Novel, low cost CaCl2 based desiccants for solar crop drying applications

Drying with solar-heated air is satisfactory so long as the sun is shining. To continue this process through the night-time and periods of cloud cover, it is necessary to either store some of this energy in a thermal mass or incorporate desiccants within the drying system. This paper reports the results from studies undertaken to develop three low cost, solar regenerative clay–CaCl₂ based solid desiccant materials; establish their moisture sorption and regeneration characteristics; assess their performance when compared with commercial desiccants; and integrate these within a low cost solar drying system for small-scale village-based crop drying. The moisture sorption and desorption performance of the desiccants were characterised in a Fison Environmental Cabinet at conditions of 85% (RH) and 25°C for 120 h for moisture sorption and 50°C and 20% (RH) for 8 h for regeneration. These conditions were representative of the environmental conditions monitored in the solar drying system. The bentonite–CaCl₂ (type 1) desiccant gave a maximum moisture sorption of 45% dry weight basis (dwb) while bentonite–CaCl₂ (type 2) and kaolinite–CaCl₂ (type 3) solid desiccants each gave moisture sorption values of 30% (dwb). It was concluded from the moisture sorption and regeneration characteristics that their application in solar crop drying and air dehumidification is highly useful due to their low regeneration temperatures, sub 100°C.     

A solar air heater with composite–absorber systems for food dehydration

Development of appropriate technologies for conversion of solar radiation to thermal energy is essential for food preservation. A solar air heater, comprising two absorber systems in a single flat-plate collector, was designed on the principles of psychrometry. The heater was integrated to a drying chamber for food dehydration. This collector design offered flexibility in manual adjustment of the thermal characteristics of the solar dryer. The performance of the dryer was evaluated by drying fresh samples of mango (Mangifera indicus). Both fresh and dried mango samples were analysed for moisture content (MC), pH and ascorbic acid. During the dehydration period, meteorological measurements were made. The air heater converted up to 21.3% of solar radiation to thermal power, and raised the temperature of the drying air from about 31.7 °C to 40.1 °C around noon. The dryer reduced the MC of sliced fresh mangoes from about 85% (w/w) to 13% (w/w) on wet basis, and retained 74% of ascorbic acid. It was found that the dryer was suitable for preservation of mangoes and other fresh foods.     

Design of mixed-mode natural convection solar crop dryers: Application of principles and rules of thumb

A mixed-mode natural convection solar crop dryer (MNCSCD) designed and used for drying cassava and other crops in an enclosed structure is presented. A prototype of the dryer was constructed to specification and used in experimental drying tests. This paper outlines the systematic combination of the application of basic design concepts, and rules of thumb resulting from numerous and several years of experimental studies used and presents the results of calculations of the design parameters. A batch of cassava 160 kg by mass, having an initial moisture content of 67% wet basis from which 100 kg of water is required to be removed to have it dried to a desired moisture content of 17% wet basis, is used as the drying load in designing the dryer. A drying time of 30–36 h is assumed for the anticipated test location (Kumasi; 6.7°N,1.6°W) with an expected average solar irradiance of 400 W/m² and ambient conditions of 25 °C and 77.8% relative humidity. A minimum of 42.4 m² of solar collection area, according to the design, is required for an expected drying efficiency of 12.5%. Under average ambient conditions of 28.2 °C and 72.1% relative humidity with solar irradiance of 340.4 W/m², a drying time of 35.5 h was realised and the drying efficiency was evaluated as 12.3% when tested under full designed load signifying that the design procedure proposed is sufficiently reliable.     

Development of a solar-assisted dryer and evaluation of energy requirement for the drying of onion

A solar-assisted forced convection dryer was developed to study the effect of airflow rate (2.43, 5.25, 8.09 kg/min), air temperature (55, 65, 75 °C), and fraction of air recycled (up to 90%) on the total energy requirement of drying of onion slices. The dryer was provided with a flat plate solar air heater having both the corrugations and triangular fins to the absorber plate. For drying of onion slices from initial moisture content of about 86% (wet basis) to final moisture content of about 7% (wet basis), the energy required per unit mass of water removed during without using recirculation of air was found between 23.548 and 62.117 MJ/kg water. The percent energy contribution by the solar air heater, electrical heater, and blower was found between 24.5% and 44.5%, 40.2% and 66.9%, and 8.6% and 16.3%, respectively. The savings in total energy due to fraction of air recycled were determined at 65 and 75 °C air temperature for the above three airflow rates. The maximum saving in total energy up to 70.7% was achieved by recycling of the exhaust air. The energy required per unit mass of water removed was found between 12.040 and 38.777 MJ/kg water. The percent energy contribution by the solar air heater, auxiliary heater, and blower was found between 22.4% and 40.9%, 33.6% and 62.6%, and 11.2% and 37.2%, respectively.     

Experimental and simulated performance of a PV-ventilated solar greenhouse dryer for drying of peeled longan and banana

This paper presents experimental and simulated performance of a PV-ventilated solar greenhouse dryer for drying of peeled longan and banana. The dryer consists of a parabolic roof structure covered with polycarbonate plates on a concrete floor. Three fans powered by a 50-W PV module ventilate the dryer. To investigate the experimental performances of the solar greenhouse dryer for drying of peeled longan and banana, 10 full scale experimental runs were conducted. Of which five experimental runs were conducted for drying of peeled longan and another five experimental runs were conducted for drying of banana. The drying air temperature varied from 31 °C to 58 °C during drying of peeled longan while it varied from 30 °C to 60 °C during drying of banana. The drying time of peeled longan in the solar greenhouse dryer was 3 days, whereas 5-6 days are required for natural sun drying under similar conditions. The drying time of banana in the solar greenhouse dryer was 4 days, while it took 5-6 days for natural sun drying under similar conditions. The quality of solar dried products in terms of colour and taste was high-quality dried products. A system of partial differential equations describing heat and moisture transfer during drying of peeled longan and banana in the solar greenhouse dryer was developed and this system of non-linear partial differential equations was solved numerically using the finite difference method. The numerical solution was programmed in Compaq Visual FORTRAN version 6.5. The simulated results reasonably agreed with the experimental data for solar drying of peeled longan and banana. This model can be used to provide the design data and is also essential for optimal design of the dryer.     

Evaluation of solar dried thyme (Thymus vulgaris Linné) herbs

Thyme (Thymus vulgaris Linné) herbs can be dried at about 50°C reached an equilibrium moisture content after 12 h and 9.5 h using the wire basket solar dryer and oven drying method respectively. The initial moisture content (wet wt. basis), (final moisture content, dry wt. basis (dwb)) determined by the Dean–Stark toluene method, oven and microwave were 75.15% (10.0%), 75.12% (11.85%) and 72.31% (12.50%) respectively. Paired t-test (α = 0.05, 10 degrees of freedom) showed no significant difference between the Dean–Stark toluene and the oven methods, but a significant difference between these two methods and the microwave method. The % essential oils extracted after drying by the oven and the wire basket solar methods were 0.5 and 0.6% (per 100 g dwb) respectively. The % oleoresin and ash content were 27% for both drying methods and 1.60, 2.03 and 2.25% for the fresh, oven dried and the wire basket solar dried herb respectively.     

Performance of a solar dryer using hot air from roof-integrated solar collectors for drying herbs and spices

A solar dryer for drying herbs and spices using hot air from roof-integrated solar collectors was developed. The dryer is a bin type with a rectangular perforated floor. The bin has a dimension of 1.0 m×2.0 m×0.7 m. Hot air is supplied to the dryer from fiberglass-covered solar collectors, which also function as the roof of a farmhouse. The total area of the solar collectors is 72 m². To investigate its performance, the dryer was used to dry four batches of rosella flowers and three batches of lemon-grasses during the year 2002–2003. The dryer can be used to dry 200 kg of rosella flowers and lemon-grasses within 4 and 3 days, respectively. The products being dried in the dryer were completely protected from rains and insects and the dried products are of high quality. The solar air heater has an average daily efficiency of 35% and it performs well both as a solar collector and a roof of a farmhouse.     

Solar-assisted smokehouse for the drying of natural rubber on small-scale Indonesian farms

Natural rubber in Indonesia is mainly produced by smallholder farmers and—being the highest foreign currency generating agricultural commodity—is of great importance for the Indonesian economy. Nevertheless, due to the lack of appropriate dryers, more than 80% of the total production has to be sold as low-grade Standard Indonesian Rubber (SIR) for a relatively low price.In order to improve the product quality, a solar-heated rubber sheet dryer was developed. It consists of a flat-plate solar air heater connected to a drying chamber. Part-recirculation of the exhaust air leads to the desired drying air temperature of 45–60°C and results in a significant reduction of the drying time. To enable drying independent of weather conditions, a biomass furnace was incorporated underneath the drying chamber.The tests have shown that during the rainy season, 320 kg of sheets can be uniformly dried to the required moisture content of 0.5% within 5 days. Compared with conventional smokehouses, the firewood consumption could be considerably reduced, from 1.0–1.5 kg per kg dry rubber to 0.3 kg when producing Ribbed Smoked Sheets (RSS). A further reduction can be expected by optimization of the heating intervals. During favourable weather conditions Air Dried Sheets (ADS) can be produced within 6–7 days without supplemented heat. The quality of the RSS and ADS fulfills the international standards.The simple design of the solar-assisted smokehouse allows local production and—after further improvements—seems to be an economical alternative for farmer groups or nucleus estates.     

Thin layer solar drying of rough rice

This paper presents a set of simple empirical equations for natural air flow solar drying of rough rice in mixed-mode type dryer, box-type dryer and open floor drying system. The moisture contents predicted by the equations were in good agreement with the observed values. The effect of drying air temperature on the drying rate constants for these three cases were found to be insignificant. The equilibrium moisture content appeared to be the most important variable controlling the drying rate. The highest drying rate was observed in case of mixed-mode dryer. The drying rate of box dryer was next to that of mixed-mode dryer. This study shows that the introduction of solar dryer for drying of rough rice is highly recommendable in Bangladesh.     

Development of a photovoltaic powered forced circulation grain dryer for use in the tropics

In tropical Africa, a high percentage of crop loss, of the order of 10–50%, occurs during the drying period due to either permanently high relative humidity and sudden rainfall periods, or such periods coinciding with the harvesting season. The traditional sun-drying technique is labour-intensive and requires a lot of land per unit throughput. Due to lack of any control, sun-dried grain is usually prone to overheating, cracking, vermin and dust contamination, rewetting during sudden rains, predation by animals and vandalism by humans. All this leads to significant product loss and impaired quality. A solar maize dryer incorporating a directly coupled photovoltaic (PV) powered d.c. fan was developed and field-tested for small scale use in Malawi, central Africa. The dryer has a capacity of 90 kg and it has been designed to utilise forced air circulation without the use of external power supplies like grid electricity, fossil fuel and batteries, which are either very expensive or non-existent in the rural areas of central Africa. A main design constraint was that the drying air temperature should not exceed 60°C, which is the international drying standard for maize grain used for human consumption. Temperatures in excess of 60°C lead to grain overheating, cracking and subsequent microbial attack. Results showed that the incorporation of a PV-driven d.c. fan provided some form of passive control over the air flow and hence the drying air temperature. The dryer was coupled to a solar air heater having a sun-tracking facility and optimised blackened sisal rope grids for improved energy collection efficiency of the order of 80%. Grain drying with this solar dryer technology, compared with sun drying, reduced the drying time by over 70%. Grain quality, texture, and flour quality and flavour improved significantly with the dryer, as grain was permanently protected from sudden rains, vermin and dust contamination. Although the capital cost of the solar dryer was found to be high (about US$900), the dryer was found to be cost-effective with a payback period of less than one year if it is used to dry grain for purchasing by the Cereal Boards.     

Solar drying of rose (Rosa sp.) petals

The rose (Rosa sp.) petals can be dried after 2 days at about 30°C reaching an equilibrium moisture content after 16 h using the solar wire basket dryer. The initial moisture content (wet wt basis) and final moisture content (dry wt basis), determined by the Dean–Stark toluene were 65.7 and 25.2% respectively. The intensity of the rose red coloured pigment (pelargonidin) decreased by a factor of 2.5 after drying. The pelargonidin ethanoic extract as an acid-base indicator, has a K_a value of 1×10⁻⁴ mol l⁻¹ and pH of end point 4 and imbibed on filter paper and allowed to air dry for 5 min showed excellent properties as acid-base test tapes.     

Design of a new solar dryer system with swirling flow for drying seeded grape

In this study solar energy supported, swirling flow new drying system is designed and artificial drying of grapes grown around Elazığ/Turkey is investigated. With the developed swirling flow dryer with airy solar collector it is examined that drying occurs homogenously and lower moisture values are obtained in when compared with classical drying system. Also it is found that with an increase in the drying air velocity decreases drying time. When air directing elements are placed inside drying chamber and rotating element to the entrance, it is examined that drying time gets shorter compared to that of natural drying. Thus, drying time which is 200 h in natural conditions decrease to 80 h with an air velocity of 1.5 m/s with the developed solar energy supported swirling flow dryer.     

Solar dryer with thermal storage and biomass-backup heater

An indirect type natural convection solar dryer with integrated collector-storage solar and biomass-backup heaters has been designed, constructed and evaluated. The major components of the dryer are biomass burner (with a rectangular duct and flue gas chimney), collector-storage thermal mass and drying chamber (with a conventional solar chimney). The thermal mass was placed in the top part of the biomass burner enclosure. The dryer was fabricated using simple materials, tools and skills, and it was tested in three modes of operation (solar, biomass and solar–biomass) by drying twelve batches of fresh pineapple (Ananas comosus), with each batch weighing about 20 kg. Meteorological conditions were monitored during the dehydration process. Moisture and vitamin C contents were determined in both fresh and dried samples. Results show that the thermal mass was capable of storing part of the absorbed solar energy and heat from the burner. It was possible to dry a batch of pineapples using solar energy only on clear days. Drying proceeded successfully even under unfavorable weather conditions in the solar–biomass mode of operation. In this operational mode, the dryer reduced the moisture content of pineapple slices from about 669 to 11% (db) and yielded a nutritious dried product. The average values of the final-day moisture-pickup efficiency were 15%, 11% and 13% in the solar, biomass and solar–biomass modes of operation respectively. It appears that the solar dryer is suitable for preservation of pineapples and other fresh foods. Further improvements to the system design are suggested.     

Thermal modeling of a natural convection greenhouse drying system for jaggery: An experimental validation

The aim of this work is to develop a thermal model so as to predict the jaggery temperature, the greenhouse air temperature and the moisture evaporated (jaggery mass during drying), during the drying of jaggery under natural convection conditions. The experiment was conducted separately for 0.75 kg and 2.0 kg of jaggery pieces having dimensions of 0.03 × 0.03 × 0.01 m³ for complete drying. The jaggery was dried in a roof-type even span greenhouse with floor area of 1.20 × 0.78 m². Experiment was carried out during February 5–8, 2004 at IIT Delhi (28°35′N 72°12′E) from 10 am to 5 pm. A computer program was developed in MATLAB software so as to calculate the jaggery temperature, the greenhouse air temperature and the moisture evaporated and was also used to predict the thermal performance of the greenhouse on the basis of solar intensity and ambient temperature. The software developed was experimentally validated. It was shown that the analytical and experimental results for jaggery drying are in good agreement.     

A large-scale solar greenhouse dryer using polycarbonate cover: Modeling and testing in a tropical environment of Lao People’s Democratic Republic

A large-scale solar greenhouse dryer with a loading capacity of 1000 kg of fruits or vegetables has been developed and tested at field levels. The dryer has a parabolic shape and the dryer is covered with polycarbonate sheets. The base of the dryer is a black concrete floor with an area of 7.5 × 20.0 m². Nine DC fans powered by three 50-W solar cell modules are used to ventilate the dryer. The dryer was installed at Champasak (15.13 °N, 105.79 °E) in Lao People’s Democratic Republic (Lao PDR). It is routinely used to dry chilli, banana and coffee. To assess the experimental performances of the dryer, air temperature, air relative humidity and product moisture contents were measured. One thousand kilograms of banana with the initial moisture content of 68% (wb) was dried within 5 days, compared to 7 days required for natural sun drying with the same weather conditions. Also three hundred kilograms of chilli with the initial moisture content of 75% (wb) was dried within 3 days while the natural sun drying needed 5 days. Two hundred kilograms of coffee with the initial moisture content of 52% (wb) was dried within 2 days as compared to 4 days required for natural sun drying. The chilli, coffee and banana dried in this dryer were completely protected from insects, animals and rain. Furthermore, good quality of dried products was obtained. The payback period of the dryer is estimated to be 2.5 years. A system of partial differential equations describing heat and moisture transfer during drying of chilli, coffee and banana in the greenhouse dryer was developed. These equations were solved by using the finite different method. The simulated results agree well with the experimental data. This model can be used to provide the design data for this type of dryer in other locations.     

Multi-shelf portable solar dryer

The solar dryer reported in this paper was developed to enable farmers to add value to their produce by drying it at farm itself. It can also be used in cottage industries in remote places. The dryer has a multi-shelf design with intermediate heating, passive, integral, direct/indirect and portable solar dryer. Intermediate heating of air in-between trays results in uniform drying in all the trays. Since the dryer at the farm is not likely to be used throughout the year, it has been made portable. A novel feature of this dryer is that the product can be dried under shade or otherwise as per requirement. The design is low cost to make it economically viable. The maximum stagnation temperature was 75 °C in the month of November at Ludhiana (31°N). During experiments on drying of fenugreek leaves the moisture evaporation on first, second and third drying day was 1.4, 0.9 and 0.4 kg/m² of aperture area. To overcome the problem of reduction in efficiency on second and third drying day, a semi-continuous mode of loading has been investigated, in which the efficiency remains almost the same on all drying days. The shelf life of the dried product is more than one year.     

Design, construction and testing of a chimney that reduces dangerous temperatures in a radiative convective solar dryer

One of the problems encountered in agricultural solar dryers that operate by natural thermo-convection is the “burn-up” of the products due to excessive solar radiation and/or low internal air velocity (insufficient chimney draft). This paper describes the design, construction and testing, in a solar dryer prototype, of a chimney of maximum draft intended to eliminate the above mentioned phenomenon. The results indicate that a slight geometry modification (keeping height constant) will increase air velocity by a factor of 2–3 with respect to a chimney of cylindrical shape, thereby decreasing product temperature approximately 10°C.     

Silk cocoon drying in forced convection type solar dryer

The thin layer silk cocoon drying was studied in a forced convection type solar dyer. The drying chamber was provided with several trays on which the cocoons loaded in thin layer. The hot air generated in the solar air heater was forced into drying chamber to avoid the direct exposure of sunlight and UV radiation on cocoons. The drying air temperature varied from 50 to 75 °C. The cocoon was dried from the initial moisture content of about 60–12% (wb). The drying data was fitted to thin layer drying models. Drying behaviour of the silk cocoon was best fitted with the Wang and Singh drying model. Good agreement was obtained between predicted and experimental values. Quality of the cocoons dried in the solar dryer was at par with the cocoons dried in the conventional electrical oven dryer in term of the silk yield and strength of the silk. Saving of electrical energy was about 0.75 kWh/kg cocoons dried. Economic analysis indicated that the NPV of the solar dryer was higher and more stable (against escalation rate of electricity) as compare to the same for electrical oven dryer. Due to simplicity in design and construction and significant saving of operational electrical energy, solar cocoon dryer seems to be a viable option.     

Performance studies of solar tunnel dryer for drying aonla (embilica officinalis) pulp

A solar tunnel dyer was constructed and evaluated the performance for drying aonla pulp. The dryer consists of a transparent UV stabilized plastic covered solar collector cum drying unit. Evaluation parameters are air temperature, solar insolation, moisture content, relative humidity and airflow rate. A minimum of 111.18 m² solar collector area is required to dry a batch 1000 kg aonla pulp in 16 hours (two days drying period). The initial and final moisture content considered were 424.93 and 10.08% dry basis, respectively. It was observed that on an average 43 per cent of higher temperature was obtained in solar tunnel dryer over the ambient temperature. The results obtained during the test period denoted that the maximum gained energy occurred at 13 o’clock hour and then gradually declined since the maximum solar radiation occurred at this time.     

Experimental investigation of a solar dryer with natural convective heat flow

A direct type natural convection solar dryer is designed. It is constructed in local materials (wood, blades of glass, metals) then tested experimentally in foodstuffs drying (cassava, bananas, mango). It is about an experimental approach which consists in analyzing the behavior of the dryer. The study relates mainly kinetics and establishment of drying heat balances. The influence of significant parameters governing heat and mass transfers, such as solar incident radiation, drying air mass flow and effectiveness, is analyzed in order to evaluate its thermal performances. Experimental data can be represented by empirical correlations of the form M(t)=M_iexp(−kt) for representation of drying process. The resolution of these drying equations makes-possible to predict total drying time of each product. Moreover, this drying process allows to reduce the moisture content of cassava and sweet banana approximately to 80% in 19 and 22 h, respectively to reach the safety threshold value of 13%. This value permits the conservation of these products about one year without deterioration. The determination of parameters, like ambient temperature, drying chamber temperature, drying air mass flow and incident heat fluxes, allow to predict the drying effectiveness for modeling and refining the dimensioning of the elaborate prototype.