Culture beds


VW


PARL

AL AA

Pumps Fans AM

IU

WUE = (WC+WP)WS

CUE = CP (CS+CR)

Workers

PARP



COP = Hh AA [9]

FUEI = IU IS [10]


Plants

Hh

HV

[12]

[2]

[4]

[1]

[3]

[8]


T KOZAI [Vol 89450




(kgm3




























NO4



or NO3














outside26)







25 and 26ndash

31

Use efficiency CPPS

Greenhouse with

ventilators closed

and enriched CO2

Greenhouse

with ventilators

open

Theoretical

maximum value

for CPPS





005 0003ndash0032



T KOZAI [Vol 89452






















ndash31)33)

ndash36)38)

ndash41)






times 1047298







300micromolm2

s1
















T KOZAI [Vol 89454




















[14B]





































T KOZAI [Vol 89456







Challenges























Conclusion





Acknowledgement


References






ndash95













T KOZAI [Vol 89458































121ndash
























42 3508ndash








Pro1047297le














T KOZAI [Vol 89460











































2 h











(kgm3




























NO4



or NO3














outside26)







25 and 26ndash

31

Use efficiency CPPS

Greenhouse with

ventilators closed

and enriched CO2

Greenhouse

with ventilators

open

Theoretical

maximum value

for CPPS





005 0003ndash0032



T KOZAI [Vol 89452






















ndash31)33)

ndash36)38)

ndash41)






times 1047298







300micromolm2

s1
















T KOZAI [Vol 89454




















[14B]





































T KOZAI [Vol 89456







Challenges























Conclusion





Acknowledgement


References






ndash95













T KOZAI [Vol 89458































121ndash
























42 3508ndash








Pro1047297le














T KOZAI [Vol 89460











































2 h
















outside26)







25 and 26ndash

31

Use efficiency CPPS

Greenhouse with

ventilators closed

and enriched CO2

Greenhouse

with ventilators

open

Theoretical

maximum value

for CPPS





005 0003ndash0032



T KOZAI [Vol 89452






















ndash31)33)

ndash36)38)

ndash41)






times 1047298







300micromolm2

s1
















T KOZAI [Vol 89454




















[14B]





































T KOZAI [Vol 89456







Challenges























Conclusion





Acknowledgement


References






ndash95













T KOZAI [Vol 89458































121ndash
























42 3508ndash








Pro1047297le














T KOZAI [Vol 89460











































2 h





























ndash31)33)

ndash36)38)

ndash41)






times 1047298







300micromolm2

s1
















T KOZAI [Vol 89454




















[14B]





































T KOZAI [Vol 89456







Challenges























Conclusion





Acknowledgement


References






ndash95













T KOZAI [Vol 89458































121ndash
























42 3508ndash








Pro1047297le














T KOZAI [Vol 89460











































2 h











times 1047298







300micromolm2

s1
















T KOZAI [Vol 89454




















[14B]





































T KOZAI [Vol 89456







Challenges























Conclusion





Acknowledgement


References






ndash95













T KOZAI [Vol 89458































121ndash
























42 3508ndash








Pro1047297le














T KOZAI [Vol 89460











































2 h



























[14B]





































T KOZAI [Vol 89456







Challenges























Conclusion





Acknowledgement


References






ndash95













T KOZAI [Vol 89458































121ndash
























42 3508ndash








Pro1047297le














T KOZAI [Vol 89460











































2 h





























T KOZAI [Vol 89456







Challenges























Conclusion





Acknowledgement


References






ndash95













T KOZAI [Vol 89458































121ndash
























42 3508ndash








Pro1047297le














T KOZAI [Vol 89460











































2 h














Challenges























Conclusion





Acknowledgement


References






ndash95













T KOZAI [Vol 89458































121ndash
























42 3508ndash








Pro1047297le














T KOZAI [Vol 89460











































2 h
















Conclusion





Acknowledgement


References






ndash95













T KOZAI [Vol 89458































121ndash
























42 3508ndash








Pro1047297le














T KOZAI [Vol 89460











































2 h






































121ndash
























42 3508ndash








Pro1047297le














T KOZAI [Vol 89460











































2 h






















42 3508ndash








Pro1047297le














T KOZAI [Vol 89460











































2 h


















































2 h








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