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Energy and Buildings 68 (2014) 183195

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Energy and Buildings

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Thermo-fluid dynamic modeling and simulation ofa bioclimatic solargreenhouse with self-cleaning and photovoltaic glasses

Paolo Sdringola, Stefania Proietti, Umberto Desideri, Giulia Giombini

Department of Industrial Engineering, University of Perugia, Via G. Duranti 67 06125, Perugia, Italy

a r t i c l e i n f o

Article history:

Received 26 September 2012

Received in revised form 8 April 2013Accepted 4 August 2013

Keywords:

Bioclimatic greenhouses

Nano-materials

Organic photovoltaic thin-film

CFD-FEM 3D modeling and simulation

a b s t r a c t

This paper describes the multifunctional complex Solaria: a development project ofan unused indus-trial area, located in a urban district in the immediate outskirts ofPerugia (Italy), conceived and designedaccording to principles ofsustainable buildings. Energy efficiency solutions and innovative experimental

components are synergically integrated in a single project, enabling to reach important results, as demon-strated by the assessment ofenvironmental achievements and the calculation ofavoided CO2emissions.

Since a quantitative evaluation of the energy savings, that can be achieved with the use of bioclimaticgreenhouses, is very complex, due to the large number of parameters,which are necessary to describe

their operation, the research work focused onthe thermo-fluid dynamic modeling ofthese systems, withthe use ofa specific CFD-FEM software, COMSOLMultiphysicsTM.

In particular a model was created, initially conceived in 2D and currently developed in 3D, whichreproduces the thermo-fluid dynamic behavior ofan experimental greenhouse in the Solaria complex.

The possibility ofchangingparameters characterizing materials and climatic conditions allowed to appre-ciate the influence on energy performance ofspecial reinforced thermal insulation, solar control glassesand external sliding sunshades. A further added value is the possibility to simulate an organic thin-film

photovoltaic device ofnanometric thickness.

2013 Elsevier B.V. All rights reserved.

1. Introduction

Both at legislation and research levels, attention is focused ontheadoption of systemsthat aimat energysaving andusing renew-able energies in buildings. Building construction involves severalenvironmental issues: the exploitation of non renewable resources,

land use, energyconsumptionin allphasesof thelife cycle ofbuild-ing, including demolition and waste disposal; but it is one of theareas withthe biggest potential of intervention. The energydemandin terms of net end-uses in Italy is steadily divided into three equal

parts (approximately 30% each) among the industrial, transportand civil sectors; the rest is consumed in agriculture, fishing, non-energy use, or it is stored. The civil sector share is divided between

tertiary (commercial and office buildings) 40% and residentialusers the remaining 60% where the distribution of typologyof uses is in line with the EU statistics. The largest demand is forheating (68%) [1].

The subject of this paper is part of a larger project, aimed at

achieving an integrated approach to solve the problems related to

Corresponding author. Tel.: +39 075 5853930; fax: +39 075 5853736.

E-mail addresses: [email protected](P. Sdringola),[email protected]

(S. Proietti), [email protected](U. Desideri), [email protected]

(G. Giombini).

ensure a comfortable and healthyliving, the sustainability of build-ings andbuildingprocess,the reduction ofenergyconsumption and

the increase of renewable energy utilization [2,3]. In order to builda sustainable building, an integrated planningis needed, providinga multiscale and integral view of the building-technical plantssystem [46]. A specific energy efficiency coordination should

be carried out to address project choices toward an integrationbetween environmental, social and economic aspects involved inthe decision-making process. This includes the following steps: abase energy project assessment; a preliminary evaluation about

energy classification and environmental sustainability; the selec-tion of certification protocols; a preliminary project on energyand environmental sustainability issues; the check about the com-

pliance with regulatory framework (heating, cooling, acousticrequirements); possible changes of envelope and plant features,aimed at improving energy saving; definitive and executive plan-ning, including optimization of renewable energy systems, activeand passive solutions for environmental sustainability; project

realization; energyand environmental sustainability certifications,in agreement with selected protocols; management choices aimedat optimizing energy consumption (e.g. participation of an EnergyService Company ESCo).

The research work described below concerns the optimizationphase, focusing on the bioclimatic greenhouses designed in a mul-tifunctional complex in Italy. This kind of passive solar systems

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184 P. Sdringola et al. / Energy and Buildings 68 (2014) 183195

Nomenclature

CFD Computational Fluid Dynamics

ENEA Italian National Agency for new technologies,energy and sustainable economic development

FEM finite element methodITO indium tin oxide

Low-E low emissivity

OPV organic photovoltaicP3HT poly (3-hexylthiophene)PCBM [6,6]-phenyl-C61-butyric acid methyl ester

PEDOT poly (3,4-ethylenedioxythiophene)

PSS poly(styrenesulfonate)

PV photovoltaicSUT services to territorial and urban level

A surface of the window between inner space andgreenhouse [m2]

cp specific heat capacity at constant pressure [J/(kgK)]cp,s specific heat capacity at constant pressure of highly

conductive layer [J/(kgK)]

ds thickness of highly conductive layer [m]

F volume forces [N/m3]

Famb ambient view factor []Fz volume forces in the z direction [N/m

3]

FLDm mean daylight factor [%]

g solar factor []G incoming radiative heat flux, or irradiation [W/m2]

Gm mutual irradiation, coming from other boundariesin the model [W/m2]

Gr Grashof number []h convective heat transfer coefficient [W/(m2K)]

I solar radiation on vertical window between innerspace and greenhouse, depending on exposure and

overhangs/obstructions [W/m2]J radiosity [W/m2]

k thermal conductivity [W/(mK)]

ks thermal conductivity of highly conductive layer[W/(mK)]

We/Whe electrical power/energy

Wp peak power

L length of linear thermal bridge [m]

m ratio of glass surface in referenceto total one, forthewindow between inner space and greenhouse []

n normalvectorof theboundary,pointed outfrom thedomain

Nu Nusselt number []p pressure [Pa]

Pr Prandtl number []

q total heat flux vector [W/m2]

q net inward radiative heat flux [W/m2]q

0 inward heat flux from external sources [W/m2]

qr net influx from radiation [W/m2]

qs heat flux transferred from other parts of thebound-

ary through a thin, highly conductive shell [W/m2]Q heat sources [W/m3]

Q0 heat flux across the window between inner spaceand greenhouse, in the absence of the greenhouse

[W/m2]t time [s]

T temperature [K]

Tamb temperature in the directions included in Famb[K]

Te outside temperature [K]Ti temperature of inner heated space (apartment) [K]

Tinf average reference temperature [K]

u velocity field (u, v, w) [m/s]u velocity in the x direction [m/s]

U thermal transmittance [W/(m2K)]

Ulim limit value of thermal transmittance, in accordance

with normative in force (Italian Legislative DecreeNo. 311/06) [W/(m2K)]

Uw thermal transmittance of glass [W/(m2K)]

v velocity in the y direction [m/s]w velocity in the z direction [m/s]grav acceleration of gravity, equal to 9.80665m/s2

p init air pressure, at the initial instant t0[Pa]

q0 finestra solar radiation on the North-East oriented win-

dow, which divides the sunspace from the nearbybalcony of the same apartment [W/m2]

q0 frontale solar radiation on the South-East oriented win-dow, facing the main street [W/m2]

q0 muro solar radiation on the South-West oriented wall,which dividesthe sunspace from thenearby balconyof the next apartment [W/m2]

t init temperature, at the initial instant t0[K]

t init aria temperature inside the greenhouse, at the initial

instant t0[K]temperature ext external temperature [K]

temperature int temperature inside the apartment adjacent

to the sunspace [K]rho aria air density [kg/m3]

rho init air density at initial conditions (t init aria, p init)[kg/m3]

coefficient of volumetric thermal expansion [K1] ratio of specific heats []

surface emissivity []

dynamic viscosity [Pa*s]

density [kg/m3]s density of highly conductive layer [kg/m3]

StefanBoltzman constant, equal to

5.67*108

W/(m2

K4

) linear transmittance of thermal bridge along the

window separating the inner space from the green-house [W/(mK)]

are characterized by several parameters necessary to describe thephysical processes; thus they were investigated by using a thermo-

fluid dynamic model built in a CFD-FEM software tool, andcouplingtwo different application modes: the integration of the incom-pressible NavierStokes equations; the solution of the general heattransfer equations.

2. Multifunctional complex Solaria

The building complex Solaria, under construction in Ponte

San Giovanni,Perugia (Italy), conceived and designed in agreementwith the above mentioned criteria, is a development project on anunused industrial area (Fig. 1). The general town-planning scheme

of the city of Perugia promotes interventions of urban transfor-mation and conversion from industrial to service/residential areaswith integrated functions. The functional mix assigned to the planzone (which almost 20,000m2 wide) is composed by the follow-

ing three uses: 65% of the covered surface dedicated to residences(115units); 30%to directional andcommercialspaces (16units); 5%to services for the territory and the city (13units). Through energyefficiency coordination an accurate study was carried out, in order

to address project choices toward a successful integration between

characteristics of the site and multiple uses of space to build, and


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P. Sdringola et al. / Energy and Buildings 68 (2014) 183195 185

Fig. 1. Aerial rendering of Solaria project in the intervention area.

toward the optimization of energyconsumptions and mitigation of

the environmental impact [79]. The project includes, for instance,(1) a centralizedsystem forthe separate accountingof consumption

equipped witha 400kWehigh-efficiency tri-generation plant and adistributionnetwork fordistrict heating/cooling;(2) a 20 kWppho-

tovoltaic generation system and solar thermal panels for domestichot water (DHW) production, able to supply over 50% of the hotwater demand; (3) recovery and reuse of rainwater (consumptionpercapita of drinking water is then reduced of approximately50%);

(4) passive solar systems (solar greenhouses and thermal bufferzones) and radiation control devices (sliding brise-soleils); (5) useof localmaterialsand components,compatiblewith well-being andhealthy living; (6) high insulation of the building envelope. The

orientation of buildings, size and shapes of windows guaranteeoptimal conditions of natural lighting, with a mean daylight fac-tor (FLDm) equal to 3.5%, and so greaterthanthe minimum value of2% imposed by Italian regulations. Moreover the acoustic design of

the building envelope, heating, technical and special systems werecarried out with an integrated approach to achieve the thermal-acoustic performance required by the legislation, and whereverpossible, with improved results.

The above-described interventions, integrated synergicallywithin a single project, helped to achieve significant certifiableresults. The complex was subjected to: a double assessment of

energy class(2008),comparing the methodologyused by the Munic-

ipality of Perugia with its own building code and the BESTClassprocedure (promoted by the Italian association SACERT Systemfor accreditation of building certification organizations). Bothmethodologies attest that the facilities belong to class A of energy

efficiency. A second assessment of energy-environmental perfor-mance by means ofsustainability evaluation protocols, introduced

in the Perugia building code (2007), reached a final score of 2.9/5compared to a reference value of a standard project (equal to 0).

Thisallowed to certify the classA of environmental sustainability.Interms of score,the assessmentwas particularly rewardingin macroareas, concerning sustainability of site, consumption of resources

(energy, water, soil,materials),quality of inner space, environmen-tal load, quality of services and management during the entire lifecycle [10,11].

The integration of some technological innovation concepts has

been considered. Besides, ensuring the desired specifications, theyshould be economicallycompatible withrequirements of the man-ufacturer; so they could potentially have a strategic disseminationin construction industry. In particular: (1) the green areas of

the complex will be equipped with photovoltaic high-tech lamps

(Stapelia

, made and patented by ENEA Italian National Agency

Fig. 2. State of Solaria project (Block G), 15th September2012.

for new technologies, energy and sustainable economic develop-ment); (2) use of reinforced thermal insulation and solar control

glasses in passive solar systems, different on the Northern andSouthern facades; (3) application of photo-catalytic products (plas-ter) to the vertical facades of the facilities, that enable to obtainsome additional benefits, including self-cleaning, passive clean-upand self-disinfection of bacterial contaminants; (4) testing of an

organic thin-film photovoltaic device (see Section 4.2) attached tothe glasses of a specific bioclimatic greenhouse [12].

The process for stepping up to a higher energy efficiency classwasanalyzed under environmentaland economic profiles, in terms

of avoided emissions (460 tCO2/year) and higher costs comparedto a standard design [13]. The Solaria complex is currently underconstruction (Fig. 2). The Block G, whose construction started inFebruary 2010, is expected to be ready before September 2013,

while the completion of theentire complex is planned in 2015.The

adoption of the energy efficient solutions, specifically designed forSolaria, makes the complex an advanced but also repeatable modelof integratedplanning, as well as a practicalapplicationof theEuro-

pean Directives on energy performance of buildings (2002/91/EC)and on energyend-use efficiency and energyservices (2006/32/EC).

3. Bioclimatic solar greenhouses

Bioclimatic greenhouses belong to the category of the passivetechnologies for the control of the thermal and hygrometric con-ditions inside a building. They are characterized by a direct and/orindirect gain,aimed at improvingcomfort and reducing energycon-

sumption. Bioclimatic greenhouses consist of an enclosed space,separatedfrom theoutside by windows andconnectedto thebuild-

ing with passages that may be opened and closed. The cover isglazed or opaque, depending on the latitude and the temperature

requirements. A greenhouse may be located adjacent to the build-ing and could be used as a living room in some periods of the year,and it increases the share of solar radiation converted into heat and

stored inside (greenhouse effect), thus contributing to the heatingof the inner space. Solar greenhouses are also called winter gar-dens for the useful and appropriate introduction of plants thatimprove the indoor air quality and regulate its humidity. During

thesummer, theshading effect of deciduous woods is oftenusedtoavoid overheating of building structures due to the excessive solarradiation [1416].

In recent years several regulatory actions were proposed both

at national and local level, aimed at disseminating technologies for

reducing energy consumption, including the passive solar systems.


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Fig. 3. Functional schemeof a direct gain solar greenhouse [18].

In Umbria, in agreement with the Regional Law 20th December

2000, No. 38, Incentives in calculation of urban parameters for theimprovement environmental comfort and energy saving in buildings,implemented by the Municipality of Perugia in its own buildingcode, the solar greenhouses are considered as volumes specifically

designed to obtain environmental comfort and energy savings, byimproving the insulation and the direct capture of solar energy.

4. Experimental solar greenhouse

Several bioclimatic greenhouses are planned in the Solaria

project, at each floor of Block G. These solar systems are excludedfrom the calculation of the urban surfaces and volumes, providingadditional 1000m3 (452m2) to the volume which was authorizedto be built.

Concerning the conditions of heat transfer and distribution, theSolaria greenhouses belong to the direct gain typology (Fig. 3) sincewalls, floors and ceilings are utilized as thermal collectors duringthe day, accumulating heat by radiation and convection, and heat

emitters during the night; the surface, which separates the innerspace from the greenhouse, is adjustable and may be removed by

displacing large mobile windows. In this way, it becomes an exten-sion of the room and the heat is directly gained inside the living

space.The Solaria greenhouses shape belongs to the lean-to category

(Fig. 4), since they share only one (the sliding windows) of thefour vertical surfaces with the building heated volume [17,18].

Fig. 4. Schemeof a lean-to solar greenhouse [18].

Fig. 5. Bioclimatic greenhouse selected forthe simulation, detailof Block G layout.

The upper side is the balcony of the upper floor, while, amongthe remaining perimeter sides, one is a wall and two are windows

(angular positions).Most greenhouses were designedto allow overlooking the main

sides of the building, because of the specific in-line typology ofstructure. The distribution of passive solar systems at different lev-

els and orientations is shown in Table 1. Further assessments allowto identify the various types and sizes of glass windows dividing

sunspaces from the apartments, as well as different plan surfaces.In order to avoid the greenhouse effect in summer conditions,

the external windows canbe manually opened, ensuring, by meansof adequate ventilation, that the indoor air temperature does notrise over the external temperature. Moreover the project plans theuse of protective and shielding devices on glass surfaces, for sum-

mer radiation control, through sliding brise-soleils. These consistof perforated aluminum plates, manually sliding, installed on thesame plane and parallel to the railings on the balconies. The holes(40.3% vacuum) enable the passage of a little amount of direct and

diffuse radiation,because of the diffractionaction of holes,ensuringgood lighting conditions during the day and the related electricitysaving.

Design criteria concerning the orientation were influenced by

the context. In fact the greenhouses lean on vertical structureswhose horizontal rotation angle is closely related to the buildinglayout, setby theBuilding Code of Perugia Municipalityand accord-ing to the integral part of the implementation plan. Besides, the

main obstructions in the Southern quadrant are other buildingsand, not having the possibility to freely choose the location of thebuilding within the project area, they cannot be corrected.

A greenhouse of the Solaria Block G was chosen as represen-

tative of the others in order to quantify, by using a simulationsoftware, the attainable energy benefits in terms of reduction ofthe thermal energy demand during the entire heating season. Forthecasestudy, a 1.60 m3.00 m sunspace was selected on the sec-

ond floor(Fig. 5), andfacing South-East, withan horizontal rotationangle of 54 from the geographic South.

From the energy point of view, it can be considered belong-ing to the lean-to typology even though only two of the three

external structures (not in contact with the heated volume) areglazed, as mentioned before. However, this feature, coupled withthe presence of lateral vertical obstructions (opaque partitions of

the balconies and/or adjacent greenhouses), makes the case studyan example of built-in greenhouse in relation to solar radiation. Theamount of incident radiation, which is maximum in the case of alean-to sunspace, decreases because of shading by vertical struc-

tures; on the other hand, a larger portion of glazed surface reducesthe envelope insulation, causing, therefore, larger heat losses.

The CFD analysis carried out on the solar greenhouse was par-ticularly interesting from a scientific and technical standpoint,

especially for the materials used in the construction process. The

surfaces definingthe sunspace aremade by thefollowing elements.


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Table 1

Distribution of bioclimatic greenhouses into Block G of Solaria complex, according to different orientations.

Level Orientation Total

South-West South-East South-East East-North-East

232 142 126 68

First 1 4 3 1 9

Second 1 4 4 1 10

Third 1 4 4 1 10

Fourth 1 4 4 0 9Penthouse 0 2 2 0 4

Total 4 18 17 3 42

Four windows equipped with a standard low-E double glazing andan aluminum frame with thermal barrier separate the inner spacefrom the greenhouse (4-16-4, Uw =1.1W/m2 K,g= 0.62). Some win-

dows equipped with a special low-E double glazing (reinforcedthermal insulation and solar control features) and an aluminumframe with a thermal barrier to separate the external environ-ment from the sunspace; for the case study the selected glass also

features self-cleaning properties (see Section 4.1) and the experi-mental application of an organic thin-film photovoltaic layer (see

Section 4.2). Finally some structural elements were considered: anexternal wall, in two cases adjacent to other sunspaces (s =41.5cm,

U=0.157W/m2 K, Ulim =0.34W/m2 K); the roof and the floor, char-

acterized by the same stratigraphy (s =46cm, U=0.401W/m2 K,

Ulim =0.43W/m2 K).

4.1. Self-cleaning glasses with reinforced thermal insulation and

solar control properties

In order to minimize theheat lossesthrough thewindows,a spe-

cial glazing is planned to be used; it couples self-cleaning propertyof its external face with the double function of reinforced thermalinsulation and solar control. It includes: a 4 mm insulating glasspane, characterized by a self-cleaning thin layer on face 1 and the

application, using cathodic vacuum deposition, of a low-E layeron face 2, consisting of noble metals (it reflects long wavelengthinfrared radiation, allowing to keep the heat inside); 16mm cav-ity, filled with 90% argon (a double barrier made of organic seals

ensures the creation of a hermetically-sealed environment andmechanical stability); a 4 mm insulating glass pane. Theglasstrans-mittance Uw, calculated according to EN 673, is 1.1 W/m2 K; lighttransmission and reflection factors are 68% and 15% respectively,

while the solar factorgis equal to 0.41.The self-cleaning glassappearssimilar to a conventionalinsulat-

ing one; it is manufactured by depositing a transparent 8 nm layerof titanium dioxide TiO2 on the external surface of a clean glass.

This material harnesses the power of both UV light from the sunand rain to efficiently combat dirt and grime that accumulates on

theoutside of thewindow(dried water marks,organic atmosphericpollutants, dust, sea spray and insect residues). Two different steps

are considered: exposure to the UV rays in daylight triggers thedecomposition of organic dirt (photocatalytic effect)and causes thesurface of the glass to turn hydrophilic; water forms a sheet on thesurface of the glass and rinses away broken-down organic dirt and

mineral material.

4.2. Organic photovoltaic thin-film

Organic solar cells include all the devices where organic carboncompounds acts as the photoactive element. Polymeric PV cellshave recently reached a maximum efficiency of 56% (laboratory

tests);the most efficient cells employa mixture of materials so that

the process of radiation absorption and charge separation is more

effective. In order to further increase their performances and espe-cially their lifespan, thus making them compatible for applicationsin the building sector, major efforts in research and development

are undertaken by industry and research laboratories, includingnew encapsulation techniques and strategies such as the introduc-tion of inorganic nano-crystals in the polymeric matrix [19].

This kind of technology has several advantages: OPV cells could

be produced as thin-film (rolls), they are flexible and lightweightand have a high energy to weight ratio; necessary materials are

abundantly available; they enable an easy scalable production andentail relatively low investment costs for the production process;

the cells are 100% recyclable, avoiding disposal problems; thanksto flexibility and transparency characteristics, they could be inte-grated into windows, cars structures, tents, or even fabrics; OPVmanufacturing couldcost far less than conventional cellsexploiting

the economies of scale.For the solar greenhouse described in Section 4, the project

involves the testing of a specific thin-film photovoltaic device,applied ona portion ofexternalwindows,with an optimal exposure

to solar radiation. The OPV material was developed at the Depart-ment of Civil and Environmental Engineering, in the Laboratory oftechnologies and material sciences of the University of Perugia. Asshown in Fig. 6, the sandwich of organic cell consists of several

layers.

- A layer of aluminum, acting as cathode.- A layer of titanium dioxide.- An active layer of P3HT:PCBM (polimer:fullerene blend). A bulk

heterojunction blend of an electron donor, P3HT or Poly(3-hexylthiophene), and an electron acceptor, PCBM fullerenederivative [6,6]-phenyl-C61-butyric acid methyl ester, allows toobtain higher conversion efficiencies than a multilayer geometry

[2022].- A layer of PEDOT:PSS or Poly(3,4-ethylenedioxythiophene):

poly(styrenesulfonate). It is the polymer mixture of twoionomers. In this application it is used as a transparent and con-

ductive polymer, improving the selectivity of the anode [23].

Fig. 6. Structure of the organic photovoltaic cell [31].


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Fig. 7. Organic thin-film photovoltaic device applied on a glass sample.

- A layer of tin-doped indium oxide, or Indium Tin OxideITO, asolid solution of indium (III) oxide (In2O3) and tin (IV) oxide

(SnO2), typically around 90%and 10% by weight; it is transparentandcolorless in theform of thin film. Electric conductivity,opticaltransparency, easy deposition process to obtain a thin-film makeITO one of the most widely used transparent conducting oxides.

In this application acts as anode.- A glass supporting base.

A sample of organic photovoltaic thin-film, deposited by thetechnique of spin coating on a glass substrate, is shown in Fig. 7,which makes evident the high transparencydegreeachieved by the

specific application [24].

5. CFD modeling

A quantitative assessment of the energy saving benefits asso-

ciated with the use of bioclimatic greenhouses is very complex,due to the number of parameters involved in the process and theireffect. Solar radiation incident on a glass surface varies accordingto latitude, day and month of year, weather conditions of the site,

presence of obstructions and/or projections. The amount of energystored into the greenhouse depends on solar radiation, angle ofincidence and transparency coefficient of the material. These lasttwo factors are mainly influenced by shape and orientation of the

greenhouse and by the characteristics of the materials used for itsconstruction. If implemented properly, the management of passive

solar systems, by individual users within the single housing units,may optimize its energy performance; otherwise the benefits are

minimized or canceled (i.e. discomfort conditions associated, forexample, to overheating phenomena in summer time).

Research activity has therefore focused on the Computational

Fluid Dynamics (CFD) modeling of solar greenhouses by COMSOLMultiphysics TM software, which allows simulating every systemthat can be described by partial differential equations. The set ofequations is solved on the basis of finite element method (FEM)

[25].The study was aimed at creating a model of the greenhouse,

where the parameters characterizing materials and climatic con-ditions may be varied. For the present case, a 3D geometry was

chosen, because the model does not have homogeneity or sym-

metry characteristics along any direction; for the discretization

of the continuous domain, linear formulation (order 1) was

selected, obtaining tetrahedral finite elements in 3D. To assess thegreenhouse behavior according to different internal and externalconditions, a transient analysis was performed.

5.1. Definition of the physics for the model

For the description of physical phenomena, two different appli-

cation modes were coupled [26].The first one is the integration of the incompressible

NavierStokes (NS) equations, using the basic module of COMSOLMulti-physicsTM, which solves for the following dependent vari-ables: pressurep and velocity vector componentsu (u, v, w) alongthe three directions (x, y, z). Convective flows into the greenhouse

are characterized by a low speed; the variations in air pressureand temperature allow to assume it as an incompressible fluid.The equation of continuityand the momentum transportequations(implied by Newtons laws of motion) are the fundamental laws

governing flow field. In particular, the NavierStokes equations areexpressed by the following:

u

t

+ (u )u = p+ u+ (u)

T

+F (1)The second application mode is the solution of the general heat

transfer equations (htgh), using the Heat Transfer module, which

solves for the following dependent variables: temperature T andradiosity J. For low-speed convective processes in inviscid fluids,some simplifying assumptions are introduced; not considering thework done by viscous forces (viscous dissipation) and the com-

pressibility effect ( =0), the heat is expressed by the followingequation [2729]:

cp

T

t

+ (kT) = Q cpu T (2)

Radiation is treated as a process that transfers energy directlybetweenboundariesand whichtherefore contributesto the bound-

ary conditions rather than to the heat equation itself. According tothe ideal gray body assumption (absorptivity and emissivity areequal), net inward radiative heat flux q is given by the differencebetween the irradiation G and the radiosityJ:

q = G J= (G T4) (3)

5.2. General settings, constants and functions

The preliminary step of the modeling phase consists in the def-inition of some constants and variables, which represent the basicparameters describing the physical system. In thiscase, the follow-

ing initial conditions (at time t0) were entered: t init, temperature[K],set equal to 281.15K (8C) in the heating season (it is the tem-

perature of an unheated space); t init aria, temperature inside thegreenhouse [K], established through an iterative process;p init, airpressure, equal to 101,325 Pa. Moreover: temperature int, temper-ature inside the apartment adjacent to the sunspace, set equal at291.15K (18 C) during the heating season; grav, acceleration of

gravity.To preliminary investigate the thermo-fluid dynamic behav-

ior of the greenhouse, different climatic conditions were takeninto account. In particular some specific days, such as 21st March

and 21st June, and mean monthly data were used to characterizeopaque surfaces and windows of the sunspace, in terms of solarradiation and external temperature (temperature ext). These func-tions were calculated by Ecotect, a software for the assessments

of energy performance and climatic integration of buildings. Once

a three-dimensional digital Block G was modeled, it was necessary


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Fig.8. Digital BlockG andurbansurroundingarea,modeledinto Ecotect software;

shadows refersto 7:00a.m. of 21st March.

to: enter the site coordinates (latitude and longitude) and its ori-entation; upload the climate files containing the weather data ofPerugia, Italy, in terms of external temperature, humidity, windspeed, etc.; model the buildings standing all around the Solaria

complex, and in particular the fixed obstacles and obstructions for

solar radiation regarding the greenhouse, as shown in Fig. 8.The simulation through Ecotect resulted in the actualvaluesof

solar radiationincident on the three external sides of the sunspace:

the largest glass surface, facing the main street on the South-Eastside (q0 frontale); the smallest glass surface, facing North-East andseparating the greenhouse from the nearby balcony of the sameapartment (q0 finestra); the South-West oriented wall, separating

the greenhouse from the nearby balcony of the next apartment(q0 muro). Solar charts for the three different exposures wereobtained as outputs of simulation; an example is in Fig. 9.

5.3. Geometry modeling

Through the use of a CAD software for 3D design, a model of the selected greenhouse was created; it reproduces the structure

to be built withinthe multi-destination Solaria complex. Each com-ponent was designed as individual parts within the program andlater assembled to form the complete structure.

Fig. 9. Solar chart of the main external glass window, South-East facing on 21st

March.

Fig. 10. Subdomainof theair inside thegreenhouse (No. 9).

5.4. Subdomain settings

The materials making up the physical model were entered intoa specific library of COMSOL Multi-physicsTM; their properties are

summarized in Table 2. The model included into the software wasused for characterizing the air inside the greenhouse; the specificheat capacity and the thermal conductivity are expressed as func-tion of the temperature T, while the density varies also with the

pressure p; using the NavierStokes equations for incompressiblefluid, this last dependence can be neglected.

After entering the new materials in COMSOL library, No. 24 sub-domains generated from the geometric model were set. As for heat

transfer analysis, the software requires the definition of the follow-ing characteristics.

Thermal properties and heat sources/sink. In order to define ther-mal properties (k, p, cp), the material making up each subdomain

was loaded from materials library; in this model heat sources orsinks are not present.

Properties of convective heat transfer. This kind of heat transfercan be enabled only for fluids and in the examined case for subdo-

main No.9 (air inside the greenhouse, Fig. 10). The characterization

concerns: fluid type (gas); velocity field, identified by the vector (u,v, w); ratio of the specific heats at constant pressure and volume (set equal to 1.4 for the air); the absolute pressure (p init).

Initial value of temperature T(t0) was set equal to the constantt init.

Fluid dynamics is automatically enabled only in fluid domains.As for subdomain No. 9, following characteristics are required.

Fluid properties. As in the previous analysis, in order to definedensity and dynamic viscosity (functions of temperature T),the air model was loaded from materials library; regarding volumeforces, theaction of gravitywas consideredon thebasisof following

expression:

Fz= (grav) =grav(rho init rho aria) (4)

The volume forces are therefore a function of density, accord-ing to the Boussinesq approximation; it assumes that variations indensity have no effect on the flow field except when they causebuoyant forces.

Initial values. At time t0, the pressurep(t0) was set equal to theconstantp init, and the flow field was assumed zero.

5.5. Boundary settings

In the fluid dynamic analysis, for each surface delimiting airvolume, the software requires the definition ofBoundary type andcondition. All the boundaries were described respectively as wall

(no openings) and no slip, i.e. absence of flow through and along

the boundary (u=0).


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Table 2

Properties of materials used for specific model simulation.

Material Specific heat capacity at constant pressure cp Thermal conductivity k Surface emissivity Density

[J/(kg K)] [W/(m K)] [kg/m3]

Aluminum frame 900 0.226 0.3 510

Glass 840 0.0325 0.03 840

Floor 856 0.198 0.92 1132Legnobloc with graphite 1259 0.067 0.85 1008

Material Specific heat capacity at constant pressure cp,s Thermal conductivity ks Thickness ds Density s[J/(kg K)] [W/(m K)] [m] [kg/m3 ]

Photovoltaic thin-film 830 11 2.25107 4239

For heat transfer analysis, the pairs of boundaries that are cre-

ated from the assembly of several subdomains (for example the airinside the greenhouse and the glass, which separates it from theoutside) behave like a single interior boundary and the same con-dition is applied. The default setting specifies continuity in both

temperature and normal heat flux across the pair, just like on anyinterior boundary. The conditions selected for the case study werethe following.

Insulation/symmetry. It was assigned to the boundaries that rep-

resent sections of opaque walls or balconies. They do not exchangewith the surroundings because they are thermally well insulated;these boundaries do not take part in radiative heat transfer.

Temperature. This condition prescribesthe temperature T0equal

to the constant temperature int, at the boundaries that separate thegreenhouse from the internal environment (Dirichlet condition).

Heat flux. It was assigned to the remaining exterior boundariesdelimiting the greenhouse and to interior boundaries delimiting air

volume, according to the following expression:

n q = q0 + qr+ qs + h(Tinf T) (5)

q0 is the inward heat flux from external sources, in this caseincoming solar radiation (hourly values), described by functions

q0 frontale, q0 finestra, q0 muro.

The heat transfer coefficient h and the reference temperature

Tinf are necessary parameters to quantify the energy transferredthrough thespecific boundary to the environment by means of con-vective heat transfer. The General Heat Transfer application mode

includes a library of predefined h coefficients for common flow sit-uations. In this case only the presence of natural convection wasconsidered, assuming that the flow is governed by buoyancy forcesgiven by the temperature difference between the fluid (air) and the

wall, thus neglecting the effect of the wind. This approximation isjustified by the fact that CFD analysis is aimed at evaluating temper-ature conditions inside the greenhouse and the energy benefit ofthis passive solar system; these aspects are weakly influenced by

wind speed in a highly urbanized areas, such as the area whereSolaria is located. In the Heat Transfer Coefficient library, the hcoefficient is based on Nusselt number Nu correlations from hand-

books and it is expressed as a function of the material properties,temperature, flow rate and geometry. For natural convection, therelationship for the Nusselt number typically has the form:

Nu = C(GrPr)n (6)

where the parameter C depends on geometry, while the exponent

n is 0.25for laminar flowand 0.33 forturbulent flow[25,30]. Forthecalculationof theheat exchangedby convection withthe surround-ings, the following inputs were assumed: Tinfequal to function

temperature ext, length scale L for natural convection on vertical

wall (3.69m) and horizontal surface (1.79 m).qr is net influx from radiation, according to Eq. (3). The irradia-

tion G is expressed by the following expression:

G = Gm + FambT4

amb

(7)

Fambis the ambient view factor; it represents a measure of how

much influence the radiosity at a given part of the boundary has tothe irradiation at some other part. Its value is equal to the frac-tion of the visual field that is not covered by other boundaries(0 Famb 1), considered to be a single boundary with constant

radiosity Jamb=Tamb4. Gm is the mutual irradiation coming from

other boundaries in the model, depending on geometry and localtemperatures. Tamb is the assumed far-away temperature in thedirections included in Famb. Assuming an ideal gray body, the equa-

tion used in the General Heat Transfer application mode to solvethe radiosityJ(in parallel with the equation for the temperature T)is:

J= (1 )[Gm + FambT4amb

] + T4 (8)

For the radiation heat transfer, two different condition typeswere used. The Surface-to-ambient radiation is appropriate forexte-rior surfaces and assumes from the outset that Gm = 0 and Famb = 1.

Eq. (3) results in:

qr= (T4amb T

4) (9)

In this case Tamb is the temperature of ambient surroundingsand it is expressed by the function temperature ext, whereas is

the emissivityof the specific boundary. Surface-to-surface radiationis the condition used for internal boundaries delimiting the air vol-ume; hemicube method,which accounts for shadowing effects, wasselected to quantity Gmand Famb.

qs represents the contribution from a thin, but highly conduct-ing, shell in contact with the boundary.The highlyconductive layerfeature is introduced by the software to model heat transfer inthin layers without the need to create a specific mesh. Because

the layer is very thin and has a high thermal conductivity, it ispossible to assume that no variation in temperature and in-planeheat flux exist along its thickness; furthermore the difference in

heat flux in the layers normal direction between its upper andlower is thought as a heat source or sink. Highly conductive layeris the characterization used to describe the organic photovoltaicthin-film.

5.6. Mesh generation and solver settings

Once geometry model subdomain/boundary conditions are

input, the software requires the characterization of the mesh. Theselection of parameters is very important because mesh nodes arethe points where the software actually calculates the solution ofpartial differential equations. In this model, a predefined setting

for a fine mesh was used (Fig. 11), resulting in 54,633 elements, fora total of 39,478 degrees of freedom.

The evaluation of thermo fluid dynamic characteristics of thebioclimatic greenhouse was performed in transient mode, in order

to consider its effect on internal temperature and energy saving at

varying solar radiation. The direct linear system solver was set for


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Fig. 11. Model domains, meshed through automatic generator of COMSOLMultiphysicsTM.

simulating an entire day (86,400s) and recording the values of thevariables every hour (3600 s).

6. Results of simulations

The simulation, carried out on March 21st and considering theuse of the reinforced thermal insulation and solar control glasses,

resulted in a 23.6 C greenhouse temperature (recorded at a centralpoint of the volume)at 1:00p.m.As shown in Fig. 12, the solar radi-ation hasa symmetrical trend with referenceto 12:00 a.m. Externalair temperature is usually characterized by a curve similar to solar

radiation, but few hours shifted because of thermal inertia of theEarths surface and atmosphere; the greenhouse temperature fol-lows a similar pattern to external air, reaching highertemperaturesduring daylight hours and maintaining them after the end of irra-

diation.3D image in Fig. 13 shows the simulation results in graphical

form: in particular 5 planes in y direction for plotting tempera-tures, while the red arrows represent the vectors of velocity field.

Air inside the greenhouse moves upwards, getting warm along theglass irradiated surfaces and drawing cooler air, thus starting acirculation that makes the greenhouse temperature uniform.

With reference to heat flux across the elements of separation

between the sunspace and the inner space, Fig. 14 summarizes

Fig. 13. Graphic simulation results in terms of temperature (surface) and velocity

field (arrow),2:00p.m. of 21st March.

the comparison between the hourly values (21st March) obtainedas result of the simulation, by integrating the total heat flux overthe window boundaries, and not considering the presence of thegreenhouse. In this last case, the energy balance of the window

is evaluated as algebraic sum of the solar heat gains and the heatlosses, according to the expression:

Q0 = [Igm+ U(Te Ti)]A+L(Te Ti) (10)

where with reference to the window dividing the inner spacefrom the greenhouse, g= 0.62, m =0.795, U=1.6W/m2 K, A =6.46m2 , = 0.09 W/(mK), L =14.5m.

The difference between those values resulted in an energy

benefit due to the presence of a passive solar system with a capac-

Fig. 12. Hourly trends of greenhouse temperature, outside temperature, temperature inside the apartment and solar radiation, 21st March.


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Fig. 14. Heat fluxacross theelementswhichdivideinnerspacefrom the greenhouse,21st March.

ity of 1.25kWh on 21st March, which includes both the positive

contributions when the greenhouse temperature is higher thanthe internal (fixed value into the apartment), and the reductionin heat losses as the greenhouse temperature is higher than theexternal one.

On the basis of the same weather conditions, another simula-tion was performed considering that the organic PV thin-film isapplied on the external glass surface facing South-East. The max-imum temperature reached inside the greenhouse is 23.55 C at

1:00p.m., and it is slightly lower (less than 1%) than the previouscase. Moreover in terms of energy contribution, the OPV film doesnot significantly influence the sunspace performance (0.1%); itspresence was considered in all subsequent simulations.

In order to evaluate the energy benefit during the entire heatingseason (183 days, from 15th Octoberto 15th April, according to theItalian legislation), a series of simulations were performed on thebasis of the average monthly climate data. For each month, from

October to April, functions of irradiation and outdoor temperaturewere then varied.The results areshown in Figs. 15 and 16; negativevalues in the first hours of the October average day are caused bya greenhouse temperature, calculated through the CFD simulation,

slightly lower than the ambient temperature.Multiplying these average values by the number of days

included in each month, the energy contribution of solar green-house was estimated at about 310kWh. The apartment chosen

as representative of the Block G of the Solaria complex is char-acterized by a useful thermal energy demand for winter heatingof about 1527kWh/year (18.4 kWh/m2year). Energy benefit due tosolargreenhouse duringthe heating seasonthus represents the20%

of the demand.Assuming to apply the photovoltaic thin-film on the external

glass surface facing South-East, for a total of 7.84m2, the potential

electricity production was estimated, for the same period, at60kWhe(7.5kWhe/m2), considering an efficiency of 3.5%.

Further simulations were conducted to investigate the behaviorof the greenhouse in summer time, when overheating phenomena

could create discomfort situations. With reference to the climatic

conditions of June 21st, the following cases were investigated.

Case 1. Traditional glasses, originally planned in the project, are

used forthe greenhouse envelope;in particular: low-E doubleglaz-ing, 5-12-5, cavity filled of argon, transmittance Uwof 1.4 W/m2 K.

Case 2. Special reinforced thermal insulation and solar controlglasses are used for the greenhouse external windows; moreovertheOPV thin-film is applied to theexternalSouth-Eastglasssurface.

Case 3. In addition to case No. 2, halved values of function

q0 frontale were entered, considering thepresenceof external slid-ing sunshades.

The results in terms of greenhouse temperature (Fig. 17) showhow the use of reinforced thermal insulation and solar control

glasses reduce the problem of overheating. A significant improve-ment, when ventilation is not sufficient (open windows, so that thetemperature is lower than theoutdoor one, and air ventsat the topand bottom of the greenhouse for operating as a solar chimney), is

guaranteedby thepresenceof sliding shadingdevices that decreasethe solar radiation through the glass: - 27% in terms of temperatureat 12:00 a.m. on 21st June.

7. Comparison with other evaluation methods

In order to use this methodology as a prediction tool, it is

necessary to validate it by comparing simulation results and exper-imental data. Response of the model is considered acceptable whenthe gap with reality is compatible with both quality of input data

and physical variations of the investigated phenomenon.Currently, experimental data are not available to validate the

model described in the previous paragraphs; in fact, the multi-functional complex is now under construction. In particular, the

BlockGwillbecompletedbySeptember2013,thusmakingpossiblethe experimental application of the OPV thin-film to the selectedgreenhouse and the construction of a data acquisition and moni-toring system.

In this phase, a comparative analysis was carried out between

the results obtained through CFD-FEM 3D modeling and other


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Fig. 15. Monthly average temperature into the sunspace.

Fig. 16. Monthly average energy contribution due to the solar greenhouse.

simplified methodologies for assessing the energy performance ofsolar greenhouses, as described below.

A preliminary method for designing passive solar systems isSunspace, based on the numeric input software Solacalc Method5000, developed for solar building applications. It divides the totalenergy benefit into four different types of solar gain: the ther-

mal radiation penetrating directly into the inner space across thewindow which divides it from the greenhouse; the thermal radia-

tion stored by the wall separating such spaces; the buffer effect,

due to the warm air inside the greenhouse, which reduces the out-wards heat losses; the pre-heating process of air changes [18]. The

monthly mean values obtained from sunspace are presented inTable 3. On the one hand the actual data, concerning solar radi-ation and features of materials defining the greenhouse volume,can be entered into thespreadsheet; on theother hand the calcula-

tion is stationary, made on the basis of the monthly average valuesabout daily solar radiation and outside temperature, thus leading

to lower energy benefits in winter and higher in autumn or spring.


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Fig. 17. Temperature into the sunspace, 21st June.

Table 3

Energy benefits due to solar greenhouse, assessed through different methodologies.

Energy benefit October November December January February March April

Sunspace kWh/day 1.0 0.6 0.5 0.6 0.8 1.2

Ecotect kWh 21.9 36.5 57.8 63.9 50.9 46.2 29.9

kWh/day 0.71 1.22 1.87 2.06 1.82 1.49 1.0

STIMA 10 TFM MJ 61 194 274 302 256 225 78

kWh 16.94 53.89 76.11 83.89 71.11 62.50 21.66

kWh/day 0.55 1.79 2.45 2.70 2.54 2.01 0.72COMSOL MultiphysicsTM kWh/day 0.42 0.99 1.88 2.29 2.15 2.01 1.39

The second tool is the software Ecotect (see Section 5.2). Afterentering site location, orientation, climate data, fixed obstruc-tions (all information necessary to evaluate solar radiation), a 3Dmodel of the greenhouse and the adjacent room was created.

So it was necessary to define the specific parameters of thermalzones and to characterize the individual items, making appropri-ate changes/integrations to the materials library. A comparativeanalysis of the situation with and without the solar greenhouse

was carried out in terms of monthly energy demand for maintain-ing the internal comfort conditions; it allowed to determine theenergy benefit in the sunspace reported in Table 3.

Finally, energy performances of the solar greenhouse wereevaluated through the use ofSTIMA10 TFM, a software aimed atcalculating winter and summer thermal loads, starting from cli-mate/geographical parameters and all the technical data describingthe investigated building. The apartment equipped with the

selected greenhousewas created into the software, entering roomsdimensions, openings with their orientation, stratigraphies ofstructures and windows delimiting the volumes (the existinglibrary was integrated). A procedure congruent with UNI TS 11300-

1 is used for assessing bioclimatic greenhouses separated fromheated spaces by a partition wall. Several aspects are evaluated, inparticular: the transmission heat losses between the inner heatedspace and the external environment through the sunspace, on the

basis of a heat-dispersion coefficient; the reduction of transmission

losses, due to both the temperature increase inside the greenhouse

and the solar radiationdirectly absorbed by the opaquecomponentof partition wall; the solar gains through the external greenhousewindows and the glass component of partition wall. Once all thedata specific for the case study are input, the software calculates

the values of energy contributions related to the solar greenhouse,reported in Table 3.

Thermo-fluid dynamic simulations allowed obtaining compara-ble results, both from a qualitative and quantitative point of view,

with other methodologies described above, in particular with theEcotect and STIMA 10 TFM software tools. The energy benefitsassociated with the presence of the bioclimatic greenhouse are:

nearly always lower than those obtained with STIMA 10 TFM, 18%on average (except for November, where the percentage increasesto 44%); lower in autumn and higher in winter and spring, if com-pared to the values provided by Ecotect.

8. Conclusions and futuredevelopments

The model described in this paper, originally conceived in2D and currently developed in 3D, reproduces the thermo-fluid

dynamic behavior of an experimental greenhouse of Solaria com-plex. The possibility to vary parameters characterizing materialsand climatic conditions allowed: to appreciate the energy per-formance improvement, related to the special reinforced thermal

insulation and solar control glasses of greenhouse envelope, and

to external sliding sunshades (27% in terms of temperature at


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12:00a.m. on 21st June); to quantify the energy benefit related

to the presence of solar greenhouse during the entire heatingseason in 310kWh, equal to about 20% of useful thermal energydemand for heating (1527kWh/year) of the representative apart-ment. Further added value was the possibility to simulate an

organic thin-film photovoltaic device of nanometric dimensions,in order to evaluate the variation of greenhouse energy perfor-mance associated to its experimental application at some exteriorglass surfaces.

Construction of Block G is expected to end within 2013, whilethe completion of the entire Solaria complex is planned in 2015.Monitoring activities have been conducted during the construc-tion process, in order to verify the compliance with the energy

efficiency requirements. In the near future several actions willbe carried on: direct measurements inside some residential units,to validate the energy class and indicators, already estimatedduring the preliminary certification phase; heat transfer testing

through the use of thermography, thermal flow meter and otherequipments.

Particular attentionwill be given to therealizationof OPVexper-imental application to the selected greenhouse, in order to analyze

thetechnicaland economic feasibility of these materialsin conven-tionalbuilding sector. A specific monitoring andacquisitionsystem

will be set up; this will allow comparing simulation results andexperimental data, and validating the model.

With reference to organic photovoltaic thin-film systems, a fur-ther in-depth study could be carried out for improving its behaviorand properties, focusing on two aspects: stability and efficiency,in fact durability is often a problem for organic materials, which

undergo oxidation processes; Life Cycle Assessment (LCA), to eval-uate the potential environmental impacts of a product, process orservice throughout its life cycle.

Acknowledgements

Authors would like to acknowledge Studio Costa & PartnersS.r.l. and SO.GES.HIT. S.r.l. for their technical support and for hav-ing provided data necessary for this study, and ESS EngineeringService System S.r.l. for the opportunity to participate at energy

efficiency coordination in design and execution phases of Solariacomplex.

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