Applied Energy 184 (2016) 1411–1431

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Applied Energy

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BIPVT systems for residential applications: An energy and economicanalysis for European climates

http://dx.doi.org/10.1016/j.apenergy.2016.02.1450306-2619/� 2016 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.

Annamaria Buonomano, Francesco Calise, Adolfo Palombo ⇑, Maria VicidominiDII – University of Naples Federico II, P.le Tecchio, 80, 80125 Naples, Italy

h i g h l i g h t s

� Dynamic simulations models of different BIPVT system layouts are developed.� BIPVT active and passive effects on the building energy consumptions are assessed.� A comprehensive parametric analysis for a residential application is carried out.� BIPVT energy performances for several European weather zones are calculated.� High energy and economic savings for heating, cooling and DHW are obtained.

a r t i c l e i n f o

Article history:Received 23 November 2015Received in revised form 19 February 2016Accepted 29 February 2016Available online 16 March 2016

Keywords:Dynamic energy performance simulationSolar energyBIPVTBuilding Integrated Solar Thermal Systems(BISTS)

a b s t r a c t

This paper analyses the energy and economic performance of roof and/or façades Building Integrated flat-plate PhotoVoltaic and Thermal (BIPVT) collectors for residential applications. Aim of the analysis is toassess the active and passive effects due to the building integration of solar technologies on the buildingenergy consumptions. In particular, a comparison among innovative building-plant system configura-tions, based on BIPVT collectors for the simultaneous production of electricity, thermal energy, anddomestic hot water, is carried out. The simulation models of the proposed system layouts are designedand implemented in TRNSYS simulation environment for the dynamic assessment of their energy andeconomic performance. By means of the developed simulation model, the occurring summer and winterbuilding passive energy effects due to the PVT building integration are also analysed.Several case studies are developed by modelling a representative multi-storey residential building and

by taking into account different European climates. For such case studies a suitable energy parametricanalysis is performed by varying the thermal resistances and capacitances of the building envelope. Bythe simulation results interesting design and economic feasibility guidelines are obtained. In particular,by varying the weather location and the building-plant configuration, the adoption of BIPVT panels pro-duces a decrease of the primary energy demands from 67% to 89%. The passive effects of the BIPVT systemin both the winter and summer season are also assessed for all the investigated climate zones. The cal-culated economic profitability resulted slightly better for roof BIPVT panels than for roof and façadeapplications. For the investigated case studies, the pay back periods appear quite long, varying from11 years for South European weather zones to 20 for North European ones.

� 2016 Elsevier Ltd. All rights reserved.

1. Introduction

The building sector accounts for approximately 40% of the totalend use of energy and is responsible for more than 30% of green-house gas emissions in OECD Countries [1]. At the same time, resi-dential and commercial buildings show a high potential of energysavings, which may contribute to a wide reduction of global energy

consumptions. To this scope, developed Countries are promotingbuilding energy efficiency policies and their implementationthrough efficient, affordable, and high impact technologies. As aconsequence of the recent global agreements regarding the reduc-tion of climate changes, new and more challenging goals for pollu-tant emission and energy consumptions are being established [1].In Europe, such targets mainly regard the greenhouse gas emissionsreduction, the energy efficiency improvement and the renewablesshare. In this framework, the European Directive 2010/31/EU forbuilding energy efficiency promoted the development of building

Nomenclature

A area, m2

c specific heat, J/(kg K)C cost, €COP Coefficient of PerformanceE energy, kWhEffT temperature efficiency modifier, �C�1

EffG radiation efficiency modifier, m2/WEER energy efficiency ratio, adimG solar irradiance, kW/m2

HDD Heating Degree Day, Kdj specific cost-price, €/kWhJ savings, €/year_m flow rate, kg/sPE primary energy, kWhP electric power, kWQ thermal power, kWSPB simple pay back, yearsT temperature, �C or KU heat transfer coefficient, kW/(m2 K)

Greek symbolsa azimuth, �e long wave emissivity, adimk conductivity, W/m Kq density, kg/m3

qs solar reflectance, adimn control function, adim

Subscripts and superscriptsa ambientAlm Almeria

cell photovoltaic cellcool coolingCB condensation boilerDHW domestic hot waterel electricE energyHE heat exchangerheat heatingHP heat pumpHVAC heating, ventilation, and air conditioningin inputMil MilanN nominalNap NaplesNG natural gasNZEB net zero energy buildingsout outputPC proposed casePES primary energy savingPLR part load ratioPS proposed systempump pumpPVT PVT solar collectorsRS reference systemref referenceSC solar collectorsSF solar fractiontot totalT thermaly year

1412 A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431

effective energy options based also on renewable energy applica-tions [2].

Among the available renewable technologies, those based onsolar energy are considered among the most promising for savingenergy and reducing carbon emissions. The solar source can besuitably exploited through suitable Solar Thermal Collectors(STC) and PhotoVoltaic (PV) panels, which today are the most prac-tical options for building applications of the renewable energysources. In EU Countries, the building integration of solar technolo-gies is often recommended or mandatory for new buildings inorder to address both the functional and aesthetic aspects [3]. Inthe next future, building envelop elements will be required to bepartly or totally replaced by the solar energy system components(integrated in building opaque and transparent/semi-transparentfacades, double skin facades, sunshades and sunscreens, balcony,etc.). Building integrated solar systems (e.g. BISTS, BIPV collectors,BIPVT panels, etc.) represents a valuable and effective measure tohelp and achieve the energy efficiency goal, by cutting energy con-sumption in buildings while boosting the share of renewables, alsorequired by the nearly zero energy building target [4].

Among the available solar technologies, nowadays the combi-nation of STC and PV panels (i.e. BIPVT system) represents the mostinteresting application for the building integration. BIPVT systemsare capable to simultaneously produce useful thermal and electri-cal energy [5]. In residential applications (where low-temperatureheat is required), the thermal energy obtained by a BIPVT system isdelivered to users through a suitable working fluid (typically air orwater) and it usually exploited for space heating purposes ordomestic hot water (DHW) preparation. At the same time, theworking fluid (by removing the absorbed heat from the PV cells)

leads to higher electrical efficiencies of PVT panels compared tothose reached by conventional PV collectors (because of loweroperating temperatures of the PVT cells) [6]. As a result, a specialeffort is today paid for developing effective BIPVT technologies,also due to their advantages with respect to stand-alone applica-tions (e.g. potential reductions of building construction capitalcost, including land costs for ground-installations).

Although building integrated solar technologies (includingBIPVT) have been used since decades, they are still far from a mas-sive commercialization, due to their technological and economicissues (e.g. perceived low reliability, lack of pre-packaged fully-integrated for plug-and-play installation, lack of well-validateddesign tools with multiple capabilities) [7]. Therefore, the develop-ment of new techniques for their building integration, emphasizingtheir active and passive effects on the overall building energydemand, is crucial in order to promote the utilization of such tech-nology [3].

To this aim, in the last decades, from both numerical and/orexperimental points of view, a number of studies focused on differ-ent aspects of the building integration of STC and PV panels werecarried out whereas only few analyses based on BIPVT applicationswere developed [8]. A detailed discussion on such applications isreported in several review papers, such as [8,9].

From the experimental point of view, several analyses on BIPVTsystems were completed with the twofold aim to aid the design ofprototypes and to validate the developed simulation models, alsouseful for assessing the BIPVT systems performance [10]. The avail-able studies are related to: (i) small scale indoor applications, e.g.BIPV Trombe wall system [11]; (ii) large outdoor applications, e.g.double-skin PVT façade [12], ventilated active façade including

A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431 1413

Phase Change Materials (PCM) [13], combination of BIPVT andunglazed transpired collector (UTC) systems for building façades[14]. On real scale experimental buildings, the influence of theBIPVT system configurations on the thermal and electrical perfor-mances has been also experimentally assessed for different roofintegration layouts [15] and air gap heights [16]. All such studiesmainly aim to investigate possible passive and active strategiesnecessary to lower the cells temperature and to maximize the heatrecovery. This is obtained through different novel and modifiedstrategies, such as: increase of the buoyancy driven natural ventila-tion [17], optimization of forced air cooled channels [18], develop-ment of suitable absorber plates for water cooled devices [19]. Theavailable literature also highlights the research effort on the designof prototypal BIPVT devices and novel systems [8]. Regarding thenovel devices, recent interesting prototypes were developed andanalysed, such as: an air cooledmultiple inlets BIPVT system, incor-porating both opaque and semi-transparent mono-crystalline sili-con PV panels [20]; a two inlets BIPVT system for enhancing theheat transfer between flowing air and PV modules [20]; an innova-tive spiral flow absorber for a water cooled BIPVT system [21].

Although the large number of BIPVT experimental prototypesfocused on experimental studies, the available literature highlightsa lack of knowledge regarding the assessment of the passive effecton the building energy demands due to the building integration ofsuch devices.

Conversely, from the numerical point of view, dynamic simula-tions were extensively adopted in many papers in order to analysethe active effect of the building integration of STC and PV panels onthe building energy consumptions and on the electricity and heatproduction. As for the experimental investigations, the majorityof such studies available in literature (carried out through severalcommercial and suitably developed dynamic energy performancesimulation models) is focused on several thermal and electricalaspects of roof and façades integration of PVT panels (i.e. the mostcommon applications of this technology [7]), without analysingtheir passive effects on the building energy demands. By meansof dynamic simulation analyses, very few studies aimed at assess-ing the performance of BIPVT system including the building inte-gration effects on the heat transfer and energy demands. Forresidential applications, the most investigated BIPVT system isthe water cooled one, due to the possibility to utilize the obtainedthermal energy for DHW preparation [22].

Several design criteria are obtained by the literature reviewregarding numerical/simulation studies: (i) the integration of PVTpanels for solar cooling applications shows that the roof integra-tion may be hailed only in case of limited availability of roof sur-faces (i.e. lower solar fractions and higher primary energyconsumptions) [23]; (ii) in case of insulated BIPVT systems reducedheat transfers (heat gain and loss) across roof and façades, areobtained (i.e. with respect to a conventional concrete wall) [24];(iii) independently of the PV cells technology, BIPVT applicationsshow an increase of electricity production and of the system ther-mal performance (especially for low-tech PVT panels) if comparedto BIPV systems [25].

It is also worth noting that the majority of the numerical studies(based on steady state models for single collector) focuses on theBIPVT system itself, disregarding the whole building-plant aspects[9]. As an example: (i) a dynamic simulation program was devel-oped for analysing and comparing the annual thermal performanceof mono-Si PV modules on a typical multi-layer façade with a nor-mal wall [26]; (ii) an analytical expressions (valid for different cli-matic conditions and different design parameters of the building)for the assessment of the electrical efficiency of thin film PV mod-ules coupled to BI opaque PVT systems were developed in [27]; (iii)the influence of the PV module technology on the energy andexergy performance of novel building roof integrated

semi-transparent PVT [28] and roof integrated PVT collectors isanalysed in [29].

Other BIPVT systems are obtained through the concentrationtechnology (Concentrating PVT, CPVT systems). In fact, in the lastdecade, besides standard stand-alone CPVT applications (Fresnel[30], Fresnel and concentrating parabolic trough system [31], para-bolic dish [32]), novel building integrated BI-CPVT ones were anal-ysed (e.g. Fresnel PVT collectors for façade integration withtracking system [33], façade and roof integrated air-gap-lens-walled compound parabolic concentrator incorporated with a sta-tic solar concentrating PVT system without tracking [34]).

Very few analyses available in literature were carried out bytaking into account the plant (e.g. BIPVT panels solar field) andthe building as a whole system. Here, the dealt with topics con-cern: (i) the active and passive effects of a roof multifunctionalBIPVT system, coupled to a radiant floor or to a PCM storage unitfor the reduction of the building energy demand [35]; (ii) a studyincluding a detailed dynamic model – predictive control of façadeintegrated PVT panels for office buildings, coupled to a radiant floorheating and active thermal storage [36].

Summarising, although the design and configurations of novelbuilding integrated solar systems (including BIPVT) have beenwidely analysed during the last years [37,38], the majority of thesimulative studies only investigated BIPV collectors and skinfaçades [39]. Too few studies are focused on building integratedsolar thermal systems (also including BIPVT technologies) [8].Therefore, the need of additional analyses, taking into accountthe whole building-plant system, are necessary [3]. In particular,at the best authors’ knowledge, in order to assess both the activeand passive effects of BIPVT systems on the whole building energydemands, the following topics are still scarcely investigated: (i)development of suitable dynamic models for the energy and eco-nomic performance analysis, (ii) implementation of comprehen-sive dynamic simulations, and (iii) development of comparativeand parametric analyses (i.e. playing a fundamental role in the cor-rect design of renewable energy systems [40]). This results is alsoin accordance with the recommendations of the COST ActionTU1205, encouraging researchers to perform new analyses, takinginto account the effect of the different weather, design and operat-ing conditions [3]. This may help to investigate novel integrationand design options of BIPVT systems and to improve their utiliza-tion in terms of applications and delivered functions [7].

1.1. Aim of the study

This paper is focused on the above outlined frameworks, withthe purpose to fill some knowledge gaps among those highlightedin literature. Specifically, in order to promote the further integra-tion of BIPVT systems in buildings, the presented analysis aims to:

� assess the active and passive effects of different BIPVT systemlayouts on the building energy demand and on the energy con-sumptions of the adopted HVAC system;

� carry out a comprehensive parametric analysis, by means ofyearly dynamic performance simulations, in order to find outthe set of building/plants design and operating parameterswhich maximize the system energy and economic performance;

� outline design guidelines related to the system layouts andbuilding envelope features as a function of the weatherconditions.

In particular, the carried out analysis is referred to: (i) differentBIPVT and PVT configurations; (ii) several building envelope fea-tures; (iii) many European weather conditions. In order to carryout yearly assessments, a dynamic simulation model for the energyand economic performance analysis is purposely developed. For

1414 A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431

comparison purposes, three different system layouts are modelledand simulated, such as: (i) stand-alone collectors; (ii) roof BIPVTpanels; (iii) roof and south-façade BIPVT collectors. A residentialapplication is analysed, by modelling a typical 3-storeys house.For this user, the modelled PVT collectors produce electricity andthermal energy for DHW preparation and space heating purposes.In particular, the solar heating is provided through a radiant floor,designed for the last building floor only.

The reported literature review showed that such analysis is com-pletely new; in fact, none of the papers available in literature inves-tigated both the above discussed active and passive effects relatedto the building integration of PVT by means of dynamic wholebuilding-plant simulations. A comprehensive detailed energy andeconomic analysis is also carried out in order to compare the threeabove mentioned building-plant system layouts vs. a conventionalone (without PVT). With the aim to study the influence of severalbuilding design parameters on the primary energy demand forspace heating and cooling, DHW and electricity production, adetailed parametric analysis is carried out for 4 different referenceweather zones. The effects of the climate conditions on the investi-gated systems are also assessed for different 14 additional Europeanweather zones. The results of this investigation are exploited asobjective functions in order to find out the optimal system configu-ration that provides the largest part of building energy needs.

The simulation model, including all the components requiredfor operating the system (mixers, valves, controllers, tanks, collec-tors, etc.), is developed in TRNSYS environment. The modelincludes suitable control strategies for the maximization of thesystem energy utilization in all the operating conditions. Systemspace heating and cooling demands are calculated on the basis ofa 3-D building model, implemented in TRNBUILD (included inthe TRNSYS package).

Through the presented analysis, interesting design guidelinesand swift feasible considerations can be obtained for the proposedBIPVT systems layouts applied to residential applications and as afunction of several weather indexes (heating/cooling degree daysand incident solar radiation).

2. System layouts

The proposed BIPVT system consists of flat-plate PVT collectors,integrated in the building roof and/or south-façade of a typicalmulti-floors residential building, consisting of three different ther-mal zones, located at ground, 1st and 2nd floor.

The building space heating and cooling demands as well as theDHW and electricity production are here assessed for several dif-ferent building-plant configurations. In the following, a descriptionof such layouts is provided.

Zone 1

Zone 2

Zone 3

N

Fig. 1. Case A: reference system.

2.1. Case A

This case corresponds to the reference system, i.e. a conven-tional building without PVT panels. For performance assessmentpurposes, all the proposed innovative system configurations, pre-sented below, are compared to Case A (Fig. 1). Here, an electricair-to-water heat pump/chiller (HP) provides space heating andcooling energy, whereas DHW preparation is obtained by a gas-fired condensation boiler.

2.2. Case B

In this building system configuration, a stand-alone PVT(unglazed) solar field (sited on a land adjacent to the building) istaken into account (Fig. 2). The hot water produced through thePVT panels is supplied to a heat storage tank (TK), through aninternal heat exchanger (HE1), Fig. 3. During winter, TK hot wateris exploited through a suitable radiant floor for the space heating ofthe thermal Zone 3 (2nd floor, Fig. 2). Depending on the availabilityof solar radiation, the radiant floor heating capacity may be alsolower than the Zone 3 heating demand. Therefore, suitable auxil-iary systems (electric heat pump units) are taken into account.The heat pump works in parallel with the radiant floor (indepen-dently by PVT panels/TK system) in order to adequately controlthe indoor air temperature. Space heating in Zones 1 and 2 (groundfloor refers to Zones 1 and 1st floor to Zones 2, Fig. 2), and spacecooling in all the three building zones, are only provided by theheat pump/chiller (similarly to the reference Case A). In otherwords, solar thermal energy is only delivered to the radiant floorof Zone 3. In the heating season, internal heat exchanger, HE2,(used for DHW preparation) is often bypassed, i.e. the solar heatis supplied in priority to the Zone 3 radiant floor. Conversely, whenthe radiant floor is not active (e.g. in summer or when the indoortemperature is higher than the selected set point), the solar ther-mal energy stored in the TK tank is used to preheat the DHW bymeans of HE2 (included in TK). In this case, tap water suppliedby the grid is preheated through HE2. Details regarding the com-plex control strategy of the system are provided in the followings.

2.3. Case C

In this building-system configuration, a south facing roofunglazed BIPVT collectors field is taken into account (Fig. 4a). ForCase C the plant operation and the solar field surface area are thesame of Case B. The main difference concerns the location of thePVT collectors, which are roof-integrated. The back surface of thecollectors coincides with the external surface of the tilted roof ofthe modelled building. Such arrangement is compliant with theItalian legislation, which requires the installation of solar collectorsas integrated (or adjacent) to the building external surfaces. Thiscase study is designed with the aim to investigate how the relatedsystem layout affects the primary energy demand for space heat-ing/cooling as well as the electricity and DHW production of thesolar collectors.

2.4. Case D

In this building system configuration, a south facing roof andsouth-façade unglazed BIPVT collectors field is taken into account(Fig. 4b). Note that, for the roofing integration the same solar fieldarea of Case B and C is assumed, whereas an additional BIPVT areais modelled for the building façade. In order to achieve an optimalcontrol of the output temperature, the modelled BIPVT collectorsare equipped with three different separate water circulating loops.

The system layout for Cases B, C and D is depicted in Fig. 3 (notethat the Case A only includes an heat pump/chiller and a gas-fired

Zone 1

Zone 2

Zone 3

N

HE1 HE2TK

PVT out

PVT in Tap water

DHW Tset,CB

Radiant floor in

Radiant floor out

Radiant floor

CB

Fig. 2. Case B: building and stand-alone PVT panels.

HWRF

PVT

TK CBP2

SCFDHW

15°C

45°C

HE1 HE2

P1

RF• PVT: PhotoVoltaic and Thermal solar collectors;• HP: air-to-water Heat Pump;• TK: water stratified vertical storage tank;• CB: gas natural Condensation Boiler;• HE1: immersed Heat Exchanger 1 into TK;• HE2: immersed Heat Exchanger 2 into TK;• RF: Radiant Floor inside the building;• P1: water constant speed pump for RF;• P2: water constant speed pump for solar loop.

tap water

from radiant floor to radiant floor

from HE1

to PVT

from HE2

HPAir inlet Air outlet: TsetHeat=20°C

TsetCool=26°C

Fig. 3. Plant layout.

(a) (b)

Zone 3

Zone 2

Zone 1

Fig. 4. (a) Case C: building with roof BIPVT panels; (b) Case D: building with façade and roof BIPVT panels.

A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431 1415

boiler for the DHW production). In particular, three main systemloops are modelled:

� SCF (Solar Collector Fluid) loop – the outlet hot water from thePVT solar field is supplied by means of a water constant speedpump (P2) to a water stratified vertical energy storage tankTK through the internal heat exchanger HE1.

� HWRF (Hot Water of the Radiant Floor) loop – the outlet hotwater from TK is supplied by means of a water constant speedpump (P1) to the Zone 3 radiant floor (RF) for indoor spaceheating.

� DHW (Domestic Hot Water) loop – the tap water from the gridis preheated through the heat exchanger HE2 and heated to theDHW set point temperature by a gas-fired condensation boiler,CB (activated if necessary).

The operating principle of the investigated cases and the relatedcontrol strategies can be summarized as follows (see Fig. 5). PVTcollectors are managed by a suitable controller by operating onthe P2 pump. Such controller receives temperature readings fromthe outlet of HE1 heat exchanger (i.e. solar collector inlet temper-ature) and the outlet pipe of the solar collector loop. Hot water

Flow rate controller

P2 on

Solar collector inlet temperature

Temperature controller

ToutSC > TinSC

ToutSC < TinSC

Solar collector outlet temperature

P2 off

boiler ON

ToutHE2 > Tset,CB

ToutHE2 < Tset,CB

Schedule demand of DHW

HE2 outlet temperature Temperature

controller

no thermal demand by the radiant floor

boiler OFF

TTK,top > TTK,top SET

Tair zone 3 < Tin,RF

flow rate

TK top temperature

Indoor air thermal zone 3

Temperature controller

P1 off

Radiant floor on

P1 on

TTK,top < TTK,top SET - 2.1°C

Tair zone 3 > Tin,RF + 2.1°C

tap water enters HE2

Fig. 5. Control strategies for the investigated case studies.

1416 A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431

produced by the solar loop is supplied to TK tank through HE1. Thecontroller stops P2 pump when solar collector outlet temperatureis lower than inlet one, in order to prevent heat dissipation.

The CB condensation boiler is activated only if the outlet watertemperature from the HE2 heat exchanger is lower than DHW setpoint one. HE2 volume is sufficiently high for fulfilling the requiredDHW storage capacity (see Table 1, VHE2/VTK). The tap water entersHE2 only if: (i) no thermal energy is required by the radiant floor(i.e. when the P1 pump is off); (ii) a simultaneous occupantsDHW demand occurs (see Table 2). Therefore, a suitable controllermanages the activation of the P1 pump. In particular, such con-troller receives temperature readings from the top of TK and fromthe indoor air of the thermal Zone 3 (where the radiant floor isinstalled). The controller produces the ON signal for the pump acti-vation if: (i) the TK top temperature is higher than TTK,topSET(Table 1); (ii) the Zone 3 indoor air temperature is lower than Tin,RF(Table 3). Top TK hot water directly supplies the radiant floor forspace heating. Then, the outlet water from the radiant floor entersthe bottom of the TK tank (i.e. return water) by means of the inletof the adopted double port. When the indoor air of Zone 3 is higherthan Tin,RF + 2.1 �C or TK top temperature is lower than TTK,topSET –1.0 �C, P1 pump is switched off and the stored thermal energy isexploited for DHW production, i.e. tap water enters HE2 heatexchanger. An auxiliary heat pump/chiller is also used for heating/-cooling each building thermal zone at the selected set-pointtemperatures.

3. System model

The performance of the investigated building-plant systemswere simulated by means of TRNSYS 17. Such software, diffuselyadopted by the academic community, enables transient energysimulations, providing a library of built-in components (e.g.pumps, mixers, diverters, valves, controllers, auxiliary heaters, heat

exchangers, etc.) often based on experimental data [41]. Thismethodology has been successfully adopted to perform dynamicanalyses of several solar systems [42,43]. For the sake of brevity,in this section only a brief description of the thermodynamic mod-els related to the most significant system components, included inthe developed simulation model, is provided (for details about themodels of the remaining devices see Ref. [41]).

3.1. BIPVT collectors

BIPVT collectors are modelled by adopting the componentincluded in the TRNSYS TESS library (Type 563, [41]). The thermalmodel is based on algorithms presented in Ref. [44]. Specifically,each of sheet and tube PVT collector includes several components,such as an absorber encapsulating the PV film, flow channels forthe cooling fluid and thermal insulation. A dual purpose is obtainedby such devices: (i) producing electricity through the embedded PVcells; (ii) generating thermal energy by transferring heat to thewater flowing in tubes bonded to an absorber plate underlyingthe cells. The model of this BIPVT collector can be connected tothe one of a multi zone building (Type 56, described in the follow-ing paragraphs) with the aim to assess the influence of the inte-grated PVT modules on the building heating and cooling loads(i.e. passive effects).

The collector model relies on linear factors linking the electric-ity efficiency of the PV cells to the related temperature and to theincident solar radiation. The cells are assumed to be operating attheir maximum power conditions. The mathematical model isbased on a derivation of the standard tube-fin solar collector algo-rithm and it assumes constant overall energy loss coefficient andabsorber absorbance [44]. Here, the collector electricity efficiencyis calculated as a function of the cell average temperature Tcelland of the incident solar radiation (Gt) [41]:

gPV ¼ g0 � ½1þ EffT � ðTcell � Tref Þ� � ½1þ EffG � ðGT � Gref Þ� ð1Þ

Table 1Main system design parameters.

Parameter Description Value Unit

Solar fieldASC Roof BIPVT Solar Collector roof aperture area (PVT for Case B) 122 m2

ASC 1 BIPVT Solar Collector façade aperture area (Zone 1) 45.0 m2

ASC 2 BIPVT Solar Collector façade aperture area (Zone 2) 37.5 m2

qP2/APVT P2 rated flow rate per unit of BIPVT aperture area 10 kg/h m2

kabs Thickness of the absorber plate 0.3 mmka Thermal conductivity of the absorber plate 385 W/m KNtubes Number of identical fluid tubes bonded to the absorber plate 700 –Wb Average bond width (between tube and absorber plate) 0.01 mkb Thermal conductivity of the bond 385 W/m KR1 Thermal resistance of the material located between the PV cells and the absorber plate 0.04 m2 K/WRb Thermal resistance of the panel back material (Case B) 2.50 m2 K/W

Thermal resistance of the panel back material (Case C, D) 0.04 m2 K/Wkp Thickness of the panel back material (Case B only) 0.10 mkp Conductivity of panel back material (Case B only) 0.04 W/m Kb0 Incidence angle modifier: 1st order coefficient 0.10 –qs Reflectance of the collector surface at normal incidence 0.15 –e Emissivity of the collector surface 0.90 –g0 PV efficiency at the reference condition 12 %bwall Collector slope (façade integrated) 90 �broof Collector slope (roof integrated) 30 �a Collector azimuth 0 �EffT Efficiency modifier temperature �0.005 1/�CEffG Efficiency modifier radiation 7.0�10�6 m2/W

TankHTK Height 2 mqHE1/APVT Heat Exchanger 1 flow rate per unit of BIPVT aperture area 10 kg/m2 hqHE2 Heat Exchanger 2 maximum flow rate 117 kg/hVHE1/2/VTK HE1 and HE2 volume per unit of TK volume 1/20 –TTK,topSET Tank TK top temperature for radiant floor activation 22 �C

BoilerPCB,rated Rated CB heat power 25,000 WgRS,DHW Efficiency of the condensation boiler (also for RS = Case A) 95 %qDHW DHW flow rate per person 65 l/p dayTset,CB CB outlet set point temperature 45 �C

Heat pump/ChillerCOPN Nominal coefficient of performance (heating) 3.5 –EERN Nominal energy efficiency ratio (cooling) 3.0 –

Milan and Freiburg Heating Cooling

Pheat/cool,rated1 Rated heating and cooling capacity for zone 1 9840 7880 WPheat/cool,rated2 Rated heating and cooling capacity for zone 2 8440 6750Pheat/cool,rated3 Rated heating and cooling capacity for zone 3 5270 4220

Naples and Almeria Heating Cooling

Pheat/cool,rated1 Rated heating and cooling capacity for zone 1 7880 6300 WPheat/cool,rated2 Rated heating and cooling capacity for zone 2 6750 5400Pheat/cool,rated3 Rated heating and cooling capacity for zone 3 4220 3370

Table 2Simulation assumptions.

Set point indoor air temperature (�C) Heating: TsetHeat = 20 Cooling:TsetCool = 26

Occupancy schedule (h) 24:00–9:00; 13:00–15:00; 18:00–24:00

Number of occupants per zone 3People heat gain (W/p) Sensible: 75 Latent: 75Light + machineries heat gains

schedule (h)18:00–23:00

Light + machineries heat gains (W/m2) 8.0Air infiltration rate (vol/h) 0.3Free cooling ventilation rate (vol/h) 2.0DHW set point temperature (�C) 45Tap water temperature (�C) 15DHW usage schedule (h) 07:00–09:00; 13:00–15:00; 20:00–

22:00

Table 3Layers features and design parameters of the radiant floor (Zone 3 only).

Layers Thickness(m)

Unit

Interior plasterboard 0.020

(m)

Thermal insulation (k = 0.023 W/m K) 0.020Concrete slab 0.150Cement based-mortar (above and below the pipes) 0.060Ceramic tiles 0.020Dpipe, Pipe outside diameter 0.018Spipe, Pipe Spacing (centre to centre) 0.20Tpipe, Pipe wall thickness 0.002

Design parameters Value Unit

Tin,RF, Zone 3 indoor air temperature forradiant floor activation

19 (�C)

qP1, P1 rated flow rate for supplying the radiant floor 1100 kg/hkpipe, Pipe wall conductivity 0.35 (W/

m K)

A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431 1417

where g0 is the PV efficiency at the reference conditions; EffT andEffG are the temperature and radiation efficiency modifiers; Tref isthe reference temperature and Gref is the reference incident solarradiation.

Several energy balances are suitably solved in order to assess:(i) the outlet temperature of the fluid (Tf,out); (ii) the temperature

1418 A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431

distribution along the PV surface (Tcell), the absorber plate and theinterface between the collector and the upper surface of the roof(Tback) [41,44]. The fluid outlet temperature is calculated as:

Tf ;out ¼ Tf ;in þ ej

� �exp

Ntubes

_m � cp �jh� L

� �� ej

ð2Þ

where Tf ;in is the inlet fluid temperature; _m is the fluid mass flowrate; Ntubes is the number of identical tubes carrying the fluidthrough the collector. e; j and h depend on the collector geometry,heat transfer coefficients, thermal resistances and temperatures ofsky, ambient and back collector surface, as detailed in [44].

As a function of Tf,out, the collector useful energy gain is calcu-lated as:

Qu ¼ _m � cpðTf ;out � Tf ;inÞ ð3ÞThe design and operating parameters of the collectors are

selected according to the data included in Ref. [23,45]. All thedesign parameters for the analysed case studies are shown inTable 1.

3.2. Energy storage tank

In order to model the dynamics of a stratified fluid storage tank,the TRNSYS built-in Type 340 is used. In this model four optionalinternal heat exchangers and ten connections (double ports), fordirect tank charge and discharge, can be modelled (necessary tothe DHW and space heating purposes) [42]. For the simulated sys-tems, one double port and two heat exchangers are taken intoaccount:

� the double port on the modelled storage tank (TK) is linked tothe radiant floor for the Zone 3 space heating;

� the HE1 heat exchanger supplies TK with the heat produced byBIPVT collectors;

� the HE2 heat exchanger is exploited for producing DHW.

The tank model is based on the assumption that the thermalstorage is subjected to thermal stratification and is divided intoN fully-mixed equal sub-volumes. The temperatures of the nodesare calculated by solving a set of differential equations, related toa virtual matrix made of a triple array of N columns. The first col-umn (j = 1) includes the data of the first heat exchanger (HE1), thesecond one (j = 2) the data of the storage tank (TK) and the last one(j = 3) the data of the second heat exchanger (HE2). One mass flowrate ( _mf ) through the double port is taken into account in each n-thnode. The temperatures of the tank nodes are calculated on thebasis of unsteady energy and mass balances, by solving a set of dif-ferential equations. The change of internal energy with the timeoccurring in the n-th node of the store (j = 2) is calculated as:

MncfdTn;j

d#¼ _mf cf n1 � ðTn�1;j � Tn;jÞ þ n2 � ðTn;j � Tnþ1;jÞ

� �þ Snk

dnðTn�1;j � Tn;jÞ þ ðTnþ1;j � Tn;jÞ� �

þ UAnðTa � Tn;jÞ þXCj¼1

c jUAjnðTn;j � Tn;jÞ ð4Þ

The first sum on the right hand side of Eq. (4) represents theheat transfer due to the mass flows through the double port (linkedto the radiant floor). Here, a positive mass flow rate from the bot-tom to the top is taken into account (n1 = 1 if _mf > 0, else n1 = 0) andvice versa (n2 = 1 if _mf < 0, else n2 = 0). The second sum takes intoaccount the conductivity between the vertical boundary layers ofthe tank. The heat losses are taken into account by the third termof Eq. (4). Here, UAn represents the heat transfer capacity rate

between the store n-th node and the outdoor environment. Thus,the fourth term on the right hand side of the same equation repre-sents the heat transfer between the heat exchanger nodes, on theboundary vertical partitions, and the store ones. Here, c j = 1 ifthe n-th store node is in contact with the n-th node of the horizon-

tal boundary of the heat exchanger, otherwise c j = 0. Similarly, UAjn

is the heat transfer capacity rate between the n-th node related theheat exchanger and the storage tank. The energy balance for an n-th heat exchanger (j = 1 and j = 3 vertical partitions) node is:

Mncf ;HEdTn;j

d#¼ _mf ;HEcf ;HE½ðTn�1;j � Tn;jÞ þ ðTn;j � Tnþ1;jÞ�

þ UAnðTa � Tn;jÞ þ c jUAjnðTn;2 � Tn;jÞ ð5Þ

where the vertical temperature boundary nodes (Tn�1;j and Tnþ1;j)represent the inlet and outlet temperatures of the considered heatexchanger. In addition, on the right hand side of Eq. (5), the secondand third terms take into account the heat loss capacity rates: (i)from the heat exchangers to the surroundings; (ii) from the heatexchanger to the TK storage tank (j = 2).

3.3. Building and radiant floor model

In order to calculate the space heating and cooling and DHWdemands, a suitable building was modelled by means of the Type56 of TRNSYS (version 17), coupled to the Google SketchUpTRNSYS3d plug-in [46]. The detailed procedure for using thesetools is reported in reference [47]. Details about both the buildingphysics and energy systems simulation models are available in[41]. A validation report about the whole Type 56, performed byusing detailed measurements from the CEC research programPASSYS, is presented in [48]. TRNSYS 17 introduces new featuresregarding radiation [49], glasses and windows (with and withoutshading devices), as successfully performed by facilities test cells[50]. In general, the outputs achieved by such software, or othersstandard simulation tools, are nowadays considered sufficientlyreliable for carrying out building energy performance simulations[51].

Through the Type 56 it is possible to model the thermal beha-viour of a radiant floor for indoor space heating. In the proposedmodel, an ‘‘active layer” is added to the floor surface of thermalZone 3 (Fig. 2). The layer is called ‘‘active” since it includes a fluidfilled pipeline for supplying heat to the system. Such layer isdescribed by several parameters referred to: floor and pipesgeometries, inlet mass flow rate, inlet fluid temperature, numberof loops (for calculating the pipeline length) and additional energygain at the fluid level (Table 3).

3.4. Energy savings and economic model

A detailed thermo-economic model was also developed in orderto assess the energy and economic profitability of the systemunder investigation. As above mentioned, in such analyses, in caseof the Reference System, (RS, corresponding to Case A), space heat-ing and cooling is provided by the electric heat pump/chiller (HP),whereas DHW is provided by the condensation boiler (CB) andelectricity by the national grid.

Primary energy savings, achieved by the proposed systems(Cases B, C and D) with respect to the reference one (Case A) aresuitably calculated. Obviously, it is assumed that all the systemsrequire the same amount of DHW and electricity, and the sameindoor set point temperatures (in all the modelled thermal zones).

The primary energy saving, calculated as a function of theannual total energy produced by reference system, is:

Table 4Building thermal zones.

Zone(Floor)

Height(m)

Volume(m3)

Floor area(m2)

Glass area (m2)

1 (Ground) 3.5 525150

19.52 (1st) 3.0 450 19.53 (2nd) 3.75 (max) 281 6.0

1.88 (average)Total 10.25 1256 450 45.0

Table 5Opaque elements features (U-values, thicknesses, solar reflectance and emissivity).

Building element U-value(W/m2 K)

Thickness(m)

qs (–) e (–)

Roof and façades without BIPVTpanels

0.31 0.30

0.400.90Internal floor/ceiling (tile flooring) 0.66 0.33

Ground floor (tile flooring) 0.81 0.18Windows glass 2.89 0.004/0.016 0.88

A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431 1419

DPE ¼Xi

EPVT;el � Eel;aux

gRS;el� QDHW;CB

gRS;DHWþ Qheat;RS

COPRS;HeatgRS;el

� Qheat;PC

COPPC;HeatgRS;elþ Qcool;RS

COPRS;CoolgRS;el� Qcool;PC

COPPC;CoolgRS;el

!i

ð6Þ

where EPVT;el is the useful electricity produced by the BIPVT; QDHW;CB

is the energy consumption of the condensation boiler; Qheat;RS is theenergy for heating and Qcool;RS is the energy for cooling, supplied bythe reference system (RS); Qheat;PC and Qcool;RS are the energies sup-plied by the heat pump/chiller for space heating and cooling,respectively; Eel;aux is the electricity supplied to the auxiliary devices(e.g. pumps); gRS;DHW is the thermal efficiency of the condensationboiler; gRS;el is the efficiency for conventional electric power produc-tion. Note that, in Eq. (6) the variable coefficients are referred toboth the Reference System (RS or Case A) and the Proposed Config-urations (PC, corresponding to Cases B, C and D). Note also that, theCoefficient of Performances of the air-to-water heat pump/chiller(COPPC;heat and COPPC;cool), are calculated by the methods recom-mended by the UNI/TS 11300 (Italian release of ISO EN 13790). Asa result, the nominal COPN and EERN given by the constructors,and their variation due to the occurring operating conditions aresuitably taken into account. In particular, COPPC;heat and COPPC;cool

are calculated as a function of the part-load ratio fPLR, the ambienttemperature Tba and the condenser/evaporator temperatures (fur-ther details are available in [52]):

COPPC;heat ¼ COPNCOPMAX

� #cþDhcð#cþDhc Þ�ðTba�Dhf Þ � ð4 � f PLRÞ=ð0:1þ 3:6 � f PLRÞ

COPPC;cool ¼ EERN �#e;out�#cþTbaþ#e

2#c�#e

� ða � f 3PLR þ b � f 2PLR þ c � f PLR þ dÞð7Þ

An economic analysis of the proposed system is also carried out.In particular, a suitable model is developed in order to assess theeconomic feasibility of the system. Here, the total capital cost(CPS,tot) of the proposed system (including BIPVT solar field orPVT collectors for Case B, pumps, valves, controllers, tank, etc.) iscalculated as a function of the solar field surface area, accordingto the data provided in Ref. [53]:

CPS;tot ¼ 660 � APVT ð8ÞThe proposed systems yearly savings are reported in terms of

operating costs with respect to those of the reference configura-tion, RS. For electricity, it is assumed that the net power productionis entirely delivered to the national grid (the producer can benefitof a feed-in tariff, jel). The thermal energy cost, jNG, was assessed interms of natural gas one.

System operating costs are due to: (i) pumps electricity con-sumption; (ii) heat pump/chiller electricity demands; (iii) conden-sation boiler natural gas consumption. As a result, the savingsobtained by the proposed system, PC, are due to: (i) produced solarelectricity; (ii) reduced use of the heat pump (due to the radiantfloor supplied by the solar collectors); (iii) DHW productionobtained by solar thermal energy. Thus, the annual savings are cal-culated as:

DC ¼ ðEPVT;el � Eel;auxÞjel �QDHW;CB

LCVNGgRS;DHWjNG

þ jelQheat;RS

COPRS;Heat� Qheat;PC

COPPC;Heatþ Qcool;RS

COPRS;Cool� Qcool;PC

COPPC;Cool

� �ð9Þ

At last, the economic profitability analysis also includes theassessment of the Simple Pay Back (SPB) period:

SPB ¼ CPS;tot

Jtotð10Þ

4. Case studies

The selection of the case studies was carefully performed inorder to analyse a residential application, which is well-representative of the next generation of buildings [54]. Therefore,a residential 3-floors building, complying with the present EUenergy efficiency regulation (low U-values, solar energy applica-tions, etc.), is simulated. For such building, a rectangular plantshape with an East-West oriented longitudinal axis is taken intoaccount, Fig. 1. Here, the south facing surface of the pitched roofis 30 degrees tilted. Three different thermal zones related to theindoor spaces of the ground, the 1st and 2nd floors, are modelled.Details about the building surfaces/volumes are provided inTable 4, whereas the envelope features are reported in Table 5.

Simulations are mainly carried out by taking into account fourdifferent European weather zones selected among those represen-tative of European climates. They refer to the climate of Freiburg,(South-Germany), Milan (North-Italy), Naples (South-Italy) andAlmeria (South-Spain), whose hourly weather data files areobtained by Meteonorm database [41]. For such weather zones, adetailed parametric analysis is also performed in order to findout the influence of the building envelope features on the perfor-mance of the examined BIPVT layouts. Additional weather zones,reported in Table 6, are also investigated in order to carefully anal-yse the relationship between performance and weather conditionsby comparing the obtained heating and cooling demands of Case C(BIPVT collectors) and Case B (PVT stand-alone collectors). Withthe purpose of performing a correlational analysis, several climaticindexes are also calculated, such as: Heating Degree Days (HDD);Cooling Degree Days (CDD) and Incident Solar Radiation (ISR). InTable 6, all the considered weather zones are sorted by decreasingHDDs. HDDs and CDDs are calculated by considering as a referencetemperature, for both heating and cooling, 18 �C [55]. Note that,according to the climate, different heating periods are taken intoaccount for the simulated weather zones [53]. Table 6 also includesthe cooling period and the HVAC system activation hours.

For all the investigated cases, the following assumptions relatedto the solar collection area are taken into account: (i) in Cases Cand D, the roof BIPVT panels are mounted on the south facing sur-face of the pitched roof. Such surface (122 m2) is 30 degrees tilted;(ii) in Case B, the sloped stand-alone PVT surface has the same sizeand tilt of the roof mounted BIPVT surface; (iii) in Case D, an addi-tional solar capture surface area, related to the south-façade BIPVT,

(air)/0.004

Table 6Weather zones indexes and HVAC system schedules.

Weather zone HDD (Kd) CDD (Kd) ISR (kWh/m2 y) Winter season Heating schedule (h) Cooling season Cooling schedule (h)

Prague (Czech Republic) 3854 150 998

15/10–15/04

06:30–08:30;12:00–15:00;17:00–20:00

01/05–30/0913:00–15:00;18:00–20:00

Copenhagen (Denmark) 3738 85 987Berlin (Germany) 3397 264 1001Vienna (Austria) 3277 269 1112London (UK) 3155 82 998Paris (France) 3122 155 1036Freiburg (Germany) 3052 255 1112Milan (Italy) 2734 436 1246Bolzano (Italy) 2646 473 1242Turin (Italy) 2585 453 1291Madrid (Spain) 2179 687 1662

01/11–15/04Pescara (Italy) 1899 606 1518Pisa (Italy) 1851 566 1447Rome (Italy) 1687 671 1563Naples (Italy) 1480 729 1512

15/11–31/03Brindisi (Italy) 1239 775 1518Larnaca (Cyprus) 842 1199 1847

01/12–15/03Almeria (Spain) 785 963 1733

Window closing (non-heating

season)

Tair zone > Tamb

Tair zone > 25°C

Outdoor airtemperature

Indoor air thermal zones (1/2/3)

Temperature controller

Window opening (non-heating

season)

Tair zone > 24°C Window opening (heating season)

Window closing (heating season)

Tair zone< 22°C

Tair zone< 23°C

Fig. 6. Windows opening strategy during the heating and the non-heating seasons.

1420 A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431

is taken into account, for a total BIPVT surface area of 205 m2 (thesouth-façade BIPVT area consists of 45 m2 at Zone 1 and 38 m2 atZone 2).

For free cooling purposes a specific windows opening strategy,described in Fig. 6, is taken into account during the no-heating sea-son and from 20:00 to 09:00, for all the investigated weather zones.During the heating season, in order to simulate the occupants beha-viour in case of overheating (indoor air temperature >25 �C), a 2 vol/h outdoor air ventilation rate (simulatedby thewindowsopening) ismodelled. Subsequently, the closing of the windows occurs whenthe indoor air temperature falls below23 �C. All the adopted simula-tion assumptions are summarized in Table 2.

Design and operating parameters of all the simulated systemcomponents are shown in Table 1, whereas those related to theradiant floor, serving only the third floor (Zone 2), are reported inTable 3. Note that the radiant floor is supplied only by solar energyand that the thermal capacity of the modelled solar field (limitedby the available surface area) is sufficient for providing space heat-ing to Zone 3 only.

Additional assumptions taken into account in the carried outthermo-economic analysis are: (i) efficiency for conventional elec-tricity production (gRS,el) equal to 46%; (ii) electricity feed-in tariff(jel) equal to 0.35 €/kWh [56]; (iii) natural gas cost (jNG) equal to1.0 €/Nm3; (iv) natural gas lower heating value equal to9.59 kWh/Nm3.

5. Results

In order to investigate the effect due to the building integrationof PVT panels on the production of electricity and the thermalenergy (for DHW and space heating purposes) as well on their pas-sive effects on the building spaces heating and cooling demands, acomprehensive analysis based on dynamic simulations is carried

out. The obtained results, related to a typical residential building,alternatively equipped with different solar energy - HVAC systems,are discussed.

This section includes a plurality of parametric analyses whichare carried out in order to assess the effect of several buildingenvelope parameters on the overall building primary energydemands. Note that, yearly dynamic simulations are carried outfor different climatic zones, in order to assess the effect of the dif-ferent weather conditions on the building-plant system energy andeconomic performance. In the following sections, yearly, weekly,and daily results are discussed with the aim to provide usefulresults for benchmarking purposes. For the sake of brevity, mostof the detailed results refer to weather zones of Freiburg, Milan,Naples, and Almeria (well representative of the European cli-mates), whereas further results are referred to the analysis relatedto a larger group of investigated weather zones.

5.1. Yearly results

The energy and economic performances of Cases B, C and D arecompared to those obtained for the reference system, Case A,where space heating and cooling are provided through an air-to-water heat pump/chiller and DHW is produced by a condensationboiler, Fig. 1.

In this session, the simulation results are referred to the climatezone of Naples. In particular, in Table 7 the calculated primaryenergy demands are reported for each investigated building-plant system layout. Regarding the traditional building configura-tion (Case A), the obtained results show that the overall primaryenergy requirement (PEtot) is mainly due to the DHW preparation(64%). This is a key point that strongly affects all the achievedresults. In fact, any energy efficiency measure aiming at reducingthe building space heating and cooling demands (including BIPVT

Table 7Primary energy demand (for heating, cooling, DHW preparation) and BIPVT system primary energy saving for the weather zone of Naples.

Case A Case B Case C Case D

(MWh/y) (kWh/m3 y) (MWh/y) (kWh/m3 y) (MWh/y) (kWh/m3 y) (MWh/y) (kWh/m3 y)

PEDHW 7.85 6.25 5.33 4.24 5.36 4.27 4.68 3.72PEAuxiliaries – – 6.69�10�2 5.33�10�2 6.66�10�2 5.30�10�2 9.14�10�2 7.27�10�2

PEel,PVT – – 37.68 29.99 37.63 29.96 52.83 42.05PEHeating 2.91 2.31 2.14 1.70 1.96 1.56 1.81 1.44PECooling 1.47 1.17 1.49 1.18 1.75 1.39 2.03 1.61PEHeat&Cool 4.38 3.48 3.62 2.88 3.71 2.95 3.84 3.06PEtot 12.23 9.73 �28.66 �22.82 �28.50 �22.69 �44.22 �35.20

A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431 1421

panels) has a minor influence on the overall energy savings. Thisresult is mainly due to the very efficient envelope of the consideredbuilding. In fact, the developed case studies are referred to a newbuilding with very low U-values and equipped with a high effi-ciency HVAC system (electric heat pump/chiller). Conversely,DHW is produced through a much less energy efficient technology(a gas-fired condensation boiler). The combination of these two cir-cumstances determines the high ratio obtained between DHW andtotal primary energy demands (PEDHW/PEtot). In the next future,new buildings with such energy features should be very commonin Europe [57]. Conversely, in case of existing buildings, the pri-mary energy required for space heating may be generally higherthan the DHW one, because of the typically scarce envelope ther-mal insulation and low energy efficiency of the heating systems.

In the carried out analysis, by comparing Cases B, C and D vs.Case A, a significant saving of primary energy for heating (PEHeating)and DHW preparation (PEDHW) is obtained. This is due to theamount of solar energy exploited for both DHW production andspace heating purposes in the three proposed building-plant sys-tem layouts (where the amount of non-renewable energy isstrongly decreased vs. that on required in Case A). The PEDHW

related to Case C is slightly higher than the one reported for CaseB, due to the adopted assumptions and the different temperaturelevel within the heat storage tank. In particular:

� solar DHW is provided only if the radiant floor does not need tobe activated (i.e. hot water from the tank is supplied to the radi-ant floor, see Fig. 5). Note that, this occurrence is rather rare forthe modelled system layouts during the heating season;

� the temperature of the hot water stored in the TK tank of Case Cis averagely higher than the one of Case B during the no-heatingmonths. This is due to lower thermal losses of BIPVT collectorsvs. stand-alone PVT panels ones (i.e. the back surfaces of thecollectors face the building indoor air Zone 3 in Case C andthe outside air in Case B).

Clearly, the larger Case D solar field area (south-roof + south-façade), compared to those of Cases B and C, leads to: (i) higherelectricity production (PEel,PVT); (ii) higher solar hot water produc-tion for the radiant floor use and DHW preparation; (iii) highercooling primary energy demand (PECooling) due to the indoor airtemperature overheating in summer. Table 7 also shows that, forthe considered building, located in the weather zone of Naples,the space heating primary energy demand (PEHeating) is signifi-cantly higher than the cooling one (PECooling) for all the investigatedcase studies. This may be interpreted as an unexpected result forthe average climatic conditions of Naples (Mediterranean climate,see Table 2). This result is due to the selected space cooling dailyschedule vs. the heating one, to the adopted free cooling strategyand windows solar shadings. Specifically, the HVAC system is acti-vated for four hours per day for cooling needs and for eight hoursper day for the heating ones (Table 6). Note that this assumption isconsistent with the typical residential occupancy and heating/cool-ing schedules in the South European weather zones.

Table 7 also shows that for all the considered proposedbuilding-plant system layouts (Cases B, C and D), significant sav-ings are achieved in terms of primary energy (PEtot) vs. Case A(from 40.7 to 56.5 MWh/y). Such savings are basically due to theelectricity produced by the BIPVT solar field (stand-alone PVT forCase B) which corresponds to a primary energy (PEel,PVT) of about37.6 MWh/y for Cases B and C and to 52.8 MWh/y for Case D. Obvi-ously, the higher amount of electricity production shown by Case Dis due to its larger BIPVT solar field area (south roof + south façade).Note that, negative values of PEtot, shown in Table 7, basically iden-tify a yearly electricity production PEel,PVT higher than the buildingdemand.

In addition, Table 7 also shows that a negligible difference isdetected between PEel,PVT of Cases B and C. This result shows thatthe building roof integration of PVT panels (Case C) determines amarginal increase of the PV cells average temperature. Therefore,a negligible reduction of the annual electricity efficiency and pro-duction is detected vs. stand-alone PVT panels.

By observing the primary energy related to the space heating(PEheating) reported in Table 7, for all the proposed building-plantsystem layouts (Cases B, C and D), an important decrease ofPEheating is detected with respect to the PEheating calculated for thereference conventional building (Case A). This result is due to theutilization of solar thermal energy, produced by BIPVT collectors(stand-alone PVT for Case B) and delivered to the radiant floor(supplying Zone 3), with a consequent reduction of the electricityrequired by the backup heat pump. Obviously, this saving is evengreater for Case D where a higher amount of solar thermal energyis available because of the larger building solar field. Table 7 alsoshows that the primary energy due to the space heating (PEHeating)is lower for Case C with respect to Case B. Therefore, it can beinferred that the building integration of PVT panels is beneficialfor the reduction of the space heating demand (useful winter pas-sive effect of BIPVT collectors).

Conversely, the opposite effect occurs for the primary energydue to the space cooling (PECooling), which in Cases C resultedhigher than that one calculated in Case B (unwanted summer pas-sive effect of BIPVT collectors causing a higher mean radiant tem-perature of Zone 3). Evidently, in case of additional south-façadeBIPVT collectors (Case D), a higher PECooling vs. Case C is obtained(higher mean radiant temperatures of Zone 1 and Zone 2).

Therefore, the building integration of PVT panels determines apassive heating effect of the indoor spaces, which decreases theheating demand and increases the cooling one. In the weather zoneof Naples, these two effects are almost counterbalanced so that theoverall primary energy consumption (PEtot) reported for Case C issimilar to the one calculated for Case B. Obviously, higher PEtot sav-ings are achieved in Case D (for the same reasons above reportedfor PEel, PVT). Note that, the primary energy required by the P1and P2 system pumps (PEAuxiliaries) is very low for all the simulatedcase studies. In order to compare the energy performance of theinvestigated case studies with those of other residential buildingsequipped with such kind of integrated solar energy devices, Table 7also reports the above discussed results in kWh/m3y (specific

1422 A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431

results are expressed per unit of heated/cooled space volumebecause of the non-standard shapes of indoor spaces).

Therefore, the following remarks can be summarised by analys-ing the results reported in Table 7 for the weather zone of Naples:

� A significant production of electricity is obtained through PVTcollectors (both integrated and stand-alone). The calculatedyearly reduction of electricity production obtained throughBIPVT panels vs. stand-alone PVT ones is marginal, due to aver-agely lower cells temperature.

� A reduction of the primary energy demand for thermal uses isalways obtained vs. traditional buildings (without solar energyapplications, Case A). This is due to the heat recovered by thesolar field and exploited for space heating and DHW prepara-tion. The integration of PVT panels in roof and roof/façades(Case C and D) causes an additional saving of the heatingdemand (vs. Case B), due to the helpful BIPVT system passiveeffect.

� During the summer season, the integration of PVT panels in roofand roof/façades (Case C and D) causes an unwanted increase ofthe cooling primary energy demand (vs. Case A and B).

� A yearly balanced effect between the occurring decrease of pri-mary energy heating demand and the increase of cooling one isobtained. In fact, no significant differences are detected for thecalculated overall heating/cooling primary energy demands ofCase B and Case C.

� Therefore, for all the above mentioned reasons, the final userappears to be slightly affected by the alternative design optionbetween integrated and non-integrated solar collectors.

5.2. Weekly results

The above discussed annual results do not allow one to betteranalyse the time-dependent variation of system performanceparameters obtained during the whole year. Therefore, in orderto show such differences, some results are also presented on aweekly basis.

The effects of the weather conditions on the BIPVT performanceare studied through a suitable comparative analysis by taking intoaccount, in addition to Naples, the weather zones of Freiburg,Milan and Almeria. It is worth noting that for all such weatherzones the simulations are performed by assuming the same panelstilt angle, without varying it as a function of the latitude.

The weekly trends of the solar radiation on the roof and facadesfor the considered weather locations are omitted for the sake ofbrevity. In Almeria and Naples the weekly incident solar radiationis higher than that one available in Milan and Freiburg. According

1 8 15 22 29 36 43 50

200

400

600

800

1000

1200

Week

Ene

rgy

(kW

h/w

eek)

E

Ecool,A,FreE

Ecool,A,MilE

Ecool,A,NapE

Ecool,A,Alm

heat,A,Fre

heat,A,Mil

heat,A,Nap

heat,A,Alm

Fig. 7. Weekly heating and cooling energy demand for Case A (le

to the weather conditions, in Freiburg and Milan the producedthermal and electrical energies are averagely lower than thoseobtained in Naples and Almeria, especially during the summermonths. Simulation results also show that in all the investigatedweather zones the electricity production obtained in Case Bresulted slightly higher than the one achieved in Case C. As men-tioned before, this is due to the averagely higher temperature ofthe hot water stored in the TK tank of Case C vs. Case B. No signif-icant differences are detected by comparing the solar thermal pro-ductions. Obviously, the overall electricity and thermal energyproduction of Case D are higher than those obtained for Cases Band C, according to the larger BIPVT solar field area.

For all the weather zones, weekly building heating and coolingdemands (Eheat and Ecool) are shown for Case A and B in Fig. 7 andfor Case C and D in Fig. 8. By comparing the results reported inthese figures, the potential heating energy saving achieved throughthe solar heated radiant floor (Cases B, C and D) and the passiveheating effect due to roof BIPVT panels (Case C) and roof/façadesBIPVT collectors (Case D) can be detected. The highest energy sav-ing for the space heating is obtained in Freiburg by comparing CaseD vs. Case A. Obviously, during the winter season, shorter heatpump running times are obtained also due to the BIPVT collectorspassive effect. Lower savings vs. those obtained in Freiburg areobserved in Milan, Naples and Almeria. According to the assump-tions adopted in the developed case studies, no differences areobserved in summer, in terms of cooling energy demands, betweenCases B and A. As expected, by comparing Fig. 8 vs. Fig. 7, the roofand roof/façades BIPVT collectors (of Cases C and D, respectively)lead to higher cooling energy requirements with respect to CaseA along all the summer season. This unwanted result is due tothe hot water flowing through the BIPVT collectors (which heatup the interior surfaces of the building roof and south-façade).The passive effect of BIPVT overheating causes an increase of thecooling energy demand (higher in Case D vs. Case C because ofthe larger building integrated solar field area). Obviously, this phe-nomenon is more evident in Almeria (black line in Fig. 8) than inthe other investigated weather zones.

5.3. Daily dynamic results

In this section, hourly simulation results are discussed. In par-ticular, in Fig. 9, for one winter sample day in Naples, the time his-tories of the occurring operating temperatures are reported for: (i)the Case B roof and the corresponding stand-alone PVT panels; (ii)the Case C roof BIPVT system. In this figure it is clearly shown thatno significant differences are detected between PVT front temper-atures (Tpv) for Case B vs. Case C. For Case B, the obtained collector

1 8 15 22 29 36 43 50

200

400

600

800

1000

1200

Week

Ene

rgy

(kW

h/w

eek)

E

Ecool,B,Fre

E

Ecool,B,MilE

Ecool,B,Nap

E

Ecool,B,Alm

heat,B,Fre

heat,B,Mil

heat,B,Nap

heat,B,Alm

ft) and Case B (right) in Freiburg, Milan, Naples and Almeria.

1 8 15 22 29 36 43 50

200

400

600

800

1000

1200

Week

Ene

rgy

(kW

h/w

eek)

Eheat,C,Fre

Ecool,C,Fre

Eheat,C,Mil

Ecool,C Mil

Eheat,C,Nap

Ecool,C,Nap

Eheat,C,Alm

Ecool,C,Alm

1 8 15 22 29 36 43 50

200

400

600

800

1000

1200

Week

Ene

rgy

(kW

h/w

eek)

Eheat,D,Fre

Ecool,D,Fre

Eheat,D,Mil

Ecool,D,Mil

Eheat,D,Nap

Ecool,D,Nap

Eheat,D,Alm

Ecool,D,Alm

Fig. 8. Weekly heating and cooling energy demand for Case C (left) and Case D (right) in Freiburg, Milan, Naples and Almeria.

0

10

20

30

40

50

60

8712 8718 8724 8730 8736

Tem

pera

ture

(°C

)

Time (h)

Tpv,BTback,SC,B Toutside,roof,B Tpv,CTback,SC,C Toutside,roof,C Tamb

24:0012:000:00

Fig. 9. Zone 3 for a sample winter day in Naples: operating temperatures of thebuilding roof and the stand-alone PVT panels (Case B) and the of the BIPVT system(Case C).

0

1

2

3

4

24

25

26

27

28

4392 4398 4404 4410 4416

Qco

ol (

kW)

Tem

pera

ture

(°C

)

Time (h)

Tindoor,air,B Tindoor,air,C Tinside,roof,B Tinside,roof,C Qcool,B Qcool,C

24:0012:000:00

Fig. 10. Zone 3 for a sample summer day in Naples: operating temperatures andcooling loads of for Case B and Case C.

A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431 1423

back surface temperatures (Tback,SC,B) are always higher than theoutdoor ambient air ones (Tamb), whereas for Case C, this behaviour(Tback,SC,C > Tamb) is observed only during the daily hours in which asignificant incident solar radiation occurs. During these hours, thetemperature of the roof external surface of Case B (Toutside,roof,B) isalways lower than the corresponding temperature of Case C, asexpected. Note that, for Case C Tback,SC,C is equal to Toutside,roof,C. Sim-ilar results are obtained during the summer season.

With regard to the passive effects of BIPVT panels on the heattransfer across the roof (Case C) with respect to the traditional roof(Case B), the following results, also shown in Fig. 9, are obtained:

� During day hours with significant incident solar radiation it isobtained: Tpv,C > Toutside,roof,B (i.e. the external BIPVT panels tem-peratures (Case C) are much higher than the corresponding onesof the Case B roof). For this reason, according to the inertialbehaviour of the building structure, in the heating (cooling) sea-son a potential helpful (unwanted) passive effect of the roofBIPVT panels vs. the traditional roof is produced.

� During all the remaining hours the opposite trend is detected:Tpv,C < Toutside,roof,B. This result is consistent with the findingsreported in literature [16,58,59]. Therefore, according to theinertial behaviour of the building structure, in the heating (cool-ing) season a potential unwanted (helpful) passive effect of theroof BIPVT panels vs. the traditional roof is produced.

Obviously, these two opposite behaviours have to be assessedon the whole year in order to find out the dominant one. Note that,the final real effect on the indoor space also depends on the build-ing envelope features (roof U-value and thermal capacitance). Inaddition, the overall result of such phenomena, in terms of heating

and cooling energy demands, depends also on the working sched-ule of the HVAC system. In all the cases, the passive effects of BIPVTpanels have to be assessed also in terms of indoor comfort whenthe HVAC system is switched off. As it is well known, the comfortdepends also on the mean radiant temperature of the buildingenvelope, which for the Zone 3 is affected by the temperature ofthe south roof internal surface (Tinside,roof).

In Fig. 10, for one summer sample day, the time histories of theZone 3 indoor air temperatures (Tindoor,air), south roof internal sur-face temperatures (Tinside,roof) and cooling loads (QCool) are reportedfor Cases B and C. In this figure, an unwanted roof BIPVTpassive effect is detected during all the afternoon hours(Tinside,roof,C > Tinside,roof,B). Note that, the peak of Tinside,roof is shifted(vs. Toutside,roof, occurring about at 12:00) late in the afternoonbecause of the thermal inertia effect of the simulated roof struc-ture. During these hours, increased cooling loads and demandsare detected for Case C vs. Case B (QCool,C > QCool,B). Conversely, dur-ing the night and early in the morning, an helpful passive effect isobtained in terms of occupants comfort (Tinside,roof,C < Tinside,roof,Band Tindoor,air,C < Tindoor, air,B). In these hours the cooling system issimulated as switched off, as typically occurs in residentialbuildings.

5.4. Parametric analysis

For the examined building-plant system layouts a suitable para-metric analysis was carried out in order to assess the effects ofsome building envelope design parameters on the electricity andDHW production as well as on the space heating and coolingdemands. The investigated parameters are:

1424 A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431

� thermal transmittance (U-value), of the building roof andfaçades. The examined U-values are: 0.32, 0.80 and 1.20 W/m2 K. Note that 0.32 W/m2 K is considered for taking intoaccount innovative energy efficient buildings (NZEBs), whereas0.80 and 1.20 W/m2 K for simulating less thermal insulated res-idential buildings equivalent to the existing ones;

� density (q) of concrete slabs included in the building roof andfaçades walls layers. The simulated densities are: 600 (verylightweight concrete), 800, 1000 and 1200 kg/m3 (common con-crete). Note that, for each investigated density a different ther-mal conductivity was adopted.

The combinations of U-values and densities taken into accountin this analysis are shown in Table 8. In all such combinations forroof and façades the thicknesses of external/internal plasterboardlayer is 0.02 m and of concrete slab layer is 0.20 m. The analysisis carried out for the weather zones data of Freiburg, Milan, Naplesand Almeria. The adopted simulation assumptions (e.g. free coolingstrategy, building envelope features, internal loads due to people,electrical devices, lighting, ventilation and infiltration ratesthrough windows, daily heating/cooling periods, etc.) are sum-marised in Table 2. In addition, in Table 8, the installed cooling

Table 8Façade/roof layers and heating/cooling capacities.

Concrete slab Thermal insulation U-value Mila

Density Conductivity Thickness Heat

(kg/m3) (W/m K) (m) (W/m2 K) (W/m

600 0.36 0.054 0.30 18.80.011 0.80 35.00.002 1.20 43.1

800 0.41 0.056 0.30 18.80.013 0.80 35.00.003 1.20 43.1

1000 0.47 0.057 0.30 18.80.014 0.80 35.00.004 1.20 43.1

1200 0.54 0.059 0.30 18.80.015 0.80 35.00.005 1.20 43.1

Fig. 11. Case A: primary energy consumptions for space heating and cooling as a fu

and heating capacities of the simulated HVAC system are alsoshown. Such HVAC system is based on a two-pipes loop supplyingseveral fan coil units. Each modelled fan coil is equipped with asingle coil which is used for heating or cooling mode by varyingthe temperature of the inlet water. Details about the heating andcooling capacities of the modelled fan coil units are reported inTable 1. Note that, for all the simulated system layouts, theinstalled heating capacity is higher than the cooling one. This isdue to the coil heat exchange capacity that is always higher forthe heating operation than for cooling one.

The first result of the carried out parametric analysis regardsthe differences of the electricity production between Case C andCase B (equal solar PV field area) in each investigated weatherzone.

In particular, a very slight growth of electricity production isobtained by increasing the roof U-values, whereas it is almost inde-pendent of the related slabs density as well as of the building inte-gration of PV panels (vs. stand-alone ones). The calculateddifferences resulted quite low, ranging from -0.12% to 0.38%.

Fig. 11 shows the heating and cooling primary energy consump-tions, PEHeating and PECooling, for Case A of the four investigatedweather zones, as a function of the U-values and densities q taken

n and Freiburg Naples Almeria

ing Cooling Heating Cooling Heating Cooling

3)

15.0 15.0 12.0 15.0 12.028.0 23.6 18.9 16.3 13.034.5 31.3 25.0 17.5 14.0

15.0 15.0 12.0 15.0 12.028.0 23.6 18.9 16.3 13.034.5 31.3 25.0 17.5 14.0

15.0 15.0 12.0 15.0 12.028.0 23.6 18.9 16.3 13.034.5 31.3 25.0 17.5 14.0

15.0 15.0 12.0 15.0 12.028.0 23.6 18.9 16.3 13.034.5 31.3 25.0 17.5 14.0

nction of the U-values, densities q of the simulated envelopes and HDD/CDD.

A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431 1425

into account for the building roof/façade concrete slab layers. Inthis figure, each bundle of lines (delimited by the coloured regions)shows the obtained trend for the different densities (600, 800,1000, 1200 kg/m3) and for a single weather zone. For each obtainedcoloured region, the bottom line is always related to the highestinvestigated density (q equal to 1200 kg/m3), while the top linerefers to the lowest one (q equal to 600 kg/m3). As expected, theU-values (ranging from 0.32 to 1.2 W/m2 K) have a high impacton the heating and cooling consumptions, whereas a very slightdependence of the building envelope density q on the primaryenergy demands is observed. On the other hand, the higher theU-value, the higher the q influence.

The carried out parametric analysis for Case A allows one toobtain an interpolating equation of the simulated results, usefulfor designers and benchmark purposes. The detected linear func-tions for the heating and cooling energy consumptions are:

PEHeating ¼ HDD � a� qb

� �h i� U � þ c � HDDþ q �HDD

10 � b þ d� �

ð11Þ

PECooling ¼ q � a � bISR

� �� U þ c � CDDþ d

CDD

� �ð12Þ

where PEHeating and PECooling are the heating and cooling primaryenergy consumptions, U is the thermal transmittance, HDD andCDD are the heating and cooling degree days, ISR is the incidentsolar radiation, a, b, c and d are the identified equation coefficients,for both the heating and cooling case, reported in Table 9. For all theanalysed cases, the obtained corresponding R2 are higher than 0.97.Despite the limitation of such approach (due to the case specificbuilding heating and cooling demands), such easy to use correla-tions can be adopted as a swift tool for the assessment of the heat-ing and cooling primary energy consumptions by simply selectingand implementing in Eqs. (11) and (12) the occurring climate indexand building envelope features.

Table 9Coefficients of interpolating Eqs. (11) and (12).

a b c d

Heating, Eq. (11) 3.0�10�3 �3.0�106 2.8�10�3 �2.8�100Cooling, Eq. (12) �4.0�10-1 3.0�10�1 1.7�10�3 �4.2�101

Table 10Primary energy differences for space heating and cooling and for DHW preparation betwe

Walls/Roof U-value DPECase A vs. Case B

DPECase A vs. Case C

Heating DHW Heating

(W/m2 K) (%)

Freiburg0.32 6.46 14.58 8.240.80 4.61 12.70 7.961.20 3.45 11.64 7.83

Milan0.32 9.54 18.06 11.790.80 7.23 16.83 11.441.20 5.56 15.61 10.99

Naples0.32 26.41 25.03 32.540.80 20.35 23.10 28.891.20 14.73 22.19 24.81

Almeria0.32 27.43 28.30 45.070.80 47.45 27.39 61.561.20 40.98 26.57 58.45

The results achieved by such equations for Case A can be alsocombined to the ones obtained by the carried out parametricanalysis for Cases B, C and D, shown in Table 10. Here, the percent-age differences of primary energy demand due to space heating,cooling and DHW production, between the reference Case Aand the other simulated cases (namely DPE Case A vs. CaseB/Case C/Case D, respectively) are shown. Note that the primaryenergy demand for the DHW preparation is equal to 7.84 MWh/yfor all investigated parameters combinations and weatherconditions. The following comments can be made by observingTable 10.

� DPE Case A vs. Case B – High primary energy savings for spaceheating are achieved. The maximum savings reached in Alme-ria, Naples, Milan and Freiburg are about 47%, 26%, 10% and6%, respectively. Essentially, the obtained savings are due tothe solar heat gain achieved through the stand-alone PVT panelsexploited by the radiant floor. Note that, the radiant floor iseffectively activated in the afternoon and evening times, i.e.when the hot water in the energy storage tank reaches theselected temperature set-point. As a consequence, the resultingenergy demand of the backup heating system (electric heatpump, HP), from about 17:00 to 22:00, is lower than the onerequired during the remaining day hours.

High energy savings are also achieved for the DHW preparation.In particular, the maximum savings range from 15% in Freiburg to28% in Almeria.

� DPE Case A vs. Case C – In all the weather zones higher primaryenergy savings for space heating are obtained vs. the previouscase (DPE Case A vs. Case B). The maximum savings obtainedin Almeria, Naples, Milan and Freiburg are about 62%, 33%,12% and 8%, respectively. In Case C, the achieved savings aredue to the solar heat gain obtained through the roof BIPVT pan-els exploited by the radiant floor (as for Case B) and to the use-ful passive effect due to the BIPVT panels.

The minimum growth of the cooling demand due to theunwanted summer passive effect of the BIPVT panels for Almeria,Naples, Milan and Freiburg is about 16%, 15%, 23% and 25%, respec-tively. Despite of such remarkable percentage growths, the coolingdemands are still rather low (see Fig. 11). The dependence of thecooling demand growth on the roof slabs density appears signifi-

en reference system (Case A) and Cases B, C and D.

DPECase A vs. Case D

Cooling DHW Heating Cooling DHW

�25.53 14.34 15.68 �58.60 20.31�42.74 12.11 14.47 �61.19 18.56�56.67 10.36 11.72 �67.51 16.85

�23.45 17.82 18.79 �49.42 24.74�34.93 15.75 18.20 �47.67 23.16�44.03 14.18 15.12 �50.56 21.85

�14.91 24.77 37.59 �27.36 34.16�22.45 22.31 37.41 �27.03 33.04�28.86 20.48 31.47 �29.26 31.09

�15.96 27.10 27.34 �26.79 35.23�21.67 25.60 56.83 �23.29 34.17�25.56 24.32 56.87 �22.78 34.52

1426 A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431

cant only in Freiburg and Milan for U = 1.20 W/m2 K. Weaker den-sity influences are detected in the remaining examined cases. Ingeneral, the higher the CDD, the lower the q influence, Fig. 11.

In all the weather zones the best energy results for the DHWpreparation are achieved for U = 0.32 W/m2 K. The correspondingenergy savings, comparable to those obtained by comparing CaseA vs. Case B, are 14%, 18%, 25% and 27% for Freiburg, Milan, Naplesand Almeria, respectively.

� DPE Case A vs. Case D – Because of the larger solar field areahigher heating primary energy savings in Naples, Milan andFreiburg (38%, 19% and 16%, respectively), are obtained vs. theprevious case (DPE Case A vs. Case C). Note that, in Almerialower savings are obtained with respect to those obtained bycomparing Case A to Case C. In this case study, the achieved sav-ings are due to the solar heat gain obtained through the roof andfaçade BIPVT panels exploited through the radiant floor and tothe useful passive effect due to the BIPVT panels. The savingdependence on the roof slabs density appears weak in Freiburgand Milan whereas rather significant for Naples and Almeriawhere the best heating energy performance is obtained forthe maximum q = 1000 and 1200 kg/m3, respectively.

The minimum growth of the cooling energy consumptions(PECooling) due to the unwanted summer passive effect of the BIPVTpanels for Almeria, Naples, Milan and Freiburg is about 23%, 27%,48% and 58%, respectively. Despite of such high percentagegrowths, the real cooling demands are still rather low. The depen-dence of the cooling demand growth on the façade and roof slabsdensity appears weak in Freiburg whereas rather significant forMilan, Naples and Almeria where the minimum cooling demandis obtained for q = 1200 kg/m3.

The maximum energy savings for the DHW production rangefrom 20% in Freiburg to 35% in Almeria.

From the above discussed remarks it is possible to observe that:

� the hot Mediterranean weather zone of Almeria (see Table 6)shows for the space heating energy savings a different resultstrend with respect to those observed in Naples, Milan and Frei-burg (cooler or colder weather zones, see Table 6). In particular,Almeria shows higher heating energy savings for Cases B and Cvs. Case A (Table 10) because of the related averagely higheroutdoor temperature and solar radiation occurring during thewinter season. In this case, the highest external heat gains areachieved with U = 0.8 W/m2 K (longest switched off time inter-vals of the electric heat pump). In Almeria, during the heatingseason, the water circuits integrated in the building façadeBIPVT panels reduce such helpful heat gains. Therefore, loweramounts of transferred solar radiation are obtained throughthe Case D building envelope vs. the Case C one. For this reasonthe heating savings of Case D vs. Case A are lower than the onesassessed for Case C vs. Case A;

� for Case A vs. Cases B, C and D (Table 10), it is observed that thebest heating energy performance is achieved for U = 0.8 W/m2 Kfor the weather zone of Almeria, and for U = 0.32 W/m2 K for theother climate zones. By comparing Case A vs. Case C, for all thefour investigated weather zones, the best cooling energy perfor-mance is achieved for U = 0.32 W/m2 K, whereas by comparingCase A vs. Case D, the higher the CDD, the higher the U-valuesthat maximizes the cooling savings. In fact, for Freiburg the bestenergy performance is achieved for U = 0.32 W/m2 K whereas inNaples for U = 0.8 W/m2 K and in Almeria for U = 1.2 W/m2 K.The maximum savings for the DHW production in all theweather zones are achieved for U = 0.32 W/m2 K, and the sav-ings dependence on the façades and roof slabs density appearsnegligible.

� due to the adopted assumptions (i.e. residential building usage),for the weather zones of Freiburg, Milan and Naples, the heatingprimary energy demands PEHeating are remarkably higher thanthe cooling ones PECooling. For the weather zone of Almeria,PEHeating and PECooling are of the same order of magnitude, espe-cially for low U-values. The total primary energy consumptionsfor space heating and cooling (PEHeating&Cooling) vs. U-values, forthe different q and for all the examined building-plant systemlayouts (Cases A, B, C and D) are shown in Fig. 12, for Freiburgand Almeria. Here, it is possible to observe that, in Freiburg(as in Milan and Naples, not reported for the sake of brevity)due to the positive passive effect (i.e. additional heat gains) ofthe building integration, passing from Case A to Case D (beingalmost coincident the demands of Cases B and C), the total pri-mary energy consumptions for space heating and coolingdecrease. Contrarily, in Almeria, due to the significant coolingdemand, the total primary energy consumptions for space heat-ing and cooling of Case C and Case D are always higher thanthose achieved in Cases B, for all the simulated U-values. Thus,the BIPVT panels passive effects causes an increase of PECoolingthat counterbalances the decrease of PEHeating. As abovereported, Fig. 12 shows that such primary energy demandsremarkably increase with the building U-values (as expected)while are rather independent of the thermal mass of the build-ing envelope.

Note that, for the considered building-plant system layouts,BIPVT panels installation shows a remarkable passive energy effectonly in the building Zone 3. For this reason a specific analysis is car-ried out for analysing the Zone 3 heating and cooling demands. Theresults of this additional investigation, in which for the opaquebuilding envelope it is assumed U = 0.32 W/m2 K and q = 1200 kg/m3, are summarized in Figs. 13–15. In particular, Fig. 13 showsthe difference (DEHeating) between the Zone 3 unitary heatingdemand for Case C with respect to Case B as a function of HDD(Heating Degree Day) and ISR (Incident Solar Radiation). In Fig. 14the same analysis is carried out for the cooling demand as a func-tion of CDD (Cooling Degree Day) and ISR. Basically, by these figuresthe energy passive effects in terms of space heating and coolingdemands (reached through the adoption of the roof BIPVT panelswith respect to the stand-alone ones) are assessed. Each markerreported in these graphs is representative of a simulated weatherzone among the 18 listed in Table 6. In particular, by observingthe graphs of Fig. 13 an almost linear trend can be detected betweenspace heating savings and weather conditions. In particular, resultsshow that such savings almost linearly increase with HDDs anddecreasewith ISRs (theminus signmeans a reduction of the heatingdemand). This is due to the enhanced winter passive heating/insu-lating effect of roof BIPVT panels in the cold winter climate zones(high HDDs and low ISRs). Fig. 14 confirms that the unitary coolingdemands (DECooling), corresponding to the occurring overheatingunwanted effects, are higher for hot climate zones (high CDDsand ISRs). Note that, the differences (DE) reported in Fig. 13 rangefrom �0.7 to nearly 0 kWh/m3 y and in Fig. 14 from 0 to0.32 kWh/m3 y. Therefore, for all the considered weather zones, aminor influence of BIPVT panels on the overall energy demandsfor space heating and cooling of Zone 3 is detected. This result isconfirmed by observing Table 11. Here, for the investigatedweatherzones of Freiburg, Milan, Naples and Almeria the heating and cool-ing energy demands of Case B Zone 3, as well as the related overallprimary heating and cooling demands, are reported.

In Fig. 15, Zone 3 differences, between Case C and Case B, interms of overall primary energy for space heating and cooling(DPEHeating&Cooling) are shown vs. CDD and HDD (the minus signdenotes a reduction of the energy demand). This figure shows thatfor the weather zones with high HDD and low CDD the roof

0

2

4

6

8

10

12

14

16

18

20

PEH

eatin

g&C

oolin

g(k

Wh/

m3 y

)

U (W/m2K)

600 800 1000 1200

Cas

e A

Cas

e B

Cas

eC

Cas

e D

ρ (kg/m3)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.32 0.80 1.20 0.32 0.8 1.2

U (W/m2K)

Cas

e A

Cas

e B

Cas

eC

Cas

e D

Fig. 12. Freiburg (left), Almeria (right): Primary energy demand for space heating and cooling vs. U-value for different densities of the concrete slabs.

Fig. 13. Zone 3: heating energy saving between Case C and B.

Fig. 14. Zone 3: cooling energy increase between Case C and Case B.

A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431 1427

integration of PVT panels leads to slight overall savings, whereasfor zones with high CDD (e.g. Larnaca, Almeria, Brindisi andNaples) the negative (unwanted) passive effect of summer over-heating is dominant with respect to the positive (useful) winterheating one. However, once again, such variations are small with

respect to the overall building primary energy consumption. Notethat, by the graphs reported in the above described Figs. 13–15:(i) very good linear correlations vs. HDD are always detected; (ii)swift energy analyses can be carried out for any location for whichthe considered climatic indexes are available.

Fig. 15. Zone 3: difference of primary energy for space heating and cooling between Case C and Case B.

Table 11Zone 3: heating and cooling energy and primary energy for space heating and coolingfor Case B.

Weather Zone EHeating ECooling PEHeating PECooling PEHeating&Cooling

(kWh/m3 y)

Freiburg 13.17 0.06 8.71 0.04 8.75Milan 11.07 0.44 7.35 0.26 7.61Naples 1.81 1.85 1.15 0.93 2.09Almeria 0.06 3.11 0.04 1.61 1.65

Table 12Differences of Primary Energy demand (DPE), total Primary Energy Savings (PES),Primary Energy Savings for space heating and cooling (PES⁄), operating costs (DC),capital cost differences (DI0), Simple Pay Back (SPB) between Case A and Cases B, Cand D.

Case DPE(MWh/y)

PES(%)

PES⁄

(%)DC(k€/y)

DI0(k€)

SPB(y)

FreiburgCase A vs. Case B 30.83 65.67 6.26 4.86 80.69 16.6Case A vs. Case C 31.58 67.32 6.96 4.94 80.69 16.3Case A vs. Case D 43.11 72.75 10.92 6.87 135.14 19.7

MilanCase A vs. Case B 34.25 69.50 8.75 5.40 80.69 15.0Case A vs. Case C 34.20 69.45 8.82 5.39 80.69 15.0Case A vs. Case D 47.92 76.91 10.63 7.57 135.14 17.8

NaplesCase A vs. Case B 40.89 81.93 17.23 6.44 80.69 12.5Case A vs. Case C 40.73 81.68 15.29 6.42 80.69 12.6Case A vs. Case D 56.44 86.77 12.35 8.91 135.14 15.2

AlmeriaCase A vs. Case B 44.88 85.50 2.00 7.07 80.69 11.4Case A vs. Case C 44.55 84.95 �5.73 7.02 80.69 11.5Case A vs. Case D 61.39 88.76 �23.35 9.70 135.14 14.0

Fig. 16. Difference of primary energy saving for space heating and cooling betweenCases A–B and Cases A–C.

1428 A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431

5.5. Thermo-economic analysis

In order to assess the economic performance of the examinedbuilding-plant system layouts, a suitable thermo-economic analy-sis is carried out. The obtained results are shown in Table 12. Here,for Freiburg, Milan, Naples and Almeria, the differences of PrimaryEnergy demand (DPE), total Primary Energy Savings (PES), PrimaryEnergy Savings for space heating and cooling (PES⁄), savings interms of operating costs (DC), extra capital costs (DI0), Simple PayBack periods (SPB) are reported [60]. All the above cited parameters

are calculated for Cases B, C and D, by taking into account Case A asreference system. In this way, a suitable comparison can be carriedout among all the investigated case studies. As an example, by com-paring Case D vs. Case A, remarkable PESs of about 73%, 77%, 87%and 89% are achieved for Freiburg, Milan, Naples and Almeria,respectively. As above discussed, these results are mainly due tothe high electricity production obtained through the façade androof BIPVT panels. Conversely, by taking into account the spaceheating and cooling demands only, the obtained primary energysavings (PES⁄ in Table 12) result much lower than the related PES.Note that they may become even negative (e.g. in the weather zoneof Almeria) if the unwanted summer overheating effects are higherthan the helpful winter ones due to the BIPVT panels.

The following remarks can be pointed out. The calculated SPBindexes resulted rather high for all the investigated weather zonesand system layouts. In particular, they range from about 11 yearsfor Almeria (Case A vs. Case B) to 20 years for Freiburg (Case Avs. Case D). As expected, the SPB indexes obtained for the exam-ined weather zones decrease as a function of the related availablesolar radiation. This result is mainly due to the achieved electricityproduction. In addition, by comparing in Table 12 the calculatedSPBs and PESs for Cases B and C (vs. Case A), minor differencesare observed among all the examined weather zones. The assessedSPBs differences are too low to address designers towards the mostprofitable system configuration between Case B and Case C. Inother words, Cases B and C are almost equivalent from the eco-nomic point of view. It is worth noting that such result wasobtained considering the conservative assumptions to neglectPVT panels landing costs for Case B and to assume the same instal-

A. Buonomano et al. / Applied Energy 184 (2016) 1411–1431 1429

lation costs of the PVT collectors between Case B and Case C. Con-sequently, if in the next future such extra costs (for Case B) andsavings for BIPVT devices will be taken into account, Case C eco-nomic profitability could be enhanced.

In Fig. 16,DPES⁄ between Cases A–B and Cases A–C is plotted vs.HDD for Freiburg, Milan, Naples and Almeria. According to thefindings shown in Fig. 15 (right), a negative DPES⁄, equal to �2%for Naples and �8% for Almeria, is detected. Therefore, in mildMediterranean or hot summer climates, stand-alone PVT panelsresult more efficient than the BIPVT ones.

In general, the adoption of façade BIPVT collectors results notmuch attractive (vs. roof applications) because of the low relatedincident solar radiation. In fact, for all the investigated weatherzones, the roof BIPVT system layout (Case C) resulted much moreeconomically convenient than the façade/roof BIPVT one (CaseD). In other words, the capital cost of the façade PV solar fields ispaid back in longer times (vs. roof applications) because of thelower thermal energy and electricity production per unit of thefaçade BIPVT surface area.

6. Conclusions

The aim of this study is to assess the energy and economic per-formance of a Building Integrated flat-plate PhotoVoltaic Thermal(BIPVT) system for a residential application. Three different novelbuilding-plant system layouts (building and stand-alone PVT pan-els, roof BIPVT collectors, façade/roof BIPVT panels) were modelledand simulated to assess the active and passive influence of the pro-posed BIPVT systems on the building space heating and coolingdemands and overall primary energy requirements. With the pur-pose of performing a comparative analysis, such system layoutswere compared to a reference traditional system, consisting of abuilding without PVT panels. In addition, in order to perform acomprehensive comparative analysis from the energy and eco-nomic point of view, the effects of the weather conditions on theenergy and economic systems performance of the proposed sys-tems were also assessed for different European weather zones.

For each novel proposed system, a dynamic simulation modelwas developed in TRNSYS environment. By means of the simula-tion models, the energy performance and economic feasibility ofeach proposed system were assessed. The models are capable tocalculate: (i) the solar thermal energy (for space heating anddomestic hot water preparation) and electricity produced throughthe BIPVT panels; (ii) the building energy demands (for heating,cooling, domestic how water purposes and electricity).

In addition to the BIPVT systems, which provide solar thermalenergy and electricity production, the proposed building-plant sys-tem layouts also include a radiant floor (driven by solar thermalenergy), suitably designed at the last building floor. Electrical heatpumps/chillers (for space heating and cooling purposes) and a gas-fired condensation boiler (for DHW production only) are also mod-elled for backup purposes and for balancing the reference systemenergy demands.

As expected, simulation results show that all the proposedinnovative systems lead to remarkable energy savings, because ofthe electricity and DHW production as well as of the space heatingobtained by means of a renewable energy source. In particular, bycomparing roof BIPVT to stand-alone collectors (with equivalentsolar field area), according to the available literature, it can bepointed out that:

� similar electricity productions are achieved. A negligible betterperformance is obtained through the stand-alone panelsbecause of their averagely lower operating temperatures, asalso outlined by the available literature, e.g. [15];

� almost similar primary energy demands of the auxiliary boilerfor the DHW preparation are required;

� slightly lower primary energy demands are obtained for spaceheating purposes (reduction of the energy demand due to theheat pump). This result is due to the helpful overall passiveeffect (additional free heat gains) obtained through the BIPVTcollectors during the winter season;

� slightly higher cooling primary energy demands are detected forspace cooling purposes (increase of the energy demand due tothe chiller). This result is due to the unwanted overall passiveeffect obtained through the BIPVT collector during the summerseason (indoor space overheating). This result is particularly evi-dent in low HDD (Heating Degree Day) zones (such as Almeria).As a result, in cooling dominated weather zones, the buildingintegration is recommended only in case of limited availabilityof land surfaces, as observed in previous studies, e.g. [13].

In general, the last the reduction of heating primary energy andthe increase of the cooling one counterbalance each other (sameorder of magnitude), thus small variations of building overall pri-mary energy demands are observed.

In order to investigate how the global energy performance ofthe BIPVT technology is dependent on the building envelope fea-tures (or it influences the indoor space behaviour in terms ofenergy passive effects), a suitable parametric analysis was also car-ried out. In particular, the parametric study was performed byvarying the building roof and façade thermal transmittances andcapacitances. The obtained results show that:

� the electricity production is practically not affected by the vari-ation of such building envelope parameters;

� the Primary Energy Saving (PES) of the different investigatedbuilding-plant systems vs. traditional buildings (without PVTpanels) for space heating production ranges from 8.3% to 62%and for DHW from 10% to 35%. The calculated increase of thecooling primary energy is between 14% and 68%. For a buildingwith stand-alone PVT panels such savings range from 3.5% to47% and from 12% to 28%;

� by decreasing the roof/façade U-values from 1.2 to 0.32 W/m2 K,an increase of primary energy saving for both space heating andDHW preparation is observed for all the investigated building-system layouts and in all the weather zones, except for Almeria.Here, in case of building with stand-alone PVT panels, the bestperformance in obtained for U = 0.8 W/m2 K;

� the minimum primary energy for space cooling is observed, inall the weather zones and for all the BIPVT system layouts, fora building envelope U-value of 0.32 W/m2 K. The only exceptionregards Almeria (low HDD), where the U-value of 1.20 W/m2 Kresulted to be the optimal one in case of roof/façade BIPVTsystem;

� a weak influence on the building-plant energy performance ismostly observed by varying the building envelope capacitance.

The results obtained through the yearly energy and economicanalyses show that by comparing residential buildings with BIPVT(or stand-alone PVT) collectors vs. traditional houses without PVTpanels, it can be observed that:

� remarkable PESs are always obtained. The highest savings areachieved by integrating PVT panels in both south-façade androof. The lower the HDD index, the higher the PES. In fact, forFreiburg, Milan, Naples and Almeria the corresponding PESresulted equal to 73%, 77%, 87% and 89%. Note that, such resultsare also affected by the standard unglazed BIPVT system takeninto account in this study;

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� fairly high SPBs (from 11 to 20 years) are achieved for all theinvestigated building-plant system layouts and weather zones.The best overall energy-economic performances are observedin low HDD zones (Almeria and Naples). Note that: (i) the SPBscalculated for buildings with roof BIPVT panels are very close tothe ones obtained with stand-alone PVT collectors; (ii) the SPBsobtained for façade and roof BIPVT collectors are remarkablylonger than those achieved with roof-only BIPVT panels.

Several indexes, such as HDD (heating degree day), CDD (cool-ing degree day) and ISR (incident solar radiation) are taken intoaccount to correlate the building heating and cooling passiveeffects, due to the BIPVT panels, to the weather conditions. Inter-esting guidelines, useful to designers, stakeholders, practitionersand policy makers, are obtained through the obtained correlationsand, in general, by the results achieved through this study. In addi-tion, a new tool useful for designers, stakeholders, and practition-ers is provided. In particular, two novel equations are presented forthe swift assessment of the heating and cooling primary energyconsumptions as a function of the building envelope features andoccurring weather indexes. By such tool simplified feasibility anal-yses of such building-plant systems can be carried out.

In conclusion, the final user appears to be slightly affected bythe alternative design option between integrated and non-integrated PVT solar collectors. Obviously, for new and refurbishedbuildings, where the installation of such solar collectors is eithermandatory or recommended, the building integration will bealways advisable also for architectural aesthetic reasons. In addi-tion, in the next future the initial cost of BIPVT systems is expectedto become even lower than the one of conventional PVT systems.Finally, in the present market framework, a suitable public fundingstrategy should be necessary for enhancing the economic prof-itability of BIPVT (and in general solar systems) technology andfor promoting its mass-market commercialization.

Acknowledgments

Authors wish to acknowledge Action TU1205 (Building Integra-tion of Solar Thermal Systems, BISTS) of the European COST (Coop-eration in Science and Technology), Transport and UrbanDevelopment (TUD), for the sponsorship and the valuable scientificsupport.

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