Agricultural sustainability estimation of the European photovoltaic greenhouses

https://doi.org/10.1016/j.eja.2020.126074Get rights and content

Highlights

  • The yield inside PV greenhouses was estimated on 14 species.

  • The evaluation identified the suitable crops inside four PV greenhouse types.

  • A PV cover ratio of 25% is compatible to all crops, with limited yield reduction.

  • A PV cover ratio of 50% is sustainable to medium and low light demanding crops.

  • Structures with 100% PV cover support only crops with optimal DLI < 10 mol m−2 d−1.

Abstract

The integration of the photovoltaic (PV) energy in the greenhouse farm has raised concerns on the agricultural sustainability of this specific agrosystem in terms of crop planning and management, due to the shading cast by the PV panels on the canopy. The PV greenhouse (PVG) can be classified on the basis of the PV cover ratio (PVR), that is the ratio of the projected area of PV panels to the ground and the total greenhouse area. In this paper, we estimated the yield of 14 greenhouse horticultural and floricultural crops inside four commercial PVG types spread in southern Europe, with PVR ranging from 25 to 100%. The aim of the work is to identify the PVG types suitable for the cultivation of the considered species, based on the best trade-off between PV shading and crop production. The daily light integral (DLI) was used to compare the light scenarios inside the PVGs to the crop light requirements, and estimate the potential yield. The structures with a PVR of 25% were compatible with the cultivation of all considered species, including the high light demanding ones (tomato, cucumber, sweet pepper), with an estimated negligible or limited yield reduction (below 25%). The medium light species (such as asparagus) with an optimal DLI lower than 17 mol m−2 d−1 and low light crops can be cultivated inside PVGs with a PVR up to 60%. Only low light demanding floricultural species with an optimal DLI lower than 10 mol m−2 d−1, such as poinsettia, kalanchoe and dracaena, were compatible inside PVGs with a PVR up to 100%. Innovative cropping systems should be considered to overcome the penalizing light scenarios of the PVGs with high PVR, also implementing LED supplementary lighting. This paper contributes to identify the sustainable PVG types for the chosen species and the alternative crop managements in terms of transplantation period and precision agriculture techniques, aimed at increasing the crop productivity and adaptability inside the PVG agrosystems.

Introduction

The environmental sustainability of the modern agriculture is strongly linked to the implementation of different strategies aimed to reducing the use of production factors, including energy. The application of this concept to intensive greenhouse systems includes the introduction of technologies based on renewable energy sources, such as photovoltaic (PV) systems, wind turbines, heat pumps, solar panels and hybrid PV thermal systems (Agrawal and Tiwari, 2015; Hassanien et al., 2016). Within the PV energy applications to protected agriculture, the PV greenhouse (PVG) is an agrosystem potentially able to combine food and energy production on the same land unit by integrating the PV systems on the greenhouse roof. The consequent main advantages are the diversification of the farmers’ income and the higher competitiveness and rural multi-functionality of the PVG farm (Marcheggiani et al., 2013; Tudisca et al., 2013).

Despite these principles that inspired the emergence of PVGs, most structures were built in marginal agricultural lands with an excessive percentage of the roof covered with PV panels. The only purpose was to maximise the PV energy production and speculate on the related income deriving from the high public subsidies, regardless of the crop light requirements (Cossu et al., 2014; Fatnassi et al., 2015). In some European countries such as Italy and France, the current regulations often prohibit the installation of ground-based PV systems in agricultural areas, due to environmental problems including soil sealing and landscape and biodiversity deterioration (Colantoni et al., 2015; Delfanti et al., 2016; Fatnassi et al., 2015). Under these circumstances, the PVG was considered a solution to bypass the current laws, by installing the PV systems on new and cheap rural buildings, such as greenhouses, specifically built for the purpose (Castellano, 2014; Marucci et al., 2018). In addition, the speculative construction of numerous PVGs in Southern European countries will require regional and national regulatory frameworks to manage the PV waste management and recycling of the high amount of PV panels at the end of their life cycle.

The agricultural sustainability of the PVGs can be defined as the optimal trade-off between energy and crop production, aimed to maximise the greenhouse crop productivity, on the basis of the actual light conditions. In fact, the shading of the PV panels on the greenhouse area affects yield, growth and development of the plants. As a result, the PVG farm can achieve a higher resource and energy use efficiency and reduce the competition for the land resources (Cuce et al., 2016; Dinesh and Pearce, 2016; Yano et al., 2010).

The cumulated global radiation inside PVGs decreases as a function of the increasing PV cover ratio (PVR), that is the ratio of the projected area of PV panels to the ground and the total greenhouse area. This reduction was found to be equal to 0.8% for each 1% increase of the PVR, as the average of the main commercial PVG types in Europe (Cossu et al., 2018). The main design criteria for the future generation of PVGs include a PVR limited to values around 20%, the use of semi-transparent or organic PV technologies, the installation pattern of the PV panels on the roof (such as the checkerboard pattern), the increase of the greenhouse height, the orientation to North (N)-South (S) instead of East (E)-West (W), or the use of the PV energy to power electrical appliances for microclimate control (Al-Shamiry et al., 2007; Emmott et al., 2015; Fatnassi et al., 2015; Minuto et al., 2009; Yano et al., 2014, 2009). Some crops require moderate shading during their cycle and the semi-transparent PV panels can be used to provide it during periods of intense irradiation through dynamic PV systems, able to adjust the tilt of the PV modules according to the crop light needs (Li et al., 2018; Marucci and Cappuccini, 2016; Moretti and Marucci, 2019a). All these technical solutions are targeted to optimize the energy and the agricultural production by varying the shading of the PV panels at canopy level and the impact on the greenhouse farm in terms of energy consumption (Moretti and Marucci, 2019b). However, a currently open issue concerns the existing PVGs with high PVR (from 50 to 100%), for which technical and agronomic solutions are required to establish a balance between energy and food production (Castellano et al., 2016; Kadowaki et al., 2012; López-Marín et al., 2012; Scognamiglio et al., 2014). For example, in Italy the current regulations impose an economic target to PVG farms, in which the income of the crops should be equal or higher than that deriving from the energy injected to the grid (Agenzia delle Entrate, 2009). The income of PVG types with high PVR is currently unbalanced towards the energy production, and the difficulties related to cultivation pose a debate on whether these PVGs can be actually considered agricultural greenhouses or power plants where most crops are precluded.

The yield reduction inside PVGs can be assumed correlated to the available solar radiation according to a rule of thumb, called the “1% rule”, that estimates roughly 1% additional production for each 1% additional light inside the greenhouse (Heuvelink, 2005; Kläring and Krumbein, 2013; Marcelis et al., 2006). An acceptable compromise between horticultural crops and energy production is usually achieved when the PVR is low (around or lower than 20%), resulting in limited yield losses and negligible impact on the fruit quality. For example, a high demanding crop such as tomato, grown in a PVG with 9.8% of the roof area covered with PV panels, did not show yield reduction due to the shading of the PV panels (Aroca-Delgado et al., 2019; Pérez-Alonso et al., 2012; Ureña-Sánchez et al., 2012). As the PVR increases, the PVG microclimate becomes affected by the reduced solar radiation to a greater extent, including a decrease of the air temperature and an increase of humidity when ventilation is not applied (Ezzaeri et al., 2018). In addition, the negative effects on growth, development and yield worsen. Plants adopt specie-specific physiological responses to shading that can include shade tolerance or shade avoidance (Gommers et al., 2013). Most species react by optimizing their photosynthetic rate. However, while shade tolerant crops can adjust to lower light levels by optimizing the radiation interception efficiency (RIE), the shade intolerant species (such as tomato) increase their vegetative growth rate and concentrate resources on stem and leaf growth instead of fruits, resulting in lower yield (Smith and Whitelam, 1997).

The PVGs are a major concern in the southern European greenhouse sector, where it is crucial to overcome or mitigate the problems related to cultivation when light is a constraint, in terms of suitable species and agricultural practices (Poncet et al., 2012). Such information is currently scarce in literature and with no general applicability, since it is available only for a limited number of species, cultivated inside specific PVG types. In this paper, the PVGs were considered as options of agricultural management for the chosen species, aimed to identify the best compromise between PVG types and crop planning. For this purpose, we introduced an innovative solar engineering approach based on the comparison between the solar radiation available at canopy level and the crop light requirements. The agricultural compatibility of four PVGs types was evaluated systematically on 14 crops (9 horticultural and 5 floricultural species), classified on the basis of their light requirements (high, medium and low), and estimating the potential yield reduction compared to a conventional greenhouse (CVG). The results identified the PVG types compatible to the cultivation of each considered species and that represent the best compromise between yield and PVR. In perspective, such findings can be used as a decision-support tool for the greenhouse growers, to adopt the best crop planning and management practices inside PVG agrosystems.

Section snippets

Solar radiation available inside the photovoltaic greenhouse types

The solar radiation distribution was calculated inside the PVGs using a specific algorithm described in detail in a previous paper (Cossu et al., 2017a, 2017b). The algorithm can calculate the direct (ID) and diffuse (Id) radiation and assess when the shadow projected by the PV array cast on specific observation points (OPs) located on the PVG area. When the OPs are under the sunlight, the algorithm attributes both ID and Id, resulting in an average global radiation on hourly basis IGP equal to:

Yield estimation inside photovoltaic greenhouses

The solar radiation distribution is heterogenous on the greenhouse area due to the shading of the PV panels, as shown by the CVs on monthly and yearly basis (Fig. 2). The average CV generally increases with the PVR: the yearly CV ranges from 31% of type 1 to 60% of type 4, indicating that a low PVR is preferable also to avoid excessive heterogeneity of light distribution, that may affect negatively the uniform growth and development on the greenhouse area. An exception is type 3, with a yearly

Photovoltaic greenhouse types compatible with crop production

While the estimated yield of high light demanding species under the ST roof was generally acceptable (Yf above 75%), the reduction was remarkable under the PV roof, as confirmed by previous experiments that highlighted the negative effects on the plant physiology such as tomato, where an increase of LAI and a reduction of the stomatal conductance, growth, photosynthetic and transpiration rate occurred, observed also on lettuce (Marrou et al., 2013b; Sirigu et al., 2013). The photosynthetic and

Conclusions

Choosing and managing crops able to adapt to the light conditions of PVGs is still a thorny issue for the greenhouse growers and researchers, especially for PVG types with high PVR. In this paper, we discussed the yield estimations and the crop planning of 14 horticultural and floricultural crops inside four common PVG types in Europe, compared to conventional greenhouses. An original method comparing the light scenarios inside PVGs and the crop light requirements was applied, to estimate the

CRediT authorship contribution statement

Marco Cossu: Conceptualization, Methodology, Software, Investigation, Writing - original draft, Writing - review & editing, Visualization. Akira Yano: Validation, Data curation, Writing - review & editing. Stefania Solinas: Investigation, Validation. Paola A. Deligios: Writing - review & editing. Maria Teresa Tiloca: Visualization. Andrea Cossu: Visualization, Resources. Luigi Ledda: Project administration, Conceptualization, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors would like to thank the Sardaflora-Cidam company of Mr. Murtas, Dr. Antonio Del Curto and Mr. Farina of the Agricola Agrisolar farm, the SerraSol Santa Lucia company for their collaboration and courtesy for the pictures shown in this manuscript. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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