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Recent developments in greenhouse solar drying: A review
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Recent developments in greenhouse solar drying: A review ⁎
Pushpendra Singha, , Vipin Shrivastavaa, Anil Kumarb,c a b c
Department of Mechanical Engineering, Lakshmi Narain College of Technology, Bhopal, India Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand Department of Energy (Energy Centre), Maulana Azad National Institute of Technology, Bhopal 462 003, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Active greenhouse Passive greenhouse Drying Solar energy Solar dryer
The world is moving towards the reduction of dependency on fossil fuels. Various innovations are undergoing to make the use of sources of renewable energy like wind, solar, tidal etc. Among these sources, solar energy is available in enormous quantity and best option that may be used for space heating and generation of electric energy. For drying agricultural and non-agricultural products, solar energy can be used directly or indirectly. But in open sun drying the products are affected by external calamities such as rain, insects and animals. To overcome the shortcomings of open sun drying various greenhouse solar dryer had been proposed so far. Some of recent researches have been discussed in the paper. This paper provides the data of already developed greenhouses so the reader can develop new and modified the greenhouse structure. To accomplish this, various researches of recent years have been studied and presented in this review.
1. Introduction Increasing population is the major problem of the entire world. Increase in the population increases the consumption of food. To fulfill this demand either that amount of food must be produced on a regular basis or produced food can be stored after some processing. Therefore, continuous production is not possible but food can be stored for a certain period by drying it. Drying is the phenomenon of reducing moisture up to a safe limit. Solar drying is the ancient method that is practiced everywhere for crop preservation. Solar drying is an effective method of utilizing energy of sun [1]. Drying of agricultural products leads to counteract the activity of various microscopic organisms. The crop after drying can be stored for a longer time without any fear of getting deteriorated [2]. The dried crop has various advantages like enhanced product quality, longer safe storage time and low post-harvest losses [3]. Heat transfer in the form of conduction, convection, thermal and radiation plays a vital role in solar drying process [4]. In open or natural sun drying, the crops are laid simply on the floor or mat in the full sunny days. As the crops being exposed directly to the sun, it gets contaminated from dirt and pest infestation and also lost by birds and beast. Researchers developed various drying techniques like spray, mechanical, electrical, solar drying etc. Such drying techniques are using around the world for drying of agricultural and non-agricultural products. Among these dryers, greenhouse solar dryer has various advantages over other types which make it a good alternative [1]. These dryers not only reduce the consumption of fossil fuels for drying purpose, but also provide the best
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quality, color and taste of the dried products [5]. The modern solar drying equipment uses optimum energy and time and occupies less area for producing better quality dried products with almost zero energy cost [6]. The working of greenhouse solar dryer is based on the principle of greenhouse effect. It allows incoming short wavelength solar radiations from the sun and traps the long wavelength solar radiations. Greenhouse dryers are used for crop cultivation, poultry, aquaculture, soil solarisation, and crop drying [7]. Greenhouse basically operates either in passive (natural convection) or active mode (forced convection) [8]. Fig. 1 shows the classification of greenhouse solar dryer on the basis of our literature review. In the passive mode of greenhouse dryer, ventilator or chimney is provided at the chimney for the natural circulation of air entering inside the dryer. While in case of the active dryer, exhaust fan is provided for moving humid air outside the dryer. Various modifications and researches have been done to improve the greenhouse dryer's performance. Some of the modifications implemented on active and passive greenhouse solar dryers in literature are:
• PV integrated greenhouse solar dryer [1,8,9,11,15,17,20–22, 24,26,31,34,36]. • Used opaque northern wall to insulate it and prevent heat loss [12,13,16,18,22,32]. • Used thermal storage materials such as sand, rock-bed, black •
painted concrete floor and PVC sheet. So that greenhouse can be used during off sun-shine period [1,12,28,34,37]. Inclined and reflecting north wall to collect maximum radiations [10].
Corresponding author. E-mail address: [email protected] (P. Singh).
http://dx.doi.org/10.1016/j.rser.2017.10.020 Received 14 April 2017; Received in revised form 6 September 2017; Accepted 26 October 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Singh, P., Renewable and Sustainable Energy Reviews (2017), http://dx.doi.org/10.1016/j.rser.2017.10.020
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Fig. 1. Classification of greenhouse solar dryer on the basis of our literature review.
2. Researches on greenhouse based solar dryer Janjai et al. [1] developed parabolic shaped PV ventilated greenhouse solar dryer with a black concrete floor. The Dryer was installed at Solar Energy Research Laboratory, Silpakorn University at Nakhon Pathom, Thailand. Fig. 2 shows the developed greenhouse dryer. The dryer floor area was 44 m2 and was enveloped with polycarbonate sheets. A 53 W rating solar panel was provided to run the 3 DC fans. Dryer was loaded with 150 kg of fresh chillies to investigate its performance. The result shows that the chillies had been dried from 80% moisture content to 10% (wet basis). The drying period varies from 2 to 1 3 2 days while in open sun drying it takes 6 days. Kumar and Tiwari [2] investigated the convective mass transfer coefficient with respect to change in mass of onion flakes in a greenhouse dryer installed at IIT Delhi, India. Three different weights 300 g, 600 g and 900 g of onion flakes were dried continuously for 33 h both in open sun and in the greenhouse. Experimental setup for drying onion under open sun drying, natural and forced convection mode is shown in Fig. 3. The even span roof type greenhouse dryer had a floor area of 1.2 × 0.78 m2 and was enveloped with UV film. The result shows that, for different modes of drying convective mass transfer of onion flakes
Fig. 2. The developed greenhouse solar dryer.
• Using greenhouse coupled with solar air heater to achieve faster drying [30,39]. • Provided additional area enhancing panels to increase drying area [35].
Fig. 3. Experimental setup for onion drying in (a) natural and (b) forced convection mode.
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behavior of green house integrated with PV/T collector. Fig. 5 shows the proposed experimental set-up installed at solar energy park, IIT Delhi campus. The 8 PV modules of 75 W rating were mounted on the wooden structure. One fan of capacity 12 W is fitted to maintain air flow rate inside a green house. Power generated by the solar module is stored in set of 12 DC batteries of 6 V rating each and an inverter of rating 2.1 KVA for converting DC to AC. Exergy efficiency (hEX) of 4% was observed for this PV/T integrated greenhouse. Janjai et al. [9] presents experimental and simulation performance by drying peeled longan and banana inside a PV integrated greenhouse dryer. The concrete floored parabolic roof type dryer having dimensions 8 m × 5.5 m × 3.5 m is enveloped with polycarbonates plates. PV module of capacity 50 W is used to run 3 fans, provided to maintain the required ventilation. The proposed setup with the positions of various instruments fitted over it is shown in Fig. 6. During drying of banana and longan, temperature varies from 30 °C to 60 °C and 31 °C to 58 °C respectively. Drying time of Longan and banana inside greenhouse were only 3 days and 4days respectively. While drying of same under open sun takes 5 days and 6 days respectively. Comparison of results shows that better color and taste were observed in greenhouse dried product. Sethi and Arora [10] modified the conventional even span type greenhouse solar dryer by using inclined reflecting north wall. The inclination angle of north wall has been optimized for tray having different width. Under natural and forced mode of drying, experimental performance of greenhouse dryer was tested in with and without inclined north wall by drying bitter gourd slices. The dryer was located in Ludhiana, Punjab, India. Outside and inside pictorial view of the improved greenhouse dryer along with inclined reflective north wall is shown in Fig. 7. Dryer floor area was 6 m × 4 m and enveloped with UV stabilized polyethylene sheet. North wall of 12 mm wooden ply board was fitted with an aluminized polyester sheet. Results showed that by using inclined north wall in natural and forced convection mode, 13.13% and 16.67% of the total drying time was saved respectively. Ganguly et al. [11] demonstrated a model of greenhouse combined with power generation and storage system. Study reveals that the integrated power arrangement gives a feasible option to power standalone greenhouses in a self-sustained way. The proposed integrated system for greenhouse application is shown in Fig. 8. The 51 solar PV modules of 75 W each were used to generate electricity. Two 480 W PEM fuel cell systems and combination of electrolyzer of 3.3 kV each were used for power back up. Some of the power generated by PV modules is utilized by greenhouse appliances and extra energy left is utilized in generating hydrogen gas by an electrolyzer. PEM fuel cell
Fig. 4. Hybrid greenhouse solar dryer integrated with PV/T module.
Fig. 5. Installed greenhouse at solar energy park, IIT Delhi.
increases by 30% − 135% with change in the mass from 300 g to 900 g. Barnwal and Tiwari [3] constructed a 30 ° inclined Roof type even Span hybrid PV/T greenhouse dryer of 100 kg capacity under forced mode at solar energy park, IIT Delhi, India. Hybrid PV/T integrated greenhouse dryer is shown in Fig. 4. The setup consists of two PV module of 75 W each and DC fan for air movement, have a dimension of 2.5 m × 2.6 m × 1.05 m. The air enters from bottom and passes through 3-tier system of perforated wire mesh trays and leaves from top. The structural frame was enveloped with UV stabilized polyethylene sheet to trap infrared radiation. Convective heat transfer coefficient of two types of grapes GR-I and GR-II was evaluated and compared to PV/T dryer in both open and greenhouse conditions. Nayak and Tiwari [8] did energy and exergy analysis to predict the
Fig. 6. Proposed setup with the positions of various instruments fitted over it.
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Fig. 7. (a) Outside and (b) inside pictorial view of the improved greenhouse dryer with (c) inclined reflective north wall.
required humidity and temperature inside the dryer. In a drying time of 4–5 h, moisture content had been reduced from 53.85% to 9.96%. Rathore and Panwar [13] built a semi-cylindrical solar tunnel dryer at College of Dairy and Food Science Technology, Udaipur, India. Performance of dryer was evaluated by drying of 320 kg of seedless grapes in a temperature of about 10 – 28 °C. In 7 days of drying period, moisture content was reduced from 85% to 16%. The schematic view of natural convection solar tunnel dryer is shown in Fig. 10. The setup was built of hemi-cylindrical metallic frame enveloped with UV stabilized polyethylene sheet. Exhaust fan and chimney is provided to maintain air flow rate. To prevent heat loss, bottom and northern wall was insulated. Almuhanna [14] constructed a gable even span type greenhouse dryer for the drying of dates at King Faisal University, Saudi Arabia. The aim of research was to evaluate thermal performance and feasibility of the dryer. Photographic view of solar air heater is shown in Fig. 11. An aluminum framed structure having dimension 2 m × 1 m × 1.2 m is enveloped with fiber glass and had a roof inclined at 30° to collect maximum solar radiation. Two plywood dryer boxes having dimension 1 m × 1 m × 0.3 m were placed in the drying area. The axial fan was fitted to maintain required air flow rate. During experimental period of dryer, the overall daily average thermal efficiency was 60.11%. Janjai [15] developed and investigated parabolic roof type greenhouse dryer for small-scale dry food industries at Nakhom Pathom in Thailand. The pictorial view of greenhouse dryer integrated with LPG burner is shown in Fig. 12. The setup had a dimension of 8 m × 20 m × 3.5 m with the loading capacity of 1000 kg. For supply hot air during off-sunshine period, 100 kW LPG burner was incorporated with it. This dryer is powered and ventilated by three 50 W PV module and 15 W DC fan. During drying of tomatoes, drying temperature varied between 35 to 65 °C and observed drying time was 2–3 days shorter than natural sun drying. Adu et al. [16] designed and built a tent type solar dryer at Nigeria. The drying tent roof frame work with transparent roof sheet is shown in Fig. 13. Drying area of the solar dryer was 29.6 ft2. The drying platform along with two long side walls are completely enveloped with a black cloth while remaining two walls are half enveloped with black cloth. The roof is enveloped with transparent polythene. The performance of dryer is obtained by drying okra. At the temperature of about 50 °C, 86.05% of initial moisture content of okra was reduced to final moisture content of 3.43% within 23 h. Kaewkiew et al. [17] evaluates a drying performance of parabolic shaped greenhouse dryer at Ubon Ratchthani, Thailand. Installed largescale polycarbonate enveloped greenhouse dryer is shown in Fig. 14.
Fig. 8. Integrated systems for greenhouse application.
Fig. 9. Semi-cylindrical shape solar tunnel dryer.
stack consumes that hydrogen to generate power for energy deficit hours. Sevda and Rathore [12] constructed a solar tunnel dryer of semicylindrical shape at M/s Cellulosic Waste Recycling Education Project, Vidya Bhawan Society, Rajasthan, India. The performance of the dryer was analyzed by drying handmade papers. Photographic view of developed solar tunnel dryer is shown in Fig. 9. The dryer floor area was 71.25 m2 and height 3.75 m and was enveloped by UV stabilized polyethylene sheet. The dryer had a capacity of drying 1500 papers/ batch. Exhaust fan and five chimneys were provided to maintain the 4
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Fig. 10. Natural convection solar tunnel dryer.
Fig. 11. Photographic view of solar air heater. Fig. 13. Drying tent roof frame work with transparent roofing sheet.
Fig. 14. Installed large-scale polycarbonate covered greenhouse dryer.
Fig. 12. Greenhouse dryer integrated with LPG burner.
drying. Reduction in drying period was observed along with better taste, color and pungency. Panwar et al. [18] constructed three semi-cylindrical solar tunnel dryers at M/s Cotton Product of India, Udaipur. Surgical cotton was dried in the dryer to carry out its thermal modeling and energy-exergy analysis. Fig. 15 shows the experimental setup of installed solar tunnel
The dryer enveloped with polycarbonate sheet has concrete floor area of 160 m2. PV modules of capacity 50 W were used to power 9 DC fans, provided to maintain required air circulation. To evaluate the performance of solar greenhouse dryer 500 kg of chillies were dried inside it. Moisture content has been reduced from 74% to 9% (wb) in 3 days in greenhouse solar dryer in comparison to 5 days taken by natural sun 5
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Fig. 15. Experimental solar tunnel dryer.
Fig. 17. Hybrid greenhouse dryer integrated with PVT.
dryers. The dryer was enveloped with a UV stabilized polythene sheet and was provided with the opaque insulated north wall. The dryer had a floor area of 67.5 m2 and loading capacity of 600 kg. Moisture content of surgical cotton had been reduced from 40% to 5% (wb) in one day. Unknown equations were computed by MATLAB 2010a. Experimental energy efficiency was found lower and experimental exergy efficiency was found higher than their predicted values. Kumar et al. [19] carried out performance evaluation of roof even type greenhouse dryer under no-load condition in both natural and forced mode. Experimental setup of dryer for natural and forced mode is shown in Fig. 16. Dryer floor area was 1.50 × 1.01 m2 and was enveloped with polycarbonate sheet of 3 mm thickness. Two vents of diameter 0.01 m were provided at the bottom of the dryer for air inlet under natural mode. Fan was attached to the sidewall for forced convection. Results reveal that forced convection increases the efficiency of the dryer by 2% as compared to natural mode of drying. Also, drying rate in forced convection is 31% higher as compared to natural convection. Nayak et al. [20] dried and tested mint leaves in PV integrated greenhouse solar dryer installed at Solar Energy Park, IIT Delhi, India. A pictorial view of hybrid solar dryer integrated with PVT is shown in Fig. 17. Dryer floor area was 2.5 m × 2.6 m and was enveloped with UV stabilized polythene sheet. The dryer had east west orientation and 30° roof inclination. Three DC fans were provided for forced convection. The moisture content had been reduced from 80% to 11% in 21 h while in open sun drying it takes 26–27 h. The dried products are converted into powdered form and then tested. The results obtained
shows that the calorific and nutritional values were retained along with the original color. Prakash and Kumar [21] built and tested the modified greenhouse solar dryer at Maulana Azad National Institute of Technology, Bhopal, India. Experimental set up of modified greenhouse solar dryer is shown in Fig. 18. The performance of experimental setup was evaluated without any load under two conditions- floor enveloped with a black PVC sheet and without enveloped floor. The dryer was tested under forced convection, provided with the help of 12 V DC fan powered by 6 W PV modules. The aim of the testing is to determine the full potential of dryer so as to dry the high moisture content crops. High temperature inside the greenhouse and decrease in relative humidity was observed in case of covered floor. Prakash and Kumar [22] developed a modified active greenhouse solar dryer with opaque northern wall and tested it with two conditions (i) black PVC sheet covered concrete floor and (ii) uncovered concrete floor. Schematic view of modified solar active dryer with and without covered inside floor is shown in Fig. 19. The 12 V DC fan run by 6 Solar cell (each of 1 W), placed in two rows with 3 cell in each row was used. Power from the solar cell is used to charge the battery. The battery gives power to operate DC fan. Concrete floor enveloped with a black PVC sheet gives a higher increase in temperature and reduction in humidity as compared to an uncovered floor. Aritesty and Wulandani [23] manufactured a rack type greenhouse solar dryer and obtained its performance by drying wild ginger in three conditions- no load and loaded with two different capacities 21 kg and
Fig. 16. Experimental setup of greenhouse dryer for (a) natural and (b) forced convection.
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Fig. 18. Modified greenhouse dryer in (a) without covered floor and (b) with covered floor.
Fig. 19. Modified greenhouse dryer (a) without covered floor and (b) with covered floor.
Nagarajan and Premkumar [26] designed and manufactured a parabolic shaped solar tunnel dryer enveloped with polycarbonate plates. A pictorial view of solar tunnel dryer is shown in Fig. 23. To investigate the performance of dryer, 1 kg of grapes were dried in drying area of 108 m2 inside a tunnel. DC fan used to circulate the air was powered by 15 W flat plate solar cell modules. In this dryer, Grapes were dehydrated from 75% to 7% moisture content within 2–3 days in a temperature range of 35–75 °C. Ramos et al. [27] developed a mixed mode type solar dryer in North of Portugal. Integrating heat and mass transfer models solved by an explicit finite difference method was used for simulation of solar drying of grapes. Picture of the solar dryer is shown in Fig. 24. Production of dried foods can be optimized, controlled and accurate drying time can be predicted from the simulations obtained by this model. The proposed simulation model can be used as a design data for the development of optimal dryer. Ayyappan et al. [28] did the thermal performance of natural convection solar greenhouse dryer integrated with thermal storage materials i.e. concrete, rock bed and sand. Thermal performance of the dryer is evaluated by drying coconuts inside the dryer. The constructed solar greenhouse dryer enveloped with UV treated polyethylene sheet is shown in Fig. 25. The dryer had semi-circular shape standing over a black painted concrete floor. Air flow inside the dryer is maintained by turbo vent fan. The decrease in drying time was observed in case of rock bed as compared to concrete and sand floor. Result shows that the concrete floor increases the day and night time temperature by 16 and
60 kg. The proposed rack type greenhouse dryer is shown in Fig. 20. Biomass stove was used to supply the hot air through four blowers of 80 W each. In a transparent building of greenhouses, Wild ginger slices are spread on 144 trays, each of dimension 0.5 m × 0.5 m. It was analyzed that at a temperature of 47.2 °C, drying of 60 kg wild ginger slice shows better performance, taking 30 h of drying time and giving 8% of drying efficiency. Janjai et al. [24] carried out performance evaluation and modeling of parabolic shaped greenhouse solar dryer installed at Loei, Thailand. Fig. 21 shows the pictorial view of drying of macadamia nuts inside the dryer. The dryer with a concrete floor had a dimension of 12.4 m × 9 m × 3.45 m and was enveloped with polycarbonate sheet. Two 50 W solar cell modules were provided to operate six DC fans, which provided the required ventilation. The performance of dryer was investigated by drying 6 batches of macadamia nuts, each batch of 730 kg. The moisture content was reduced from 14% to 16% (wb) to 2–3% (wb) in 5 days. Also the modeling of dryer had been carried out. Dhanore and Jibhakate [25] designed, developed and installed a solar tunnel dryer at Kavikulguru Institute of Technology and Science, Nagpur, India for drying of red chilly. Semi-cylindrical shaped type solar tunnel dryer is shown in Fig. 22. Installed dryer had a wooden frame with a dimension 2 ft × 6 ft × 1 ft covered with UV stabilized polyethylene. Two trays of size 2 ft × 2 ft were used for drying product in the heating chamber. Fan and chimney are provided to maintain the constant air flow rate. Higher drying rate was observed in a solar tunnel dryer than natural sun drying. 7
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Fig. 23. Solar tunnel dryer.
Fig. 20. Rack type greenhouse-effect solar dryer.
Fig. 24. Mixed mode type solar dryer.
Fig. 21. Drying of Macadamia nuts inside the dryer.
Fig. 25. Solar greenhouse dryer integrated with thermal storage materials.
average efficiency of 9.5%. Sallam et al. [29] installed two same dimensioned direct and indirect prototype solar dryers at Department of Food Science and Technology building, Faculty of Agriculture, Cairo University, Giza. Mint was dried under both forced and natural convection mode. Photograph of the direct solar dryer is shown in Fig. 26. The transparent polyethylene film was used to cover the direct prototype solar dryer while black polyethylene film was used to cover the indirect prototype solar dryer. Six perforated galvanized steel trays of dimension 1 m × 0.9 m × 0.04 m were used in each dryer. Forced convection gives higher drying rate of mint than natural convection. In case of forced convection drying rate was observed same in both direct and indirect drying. Effective diffusion coefficients for the drying of mint varied between 1.2 × 10–11 and 1.33 × 10–11 m2/s. Chan et al. [30] developed an integrated collector drying chamber solar dryer. It consists of a transparent structure, centrifugal blower,
Fig. 22. Installed solar tunnel dryer.
3 °C respectively as compared with ambient temperature. Moisture content of coconuts had been reduced from 52% to 7% (w.b.) in period of 78 h. Thus saving drying period by 55% as in natural sun drying 174 h were taken to reach the same moisture level. The dryer had an 8
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Fig. 28. Hybrid solar drying system for drying salted silver jewfish.
drying. The fabricated dryer was tested in two conditions (i) with solar collector and (ii) without solar collector inside the dryer. Thermocol sheet along with nickel polished aluminum sheet was provided to insulate the north wall and to utilize the maximum solar radiation. Schematic view of dryer with and without solar collector is shown in Fig. 29. The even span roof type greenhouse solar dryer having a dimension of 1.5 m × 1 m × 0.5 m was enveloped with 3 mm thick UV treated polycarbonate sheet. Roof inclination was 23.5°. The experimental result shows that greenhouse solar dryer with the solar collector is more effective than greenhouse solar dryer without solar collector. The maximum inside room temperature was 65.2 °C and 4.4% as lower inside room relative humidity. Prakash et al. [34] developed the modified greenhouse solar dryer working under passive and active mode at Maulana Azad National Institute of Technology, Bhopal, India. Performance of dryer was evaluated in three different floor conditions namely barren floor, floor enveloped with black PVC sheet and black painted concrete floor. The dryer was tested in no load and loaded conditions both. Tomato flakes, potato chips and capsicum flakes were used as crop in loaded condition. Modified greenhouse dryer under passive and active mode is shown in Fig. 30. Both modified even span type greenhouse dryer having dimension 1.5 m × 1 m × 0.5 m was built up of aluminum frame and
Fig. 26. Direct solar dryer.
feed hopper, pneumatic conveyor and hopper with vortex at the top. A gas stove heats up the air supplied to the drying chamber through a blower. The experimental setup of dryer is shown in Fig. 27. Rough rice is used for testing purpose. Results show that drying of 104 kg rough rice from initial moisture content 28.4% (w.b.) to the final moisture of 14.3% (w.b.) was taken 5 h. Drying efficiency for 104 kg and 200 kg conditions was 22.4% and 31.7% respectively. Fudholi et al. [31] did energy and exergy analysis of a hybrid solar drying system for drying salted silver jewfish. Photograph of constructed hybrid solar drying system is shown in Fig. 28. Experimental setup was made up of V-grove solar air collector, rotating rack type drying chamber, PV array and fans. 66% of energy requirements are fulfilled by solar energy. Diesel burner is provided to supply the hot air. Salted silver jewfish was dried in 8 h from moisture content of 64–10%. Average exergy efficiency of 31% was observed. Chauhan and Kumar [32,33] fabricate and tested north wall insulated greenhouse dryer in no-load conditions under natural mode of
Fig. 27. Experimental set up of solar dryer with its major components.
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Fig. 29. Greenhouse dryer (a) with solar collector and (b) without solar collector.
Fig. 30. Modified greenhouse solar dryer under (a) passive mode and (b) active mode for potato chips drying.
Fig. 31. The pictorial view of (a) Parabolic Greenhouse and (b) Parabolic greenhouse with the additional area-enhanced panels.
drying. The pictorial view of Parabolic Greenhouse and Parabolic greenhouse with the additional area-enhanced panels is shown in Fig. 31. The dimension of parabolic shaped greenhouse and parabolic greenhouse with enhanced panels was 3.5 m × 2 m × 1.5 m and 3.5 m × 3 m × 1.5 m respectively. The greenhouse had a black painted floor and was enveloped with transparent polycarbonate sheets. Results show that the cost of the parabolic greenhouse with the additional area-enhanced panels increases only by 7% but average efficiency increases by 15% and the internal temperature were 11 °C higher than the parabolic greenhouse without additional area. Tiwari et al. [36] constructed a mixed mode hybrid PV/T solar greenhouse dryer at IIT, Delhi. Thermal modeling was carried out to evaluate the energy and exergy analysis. MATLAB 2013a was used for
was enveloped with transparent polycarbonate sheet except at the north wall. The opaque mirror was placed at north wall. Exhaust fan operated by solar panel (rating 6 W) with battery backup was used in active mode. The experimental study shows that in load condition, PVC enveloped concrete floor gives better result than other two floor conditions. It is due to high thermal storage. Modified greenhouse dryer gives better drying performance under active mode than the passive mode for drying high moisture content crop. Jitjack et al. [35] built two structures of a greenhouse (i) Parabolic greenhouse and (ii) Parabolic greenhouse with the additional area-enhanced panels at Rajamangala University of Technology Tawan - Ok in Chonburi, Thailand. The experimental setup was tested under two conditions (i) empty greenhouse and (ii) greenhouse with rubber sheets 10
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maintained by Two DC fans run by 3 PV modules. To avoid decolouration of crop, the roof was enveloped with PV panels. Results show that with the increase in room and crop temperature, system and drying efficiency both increases. Theoretical and experimental values of overall thermal energy were found to be 1.92 kWh and 2.03 kWh respectively while values of overall thermal exergy were 0.532 kWh and 0.535 kWh respectively. Belloulid et al. [37] dried mechanically dewatered sludge in an open greenhouse pilot located at Marrakesh city. Two types of samples, Sludge I and Sludge II were molded into 30 cylindrical cakes each having 5 cm diameter, 1.5 cm height and 30 cm3 volume. Fig. 33 shows the 2D and 3D sketch of experimental drying plant. The dimension of open greenhouse solar dryer was 160 cm × 60 cm. The dryer had a cemented floor and was enveloped with transparent polycarbonate sheet of thickness 1 cm. The dryer was operated under natural convection. In hot and cold seasons, the moisture content had been decreased from 4 kg water/kg DS to 0.08 kg water/kg DS and 0.2 kg water/kg DS, respectively, in 72 h. At least 80% volume reduction is obtained in both the seasons. Morad et al. [38] constructed three solar tunnel greenhouse dryer to dry peppermint plants. A photographic view of greenhouse solar tunnel dryer is shown in Fig. 34. The dryers have a dimension of 2 m × 1 m ×
Fig. 32. Photovoltaic integrated greenhouse solar dryer.
numerical computations. Mixed mode hybrid PV/T solar greenhouse dryer is shown in Fig. 32. The experimental setup having the floor area of 1.066 m2 with roof inclination of 30° consists of aluminum frame and was enclosed with 3 mm thick glass. Required air circulation was
Fig. 33. (a) 2D and (b) 3D sketch of experimental drying plant.
Fig. 34. Greenhouse solar tunnel dryer for drying peppermint.
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Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.rser.2017.10.020. References [1] Janjai S, Khamvongsa V, Bala BK. Development, design, and performance of a pv ventilated greenhouse dryer. Int Energy J 2007;8:249–58. [2] Kumar A, Tiwari GN. Effect of mass on convective mass transfer coefficient during open sun and greenhouse drying of onion flakes. J Food Eng 2007;79:1337–50. [3] Barnwal P, Tiwari GN. Grape drying by using hybrid photovoltaic-thermal (PV/T) greenhouse dryer: an experimental study. Sol Energy 2008;82:1131–44. [4] Shrivastava V, Kumar A. Experimental investigation on the comparison of fenugreek drying in an indirect solar dryer and under open sun. Heat Mass Transf 2016;52:1963–72. [5] Patil R, Gawande R. A review on solar tunnel greenhouse drying system. Renew Sustain Energy Rev 2016;56:196–214. [6] Prakash O, Kumar A. Historical review and recent trends in solar drying systems. Int J Green Energy 2013;10:690–738. [7] Chauhan PS, Kumar A, Gupta B. A review on thermal models for greenhouse dryers. Renew Sustain Energy Rev 2016;75:548–58. [8] Nayak S, Tiwari GN. Energy and exergy analysis of photovoltaic/thermal integrated with a solar greenhouse. Energy Build 2008;40:2015–21. [9] Janjai S, Lamlert N, Intawee P, Mahayothee B, Bala BK, Nagle M, Müller J. Experimental and simulated performance of a pv-ventilated solar greenhouse dryer for drying of peeled longan and banana. Sol Energy 2009;83:1550–65. [10] Sethi VP, Arora S. Improvement in greenhouse solar drying using inclined north wall reflection. Sol Energy 2009;83:1472–84. [11] Ganguly A, Misra D, Ghosh S. Modeling and analysis of solar photovoltaic-electrolyzer-fuel cell hybrid power system integrated with a floriculture greenhouse. Energy Build 2010;42:2036–43. [12] Sevda MS, Rathore NS. Performance evaluation of the semicylindrical solar tunnel dryer for drying handmade paper. J Renew Sustain Energy 2010;2(013107):1–18. [13] Rathore NS, Panwar NL. Experimental studies on hemi cylindrical walk-in type solar tunnel dryer for grape drying. Appl Energy 2010;87:2764–7. [14] Almuhanna EA. Utilization of a solar greenhouse as a solar dryer for drying dates under the climatic conditions of the eastern province of Saudi Arabia. J Agric Sci 2012;4:237–46. [15] Janjai S. A greenhouse type solar dryer for small-scale dried food industries: development and dissemination. Int J Energy Environ 2012;3:383–98. [16] Adu EA, Bodunde AA, Awagu EF, Olayemi FF. Design, construction and performance evaluation of a solar agricultural drying tent. Int J Eng Res Technol 2012;1:1–11. [17] Kaewkiew J, Nabnean S, Janjai S. Experimental investigation of the performance of a large-scale greenhouse type solar dryer for drying chilli in Thailand. Procedia Eng 2012;32:433–9. [18] Panwar NL, Kaushik SC, Kothari S. Thermal modeling and experimental validation of solar tunnel dryer: a clean energy option for drying surgical cotton. Int J LowCarbon Technol 2013;11:16–28. [19] Kumar A, Prakash O, Kaviti A, Tomar A. Experimental analysis of greenhouse dryer in no-load conditions. J Environ Res Dev 2013;7:1399–406. [20] Nayak S, Kumar A, Mishra J, Tiwari GN. Drying and testing of mint (Mentha piperita) by a hybrid photovoltaic-thermal (PVT)-based greenhouse dryer. Dry Technol 2011;29:1002–9. [21] Prakash O, Kumar A. Thermal performance evaluation of modified active greenhouse dryer. J Build Phys 2014;37:395–402. [22] Prakash O, Kumar A. Performance evaluation of greenhouse dryer with opaque north wall. Heat Mass Transf 2014;50:493–500. [23] Aritesty E, Wulandani D. Performance of the rack type-greenhouse effect solar dryer for wild ginger (curcuma xanthorizza roxb.) drying. Energy Procedia 2014;47:94–100. [24] Janjai S, Phusampao C, Nilnont W, Pankaew P. Experimental performance and modeling of a greenhouse solar dryer for drying macadamia nuts. Int J Sci Eng Res 2014;5:1155–61. [25] Dhanore RT, Jibhakate YM. A solar tunnel dryer for drying red chilly as an agricultural product. Int J Eng Res Technol 2014;3:310–4. [26] Nagarajan S, Premkumar M. Analysis of thermal performance in solar dryer. IOSRJournal Mech Civil Eng 2014;11:71–4. [27] RamosInês N, Teresa RSB, Silva CLM. Simulation of solar drying of grapes using an integrated heat and mass transfer model. Renew Energy 2015;81:896–902. [28] Ayyappan S, Mayilsamy K, Sreenarayanan VV. Performance improvement studies in a solar greenhouse dryer using sensible heat storage materials. Heat Mass Transf 2016;52:459–67. [29] Sallam YI, Aly MH, Nassar AF, Mohamed EA. Solar drying of whole mint plant under natural and forced convection. J Adv Res 2015;6:171–8. [30] Chan Y, Dyah N, Abdullah K. Performance of a recirculation type integrated collector drying chamber (ICDC) solar dryer. Energy Procedia 2015;68:53–9. [31] Fudholi A, Yendra R, Basri DF, Ruslan Mohd H, Kamaruzzaman S. Energy and exergy analysis of hybrid solar drying system. Contemp Eng Sci 2016;9:215–23.
Fig. 35. Chapel shaped greenhouse solar dryer.
0.8 m, enveloped with a transparent plastic film of 200 mm thickness. For required air circulation, each dryer were provided with a fan driven by electric motor. The thermostat was provided to maintain the greenhouse temperature at or below 50 °C by controlling air flow rate. The solar tunnel was tested in four different operating conditions namely (i) Two different plant conditions (whole plants and leaves) (ii) Three different peppermint loads (2, 4 and 6 kg/m2) (iii) Three different air flow rates (1.05, 2.10 and 3.15 m3/min) (iv) Two different fan operating systems. Comparison among above conditions shows that load of 4 kg/m2 with air flow of 2.10 m3/min and continuous fan operating condition gives an optimal condition for drying of peppermint plant. Zaineb et al. [39] installed a chapel shaped greenhouse solar dryer attached with flat plate solar air collector at Research and Technology Centre of Energy in Borj Cedria (North of Tunisia). The performance of dryer was studied by drying red pepper. TRNSYS simulation program was used for developing mathematical model to predict the effect of the area of the product to be dried, the exhaust air flow rate and the collector area on drying kinetics. The pictorial view of solar greenhouse drying system is shown in Fig. 35. Experimental setup enveloped with plexi-glass had a floor area of 14.8 m2. The simulation results showed that collector area of 2 m2 is sufficient to compensate heat losses via centrifugal fans exhausting moist air. An increase in airflow rate higher than 250 kg/h does not affect drying kinetics. Water evaporation slows down due to increase in area of product which lowers the drying speed and increases the drying time. 3. Conclusion Various outcomes of this review are listed below-
• Greenhouse solar dryer operating in active mode is better as compared to passive mode. • Forced convection is suitable for high moisture content crops while natural convection can be used for crops with low moisture content. • Color, quality, taste and nutritious value of the dried product are better in greenhouse solar drying than open sun drying. • PV/T integrated greenhouse dryer are the best option for remote locations where electricity is not easily available. • Use of thermal storage material inside the greenhouse solar dryer • • •
increases the greenhouse inside temperature that reduces drying period. The insulated north wall prevents the heat loss to surroundings and improves the performance of greenhouse dryer. Pre-heating of air by using LPG burner, biomass or solar collector could be used to increases the dryer efficiency. For selecting a suitable dryer for drying any crop in any location design data given by simulated model can be used.
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(PVT) mixed mode greenhouse solar dryer. Sol Energy 2016;133:421–8. [37] Belloulid MO, Hassan Hamdi, Laila Mandi, Naaila Ouazzani. Solar greenhouse drying of waste water sludges under arid climate. Waste Biomass- Valoriz 2017;8:193–202. [38] Morad MM, El-Shazly MA, Wasfy KI, El-Maghawry Hend AM. Thermal analysis and performance evaluation of a solar tunnel greenhouse dryer for drying peppermint plants. Renew Energy 2017;101:992–1004. [39] Azaizia Z, Kooli S, Elkhadraoui A, Hamdi I, Guizani A. Investigation of a new solar greenhouse drying system for peppers. Int J Hydrog Energy 2017;42:8818–26.
[32] Chauhan PS, Kumar A. Heat transfer analysis of north wall insulated greenhouse dryer under natural convection mode. Energy 2017;118:1264–74. [33] Chauhan PS, Kumar A. Performance analysis of greenhouse dryer by using insulated north-wall under natural convection mode. Energy Rep 2016;2:107–16. [34] Prakash O, Kumar A, Laguri V. Performance of modified greenhouse dryer with thermal energy storage. Energy Rep 2016;2:155–62. [35] Jitjack K, Thepa S, Sudaprasert K, Namprakai P. Improvement of a rubber drying greenhouse with a parabolic cover and enhanced panels. Energy Build 2016;124:178–93. [36] Tiwari S, Tiwari GN, Al-Helal IM. Performance analysis of photovoltaic–thermal
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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
Recent developments in greenhouse solar drying: A review ⁎
Pushpendra Singha, , Vipin Shrivastavaa, Anil Kumarb,c a b c
Department of Mechanical Engineering, Lakshmi Narain College of Technology, Bhopal, India Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand Department of Energy (Energy Centre), Maulana Azad National Institute of Technology, Bhopal 462 003, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Active greenhouse Passive greenhouse Drying Solar energy Solar dryer
The world is moving towards the reduction of dependency on fossil fuels. Various innovations are undergoing to make the use of sources of renewable energy like wind, solar, tidal etc. Among these sources, solar energy is available in enormous quantity and best option that may be used for space heating and generation of electric energy. For drying agricultural and non-agricultural products, solar energy can be used directly or indirectly. But in open sun drying the products are affected by external calamities such as rain, insects and animals. To overcome the shortcomings of open sun drying various greenhouse solar dryer had been proposed so far. Some of recent researches have been discussed in the paper. This paper provides the data of already developed greenhouses so the reader can develop new and modified the greenhouse structure. To accomplish this, various researches of recent years have been studied and presented in this review.
1. Introduction Increasing population is the major problem of the entire world. Increase in the population increases the consumption of food. To fulfill this demand either that amount of food must be produced on a regular basis or produced food can be stored after some processing. Therefore, continuous production is not possible but food can be stored for a certain period by drying it. Drying is the phenomenon of reducing moisture up to a safe limit. Solar drying is the ancient method that is practiced everywhere for crop preservation. Solar drying is an effective method of utilizing energy of sun [1]. Drying of agricultural products leads to counteract the activity of various microscopic organisms. The crop after drying can be stored for a longer time without any fear of getting deteriorated [2]. The dried crop has various advantages like enhanced product quality, longer safe storage time and low post-harvest losses [3]. Heat transfer in the form of conduction, convection, thermal and radiation plays a vital role in solar drying process [4]. In open or natural sun drying, the crops are laid simply on the floor or mat in the full sunny days. As the crops being exposed directly to the sun, it gets contaminated from dirt and pest infestation and also lost by birds and beast. Researchers developed various drying techniques like spray, mechanical, electrical, solar drying etc. Such drying techniques are using around the world for drying of agricultural and non-agricultural products. Among these dryers, greenhouse solar dryer has various advantages over other types which make it a good alternative [1]. These dryers not only reduce the consumption of fossil fuels for drying purpose, but also provide the best
⁎
quality, color and taste of the dried products [5]. The modern solar drying equipment uses optimum energy and time and occupies less area for producing better quality dried products with almost zero energy cost [6]. The working of greenhouse solar dryer is based on the principle of greenhouse effect. It allows incoming short wavelength solar radiations from the sun and traps the long wavelength solar radiations. Greenhouse dryers are used for crop cultivation, poultry, aquaculture, soil solarisation, and crop drying [7]. Greenhouse basically operates either in passive (natural convection) or active mode (forced convection) [8]. Fig. 1 shows the classification of greenhouse solar dryer on the basis of our literature review. In the passive mode of greenhouse dryer, ventilator or chimney is provided at the chimney for the natural circulation of air entering inside the dryer. While in case of the active dryer, exhaust fan is provided for moving humid air outside the dryer. Various modifications and researches have been done to improve the greenhouse dryer's performance. Some of the modifications implemented on active and passive greenhouse solar dryers in literature are:
• PV integrated greenhouse solar dryer [1,8,9,11,15,17,20–22, 24,26,31,34,36]. • Used opaque northern wall to insulate it and prevent heat loss [12,13,16,18,22,32]. • Used thermal storage materials such as sand, rock-bed, black •
painted concrete floor and PVC sheet. So that greenhouse can be used during off sun-shine period [1,12,28,34,37]. Inclined and reflecting north wall to collect maximum radiations [10].
Corresponding author. E-mail address: [email protected] (P. Singh).
http://dx.doi.org/10.1016/j.rser.2017.10.020 Received 14 April 2017; Received in revised form 6 September 2017; Accepted 26 October 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Singh, P., Renewable and Sustainable Energy Reviews (2017), http://dx.doi.org/10.1016/j.rser.2017.10.020
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Fig. 1. Classification of greenhouse solar dryer on the basis of our literature review.
2. Researches on greenhouse based solar dryer Janjai et al. [1] developed parabolic shaped PV ventilated greenhouse solar dryer with a black concrete floor. The Dryer was installed at Solar Energy Research Laboratory, Silpakorn University at Nakhon Pathom, Thailand. Fig. 2 shows the developed greenhouse dryer. The dryer floor area was 44 m2 and was enveloped with polycarbonate sheets. A 53 W rating solar panel was provided to run the 3 DC fans. Dryer was loaded with 150 kg of fresh chillies to investigate its performance. The result shows that the chillies had been dried from 80% moisture content to 10% (wet basis). The drying period varies from 2 to 1 3 2 days while in open sun drying it takes 6 days. Kumar and Tiwari [2] investigated the convective mass transfer coefficient with respect to change in mass of onion flakes in a greenhouse dryer installed at IIT Delhi, India. Three different weights 300 g, 600 g and 900 g of onion flakes were dried continuously for 33 h both in open sun and in the greenhouse. Experimental setup for drying onion under open sun drying, natural and forced convection mode is shown in Fig. 3. The even span roof type greenhouse dryer had a floor area of 1.2 × 0.78 m2 and was enveloped with UV film. The result shows that, for different modes of drying convective mass transfer of onion flakes
Fig. 2. The developed greenhouse solar dryer.
• Using greenhouse coupled with solar air heater to achieve faster drying [30,39]. • Provided additional area enhancing panels to increase drying area [35].
Fig. 3. Experimental setup for onion drying in (a) natural and (b) forced convection mode.
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behavior of green house integrated with PV/T collector. Fig. 5 shows the proposed experimental set-up installed at solar energy park, IIT Delhi campus. The 8 PV modules of 75 W rating were mounted on the wooden structure. One fan of capacity 12 W is fitted to maintain air flow rate inside a green house. Power generated by the solar module is stored in set of 12 DC batteries of 6 V rating each and an inverter of rating 2.1 KVA for converting DC to AC. Exergy efficiency (hEX) of 4% was observed for this PV/T integrated greenhouse. Janjai et al. [9] presents experimental and simulation performance by drying peeled longan and banana inside a PV integrated greenhouse dryer. The concrete floored parabolic roof type dryer having dimensions 8 m × 5.5 m × 3.5 m is enveloped with polycarbonates plates. PV module of capacity 50 W is used to run 3 fans, provided to maintain the required ventilation. The proposed setup with the positions of various instruments fitted over it is shown in Fig. 6. During drying of banana and longan, temperature varies from 30 °C to 60 °C and 31 °C to 58 °C respectively. Drying time of Longan and banana inside greenhouse were only 3 days and 4days respectively. While drying of same under open sun takes 5 days and 6 days respectively. Comparison of results shows that better color and taste were observed in greenhouse dried product. Sethi and Arora [10] modified the conventional even span type greenhouse solar dryer by using inclined reflecting north wall. The inclination angle of north wall has been optimized for tray having different width. Under natural and forced mode of drying, experimental performance of greenhouse dryer was tested in with and without inclined north wall by drying bitter gourd slices. The dryer was located in Ludhiana, Punjab, India. Outside and inside pictorial view of the improved greenhouse dryer along with inclined reflective north wall is shown in Fig. 7. Dryer floor area was 6 m × 4 m and enveloped with UV stabilized polyethylene sheet. North wall of 12 mm wooden ply board was fitted with an aluminized polyester sheet. Results showed that by using inclined north wall in natural and forced convection mode, 13.13% and 16.67% of the total drying time was saved respectively. Ganguly et al. [11] demonstrated a model of greenhouse combined with power generation and storage system. Study reveals that the integrated power arrangement gives a feasible option to power standalone greenhouses in a self-sustained way. The proposed integrated system for greenhouse application is shown in Fig. 8. The 51 solar PV modules of 75 W each were used to generate electricity. Two 480 W PEM fuel cell systems and combination of electrolyzer of 3.3 kV each were used for power back up. Some of the power generated by PV modules is utilized by greenhouse appliances and extra energy left is utilized in generating hydrogen gas by an electrolyzer. PEM fuel cell
Fig. 4. Hybrid greenhouse solar dryer integrated with PV/T module.
Fig. 5. Installed greenhouse at solar energy park, IIT Delhi.
increases by 30% − 135% with change in the mass from 300 g to 900 g. Barnwal and Tiwari [3] constructed a 30 ° inclined Roof type even Span hybrid PV/T greenhouse dryer of 100 kg capacity under forced mode at solar energy park, IIT Delhi, India. Hybrid PV/T integrated greenhouse dryer is shown in Fig. 4. The setup consists of two PV module of 75 W each and DC fan for air movement, have a dimension of 2.5 m × 2.6 m × 1.05 m. The air enters from bottom and passes through 3-tier system of perforated wire mesh trays and leaves from top. The structural frame was enveloped with UV stabilized polyethylene sheet to trap infrared radiation. Convective heat transfer coefficient of two types of grapes GR-I and GR-II was evaluated and compared to PV/T dryer in both open and greenhouse conditions. Nayak and Tiwari [8] did energy and exergy analysis to predict the
Fig. 6. Proposed setup with the positions of various instruments fitted over it.
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Fig. 7. (a) Outside and (b) inside pictorial view of the improved greenhouse dryer with (c) inclined reflective north wall.
required humidity and temperature inside the dryer. In a drying time of 4–5 h, moisture content had been reduced from 53.85% to 9.96%. Rathore and Panwar [13] built a semi-cylindrical solar tunnel dryer at College of Dairy and Food Science Technology, Udaipur, India. Performance of dryer was evaluated by drying of 320 kg of seedless grapes in a temperature of about 10 – 28 °C. In 7 days of drying period, moisture content was reduced from 85% to 16%. The schematic view of natural convection solar tunnel dryer is shown in Fig. 10. The setup was built of hemi-cylindrical metallic frame enveloped with UV stabilized polyethylene sheet. Exhaust fan and chimney is provided to maintain air flow rate. To prevent heat loss, bottom and northern wall was insulated. Almuhanna [14] constructed a gable even span type greenhouse dryer for the drying of dates at King Faisal University, Saudi Arabia. The aim of research was to evaluate thermal performance and feasibility of the dryer. Photographic view of solar air heater is shown in Fig. 11. An aluminum framed structure having dimension 2 m × 1 m × 1.2 m is enveloped with fiber glass and had a roof inclined at 30° to collect maximum solar radiation. Two plywood dryer boxes having dimension 1 m × 1 m × 0.3 m were placed in the drying area. The axial fan was fitted to maintain required air flow rate. During experimental period of dryer, the overall daily average thermal efficiency was 60.11%. Janjai [15] developed and investigated parabolic roof type greenhouse dryer for small-scale dry food industries at Nakhom Pathom in Thailand. The pictorial view of greenhouse dryer integrated with LPG burner is shown in Fig. 12. The setup had a dimension of 8 m × 20 m × 3.5 m with the loading capacity of 1000 kg. For supply hot air during off-sunshine period, 100 kW LPG burner was incorporated with it. This dryer is powered and ventilated by three 50 W PV module and 15 W DC fan. During drying of tomatoes, drying temperature varied between 35 to 65 °C and observed drying time was 2–3 days shorter than natural sun drying. Adu et al. [16] designed and built a tent type solar dryer at Nigeria. The drying tent roof frame work with transparent roof sheet is shown in Fig. 13. Drying area of the solar dryer was 29.6 ft2. The drying platform along with two long side walls are completely enveloped with a black cloth while remaining two walls are half enveloped with black cloth. The roof is enveloped with transparent polythene. The performance of dryer is obtained by drying okra. At the temperature of about 50 °C, 86.05% of initial moisture content of okra was reduced to final moisture content of 3.43% within 23 h. Kaewkiew et al. [17] evaluates a drying performance of parabolic shaped greenhouse dryer at Ubon Ratchthani, Thailand. Installed largescale polycarbonate enveloped greenhouse dryer is shown in Fig. 14.
Fig. 8. Integrated systems for greenhouse application.
Fig. 9. Semi-cylindrical shape solar tunnel dryer.
stack consumes that hydrogen to generate power for energy deficit hours. Sevda and Rathore [12] constructed a solar tunnel dryer of semicylindrical shape at M/s Cellulosic Waste Recycling Education Project, Vidya Bhawan Society, Rajasthan, India. The performance of the dryer was analyzed by drying handmade papers. Photographic view of developed solar tunnel dryer is shown in Fig. 9. The dryer floor area was 71.25 m2 and height 3.75 m and was enveloped by UV stabilized polyethylene sheet. The dryer had a capacity of drying 1500 papers/ batch. Exhaust fan and five chimneys were provided to maintain the 4
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Fig. 10. Natural convection solar tunnel dryer.
Fig. 11. Photographic view of solar air heater. Fig. 13. Drying tent roof frame work with transparent roofing sheet.
Fig. 14. Installed large-scale polycarbonate covered greenhouse dryer.
Fig. 12. Greenhouse dryer integrated with LPG burner.
drying. Reduction in drying period was observed along with better taste, color and pungency. Panwar et al. [18] constructed three semi-cylindrical solar tunnel dryers at M/s Cotton Product of India, Udaipur. Surgical cotton was dried in the dryer to carry out its thermal modeling and energy-exergy analysis. Fig. 15 shows the experimental setup of installed solar tunnel
The dryer enveloped with polycarbonate sheet has concrete floor area of 160 m2. PV modules of capacity 50 W were used to power 9 DC fans, provided to maintain required air circulation. To evaluate the performance of solar greenhouse dryer 500 kg of chillies were dried inside it. Moisture content has been reduced from 74% to 9% (wb) in 3 days in greenhouse solar dryer in comparison to 5 days taken by natural sun 5
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Fig. 15. Experimental solar tunnel dryer.
Fig. 17. Hybrid greenhouse dryer integrated with PVT.
dryers. The dryer was enveloped with a UV stabilized polythene sheet and was provided with the opaque insulated north wall. The dryer had a floor area of 67.5 m2 and loading capacity of 600 kg. Moisture content of surgical cotton had been reduced from 40% to 5% (wb) in one day. Unknown equations were computed by MATLAB 2010a. Experimental energy efficiency was found lower and experimental exergy efficiency was found higher than their predicted values. Kumar et al. [19] carried out performance evaluation of roof even type greenhouse dryer under no-load condition in both natural and forced mode. Experimental setup of dryer for natural and forced mode is shown in Fig. 16. Dryer floor area was 1.50 × 1.01 m2 and was enveloped with polycarbonate sheet of 3 mm thickness. Two vents of diameter 0.01 m were provided at the bottom of the dryer for air inlet under natural mode. Fan was attached to the sidewall for forced convection. Results reveal that forced convection increases the efficiency of the dryer by 2% as compared to natural mode of drying. Also, drying rate in forced convection is 31% higher as compared to natural convection. Nayak et al. [20] dried and tested mint leaves in PV integrated greenhouse solar dryer installed at Solar Energy Park, IIT Delhi, India. A pictorial view of hybrid solar dryer integrated with PVT is shown in Fig. 17. Dryer floor area was 2.5 m × 2.6 m and was enveloped with UV stabilized polythene sheet. The dryer had east west orientation and 30° roof inclination. Three DC fans were provided for forced convection. The moisture content had been reduced from 80% to 11% in 21 h while in open sun drying it takes 26–27 h. The dried products are converted into powdered form and then tested. The results obtained
shows that the calorific and nutritional values were retained along with the original color. Prakash and Kumar [21] built and tested the modified greenhouse solar dryer at Maulana Azad National Institute of Technology, Bhopal, India. Experimental set up of modified greenhouse solar dryer is shown in Fig. 18. The performance of experimental setup was evaluated without any load under two conditions- floor enveloped with a black PVC sheet and without enveloped floor. The dryer was tested under forced convection, provided with the help of 12 V DC fan powered by 6 W PV modules. The aim of the testing is to determine the full potential of dryer so as to dry the high moisture content crops. High temperature inside the greenhouse and decrease in relative humidity was observed in case of covered floor. Prakash and Kumar [22] developed a modified active greenhouse solar dryer with opaque northern wall and tested it with two conditions (i) black PVC sheet covered concrete floor and (ii) uncovered concrete floor. Schematic view of modified solar active dryer with and without covered inside floor is shown in Fig. 19. The 12 V DC fan run by 6 Solar cell (each of 1 W), placed in two rows with 3 cell in each row was used. Power from the solar cell is used to charge the battery. The battery gives power to operate DC fan. Concrete floor enveloped with a black PVC sheet gives a higher increase in temperature and reduction in humidity as compared to an uncovered floor. Aritesty and Wulandani [23] manufactured a rack type greenhouse solar dryer and obtained its performance by drying wild ginger in three conditions- no load and loaded with two different capacities 21 kg and
Fig. 16. Experimental setup of greenhouse dryer for (a) natural and (b) forced convection.
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Fig. 18. Modified greenhouse dryer in (a) without covered floor and (b) with covered floor.
Fig. 19. Modified greenhouse dryer (a) without covered floor and (b) with covered floor.
Nagarajan and Premkumar [26] designed and manufactured a parabolic shaped solar tunnel dryer enveloped with polycarbonate plates. A pictorial view of solar tunnel dryer is shown in Fig. 23. To investigate the performance of dryer, 1 kg of grapes were dried in drying area of 108 m2 inside a tunnel. DC fan used to circulate the air was powered by 15 W flat plate solar cell modules. In this dryer, Grapes were dehydrated from 75% to 7% moisture content within 2–3 days in a temperature range of 35–75 °C. Ramos et al. [27] developed a mixed mode type solar dryer in North of Portugal. Integrating heat and mass transfer models solved by an explicit finite difference method was used for simulation of solar drying of grapes. Picture of the solar dryer is shown in Fig. 24. Production of dried foods can be optimized, controlled and accurate drying time can be predicted from the simulations obtained by this model. The proposed simulation model can be used as a design data for the development of optimal dryer. Ayyappan et al. [28] did the thermal performance of natural convection solar greenhouse dryer integrated with thermal storage materials i.e. concrete, rock bed and sand. Thermal performance of the dryer is evaluated by drying coconuts inside the dryer. The constructed solar greenhouse dryer enveloped with UV treated polyethylene sheet is shown in Fig. 25. The dryer had semi-circular shape standing over a black painted concrete floor. Air flow inside the dryer is maintained by turbo vent fan. The decrease in drying time was observed in case of rock bed as compared to concrete and sand floor. Result shows that the concrete floor increases the day and night time temperature by 16 and
60 kg. The proposed rack type greenhouse dryer is shown in Fig. 20. Biomass stove was used to supply the hot air through four blowers of 80 W each. In a transparent building of greenhouses, Wild ginger slices are spread on 144 trays, each of dimension 0.5 m × 0.5 m. It was analyzed that at a temperature of 47.2 °C, drying of 60 kg wild ginger slice shows better performance, taking 30 h of drying time and giving 8% of drying efficiency. Janjai et al. [24] carried out performance evaluation and modeling of parabolic shaped greenhouse solar dryer installed at Loei, Thailand. Fig. 21 shows the pictorial view of drying of macadamia nuts inside the dryer. The dryer with a concrete floor had a dimension of 12.4 m × 9 m × 3.45 m and was enveloped with polycarbonate sheet. Two 50 W solar cell modules were provided to operate six DC fans, which provided the required ventilation. The performance of dryer was investigated by drying 6 batches of macadamia nuts, each batch of 730 kg. The moisture content was reduced from 14% to 16% (wb) to 2–3% (wb) in 5 days. Also the modeling of dryer had been carried out. Dhanore and Jibhakate [25] designed, developed and installed a solar tunnel dryer at Kavikulguru Institute of Technology and Science, Nagpur, India for drying of red chilly. Semi-cylindrical shaped type solar tunnel dryer is shown in Fig. 22. Installed dryer had a wooden frame with a dimension 2 ft × 6 ft × 1 ft covered with UV stabilized polyethylene. Two trays of size 2 ft × 2 ft were used for drying product in the heating chamber. Fan and chimney are provided to maintain the constant air flow rate. Higher drying rate was observed in a solar tunnel dryer than natural sun drying. 7
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Fig. 23. Solar tunnel dryer.
Fig. 20. Rack type greenhouse-effect solar dryer.
Fig. 24. Mixed mode type solar dryer.
Fig. 21. Drying of Macadamia nuts inside the dryer.
Fig. 25. Solar greenhouse dryer integrated with thermal storage materials.
average efficiency of 9.5%. Sallam et al. [29] installed two same dimensioned direct and indirect prototype solar dryers at Department of Food Science and Technology building, Faculty of Agriculture, Cairo University, Giza. Mint was dried under both forced and natural convection mode. Photograph of the direct solar dryer is shown in Fig. 26. The transparent polyethylene film was used to cover the direct prototype solar dryer while black polyethylene film was used to cover the indirect prototype solar dryer. Six perforated galvanized steel trays of dimension 1 m × 0.9 m × 0.04 m were used in each dryer. Forced convection gives higher drying rate of mint than natural convection. In case of forced convection drying rate was observed same in both direct and indirect drying. Effective diffusion coefficients for the drying of mint varied between 1.2 × 10–11 and 1.33 × 10–11 m2/s. Chan et al. [30] developed an integrated collector drying chamber solar dryer. It consists of a transparent structure, centrifugal blower,
Fig. 22. Installed solar tunnel dryer.
3 °C respectively as compared with ambient temperature. Moisture content of coconuts had been reduced from 52% to 7% (w.b.) in period of 78 h. Thus saving drying period by 55% as in natural sun drying 174 h were taken to reach the same moisture level. The dryer had an 8
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Fig. 28. Hybrid solar drying system for drying salted silver jewfish.
drying. The fabricated dryer was tested in two conditions (i) with solar collector and (ii) without solar collector inside the dryer. Thermocol sheet along with nickel polished aluminum sheet was provided to insulate the north wall and to utilize the maximum solar radiation. Schematic view of dryer with and without solar collector is shown in Fig. 29. The even span roof type greenhouse solar dryer having a dimension of 1.5 m × 1 m × 0.5 m was enveloped with 3 mm thick UV treated polycarbonate sheet. Roof inclination was 23.5°. The experimental result shows that greenhouse solar dryer with the solar collector is more effective than greenhouse solar dryer without solar collector. The maximum inside room temperature was 65.2 °C and 4.4% as lower inside room relative humidity. Prakash et al. [34] developed the modified greenhouse solar dryer working under passive and active mode at Maulana Azad National Institute of Technology, Bhopal, India. Performance of dryer was evaluated in three different floor conditions namely barren floor, floor enveloped with black PVC sheet and black painted concrete floor. The dryer was tested in no load and loaded conditions both. Tomato flakes, potato chips and capsicum flakes were used as crop in loaded condition. Modified greenhouse dryer under passive and active mode is shown in Fig. 30. Both modified even span type greenhouse dryer having dimension 1.5 m × 1 m × 0.5 m was built up of aluminum frame and
Fig. 26. Direct solar dryer.
feed hopper, pneumatic conveyor and hopper with vortex at the top. A gas stove heats up the air supplied to the drying chamber through a blower. The experimental setup of dryer is shown in Fig. 27. Rough rice is used for testing purpose. Results show that drying of 104 kg rough rice from initial moisture content 28.4% (w.b.) to the final moisture of 14.3% (w.b.) was taken 5 h. Drying efficiency for 104 kg and 200 kg conditions was 22.4% and 31.7% respectively. Fudholi et al. [31] did energy and exergy analysis of a hybrid solar drying system for drying salted silver jewfish. Photograph of constructed hybrid solar drying system is shown in Fig. 28. Experimental setup was made up of V-grove solar air collector, rotating rack type drying chamber, PV array and fans. 66% of energy requirements are fulfilled by solar energy. Diesel burner is provided to supply the hot air. Salted silver jewfish was dried in 8 h from moisture content of 64–10%. Average exergy efficiency of 31% was observed. Chauhan and Kumar [32,33] fabricate and tested north wall insulated greenhouse dryer in no-load conditions under natural mode of
Fig. 27. Experimental set up of solar dryer with its major components.
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Fig. 29. Greenhouse dryer (a) with solar collector and (b) without solar collector.
Fig. 30. Modified greenhouse solar dryer under (a) passive mode and (b) active mode for potato chips drying.
Fig. 31. The pictorial view of (a) Parabolic Greenhouse and (b) Parabolic greenhouse with the additional area-enhanced panels.
drying. The pictorial view of Parabolic Greenhouse and Parabolic greenhouse with the additional area-enhanced panels is shown in Fig. 31. The dimension of parabolic shaped greenhouse and parabolic greenhouse with enhanced panels was 3.5 m × 2 m × 1.5 m and 3.5 m × 3 m × 1.5 m respectively. The greenhouse had a black painted floor and was enveloped with transparent polycarbonate sheets. Results show that the cost of the parabolic greenhouse with the additional area-enhanced panels increases only by 7% but average efficiency increases by 15% and the internal temperature were 11 °C higher than the parabolic greenhouse without additional area. Tiwari et al. [36] constructed a mixed mode hybrid PV/T solar greenhouse dryer at IIT, Delhi. Thermal modeling was carried out to evaluate the energy and exergy analysis. MATLAB 2013a was used for
was enveloped with transparent polycarbonate sheet except at the north wall. The opaque mirror was placed at north wall. Exhaust fan operated by solar panel (rating 6 W) with battery backup was used in active mode. The experimental study shows that in load condition, PVC enveloped concrete floor gives better result than other two floor conditions. It is due to high thermal storage. Modified greenhouse dryer gives better drying performance under active mode than the passive mode for drying high moisture content crop. Jitjack et al. [35] built two structures of a greenhouse (i) Parabolic greenhouse and (ii) Parabolic greenhouse with the additional area-enhanced panels at Rajamangala University of Technology Tawan - Ok in Chonburi, Thailand. The experimental setup was tested under two conditions (i) empty greenhouse and (ii) greenhouse with rubber sheets 10
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maintained by Two DC fans run by 3 PV modules. To avoid decolouration of crop, the roof was enveloped with PV panels. Results show that with the increase in room and crop temperature, system and drying efficiency both increases. Theoretical and experimental values of overall thermal energy were found to be 1.92 kWh and 2.03 kWh respectively while values of overall thermal exergy were 0.532 kWh and 0.535 kWh respectively. Belloulid et al. [37] dried mechanically dewatered sludge in an open greenhouse pilot located at Marrakesh city. Two types of samples, Sludge I and Sludge II were molded into 30 cylindrical cakes each having 5 cm diameter, 1.5 cm height and 30 cm3 volume. Fig. 33 shows the 2D and 3D sketch of experimental drying plant. The dimension of open greenhouse solar dryer was 160 cm × 60 cm. The dryer had a cemented floor and was enveloped with transparent polycarbonate sheet of thickness 1 cm. The dryer was operated under natural convection. In hot and cold seasons, the moisture content had been decreased from 4 kg water/kg DS to 0.08 kg water/kg DS and 0.2 kg water/kg DS, respectively, in 72 h. At least 80% volume reduction is obtained in both the seasons. Morad et al. [38] constructed three solar tunnel greenhouse dryer to dry peppermint plants. A photographic view of greenhouse solar tunnel dryer is shown in Fig. 34. The dryers have a dimension of 2 m × 1 m ×
Fig. 32. Photovoltaic integrated greenhouse solar dryer.
numerical computations. Mixed mode hybrid PV/T solar greenhouse dryer is shown in Fig. 32. The experimental setup having the floor area of 1.066 m2 with roof inclination of 30° consists of aluminum frame and was enclosed with 3 mm thick glass. Required air circulation was
Fig. 33. (a) 2D and (b) 3D sketch of experimental drying plant.
Fig. 34. Greenhouse solar tunnel dryer for drying peppermint.
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Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.rser.2017.10.020. References [1] Janjai S, Khamvongsa V, Bala BK. Development, design, and performance of a pv ventilated greenhouse dryer. Int Energy J 2007;8:249–58. [2] Kumar A, Tiwari GN. Effect of mass on convective mass transfer coefficient during open sun and greenhouse drying of onion flakes. J Food Eng 2007;79:1337–50. [3] Barnwal P, Tiwari GN. Grape drying by using hybrid photovoltaic-thermal (PV/T) greenhouse dryer: an experimental study. Sol Energy 2008;82:1131–44. [4] Shrivastava V, Kumar A. Experimental investigation on the comparison of fenugreek drying in an indirect solar dryer and under open sun. Heat Mass Transf 2016;52:1963–72. [5] Patil R, Gawande R. A review on solar tunnel greenhouse drying system. Renew Sustain Energy Rev 2016;56:196–214. [6] Prakash O, Kumar A. Historical review and recent trends in solar drying systems. Int J Green Energy 2013;10:690–738. [7] Chauhan PS, Kumar A, Gupta B. A review on thermal models for greenhouse dryers. Renew Sustain Energy Rev 2016;75:548–58. [8] Nayak S, Tiwari GN. Energy and exergy analysis of photovoltaic/thermal integrated with a solar greenhouse. Energy Build 2008;40:2015–21. [9] Janjai S, Lamlert N, Intawee P, Mahayothee B, Bala BK, Nagle M, Müller J. Experimental and simulated performance of a pv-ventilated solar greenhouse dryer for drying of peeled longan and banana. Sol Energy 2009;83:1550–65. [10] Sethi VP, Arora S. Improvement in greenhouse solar drying using inclined north wall reflection. Sol Energy 2009;83:1472–84. [11] Ganguly A, Misra D, Ghosh S. Modeling and analysis of solar photovoltaic-electrolyzer-fuel cell hybrid power system integrated with a floriculture greenhouse. Energy Build 2010;42:2036–43. [12] Sevda MS, Rathore NS. Performance evaluation of the semicylindrical solar tunnel dryer for drying handmade paper. J Renew Sustain Energy 2010;2(013107):1–18. [13] Rathore NS, Panwar NL. Experimental studies on hemi cylindrical walk-in type solar tunnel dryer for grape drying. Appl Energy 2010;87:2764–7. [14] Almuhanna EA. Utilization of a solar greenhouse as a solar dryer for drying dates under the climatic conditions of the eastern province of Saudi Arabia. J Agric Sci 2012;4:237–46. [15] Janjai S. A greenhouse type solar dryer for small-scale dried food industries: development and dissemination. Int J Energy Environ 2012;3:383–98. [16] Adu EA, Bodunde AA, Awagu EF, Olayemi FF. Design, construction and performance evaluation of a solar agricultural drying tent. Int J Eng Res Technol 2012;1:1–11. [17] Kaewkiew J, Nabnean S, Janjai S. Experimental investigation of the performance of a large-scale greenhouse type solar dryer for drying chilli in Thailand. Procedia Eng 2012;32:433–9. [18] Panwar NL, Kaushik SC, Kothari S. Thermal modeling and experimental validation of solar tunnel dryer: a clean energy option for drying surgical cotton. Int J LowCarbon Technol 2013;11:16–28. [19] Kumar A, Prakash O, Kaviti A, Tomar A. Experimental analysis of greenhouse dryer in no-load conditions. J Environ Res Dev 2013;7:1399–406. [20] Nayak S, Kumar A, Mishra J, Tiwari GN. Drying and testing of mint (Mentha piperita) by a hybrid photovoltaic-thermal (PVT)-based greenhouse dryer. Dry Technol 2011;29:1002–9. [21] Prakash O, Kumar A. Thermal performance evaluation of modified active greenhouse dryer. J Build Phys 2014;37:395–402. [22] Prakash O, Kumar A. Performance evaluation of greenhouse dryer with opaque north wall. Heat Mass Transf 2014;50:493–500. [23] Aritesty E, Wulandani D. Performance of the rack type-greenhouse effect solar dryer for wild ginger (curcuma xanthorizza roxb.) drying. Energy Procedia 2014;47:94–100. [24] Janjai S, Phusampao C, Nilnont W, Pankaew P. Experimental performance and modeling of a greenhouse solar dryer for drying macadamia nuts. Int J Sci Eng Res 2014;5:1155–61. [25] Dhanore RT, Jibhakate YM. A solar tunnel dryer for drying red chilly as an agricultural product. Int J Eng Res Technol 2014;3:310–4. [26] Nagarajan S, Premkumar M. Analysis of thermal performance in solar dryer. IOSRJournal Mech Civil Eng 2014;11:71–4. [27] RamosInês N, Teresa RSB, Silva CLM. Simulation of solar drying of grapes using an integrated heat and mass transfer model. Renew Energy 2015;81:896–902. [28] Ayyappan S, Mayilsamy K, Sreenarayanan VV. Performance improvement studies in a solar greenhouse dryer using sensible heat storage materials. Heat Mass Transf 2016;52:459–67. [29] Sallam YI, Aly MH, Nassar AF, Mohamed EA. Solar drying of whole mint plant under natural and forced convection. J Adv Res 2015;6:171–8. [30] Chan Y, Dyah N, Abdullah K. Performance of a recirculation type integrated collector drying chamber (ICDC) solar dryer. Energy Procedia 2015;68:53–9. [31] Fudholi A, Yendra R, Basri DF, Ruslan Mohd H, Kamaruzzaman S. Energy and exergy analysis of hybrid solar drying system. Contemp Eng Sci 2016;9:215–23.
Fig. 35. Chapel shaped greenhouse solar dryer.
0.8 m, enveloped with a transparent plastic film of 200 mm thickness. For required air circulation, each dryer were provided with a fan driven by electric motor. The thermostat was provided to maintain the greenhouse temperature at or below 50 °C by controlling air flow rate. The solar tunnel was tested in four different operating conditions namely (i) Two different plant conditions (whole plants and leaves) (ii) Three different peppermint loads (2, 4 and 6 kg/m2) (iii) Three different air flow rates (1.05, 2.10 and 3.15 m3/min) (iv) Two different fan operating systems. Comparison among above conditions shows that load of 4 kg/m2 with air flow of 2.10 m3/min and continuous fan operating condition gives an optimal condition for drying of peppermint plant. Zaineb et al. [39] installed a chapel shaped greenhouse solar dryer attached with flat plate solar air collector at Research and Technology Centre of Energy in Borj Cedria (North of Tunisia). The performance of dryer was studied by drying red pepper. TRNSYS simulation program was used for developing mathematical model to predict the effect of the area of the product to be dried, the exhaust air flow rate and the collector area on drying kinetics. The pictorial view of solar greenhouse drying system is shown in Fig. 35. Experimental setup enveloped with plexi-glass had a floor area of 14.8 m2. The simulation results showed that collector area of 2 m2 is sufficient to compensate heat losses via centrifugal fans exhausting moist air. An increase in airflow rate higher than 250 kg/h does not affect drying kinetics. Water evaporation slows down due to increase in area of product which lowers the drying speed and increases the drying time. 3. Conclusion Various outcomes of this review are listed below-
• Greenhouse solar dryer operating in active mode is better as compared to passive mode. • Forced convection is suitable for high moisture content crops while natural convection can be used for crops with low moisture content. • Color, quality, taste and nutritious value of the dried product are better in greenhouse solar drying than open sun drying. • PV/T integrated greenhouse dryer are the best option for remote locations where electricity is not easily available. • Use of thermal storage material inside the greenhouse solar dryer • • •
increases the greenhouse inside temperature that reduces drying period. The insulated north wall prevents the heat loss to surroundings and improves the performance of greenhouse dryer. Pre-heating of air by using LPG burner, biomass or solar collector could be used to increases the dryer efficiency. For selecting a suitable dryer for drying any crop in any location design data given by simulated model can be used.
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(PVT) mixed mode greenhouse solar dryer. Sol Energy 2016;133:421–8. [37] Belloulid MO, Hassan Hamdi, Laila Mandi, Naaila Ouazzani. Solar greenhouse drying of waste water sludges under arid climate. Waste Biomass- Valoriz 2017;8:193–202. [38] Morad MM, El-Shazly MA, Wasfy KI, El-Maghawry Hend AM. Thermal analysis and performance evaluation of a solar tunnel greenhouse dryer for drying peppermint plants. Renew Energy 2017;101:992–1004. [39] Azaizia Z, Kooli S, Elkhadraoui A, Hamdi I, Guizani A. Investigation of a new solar greenhouse drying system for peppers. Int J Hydrog Energy 2017;42:8818–26.
[32] Chauhan PS, Kumar A. Heat transfer analysis of north wall insulated greenhouse dryer under natural convection mode. Energy 2017;118:1264–74. [33] Chauhan PS, Kumar A. Performance analysis of greenhouse dryer by using insulated north-wall under natural convection mode. Energy Rep 2016;2:107–16. [34] Prakash O, Kumar A, Laguri V. Performance of modified greenhouse dryer with thermal energy storage. Energy Rep 2016;2:155–62. [35] Jitjack K, Thepa S, Sudaprasert K, Namprakai P. Improvement of a rubber drying greenhouse with a parabolic cover and enhanced panels. Energy Build 2016;124:178–93. [36] Tiwari S, Tiwari GN, Al-Helal IM. Performance analysis of photovoltaic–thermal
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