Building Façade Retrofit with Solar Passive Technologies

energies

Review Building Façade Retroﬁt with Solar Passive Technologies: A Literature Review

Sara Brito-Coimbra 1 , Daniel Aelenei 1,2,* , Maria Gloria Gomes 3 and Antonio Moret Rodrigues 3

1 NOVA School of Science and Technology, NOVA University of Lisbon, 2829-516 Caparica, Portugal; [email protected] 2 Centre of Technology and Systems/UNINOVA, FCT Campus, 2829-516 Caparica, Portugal 3 CERIS, Department of Civil Engineering, Architecture and Georresources (DECivil), Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal; [email protected] (M.G.G.); [email protected] (A.M.R.) * Correspondence: [email protected]

Abstract: Worldwide, buildings have been presented as one of the main energy consumers and, for that matter, there is an increased tendency to invest in policies and measures that promote more efficient buildings. Among the chosen strategies, the need to promote the use of passive solutions and retrofit the existing building stock is often pointed out. Portuguese building stock has proven to be obsolete in terms of thermal comfort, which can directly affect the energy demand for climatization purposes. Considering the great solar availability in the country, when compared to other European locations, building retrofit with solar passive technologies can be a suitable solution. This paper aims to review studies on the application of solar passive technologies to retrofit façades in the Mediterranean climate context, with a special focus on Portugal. Four retrofit passive solar technologies were reviewed, namely glazing, sun shading, sunspaces and Trombe wall technologies.

Citation: Brito-Coimbra, S.; Aelenei, Keywords: solar passive technologies; building retrofit; façade; Mediterranean climate D.; Gloria Gomes, M.; Moret Rodrigues, A. Building Façade Retrofit with Solar Passive Technologies: A Literature Review. 1. Introduction Energies 2021, 14, 1774. https:// doi.org/10.3390/en14061774 Worldwide, buildings are presented as one of the main energy consumers [1]. Poor design of existing buildings, along with the improvement of living over the years, has Academic Editor: Chi-Ming Lai contributed to a significant increase in energy consumption for space conditioning [2]. Consequently, several policies and regulations have been designed to promote more ef- Received: 28 February 2021 ficient buildings. The Energy Performance of Buildings Directive, also known as EPBD, Accepted: 19 March 2021 is an example of policies [3–5]. It targets not only new but also existing buildings as the Published: 23 March 2021 European building stock is mostly represented by the existing buildings. To counter the aging of the existing European building stock, the best solution is to Publisher’s Note: MDPI stays neutral building retrofit [6], which can be seen as an opportunity to reduce energy consumption and with regard to jurisdictional claims in improve the thermal comfort and well-being of the occupants [7]. Regarding the Portuguese published maps and institutional affil- context, according to the last Census [8], nearly 70% of the residential building stock was iations. built prior to 1990, when the first thermal regulation was set up [9]. As a result, due to insufficient thermal insulation at the opaque building envelope, inappropriate windows (single-glazed windows with timber or aluminum without a thermal break frame) and lack of care with the regard to correction of thermal bridges, most of the buildings are in Copyright: © 2021 by the authors. poor condition regarding thermal performance and energy efficiency. The last Portuguese Licensee MDPI, Basel, Switzerland. Census [8] also showed that most of the needs to repair occur at the envelope level (walls, This article is an open access article window frames and roof), which can directly impact the buildings’ thermal performance. distributed under the terms and Indeed, buildings built prior to 1990 are responsible for higher energy demands for heating, conditions of the Creative Commons ranging from 265 to 334 kWh/m2.year, while those built between 1990 and 2010 require Attribution (CC BY) license (https:// between 112 and 265 kWh/m2.year [10]. According to a study performed in 2014 [11], creativecommons.org/licenses/by/ approximately 80% of residential buildings had energy demands for heating higher than 4.0/).

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100 kWh/m2.year, which is somehow concerning when analyzed in the context of often- referred “mild climates.” Other studies have also shown that other Southern European countries with Mediterranean climates present similar needs to building retrofit to keep up with current building standards [12–17]. As stated by the updated World Map of the Köppen–Geiger climate classification [18], Mediterranean climates are characterized by cool and wet winters. Additionally, they are divided into two climate zones based on their summer characteristics: Csa (hot and dry summer) and Csb (warm and dry summer). Regarding mainland Portugal, the south is classified as Csa, while the north and center are classified as Csb. Following the EPBD recast [4], Portuguese legislation [19] demands that the retrofit of the existing building stock should be progressively done toward implementing the concept of nearly zero-energy buildings (nZEB). These are buildings with very high energy performance due to passive strategies and renewable technologies, where the very low amount of energy that is required for their functioning is mainly covered by energy from renewable sources [4]. Generally, the strategy followed in implementing nZEB is divided into three different categories: Passive Strategies (e.g., Passive Solar Heat Gain), Energy Efficiency (e.g., Radiant Heating) and Renewable Energy Systems (e.g., Solar Thermal) [20]. Passive Strategies are normally the starting point for building designers as they can be applied to both residential and nonresidential buildings [20]. They are characterized by the direct interaction between the building envelope and the environment, and they address the heating, cooling and lighting needs of a building with the intent to prevent, reject/collect or control heat gains and/or natural lighting, without the use of electrical or mechanical equipment [19–21]. Among the Passive Strategies, solar passive technologies can be set as a viable solution to building retrofit under Mediterranean climates, as this climate is mostly known for its solar availability (solar hours and solar irradiance). The literature on solar passive technologies is already very widespread. Some papers focused on the design optimization of passive solar strategies [21], while others focus on multiple technologies [22–25] or targeted a specific technology, such as Glazing [26], Sun Shading [27,28] and Trombe walls [29,30]. However, the literature review showed that there is an apparent lack of review papers on the application of solar passive technologies for building retrofit, especially in the Portuguese context. For this reason, the aim of this paper is to provide a literature review on the application of solar passive technologies to retrofit the façades of Portuguese buildings, considering the Mediterranean climate. This paper is organized into four sections. The first section presents the motivation and the goal of the research. The second section briefly discussed solar availability in the Portuguese context and compared it to other European contexts. The third section describes the findings on studies about the application of solar passive technologies to retrofit Portuguese façades. Section four provides a brief discussion on the feasibility of retrofit actions with solar passive technologies. Finally, the fifth section draws conclusions from the previous sections.

2. Solar Availability The potential of solar passive systems mainly relies on the availability of solar radiation, which is determined by the climate and latitude of the location and by the placement and position of the system [31,32]. Solar availability on urban façades is also impacted by the obstruction caused by nearby buildings [32]. Studies reported in the reviewed literature characterize solar availability either using the global vertical irradiation on the south façade or the global horizontal irradiation [33], [34]. In the Portuguese context, a study based on solar irradiation data devised by Brito et al. [35] analyzed the solar potential of roofs and façades of two different neighbors in Lisbon. According to their study, the annual solar irradiation of roofs varied between 1000 and 1800 kWh/m2.year, while façades varied between 100 and 1000 kWh/m2.year, depending on the façade orientation. The study also showed that façades facing east and Energies 2021, 14, x FOR PEER REVIEW 3 of 20

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and 1800 kWh/m2.year, while façades varied between 100 and 1000 kWh/m2.year, depend- and 1800 ingkWh/m on the2.year, façade while orientation. façades varied The betweenstudy also 100 showed and 1000 that kWh/m façades2.year, facing depend- east and west west present a wide range of irradiation values (100–800 kWh/m2/year), and the sum of ing on thepresent façade a orientation. wide range Theof irradiation study also values showed (100–800 that façades kWh/m facing2/year), east and and the west sum of their their potential could surpass those facing south. present apotential wide range could of irradiationsurpass those values facing (100–800 south. kWh/m2/year), and the sum of their In this study, the annual Global Solar Irradiation (GSI) is used as an indicator to In this study, the annual Global Solar Irradiation (GSI) is used as an indicator to eval- potentialevaluate could surpass solar availability. those facing Figuresouth.1 shows the GSI of Lisbon when compared to other uate solar availability. Figure 1 shows the GSI of Lisbon when compared to other Euro- In thisEuropean study, locations,the annual namely GlobalAncona Solar Irradiation (Italy), Barcelona (GSI) is used (Spain), as an London indicator (United to eval- Kingdom), pean locations, namely Ancona (Italy), Barcelona (Spain), London (United Kingdom), uate solarMünchen availability. (Germany), Figure 1 Paris shows (France) the GS andI of StockholmLisbon when (Sweden). compared The to dataother correspond Euro- to München (Germany), Paris (France) and Stockholm (Sweden). The data correspond to pean locations,2015 and namely were obtained Ancona from (Italy), the PhotovoltaicBarcelona (Spain), Geographical London Information (United Kingdom), System of the EU 2015 and were obtained from the Photovoltaic Geographical Information System of the MünchenScience (Germany), Hub of Paris the European (France) and Commission Stockholm [36 (Sweden).]. As can beThe seen data in correspond Figure1, high to annual EU Science Hub of the European Commission [36]. As can be seen in Figure 1, high annual 2015 andvalues were wereobtained recorded from the in all Photovolta locationsic of Geographical Mediterranean Information climates (Lisbon,System of Ancona the and EU ScienceBarcelona),values Hub wereof the the recorded European highest in value Commission all locations being associated [36]of Me. Asditerranean can with be Lisbonseen climates in Figure (1789 (Lisbon, kWh/m 1, high Ancona annual2.year). and When Bar- 2 values werecomparedcelona), recorded the to Paris, highestin all locations Munich, value being Londonof Me associatedditerranean and Stockholm, with climates Lisbon Lisbon’s (Lisbon, (1789 GSIkWh/m Ancona is 31%,.year). and 33%, Bar- When 41% andcom- celona), 48%thepared highest higher to Paris, thanvalue Munich, the being corresponding associated London and values,with Stockholm, Lisbon respectively. (1789 Lisbon’s kWh/m GSI2.year). is 31%, When 33%, com- 41% and 48% pared to Paris,higher Munich, than the London corresponding and Stockholm, values, respectively. Lisbon’s GSI is 31%, 33%, 41% and 48% higher than the corresponding values, respectively.

Figure 1. Annual sum of Global Solar Irradiation (GSI) in 2015. Figure 1. Annual sum of Global Solar Irradiation (GSI) in 2015. Figure 1. Annual sum of Global Solar Irradiation (GSI) in 2015. 3.3. Solar Solar Passive Passive Technologies Technologies 3. Solar PassiveInIn this Technologiesthis section, section, thethe keykey findings ﬁndings of of the the studies studies devised devised on on the the application application of solar of solar pas- In thispassivesive section, technologies technologies the key usedfindings used to retrofit to of retroﬁtthe studiesfaçades façades devised of Portuguese of Portuguese on the applicationbuildings, buildings, considering of solar considering pas- the Medi- the sive technologiesMediterraneanterranean used climate, climate,to retrofit are presented. are façades presented. of Portuguese buildings, considering the Medi- terranean climate,AsAs Passive Passive are presented. Strategies, Strategies, solar solar technologies technologies can can address address both, both, heating heating and and cooling cooling needs needs As byPassiveby preventing, preventing, Strategies, rejecting/collecting,rejecting/collecting, solar technologies or can or controlling controlling address both, heat heat gains.heat gains.ing Solar and Solar technologiescooling technologies needs can be can cat- by preventing,beegorized categorized rejecting/collecting, as Direct as Direct and Indirect and or controlling Indirect Solar Gain Solar heat Te Gaingains.chnologies Technologies Solar [37].technologies Direct [37]. Gain can Direct Solarbe cat- Gain Technolo- Solar egorizedTechnologies giesas Direct absorb, and absorb,store Indirect and store Solarrelease and Gain releasesolar Te chnologiesenergy solar energydirectly [37]. directly Directinto the Gain into indoor the Solar indoor environment Technolo- environment (Figure gies absorb,(Figure2a) store[38].2a) and [38]. release solar energy directly into the indoor environment (Figure 2a) [38].

(a) (b) Figure 2. Examples(a) of solar passive technologies: (a) direct solar(b gain) through a window; (b) indi- Figure 2. Examplesrect solar ofgain solar through passive a Trombetechnologies: wall system. (a) direct solar gain through a window; (b) indi- Figure 2. Examples of solar passive technologies: (a) direct solar gain through a window; (b) indirect rect solar gain through a Trombe wall system. solar gain through a Trombe wall system.

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In the case of indirect solar gain technologies, solar energy is first absorbed by a thermal mass and then transferred into the indoor environment for later use (Figure2b )[37]. In the case of indirect solar gain technologies, solar energy is first absorbed by a ther- A matrix of suitable retrofit solutions for each case is presented in Table1, and the retrofit mal mass and then transferred into the indoor environment for later use (Figure 2b) [37]. A matrixsolutions of suitable are retrofit described solutions in detail for each in Sections case is presented3.1–3.4. in Table 1, and the retrofit Table 1.solutionsSolar technologies are described used in to detail retrofit in according Sections to3.1–3.4. the type of solar gain and target season.

Table 1. Solar technologies used to retrofit according to the type of solar gainTargets and target season. Type of Solar Gain Solar Technology Heating SeasonTargets Cooling Season Type of Solar Gain Solar Technology √ √ Glazing technologies Heating Season Cooling Season Direct √ Sun shading technologiesGlazing technologies √ √ Direct √ Sunspace technologiesSun shading technologies √ Indirect √ Trombe wall technologiesSunspace technologies √ Indirect Trombe wall technologies √ At the end of the section, a table is given that summarizes the main findings from the At literaturethe end of reviewthe section, (Table a table2). is given that summarizes the main findings from the literature review (Table 2). 3.1. Glazing Technologies 3.1. Glazing TechnologiesWorldwide, window retrofit is, without any doubt, the most popular retrofit action. Worldwide,The reasons window behind retrofit this successis, without are any threefold. doubt, the Firstly, most windowspopular retrofit usually action. present higher The reasonsthermal behind transmittance this success values are threefol (U-values)d. Firstly, than thewindows opaque usually façade [present26], and, higher as consequence, thermalmost transmittance of the heat values exchange (U-values) between than the the exterior opaque andfaçade interior [26], and, environment as consequence, occurs via glazed most ofspans the heat [39 ].exchange Secondly, between financial the supportexterior programsand interior often environment encourage occurs window via retrofit. In glazed spansthe Portuguese [39]. Secondly, context, financial the “Renewsupport programs Home &Office” often encourage program window [40] is an retrofit. example of such In the Portugueseprograms. context, Thirdly, the windows “Renew areHome the & simplest Office” solarprogram passive [40] technologyis an example and of the easiest such programs.building Thirdly, element windows to retrofit are [20 the,37 simplest]. solar passive technology and the easiest building elementIn the Portuguese to retrofit [20, context, 37]. window retrofit is either done by: In the• PortugueseInstalling context, a secondary window window retrofit in is addition either done to the by: existing single-glazed window; • Installing• Replacing a secondary the window entire window in addition system; to the existing single-glazed window; • Replacing• Adding the entire Solar window Control system; Films (SCFs) to the existing window. • Adding Solar Control Films (SCFs) to the existing window. Window retrofit through the installation of a secondary window is based on the inter- Windownal addition retrofit ofthrough a second the pane installation of glass of or a plastic,secondary while window keeping is based the original on the in- single-glazed ternal additionwindow, of to a improve second thepane thermal of glass performance or plastic, while of the windowkeeping systemthe original during single- wintertime [40]. glazed window,This solution to improve can also the maintain thermal perf theormance external of original the window aesthetics system of theduring renovated win- window tertime system,[40]. This which solution is usefulcan also in maintain the case the of retrofittingexternal original the historical aesthetics buildings, of the reno- as shown in vated windowFigure3 system,, which which illustrates is useful the in impact the case of theof retrofitting retrofitting the on historical a building buildings, located in the UN- as shownESCO’s in Figure Historic 3, which Centre illustrates of Porto the [41 impact]. No measurableof the retrofitting impact on of a thisbuilding type located of retrofit solution in the UNESCO’swas found Historic in the literature Centre offor Porto the [41]. Portuguese No measurable context. impact of this type of retrofit solution was found in the literature for the Portuguese context.

(a) (b)

Figure 3.Figure Example 3. Exampleshowing the showing impact the of window impact of retrofi windowtting retroﬁttingon a building on located a building in the located Historic in the Historic Centre of Porto (a) before and (b) after the intervention [41]. Centre of Porto (a) before and (b) after the intervention [41].

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Regarding the replacement of window systems, most Portuguese studies considered the replacement of single glazing with multilayer glazings. On the whole, triple-glazed Energies 2021,windows14, 1774 were usually chosen for locations in the north and center-north of the mainland, 5 of 18 as shown in [42]. However, the replacement with double-glazed windows was the most common solution nationwide for both residential and nonresidential buildings, as shown in [41,43–49] and [50,51],Regarding respectively. the replacement It is estimated of window that systems, replacing most single- Portuguese with double- studies considered glazed windows isthe an replacement investment of that single can glazing cost between with multilayer 200 and glazings. 1500 EUR/unit, On the whole, whereas triple-glazed the investment in windowsretrofit measures were usually of the chosen opaque for locations walls with in the traditional north and External center-north Thermal of the mainland, as shown in [42]. However, the replacement with double-glazed windows was the most Insulation Composite Systems (ETICSs) is between 30 and 60 EUR/m2. The measurable common solution nationwide for both residential and nonresidential buildings, as shown impact of replacing single- with double-glazed windows was studied by Eskander et al. in [41,43–49] and [50,51], respectively. It is estimated that replacing single- with double- [47], who verified glazedthat the windows heat transferred is an investment through that the can windows cost between could 200 be and lowered 1500 EUR/unit, to near whereas 40% following thethe retrofit. investment Eicker in et retrofit al. [52] measures analyzed of the the opaque impact walls of replacing with traditional glazing External sys- Thermal tems with ones withInsulation lower total Composite solar Systemsenergy transmittance (ETICSs) is between (g-value), 30 and a 60parameter EUR/m2 .that The is measurable also known as theimpact solar offactor. replacing According single- with to [53], double-glazed the g-value windows represents was studied the quotient by Eskander be- et al. [47], tween the energy whotransmitted verified that to the the interior heat transferred through through a glazed the gap windows and the could energy be lowered of the to near 40% solar radiation thatfollowing affects it the. The retrofit. researchers Eicker et concluded al. [52] analyzed that the the cooling impact of energy replacing needs glazing of systems a residential buildingwith onesin Almada with lower (a city total and solar a energymunicipality transmittance in Portugal (g-value), located a parameter on the that is also known as the solar factor. According to [53], the g-value represents the quotient between the southern margin of the Tagus River from Lisbon) could be lowered to 11% if single-glazed energy transmitted to the interior through a glazed gap and the energy of the solar radiation windows with a gthat-value affects of 0.8 it. were The researchersreplaced with concluded double-glazed that the cooling windows energy with needs a g-value of a residential of 0.51 [52]. building in Almada (a city and a municipality in Portugal located on the southern margin Other studiesof considered the Tagus River the fromreplacement Lisbon) could of single-glazed be lowered to 11%windows if single-glazed with double- windows with a glazed and low-emissiveg-value of(e-low) 0.8 were glazing replaced windows with double-glazed [42,54]. The windows literature with pointed a g-value out of that 0.51 [52]. e-low glazing is a goodOther option studies for a considered mild-temperate the replacement climate (Portugal of single-glazed mainland) windows because with double- it provides good thermalglazed and insulation low-emissive during (e-low) wintertime glazing windows and reduces [42,54 the]. The entrance literature of pointed heat out that during summertime,e-low allowing glazing isthe a goodpassage option of visible for a mild-temperate light for daylight climate [38,55]. (Portugal mainland) because it provides good thermal insulation during wintertime and reduces the entrance of heat As stated by Moretti and Belloni [56], to encourage visual comfort and daylighting during summertime, allowing the passage of visible light for daylight [38,55]. for energy savings, nonresidentialAs stated by Moretti building ands Bellonitend to [56 have], to encouragehigh window-to-wall visual comfort ratios. and daylighting However, this designfor energy option savings, may also nonresidential lead to an increase buildings in tend energy to have demand high window-to-wallfor heating ratios. or cooling purposes.However, For that this reason, design in option recent may years, also many lead to studies an increase have in been energy devised demand on for heating glazing retrofit withor cooling Solar Control purposes. Films For that (SCFs) reason, [57–59]. in recent In years,Portugal, many Teixeira studies haveet al. been [59] devised on studied an office buildingglazing retrofit with large with Solarglazed Control areas Filmsin Lisbon (SCFs) and [57 concluded–59]. In Portugal, that SCFs Teixeira can et al. [59] be a suitable retrofitstudied measure an office as they building can withreduce large solar glazed heat areas gains, in Lisbon the peak and demand concluded load that SCFs can and the annual energybe a suitable consumption. retrofit measure These find as theyings can are reduce pertinent solar because, heat gains, as theCausone peak demand et load and the annual energy consumption. These findings are pertinent because, as Causone al. [60] pointed out, large glazed surfaces can cause a significant increase of cooling loads et al. [60] pointed out, large glazed surfaces can cause a significant increase of cooling loads in warm climates indue warm to the climates increased due tosolar the increasedheat gain. solar Furthermore, heat gain. Furthermore, Pereira et al. Pereira [57] also et al. [57] also studied the applicationstudied of the SCF application in the Lisbon of SCF context in the Lisbon and verified context that and verifiedthis retrofit that thissolution retrofit solution has the potential tohas promote the potential daylight to promote availability daylight while availability limiting while the visual limiting discomfort the visual discomfort in in office buildings (Figureoffice buildings 4). (Figure4).

(a) (b)

FigureFigure 4. Glazing 4. Glazing retroﬁt retrofit with Solar with Control Solar Films:Control (a) Films: small-scale (a) small-scale model (1:10); model (b) standard (1:10); (b structure) standard with struc- an internal applicationture with on the an glass internal surface application [57]. on the glass surface [57].

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3.2. Sun Shading Technologies 3.2. Sun Shading TechnologiesTechnologies Portuguese cities such as Évora and Beja are characterized by very hot summers, with Portuguese cities such as Évora and Beja are characterized by very hot summers, with averagePortuguese temperatures cities ofsuch 30.2 as CÉvora and and 32.6 Beja◦C, are respectively, characterized while by very Lisbon hot hassummers, an average with average temperatures of 30.2 C and 32.6 °C, respectively, while Lisbon has an average maximumaverage temperatures temperature of 27.430.2 ◦CC and in July 32.6 [ 61°C,,62 respectively,]. Thermal discomfort while Lisbon due has to overheating, an average maximum temperature of 27.4 °C in July [61,62]. Thermal discomfort due to overheating, duringmaximum the cooling temperature season, of is 27.4 often °C pointedin July [61,62]. as a key Thermal issue, as discomfort shown in [due63]. to For overheating, this reason, during the cooling season, is often pointed as a key issue, as shown in [63]. For this reason, theduring control the ofcooling solar heatingseason, is gains often is pointed a key retrofit as a key measure issue, as in shown the Portuguese in [63]. For contextthis reason, [64]. the control of solar heating gains is a key retrofit measure in the Portuguese context [64]. Thethe mostcontrol effective of solar way heating to control gains is solar a key heat retrofit gain measure is by intercepting in the Portuguese it before context it reaches [64]. The most effective way to control solar heat gain is by intercepting it before it reaches the theThe building most effective envelope way while to control ensuring solar that heat the gain strategy is by intercepting does not interfere it before with it reaches solar gains the building envelope while ensuring that the strategy does not interfere with solar gains dur- duringbuilding wintertime envelope when while it ensuring is required that [65 the]. strategy This control does can not be interfere achieved with using solar sun gains shading during wintertime when it is required [65]. This control can be achieved using sun shading devices,ing wintertime as shown when in [66 it, 67is ].required This solar [65]. technology This control is often can be considered achieved andusing recommended sun shading indevices, building as retrofit shown studies,inin [66,67].[66,67]. as ThisThis shown solar solar in tec [tec42hnology,hnology68,69]. is is often often considered considered and and recommended recommended in buildingCapeluto retrofitretrofit et al. studies, [studies,70] analyzed asas shownshown the in retrofitin [42,68,69]. [42,68,69]. of façades in multiple European cities and concludedCapeluto that, et when al.al. [70][70] a singleanalyzedanalyzed strategy thethe retrofitretrofit was chosenof of façades façades to retrofitin in multiple multiple a south-facing European European cities façade cities and and in Porto,concluded sun shading that, whenwhen was theaa singlesingle best option.strategystrategy The waswas study chosenchosen also to to showed retrofit retrofit that,a asouth-facing south-facing even when façade multiplefaçade in in strategiesPorto, sun are shading chosen, waswas sun thethe shading bestbest option.option. appears The The twice st studyudy in thealso also top showed showed three that, combined that, even even when measures, when multiple multiple first combinedstrategies with are chosen, glazing sunsun and shadingshading in third appears appears combined twic twic withee in in the ventilation.the top top three three combined combined measures, measures, first first combinedStudies with on the glazingglazing individual andand inin impact thirdthird combined combined of sun shading with with ventilation. retrofittingventilation. in the Portuguese context are scarce.Studies It appears on the individualindividual from the literatureimpact impact of of that,sun sun shad withshading verying retrofitting retrofitting few exceptions, in in the the Portuguese thisPortuguese retrofit context optioncontext wasare alwaysscarce. It considered appears fromfrom in conjunction thethe literature literature with that, that, otherwith with very retrofitvery few few measures. exceptions exceptions In, this, otherthis retrofit retrofit words, option option the specificwas always measurable considered impact inin conjunctionconjunction of sun shading withwith retrofitot otherher retrofit wasretrofit rarely measures. measures. presented In In other other and words, discussed.words, the the Onespecific of the measurable exceptions impact wasimpact presented ofof sunsun shading byshading Pereira retrofit retrofit Tavares was was et rarely al.rarely [38 ],presented presented who assessed and and discussed. the discussed. impact ofOne adding of the horizontal exceptionsbrise-soleils waswas presentedpresentedof different byby sizesPePereirareira to housing TavaresTavares unitset et al. al. in[38], [38], Lisbon who who (Figure assessed assessed5). Thethe the authorsimpact of concluded adding horizontalhorizontal that by adding brise-soleilsbrise-soleils horizontal of of different different brise-soleils sizes sizes to to housing housing to buildings units units inwith in Lisbon Lisbon large (Figure (Figure glazing 5). 5). areas,The authors the cooling concluded energy thatthat needs by by adding adding could behorizontal horizontal lowered brise-soleils bybrise-soleils up to 55%, to to buildings depending buildings with with on large the large façade glaz- glaz- orientationing areas, the and coolingcooling the type energyenergy of glass needsneeds of the couldcould window. bebe lowered lowered The study by by up alsoup to to showed55%, 55%, depending depending that the greateron on the the thefaçade width orientation of the used andand shading thethe typetype devices, ofof glassglass theofof the the lower window. window. the influence The The study study of also thealso showed type showed of that glass that the ofthe thegreater window. the width ofof thethe usedused shadingshading devices, devices, the the lower lower the the influence influence of of the the type type of of glass glass of the window.

Figure 5. Sun shading retrofit with horizontal brise-soleils with different sizes [38]. FigureFigure 5. 5.Sun Sun shading shading retroﬁt retrofit with with horizontal horizontal brise-soleils brise-soleils with with different different sizes sizes [38 [38].]. Another exception was presented by Palmero-Marrero et al. [71], who analyzed the AnotherAnother exception exception was was presented presented by by Palmero-Marrero Palmero-Marrero et et al. al. [ 71[71],], who who analyzed analyzed the the effect of louver shading devices on building energy requirements of multiple cities across effecteffect of of louver louver shading shading devices devices on on building building energy energy requirements requirements of of multiple multiple cities cities across across the world, including Lisbon. The authors showed that 50% of the total energy consump- thethe world, world, including including Lisbon. Lisbon. The The authors authors showed showed that that 50% 50% of the of the total total energy energy consumption consump- couldtion could be saved be saved by using by using louvers louvers (Figure (Figur6), comparede 6), compared with no with shading no shading case. Nevertheless, case. Never- tion could be saved by using louvers (Figure 6), compared with no shading case. Never- boththeless, studies both pointed studies thatpointed adding that shading adding devicesshadingcan devices lead can to higher lead to energy higher expenditure energy ex- theless, both studies pointed that adding shading devices can lead to higher energy ex- duringpenditure the heatingduring the season. heating season. penditure during the heating season.

Figure 6. Sun shading retrofit with louvers (horizontal layout in a south façade and vertical layout FigureFigurein east 6. or6.Sun Sunwest shading shading façades) retroﬁt retrofit [71]. with with louvers louvers (horizontal (horizontal layout layout in ain south a south façade façade and and vertical vertical layout layout in in east or west façades) [71]. east or west façades) [71].

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Regarding the design of sun shading, the literature emphasized several aspects. The adaptabilityRegarding should the design be ofweighed, sun shading, namely the literature if the shading emphasized device several is fixed aspects. or movable. The The adaptabilityaesthetics and should handling be weighed, of shading namely devices if the shading should devicealso be is taken ﬁxed into or movable. account Thebecause they aestheticsare often and pointed handling as ofkey shading factors devices for users should [72,73]. also be As taken shading into account devices because can reduce they natural arelight, often designers pointed as should key factors assess for the users balance [72,73 ].be Astween shading the devicesenergy cansavings reduce from natural cooling (due light,to shading designers devices) should assessand the the energy balance co betweennsumption the energy due to savings artificial from light. cooling (due to shading devices) and the energy consumption due to artiﬁcial light.

3.3.3.3. Sunspace Sunspace Technologies Technologies SunspaceSunspace is is a solara solar technology technology designed designed to collectto collect heat heat gains. gains. It isIt basedis based on on a a glass- glasshousehouse (Figure (Figure 7)7 that) that absorbs absorbs solar solar radiation radiation and and partially partially converts itit intointo energyenergy for heat- foring heating the indoor the indoor environment environment [74–77]. [74–77 ].

FigureFigure 7. 7.Illustration Illustration of a of sunspace a sunspace [77]. [77].

ItIt recreates recreates on aon small a small scale scale the macroscopic the macrosco phenomenonpic phenomenon of the greenhouse of the greenhouse effect effect thatthat happens happens between between the Earththe Earth and theand atmosphere, the atmosphere, and it worksand it as works a buffer as zonea buffer against zone against heatheat losses losses toward toward the the outdoor outdoor environment environment [74,75 ].[74,75]. Portuguese Portuguese sunspaces sunspaces are usually are usually south oriented, with single glazing and a high glazing to floor surface ratio (between 60% south oriented, with single glazing and a high glazing to floor surface ratio (between 60% and 250%) [78]. andCase 250%) studies [78]. on Portuguese retrofitted sunspaces are scarce, even though it is common to closeCase existing studies balconies on Portuguese so that they retrofitted can perform sunspaces as sunspaces. are scarce, Such an even example though was it is com- studiedmon to by close Macieira existing et al. [balconies79], who proposed so that they the use can of perform transparent as membranesunspaces. screens Such an to example closewas existing studied balconies by Macieira (Figure et8a). al. To [79], optimize who theproposed thermal performance,the use of transparent the proposed membrane retrofitscreens solution to close can existing be completely balconies open (Figure all day 8a). or nightTo optimize time, and the it canthermal also integrateperformance, the anproposed additional retrofit opaque/reflective solution can membrane be completely to work open as a verticalall day shade or night (with time, adjustable and it can also height),integrate as shown an additional in Figure opaque/reflective8b. As the proposed membrane retrofit solution to work uses as membranes a vertical of shade PVC (with ad- or EFTE as an alternative to conventional glazing, the technology was named Membrane justable height), as shown in Figure 8b. As the proposed retrofit solution uses membranes Alternative Sunspace (MAS). The study concluded that MAS is an efficient alternative to traditionalof PVC or sunspaces, EFTE as according an alternative to constructive, to conventional environmental glazing, and economicthe technology aspects. was named Membrane Alternative Sunspace (MAS). The study concluded that MAS is an efficient alternative to traditional sunspaces, according to constructive, environmental and economic aspects.

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Energies 2021, 14, x FOR PEER REVIEW 8 of 20

(a) (a) (b(b) )

FigureFigure 8. FigureRetrofitted 8. Retrofitted 8. Retroﬁtted sunspace sunspace sunspace during during th during the daytimee daytime the daytime period periodperiod in the: in((aa) the:)heating heating (a) heatingseason; season; season;(b )( bcooling) cooling (b) cooling seasonseason withseason withan open with an open an(adjustable) open (adjustable) (adjustable) membrane membrane membrane [79]. [79]. [79].

AeleneiAelenei Aeleneiet al. et [80] al. et[80] studied al. studied [80] the studied the use use of theof different different use of differentattached-sunspaceattached-sunspace attached-sunspace configurations configurations configurations (Figure (Figure 9 9 in retrofittingin retrofitting(Figure design9) designin retrofitting and and concluded concluded design that and that concludedfu fullylly integrated integrated that fully sunspacessunspaces integrated led led sunspaces to to better-perform- better-perform- led to better- ing resultsing resultsperforming because because resultsthey they have because have a smallera smaller they have glazing glazing a smaller areaarea glazingin contactcontact area with with in contactthe the external external with environ- the environ- external environment. The study also pointed that sunspaces may be a very efficient solution in ment.ment. The Thestudy study also also pointed pointed that that sunspaces sunspaces ma mayy bebe a very efficient efficient solution solution in inthe the south south of mainlandthe south Portugal of mainland (e.g., Faro Portugal region), (e.g., as Faroit evidenced region), a aspotential it evidenced of 100% a potentialenergy saving of 100% of mainland Portugal (e.g., Faro region), as it evidenced a potential of 100% energy saving duringenergy wintertime. saving during However, wintertime. in locations However, with more in locations severe winters, with more such severe as Bragança, winters, suchthe as duringpotential wintertime.Bragança, energy the However, savings potential is in energylowered locations savings to 48%. with is Furthermore, lowered more severe to 48%. sunspaces winters, Furthermore, suchmight as sunspacesbe Bragança, a very at- might the potentialtractivebe energy a solution very attractivesavings to retrofit is solution lowered dwellings to retrofitto built 48%. dwellingsbe Furthermore,fore the built1990s before whensunspacesthe compared 1990s might when to retrofitbe compared a very ap- at- to tractiveproaches solutionretrofit based approaches to retrofiton thermal based dwellings insulation on thermal built dueinsulation be tofore the thegood due 1990s solar to the whenpotential good compared solar in Portugal potential to retrofit and in Portugal the ap- proacheseconomicand based the benefits economicon thermal of sunspace benefits insulation ofsystems sunspace due [80]. to systems the good [80 ].solar potential in Portugal and the economic benefits of sunspace systems [80].

(a) (b) (c) (d)

FigureFigure 9. Types 9. Typesof sunspaces: of sunspaces: (a) adjacent; (a) adjacent;(b) adjacent; (b) (c adjacent;) partially (integrated;c) partially (d integrated;) totally inte- (d) totally (a) (b) (c) (d) gratedintegrated [80]. [80]. Figure 9. Types of sunspaces: (a) adjacent; (b) adjacent; (c) partially integrated; (d) totally inte- 3.4. Trombe Wall Technologies grated3.4. [80]. Trombe Wall Technologies TrombeTrombe wall, wall, also also known known as a as storage a storage wall wall or ora solar a solar heating heating wall, wall, is isa asolar solar technol- technology 3.4. Trombeogy mainlymainly Wall designeddesignedTechnologies to collect collect heat heat gains. gains. It Itconsists consists of ofan anexternal external transparent transparent layer layer in in frontfront of the of external the external wall (known wall (known as a massive as a massive wall), with wall), a small with air a smallcavity air between cavity them. between TheTrombe them.system wall, Theworks also system by known absorbing works as by asolar storage absorbing energy wall through solar or a energy solar the external heating through transparent wall, the external is a skin,solar transparent which technology mainlyresultsskin, in designed a which buffer results effect to collect in in the a bufferair heat cavity, effectgains. wher in Ite the theconsists air heat cavity, is ofstored wherean externalduring the the heat transparent daytime is stored [81–83]. during layerIn thein frontthe of thehoursdaytime external following[81– 83wall]. the In (known the sunset, hours as the followinga massiveenergy theis wall), transmitted sunset, with the a energyas small heat isairto transmitted thecavity indoor between asenviron- heat them. to the The systemment,indoor which works environment, enhances by absorbing the which ind solaroor enhances air energy temperature the through indoor [81–83]. airthe temperature external transparent [81–83]. skin, which results in aAccording bufferAccording effect to the toin theventilationthe ventilationair cavity, mode modewher of ethe of the the air heat aircavity cavityis stored of ofthe the duringTrombe Trombe the wall wall daytime system, system, [81–83]. it it can can In be the hoursbe classifiedclassified following as as nonventilated nonventilated the sunset, (orthe (or unvented), unvented),energy is ventilatedtransmitted ventilated or or double doubleas heat ventilated. ventilated. to the indoor The The ventila- ventilation environ- mode relies on the existence of vents (ventilation openings) on the external transparent ment,tion which mode enhances relies on the existenceindoor air of temperaturevents (ventilation [81–83]. openings) on the external transparent skin or thethe massivemassive wall.wall. WhenWhen thethe TrombeTrombe wall wall is is designed designed without without vents, vents, or or the the vents Accordingare closed, to the the ventilation system is classified mode of as the unvented. air cavity If theof the Trombe Trombe wall wall has (open)system, vents it can in be classifiedvents are as closed, nonventilated the system (or is classified unvented), as unvented. ventilated If theor doubleTrombe ventilated. wall has (open) The ventsventila- in oneone layer layer of of the the system system (e.g., (e.g., external external transp transparentarent skin skin or or massive massive wall), wall), it it is is classified classified as tion mode relies on the existence of vents (ventilation openings) on the external transpar- as ventilated.ventilated. When When the the Trombe Trombe wall wall has has (ope (open)n) vents vents in in different different two two layers layers of of the the sys- system, ent skintem, namely ornamely the onmassive on the the external external wall. transparent Whentransparent the skinTrombe skin and and on wallon the the is massive massive designed wall, wall, without the the system system vents, is is classified clas- or the ventssified are closed, as double the ventilated. system is classifiedEach one of as the unvented. described If ventilationthe Trombe modes wall ofhas the (open) Trombe vents in onewall layer is illustrated of the system in Figure (e.g., 10. external transparent skin or massive wall), it is classified as ventilated. When the Trombe wall has (open) vents in different two layers of the system, namely on the external transparent skin and on the massive wall, the system is classified as double ventilated. Each one of the described ventilation modes of the Trombe

wall is illustrated in Figure 10.

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Energies 2021, 14, x FOR PEER REVIEWasdouble ventilated. Each one of the described ventilation modes of the Trombe9 of wall 20 is illustrated in Figure 10.

(a) (b) (c)

FigureFigure 10.10. ConfigurationConfiguration of of different different Trombe Trombe walls: walls: (a) nonventilated (a) nonventilated or unvented; or unvented; (b) classic (b) classicor or ventilated; (c) double ventilated: 1—shading device; 2—ventilation openings in the massive wall; ventilated; (c) double ventilated: 1—shading device; 2—ventilation openings in the massive wall; 3—glazing; 4—air layer; 5—massive wall; 6—ventilation openings in the glazing [84]. 3—glazing; 4—air layer; 5—massive wall; 6—ventilation openings in the glazing [84]. Trombe wall technologies are not commonly used to retrofit residential buildings in Trombe wall technologies are not commonly used to retrofit residential buildings in Portugal, perhaps due to the lack of information among Portuguese building designers Portugal, perhaps due to the lack of information among Portuguese building designers on on the application and efficiency of this constructive solution [85]. Nevertheless, among the application and efficiency of this constructive solution [85]. Nevertheless, among solar solar technologies, a Trombe wall seems to be a suitable retrofit solution for residential technologies, a Trombe wall seems to be a suitable retrofit solution for residential buildings buildings due to its simple configuration, zero running costs, among other reasons [86,87]. due to its simple configuration, zero running costs, among other reasons [86,87]. As Trombe wall systems are not commonly used in Portugal, studies on their appli- cationAs and Trombe impact wall are systems also scarce. are The not commonlystudies developed used in by Portugal, Briga-Sá studieset al. [84,85,88] on their are appli- cationone of andthe few impact exceptions. are also With scarce. regard The to studies the performance, developed the by researchers Briga-Sá et concluded al. [84,85 ,that88] are onethe ofintegration the few exceptions.of a Trombe Withwall is regard beneficial to the for performance, the performance the of researchers Portuguese concluded buildings, that thegiven integration the fact that of the a Trombe energy wallneeds is for beneficial heating can for be the reduced performance up to approximately of Portuguese 16% buildings,[85]. givenOther thekey factfindings that thehave energy also been needs presen for heatingted in the can literature be reduced with up regard to approximately to the ap- 16%plication [85]. of Other Trombe key wall findings systems have in alsoEuropean been presentedlocations with in the Mediterranean literature with climates. regard For to the applicationexample, the of optimized Trombe wall design systems of a Trombe in European wall is often locations a double-glazed with Mediterranean layer, as seen climates. in For[33,84] example, and [88–91], the optimized with air layer design depths of a lower Trombe than wall 10 cm, is oftenas seen a double-glazedin [88,89,92]. In addi- layer, as seention, induring [33,84 summertime,88–91], with is airstrongly layer depthsrecommended lower thanthe use 10 of cm, sun as shading seen in [to88 counter,89,92]. In addition,overheating. during This summertime statement was is referred strongly to recommended both in Portuguese the use studies of sun [84,90] shading and studies to counter overheating.devised on other This statementEuropean wasregions referred with toMedi bothterranean in Portuguese climates studies [89,93]. [84 For,90 ]example, and studies devisedBriga-Sá onet al. other [84] Europeannoticed that regions the outer with surface Mediterranean of the Trombe climates wall can [89 ,93reach]. For tempera- example, Briga-Stures aboveá et al. 60 [ 84°C] when noticed it is that unscreened the outer but surface under of 30 the °C Trombe when it wall is shaded. can reach temperatures aboveIn 60 recent◦C when years, it the is unscreened literature also but presented under 30 some◦C when innovative it is shaded. Trombe wall technologies Inconsidering recent years, the thePortuguese literature context. also presented Sacht et someal. [90] innovative investigated Trombe the performance wall technologies of consideringa modular Trombe the Portuguese wall (Figure context. 11) for Sacht building et al. retrofit [90] investigated under different the Portuguese performance cli- of a modularmates. The Trombe proposed wall retrofit (Figure solution 11) for buildingis innovative retrofit for two under reasons. different Firstly, Portuguese it is based climates. on Themultiple proposed modules retrofit of a solution Trombe iswall, innovative instead forof a two single reasons. zone, Firstly,and modules it is based are normally on multiple modulesused in active of a Trombesystem devices wall, instead (e.g., PV of modu a singleles). zone, In addition, and modules the study are showed normally that, used re- in activegardless system of the devices location, (e.g., when PV the modules). modular In Trombe addition, wall the was study considered, showed the that, heating regardless en- of theergy location, demand when met the the Portuguese modular Trombe building wall code was requirements. considered, the heating energy demand met the Portuguese building code requirements. Another innovative technology within Trombe wall systems was presented in the literature by Aelenei et al. [94]. They proposed a complementary solution to a traditional External Thermal Insulation Composite System (ETICS), which targets thermal bridge areas with the intent to mitigate winter heat losses (Figure 12). The proposed solution relies on the principle of unvented Trombe walls, but it considers the external transparent skin in front of a structural thermal bridge area (boundary between concrete columns and masonry walls). The design of this innovative concept, named Solar Bridge Retrofit System (SBRS), enables heavy structures of the building (e.g., concrete columns) to absorb solar radiation and conduct the heat slowly inward or to the adjacent structure [94].

Energies 2021, 14, x FOR PEER REVIEW 10 of 20

Energies 2021, 14, 1774 10 of 18 Energies 2021, 14, x FOR PEER REVIEW 10 of 20

Figure 11. Trombe wall technology: modular Trombe wall. Reproduced with permission from [90].

Another innovative technology within Trombe wall systems was presented in the literature by Aelenei et al. [94]. They proposed a complementary solution to a traditional External Thermal Insulation Composite System (ETICS), which targets thermal bridge areas with the intent to mitigate winter heat losses (Figure 12). The proposed solution relies on the principle of unvented Trombe walls, but it considers the external transparent skin in front of a structural thermal bridge area (boundary between concrete columns and masonry walls). The design of this innovative concept, named Solar Bridge Retrofit System (SBRS),Figure enables11. Trombe heavy wall structurestechnology: of modular the buildin Trombeg (e.g., wall. concreteReproduced columns) with permission to absorb from solar Figure 11. Trombe wall technology: modular Trombe wall. Reproduced with permission from [90]. radiation[90]. and conduct the heat slowly inward or to the adjacent structure [94].

Another innovative technology within Trombe wall systems was presented in the literature by Aelenei et al. [94]. They proposed a complementary solution to a traditional External Thermal Insulation Composite System (ETICS), which targets thermal bridge areas with the intent to mitigate winter heat losses (Figure 12). The proposed solution relies on the principle of unvented Trombe walls, but it considers the external transparent skin in front of a structural thermal bridge area (boundary between concrete columns and masonry walls). The design of this innovative concept, named Solar Bridge Retrofit System (SBRS), enables heavy structures of the building (e.g., concrete columns) to absorb solar radiation and conduct the heat slowly inward or to the adjacent structure [94].

(a) (b) Figure 12. Conceptual drawing of the Solar Bridge Retrofit System (SBRS) designed to correct the Figure 12. Conceptual drawing of the Solar Bridge Retroﬁt System (SBRS) designed to correct thermal bridges introduced by concrete columns: (a) Horizontal cross-section; (b) 3D View (right- the thermal bridges introduced by concrete columns: (a) Horizontal cross-section; (b) 3D View hand side): 1—external transparent skin (glazing); 2—External Thermal Insulation Composite Sys- tem(right-hand (ETICS); side): 3—frame 1—external of the glazing; transparent 4—air skin cavi (glazing);ty of the 2—External SBRS; 5—masonry Thermal wall; Insulation 6—concrete Composite columnSystem [94]. (ETICS); 3—frame of the glazing; 4—air cavity of the SBRS; 5—masonry wall; 6—concrete column [94]. Table 2 summarizes the main findings reported in the literature regarding the re- viewedTable solar2 summarizes passive technologies. the main ﬁndings reported in the literature regarding the reviewed solar passive technologies. (a) (b) Figure 12. Conceptual drawing of the Solar Bridge Retrofit System (SBRS) designed to correct the thermal bridges introduced by concrete columns: (a) Horizontal cross-section; (b) 3D View (right- hand side): 1—external transparent skin (glazing); 2—External Thermal Insulation Composite Sys- tem (ETICS); 3—frame of the glazing; 4—air cavity of the SBRS; 5—masonry wall; 6—concrete column [94].

Table 2 summarizes the main findings reported in the literature regarding the reviewed solar passive technologies.

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Table 2. Literature review matrix of the reviewed studies on solar technologies used to building retroﬁt Portuguese buildings.

Solar Passive Technology Location Type of Study Type of Building Energy Analysis General Findings: General Findings: Ref. Benefits Drawbacks and Recommendations Multiple locations, Numerical Residential [52] Glazing technologies I Replacing single-glazed I N.A. I N.A. including Portugal windows with (Almada) double-glazed can lower cooling energy needs to 11%. Portugal (Lisbon) Experimental and Office [59] I Retrofitting large glazing I N.A. I N.A. numerical areas with Solar Control Films can reduce solar heat gains, the peak demand load and the annual energy consumption.

Portugal (Lisbon) Numerical Residential I I I [38] Sun shading technologies Horizontal brise-soleils can N.A. Applied shading devices lower up to 55% of the can lead to higher energy cooling energy needs, expenditure during the depending on the façade heating season. orientation and the type of glass of the window. Multiple locations, Numerical Residential [70] I N.A. I Regarding the retroﬁt of I N.A. including Portugal (Porto) a south face, sun shading is the best single strategy but also appears twice in the top 3 combined strategies (1st shading + glazing; 3rd ventilation + shading). Multiple locations, Numerical Residential [71] I 50% of the total energy I N.A. I Louvers can increase including Portugal consumption could be heating energy needs. (Lisbon) saved by using louvers, compared with no shading.

Portugal (Porto) Experimental and Residential [79] Sunspace technologies I N.A. I Retrofit sunspaces with I Not all intervention numerical membranes (instead of scenarios can be glazing) allow design retrofitted with customization. membranes (although I Compared to glass: (a) membranes are easier and quicker interesting alternatives retrofit with membranes; when the use of glass is (b) easier to repair or limited due to its size, replace membranes weight and/or cost). when damaged; (c) provide protection against material projection in the case of a seismic event). Energies 2021, 14, 1774 12 of 18

Table 2. Cont.

Solar Passive Technology Location Type of Study Type of Building Energy Analysis General Findings: General Findings: Ref. Beneﬁts Drawbacks and Recommendations Portugal (multiple Numerical Residential [80] I Sunspaces are very efﬁcient I Totally integrated I Designing sunspaces locations on the mainland) solutions to south sunspaces perform better with opaque glazing can Portuguese locations as results than adjacent and be a good solution to they can lead to 100% partially integrated avoid overheating energy saving during because they have a during the cooling wintertime. smaller glazing area in season. contact with the external environment. Portugal (Vila Real) Experimental and Test cell [84,85,88] I The integration of a vented I The heat takes near 3 I Cooling season: shading Trombe wall numerical or an unvented Trombe times more to reach the devices are strongly wall in the façade can indoor environment if recommended because decrease the heating there is no ventilation in the outer surface of the energy needs up to nearly the massive wall. Trombe wall can exceed ◦ 16% and 9%, respectively. 60 C when unshaded. If shaded, the values are reduced to 30 ◦C or less. I During the cooling season, it may be necessary to combine a Trombe wall with night ventilation and/or shading devices to improve the overall performance. Portugal (Caparica) Experimental Test cell [94] I N.A. I Trombe wall systems I N.A. applied to structural thermal bridge areas can help promote heat gain (instead of heat losses due to the thermal bridge phenomenon). Portugal (Lisbon, Porto, Numerical Test cell [90] I Trombe wall modules I N.A. I It is also very important Lajes, Funchal) lower the heating energy to consider the cooling needs. energy needs when I Trombe wall in the south considering Trombe façade led to lower heating wall systems. than in east, west and north façades, regardless of the geographic location. Energies 2021, 14, 1774 13 of 18

4. Feasibility of Retrofit Works with Solar Passive Technologies Each building is unique and, thus, it is the nature of its retrofit project, which makes it difficult to define general action guidelines and implies that the optimal solution should be defined for each retrofit case [6]. To facilitate the process of finding the optimal retrofit solutions for each case, the literature presented assessment methodologies on the feasibility of the retrofit works. Those methodologies, such as those presented in [95,96], were designed to assess any kind of retrofit works and not specifically the retrofit works with solar passive technologies. Nevertheless, the performance indicators associated with those methodologies may be used to assess the feasibility of the solar passive technologies listed in Section3 as well. Table3 presents a list of indicators that can help assess the feasibility of retrofit projects with solar passive technologies.

Table 3. Performance indicators that can be used to assess the feasibility of retroﬁt projects with solar passive technologies.

Scope Indicator Unit Solar irradiation on the façade surface kWh/m2/year Solar availability Hours of solar exposure Hours Discomfort period Hours; % Heat gain MJ/m2; kW/m2 Heat transmission (Heat ﬂux) W/m2 Thermal performance and comfort Predicted Mean Vote (PMV) – Relative humidity % Temperature (surface: glazing and massive wall; cavity) ◦C Time lag Hours CO concentration ppm Indoor air quality and acoustics 2 Acoustics −dB; RT60 Cooling energy needs (or cooling energy needs savings) kWh; kWh/year; % Energy efﬁciency Energy storage (and release) Hours Heating energy needs (or heating energy needs savings) kWh; kWh/year; % CO emissions (production and operational phase) kg CO eq. Environment 2 2 Energy demand (production and operational phase) kWh; kWh/m2 Annual savings €;% Internal rate of return (IRR) – Investment costs € Economic Net present value (NPV) – Payback period (PP) Years Savings-to-investment ratio (SIR) –

It should be noted that retrofit measures usually lead to lower energy needs, but that does not always imply a lower global cost [95]. Therefore, studies on the feasibility should not only be based on energy efficiency indicators but also thermal performance and comfort, indoor air quality and acoustics, fire protection and environment and economic indicators. Furthermore, retrofit design projects should also include consideration of the following aspects: • Retrofit conditions—questions such as the disturbance levels to the inhabitants or site and the possibility of working from the inside of the building vs. the need for scaffoldings in high-rise buildings should be holistically weighed, as shown in [97]. • Compatibility between the existing constructive solutions and the proposing retrofit technologies—the retrofit design project should be compatible with the existing solution (e.g., materials properties), and possible consequences should be assessed (e.g., reduction of air permeability and the creation of new thermal bridge areas following the fixation of new constructive systems), as well as how to mitigate them [98]; Energies 2021, 14, 1774 14 of 18

• Impact on cultural heritage—the retroﬁt design project should involve a strong di- chotomy between aesthetic–architectonical value and energy efﬁciency goals, especially in the context of the rehabilitation of historic buildings [99].

5. Conclusions This paper presents an overview of the state-of-art solar passive technologies used to retrofit Portuguese façades. The final remarks and recommendations for future works are presented as follows: 1. Direct solar technologies (glazing and sun shading) are commonly used strategies to thermally retrofit Portuguese façades, and they are often applied together. Their widespread application is assisted by: (a) their simplicity (easy elements to retrofit); (b) the Portuguese regulation that presents design parameters (which facilitates the job of building designers); (c) national financial support programs (incentives). Sun shading is often recommended in the literature, but its specific impact is usually not indicated. 2. There is an apparent scarcity of studies on indirect solar technologies used to retrofit Portuguese buildings. Regarding sunspaces, even though balconies are often con- verted into sunspaces, there are no sufficient available data on the optimization design parameters leading to the upgrade of the energy performance. Regarding Trombe wall systems, most of the studies found were devised in the last decade, meaning that there is a growing interest in this subject. All in all, more detailed studies and research (on optimization design, operation, and efficiency) should be devised on Trombe wall systems to encourage building designers and owners to consider this technology. 3. Studies have shown that solar passive technologies usually lead to energy savings. However, more detailed feasibility studies should be conducted, especially concerning cost and environmental indicators. Specific studies on the feasibility of solar passive technologies compared to conventional retrofit solutions should also be conducted to further increase the knowledge on solar passive technologies in the Portuguese context. 4. Overall, it was concluded that building retrofit with solar passive technologies may be a viable way to (a) improve the thermal comfort and efficiency (more energy savings and lower greenhouse gas emissions) of buildings in the Mediterranean climate context, (b) further improve the technical and scientific knowledge, (c) increase the building value as it updates it toward current living standards and (d) contribute toward achieving greener buildings and cities.

Author Contributions: Conceptualization, S.B.-C. and D.A.; methodology S.B.-C., D.A., M.G.G. and A.M.R.; investigation, S.B.-C.; supervision, D.A., M.G.G. and A.M.R.; writing—original draft, S.B.-C.; writing—review and editing D.A., M.G.G. and A.M.R. All authors have read and agreed to the published version of the manuscript. Funding: This study was supported by the Portuguese Foundation of Science and Technology (FCT), doctoral grant PD/BD/135171/2017, under the Eco Construction and Rehabilitation (Eco-CoRe) doctoral program. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest. Energies 2021, 14, 1774 15 of 18

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