1. Introduction
Increasing global temperatures amplify the necessity for climate protection strategies and the utilization of renewable energies to meet climate protection objectives and minimize the impact of global warming. In urban areas, solar energy represents the most abundant renewable energy source, which can be flexibly harnessed by incorporating photovoltaic panels into building surfaces.
At the same time, cities face the consequences of climate change, specifically the urban heat island (UHI) effect, which refers to the disparity in the ambient air temperature between urban and rural areas. The growth of metropolitan populations is expected to exacerbate the UHI, resulting in adverse effects on human health: Heatwaves lead to higher mortality rates and increased health issues, especially among the elderly population. Additionally, reduced night-time cooling impairs sleep quality and reduces overall performance [
1,
2].
At the local scale, the thermal microclimate is particularly influenced by the surrounding surfaces, materials, and physical characteristics. Intelligently controlled, the installation of photovoltaics is a common strategy to reduce the carbon emissions and improve the dynamic emission balance of a building energy system [
3,
4]. Adding photovoltaic modules—driven by climate protection efforts—raises questions about how this new construction material interacts with the surrounding thermal microclimate [
5,
6].
The significance of this topic has only recently gained importance in urban planning practices and research. A limited number of experimental and simulation-based studies has been conducted thus far with diverging results: Some studies indicate that photovoltaic panels elevate the surrounding ambient air temperature [
7,
8,
9,
10], whereas further studies suggest a phenomenon of ‘solar cooling’ due to the conversion of absorbed solar radiation into electricity [
11,
12,
13,
14,
15,
16]. The differing results between different simulation-based studies can be primarily traced back to simplifications in modeling the photovoltaic cell temperatures. For instance, in [
12,
13], a photovoltaic model computes the module temperature by adding the ambient air temperature to the irradiance at the module level, multiplied by a constant factor. Consequently, this approach does not consider the impact of wind speed, wind direction, and module efficiency on photovoltaic temperatures. The effective albedo approach employed in [
11,
14] also involves significant simplifications and assumptions, which have not yet undergone validation. In [
17], longwave and diffuse radiation are excluded from the energy balance calculations, which are only performed when the Sun is at its zenith. Further studies assume that the albedo of the surrounding building surfaces is equivalent to the albedo of the photovoltaic modules [
11,
16]. In [
13], it is assumed that each building has mechanical air conditioning, and the waste heat generated is released into the environment as sensible heat. Consequently, the observed reduction in ambient air temperature primarily stems from the decreased demand for cooling energy, leading to a decrease in anthropogenic heat emissions.
Moreover, these studies mainly investigate rooftop photovoltaic installations and focus on ambient air and surface temperatures. Although they directly influence human well-being, façade-integrated photovoltaics are rarely analyzed, and only one study has considered the mean radiant temperature and metrics describing human thermal comfort [
18]. However, enhancing the albedo of surfaces can result in a decrease in outdoor thermal comfort [
19]. Therefore, failing to account for outdoor thermal comfort aspects, including the mean radiant temperature, leads to an incomplete characterization of the impacts on the thermal microclimate when comparing façade-integrated photovoltaics to conventional building surfaces.
Hence, the extent to which and the specific circumstances under which implementing photovoltaics in building façades impacts the local microclimate, particularly in terms of the ambient air temperature and mean radiant temperature, as well as outdoor thermal comfort, remain to be determined. It is also uncertain whether PV applications lead to overall warming of the urban climate within a city or district context or rather provide urban solar cooling, potentially making them a viable climate adaptation measure.
This research gap leads to the questions, how do façade-integrated photovoltaic installations impact the surrounding urban thermal microclimate and human outdoor thermal comfort? What parameters influence this impact of photovoltaics on its surroundings?
A twofold simulation-based methodology has been developed to comprehensively answer the research questions: First, the impacts of façade-integrated photovoltaics on the urban thermal microclimate are investigated using a case study in Munich, Germany. The simulation results for the PV or reference façade temperature, urban ambient air temperature, mean radiant temperature, and outdoor thermal comfort for a retrofitted district with PV installations are compared to those for a district with conventional building materials. A subsequent parameter study allows us to gain further insights into the relevance of several parameters to the microthermal impact. This study focuses on the outdoor thermal microclimate. Hence, investigations into the effect of photovoltaics on the indoor climate and building energy consumption, as investigated in [
12,
13,
20], are excluded from the present research.
5. Discussion
The present study investigates the impact of photovoltaic façades on the urban thermal microclimate and outdoor thermal comfort with the following key results:
The PV modules replacing a ventilated façade cladding reach higher cell temperatures than the surface temperature of a rendered façade, particularly during the daytime. In summer, the temperature difference reaches up to +8.60 K; in winter, the cell temperature is up to +13.75 K warmer. Hence, converting solar radiation into electricity instead of heat can only partly compensate for the higher solar absorption. The daytime results correlate with the findings of Cortes et al. [
16]. However—contrary to our results—they identified the cell temperatures to be lower than the conventional building surface temperatures during night-time. In contrast, Tian et al. [
13] simulated a perpetual surface temperature reduction by installing PV modules. This result, however, is due to the assumption of a low albedo of the reference façade of 0.1, leading to high surface temperatures. Analyzing the temperature difference between the ambient air temperature and cell or surface temperature, respectively, provides insights into the convective heat fluxes between the building façade and the surrounding air. As the temperature difference between the cell and ambient air temperature, reaching a maximum of +9.99 K, is higher than the difference between the building surface and ambient air temperature, with a maximum difference of +1.39 K, the convective heat fluxes from the PV modules to their surroundings are considerably higher. These results correlate with the findings of Brown et al. [
64], Scherba et al. [
65], Pham et al. [
66], and Fassbender et al. [
6,
67].
The higher temperature of the outer building layer leads to an elevated mean radiant temperature in front of the building with a PV installation compared to the reference case, reaching a maximum difference of +5.47 K in summer and +6.27 K in winter at a distance of 0.5 m from the façade. While the daytime warming effect is most pronounced directly in front of the façade, the elevated mean radiant temperature is still perceptible by up to +3.72 K at a distance of 2.5 m from the building façade, due to the relevance of longwave radiative heat fluxes to the determination of the T
MRT. Hence, the PV-induced increased mean radiant temperature is perceptible to pedestrians directly in front of the façade and to passersby in the urban canyon. The simulation results correlate with the studies of Herrmann and Matzarakis [
31] and Evola et al. [
38]; however, the simulated mean radiant temperature in our study is considerably lower than in other studies [
14,
19]. The discrepancies between the studies rely on the complexity of calculating the mean radiant temperature due to the superimposed radiant flux densities, particularly in complex urban configurations [
28,
33,
37]. The algorithm applied using Ladybug, and hence applied in our simulations, is known to underestimate the mean radiant temperature [
29].
The higher daytime surface temperature and hence the higher mean radiant temperature in the PV case also impacts human outdoor thermal comfort, expressed using the Universal Thermal Climate Index. As the UTCI is affected by several factors (ambient air temperature, wind velocity, humidity, and mean radiant temperature), the PV warming effect is less pronounced in the UTCI than in the TMRT, reaching a difference of up to +1.46 K in summer and +2.21 K in winter. Hence, the PV installation affects the UTCI negatively in summer and positively in winter but to a lesser extent than the mean radiant temperature. The higher UTCI values in winter lead to lower cold stress, hence resulting in a higher annual outdoor thermal comfort autonomy in the PV case by 0.91%. In summer, when thermal stress can negatively affect human health, installing PV modules on the building façades increases the hours of heat stress by 86. However, the higher UTCI values solely occur during the daytime, while the temperature during the night-time hours relevant to human recovery remains identical.
The parameter study reveals that the T
cell reacts more sensitively to changes I then PV efficiency (up to +/− 8.73 K) than to changes in the mounting situation of the PV modules (up to +/− 3.78 K). The surface temperatures of the reference façade vary by up to +/− 11.81 K depending on its solar absorption. In accordance with the case study results, the variations in the cell and surface temperature are significant, while they are less pronounced for the T
MRT and even less for the UTCI. All the simulation cases considered, the PV façades consistently lead to higher surface temperatures (+1.74 K to 14.44 K), mean radiant temperatures (+3.14 K to +8.22 K), and a higher Universal Thermal Comfort Index (+0.79 K to +1.96 K) during the hottest week at noon. Thus, even with PV efficiencies of 30%, the energy removal due to electricity production could not entirely compensate for the higher solar absorption. Within the next ten years, PV efficiencies are expected to stay below 30% on average [
68]—hence, a leap in innovation is required (e.g., through silicon-perovskite tandem cells) to achieve a solar cooling effect by replacing low-albedo façade materials with highly efficient PV modules. As a limitation, it needs to be mentioned that the parameter study only investigates the daytime temperatures but neglects the night-time effects.
To summarize, we see a daytime heating impact of the PV modules on the surface and mean radiant temperatures, as well as on the UTCI. In contrast, the temperatures are in the same order of magnitude during night-time. While the simulation results are in accordance with several metrological studies on the impact of PV on the thermal microclimate [
9,
10,
65,
67], they contradict most simulation-based studies identifying PV cooling effects [
11,
12,
13,
14,
16,
69]. The discrepancies primarily rely on simplifications, particularly concerning calculating the PV cell temperatures in the existing simulation-based studies. For instance, Masson et al. [
12] and Salamanca et al. [
13] utilize a photovoltaic model that estimates the module temperature by adding ambient air temperature to irradiance at the module level and then multiplying by a constant factor. Consequently, the impact of wind speed and module efficiency on the photovoltaic temperatures is disregarded. Similarly, the effective albedo method employed by Taha [
11] and Berardi and Graham [
14] involves notable simplifications and assumptions that have not undergone validation. The applied the NOCT cell temperature model to calculating PV cell temperatures, accounting for the ambient air temperature, the plane of array irradiance, the module’s nominal operating cell temperature, the mounting situation, the module efficiency, transmittance and absorptance effects, and wind speed. However, it must be mentioned that the NOCT cell temperature model was initially conceived for rooftop or freestanding PV. Hence, the cell temperatures might be slightly underestimated due to the restricted rear ventilation in the case of PV façades compared to rooftop photovoltaics. Moreover, the present simulations using EnergyPlus might underestimate the thermal mass of the building façade, resulting in lower surface temperatures of the reference building during night-time. Consideration of a higher thermal storage capacity whereas might induce a nocturnal cooling effect in PV façades.
6. Conclusions
The first research question, ‘how do façade-integrated photovoltaic installations impact the surrounding urban thermal microclimate and human outdoor thermal comfort?’, can be answered twofold for day- and night-time and winter and summer periods, respectively.
During the day, PV façades reach higher surface temperatures, warming the adjacent mean radiant temperature and increasing, to a lesser extent, the Universal Thermal Climate Index. With an increasing distance from the building façade, the warming effect is attenuated, but even with a distance of 2.5 m from the building, it is still perceptible as an up to +3.72 K higher mean radiant temperature and an up to +1.38 K higher UTCI. We see no directly quantifiable impact on the adjacent ambient air temperature. For night-time periods, we need to distinguish between summer and winter: In summer, PV installations lead to a moderate warming effect (<1 K) on the mean radiant temperature. This effect, however, might be due to an underestimation of the buildings’ thermal mass in the simulation. In winter, the mean radiant temperature in front of the PV façade is slightly lower than in front of the reference façade. In neither case do the PV modules affect the UTCI at night. Despite extended human exposure to thermal heat stress in summer, PV façades improve the annual outdoor thermal comfort autonomy by 0.91% due to lower cold stress in winter.
The parameter study answers the second research question, ‘What parameters influence the impact of photovoltaics on its surroundings?’: The most critical parameters affecting the impact of PV modules on the surrounding microclimate are the albedo of the reference building surface—due to the sensitivity of the surface temperatures to changes in solar absorption—and the efficiency of the PV modules—due to a higher amount of energy conversion with higher efficiencies. While the cell and building surface temperatures react sensitively to changes in the input parameters, this effect is attenuated for the mean radiant temperature and even more for the Universal Thermal Climate Index. In all the simulated cases, we identify the heating effect caused by installing PV modules.
The present study contributes to closing the research gap between metrological and simulation-based studies. While PV façades have a warming impact on the surrounding mean radiant temperature and reduce human thermal comfort, these effects are restricted to the daytime. At night, the effects are marginal. Hence, nocturnal recovery, as a key element for human health, is not affected. As technological advancements continue and the photovoltaic efficiencies improve, more energy will be extracted from the system as electricity. Consequently, this will mitigate or even eliminate the heating effect caused by elevated module temperatures and subsequently increase the convective and radiative heat transfer. Thus, photovoltaics in an urban context should still be considered a vital measure for the decentralized supply of renewable energy and hence for achieving climate protection goals. However, the microthermal impact must be considered when planning urban PV installations. Therefore, further research analyzing the installation of PV in different urban contexts and various climate zones is vital to provide deeper insights into the effects of photovoltaics on the urban thermal microclimate and outdoor thermal comfort. Moreover, implementing a temperature model conceived for building-integrated photovoltaics while considering the PV efficiency for estimating PV cell temperatures is required for even more precise results.