*3.3. Others*

Klaring [10] found that broccoli yield is reduced by 1% for every 1% reduction in irradiation by shading.

Abdel-Mawgoud et al. [8] found that, in tomatoes with 30% shade, the yield was not reduced, although the total dry matter did decrease; the shade can serve to improve the commercial quality of the fruit by reducing burns.

Rosales et al. [20] comment that the increase in temperature and solar radiation in the cherry tomato during May in Spain diminishes the nutritional quality of the fruits.

Sato et al. [23] claim that as the shading level increases, the dry weights of tomato plants decrease, but no differences occur in the distribution of the organic dry matter.

Hernández et al. [33] state that in tomatoes with 50% of the sunlight attenuated by shading, a higher yield, as well as a higher concentration of lycopene, was obtained with lower doses of irrigated nitrogen (7 mM of N), regardless of the doses of N to tomatoes without shade.

Papadopoulos and Pararajasingham [9] explored the consequences of spacing between plants and light penetration for tomato productivity.

Kittas et al. [11] evaluated the quality of light that is received by plants by comparing three types of shading: bleaching, external mesh, and internal aluminized screens. Their results indicate the need to better control the characteristics of the light that is caused by the shading system used.

Abreu and Meneses [13] assert that roof bleaching reduces radiation transmission by 50%, which in turn reduces periods with temperatures above 30 ◦C.

Bartzanas and Kittas [18] took different measurements in a partially shaded greenhouse with a cooling system, finding that in the shaded part, greater transpiration occurred.

Aberkani et al. [24] comment that differences in air temperature of up to 6 ◦C, a humidity increase of 10% and reduction in the need for ventilation are possible when polyethylene liquid foam is used in greenhouse ceilings.

Holcman and Sentelhas [39] maintain that the lowest transmission of solar radiation is achieved with black polyethylene sheets ascompared with the use of red or blue sheets or with thermo-reflective sheets; the highest temperatures are reached with blue sheets.

Priarone et al. [40] investigated the selection of the most favourable solutions for ventilation, heating, cooling and thermo-hygrometric control of a greenhouse, and they propose, as optimal, the shading of the glazed surfaces, the natural ventilation and the forced convection of the external air.

### **4. Photovoltaic Modules in Greenhouses**

The application of photovoltaic modules (PM) to agricultural environments has been analysed by a large number of authors (Table 2).


**Table 2.** Studies related to photovoltaic modules in agriculture.


**Table 2.** *Cont*.

Kozai et al. [41] explain that considerable amounts of electricity can be generated without significantly affecting the transmission of solar radiation if the number of photovoltaic modules and the orientation of the greenhouse are correctly chosen for the latitude and the time of year.

Yano et al. [42] made use of the energy that is generated by photovoltaic modules to operate an autonomous lateral ventilation system. Later, Yano et al. [44] verified that when photovoltaic modules are mounted on the interior surface of greenhouses in Japan, greater energy efficiency is obtained with inclination angles of 20 degrees than with angles of 28 degrees.

Campiotti et al. [43] describe a prototype photovoltaic greenhouse being built in southern Italy.

Yano et al. [46] and Fatnassi et al. [69] found that the arrangemen<sup>t</sup> of panels in the form of a chessboard compared to panels placed in other arrangements improves the distribution of sunlight within the greenhouses. Kadowaki et al. [55] found that the placement of photovoltaic modules in this arrangemen<sup>t</sup> is desired to reduce the effects of shading. Additionally, Cossu et al. [85] sugges<sup>t</sup> new design criteria for PV greenhouses, concerning the decrease of the PV array coverage and different installation patterns of the PV panels on the roof.

Qoaider and Steinbrecht [47] demonstrated that providing power for entire farming populations is feasible with photovoltaic energy.

Carlini et al. [48] used TRNSYS 16 software to simulate temperatures and humidity in a greenhouse with photovoltaic modules in order to determine the performance of the greenhouse.

Sonneveld et al. [49] developed a hybrid system of photovoltaic and thermal panels together with the reflection of near infrared radiation to improve climate conditions in a greenhouse and avoid the use of fossil fuels.

Dupraz et al. [50] propose that an agrovoltaic system (using agricultural land for the generation of solar energy) may be the best solution in countries with few areas conducive to agriculture. One year later, Poncet et al. [58] stated that the main challenge for the agrovoltaic systems is to achieve higher productivity and quality, while reducing the environmental impact. However, Marrou et al. [60] note that moving from an open crop to an agrovoltaic system requires small modifications focused on the mitigation of shaded areas and the selection of plants adapted to fluctuating shadows. More recently, Dinesh and Pearce [83] affirmed that the value of electricity that is generated by solar energy and the production of shade-adapted crops creates an increase of more than 30% of the economic value of the lands that deploy an agrovoltaic system.

Campiotti et al. [51], in an experiment that was carried out in southern Italy with a greenhouse with rooftop photovoltaic panels, found that the energy requirements of 21 tomato plants for 120 days were 19.48 kW·h, and the modules produced a total of 333.6 kW·h from September to December. Later, Pérez-Alonso et al. [57] and Pérez-García et al. [66] conducted two experiments in south-eastern Spain, in which an annual energy yield of 8.25 kW·h·m<sup>−</sup><sup>2</sup> was achieved through 24 flexible modules.

Pérez-Alonso et al. [52] did not obtain significant differences between tomatoes shaded by flexible photovoltaic panels and non-shaded tomatoes; however, Klaring and Krumbein [59] maintain that restricting the intensity of solar radiation through permanent shading leads to a reduction in the growth and yield of the tomato plant but not the quality of the fruit.

Ganguly et al. [53] managed to maintain an optimum temperature for the cultivation of flowers in a greenhouse in India from energy provided by panels installed in the roof and a support system for critical hours, providing a clear example of agronomic compatibility.

Carlini et al. [54] proved that solar greenhouses with photovoltaic modules manage to save energy in both cooling and heating tasks.

Castellano [61] discusses different configurations for the placement of photovoltaic modules in greenhouses and analyses some parameters with Autodesk Ecotect Analysis software.

In South Korea, Juang and Kacira [62] propose adding integrated photovoltaic systems to the structure of greenhouses to alleviate the energy and food problems of certain populations that have difficulty accessing electricity, fertilizers, or good quality water.

Cossu et al. [63], in a greenhouse with 50% of the roof surface being occupied by photovoltaic modules, included supplementary lighting with the energy that was generated by the modules, but the plantation was too shaded and did not obtain benefits.

Tani et al. [64] claim that light diffusion films can be applied to improve productivity in crops shaded by photovoltaic panels.

Pérez-Alonso et al. [65] state that the commercial production of tomatoes is compatible with 9.8% shading that is produced by flexible photovoltaic modules. Tripanagnostopoulos et al. [92] propose that a PV system covering only 6.5% of the roof surface could be enough to completely cover the electricity needs for the auxiliary processes of a greenhouse. Liu et al. [95] have developed new types of photovoltaic sheets that shade on the field can be reduced.

Serrano et al. [67] made use of flexible panels to supply energy to autonomous systems and to replace the shading elements, thus achieving normal crop development.

Marucci et al. [70] used dynamic panels, which move along the longitudinal axis, in order to vary the degree of shading.

Bulgari et al. [71] reveal that the efficiency of the use of solar radiation by tomato plants is greater in greenhouses with solar panel shading, but the fruit tends to have lower lycopene, beta-carotene, sucrose, reducing sugars, and total sugar content.

Castellano and Tsirogiannis [73] performed an analysis of different photovoltaic configurations in the greenhouse to determine the effects of shading and energy efficiency.

Marucci and Capuccini [75] reported that it is possible to combine the production of electricity and agricultural production if the type of crop, the latitude, and the characteristics of the greenhouse are taken into account. That same year, Marucci and Capuccini [76] affirmed that the use of photovoltaic panelsisaviablealternativebothforshadinggreenhousesandforelectricityproductioninwarmareas.

Hassanien et al. [77] conducted a small discussion on photovoltaic technology and the challenges it faces in the agricultural environment.

Castellano et al. [79] used a model that predicts the density distribution of photosynthetic photons in photovoltaic greenhouses with an error of 19%. That same year, Castellano et al. [80] developed a model to predict the effect of shading within a photovoltaic greenhouse.

Cuce et al. [81] managed to save up to 80% in energy consumption in greenhouses by combining solar and thermal energy with new insulation materials.

Cossu et al. [84] developed an algorithm to estimate the global radiation that had accumulated within photovoltaic greenhouses to aid in the selection of the most suitable plant species according to their light needs.

Carreño-Ortega et al. [86] estimated that the use of photovoltaic modules in the agricultural environment can increase the profitability of the farms up to 52.78% and that environmental and economic improvements would also be obtained.

Marucci et al. [87,94] analysed the shading variation produced by the application of flexible and semi-transparent photovoltaic panels in a tunnel-type greenhouse, where the percentage of shading during the year never exceeded 40%.

Valle et al. [88] demonstrated that an agrivoltaic system achieved high productivity per unit of land area using solar trackers instead of stationary photovoltaic panels, whereas the production of lettuce biomass was maintained close to or even similar to that obtained under full sun conditions.

Trypanagnostopoulos et al. [89,93] explained that the use of photovoltaic panels in a lettuce crop produced a 20% shading of the greenhouse, and plant growth was the same as that of the reference greenhouse, without photovoltaic panels on the roof.

Loik et al. [90] reported that in a trial conducted on a tomato crop, the wavelength-selective photovoltaic systems produced a small decrease in water use, whereas minimal effects were observed on the number and fresh weight of the fruit for several commercial species.

Yildirim et al. [91] conducted an economic and environmental assessment for tomato, cucumber and lettuce crops using photovoltaic solar panels on the roof of the greenhouse and connected to the grid to support a heat pump and generate electricity.

Minuto et al. [45] conducted an experiment with semi-transparent photovoltaic panels on a glass greenhouse and did not find large differences in the behaviour of the tomato plants due to shade.

Marucci et al. [56] explored the possibility of using semi-transparent photovoltaic materials to avoid the loss of solar radiation by shading.

Yano et al. [68] revealed that the electrical energy produced by semi-transparent modules with a cell density of 39% is sufficient for regions with high demand in summer and low demand in winter.

Yang et al. [72] optimized the use of sunlight by manipulating the photonic crystals in transparent organic photovoltaic cells.

Buttaro et al. [78], in a greenhouse arugula plantation, found that semi-transparent modules can satisfy all of the required electricity demand and that the yield of the plantation decreases if traditional modules are used.

Cossu et al. [74] claim that semi-transparent photovoltaic technology with spherical microcells can be used to contribute to the sustainability of greenhouses.

Saifultah et al. [82] conducted a review of materials used for manufacturing semi-transparent modules.

### **5. Other Related Studies**

Table 3 shows articles that are related to greenhouses, renewable energies and/or shading but do not fit into the categories described above.


### **Table 3.** Other related studies.


**Table 3.** *Cont*.

Reca et al. [114] verified that the profitability and energy efficiency of a photovoltaic system for irrigation is relatively low, although it can be improved by using excess energy for other tasks.

Bot et al. [96] developed Dutch-type greenhouses that do not use fossil fuels, thus improving the insulation value of the roofs and capturing solar energy for storage.

Marcelis et al. [97] showed that light has a positive effect on the yield and quality of greenhouse crops, but this effect is more noticeable when the amount of light is lower.

Verheul [105] states that an increase in the intensity of the light increases the tomato yield, but not the quality.

In Holland, Hemming et al. [98] observed that covering greenhouses with light-diffusing materials led to increases in production in the summer months by 6%. Later, in Dutch greenhouses, Hemming et al. [100] and Bibi et al. [104] obtained grea<sup>t</sup> results in cucumbers, proving that diffuse light improves photosynthesis in the middle zones of plants.

Suri et al. [99] state that photovoltaic energy is already in a position to make a significant contribution to the European Union's energy landscape.

Sonneveld et al. [101] presented the possibility of taking advantage of excess solar energy in summer to convert it into high-grade electricity and use it for cooling or heating.

Abdel-Ghanyand Al-Helal [102] developed an improved thermal model for greenhouses.

Abdel-Ghany [103] states that at a density of plants corresponding to a leaf area index of 3, 54% of the solar radiation used by the greenhouse is converted into sensible heat and 46% into latent heat through evapotranspiration.

Schuch et al. [106] reduced the consumption of fossil fuels by 81% with a system to capture solar thermal energy in a tomato greenhouse.

Klaring et al. [107] reported that carbon dioxide emissions from tomato crops can be reduced by lowering the heating temperature without affecting the fruit, but harvest times are increased.

Bian et al. [108] discussed the advantages of LED technology to modify the accumulation of phytochemicals with light.

El-Maghlany et al. [109] performed an efficiency analysis of solar energy capture and energy savings according to the type of greenhouse.

Cakir and Sahin [110] analysed different greenhouse types and found the elliptical greenhouse to be the most appropriate for the cold climate and latitude of Bayburt, Turkey.

Attar and Farhat [111] explain that the cost of heating in a 1000-m<sup>3</sup> greenhouse can be reduced by 51.8% if a heated water system is integrated.

Shyam et al. [112] and Elkhadraoui et al. [113] developed greenhouses as biomass dryers with electricity that is supplied from solar panels.

Ziapour and Hashtroudi [115] modified the roof of a greenhouse to partially reflect sunlight in a collector, and thus save on energy expenditure.

Arabkooshar et al. [116] used thermal panels and geothermal wells for heating, thus reducing the diesel consumption in winter.

Xue [117] states that photovoltaic greenhouses that occupy a large area of land require large outlays, which are not available to farmers or even to large companies.

Anifantis et al. [118] analyses the performance of an independent renewable energy system for greenhouse heating by using photovoltaic panels that are connected to an electrolyser, which produces hydrogen by electrolysis during the day and stores it in a pressure tank.

### **6. Studies Related to Shading and Photovoltaic Panels in Greenhouses by Country**

The country of the main author of each of the publications analysed has been used to create Figure 2.

**Figure 2.** Publications by country.

The 113 publications analysed, which were related in some way to greenhouses, solar panels, tomato, and/or shade, represent 27 countries. If we look at the number of investigations in each country, the one with the most publications is Italy, with a total of 21, followed by Spain with 13; Japan with 12; Saudi Arabia with 8; Greece with 7; Holland with 6; France and Germany with 5; Brazil and China with 4; Egypt and the United States with 3; Canada, South Korea, India, Iran, United Kingdom, Tunisia, and Turkey with 2, and Belgium, Denmark, Hungary, Norway, Pakistan, Portugal, Serbia, and Taiwan with 1 (Figure 2).

In Figure 3, the research carried out in each section of the review by each country is indicated.

**Figure 3.** Number of publications in each field per country.

### **7. Studies Related to Shading and Photovoltaic Modules in Greenhouses by Year**

Figure 4 shows the number of studies per year, as well as the fields studied.

**Figure 4.** Number of publications in each field of knowledge per year.

A clear tendency to increase the number of publications can be observed. A notable difference is apparent in the number of studies that have carried out since 2010, with the exception of 2013, when only two studies that were related to the subject were analysed; however, 2017 had the most publications, with a total of 20, followed by 2015 with 14.

### **8. Studies Related to Authors and the Number of Citations**

Table 4 shows the authors that appear in at least two citations.


**Table 4.** Authors and the number of documents.

If we observe the number of documents per each author, the ones with the most publications are "Yano A" and "Abdel-Ghany AM", with a total of 8, followed by "Al-Helal IM" and "Marucci A" with 6 (see Table 4).

### **9. Studies Related to Journals and the Number of Citations**

Table 5 shows the journals that appear in at least two articles.


**Table 5.** Journals/Conferences and the number of documents.

The most important conference document is Acta Horticulturae with 19 documents. The most important journal is Biosystems Engineering, with a total of eight documents, followed by Renewable Energy with 6; Solar Energy, Renewable & Sustainable Energy Reviews, Applied Energy and Journal of Agricultural Engineering with five. It should be noted that Acta Horticulturae is not a journal in the true sense of the word.

### **10. Conclusions of the Review**

The countries with the highest number of publications concerning solar panels and crops were Italy, with a total of 21; Spain with 13; and Japan with 12. During the past decade, the number of relevant publications has increased. These three countries are in the same latitude range, although the studies are very different depending on the type of crop selected and the type of greenhouse structure.

The most important authors of this topic are "Yano A" and "Abdel-Ghany AM", and regarding the number of documents that are cited, the most important journal is Biosystems Engineering.

Most of the studies justify the use of photovoltaic panels alongside agricultural production, although others cast general doubt on the economics of using these panels. Further technological development of photovoltaic panels with respect to transparency and energy efficiency could make their coexistence with greenhouse crops more economically viable. The trend of research on this subject is the search for the percentage of shading that makes the shading compatible for each type of crop.

**Acknowledgments:** To the 'Consejería de Innovación, Ciencia y Empresa de la Junta de Andalucía (Spain)' and the 'Ministerio de Ciencia e Innovación del Gobierno de España' as well as to the 'Fondos Europeos de Desarrollo Regional (FEDER)' for financing the present work through the research projects P08-AGR-04231 and AGL2006-11186, respectively.

**Author Contributions:** All authors contributed equally to the manuscript, and have approved the final manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
