Renewable and Sustainable Energy Reviews 54 (2016) 989–1001
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Renewable and Sustainable Energy Reviews
journal homepage: www.elsevier.com/locate/rser
Advanced applications of solar energy in agricultural greenhouses
Reda Hassanien Emam Hassanien a,b, Ming Li b,n, Wei Dong Lin b a Agricultural Engineering Department, Faculty of Agriculture, Cairo University, Cairo 12613, Egypt b Solar Energy Research Institute, Yunnan Normal University, Kunming 650500, China article info abstract
Article history: Energy is the largest overhead cost in the production of agricultural greenhouse crops in temperate Received 25 March 2015 climates. Moreover, the initial cost of fossil fuels and traditional energy are dramatically increasing. The Received in revised form negative environmental impacts, limited sources of fossil fuels and a high consumption of energy and 4 September 2015 food have caused the increase in demand for solar energy as a green and sustainable choice. Therefore, Accepted 21 October 2015 this paper reviews the solar energy application technologies in the environmental control systems of greenhouses (cooling, heating and lighting) mainly the generated energy of photovoltaic (PV) and solar Keywords: collectors, as well as the PV water pumping for irrigation. Furthermore, this paper briefly discusses the PV greenhouse economic analyses and the challenges for this technology. Solar energy & 2015 Elsevier Ltd. All rights reserved. Cooling systems Heating systems PV pump
Contents
1. Introduction ...... 989 2. Environmental control systems in greenhouses...... 990 2.1. Heating systems...... 990 2.2. Cooling systems ...... 993 2.3. Lighting systems ...... 994 3. The photovoltaic water pumping in irrigation systems ...... 996 4. Economic analyses...... 997 5. Conclusion and challenges of solar energy in greenhouses...... 998 Acknowledgments...... 999 References...... 999
1. Introduction of food and energy technology by using alternative sources. In addition, climate changes and poor water resources reveal that The increase of world population and energy consumption has protected cultivation in greenhouses has become the favored way directed researchers and scientists to provide a sufficient amount to develop the agricultural sector. Greenhouse production is car- ried out by taking advantage of favorable climate (air temperature, relative humidity and lighting) while keeping the operational cost Abbreviations: PVs, Photovoltaic array for straight-line shading; PVc, Photovoltaic array for checkerboard shading; APV, Agrophotovoltaic; HETS, Horizontal Earth at a minimum [1,2]. Heating and cooling systems are two major Tube System; PEM, Polymer Electrolyte Membrane; EAHE, Earth-to-Air Heat costs involved in greenhouses production. Heating is usually pro- Exchanger; LLP, loss-of-load probability; SWHS, Solar Water Heating System; NIR, vided by burning fossil fuels (diesel, fuel oil, liquid petroleum, gas) Near Infrared Radiation; SAH, Solar Air Heater; SATE, Surplus Air Thermal Energy; UWTE, Underground Water Thermal Energy; SAHLSC, Solar Air Heater With Latent which increase carbon dioxide (CO2) emission, or by using electric Heat Storage; PVWP, Photovoltaic water pumping; PV-GHP, Photovoltaic- heaters, which consume more energy [3]. Tong et al. [4] conducted Geothermal Heat Pump an experiment in Japan and reported that the hourly energy con- n Corresponding author. Tel./fax: þ86 871 65517266. E-mail addresses: [email protected] (R.H.E. Hassanien), sumption for heating from January to March in the greenhouse 2 [email protected] (M. Li). with heat pumps was in the range of 0.22 to 0.56 MJ m , while http://dx.doi.org/10.1016/j.rser.2015.10.095 1364-0321/& 2015 Elsevier Ltd. All rights reserved. 990 R.H.E. Hassanien et al. / Renewable and Sustainable Energy Reviews 54 (2016) 989–1001 heating with a kerosene heater was in the range of 0.42–0.76 the short waves of solar irradiation to enter the greenhouse. The 2 MJ m .Concurrently, the hourly CO2 emissions in the greenhouse materials inside the greenhouse then re-radiate these waves as with heat pumps were in the range of 9.5–24 g m 2, while that in infrared radiation (IR), which is then detained inside the green- the greenhouse with the kerosene heater was in the range of 31– house by the transparent cover materials [17–20]. The solar irra- 55 g m 2 . The prices of fossil fuels are rising rapidly [5] thereby, diation inside the greenhouse depends upon its orientation as the the major challenge for agricultural greenhouses is to find ways East-west oriented greenhouse is more efficient in collecting solar that improve energy efficiency combined with an absolute radiation in winter than in summer [21,22]. The energy con- reduction of the overall energy consumption and related CO2 servation in greenhouses could be improved by ‘‘Double thermal emissions of the greenhouse industry [6]. Energy is the backbone screen’’ and ‘‘Double glazing’’ with 60% reduction in energy of the modern world in terms of economic growth and solar demand. However, the highest improvement (80%) was observed energy is the main source of other renewable energies applied in by using a fully closed greenhouse without ventilation [23,24]. agricultural and industrial sectors. Subsequently, concerns over Solar cooling and solar heating applications in buildings have been energy security are mandating the use of green energy, such as intensively conducted and reviewed [25–35]. Nevertheless, certain solar energy sources, to reduce both CO2 emissions and heating studies have been implemented in agricultural greenhouses. Those costs. On the other hand, paying more attention to food, envir- studies were performed to develop the environmental control onment and energy is more urgent than ever for sustainable systems in greenhouses by thermal, dynamic and energy predic- greenhouse crop production. In the last few decades, solar energy tion modules to reduce energy consumption [36–40], a few has been developed intensively due to both technological applications have been conducted e.g. Yano et al.[41] developed a improvements and government policies supportive of renewable prototype of a microspherical semi-transparent solar module for energy development and utilization [7]. Nevertheless, solar energy greenhouse roof applications by using two modules. Firstly, they technologies have a relatively high initial cost; they do not require used 1500 spherical solar microcells (1.8 mm diameter, crystalline fuel, they have low carbon emission, long term solar resources, less silicon) with 15.4 cells cm 2 density in (108 mm 90 mm) area; payback time, and they often require little maintenance [8,9]. The 39% of the area of this module was covered with the spherical main purpose of greenhouses is to provide ideal growth conditions solar microcells and the remaining 61% was transparent to allow sustainable of microclimate for optimum plant growth and for the most sunlight to enter the greenhouse for promising plant early marketing of ornamental and vegetable crops [10] or early/ photosynthesis. Secondly, 500 cells with 5.1 cells cm 2 density year-round production. Most hot-season greenhouse vegetable when 30% of the area of this module was covered with the cells. plants grow rapidly at daily temperatures between 20–30 °C and They concluded that according to the annual electrical energy 14–18 °C at night. In those areas of the world where winters are production per unit of greenhouse land area, the semi-transparent cold, with ambient temperatures less than 0 ºC, heating systems PV modules were potentially suitable for greenhouses with basic are required to provide certain internal air temperatures for electrical environmental control systems in high-irradiation greenhouses [11]. Conversely, in tropical climates, where ambient regions where electricity production could be high and winter temperatures in summer exceed 40 °C, cooling systems are demand low. The experiences in the integration of PV and required to prevent crops in greenhouses from high air tempera- greenhouse carried out in South Eastern Spain have been inves- ture or the accumulation of heat during daytime higher than the tigated in a greenhouse roof with 9.8% coverage area by means of optimal level [12]. There are numerous interrelated parameters 24 flexible thin film PV modules. The results indicated that the that could influence the environmental control system in green- yearly electricity production normalized to the greenhouse ground houses such as the size of the greenhouse, its location, the type of surface was 8.25 kWh m 2. The effect of flexible solar panels covering material, heat storage method, quantity and quality of mounted on top of a greenhouse for electricity production on yield materials used, type of cultivation, desired day and night tem- and fruit quality of tomatoes has been also revealed that there perature of the inside air, and outside ambient conditions. The low were no differences found in terms of total or marketable pro- valued energy, which is delivered by sustainable energy sources duction under solar panels and control greenhouses [42]. Subse- such as heat pumps, solar collectors and energy storage, has been quently, solar panels did not affect the yield and price of tomatoes successfully used in heating and cooling systems and is still being despite their negative effect on fruit size and color. The simulation used in sustainable buildings [13,14]. As a consequence, the bar- of PV energy to predict its performance in greenhouses indicated riers to solar energy utilization in the agricultural sector require that the panel with two axes provided the best performance, and urgent attention and further research [15]. Recently, the conver- both sensors supplied their best returns in the case of a dark blue sion of cropland into PV plants has been increased. Thereby, sky. Moreover, the total satisfaction of the load was the only combining PV panels and crops on the same area unit of land determining factor in choosing the components of an installation could alleviate the increasing competition for land between food [43]. Van Beveren et al. [44] proposed an energy minimizing and energy production. Out of all the reviewed papers, this paper module to minimize the energy consumption of a commercial rose briefly reviewed and discussed numerous new and feasible tech- greenhouse especially for heating and cooling systems. They found nologies for solar energy applications in the environmental control that the energy saving potential as compared to the actual grow- of greenhouses microclimate and its irrigation systems. er's practice was substantial because of less natural ventilation on colder days and more natural ventilation and less heating on warmer days. Concurrently, relaxing the temperature and 2. Environmental control systems in greenhouses humidity decreases the energy input to the greenhouse.
Environmental control in greenhouses is meant to achieve 2.1. Heating systems indoor temperatures, relative humidity, light and CO2, which are as close as possible to optimal growth conditions for plants by It has been confirmed that the growth, yield and quality of using heating, cooling, ventilation, variable shading, and CO2 greenhouse plants were affected when temperatures were below enrichment and lighting systems as shown in Fig. 1. A greenhouse 12 °C or above 30 °C and the optimal temperature range is is a structure covered with transparent materials that utilize solar between 22 and 28 °C in the daytime and 15 20 °C at night [45]. radiant energy and provide optimum growing conditions for The structure of a greenhouse is usually not sufficient to keep the plants [16]. The cover materials (plastic, glass and fiberglass) allow inside air temperature at an appropriate level for the optimum R.H.E. Hassanien et al. / Renewable and Sustainable Energy Reviews 54 (2016) 989–1001 991
Fig. 1. Classification of various environmental control systems for agricultural greenhouses Sethi and Sharma with additional modification and permission of the copyright owner [11]. growth especially in cold climate. In addition, the shortage of was better than the performance of common heat pump heating heating systems has adverse effects on the yield, cultivation time, system [51]. The energy storage system was utilizing 4970 m3 of quality and quantity of the products in greenhouses; thereby, underground soil to store the heat captured by a 500 m2 solar auxiliary heating systems are required. Nevertheless, due to high collector in non-heating seasons through U-tube heat exchangers. relative cost of energy, only a small number of greenhouse owners It was found that solar energy covered all the heating loads will be able to use auxiliary heating systems. There are two types directly or indirectly and no auxiliary heating equipment was of agricultural solar greenhouses which utilize solar energy for installed. Concurrently, this system was capable of maintaining an heating purposes. Firstly, the passive greenhouses, which are uti- interior air temperature 13 °C higher than that of the ambient lized as collectors and designed for maximizing the solar heat temperature. In another study, a seasonal thermal energy storage gains by using a special cover and structure materials [2]. Sec- using paraffin wax as a Phase Change Material (PCM) with the ondly, the active greenhouses, which are equipped with solar latent heat storage technique had been investigated to heat a systems that utilize a separated collecting system from the greenhouse with a 180 m2 floor area [52] .The system consisted of greenhouse with an independent heat storage system, such as flat plate solar air collectors of 27 m2, latent heat storage (LHS) adding thermal energy inside the greenhouse from an air heating tank of 11.6 m3, experimental greenhouse, heat transfer unit and system in addition to direct thermal heating [36,46–48]. The free data acquisition unit. The LHS unit was filled with 6000 kg of energy from the sun can be used to heat the greenhouse by col- paraffin, equivalent to 33.33 kg of PCM per square meter of the lecting and storing heat in summer in a semi-closed greenhouse greenhouse ground surface area as shown in Fig. 2. PCM is a and in winter for heating. On top of that, it can be used to generate substance with a high heat of fusion which melting and solidifying electricity by integrating PV panels on the roof of the greenhouse at a certain temperature. It is capable of storing and releasing large [49]. It has been predicted by a simulation model that the monthly amounts of energy, PCMs are classified as latent heat storage (LHS) solar heating fraction may be correlated with a dimensionless units. Therefore, it was indicated that the average net energy and variable which involved mean daily solar radiation, total capture exergy energy efficiencies were 40.4% and 4.2%, respectively. factor of the greenhouse cover, and night-time heat load [50]. The It has been reported that using geothermal energy by earth- performance of a demonstrated 2304 m2 solar-heated greenhouse tube heat exchanger systems, heat pipe heating systems and equipped with a seasonal thermal energy storage system in China ground source heat pumps systems could reduce the heating loads 992 R.H.E. Hassanien et al. / Renewable and Sustainable Energy Reviews 54 (2016) 989–1001
Fig. 2. Heating greenhouse by solar air collectors with PCM (with permission of the copyright owner) [52]. by 20–70% at various locations [36,53]. The effective use of ground new solar air heater using a packed bed of spherical capsules with source heat pumping systems with suitable technology in the the latent heat storage system has been investigated in east–west modern greenhouses could play a leading role in the future for oriented greenhouses in Tunisia [60]. It was observed that the heating, cooling and dehumidification of greenhouses. Recently, nocturnal temperature inside the heated greenhouse was higher there is a composite system; same system could be used for than that of the conventional greenhouse by 5 °C. The relative heating the greenhouses in winter and cooling them in summer humidity decreased at night time inside the heated greenhouse [54,55]. Yang et al. [56] developed a heating and cooling system to from 10–20%. Therefore, the solar air heater system attained 31% of utilize surplus air thermal energy (SATE) and underground water the total heat requirements at nighttime and reduced CO2 emis- thermal energy (UWTE) as heat sources. The system consisted of a sions. Benli et al. [61] analyzed and discussed the thermal per- heat pump system including high and low temperature heat sto- formance of a PCM thermal storage unit consisting of a 10-pieces rage tanks, fan coil units (FCU) in the greenhouse, and an under- solar air collector for space heating of a greenhouse and charging ground water source. They successfully recovered and utilized of PCM as CaCl26H2O with a melting temperature of 29 °C. The hot SATE and UWTE for cooling and heating when operating this air delivered by ten-piece solar air collector passed through the system in February and March. On a clear day, the SATE and UWTE PCM to charge the storage unit then to heat the ambient air before were measured to be 878.2 MJ and 603.1 MJ, respectively. SATE being admitted to a greenhouse. They reported that the proposed and UWTE are natural and sustainable energy sources which save size of collectors integrated PCM provided about 18–23% of the fi energy and increase agricultural bene ts. Various renewable total daily thermal energy requirements of the greenhouse for energy sources (biogas, soil and sun) have been investigated for 3–4 h compared to the conventional heating device. Moreover, the heating a greenhouse in Turkey including a flat plate solar col- nocturnal temperature inside a greenhouse equipped with a solar lector with water antifreeze solution (% 20), aluminum plate, and air heater collector with latent heat storage and nocturnal shutter expansion tank [57]. Results illustrated that solar energy can be was higher than that of the outside temperature by approximately stored underground and then used to raise soil temperature and to 7 °C [62,63]. The solar air collector stores solar energy during heat biogas reactors. Moreover, solar energy system as a standa- daytime by reducing the internal temperature during the day and lone solution could be feasible with high storage temperatures. supplies it for heating at night. On top of that, It has been observed The efficiency of a photovoltaic-geothermal heat pump integrated that the temperatures of greenhouse air can be increased by system (PV-GHP) as a greenhouse heating system and a conven- around 7–8 ºC during winter season, when the photovoltaic/ther- tional hot air generator using liquefied petroleum gas (LPG-HG) mal (PV/T) operated and coupled with the earth air heat exchanger were compared in terms of environmental impact analysis in Italy [58]. The microclimatic conditions, the thermal energy produced (EAHE) at night [64]. Mehrpooya et al. [65] investigated the opti- and the electricity consumption were analyzed. Consequently, the mum performance of the combined solar collector and geothermal PV-GHP was preferable because it has a good impact on the heat pump system to meet the heat load for greenhouses in terms environment. Due to the plant with the PV-GHP reduces carbon of economic and technical points of view. Results indicated that fi emissions by 50%. Subsequently, to assess the sustainability of the the selected model has mean seasonal coef cient of performance geothermal heat pump plant, the estimated payback-time for of 4.14, borehole length of 50 m, and 3 boreholes. Moreover, the 2 energy and for carbon emissions were 1 year and 2.25 years, total collector area of the gained model was 9.42 m . Sonneveld respectively. A solar air heater (SAH) system was investigated et al. [66] studied the performance of a new type of greenhouse, experimentally for heating a greenhouse in Iraq [59]. Six solar air which combines reflection of near infrared radiation (NIR) with heaters with a single glass cover and a ‘V’ corrugated absorber electrical power generation using hybrid PV/T collector modules as plate connected in parallel were mounted on the greenhouse roof. shown in Fig. 3. In addition to the generation of electrical and The mass flux of air through the collectors was varied from 0.006 thermal energy, the reflection of the NIR resulted in improved to 0.012 kg s 1 m 2. An air mass flux of 0.012 kg s 1 m 2 was climate conditions in the greenhouse. The typical yearly yield of found to provide about 84% of the daily heat demand to keep the this greenhouse system was as a total electrical energy of inside air temperature of greenhouses at 18 °C during winter, SAH 20 kW h m 2 and a thermal energy of 160 kW h m 2. Thus, the could cover all the daily heating demand with an excess of heating system could operate independent of fossil fuels for this approximately 46%. Additionally, the thermal performance of a greenhouse. R.H.E. Hassanien et al. / Renewable and Sustainable Energy Reviews 54 (2016) 989–1001 993
Fig. 3. Greenhouse concept (A) with a spectral selective cylindrical mirror and a collector in the focal line (Yellow line indicates visual light and red line indicates NIR radiation). (B)The complete greenhouse (with permission of the copyright owner) [66].
Attar et al. [67] used a TRNSYS simulation to evaluate the the production of soil-less tomatoes in three different Italian areas performances of a solar water heating system (SWHS) for green- have also been investigated with computational aspects and houses according to Tunisian weather. The SWHS were two solar methods of the TRNSYS simulation [71]. It was observed that the collectors, with a total surface of 4 m2; a storage tank of 200 L and integrated PV roof saved energy in air conditioning for all three a capillary polypropylene heat exchanger integrated in the locations with an average of 30% for summer cooling and 11% for greenhouse. Results of simulation revealed that the temperature at winter heating. In another study, the energy performance of a the collector outlet decreases when the tank volume increases. solar PV system-assisted earth-to-air heat exchanger (under- Moreover, decreasing the exchanger inlet flow rate was a good ground air tunnel) for greenhouse cooling has been investigated. It solution to reduce heating losses. They suggested a flat plate col- was found that the average value of temperature difference lector and a 200 L tank as the best storage system to increase the between inlet and outlet of earth-to-air heat exchanger system inside air temperature of the greenhouse by 5 °C and provide (EAHE) was 8.29 °C. The total electric energy consumption of this suitable conditions for growing tomatoes. Additionally, Kıyan et al. system was 8.10 kWh by operating it for about 11 h/day, when [68] developed a mathematical model (Matlab/Simulink) to 34.55% of this energy demand was provided from PV cells [13].In investigate the thermal behavior of a greenhouse, which is heated an Indian study, Ganguly et al. [72] modulated and analyzed a by a hybrid solar collector system. This hybrid system contains an greenhouse-integrated power system consisting of solar PV panels, evacuated tube solar heat collector unit, an auxiliary fossil fuel electrolyzer bank and polymer electrolyte membrane (PEM) fuel heating unit, a hot water tank, control unit and piping units. This cell stacks. They found that 51 PV modules, each of 75 Wp along model has been developed to predict the storage water tempera- with a 3.3 kW electrolyzer and 2 PEM fuel cell stacks, each of ture, the indoor temperature and the amount of auxiliary fuel, as a 480 W, can support the energy requirement of a 90 m2 floriculture function of various design parameters of the greenhouse such as greenhouse with a fan-pad ventilated system. Whereas, under the location, dimensions and meteorological data of the region. Saudi Arabian arid climate, the solar radiation availability and the Results of simulations indicated that revising the existing fossil demand for electricity were studied and quantified to optimize the fuel system with the proposed hybrid system was economically PV greenhouse for desert climate operation [73]. The PV green- feasible for most cases and it has a positive environmental impact; house system consisted of the 14.72 kW PV arrays, a 3000 A h however, it requires a slightly longer payback period than battery storage system, a 15 kW power conditioning system and expected. data measurement collection system in a 9 m wide and 39 m length of fiberglass greenhouse. Results showed that the PV 2.2. Cooling systems greenhouse subsystem met the required load of the cooling and pumping equipment. Moreover, its performance was satisfactory. A greenhouse cooling system is used to decrease the accumu- In a Japanese study, a greenhouse side window ventilation con- lated heat energy from inside to outside the greenhouse by various troller, driven by PV energy, has been developed for saving power techniques such as ventilation (natural and forced), shading/ [74]. The side window controller was moved using power from a reflection, evaporative cooling (fan-pad system, mist/fog and roof combination of an amorphous silicon PV module of 0.078 m2 with cooling). Subsequently, a PV hybrid system for cooling a tropical rated maximum power of 3.2 W and a battery capacity of 28 A h in greenhouse has been investigated in Selangor, Malaysia [69]. This a 5 h rate (12V). The greenhouse side windows were well operated system included 48 solar PV Panels with 18.75 W for each, one according to the greenhouse temperature without outage. Conse- inverter, 1 charge controller and a battery bank (including 12 quently, the interior temperatures of the greenhouse were con- batteries). The load consisted of two misting fans for cooling a centrated between the side-vent opening and shutting tempera- greenhouse with 400 W electric power at a five hours daily tures. Therefore, PV power systems are applicable for a greenhouse operation. It was found that the PV system could be suitable in environmental control system. An experiment was performed by supplying electricity to cover the loads requirement demands Mongkon et al. [75] which investigated the cooling performance of without using extra energy from the grid. Moreover, the passive the horizontal earth tube system (HETS) when placed horizontally cooling system has been used as a low energy consuming tech- at a depth of 1m in loam soil under the lawn surface to the North nique. Consequently, solar chimney (SC) and underground earth to of the agricultural greenhouse under the tropical climate. They air heat exchanger (EAHE) have been used for the cooling and found that increasing tube diameter and air velocity could enlarge ventilation of solar houses [70]. The results revealed that the solar the coefficient of performance more than other parameters. chimney can be perfectly used to power the underground cooling Moreover, operating the HETS during day time with a clear sky system during the daytime, without any auxiliary electricity. could decrease the temperature and lead to better results in the Thereafter, the comparison of optical and thermal behavior of a greenhouse during summer months. On the other hand, the fea- solar PV greenhouse and a similar glass greenhouse, devoted to sibility for combining cooling and high grade energy production in 994 R.H.E. Hassanien et al. / Renewable and Sustainable Energy Reviews 54 (2016) 989–1001
Fig. 4. Schematic of potential solar PV cooling system (with permission of the copyright owner) [77].
Fig. 5. Schematic of potential solar thermal cooling system (with permission of the copyright owner) [77]. a new design for solar greenhouses has been investigated [76] as a different plants require different amounts of light for optimal solution for energy supply in winter and cooling in summer by growth. Plants are classified into high energy and long-day (LD) or applying seasonal storage of excess solar energy and exploiting low energy and short-day (SD) plants, depending on the intensity this for heating in winter. Meanwhile, the excess solar energy was of light they need. directly converted to high grade electric energy for heating. Supplemental lighting is essential to produce greenhouse crops Therefore, this system was cheaper with energy savings of about during cloudy, low-light days, to increase total daily light levels 35% compared to heating by furnace. during short days of natural light and to extend the day length or There are a number of solar driven cooling technologies such as photoperiod to initiate or prevent flowering. However, adding solar-PV-driven air conditioning technologies and solar-thermal supplemental lighting to a greenhouse is a relatively expensive systems. The PV-driven air conditioning uses a conventional vapor investment. Operational costs are relatively high, approaching or compression air conditioning cycle in which the electrical input is surpassing the cost of heating in some applications, and different provided by PV panels. The solar–thermal system has three dif- sources of light produce different amounts of light at different ferent types. The first is based on a solid desiccant. The second wavelengths. Nevertheless, supplemental lighting is usually not type of thermally driven system is absorption cooling and the required in greenhouses in many regions from May through third is the adsorption cycle [77, 78] as shown in Figs. 4 and 5. November. The minimum recommended light intensity for flor- iculture crops is 2 μmol m 2 s 1 (100 lux) at plant height. Fur- 2.3. Lighting systems thermore, the movable shade curtains can reduce the energy load in the greenhouse crop during warm and sunny conditions and Plant growth requires an appropriate light intensity. Exces- they reduce heat radiation losses at night. Under the sunny con- sively high or low intensity could disrupt photosynthesis in the ditions of winter seasons, the structures covered with shading plant. In addition, light not only has effects on plant morphology, materials are suitable for growing crops that can grow at photo- physiology and microstructure but also has an important impact synthetically active radiation (PAR) less than 150 W m 2. How- on the plant production, because plants produce food for them- ever, under cloudy conditions few plants can grow at PAR less than selves and others when plant use the energy from light to convert 30 W m 2 [85]. Therefore, integrating the transparent PV modules
CO2 and water into carbohydrates and oxygen [79–84]. Normally, on the roof of greenhouses not only decreases the energy load, but R.H.E. Hassanien et al. / Renewable and Sustainable Energy Reviews 54 (2016) 989–1001 995 also generates appropriate energy for artificial lighting in winter energy and crop production. Solar-LED was used alternatively to and forced ventilation in summer. Additionally, agrivoltaic or fuel-based lighting for night fishing in Tanzania [92]. Subsequently, Agrophotovoltaic (APV) is to produce agricultural commodities solar energy could be used in greenhouses for supplementary under the photovoltaic modules in open fields such as, potatoes, lighting at night for growing long-day floriculture crops. Yano et al. onions, vegetables, salads, herbs, oil plants, fruits (grapes, apple, [93] investigated the generated electrical energy of different tilt pear, cherry, etc.) and animals (sheep, chicken, cattle, etc.). Agro- angles for four amorphous-silicon PV modules arranged in an arch photovoltaic could protect the crops against high temperatures, or formation at the Northern end of the Gothic-arch style roof of a increased water availability for the crops if the rainfall is con- north-south oriented greenhouse. Two were attached to the inner centrated and infiltrated on a limited cropped area. The height surface of the East roof with tilted angles of 20° and 28°. The other level of the solar panels has no impact on the total quantity of two were attached to the inner surface of the west roof with tilted radiation available at the soil level but the low level has a very angles of 20° and 28°. The total area of PVs was 1.7 m2, which were large impact on the heterogeneity of radiation at ground level. 1.5% of the roof area as shown in Fig. 6B. The results illustrated that Agrivoltaic systems could maximize land use and productivity the low tilt angle module generates more electrical energy when it fi – ef ciently by a 35 73% under shading density of 50% from the was mounted inside the roof of a north–south oriented green- incoming radiation of PV panels [86,87]. Crops can achieve high house. Furthermore, Kadowaki et al. [94] used PV arrays to supply yield under the fluctuating shade of PV panels. Subsequently, electricity for a greenhouse. The PV array was mounted inside the under a dry Mediterranean climate, microclimate measurements South roof of an east-west oriented single-span greenhouse, with at crop level below PV panels indicated that these systems could a hydroponic Welsh onion (Allium fistulosum L.) as shown in contribute to alleviate climatic stress and to save water [88].PV Fig. 6C. Meanwhile, effects of PV array shading (straight-line and systems require land to intercept incoming solar radiation. Con- checkerboard of array distribution 12.9% of the roof area for each) sequently, the area required for both food and energy supply on the onion growth were investigated. The results showed that depends on land use benefits per square meter [89]. There are a the PV array for straight-line shading decreased the fresh weight number of opportunities for APV in the agricultural sector such as (FW) and dry matter weight (DW) of the Welsh onion comparing to protection from radiation stress, lower water demand, less salini- the checkerboard and control. Nevertheless, the electrical energy zation due to less irrigation, soil quality improvement, higher crop yields, different species adapted to lower radiation conditions can generated in the shading of greenhouses by PV arrays was more fi be cultivated, existing PV structures can be used for shading, bene cial than that of the control greenhouse. Concurrently, the product diversification for farmers and higher income security. distribution of sunlight energy in an east-west oriented single-span Moreover, APV can be used for water desalination of groundwater greenhouse equipped with a PV array (12.9% of the roof area) inside in the poor land [90]. Cossu et al. [91] investigated the effects of a Gothic-arch style roof and the electrical energy generated by the shading from the PV array on tomato plants in an east-west PV array were studied [95] when the PV array was straight-line (PVs oriented greenhouse of which 50% of its roof area was replaced array) and checkerboard (PVc array) with 30 PV modules facing the with PV modules. Meanwhile, they integrated the natural radia- southern sky for each. It was found that under the assumption of a tion with supplementary lighting powered by PV energy as shown cloudless sky, the PVs and PVc arrays are estimated to generate 1 1 in Fig. 6A. They found that PV array reduced the availability of electricity of 4.08 GJ y and 4.06 GJ y , respectively. Additionally, solar radiation inside the greenhouse by 64%. However, the sup- the checkerboard arrangement improved the unbalanced spatial plementary lighting, powered without exceeding the energy pro- distribution of received sunlight energy in the greenhouse. More- duced by the PV array, was not enough to affect the crop pro- over, a greenhouse shading screen control system has been devel- duction, whose revenue was lower than the cost for heating and oped and operated by a stand-alone power system with 4.8 W lighting. Therefore, designing PV greenhouses is useful for both amorphous silicon PV module and a battery of 28 A h capacity in a
Fig. 6. Integration of PV panels with greenhouses .(A) 50% of its roof area covered with PV in Italy [91]. (B) 1.5 % of the greenhouse roof area, (C)) the PV array covered 12.9% of the greenhouse roof area. B and C, respectively in Japan (with permission of the copyright owner) [93–95]. 996 R.H.E. Hassanien et al. / Renewable and Sustainable Energy Reviews 54 (2016) 989–1001
5hrate[96]. It was observed that the capacity of this system was 3. The photovoltaic water pumping in irrigation systems sufficient for shading screen operation In recent years, Liu et al. [97] reported that the Chinese gov- Historically, solar water pumps have not been widely used in ernment has supported many projects for the applications of the greenhouses. Nevertheless, the utilization of PV systems in all irri- thin film PV solar greenhouses at different provinces in China such gation applications has increased in the last decades especially in as Hebei project in an area of 6667 hm2 for generating electric remote and desert areas, where no reliable electricity supply is power of 120 MW Fig. 7. available. The studies of PV-water pumping systems related to fl The intensive study of solar energy applications in greenhouses irrigation applications were extensively reported and the water ow rate has been predicted to achieve an efficient management of worldwide showed that cooling and lighting systems were con- water demand in remote and desert areas [101–110],andthewhole ducted by the stand alone/ integrated PV technologies and both possible methods for developing this system have been reviewed systems were more viable in hot climates than in moderate cli- [111] as shown in Figs. 8 and 9. The prediction of water flow rate by mates. Using PV on the roof of greenhouses for shading in hot theoretical and experimental analysis in a PV water pumping sys- climate not only decreases the inside air temperatures of green- tem has been investigated for achieving the optimal sizing of houses and the high solar radiation, but also generates energy for PV-based water pumping system and optimum utilization of solar operating the cooling equipment. On the other hand, a heating energy and water management in remote and desert areas. It was system was applied by the integrated systems of flat plates or indicated that developed models can predict accurately the hourly evacuated tubes as solar heaters and a hybrid PV cells with ther- flow rate based on measured hourly air temperature and solar irra- mal collectors as shown in Table 1. diation [103,112–115]. On the other hand, It was reported that the PV
Fig. 7. Photos of solar energy applications in agricultural greenhouses. a. Thin film PV solar glass greenhouses (with permission of the copyright owner) [97], b. solar collector of evacuated tube, China.
Table 1 Solar energy applications in the agricultural greenhouses worldwide.
Environmental control systems in Solar energy applications Country References greenhouses
Heating and cooling A semi-transparent PV modules Japan [41] A flexible thin film PV modules (9.8 % of the roof area) Spain [42,98] Heating Seasonal thermal energy storage China [51] A flat plate solar air collectors with the latent heat storage technique Turkey [52] Earth-tube heat exchanger systems ( thermal energy) India [53] A flat plate solar collector with water antifreeze solution (% 20); aluminum Turkey [57] plate A Photovoltaic-geothermal heat pump integrated system (PV-GHP) Italy [58] Solar air heaters on the roof Iraq [59] Solar air heater using a packed bed of spherical capsules with the latent heat Tunisia [60] storage system a phase change thermal storage unit consisted of ten pieced solar air Turkey [61] collectors Hybrid PV/T collector Netherlands [66] Cooling PV hybrid system Malaysia [69] A solar flat-plate collector Morocco [99] Solar chimney (SC) and underground (EAHE) Iran [70] PV greenhouse Italy [71] A greenhouse-integrated power system with India [72] PV-greenhouse Saudi Arabia [73] Side ventilation driven by PV energy Japan [74] HETS bedded underground of 1 m Thailand [75] Seasonal storage Netherlands [76] Lighting and cooling by shading APV in open field for shading Germany , Italy and France [86,87,100]. PV array (50% of the roof area) Italy [91] PV array (1.5% of the roof area ) Japan [93] PV array (12.9% of the roof area) Japan [94,95] A stand-alone PV for shading screen control Japan [96] Thin film PV solar glass greenhouses China [97] R.H.E. Hassanien et al. / Renewable and Sustainable Energy Reviews 54 (2016) 989–1001 997
Fig.8. Schematic diagram of PV water pumping system (with permission of the copyright owner) [110].
Fig. 9. Layout of PV irrigation system (with permission of the copyright owner) [106, 107]. array and the storage tank are the most important elements for the desert areas to increase the reclamation of agricultural lands and to optimal designing of the PV water pumping [105,116]. The loss-of- maximize the utilization of solar energy, as well as to guarantee the load probability (LLP) of a PV water pumping system has been esti- sustainability of food and energy production. mated by drawing LLP maps with normalized parameters using long The performance of the directly coupled PV water pumping term observed or generated sequences of meteorological data, which system has been investigated at different conditions of varying provides a generalized and practical graphical tool for systems sizing solar irradiance values and two static head hydraulic system [117]. Therefore, a poor design of the PV array and/or the storage tank configurations [110]. The system consisted of a 1.5 kW PV array, DC could affect system reliability and create a deficit of daily water motor and a centrifugal pump as shown in Fig.8. It was observed demand for the population. On the other hand, the effect of pumping that the system is suitable for low delivery flow rate applications headonsolarwaterpumpingsystembyusingtheoptimumPVarray and it was a low cost design, without any complex electronic configuration, adequate to supply a DC Helical pump with an opti- control and auxiliary systems. The performance of four sub- mum energy amount, under the outdoor conditions of the Madinah mersible PV water pumping systems in Tunisia has been investi- site in Saudi Arabia has been conducted [118]. The experimental gated [120] and results showed that the maximum overall effi- results revealed that the efficiency increased with the increasing of ciency of the system was 3.7% and the mean efficiency of the solar radiation until the pump reached their maximum power. Fur- whole system was 2.5% at constant head pumping. Whereas, the thermore, the system efficiency increased with decreasing pumping potential of solar irrigation systems for sustaining pasture lands in heads during low solar radiation, and the optimum pumping head arid regions in China [121] reported that PV pumping could cover corresponds to the best average system efficiency at the head of about 8.145 million ha appropriately for the grasslands. 80 m. A solar water pumping system has numerous advantages; for example, besides no cost for fuel and maintenance, it has no noise and pollution for the environment. Meanwhile, many parts of the 4. Economic analyses world are rural in nature and consequently do not have electrical distribution lines in many parts of villages, farms, and ranches. The economic analyses include the initial costs, energy cost and Thereby, using small scale applications of PV powered water payback period in comparison with the expected service life cycle pumping is more cost effective in these areas and in its greenhouses of any solar system installation [122–125]. Subsequently, to size [119]. Recently, the Governments of some developing countries such the solar energy for cooling, heating and lighting systems, a as Egypt started to support the applications of solar energy in the number of parameters should be considered such as the energy 998 R.H.E. Hassanien et al. / Renewable and Sustainable Energy Reviews 54 (2016) 989–1001 consumption for a single system and equipment for environmental the best overall performance. Furthermore, the integrated photo- control, the required air temperature and relative humidity inside voltaics in greenhouse roofs have also been presented, in terms of the greenhouse, ambient temperature, and daily use. Conse- energy generated, shading effects on plants, and the environ- quently, in order to evaluate the economic analysis, the solar mental impact by decreasing CO2 emissions. energy applications in the environmental control systems of Subsequently, studies from various regions of the world have greenhouses should be compared to conventional systems. For conducted many experimental applications and theoretical inves- example, the heating cost in Tunisia and payback times of solar air tigations of solar energy on the agricultural greenhouses. Thus, heater with latent heat storage collectors and the conventional different types of solar application systems in the environmental systems revealed that the payback period of solar air heater with control of greenhouses were discussed. The previous studies latent storage was 5 years if the system operates only 3 months revealed that PV systems and/or solar thermal system would be per year as shown in Table 2. Furthermore, it has been reported the suitable options in greenhouse application especially for that PV module prices have had dramatic reductions in the past remote and desert areas. Moreover, solar technologies are envir- decade by 80%, while the prices for competing gasoline or diesel onment friendly, low maintenance, have no fuel cost, and increase fuel have been increased by over 250% [126]. PV water pumping the overall land productivity by integrated PV panels in agri- technology is the most cost-effective for steady pumping needs cultural land or greenhouses such as APV systems. The decrease of such as community water supply or livestock watering, and it is PV module prices will make the PV greenhouses and PV powered considered as a sustainable and economical solution to provide water pumping systems more feasible in the near future and it can water for land irrigation [127]. In a word, the clean environmental pay back the initial cost in a short time. Climate changes increase impact of solar energy could also be considered as a great benefit. the energy consumptions in greenhouses along with the increas- The economic study of solar thermal cooling systems which ing of the conventional fuels prices (coal and diesel). Canakci et al. covered most climatic regions worldwide showed that solar [5] estimated the heating requirements and their costs for a gothic thermal cooling is more viable in hot climates than in moderate roofed and coal heated, plastic model greenhouse located in an European climates. The specific costs per kWh cooling in German area with 1 ha representing modern greenhouses in Turkey. They locations vary between 0.25 and 1.01 €/kWh, in Spanish locations reported that the total annual heating requirement was between between 0.13 and 0.30 €/kWh. In hot climates like Jakarta and 3,592,848 and 10,459,688 MJ/ha. The calculated total annual and Riyadh, the specific costs were as low as 0.09–0.15 €/kWh. Fur- hourly costs per ha were 65,891.5–151,220.6 $/year and 23.8–34.2 thermore, the maximum investment costs were calculated and got $/h, respectively. Whereas, the energy requirements for seasonal a payback time of 10 years [128]. Fathoni et al. [129] studied the heating (October 15th to March 15th) at Turin and Remo, in Italy technical and economic potential of solar energy application in ranges from 134 kW h m 2 to 209 kW h m 2 which included the Indonesia. They found that Makassar got the shortest payback losses of air infiltration and the heating cost can be valued into period for 11 years and Banjarmasin got the longest payback 15.7 € m 2 for Turin and 10.5€ m 2 for Sanremo [8]. Furthermore, period of 17.6 years. Similar results were found in Spain which in [4,6,8,41] a collected literatures survey of annual electrical indicated that the payback time to return the investment capital of energy consumption per unit greenhouse area at different loca- integrated PV panels on greenhouses would be about 18 years tions with electrical load as shown in Table 3 reported that energy [42]. Furthermore, solar energy application can reduce the consumption deviated from (0.1–528 kW h m 2 yr 1) according greenhouse gas emission by up to 243,252 tons per year in par- to the location of greenhouses and the level of applied technology, ticular selected locations. In addition, the payback period of a e.g. for maintaining the optimal microclimate conditions, cooling mixed mode solar greenhouse dryer with forced convection, used and heating systems are required. Therefore, solar energy appli- to dry red pepper and sultana grape in Tunisia, was found to be cations in agricultural greenhouses should be given a high priority 1.6 years, which was significantly less than the estimated life of in future studies to reduce energy consumption. Nevertheless, the the system (20 years) [130]. solar energy technologies in agricultural greenhouses have sig- nificant advantages; a lot of challenges are remaining. Firstly, PV water pumping operation and maintenance, as well as the dis- 5. Conclusion and challenges of solar energy in greenhouses tances to the grid, the water elevation, and water storage, are critical factors when determining the economic feasibility of PV This paper has reviewed state-of-the-art solar energy applica- pumping technologies [131,132]. tions in agricultural greenhouses, with the focus on the environ- Secondly, the high humidity rates and the agrochemicals could mental control systems, particularly heating, cooling, lighting and affect the long-term life of the PV arrays integrated on the irrigation systems. A variety of solar energy heating systems have greenhouse roofs. Additionally, the variability of the microclimatic been discussed, including solar air heaters and solar thermal col- conditions in PV greenhouses, particularly the reduction of light lectors. Among solar heating systems, the solar air heaters show and solar radiation under PV panels could also have an effect. Since shading not only influences the amount of light received by Table 2 plants but also changes microclimatic conditions, such as air and Comparison of the cost and the payback times between solar air heating and the ground temperature, humidity and CO2 concentrations, which are fuel boiler [60]. important factors for plant growth [137,138]. Subsequently, to integrate PV arrays with greenhouses, a specific and new PV System cost ($) Hybrid systems Conventional systems (SAHLSCþFuel boiler (Fuel boiler) greenhouse design or adaptation of existing structures are required. It has been reported that the position and orientation of Operation cost ($) 3600 1800 PV modules on greenhouses roofs must be considered carefully to Maintenance cost ($) 4.75 116.55 provide a sufficient electrical energy with minimum shading of Annual energy 150 150 – Consumption (kWh) 1200 3000 plants [93 95]. Consequently, for a north-south oriented green- Gain of energy (kWh) 1800 – house in the Northern hemisphere, a PV module mounted on the Energy cost ($) 259.2 651.25 North end of the greenhouse roof casts the smallest shadow in the Total cost ($) 4014 2717.8 greenhouse. Whereas, for an east-west oriented greenhouse, the Payback of the system 5 – South-facing roof is suitable for generating electricity by PV (year) modules, but the effects of shading by the PV modules will be R.H.E. Hassanien et al. / Renewable and Sustainable Energy Reviews 54 (2016) 989–1001 999
Table 3 Annual electrical energy consumption per unit greenhouse area at different countries with electrical load (after [41] with additional data).
Location Electrical load Annual electrical energy consumption per unit References greenhouse area (kW h m 2 yr 1)
Mediterranean heating, cooling, ventilation 2–9 [133] Spain window operation, pumps fans, irrigation and fertilization equipment, fuel burner, 3 (for only 8 months ) [42] Spain window-opening and screen motors, automatism for climate control, compressor, 7 [134] electric resistance of the fuel reservoir Greece ventilation, cooling, lighting 20 [135] Saudi Arabia fans, cooling pump, PC 56 [73] Sweden ventilation, pumps, lighting and other devices 140 [136] Finland Heating, cooling/ ventilation, lighting and other devices 527.78 [6] Netherlands Heating, cooling/ ventilation, lighting and other devices 416.67 [6] southern France Heating, cooling/ ventilation, lighting and other devices 138.8- 444.44 [6] Japan Heating 0.1 - 0.2 (for 2 months) [4] Italy Heating 134 - 209 (for only 5 months) [8]
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