Review of solar dryers for agricultural and marine products

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Abstract

Drying for agricultural and marine products are one of the most attractive and cost-effective application of solar energy. Numerous types of solar dryers have been designed and developed in various parts of the world, yielding varying degrees of technical performance. Basically, there are four types of solar dryers; (1) direct solar dryers, (2) indirect solar dryers, (3) mixed-mode dryers and (4) hybrid solar dryers. This paper is a review of these types of solar dryers with aspect to the product being dried, technical and economical aspects. The technical directions in the development of solar-assisted drying systems for agricultural produce are compact collector design, high efficiency, integrated storage, and long-life drying system. Air-based solar collectors are not the only available systems. Water-based collectors can also be used whereby water to air heat exchanger can be used. The hot air for drying of agricultural produce can be forced to flow in the water to air heat exchanger. The hot water tank acts as heat storage of the solar drying system.

Introduction

The potential of using solar energy in the agricultural sector has increased due to fluctuation in the price of fossil fuel, environmental concerns and expected depletion of conventional fossil fuels. Solar assisted drying system is one of the most attractive and promising applications of solar energy systems in tropical and subtropical countries. Traditionally all the agricultural crops were dried in the sun. Drying is one of an important post handling process of agricultural produce. It can extend shelf life of the harvested products, improve quality, improve the bargaining position of the farmer to maintain relatively constant price of his products and reduces post harvest losses and lower transportation costs since most of the water are taken out from the product during the drying process. Direct sun drying requires large open space area, and very much dependent on the availability of sunshine, susceptible to contamination with foreign materials such as dusts, litters and are exposed to birds, insect and rodents. Hence, most agricultural produce that is intended to be stored must be dried first. Otherwise insects and fungi, which thrive in moist conditions, render them unusable.

Other limitations were given by the availability of appropriate drying equipment which is technically and economically feasible and the lack of knowledge how to process agricultural products. Up to now only a few solar dryers who meet the technical, economical and socio-economical requirements are commercially available. The technical development of solar drying systems can proceed in two directions. Firstly, simple, low power, short life, and comparatively low efficiency-drying system. Secondly, high efficiency, high power, long life expensive drying system [1], [2]. Various solar dryers have been developed in the past for the efficient utilization of solar energy. Many studied have been reported on solar drying of agricultural products [3], [4]. Several studied have been done in the tropics and subtropics to develop solar dryers for agricultural products. Basically, there are four types of solar dryers [5]; direct solar dryers, indirect solar dryers, mixed-mode dryers, and hybrid solar dryers.

The energy requirement for agricultural products can be determined from the initial and final moisture content of each product. Products have different drying rate and maximum allowable temperatures, as given in Table 1 [6], [7], [8].

Section snippets

Classification of solar drying systems

Table 2 shows a systematic classification of available solar dryers for agricultural products, based on the design of system components and mode of utilization of solar energy.

Natural convection solar dryers (passive dryers)

The simplest of solar cabinet dryer (Fig. 1) was presented by Othieno [9], it was very simple, and consists essentially of a small wooden hot box. Dimensions of this drier is 2 m × 1 m (long and width). The sides and bottom can be portable and can be constructed from wood or metal sheet. A transparent polyethylene sheet was used as cover at upper surface. Air holes are located on the sides of the drier for circulation.

Othieno [9] designed a portable direct type natural convection solar dryer (Fig. 2

Apple

Elicin and Sacilik et al. [31] has been developed a solar tunnel dryer (Fig. 23b) capable of dehydration of the apples. The moisture content was reduced from 82 to 11% in 32 h for the open sun drying, whereas the solar tunnel dryer took only 28 h. On base weather conditions, solar tunnel dryer resulted in a reduction in the drying time to an extent about 14% in comparison to open sun drying. The apple dried in the solar tunnel drier was completely protected from rain, insects, dust, bird, and the

Conclusions

The use of solar drying for agricultural and marine products has a large potential from the technical and energy saving point of view. Numerous types of solar dryers have been designed and developed in various parts of the world, yielding varying degrees of technical performance. Table 3 illustrates the general categories of solar dryers along with advantages and disadvantages of each. Table 4 gives examples of several solar driers and the possible price for local manufacture.

In conclusion the

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