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Thermal and cost analysis of various air filled double glazed reflective windows for energy efficient buildings
Journal Pre-proof Thermal and cost analysis of various air filled double glazed reflective windows for energy efficient buildings Gorantla Kirankumar, Shaik Saboor, Shaik Sharmas Vali, Debasish Mahapatra, Ashok Babu Talanki Puttaranga Setty, Ki-Hyun Kim PII:
S2352-7102(19)31689-4
DOI:
https://doi.org/10.1016/j.jobe.2019.101055
Reference:
JOBE 101055
To appear in:
Journal of Building Engineering
Received Date: 23 August 2019 Revised Date:
9 October 2019
Accepted Date: 4 November 2019
Please cite this article as: G. Kirankumar, S. Saboor, S.S. Vali, D. Mahapatra, A.B. Talanki Puttaranga Setty, K.-H. Kim, Thermal and cost analysis of various air filled double glazed reflective windows for energy efficient buildings, Journal of Building Engineering (2019), doi: https://doi.org/10.1016/ j.jobe.2019.101055. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
THERMAL AND COST ANALYSIS OF VARIOUS AIR FILLED DOUBLE GLAZED REFLECTIVE WINDOWS FOR ENERGY EFFICIENT BUILDINGS Gorantla Kirankumara, Shaik Saboorb*, Shaik Sharmas Valic, Debasish Mahapatrad Ashok Babu Talanki Puttaranga Settye, Ki-Hyun Kimf a
Department of Mechanical Engineering, Sasi Institute of Technology & Engineering Tadepalligudem-534101, Andhra Pradesh, India b* School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India. c,d,e
Department of Mechanical Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore, India.
f
Department of Civil & Environmental Engineering, Hanyang University, 222 Wangsimni-Ro, Seoul 133-791, Republic of Korea. (*Corresponding Author: [email protected] (Saboor S.))
Abstract The enormous amount of energy is being consumed by buildings in an attempt to deliver thermal comfort in buildings. This paper aims at reducing/increasing the total solar heat gain through various combinations of double glazed reflective windows of buildings. The spectral characteristics of six reflective glasses namely bronze, green, grey, opal blue, sapphire blue and gold-reflective glasses at a normal angle of incidence by using UV-3600 Shimadzu spectrophotometer according to ASTM E 424 standards were experimentally measured. The solar optical properties of the glasses were deduced by developing a MATLAB code using spectral data which was obtained from experiments in the solar spectrum wavelength range of 300 nm to 2500 nm. Thirty air-filled double-glazed reflective windows have been studied for both thermal and cost analysis in the Indian composite climatic zone (New Delhi 28.580N, 77.200E). The configuration C13 (Grey reflective glass-Air gap 10 mm-Gold reflective glass) is observed to be the best air-filled double glazed window from the highest annual cost savings ($ 79.29 per annum in SE direction) and lower payback period (1.42 years) point of views among thirty double-glazed reflective glasses studied. The results of this paper are useful in the design of sustainable passive solar buildings.
Keywords: Air-filled double-glazed reflective windows, Solar optical properties, Annual cost saving double glazed glass, Glass used for reducing cooling and heating loads
1. Introduction Buildings use an enormous amount of energy to provide daylighting, cooling, and heating in their interior environments. Window glasses play a major role as building enclosures and they provide visual comfort to the occupants. Window glasses also allow more solar radiation in comparison to other building enclosures. Of late, all commercial complexes and multinational companies are making use of a wide range of glass enclosures to provide architectural beauty and ambiance to the buildings. These building envelopes contribute to an increase in heating and cooling loads inside the buildings. With regard to thermal comfort, substantial use of glass envelopes causes more amount of heat addition in the building, giving rise to thermally uncomfortable conditions. Researchers, architects, and building engineers are investigating the use of different solar control window glasses to ensure building envelopes provide essential visual as well as thermal comfort to the occupants with minimum energy consumption. From this viewpoint, investigations on different window glasses to calculate solar heat gain through fenestration are carried out by researchers. Thermal performance of gas-filled (air, xenon, and krypton) tinted double-glazed windows using TRANSYS was reported in the literature [1]. Designing of inward tilting glass fenestration to reduce transmission of solar radiation through the clear and brown glass during summer and winter season of Baghdad city in Iraq was reported [2]. Different window glasses with and without overhang shading devices at the different window to wall ratios were studied to allow the least possible heat into the building in Indian climatic zones. [3,4].Sujoy et al. (2009) developed and used a numerical model to compute the amount of solar radiation through a clear glass fenestration and compared the analytical results with the experimental results [5]. Heat gain into the buildings which were modeled with four different wall and window glasses was studied and fly ash brick house with grey tint glass was reported to be the best option for reduced cooling loads [6]. Researchers have also reported detailed studies on the measurement and evaluation of spectral properties of clear, double and triple low-E glasses, aimed at reducing the solar radiation in buildings [7,8]. A mathematical model of steady-state heat transfer through hollow double glazing was presented [9].
Extensive research on the determination of direct solar radiation into buildings through different windows having inward glass tilts was carried out at different Indian latitudes (Kirankumar & Ashok Babu, 2015). Solar radiation passing through various single glazed reflective glasses has been computed and reported in the literature by the authors of this manuscript [10]. The CFD analysis was carried out to study the heat transfer through a double-glazed unit with louvered blinds [11]. Influence of various roof construction, sun shields and Low-E glass on monthly and annual energy consumption in Taiwan using EQUEST software was reported [12]. Research on reducing the solar radiation through double glazed embedded with cooling pipes was carried out numerically and the results were validated with CFD simulation [13]. A theoretical study was carried out to propose a model and compute a method for the accumulation of seasonal solar heat gain passing through a window of various tilt angles in different regions of Japan [14]. Anuranjan and Sudhir compared experimental and simulation (WINDOW 6) results and reported the universal heat transfer coefficient for double-glazed windows with interpane blinds [15]. Studies on various parameters like window type, orientation, size, multi-pane glazing and shadowing effect on thermal assessment were determined and reported [16-18]. A detailed investigation on the use of different combinations of double glass windows with float and tinted glasses was carried out with the aid of Design builder simulation software to determine the amount of heat gain through the windows for various Indian climatic regions [19]. A detailed study on the minimization of direct solar radiation through the window into the building was studied with the help of a 3 mm clear glass window at different window tilt angles for all the eight orientations in New Delhi climatic region [20]. Thermal and energy performance analysis were carried out at the various window to wall area ratios of school building consisting of east and west windows using multiple simulation software tools [21]. An analytical and experimental study of a single transparent glass window and a ventilated double glass window in three climatic regions of Portugal was reported by Jorge & Helena [22]. Maatouk and Shigenao determined heat flux and temperature distribution in winters by using single- and double-glazed windows with the help of radiation element and ray emission model [23]. The correlation of window to wall ratio and window orientation on the energy consumption of the building was found by Alghoul, S.K. et al. [24]. From their study it was found that the addition of a window to the wall increases the energy consumption required for cooling. Their study concluded that the addition of a window in the south direction wall reduces the heating load. Hilliaho, K. et al.
analyzed the effect of glazing on the indoor climate and energy saving of a building. The heating energy consumption was reduced by 5.6% after the installation of glazing in a brick-walled building [25]. It is evident from the above literature that there are no momentous reports available on investigating the thermal performance of double reflective window glasses and the cost savings of the net cooling and heating loads of these glasses. To address this gap, this paper reports a detailed study on the thermal performance and cost analysis of double reflective glasses as a building envelope in all eight orientations. This paper aims at determining the solar radiation through various air-filled double-glazed reflective glasses into the buildings located in the composite climatic region of India (New Delhi). This paper proposes the best combination of reflective glasses for double glazed windows for reducing annual net cooling and heating cost. This paper also presents the annual net savings and payback period of thirty air-filled double glazed window combinations. The results of this paper help to identify the most critical component in double glazed window glass. The critical component could be either outer reflective glass, the air gap or inner reflective glass. It also recommends the most influential solar optical property for the heat gained/lost among three solar optical properties for reduced cooling or heating loads through a double-glazed window. The optical properties of glasses vary based upon the type of metal oxide coating applied on the glass surface. In this report, the thermal performance of thirty combinations of double-glazed reflective glasses with six types of commercial glasses available in the Indian market was investigated. A total of six reflective glasses are considered in this paper to find the heat gain inside the buildings and these glasses are bronze-reflective (Titanium oxide coated), green reflective (Iron oxide coated), grey reflective (Vanadium oxide coated), opal blue reflective (Copper compounds), sapphire blue reflective (Cobalt oxide coated) and gold reflective glass (Cadmium sulfide). This paper proposes the best combination of reflective glasses for a double-glazed window to serve the purpose of reducing yearly net cooling and heating cost. It also presents a payback period for thirty air-filled double glazed window glasses. The results of this paper are useful in designing sustainable and solar passive buildings. 2. Experimental methodology Glass window’s optical properties are needed to be known to find the radiation passing through it as the optical properties affect the heat gain into building up to a great extent. The spectral characteristics of glasses were measured experimentally using the UV-3600 Shimadzu
spectrophotometer. In this work, the analysis of spectral characteristics of six reflective glasses such (SPBRGW) and gold reflective (GLDRGW) glasses with a size of 30 mm × 30 mm and a thickness
as bronze (BZRGW), green (GRGW), grey (GrRGW), opal blue (OPBRGW), sapphire blue
of 5 mm was carried out. The analysis was done at a normal angle of incidence. The spectral transmission and spectral reflections have been measured in normal-diffuse or diffuse normal mode by placing specimens in direct contact with a 150 mm integrated sphere aperture as per standards [26]. The solar optical properties of glasses were measured at a wavelength interval of 2 nm in the wavelength range of 300-2500 nm (Ultraviolet, Visible and Near Infrared) for better accuracy. The solar optical properties were evaluated in the range of 300-2500 nm by using a MATLAB code. The weighted average of these spectral curves is obtained as per standards using equations (1) to (3) [2728]. To find the solar radiation passing through glass windows and to calculate net annual cost savings another MATLAB code was used. Fig. 1 (a) shows the building with a glass window and Fig. 1 (b) depicts the images of glass windows. The solar heat gain coefficient and solar optical properties of six reflective glasses are tabulated in Table 1. Figure 1. (a) Building with glass window (b) Images of reflective glasses. T
=
S τ(λ)Δλ
S Δλ
R
=
S ρ(λ)Δλ
S Δλ
(1)
(2)
Solar absorbtance of the glass can be obtained by Eq. (3). A
(∑
= 1 − (∑
S ρ(λ)Δλ ∑
S τ(λ)Δλ ∑
S Δλ)
S Δλ) −
(3)
Table 1. Properties of 5 mm thick reflective glasses
Fig. 2 depicts the spectral transmission and spectral reflection of reflective glass windows,
respectively. It can be noted that gold reflective glass has the highest transmission value and grey reflective glass has the lowest transmission value in comparison with other reflective glass windows. These spectral characteristics are significant in estimating the transmittance, reflectance and absorbtance values. The solar optical properties obtained are useful in computing the solar heat gain coefficient and solar heat gain through the fenestration.
Figure 2. Spectral characteristics of reflective glass windows (a) Transmission (b) Reflection 3. Numerical model Solar radiation impinging on the earth’s surface is called extra-terrestrial radiation. Solar radiation is in the form of electromagnetic waves with wavelengths ranging between 0.2 µm-10 µm. It can be classified into ultraviolet region (0.3µm-0.38µm), visible region (0.38 µm-0.78 µm), near-infrared region (0.78µm-2.5µm), and infrared region (2.5 µm-10 µm). The solar radiation enters into the building through glazing is the sum of direct, diffuse and reflected radiation. The total solar radiation (direct + diffused + reflected ground radiation) falling on building enclosures was found by considering the solar spectrum region range from 0.3 µm to 2.5 µm [29]. To find these three types of radiation, several solar angles such as hour, declination, solar altitude, solar azimuth, surface solar azimuth, and incidence angles are to be considered. Through this work, we attempted to find the total solar radiation through double-glazed reflective glasses with an unventilated air space of 10 mm between the glasses. A composite climatic zone was observed and analyzed as per Indian standards, from 6:00 AM to evening 6:00 PM at summer solstice i.e. on a peak summer day and 7:00 am to 5:00 pm at winter solstice i.e. on a peak winter day [30-32]. The clear and intermediate sky conditions with eight cardinal locations like E, W, N, S, SE, SW, NW, and NE were considered. In addition to this, the total solar radiation through all double-glazed reflective glass window combinations in the composite climatic region of New Delhi (28.580N, 77.200E), India was computed. Building models of measurements 3.5 m × 3.5 m × 3.5 m were considered, and a 40%
window to wall ratio (2.45 m × 2 m) was maintained as per standards (ECBC, 2009 [33]). The
glasses were aligned such that, thirty types of double-glazed reflective window combinations resulted (C1 to C30), as presented in Fig. 3. These combinations were placed one after the other in all the eight orientations resulting in a total of two hundred and forty combinations being tested to
find the total solar radiation passing through them in the selected composite climatic zone (New Delhi). ASHRAE clear and intermediate sky models at atmospheric conditions were used for this analysis.
Figure 3. Window systems of double reflective glass combinations with an air space of 10mm (C1 to C30).
The following steps were followed to find the total solar radiation passing through the window glass into buildings of any latitude by using ASHRAE clear sky and intermediate sky model [34,35]. Declination angle is the inclination of the earth’s axis measured from the perpendicular to the sun’s rays. It can be computed by Eq. (4) Declination angle !"
= #$. &'(!)
$*+(#,& + ) ) $*'
Solar altitude angle (!). = /0(1/0( !" /0(2 + (!)1(!) Solar azimuth angle
/0(ɸ =
(4) !"
(!).(!)1 − (!) /0(./0(1
(5) !"
Surface solar Azimuth angle γ= ɸ−Ψ
(6)
(7)
The surface azimuth measured from the south for the orientations N, NE, E, SE, S, SW, W, and NW are 1800, -1350, -900, -450, 00, 450, 900, and 1350, respectively (ASHRAE, 2003 [34]). Angle of incidence /0(6 = /0(./0(7/0(8 − (!).(!)8
Terrestrial solar irradiance on a clear atmosphere day is given by < 9:; = =>?(@⁄(!).)
(8)
(9)
Incident direct solar radiation on glazing is given by Eq. (10) IC = ICD cos θ II
= CICD
(10)
KLMN O
Incident diffused solar radiation from the sky onto the glazing can be computed by Eq. (11)
(11) The incident reflected radiation from the ground surface onto the glazing is given by Eq. (12) IP
C
= (C + sin β)ICD ρT
KLMN O
(12)
Where, the constants A, B, and C are used for calculating solar radiation per hour in Indian climates [36,37] Incident total solar radiation onto the glazing is presented by Eq. (13) IU = (IC + II + IP C ) (13)
Total solar radiation passing through a single glazing window can be obtained from Eq. (14) I
PV
= (IC
+ II
I
CPV
= (IC
+ II
+ IP
C ). WT
+
U A h[
\ . AP
(14) Total solar radiation passing through double glazed window glass is given by Eq. (15) + IP
C ). WT
α N + α[ + U( + αN XCT )\ . AP h[
Where, U = 1⁄_1⁄h[ + dx1⁄K1 + CT + dx2⁄K2 + 1⁄hN d CeT = 1
(15)
CT is the thermal resistance of the air gap between two glasses t eT t eT h x g1.25 + j2.32X lmn1 + ow rs − w tuw eT
eT
f v Where (CT , hN , h[ are considered as per standards (CIBSE, 2006 [38]))
(16)
The results obtained were compared with results found in the literature Ishwar et.al (2011) on this
subject for validation purposes. The validation of the MATLAB program was carried out for a 3 mm clear glass window in the composite climatic zone of New Delhi (28.580N, 77.200E). The deviation of the validation results was within ±1%. Hence, the program was considered to be reliable for studying other glasses [10, 20]. The mathematical model computes heat gain through the glass (without frame) and it does not consider the glass frame. This model does not consider the infiltration loads and internal loads of the building. 4. Cost Analysis Methodology The savings in yearly net cooling and heating cost of double glazed glass combinations in all eight orientations were determined for the composite climatic region of New Delhi. For computing net cost savings annually, the following procedure has been followed [39]. The mean diurnal incident total solar radiation onto the glazing during any season can be computed by Eq. (13). For computing incident solar radiation on glazing in both summer and winter seasons, the following months are considered in the study: i.) Summer season is from April to August ii.) Winter season is from September to March. The no. of days in each month is also considered for computing average daily incident total solar radiation onto the glazing. The total summer solar radiation incident on the glazing (QSummer) can be obtained from Eqs. (17). Q
{
|}
= (IU X30)
•}N€
+ (IU X31)•e‚ + (IU X30)ƒ{
(IU X31)ƒ{€‚ + (IU X31)
{T{M„
|
+
(17)
Where, ITS is the summer diurnal mean solar radiation incident on glazing (Direct + Diffuse + Reflected radiation). The total winter solar radiation incident on the glazing (QWinter) can be computed using Eqs. (18). …†!)‡=ˆ = (9‰† Š$+)‹=?‡=Œ•=ˆ + (9‰† Š$Ž)•/‡0•=ˆ + (9‰† Š$+);0•=Œ•=ˆ
+ (9‰† Š$Ž):=/=Œ•=ˆ + (9‰† Š$Ž)‘")’"ˆ“ + (9‰† Š#”)•=•ˆ’"ˆ“ + (…‰† Š$Ž)–"ˆ/2
(18)
Where, ITW is the winter daily average solar radiation incident on glazing The reduced annual cooling load and increased annual heating load can be computed by using Eqs. (19) and (20), respectively.
Reduced cooling load = Q
{
Increased heating load = Q VN
|}
× AP × (SHGCCPžP − SHGCCP
„|} ×
AP × (SHGCCPžP − SHGCCP
P)
P)
(19)
(20)
Where, SHGCDGCG and SHGCDGRG are SHGC of double-glazed clear glass and double-glazed reflective glasses, respectively. The solar optical properties of double glazed clear glass: transmittance, reflectance, and absorbtance are 68%, 13%, and 19%, respectively. The unit cost of natural gas and electricity is $ 0.45/therm and $ 0.073 kWh, respectively. The efficiency of the furnace and COP of the cooling system are taken as 0.8 and 2.5, respectively. 1Therm =29.31 kWh. Reduced cooling costs
= (Reduced cooling load) (Electricity cost per unit) / (COP)
Increased heating costs = (Increased heating load) (unit cost of fuel) /
(Efficiency)
The net yearly cost savings
= Reduced cooling costs – Increased heating costs Simple payback period =
© •€| | „e„N[ ª[M„ {e€ ª[M„ Me«N TM
Implementation cost = Unit price of glazing ($/m2) × Glass area (m2)
(21)
(22)
(23)
(24) (25)
5. Results and discussions 5.1 Solar heat gain through a window system of double glazed bronze reflective glass combinations Fig. 4 shows the alignment of double bronze reflective glass window combinations. They are arranged from C1 to C5 as shown in fig. The bronze reflective glass window was exposed to the
outer environment and other reflective glasses were kept toward the inner side one after the other as presented below. Total solar radiation passing through these combinations of glasses was computed on a peak hot summer and on a peak cold winter day in New Delhi climatic conditions and the results are tabulated in Table 2. Similarly, various combinations of other reflective glasses have also been considered for the study from C6 to C30 as presented in Fig. 5.
Figure 4. Double bronze reflective glass window combinations from C1 to C5 with an air space of 10mm
Tables 2 and 3 present the solar heat gain through a window system of double glazed bronze reflective glass combinations in all eight orientations during summer and winter climates of New Delhi climatic region. They are graphically presented in Fig. 5.
Table 2. Solar heat gain through a window system of double-glazed bronze reflective glass combinations during summer and winter (kW)
From the graphs, it is noticed that the south-oriented double-glazed bronze reflective glass gains the lowest and highest heat during both summer and winter, respectively among all the orientations studied. The double bronze reflective glass combination C5 (Bronze reflective glass with gold reflective glass) is observed to be the best option for reduced heat gain inside the building among all other bronze reflective glass combinations in all orientations during summer. During winter, C4 is observed to be the best option for reduced heating loads among all the bronze reflective glasses studied in all orientations.
Figure 5. Solar heat gain through a window system of double glazed bronze reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
5.2 Yearly cost savings of a window system of double glazed bronzed reflective glass combinations in the composite climatic zone of India Fig. 6 depicts the graph between a window system of double-glazed bronze reflective glass combinations and yearly cost savings ($/annum), in all eight orientations of New Delhi climatic region. From the figure, it can be seen that the window system of bronze reflective glass combination C5 is the most energy-efficient and saves the maximum cost when compared to other double glazed bronze window combinations. It accounts for an annual cost saving of $ 72.6 when compared to double glazed clear glass in the southeast direction. The best window orientation from the point of view of maximum annual cost saving (cooling cost +heating cost) is in the order of SE, SW, S, NE, NW, N, E, and W. From the results, it is observed that the placing of windows in SE, SW, and S saves the highest yearly net cooling and heating cost among all eight orientations studied. The deviation in the yearly net cooling and heating cost of SE, SW, and S orientations is very less as presented in Fig. 6. Figure 6. Yearly cost savings of double-glazed bronze-reflective glass combinations in New Delhi climatic region
5.3 Solar heat gain through a window system of double glazed green reflective glass combinations Solar heat gain through a window system of double glazed green reflective glass combinations (C6 to C10), for New Delhi climatic region on a peak summer day and on a peak winter day, in all eight cardinal directions was estimated.
Figure. 7. Solar heat gain through a window system of double glazed green reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
It is noticed from the graphs that, among all orientations studied the south-oriented window system of double glazed green reflective glass gains the lowest and the highest heat during summer and
winter, respectively. The window system of double glazed green reflective glass combination C9 (Green reflective glass with gold reflective double glazing) is observed to be the best option among all other green reflective glass combinations in all orientations during summer for reducing cooling load inside the building. During winter, C8 is observed to be the best option among all the other green reflective glasses studied in all orientations for reducing heating loads. 5.4 Yearly cost savings of a window system of double glazed green reflective glass combinations in the composite climatic zone of India Fig. 8 shows yearly cost saving of double-glazed green-reflective glass combinations, in all eight orientations in New Delhi climatic region. From the figure, it is noticed that the window system of green reflective glass combination C9 is the most energy-efficient when compared to all other double green reflective window glass combinations. It reports the highest net cost-saving (cooling + heating) of $ 76.85 (per annum) in comparison with the window system of double glazed clear glass in the southeast direction. From the results, it is noticed that the placing of windows in SE, SW and S direction saves the highest yearly net cooling and heating cost among all eight orientations studied. The deviation in the yearly net cooling and heating cost savings of SE, SW and S orientations is very less as presented in Fig. 8. Figure 8. Yearly cost savings of double-glazed green-reflective glass combinations in New Delhi climatic region
5.5 Solar heat gain through a window system of double glazed grey reflective glass combinations Solar heat gain through a window system of double glazed grey reflective glass combinations (C11 to C15) for New Delhi climatic region on a peak summer day and on a peak winter day, in all eight cardinal directions was estimated.
Figure 9. Solar heat gain through a window system of double glazed grey reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
From the graphs, it is evident that the south-oriented window system of double glazed grey reflective glass gains the lowest and highest heat during summer and winter, respectively among all orientations studied. The window system of double grey reflective glass combination C13 (Grey reflective glass with gold reflective double glazing) is reported to be the best option among the other grey reflective glass combinations for reducing cooling load inside the building in all orientations during summer. During winter, C12 is observed to be the best option among other grey reflective glasses studied for reducing heating loads in all orientations. 5.6 Yearly cost savings of a window system of double glazed grey reflective glass combinations in the composite climatic zone of India
Fig. 10 depicts yearly cost savings of double glazed grey reflective glass, in all eight orientations of New Delhi climatic region. From the graph, it is evident that the window system of grey reflective glass combination C13 is the most energy-efficient among all other double glazed grey reflective glass combinations and it reports the highest net annual cost saving (cooling cost +heating cost) of $79.292 in comparison with the window system of double glazed clear reflective glass in the southeast direction.
Figure 10. Yearly cost savings of double-glazed grey-reflective glass combinations in New Delhi climatic region
5.7 Solar heat gain through a window system of double glazed opal blue reflective glass combinations
Solar heat gain through a window system of double glazed opal blue reflective glass combinations (C16 to C20) for New Delhi climatic region on a peak hot summer day and on a peak winter day, from all eight cardinal directions, was estimated.
Figure 11. Solar heat gain through a window system of double glazed opal blue reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations From Fig. 11, it is evident that the south-oriented window system of double-glazed opal blue reflective glass gains the lowest and highest heat during summer and winter, respectively among all orientations studied. The window system of double opal blue reflective glass combination C17 (Opal blue reflective glass with gold reflective double glazing) is reported to be the best selection for reducing cooling load inside the building among all another opal blue reflective glass combinations in all orientations during summer. During winter, C16 is observed to be the best option for reducing heating load among all the other opal blue reflective glasses studied in all orientations. 5.8 Yearly cost savings of a window system of double glazed opal blue reflective glass combinations in the composite climatic zone of India
Fig. 12 presents annual cost savings of double glazed opal blue reflective glass combinations, in all eight orientations of New Delhi climatic zone. From the result, it is evident that the window system of double-glazed opal blue reflective glass combination C17 is the most energy-efficient when compared with another double-glazed opal blue reflective glass combinations studied. It accounts for the highest net cost-saving (cooling + heating) of $ 76.81 (per annum) in comparison with the window system of double glazed normal clear reflective glass in the southeast direction. Figure 12. Yearly cost savings of double-glazed opal blue-reflective glass combinations in New Delhi climatic region.
5.9 Solar heat gain through a window system of double glazed sapphire blue reflective glass combinations Solar heat gain through a window system of double glazed sapphire blue reflective glass combinations (C21 to C25) for New Delhi climatic region on a peak summer day and on a peak winter day, in all eight cardinal directions, was reported in this section.
From Fig.13, it is evident that the south-oriented window system of double sapphire blue reflective glass gains the lowest and highest heat during summer and winter, respectively among all orientations studied. The window system of double sapphire blue reflective glass combination C21 (Sapphire blue reflective glass with gold reflective double glazing) is reported to be the best choice for reducing cooling load inside the building among all other sapphire blue reflective glass combinations in all orientations during summer. During winter, C24 is observed to be the best choice for reducing heating load among all the other sapphire blue reflective glasses studied in all orientations. Figure 13. Solar heat gain through a window system of double glazed sapphire blue reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
5.10 Yearly cost savings of a window system of double glazed sapphire blue reflective glass combinations in the composite climatic zone of India
Fig. 14 depicts yearly cost saving of double glazed sapphire blue reflective glass combinations, in all eight orientations of New Delhi climatic zone. From the results, it is noticed that the window system of sapphire blue reflective glass combination C21 is the most energy-efficient among all other double glazed sapphire blue reflective glass combinations. It accounts for the highest net cost saving of (cooling + heating) of $ 64.96 (per annum), in comparison with the double-glazed window system of clear reflective glass in the southeast direction. Figure 14. Yearly cost savings of double-glazed sapphire blue-reflective glass combinations in New Delhi climatic region 5.11 Solar heat gain through a window system of double glazed gold reflective glass combinations
Solar heat gain through a window system of double glazed gold reflective glass combinations (C26 to C30) for New Delhi climatic region on a peak summer day and on a peak winter day, for all eight cardinal directions was computed and presented in this section.
Figure 15. Solar heat gain through a window system of double glazed gold reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
From Fig. 15, it is evident that the south-oriented window system of double gold reflective glass, gains the lowest and highest heat during summer and winter, respectively among all orientations studied. The window system of double gold reflective glass combination C29 (Gold reflective glass with opal blue reflective double glazing) is reported to be the best choice among all other gold reflective glass combinations in all orientations for reducing cooling load inside the building during summer. During winter, C30 is reported to be the best choice among all other gold reflective glasses studied for reducing the heating load in all orientations. 5.12 Yearly cost savings of a window system of double glazed gold reflective glass combinations in the composite climatic zone of India
Fig. 16 presents yearly cost savings of double-glazed gold reflective glass combinations, in all eight orientations of New Delhi climatic zone. From the results, it is evident that the window system of gold reflective glass combination C27 is the most energy-efficient in comparison with all other double-glazed gold reflective glass combinations. It accounts for the highest net annual cost saving (cooling + heating) of $ 52.226 in comparison with the double-glazed window system of clear reflective glass in the southeast direction. Figure 16. Yearly cost savings of double-glazed gold-reflective glass combinations in New Delhi climatic region
Table 3. Payback period of double-glazed reflective glass window combinations
Figure 17. Payback period of double glazed windows for New Delhi climatic region In summer, the south-facing window is observed to be the best position to place windows, as it receives the least radiation among the eight orientations studied. In winter, the south-facing is
noted to be the best position to place windows, as it gains the highest radiation among the eight orientations studied. The arrangement of window orientations from the best to the worst during summer season, from the viewpoint of least heat gain is S, N, SE, SW, NE, NW, E, and W. The arrangement of window orientations from the best to the worst during winter season, from the viewpoint of highest heat gain is S, SE, SW, E, W, NE, NW, and N. The summer solar heat gain in buildings through the window system of double reflective glass combinations arranged in the order of the best to the worst in south orientation is as follows: C13 (1.784 kW), C15 (1.835 kW), C11 (1.847 kW), C14 (1.884 kW), C9 (1.904 kW), C17(1.906 kW), C12 (1.95kW), C19 (1.952 kW), C7 (1.96 kW), C6(2.001 kW), C10 (2.003 kW), C20 (2.006 kW), C18 (2.008 kW), C8 (2.074 kW), C16 (2.079 kW), C5 (2.265 kW), C1 (2.325 kW), C3 (2.341 kW), C2 (2.394 kW), C4 (2.487 kW), C21 (2.49 kW), C23 (2.571 kW), C25 (2.59 kW), C22 (2.651 kW), C24 (2.654 kW), C27 (3.118 kW), C29 (3.138 kW), C28 (3.196 kW), C26 (3.226 kW), and C30 (3.351 kW) The winter solar heat gain in buildings through the window system of double reflective glass combinations arranged in the order of the best to the worst in south orientation is as follows: C30 (12.279kW), C26 (11.821 kW), C28 (11.712 kW), C29 (11.499 kW), C27 (11.426 kW), C24 (9.725 kW), C22 (9.715kW), C25 (9.492 kW), C23 (9.423 kW), C21 (9.125 kW), C4 (9.113 kW), C2 (8.772 kW), C3 (8.578 kW), C1 (8.519 kW), C5 (8.3 kW), C16 (7.619 kW), C8 (7.601 kW), C18 (7.357 kW), C20 (7.352 kW), C10 (7.34 kW), C6 (7.33 kW), C7 (7.182 kW), C19 (7.154 kW), C12 (7.164 kW), C17 (6.984 kW), C9 (6.976 kW), C14 (6.905 kW), C11 (6.768kW), C15 (6.725 kW), and C13 (6.536 kW). The best double reflective glass in summer is not the best choice in winter. Therefore, the highest annual net cooling and heating cost savings of double reflective glasses have been identified in this manuscript. The annual net cooling and heating cost saving depends on the orientation of window placement and the type of glass combination. The best window orientations from the highest annual net cooling and heating cost to the lowest annual net cooling and heating cost for any glass window can be arranged in an order as SE, SW, S, NE, NW, N, E, and W. From the results, it is noticed that the placing of any glass window in SE, SW, and S saves highest annual net cooling and heating cost among all eight orientations studied. The deviation in yearly net cooling and heating cost saving of SE, SW, and S orientations is very less. From the results, it is evident that the outer reflective glass is the most critical component in the double-glazed window glass arrangement (outer reflective glass + air gap + inner reflective glass) for the heat gained/lost inside a building compared to the inner
reflective glass. The solar transmittance of the outer reflective glass plays an important role in heat gained/lost into building among other solar optical properties of glass. The higher the solar transmittance of the outer reflective glass, the higher is the heat gain into the building. Fig. 17 shows a simple payback of thirty air-filled double glazed windows. From Table 3, it is observed that the payback period for the C17 is the lowest (1.34 years) whereas, the payback period is the highest for the C24 configuration. The configuration C13 is observed to be the best configuration from the highest annual cost savings point of view and it has a slightly higher payback period (1.42 years) than C17 (1.34 years). Table 3 presents a detailed table of annual cost savings and the payback period of thirty double glazed windows. This would help to identify the best airfilled double glazed window glasses for energy-efficient buildings. 6. Conclusion The present work presents thermal and cost analysis of thirty air-filled double-glazed reflective glass windows (C1 to C30) in the composite climatic zone of India (New Delhi (28.580N, 77.200E)) during peak summer and winter days. This paper signifies the annual net cooling and heating cost savings of using these thirty air-filled double-glazed reflective glass windows as a replacement to the conventional glass windows. This paper also presents a payback period of thirty air-filled double-glazed reflective glass windows. This work proposes the optimum window orientation from the perspective of the highest annual net cooling and heating cost savings. The best combination of reflective glasses for double glazed window arrangement has also been proposed. •
From Fig. 10, the window system of double glazed grey reflective glass combination C13 (Grey reflective glass-Air gap 10 mm-Gold reflective glass) is observed to save the highest annual net cooling and heating cost ($ 79.292 per annum in SE direction) combination, in all eight orientations of window location among all the thirty studied configurations of doubleglazed window glasses in the composite climatic zone of India.
•
From the results, it is evident that the outer reflective glass is the most critical component in the double-glazed window glass arrangement (outer reflective glass + air gap + inner reflective glass) for the heat gained/lost inside a building compared to the inner reflective glass. The most influencing solar optical property which decides heat gained/lost in a building is solar transmittance of the outer reflective glass.
•
From Figs. 6, 8, 10, 12, 14, and 16, the window system of double-glazed reflective glass
combinations can be arranged in an order, from the highest energy-efficient point of view to the lowest energy efficient point of view in any of the eight orientations as follows: C13, C15, C11, C14, C9, C17, C12, C19, C7, C6, C10, C20, C18, C8, C16, C5, C1, C3, C2, C4, C21, C23, C25, C22, C24, C27, C29, C28, C26, and C30. •
From Fig. 17, it is observed that the payback period for the C17 is the lowest (1.34 years) whereas, the payback period is the highest for the C24 configuration. The configuration C13 is observed to be the best configuration from the highest annual cost savings point of view and it has a slightly higher payback period (1.42 years) than C17 (1.34 years).
•
The best order of window orientations from the highest annual net cooling and heating cost savings point of view to the lowest annual net cooling and heating cost savings point of view for any glass window is SE, SW, S, NE, NW, N, E, and W.
The results of this paper are beneficial in the design of energy-efficient window systems for reducing air-conditioning loads in buildings. The results are also useful for energy-efficient retrofitting in buildings.
Nomenclature AG
Area of the glass [m2]
A
Solar radiation in the absence of atmosphere [W/m2]
B
Atmospheric extinction coefficient [-]
C
Sky radiation coefficient [-]
Cg
Thermal resistance of the air gap [m2.K/W]
dia
Declination angle [Deg]
h
Hour angle [Deg]
k
Angle of window glass from vertical [Deg]
l
Latitude [Deg]
IDN
Solar radiation at normal incidence [W/m2]
IDSR
Direct solar radiation from the sun [W/m2]
IdSR
Diffuse solar radiation from the sky [W/m2]
IGRD
Ground reflected solar radiation [W/m2]
IT
Total incident solar radiation [W/m2]
Rsi
Inside surface resistance film coefficient [m2.K/W]
Rso
Outside surface resistance film coefficient [m2.K/W]
dx1
Thickness of the outer glass [m]
dx2
Thickness of the inner glass [m]
K1
Thermal conductivity of outside glass [W/m.K]
K2
Thermal conductivity of inside glass [W/m.K]
tag
Thickness of the air space between glasses [m]
nd
Number of days from January 1st
TSOLAR
Solar transmittance [%]
RSOLAR
Solar reflectance [%]
ASOLAR
Solar absorptance [%]
Wag
Width of the air space between glasses [m]
U
Overall heat transfer coefficient [W/m2.K]
C1
BZRGW–Air gap10mm–GRGW
C2
BZRGW–Air gap10mm–GrRGW
C3
BZRGW–Air gap10mm–OPBRGW
C4
BZRGW–Air gap10mm–SPBRGW
C5
BZRGW–Air gap10mm–GLDRGW
C6
GRGW–Air gap10mm–GrRGW
C7
GRGW–Air gap10mm–OPBRGW
C8
GRGW–Air gap10mm–SPBRGW
C9
GRGW–Air gap10mm–GLDRGW
C10
GRGW–Air gap10mm–BZRGW
C11
GrRGW–Air gap10m–OPBRGW
C12
GrRGW–Air gap10mm–SPBRGW
C13
GrRGW–Air gap10m–GLDRGW
C14
GrRGW–Air gap10mm–BZRGW
C15
GrRGW–Air gap10mm–GRGW
C16
OPBRGW–Air gap10mm–SPBRGW
C17
OPBRGW–Air gap10mm–GLDRGW
C18
OPBRGW–Air gap10mm–BZRGW
C19
OPBRGW–Air gap10mm–GRGW
C20
OPBRGW–Air gap10mm–GrRGW
C21
SPBRGW–Air gap10mm–GLDRGW
C22
SPBRGW–Air gap10mm–BZRGW
C23
SPBRGW–Air gap10mm–GRGW
C24
SPBRGW–Air gap10mm–GrRGW
C25
SPBRGW–Air gap10mm–OPBRGW
C26
GLDRGW–Air gap10mm–BZRGW
C27
GLDRGW–Air gap10mm–GRGW
C28
GLDRGW–Air gap10mm–GrRGW
C29
GLDRGW–Air gap10mm–OPBRGW
C30
GLDRGW–Air gap10mm–SPBRGW
BZRGW Bronze reflective glass window GRGW
Green reflective glass window
GrRGW Grey reflective glass window OPBRGW Opal blue reflective glass window SPBRGW Sapphire blue reflective glass window GLDRGW Gold reflective glass window UV
Ultraviolet region
VIS
Visible region
NIR
Near infrared region
Greek letters λ
Wavelength [nm]
∆λ
Wavelength interval [nm]
β
Solar altitude angle [Deg]
Sλ
Relative spectral distribution of the solar radiation [W/m2]
θ
Solar incidence angle [Deg]
ϕ
Solar azimuth angle [Deg]
Ψ
Surface azimuth angle [Deg]
γ
Surface solar azimuth angle [Deg]
ρg
Ground reflectance factor [-]
?? (λ) ρ (λ)
Spectral reflection [%]
α(λ)
Spectral absorption [%]
αo
Solar absorptance of the outside glass [%]
αi
Solar absorptance of the inside glass [%]
Spectral transmission [%]
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LIST OF TABLES:
Table 1. Properties of 5 mm thick reflective glasses S.No. 1. 2. 3. 4. 5. 6.
Window Glass BZRGW GRGW GrRGW OPBRGW SPBRGW GLDRGW
TSOLAR 0.37 0.29 0.26 0.29 0.42 0.55
RSOLAR 0.14 0.14 0.08 0.13 0.11 0.32
ASOLAR 0.49 0.57 0.66 0.58 0.47 0.13
SHGC 0.48 0.42 0.41 0.42 0.53 0.58
Table 2. Solar heat gain through a window system of double-glazed bronze reflective glass combinations during summer and winter (kW) Direction N NE E SE S SW W NW
C1 2.433 2.847 3.034 2.77 2.325 2.77 3.034 2.847
Window system of double-glazed bronze reflective glass combinations Summer Winter C2 C3 C4 C5 C1 C2 C3 C4 2.505 2.45 2.603 2.371 1.86 1.915 1.873 1.99 2.931 2.866 3.045 2.773 2.285 2.353 2.301 2.444 3.124 3.055 3.245 2.956 4.661 4.799 4.693 4.986 2.852 2.789 2.963 2.698 7.149 7.361 7.198 7.647 2.394 2.341 2.487 2.265 8.519 8.772 8.578 9.113 2.852 2.789 2.963 2.698 6.838 7.041 6.885 7.315 3.124 3.055 3.245 2.956 4.441 4.573 4.472 4.751 2.931 2.866 3.045 2.773 2.285 2.353 2.301 2.444
C5 1.812 2.226 4.541 6.965 8.3 6.662 4.327 2.226
Table 3. Payback period of double-glazed reflective glass window combinations Glazing
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 C29 C30
Local price rate Implementation excluding tax at the time cost ($) 2 of procurement ($/m ) 24 117.6 25 122.5 24 117.6 31 151.9 20 98 26 127.4 25 122.5 33 161.7 22 107.8 24 117.6 26 127.4 34 166.6 23 112.7 25 122.5 26 127.4 33 161.7 21 102.9 24 117.6 25 122.5 26 127.4 29 142.1 31 151.9 33 161.7 34 166.6 33 161.7 20 98 22 107.8 23 112.7 21 102.9 29 142.1
Annual cost savings ($)
Payback period (Years)
71.3 69.9 71.0 67.9 72.6 74.9 75.7 73.4 76.9 74.8 78.0 75.9 79.3 77.3 78.2 73.3 76.8 74.8 75.9 74.8 65.0 61.7 63.3 61.6 62.9 50.0 52.2 50.6 51.8 47.5
1.65 1.75 1.66 2.24 1.35 1.70 1.62 2.20 1.40 1.57 1.63 2.19 1.42 1.58 1.63 2.21 1.34 1.57 1.61 1.70 2.19 2.46 2.55 2.70 2.57 1.96 2.07 2.23 1.99 2.99
FIGURES:
(a)
(b)
Figure 1. (a) Building with glass window (b) Images of reflective glasses.
(a)
(b)
Figure 2. Spectral characteristics of reflective glass windows (a) Transmission (b) Reflection
Figure 3. Window systems of double reflective glass combinations with an air space of 10mm (C1 to C30).
Figure 4. Double bronze reflective glass window combinations from C1 to C5 with an air space of 10mm
(a)
(b)
Figure 5. Solar heat gain through a window system of double glazed bronze reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
Figure 6. Yearly cost savings of double-glazed bronze-reflective glass combinations in New Delhi climatic region
(a)
(b)
Figure. 7. Solar heat gain through a window system of double glazed green reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
Figure 8. Yearly cost savings of double-glazed green-reflective glass combinations in New Delhi climatic region
(a)
(b)
Figure 9. Solar heat gain through a window system of double glazed grey reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
Figure 10. Yearly cost savings of double-glazed grey-reflective glass combinations in New Delhi climatic region
(a)
(b)
Figure 11. Solar heat gain through a window system of double glazed opal blue reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
Figure 12. Yearly cost savings of double-glazed opal blue-reflective glass combinations in New Delhi climatic region.
(a)
(b)
Figure 13. Solar heat gain through a window system of double glazed sapphire blue reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
Figure 14. Yearly cost savings of double-glazed sapphire blue-reflective glass combinations in New Delhi climatic region
(a)
(b)
Figure 15. Solar heat gain through a window system of double glazed gold reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
Figure 16. Yearly cost savings of double-glazed gold-reflective glass combinations in New Delhi climatic region
Figure 17. Simple payback period of double glazed windows for New Delhi climatic region
Manuscript Highlights •
Thermal performance of various air filled double glazed reflective windows
•
Annual cost saving and payback periods of various air filled double glazed reflective windows
•
Optimal combination of reflective glasses for double glazing for reduced cooling and heating loads
•
Solar heat gained/lost through double glazed reflective glass windows
•
Windows for sustainable and energy efficient buildings
OCTOBER 09, 2019 To, Editors-in-Chief J. de Brito, J. M. LaFave, and R. Yao Journal of Building Engineering Dear Sir, Authors would like to declare no conflict of interest for manuscript Ms.: JOB 2019 Ref no:1678, entitled “Thermal and Cost Analysis of Various Air Filled Double Glazed Reflective Windows for Energy Efficient Buildings” for Journal of Building Engineering.
Thanks and Best Regards, Yours Sincerely, Dr. SABOOR S
S2352-7102(19)31689-4
DOI:
https://doi.org/10.1016/j.jobe.2019.101055
Reference:
JOBE 101055
To appear in:
Journal of Building Engineering
Received Date: 23 August 2019 Revised Date:
9 October 2019
Accepted Date: 4 November 2019
Please cite this article as: G. Kirankumar, S. Saboor, S.S. Vali, D. Mahapatra, A.B. Talanki Puttaranga Setty, K.-H. Kim, Thermal and cost analysis of various air filled double glazed reflective windows for energy efficient buildings, Journal of Building Engineering (2019), doi: https://doi.org/10.1016/ j.jobe.2019.101055. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
THERMAL AND COST ANALYSIS OF VARIOUS AIR FILLED DOUBLE GLAZED REFLECTIVE WINDOWS FOR ENERGY EFFICIENT BUILDINGS Gorantla Kirankumara, Shaik Saboorb*, Shaik Sharmas Valic, Debasish Mahapatrad Ashok Babu Talanki Puttaranga Settye, Ki-Hyun Kimf a
Department of Mechanical Engineering, Sasi Institute of Technology & Engineering Tadepalligudem-534101, Andhra Pradesh, India b* School of Mechanical Engineering, Vellore Institute of Technology, Vellore, Tamil Nadu, India. c,d,e
Department of Mechanical Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore, India.
f
Department of Civil & Environmental Engineering, Hanyang University, 222 Wangsimni-Ro, Seoul 133-791, Republic of Korea. (*Corresponding Author: [email protected] (Saboor S.))
Abstract The enormous amount of energy is being consumed by buildings in an attempt to deliver thermal comfort in buildings. This paper aims at reducing/increasing the total solar heat gain through various combinations of double glazed reflective windows of buildings. The spectral characteristics of six reflective glasses namely bronze, green, grey, opal blue, sapphire blue and gold-reflective glasses at a normal angle of incidence by using UV-3600 Shimadzu spectrophotometer according to ASTM E 424 standards were experimentally measured. The solar optical properties of the glasses were deduced by developing a MATLAB code using spectral data which was obtained from experiments in the solar spectrum wavelength range of 300 nm to 2500 nm. Thirty air-filled double-glazed reflective windows have been studied for both thermal and cost analysis in the Indian composite climatic zone (New Delhi 28.580N, 77.200E). The configuration C13 (Grey reflective glass-Air gap 10 mm-Gold reflective glass) is observed to be the best air-filled double glazed window from the highest annual cost savings ($ 79.29 per annum in SE direction) and lower payback period (1.42 years) point of views among thirty double-glazed reflective glasses studied. The results of this paper are useful in the design of sustainable passive solar buildings.
Keywords: Air-filled double-glazed reflective windows, Solar optical properties, Annual cost saving double glazed glass, Glass used for reducing cooling and heating loads
1. Introduction Buildings use an enormous amount of energy to provide daylighting, cooling, and heating in their interior environments. Window glasses play a major role as building enclosures and they provide visual comfort to the occupants. Window glasses also allow more solar radiation in comparison to other building enclosures. Of late, all commercial complexes and multinational companies are making use of a wide range of glass enclosures to provide architectural beauty and ambiance to the buildings. These building envelopes contribute to an increase in heating and cooling loads inside the buildings. With regard to thermal comfort, substantial use of glass envelopes causes more amount of heat addition in the building, giving rise to thermally uncomfortable conditions. Researchers, architects, and building engineers are investigating the use of different solar control window glasses to ensure building envelopes provide essential visual as well as thermal comfort to the occupants with minimum energy consumption. From this viewpoint, investigations on different window glasses to calculate solar heat gain through fenestration are carried out by researchers. Thermal performance of gas-filled (air, xenon, and krypton) tinted double-glazed windows using TRANSYS was reported in the literature [1]. Designing of inward tilting glass fenestration to reduce transmission of solar radiation through the clear and brown glass during summer and winter season of Baghdad city in Iraq was reported [2]. Different window glasses with and without overhang shading devices at the different window to wall ratios were studied to allow the least possible heat into the building in Indian climatic zones. [3,4].Sujoy et al. (2009) developed and used a numerical model to compute the amount of solar radiation through a clear glass fenestration and compared the analytical results with the experimental results [5]. Heat gain into the buildings which were modeled with four different wall and window glasses was studied and fly ash brick house with grey tint glass was reported to be the best option for reduced cooling loads [6]. Researchers have also reported detailed studies on the measurement and evaluation of spectral properties of clear, double and triple low-E glasses, aimed at reducing the solar radiation in buildings [7,8]. A mathematical model of steady-state heat transfer through hollow double glazing was presented [9].
Extensive research on the determination of direct solar radiation into buildings through different windows having inward glass tilts was carried out at different Indian latitudes (Kirankumar & Ashok Babu, 2015). Solar radiation passing through various single glazed reflective glasses has been computed and reported in the literature by the authors of this manuscript [10]. The CFD analysis was carried out to study the heat transfer through a double-glazed unit with louvered blinds [11]. Influence of various roof construction, sun shields and Low-E glass on monthly and annual energy consumption in Taiwan using EQUEST software was reported [12]. Research on reducing the solar radiation through double glazed embedded with cooling pipes was carried out numerically and the results were validated with CFD simulation [13]. A theoretical study was carried out to propose a model and compute a method for the accumulation of seasonal solar heat gain passing through a window of various tilt angles in different regions of Japan [14]. Anuranjan and Sudhir compared experimental and simulation (WINDOW 6) results and reported the universal heat transfer coefficient for double-glazed windows with interpane blinds [15]. Studies on various parameters like window type, orientation, size, multi-pane glazing and shadowing effect on thermal assessment were determined and reported [16-18]. A detailed investigation on the use of different combinations of double glass windows with float and tinted glasses was carried out with the aid of Design builder simulation software to determine the amount of heat gain through the windows for various Indian climatic regions [19]. A detailed study on the minimization of direct solar radiation through the window into the building was studied with the help of a 3 mm clear glass window at different window tilt angles for all the eight orientations in New Delhi climatic region [20]. Thermal and energy performance analysis were carried out at the various window to wall area ratios of school building consisting of east and west windows using multiple simulation software tools [21]. An analytical and experimental study of a single transparent glass window and a ventilated double glass window in three climatic regions of Portugal was reported by Jorge & Helena [22]. Maatouk and Shigenao determined heat flux and temperature distribution in winters by using single- and double-glazed windows with the help of radiation element and ray emission model [23]. The correlation of window to wall ratio and window orientation on the energy consumption of the building was found by Alghoul, S.K. et al. [24]. From their study it was found that the addition of a window to the wall increases the energy consumption required for cooling. Their study concluded that the addition of a window in the south direction wall reduces the heating load. Hilliaho, K. et al.
analyzed the effect of glazing on the indoor climate and energy saving of a building. The heating energy consumption was reduced by 5.6% after the installation of glazing in a brick-walled building [25]. It is evident from the above literature that there are no momentous reports available on investigating the thermal performance of double reflective window glasses and the cost savings of the net cooling and heating loads of these glasses. To address this gap, this paper reports a detailed study on the thermal performance and cost analysis of double reflective glasses as a building envelope in all eight orientations. This paper aims at determining the solar radiation through various air-filled double-glazed reflective glasses into the buildings located in the composite climatic region of India (New Delhi). This paper proposes the best combination of reflective glasses for double glazed windows for reducing annual net cooling and heating cost. This paper also presents the annual net savings and payback period of thirty air-filled double glazed window combinations. The results of this paper help to identify the most critical component in double glazed window glass. The critical component could be either outer reflective glass, the air gap or inner reflective glass. It also recommends the most influential solar optical property for the heat gained/lost among three solar optical properties for reduced cooling or heating loads through a double-glazed window. The optical properties of glasses vary based upon the type of metal oxide coating applied on the glass surface. In this report, the thermal performance of thirty combinations of double-glazed reflective glasses with six types of commercial glasses available in the Indian market was investigated. A total of six reflective glasses are considered in this paper to find the heat gain inside the buildings and these glasses are bronze-reflective (Titanium oxide coated), green reflective (Iron oxide coated), grey reflective (Vanadium oxide coated), opal blue reflective (Copper compounds), sapphire blue reflective (Cobalt oxide coated) and gold reflective glass (Cadmium sulfide). This paper proposes the best combination of reflective glasses for a double-glazed window to serve the purpose of reducing yearly net cooling and heating cost. It also presents a payback period for thirty air-filled double glazed window glasses. The results of this paper are useful in designing sustainable and solar passive buildings. 2. Experimental methodology Glass window’s optical properties are needed to be known to find the radiation passing through it as the optical properties affect the heat gain into building up to a great extent. The spectral characteristics of glasses were measured experimentally using the UV-3600 Shimadzu
spectrophotometer. In this work, the analysis of spectral characteristics of six reflective glasses such (SPBRGW) and gold reflective (GLDRGW) glasses with a size of 30 mm × 30 mm and a thickness
as bronze (BZRGW), green (GRGW), grey (GrRGW), opal blue (OPBRGW), sapphire blue
of 5 mm was carried out. The analysis was done at a normal angle of incidence. The spectral transmission and spectral reflections have been measured in normal-diffuse or diffuse normal mode by placing specimens in direct contact with a 150 mm integrated sphere aperture as per standards [26]. The solar optical properties of glasses were measured at a wavelength interval of 2 nm in the wavelength range of 300-2500 nm (Ultraviolet, Visible and Near Infrared) for better accuracy. The solar optical properties were evaluated in the range of 300-2500 nm by using a MATLAB code. The weighted average of these spectral curves is obtained as per standards using equations (1) to (3) [2728]. To find the solar radiation passing through glass windows and to calculate net annual cost savings another MATLAB code was used. Fig. 1 (a) shows the building with a glass window and Fig. 1 (b) depicts the images of glass windows. The solar heat gain coefficient and solar optical properties of six reflective glasses are tabulated in Table 1. Figure 1. (a) Building with glass window (b) Images of reflective glasses. T
=
S τ(λ)Δλ
S Δλ
R
=
S ρ(λ)Δλ
S Δλ
(1)
(2)
Solar absorbtance of the glass can be obtained by Eq. (3). A
(∑
= 1 − (∑
S ρ(λ)Δλ ∑
S τ(λ)Δλ ∑
S Δλ)
S Δλ) −
(3)
Table 1. Properties of 5 mm thick reflective glasses
Fig. 2 depicts the spectral transmission and spectral reflection of reflective glass windows,
respectively. It can be noted that gold reflective glass has the highest transmission value and grey reflective glass has the lowest transmission value in comparison with other reflective glass windows. These spectral characteristics are significant in estimating the transmittance, reflectance and absorbtance values. The solar optical properties obtained are useful in computing the solar heat gain coefficient and solar heat gain through the fenestration.
Figure 2. Spectral characteristics of reflective glass windows (a) Transmission (b) Reflection 3. Numerical model Solar radiation impinging on the earth’s surface is called extra-terrestrial radiation. Solar radiation is in the form of electromagnetic waves with wavelengths ranging between 0.2 µm-10 µm. It can be classified into ultraviolet region (0.3µm-0.38µm), visible region (0.38 µm-0.78 µm), near-infrared region (0.78µm-2.5µm), and infrared region (2.5 µm-10 µm). The solar radiation enters into the building through glazing is the sum of direct, diffuse and reflected radiation. The total solar radiation (direct + diffused + reflected ground radiation) falling on building enclosures was found by considering the solar spectrum region range from 0.3 µm to 2.5 µm [29]. To find these three types of radiation, several solar angles such as hour, declination, solar altitude, solar azimuth, surface solar azimuth, and incidence angles are to be considered. Through this work, we attempted to find the total solar radiation through double-glazed reflective glasses with an unventilated air space of 10 mm between the glasses. A composite climatic zone was observed and analyzed as per Indian standards, from 6:00 AM to evening 6:00 PM at summer solstice i.e. on a peak summer day and 7:00 am to 5:00 pm at winter solstice i.e. on a peak winter day [30-32]. The clear and intermediate sky conditions with eight cardinal locations like E, W, N, S, SE, SW, NW, and NE were considered. In addition to this, the total solar radiation through all double-glazed reflective glass window combinations in the composite climatic region of New Delhi (28.580N, 77.200E), India was computed. Building models of measurements 3.5 m × 3.5 m × 3.5 m were considered, and a 40%
window to wall ratio (2.45 m × 2 m) was maintained as per standards (ECBC, 2009 [33]). The
glasses were aligned such that, thirty types of double-glazed reflective window combinations resulted (C1 to C30), as presented in Fig. 3. These combinations were placed one after the other in all the eight orientations resulting in a total of two hundred and forty combinations being tested to
find the total solar radiation passing through them in the selected composite climatic zone (New Delhi). ASHRAE clear and intermediate sky models at atmospheric conditions were used for this analysis.
Figure 3. Window systems of double reflective glass combinations with an air space of 10mm (C1 to C30).
The following steps were followed to find the total solar radiation passing through the window glass into buildings of any latitude by using ASHRAE clear sky and intermediate sky model [34,35]. Declination angle is the inclination of the earth’s axis measured from the perpendicular to the sun’s rays. It can be computed by Eq. (4) Declination angle !"
= #$. &'(!)
$*+(#,& + ) ) $*'
Solar altitude angle (!). = /0(1/0( !" /0(2 + (!)1(!) Solar azimuth angle
/0(ɸ =
(4) !"
(!).(!)1 − (!) /0(./0(1
(5) !"
Surface solar Azimuth angle γ= ɸ−Ψ
(6)
(7)
The surface azimuth measured from the south for the orientations N, NE, E, SE, S, SW, W, and NW are 1800, -1350, -900, -450, 00, 450, 900, and 1350, respectively (ASHRAE, 2003 [34]). Angle of incidence /0(6 = /0(./0(7/0(8 − (!).(!)8
Terrestrial solar irradiance on a clear atmosphere day is given by < 9:; = =>?(@⁄(!).)
(8)
(9)
Incident direct solar radiation on glazing is given by Eq. (10) IC = ICD cos θ II
= CICD
(10)
KLMN O
Incident diffused solar radiation from the sky onto the glazing can be computed by Eq. (11)
(11) The incident reflected radiation from the ground surface onto the glazing is given by Eq. (12) IP
C
= (C + sin β)ICD ρT
KLMN O
(12)
Where, the constants A, B, and C are used for calculating solar radiation per hour in Indian climates [36,37] Incident total solar radiation onto the glazing is presented by Eq. (13) IU = (IC + II + IP C ) (13)
Total solar radiation passing through a single glazing window can be obtained from Eq. (14) I
PV
= (IC
+ II
I
CPV
= (IC
+ II
+ IP
C ). WT
+
U A h[
\ . AP
(14) Total solar radiation passing through double glazed window glass is given by Eq. (15) + IP
C ). WT
α N + α[ + U( + αN XCT )\ . AP h[
Where, U = 1⁄_1⁄h[ + dx1⁄K1 + CT + dx2⁄K2 + 1⁄hN d CeT = 1
(15)
CT is the thermal resistance of the air gap between two glasses t eT t eT h x g1.25 + j2.32X lmn1 + ow rs − w tuw eT
eT
f v Where (CT , hN , h[ are considered as per standards (CIBSE, 2006 [38]))
(16)
The results obtained were compared with results found in the literature Ishwar et.al (2011) on this
subject for validation purposes. The validation of the MATLAB program was carried out for a 3 mm clear glass window in the composite climatic zone of New Delhi (28.580N, 77.200E). The deviation of the validation results was within ±1%. Hence, the program was considered to be reliable for studying other glasses [10, 20]. The mathematical model computes heat gain through the glass (without frame) and it does not consider the glass frame. This model does not consider the infiltration loads and internal loads of the building. 4. Cost Analysis Methodology The savings in yearly net cooling and heating cost of double glazed glass combinations in all eight orientations were determined for the composite climatic region of New Delhi. For computing net cost savings annually, the following procedure has been followed [39]. The mean diurnal incident total solar radiation onto the glazing during any season can be computed by Eq. (13). For computing incident solar radiation on glazing in both summer and winter seasons, the following months are considered in the study: i.) Summer season is from April to August ii.) Winter season is from September to March. The no. of days in each month is also considered for computing average daily incident total solar radiation onto the glazing. The total summer solar radiation incident on the glazing (QSummer) can be obtained from Eqs. (17). Q
{
|}
= (IU X30)
•}N€
+ (IU X31)•e‚ + (IU X30)ƒ{
(IU X31)ƒ{€‚ + (IU X31)
{T{M„
|
+
(17)
Where, ITS is the summer diurnal mean solar radiation incident on glazing (Direct + Diffuse + Reflected radiation). The total winter solar radiation incident on the glazing (QWinter) can be computed using Eqs. (18). …†!)‡=ˆ = (9‰† Š$+)‹=?‡=Œ•=ˆ + (9‰† Š$Ž)•/‡0•=ˆ + (9‰† Š$+);0•=Œ•=ˆ
+ (9‰† Š$Ž):=/=Œ•=ˆ + (9‰† Š$Ž)‘")’"ˆ“ + (9‰† Š#”)•=•ˆ’"ˆ“ + (…‰† Š$Ž)–"ˆ/2
(18)
Where, ITW is the winter daily average solar radiation incident on glazing The reduced annual cooling load and increased annual heating load can be computed by using Eqs. (19) and (20), respectively.
Reduced cooling load = Q
{
Increased heating load = Q VN
|}
× AP × (SHGCCPžP − SHGCCP
„|} ×
AP × (SHGCCPžP − SHGCCP
P)
P)
(19)
(20)
Where, SHGCDGCG and SHGCDGRG are SHGC of double-glazed clear glass and double-glazed reflective glasses, respectively. The solar optical properties of double glazed clear glass: transmittance, reflectance, and absorbtance are 68%, 13%, and 19%, respectively. The unit cost of natural gas and electricity is $ 0.45/therm and $ 0.073 kWh, respectively. The efficiency of the furnace and COP of the cooling system are taken as 0.8 and 2.5, respectively. 1Therm =29.31 kWh. Reduced cooling costs
= (Reduced cooling load) (Electricity cost per unit) / (COP)
Increased heating costs = (Increased heating load) (unit cost of fuel) /
(Efficiency)
The net yearly cost savings
= Reduced cooling costs – Increased heating costs Simple payback period =
© •€| | „e„N[ ª[M„ {e€ ª[M„ Me«N TM
Implementation cost = Unit price of glazing ($/m2) × Glass area (m2)
(21)
(22)
(23)
(24) (25)
5. Results and discussions 5.1 Solar heat gain through a window system of double glazed bronze reflective glass combinations Fig. 4 shows the alignment of double bronze reflective glass window combinations. They are arranged from C1 to C5 as shown in fig. The bronze reflective glass window was exposed to the
outer environment and other reflective glasses were kept toward the inner side one after the other as presented below. Total solar radiation passing through these combinations of glasses was computed on a peak hot summer and on a peak cold winter day in New Delhi climatic conditions and the results are tabulated in Table 2. Similarly, various combinations of other reflective glasses have also been considered for the study from C6 to C30 as presented in Fig. 5.
Figure 4. Double bronze reflective glass window combinations from C1 to C5 with an air space of 10mm
Tables 2 and 3 present the solar heat gain through a window system of double glazed bronze reflective glass combinations in all eight orientations during summer and winter climates of New Delhi climatic region. They are graphically presented in Fig. 5.
Table 2. Solar heat gain through a window system of double-glazed bronze reflective glass combinations during summer and winter (kW)
From the graphs, it is noticed that the south-oriented double-glazed bronze reflective glass gains the lowest and highest heat during both summer and winter, respectively among all the orientations studied. The double bronze reflective glass combination C5 (Bronze reflective glass with gold reflective glass) is observed to be the best option for reduced heat gain inside the building among all other bronze reflective glass combinations in all orientations during summer. During winter, C4 is observed to be the best option for reduced heating loads among all the bronze reflective glasses studied in all orientations.
Figure 5. Solar heat gain through a window system of double glazed bronze reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
5.2 Yearly cost savings of a window system of double glazed bronzed reflective glass combinations in the composite climatic zone of India Fig. 6 depicts the graph between a window system of double-glazed bronze reflective glass combinations and yearly cost savings ($/annum), in all eight orientations of New Delhi climatic region. From the figure, it can be seen that the window system of bronze reflective glass combination C5 is the most energy-efficient and saves the maximum cost when compared to other double glazed bronze window combinations. It accounts for an annual cost saving of $ 72.6 when compared to double glazed clear glass in the southeast direction. The best window orientation from the point of view of maximum annual cost saving (cooling cost +heating cost) is in the order of SE, SW, S, NE, NW, N, E, and W. From the results, it is observed that the placing of windows in SE, SW, and S saves the highest yearly net cooling and heating cost among all eight orientations studied. The deviation in the yearly net cooling and heating cost of SE, SW, and S orientations is very less as presented in Fig. 6. Figure 6. Yearly cost savings of double-glazed bronze-reflective glass combinations in New Delhi climatic region
5.3 Solar heat gain through a window system of double glazed green reflective glass combinations Solar heat gain through a window system of double glazed green reflective glass combinations (C6 to C10), for New Delhi climatic region on a peak summer day and on a peak winter day, in all eight cardinal directions was estimated.
Figure. 7. Solar heat gain through a window system of double glazed green reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
It is noticed from the graphs that, among all orientations studied the south-oriented window system of double glazed green reflective glass gains the lowest and the highest heat during summer and
winter, respectively. The window system of double glazed green reflective glass combination C9 (Green reflective glass with gold reflective double glazing) is observed to be the best option among all other green reflective glass combinations in all orientations during summer for reducing cooling load inside the building. During winter, C8 is observed to be the best option among all the other green reflective glasses studied in all orientations for reducing heating loads. 5.4 Yearly cost savings of a window system of double glazed green reflective glass combinations in the composite climatic zone of India Fig. 8 shows yearly cost saving of double-glazed green-reflective glass combinations, in all eight orientations in New Delhi climatic region. From the figure, it is noticed that the window system of green reflective glass combination C9 is the most energy-efficient when compared to all other double green reflective window glass combinations. It reports the highest net cost-saving (cooling + heating) of $ 76.85 (per annum) in comparison with the window system of double glazed clear glass in the southeast direction. From the results, it is noticed that the placing of windows in SE, SW and S direction saves the highest yearly net cooling and heating cost among all eight orientations studied. The deviation in the yearly net cooling and heating cost savings of SE, SW and S orientations is very less as presented in Fig. 8. Figure 8. Yearly cost savings of double-glazed green-reflective glass combinations in New Delhi climatic region
5.5 Solar heat gain through a window system of double glazed grey reflective glass combinations Solar heat gain through a window system of double glazed grey reflective glass combinations (C11 to C15) for New Delhi climatic region on a peak summer day and on a peak winter day, in all eight cardinal directions was estimated.
Figure 9. Solar heat gain through a window system of double glazed grey reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
From the graphs, it is evident that the south-oriented window system of double glazed grey reflective glass gains the lowest and highest heat during summer and winter, respectively among all orientations studied. The window system of double grey reflective glass combination C13 (Grey reflective glass with gold reflective double glazing) is reported to be the best option among the other grey reflective glass combinations for reducing cooling load inside the building in all orientations during summer. During winter, C12 is observed to be the best option among other grey reflective glasses studied for reducing heating loads in all orientations. 5.6 Yearly cost savings of a window system of double glazed grey reflective glass combinations in the composite climatic zone of India
Fig. 10 depicts yearly cost savings of double glazed grey reflective glass, in all eight orientations of New Delhi climatic region. From the graph, it is evident that the window system of grey reflective glass combination C13 is the most energy-efficient among all other double glazed grey reflective glass combinations and it reports the highest net annual cost saving (cooling cost +heating cost) of $79.292 in comparison with the window system of double glazed clear reflective glass in the southeast direction.
Figure 10. Yearly cost savings of double-glazed grey-reflective glass combinations in New Delhi climatic region
5.7 Solar heat gain through a window system of double glazed opal blue reflective glass combinations
Solar heat gain through a window system of double glazed opal blue reflective glass combinations (C16 to C20) for New Delhi climatic region on a peak hot summer day and on a peak winter day, from all eight cardinal directions, was estimated.
Figure 11. Solar heat gain through a window system of double glazed opal blue reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations From Fig. 11, it is evident that the south-oriented window system of double-glazed opal blue reflective glass gains the lowest and highest heat during summer and winter, respectively among all orientations studied. The window system of double opal blue reflective glass combination C17 (Opal blue reflective glass with gold reflective double glazing) is reported to be the best selection for reducing cooling load inside the building among all another opal blue reflective glass combinations in all orientations during summer. During winter, C16 is observed to be the best option for reducing heating load among all the other opal blue reflective glasses studied in all orientations. 5.8 Yearly cost savings of a window system of double glazed opal blue reflective glass combinations in the composite climatic zone of India
Fig. 12 presents annual cost savings of double glazed opal blue reflective glass combinations, in all eight orientations of New Delhi climatic zone. From the result, it is evident that the window system of double-glazed opal blue reflective glass combination C17 is the most energy-efficient when compared with another double-glazed opal blue reflective glass combinations studied. It accounts for the highest net cost-saving (cooling + heating) of $ 76.81 (per annum) in comparison with the window system of double glazed normal clear reflective glass in the southeast direction. Figure 12. Yearly cost savings of double-glazed opal blue-reflective glass combinations in New Delhi climatic region.
5.9 Solar heat gain through a window system of double glazed sapphire blue reflective glass combinations Solar heat gain through a window system of double glazed sapphire blue reflective glass combinations (C21 to C25) for New Delhi climatic region on a peak summer day and on a peak winter day, in all eight cardinal directions, was reported in this section.
From Fig.13, it is evident that the south-oriented window system of double sapphire blue reflective glass gains the lowest and highest heat during summer and winter, respectively among all orientations studied. The window system of double sapphire blue reflective glass combination C21 (Sapphire blue reflective glass with gold reflective double glazing) is reported to be the best choice for reducing cooling load inside the building among all other sapphire blue reflective glass combinations in all orientations during summer. During winter, C24 is observed to be the best choice for reducing heating load among all the other sapphire blue reflective glasses studied in all orientations. Figure 13. Solar heat gain through a window system of double glazed sapphire blue reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
5.10 Yearly cost savings of a window system of double glazed sapphire blue reflective glass combinations in the composite climatic zone of India
Fig. 14 depicts yearly cost saving of double glazed sapphire blue reflective glass combinations, in all eight orientations of New Delhi climatic zone. From the results, it is noticed that the window system of sapphire blue reflective glass combination C21 is the most energy-efficient among all other double glazed sapphire blue reflective glass combinations. It accounts for the highest net cost saving of (cooling + heating) of $ 64.96 (per annum), in comparison with the double-glazed window system of clear reflective glass in the southeast direction. Figure 14. Yearly cost savings of double-glazed sapphire blue-reflective glass combinations in New Delhi climatic region 5.11 Solar heat gain through a window system of double glazed gold reflective glass combinations
Solar heat gain through a window system of double glazed gold reflective glass combinations (C26 to C30) for New Delhi climatic region on a peak summer day and on a peak winter day, for all eight cardinal directions was computed and presented in this section.
Figure 15. Solar heat gain through a window system of double glazed gold reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
From Fig. 15, it is evident that the south-oriented window system of double gold reflective glass, gains the lowest and highest heat during summer and winter, respectively among all orientations studied. The window system of double gold reflective glass combination C29 (Gold reflective glass with opal blue reflective double glazing) is reported to be the best choice among all other gold reflective glass combinations in all orientations for reducing cooling load inside the building during summer. During winter, C30 is reported to be the best choice among all other gold reflective glasses studied for reducing the heating load in all orientations. 5.12 Yearly cost savings of a window system of double glazed gold reflective glass combinations in the composite climatic zone of India
Fig. 16 presents yearly cost savings of double-glazed gold reflective glass combinations, in all eight orientations of New Delhi climatic zone. From the results, it is evident that the window system of gold reflective glass combination C27 is the most energy-efficient in comparison with all other double-glazed gold reflective glass combinations. It accounts for the highest net annual cost saving (cooling + heating) of $ 52.226 in comparison with the double-glazed window system of clear reflective glass in the southeast direction. Figure 16. Yearly cost savings of double-glazed gold-reflective glass combinations in New Delhi climatic region
Table 3. Payback period of double-glazed reflective glass window combinations
Figure 17. Payback period of double glazed windows for New Delhi climatic region In summer, the south-facing window is observed to be the best position to place windows, as it receives the least radiation among the eight orientations studied. In winter, the south-facing is
noted to be the best position to place windows, as it gains the highest radiation among the eight orientations studied. The arrangement of window orientations from the best to the worst during summer season, from the viewpoint of least heat gain is S, N, SE, SW, NE, NW, E, and W. The arrangement of window orientations from the best to the worst during winter season, from the viewpoint of highest heat gain is S, SE, SW, E, W, NE, NW, and N. The summer solar heat gain in buildings through the window system of double reflective glass combinations arranged in the order of the best to the worst in south orientation is as follows: C13 (1.784 kW), C15 (1.835 kW), C11 (1.847 kW), C14 (1.884 kW), C9 (1.904 kW), C17(1.906 kW), C12 (1.95kW), C19 (1.952 kW), C7 (1.96 kW), C6(2.001 kW), C10 (2.003 kW), C20 (2.006 kW), C18 (2.008 kW), C8 (2.074 kW), C16 (2.079 kW), C5 (2.265 kW), C1 (2.325 kW), C3 (2.341 kW), C2 (2.394 kW), C4 (2.487 kW), C21 (2.49 kW), C23 (2.571 kW), C25 (2.59 kW), C22 (2.651 kW), C24 (2.654 kW), C27 (3.118 kW), C29 (3.138 kW), C28 (3.196 kW), C26 (3.226 kW), and C30 (3.351 kW) The winter solar heat gain in buildings through the window system of double reflective glass combinations arranged in the order of the best to the worst in south orientation is as follows: C30 (12.279kW), C26 (11.821 kW), C28 (11.712 kW), C29 (11.499 kW), C27 (11.426 kW), C24 (9.725 kW), C22 (9.715kW), C25 (9.492 kW), C23 (9.423 kW), C21 (9.125 kW), C4 (9.113 kW), C2 (8.772 kW), C3 (8.578 kW), C1 (8.519 kW), C5 (8.3 kW), C16 (7.619 kW), C8 (7.601 kW), C18 (7.357 kW), C20 (7.352 kW), C10 (7.34 kW), C6 (7.33 kW), C7 (7.182 kW), C19 (7.154 kW), C12 (7.164 kW), C17 (6.984 kW), C9 (6.976 kW), C14 (6.905 kW), C11 (6.768kW), C15 (6.725 kW), and C13 (6.536 kW). The best double reflective glass in summer is not the best choice in winter. Therefore, the highest annual net cooling and heating cost savings of double reflective glasses have been identified in this manuscript. The annual net cooling and heating cost saving depends on the orientation of window placement and the type of glass combination. The best window orientations from the highest annual net cooling and heating cost to the lowest annual net cooling and heating cost for any glass window can be arranged in an order as SE, SW, S, NE, NW, N, E, and W. From the results, it is noticed that the placing of any glass window in SE, SW, and S saves highest annual net cooling and heating cost among all eight orientations studied. The deviation in yearly net cooling and heating cost saving of SE, SW, and S orientations is very less. From the results, it is evident that the outer reflective glass is the most critical component in the double-glazed window glass arrangement (outer reflective glass + air gap + inner reflective glass) for the heat gained/lost inside a building compared to the inner
reflective glass. The solar transmittance of the outer reflective glass plays an important role in heat gained/lost into building among other solar optical properties of glass. The higher the solar transmittance of the outer reflective glass, the higher is the heat gain into the building. Fig. 17 shows a simple payback of thirty air-filled double glazed windows. From Table 3, it is observed that the payback period for the C17 is the lowest (1.34 years) whereas, the payback period is the highest for the C24 configuration. The configuration C13 is observed to be the best configuration from the highest annual cost savings point of view and it has a slightly higher payback period (1.42 years) than C17 (1.34 years). Table 3 presents a detailed table of annual cost savings and the payback period of thirty double glazed windows. This would help to identify the best airfilled double glazed window glasses for energy-efficient buildings. 6. Conclusion The present work presents thermal and cost analysis of thirty air-filled double-glazed reflective glass windows (C1 to C30) in the composite climatic zone of India (New Delhi (28.580N, 77.200E)) during peak summer and winter days. This paper signifies the annual net cooling and heating cost savings of using these thirty air-filled double-glazed reflective glass windows as a replacement to the conventional glass windows. This paper also presents a payback period of thirty air-filled double-glazed reflective glass windows. This work proposes the optimum window orientation from the perspective of the highest annual net cooling and heating cost savings. The best combination of reflective glasses for double glazed window arrangement has also been proposed. •
From Fig. 10, the window system of double glazed grey reflective glass combination C13 (Grey reflective glass-Air gap 10 mm-Gold reflective glass) is observed to save the highest annual net cooling and heating cost ($ 79.292 per annum in SE direction) combination, in all eight orientations of window location among all the thirty studied configurations of doubleglazed window glasses in the composite climatic zone of India.
•
From the results, it is evident that the outer reflective glass is the most critical component in the double-glazed window glass arrangement (outer reflective glass + air gap + inner reflective glass) for the heat gained/lost inside a building compared to the inner reflective glass. The most influencing solar optical property which decides heat gained/lost in a building is solar transmittance of the outer reflective glass.
•
From Figs. 6, 8, 10, 12, 14, and 16, the window system of double-glazed reflective glass
combinations can be arranged in an order, from the highest energy-efficient point of view to the lowest energy efficient point of view in any of the eight orientations as follows: C13, C15, C11, C14, C9, C17, C12, C19, C7, C6, C10, C20, C18, C8, C16, C5, C1, C3, C2, C4, C21, C23, C25, C22, C24, C27, C29, C28, C26, and C30. •
From Fig. 17, it is observed that the payback period for the C17 is the lowest (1.34 years) whereas, the payback period is the highest for the C24 configuration. The configuration C13 is observed to be the best configuration from the highest annual cost savings point of view and it has a slightly higher payback period (1.42 years) than C17 (1.34 years).
•
The best order of window orientations from the highest annual net cooling and heating cost savings point of view to the lowest annual net cooling and heating cost savings point of view for any glass window is SE, SW, S, NE, NW, N, E, and W.
The results of this paper are beneficial in the design of energy-efficient window systems for reducing air-conditioning loads in buildings. The results are also useful for energy-efficient retrofitting in buildings.
Nomenclature AG
Area of the glass [m2]
A
Solar radiation in the absence of atmosphere [W/m2]
B
Atmospheric extinction coefficient [-]
C
Sky radiation coefficient [-]
Cg
Thermal resistance of the air gap [m2.K/W]
dia
Declination angle [Deg]
h
Hour angle [Deg]
k
Angle of window glass from vertical [Deg]
l
Latitude [Deg]
IDN
Solar radiation at normal incidence [W/m2]
IDSR
Direct solar radiation from the sun [W/m2]
IdSR
Diffuse solar radiation from the sky [W/m2]
IGRD
Ground reflected solar radiation [W/m2]
IT
Total incident solar radiation [W/m2]
Rsi
Inside surface resistance film coefficient [m2.K/W]
Rso
Outside surface resistance film coefficient [m2.K/W]
dx1
Thickness of the outer glass [m]
dx2
Thickness of the inner glass [m]
K1
Thermal conductivity of outside glass [W/m.K]
K2
Thermal conductivity of inside glass [W/m.K]
tag
Thickness of the air space between glasses [m]
nd
Number of days from January 1st
TSOLAR
Solar transmittance [%]
RSOLAR
Solar reflectance [%]
ASOLAR
Solar absorptance [%]
Wag
Width of the air space between glasses [m]
U
Overall heat transfer coefficient [W/m2.K]
C1
BZRGW–Air gap10mm–GRGW
C2
BZRGW–Air gap10mm–GrRGW
C3
BZRGW–Air gap10mm–OPBRGW
C4
BZRGW–Air gap10mm–SPBRGW
C5
BZRGW–Air gap10mm–GLDRGW
C6
GRGW–Air gap10mm–GrRGW
C7
GRGW–Air gap10mm–OPBRGW
C8
GRGW–Air gap10mm–SPBRGW
C9
GRGW–Air gap10mm–GLDRGW
C10
GRGW–Air gap10mm–BZRGW
C11
GrRGW–Air gap10m–OPBRGW
C12
GrRGW–Air gap10mm–SPBRGW
C13
GrRGW–Air gap10m–GLDRGW
C14
GrRGW–Air gap10mm–BZRGW
C15
GrRGW–Air gap10mm–GRGW
C16
OPBRGW–Air gap10mm–SPBRGW
C17
OPBRGW–Air gap10mm–GLDRGW
C18
OPBRGW–Air gap10mm–BZRGW
C19
OPBRGW–Air gap10mm–GRGW
C20
OPBRGW–Air gap10mm–GrRGW
C21
SPBRGW–Air gap10mm–GLDRGW
C22
SPBRGW–Air gap10mm–BZRGW
C23
SPBRGW–Air gap10mm–GRGW
C24
SPBRGW–Air gap10mm–GrRGW
C25
SPBRGW–Air gap10mm–OPBRGW
C26
GLDRGW–Air gap10mm–BZRGW
C27
GLDRGW–Air gap10mm–GRGW
C28
GLDRGW–Air gap10mm–GrRGW
C29
GLDRGW–Air gap10mm–OPBRGW
C30
GLDRGW–Air gap10mm–SPBRGW
BZRGW Bronze reflective glass window GRGW
Green reflective glass window
GrRGW Grey reflective glass window OPBRGW Opal blue reflective glass window SPBRGW Sapphire blue reflective glass window GLDRGW Gold reflective glass window UV
Ultraviolet region
VIS
Visible region
NIR
Near infrared region
Greek letters λ
Wavelength [nm]
∆λ
Wavelength interval [nm]
β
Solar altitude angle [Deg]
Sλ
Relative spectral distribution of the solar radiation [W/m2]
θ
Solar incidence angle [Deg]
ϕ
Solar azimuth angle [Deg]
Ψ
Surface azimuth angle [Deg]
γ
Surface solar azimuth angle [Deg]
ρg
Ground reflectance factor [-]
?? (λ) ρ (λ)
Spectral reflection [%]
α(λ)
Spectral absorption [%]
αo
Solar absorptance of the outside glass [%]
αi
Solar absorptance of the inside glass [%]
Spectral transmission [%]
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LIST OF TABLES:
Table 1. Properties of 5 mm thick reflective glasses S.No. 1. 2. 3. 4. 5. 6.
Window Glass BZRGW GRGW GrRGW OPBRGW SPBRGW GLDRGW
TSOLAR 0.37 0.29 0.26 0.29 0.42 0.55
RSOLAR 0.14 0.14 0.08 0.13 0.11 0.32
ASOLAR 0.49 0.57 0.66 0.58 0.47 0.13
SHGC 0.48 0.42 0.41 0.42 0.53 0.58
Table 2. Solar heat gain through a window system of double-glazed bronze reflective glass combinations during summer and winter (kW) Direction N NE E SE S SW W NW
C1 2.433 2.847 3.034 2.77 2.325 2.77 3.034 2.847
Window system of double-glazed bronze reflective glass combinations Summer Winter C2 C3 C4 C5 C1 C2 C3 C4 2.505 2.45 2.603 2.371 1.86 1.915 1.873 1.99 2.931 2.866 3.045 2.773 2.285 2.353 2.301 2.444 3.124 3.055 3.245 2.956 4.661 4.799 4.693 4.986 2.852 2.789 2.963 2.698 7.149 7.361 7.198 7.647 2.394 2.341 2.487 2.265 8.519 8.772 8.578 9.113 2.852 2.789 2.963 2.698 6.838 7.041 6.885 7.315 3.124 3.055 3.245 2.956 4.441 4.573 4.472 4.751 2.931 2.866 3.045 2.773 2.285 2.353 2.301 2.444
C5 1.812 2.226 4.541 6.965 8.3 6.662 4.327 2.226
Table 3. Payback period of double-glazed reflective glass window combinations Glazing
C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26 C27 C28 C29 C30
Local price rate Implementation excluding tax at the time cost ($) 2 of procurement ($/m ) 24 117.6 25 122.5 24 117.6 31 151.9 20 98 26 127.4 25 122.5 33 161.7 22 107.8 24 117.6 26 127.4 34 166.6 23 112.7 25 122.5 26 127.4 33 161.7 21 102.9 24 117.6 25 122.5 26 127.4 29 142.1 31 151.9 33 161.7 34 166.6 33 161.7 20 98 22 107.8 23 112.7 21 102.9 29 142.1
Annual cost savings ($)
Payback period (Years)
71.3 69.9 71.0 67.9 72.6 74.9 75.7 73.4 76.9 74.8 78.0 75.9 79.3 77.3 78.2 73.3 76.8 74.8 75.9 74.8 65.0 61.7 63.3 61.6 62.9 50.0 52.2 50.6 51.8 47.5
1.65 1.75 1.66 2.24 1.35 1.70 1.62 2.20 1.40 1.57 1.63 2.19 1.42 1.58 1.63 2.21 1.34 1.57 1.61 1.70 2.19 2.46 2.55 2.70 2.57 1.96 2.07 2.23 1.99 2.99
FIGURES:
(a)
(b)
Figure 1. (a) Building with glass window (b) Images of reflective glasses.
(a)
(b)
Figure 2. Spectral characteristics of reflective glass windows (a) Transmission (b) Reflection
Figure 3. Window systems of double reflective glass combinations with an air space of 10mm (C1 to C30).
Figure 4. Double bronze reflective glass window combinations from C1 to C5 with an air space of 10mm
(a)
(b)
Figure 5. Solar heat gain through a window system of double glazed bronze reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
Figure 6. Yearly cost savings of double-glazed bronze-reflective glass combinations in New Delhi climatic region
(a)
(b)
Figure. 7. Solar heat gain through a window system of double glazed green reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
Figure 8. Yearly cost savings of double-glazed green-reflective glass combinations in New Delhi climatic region
(a)
(b)
Figure 9. Solar heat gain through a window system of double glazed grey reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
Figure 10. Yearly cost savings of double-glazed grey-reflective glass combinations in New Delhi climatic region
(a)
(b)
Figure 11. Solar heat gain through a window system of double glazed opal blue reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
Figure 12. Yearly cost savings of double-glazed opal blue-reflective glass combinations in New Delhi climatic region.
(a)
(b)
Figure 13. Solar heat gain through a window system of double glazed sapphire blue reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
Figure 14. Yearly cost savings of double-glazed sapphire blue-reflective glass combinations in New Delhi climatic region
(a)
(b)
Figure 15. Solar heat gain through a window system of double glazed gold reflective glass combinations in buildings during (a) summer and (b) winter season in all orientations
Figure 16. Yearly cost savings of double-glazed gold-reflective glass combinations in New Delhi climatic region
Figure 17. Simple payback period of double glazed windows for New Delhi climatic region
Manuscript Highlights •
Thermal performance of various air filled double glazed reflective windows
•
Annual cost saving and payback periods of various air filled double glazed reflective windows
•
Optimal combination of reflective glasses for double glazing for reduced cooling and heating loads
•
Solar heat gained/lost through double glazed reflective glass windows
•
Windows for sustainable and energy efficient buildings
OCTOBER 09, 2019 To, Editors-in-Chief J. de Brito, J. M. LaFave, and R. Yao Journal of Building Engineering Dear Sir, Authors would like to declare no conflict of interest for manuscript Ms.: JOB 2019 Ref no:1678, entitled “Thermal and Cost Analysis of Various Air Filled Double Glazed Reflective Windows for Energy Efficient Buildings” for Journal of Building Engineering.
Thanks and Best Regards, Yours Sincerely, Dr. SABOOR S