Energy sustainable greenhouse crop cultivation using photovoltaic technologies
Introduction
Photosynthesis converts sunlight energy into biochemical energy that is then transferred to biological energy flowing along the food chain. Most biological activities therefore depend fundamentally on photosynthesis. Electrical charges in photosystems embedded in thylakoid membranes in plant cell chloroplasts are separated when photons from the sun hit chlorophylls in the photosystems. The separated electrons and protons work to produce adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH), which are then used to produce carbohydrates in the Calvin–Benson cycle. Consequently, photon energy from the sun is converted into chemical energy in the form of carbohydrates in plants [1]. Photon yields of photosynthetic CO2 fixation among several vascular plants of diverse origins are fairly constant, equal to 9.3 photons used per CO2 fixation [2,3]. Best-case solar-to-biomass energy conversion efficiency is estimated as 8–10% [3,4].
The matrix of solar cells of photovoltaic (PV) systems comprises semiconductors, organic molecules, and inorganic molecules [5]. Electrical charges in the solar cells are separated when photons from the sun collide with the materials. Negative charges move to the direction of positive charges through an external circuit. Thereby, electricity is extracted from the solar cells [6]. The best efficiencies of crystalline Si, amorphous Si, perovskite, dye sensitized, and organic cells are reported respectively as 26.7, 10.2, 19.7, 11.9, and 11.2% [7].
Although photosynthesis and PV share a similar process of energy transfer from photons to charges, they play different roles for bio-production and electricity production [8]. Plants have not only served directly or indirectly as food for humans; they have also been exploited as energy resources throughout the history of human activities [9]. Demand for plants for use as an energy resource is increasing today under pressures of underground resource depletion and increasing atmospheric-CO2 concentrations [10]. What about the duality of PV? Of course PV cannot be eaten, but it can support various activities related to food production and subsequent supply chains by providing electricity converted from sunlight [11,12].
Greenhouse plant production is a cultivation practice that controls the interior cultivation environment, optimizing it for crop growth and development [13]. Vegetables, fruits, and flowers are cultivated in greenhouses. Fuel and electricity are applied to control the greenhouse interior environment, aiming to improve or stabilize crop yields and quality, but their increasing prices reduce grower profits [[14], [15], [16], [17], [18]]. Accordingly, growers strive to increase crop production efficiency while minimizing fuel and electricity consumption [15,16,[19], [20], [21], [22], [23], [24], [25], [26]]. If renewable energy resources could be used actively in greenhouses, then they could decrease the consumption of fossil fuels and grid electricity [14,22,[27], [28], [29], [30], [31], [32]]. The consequent diminished dependence on traditional energy resources can further mitigate greenhouse gas emissions from the agricultural sector [33].
Greenhouses are typically built on open fields with good sunshine availability because of the fundamentally important demand of sunlight for crop photosynthesis. Therefore, such locations are invariably suitable for PV electricity production [34]. An ingenious and energy-saving plant production system might be achieved if a greenhouse and PV could be integrated appropriately on the same land unit. Nevertheless, a delicate occupancy balance between the crops and PV must be achieved to use sunlight for electricity production in the greenhouse because the sunlight is necessary for plant photosynthesis [34]. Some PV technologies have been deployed already in greenhouse industries, although balance optimization on crop and electricity production remains the subject of intensive investigation.
This review first presents basic aspects of cultivation and electricity demand in greenhouses. Then, PV technology applications to greenhouses to date are summarized. Also, PV shading effects on greenhouse plants are discussed. Finally, possibilities for the additional utilization of PV technologies in greenhouses are explored.
Section snippets
Greenhouse overview
Vegetables, fruits, and flowers are the major crops produced through greenhouse systems [35,36]. Greenhouse walls and roofs are made of transparent glass or plastic, enabling cultivation even when low temperatures restrict open field crop growth [25,37,38]. This merit is particularly useful in temperate zones [[38], [39], [40]]. In addition, the greenhouse extends the cultivation season and broadens the choices of crop species [35,38]. Actually, greenhouses can shorten the cultivation duration,
Temperature
Sunlight penetrates easily into a greenhouse because of roof and wall transparency. The cover materials block thermal leakage. Consequently, the internal temperature becomes higher than that outside [18,37]. By exploiting this thermal property, various technologies related to nighttime heating have been applied. Mainly, such applications are based on the principle of thermal energy storage in walls, soil, or water tanks during daytime, with energy released into the greenhouse during nighttime [
Electrical energy demand for greenhouse environment management
Crop yields and quality can be improved by controlling the greenhouse internal environment using fuels and electricity [70]. Therefore, reducing fuel and electricity consumption to achieve a better growth environment constitutes a major theme of greenhouse cultivation [19,65,109].
Table 1 presents electrical energy demand in greenhouses recorded in the literature across wide geographical regions. Reported original data have various units of electrical energy. To compare all data based on the
Application of stand-alone PV technologies to the greenhouse environment management
Electricity plays crucially important roles in greenhouse management, whereas growers intending to minimize the use of commercial electricity or greenhouses at remote areas are often inaccessible to power lines. In such situations, PV can facilitate greenhouse management. In actuality, the application of solar PV technologies to meet all or part of the greenhouse electricity demand has been attempted (Table 2).
Stand-alone PV power systems are useful where commercial power grids are not
Grid-connected greenhouse PV systems
In southern Europe, solar radiation is excessive during summer [117]. Shading and active cooling using methods such as fogging are necessary. Alternatively, agricultural activity must be suspended [117]. This situation demands exploration of solutions to exploit solar energy in various ways, such as for cooling system operations or for direct sale to commercial grids [117]. If a large-scale greenhouse PV system is connected to the grid, then a large amount of the generated electricity can be
Mitigation of crop shading using PV array spacing and semi-transparent technologies
The whole greenhouse roof area is often covered with opaque conventional PV panels to maximize energy production. However, this scenario is unsuitable for green plant cultivation [34] (Fig. 1a). In fact, the annual global radiation decreases by 0.8% for each additional 1.0% of PV coverage on the roof, as the average of the most common PV greenhouse types [165]. The greenhouse internal light environment varies greatly according to whether the PV modules are concentrated as a single array (Fig. 1
Possible crop yield and quality improvement using dynamic PV shading controls
Photosynthetic photon flux density (PPFD) exceeds 2000 μmol m−2 s−1 around noon on sunny summer days [187,188], whereas photosynthetic light saturation points of major agricultural C3 plants are 500–1500 μmol m−2 s−1 [93,189] (Table 4). Actually, many crop species do not grow optimally at the maximum solar irradiance available in the habitat [3,[190], [191], [192]]. In leaves under full sunlight, up to 80% of the absorbed solar energy must be dissipated to prevent severe damage to
PV power generation using solar irradiance outside the PAR wavelength range
PV power generation and plant cultivation can coexist if the PV cell uses only solar irradiance outside the wavelength range of PAR. Actually, more than 50% of ground-level solar spectrum is not used for plant photosynthesis [3,8,146,190,193]. Near-infrared (NIR) is therefore an attractive energy resource for electricity production in PV greenhouses.
By coating a greenhouse roof with NIR reflective sheets, the roof reflects the NIR-part of solar irradiance to the sky, but it lets the PAR enter
Prospects for energy-sustainable greenhouse cultivation using PV technologies
In light of all the features of greenhouse crop production and PV electricity generation explained in the preceding sections, prospects for PV application to greenhouse cultivation can be inferred.
Using PV energy to compensate for electricity demand in high-latitude greenhouses is actually difficult because of greater needs for energy consumption for heating in winter, a time with less insolation. Improvements of the energy efficiencies of greenhouse electrical appliances and PV cells are key
Conclusions
This review elucidated greenhouse features, the use of electricity for greenhouse environment management, the applications of various PV systems to greenhouses, and the effects of PV shading on plants. Prospects for energy-sustainable greenhouse PV technologies were addressed in the preceding section.
The starting point of energy circulation for life on Earth is the chemical energy converted from sunlight energy by photosynthesis. The availability of solar energy and the distribution of plants,
Declaration of interests
None.
Acknowledgment
This study was partly supported by JSPS KAKENHI Grant Number JP18K05903, Japan.
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