Available online at www.sciencedirect.com

www.elsevier.com/locate/solener

ScienceDirect

Solar Energy 122 (2015) 1100–1116

Integrated solar thermal facade system for building retrofit

A. Giovanardi a,b,⇑, A. Passera a, F. Zottele c, R. Lollini a

aEURAC Research, Institute for Renewable Energy, Bolzano, ItalybUniversita degli Studi di Trento, Italy

cFondazione Edmund Mach, Technology Transfer Centre, Italy

Received 27 December 2014; received in revised form 17 August 2015; accepted 11 October 2015

Communicated by: Associate Editor Ursula Eicker

Abstract

In the perspective of the Net Zero Energy Buildings as specified in the EPBD 2010/31/EU, we propose the concept and design of amodular unglazed solar thermal (UST) facade component for facilitating the installation of active solar facades. The renovation of exist-ing buildings offers an opportunity to improve the energy efficiency when using such a system and a novel design methodology tackled viaa parametric approach is here proposed. We analysed a variety of building typologies as potential application targets of the UST col-lector, properly sizing the collector field for each typology to match the heat loads profile. We investigated the thermal behaviour of thenovel thermal facade component and the energy potentiality in covering the heat demand using the TRNSYS software’s model of theUST collector field as a part of a combisystem. We concluded with the definition of rules of thumb for early design stage. The work herepresented demonstrates that the low-cost, the versatile modularity and the easy installation make this active solar facade an innovativeand promising technology for the building stock transformation, despite of the low quality of the produced energy due to the low out-come temperature of the unglazed collector.� 2015 Elsevier Ltd. All rights reserved.

Keywords: Unglazed solar thermal collector; Metal cladding; Facade; Building energy retrofit; Trnsys; EnergyPlus

1. Introduction

Given their mature and technical reliability, Solar Ther-mal (ST) technologies are used more and more to coverbuilding heat loads, mainly Domestic Hot Water (DHW)

http://dx.doi.org/10.1016/j.solener.2015.10.034

0038-092X/� 2015 Elsevier Ltd. All rights reserved.

Abbreviations: a, absorbance; AC, collector area; e, emittance; AL, activelayer; BIST, building integrated solar thermal; DHW, domestic hot water;DOE, department of energy; _m, collector mass flow rate; _m1, flow rate ofthe primary solar loop; QLOADS, heat loads for DHW and SH; QSUN,useful energy gain of collector field; SH, space heating; SF, solar fraction;ST, solar thermal; UST, unglazed solar thermal; VC, storage tank volume.⇑ Corresponding author at: Via Piave 4, 37017 Lazise, Verona, Italy.

Tel.: +39 3497130192.E-mail address: alessia.giova@gmail.com (A. Giovanardi).

preparation and Space Heating (SH) (D’Antoni andSaro, 2012). Nowadays, there is still a lack of optimisedST technologies, conceived specifically for the existingbuilding stock, due to high complexity and number ofbuilding/urban contexts (EA ECBCS Annex50, 2011).However, the envelope system has a significant impact onthe building energy performances and on the overall valuewhen retrofitted or replaced. More specifically, the renova-tion of the facade allows the buildings not only to changeaccording to modern aesthetic perception, but also toimprove their energy behaviour.

The Building Performance Institute Europe (Nolte andStrong, 2011) reports that the non-residential building

A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116 1101

stock accounts for 25% in floor space of the European totalstock, and it represents the most complex and heteroge-neous sector compared to the residential one. Accordingto Balaras et al. (2005), about 70% of the European resi-dential stock was built before 1970, when energy efficiencywas not an issue neither for opaque (walls, roofs) nor trans-parent (windows) envelope parts, and so the buildings builtbetween 1945 and 1970 should be the priority for energyretrofit actions. Therefore, the development of novel solarthermal components represent a promising choice for therenovation market when they are conceived as integratedin the building envelope and no longer as mere additionaltechnical elements (see Bergmann, 2002; Hestnes, 2000;Munari Probst and Roecker, 2007).

In these last decades the European policies actively pushto improve the production of energy for building needs byusing solar technologies and Buildings Integrated SolarThermal (BIST) are becoming more interesting and attrac-tive (Bergmann, 2002; Hestnes, 2000). Specifically, thefacade represents a fairly easy option for the installationof collectors as it provides a further potential envelope sur-face for solar thermal integration to supply hot water fordomestic use, space heating and cooling. Although theamount of incident solar radiation on the vertical surfaceis about 30% lower than the amount hitting an optimaltilted surface, the implementation of these technologiesinto the facade avoids heat overproduction and collectors’overheating in summer-time, and allows to properly sizethe building energy system according to the actual heatdemand (Munari Probst and Roecker, 2007). Moreover,when ST collectors are mounted in vertical, they are lesssensitive to the weather conditions and dust, rain and snowwill not damage them.

Nevertheless, buildings integrated solar thermal intofacade are still rare since the design of an active facade rep-resents a tricky phase for architects and engineers dealingwith both aesthetic and functional issues (higher visibilityof the collectors) and with energy and technical aspects(support structure typology and presence of hydraulic con-nections) as investigated by Munari Probst and Roecker(2007). Generally, the integration of such solar technolo-gies in the building envelope is much easier to achieve forthe new constructions. The facade retrofitting with ST col-lector integration opens up a new challenge for designersand manufacturers, requiring greater design efforts fromthe architectural and engineering point of view and needingappropriate components.

1.1. ST integration into facade for energy retrofit

The solar thermal integration entails that the buildingenvelope, specifically the facade, acquires the feature ofmultifunctionality, since (1) the ST collector must guar-antee the same envelope functions of the replaced ele-ment(s), and (2) the envelope takes up the newfunction of collecting solar irradiation, producing anddistributing hot water.

The solar thermal facade solutions available on the mar-ket still focus on the maximisation of energy productionand generally do not match neither the multifunctionalitynor the possibility of being integrated in the facade systems(module size, colour, texture, jointing typology, collectorfield dimension). As found in IEA SHC Task 41 –SubtaskA (2012), there are still few commercial applica-tions of solar thermal collector in the refurbishment marketand the few available products are mainly conceived for theimplementation in new buildings.

1.2. Metal unglazed solar thermal collectors

Among the available ST technologies, the metalunglazed solar thermal (UST) collectors have a lower effi-ciency and a lower outlet temperature compared to the per-formances of the flat plate glazed collectors. Nevertheless,the metal UST collectors are simpler, cheaper and aesthet-ically appealing. The European project Solabs (Solabs,2006) exploited the idea of developing an unglazed colouredmetal solar absorber for building facades and for the inte-gration into heating systems. Later the European projectBionicol (Bionicol, 2011) developed an aluminium roll-bond solar absorber prototype and a novel approach forthe optimisation of the absorber channels layout.

1.3. Research objectives

We present here a novel concept of a solar facade sys-tem, which integrates an unglazed solar collector, to beused in building retrofit actions. Merging two differenttechnologies – roll-bond technology and metal claddingsystem – into one component may solve both the engineer-ing and architectural issues, only when they are correctlytackled.

We developed a methodological approach to address thedesign of an unglazed-solar–thermal/facade-cladding sys-tem investigating its application in selected reference build-ings and analysing the thermal and energy potentialities ofthis low temperature solar thermal technology by simula-tions. The outcome is a UST collector conceived as aunique, active, functional module both for the thermalenergy production (as an absorber) and for the existingfacade re-cladding. The potential development as an indus-trial product is based on three factors: (1) high modularityby easily installable elements to be sized for the specificneeds of the buildings, (2) low-price technology, given bythe industrial process already developed for the fridgeevaporators, and (3) versatile modules to be used bothfor new buildings and for energy retrofitting of existingbuildings. Fig. 1 shows the methodological approach devel-oped in the present work.

2. Integrating the technologies: a novel solar facade

The present work aims to facilitate the process of inte-gration of solar technologies in the envelope, as previously

Fig. 1. Chart of the methodological approach to address the concept and design of the solar active facade system.

1102 A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116

A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116 1103

pursed by the IEA SHC Task 41 (2012), by conceiving anddesigning a new solar thermal technology integrated infacade, for both new and existing buildings in need ofenergy retrofit actions and facade renovation. In this workwe present the possibility to merge two technologies intoone product:

� roll-bond technology represents a high potential, lowcost technical solution when used to produce absorberof UST collectors to answer the energy aspect. Thanksto the lightness and the manufacturing easiness of alu-minium, the roll-bond technology shows a very goodflexibility in the absorber design, channel cross sectionand layout, according to any custom request. Clearly,the channels configuration has a significant influenceon the collector efficiency as investigated in Bionicolproject (Bionicol, 2011).

� metal cladding system (cassette or plank) for the facadeapplication is already used by architects for its aestheticfeature, modularity, easiness of installation (plug &play) and maintenance. Certainly, the use of metal forthe ‘‘over skins” of buildings is a proper means for con-veying a ‘‘high-tech” aspect and has been widely usedfor both new buildings and for the refurbishment. Thefacade metal cladding systems have been indeed alreadyevaluated in deep detail in Solabs Project (Solabs, 2006).

This integration must be coherent and controlled fromthe functional, constructive and formal point of view asdescribed in detail in Munari Probst and Roecker (2007)and IEA SHC Task 41 – SubtaskA (2012).

2.1. Configuration of the active facade system

To couple the two technologies, roll-bond and metalcladding, we must take into account some technical andaesthetic requirements and some constraints in the selec-tion phase of the cladding systems:

� limits in size, thickness and material: attention isrequired for the mechanical resistance of the plate whenit is bended;

� presence of a ‘‘non-active” space around the ‘‘active”part of the absorber where the channels are placed;

The integration into facade has to meet standard build-ing constructive constraints and must be compatible withthe materials, shapes and structure of the envelope. Fixingdetails and jointing must be designed carefully and must becompatible with the materials in order to ensure the dura-bility of the envelope. Moreover the solar facade systemshould be easily accessible for maintenance, and the visualaspect has to be taken into account given the strong impactof the facade on the building architecture. So, the overallconfiguration of the solar collector field, the shape and sizeof the module, the colour, the surface texture and the

jointing typology, define the level of flexibility, and thusthe final potential application of the solar facade system.

According to the communication purpose of the build-ing facade, and to the final use of the building, the designercan play with alternation of elements to create rhythm andequilibrium in the facade, by choosing among a wide rangeof configuration options:

� absorber channels (geometry and presence on one side oron both the sides);

� visible or concealed jointing among claddings (for exam-ple negative jointing);

� colour of the selective paint coating on the sight side ofthe UST collector cladding system, chosen in a selectiveT.I.S.S. paints palette (Orel et al., 2007).

Following the same approach of Solabs Project (Solabs,2006), we compared the cassette and the plank claddingsystems since these two systems are used for easy, quickand cost-effective installation and are both potentiallyeasily adaptable as valid options to be UST collectors.We selected the cassette option balancing all the technicaland architectural aspects, from roll-bond constraints andgeometrical specifications, assembly and maintenance easi-ness of claddings to the variable aesthetic preferences of thedesigners and building holders. This technology showsmore flexibility for the maintenance and when dismantledand until now, it has not been developed as a solar elementfor the market and in the research field.

Moreover, the whole active facade has been conceivedas a system for both the generation and the distributionof the thermal energy. This energy system directly couplesthe UST collector, as base component of the outside clad-ding system, with a radiant wall system exchanging heatwith the indoor environment through the existing wall.The facade system can be used both for heating, with adirect distribution of the produced hot water, and for cool-ing, when the radiant element absorbs energy from theindoor environment (mainly during the night), rejecting itthanks to the radiative exchange of the unglazed collectorwith the sky. If the building does not need thermal energy,the produced hot water is stored and used then to coverDHW demand or SH when solar radiation is not available.

A first configuration of the solar cassette is proposed asa modular element folded on the four edges (Fig. 2). Theabsorber has a geometrical proportion of 1:2 width/lengthand negative jointing variable in width, according to thesupport structure typology. The roll-bond channels arepresent only on one side of the absorber and the sight sideis flat. Moreover, the active solar facade is an outside layerdirectly connected to a radiant wall system placed on theexternal surface of the existing building wall. The cassette/-solar collector is installed on a support structure, while theradiant wall system is fixed on the existing wall. The insu-lation material is directly installed on the radiant wall sys-tem or on the existing building wall, when the radiant wallsystem is not present. The plug & play functionality of the

Fig. 2. Active solar facade system components: the solar cassette (blue), the insulation layer (yellow), the radiant wall system (red), the aluminiumsubstructure and the existing masonry structure. (For interpretation of the references to colour in this figure legend, the reader is referred to the webversion of this article.)

1104 A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116

cassette facilitates the hydraulic connections between inlet/outlet pipes among collector absorbers and allows the pres-ence of a thin air cavity between insulation layer and thecassette system. Since there is not a real gap continuity,the insulation layer is not ventilated, but the humidity dif-fusion is guaranteed from inside the building towards out-side, while with an insulation layer directly fixed behind thecladding system, the diffusion of the humidity would not bepossible. A more detailed description of the installation ofthe active facade system is given in Appendix A.

Depending on the facade’s designer aesthetic needs, it ispossible either (1) to hide the added function of the cassetteto heat water or (2) to increase the value of such a newfunction by emphasizing the functional elements (pipesand connections among pipes) and then giving the facadea technology aspect. Clearly, for each aesthetic optionselected by the designer, proper technical solutions mightbe developed, such as the hydraulic circuit scheme – coldand hot water distribution pipes – and the substructure,considering as well the advantages to dispose of non-active elements known as dummies, where the UST cas-settes cannot be applied.

3. Reference buildings for the solar facade implementation

Some residential and non-residential buildings havebeen selected from the American Department of Energy(DOE) benchmark (DOE, n.d.) by matching the solaractive facade with both the architectural design and theenergy load.

The design and development of a new technology in thethermal energy field requires a high sampling rate of thebuilding heat consumption. For the peak consumptiondemand a short sampling period is crucial to the properdesign and sizing of the building’s energy plant systems.However, in Europe there are no up-to-date statistical pub-lications or complete databases of hourly energy load pro-file relative to a reference building for each category, e.g.single and multifamily houses, offices, commercial build-ings, hospitals, hotels, schools and sports centres, and foreach European climate zone.

To estimate their annual, monthly and hourly DHWand SH demand profiles, we used the model developedby the DOE to run a set of energy simulations in Energy-

Plus on multifamily houses, hospitals, hotels, schools andoffices (DOE, n.d.). Since we were more interested at con-structive typologies and relative energy demand, ratherthan HVAC system and its energy consumption, weassumed the models as applicable also in the Europeancontext.

We focused the analysis on four building typologies –a high and a low category hotel, a secondary school anda residential building with eight flats. We investigatedtheir behaviours using EnergyPlus in three European cli-mates (Stockholm, Zurich and Rome), also by analogywith the choice made within IEA Task 26 Solar Com-bisystem (Weiss, 2003) as proposed by Bonhote et al.(2009). The results of the simulation run in EnergyPlus

on the four typologies of building in Zurich are showedin Fig. 3.

Fig. 3. Monthly average thermal energy demand predicted by EnergyPlus using DOE benchmark model of a high category hotel, a secondary school, amultifamily house and a low category hotel in Zurich.

A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116 1105

3.1. Selected buildings: multifamily house and low category

hotel

Because the developed solar facade system has the dou-ble function of covering the building facade and producinghot water – matching both aesthetic and energy aspects –the multifamily house and the low category hotel shouldbe taken as reference buildings for the implementation ofthis modern solar facade component because (1) they canbe renovated with a modern stylish, and (2) their heatdemand for DHW is quite constant during the whole year.

Once the two building typologies were selected, it wasimportant to individuate the amount of the facade surfacepotentially available for the unglazed solar cassette imple-mentation, and thus available to be converted into solarsurface. The opaque facade available with south expositionwas respectively 300 m2 for the hotel and 240 m2 for themultifamily building.

3.1.1. Domestic hot water load profileThe hot water is requested mainly in the morning and in

the evening, when people shower. One more important

factor for the evaluation of the DHW load is the tempera-ture of water, delivered to the building plant equipmentfrom the waterworks. Although the fresh water temperatureis a function of the context and of the season, we assumed aconstant value of 10 �C in order to simplify the analyses per-formed with EnergyPlus. The resulting load profiles wereused as input for the TRNSYS simulations (see Section 4.4),where a sinusoidal function representing the fresh watertemperature for combisystem simulations has been used,according to the approach chosen by Weiss (2003).

3.1.2. Space heating load profileThe SH load of the two buildings has been obtained by

using EnergyPlus simplifying the thermal zones of thebuildings taking into account only the ones more fre-quently used by the occupants, e.g. guest rooms for thehotel.

With this output, we sized a low temperature heatingsystem with a radiant floor distribution accordingly tothe standard EN 1264-2:2009 (see Section 4.4). In addition,we calculated the hourly flow rate using the floor character-istics and the design data (water delivery temperature and

1106 A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116

temperature drop) of the heating system. The hourly flowrate value is variable and linearly proportional to therequired heat load.

4. Modelling

4.1. Facade modelling

A simple thermal zone has been modelled by TRNSYS

in order to assess the thermal energy exchanged by thesolar facade with the radiant wall system towards inside,and with the external environment towards outside. Wemainly investigated the summer night cooling potentialityof the active solar facade with a south orientation.

The unglazed metal solar collector has been identified asan active surface to potentially reduce both the heating andcooling space in buildings. In winter sunny days, the heatproduced by the vertical UST collector is transferred tothe radiative unit of the radiant wall system, which releasesthe heat to the masonry making the internal wall surfacewarmer than the indoor air. During the summer nights,the system behaviour is inverted and the solar facadeactively contributes to reduce inside temperature beingthe temperature of the UST collector lower than the skytemperature, and so being able to reject heat. The heatstored inside the building is released to the existing walland then intercepted by the radiative unit directly coupledin a closed water loop with the collector, which finallyrejects the heat to the external ambient.

The model is described in Appendix B. Since the loca-tions of Stockholm and Zurich do not have significantcooling load in residential buildings, we focused the analy-sis on the residential building located in Rome.

We considered 5 cm of brickwork wall and 15 cm ofmineral wool as thermal insulation directly installed onthe radiant wall system. As first step, the solar thermal-active layer circuit was supposed to work in a continuousway, without any kind of control. Fig. 4-above shows thetemperature profiles from the 10th to the 13th of August.The results show that the indoor temperatures are quitehigh over the day (around 35 �C in August). During thenight, the internal wall surface temperature is lower thanthe air zone temperature of about 3 �C. This means thatthe system has an effect of cooling, but due to the high tem-perature inside the masonry generated by the radiative unitduring the day, the temperatures of the zone are still high,around 33 �C.

Next, we applied a simple control of the active systemoperation in order to avoid the day overheating: the circu-lating pump is ON if solar radiation does not hit thefacade. On the contrary, during the day when the solarradiation does hit the facade, the thermal energy producedby the collectors is delivered to a storage system, modellinga continuous DHW demand, namely setting the storageoutlet temperature back to the collector at 35 �C. This isa realistic assumption for a hotel with an intensive use ofdomestic hot water. This system leads to a temperature

reduction of both the air zone and the inner wall surface.As shown in Fig. 4-below, the air zone temperature (blackthick dashed line) is lower than the first scenario (from 33to 27 �C) and, due to the lower thermal exchange throughthe wall, the difference with the surface temperature (blackthick line) during the night is reduced (surface temperatureis lower than the air temperature of nearly 1 �C). Withrespect to the same zone without an active system inte-grated in facade, the temperatures are lower and the cool-ing effect occurs and part of the heat inside the room istransferred outside through the facade system.

The maximum cooling power achieved during the sum-mer season is around 275 W.

4.2. Assessment of the prototype thermal transmittance

The calculation of the thermal transmittance has beenperformed by the Delphin software and then used as inputin the TRNSYS modelling of the UST collector (Sec-tion 4.3.1.1). We run simulations of a facade module oftwo adjacent cassettes with the hypothesis of a steady stateair conditions and considering a distance of 20 mmbetween two cassettes, 1.5 mm in thickness of aluminiumabsorber and 20 mm of back insulation of the absorber.

Weobtained a thermal transmittance of 1.56 W/(m2 K)�1

for the whole facade module. We rounded it to 2W/(m2 K)�1 and we used it as solar collector heat loss coef-ficient back and edges.

4.3. Energy system modelling

We defined a novel energy concept coupling the USTcollector with a storage tank and a back-up source. Weinvestigated the potentiality of the BIST in covering theheat demand for DHW and SH (combisystem) of the tworeference buildings in three different European climates.The combisystem has been modelled by the TRNSYS soft-ware to evaluate the energy performance and to define asizing procedure that minimises the use of the back-up bymeans of an optimal solar source’s utilization.

4.3.1. Parametric study of the UST collector

Although UST collectors represent the simplest andlowest cost typology of solar thermal converters, the highdegree of thermal losses reduces their efficiency and theoutput temperature.

An initial evaluation of the behaviour of a singleunglazed collector has been studied by means of a paramet-ric analysis: (1) to investigate how the UST collectorexchanges heat with the external environment and underwhich weather conditions it absorbs and losses heat and(2) to select the flow rate-to-collector area ratio, givingthe best compromise between collector efficiency and outlettemperatures. We performed the analysis by usingTRNSYS and its TESS library to identify which compo-nent can properly model the UST collector. Type 559 hasbeen chosen because it implements a calculation of the

Fig. 4. Temperature profiles during three summer days (August 10th–August 13th) in Rome with a 5 cm brickwork wall and 15 cm mineral wool andcontinuous running system (above) and controlled running system (below).

A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116 1107

convective and radiation heat transfer coefficients based onHotel–Whillier–Bliss equation (Duffie and Beckman,2006), and therefore can properly model the UST collectorheat loss coefficient.

4.3.1.1. Type 559 equation and TRNSYS deck description.

The aim of this study on the UST collector is to investigatethe influence of some design parameters and therefore toindividuate the flow rate-to-collector area. For the Type559, we used technical and thermal parameters and inputdata given in Table 1 that are based on the previous anal-ysis of the reference buildings’ models (EnergyPlus), addi-tional hypotheses for the prototype and data sheets ofUST collectors with similar characteristics present on themarket.

The parametric analysis of the UST collector model wasperformed using a simple TRNSYS deck, including the col-lector (Type 559), a pumpwith constant flow rate (Type 3b),a weather file reader (Type 15-6), an equation providing the

Table 1Some input data of Type 559.

Value

Surface area (m2) 1Heat loss coefficient back and edges (W/(m2 K)) 2Efficiency factor F’ (–) 0.84Fluid specific heat (kJ/(kg K)) 3.72

inlet water temperature value and a general forcing functionfor the flow rate value (Type 14). If the solar irradiation hit-ting the vertical surface is higher than 100 W/m2, a controlsignal switches the pump on (see Table 2).

Solar absorbance (a) and thermal emittance (e) are char-acteristic values of thickness insensitive spectrally selective(T.I.S.S.) paints used for solar absorbers. Orel et al. (2007)proposed some coloured T.I.S.S. paint coatings forunglazed solar facade absorbers in order to face both thearchitects’ aesthetic and the energy efficiency needs. Thesevalues were set as a = 0.77 and e = 0.31 (blue colour).

According to the analysis carried out by Medved et al.(2003), we run a set of simulations, with varying area (Atot)and mass flow rate ( _m) to verify how efficiency and outlettemperature change in Stockholm, Zurich and Rome.The collector was considered as installed vertically (slope90�) and south oriented (azimuth 0�).

We run the simulations with all the combinationsbetween different collector areas (Atot) and different valuesof flow rate ( _m): 300, 200, 150, 120, 80 and 60 m2, depend-ing on the available facade surface exposed to the South,and 1000, 5000, 10,000 kg/h. The results show a strong cor-relation of both the collector thermal energy productionand the efficiency to the mass flow rate, whereas the collec-tor area has a great effect on the temperatures. Fig. 5 showshow the specific mass flow rate ( _m/Atot) is correlated to theefficiency coefficient (eta) up to 40 kg/(h m2

coll), while thereis no significant increase of efficiency with values higher

Table 2South facade wall construction layers.

Layer (–) Density (kg/m3) Specific heat (kJ/kg K) Conductivity (W/m K) Thickness (m)

Inside plaster (gypsum fiberboard) 1200.00 1.10 0.20 0.015Masonry (brickwork) 1250 0.84 0.50 0.200Conductive layer 0.10 0.10 2.78 0.060Active layer (serpentine) – – – –Conductive layer 0.10 0.10 2.78 0.030Insulation (mineral wool) 80.00 0.90 0.04 0.150Air cavity – 1.00 0.6 0.025Cladding system 1200.00 0.90 0.40 0.005

1108 A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116

than 60 kg/(h m2). Since the objective is a good balancebetween (1) a useful energy produced by the solar collectorsand (2) an outlet temperature as high as possible, the valuesof _m/Atot > 40 kg/(h m2) have been discarded. Specifically,_m/Atot < 10 kg/(h m2

coll) (Fig. 5) allows higher temperaturesand higher temperature drops: the water with higher poten-tial use matches better the specific loads in term of power.As expected, in warmer European climates like in Rome,the outlet temperatures are higher than in middle and

Fig. 5. Specific mass flow rate trend on the collector efficiency (above) and the cin Rome.

colder European locations (Zurich and Stockholm). Hence,large collector areas and low flow rates should be takeninto consideration for the sizing of the building thermalsystem when coupled with facade integrated USTcollectors.

Summarising, since the southern facade of the two refer-ence buildings �300 m2 of the low category hotel and240 m2 of the multifamily house respectively – has a largesurface available, one possible configuration of the solar

ollector outlet temperature (below), at 12 am in the coldest and hottest day

A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116 1109

collector field could be covering the whole facade surfacewith the unglazed collectors and then varying _m/Atot inthe range from 3 to 40 kg/(h m2

coll). To identify the opti-mum values of flow rate for the reference buildings, whenthe whole available facade is used, is the task of the studyon the building energy system.

4.4. UST collector integrated with the building energy

system: configuration of a combisystem

We describe here the investigation of the potentialities ofthe UST collector when integrated in the building com-bisystem (SH and DHW production). The objectives ofthe study are (1) to evaluate the energy performance and(2) to define preliminary sizing procedures for the com-bisystem, in order to decrease the auxiliary heater andincrease the solar collectors’ utilization.

As first step, we modelled a standard combisystem con-figuration (Fig. 6) with the following components:

� UST collector with vertical installation;� external heat exchanger;� storage tank;� auxiliary heater as back-up;� controllers for the system operation (solar loop and aux-iliary heater).

The UST collectors deliver heat, across a counter flowheat exchanger, to the storage tank, used to cover the totalheat for SH and DHW demand. When the solar energydoes not cover the loads, the back-up system starts towork.

All the components – UST collector, external heatexchanger, storage tank and back-up, and the control strat-egy – have been modelled using a TRNSYS deck with fourmain loops: the space heating, the domestic hot water, theauxiliary and the solar. A more detailed description of theTRNSYS model is available in the Appendix C. The sizingof the tank volume and the flow rate of the solar loop hasbeen optimised by simulations on the reference buildings,

Fig. 6. Hydraulic scheme of the combisystem with the main components as debeen calculated at the red dotted line. (For interpretation of the references to carticle.)

placed in the three locations, minimising the auxiliary hea-ter use.

4.4.1. Storage tank volume sizing

In the literature, some authors propose to size the stor-age tank capacity with a ratio in litre per m2 of collectorarea: 50 l/m2

coll (Thur, 2007), 50–70 l/m2coll for solar fraction

around 50%, 30–50 l/m2coll for solar fraction around 25%

(German Solar Energy society, 2010) and 50–100 l/m2coll

(Weiss, 2003). Solar fraction (SF) is the percentage of thetotal thermal load satisfied with solar energy. These valuesare valid for more efficient technologies than UST and witha proper collector inclination depending on the climatelocation (for example 50� in Stockholm and 30� in Rome).The facade system is characterized by a low efficiency and alow solar energy production if compared with the optimalslope of the ST systems and so there is a lower possibility tostore heat per unit of collector area. Given these consider-ations, we selected the storage volume in the range 10–50l/m2

coll of tank volume-to-collector area ratio, with an over-all heat loss coefficient of the tank equal to 3 W/K.

4.4.2. Combisystem simulation description

We performed a parametric analysis following theapproach of Hobbi and Siddiqui (2009). We estimatedthe influence of the hot water storage tank capacity onthe solar fraction (SF, considering SH and DHW thermalload and accounting for the heat losses of the combisystem)and efficiencies (eta, the percentage of the solar energyobtained by solar irradiation hitting the facade) of the com-bisystem. We used SF of the solar water heating system asan optimisation parameter to determine the set of optimumvalues for the design parameters of both the collector andthe system: the collector area (AC), the collector mass flowrate ( _m) and the storage tank volume (VC).

To estimate the annual and monthly solar fraction andto determine the adequate collector area, a first set of sim-ulations in TRNSYS were run varying the collector areawith a fixed value of tank volume-to-collector area ratio(VC/AC) and of the mass flow rate ( _m). Furthermore, we

scribed in Section 4.4. The solar fraction and the efficiency coefficient hasolour in this figure legend, the reader is referred to the web version of this

Fig. 7. Annual efficiency (above) and solar fraction (below) values varying the storage volume-to-collector area for the low category hotel (left) and themultifamily house (right).

1110 A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116

run several simulations to determine the effect of the collec-tor mass flow rate on the annual and monthly solar frac-tion, keeping a constant value for the collector area (AC)and for the tank volume-to-collector area ratio (VC/AC)and so we determined the effect of the tank volume onthe system performance for the values of the collector areaand the mass flow rate.

Firstly, we analysed the specific flow rate of the solarloop in the range 3–40 kg/(h m2

coll), as defined in Sec-tion 4.3.1.1. Once fixed at 0.75 the effectiveness e-NTU ofthe external heat exchanger, placed between thesolar source and the storage tank, we obtained a value of4 kg/(h m2

coll) for the flow rate of the primary solar loop( _m1) for all the reference buildings, and for all the threeEuropean locations.

Therefore, we varied the tank volume-to-collector arearatio in the range from 10 to 50 l/m2

coll, with a fixed surfacearea and a selected value of specific mass flow rate at 4kg/(h m2

coll). In this range, the annual SF increases rapidly

from 57% to 66% as reported by Hobbi and Siddiqui(2009).

4.4.3. Results of the simulations

We here provide the annual averages plots of solar frac-tion and efficiencies versus the tank volume-to-collectorarea ratio, obtained by TRNSYS simulations.

(a) Low category hotel (300 m2): the highest SF valuesare obtained in the range for tank volume-to-collector area ratio going from 26.7 l/m2

coll to 33.3l/m2

coll with the value of 26% for Zurich and 18%for Stockholm. In Rome, the SF increases rapidlyto the maximum value of 55% at 40 l/m2

coll. In thesame range 26.7–33.3 l/m2

coll, the efficiency coefficientis around 12% for Stockholm and 14% for Zurich,whereas for Rome it is around 15%, with the maxi-mum value of 16% in the range 33.3–40 l/m2

coll

(Fig. 7-left).

Fig. 8. Solar contribution for the heat load, in winter and summer, for the multifamily house in Stockholm, Zurich and Rome.

A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116 1111

(b) Multifamily house (240 m2): the highest SF isobtained for storage tank volume-to-collector areain the range from 33.3 to 41.7 l/m2

coll, providing amaximum value of 22% for Stockholm, 26% forZurich and 51% for Rome, with a minimal differencewith the other values of volume (Fig. 7-right). Themaximum values of efficiency coefficient occur above41.7 l/m2

coll, 11% for Stockholm and 12% for Zurich,whereas the energy system simulated in Rome gives aconstant increasing value of efficiency in the wholerange from 25 to 50 l/m2

coll with a maximum of14.5% (Fig. 7-right).

Summarising, a storage tank volume-to-collector areaaround 30 l/m2

coll gives values of SF close to the opti-mum. As long as the storage tank volume-to-collectorarea is in the range 20–40 l/m2

coll, the SF level is stillappreciable, given also the relatively small differencewith the maximum. Moreover, because of high costsof installation and maintenance of big storages, a rangevolume-to-collector area of 20–25 l/m2

coll could be takeninto consideration as well.

(c) Solar contribution for season: the solar contributionin winter and summer, for both the typologies ofbuilding heat needs, has been estimated by varyingthe storage volume-to-collector area ratio in therange 10–50 l/m2

coll.

Focusing on the multifamily house results (Fig. 8), themaximum value for the winter season occurs with a storagevolume around 33.3 l/m2

coll, for all the three locations. Insummer, when the solar energy is used just to cover theDHW, 33.3 l/m2

coll is still valid for Stockholm, whereas inZurich and Rome increasing values of storage volume-to-collector area give an increasing solar contribution, witha maximum in correspondence of 50 l/m2

coll. For the south-ern European locations the available solar energy is higherand so the solar contribution, which requires higher stor-age tank volumetric capacity.

4.4.4. Rules of thumb for the combisystemIn the light of the parametrical analysis with TRNSYS,

we proposed some ‘‘rules of thumb” for a standard

Fig. 9. Horizontal section of the active solar facade (dimensions in mm).

1112 A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116

configuration of a solar combisystem coupled with USTcollectors vertically installed.

We recommend a flow rate running in the primarysolar loop of 4 kg/(h m2

coll), in order to maintain waterwith a high energy potential to be delivered to the storagetank and an effectiveness value of the external heatexchanger around 0.75. In locations placed in southernEurope like Rome, given the higher solar contribution,it could be possible to get higher values of flow rate until20 kg/(h m2

coll).The parametric study carried out on the combisystem

for the multifamily house and the hotel, shows that thestorage sizing depends significantly on the climate and onthe users. The results show that high annual values of SFand efficiency coefficient can be generally reached with avolume between 25 and 40 l/m2

coll. In the summer-time,50 l/m2

coll would allow to easily achieve a higher SF but avolume-to-collector area of 20–25 l/m2

coll could be enoughto have acceptable percentages of SF and to limit the costsand the heat loss from the storage tank.

A finer-tuning of this configuration must take into con-sideration two separated storages connected in cascade forDHW and SH demands in order to improve the overallperformance of the combisystem. Once the target tempera-ture is reached in the first storage for DHW, the valve forthe second storage is switched on to supply hot water forthe SH at a lower temperature.

5. Conclusion

This work presents a new unglazed solar thermal collec-tor concept for facade integration in building energy retro-fit actions and at the same time addresses the future designand development opportunities of facades integratingactive energy systems in the perspective of the Net ZeroEnergy Buildings.

We proposed to merge two technologies, the roll-bondand the metal cladding system, into one active facade com-ponent, analysing both the potentialities and the integra-tion problems. In order to be competitive with otherproducts for the building energy retrofit, the componenthas been conceived to be easily installable and with a costcomparable with a traditional metal cladding.

This concept is also part of a solar combisystem tomatch SH and DWH demand. Based on our simulations,we defined the rules of thumb for a preliminary sizing ofthis combisystem.

We developed the study of the facade concept through amethodological approach following four main steps:

� analysis of technical and architectural integration issuesto select the proper suitable cladding system: the cassetteshows more flexibility and until now it has not beendeveloped as a solar element for the market and in theresearch field;

� selection of typologies of building suitable for the appli-cation of the solar active facade from the energy and aes-thetic standpoint: the multifamily house and the lowcategory hotel were taken as reference buildings for theimplementation of the solar facade because (1) they canbe renovated with a modern stylish and (2) their DHWdemand is quite constant during the whole year;

� modelling of the UST collector coupled with an activewall layer and integrated into a south oriented facadeby using the TRNSYS dynamic simulation tool in orderto investigate the thermal behaviour of the whole facadesystem and specifically the summer night coolingpotentiality;

� modelling of the Building Integrated Solar Thermal(BIST), as part of the building energy system, by usingTRNSYS in order to define some rules of thumb for astandard configuration of a solar combisystem for a

Fig. 10. Vertical sections of the active solar facade (dimensions in mm). During summer nights, sky temperature is lower than the external facade surfacetemperature: therefore, the thermal exchange between the UST collector and the sky occurs. The activation of the combined system allows to catch theheat stored inside the building thank to the radiant wall system, then the circulating fluid brings the heat towards the solar collector, which releases theheat to the ambient.

Fig. 11. Graphic model of the thermal zone implemented in TRNSYS.

A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116 1113

multifamily house and a hotel. We recommend a flowrate running in the primary solar loop of 4 kg/(h m2

coll),in order to get water with a high energy potential to bedelivered to the storage tank and an effectiveness valueof the external heat exchanger around 0.75. Higher val-ues of flow rate until 20 kg/(h m2

coll) could be obtained in

locations placed in southern Europe like Rome. More-over, the parametric study of the combisystem showedthat acceptable percentage of annual values of SF canbe reached with a volume-to-collector area ratio of 20–25 l/m2

coll, to limit the costs and the heat loss from thestorage tank.

Table 3Parameters describing the AL TRNSYS function.

Active layer parameters

Fluid specific heat (kJ/kg K) 4.186 (water)Pipes spacing (m) VariablePipes diameter (m) 0.020Pipes thickness 0.002Pipes thermal conductivity (W/m K) 0.35 (plastic)

1114 A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116

The achievements of this work give the opportunity forfurther developments: (1) to develop an UST collectorfacade system prototype for an experimental campaignaimed at the model validation and so to generalise theresults for different buildings and different climate contexts,(2) to assess through dynamic simulations the thermalbehaviour of the UST collector integrated in the existingbuilding facade by varying some parameters related tothe wall construction (materials and thickness) and to theradiative unit (active layer) features (pipe spacing and massflow rate), (3) to investigate through dynamic simulationsthe energy performance of a system equipped with an heatpump and the UST collector as component to pre-heat thewater, and (4) to develop the solar facade system, includingUST cladding component and radiant wall system, as acommercial energy retrofit package to contribute to thebuilding stock transformation.

Acknowledgments

The authors wish to thank Stefano Avesani of the Euracstaff who has contributed with the simulations. We are alsograteful to the editor and to the reviewers for their thought-ful comments on the manuscript.

Appendix A

The active facade system, as showed in Figs. 9 and 10,consists of:

� UST collector/Cassette cladding system.� Insulation layer, directly installed on the radiant wallsystem or on the existing building wall (where the ser-pentine is not present).

� Radiant wall system, fixed on the existing building wall.� Aluminium support structure.

The installation of the facade system should start from(i) the fastening of a substructure fixed to the buildingstructure, (ii) the application of a radiant system, whichis a radiative unit incorporated in a conductive layer, adja-cent to the wall, (iii) the installation of an insulation layerand (iv) finally the mounting of the UST collectors/cas-settes with a plug & play assembly, facilitating the hydrau-lic connections between the solar cassette and the radiantsystem, and among the solar modular elements.

Appendix B

The study focuses on a single thermal zone, of the refer-ence multifamily building, modelled by TRNSYS (seeFig. 11). More in detail, the total heated floor surface areaof the room is 24 m2, the facade has a net opaque surface of13.7 m2 and a window area of 2.5 m2. The window has athermal transmittance of 1.8 W/m2 K and a g-value of0.6. In the present analysis, we considered a solar field of12 m2 surface area, which produces thermal energy to bedelivered to a 13.5 m2 radiant wall area. We consideredthe climate of Rome, since the cooling demand in residen-tial buildings located in Stockholm and Zurich are notsignificant.

Since the energy balance is strongly affected by internalgains, we defined a hypothetical occupation profile of theroom. The calculation model was developed according tothe following data:

� Occupation: from 0 to 3 people depending on the hourand the day (weekday, Saturday, Sunday).

� Lighting: 10 W/m2 – convective part 30% (ON if zone isoccupied and solar radiation on horizontal < 300W/m2).

� Equipment: 80 W (corresponding to a TV screen).� Shadings control: depending on the exposure and solarradiation on surface (ON if solar radiation on verticalsurface >200 W/m2).

� Air change for natural ventilation (windows opening):depending on the number of people and the air temper-ature (ON if outdoor temperature is lower than theindoor temperature and if indoor temperature is biggerthan 25 �C; criteria: 0.5 vol/h).

� Infiltration rate: 0.20 vol/h.

The TRNSYS Type56 implements a steady-state 1Dsimplified model of the complex 3D-dependent behaviourof a radiant system. This function is called Active Layer(AL) and it is based on a network of thermal resistanceand on the assumption of linear temperature trend of thefluid in the pipe. The model is described in TRNSYS 16manual (TRANSSOLAR, 2007). The AL TRNSYS func-tion is described by five parameters, typical of floor radiantheating systems given in Table 3.

Due to an automatic setting of TRNSYS, the radiantlayer has to stay between two layers (made of the samematerial) and with a thickness bigger or equal to 0.3 timesthe pipes spacing. This is the reason why we set two ‘con-ductive layers’. We hypothesized a layer with high conduc-tivity (10 kJ/h m K) and low density (0.1 kg/m3) in such away to remedy to the minimum thickness to be set, whichwould give a slow system in terms of reactivity.

The UST collector (modelled by Type 559) receives theflow coming from the active layer. Hence, outlet tempera-ture from Type 56 is directly connected to the UST collec-tor as inlet temperature. The same connection concerns themass flow rate. Once the fluid is passed through the solar

A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116 1115

thermal collector, it returns to the active layer. Outlet massflow and outlet temperature of the solar collector fieldbecome inlet mass flow and inlet temperature of the radiantsystem. The solar collector is implemented as externalfacade layer by imposing the mean plate temperature asboundary condition of the wall construction.

The control of the active system was modelled depend-ing on the season. During heating season (from October15th to April 14th), the loop connecting the solar fieldand the radiant wall system works only when solar radia-tion hitting facade is available in order to release the col-lected heat inside the building. In cooling season thefacade system is active in the night hours, so when solarradiation is not available.

Appendix C

The TRNSYS deck comprises four main loops: thespace heating, the domestic hot water, the auxiliary andthe solar.

C.1. Space heating loop

We sized the SH considering the low temperature distri-bution system of the heating radiant floor according to thestandard EN 1264-2:2009 (see Section 3.1.2). The deliverytemperature of the distribution system depends on theexternal temperature. We estimated the heating demandfor the two reference buildings with simulations inEnergyPlus.

In the TRNSYS deck, a logical controller calculates theproper value of the set point temperature of a temperingvalve – Type 11b – and generates the on/off signal operatingthe pump, according to hourly and seasonal scheduling(h 07:00–21:00 during the heating season) and to heatingneed. The flow rate of the pump is variable and proportion-ate to the heat load needed. Part of the fluid on the return sideis sent to a three way tee-piece– Type 11 h– to cool the waterif the delivery temperature coming from the tank is too high.

C.2. Domestic hot water loop

We considered as inputs for the DHW profile (1) thefresh water temperature and (2) the hourly DHW load frac-tion profile. This last is multiplied by the daily peak demandin order to get the rate requested by the building. Type 805represents the heart of the DHW circuit. The primary sideflow, between Type 805 and Type 340 – storage, is calcu-lated iteratively until the set point temperature (45 �C) onthe secondary side at the given draw-off flow rate or themaximum flow rate on the primary side is reached.

C.3. Auxiliary heater/back-up system

As it is not reliable to fully depend on the solar energy,the solar combisystem needs a back-up heating system sucha gas heater or a heat pump. The auxiliary heater ensures

hot water when solar energy contribution is not enough,e.g. bad weather or when there is high hot water consump-tion. In the TRNSYS model, the auxiliary loop consists ofa pump and a heater, both controlled by a signal in orderto keep the upper part of the storage at a certain tempera-ture level (45 �C).

C.4. Solar loop

The solar loop connects the thermal collectors to the stor-age tank. It consists mainly of a primary and a secondarysolar loop. An external heat exchanger, modelled with Type5b, is connected to the primary solar loop composed of thecollector circuit, simulated using the Type 559 and a variablespeed pump, Type 110. The secondary solar loop comprisesthe storage circuit, consisting of the solar heat exchanger, asecond variable pump and the storage tank, modelled withthe Type 340. Given the large collector field area, we pre-ferred to use an external heat exchanger because it canexchange higher thermal capacities with small temperaturedegrees. The flow in the primary solar loop is assumed tobe an antifreeze solution of 30% glycol in water.

We implemented the control strategy in the loop for theoperation of the two variable pumps. The solar radiationhitting the vertical surface and the water temperature dif-ference at the bottom of the storage and at the outlet ofthe collectors do control the operations of the variablepumps. If the solar radiation rate is between 100 and350 W/m2, the pumps can work between 0% and 100% oftheir nominal power. Over 350 W/m2, they work at themaximum power.

References

Balaras, C.A., Droutsa, K., Dascalaki, E., Kontoyiannidis, S., 2005.Deterioration of European apartment buildings. Energy Build 37 (5),515–527. http://dx.doi.org/10.1016/j.enbuild.2004.09.010.

Bergmann, 2002. Facade integration of solar thermal collectors a newopportunity for planners and architects. Renew Energy World 5 (3),89–97.

Bionicol, 2011. BIONICOL – Project final report. Retrieved from www.bionicol.eu.

Bonhote, P., Eperon, Y., Renaud, P., 2009. Unglazed coloured solarabsorbers on facade: modelling and performance evaluation. Sol.Energy 83 (6), 799–811. http://dx.doi.org/10.1016/j.solener.2008.11.014.

D’Antoni, M., Saro, O., 2012. Massive solar-thermal collectors: a criticalliterature review. Renew. Sustain. Energy Rev. 16 (6), 3666–3679.http://dx.doi.org/10.1016/j.rser.2012.02.076.

DOE, n.d. Report – DOE benchmark models for building energysimulations. Retrieved February 6, 2015, from http://energy.gov/eere/buildings/commercial-reference-buildings.

Duffie, Beckman, 2006. Solar Engineering of Thermal Processes. JohnWiley & Sons.

EA ECBCS Annex50 (Ed.), 2011. Retrofit Module Design Guide.German Solar Energy Society, 2010. Planning and Installing Solar

Thermal Systems: A Guide for Installers. In: Architects and Engineers.Earthscan Ltd.

Hestnes, A.G., 2000. Building Integration Of Solar Energy Systems. Sol.Energy 67 (4–6), 181–187. http://dx.doi.org/10.1016/S0038-092X(00)00065-7.

1116 A. Giovanardi et al. / Solar Energy 122 (2015) 1100–1116

Hobbi, A., Siddiqui, K., 2009. Optimal design of a forced circulation solarwater heating system for a residential unit in cold climate usingTRNSYS. Sol. Energy 83 (5), 700–714. http://dx.doi.org/10.1016/j.solener.2008.10.018.

IEA SHC Task 41, 2012. Retrieved from http://task41.iea-shc.org/.IEA SHC Task 41 – SubtaskA, 2012.Medved, S., Arkar, C., Cerne, B., 2003. A large-panel unglazed roof-

integrated liquid solar collector-energy and economic evaluation. Sol.Energy 75 (6), 455–467. http://dx.doi.org/10.1016/j.solener.2003.09.009.

Munari Probst, M., Roecker, C., 2007. Towards an improved architecturalquality of building integrated solar thermal systems (BIST). Sol. Energy81 (9), 1104–1116. http://dx.doi.org/10.1016/j.solener.2007.02.009.

Nolte, I., Strong, D., 2011. Europe’ s Buildings Under the Microscope. ACountry-by-country Review of the Energy Performance of Building.Buildings Performance Institute Europe (BPIE).

Orel, B., Spreizer, H., Slemenik Perse, L., Fir, M., Surca Vuk, A., Merlini,D., Kohl, M., 2007. Silicone-based thickness insensitive spectrallyselective (TISS) paints as selective paint coatings for coloured solarabsorbers (Part I). Sol. Energy Mater. Sol. Cells 91 (2–3), 93–107.http://dx.doi.org/10.1016/j.solmat.2006.07.013.

Solabs, 2006. SOLABS. Development of unglazed solar absorbers(resorting to coloured selective coatings on steel material) for buildingfacades, and integration into heating systems (solabs) – Projectreference: ENK6-CT-2002-00679. Retrieved from http://cordis.eu-ropa.eu/project/rcn/67210_en.html.

Thur, A., 2007. High Efficiency by Minimizing Temperatures HighEfficiency by Minimizing Temperatures – PhD thesis, BGY DTU.

Weiss, W. (Ed.), 2003. Solar Heating for Houses – a Design Handbook forSolar Combisystems. James & James.