Elsevier

Solar Energy

Volume 83, Issue 9, September 2009, Pages 1472-1484
Solar Energy

Improvement in greenhouse solar drying using inclined north wall reflection

https://doi.org/10.1016/j.solener.2009.04.001Get rights and content

Abstract

A conventional greenhouse solar dryer of 6 m2 × 4 m2 floor area (east–west orientation) was improved for faster drying using inclined north wall reflection (INWR) under natural as well as forced convection mode. To increase the solar radiation availability onto the product (to be dried) during extreme summer months, a temporary inclined wall covered with aluminized reflector sheet (of 50 μm thickness and reflectance 0.93) was raised inside the greenhouse just in front of the vertical transparent north wall. By doing so, product fully received the reflected beam radiation (which otherwise leaves through the north wall) in addition to the direct total solar radiation available on the horizontal surface during different hours of drying. The increment in total solar radiation input enhanced the drying rate of the product by increasing the inside air and crop temperature of the dryer. Inclination angle of the reflective north wall with vertical (β) was optimized for various selective widths of the tray W (1.5, 2, 2.5 and 3 m) and for different realistic heights of existing vertical north wall (h) at 25°N, 30°N and 35°N latitudes (hot climatic zones). Experimental performance of the improved dryer was tested during the month of May 2008 at Ludhiana (30.56°N) climatic conditions, India by drying bitter gourd (Momordica charantia Linn) slices. Results showed that by using INWR under natural convection mode of drying, greenhouse air and crop temperature increased by 1–6.7 °C and 1–4 °C, respectively, during different drying hours as compared to, when INWR was not used and saved 13.13% of the total drying time. By using INWR under forced convection mode of drying, greenhouse air and crop temperature increased by 1–4.5 °C and 1–3 °C, respectively, during different drying hours as compared to, when INWR was not used and saved 16.67% of the total drying time.

Introduction

Agricultural greenhouses are primarily used for increasing crop production during off-season. However, due to higher air temperature inside these greenhouses in hot climatic zones, these cannot be used for raising crops during extreme summer months and remain unused for about three months of May, June and July. During these months, this empty greenhouse can be used as a crop dryer (Condori and Saravia, 1998). This dual use of greenhouse for crop production and as a crop dryer improves its economic viability (Condori et al., 2001). Greenhouse drying has been practiced since more than two decades. Naturally ventilated greenhouses for drying applications have been reported in the past (Sadykov and Khairiddinov, 1982, Muthuveerapan et al., 1985). A solar tunnel dryer was used for drying grapes (Muhlbauer, 1983). It was reported that the dryer produced high quality dried grapes up to the desired moisture content level. In another study, a greenhouse type dryer for multi-crop solar drying was used in natural as well as forced convection mode for drying bamboo (Ong, 1996). It was reported that the moisture content of bamboo could be brought down to about 19% wb from 90% wb in 17 days by operating the dryer over 8 h each day. However, under natural ventilation drying conditions, the final moisture content reached to only 22% wb. Similarly, a greenhouse solar dryer was used to dry vanilla pods (Abdullah and Mursalim, 1997). A fiber reinforced plastic (FRP) hybrid solar drying house was effectively used for brown rice drying (Rachmat et al., 1998). In another study, a greenhouse was used for alfalfa drying considering the differences in drying behavior between stems and leaves of alfalfa (Rachmat and Horible, 1999). A three-tier drying rack was used in a greenhouse type solar dryer for multi-tiered drying of mustard under natural as well as forced convection conditions (Manohar and Chandra, 2000). The performance of another solar tunnel dryer was also studied for drying pineapple slices (Bala et al., 2003). The proximate analysis indicated that the pineapple dried in a solar tunnel dryer was a good quality product for human consumption. Peas and cabbage were dried inside the greenhouse under natural and forced convection and mathematical models were also developed. The predictions of crop temperature, greenhouse room air temperature and rate of moisture evaporation were made on the basis of solar intensity and ambient temperature (Jain and Tiwari, 2004). In another study, a thermal model of a natural convection greenhouse drying for jaggery was also developed (Kumar and Tiwari, 2006). It was shown that analytical and experimental results matched well. Recently a mixed mode type forced convection solar tunnel dryer was developed to dry hot red and green chillies (Hossain and Bala, 2007).

Experiments inside a wind tunnel were conducted to study the drying of red pepper in open sun and greenhouse conditions (Kooli et al., 2007) where solar radiation was simulated by a 1000 W lamp. Effect of drying parameters on moisture content and drying time were determined. A simple drying model of red pepper related to water evaporation process was developed and verified.

A mathematical model for drying agricultural products in a mixed-mode natural convection solar crop dryer was presented (Forson et al., 2007). The governing equations of the drying air temperature and humidity ratio; the material temperature and its moisture content; and performance criteria indicators were derived. Results of simulation runs using the model were presented and compared with the experimental data. It was shown that the model could predict the performance fairly accurately.

Feasibility studies of a solar chimney to dry agricultural products were also performed (Ferreira et al., 2008). A prototype solar chimney was built. The constructed chimney generated a hot airflow with a yearly average rise in temperature (compared to the ambient air temperature) of 13 ± 1 °C. In the prototype, the yearly average mass flow was found to be 1.40 ± 0.08 kg/s, which allowed a drying capacity of approximately 440 kg.

It is known that inside a fully closed single polyethylene (PE) cover greenhouse (east–west orientation), air temperature rises about 12–16 °C above the ambient air temperature (38–42 °C) during the extreme summer months (Sethi, 2009). However, for lowering the inside moisture, side and top ventilators have to be opened in natural convection mode or an exhaust fan has to be used in forced convection mode. Due to the opening of ventilators, rise in the inside air temperature of the greenhouse is limited to about 10–12 °C (under natural convection mode). On the other hand, use of exhaust fans for expelling the inside hot and moist air further lowers the inside air temperature rise to only 7–9 °C (under forced convection mode) (Sethi and Sharma, 2007). This increase in the inside air temperature is generally insufficient for effective crop drying and the crop takes comparatively longer time to reach up to the desired moisture content of the dried product.

It is observed from the above literature study that researchers around the world have made use of only the direct transmitted solar radiation for generating the greenhouse effect responsible for drying the product placed inside the greenhouse. However, a part of solar radiation incident on the transparent north wall is lost to the environment and is not used at all. Hence, in this study, an even span east–west orientation greenhouse (of 6 × 4 m2 area) has been modified for faster solar drying by using aluminized reflector sheet on the temporarily raised inclined north wall inside the greenhouse. The inclined wall blocks the fraction of solar radiation otherwise leaving from the north wall and reflects it back onto the product (placed adjacent to the north wall on a tray). In this way, inside product absorbs reflected beam radiation along with the direct global radiation which helps to further raise its temperature as well as inside air temperature under natural as well as forced convection mode. The angle of the reflective north wall with vertical (β) can be adjusted depending upon the zenith angle of the sun, width of the tray (W) and height of the vertical north wall (h). The literature survey shows that so far no such study for greenhouse crop drying using the reflective inclined north wall has been conducted to increase the crop drying rate. The inclination angle of the proposed reflective inclined wall has been optimized for the summer months at selected latitudes (of hot climates only) and for different tray widths. Experimental performance of the proposed greenhouse type solar dryer has been studied using INWR and without using INWR. The complete experiment has been performed under natural as well as forced convection modes of drying in summer conditions (May, 2008) at Ludhiana climate.

Section snippets

Description of improved greenhouse crop dryer

An east–west orientation, even span greenhouse of 6 m × 4 m floor area located at Ludhiana (30.56°N latitude, 247 m above sea level), Punjab, India was modified and converted into a dryer (Figs. 1a and 2, pictorial views). A door (1 m × 1.8 m) on the west wall and a ventilator (0.8 m × 0.8 m each) with screw opening and closing mechanism was provided on the center of each inclined north and south roof. The slope of both the roofs was 23.56° with the horizontal. Central and side height of the greenhouse was

Optimization of inclination angle for north wall and width of the tray

Zenith angle of the sun ‘θz’ at any time of the day and for any day of the year (Fig. 1a) can be determined at any latitude (Duffie and Beckman, 1991) asθz=cos-1(sinδsinϕ+cosδcosϕcosω)ω is equal to 15° times the number of hours from solar noon. It is negative before noon, zero at noon and positive after 12 noon and is given asω=15tsolar-12Declination angle of the sun in degrees is given byδ=23.45×sin[360(284+n)/365]

It is known that θz becomes smaller during summer months at 25°N, 30°N and 35°N

Experimental procedure

Pickles form an integral part of daily food intake in India. Bitter gourd (Momordica charantia Linn) is considered as one of the favorite pickles as it is a rich source of vitamins and minerals. This product is available in the market from April onwards and is available up to mid September. During this period, greenhouse can also be spared for crop drying applications as winter cropping is almost over. During the peak season, it can be dried and stored for rest of the year. So bitter gourd

Variation of solar radiation intensity and air velocity during experimentation

Variation of air velocity and global solar radiation intensity on horizontal (inside and outside) surface during each hour of the first experiment under natural convection mode (without using INWR) is shown in Fig. 3. It was observed that the variation in outside air velocity (Va) was between 0.35 and 1.25 m s−1 with average of 0.62 m s−1 during the three drying days. The measured global solar radiation intensity (beam and diffuse) on the horizontal surface (Ig) outside the greenhouse dryer varied

Conclusions

Based on the results it can be concluded that optimally inclined reflective north wall inside the greenhouse dryer (at an angle β) can significantly increase the total solar radiation incident onto the crop which can further enhance the air and crop temperature of the dryer from 10 am to 2 pm under forced as well as natural convection mode of drying. By using INWR under natural convection mode of drying, greenhouse air and crop temperature increased by 1–6.7 °C and 1–4 °C, respectively, during

Acknowledgements

The authors are thankful to the authorities of Punjab Agricultural University, Ludhiana and Punjab Govt. for providing the financial support under the scheme NP-43 entitled “Utilization of solar energy for creating optimum environment for plant and animal production systems”. The authors are also thankful to the Heads of the Departments of Mechanical Engineering and Processing and Food Engineering for providing the necessary facilities and manpower for fabrication work and conducting the

Cited by (0)

View full text