Performance analysis of photovoltaic plants installed in dairy cattle farms

Electric production from renewable resources, such as solar photovoltaic (PV), is playing an increasingly essential role in the agricultural industry because of the progressive increase in the energy price from fossil fuels and the simultaneous decrease in the income deriving from farming activities. A central issue in the sustainable diffusion of PV technologies is represented by the actual energy efficiency of a PV system. For these reasons, a performance analysis has been carried out in order to assess the potentials offered by different PV plants within a defined geographical context with the aim of investigating the impact of each component has on the PV generator global efficiency and defining the main technical parameters that allow to maximise the annual specific electric energy yield of an architectonically integrated plant, installed in a dairy house, compared to a ground-mounted plant. The annual performances of three grid connected PV plants installed in the same dairy cattle farm have been analysed: two are architectonically integrated plants - i.e., a rooftop unidirectional and a multi-field systems (both 99 kW_p) - and the other is a ground-mounted plant (480 kW_p). Furthermore, the electrical performances, estimated by the photovoltaic geographical information system (PVGIS), developed by the EU Joint Research Centre, and by an analytical estimation procedure (AEP), developed on the basis of a meteo-climatic database related to the records of the nearest weather station and integrated by the components’ technical specifications, have been compared with the actual yields. The best annual performance has been given by the ground-mounted PV system, with an actual increase of 26% and in the range of 6÷12% according to different estimations, compared to the integrated systems, which were globally less efficient (average total loss of 26÷27% compared to 24% of the ground-mounted system). The AEP and PVGIS software estimates showed a good level of reliability for mean deviations between the annual actual and estimated electrical power yields have been equal to 11.5% for each PV system given the actual irradiation’ s uncertainty during the examined year. The main technical parameters, crucial to maximise the energy yield from a ground-mounted PV system to an integrated one, have been identified in the Tilt and Azimuth angles. Indeed, once a variance of 3÷4% in the global efficiency has been confirmed when the type of PV system is changed, in the case of the unidirectional integrated PV plant, the high roof pitch and the almost South orientation guarantee a solar energy increase up to 18% higher than that obtainable on the horizontal plane and similar to the increase estimated for the ground-mounted generator (+20%). Hence, integrated PV systems, besides reaching the same levels of energy efficiency as those ground-mounted, are also more sustainable than the latter. This is true providing that there are both a suitable orientation and an accurate design, especially to prevent the PV panels’ warming during summer, on an already available surface that is, however, functional to the roof’s architecture.