*3.2. Impact of the Local Zoning Plans and Building Location on the Energy E*ffi*ciency of PV Panels*

Decisions concerning the implementation of solar systems at the urban scale should be based on the local conditions of solar irradiation and these values defined in solar maps. Solar maps provide data about the position and system size of PV systems on roofs, the produced amount of electricity, the installation size, and the financial payback time. In this case, the output of the PV system is, besides the efficiency and additional losses, calculated by considering the air temperatures near urban rooftops [25] (p. 44). The appropriate configuration of PV panels with a building and its components is a basic requirement for the energy efficiency of PV systems. In many cases, it is the built or natural elements surrounding a building that may be crucial for solar harvesting on building facades and roofs. They can impair the solar radiation incident on the building and PV panels. Therefore, the building designers should carefully analyze the proposed site plan in terms of the potential obstacles to the undisturbed flow of radiation toward a building. There are two determining factors in this regard: (1) regulations in development (zoning) plans and (2) site planning solutions unrestricted by building regulations.

Zoning plans contain various regulations that must be respected by architects when designing buildings and implementing site plans. Especially important are stipulations regarding the orientation of buildings, their position on the lot, and their relation to the urban grid. They all impact the efficiency of PV panels installed on buildings. Even if respecting the local ordinance does not result in some impairments to solar systems, other elements of site plans, of which their location is unrestricted or undefined therein, may conflict with the efficiency of panels. The built structures located on the same or adjacent tract of land, as well as vegetation, can aggravate their yield or even thoroughly render them inefficient or useless if they are an obstacle intercepting the path of solar rays. There are some typical situations in zoning plans that could be indicated as conflicting with the rules for PV panel positioning to ensure an acceptable electricity yield in these installations. In the case of the pitched roofs of houses, the optimum location for the installation of panels is the south-inclined roof surface. A problem appears when the zoning plan determines the north–south axis of a building as compulsory for the building's orientation. The panels, for obvious reasons, must be exposed to the east or west, which are not optimal situations (Figure 2). Similar problems can occur in the case of a north–south street orientation (Figure 3). NW–SE orientations can also be disadvantageous for a similar reason (Figure 4).

**Figure 2.** Possible positions of photovoltaic (PV) panels as a function of a W–E street orientation (diagram by W. Celadyn, P. Filipek).

**Figure 3.** Possible positions of PV panels as function of an N–S street orientation (diagram by W. Celadyn, P. Filipek).

**Figure 4.** Possible positions of PV panels as function of an NW–SE street orientation (diagram by W. Celadyn, P. Filipek).

Another aspect of zoning plan regulations and their consequence on PV panels' efficiency is the obligation to respect the building lines comprised therein. There are usually two basic types of building lines that determine the location of constructions on building lots: the build-to line and unsurpassable building line. In the first case, a building should be located on a building lot so that its main facade is contiguous to the line. The compulsory character of a building's location permits the design of surrounding vegetation, if present, to ensure that the intensity of solar radiation incident on PV panels is not impaired (Figure 5). In the second variant, a building can be located to ensure the defined line is not surpassed. The first option ensures that the spatial situation is controllable, whereas, in the second, it is impossible to predict the final location, which is dependent on the architect's decision. This occurrence makes the issue of the reasonable configuration of adjacent buildings and the vegetation existing on-site prior to their construction unpredictable, as it does for the efficiency of the potential solar systems installed on buildings (Figure 6).

**Figure 5.** Predictable positions of buildings and PV panels because of a compulsory built-to line defined in the zoning plan (diagram by W. Celadyn, P. Filipek).

**Figure 6.** Probable positions of buildings and PV panels because of noncompulsory unsurpassable building line defined in the zoning plan (diagram by W. Celadyn, P. Filipek).

This analysis indicates that the zoning plan regulations have meaningful relationships with building solar systems. Their mutual dependence should be seriously considered and carefully analyzed by both urban planners and architects to ensure the systems work in terms of the potential electrical energy generation. Zoning plans generally do not envisage such analyses. This creates challenges for the installation of PV panels, rendering them frequently useless. This occurs with existing buildings and their surroundings. The property relations and adjacent parcels being built and arranged with high vegetation make the situation difficult to resolve. So far, the awareness of planning officers and urban planners in this regard appears insufficient, if not absent.

### *3.3. Spatial Position of PV Panels and Their Energy E*ffi*ciency*

Whereas solar thermal systems have always been closely tied to the planning of buildings, developments in photovoltaic technology allowed photovoltaic elements to be integrated in the building envelope since the early 1980s [26] (p. 106). However, the yield from a vertical facade panel is much lower. Unlike with thermal collectors, even an incident energy <200 W/m2 can still contribute to generating electricity [15] (p. 291). This is why photovoltaic systems are less dependent on the orientation

of building components. However, to be energy efficient, the mutual configuration of the panel surfaces and the angles of incidence of solar rays must be optimized. This is a factor that, in practice, determines the possibility of installing effective solar systems on buildings. Therefore, it is important for the configuration to avoid any disturbances in the accessibility of solar rays. In contrast to solar thermal applications, in photovoltaics, even relatively little shading of the solar cells can lead to a considerable reduction in the energy yield [26] (p. 106).

The highest transmission of solar radiation through glass occurs when the angle of incidence of the solar rays on the glass surface is perpendicular. Research indicated that within the range of 0–60◦, the deviation from perpendicular gives a transmission loss of energy between 8 and 10% [27], or even more. This loss (technically an incident angle modifier) is due to the glass internal transmission (due to a longer light path length) and glass surface reflection, not attributed to the total irradiance on the panel and PV electricity generation. The solar radiation (the beam component) is reduced to 50% for an incident angle of 60◦, and abruptly drops down to 0% by the direct radiation angle of incidence approaching 90◦. Behind this range, the intensity of the transmitted solar energy abruptly drops down to 0% when the angle of incidence approaches 90◦. This does not mean that below this range that the electricity is not generated. About 50% of the radiation occurs in the form of diffuse radiation [15] (p. 291), so it still can generate some amount of energy; this even occurs under an overcast sky. PV cells can be mounted on movable panels programmed to track the sun so that the cells are always perpendicular to the sun's rays for the maximum interception of solar radiation [4] (p. 64).

The issue of the relationship between the angle of incidence of solar rays and the plane of photovoltaic panels is less important in the case of photovoltaic cells mounted on the membrane that absorb all incident sun rays from any direction at any time of the year without the need for any manual or automatic override [28] (p. 34) (Figures 7 and 8).

Solar radiation varies widely over the course of a day and a year and is strongly influenced by the prevailing weather conditions. Radiated energy can differ up to a factor of 10 on two consecutive days, being, at times, up to 50 times higher values on a clear summer day than on an overcast winter day [29] (p. 49). Some sources suggest that the highest annual radiation volume in Central Europe is available to south-facing fixed systems installed at an angle of 30 degrees or less to the horizontal [15] (p. 291). A south–west orientation by the same inclination reduces the yield to only 96% [29] (p. 54). The case of a house in Figures 7 and 8 proves that PV panels with an energy efficiency of 5.5 kWp installed on its roof and deviating by 37◦ from the north–south towards the south–east can still produce electricity in significant quantities (Figure 9). On the winter day of 15 March, the electricity generation reached its peak of 4.2 kW, and was registered as late at 6 pm.

**Figure 7.** A house oriented at an angle of 37◦ from the N–S direction with PV panels installed on a stepped roof (diagram by W. Celadyn, P. Filipek).

**Figure 8.** Stepped roof with PV monocrystalline photovoltaic panels installed at the recommended angle of 15◦. Location: 49◦57- N, 19◦55-E (photo by W. Celadyn).

**Figure 9.** A chart indicating the solar efficiency of the above house on a sunny day (15 March 2020). Location: 49◦57- N, 19◦55-E (source: sunnyportal.com).

Computer tools are available for the calculation of the efficiency of photovoltaic systems; an example is PVSYST, used by, e.g., Fartaria [30] (pp. 93–101), to calculate the mutual shading of direct normal and diffuse radiation. Building envelopes can lend walls and roofs to photovoltaic installations, and substantial differences exist between their solar conditions. In the latter, pitched and flat roofs are also differentiated in this regard. Flat roofs are a particularly good place for the location of PV panels as their arrangement is independent of the roof pitch. However, the problem of self-shading occurs due to adjacent tilt panels on flat surfaces or due to them tilting away from low sloped roofs. As a rule of thumb, there must not be any shading on 21 December. The calculated minimal module spacing (in the Northern Hemisphere) is defined by the equation previously reported by [31] (p. 227). To accommodate as much PV power as possible, the optimum angle of attack, β, equal to 15◦ to the horizontal, is often changed to 20◦, as the energy yield is then only reduced by 2% [31] (p. 227). This angle varies with the latitude of installation. The optimal tilt angle is within the latitude angle plus or minus 10–15◦. The lower the angle of inclination of the module surface, the higher the usable incident radiation. When the modules are installed over the entire roof surface, almost horizontal, the overall

energy yield is maximized [26] (p. 107). The energy output of PV panels can be compromised by their low tilt as the cleaning of their surfaces by rainwater is less efficient.

Most of the absorbed solar radiation becomes thermal energy that can heat up the PV panels. An increase in the temperature of the panels over 25 ◦C is disadvantageous as the PV panel will produce less than the rated generation efficiency. This efficiency loss due to an increased temperature depends on the types of solar cells involved. An effective ventilation of the back of panels or the use of generated thermal energy to heat the interiors or water can be helpful.
