Photovoltaics and Electriﬁcation in Agriculture

The editorial introduces a Special Issue entitled “Photovoltaics and Electrification in Agriculture” [...]

Electricity also makes it possible to technify crops in so-called precision agriculture. In this way, agriculture optimizes the resources used, reduces the carbon footprint and the environmental impact, and manages to produce more, better, and more efficiently. Precision agriculture is the evolution of agriculture towards sustainability. All of this requires efficient technology and energy with the least possible impact, such as photovoltaic energy.

The Use of Photovoltaics in Greenhouses
The new context of climate change, in turn, generates new forms of production where the environmental parameters in which it takes place must be controlled more and better. The paradigm of this type of system is greenhouse cultivation. In this type of cultivation, not only the interior temperature is controlled, or parameters such as relative humidity, the amount of CO 2 and the nutrients applied in the case of hydroponic cultivation but also the amount of radiation that reaches the plants. In countries with high insolation, shading is used either by means of meshes or by "liming" the roof. Photovoltaic solar energy as an energy source in greenhouses can also be used as a shading element for certain crops. However, the analysis of the impact of the solar panels on the crop is a topic being studied nowadays and a topic of great interest for the future development of solar energy applied to greenhouses.
The use of alternative systems, such as semi-transparent photovoltaic panels, can add value to greenhouse cultivation. This type of panel lets a part of the radiation pass through, as it does not include a reflective layer on the back. Consequently, the panel may lose a small percentage of efficiency, which is offset by the benefits to the crop. In the work of Aira et al. [3], the influence of radiation loss on production and crop quality is analyzed. In their conclusions, the shaded crop adapted to the new conditions, partially compensating for the decrease of radiation.
The application of agrovoltaic technology in arid areas, such as the southwestern United States, using organic solar panels, was carried out by Waller et al. [4] in a study exploring the influence of certain photovoltaic materials that do not completely block socalled photo active radiation (PAR), applied to hydroponic cultivation under greenhouse, something that makes it a very interesting case and with a great future for its high efficiency not only in terms of energy but also in the application of water and nutrients. Their conclusions show that, as in other cases of similar studies, the crop adapts to the decrease in radiation so that finally, the amount of harvest by weight is similar.
In greenhouse cultivation, fixed-plane surfaces are available with a certain orientation. The estimation of radiation on such plane, depending on the location, must be carefully analyzed to determine the impact and viability on the crop and on the photovoltaic production itself, something that has been considered in the work of Díez et al. [5], applying new methodology, including anisotropic models of the sky. The model is able to calculate the incidence angle of the radiation at any time besides the irradiation of a considered placement, with increased accuracy. As one of the conclusions, it allows predicting the hourly radiation reaching the crop and then the expected production.

Viability of Photovoltaics Coexisting with Traditional Agriculture
An agrovoltaic system has certain benefits in terms of the temperature of both the panels and the crop itself. In the study by Othman et al. [6], it is analyzed how the crop itself, with its evapotranspiration, reduces the temperature of the solar panels, which, as is known, is the main responsible for their loss of efficiency. Likewise, the placement of panels reduces the evapotranspiration itself and therefore, the hydric stress of the crop, especially in areas with high solar radiation. All these topics are currently the object of analysis and research, and each new study contributes to the improvement of the knowledge in this area.
The areas in countries with desert climates in Africa and the Middle East present peculiarities that make the analysis of the viability of photovoltaic systems on crops interesting. The case of Africa is especially interesting since, on the one hand, climate change can affect this continent much more intensely and, on the other, population growth is significantly higher than on the rest of the planet. Therefore, optimizing the use of resources is extremely necessary in order to achieve the necessary social and political stability for a region where the only option many young people find is to emigrate. The Srijana et al. study [7] shows how traditional farming systems based on the use of diesel for irrigation and those that only use rainwater are at a clear disadvantage compared to agrovoltaic systems that use solar energy for irrigation. Moreover, the concept of land equivalent ratio (LER) is calculated with positive results that demonstrate how the combination of photovoltaics and agriculture results in better use of land.
In the Moreda et al. study [8], an exhaustive theoretical study was carried out, including nine types of rotating crops on a surface of 24 ha on the hypothesis of crops in the southern area of Spain, one of the most representative areas of Mediterranean agriculture. Two systems for obtaining irrigation water were compared: Surface water and extracted from wells. The conservative analysis of the profitability of the shared use of the land for solar panels and rotary cutters was carried out in periods of four years. The conclusion is moderately optimistic, however, the rise in fossil electricity prices, as well as the emission reduction targets, make the results more than reasonable.
Not only the regions that until now were classified as arid, but also those semi-arid or even those that were less warm, are affected by the increase in temperatures due to climate change as well as the decrease in average rainfall. This forces other regions like the rest of the Mediterranean areas to adapt their crops to this new situation.
The economic feasibility of agrovoltaic systems is an issue depending not only on the type of crop and photovoltaic system but also on the area of the analysis. Pascaris et al. [9] analyzed the agrovolvaic systems from a human point of view, interviewing north-American farmers and detecting in their conclusions how farmers are concerned not only with the present situation but also the long-term issues like productivity, market potential, just compensation, and system flexibility, in order to decide to implement or not the new energy systems over their crops. Although the lifetime of the systems has been extended to 25 or 30 years, and the reliability has been increased, there are other considerations related to the market that can affect the profitability and therefore, the viability of these systems. However, this is an issue that affects not only agrovoltaic systems but everything that concerns the agricultural market in general, generating increasing uncertainties that do not have to do with technology itself but also with the geopolitical situation and global market tensions.
We can also find studies in areas with high humidity, such as central Europe. In the study by Wexelex et al. [10], trials were carried out for two different years with the cultivation of celery under solar panels in irrigated cultivation. Although the years were not conducive to the analysis, it can be observed that in any case, the agrovoltaic system does not pose a significant problem for the crop, which would allow concluding that the use of the soil can be shared with the consequent improvement in efficiency of land use.

Precision Agriculture and Photovoltaics
The electrification of the tasks associated with agriculture encompasses not only the more traditional methods, such as tillage or pumping, but also those related to the so-called precision agriculture.
There are many activities that benefit from the application of intelligent systems, such as selective spraying and fertilization, which can be applied with robots that include an internet connection (internet of things IoT), and which benefit from autonomy that provides them with a photovoltaic power system like the one proposed by Chand et al. in their study [11]. The document concludes that this field still has a long way to go and that there are still few experiences, which is why it is an area of great interest for research and the application of results to the automation of cultivation tasks and the improvement of precision in agriculture.
In agricultural applications, an important aspect is the remoteness from the connection points when implementing the Internet of Things (IoT), which influences the energy consumption of transmission systems and forces to study strategies to optimize the use of data and power. This aspect is addressed by Swain et al. in their work [12], where they study a long-range system applying Matlab algorithms. In their conclusions, they determine the improvement that occurs by including hybrid systems in terms of range and then addressing real-time systems. All this is applicable to the new generation of agriculture 4.0.

Research in Electrification Applied to Rural Areas
Not only photovoltaics is covered in this Special Issue, but also cases of research in electricity production and transport applied to rural areas. This is the case of Pindado et al. in the work about the importance of a precise selection of the protections as fuses [13] in the context of a rapid growth of renewable connected plants with a distributed scheme. In this kind of topology, the fault of one of the plants can affect the others and, with the increasing number of plants, the adequate selection of the fuses to operate when necessary is of vital importance.
Rural electrification is not yet complete in many areas, such as areas in Palestine, limiting local development that is fundamentally agronomic. The contribution of photovoltaic energy to solve the problem of rural electrification is crucial due to lower prices, modularity, and increased reliability, something that is reflected in the Ibrik study [14], something also reinforced by the low impact on greenhouse gas emissions of this solution (which we remember is not null, since it exists in the manufacture, installation, and dismantling of the plant). In addition, electrical micro-grids are a boost both to local development in rural areas and to the elimination of the need for new power lines.

Conclusions
The electrification of the tasks associated with agriculture is an unstoppable and necessary process. The use of photovoltaic energy will be one of the energy sources with the greatest impact on this process. In the context of the decarbonization of all sectors, including agricultural and livestock production and the rural world, photovoltaic energy is undoubtedly one of the mechanisms with the easiest implementation.
In summary, the abovementioned works demonstrate in their analysis and conclusions that: -Photovoltaic energy is the most competitive electrical energy option for the agricultural sector at the present time due to the drastic drop in component prices. - The cultivation can be developed under photovoltaic panels coexisting in the so-called "agrovoltaics" with an increase in profitability or land use. -Precision agriculture or agriculture 4.0, based on the Internet of Things (IoT), benefits from the use of photovoltaic solar energy for its purposes. -Greenhouses are an ideal agricultural production system for the integration of photovoltaic panels of different technologies, such as organic, semi-transparent, or amorphous silicon panels.