Improvement in greenhouse solar drying using inclined north wall reflection

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.     

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.     

Performance of a solar crop dryer under PNG climatic conditions

A small solar crop dryer, consisting of a drying unit, thermal storage and solar collector, was designed for the climatic conditions of Papua New Guinea, and was constructed and tested at the Energy Research site of the University of Papua New Guinea. Detailed experimental studies were carried out for drying of tapioca, as well as the testing of the drying unit with and without thermal storage. Based on economic analysis, it is observed that the annual cost per kilogram of dried tapioca is about K0.27 ≠.     

Solar dryers in the Asia-Pacific region

The traditional method of solar drying in the Asia-Pacific region is by open-air drying where the product to be dried is exposed directly to the sun. The disadvantages of this traditional form of drying are many. Dirt, dust, and wind-blown debris, insect infestation, human and animal interference, will result in contamination of the product. Speed of drying and quality of dried product will be reduced due to over/under drying, intermittent sunshine, interruption and wetting by rain. Mechanical solar drying using natural convection or forced circulation would minimise these disadvantages. The author of this paper visited a number of countries and studied the numerous designs available for solar drying in the region. Three types of solar dryers considered by the author as having the best potential for development in the Asia-Pacific Region are discussed. These are the natural convection cabinet-type solar dryer, the forced convection indirect solar dryer, and the greenhouse-type solar dryer.     

Solar drying system for energy conservation

A low cost portable farm solar dryer was evaluated for drying goose berry candy in the conditions of Vidarbha region of Maharashtra state. Temperature profile at top, middle and bottom in its seven trays loaded with candy was studied with respect to ambient temperature during the course of drying and maximum solar radiation of 1120 W/m² was observed at 11.30 to 12 h. The solar radiation was varied from 720–500 W/m² at 9.00 h to 16.00 h. The minimum temperature of 27°C was observed at bottom tray of the dryer and maximum of 44°C in top tray at 9.00 h. The maximum temperature of 70°C was attained at 11.30 h. The conventional drying method took 8 days to dry the product. The moisture content was reduced from 36.38 to 8.33 per cent (wb) in three days in solar drying method. The product recovery was 71.55 per cent as compare to 35 per cent in conventional drying method. The drying period was reduced by 62 per cent and product recovery was doubled using portable farm solar dryer. The goose berry candy was also dried with and without shade drying methods. The temperature variation of dryer without shade was found, in the range of 23–36°C, 31–48°C and 38–55°C in bottom, middle and top trays respectively of dryer. The weight loss of 810, 870 and 820 g were observed in three days at bottom, middle and top trays of the dryer respectively. The thermal efficiency of the dryer onepy first day drying was found 15.55 and 15.23 per cent in shade and without shade drying methods respectively. Appearance, taste and flavour of goose berry candy dried in farm solar dryer with shade were superior to conventional drying. The cost of final product was Rs 114/kg. The profit from a single unit of farm solar dryer per year was Rs 57588/-.     

Development of a new type of solar dryer: Its mathematical modelling and experimental evaluation

A solar dryer fitted with a novel design of absorber having inbuilt thermal storage capabilities was designed, fabricated, simulated and also tested at Rajiv Gandhi College of Engineering Research & Technology, Chandrapur (MS) India. Thermic oil was used as a storage material. The main objective of the study was to reduce the drying period and enhance the quality of dried product mainly chillies and fenugreek leaves. The products were laid in a single layer. The dimensions of the dryer were arrived at using the well-defined procedure available in literature. The mass of thermic oil needed in the absorber and mass of product to be dried in trays were optimized using simulation techniques. The maximum drying air temperature required for drying agricultural products was around 65°C. The ambient conditions at the location were 25–40°C, 16–43% RH and solar radiation 105–1024 W m⁻². Experimental studies based on temperature and humidity measurements were performed on the dryer. The research concluded that the desired drying air temperature was achieved and maintained for a longer period. The length of operation of the solar air heater and the efficiency of the dryer were increased, and better quality of agricultural products in terms of colour value were obtained compared with open sun drying. Copyright © 2007 John Wiley & Sons, Ltd.     

Performance of indirect through pass natural convective solar crop dryer with phase change thermal energy storage

A state-of-the-art solar crop dryer was developed with thermal energy storage to maintain continuity of drying of herbs for their colour and flavour vulnerability. The dryer consists of flat plate solar collector, packed bed phase change energy storage, drying plenum with crop trays and natural ventilation system. Dryer is designed with a maximum collector area of 1.5 m², six crop trays with an effective area of 0.50 × 0.75 m², can hold 12 kg of fresh leafy herbs. The dryer is attached with a packed bed thermal energy storage having capacity of 50 kg phase change material (PCM). The drying system works in such a manner that phase change material stores the thermal energy during sun shine hours and releases the latent and sensible heat after sunset, thus dryer is effectively operative for next 5–6 h. The temperature in drying chamber was observed 6 °C higher than the ambient temperature after sunshine hours till the mid night during the month of June at Jodhpur. Economic performance of the dryer was analysed with return on capital and simple payback period as 0.65 and 1.57 year respectively on optimum cost of raw material and product sale price.     

Optimization of mixed-mode and indirect-mode natural convection solar dryers

This paper presents a comparison of optimized mixed-mode and indirect-mode natural convection solar dryers for maize. The mixed-mode and indirect-mode solar drying simulation models were validated against results from a laboratory solar dryer with experiments carried out under a solar simulator at the University of Newcastle upon Tyne, UK. The models are now run under variable solar conditions in order to optimize the dryers and compare their performance. The inputs to the simulation models were Zambian weather conditions and materials. The solar drying simulations are combined with the cost of the dryer materials and a search technique that finds the dryer dimensions at the minimum drying cost. Optimization gave a shorter collector length for the mixed-mode solar dryer (1.8 m) than for the indirect-mode dryer (3.34 m) of the same grain capacity (90 kg). The drying cost, annual cost and initial cost of the mixed-mode dryer are lower than those of the indirect-mode although the quantity of dry grain obtained from the mixed-mode for the whole year is less than for the indirect-mode; the drying costs are 12.76 and 16.05 US$/ton for mixed-mode and indirect-mode dryers, respectively.     

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.     

Experimental and modelling performances of a roof-integrated solar drying system for drying herbs and spices

This paper presents experimental performance of solar drying of rosella flower and chili using roof-integrated solar dryer and also presents modelling of the roof-integrated solar dryer for drying of chili. Field-level tests for deep bed drying of rosella flower and chili demonstrated that drying in the roof-integrated solar dryer results in significant reduction in drying time compared to the traditional sun drying method and the dry product is a quality dry product compared to the quality products in the markets. The payback period of the roof-integrated solar dryer is about 5 years. To simulate the performance of the roof-integrated solar dryer for drying herbs and spices using hot air from roof-integrated solar collectors, two sets of equations were developed. The first set of equations was solved implicitly and the second set of equations was solved explicitly using finite difference technique. The simulated air temperatures at the collector outlet agreed well with the observed air temperatures. Good agreement was also found between experimental and simulated moisture contents.     

Some mechanical aspects of low cost augmented integrated rock system

The development of an augmented integrated rock system is described in this paper. A unique AIRS (Flat Plate air heater connected in series with an integrated rock storage and collection system) concept is used as the absorber plate-to-air stream and then air stream-to-rock bed storage and collection unit heat transfer mechanism. The intention was to increase the efficiency of the air heater, over that of a ‘conventional’ type solar air heater by increasing the absorber area which includes the metallic absorber plate and the rock surface exposed directly to the sun. The programme objective was to design a device which can store the energy during its operation and supply it during the evening and late night hours when there is no sun in the sky. In order to achieve this goal, we have fabricated, tested and evaluated ‘AIRS’, and the results obtained are presented in this paper.     

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.     

Experimental performance of a new design of solar dryer for drying osmotically dehydrated cherry tomatoes

The performance of a new design of a solar dryer for drying osmotically dehydrated cherry tomatoes is presented. The dryer consists of drying cabinet, heat exchanger, 16-m² water type solar collector, and water type heat storage unit. The cabinet size is 1.0 m wide × 3.0 m long × 1.4 m high with the load capacity of 100 kg for osmotically dehydrated cherry tomatoes. Three batches of osmotically dehydrated cherry tomatoes were dried in this dryer during May–June, 2014. For each batch, 100 kg of osmotically dehydrated cherry tomatoes were dried. There was a considerable reduction in drying time in the new solar dryer as compared to natural sun drying. The dried products were completely protected from rains and insects and were of high quality dried products. The efficiencies of the solar collector was 21%–69%. The pay-back period of the dryer is estimated to be 1.37 years.     

Experimental and simulation studies on the no-load performance assessment of a domestic solar dryer

Solar dryers have been tested with various simple and complex design modifications for better efficiencies. This article attempts to investigate the effect of a very simple design modification on the no-load performance of a natural convection domestic solar dryer (NCDSD). A direct-type domestic solar dryer has been developed with an air cavity surrounding the drying chamber. To compare the effect of air cavity, a domestic solar dryer without air cavity has also been developed and both the dryers were tested simultaneously under the climate of Hisar, India. The values of thermal efficiency were calculated along with convective heat transfer coefficient from absorber plate to the drying air. Both the dryers were also analyzed by developing finite element models in COMSOL multiphysics computer software. The no-load thermal efficiency for the domestic solar dryer without and with air cavity was found to be 22.68% and 34.08%, respectively. The values of coefficient of correlation for modeled and actual drying tray temperatures for dryer without and with air cavity were 0.980466 and 0.9833917, respectively. The proposed finite element model would be helpful in the design and development of NCDSDs.     

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.     

Design and experimental evaluation of a solar dryer for commercial high-quality hay production

Design, experimental functional performance and economic evaluation of an energy efficient commercial-type solar energy dryer for production of high-quality hay, especially for the export market, are presented. The newly developed solar hay dryer consists of a solar collector with aluminum absorber plate and spaced fins, a drying shed with perforated metal grate floor above the ground level, swing-away plywood frames and polyethylene curtains for effectively sealing the hay stack during drying operations, an insulated duct, and a crawl space below the floor where a 3-hp in-line centrifugal fan is housed for air circulation by suction. In late August and in early September, 1996, 160 small rectangular bales of alfalfa hay with about 25% bromegrass were successfully dried from 33% initial moisture content to 13% moisture and from 25% to 11% moisture in 4 and 3 days, respectively, under average weather conditions in Saskatoon, Canada. The air temperature rise above ambient was 13–15 °C during peak bright sunshine hours in August and 10–13 °C in September. Ambient relative humidities ranged from 30–90%. Unlike field-cured hay, the hay produced by the dryer was of high-quality and remained green in colour and attractive after drying. Compared to field drying or conventional natural gas drying system, the payback period on investment in full-scale solar hay drying system may be just one to two years.     

The effect of plane booster reflectors on the performance of a solar air heater with solar cells suitable for a solar dryer

We present a theoretical study of a solar photovoltaic-thermal (hybrid) system consisting of a flat-plate solar air heater mounted with solar cells and a plane booster. A conventional flat-plate collector is converted into a hybrid system by mounting solar cells directly on the absorber plate. A hybrid system is self-sufficient in the sense that the electrical energy required by the pump is supplied by the panel. Such systems are well suited to applications such as solar drying. The combined system is analysed for the case when the radiative and absorptive properties of the cell surface and the absorber plate are nearly the same. The solar cell efficiency is a linearly-decreasing function of the absorber plate temperature. The performance of the system has been evaluated for various combinations of boosters. The minimum area of the solar cells required to run the pump at a given flow rate has been calculated as a function of time, with and without boosters. The minimum cell area required decreases with the use of boosters. High cost cells may be replaced by low cost reflectors. The solar air heaters presently available on the market are not suitable for direct conversion to hybrid systems.     

Crop drying by indirect active hybrid solar – Electrical dryer in the eastern Algerian Septentrional Sahara

In the present work, a new specific prototype of an indirect active hybrid solar–electrical dryer for agricultural products was constructed and investigated at LENREZA Laboratory, University of Ouargla (Algerian Sahara). In the new configuration of air drying passage; the study was done in a somewhat high range of mass flow rate between 0.04 and 0.08 kg/m² s a range not properly investigated by most researchers. Experimental tests with and without load were performed in winter season in order to study the thermal behavior of the dryer and the effect of high air masse flow on the collector and system drying efficiency. The fraction of electrical and solar energy contribution versus air mass flow rate was investigated. Slice tomato was studied with different temperatures and velocities of drying air in order to study the influence of these parameters on the removal moisture content from the product and on the kinetics drying and also to determine their suitable values. Many different thin layer mathematical drying models were compared according to their coefficient of determination (R²) and reduced chi square (χ²) to estimate experimental drying curves. The Middli model in this condition proved to be the best for predicting drying behavior of tomato slice with (R² = 0.9995, χ² = 0.0001). Finally an economic evaluation was calculated using the criterion of payback period which is found very small 1.27 years compared to the life of the dryer 15 years.     

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.