## 3 The Influence of Soiling on Photovoltaic Energy
### Experimental Assessment of PV Sailing Losses  
The \"Short-term forecasting of PV energy output including the soiling effect (PVCastSOIL)\" project includes the objective of monitoring and modeling soiling under Mediterranean climate conditions. To achieve this, an off-grid PV system was installed on a 10m-high rooftop at the Research Centre for Energy, Environment and Technology (CIEMAT), Madrid, Spain (Csa climate). Located within a semi-urban area, CIEMAT's facilities are surrounded by a gardened university campus, a forest park, and some paved roads. The PV system is installed above a building surrounded by well-developed conifers: cedars, cypresses, and pines (Figure 4). This PV system consists of twelve commercial PV modules (i.e., six 60-cell crystalline silicon modules and six cadmium telluride modules with peak powers of 365 W and 80 W, respectively) with their corresponding temperature sensors, a PVPM2540C (PVE Photovoltaik Engineering) I-V curve tracer connected to a multiplexer, a DustIQ (Kipp & Zonen, Delft, The Netherlands) soiling monitoring sensor, and some additional apparatus to monitor the meteorological variables (e.g., solar irradiance, temperature, wind speed, precipitation, relative humidity); a series of coated and uncoated float glass samples are exposed alongside as well. The design of this PV system will allow us to determine the local soiling ratio and the influence of the local meteorology, as well as the dependence of soiling on several technical parameters such as the PV technology (i.e., crystalline silicon versus cadmium telluride), the module inclination (i.e., 35\({}^{\circ}\) versus 8\({}^{\circ}\), which represent the inclination of latitude-optimized fixed-tilt and building-integrated PV systems, respectively), the front glass surface (i.e., uncoated glass versus glass coated with an anti-soiling commercial product), and the maintenance schedule (i.e., no cleaning versus periodic cleaning). Figure 4: Panoramic view of the rooftop PV system installed for soiling monitoring. During the first months of 2019, a conclusion from regular visual inspection was that the soiling layer mainly contained fine dust, although pollen deposition was prevalent during January and February. In accordance with data from a nearby pollen monitoring station (located less than 1 km away) from the regional palynological network PALINOCAM [100], it could only be identified as Cupressaceae pollen at that time of the year. Pollen deposition on the PV modules was less conspicuous at later dates. In addition to the presence of dust and pollen, bird droppings were regularly noticed on the PV modules. Our findings were in agreement with other authors reporting that broadly cultivated ornamental species (e.g., cypresses, plane trees, etc.) have a major impact on the airborne pollen spectrum of many populated areas [15; 17]. The local pollen calendar confirmed that: (1) the airborne total pollen concentration maintains medium-high levels during the first semester of the year and low levels during the second one (see Figure 5) and (2) Cupressaceae pollen is an important contributor to the airborne total pollen from late winter to early spring [101]. In addition, data gathered from the nearby pollen monitoring station during that period showed that Cupressaceae (i.e., in our location, cypresses) and plane tree (_Platanus_ sp.) pollen were almost exclusively responsible for the local airborne pollen from January to mid-March and from mid-March to early April, respectively (Figure 6), while it consisted of a mixture of pollen from other plants (e.g., oak, pine, olive, grass, plantain, etc.) in the following months. Indeed, soiling by cypress pollen was observed when visiting the PV installation from January to mid-March. Contrarily to the local-scale trend indicated by Figure 6, more soiling was noticed during that period than later on. The reason why Cupressaceae pollen prevailed over plane tree pollen as a soiling agent was the proximity of several cypress specimens exceeding the rooftop height.