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The influence of greenhouse-integrated photovoltaics on crop production
Solar Energy 155 (2017) 517–522
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Review
The influence of greenhouse-integrated photovoltaics on crop production Claire S. Allardyce a,⇑, Christian Fankhauser b, Shaik M. Zakeeruddin a, Michael Grätzel a, Paul J. Dyson a a b
Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), EPFL–BCH, CH–1015 Lausanne, Switzerland Center for Integrative Genomics, Faculty of Biology and Medicine, University of Lausanne, 1015 Lausanne, Switzerland
a r t i c l e
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Article history: Received 7 September 2016 Received in revised form 6 June 2017 Accepted 16 June 2017
Keywords: Photovoltaics Dye-sensitized solar cells Agriculture Greenhouse-integrated photovoltaics Photobiomodulation
a b s t r a c t Photovoltaics (PVs) have been particularly successful in many domestic and industrial settings where opaque PV-covered roofs provide renewable electricity. Modern farming, for an ever growing population, employs vast areas of greenhouses consuming considerable amounts of energy. The majority of greenhouses are not suited to coverage by opaque PVs. Herein, we describe the current-state-of-the-art in greenhouse-integrated opaque PVs and their limitations, particularly with respect to the compatibility with certain plant cultivars. We propose semi-transparent PVs (Dye-Sensitized solar Cells, DSCs) as alternative greenhouse glazing that, compared to conventional greenhouse glazing and currently marketed greenhouse integrated opaque PV materials, offers advantages including enhanced thermal stabilisation and similar or improved edible biomass yields. Large-scale validation of DSCs in solar sharing for crop production (yield, appearance and nutritional content) is now in progress. Ó 2017 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3. 4. 5. 6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Integration of opaque PVs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Constraints of sharing solar energy between electricity generation and cultivation . Potential advantages of integrated PVs beyond electricity generation . . . . . . . . . . . . The future technology: DSCs and their integration into greenhouses . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Land is a valuable resource and about 38% of the dry land area of the planet is used for agriculture (Ramankutty and Foley, 1999; Ramankutty et al., 2008); a figure that has remained stable for several years because it is near to the maximum available area for this practice. Sites not used for agriculture or other human needs are either uneconomical to farm or serve as important ecological resources and should be preserved. An increased food supply, therefore, demands higher production from existing sites, which generally requires increased energy input directly (for temperature ⇑ Corresponding author. E-mail address: [email protected] (C.S. Allardyce). http://dx.doi.org/10.1016/j.solener.2017.06.044 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.
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regulation, irrigation and artificial lighting) and indirectly (for nitrogen fixation technology, phosphate fertiliser acquisition (mining and transport) and increased chemical treatments, including pest control due to high density planting). In the past few decades, the energy expenditure per kilo of crop yield has increased due to the expansion of controlled environment farming (CEF). Traditionally in greenhouses, but increasingly in closed hangers where even the light is artificial, CEF leads to higher crop yields over the same land area such that, despite increased costs due to investment in infrastructure and energy, CEF is becoming the norm in the industrial world (Beyhan et al., 2013; Bot et al., 2005). Currently, much of the energy for CEF comes from fossil fuels sources. As an example, in terms of crop yield for lettuce production CEF increases yield by more than an order of magnitude, but
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to achieve this increase requires much more energy compared to conventionally produced lettuce. The energy input is equivalent to a net yield of 0.46 g of lettuce per kJ of energy for CEF methods versus 3.5 g/kJ for conventional methods (Barbosa et al., 2015). The requirements are such that there is mounting opposition to CEF – even from scientists who developed CEF – and a 2014 estimate of the carbon footprint was 3.6 kg of CO2 per kg of lettuce (Shackford, 2014). This awareness has led to monitoring (for example, Eurostat, 2015) and some legislation for reduced energy consumption. In addition to the environmental considerations of highly energy-dependant food production, there are socioeconomic issues: the higher the dependence of food production on fossil fuels, the stronger the links between their prices. In many areas of the world, increases in oil prices directly impact food accessibility, particularly for the poorest people (Bakhat and Würzburg, 2013). To break this cycle, renewable energy capacity needs to be developed. Photovoltaic (PV) materials are a particularly interesting renewable energy technology as despite the relatively high initial investment they often require little maintenance over the long-term (Fabrizio, 2012; Ould-Amrouche et al., 2010). One of the most familiar PV materials are the opaque crystalline silicone solar cells (CSCs) that have undergone significant technological improvements (Timilsina et al., 2012) and are now common over roofs of domestic and industrial buildings. Integration of PVs into agricultural settings is known as, for example, ‘‘agrivoltaics” by Dupraz et al., 2011, or ‘‘agrinergy” by Bölük (2013). These fields are growing as the pressure to reduce the carbon footprint of farming focuses on integrating renewable energies into CEF. Whilst CSCs can be used on the roof of closed hangers in the same way as other buildings, greenhouses require a different approach due to the required transparency of any cladding. A number of models have been proposed based on CSC partial coverage and semitransparent PVs, including Dye-Sensitized solar Cells (DSCs) and organic PVs (OPVs). These approaches are summarised in Fig. 1. The benefit-cost ratio of integrating PVs into greenhouses appears favourable in certain environments.
2. Integration of opaque PVs The integration of CSCs into greenhouses has been described for both traditional and thin-film CSC modules (Al-Ibrahim et al., 2006; Campiotti et al., 2011; Cossu et al., 2014; Kadowaki et al., 2012; Pérez-Alonso et al., 2012; Ureña-Sánchez et al., 2012; Yano et al., 2009, 2010, 2014). As the CSCs alter the light entering the greenhouse (see Fig. 1), having a knock-on effect on plant growth,
Fig. 2. Photograph of CSCs on a greenhouse roof. (Photograph courtesy of Swissradies Kerzers, Switzerland).
various products and formulations have been developed where the CSCs are used to partially cover the greenhouse roof and thereby balance energy production with plant growth. For example, Fig. 2 shows a photograph of CSC integrated into a greenhouse for radish production. The launch of these products has been met with high interest and success. The process of validating the application of CSCs in greenhouses can include both field trials and computer modelling. The Land Equivalent Ratio (LER) is generally used to compare the output of land when used simultaneously for multiple purposes, such as multiple crops systems (Mead and Willey, 1980) or agrivoltaics (Dupraz et al., 2011; Pérez-Alonso et al., 2012). The LER is the relative area of a sole crop required to give the same yield as achieved with intercropping. As such, a LER over 1 is interpreted as beneficial. However, caution must be taken in this type of calculation as it does not differentiate the value – monetary or otherwise – of one yield over the other. For example, in agrivoltaics, a LER over 1 would be obtained where PVs were used to completely cover the surface area even if the crop yield was only a small fraction of the monosystem. As such, a higher LER despite a potentially lower crop yield is one weakness of relying on this measure to gauge the success of the agrivoltaic systems: the LER does not gauge the real value of the relative yields of food and power. The decline in food harvest due to shade response of crops is one factor limiting the integration of CSCs into greenhouses. As a result, independent of LER favouring total greenhouse coverage with CSCs, the balance between electricity generation and crop
Fig. 1. Diagrams summarising potential greenhouse glazing approaches. Left: traditional greenhouse glass transmits the majority of visible light and a portion of infrared. Middle: Opaque CSCs materials have been successfully positioned on greenhouse roofs. Despite having almost no light transmission, if only a portion of the roof is covered the greenhouses may still be used to cultivate some types of crops. Right: Using specifically coloured semi-transparent PVs, including DSCs, OPVs and PVs based on luminescent solar concentrators, the incident light can be filtered to share the spectrum between plant growth and electricity generation.
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harvest has led to estimations of an optimal 30% maximum coverage of the roof with CSCs (Kadowaki et al., 2012; Marrou et al., 2013); although in 2013 this coverage was later suggested to have an unprofitable effect on food production (Marrou, 2013). In many cases, such as the greenhouses shown in Fig. 2, the CSCs are a considerable height above the crop maximising light entrance from the sides of the structure, but increasing greenhouse construction and maintenance costs. Even in this form, greenhouse integrated CSCs are incompatible with crops of high economic importance, such as tomato. Generally, where food yield is weighted as the priority over electricity generation <10% of roof space can be covered with CSCs (Pérez-Alonso, 2012; Ureña-Sánchez, 2012). CSCs were initially projected to be profitable as most plants only require 40–50% of full sunlight for saturating amounts of photosynthetically active radiation (PAR) (Fankhauser and Batschauer, 2016). PAR is the part of the light spectrum used for photosynthesis. But light is used for other plant functions including growth regulation. As CSCs filter all wavelengths more or less equally, they essentially provide shade all year around reducing PAR, growth regulating light and non-essential light in all weather conditions. As such permanent CSC structures must be optimised to allow cultivation during the significant part of the day and of the year when sunlight is sub-optimal. Many plants need ‘‘full sun” conditions and the light spectrum under full or partial shade, such as the environment created by integrating CSC into greenhouses, causes differences in growth and development. For example, tomatoes grown under CSCs ripen later and yield fruit with significantly lower mean mass and maximum yield compared to traditional greenhouse crops (Marrou, 2013). As a second example, for passion fruit production, flowering delay was observed in the CSC-greenhouse system accompanied by increased vegetative growth (Kadowaki et al., 2012; Scognamiglio et al., 2015). Interestingly, although the yield of passion fruit was generally lower than in an open field system, the fruit was larger and of better quality due to improved pest control. However, given that the CSC-integrated greenhouse crop yield was not compared to traditional greenhouse conditions, it is currently unknown whether the differences with open field cultivation are due to CSC shading or the greenhouse. In studies with other crops, yields under full-shade conditions compared to traditional greenhouse conditions were reported as 41% for French beans, 48% cucumber and 59% for wheat (Marrou, 2013) and up to 48% for lettuce (Yano et al., 2010). It is worth noting that lettuce was considered as a priori crop for testing CSC-integrated greenhouses as lettuce can be planted at any season of the year and in many different conditions including open fields and closed environments with different light qualities (Thicoïpé et al., 1997). This plasticity demonstrates that the crop is adapted to a wide range of light environments, but not that of CSC-integrated greenhouses. Shade intolerance is the main ecophysical constraint for integrating PVs into greenhouses. Mapping plant responses to shade is species-specific with some plants better able to adapt to lower than optimal levels of light (Gommers et al., 2013). Others, such as tomato, initiate shade avoidance strategies, where rapid growth aims to push leaves above the canopy (Smith and Whitelam, 1997). In this response, resources are channelled into stem and leaf growth rather than the fruit, reducing crop yields. In other crops, such as lettuce and maize, the shade response involves a general down-regulation of growth (Cantagallo et al., 2004; Dinesh et al., 2016; Fu et al., 2009; Worku et al., 2004), often resulting in a reduction of total biomass production at maturity (Dapoigny et al., 2000; Kitaya et al., 1998). For lettuce, different responses were even observed for different varieties, but there are some general trends. For example, when the lettuce is grown in greenhouses completely or half covered with CSCs there are generally fewer leaves. These leaves are thinner, but larger than those grown in
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conventional greenhouses (Bensink, 1971). The leaf arrangement is also different (Marroua et al., 2013). All these changes alter the photosynthetic capacity of the plant and have also been reported on pastureland where intermittent shading is caused by a tree and shrub layer (Peri et al., 2007). The changes are explained as adaptations to improve the capacity of the plant to intercept light (Gimenez et al., 2002; Marroua et al., 2013; Sinclair et al., 1999). Other detrimental affects of integrating CSCs into greenhouse have been reported. For tomato grown in greenhouses with integrated CSCs, difficulties in controlling both the internal environment of the greenhouse, particularly humidity, and the quality of pollen led to a low yield (Scognamiglio et al., 2015). Humidity control was one of the main challenges in the optimisation of CSCintegrated greenhouses for radish production in Switzerland.
3. Constraints of sharing solar energy between electricity generation and cultivation Different regions of the light spectrum have various effects on plant growth. PAR encompasses all light in the 400–700 nm wavelength range; however, not all wavelengths are utilised to the same efficiency. Peak rates of photosynthesis are observed in response to light in the red and blue regions of the spectrum, whereas green light is poorly absorbed. In addition, UV and far-red light (near IR from 700-800 nm) are not used for photosynthesis, but have profound effects on plant growth and development (Galvão et al., 2015; Fraser et al., 2016). Plants adapt to the available light by responding to absolute and relative amounts of incident light over specific wavebands (de Wit et al., 2016; Galvão et al., 2015). For example, the amount of blue light can regulate development (Casal, 2013; Song et al., 2013), whereas the ratio of red and far-red light (700–800 nm) initiates the shade avoidance response (Gommers et al., 2013). Many developmental and morphological processes are mediated by a set of blue, red and far-red photoreceptors (Wang and Folta, 2013; Galvão et al., 2015; Fraser et al., 2016). This same set of receptors regulate processes important for the productivity of the plant. For example, light-regulated processes, such as internode and petiole elongation growth and leaf expansion, determine the leaf area for photosynthesis (Tsukaya, 2005). Consequently, poor light quality can induce leaf phenotypes that have reduced photosynthetic capacity. Light quality also affects plant morphology, determining, for example density and aperture control of stomata, ultimately regulating photosynthesis and water uptake of the plant through the rate of transpiration. In addition to the gross changes in morphology, light quality triggers molecular responses, including adaptions of the photosynthetic system (Senevirathna et al., 2008) and modifications of the radiation use efficiency (i.e. light to biomass conversion) (Dapoigny et al., 2000; Rizzalli et al., 2002). Importantly, the quality and quantity of light not only determines the amount of available resources, but also how they are distributed through the plant such that the edible crop yield may be reduced even if total biomass is increased. Intuitively, as green light is poorly absorbed by plants and, therefore, is likely to have the least effect on development if removed from the spectrum, any type of PVs – CSCs or Dye-Sensitized solar Cells (DSCs) – that use just green light may be compatible with shade-sensitive crop production. Recent studies have highlighted the role of light in the plant’s defence system (Ballaré, 2014). This observation is particularly important in CEF, where crops are densely packed and disease can spread quickly. The dense planting increases the relative amounts of far-red light, which is suggested to diminish defence: a phenomenon known as the growth-defence trade-off (Ballaré, 2014). Therefore, light quality, resulting from the combined effects
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Fig. 3. Photograph of tinted polytunnels currently being validated in Switzerland for cultivation of ornamental plant with improved growth characteristics.
of incident light and how it is propagated within the greenhouse environment, can impact crop yield directly through distribution of resources and indirectly through disease resistance. Indeed, the use of high-pressure sodium (HPS) lamps for light supplementation and, later, light emitting diodes (LEDs) centres not only on increasing PAR, but increasing the intensity of specific wavebands of light to improve development. LEDs offer more opportunities to precisely control light quality, in turn improving crop quality. However, similar benefits can be realised with energy-passive light filters (Fig. 3). Currently, the filter systems are dyed materials and fluorescent dyes have also been proposed (Novoplansky, 1990). The fluorescent system converts green wavebands to red and was shown to increase tomato fruit yields and increase the number of flowering branches on rose bushes compared to conventional, non-dyed systems. The positive benefits were attributed to both morphogenic and photosynthetic responses. It is therefore logical to assume that incorporating a photosensitive dye, as in DCSs, the filtered light can be used for energy generation. Sharing solar energy between plant growth and electricity generation (Fig. 1, Right panel) should be optimised such that plant growth is not compromised, considering both the morphogenic and photosynthetic responses of the plants. This type of light shar-
ing has been explored by reflecting specific wavelengths of light back towards CSC collectors (Sonneveld et al., 2010) or incident light being filtered through specifically coloured semitransparent PVs, including DSCs and organic PV (OPV) (Lau et al., 2014). The components of DSCs and OPVs can be tuned to allow only the wavebands of light important for plant growth to enter the greenhouse (Fig. 4). Specifically, crops have been successfully cultivated under blue-red diodes (Yorio et al., 2001), suggesting green light can be channelled into electricity production with no loss of biomass and without inducing shade responses. Luminscent solar concentrators have also been developed that transmit blue and red light wavebands and, in addition, absorb green light wavebands re-emitting the energy as red light wavebands. The system has been shown to be compatible with algae growth without notable reduction in growth rate or achievable biomass density (Allen et al., 2015). The green light waveband of the sunlight spectrum contains enough energy to consider integrating semi-transparent PVs as an interesting possibility (Sonneveld et al., 2010). Numerical modelling has demonstrated the compatibility of OPVs light absorption spectrum with 27 herbaceous plants, including common greenhouse crops (Emmott et al., 2015). The studies used the averaged action spectrum of the plants’ biomass as an indicator of crop growth – focusing on biomass changes and not photomorphological responses – to model the impact of integrating PV materials into a greenhouse, including an economic analysis. The data from these studies confirm that the high absorption in the PAR region means a limited area can be covered by the CSCs (Al-Ibrahim et al., 2006; Kadowaki et al., 2012; Marroua et al., 2013; PérezAlonso, 2012; Ureña-Sánchez, 2012) reducing electricity production, yet the transmission of light through semi-transparent PVs, in this case OPVs, can be tailored such that PAR absorption is minimal and the entire greenhouse surface can be covered without a significant reduction in the crop yield, overall leading to a favourable benefit-cost ratio based on existing product efficiency (Emmott et al., 2015; Yang et al., 2015). The same rational could be applied to other semi-transparent PVs, including DSCs. Dyes can be rationally designed with specific properties (Kim et al., 2014). As OPVs and DSCs work in diffuse lighting they are suitable for climates where light is not intense. Furthermore, their relatively high efficiency independent of the direction of the incident sunlight allows the sides of the greenhouses to be covered and the roof pitch to be optimised for other features, such as strength, weather resistance, and thermal regulation, rather than solely the optimal angle for solar capture of the integrated PVs.
4. Potential advantages of integrated PVs beyond electricity generation
Fig. 4. UV–VIS spectra of four different marketed DSC products showing the absorption profile compared to the photosynthetic absorption of plants (ethanol extract of whole spinach leaves). Plants absorb light in the blue and red regions, but photomorphic responses are often in response to the relative amount of two different light wavebands. The absorption of DSCs depends on the combined properties of the amount and type of dye and electrolyte used along with contributions from the support and its coatings. The three red spectra use the same dye, but have considerably different absorption profiles due to the other components of the DSC, particularly in the 400–550 nm range and these differences should be considered for greenhouse-integrated photovolatics.
Taking design a step further, PVs may be designed for greenhouse integration that are not only compatible with a greater number of cultivars, but have other benefits. Light quality has been shown, for example, to control the amount of phytochemicals in different crops (Lia and Kubot, 2009). In lettuce, the concentrations of the stress response compound, anthocyanin, can be significantly increased with supplements of light from UV sources; a waveband currently removed from solar radiation by the filtering effects of standard greenhouse glass. Similarly, carotenoids can be increased with blue light and phenolics with red light, compared to those in a white light control. Interestingly, although the lettuce contained fewer important phytochemicals per gram, the overall biomass was increased with supplemental far-red light compare to white light. These studies suggest that filtering light using semitransparent PVs and/or using the electricity generated from
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semi-transparent PVs or CSCs to supplement the internal light environment with artificial light sources could improve crop quality. Although the mechanisms of the changes in phytochemicals levels under different light environments are not well understood, there is a clear need to monitor resultant nutritional changes in the crop and not rely on biomass alone as a marker of quality. Selective filtering of light can also be used to stabilise the temperature in the greenhouse. The main heating component of light is infra-red. By reflecting this light component, the greenhouse becomes more insulated from external changes in temperature. Most greenhouse vegetable plants have optimal growth temperatures of between 20–25 °C during the day and 14–18 °C at night. Therefore, in colder climates, particularly where winter temperatures are less than 4 °C, heating systems are employed to raise the internal temperature (Sethi and Sharma, 2008). Conversely, in warmer climates, where ambient temperatures in the greenhouse may exceed 30 °C, cooling systems are used (Sethi and Sharma, 2007). Given plants can rapidly wilt with day-to-day fluctuations in temperature and humidity, cooling/heating are main components of fuel consumption in CEF. Accordingly, insulation is becoming an increasingly important and legislated demand for greenhouses. Since DSCs are constructed from TiO2-coated glass they provide intrinsic insulation properties that are attractive for greenhouse applications.
5. The future technology: DSCs and their integration into greenhouses
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6. Conclusions PVs can be integrated into greenhouses to provide electricity generation over the same land area as crop growth. In addition, there is suggestion that the shading effects of greenhouseintegrated PVs could reduce irrigation requirements. However, the application of opaque CSCs is limited due compatibility with crop growth. Semi-transparent PVs potentially widen the application and provide additional benefits including improved crop quality and reduced energy use due to integral thermal regulation properties. To date DSCs and OPVs have not been optimised for these features and, therefore, there is considerable potential to design and optimize PV materials to make greenhouses energyautomatous, and possibly even energy-positive feeding electricity into the grid, while simultaneously improving crop quality and yield. In line with this idea, recently luminescent solar concentrators integrated into greenhouses have been studied over a one-year cycle (Corrado et al., 2016) and the system is being marketed in the US. Acknowledgements This work was supported by the Integrative Food Science and Nutrition Center (IFNC), Ecole polytechnique fédérale de Lausanne (EPFL), the University of Lausanne and the Swiss National Science Foundation (FNS 31003A_160326, Sinergia Grant CRSII3_154438, SystemsX Grant PlantMechanix 51RT-0_145716). References
While there is still a great deal to discover about the influence of the different components of light on crop production, in terms of both quantity and nutritional quality, it is evident that DSCs can have beneficial effects beyond electricity generation to cover the energy demand of the greenhouse and the future for the integration of this technology into greenhouses may depend on design review to consider these additional features. Notably, considerable efforts have been made to develop both metal-based (Antony et al., 2015; Mishra et al., 2011; Stengel et al., 2012) and organic dyes (Antony et al., 2015) for semi-transparent PVs focusing on maximum conversion efficiencies; however, integrating semitransparent PVs into greenhouses requires consideration of different properties, including but not limited to insulation, light transmission, and weight, and to date only cursory studies have been made to optimize semi-transparent photovoltaic for crop production. The optimisation process should also consider a cost benefitanalysis as farming is a zero profit industry. In this respect, extensive investigations under real outdoor conditions and accelerated tests in laboratories have established that DSC panels endowed with state-of-the art ionic liquid electrolytes and dyes are stable over a service time of at least 20 years. They have been shown to pass the stringent tests applied as a norm to PV panels for outdoor deployment (De Silvestro et al., 2010; Grätzel and Durrant, 2008). However, for OPV the expected lifetimes are still limited to the 1– 2 year realm. As of January 2017, we have opened a research greenhouse with integrated DSCs where complete biochemical characterisation of crops will be performed along side the standard electricity production studies for building integrated PVs. Given the foundation studies on light filtering effects and plant growth, we are confident that the selected DSCs will have positive effects on plant biomass and will influence morphology. In addition, as light quality can affect a plants defence system, the relative need for agrochemicals, such as pesticides and fungicides, will be monitored along with the nutritional value of the crop.
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Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Review
The influence of greenhouse-integrated photovoltaics on crop production Claire S. Allardyce a,⇑, Christian Fankhauser b, Shaik M. Zakeeruddin a, Michael Grätzel a, Paul J. Dyson a a b
Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), EPFL–BCH, CH–1015 Lausanne, Switzerland Center for Integrative Genomics, Faculty of Biology and Medicine, University of Lausanne, 1015 Lausanne, Switzerland
a r t i c l e
i n f o
Article history: Received 7 September 2016 Received in revised form 6 June 2017 Accepted 16 June 2017
Keywords: Photovoltaics Dye-sensitized solar cells Agriculture Greenhouse-integrated photovoltaics Photobiomodulation
a b s t r a c t Photovoltaics (PVs) have been particularly successful in many domestic and industrial settings where opaque PV-covered roofs provide renewable electricity. Modern farming, for an ever growing population, employs vast areas of greenhouses consuming considerable amounts of energy. The majority of greenhouses are not suited to coverage by opaque PVs. Herein, we describe the current-state-of-the-art in greenhouse-integrated opaque PVs and their limitations, particularly with respect to the compatibility with certain plant cultivars. We propose semi-transparent PVs (Dye-Sensitized solar Cells, DSCs) as alternative greenhouse glazing that, compared to conventional greenhouse glazing and currently marketed greenhouse integrated opaque PV materials, offers advantages including enhanced thermal stabilisation and similar or improved edible biomass yields. Large-scale validation of DSCs in solar sharing for crop production (yield, appearance and nutritional content) is now in progress. Ó 2017 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3. 4. 5. 6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Integration of opaque PVs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Constraints of sharing solar energy between electricity generation and cultivation . Potential advantages of integrated PVs beyond electricity generation . . . . . . . . . . . . The future technology: DSCs and their integration into greenhouses . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Land is a valuable resource and about 38% of the dry land area of the planet is used for agriculture (Ramankutty and Foley, 1999; Ramankutty et al., 2008); a figure that has remained stable for several years because it is near to the maximum available area for this practice. Sites not used for agriculture or other human needs are either uneconomical to farm or serve as important ecological resources and should be preserved. An increased food supply, therefore, demands higher production from existing sites, which generally requires increased energy input directly (for temperature ⇑ Corresponding author. E-mail address: [email protected] (C.S. Allardyce). http://dx.doi.org/10.1016/j.solener.2017.06.044 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.
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regulation, irrigation and artificial lighting) and indirectly (for nitrogen fixation technology, phosphate fertiliser acquisition (mining and transport) and increased chemical treatments, including pest control due to high density planting). In the past few decades, the energy expenditure per kilo of crop yield has increased due to the expansion of controlled environment farming (CEF). Traditionally in greenhouses, but increasingly in closed hangers where even the light is artificial, CEF leads to higher crop yields over the same land area such that, despite increased costs due to investment in infrastructure and energy, CEF is becoming the norm in the industrial world (Beyhan et al., 2013; Bot et al., 2005). Currently, much of the energy for CEF comes from fossil fuels sources. As an example, in terms of crop yield for lettuce production CEF increases yield by more than an order of magnitude, but
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to achieve this increase requires much more energy compared to conventionally produced lettuce. The energy input is equivalent to a net yield of 0.46 g of lettuce per kJ of energy for CEF methods versus 3.5 g/kJ for conventional methods (Barbosa et al., 2015). The requirements are such that there is mounting opposition to CEF – even from scientists who developed CEF – and a 2014 estimate of the carbon footprint was 3.6 kg of CO2 per kg of lettuce (Shackford, 2014). This awareness has led to monitoring (for example, Eurostat, 2015) and some legislation for reduced energy consumption. In addition to the environmental considerations of highly energy-dependant food production, there are socioeconomic issues: the higher the dependence of food production on fossil fuels, the stronger the links between their prices. In many areas of the world, increases in oil prices directly impact food accessibility, particularly for the poorest people (Bakhat and Würzburg, 2013). To break this cycle, renewable energy capacity needs to be developed. Photovoltaic (PV) materials are a particularly interesting renewable energy technology as despite the relatively high initial investment they often require little maintenance over the long-term (Fabrizio, 2012; Ould-Amrouche et al., 2010). One of the most familiar PV materials are the opaque crystalline silicone solar cells (CSCs) that have undergone significant technological improvements (Timilsina et al., 2012) and are now common over roofs of domestic and industrial buildings. Integration of PVs into agricultural settings is known as, for example, ‘‘agrivoltaics” by Dupraz et al., 2011, or ‘‘agrinergy” by Bölük (2013). These fields are growing as the pressure to reduce the carbon footprint of farming focuses on integrating renewable energies into CEF. Whilst CSCs can be used on the roof of closed hangers in the same way as other buildings, greenhouses require a different approach due to the required transparency of any cladding. A number of models have been proposed based on CSC partial coverage and semitransparent PVs, including Dye-Sensitized solar Cells (DSCs) and organic PVs (OPVs). These approaches are summarised in Fig. 1. The benefit-cost ratio of integrating PVs into greenhouses appears favourable in certain environments.
2. Integration of opaque PVs The integration of CSCs into greenhouses has been described for both traditional and thin-film CSC modules (Al-Ibrahim et al., 2006; Campiotti et al., 2011; Cossu et al., 2014; Kadowaki et al., 2012; Pérez-Alonso et al., 2012; Ureña-Sánchez et al., 2012; Yano et al., 2009, 2010, 2014). As the CSCs alter the light entering the greenhouse (see Fig. 1), having a knock-on effect on plant growth,
Fig. 2. Photograph of CSCs on a greenhouse roof. (Photograph courtesy of Swissradies Kerzers, Switzerland).
various products and formulations have been developed where the CSCs are used to partially cover the greenhouse roof and thereby balance energy production with plant growth. For example, Fig. 2 shows a photograph of CSC integrated into a greenhouse for radish production. The launch of these products has been met with high interest and success. The process of validating the application of CSCs in greenhouses can include both field trials and computer modelling. The Land Equivalent Ratio (LER) is generally used to compare the output of land when used simultaneously for multiple purposes, such as multiple crops systems (Mead and Willey, 1980) or agrivoltaics (Dupraz et al., 2011; Pérez-Alonso et al., 2012). The LER is the relative area of a sole crop required to give the same yield as achieved with intercropping. As such, a LER over 1 is interpreted as beneficial. However, caution must be taken in this type of calculation as it does not differentiate the value – monetary or otherwise – of one yield over the other. For example, in agrivoltaics, a LER over 1 would be obtained where PVs were used to completely cover the surface area even if the crop yield was only a small fraction of the monosystem. As such, a higher LER despite a potentially lower crop yield is one weakness of relying on this measure to gauge the success of the agrivoltaic systems: the LER does not gauge the real value of the relative yields of food and power. The decline in food harvest due to shade response of crops is one factor limiting the integration of CSCs into greenhouses. As a result, independent of LER favouring total greenhouse coverage with CSCs, the balance between electricity generation and crop
Fig. 1. Diagrams summarising potential greenhouse glazing approaches. Left: traditional greenhouse glass transmits the majority of visible light and a portion of infrared. Middle: Opaque CSCs materials have been successfully positioned on greenhouse roofs. Despite having almost no light transmission, if only a portion of the roof is covered the greenhouses may still be used to cultivate some types of crops. Right: Using specifically coloured semi-transparent PVs, including DSCs, OPVs and PVs based on luminescent solar concentrators, the incident light can be filtered to share the spectrum between plant growth and electricity generation.
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harvest has led to estimations of an optimal 30% maximum coverage of the roof with CSCs (Kadowaki et al., 2012; Marrou et al., 2013); although in 2013 this coverage was later suggested to have an unprofitable effect on food production (Marrou, 2013). In many cases, such as the greenhouses shown in Fig. 2, the CSCs are a considerable height above the crop maximising light entrance from the sides of the structure, but increasing greenhouse construction and maintenance costs. Even in this form, greenhouse integrated CSCs are incompatible with crops of high economic importance, such as tomato. Generally, where food yield is weighted as the priority over electricity generation <10% of roof space can be covered with CSCs (Pérez-Alonso, 2012; Ureña-Sánchez, 2012). CSCs were initially projected to be profitable as most plants only require 40–50% of full sunlight for saturating amounts of photosynthetically active radiation (PAR) (Fankhauser and Batschauer, 2016). PAR is the part of the light spectrum used for photosynthesis. But light is used for other plant functions including growth regulation. As CSCs filter all wavelengths more or less equally, they essentially provide shade all year around reducing PAR, growth regulating light and non-essential light in all weather conditions. As such permanent CSC structures must be optimised to allow cultivation during the significant part of the day and of the year when sunlight is sub-optimal. Many plants need ‘‘full sun” conditions and the light spectrum under full or partial shade, such as the environment created by integrating CSC into greenhouses, causes differences in growth and development. For example, tomatoes grown under CSCs ripen later and yield fruit with significantly lower mean mass and maximum yield compared to traditional greenhouse crops (Marrou, 2013). As a second example, for passion fruit production, flowering delay was observed in the CSC-greenhouse system accompanied by increased vegetative growth (Kadowaki et al., 2012; Scognamiglio et al., 2015). Interestingly, although the yield of passion fruit was generally lower than in an open field system, the fruit was larger and of better quality due to improved pest control. However, given that the CSC-integrated greenhouse crop yield was not compared to traditional greenhouse conditions, it is currently unknown whether the differences with open field cultivation are due to CSC shading or the greenhouse. In studies with other crops, yields under full-shade conditions compared to traditional greenhouse conditions were reported as 41% for French beans, 48% cucumber and 59% for wheat (Marrou, 2013) and up to 48% for lettuce (Yano et al., 2010). It is worth noting that lettuce was considered as a priori crop for testing CSC-integrated greenhouses as lettuce can be planted at any season of the year and in many different conditions including open fields and closed environments with different light qualities (Thicoïpé et al., 1997). This plasticity demonstrates that the crop is adapted to a wide range of light environments, but not that of CSC-integrated greenhouses. Shade intolerance is the main ecophysical constraint for integrating PVs into greenhouses. Mapping plant responses to shade is species-specific with some plants better able to adapt to lower than optimal levels of light (Gommers et al., 2013). Others, such as tomato, initiate shade avoidance strategies, where rapid growth aims to push leaves above the canopy (Smith and Whitelam, 1997). In this response, resources are channelled into stem and leaf growth rather than the fruit, reducing crop yields. In other crops, such as lettuce and maize, the shade response involves a general down-regulation of growth (Cantagallo et al., 2004; Dinesh et al., 2016; Fu et al., 2009; Worku et al., 2004), often resulting in a reduction of total biomass production at maturity (Dapoigny et al., 2000; Kitaya et al., 1998). For lettuce, different responses were even observed for different varieties, but there are some general trends. For example, when the lettuce is grown in greenhouses completely or half covered with CSCs there are generally fewer leaves. These leaves are thinner, but larger than those grown in
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conventional greenhouses (Bensink, 1971). The leaf arrangement is also different (Marroua et al., 2013). All these changes alter the photosynthetic capacity of the plant and have also been reported on pastureland where intermittent shading is caused by a tree and shrub layer (Peri et al., 2007). The changes are explained as adaptations to improve the capacity of the plant to intercept light (Gimenez et al., 2002; Marroua et al., 2013; Sinclair et al., 1999). Other detrimental affects of integrating CSCs into greenhouse have been reported. For tomato grown in greenhouses with integrated CSCs, difficulties in controlling both the internal environment of the greenhouse, particularly humidity, and the quality of pollen led to a low yield (Scognamiglio et al., 2015). Humidity control was one of the main challenges in the optimisation of CSCintegrated greenhouses for radish production in Switzerland.
3. Constraints of sharing solar energy between electricity generation and cultivation Different regions of the light spectrum have various effects on plant growth. PAR encompasses all light in the 400–700 nm wavelength range; however, not all wavelengths are utilised to the same efficiency. Peak rates of photosynthesis are observed in response to light in the red and blue regions of the spectrum, whereas green light is poorly absorbed. In addition, UV and far-red light (near IR from 700-800 nm) are not used for photosynthesis, but have profound effects on plant growth and development (Galvão et al., 2015; Fraser et al., 2016). Plants adapt to the available light by responding to absolute and relative amounts of incident light over specific wavebands (de Wit et al., 2016; Galvão et al., 2015). For example, the amount of blue light can regulate development (Casal, 2013; Song et al., 2013), whereas the ratio of red and far-red light (700–800 nm) initiates the shade avoidance response (Gommers et al., 2013). Many developmental and morphological processes are mediated by a set of blue, red and far-red photoreceptors (Wang and Folta, 2013; Galvão et al., 2015; Fraser et al., 2016). This same set of receptors regulate processes important for the productivity of the plant. For example, light-regulated processes, such as internode and petiole elongation growth and leaf expansion, determine the leaf area for photosynthesis (Tsukaya, 2005). Consequently, poor light quality can induce leaf phenotypes that have reduced photosynthetic capacity. Light quality also affects plant morphology, determining, for example density and aperture control of stomata, ultimately regulating photosynthesis and water uptake of the plant through the rate of transpiration. In addition to the gross changes in morphology, light quality triggers molecular responses, including adaptions of the photosynthetic system (Senevirathna et al., 2008) and modifications of the radiation use efficiency (i.e. light to biomass conversion) (Dapoigny et al., 2000; Rizzalli et al., 2002). Importantly, the quality and quantity of light not only determines the amount of available resources, but also how they are distributed through the plant such that the edible crop yield may be reduced even if total biomass is increased. Intuitively, as green light is poorly absorbed by plants and, therefore, is likely to have the least effect on development if removed from the spectrum, any type of PVs – CSCs or Dye-Sensitized solar Cells (DSCs) – that use just green light may be compatible with shade-sensitive crop production. Recent studies have highlighted the role of light in the plant’s defence system (Ballaré, 2014). This observation is particularly important in CEF, where crops are densely packed and disease can spread quickly. The dense planting increases the relative amounts of far-red light, which is suggested to diminish defence: a phenomenon known as the growth-defence trade-off (Ballaré, 2014). Therefore, light quality, resulting from the combined effects
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Fig. 3. Photograph of tinted polytunnels currently being validated in Switzerland for cultivation of ornamental plant with improved growth characteristics.
of incident light and how it is propagated within the greenhouse environment, can impact crop yield directly through distribution of resources and indirectly through disease resistance. Indeed, the use of high-pressure sodium (HPS) lamps for light supplementation and, later, light emitting diodes (LEDs) centres not only on increasing PAR, but increasing the intensity of specific wavebands of light to improve development. LEDs offer more opportunities to precisely control light quality, in turn improving crop quality. However, similar benefits can be realised with energy-passive light filters (Fig. 3). Currently, the filter systems are dyed materials and fluorescent dyes have also been proposed (Novoplansky, 1990). The fluorescent system converts green wavebands to red and was shown to increase tomato fruit yields and increase the number of flowering branches on rose bushes compared to conventional, non-dyed systems. The positive benefits were attributed to both morphogenic and photosynthetic responses. It is therefore logical to assume that incorporating a photosensitive dye, as in DCSs, the filtered light can be used for energy generation. Sharing solar energy between plant growth and electricity generation (Fig. 1, Right panel) should be optimised such that plant growth is not compromised, considering both the morphogenic and photosynthetic responses of the plants. This type of light shar-
ing has been explored by reflecting specific wavelengths of light back towards CSC collectors (Sonneveld et al., 2010) or incident light being filtered through specifically coloured semitransparent PVs, including DSCs and organic PV (OPV) (Lau et al., 2014). The components of DSCs and OPVs can be tuned to allow only the wavebands of light important for plant growth to enter the greenhouse (Fig. 4). Specifically, crops have been successfully cultivated under blue-red diodes (Yorio et al., 2001), suggesting green light can be channelled into electricity production with no loss of biomass and without inducing shade responses. Luminscent solar concentrators have also been developed that transmit blue and red light wavebands and, in addition, absorb green light wavebands re-emitting the energy as red light wavebands. The system has been shown to be compatible with algae growth without notable reduction in growth rate or achievable biomass density (Allen et al., 2015). The green light waveband of the sunlight spectrum contains enough energy to consider integrating semi-transparent PVs as an interesting possibility (Sonneveld et al., 2010). Numerical modelling has demonstrated the compatibility of OPVs light absorption spectrum with 27 herbaceous plants, including common greenhouse crops (Emmott et al., 2015). The studies used the averaged action spectrum of the plants’ biomass as an indicator of crop growth – focusing on biomass changes and not photomorphological responses – to model the impact of integrating PV materials into a greenhouse, including an economic analysis. The data from these studies confirm that the high absorption in the PAR region means a limited area can be covered by the CSCs (Al-Ibrahim et al., 2006; Kadowaki et al., 2012; Marroua et al., 2013; PérezAlonso, 2012; Ureña-Sánchez, 2012) reducing electricity production, yet the transmission of light through semi-transparent PVs, in this case OPVs, can be tailored such that PAR absorption is minimal and the entire greenhouse surface can be covered without a significant reduction in the crop yield, overall leading to a favourable benefit-cost ratio based on existing product efficiency (Emmott et al., 2015; Yang et al., 2015). The same rational could be applied to other semi-transparent PVs, including DSCs. Dyes can be rationally designed with specific properties (Kim et al., 2014). As OPVs and DSCs work in diffuse lighting they are suitable for climates where light is not intense. Furthermore, their relatively high efficiency independent of the direction of the incident sunlight allows the sides of the greenhouses to be covered and the roof pitch to be optimised for other features, such as strength, weather resistance, and thermal regulation, rather than solely the optimal angle for solar capture of the integrated PVs.
4. Potential advantages of integrated PVs beyond electricity generation
Fig. 4. UV–VIS spectra of four different marketed DSC products showing the absorption profile compared to the photosynthetic absorption of plants (ethanol extract of whole spinach leaves). Plants absorb light in the blue and red regions, but photomorphic responses are often in response to the relative amount of two different light wavebands. The absorption of DSCs depends on the combined properties of the amount and type of dye and electrolyte used along with contributions from the support and its coatings. The three red spectra use the same dye, but have considerably different absorption profiles due to the other components of the DSC, particularly in the 400–550 nm range and these differences should be considered for greenhouse-integrated photovolatics.
Taking design a step further, PVs may be designed for greenhouse integration that are not only compatible with a greater number of cultivars, but have other benefits. Light quality has been shown, for example, to control the amount of phytochemicals in different crops (Lia and Kubot, 2009). In lettuce, the concentrations of the stress response compound, anthocyanin, can be significantly increased with supplements of light from UV sources; a waveband currently removed from solar radiation by the filtering effects of standard greenhouse glass. Similarly, carotenoids can be increased with blue light and phenolics with red light, compared to those in a white light control. Interestingly, although the lettuce contained fewer important phytochemicals per gram, the overall biomass was increased with supplemental far-red light compare to white light. These studies suggest that filtering light using semitransparent PVs and/or using the electricity generated from
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semi-transparent PVs or CSCs to supplement the internal light environment with artificial light sources could improve crop quality. Although the mechanisms of the changes in phytochemicals levels under different light environments are not well understood, there is a clear need to monitor resultant nutritional changes in the crop and not rely on biomass alone as a marker of quality. Selective filtering of light can also be used to stabilise the temperature in the greenhouse. The main heating component of light is infra-red. By reflecting this light component, the greenhouse becomes more insulated from external changes in temperature. Most greenhouse vegetable plants have optimal growth temperatures of between 20–25 °C during the day and 14–18 °C at night. Therefore, in colder climates, particularly where winter temperatures are less than 4 °C, heating systems are employed to raise the internal temperature (Sethi and Sharma, 2008). Conversely, in warmer climates, where ambient temperatures in the greenhouse may exceed 30 °C, cooling systems are used (Sethi and Sharma, 2007). Given plants can rapidly wilt with day-to-day fluctuations in temperature and humidity, cooling/heating are main components of fuel consumption in CEF. Accordingly, insulation is becoming an increasingly important and legislated demand for greenhouses. Since DSCs are constructed from TiO2-coated glass they provide intrinsic insulation properties that are attractive for greenhouse applications.
5. The future technology: DSCs and their integration into greenhouses
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6. Conclusions PVs can be integrated into greenhouses to provide electricity generation over the same land area as crop growth. In addition, there is suggestion that the shading effects of greenhouseintegrated PVs could reduce irrigation requirements. However, the application of opaque CSCs is limited due compatibility with crop growth. Semi-transparent PVs potentially widen the application and provide additional benefits including improved crop quality and reduced energy use due to integral thermal regulation properties. To date DSCs and OPVs have not been optimised for these features and, therefore, there is considerable potential to design and optimize PV materials to make greenhouses energyautomatous, and possibly even energy-positive feeding electricity into the grid, while simultaneously improving crop quality and yield. In line with this idea, recently luminescent solar concentrators integrated into greenhouses have been studied over a one-year cycle (Corrado et al., 2016) and the system is being marketed in the US. Acknowledgements This work was supported by the Integrative Food Science and Nutrition Center (IFNC), Ecole polytechnique fédérale de Lausanne (EPFL), the University of Lausanne and the Swiss National Science Foundation (FNS 31003A_160326, Sinergia Grant CRSII3_154438, SystemsX Grant PlantMechanix 51RT-0_145716). References
While there is still a great deal to discover about the influence of the different components of light on crop production, in terms of both quantity and nutritional quality, it is evident that DSCs can have beneficial effects beyond electricity generation to cover the energy demand of the greenhouse and the future for the integration of this technology into greenhouses may depend on design review to consider these additional features. Notably, considerable efforts have been made to develop both metal-based (Antony et al., 2015; Mishra et al., 2011; Stengel et al., 2012) and organic dyes (Antony et al., 2015) for semi-transparent PVs focusing on maximum conversion efficiencies; however, integrating semitransparent PVs into greenhouses requires consideration of different properties, including but not limited to insulation, light transmission, and weight, and to date only cursory studies have been made to optimize semi-transparent photovoltaic for crop production. The optimisation process should also consider a cost benefitanalysis as farming is a zero profit industry. In this respect, extensive investigations under real outdoor conditions and accelerated tests in laboratories have established that DSC panels endowed with state-of-the art ionic liquid electrolytes and dyes are stable over a service time of at least 20 years. They have been shown to pass the stringent tests applied as a norm to PV panels for outdoor deployment (De Silvestro et al., 2010; Grätzel and Durrant, 2008). However, for OPV the expected lifetimes are still limited to the 1– 2 year realm. As of January 2017, we have opened a research greenhouse with integrated DSCs where complete biochemical characterisation of crops will be performed along side the standard electricity production studies for building integrated PVs. Given the foundation studies on light filtering effects and plant growth, we are confident that the selected DSCs will have positive effects on plant biomass and will influence morphology. In addition, as light quality can affect a plants defence system, the relative need for agrochemicals, such as pesticides and fungicides, will be monitored along with the nutritional value of the crop.
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