The effect of CIS BIPV as a shading device on building life cycle energy performance

ABSTRACT

The purpose of this study is to evaluate the effect of CIS Building Integrated Photovoltaic as a shading device on building energy performance and PV life cycle. A multi-faceted BIPV energy evaluation approach is created. Energy performance is evaluated in two climate regions, in three scenarios for two transparency ratios of an office building. The case study indicated that when BIPV was used, in mild climates there was 24.82–41.05%, and in hot climates, 35.21–56.18% net energy consumption decreased. The PVs can produce 6.5–9.1 times the amount of energy consumed during manufacturing over the usage period. Finally, it was shown that the CIS technology contributes to sustainable architecture by reducing the integrated design difficulties in front of BIPV.

KEYWORDS:

• BIPV
• CIS PV
• shading device
• building net energy needs
• CIS PV life cycle energy analyses
• sustainability

1. Introduction

Buildings consume a large amount of the energy produced on Earth. In this study, CIS (Copper indium (di)selenide) PV systems as a shading device in the building have been studied as a proposal on a building scale. Shading devices integrated with PV systems have two cohesive functions such as PV as an active system and a shading device as a passive system. The PV system designed as a shading device is BIPV (Building Integrated PV) system. The BIPV system is an integrated design of PV system with a secondary function on the building envelope and this system is a part of the building envelope (Peng, Huang, and Wu 2011; Aaditya and Mani 2018). It was predicted that shading devices on the building will decrease the cooling load during summer and the PV system will produce electric energy throughout the year and decrease fossil fuel-based energy consumption (buying energy from the grid).

BIPV_shading (the BIPV system as a shading device) should be considered as two separate sub-systems: shading and PV. There are shading devices studies as a reference: Shading device type selection is based on direction (Zorer Gedik 2002; Grobman, Capeluto, and Austern 2016; Elzeyadi 2017), the effect of shading devices on building energy (thermal, lighting) loads (Huang, Niu, and Chung 2011; Zorer Gedik 2001; Ahmad and Reffat 2018; Panteli et al. 2018; Touma and 2Ouahrani 2018).

Many studies evaluated different parameters that affect the efficiency of PV systems (Hwang, Kang, and Kim 2011; Wittkopf et al. 2012; Kalogirou, Agathokleous, and Panayiotou 2013), determining optimum PV module tilt angle (Wang and Chang 2010; Mehleri et al. 2010; Badescu and Iacobescu 2013), building net energy load (Bot et al. 2019) and studies on the integration of the PV system (BIPV systems) with a building envelope (Peng, Huang, and Wu 2011; Hagemann 2004; Biyik et al. 2017; Lovati et al. 2019) and effect on building net energy load (Vincenzo, Kesten, and Infield 2012; Peng et al. 2015; Ekoe A Akata, Njomo, and Agrawal 2017; Dehwah and Asif 2019; Costanzo et al. 2018; Vassiliades et al. 2018; Braun and Rüther 2010; Maghrabie et al. 2021).

In BIPV systems used as shading devices, topics and studies could be summarised as follows. The integration of PV material with the shading element was first proposed in 1998, in Korea (Yoo, Lee, and Lee 1998). One of the topics studied in this field was BIPV_shading evaluation with energy criteria (Vincenzo, Kesten, and Infield 2012; Peng et al. 2015; Ekoe A Akata, Njomo, and Agrawal 2017; Dehwah and Asif 2019; Costanzo et al. 2018; Vassiliades et al. 2018; Braun and Rüther 2010; Maghrabie et al. 2021; Yoo, Lee, and Lee 1998; Yoo and Lee 2002). There were optimisation studies about the BIPV_shading. Some of them were carried out with the criteria of energy use, PV energy production and daylight illumination (Dino 2017; Taveres-Cachat et al. 2019). The dynamic BIPV_shading (Jayathissa et al. 2017) and the movable BIPV_shading (Akbari Paydar 2020) subjects were studied in the field of optimisation. The effect of BIPV_shading on visual comfort (useful daylight illuminance, daylight glare index) with energy criteria was investigated by Mandalaki, Tsoutsos, and Papamanolis (2014). The studies by Mandalaki et al. (2012) and by Fouad, Mohamed, and Shihata (2018) examined the effect of BIPV_shading on daylight. The energy effect of the BIPV_shading with different tilt angles on the building was analysed by Sun and Yang (2009). Studies to select the most suitable BIPV_shading type according to the façade directions are limited to the south façade and a single volume. Different types of BIPV_shading were investigated on the south side with the energy efficiency criterion (Mandalaki et al. 2012). Various types of shading elements integrated with PV have been investigated with many quantitative (energy) and qualitative (visual-comfort) criteria (Stamatakis, Mandalaki, and Tsoutsos 2016). Martinopoulos et al. evaluate in terms of energy consumption and indoor environmental conditions BIPV_shading options for an office building (Martinopoulos et al. 2018). The life cycle assessment for the photovoltaic integrated shading system was researched by Fouad et al. (2019). Some other studies have been done to evaluate the quality of PV integration into architecture (Custódio et al. 2022; Devetaković et al. 2020). In their comprehensive review of the BIPV system design, A. K. Shukla, Sudhakar, and Baredar (2016) presented a twelve-step BIPV system design path. However, the step about ‘PV shading devices’ does not include ‘a way to design BIPV shading’. As seen in this review study, it can be said that there is a need for a guiding study on ‘designing BIPV shading’.

Several studies on BIPV_shading are similar repeated for different locations (Jung 2014; Chang 2010; Kim et al. 2010). The solar data vary according to the location that is used by the PV system and the shading device design. Since the quality and quantity of incident solar radiation changes according to location, it is hard to make universal assumptions and for that reason, local studies become more important.

The main purpose of this study is to explain the effect of CIS BIPV_shading on building energy demand in different climates. In addition, the shadowing effect of this system on the building energy demand is intended to be shown depending on the window-to-wall ratio (WWR). The effect of BIPV_shading on building energy consumption is investigated as a whole and separate manner such as the PV function and shading function. Contrary to many studies with a single zone sample, a whole building sample is studied. Since the shading function of the system affects the heating–cooling loads of the building and the zones of the building are in thermal interaction with each other, the building is considered as a whole. One more important point (scope) of this study is working with CIS thin-film PV. CIS is currently regarded as the most innovative technology associated with the serial production of solar modules (Solar Frontier 2021). The CIS cells, because of their high efficiency and properties, become more and more popular in BIPV applications (Frydrychowicz–Jastrzębska and Bugała 2016). In this study, the most important reasons for choosing CIS technology are the high colour homogeneity (pv Europe 2016) and high shadow tolerance (Solar Frontier 2021) features that facilitate adaptation to the architecture. CIS technology, with its colour homogeneity, gives PVs the freedom to integrate with architectural design in terms of aesthetics. The shade tolerance feature of CIS technology creates flexibility in positioning the PV and in architectural design against the shading effects of the surroundings. In the meantime, a review article (Zhang et al. 2018) on PVs (PVSD) used as shading elements identified eight studies on CIGS (copper indium gallium diselenide, like CIS) PV type and aligned that there is limited knowledge about PVSD. Moreover, a significant feature of the study is the comprehensive analysis and evaluation of energy from the building and PV system aspects of the BIPV system. The main energy indicators such as energy payback time (EPBT) and energy return factor (ERF) were used to evaluate the energy sustainability of the PV system. To sum up, this study takes into consideration the passive systems of the building, energy demand, and energy generated by the photovoltaic, and how they interact in different scenarios. Under energy policies in the World, the importance and request for PV systems are increasing. Therefore, the growing interest in PV systems and the need for local studies are growing the importance of this study. Additionally, unlike other studies, it is shown that the use of per unit BIPV_shading area in a building designed according to the climate will add a considerable contribution to decreasing energy consumption. The methodology presented in this study can be used by architects who want to integrate a renewable energy system (PV) into the buildings at the early design phase, and those working in areas such as green buildings, low-energy building, sustainable cities and sustainable buildings. This study is also important for those who want to obtain a green building certificate, as BIPV_shading systems can provide points from the parts of green building certification rating systems related to renewable energy and building energy consumption.

2. Methodology

The main purpose of this approach is to evaluate the energy performance of the design with (approximate) ∼100% window-to-wall (transparency) ratio by comparing it with the reference design of an example building with PV shading devices in different climatic regions. The flow diagram of the method is given in Figure 1.

Figure 1. The flow diagram of the study.

The flow diagram in Figure 1 is applied to two different climate regions in this study. This flow diagram can be applied to any number of different climate regions and different building types.

Building energy performance is evaluated under three scenarios with reference building and building with ∼100% window-to-wall ratio (building with ∼100% WWR). The building scenario with a window-to- wall ratio of ∼100% is designed with a glass curtain wall system. This scenario is named a ‘building with ∼100% WWR’, taking into account that there are columns close to the glass curtain wall on the perimeter of the building. Three scenarios are neutral position, using a shading device on the building (position with shading device), and using a shading device integrated with the PV system on the building (position with BIPV), respectively. The position with the shading device scenario is the scenario that uses the shading device in the designed BIPV module dimensions. Building scenarios are shown in Table 1.

Table 1. The building scenarios.

	Reference building and Building with ∼100% WWR
İstanbul and Antalya	Neutral position (building without BIPV) Position with a shading device (building with only a shading device applied) Position with BIPV (BIPV applied to build)

Softwares accepted in the field (PVsyst and DesignBuilder-EnergyPlus) (Pabasara et al. 2019) are used for the implementation of the methodology. Studies were examined to show the accuracy of the results of the PV software used. In these studies, energy generation measurements and simulation results of the PV installations are compared. These validation studies are given in Table 2 with PV type, measurement period, location and accuracy parameters.

Table 2. The studies on the accuracy of the PV software used.

Accuracy	Location	Measurement period	PV type	References
Energy-average annual error 3.84% (min 3.77%, max 3.91%).	Spain	3 years	Monocrystalline, Polycrystalline	Nicolás-Martín, Eleftheriadis, and Santos-Martín (2020)
The average deviation was maintained at 1.6%. Deviation from the simulation between 5.0% −0.6% for 6 months.	Sri Lanka	6 months	Unmentioned	Viduruwan and Induranga (2021)
−0.95% deviation (Energy generated in a year) 12-month average deviation is 0.5% (Monthly average energy production)	Palencia, Spain	2008 to 2020	Model FOTONA 180D	González-Peña et al. (2021)
The average difference is at 1%	Switzerland	2002 until 2014	Mono-crystalline	Wittmer, Mermoud, and Schott (2015)
The accuracy is 1.2%.	Switzerland	6 years	CIS module (Shell ST40)	Mermoud and Lejeune (2010)
Differ 9%	Southern Brazil	August 27 and October 31, 2020	Multi-crystalline silicon	Chepp, Gasparin, and Krenzinger (2021)
Yearly average energy was under-estimated by 3.3%	Southern Italy	43 months	Monocrystalline silicon-Sunpower E19/320	Malvoni et al. (2017)
An uncertainty of 0.47% and 0.32%	Southern India	1 year	CIS p-Si	Ramanan, Kalidasa Murugavel, and Karthick (2019)

These studies, which show the differences between the PVsyst software simulation results and the measurements, have been made in different PV types, in various locations, and in measurement periods of up to 12 years. While most are less than 5.0%, their accuracy ranges from 9% to 0.32%. More importantly, in studies with CIS technology, this difference is between 1.2% and 0.47%, so simulation results and measurements are quite close.

2.1. Definition of climate region

Two different cities are chosen in this study to evaluate how the BIPV system effectiveness may change according to the different climate regions. These cities are İstanbul (41° north latitude – 28.8° longittude) and Antalya (36.8° north latitude – 30.7° longitude) in Türkiye. The climate is warm-humid in İstanbul and hot-humid in Antalya. Also, these locations are in a Temperate-Dry summer-Hot summer (Csa) climate according to the Koppen climate classification. Solar radiation annual averages were 1168 kWh/m² for İstanbul and 1390 kWh/m² for Antalya (Özkiliç Keleş 2008). The average highest temperature in İstanbul is 26.9°C (Aug), the average lowest temperature is 3.1°C (Feb) and the average temperature is 14.4°C (Annual). The average highest temperature in Antalya is 34.1°C (Jul), the average lowest temperature is 6.0°C (Jan) and the average temperature is 18.8°C (Annual) (Turkish State Meteorological Service 2019). İstanbul and Antalya global radiation values (Republic of Turkey Ministry of Energy and Natural Resources 2023) by month are shown in Table 3.

Table 3. The city’s global radiation values (KWh/m²-day) by months (Republic of Turkey Ministry of Energy and Natural Resources 2023).

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
İstanbul	2.00	2.57	4.20	5.28	6.30	6.79	6.79	6.07	5.09	3.74	2.37	1.8
Antalya	2.12	2.57	4.37	5.47	6.36	6.93	6.65	6.14	5.16	3.93	2.51	1.92

2.2. Case study building

Within the scope of this study, the building type is accepted as an office building considering the working hours of the day. Both a ∼100% WWR office building and a climate-adapted building (determined as a reference building) have been analysed.

It is assumed that the office building is detached and there is no shadow effect of surroundings. To constitute climatic design, building shape ratios presented by Olgyay for 25°–45° north latitude climate regions are adopted (Olgyay 1963; Yaşa 2013). In İstanbul (range 1/1–2.4 and best 1/1.6) and in Antalya (range 1/1–3 and best 1/1.7), a 1/1.7 aspect ratio which was within recommended range is adopted. To facilitate the comparison of climate scenarios, buildings in İstanbul and Antalya are designed with the same aspect ratio. In addition, reference building design has been made by considering the building bylaw requirements (IBB 2007) and TS 825 thermal insulation requirements for buildings (TSI 2008). The open-planned office building is designed. For climatic design, the building is located in the east–west direction, with the long edge of the building facing south. Dimensions of the reference building are 15.5 m in width, 26.4 m in length, and 3.6 m in storey height. The building consists of four identical storeys. The gross area of the building is 1636 m², and the gross volume is 5892 m³.

In reference building, WWR for the north facade is determined according to at least 20% WWR for daylight utilisation in different studies. 50% WWR in the south facade, and 30% WWR in the east and west facades are adopted. Normal floor plans and elevations are given in Figure 2. Window distribution design on facades is made compliant with interior furnishing depending on window-to-wall ratios. This way, the comfort and efficiency of daylight are increased.

Figure 2. Normal floor plans and elevations.

The U values of building envelope constructions are determined according to U value upper limits suggested by TS 825 thermal insulation requirements for building regulation degree-day region (TSI 2008). The U values of the building envelope constructions of the buildings located in different regulation degree-day regions are indicated in Table 4. Other building constructions are used with default U values. The U values of the internal floor and partition construction of the buildings are 2.93 and 1.64 W/m²K, respectively. Double glass is used in windows. The solar heat gain coefficient (SHGC) of glazing is 0.802 and light transmission is 0.818.

Table 4. The U values of building envelope constructions in İstanbul and Ankara.

	Roof	External wall	Ground floor	Window glazing	Window frame
İstanbul	0.4 W/m^2K	0.6 W/m^2K	0.6 W/m^2K	2.4 W/m^2K	1.75 W/m^2K
Antalya	0.45W/m^2K	0.7 W/m^2K	0.7 W/m^2K	2.4 W/m^2K	1.75 W/m^2K

In the energy model, the shading devices are modelled with 100% opaque material and their thermal and reflective properties are not considered.

Activity variables that should be defined in the simulation programme and HVAC (Heating, Ventilation, and Cooling) system variables are given in Table 5. The variables indicated in Table 5 belong to open-office space, and variable selections for other spaces are different.

Table 5. The Activity and HVAC system variable selections in the building simulation.

Activity		HVAC
Occupancy (density) Working hours (schedule)	0.12 people/m^2 8:30–18:00 (only on weekdays)	Building HVAC systems	Fan Coil Unit (4 pipes), Air-cooled chiller
Activity Metabolic factor Clothing	light office work, standing, walking 0.90 met in winter 1.00 clo, in summer 0.50 clo	Mechanical ventilation Economiser Heat recovery, type	on off on, enthalpy
Heating setpoint temperatures Cooling setpoint temperatures Minimum fresh air Natural ventilation setpoint temperatures (indoor min temperature control)	heating 20°C, heating set back 12°C cooling 26°C, cooling set back 28°C 10 l/s-person 24°C (min)	Heating Fuel Heating system seasonal CoP	on natural gas 0.83
Lighting-target illuminance Lighting control	400 lux On	Cooling Fuel Cooling system seasonal CoP	on electricity 1.67
Internal gains	10 W/m^2 (5W/m^2 computers and 5W/m^2 office equipments)	Natural ventilation Outside air (ac/h) Outdoor temperature limits	on 1.50 on

2.3. The design of fixed shading device integrated with PV system module

Due to the shading function of the BIPV_shading, the type is selected according to the directions and the BIPV_shading is designed as horizontal on the south side and vertically on the east and west sides (Dino 2017).

Horizontal BIPV_shading module design:

In this article, additional studies that determine the optimum PV angle were examined. SOLAREC (European Union solar power electricity operation) research centre data and the annual tilt angle according to a different study in the literature is between 29 ° and 33° for Turkey’s cities (Kisa Ovali 2009; Moral Uğur 2006). The annual performance difference between 10° and 30° in Turkey’s conditions will not exceed 15%. In Turkey, considering the summer and winter seasonal average, the optimum PV tilt angle is acceptable as 30° (Özkiliç Keleş 2008; Yanardağ 2015; Turhan and Çetiner 2012). As a result, an optimum PV tilt angle of the scope of this study throughout the year in Turkey (36°–42° northern latitudes) is accepted 30°.

To determine the desired period of sun protection, the outdoor temperature curves of the relevant city are used (Berköz et al. 1995). To define the BIPV module length required for protection from the sun, the design day when the solar radiation comes the most oblique in the period for which protection is desired is determined. The design hour, on the other hand, is noon local time, when the directional solar radiation is highest in the horizontal plane.

The width of the BIPV is limited by the window widths (1.5 m) (Fouad, Mohamed, and Shihata 2018). The length of the BIPV is found by intersecting the optimum PV tilt angle with the direction of the solar radiation at the elevation angle at the determined day and hour (Figure 3).

Figure 3. The horizontal BIPV system design in the reference building in İstanbul.

Since the PV size is considered as the active area (real area) in the 3D model of the PV software used and this study evaluates the area calculation, a PV module size is assumed to be 0.75 m × 0.56 m. As an example in Figure 3, the PV module placement diagram on the horizontal BIPV module in the reference building in İstanbul is shown. In the BIPV system it is assumed that the areas outside the PV modules are completed with a different material.

In Table 6, the horizontal BIPV system design information in the building scenarios is shown.

Table 6. The horizontal BIPV system design information designed for building scenarios.

Building scenarios	Protection period and design date	BIPV module length	PV modules in a BIPV module
İstanbul Reference building	21°C and over a period: 1 June–1 October	2.22m	8 units
İstanbul Building with ∼100% WWR	21°C and over a period: 1 June–1 October	2.50 m	140 units
Antalya Reference building	21°C and over a period: 1 May–1 November	2.80 m	10 units
Antalya Building with ∼100% WWR	21°C and over a period: 1 May–1 November	3.14 m	188 units

Vertical BIPV_shading module design:

The time ranges from morning to noon on the eastern facade (8:30–12:00) and from noon to afternoon on the west facade (12:00–18:00), are the periods for which sun protection is desired. By studying the shadow curve diagram, the sun protection effect ranges of different BIPV system designs are examined and the vertical BIPV_shading module layout in Figure 4 is considered. The diagram in Figure 4 shows the width of the BIPV modules, the angle between the facade and the distance parameters between each other. The height (h) of the vertical BIPV is limited by the height of the glass surface.

Figure 4. The vertical BIPV system design in the reference building in İstanbul.

In this study, the disadvantages of shading are reduced by using CIS technology, which is less affected by shadows (Solar Frontier 2021). The vertical BIPV system is designed with the same PV module described in the horizontal BIPV system design. As an example in Figure 4, vertical BIPV system design and module placement diagrams are shown in the reference building in İstanbul.

In Table 7, the vertical BIPV system design information in the building scenarios is shown. Model perspectives of the three scenarios are presented in Figure 5.

Figure 5. Perspectives of three scenarios from the İstanbul reference building model (neutral position, position with shading device and position with BIPV).

Table 7. The vertical BIPV system design information designed for building scenarios.

Building scenarios	BIPV-facade angle	Dimensions	PV modules in a BIPV module
Reference building	east facade: + 45° west facade: −45°	d = 0.75m w = 0.75m h = 1.85m	3 units
Building with ∼100% WWR	east facade: + 45° west facade: −45°	d = 0.75m w = 0.75m h = 14.40m	24 units

2.4. Determining the energy needs and energy production

Building energy needs have been modelled using the dynamic thermal simulation software DesignBuilder, which is actually an interface programme using the EnergyPlus simulation engine. An annual energy analysis of the building scenarios in both İstanbul and Antalya is conducted. Net energy and energy consumption per unit area of conditioned building area are used in the evaluation of the energy analyses. Net energy indicates energy amount from grid after subtracting generated energy amount from consumed energy amount of the building.

PV energy generations in this study are calculated with the PVsyst software package. The building model with BIPV in the dynamic thermal simulation software is also created in the PV programme. The PV systems on the east, south and west facades are designed separately and for this reason, a PV system is formed on each of the 3 facades. PVsyst models of the scenarios with BIPV in Istanbul are shown in Figure 6.

Figure 6. PVsyst models of the scenarios with BIPV in Istanbul (the reference building on the left and the building with ∼100% WWR on the right).

The used CIS thin film (cells) technology advantages are good low-light behaviour, high shadow tolerance, high-temperature stability and light soaking effect (Solar Frontier 2021). It can generate electricity not only in sunny weather but also in diffused light (Raf 2021). The nominal conversion efficiency of the modules id equal to 11% for CIS, which is a typical value (Raugei, Bargigli, and Ulgiati 2007).

It is considered that BIPV modules shade their own PV modules, in other words, self-shading. To account for the shadows of objects (building and BIPV) in the PV programme, a model of the PV installation and its surroundings is created and the linear shading calculation method is used. In thin-film modules, there is no electrical mismatch loss if the shades affect all cells of each module in the same way. Thus the ‘Linear shadings’ are representative for calculating shading losses in thin film modules. PVsyst uses a PV model that is very compatible with CIS (PVsyst 2021). The simulation is based on an hourly data file.

The specifications of the PV system and simulation parameters are shown in Table 8.

Table 8. PV system specifications and simulation parameters.

Specifications	Rating/details	Specifications	Rating/details
System type	Grid-Connected Building system	Simulation parameters
Characteristics of a PV module		Models used	Transposition: Perez Diffuse: Perez, Meteonorm Circumsolar: separate
Rough module area	0.42 m^2
STC power	Pnom 40 Wp
Open circuit voltage	Voc 23.3 V
Short circuit current	Isc 2.68 A	Horizon	Free horizon
Characteristics of a grid inverter		Albedo	0.2
	monophased	Nominal PV power-Total number of PV modules/ Total number of inverters
Operating mode	MPTT
Maximum efficiency	Max Eff. 97.0%	Antalya Ref. Build. Antalya ∼100% WWR Build. İstanbul Ref. Build. İstanbul ∼100% WWR Build.	30.4 kWp – 760/9 68.5 kWp – 1712/19 27.2 kWp – 680/8 60.8 kWp – 1520/17
European average efficiency	Euro Eff. 96.3%
Nominal AC Power	Pnom AC 3.0 kWac
Technology	25 kHz, IGBT
Frequency	50–60 Hz	PV modules’ connection type	in series modules and in parallel strings (on each facade)
Oper. Voltage	125–440 V
Power threshold	15W

The energy cover factor (C_PV) is used to determine the ratio of the amount of energy produced by the PV system to meet the electrical energy consumption of the building. C_PV can be calculated with the equation given below (Cellura et al. 2012). (1) $C_{PV}={\displaystyle \frac{E_{PV}}{E_{T,e}}\times 100}$(1) E_PV is the annual energy amount (kWh/y) generated by the PV system and E_T,e is the annual electrical energy consumption of the building (kWh/y).

2.5. Calculation of the PV system energy performance indicators

Considering the whole life cycle of the PV system, energy is also consumed for the production of system components. To evaluate the energy performance of PV systems, life cycle energy analyses related to manufacturing and operation phases are made.

Energy input (E_in) is the total amount of energy needed during the PV system life cycle. The amount of energy (embodied energy values of PV systems) consumed during the production of PV modules and other system components is taken into account as energy input (Nawaz and Tiwari 2006; IEA 2006). Within the scope of other system components (BOS), supporting substructure, inverter and cabling are discussed. The gross energy requirement (total primary energy) of a CIS PV module is 2870 MJ/m² (with frame) (Alsema 2012; Knapp and Jester 2001) and that of BOS is 700 MJ/m² (Frankl et al. 1998). Energy output (E_out) is the amount of energy generated annually by the PV system. E_out is calculated with PVsyst software.

Energy payback time (EPBT) is the ratio of the total energy consumed during the PV system life cycle (E_in) to the annual energy amount generated during the operation phase (E_out) (IEA 2006). The energy return factor (ERF) is the ratio of the amount of energy generated during the PV system operation phase (E_out) to the total amount of energy consumed during the life cycle (E_in) (IEA 2006). EPBT (Alsema 2012, 1998) and ERF (Alsema 1998) are calculated with the following equations: (2) $EPBT={\displaystyle \frac{E_{in}}{E_{out}}}$(2) (3) $ERF={\displaystyle \frac{E_{out}\times N}{E_{in}}}$(3)

Within the scope of the study, the lifetime of the PV system (N) is taken into account as 30 years (Knapp and Jester 2001; IEA 2020). A linear degradation of 0.7%-points per year is assumed in the performance of PV modules over their lifetime (IEA 2020). In this study, the grid efficiency value (31%) determined for the Continental European electricity grid is used (IEA 2006).

The Performance Ratio (PR) is the ratio of the energy effectively produced, for the energy produced if the system was continuously working at its nominal STC efficiency. The PR is defined in the norm IEC EN 61724 and the PR formula is shown below for a grid-connected system: (PVsyst 2021) (4) $PR=E\mathrm{\_}Grid/\left(GlobInc\ast PnomPV\right)$(4) E_Grid is the available energy and PnomPV is the STC-installed power. The PR is used as a performance metric to quantify the overall system losses due to temperature effects, shading and inefficiency of its components (IEA 2020).

3. Results

Energy analysis results of the building scenarios for İstanbul and Antalya are given in Figures 7 and 8, respectively.

Figure 7. The energy analysis results of the building scenarios in İstanbul.

Figure 8. The energy analysis results of the building scenarios in Antalya.

İstanbul and Antalya energy results are explained in terms of ‘Reference building’ and ‘Building with ∼100% WWR’. In addition, some energy ratio results are obtained by comparing the scenarios. The details are shared below.

As shown in Figure 7, using BIPV_shading on a reference building in İstanbul, the grid-based net energy consumption of the building decreased 24.82% compared to the neutral state of the building and 11.09% of this ratio is generated with PV system function. In the scenario where only shading devices are used, it can be seen that a 13.73% decrease in net energy consumption is achieved compared to the neutral state.

As shown in Figure 8, by using BIPV_shading on the reference building in Antalya, the grid-based net energy consumption of the building decreased by 35.21% compared to the neutral state of the building, 18.93% of this ratio is generated with shading function, and 16.28% is generated with PV system function.

As shown in Figure 7, using BIPV_shading on a building with ∼100% WWR in İstanbul, the grid-based net energy consumption of the building decreased by 41.05% compared to the neutral state of the building. In the scenario where only shading devices are used, it can be seen that a 27.09% decrease in net energy consumption is achieved compared to the neutral state. When the positions with the shading device and the position with BIPV_shading are compared, it is seen that the PV system function decreased the energy consumption of the building by 13.96%.

As shown in Figure 8, using BIPV_shading on a building with ∼100% WWR in Antalya, the grid-based net energy consumption of the building decreased by 56.18% compared to the neutral state of the building, 36.12% of this ratio is generated with shading function, and 20.06% is generated with PV system function.

The performance Ratio is given separately (Eranki and Mani 2020) according to the facades of each building scenario. In this way, performance ratios of PV systems can be evaluated separately according to BIPV_shading type (horizontal, vertical) and orientation.

The electrical energy consumption of the building, considered in the C_PV calculation, consists of cooling, lighting and interior equipment (other) energy needs. The calculation results of the energy indicators and energy cover factor of PV systems are given in Table 9.

Table 9. The calculation results of the energy indicators and C_PV of PV systems.

Building scenarios	Ein (kWh) (primary energy)	Eout (kWh/y) (primary energy)	EPBT (year)	ERF	Performance ratio	CPV
İstanbul Reference building	283220	67493.548	4.2	6.5	West 55% East 56% South 66%	18.087%
Antalya Reference building	316540	106032.258	3	9.1	West 55% East 56% South 63%	22.324%
İstanbul Building with ∼100% WWR	633080	128870.967	4.9	7.4	West 50% East 51% South 61%	25.726%
Antalya Building with ∼100% WWR	713048	203774.193	3.5	7.8	West 50% East 51% South 56%	34.557%

4. Discussion

The following inferences were made by comparing building scenarios in terms of climate regions and building envelopes.

When the neutral state of building scenarios in Antalya is compared with the same scenarios in İstanbul, it is seen that the net energy load is higher in Antalya. This is principally due to the higher cooling loads in Antalya than in Istanbul. Net energy loads of BIPV and shading device scenarios in Antalya decreased more than those in Istanbul. One of the reasons for such results is that more electrical energy is produced by the PV system due to the higher sunshine hours and solar radiation values in the hot climate region. In addition, another factor is larger PV areas in BIPV design in Antalya. One more reason is in hot climate regions with a higher cooling load, the shading function has a higher energy consumption decrease. As a result of the comparison of the places in these different latitudes and different climate regions, it has been seen that BIPV_shading efficiency decreases from hot climate regions to warm climate regions.

The energy consumption difference between buildings with ∼100% WWR and reference buildings in a neutral state in both cities is dominantly caused by the cooling load. Since the building with ∼100% WWR had higher daylight usage, the lighting load is lower than the reference building. According to the energy analysis of the building scenarios, the share of PV function in the total reduction rate in the grid-based net energy consumption of the building is high in the reference building.

When BIPV is evaluated separately according to system functions, the following results are as follows:

While BIPV system shading function decreased the cooling load in both the reference building and ∼100% WWR building, the PV system function generated energy and decreased fossil fuel-based grid energy consumption of the building. BIPV_shading reduces the benefit from sunlight and increases lighting load at certain levels. Additionally, during the heating period, BIPV_shading decreases solar heat gain and increases building heating load in some circumstances. When a decrease in cooling load and energy generation by BIPV_shading are considered, such an increase in heating load is insignificant. A decrease in building energy load is generated by the shading function during the cooling period of BIPV_shading. A decrease in net grid-based energy consumption is the total gain of shading and energy production functions. The shading function affecting cooling load is more effective on (∼100% WWR) buildings that had a higher cooling load. Also, the bigger size of the BIPV_shading in ∼100% WWR building is effective on this result.

The shading function had a higher contribution to the total gain provided by BIPV_shading. Given the low PV yields according to the current PV technology, the BIPV_shading gain will increase as the efficiency of PV technologies increases.

The study is evaluated in terms of BIPV energy gain and the following results are obtained:

In Table 10, the energy gain of BIPV_shading and only the energy production of the PV function are presented for the unit area. PV energy production indicates the energy amount produced by only the PV system. BIPV_shading energy gain (Sun, Lu, and Yang 2012) represents energy gain due to decreased cooling load caused by the shading effect of the system and PVs’ energy production. In short, this gain value is the total decrease value provided by the net energy load of the building in the neutral state. Information regarded PV energy production and BIPV_shading energy gain are average of all PV or BIPV_shading on the east, south, and west facade.

Table 10. PV energy production and BIPV_shading energy gain based on building scenario.

Scenarios	İstanbul		Antalya
	Reference building	Building with ∼100% WWR	Reference building	Building with ∼100% WWR
1 m^2 PV energy production	73.260 kWh	62.578 kWh	102.976 kWh	87.853 kWh
1 m^2 BIPVshading energy gain	150.459 kWh	161.527 kWh	205.423 kWh	222.582 kWh

A higher energy gain of 1 m² BIPV_shading in a building with ∼100% WWR compared to a reference building is caused by the fact that the shading effect is more effective since the cooling load is higher in a building with ∼100% WWR. However, the ratio of the energy gain from 1 m² BIPV_shading to the total energy load of the building in the neutral state should be considered. This reduction rate is lower in the ∼100% WWR building than in the reference building because the energy load in the ∼100% WWR building is higher.

The rate (ratio) which provides building energy needs with energy gain obtained from 1 m² BIPV_shading is higher in a reference building adapted to climate compared to ∼100% WWR building. It is seen that BIPV_shading is more effective in climatic building design (in terms of affecting the building total net energy load). Accordingly, it can be suggested that buildings should be designed and adapted to climate and then equipped with active systems such as PV (BIPV).

The energy cover factor is between 18.1% and 34.6% in scenarios and is a significant share in meeting the electricity needs of buildings.

The PV part of the BIPV system is evaluated with the PV system energy performance indicators.

The EPBT values of the building scenarios are calculated between 3 and 4.9 years. The PV system pays back the energy spent in its manufacturing phase within 30 years of system life. In both cities, the EPBT is longer at ∼100% WWR building compared to the reference building.

It is determined that the PV systems can produce 6.5 to 9.1 (ERF values) times the amount of energy consumed during the manufacturing stages over a 30-year usage period. The ERF value in Antalya is more than the ERF value in İstanbul.

PR is lower in ∼100% WWR building scenarios compared to reference building scenarios. When the same scenarios of İstanbul and Antalya are compared, it is seen that the PR on the south facades of Antalya are less than İstanbul. The PV performance ratios on the east facades (and between the west facades) of İstanbul and Antalya building scenarios in the same building envelope are the same. There is a strong temperature dependence on the PR (Reich et al. 2012). Performance ratios lower than 66% may be due to temperature effects and shading.

5. Conclusions

In this paper, the BIPV_shading design approach is presented and applied in an office building example in different climates and different building envelopes. Furthermore, it focuses on ‘the impact of BIPV_shading on building net energy needs’, ‘PV system life cycle’ and ‘CIS type PV use’. It is a study that has results in line with directly and indirectly many of the United Nations Sustainable Development Goals. Detailed results of the study are listed below.

A unique BIPV_shading design approach represented with cases that can be used by BIPV_shading designers is presented. The effect of BIPV_shading on building energy use is exemplified by different scenarios. In this way, the BIPV_shading energy gains that will be most beneficial in the architectural design have been explored.

The study suggests that CIS PV panels can function as shading devices, not only on the roof but also on glazed facades, serving a dual function: generating electricity and shading fenestration. In this way, the building’s surface area where photovoltaics can be used increases.

One of the contributions of this study is to show how effective the PV and shading functions of the BIPV_shading system are in different climates and different building envelopes.

In terms of reducing the building energy load, CIS BIPV_shading is effective mainly in sunny and warm (hot) climates.

To increase the effect of (per unit area) BIPV_shading on the role of reducing the building energy load, it has been observed that the energy load must be reduced by building design. This study leads how BIPV_shading can be dealt with in the building design process.

The ratio of 1 m² BIPV_shading energy gain to building energy requirement is a concept presented in this study. This concept, described in the context of the study, shows the extent to which the BIPV_shading can be effective in reducing the building's energy load. Also, BIPV_shading is the way of displaying the function of PV in a building in subscript format and is offered here.

A multi-faceted BIPV energy evaluation approach that covers the effect of the BIPV system on the building energy need, the effect on the grid energy demand and the PV system life cycle energy analysis is created.

With the energy gain of the BIPV system, the grid electricity demand is reduced. In addition, since the CIS PV system produces more than the energy input, CO2 emissions can also be reduced.

With the effect of the shade tolerance feature of CIS technology, the case study showed successful energy performance, despite the self-shading of BIPVshadings. The shade tolerance feature of CIS technology, and colour homogeneity feature, bring CIS-type PV to the fore by providing flexibility in terms of the freedom of architectural design that integrates with the PV. Most importantly, all this contributes to sustainable architecture by reducing the integrated design difficulties in front of BIPV.

As a result, the use of CIS BIPVshading in the sample office building can sustainably integrate the renewable energy system (PV) into the building with successful energy performance, in addition to providing energy gain in all the scenarios discussed.

In future studies, the BIPV_shading design approach can be improved in terms of glare and daylight illuminance level.

Disclosure statement

No potential conflict of interest was reported by the author(s).