Optimization of glazing systems in Non-Residential buildings: The role of the optical properties of air-conditioned environments

Accepted Manuscript Optimization of glazing systems in non-residential buildings: The role of the optical properties of air-conditioned environments Roberto Bruno PII:

S0360-1323(17)30425-0

DOI:

10.1016/j.buildenv.2017.09.011

Reference:

BAE 5090

To appear in:

Building and Environment

Received Date: 13 April 2017 Revised Date:

7 September 2017

Accepted Date: 8 September 2017

Please cite this article as: Bruno R, Optimization of glazing systems in non-residential buildings: The role of the optical properties of air-conditioned environments, Building and Environment (2017), doi: 10.1016/ j.buildenv.2017.09.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT OPTIMIZATION OF GLAZING SYSTEMS IN NON-RESIDENTIAL BUILDINGS: THE ROLE OF THE OPTICAL PROPERTIES OF AIR-CONDITIONED ENVIRONMENTS Roberto Bruno Mechanical, Energetic and Management Engineering Department – University of Calabria

e-mail: [email protected]

ABSTRACT

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V. P. Bucci 46/c – ZIP 87036 – Arcavacata di Rende (CS) – ITALY

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Highly glazed envelope in non-residential buildings are usually designed to have more access to daylight and to offer better visual comfort. However, large windows allow for the transfer of remarkable heat gains and thermal losses that affect the building energy consumptions. Because the latter represent the majority of operating expenses, in the design phase an appropriate window size and a suitable glass typology have to be identified to achieve energy and economy savings. In this paper, a simplified procedure to determine the optimal glazed system in function of the sensible and latent energy requirements, is proposed. The calculation of the involved energy requirements has been carried out by the quasi-steady procedures described in the international standard ISO 13790, but employing a novel steady-state model to calculate the transmitted solar radiation through windowed surface. An absorption coefficient of the indoor cavity, in fact, was introduced to take into account the fraction of the solar radiation transmitted through the windows that, after mutual internal reflections, escapes outside from the same glazed surfaces. In building envelopes equipped with large glazed surface, this fraction cannot be neglected because it does not become a thermal load for the indoor environment. The same calculations were carried out considering also the air-conditioned volume as “black” to the transmitted solar radiation in order to evaluate the weight of the cavity absorption coefficient on the choice of the glazed system.

keywords: window, solar gains, optical properties, energy savings, quasi-steady procedure

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Nomenclature

Surface area [m²] Reduction factor [-] Operation time factor internal water vapor mass flow rate [gWV/h] total solar energy transmittance or solar factor [-] (from EN ISO 410) specific enthalpy [kWh/kg] Incident solar irradiation [kWh/m²] per day Number of days per month[-] secondary heat flux coefficient [-] Energy demand [kWh] temperature [°C] time constant [h] thermal transmittance [W/m²K]

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Greek symbols

η τ

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ι

absorption coefficient [-] heat balance ratio [-] heat flux [W/m²] incident angle [rad] utilization factor [-] transmission coefficient [-]

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α γ Φ

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beam cooling cavity diffuse external effective frame floor glass gain heating thermal losses internal k-th surface latent mean normal need outdoor air opaque illuminated floor by daylight reflected shading solar visible window exposition correction water vapor curtains deployed

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b C cav d e eff F f gl gn H ht i k lat m n nd oa op p r sh sol vis w W wv with

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Subscripts

1. Introduction

The residential and non-residential sectors are responsible of a remarkable part of the global energy consumptions [1-4]; the majority of this energy is required to satisfy heating, cooling, domestic hot water and illumination needs [5]. In non-residential buildings, energy consumptions represent the largest part of the operating expenses, therefore a consistent reduction is achievable by means of accurate design approaches [6, 7]. Energy-efficient buildings are obtained by choosing optimized design variables and construction parameters affecting the energy consumptions [8, 9]. Among the several design parameters of non-residential buildings, in [10, 11] the transparent surfaces, the orientation and the thermos-physical properties of the involved materials, are mentioned. Windows have a great influence on building energy performances, by participating directly to the global energy balance of indoor environments by means of

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thermal and solar transmittances. Moreover, in non-residential buildings also electric needs for lighting are affected by windowed surface area [12]. Usually, simulation codes are used for energy performances evaluations, but require lengthy inputs, graphic interface difficult to understand and especially are not indicated for design evaluations. Contrarily, simplified models are easier to use and they allow for design evaluations, though they do not consider in detail the actual response of the building-plant system to the external forcings. In literature, simplified quasi-steady model for the evaluation of heating and cooling energy requirements are available [13] and the same approach has been adopted by international reference standards [14,15]. Therefore, designers can employ common standardized calculation methods that, despite a certain degree of approximation, can be used as alternative to complex dynamic simulation codes providing reliable results. Moreover, the standardized procedures are usually employed to define the energy label of residential and non-residential buildings [16]. Nowadays, thermal performances of windowed systems have been investigated especially for continental climatic context [17-19] where the role of thermal losses prevails on the solar gains; in warm climatic context and in presence of large glazed building envelopes, the opposite occurs. In Mediterranean climatic context, cooling energy performances of windows have been investigated for residential and non–residential buildings [20, 21]. A Window Energy Rating System (WERS) was developed in Spain considering thermal losses, solar gains and latent loads due to indoor air-change [22]. The Canadian Standard Association proposed a specific WERS for residential buildings [23], whereas other WERS were formulated for US [24], Australia [25], New Zealand [26] and Denmark [27]. In Italy, a WERS developed only for residential buildings is available, neglecting the role of lighting [28]. In this paper, simplified procedures to determine the monthly energy demands for heating, cooling, and latent needs, are presented. The heating and cooling energy requirements were evaluated by a quasisteady approach that uses the concept of “utilization factors” applied to the steady-state monthly energy [15]. With reference to the transmitted solar radiation through the windows, the calculation procedure described in the standard ISO 13790 considers the indoor environment as a “black body”, therefore all the incoming solar gains are involved in the calculation of thermal energy requirements. However, in presence of large glazed surface, usually employed in non-residential buildings to achieve better visual comfort, a fraction of the transmitted solar radiation is reflected by inner surface, escaping newly from the same glazed system. This fraction cannot be neglected because it does not became a thermal load for the airconditioned environment, producing substantial deviations in the calculation of heating and cooling requirements. In order to achieve precise results in the evaluation of heating and cooling demands, an absorption coefficient of the indoor environment, or cavity absorption coefficient, is proposed [29]. This coefficient is employed in a novel steady-state procedure for the evaluation of the solar gains transmitted through transparent surfaces that actually remain in the air-conditioned volume, to employ in the quasisteady model for the calculation of the thermal energy requirements, obtaining results close to those determined by transient simulation codes [30]. The seasonal energy demands for heating, cooling and the latent needs have been determined as sum of the monthly values. Regarding a non-residential building, the optimal glazed system has been identified by varying WWR (Window to vertical Wall area Ratio) and the typology of glass. By means of a parametric study, the optimal building envelope configuration that minimizes the global thermal energy requirements, was recognized. For a better consideration on the role of the cavity absorption coefficient, the reference building was located in the Mediterranean area, where solar gains are a key element for the determination of heating and cooling requirements. Moreover, the weight of the cavity absorption coefficient on the windows choice was evaluated by comparing the results with those determined supposing the indoor environments as black to the transmitted solar radiation. Finally, in order to identify the optimal glazed surface in function of the dispersing characteristics of the building envelope, two different values of the thermal transmittance of opaque walls, were considered.

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2. Calculation Methodology Regarding the heating and cooling needs, the calculation model is based on a monthly energy balance between thermal losses and heat gains determined in steady-state conditions. Successively, the dynamic effects on the energy needs are taken into account by the gain utilization factors, making the calculation procedure “quasi-steady” [15]:

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QH ,nd = QH ,ht − η H , gn ⋅ QH , gn QC ,nd = (1 − ηC , gn ) ⋅ QC , gn

(1) (2)

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Thermal losses involve transmission and ventilation heat exchanges and both are proportional to the difference temperature between indoor and outdoor air. Utilization factors depend on the building time constant (t) and on the monthly heat balance ratios (γ), defined as ratio between energy gains and thermal losses. Energy gains are determined as sum between endogenous gains and solar gains through opaque and transparent surfaces. For a detailed evaluation of the latter, in order to quantify the actual rate that remains in the indoor environment, the optical behaviour of the indoor environment has to be considered [29, 31]. For this purpose, the following relation to use on monthly basis for the kth windowed surface, is proposed:

Qsol = ∑ Fsh, gl ,k ⋅ (1 − FF ,k ) ⋅ Aw,k ⋅ I sol ,k ⋅ FW ,k ⋅ [τ b ,n ⋅ α cav + qi + τ b ,n ⋅ (1 − α cav ) ⋅ qe ],k ⋅ 86400 ⋅ N k

Fsh,gl is the reduction factor for shading produced by mobile devices; FF is the fraction of the frame surface (set to 20%) on the gross window area Aw,k; Isol,k is the monthly average daily solar irradiation (in kWh/m² per day); Fw,k is the corrective factor of the normal solar factor.; N is the number of days of the month belonging to the heating/cooling period.

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where:

(3)

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The square brackets of Eq. (3) can be assumed as the effective solar factor for normal incidence of the glass (ggl,n,eff), that considers the actual solar radiation that participates to the indoor energy balance. It depends on the beam solar transmission coefficient for normal incidence (τb,n), the secondary heat flux factors linked to the radiative-convective heat exchange of inner and external panes (qi and qe) and the cavity absorption coefficient (αcav). The first term represents the direct optical fraction of solar radiation instantly absorbed by the indoor environment, the last accounts for the glazing absorption of the fraction of solar radiation reflected by internal surfaces, before that it escapes towards outside. Regarding the solar radiation entering through the glazed surface, the cavity absorption coefficient characterises the indoor environment while the effective solar factor characterises its absorption in the system constituted by indoor environment and glazed surfaces. Optical parameters of the glasses could be deduced from data sheets or from the EN 410 Standard [32]. The cavity absorption coefficient is calculated by Eq. (4) and it is function of the ratio of the total windowed surface to the total opaque surface (Aw/Aop) and the optical properties of the indoor environment. The latter involve the mean absorption coefficient of the solar radiation of internal walls (αm) and the solar transmission coefficient for the diffuse component of the glass (τd), considering the transmitted solar radiation completely diffuse for the presence of shading devices [29]:

ACCEPTED MANUSCRIPT α cav = 1 − k1 ⋅ e

 αm [ − k 2 ⋅  AW / Aop 

k

3  ]  

(4) where τd determines the values of three constants k: k1 = 3.500 − 5.453 ⋅ τ d + 4.516 ⋅ τ d2 k 2 = 3.700 − 5.388 ⋅ τ d + 3.462 ⋅ τ d2

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k3 = 0.124 + 0.545 ⋅ τ d − 0.355 ⋅ τ

(5)

2 d

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The hypothesis to consider the presence of internal shading device is mandatory because in non-residential buildings are usually employed to avoid glare effects. For a double clear glass (τd=0.59), a mean absorption coefficient of 0.35 (correspondent to white and smooth internal surfaces) and a ratio Aw/Aop of 0.1 (corresponding usually with a whole glazed wall) the cavity absorption coefficient is about 0.70, therefore the 30% of the entering solar radiation does not participate to the indoor energy balance. Contrarily, for Aw/Aop ratio lower than 0.05 (typical of residential buildings), the role of the cavity absorption coefficient can be neglected. In Fig.1, the trends of the cavity absorption coefficient in function of the ratio AW/Aop, for αm =0.35 and for three types of windowed surfaces equipped with different types of clear glazed system, are reported. 1.00 αcav 0.90

αm=0.35

0.80

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0.70

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0.60 0.50

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0.40 0.30

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0.20 0.10 0.00

0

0.05

0.1

0.15

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Fig. 1 – Cavity absorption coefficient trends in function of the ratio Aw/Aop and for three different types of glazed surfaces, supposing a mean solar absorption coefficient of internal opaque surfaces of 0.35 The monthly values of FW,k appearing in Eq. (3) are determined by the procedure described in [33], in function of the beam (Ib), diffuse (Id) and reflected (Ir) components of the incident solar radiation on the glazed surface, by using the following relation:

ACCEPTED MANUSCRIPT FW ,k =

gb g gl ,n

  gd Ib ⋅ +  ( I b + I d + I r )  g gl ,n

 Id + Ir  ⋅   (Ib + I d + I r ) 

(6)

The values of the normalized beam component of the solar factorg(b g gl ,n ) vary with the time because they depend on the monthly average daily incidence angle (ι ) of the beam component: w gb = u − e[ v⋅ι ] g gl ,n

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(7)

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The monthly average daily values of the incident angle for vertical surfaces in function of the exposure, of the site latitude, of the period of the year and for glazed system equipped with single, double and triple panes, can be found in [33]. The values of the normalized diffuse component of the solar factor (g d g gl ,n) and of the three coefficient u, v, and w, (Tab. 1) are constant and they depend only on the number of panes in the glazed system:

Glazed system Single Double Triple

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Tab. 1 – Coefficients for the evaluation of the solar transmittance correction factor by equation (6) and (7)

gd g gl ,n

u

v

w

0.919 0.877 0.855

2.00 2.05 2.09

0.085 0.180 0.240

4.65 3.15 2.50

Fsh, gl =

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The coefficients Fsh,gl in Eq. (3) take into account the effect of internal movable shading devices on the transmitted solar radiation through windows and it can be calculated by the relation:

(1 − f sh,with ) ⋅ g gl + f sh,with ⋅ g gl ,sh g gl

(8)

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If the incident solar radiation exceeds a set point value, for instance 300 W/m² in accordance to EN ISO 13790 [15], the term fsh,with is a weighted fraction that considers the operating time of solar shadings during daytime. It is calculated as the ratio between the monthly hours with incident solar radiation greater than the set point and the global number of monthly hours with incident solar radiation. The term ggl,sh is the modified solar factor to consider the presence of internal curtains, usually determined as product between the normal solar transmittance and a correction factor depending on the curtain typology. When employed, the ggl value appearing in Eq. (3) corresponds to the product between FW and the square brackets of Eq. (3). In non-residential buildings, HVAC system are often used for mechanical ventilation and humidity level control, therefore latent requirements have to be calculated in the heating and cooling periods. By supposing an all-air plant operating in continuous regime, in design evaluations the water vapor exchanged by ventilation with the external environment and the ventilation losses can be neglected; the incoming air has to remove the water vapor provided by internal sources (people, equipment) and the energy consumption due to endogenous latent loads is [34]:

Qlat = hwv ⋅ Gi ⋅ 86400⋅ N

(8)

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where hwv can be set to 0.707 kWh/kg and N considers only the number of days in the months included in the heating or cooling periods. Finally, the sums of heating, cooling and latent needs allow for the calculation of the annual thermal energy requirements varying WWR, windowed typology and thermal transmittance of opaque walls. The minimal value calculated among the different building envelope configuration allows for the identification of the best windowed system and the correspondent surface. The same calculations concerning the transmitted solar radiation have been repeated by using the standard procedure (EN ISO 13790) in order to evaluate the weight of the cavity absorption coefficient on the choice of the windowed system.

3. Cases study 3.1 The reference building

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The energy evaluations have been carried out on the reference non-residential building shown in Fig. 2. It consists of ten floors above ground, the first with pilotis to insulate the envelope from the ground, and the remaining nine floors with the same size to host open-space offices. The building floor has a rectangular form (10 × 30 m) with a longitudinal development in the east-west direction, an overall height of 33 m with an external stairway in the North façade to avoid the presence of non air-conditioned spaces. The thermal and optical properties of the windowed system considered for the calculation of the annual thermal energy requirements are listed in Tab.2. Windows with a clear single pane (W1), double clear pane (W2), double pane with a glass equipped with a low-emission treatment (W3), a clear triple pane (W4) and a triple pane with low-emission glasses (W5), were considered. The UW values represent the overall thermal transmittance of the windows considering both the frame and the glazed system.

Fig. 2 – Non-residential reference building with WWR of 80%

Tab. 2 – Thermal and optical characteristics of the five considered windowed system

Id. W1 W2 W3 W4 W5 * **

Windows Single pane Double pane Double pane Triple pane Triple pane

Glazed system 4 4/16/4 * 4/16/4 4/8/4/8/4 ** * 4 /16/4/16/4

Gas filled Air Argon Air Argon

panes with low-ε treatment on the inner side of internal pane panes with low-ε treatment on the inner side of external pane

Uw [W/m²K] 5.68 2.83 1.24 2.26 0.70

ggl,n 0.855 0.755 0.642 0.678 0.501

τd 0.749 0.59 0.449 0.475 0.321

τb,n 0.830 0.693 0.516 0.580 0.387

τvis 0.901 0.817 0.760 0.742 0.643

qi 0.025 0.062 0.126 0.098 0.114

qe 0.070 0.119 0.144 0.147 0.228

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Because all the building floors have the same geometrical and optical characteristics, also the cavity absorption coefficient evaluated by Eq. (4) is the same for each floor. The obtained values of the absorption coefficient of the indoor environment, in function of WWR, of the windowed system (τd) and by setting an average absorption coefficient of inner surfaces of 0.35, are listed in Tab. 3.

Tab. 3 – Absorption coefficients αcav and effective solar transmittance for normal incidence of the indoor environment in function of the ratio between transparent and vertical opaque area and for the considered window systems W3 k1 k2 k3 1.962 1.979 0.297 ggl,n,eff αcav 1 0.642 0.979 0.554 0.951 0.544 0.924 0.535 0.899 0.526 0.875 0.518 0.853 0.510 0.832 0.503 0.812 0.496

W4 k1 k2 k3 1.929 1.922 0.303 ggl,n,eff αcav 1 0.678 0.979 0.639 0.949 0.626 0.920 0.613 0.894 0.601 0.869 0.590 0.846 0.580 0.824 0.570 0.804 0.561

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W2 k1 k2 k3 1.855 1.726 0.322 ggl,n,eff αcav 1 0.755 0.974 0.745 0.937 0.723 0.903 0.703 0.872 0.684 0.843 0.667 0.816 0.651 0.791 0.635 0.767 0.621

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WWR 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

W1 k1 k2 k3 1.949 1.607 0.333 ggl,n,eff αcav 1 0.855 0.968 0.832 0.923 0.798 0.882 0.766 0.844 0.737 0.810 0.711 0.778 0.686 0.749 0.664 0.721 0.642

W5 k1 k2 k3 2.215 2.327 0.262 ggl,n,eff αcav 1 0.501 0.983 0.441 0.961 0.436 0.940 0.432 0.920 0.428 0.902 0.424 0.884 0.420 0.868 0.417 0.852 0.414

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The cavity absorption coefficients decrease with the transparent surface growth, therefore the transmitted solar radiation calculated by the standard procedure is strongly overestimated in presence of highly glazed building envelopes. In particular, passing WWR from 0 to 80%, the effective solar factor for normal incident decreases of 21.3, 13.4, 14.6, 11.7 and 8.7 percentage points respectively for the windowed systems W1, W2, W3, W4 and W5. Despite the indoor environment is the same, the results are strongly dissimilar among the considered windowed systems because the different optical properties of the glazed surfaces (τd) determine different amount of solar radiation entering and escaping the indoor environment.

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3.2 Parameters depending on climatic data

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Energy evaluations have been carried out by using climatic data of Rome (Lat. 41.9°N, Long. 12.4 °E); the calculation of the correction factors concerning the normal solar transmittance and the correction factors related to internal shading devices depend on solar radiation data [35]. Beam and diffuse components and the mean incident angle are reported at monthly average daily level in Tab. 4 for vertical surfaces, in function of the exposure and of the month of the year [33, 35]. The normalized beam component of the solar factor appearing in Eq. (7) and the final values of the correction factor FW calculated in accordance to Eq. (6), are listed in Tab. 5. The results show that: -

for the South exposure, the maximum reduction of the transmitted solar radiation through the windows is 21%, 28% and 31% respectively for single, double and triple pane in July; for the East/West exposure, the percentage varies between 10% and 19% passing from single to triple pane in December; for the North exposure, the directional aspects of the beam component of the solar radiation are involved only in summer and the maximum reductions of solar gains are 10%, 15% and 18% for single, double and triple pane.

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In Tab. 6, the values of the weighted fraction fsh,with provided by EN ISO 13790 and the correspondent correction factor for shading produced by internal curtains Fsh,gl, are reported. The latter have been determined assuming the employment of internal white venetian blinds, for which the same standard suggests a reduction factor of 0.45 to calculate the modified solar factor ggl,sh. Unitary correction factors for the shading devices located in the North exposure were detected, therefore these values have not been reported. Tab. 4 –Rome: beam (Ib) and diffuse (Id+Ir) solar radiation (in kWh/m² per day) and incidence angle (ι) on vertical surfaces at monthly average daily level, in function of the exposure and of the month of the year

Id+Ir

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East/West Ib ι 0.52 59.9 0.75 55.5 1.02 51.2 1.45 48.8 1.81 48.9 1.99 49.6 2.20 49.3 1.95 48.6 1.45 49.9 0.94 53.8 0.62 58.6 0.44 61.2

0.60 0.85 1.21 1.58 1.89 2.01 1.97 1.75 1.41 1.02 0.67 0.52

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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

South Ib ι 1.98 41.8 2.18 50.6 2.12 62.2 1.73 69.7 1.24 75.2 0.96 77.8 1.26 76.7 1.79 72.2 2.33 66.0 2.63 54.5 2.14 44.1 1.88 39.4

The effects linked to the reduction of the transmitted solar radiation due to internal curtains can be summarized as follow:

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for the South exposure, solar radiation is reduced from a factor of 0.53 in October and December and a value of 0.69 detected in June; for East exposure, solar radiation is mainly attenuated during summer (minimal factor of 0.59) while in winter the corrector factor is often over 0.65; for the West exposure, the correction factor ranges between 0.60 in summer and 0.84 in November.

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The calculation of heating and cooling demands, in accordance to Eq. (1) and Eq. (2), require the evaluation of the thermal losses through the building envelope and of the energy gains. Transmission losses were determined in function of the thermal transmittance and surface areas of opaque and transparent surfaces. Two different values of the thermal transmittance of the opaque envelope were considered: 0.3 W/m²K for well-insulated envelope (representative of new buildings) and 2 W/m²K for dispersing envelope (representative of existing buildings). Because the thermal losses depend on the outdoor air temperature, in Tab. 7 the values concerning the monthly average daily levels of outside temperature for Rome, are reported [35]. Internal gains consider an endogenous heat flux (φi) of 6 W/m² in addition to solar gains. The utilization factors appearing in equations (1) and (2) are calculated in function of the heat balance ratio and of the time constant of the building: the latter were evaluated considering a monocapacitive model with a specific thermal capacity of the building opaque components set to 155 kJ/m²K. Since time constants vary in function of thermal transmittance and of the surface of walls and windows, in the parametric study different values have been detected in function of the WWR and in function of the

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typologies of windows and opaque walls, providing different length of the heating and cooling periods as well as different values of the utilization factors. Finally, latent requirements have been determined supposing an internal water vapor mass flow rate of 1980 gWV/h [34].

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Tab. 5 – Rome: Values concerning the normalized beam component of the solar transmittance and correction factor of the total solar energy transmittance in function of the exposure and of the number of panes in the glazed system

0.974 0.897 0.747 0.609 0.482 0.413 0.443 0.556 0.682 0.854 0.956 0.988

0.947 0.885 0.786 0.726 0.707 0.712 0.694 0.704 0.747 0.854 0.932 0.959

0.889 0.924 0.949 0.959 0.959 0.956 0.957 0.960 0.954 0.934 0.901 0.877

0.905 0.921 0.933 0.938 0.938 0.937 0.939 0.940 0.937 0.926 0.910 0.900

0.820 0.873 0.915 0.936 0.935 0.929 0.932 0.937 0.926 0.891 0.837 0.802

Tab. 6 –Rome: monthly values of the weighted fraction (fsh,with) and correction factor Fsh,gl in function of the exposure

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

South 0.81 0.82 0.81 0.74 0.62 0.56 0.62 0.76 0.82 0.86 0.84 0.86

fsh,with East 0.52 0.48 0.66 0.71 0.71 0.75 0.74 0.75 0.73 0.72 0.62 0.50

West 0.39 0.55 0.63 0.62 0.64 0.68 0.73 0.72 0.67 0.60 0.30 0.42

South 0.56 0.55 0.56 0.59 0.66 0.69 0.66 0.58 0.55 0.53 0.54 0.53

Fsh,gl East 0.71 0.74 0.64 0.61 0.61 0.59 0.59 0.59 0.60 0.60 0.66 0.73

FW

gb g gl , n

0.851 0.875 0.894 0.905 0.905 0.903 0.906 0.908 0.902 0.884 0.858 0.843

0.782 0.843 0.891 0.916 0.915 0.908 0.911 0.918 0.905 0.862 0.801 0.763

West 0.79 0.70 0.65 0.66 0.65 0.63 0.60 0.60 0.63 0.67 0.84 0.77

FW

gb g gl , n

0.821 0.849 0.872 0.884 0.884 0.881 0.885 0.888 0.881 0.859 0.829 0.813

0.000 0.000 0.000 0.499 0.753 0.825 0.797 0.632 0.158 0.000 0.000 0.000

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0.957 0.908 0.820 0.759 0.735 0.737 0.721 0.736 0.782 0.882 0.946 0.966

FW

gb g gl , n

North Double

Single

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0.981 0.920 0.787 0.652 0.520 0.444 0.477 0.597 0.724 0.883 0.968 0.990

FW

gb g gl , n

Triple

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0.966 0.941 0.886 0.837 0.811 0.808 0.797 0.817 0.858 0.927 0.961 0.971

FW

gb g gl , n

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0.980 0.950 0.867 0.763 0.646 0.574 0.606 0.717 0.821 0.930 0.974 0.985

FW

gb g gl , n

Est/West Double

Single

EP

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

gb g gl , n

Triple

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Month

South Double

Single

FW

gb g gl , n

0.919 0.919 0.919 0.913 0.902 0.903 0.899 0.906 0.918 0.919 0.919 0.919

0.000 0.000 0.000 0.369 0.641 0.731 0.695 0.504 0.067 0.000 0.000 0.000

Triple

FW

gb g gl , n

FW

0.877 0.877 0.877 0.869 0.853 0.853 0.848 0.860 0.876 0.877 0.877 0.877

0.000 0.000 0.000 0.345 0.598 0.688 0.651 0.468 0.091 0.000 0.000 0.000

0.855 0.855 0.855 0.847 0.829 0.827 0.822 0.837 0.854 0.855 0.855 0.855

Tab. 7 – Monthly average daily values of the outdoor air temperature

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Toa [°C] 7.6 8.7 11.4 14.7 18.5 22.9 25.7 25.3 22.4 17.4 12.6 8.9

ACCEPTED MANUSCRIPT 4. Results and Discussion 4.1 Energy performances for a well-insulated opaque envelope

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In Fig. 3, the trends of the seasonal heating requirements of a single floor of the reference building determined with the proposed procedure and for a well-insulated envelope, in function of WWR and of the considered windowed system, are reported. The office building equipped with the windowed systems W3 and W5 provides null heating needs, because the exploitation of these systems determine a building envelope configuration where solar gains prevail on thermal losses. Contrarily, W1, W2 and W4 present heating requirements that increase with the WWR growth, therefore the opposite condition occurs. In Fig. 4, the same trends are reported considering the indoor environment “black” to the transmitted solar radiation (αcav=1). In the latter case, passing from WWR=0.1 to WWR=1, the heating demands are considerably underestimated. In particular, comparing the results with those obtained by the proposed procedure, the windowed system W1 shows deviations that range between 100 kWhth and 5700 kWhth for a single floor, whereas W2 and W4 systems provide deviation varying between 200 kWhth and 1700 kWhth. By supposing an electrical heat pump system to supply the air-conditioning plant with a seasonal coefficient of performance of 3, the involvement of the indoor optical properties in the calculation of the heating needs provides greater electric consumption. For instance, the building equipped with W1 system determines (in continuous operation regime) an extra electric consumption varying between 2700 kWhe (WWR=0.4) and 17000 kWhe (WWR=1) per year referring the whole structure.

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Fig. 3 – Rome: seasonal heating requirements for a single floor of the reference building determined with the proposed procedure, in function of WWR for the considered windowed system in presence of a well-insulated envelope

In Fig. 5 the seasonal cooling requirements of a single floor of the reference building, are reported. As expected, in the considered climatic context the results highlight that cooling demands are more significant than heating needs. Moreover, the effect of the cavity absorption coefficient on the obtained results is more marked than the heating period, comparing the same trends with those reported in Fig. 7, where the standard procedure was applied.

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Fig. 4 – Rome: seasonal heating requirements for a single floor of the reference building determined with the standard procedure, in function of WWR for the considered windowed system in presence of a well-insulated envelope

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In the first case, the windowed system W1 is more indicated for every values of WWR because the summer thermal losses allow for a substantial reduction of the cooling period, despite the highest value of solar transmittance. The systems W4 and W2 provide similar results, however the latter is preferable because it is cheaper than the first one. The cooling requirements determined with the W3 system are greater than those determined for the W5 system, for every WWR value. If cooling needs are calculated without consider the optical properties of the air-conditioned volumes, see Fig. 6, the obtained results are strongly different. The W3 system continues to be the worst choice, whereas W1 is the best but only if the WWR is lower than 0.5. For higher WWR values, in fact, the W4 system becomes more efficient providing the lowest cooling energy requirements. In addition, W5 system becomes more efficient than W1 when WWR is higher than 0.8, while W2 provides similar results to the W4 system, but only if WWR is lower than 0.3. The cooling energy requirements determined for a single floor by the standard procedure are always overestimated: the maximum deviations are observable when WWR=1 and they are about 18000 kWhth, 14500 kWhth, 9500 kWhth, 10500 kWhth and 5000 kWhth respectively for the systems W1, W2, W3, W4 and W5. If the seasonal EER of a hypothetical heat pump employed to satisfy cooling needs is set to 3, the hypothesis to consider “black” the indoor environment to the transmitted solar radiation, determines for the whole building an extra-electricity consumption (in continuous operation regime) varying between 15000 kWhe (W5) and 54000 kWhe (W1) per year. Latent energy requirements for the well-insulated reference building are slightly affected by the cavity absorption coefficient, and reduced variations with the window typology are mainly due to the different length of the cooling period. However, these requirements have been involved in the calculation of the annual thermal energy demands, reported in Fig. 7 and Fig. 8 for the two calculation procedures used to determine transmitted solar gains.

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Fig. 5 – Rome: seasonal cooling requirements for a single floor of the reference building determined with the proposed procedure, in function of WWR for the considered windowed system in presence of a well-insulated envelope

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Fig. 6 – Rome: seasonal cooling requirements for a single floor of the reference building determined with the standard procedure, in function of WWR for the considered windowed system in presence of a well-insulated envelope

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Annual Thermal Energy Requirements (Uop=0.3 W/m²K) 70000 kWhth

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Fig. 7 – Rome: annual thermal energy requirements for a single floor of the reference building determined with the proposed procedure, in function of WWR for the considered windowed system in presence of a well-insulated envelope

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Fig. 8 – Rome: annual thermal energy requirements for a single floor of the reference building determined with the standard procedure, in function of WWR for the considered windowed system in presence of a well-insulated envelope

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− a⋅τ vis ⋅WWR⋅( Ap −2 )

f d = b ⋅ [1 − e

Ap

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By considering the actual optical behavior of the indoor environment, the best building envelope configuration requires the W1 system if WWR is lower than 0.2, successively the same glazed system is not suggested cause the increment of the heating energy requirements that prevail on the reduction of the cooling needs. For WWR greater than 0.2, the W4 system becomes the best solution though its performance are similar to those determined for the W2 system, with WWR lower than 0.6. Contrarily, the W3 system has to be avoided if WWR is lower than 0.4; successively, W1 becomes the most penalized windowed system. Finally, W5 is preferable compared to W2 performances, but only if WWR is greater than 0.9. Contrarily, if the optical properties of the indoor environments are neglected, the W1 system continues to be the best choice with WWR lower than 0.2, but this time W4 becomes more efficient if WWR ranges between 0.2 and 0.75; for higher percentage, W5 is preferable to W4. Moreover, in highly glazed envelope the employment of W1 and W3 have to be still avoided, but the standard procedure provides the W3 system preferable than W2 when WWR is greater than 0.85. However, in every case the annual thermal energy requirements increase with the WWR growth, therefore the adoption of large glazed surfaces seem to be not appropriate due to an excessive increment of the building operative expense. If electricity savings due to daylight have to be achieved in the lighting plant, WWR greater than 0.3 are not appropriate, as indicated in Fig. 9, where the trends of the saved electric energy in function of the windowed surface and for the considered glazing systems, were reported. These results have been obtained using the following relation, as described in [36]:

Ap Af

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where a and b are two coefficients related to the considered locality (16.03 and 72.44 respectively for Rome [36]), τvis is the visible solar transmission coefficient of the glasses (see Tab. 1), Ap is the floor area and the ratio Ap/Af is connected to the day-lit area. For the latter, a unitary value indicates a floor entirely illuminated.

Fraction of electric energy saved by daylight 100

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4.2 Energy performances for a dispersing opaque envelope

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The Fig. 9 suggests that the electric consumptions due to artificial lighting are affected by WWR only in the range 0 - 0.3, while for higher ratios the same consumptions stabilize. Therefore, a WWR of 0.3 minimizes the illumination consumptions allowing for the achievement of suitable visual comfort. With the latter building envelope configuration, the proposed procedure on the transmitted solar gains returns the windowed systems W2 or W4 as more appropriate to minimize also energy consumption concerning the air-conditioning plant. If aesthetic criteria impose the employment of greater WWR, the optimal glazed system have to be identified by the annual thermal energy requirements shown in Fig. 7. If an economic criteria is followed, the W2 system is suggested for WWR lower than 0.6 because its performances are very close to those determined for W4. Contrarily, if WWR is over 0.8 the energy evaluations carried out by the standard procedure provides a different windowed system as the best choice (W5) with an annual thermal energy requirements overestimation of about 6000 kWhth per year for a single floor of the building.

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Regarding the reference building equipped with dispersing opaque walls, the trends concerning heating and cooling demands are completely different. In the first case, by employing the cavity absorption coefficient (Fig. 10), only for W1 the thermal losses prevails on solar gains with the WWR growth, while for W3 and W5 systems the opposite occurs. Interesting trends were determined for W2 and W4: the first provides an optimal WWR in the range 0.4-0.6, for the second the heating needs stabilize if WWR is greater than 0.7. If the WWR is lower than 0.2, W3 and W5 present similar trends. In Fig. 11 the same calculations have been reported by using the standard procedure: the trends are similar but the results are strongly underestimated. By considering WWR=1, deviations greater than 5700 kWhth (W1) and 1650 kWhth (W2 and W4) have been detected. Generally, the role of the cavity absorption coefficient in the calculation of the heating needs in the dispersing envelope is less pronounced if compared with the well-insulated opaque envelope.

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ACCEPTED MANUSCRIPT Fig. 10 – Rome: seasonal heating requirements for a single floor of the reference building determined with the proposed procedure, in function of WWR for the considered windowed system in presence of a dispersing envelope

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Fig. 11 – Rome: seasonal heating requirements for a single floor of the reference building determined with the standard procedure, in function of WWR for the considered windowed system in presence of a dispersing envelope

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In Fig. 12, the seasonal cooling requirements calculated considering the proposed calculation procedure for the transmitted solar gains, are shown whereas in Fig. 13 the same trends have been reported by using the standard procedure. In the first case, W5 is the best choice if WWR is lower than 0.6, contrarily W1 is preferable for greater values of WWR. W3 is the worst solution if WWR is greater than 0.35; for lower values, W1, W2, W3 and W4 systems provide similar results. In the hypothesis of “black” cavity to the transmitted solar radiation, W5 becomes the best choice for every WWR, followed respectively by W4 and W3 if WWR is lower than 0.5. The latter is the worst choice if WWR is greater than 0.6, while W1 and W2 provide similar results for every WWR. Generally, the standard procedure determines overestimated cooling energy requirements, and consequently the latent needs that depend mainly on the cooling period length. If WWR is set to unity, the detected maximum deviations range between 5000 kWhth (W5) and 18000 kWhth (W1) in continuous operation regime. With reference to the whole building, by converting the cooling thermal needs in electric energy, correspondent extra-consumptions of 15000 kWhe and 54000 kWhe were determined. The annual energy thermal requirements are reported in Fig 14 and in Fig. 15 for both the calculation procedures. In the first case, the involvement of the indoor optical properties makes W5 preferable, but with WWR greater than 0.4, a noticeable increment of the annual thermal energy requirements was detected. Moreover, W5 becomes less performant than W4 in the extreme case of WWR greater than 0.95. The windowed systems W3 is better than W4 but only if WWR is lower than 0.6, while W2 is always preferable than W1, that presents the worst performances for every WWR value. If internal optical properties are neglected, similar trends were obtained with higher annual energy demands. Usually, in the existing buildings refurbishment the WWR value is set, therefore the employment of W5 is suggested. If economic reasons lead to the use cheaper systems, W3 has to be employed when WWR is lower than 0.6, otherwise W4 becomes more performant. Generally, the proposed calculation procedure

ACCEPTED MANUSCRIPT determines slightly variations of the annual thermal energy requirements in the optimal building configuration. A rational exploitation of solar gains happens for dispersing envelopes equipped with insulated glazed system and for reduced WWR.

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Fig. 12 – Rome: seasonal cooling requirements for a single floor of the reference building determined with the proposed procedure, in function of WWR for the considered windowed system in presence of a dispersing envelope

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Fig. 13 – Rome: seasonal cooling requirements for a single floor of the reference building determined with the standard procedure, in function of WWR for the considered windowed system in presence of a dispersing envelope

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Fig. 14 – Rome: annual thermal energy requirements for a single floor of the reference building determined with the proposed procedure, in function of WWR for the considered windowed system in presence of a dispersing envelope

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Fig. 15 – Rome: annual thermal energy requirements for a single floor of the reference building determined with the standard procedure, in function of WWR for the considered windowed system in presence of a dispersing envelope

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5. Conclusions

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The influence of the optical properties of air-conditioned volumes on the choice of the suitable windowed system to use in non-residential buildings, was evaluated. Windows have a great influence on the building energy consumptions, participating directly on the energy balance by means of thermal and solar transmittances. Regarding the simplified quasi-steady procedure suggested by international standards to calculate heating and cooling demands, the solar gains transmitted through the windows are considered completely absorbed by the indoor cavity. Actually, in large glazed building envelopes, a fraction of the transmitted solar radiation does not participate to the indoor energy balance because it escapes newly from the same glazed system. In this paper, a cavity absorption coefficient to determine the actual solar gains absorbed by inner surfaces, depending on the geometrical and optical characteristics of the airconditioned volumes, has been introduced. The involvement of this coefficient in the calculation of the transmitted solar gains proves that the “escaping” solar radiation cannot be neglected, because it provides an overestimation of the annual thermal energy requirements. The cavity absorption coefficient has been applied to a steady-state formulation of the transmitted solar gains through windows, introducing the concept of the “effective” solar factor for normal incidence, which characterizes the absorption of the solar radiation in the system constituted by the air-conditioned volume and the windowed surfaces. The dynamic effects in the calculation of heating and cooling requirements have been taken into account by the gains utilization factors. Besides cooling and heating needs, by supposing an all-air air-conditioning plant also the monthly latent energy requested to regulate the indoor humidity level, was computed. The proposed procedure was applied to a reference non-residential building located in Rome, where climatic data makes solar gains more influential than thermal losses, calculating the annual thermal energy requirements in function of the percentage of glazed surface to the vertical opaque area (WWR) and of the typology of the windowed system. Five glazed systems were investigated: a single pane (W1), double clear panes (W2), double pane with low-emissive treatment (W3), triple clear panes (W4) and triple panes with low-emissive

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treatments (W5). Moreover, a well-insulated and a dispersing opaque envelopes have been considered, in order to identify the best windowed system in buildings equipped with different opaque walls. Regarding the reference well-insulated opaque envelope and by using climatic data of Rome, the involvement of the indoor optical properties in the calculation procedures provides the W1 windowed system as the best choice, but only if WWR is lower than 0.20. For greater values, the W4 system becomes more performant, however the annual energy requirements are similar to those determined for W2 if WWR is lower than 0.6. Contrarily, W3 has to be avoided if WWR is lower than 0.40, successively is W1 the most penalized system. By assuming completely absorbed the transmitted solar radiation, W1 is still the best choice for reduced windowed surfaces, but this time W4 is the best solution when WWR ranges between 0.20 and 0.75; for higher percentage, W5 is the preferable windowed system. In order to achieve also the maximum electricity savings by exploiting daylight, a WWR greater than 0.3 is not recommended because beyond this percentage the fraction of saved electric energy stabilizes. The results obtained on the well-insulated opaque envelope highlights that indoor optical properties affect the choice of the windowed system. Moreover, the overestimation of the energy needs due to the hypothesis of considering a “black” indoor environments lead to deviations which vary between 45000 kWhth (W5) and 120000 kWhth (W2) by setting WWR=1. Different results were detected for the reference dispersing building envelope: the involvement of the optical properties of the indoor environment make this time the W5 system more performant, but if WWR is over 0.40 the annual thermal requirements increase noticeable becoming less performant than W4 if WWR is greater than 0.95. If internal optical properties are neglected, the trend are similar but the annual energy requirements are obviously higher. Also in this case, the differences between the two calculation procedures lead to similar electric extra-consumptions detected for the well-insulated building. Therefore, the obtained results shown that the absorption coefficient is more influential on the choice of the windowed system when the building envelope is equipped with well-insulated opaque walls in the considered climatic context. Moreover, for well-insulated and dispersing envelopes the standard procedure provides a noticeable overestimation of the annual thermal energy requirements, especially due to the deviations obtained in the cooling demands, independently to the glasses solar transmittance. Contrarily, heating needs are underestimated, especially in building envelopes employing windows with low thermal transmittance.

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Concluding, in Rome the role of the windows with single panes in envelopes with low WWR has to be revalued because these systems allow for a reduction of the cooling requirements that prevails on the augment of the winter thermal losses, but only in presence of well-insulated opaque walls. Windows with triple panes and low emissive treatments are not suggested for effect of the excessive increment of the cooling requirements, despite the reduction of the heating needs. For large glazed envelopes W2 and W4 are suggested, but only if the cavity absorption coefficient is employed in the calculation procedure of the transmitted solar radiation. The standard procedure, instead, provides the W5 system as the best choice for very highly glazed envelopes. In presence of dispersing opaque walls, insulated windows such as W3 and W5 represent the best solution because allow for the achievement of a good compromise between reduction of heating needs and increment of cooling demands. The role of latent needs on the calculation results is not significant, because their variation with WWR and glazing typology depend mainly on a different length of the cooling period. In the next future, envelopes thermally insulated will represent the standard of building, therefore an accurate assessment of optical properties of indoor environments is required if the optimal window typology and the correspondent transparent surface, have to be identified.

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[25] Ballinge J.A., Lyons P.R., Advanced glazing technology for Australia – research and application, Renewable Energy (8), 1996, pp 61–65 [26] Burgess J.C., Skates H., A New Zealand window efficiency rating system. In: Proceedings of International Conference on Building Envelope Systems and Technology, Ottawa, 2001, pp. 94–102 [27] Duer K., Svendsen S., Mogensen M.M., Laustsen J.B., Energy labeling of glazings and windows in Denmark: calculated and measured value, Solar Energy, N° 73 (1), 2002, pp. 23–31

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[31] Arcuri N., Bruno R., Bevilacqua P., Influence of the optical and geometrical properties of indoor environments for the thermal performances of chilled ceilings, Energy and Buildings, N° 88, 2015, pp. 229237 [32] EN 410:2011, Glass in building – Determination of luminous and solar characteristics of glazing [33] Bruno R., Oliveti G., Arcuri N., An analytical model for the evaluation of the correction factor FW of solar gains through glazed surfaces defined in EN ISO 13790, Energy Build., N° 96, 2015, pp. 1-19 [34] UNI 11300-1, Italian Unification Institution, Energy performance of buildings, Part 1: Evaluation of energy needs for space heating and cooling, 2014 [35] UNI 10349-1, Italian Unification institution, Heating and cooling of buildings - Climatic data - Part 1: Monthly means for evaluation of energy need for space heating and cooling and methods for splitting global solar irradiance into the direct and diffuse parts and for calculate the solar irradiance on tilted planes, 2016

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[36] Ihm P., Nemri A., Krarti M., Estimation of lighting energy savings from daylighting, Building and Environment, N° 44, 2009, pp. 509-514

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Solar radiation transmitted through glazed surfaces has been studied. Indoor optical properties have been involved for the transmitted solar radiation. Different envelope configurations have been investigated. The best windowed system and correspondent surface have been identified. Minimal thermal energy consumption have been determined.

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