**3. Analyses of PV Technologies and Important Design Issues**

### *3.1. PV Panel Systems and Their Development*

PV panels, the most popular system for providing renewable energy to buildings on the market, have developed rapidly, bringing down their prices. The low performance of PV panels has increased gradually up to a 17% energy effectiveness, on average. Some of the newest systems boast a 40% energy effectiveness. The development of these systems enhanced not only the higher energy efficiency of panels, but also made some breakthroughs in terms of basic materials. The systems are classified into three generations [13] (p. 24). The first one is the most popular monocrystalline silicon cells (SCPV, m-Si) which have the highest efficiency (13–26.1% [14]), and are durable and expensive, as well as polycrystalline silicon cells (PCPV, p-Si) which have a lower energy efficiency of 10–23.3%, typically 11–14%, and are less expensive, as evidenced by [14,15] (p. 291), and [1] (pp. 76–77). The second generation are the transparent thin-film cells, which offer flexible systems, are less durable, less expensive and have lower efficiencies (5–7%), as evidenced by [1] (pp. 77–78), due to other sources having a maximum of 14% for amorphous Si:H stabilized solar cells [14]. They can be installed on curved surfaces. An attractive option is the possibility of their integration with membrane flexible constructions. The third-generation systems are of very different technologies. One novel solution is a polymer-based system (DSSC—dye-sensitized solar cells) which has a 12.3% efficiency (as can been seen in [14] and [16] (pp. 1–11)). Perovskite/Si tandem (monolithic) cells achieve a 29.1% energy yield [14]. The modules of mass production have somewhat lower efficiencies. The available maximum energy efficiency of multijunction solar cells is about 47.1% [14]. The solar cells at more than 40% efficiencies are mostly of very small dimensions and are used mostly in concentrated solar modules and in rigid frames (not flexible), which require heliostats for installations and operations. It should be mentioned that some current top solar cells are not ready or appropriate for applications in buildings now. The focus of the innovative third-generation PV systems is on thin-film technologies that combine the high electrical efficiency of monocrystalline cells with the flexibility and lower costs of thin-film manufacturing [8] (p. 106). Monocrystalline panels are black or deep-blue, which is a serious drawback from an architect's viewpoint (Figure 1a). Polycrystalline panels offer a wider palette of accessible colors, which was welcomed by architects (Figure 1b). However, an even better and more attractive solution is a new generation of PV cells that come in a much more diversified scale of colors (Figure 1c). In this case, there is an interrelation between a given color and the efficiency of a panel. Generally, it has been proved that the most energy efficient panels are black PV panels, and lighter colors reduce the efficiency [17].

**Figure 1.** Photovoltaic panels: (**a**) monocrystalline, (**b**) polycrystalline, (**c**) dye-sensitized colored cell (photo by Celadyn, W.).

PV panels can be mounted on roofs, facades or be stand-alone frames on the ground. When they are in some way integrated with building components, they are termed building-integrated PV (BIPV) systems. Especially exciting are the building-integrated photovoltaic technologies integrating solar cells directly into building materials, such as semitransparent insulated glass windows, skylights, spandrel panels, flexible shingles, and raised-seam metal roofing [18] (p. 309), [19] (p. 3) and [20]. Well-integrated PV modules are suitable to contribute to the comfort of the building: they serve as weather protection, heat insulation, shading modulation, noise protection, thermal isolation and electromagnetic shielding, etc. [11] (p. 126). Holistically designed BIPV systems will reduce a building's energy demand from the electric utility grid while generating electricity on-site and performing as the weathering skin of the building [6] (p. 2). The first pioneering building with a BIPV installation was a multifamily residence designed by T. Herzog and B. Schilling and constructed in Munich in 1982 [21].

In future cities, solar cells and BIPV systems will evermore play an increasingly significant role in facade forming and electrical energy generation in the residential and other types of objects [8] (p. 104). The comparable prices between BIPV systems and conventional building materials confirm this assumption [20,22] (p. 9).

Retrofitting historical buildings, which aims at improving their energy-related parameters, can make use of photovoltaic systems. However, in this case the installation of these systems on facades or rooftops can be more difficult and controversial than in contemporary buildings as it can involve interventions in a building's valuable historic appearance. Therefore, instead of considering BIPV as a technical constraint for designers, a new approach based on the integration of BIPV solutions as a new "raw material" for architectural renewal projects is a good option avoiding conspicuous disfigurations of the building envelope [10] (pp. 1–2).

PV solar systems can store the produced energy, locally converted from a DC to AC current, in home batteries, or send it into the utility grid—the community's electrical wires—to be distributed to others [23] (p. 223). Panel installations can be fixed or track the sun, usually on one axis only.

Not everyone likes the appearance of typical solar electric systems, which are usually mounted on roofs. Therefore, solar manufacturers have begun to produce less conspicuous systems like thin-film solar electric materials using a noncrystalline sun-absorbing layer, which use a fraction of the semiconductor material of their predecessors. This amorphous silicon is, however, only 5% efficient. Despite its evident positive characteristics permitting its use in windows and skylights to produce electricity, it has some significant disadvantages [23] (p. 221).

Meeting the sustainability paradigm requires building resiliency, which can be achieved using a diversity of energy sources. This option enhances the system's ability to function under a wide variety of conditions and withstand many kinds of disturbances. Individual homeowners and businesses are encouraged to install small-scale wind turbines, photovoltaic panels, and other devices to produce renewable energy [24] (p. 158). PV panels are considered an indispensable, easy to install, and relatively inexpensive solution to such systems. Therefore, they play a significant role in increasing the sustainability of buildings of all kinds.
