energies

Article A Photovoltaic with Variable Shading for the Optimization of Agricultural and Energy Production

Simona Moretti and Alvaro Marucci *

Department of Agricultural and Forest Sciences, University of Tuscia, Via San Camillo de Lellis, s.n.c., 01100 Viterbo, Italy * Correspondence: [email protected]; Tel.: +39-0761-357-365

 Received: 11 June 2019; Accepted: 3 July 2019; Published: 5 July 2019 

Abstract: The cultivation of plants in currently plays a role of primary importance in modern agriculture, both for the value obtained with the products made and because it favors the development of highly innovative technologies and production techniques. An intense research effort in the field of energy production from renewable sources has increasingly led to the development of greenhouses which are partially covered by photovoltaic elements. The purpose of this study is to present the potentiality of an innovative prototype photovoltaic greenhouse with variable shading to optimize energy production by photovoltaic panels and agricultural production. With this prototype, it is possible to vary the shading inside the greenhouse by panel rotation, in relation to the climatic conditions external to the greenhouse. An analysis was made for the solar radiation available during the year, for cases of completely clear sky and partial cloud, by considering the 15th day of each month. In this paper, the results show how the shading variation enabled regulation of the internal radiation, choosing the minimum value of necessary radiation, because the internal microclimatic parameters must be compatible with the needs of the plant species grown in the greenhouses.

Keywords: dynamic photovoltaic greenhouse; variable shading; renewable source; passive cooling system

1. Introduction Cultivation in greenhouses allows us to satisfy the growing demand for vegetables and fruits by the growing global population by extending the production window both geographically and seasonally [1]. The use of greenhouses allows for the control of the microclimatic parameters that characterize the internal environment (i.e., air temperature, relative humidity, lighting level, CO2 concentration) [2–5], influencing both crop quality and quantity [6,7] and the spread of pathogens and diseases [8]. Greenhouses use solar radiation input, but the control of the internal environment takes place thanks to energy supplied from different sources, including fossil fuels and the electricity network [9]. These systems that take advantage of the greenhouse effect require high amounts of energy for the operation of forced cooling, heating, and lighting systems [10]. Given the need to reduce human pollution, the combination and use of renewable energy sources (e.g., ) seems to be the best solution to follow [11,12]. Some studies have shown that solar panels can produce energy for the operation of heat pumps in Italy [13,14], fog cooling systems in Malaysia [15] and in Saudi Arabia [16], and for fan and pad cooling systems in Arizona [17]. By appropriately positioning solar panels on greenhouse roofs, it is possible to obtain multiple advantages: using the solar energy produced to make the agricultural production independent of

Energies 2019, 12, 2589; doi:10.3390/en12132589 www.mdpi.com/journal/energies Energies 2019, 12, 2589 2 of 15 traditional energy sources [18,19]; reducing the environmental impact and production costs; not subtracting from the land useful for crops [20,21] (since both agriculture and photovoltaic panels need sunlight and available land, and these two practices are in conflict [22]); and using the panels as a passive cooling system thanks to the shading produced [23,24], as an alternative to special shading systems such as nets [25–27] and reflective coatings [25,28]. Depending on the latitude and weather conditions, cultures must be protected from low temperatures in winter and/or autumn and at night and from high temperatures in spring and/or summer. These low- and high-temperature conditions occur easily in Mediterranean countries [25]. To correct the excessively high temperature values that can occur inside the greenhouse, there are several methods; among them, shading is an advantageous practice—especially in regions of high insolation such as Spain [29] and Saudi Arabia [30]. Shading reduces the level of solar radiation, the air temperature, and the rate of evapotranspiration, reducing water consumption—a fundamental aspect for countries where this resource is scarce [31,32]. Furthermore, it has been shown that shading combined with evaporative cooling is more effective in arid regions and in hot seasons, while shading combined with thermal screens reduces the energy consumption used for heating in cold regions, maintaining the temperature of the internal air at 5 ◦C higher than outside air [33]. Shading is important to improving crop growth, extending the crop cycle, and delaying ripening [33,34], and its benefits have been demonstrated by some studies conducted in Japan [35] and in the Mediterranean regions [26]. When placing the solar panels on the roof of the greenhouse, the shading depends on the inclination of the solar rays (and therefore on the latitude, altitude, time of day, and season) [36], their arrangement on the roof, their degree of transparency, and their inclination. Some studies have analyzed the effect of the panel arrangement on cultures, and highlighted how this aspect drastically influences plant growth and energy production [7,37–39]. Controlling the panels’ disposition seems to be a good solution to reducing the lack of lighting uniformity [35]. Photovoltaic panels can be opaque, semitransparent, or transparent, and can let different amounts of solar radiation pass, influencing crop growth [25]. Opaque panels have negative effects on production, reducing the crop growth in the case of tomatoes or reducing the amount of biomass in the case of maize [40–45]. Meanwhile, in the experiments conducted so far, semi-transparent panels such as DSCs (dye-sensitized solar cells) [46,47], OPVs (organic ) [48–50], and PVs based on the latest generation of luminescent solar concentrators have been shown to have great potential for improvement in terms of biomass production, plant morphology, and nutritional content, since they allow the wavelength used by plants for photosynthesis and morphogenesis to pass through [49]. Li et al. say that “the installation of semi-transparent PV modules on a greenhouse roof surface can be beneficial when crops require moderate shading under high-irradiation conditions” [9] and, for example, tomato [10,38], lettuce [51,52], wild rocket [53], and Welsh onion [37] were cultivated correctly under semi-transparent panels [9]. The arrangement of the semi-transparent panels varies from checkerboard to conventional planar PV modules [7,54,55] or cells [39,56,57] to dispersed PV micro-cells [58,59]. One important aspect to be analyzed for shading concerns the inclination of the panels [60]. By placing the solar panels on the roof of the greenhouse and creating a static system, independent of time, their inclination will be equal to that of the greenhouse cover. In this way, the shading system is not very dynamic and not very adaptable to the needs of the plants, which are influenced by excessive shadowing when it is not necessary [36,61]. Considering that the amount of solar radiation reaching the Earth varies according to the time of day, season, latitude, weather conditions, and altitude, a compromise must be found between electricity production and agriculture, making the system dynamic over time, exactly like irradiation. In other words, until today the research has developed fixed-shading photovoltaic greenhouses. Energies 2019, 12, 2589 3 of 15

A fixed shading value can work well in some months of the year, but not others; it can be optimal in clear skies but not in partially or totally cloudy sky conditions. Additionally, during the day, the shading should vary with higher values in the central hours of the day. The research gap can be filled only with photovoltaic greenhouses with variable shading by PV panels. This article describes the experimental results obtained by applying a dynamic photovoltaic greenhouse prototype where shading inside the greenhouse was continuously changeable according to the demand of the crops for light and external weather conditions [62]. The panels’ position could change at any time of the day, optimizing the production of electricity and agriculture. When the irradiation was at a maximum, during the hot season and in the central hours of the day, the panels could be positioned in such a way as to shade the crop and obtain maximum energy production; when the level of radiation was reduced, such as in the cold season and in the first and last hours of the day, the panels could be positioned in such a way as to allow more solar radiation to pass through. Thanks to the rotation of the panels, it was possible to vary the degree of shading from a minimum of 0 degrees to a maximum of 78 degrees. Furthermore, it is important to underline that with this panel movement system, when it was necessary to let in more solar radiation because the sky was covered, the roof remained closed and the crop continued to be protected from any precipitation. When the optimal inclination of the solar panels to produce electric energy was lost, causing a reduction in energy production, reflective aluminum mirrors reflected the otherwise-lost radiation back on the panels and in this way the electricity production was improved [62]. This innovative system is able to

(1) Continuously vary the shading according to the weather conditions, the period of the year, the time of day, and the type of harvest. The mobile system (panels and mirrors) makes the structure dynamic and flexible based on the needs of the situation; (2) Optimize energy and agricultural production consuming the same land unit, without filling areas adjacent to the greenhouse for the positioning of the panels at the expense of crops; (3) Totally recover the amount of energy lost by reflection when the inclination of the direct solar radiation moves away from the optimal one thanks to the use of aluminum mirrors constantly aligned with the sun’s rays; (4) Use a large part of the greenhouse roof for the installation of photovoltaic panels, leaving the crop protected from precipitation.

In this paper the variation in shading obtained from the dynamic prototype of the photovoltaic greenhouse is analyzed and discussed in order to maintain certain minimum internal solar radiation 2 1 thresholds expressed in terms of both global daily radiation (5 MJ m− day− ) and energy flow 2 (400 Wm− ).

2. Materials and Methods The prototype used in this research was realized at the Tuscia University in Viterbo, Italy. The length and width of the greenhouse were 3.79 and 2.41 m, respectively; it had an asymmetric cross section specifically designed for photovoltaic energy generation. The photovoltaic panels occupied an area of 8.15 m2 and were positioned on the south pitch, inclined at 33◦. The north pitch without photovoltaic panels had a slope of 51◦ (Figure1). Energies 2019, 12, x FOR PEER REVIEW 4 of 15 Energies 2019, 12, x FOR PEER REVIEW 4 of 15 Energies 2019, 12, 2589 4 of 15

Figure 1. The experimental prototype used for this search. Figure 1. The experimental prototype used for this search. The prototype was made of iron and glass, with a transverse vertical polycarbonate wall. MoreThe prototype details about was made the geometricalof iron andand glass,glass characteristics, withwith aa transversetransverse and technical verticalvertical data polycarbonatepolycarbonate of the prototype wall.wall. are reportedMore in details detailsMarucci about about et al. the the[6 geometrical2] . geometrical characteristics characteristics and technical and technical data of data the prototype of the prototype are reported are inreported MarucciThis ininnovative etMarucci al. [62]. etsystem al. [62 ]arose. from the observation of shaded greenhouses where the pitch of the southThis - innovativeinnovativefacing roof systemsystem was partially arosearose from from covered the the observation observaby photovoltaiction of of shaded shaded panels. greenhouses greenhouses This solution where where has the the become pitch pitch of the of south-facingmostthe south popular-facing roofone roof. was If we was partially consider partially covered the covered latitude by photovoltaic byof thephotovoltaic Mediterranean panels. panels. This areas solutionThis, knowing solution has becomethe has inclination become the most the of popularthemost sun popular’s one. rays If oneat we 12:00,. If consider we it consider is possible the latitude the to latitude observe of the of Mediterranean thatthe Mediterraneanthe portion areas, of theareas knowing illuminated, knowing the inclinationfloorthe inclination is poor of and the of sun’snotthe uniform.sun rays’s ra atys Figure 12:00, at 12:00, it2 isshows it possible is possible the distribution to observe to observe that of thethat the partial portionthe portion shadows of the of illuminatedthe from illuminated June 21st. floor floorWhen is poor is the poor and height and not uniform.anglenot uniform. of the Figure solar Figure2 shows rays 2 shows decreases, the distribution the distribution the internal of the of partialsolar the partialradiation shadows shadows is from also June fromreduced, 21st. June When as21st. shown When the heightin the Figures height angle 3 ofandangle the 4. ofsolar the rays solar decreases, rays decreases, the internal the internal solar radiation solar radiation is also reduced,is also reduced, as shown as inshown Figures in3 F andigures4. 3 and 4.

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Figure 2. Direct solar radiation at Mediterranean latitudes: 21 June June,, 12:00 p pm.m. ( (PV:PV: photovoltaic) photovoltaic).. Figure 2. Direct solar radiation at Mediterranean latitudes: 21 June, 12:00 pm. (PV: photovoltaic).

Energies 2019, 12, 2589 5 of 15 EnergiesEnergies 2019 2019, 12, 12, x, xFOR FOR PEER PEER REVIEW REVIEW 5 5of of 15 15 Energies 2019, 12, x FOR PEER REVIEW 5 of 15

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FigureFigure 3. 3. Direct Direct solar solar radiation radiation at at Mediterranean Mediterranean latitudes: latitudes: 21 21 September September, 12:00, 12:00 p pm.m. Figure 3. DirectDirect solar solar radiation radiation at at Mediterranean Mediterranean latitudes: 21 21 September September,, 12:00 p pm.m.

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FFigureFigureigure 4.4. 4. DirectDirect Direct solar solarsolar radiation radiationradiation at atat Mediterranean MediterraneanMediterranean latitudes: latitudes:latitudes: 21 2121 December December,December, 12:00,12:00 12:00 ppm. pm.m. Figure 4. Direct solar radiation at Mediterranean latitudes: 21 December, 12:00 pm. UnderUnderUnder these these conditions, conditions conditions, most,most most of of ofthe the the direct direct direct solar solar solar radiation radiation radiation enters enters enters the protectedthe the protec protec environmenttedted environment environment from Under these conditions, most of the direct solar radiation enters the protected environment fromthefrom vertical the the vertical vertical wall facingwall wall facing south.facing south. south. Finally, Finally, Finally, Figure F5 Figure showsigure 5 5 thatshows shows the that that checkerboard the the checkerboard checkerboard formation formation formation allows greaterallows allows from the vertical wall facing south. Finally, Figure 5 shows that the checkerboard formation allows greateruniformitygreater uniformity uniformity of lighting of of lighting lighting and therefore and and therefore therefore is much is moreis much much effi more morecient efficient forefficient the growthfor for the the growth of growth crops. of of crops. crops. greater uniformity of lighting and therefore is much more efficient for the growth of crops.

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Figure 5. Direct solar radiation at Mediterranean latitudes for a different checkerboard formation of FigureFigure 5. 5. Direct Direct solar solar radiation radiation at at Mediterranean Mediterranean latitudes latitudes for for a adifferent different checkerboard checkerboard formation formation of of Figurephotovoltaic 5. Direct panels: solar 21 radiation June, 12:00 at Mediterranean pm. latitudes for a different checkerboard formation of photovoltaicphotovoltaic panels: panels: 21 21 June June, 12:, 12:0000 pm pm. . photovoltaic panels: 21 June, 12:00 pm. These solutions are expensive to implement, and as they are “static”, they are good for limited These solutions are expensive to implement, and as they are “static”, they are good for limited periodsThese of solutions the year andare day,expensive and only to implement for certain, and crops. as they It is necessaryare “static to”, they consider are good that underfor limited clear periodsThese of solutionsthe year andare expensiveday, and onlyto implement for certain, and crops. as theyIt is arenecessary “static ”,to theyconsider are good that underfor limited clear periodssky conditions of the year solar and radiation day, and varies only considerably for certain duringcrops. It the is yearnecessary in terms to ofconsider intensity that of under energy clear flow periodssky conditions of the year solar and radiation day, and varies only considerablyfor certain crops. during It is the necessary year in toterms consider of intensity that under of energy clear sky(Wm conditions2) and in solar terms radiation of duration varies of solar considerably radiation (dayduring length). the year in terms of intensity of energy sky conditions− −2 solar radiation varies considerably during the year in terms of intensity of energy flowflow ( Wm(Wm−2) )and and in in terms terms of of duration duration of of solar solar radiation radiation (day (day length). length). flow (Wm−2) and in terms of duration of solar radiation (day length).

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Energies 2019, 12, x FOR PEER REVIEW 6 of 15 For example, in the absence of clouds, at the latitude of 42◦ north and at an altitude of 300 m, the Energies 2019, 12, x FOR PEER REVIEW 6 of 15 2 1 2 1 global dailyFor radiationexample, in ranges the absence from o 7.3f clouds, MJ m −at dthe− latitudeon 21 December of 42° north to and 29.4 at MJan altitude m− d− ofon 300 21 m, June—an the approximatelyglobalFor daily example, four-fold radiation in the increase.ranges absence from of clouds,7.3 MJ mat− 2thed−1 latitudeon 21 December of 42° north to 29.4and atMJ an m altitude−2d−1 on of21 300June m,— thean Toapproximatelyglobal make daily the radiation dynamic four-fold ranges system,increase. fromit was7.3 MJ decided m−2d−1 on to move21 December the panels. to 29.4 MJ m−2d−1 on 21 June—an Aapproximately mechanicalTo make the systemfour dynamic-fold allowed increase. system the , it rotationwas decided of the to move panels the along panels. the longitudinal axis and regulated the shadingATo mechanical insidemake the the dynamic structuresystem system allowed (Figure, it was the6). dec rotationided to of move the panelsthe panels. along the longitudinal axis and regulatedA mechanical the shading system inside allowedthe structure the rotation(Figure 6). of the panels along the longitudinal axis and regulated the shading inside the structure (Figure 6).

Figure 6. PV panels and mirrors on the prototype. Figure 6. PV panels and mirrors on the prototype. Figure 6. PV panels and mirrors on the prototype. However,However, the t dynamiche dynamic system system caused caused thethe loss of of the the optimal optimal inclination inclination for forthe theproduction production of of energy with photovoltaic panels, resulting in losing part of the solar radiation useful for reflection. energy withHowever, photovoltaic the dynamic panels, system resulting caused in the losing loss partof the of optimal the solar inclination radiation for useful the production for reflection. of energyThis with problem photovoltaic was partially panels, solved resulting by placing in losing mirrors part of (F theigures solar 6 radiationand 7) [6 2useful]. for reflection. This problem was partially solved by placing mirrors (Figures6 and7)[62]. This problem was partially solved by placing mirrors (Figures 6 and 7) [62].

Figure 7. Detail of the movement system of the PV panels and mirrors.

FigureFigure 7. Detail7. Detail of of the the movement movement system of of the the PV PV panels panels and and mirrors. mirrors.

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Energies 2019, 12, x FOR PEER REVIEW 7 of 15 To reflect part of the lost radiation, the mirrors must always be aligned with the sun’s rays. Defined withToα thereflect angle part between of the thelost mirrorsradiation, and the the mirrors panels, must the importancealways be aligned of the mirrors with the is greatersun’s rays. the Definedsmaller the with angle α theα. At angle 55◦, betweenthe area of the the mirrors photovoltaic and the panel panels, that allows the importance the mirrors of to the be e mirrorfficients for is greaterthe recovery the smaller of solar the radiation angle α. is At 30%. 55°, Atthe 45 area◦, only of the a reflection photovoltaic occurs panel on thethat mirror, allows butthe themirrors area to of bethe efficient panel that for allowsthe recovery the mirror of solar to recover radiation the is solar 30%. radiation At 45°, only is 100%. a reflection As studied occurs by o Maruccin the mirror, et al. “Forbut the angles area smallerof the panel than 45that◦, theallows number the m ofirror reflections to recover increases the solar proportionally radiation is to100%. the decreaseAs studied in theby Marucciincidence et angle” al. “For [62 angles]. smaller than 45°, the number of reflections increases proportionally to the decreaseIf the in photovoltaic the incidence panels angle are” [6 parallel2]. to the roof, they allow a maximum shading degree of 78%. MarucciIf the et photovoltaic al. defines the panels degree are of parallel shading to as the “as roof the, ratiothey allow between a maximum the projection shading of the degree length of of 78%. the Maruccipanel including et al. defines the frame the degree (21 cm) of on shading the pitch as and“as thethe distanceratio between between the theprojection points of of rotation the length of the of thepanel panel (27 cm)”including [62]. the frame (21 cm) on the pitch and the distance between the points of rotation of the panTheel percentage(27 cm)” [6 of2]. shading could be modified thanks to the panel movement system. If the portion of theThe panels percentage projected of on shading the ground could is be reduced, modified the shadingthanks to is thereduced. panel movement system. If the portionThe of degree the panels of shading projected must on be the selected ground based is reduced, on the shading is reduced. The degree of shading must be selected based on (1) The1) kind The of kind crop; of crop; (2) Latitude;2) Latitude; (3) The3) time The of day;time andof day; and (4) External4) External weather weather conditions. conditions.

3. Results Results and and D Discussioniscussion Figure 88 showsshows thethe valuesvalues ofof thethe externalexternal andand internalinternal globalglobal solarsolar radiationradiation measuredmeasured onon clearclear days and with different different values of shading by the PV panels and the values of the transmittance of the

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% 2 - 80 25 70 MJ m MJ 20 60 50 15 40 10 30 20 5 10 0 0 0 10 20 30 40 50 60 70 80 Shading rate

External solar radiation Internal solar radiation Average transmittance %

Figure 8. Outside and inside global solar radiation at different shading percentages. Figure 8. Outside and inside global solar radiation at different shading percentages. The values of internal solar radiation were obtained by processing the data measured by internalThe sensors values [ of62 ].internal It can besolar seen radiation that the internal were obtained solar radiation by processing decreased the as data the percentagemeasured by of internalshading sensors increased, [62]. and It can that be the seen system that wasthe internal very eff ectivesolar radiation in adapting decreased to external as the radiation percentage levels. of shadingFrom the increase measuredd, and data that shown the system in Figure wa8s andvery with effective the directin adapting and di toff useexternal solar radiation radiation levels. levels Fromcalculated the measured [36,63], the data annual shown trends in ofFigure external 8 and and with internal the solar direct radiation and diffuse were simulatedsolar radiation for di fflevelserent calculatedshading percentages [36,63], the (20%, annual 40%, trend 60%,s andof external 78%). For and this internal simulation, solar the radiation 15th of wereeach month simulated in clear for differentsky conditions shading was perc consideredentages (20%, (Figure 40%,9). 60%, and 78%). For this simulation, the 15th of each month in clear sky conditions was considered (Figure 9).

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1 -

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MJ m MJ 23

20

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13

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0 January February March 15th April 15th May 15th June 15th July 15th August September October November December 15th 15th 15th 15th 15th 15th 15th 20% 40% 60% 78% External solar radiation Internal solar radiation (0% shading) limit 5 MJm^-2day^-1

Figure 9. Levels of external and internal internal solar solar radiation radiation under under different different levels levels of of shading shading (0%, (0%, 20%, 20%, 40%, 60% 60%,, and 78%).

2 1 VValuesalues of internal solarsolar radiationradiation belowbelow 55 MJMJ mm−−2 day−1 areare not not sufficient sufficient for for the the optimal optimal growth growth of most crops crops,, and this is is reflected reflected in in the the poor poor quality quality of of the the final final products products in in terms terms of of size, size, quantity, quantity, colorcolor,, and nutritional properties. This threshold value w wouldould never be reached if the shading percentage reached 78%. According to the the simulation, simulation, for for 60% 60% of the of shading, the shading, the critical the critical months months are January, are January, February, February, October, November, October, Novemberand December., and For December. the months For of the January, months November, of January, and November December,, and 40% Decemberof the shading, 40% was of notthe 2 1 shadingallowed was to reach not allow the lowered tolimit reach of the 5 MJ lower m− limitday− of. 5 Finally, MJ m−2 the day month−1. Finally, of December the month did of notDecember exceed didthe threshold,not exceed even the withthreshold the shading, even with percentage the shading at 20%. percentage Thus, in the at 20%. months Thus, of January, in the months February, of January,October, February, November, October, and December, November thanks, and toDecember, the mobile thanks panels, to the it ismobile possible panels, to allow it is morepossible solar to allowradiation more to pass,solar reducingradiation theto pass, shading. reducing During the spring shading. and summer,During sp onring the and other summer, hand, it ison possible the other to hand,use the it panelsis possible as if theyto use were the apanels passive as coolingif they were system, a passive because cooling the available system solar, because radiation the available exceeds solarthe demand radiation of mostexceed plants,s the demand at the same of most time plants, producing at the electricity same time that producing could eventually electricity be that entered could in eventuallythe electricity be network.entered in Figure the 9electricity clearly shows network. that in Figure the spring 9 clearly and autumn shows seasonthat in it the is su springfficient and to autumnshade to season20%, while it is in sufficient the summer to shade period to a value 20%, of while 50% in is su theffi cient. summer period a value of 50% is sufficient.Thisnew dynamic photovoltaic greenhouse was also designed and built to achieve instantaneous shadingThis variations. new dynamic When wephotovoltaic set an optimal greenhouse limit (constant was or also variable) designed of internal and solarbuilt radiation to achie forve 2 instantaneousthe plant cultivated shading (for variations.example, 400 When Wm − we), allset the an PV optimal panels, limit initially (constant aligned or with variable) the mirrors of internal to the solar rays,radiation could for begin the plant to rotate cultivated to increase (for the example shading, 400 in suchWm− a2), way all the that PV the panels, internal initially solar radiation aligned withwas maintainedthe mirrors atto thethe establishedsolar rays, could optimal begin value. to rotate to increase the shading in such a way that the internalOn solar June 21,radiation with clear was skymaintained and at 42 at◦ thenorth established latitude, inoptimal order tovalue. maintain the greenhouse at the 2 fixedOn level June of 21, solar with radiation clear sky (400 and Wm at −42),° thenorth PV latitude, panels should in order begin to maintain to produce the shadegreenhouse three hoursat the fixedafter sunriselevel of andsolar stop radiation shading (400 three Wm hours−2), the before PV panels sunset should (Figure begin 10). Into addition,produce shade the shading three hours is not afterconstant sunrise and and must stop gradually shading increase three hours up to before a maximum sunset of (Figure 47% in 1 the0). In middle addition, of the the day. shading is not constant and must gradually increase up to a maximum of 47% in the middle of the day.

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1000 100 1000 100 21000 100 W/m2 W/m2 900 90 W/m 900 90 900 90 % 800 80 % 800 80 700 70 700 70 600 60 600 60 46 47 47 47 47 500 44 46 47 47 47 47 44 50 500 42 44 46 44 42 50 500 38 42 42 38 50 38 38 400 33 38 38 33 40 400 33 33 40 25 25 300 25 25 30 300 25 25 30 200 14 14 20 200 14 14 20 100 10 100 0 0 0 0 0 0 0 0 0 0 0 0 0 0 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 h Internal solar radiation External solar radiation Internal solar radiation (0% shading) Shading percentage PV energy production h Internal solar radiation External solar radiation Internal solar radiation (0% shading) Shading percentage PV energy production Internal solar radiation External solar radiation Internal solar radiation (0% shading) Shading percentage PV energy production FigureFigure 10.10. DailyDaily trendtrend ofof shadingshading percentagespercentages so so thatthat thethethe internal internalinternal solar solarsolar radiation radiationradiation is iis kept kept lower lowerlower than thanthan or oror equalequal toto thethethe set set limitlimit onon JuneJune 21.21.

OnOn MarchMarch 21,21,, thethethe PV PV panelspanels shouldshould beginbegin toto produceproduce shadeshade threethree hourshours afterafter sunrisesunrise andand stopstop producingproducing shadeshade threethree hourshours beforebefore sunsetsunset (Figure(Figure 1111).). TheThe shadingshading inin thisthis casecase shouldshould graduallygradually increaseincreaseincrease to toto a aa maximum maximummaximum of ofof 35% 35%35% in inin the thethe middle middlemiddle of ofof the thethe day. day.day.

1000 100 1000 100 21000 100 W/m2 W/m2 900 90 W/m 900 90 900 90 % 800 80 % 800 80 700 70 700 70 600 60 600 60 500 50 500 50 35 400 34 35 34 40 400 32 34 35 34 32 40 28 32 32 28 28 28 300 23 28 28 23 30 300 23 23 30 200 14 14 20 200 14 14 20 100 10 100 0 0 0 0 0 0 0 0 0 0 0 0 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 h Internal solar radiation External solar radiation Internal solar radiation (0% shading) Shading percentage PV energy production h Internal solar radiation External solar radiation Internal solar radiation (0% shading) Shading percentage PV energy production

FigureFigure 11.11.Daily Daily trend trend of of shading shading percentages percentage sos thatso that the internalthethe internalinternal solar solarsolar radiation radiationradiation is lower isis lowerlower thanor thanthan equal oror toeq theual setto thethe limit set on limit March on March 21. 21.

Lastly,Lastly, on on February February 21, 21,, the thethe maximum maximummaximum shading shadingshading must mustmust not notnot exceed exceedexceed 18% 18%18% (Figure (Fig(Figure 12 12).).

1000 100 1000 100 21000 100 W/m2 W/m2 900 90 W/m 900 90 900 90 % 800 80 % 800 80 700 70 700 70 600 60 600 60 500 50 500 50 400 40 400 40 300 30 300 30 300 18 30 17 18 17 200 13 17 18 17 13 20 200 13 13 20 6 6 100 6 6 10 100 0 0 0 0 0 0 0 0 0 0 0 0 0 0 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 h Internal solar radiation External solar radiation Internal solar radiation (0% shading) Shading percentage PV energy production h Internal solar radiation External solar radiation Internal solar radiation (0% shading) Shading percentage PV energy production Internal solar radiation External solar radiation Internal solar radiation (0% shading) Shading percentage PV energy production Figure 12. Daily trend of shading percentages so that the internal solar radiation is lower than or FigureFigure 12.12.Daily Daily trend trend of of shading shading percentages percentage sos thatso that the internalthe internal solar solar radiation radiation is lower is lower thanor than equal or equal to the set limit on February 21. toequal the setto the limit set on limit February on February 21. 21.

On thethe threethree daysdays considered,considered, thethe panelspanels mustmust beginbegin toto shadeshade aboutabout threethree hours after sunrise and stop shadinging about threethree hours before sunset;; thesethese intervalsintervals do not vary much during the year, but theythey can be changed by modifying the limit for the internal solar radiation. However,, one factor

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EnergiesOn 2019 the, 12 three, x FOR days PEER considered, REVIEW the panels must begin to shade about three hours after sunrise10 of 15 and stop shading about three hours before sunset; these intervals do not vary much during the year, butthat they does can var bey changedgreatly duringby modifying the year the is limit the for percentage the internal of solar shading radiation. required However, to have one thefactor same thatinternal does solar vary radiation. greatly during On days the year with is a the partly percentage cloudy ofsky shading (Figure required 13), the shading to have the by samethe PV internal panels solarmust radiation. be instantly On daysadapted with to a thepartly external cloudy solar sky (Figure radiation 13 ), and the the shading set limit by the for PV the panels internal must solar be instantlyradiation. adapted to the external solar radiation and the set limit for the internal solar radiation.

1000 100 W/m2 900 90 % 800 80

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600 60

500 43 50 41 41 37 400 40 31 26 300 22 24 30

200 13 20 9 11 100 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 h Internal solar radiation External solar radiation Internal solar radiation (0% shading) Shading percentage PV energy production FigureFigure 13.13.Daily Daily trendtrend ofof thetheshading shading percentagepercentagewith withpartly partlycloudy cloudysky sky on on May May 23. 23.

TheThe shading shading of of the the PV PV panels panels adapts adapts instantaneously instantaneously to to the the variations variations inin thethe external external solar solar radiation:radiation: itit mustmust bebe nullnull whenwhen thethe internalinternal solarsolar radiationradiation isis lessless thanthan oror equalequal toto thethe setset limit,limit,and and mustmust graduallygradually increase increase to to remain remain equal equal to to the the set set limit limit for for the the internal internal solar solar radiation. radiation. TheThe trendstrendsin inthe theelectricity electricity productionproduction ofof thethe PVPV panelspanels onon representativerepresentative daysdays (sunny(sunny daysdays ofof didifferentfferent lengths lengths and and partlypartly cloudycloudydays) days)are are shownshown inin thethe correspondingcorresponding figures,figures, dependingdepending onon thethe externalexternal solar solar radiation radiation and and above above all all on on the the shading shading of theof the panels. panels. The Th electricitye electricity production production of the of PVthe panelsPV panels is null is null in the in firstthe first and lastand hourslast hours of the of day,the day, where where the panels the panels are aligned are aligned to the to rays the ofrays the of sun the 2 likesun mirrors;like mirrors; it grows it grows with increasing with increasing shading shading up to about up to 65 about Wm −65in Wm the−2 central in the hourscentral of hours the day of onthe theday summer on the summer solstice. Tablesolstice.1 shows Table the 1 shows total daily the total values daily of the values electricity of the producedelectricity by produced the PV panels by the 2 1 expressedPV panels in expressed MJ m− d in− .MJ m−2d−1.

Table 1. External and internal solar daily radiation of the prototype and electricity production of the Table 1. External and internal solar daily radiation of the prototype and electricity production of the PV panels. PV panels.

ExternalExternal S Solarolar InternalInternal Solar Solar PV EPVnergy Energy PV EPVnergy Energy Day Radiation Radiation Production Efficiency Day Radiation2 (MJ1 Radiation (MJ2 1 Production (MJ2 1 Efficiency (MJ m− d− ) (MJ m− d− ) (MJ m− d− ) (%) m−2d−1) m−2d−1) m−2d−1) (%) June 21, clear sky 29.5 16.8 1.44 4.9 MarchJune 21,21, clear clear sky sky 29.5 18.8 16.8 12.7 1.44 0.634.9 3.3 FebruaryMarch 21, 21, clear clear sky 13.3 10.4 0.25 1.9 18.8 12.7 0.63 3.3 May 23, partlysky cloudy sky 20.6 13.8 0.67 3.2 February 21, clear 13.3 10.4 2 0.25 1.9 On clear days,sky with the internal threshold of 400 Wm− , the daily electricity production varied 2 1 from 0.25May to 1.4423, partly MJ m − d− with a maximum efficiency percentage of 4.9%. In cloudy conditions 20.6 13.8 0.67 3.2 the productioncloudy of sky electricity was lower and in any case extremely variable with the distribution and intensity of cloudiness. To immediately adapt the shading to the required conditions, the panel movementOn clear system days, (manual with the in internal the prototype) threshold could of 400 be Wm easily−2, the automated daily electricity and associated production with varied the centralizedfrom 0.25 to greenhouse 1.44 MJ m−2 managementd−1 with a maximum system. efficiency The value percentage of internal of solar 4.9%. radiation In cloudy was conditions established the basedproduction on the of needs electricity of the cultivatedwas lower plants. and in Based any case on the extremely value of variable external solarwith radiationthe distribution measured, and theintensity automatic of cloudiness. control system To obtained immediately the percentage adapt the of shading shading to and the therefore required the conditions, angle that the the panels panel mustmovement take with system respect (manual to the inpitch. the The prototype) automated could management be easily automated system also and controlled associated the anglewith the of thecentralized mirrors, whichgreenhouse must alwaysmanagement be aligned system. with The the value sun’s rays.of internal solar radiation was established based on the needs of the cultivated plants. Based on the value of external solar radiation measured, the automatic control system obtained the percentage of shading and therefore the angle that the panels must take with respect to the pitch. The automated management system also controlled the angle of the mirrors, which must always be aligned with the sun’s rays.

Energies 2019, 12, 2589 11 of 15

Energies 2019, 12, x FOR PEER REVIEW 11 of 15 The energy exchanges of the proposed prototype, including the radiative exchanges in the night period,The were energy studied exchanges in theof the energy proposed balance prototype, [36]. in Including the winter the radiative months, exchanges shading in the and night electricity productionperiod, were were drastically studied in reduced the energy to become balance [36]. negligible, In the winter as the months,solar energy shading that and reaches electricity the ground is just enoughproduction for were cultivated drastically plants. reduced Without to become plant negligible cultivation,, as the however, solar energy the shadingthat reaches would the be the maximumground possible is just enough (78%), asfor wouldcultivated the plants. production Without of plant electricity. cultivation, however, the shading would be the maximum possible (78%), as would the production of electricity. AnotherAnother possible possible use use of thisof this prototype prototype couldcould be be to to entrust entrust the themodification modification of the ofshading the shading to the to the strong variationstrong variation of the of anglethe angle of theof the solar solar rays, rays, keepingkeeping the the photovoltaic photovoltaic panels panels fixed fixedand horizontal and horizontal.. This workingThis working method method of theof the prototype prototype willwill be the the subject subject of offurther further studies studies and research. and research. RotationRotation is still is necessary still necessary for the for mirrors, the mirrors which, which must must always always be bealigned aligne tod to sunlight. sunlight. It It is is a avariation of passivevariation shading, of passive without shading, energy without consumption energy consumption for the control for the and control handling and handling systems systems (Figure 14). (Figure 14).

FigureFigure 14. 14.Shading Shading in in clear clear skysky conditions with with horizontal horizontal PV panels. PV panels.

The shading obtained is favorable for crop requirements: maximum in summer, minimum during the winter. Energies 2019, 12, 2589 12 of 15

4. Conclusions This study shows the feasibility of changing the degree of shading inside a greenhouse based on the available solar radiation and on plant needs, thanks to a dynamic system of photovoltaic panels. The advantages of this innovation are evident, because the mobile photovoltaic panels positioned on the roof allow optimization the agricultural production (varying the amount of solar radiation that reaches the plants) and the production of electrical energy (thanks also to the presence of the mirrors). Finally, the shading can be almost completely removed: the photovoltaic panels can rotate to cancel their projection on the floor and not interfere with the lighting. With a set optimal limit of internal solar radiation for the plant cultivated, all the PV panels, initially aligned with the mirrors to the solar rays, can begin to rotate to increase the shading in such a way that the internal solar radiation is maintained at the established optimal value (constant or variable). With a partly cloudy sky, the shading by the PV panels can be instantly adapted to the external solar radiation and the set limit for the internal solar radiation. Therefore, this prototype is a dynamic and flexible system that easily adapts to agricultural and energy production demands.

Author Contributions: Conceptualization, S.M. and A.M.; methodology, A.M.; software, S.M.; validation, S.M.; formal analysis, S.M. and A.M.; investigation, S.M. and A.M.; resources, A.M.; data curation, S.M.; writing—original draft preparation, S.M.; writing—review and editing, S.M.; visualization, S.M.; supervision, A.M.; project administration, A.M.; funding acquisition, A.M.. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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