I have reviewed some papers in solar dryers, especially for which are designed for drying agricultural products. The following is the summarize of my review:
Enein et al. (2000) reported a parametric study of a solar air heater with and without thermal storage for solar drying applications. An optimization process for a flat-plate solar air heater with and without thermal storage was carried out. Three kinds of material for thermal storage were used, i.e. water, stones and sand. The average temperature of flowing air increases with the increase of the collector length and width up to typical values for these parameters. The outlet temperature of flowing air was found to decrease with an increase of the airflow channel spacing and mass flow rate. The thermal performance of the air heater with sensible storage materials is considerably higher than that without the storage. An optimal thickness of the storage material of about 0.12 m was found to be convenient for drying various agriculture products. In addition, the proposed mathematical model may be used for estimating of the thermal performance of flat plate-solar air heater with and without thermal storage.
Pangavhen et al. (2002) proposed a design, development and performance testing of a new convection solar dryerThe solar dryer is capable of producing average temperature between 50 and 55°C, which was optimal for dehydration of grapes as well as for most of the fruits and vegetables. This system was capable of generating an adequate natural flow of hot air to enhance the drying rate. The drying airflow rate increases with ambient temperature by the thermal buoyancy in the collector. The collector efficiencies ranged between 26% for mass flow rate of 0.0126 kg/s of air and 65% for mass flow rate of 0.0246 kg/s. This was sufficient for heating the drying air. The drying time of grapes was reduced by 43% compared to the open sun drying.
Bena and Fuller (2002) developed a direct-type natural convection solar dryer with simple biomass burner. It was expected to be suitable for small-scale processors of dried fruits and vegetables in non-electrified areas of developing countries. The capacity of the dryer was found to be 20–22 kg of fresh pineapple arranged in a single layer of 1-cm-thick slices. The key features of the biomass burner were found to be the addition of thermal mass on the upper surface, an internal baffle plate to lengthen the exhaust gas exit path and a variable air inlet valve. The author also suggested some modifications to further improve the performance of both the solar and biomass components of the dryer.
Ekechukwu and Norton (1999) presented a comprehensive review of the various designs, details of construction and operational principles for a variety of practical solar-energy drying systems. The appropriateness of each design type for applications used by rural farmers in developing countries was discussed.
Mumba (1995) developed a photovoltaic-powered forced-circulation grain dryer for use in the tropics. From performance testing results, it was indicated that the dryer has a capacity of 90 kg (maize) for drying from an initial moisture content 33.3% to under 20% during one day. The important feature of this dryer is the use of photovoltaic solar cell incorporated in the solar air heater section to power a D.C. fan.
Salom et al.( 1996) proposed a mathematical model for indirect solar dryers where the drying chamber is assumed thermally insulated and opaque from the incident radiation. The solar dryer model is composed of a solar collector, a drying chamber and a chimney. The modeled dryer system is based on the chimney effect and natural convection flow. The mathematical model and solution procedure for the prediction of the thermal behavior of indirect solar dryers was presented in order to develop and improve dryer designs. However, the author suggested that the proposed model should be further validated. More information is needed with respect to the values of the chamber pressure drop and the drying rates for indirect solar dryers.
Bala et al. (1995) presented a technique for optimization of natural-convection, solar dryers. The optimal design was specified for the condition of Bangladesh. The physical simulation was combined with a cost prediction and experimental techniques, which found the constrain minimum of total cost per unit moisture removal. The optimum design was a relatively long collector, a thin grain bed and negligible chimney height. The result of sensitivity analysis indicated that the design geometry is insensitive to material or fixed costs, but grain capacity has some effects. Turning the grain has almost no effect on drying but greatly reduces over-drying at the bottom of the bed. In addition, the authors suggested that it would be useful if the theory were applied to a wider range of meteorological conditions. It is essential that the optimized drying be verified experimentally.
Bennamoun and Azeddine (2003) studied a simple, efficient and inexpensive solar batch dryer for agricultural products through simulations. They used onion as the dried product, and the shrinking effect was taken into account. In addition, it was suggested that the study could be developed for other agricultural products and for the behavior of solar dryer in different seasons.
Sebaii et al. (2002) reported a study of an indirect type natural convection solar which investigated experimentally and theoretically for drying grapes, figs, onions, apples, tomatoes and green peas. The drying constants for the selected crops were obtained from the experimental results and were then correlated with the drying product temperature. Linear correlation between drying constant and product temperature were proposed for the selected crops. The empirical constants of Henderson’s equation were obtained for all the materials from investigation, which are not available in the literature. The proposed empirical correlation suggested that it could well describe the drying kinetics of the selected crops.
Gallali et al. (2000) reported the result of an investigation of some dried fruit and vegetables (grapes, figs, tomatoes and onions) based on chemical analysis (vitamin C, total reducing sugars, acidity, moisture, and ash content) and sensory evaluation data (color, flavor, and texture). They compared products dried by solar dryers and natural sun drying. The study indicated that using solar dryers gives more advantages than natural sun drying, especially in terms of drying time.
Karathanos and Belessiotis (1997) reported the sun and solar air drying kinetics of some agricultural products, i.e. sultana grapes, currants, figs, plums and apricots. The drying rates were found for both solar and industrial drying operations. Air and product temperatures were measured for the entire industrial drying process. It was shown that most materials were dried in the falling rate period. Currants, plums, apricots and jigs exhibited two drying rate periods, a first slowly decreasing (almost constant) and a second fast decreasing (falling) drying rate period. In addition, they indicated that the industrial drying operation resulted in a product of superior quality compared to products dried by solar dehydration.
Leon et al., (2002) presented a review of existing evaluation methods and the parameters generally considered for evaluation of solar food dryers. These parameters can be classified as : (i) physical features of the dryers; (ii) thermal performance; (iii) quality of dried product; (iv) cost of dryer and payback period.
a. Physical features of dryer
- type, size, and shape
- collector area and solar aperture
- drying capacity/loading density (kg/unit aperture area)
- tray area and number of layers
- loading/unloading convenience
- loading/unloading time
- handling, cleaning, maintenance convenience, and ease of construction
b. Thermal performance
- drying time/drying rate up to 10% product moisture content (d.b.), (this may, however, vary from product to product)
- dryer/drying efficiency until product moisture content reaches 10% (d.b.)
- first day drying efficiency
- drying air temperature and relative humidity
- maximum drying temperature at no-load and with load
- duration of drying air temperature10oC above ambient
- airflow rate
c. Quality of dried products
- sensory quality (color, flavor, taste, texture, aroma)
- nutritional attributes - quantified for easy comparison
- rehydration capacity - consistency in presentation
- uniformity of drying
d. Economics and other parameters
- cost of drying and payback periods
- floor space requirements, skilled technician and operator requirements, and safety and reliability
In addition, a comprehensive procedure for testing solar dryers has been also proposed.
(from many papers)
