1. Introduction
Sustainable building envelopes have recently received increasing attention due to their benefits in reducing the environmental impact of building development [
1]. The building sector contributes over a third of the world’s total energy consumption and greenhouse gas emissions impact [
2,
3]. Today, many countries worldwide have enacted energy policies to meet Net-Zero Energy Buildings (NZEBs) criteria, responding to such global energy and environmental issues [
1]. The design and development of NZEBs with a sustainable envelope has become a challenge for building architects and designers [
4]. For example, the European Union initiated the Energy Performance of Buildings Directive (EPBD), according to which entire newly developed buildings will be ‘’nearly zero-energy buildings’’ from 2021 [
5]. The US Army issued a policy in January 2014 directing all facilities to adopt net-zero energy policies by reducing energy use and increasing renewable energy production. Gibson [
6] compared and concluded with persistent and successful anchoring of a change in Army culture towards net-zero energy strategies. Sustainable construction is recognized as the best solution for the construction industry to minimize the negative impact of work to achieve the goal of sustainable development and balance environmental, social, and economic factors [
7,
8].
For many years, solar PV systems have been one of the most prominent renewable technologies for building applications [
9]. The advantages of PV systems include green technologies with no noise or pollution and adaptability to various applications. Such benefits make PV systems durable and dependable, with greatly reduced maintenance requirements. On the other hand, PV systems also have some disadvantages, such as high initial costs compared to competing power generation technologies, requiring a relatively large array area to generate a significant amount of power, and the availability of solar radiation resources at a given location [
10]. PV integration into a building is imperative through building-integrated PV (BIPV) or building-attached/applied PV (BAPV) techniques to achieve high-energy-efficient building performance [
11]. BIPV replaces the building envelope with components such as PV modules and directly absorbs solar radiation to generate electrical energy on-site [
12], which is suitable for new construction. BAPV does not replace the structural component, but can be installed directly on the shell or be rooftop mounted. It has a shielding effect in summer and contributes to some impacts such as lowering indoor temperature [
11], which is suitable for the energetic optimization of existing buildings [
13]. Moschetti et al. [
14] studied a Norwegian NZEB with PV modules and low-carbon insulation materials. The results showed that using PV modules was the most effective in reducing operational energy, and the embodied energy and emissions from the materials for NZEBs were significant.
There is already a significant amount of research on the potential of applying PV to rooftop areas [
15,
16,
17,
18]. However, research on the application of PV to building façades only started a few years ago [
19,
20,
21,
22,
23]. The façade, as one of the most important and largest components of a building, could have a significant impact on the sustainability performance of the entire building [
24]. According to the previous literature, façades can help minimize buildings’ harmful environmental effects [
25]. In other studies, façades, floors, and roofs bring large heat losses to the building. The façade causes 60% of the heat losses, while the floor and roof only account for 15% and 25% [
26]. Adding and integrating passive strategies into building envelopes is a step towards achieving NZEBs [
27]. Changying et al. [
28] researched and developed an overall architectural approach to support the design of typical residential high-rise buildings with façade-integrated photovoltaics (FIPV) in Trondheim, Norway. The results showed that roof and façade areas integrated with PV could cover up to 60% of the household energy consumption of an 11-story high-rise building. Adi et al. [
29] explored the power generation potential of building-integrated PV in typical residential building types in Rishon Lezion, Israel, by evaluating the shadows cast on façades and roofs. The results showed that some façades (mainly south- and east-facing) could still significantly contribute to the total solar potential of urban buildings. It is predicted that by 2050 more than half of the world’s PV capacity will be installed on building envelopes [
30].
Multi-skin façade (MSF) technologies with PV systems integrated into building façades have also been considered to improve the indoor thermal environment and reduce cooling and heating demands because they have a high energy-saving potential for building applications. Engineers and architects have introduced the air layer as an internal structure in façade construction in the last decade to create energy-efficient façades. This strategy is widely used in buildings with glass façades to reduce energy consumption [
31]. Several options for incorporating air layers in building envelopes include Trombe walls, solar chimneys, unglazed transpired collectors, double-skin façades (DSFs), three-skin façades, and quadruple-skin façades [
32]. Among them, DSFs have been considered one of the most promising responsive building elements as one of the building retrofitting options [
33,
34,
35]. However, the MSF concept is used in this study to evaluate their performance when applied to commercial office building façades. In principle, the structure and operating mechanism of DSF and MSF are closely similar. The MSF model in this study adds a layer of foam insulation in the system’s innermost layer.
In the literature, although many studies have been published on the thermal and electrical performance of DSF systems [
36,
37,
38,
39], the evaluation of the MSF with renewable systems is still insufficient. The review of existing studies shows that the DSF and MSF systems in this study are similar, so the previous studies on DSF technologies can also be used for the literature review. A DSF consists of two or more layered structures with outer and inner spaces separated by the air cavity thickness [
40]. Moreover, the DSF system also brings aesthetic appearance benefits and sound insulation to the building envelope [
41]. In the case of the south-facing façade, the solar-heated air is used for heating purposes in winter but must be removed in summer to prevent the building from overheating [
42]. As confirmed by several studies, Pomponi et al. [
43] investigated many DSF systems in temperate climates. The cavity size can vary between 0.20 m and 2 m according to building features and circumstances. Such systems can reduce energy consumption by 90% and 30% for heating and cooling, respectively, in buildings. Gratia and De Herde [
44] extensively studied the effects of natural ventilation on DSFs. In another study [
45], the performance of a DSF south façade was optimized by considering multiple configurations and factors. An open configuration has solved the overheating effect in the air cavity to allow air to escape from the cavity [
46]. DSF systems have been studied and adopted as promising passive building technology with renewables. For example, the systems can also be integrated into building-integrated photovoltaic (BIPV) windows, called PV-DSF [
47]. These PV windows can replace the outer layer windows, generating renewable energy. Peng et al. [
48] developed a type of ventilated façade (BIPV) with a DSF. This PV-DSF model can generate electricity on-site and reduce heat gains and heat losses through the building façades. Kim et al. [
49] compared the thermal and daylighting effects of a DSF system with interior and exterior blinds and an office space where no passive technologies were applied. Results showed that the simulated DSF model could save up to 40% on heating, 2% on cooling, and 5% on total consumption, compared to the base case with no blinds or controls. Zomorodian and Tahsildoost [
50] applied the optimal DSF system to reduce the building’s energy consumption. According to the results, DSF configurations reduce 14.8% of the energy consumption of the building.
Commercial buildings typically have a large façade-to-roof area ratio in the buildings category, with façades considered, relatively, more attractive than the roof as possible surfaces for energy production [
51]. Office buildings have one of the highest energy consumption values compared to other building areas [
52]. This study was conducted to evaluate the performance of PV multi-skin façade (PV-MSF) systems integrated into the façade of a medium-sized office building prototype model [
53].
Several factors that affect the performance of the PV system include the solar radiation incident on solar panels, ambient temperature, cell temperature, shading effect, tilt angle, direction, etc. [
10]. Factors that cannot be controlled include solar radiation, ambient temperature, dirt, etc. Factors that can be controlled include tilt angle and direction, installation techniques, etc. For medium/large PV systems, the tilt angle and orientation angle significantly impact the energy and the specific yield [
54]. Some studies on tropical countries have shown that PV arrays facing east receive higher irradiance than those facing the equator [
55], and the orientation influence at a low tilt angle is assumed to be negligible [
56]. Jafarkazemi and Saadabadi [
57] used a simulation method to assess the effect of orientation on the optimal tilt angle of solar panels on power generation. The results showed that the optimal orientation angle is to the south.
Although many studies on PV-DSF or PV-MSF technology have been reported, there are very few studies on changing the tilt angle of PV modules integrated into the skin layer. The differences between the geometric configurations of the system also affect the efficiency and potential of power generation. Hachem et al. [
58] studied the impact of the geometric design of equatorially oriented DSF on energy efficiency. A base case was an office model with a modular area of 3 × 3 m², with a south orientation, in the middle of a twelve-story office building. Results showed that the fold position and the cavity depth significantly impact thermal load and power generation potential, with the total annual power generation potential from the multifold configurations exceeding that of the flat façade by up to 80%. Another study [
59] examined the impact of equatorially oriented façade design on energy efficiency. The author has studied the geometric configuration equivalent to two units (upper plates and lower plates) of the module system by changing the tilt angle and the orientation angle (70°/15° and 60°/20°) of the surface-integrated PV. The results show that the electricity generated by the PV system is integrated into 50% of the façade surface in the form of folded plates, increasing by up to 56% compared to a south-facing flat.
Existing studies have mainly examined the creation of geometric configurations with PV integrated and applied to a space of a given size (module), enclosed and surrounded by adjacent rooms. There are also studies using such modules for attachment to building façades, but the variety of geometries and differences in angular variation of PV panels are still limited. The simulation of certain size modules in a certain space has the advantage that the output data are extracted accurately and intensively, because the installation data are not large. However, the limitation is that if these modules are attached to a building with a large façade area, the output will be inaccurate compared to the module size because the sample installation data are large. In this case, the strong shading effect when many similar modules are installed on building façades leads to strongly deviating result data compared to a module with a specific sample size.
This study proposes various geometric configurations of the PV-MSF system and arranges them uniformly over the south façade of a medium-sized model office building [
53]; the building’s orientation angle is set to south. Changing the tilt angle for PV arrays is the main objective to compare the power generation efficiency of different MSF configurations. The MSF system model of the test facility is used for validation purposes, with the simulation environment set up so that the output parameters match the actual measurement parameters. The validated MSF model is used to assemble the entire south façade of the office building. An additional result is a shading effect on the building energy performance when a row of identical modules is arranged on the building façade. Based on the results, the options between the geometric configurations are compared to find the optimal solution for the power generation potential.
5. Conclusions
A comparative study of heating and cooling energy consumption, power generation potential, and energy yield rate per unit area were quantitatively conducted to examine the impact of different multi-skin façade design integrated photovoltaic (PV-MSF) surfaces installed on the south face of a medium-sized office building, based on the ASHRAE Standard 90.1-2019. The PV-MSF system model was based on the basic geometric configurations developed as pyramids: triangular pyramid (TP) and rectangular pyramid (RP). Twenty-four different patterns were created based on these two configurations, dividing the surfaces into equal units with numbers ranging from one, four, nine and sixteen units. Combined with changing the inclination angle of the PV integrated roof surface based on the local latitude of 36.35 (Daejeon City, South Korea), the remaining two tilt angles were 51.35° and 21.35° (corresponding to the local latitude degrees ±15°).
All the above 24 patterns were compared with the base case, where the PV external surface was installed at an angle of 90° which resulted in the optimal PV installation angle plan for the roof surface of the configurations. Cavity depth had also been studied and compared, and the option had optimal power generation efficiency which could save on heating and cooling demands. Before performing simulations for the analytical investigation, the MSF system was validated with measurement data from an experimental facility. Additionally, the airflow network (AFN) model in EnergyPlus was used to study thermal analysis and power generation performance. The key findings of this study are as follows:
The heating and cooling energy were significantly reduced in all TP and RP configurations compared to the base case. The heating and cooling demands difference between the TP and RP patterns was insignificant. The designs of the above two arrangements acted as window awnings, providing positive effects in the form of effective shading and reducing the building’s energy end uses.
Creating only one unit in one pattern for the TP configuration was most efficient. The highest electricity generation potential pattern was the 1TP_36.35 pattern, with the inclination of the roof-integrated PV corresponding with the local latitude (the local degree 36.35°) since the reach of the peak point was wide enough with the cavity depth of 1.41 m.
For the RP configuration, the roof PV-integrated surface angle according to the local latitude did not significantly impact the power generation potential. The power generation potential of RP patterns tended to increase with a roof slope of 21.35°, 36.35° and 51.35°. Generating nine units in one design was the most efficient power generation potential.
The base case had a much smaller PV module area than the TP patterns, so the energy yield was also lower. However, in terms of energy yield rate per unit area, the base case was slightly more dominant, 17.5% higher than the 1TP_36.35 pattern—which was the highest performance pattern in the TP patterns. The base case had a PV module area close to that of the RP patterns, but the PV power generation potential and energy yield per unit area were the lowest. Specifically, the plan with the highest energy yield value of RP patterns was 9RP_51.35, which was 49.4% higher than the base case and 46.6% higher in comparison based on the energy yield rate per unit area.
Considering the power generation potential and energy yield rate per unit area of the entire patterns of the two TP and RP configurations with the base case, the RP patterns accounted for the highest and most optimal proportion, with the conclusion that RP patterns had a great potential to achieve high efficiency in the design and installation of the MSF systems. RP patterns also improved practical efficiency for smaller PV installation areas, and maintainability was highly appreciated in the event of power generation problems.
This result could help to find specific solutions to increase the number of units (up to nine units) on a surface of a pattern (RP configuration) to decrease the cavity depth and maintain the best-performing PV installation area while keeping the heating and cooling loads of the perimeter zone not significantly affected. As for the TP configuration, it was necessary to increase the cavity depth and install the PV with a roof slope according to the local latitude for power generation efficiency and overall energy efficiency.
The research presented in this study fills the gap in the existing studies regarding applying the PV-MSF system to a large area of a medium-sized office building. We provide an objective assessment result with a large sample file that can be applied to solve energy-saving problems and highlight the role of geometric patterns designed with different built-in complexities used on the building façade, compared to the conventional flat façade module.
The workflow used in this study helps architects, engineers, and investors build and package options earlier to create an energy-saving office building model. In the future, the results can also guide the design of various geometric configurations to further develop kinetic façade systems with renewables.