**1. Introduction**

Buildings in Europe are responsible for 36% of all greenhouse gas emissions. To fulfil targets presented in the Paris climate agreement, emissions in the building sector must be decreased by 90% [1]. Building integrated photovoltaics (BIPV) in form of façade structures are solutions that can significantly contribute to this goal, as well as increase the share of renewable energy in the final energy supply. As such, BIPV are one of the essential technologies for the nearly zero energy buildings. Regarding to the structure of building stock and for ensuring the cost effectiveness of BIPV in general, solutions for refurbishment of buildings are of grea<sup>t</sup> interest [2]. Among several design options, BIPV glazed façade with a natural or forced ventilated air gap has several comparative advantages and are from the architectural perspective upgraded double ventilated façades [3].

One way to fulfil this goal is in the multi-functionality of BIPV solutions. Relative low efficiency of solar energy utilization with PV cells can be improved by solar concentrators or tracking devices, although in case of BIPV applications, it is more convenient to upgrade PV modules to combine power and heat generators to so called photovoltaic thermal building structures (BIPV/T). The liquid heat transfer media can be used to supply the heat to the buildings [4,5], although preheating of ventilation air for building ventilation is a better option because such applications operate as a low exergy system [6]. In [7] opaque PV modules are cooled by air flowing through the forced ventilated air gap and authors report that the overall energy efficiency during the winter months was in the range between 48% to 52% on the monthly basis. Analysis of natural ventilated semitransparent BIPV designed as a double façade structure is shown in [8] showing energy and environmental advantages of semitransparent BIPV over the opaque PV modules. In [9] authors propose solutions for forced ventilated close loop BIPV/T based on the analytical modeling and point out the need for experimental validation.

The second most common cited advantage of ventilated BIPV structures is the increase of the PV cell e fficiency by cooling. Buoyancy driven natural ventilated semitransparent BIPV consisting of see-through a-Si PV cells was studied by [10]. Authors have shown that daily energy output can be increased by 1.9% to 3% due to the lower operating PV cell temperature. Ventilated BIPV façade was studied by [11] and authors claim that the PV modules e fficiency can be increased by 2.2% on the annual basis in case of natural ventilation and up to 4.7% to 5.7% in cases of forced ventilation with di fferent air flow rates. Similar results, 2.5% increase in annual electricity production by ventilated façade mounted opaque PV modules, are reported in [12].

Ventilation of buildings significantly contributes to the wellbeing in buildings. Not only bioe ffluents, but pollutants such as formaldehyde and odors, can be e fficiently removed from indoor air [13]. Proper indoor air quality (IAQ) increases the occupant's productivity as well [14]. Mechanical ventilation systems with heat recovery could e fficiently reduce heat demand, but significantly increase the electricity demand, especially in commercial buildings. Nevertheless, in [15] it is shown that in all-glass buildings with BIPV façade structures, electricity demand for ventilation, even in case of the ventilation systems that fulfill present requirements about energy e fficiency, is dominant compared to heating, cooling, and lighting systems, measured by primary energy needed. Central mechanical ventilation systems are di fficult to adjust to the presence of occupancies and their personal physiological needs. Furthermore, decentralized ventilation can be energy e fficient in most European climate regions [16]. Dynamic insulation is a part of the building envelope where outdoor air passes through a porous thermal insulation layer towards the interior and redirects heat loss flux. Research on this technique is shown in [17]. The authors have shown that dynamic thermal transmittance of a ventilated structure having static *U* 0.3 <sup>W</sup>/m2K decreases to 0.15 <sup>W</sup>/m2K at air flow rate 0.75 1/s per m<sup>2</sup> of the building structure area. Similarly, transmission heat losses of the envelope building structure can be decreased if the air gap, designed inside the building structures, is ventilated by outdoor air, and then transferred into the buildings as preheated air. In [18] building ventilated opaque BIPV was studied at a steady state outdoor temperature, solar irradiation, and wind velocity. Indoor heat gains were investigated using computational fluid dynamics (CFD) techniques and compared to static heat losses expressed by thermal transmittance *U* of the building envelope structure.

In the presented article, a multi-functional modular semitransparent BIPV glazed façade structure was designed, built, in-situ tested, and analyzed during winter-time conditions. M-Si PV cells are built in the BIPV in form of double side glass laminated PV module with 60% PV cell packing factor, which make the BIPV is semitransparent. Modular units can be multiplied according to needs of particular flat/office/building in new, as well as in renovated buildings. Thick temperate glass layers (4 mm each) of BIPV also result in the specific thermal response of the structure. Several aspects of functionality of solar energy utilization are addressed and evaluated, such as (a) electricity production, preheating of air for space ventilation and dynamic thermal insulation performance; (b) overall e fficiency of solar energy utilization is determined in form of approximation multi-parametric model, providing a tool for evaluation of such BIPV glazed façade structure in di fferent climate conditions; and (c) all models are developed on the base of diurnal averaging of independent variables, which enables integration in buildings thermal response models.

#### **2. Object of Research and Research Methods**

#### *2.1. Semitransparent BIPV Glazed Façade Structure with Forced Ventilated Air Gap*

In general term, BIPV are multifunctional devices because they incorporate passive functions of ordinary façade (or roof) structures, for example, precipitation and sound protection with active renewable energy utilization. In this way lower production of electricity compared to self-stand systems, as a consequence of the position of installation defined by the building envelope, can be compensated at least in terms of investment. Nevertheless, in the presented article, the multifunctional nature of the examined BIPV glazed façade structure is evaluated in terms of utilization of solar energy thereby increasing energy efficiency of the building, while improving indoor living comfort. A pilot BIPV glazed façade structure was designed to fit new buildings and could be used for energy refurbishment as well, possibly eliminating the need for additional thermal insulation. It consisted of a transparent glass façade with integrated PV cells and forced ventilated air gap which, beside electricity production, enabled preheating of fresh supply air and it to act as dynamic thermal insulation (Figure 1). In this way, the heat losses of façade envelope decrease as consequence of the lower (static) thermal transmittance *Ust* and because part of transition heat losses preheats the air that flows inside the ventilated air gap. Both effects are evaluated with dynamic thermal transmittance *Ue*ff. To increase the efficiency of solar energy utilization, the BIPV was designed as a semitransparent structure with 60% of opaque PV cell area. Another reason for this BIPV design follows from optimization of multilayer glazing according to the natural heating, shading, daylight, and occupancies view towards the outdoor environment [15]. A similar conclusion is presented in [8]. In this way, both the ventilated BIPV on the opaque façade structure and BIPV glazing can be combined with the same architectural appearance of the building. BIPV were produced by [19] and consist of two 4 mm hard glass panes and encapsulated layer. Monocrystalline silicon cells with reference efficiency 18.5% [19] and size 156 × 156 mm are installed in the BIPV glazed structure.

**Figure 1.** Experimental semitransparent building integrated photovoltaics (BIPV) glazed façade structure (shown by the rectangle section). (**a**) The façade wall with two ventilation openings, each of them was enclosed by a fan; (**b**) BIPV glazed structure installed 80 mm in front of the façade wall, and (**c**) interior of experimental BIPV façade structure—air channels were thermal insulated during the experiment.

The BIPV glazed façade structure was installed in front of the opaque south orientated façade wall of laboratory unit in the way, and an 80 mm thick air gap, which was between BIPV glazed structure and façade wall, was formed. The façade wall (Figure 1a) consisted of a thermal insulation layer (*d* 0.035 m, λ 0.035 W/mK), lined by an inner and outer layer of solid wood (*d* 0.005 m, λ 0.14 W/mK) made of two transversely glued soft wood plates. Such building structures are often installed as opaque parapet as part of glazed building façade structures, since its thermal transmittance *Ust* (1.027 <sup>W</sup>/m2K) does not exceed the common required level for glazed façades (e.g., in Slovenia *Umax* equals 1.3 <sup>W</sup>/m2K). The surface of the structure has absorptivity of solar irradiation α*s* 0.65 and emissivity of IR irradiation ε*IR* 0.9, which is close to that of concrete façade structures. The section of the experimental BIPV glazed façade structure is 0.435 m wide and 1.167 m high, with area *ABIPV* 0,508 m2. All support structures, such as the installing frame and outdoor and indoor air channels were thermal insulated to keep heat transfer close to the 2D problem.

The air gap was forced ventilated by two DC fans with power of 2 W at a supply voltage of 12 V. By changing the supply voltage, ventilation air flow rate . *Va*,*in* was set on a daily basis to the value between 0 m<sup>3</sup>/<sup>h</sup> and 61.5 m<sup>3</sup>/<sup>h</sup> and kept constant all day long. If reverse flow fans were used instead, the overheating protection by forced ventilation of the air gap with cooler indoor air instead of warmer outdoor air, could be applied. The thermal response of the BIPV glazed façade structure in case of the non-ventilated air gap was tested to examine the static thermal transmittance *Ust* of the structure, while other discrete values of ventilation air flow rates were selected based on the indoor air quality (IAQ). It was assumed that the o ffice with a net volume *Vn* of 35 m<sup>3</sup> and a useful area *Au* 14 m<sup>2</sup> will be equipped by the pilot BIPV shown in Figure 1. One person (1.2 met, 1 clo) occupied the o ffice between 8:00 and 17:00 and emitted *SCO2* 800 mg of CO2 per minute [20]. No brake or leaving from the o ffice was assumed. When the person started to work, the *CO2,8:00* concentration was equal to the outdoor concentration 500 ppm. The first order concentration model was used to determine transient *CO2,t* concentrations assuming constant conservative (decay factor *k* = *0*) pollutant source *SCO2* [21]:

$$\mathcal{C}\_{\text{CO2},t\to\text{oo}} = \frac{S\_{\text{CO2}} + \mathcal{C}\_{\text{CO2},8.00} \cdot \dot{V}\_{a,i}}{\dot{V}\_{a,i} + \underbrace{k \, V\_n}\_{\longleftrightarrow}} \left(\frac{\text{mg}}{\text{m}^3}\right) \tag{1}$$

$$\overset{\text{decay}}{\longleftrightarrow}\_{0}$$

$$\mathcal{C}\_{\text{CO2},t} = \mathcal{C}\_{\text{CO2},t\to\text{oo}} + \left(\mathcal{C}\_{\text{CO2},t=0} - \mathcal{C}\_{\text{CO2},t\to\text{oo}}\right) \cdot e^{-\frac{\dot{V}\_{a,i}}{\text{T}\_{\text{T}}}t} \left(\frac{\text{mg}}{\text{m}^3}\right) . \tag{2}$$

Taking into account IAQ quality categories as defined in [22], the *CO2,17:00* concentration that appeared in the o ffice at the end of the workday (at 17:00) did not exceed class III (1350 ppm above outdoor concentration) if o ffice was ventilated with constant air flow rate . *Va*,*in* 19.5 m<sup>3</sup>/h, and class I requirements (550 ppm above outdoor concentration) were achieved if o ffice was ventilated with constant air flow rate . *Va*,*in* 62 m<sup>3</sup>/h.
