3.1. Crop Selection
The microclimate aspect under the solar PV structure should be taken into account in the selection of suitable crops to be cultivated in solar farms
[5][46][116][101]. A setup by Refs.
[42][117] in particular divided the area beneath the solar panel into three sub-treatments: (1) sky fully open area between panels (SFO); (2) Solar partially open between panels (SPO); (3) solar fully covered area under panels (SFC) as illustrated in
Figure 3A. SFO zones are located between the edges of mounted photovoltaic panels and areas that have received full light
[28]. No shade covers the SFO zone, according to the shadow length estimate
[113], while SPO areas are situated in the penumbra and have been subjected to episodic shade
[118]. SFC areas are located immediately under the photovoltaic panels and receive complete shade
[73]. However, the division of the sub-treatments are subjective and subjected to the solar photovoltaic design
[5][19][38][97]. For example, the AVS design at Montpellier Experimental Agrivoltaic Station by
[4][6][18][69] (as described on page 9) is high in length, and is thus less suitable to be divided into SFO, SPO, and SFC. Previous researchers at that station classified the area beneath the solar panel as FD and HD. The average proportion of daily radiation emitted below the solar panel (FD and HD treatments) relative to the FS treatment varies with the growing season. In general, AVS is not recommended in crop rotation systems
[41]. However, crop rotation could increase the production of agriculture in AVS farms
[69], especially in regions that experience different seasons throughout the year
[104]. Furthermore, when used in conjunction with permanent cultures—such as berries, bananas, or wine grapes—the cost of these types of applications is smaller, thus delivering increased efficiency through the optimization of techno-ecological synergies
[41].
Figure 3. Schematic Drawing of Shade Zones. (
A) SFO, SPO and SFC. (
B) FD and HD. (Modified
Figure 3A), source:
[32]; The illustration 3(B), source:
[4]). *H is object height and L is shadow length.
Next, the heat or thermal energy dissipated under the photovoltaic array is also a critical factor to consider in relation to the continuous development of the crops beneath
[19][54][119][90]. In this case, the open field AVS is better than the closed greenhouse AVS
[120], as its key characteristics are the mean daily reduction in light access for plants
[28][38] without major changes in other microclimate parameters such as relative humidity, wind speed and direction, and soil moisture
[26][121][113] at the level of the canopy
[69][103]. If the AVS design were able to regulate adequate air circulation below the open structure, the air temperature, VPD
[69], mean relative humidity, and wind speed
[113] might be insignificantly different
[122], or optimized
[11], compared to the ambient surrounding; however, it depends on the structural design
[21][54][62] and regions
[10][116][96]. Enclosed structures, on the other hand, offer the advantage of being able to regulate the temperature inside the structure to meet the demands of the crop
[97][116][101]. Furthermore, in a study conducted by
[31], it was found that reduced light is not often harmful to crop quality, as improved Radiation Interception Efficiency (RIE) has been shown in the shade; however, a specific arrangement of the solar panel is needed to compromise between agriculture production and electricity generation in a way that can improve the production of both. However, solar management is not amenable to all types of crops, and there is a need for further research before an economically viable approachable system using PV technology can be designed
[7][12][26][113][123].
There are several factors suggested by the author, based on reviews, to facilitate the crop selection for AVS: (1) the design of solar PV structure
[8][54][62][63]; (2) the location of sub-treatments
[6][69][113]; (3) the approaches of AVS
[44][98]. For the first factor, types of design considered for solar PV structure have been described in
Section 3. In case the introduction of agriculture production is on the existing solar farms or an unaltered solar panel structure, the approach used by
[50] could be the sustainable solution to combine both productions. They suggested the planting of high-value herbal crops in solar farms with zero or minimal modification of the solar PV structure. The authenticity of growing herbal crops under solar photovoltaic arrays is justified by the sustainability and morphological aspect of the arrangement as a way of using unused land. For example, the maximum height of the Java Tea Plant (high-value herbal crops), which is less than three feet (from the ground) and grows in a regulated manner, is considered suitable and will not interfere with the PV panel electricity generation operation. The chosen plant is also classified as a shade-loving herb, and the temperature beneath the solar PV structure measured is within an acceptable range for ornamental herbal plants. The solar farm project’s maintenance requirements are met by field arrangements of herbal polybags and a manual irrigation solution
[59].
For the location of sub-treatments which are SFO, SPO and SFC
[42][113], or FD and HD
[4][6][18][69], a wide range of crops can be selected to be planted based on their physiological and morphological traits
[4][28][71]. Besides that, the selection of suitable crops for AVS should also be identified based on local climate and weather conditions
[5][12][21][28]. In general, shade-loving plants are best suited for planting in less sunlit areas, while sun-loving plants are better suited to seeding in sunlight areas
[64][98][94]. For the arid region, such spices may be successfully grown between two rows of solar PV, as these are short in nature, for example:
Trigonella foenum-graecum Linn. (‘methi’),
Plantago ovata Forsk. (‘isabgol’), Coriandrum sativum Linn. (coriander or ‘dhania’), etc
[25]. The following vegetable crops may also be grown:
Brassica oleracea var. botrytis (cauliflower),
Brassica oleracea var. capitate (cabbage),
Allium cepa Linn. (onion),
Allium sativum (garlic),
Capsicum annum Linn. (chilli), etc.
[25]. The land area beneath photovoltaic panels can also be used to grow vegetable crops of the Cucurbitaceae family, such as
Cucurbita pepo Linn. (‘kakri’),
Lagenaria siceraria (‘lauki’),
Citrullus fistulosus Stock (‘tinda’), etc.
[25]. Cultivating crops in areas below the photovoltaic panel has the added benefit of reducing the heat load on the bottom surface of the photovoltaic panel by modifying the microclimate and thus assisting in generating the maximum amount of electricity
[21][50]. Additionally,
[59] also proposed that herbs could be planted in tropic areas using AVS applications with minor modification of the solar panel structures. Herbal plants such as
Orthosiphon stamnieus is suitable in the tropical region
[50], while
Cassia angustifolia (senna), Aloe vera (‘gwarpatha’), and others may also be considered as potential crops if the PV structure is in rocky scrubs or degraded lands, depending on the region
[25]. Next, some studies performed in various regions of the world indicate different kinds of crops, such as semi-arid pasture (Oregon, USA;
[113]), Maize (Po Valley, Northern Italy;
[53]), lettuce (short cycle crop), cucumbers (short cycle crop), durum wheat (long cycle crop) cultivated at Montpellier Experimental Agrivoltaic Station, France
[6][31], and potato and wheat (Demeter-certified farm community Heggelbach, Germany;
[41]). In addition,
[5] stated that, in some regions, certain crops such as fruit trees (i.e., kiwi, apple, pear, cherry), berries (i.e., raspberries, blackberries), tomatoes, sweet peppers, coffee, and ginseng, are among the crops that are also able to cope with a reduction of more than 50% in the light source. Based on these findings, it is possible to conclude that the selection of suitable crops for integration into the AV system is subjective, depending on local weather and the architecture of the PV structure
[8][28][33][85][116][124].
However, a suggestion from
[98], as shown in the table below, to include the AVS approaches may be able to further facilitate the selection of suitable crops. The table illustrates how solar farms and crops can be combined according on the land-use type and AVS strategies. Next, several modifications of the solar PV structure and types of the crop cultivated will be recommended. For example, the suggestions for short crop planting area with agriculture centric approach are as follows: (1) plant mix of sun-loving and shade-tolerant crops, (2) raised solar PV structures, and (3) space solar PV structures. Other options are covered in
Table 4.
Table 4. Opportunities for Solar PV and Agricultural Integration by Land-Use Type. (Source:
[98]).
3.2. Agronomic Practices
Solar energy is the most plentiful and readily available source of energy
[25][28][47][122]. The use of AVS technologies in areas where a solar farm and agriculture coexist
[51][125][99] could have synergistic effects that aid in the production of ecosystem services such as crop production
[9][20][38], local climate regulation
[34][96][126], water conservation
[13][18][56], and renewable energy production
[21][85][127]; and it also aligns with food-energy-water (FEW) nexus
[34][63][126].
Thus, the integration should potentially influence the microclimate and soil moisture
[21][51][116]; hence, it may provide suitable environmental conditions
[18][34][43] and increase the water-use efficiency for agricultural production
[3][120][102] while maintaining the renewable energy production
[41][44]. As mentioned in 2.1, the photosynthesis process requires light, carbon dioxide, and water to produce glucose as the source of energy for plants. If the sources of light and carbon dioxide are not limited, an optimum amount of irrigation water is needed to enhance the photosynthesis rate. Thus, regions with insufficient water resources are most likely to benefit as solar management decreases potential evapotranspiration (PET) and water demand
[26][51][113][126]. The reducing amount of irrigation water needed without compromising crop-water requirements can make a significant contribution to reducing agricultural production costs, making the industry more competitive and sustainable
[18][21][65]. However, a systematic or proper irrigation schedule is a must in AVS sites
[120][128] to minimize the environmental impacts caused by excess water and leaching of subsequent agrichemicals
[129] that might affect the structure of solar PV. Water-use efficiency can be improved
[3][8][120] by understanding the concept of evaporation, evapotranspiration (ET), and irrigation water requirements
[130]. ET is the mechanism by which water originates from a wide range of sources such as soil compartment and/or layer of vegetation and is transferred to the atmosphere
[131]. Also, ET involves evaporation from bodies of surface water, surface of land, sublimation of snow and ice, plant transpiration, and intercepted canopy water
[120]. Besides that, the evaporation process that happens also significantly reduces the percentage of soil moisture content
[19][41][126]. On the other hand, irrigation water requirements are defined as the quantity of water necessary for crop growth
[130][132]. In addition, the loss of electrical output due to dust accumulation on the panel surface as a result of agricultural management, such as tillage and harvesting, is also a source of concern
[5][7][10][21]. In regions with low precipitation or long stretches of dry weather (e.g., monsoon climates), periodic cleaning of the module surface should be considered to prevent decreasing electricity yields due to dust accumulation
[85]. This could be done by combining irrigation systems and PV cleaning to reduce increased water use
[51]; however, without a small water distributor under the panels, it may result in inconsistent watering of crops
[119]. Hence, proper assessment of evapotranspiration
[128], soil moisture content
[129], and PV cleaning processes
[10][119] are needed before designing the irrigation system for agricultural production in AVS.
Another aspect is that extreme heterogeneity and spatial gradients in biomass production
[5] and soil moisture
[45][120] were observed as a result of the heterogeneous shade pattern of the PV array
[113]. In the studies conducted at Montpellier Experimental Agrivoltaic Station by
[4][6][18][69], the shadow effect of the PV array can be seen from the agricultural yield, where the HD structure produces more yield than the FD structure. The results show that, with the improvement of PV panel arrangement, LER may potentially exceed 1
[38][85]. Next, a solar tracker controller developed by
[53] found that maize grown under the AVS plots tended to have more stabilized and higher yields in drought stressors and rainfed conditions. Besides that, crop selection can also reduce the effect of the heterogeneous shade pattern of the PV array
[11][25][28]. This can be seen in the experiment conducted by
[61] using Java Plant Tea in Malaysia. The result obtained shows a good agreement between the selected crop and the PV panels above them that act as their artificial shading. To sum up, acts to reduce the extreme heterogeneity and spatial gradients in agricultural production are: (1) optimize PV array placement to create a spatially uniform shadow pattern
[4][10][125]; (2) improve the solar tracker controller that considers the need for solar radiation for both productions (electricity and agriculture)
[53][97][111]; (3) select a suitable crop to be planted with a minimal light source (due to shading effect of solar PV structure)
[25][54][85]. Besides that, as suggested by
[5][94], the PV structure can be raised to reduce the heterogeneity effect, while allowing the conventional agricultural machines to pass
[4][28][38], and reducing the back pain
[68] while doing agricultural work due to low PV structure
[86]. The gap between the pillars also needs to be suitable for planting distances and working widths of the machinery to avoid the loss of utilizable land
[5][41]. Careful planning is essential, since the space required for the machine to pass might restrict the amount of land available for solar panels
[19][68]. Also, ram protection should be installed to avoid collisions between agricultural machines and the solar PV pillars
[68].
Other than that, the agronomic practices for agricultural production at AVS, likely similar to standard and common agronomic practices
[62][101], include the steps listed in
Figure 4. More information on standard practices can be found in documents such as the ones written by
[133], which specifically address cropping systems and agronomic management. However, precision agriculture methods such as site-specific crop management (SSCM), for which decisions on resource application and agronomic procedures are being improvised, can be developed to better meet crop requirements based on soil heterogeneity in the field
[33].
Figure 4. Common Agronomic Practices.