**1. Introduction**

Depending on the latitude, the weather conditions (temperature, humidity, and CO2) often are not optimal for crops. For this reason, crops are usually protected by structures (greenhouses). Even so, climate control becomes necessary—in winter/fall due to low temperatures during the night and in spring/summer due to high temperatures during the day (Callejón-Ferre et al. [1]; Castilla [2]). A clear example of this situation occurs in Mediterranean countries.

To correct high and low temperatures, several shading systems in greenhouses are available: bleaching, mesh, screens, and photovoltaic panels (Figure 1).

Bleaching is the simplest and most economical technique that is used as a shading system. It consists of applying a solution of water and calcium carbonate to the roof of the greenhouse [1]. The other systems that were used (mesh and screens) can be used inside or outside of the greenhouse, and can be permanent (fixed) or mobile (displaceable; Figure 1).

Recently, photovoltaic panels have been used on the roofs of the greenhouses. These can be opaque, semi-transparent, or transparent, allowing for less solar radiation to pass through, which can intentionally affect or not affect the crop development. This situation, supposedly, would allow for the compatible generation of electrical energy and agricultural production (Ureña-Sánchez et al. [3]). The integration of semi-transparent photovoltaic panels can decrease the solar irradiation and the internal air temperatures, as well as generate electric energy for environmental control systems (Hassanien et al. [4]).

Some concern remains about the impact that solar panels could have on crop yield and fruit quality, as a direct relation exists between the solar radiation that is received by the plants and decreased crop yields (kg·m<sup>−</sup>2) and smaller fruit sizes [5].

The objective of this work is to review the literature for applications of selective shading systems on crops, highlighting the use of photovoltaic panels.

**Figure 1.** Shading systems in greenhouses typical of Mediterranean countries.

### **2. Material and Methods**

The analysed articles were obtained electronically from the Library of the University of Almeria with a license from the Spanish Foundation for Science and Technology (FECYT) of Web of Science Core Collection (Wos) provided by Clarivate Analytics [6] and Scopus [7]. Through the 'Advanced search' option, terms such as 'greenhouse', 'solar', 'roof', 'energy', 'covering material', and 'shading' were used. A total of 113 articles (articles from scientific journals and conferences) that were directly or indirectly related to the previous terms have been analysed in the period from January 1990 to February 2018.

Only two databases have been used, which could limit the number of bibliographic citations obtained. Likewise, because only articles and congresses were considered, information from books, book chapters, and other similar formats is excluded.

### **3. Shading, Mesh, Screens and Others**

Shading produced by different systems and used in greenhouses has been investigated by numerous authors, mainly from the 1990s to the present (Table 1).




**Table 1.** *Cont*.
