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Review of solar-energy drying systems II: an overview of

solar drying technology

O.V. Ekechukwu

a,

*, B. Norton

b

aEnergy Research Centre, University of Nigeria, Nsukka, Nigeria

bPROBE, Centre for Performance Research on the Built Environment, Department of Building and Environmental Engineering, University of Ulster, Newtownabbey BT37 0QB, Northern Ireland, U.K.

Received 29 August 1997

Abstract

A comprehensive review of the various designs, details of construction and operational principles of the wide variety of practically-realised designs of solar-energy drying systems reported previously is presented. A systematic approach for the classi®cation of solar-energy dryers has been evolved. Two generic groups of solar-energy dryers can be identi®ed, viz passive or natural-circulation solar-energy dryers and active or forced-convection solar-energy dryers (often referred to as hybrid solar dryers). Three sub-groups of these can also be identi®ed, viz integral-type (direct mode), distributed-type (indirect mode) and the mixed-mode type. The appropriateness of each design type for application by rural farmers in developing countries is discussed. Some very recent developments in solar drying

technology are highlighted.#1998 Elsevier Science Ltd. All rights reserved.

Keywords:Solar-energy drying systems; Systematic classi®cation; High temperature dryers; Low temperature dryers; Open-to-sun drying; Passive solar dryers; Natural-circulation solar dryers; Active solar dryers; Forced-convection solar dryers; Hybrid solar dryers; Integral-type solar dryers; Direct solar dryers; Distributed-type solar dryers; Indir-ect solar dryers; Mixed-mode solar dryers; Application by rural farmers

1. Introduction

In many rural locations in Africa and most developing countries, grid-connected electricity and supplies of other non-renewable sources of energy are either unavailable, unreliable or, for many farmers, too expensive. Thus, in such areas, crop drying systems that employ motorised fans and/or electrical heating are inappropriate. The large initial and running costs of fossil fuel powered dryers present such barriers that they are rarely adopted by small scale farmers.

0196-8904/99/$ - see front matter#1998 Elsevier Science Ltd. All rights reserved. PII: S0196-8904(98)00093-4

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The traditional open sun drying utilised widely by rural farmers has inherent limitations: high crop losses ensue from inadequate drying, fungal attacks, insects, birds and rodents encroachment, unexpected down pour of rain and other weathering e€ects. In such conditions, solar-energy crop dryers appear increasingly to be attractive as commercial propositions.

Climatic conditions have a great in¯uence on the extent of crop losses and deterioration during sun drying. If a climate is warm and dry, crops can be ®eld dried. For this to be feasible, the ambient relative humidity during the harvest period must be low enough to ensure that the crop, when dried to its equilibrium moisture content, can be stored safely. Meterological data, even for the ``most favoured'' areas, show that this is not always feasible [1]. The crop also requires an undesirably long period to reach this equilibrium moisture content. In hot and humid climates, crop deterioration is obviously worse, as both warmth and high moisture contents promote the growth of fungi, bacteria, mites and insects in crops. Warmth and siccity are propitious conditions for natural open sun drying. Unfortunately, the tropics are characterised by hot damp climates. If the relative humidity of ambient air is too high to facilitate drying in the ®eld, such air would obviously be of limited value for drying the harvested crop [1]. Thus, these climatic conditions dictate the need for more e€ective drying methods.

The basic essence of drying is to reduce the moisture content of the product to a level that prevents deterioration within a certain period of time, normally regarded as the ``safe storage period'' [2]. Drying is a dual process of,

. heat transfer to the product from the heating source and

. mass transfer of moisture from the interior of the product to its surface and from the surface to the surrounding air.

Drying involves the extraction of moisture from the product by heating and the passage of air mass around it to carry away the released vapour. Under ambient conditions, these processes continue until the vapour pressure of the moisture held in the product equals that held in the atmosphere [1, 2]. Thus, the rate of moisture desorption from the product to the environment and absorption from the environment are in equilibrium, and the crop moisture content at this condition is known as the equilibrium moisture content. Under ambient conditions, the drying process is slow, and in environments of high relative humidity, the equilibrium moisture content is insuciently low for safe storage [1, 2]. The objective of a dryer is to supply the product with more heat than is available under ambient conditions, thereby increasing suciently the vapour pressure of the moisture held within the crop and decreasing signi®cantly the relative humidity of the drying air and thereby increasing its moisture carrying capacity and ensuring a suciently low equilibrium moisture content [2].

In solar drying, solar-energy is used as either the sole source of the required heat or as a supplemental source. The air ¯ow can be generated by either natural or forced-convection. The heating procedure could involve the passage of preheated air through the product or by directly exposing the product to solar radiation or a combination of both [2]. The major requirement is the transfer of heat to the moist product by convection and conduction from the surrounding air mass at temperatures above that of the product or by radiation, mainly from the sun and to a little extent from surrounding hot surfaces, or conduction from heated surfaces in contact with the product [2]. Absorption of heat by the product supplies the energy

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necessary for the vapourization of water from the product, some 2.5 kJ of water evaporated [1]. The process that occurs at the surface of the product is simply evaporation. Water starts to vapourise from the surface of the moist product when the absorbed energy has increased its temperature suciently for the water vapour pressure of the product moisture to exceed the vapour pressure of the surrounding air. Moisture replenishment to the surface is by di€usion from the interior, and this process depends on the nature of the product and its moisture content. If the di€usion rate is slow, it becomes the limiting factor in the drying process, but if it is suciently fast, the controlling factor may be the rate of evaporation at the surface [2]. The latter is the case at the commencement of the drying process. In direct radiation drying, part of the solar radiation may penetrate the material and be absorbed within the product itself, thereby generating heat in the interior of the product as well as at its surface, and thereby enhancing heat transfer. The solar radiation absorptance of the product is an important factor in direct solar drying. Fortunately, most agricultural materials have relatively high absorptances (between 0.67 and 0.90) [3] which may increase or decrease as the drying progresses. The thermal conductivity of the crop is also important, particularly if the drying layer is deep enough to require heat conduction between particles.

For economic reasons, maximum drying rates are desired though product quality must be considered. In addition, excessive temperature (which may adversely a€ect crop properties like germinability) must be avoided when drying certain crops. During drying, some crops have a tendency to form dry surface layers which are impervious to subsequent moisture transfer if the drying rate is very rapid. To avoid this requires e€ective control of the drying process. The heat transfer and evaporation rates must be closely controlled to guarantee optimum drying rates. The control of the drying process in natural-circulation dryers presents a major problem, as such dryers are designed to minimise capital and running costs. Thus, special control mechanisms are inappropriate. The best approach is to incorporate into the design of the dryers such structural features that would guarantee that extreme conditions to not prevail in the dryer under the envisaged climatic conditions and crop properties [2]. One such approach, which regulates the residency period of the drying air within the drying chamber for natural-convection solar dryers is the incorporation of ``chimneys''.

2. A systematic classi®cation of drying systems

All drying systems can be classi®ed primarily according to their operating temperature ranges into two main groups of high temperature dryers and low temperature dryers. However, dryers are more commonly classi®ed broadly according to their heating sources into fossil fuel dryers (more commonly known as conventional dryers) and solar-energy dryers. Strictly, all practically-realised designs of high temperature dryers are fossil fuel powered, while the low temperature dryers are either fossil fuel or solar-energy based systems.

2.1. High temperature dryers

High temperature dryers are necessary when very fast drying is desired. They are usually employed when the products require a short exposure to the drying air. Their operating

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temperatures are such that, if the drying air remains in contact with the product until equilibrium moisture content is reached, serious over drying will occur. Thus, the products are only dried to the required moisture contents and later cooled [1]. High temperature dryers are usually classi®ed into batch dryers and continuous-¯ow dryers [1, 4, 5]. In batch dryers, the products are dried in a bin and subsequently moved to storage. Thus, they are usually known as batch-in-bin dryers [4]. Continuous-¯ow dryers are heated columns through which the product ¯ows under gravity and is exposed to heated air while descending [5]. Because of the temperature ranges prevalent in high temperature dryers, most known designs are electricity or fossil-fuel powered. Only a very few practically-realised designs of high temperature drying systems are solar-energy heated [2].

2.2. Low temperature dryers

In low temperature drying systems, the moisture content of the product is usually brought in equilibrium with the drying air by constant ventilation. Thus, they do tolerate intermittent or variable heat input. Low temperature drying enables crops to be dried in bulk and is most suited also for long term storage systems. Thus, they are usually known as bulk or storage dryers [1]. Their ability to accommodate intermittent heat input makes low temperature drying most appropriate for solar-energy applications. Thus, some conventional dryers and most practically-realised designs of solar-energy dryers are of the low temperature type.

2.3. Classi®cation of solar-energy drying systems

Fig. 1 illustrates a systematic classi®cation of drying systems, indicating the sub-classes and the group lineage of solar drying systems. Solar-energy drying systems are classi®ed primarily according to their heating modes and the manner in which the solar heat is utilised.

In broad terms, they can be classi®ed into two major groups, namely [2]:

. active solar-energy drying systems (most types of which are often termed hybrid solar dryers); and

. passive solar-energy drying systems (conventionally termed natural-circulation solar drying systems).

Three distinct sub-classes of either the active or passive solar drying systems can be identi®ed (which vary mainly in the design arrangement of system components and the mode of utilisation of the solar heat, namely [2]:

. integral-type solar dryers;

. distributed-type solar dryers; and

. mixed-mode solar dryers.

The main features of typical designs of the various classes of solar-energy dryers are illustrated in Fig. 2.

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Fig. 1. Classi®cation of dryers and drying modes.

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3. Passive solar drying systems

3.1. Open-to-sun drying

There are two traditional approaches by which passive solar crop drying is undertaken in tropical countries, namely [2]:

. The plant bearing the crop is allowed to die, either in contact with the soil or is cut down but not removed, thus the crop is dried ``in situ''.

. The crop is spread on the ground, mat, cemented ¯oor or placed on either horizontal or vertical shelves exposed to solar radiation and to natural air currents. The crop is usually stirred occasionally in order to expose di€erent parts of it to the sun and thereby encourage more rapid removal of the saturated air.

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Despite the rudimentary nature of the processes involved, such techniques still remain in common use. Because the power requirements (i.e. from the solar radiation and the air's enthalpy) are readily available in the ambient environment, and as little or no capital cost is required and running costs low (often labour only), these are frequently the only commercially viable methods in which to dry agricultural produce in developing countries. Though utilised widely, natural open-to-sun drying techniques have inherent limitations: high crop losses ensue from inadequate drying, fungal and insect infestation, birds and rodent encroachment and weathering e€ects. The process is intermittent, being a€ected by cloudiness and unexpected rain. Output is low and can be of very poor quality.

For tropical climates, sun drying poses serious practical problems during the wet season, as periodically but irregularly, the crop has to be removed to storage or protected from rain. The quality of the dried product is often degraded seriously, sometimes beyond edibility. Thus, at present, a large proportion of the world's supply of dried fruits and vegetables continue to be ``sun dried'' in the open under primitive conditions. Whilst more ecient solar drying methods are being developed, the traditional drying methods do have the following positive attributes:

. small capital cost;

. low running cost;

. independence from fuel supplies.

3.2. Natural-circulation solar-energy crop dryers

Natural-circulation solar-energy dryers depend for their operation entirely on solar-energy. In such systems, solar-heated air is circulated through the crop by bouyancy forces or as a result of wind pressure, acting either singly or in combination. These dryers are often called ``passive'' in order to distinguish them from systems that employ fans to convey the air through the crop. The latter are termed ``active'' solar dryers. Natural-circulation solar-energy dryers appear the most attractive option for use in remote rural locations. They are superior operationally and competitive economically to natural open-to-sun drying. The advantages of natural-circulation solar-energy tropical dryers that enable them to compete economically with traditional drying techniques are:

. they require a smaller area of land in order to dry similar quantities of crop that would have been dried traditionally over large land areas in the open;

. they yield a relatively high quantity and quality of dry crops because fungi, insects and rodents are unlikely to infest the crop during drying;

. the drying period is shortened compared with open air drying, thus attaining higher rates of product throughput;

. protection is a€orded the crop from sudden down pours of rain; and

. commercial viability, i.e. their relatively low capital and maintenance costs because of the use of readily available indigenous labour and materials for construction.

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Three generic types of natural-circulation solar-energy dryers have evolved and both retain many of the advantages of traditional open-to-sun drying. These are:

. integral-type natural-circulation solar-energy dryers;

. distributed-type natural-circulation solar-energy dryers; and

. mixed-mode natural-circulation solar-energy dryers.

3.2.1. Distributed-type natural-circulation solar-energy dryers

These are often termed indirect passive solar dryers. Here, the crop is located in trays or shelves inside an opaque drying chamber and heated by circulating air, warmed during its ¯ow through a low pressure drop thermosyphonic solar collector [6]. Because solar radiation is not incident directly on the crop, caramelization and localised heat damage do not occur [2, 7]. These dryers are also recommended generally for some perishables and fruits for which their vitamin content are reduced considerably by direct exposure to sunlight and for colour retention in some highly pigmented commodities that are also very adversely a€ected by direct exposure to the sun [7].

Distributed passive solar dryers have higher operating temperatures than direct dryers or sun drying and can produce higher quality products. Thus, they are recommended for relatively deep layer drying [8]. Their shortcomings, however, are the ¯uctuations in temperatures of the air leaving the air heaters, thereby making it dicult to maintain constant operating conditions within the drying chamber, and the operational diculties of loading and unloading the trays and occasional stirring of the product [8]. Distributed-type dryers, though, have an inherent tendency towards greater eciency, as the component units can be designed for optimal eciency of their respective functions [7]. They are, however, relatively elaborate structures requiring more capital investment in equipment and incur larger running (i.e. maintenance) costs than the integral units [2].

A typical distributed natural-circulation solar-energy dryer (Fig. 3) would be comprised of the following basic units:

. an air-heating solar-energy collector;

. appropriately insulated ducting;

. a drying chamber; and

. a chimney.

Though no detailed side-by-side tests have been reported, it is generally agreed that well designed forced-convection distributed solar dryers are more e€ective and more controllable than the natural-circulation types [7]. Thus, most practically-realised distributed solar dryers are of the active (forced-convection) type. Of the natural-circulation types built, most are of the mixed-mode design (which retain most of the features of distributed dryers). Thus, few practically-realised typical distributed type-passive solar dryers are reported in the literature [2]. Fig. 4 illustrates an indirect solar maize dryer by Othieno, Grainger and Twidell [9±12]. The dryer consisted of a single-glazed passive solar air heater with a 1 m2 single ¯at-plate absorber

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bin equipped with a chimney. The entire dryer assembly was made from hardboard. To improve eciency, the air heater was modi®ed with a wider air gap (15 cm) to accommodate three layers of wire-mesh absorber between the glazing and the ¯at-plate absorber. The dryer was capable of drying 90 kg of wet maize from a moisture content of about 20% wet basis to 12% within 3 days on a bright day.

A bigger, but similar, design to the one described above, constructed for timber drying, was reported by Akachukwu [13]. Reported modi®cations to the typical indirect dryer design include absorbers equipped with thermal storage, either of rock bed [14] or water [15, 16], and

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the ventilated greenhouse solar air heaters made of clear plastic covers and black plastic absorbers [17]. A few other designs are equipped with re¯ectors or concentrators [15, 16, 18]. One unique design [19] consisted of a freely-ventilated cylindrical crib (made from chicken wire mesh and local bush stems as the drying chamber) attached to a solar air heater. Moist air exit was via the numerous vents of the mesh work. Performance studies of other practically-realised designs of distributed-type natural-circulation solar dryers have also been reported [20±29].

Emphasis on the design and construction of distributed passive solar dryers has tended to be on improved eciency of the air heaters and air circulation. Little attention has been paid to the ecient design of the drying chambers. Drying chambers are usually constructed from wooden materials (sometimes without additional insulation) and thus are highly susceptible to damage under harsh weathering e€ects. Drying chambers should be insulated properly to minimise heat losses and made durable (within economically justi®able limits). Construction from metal sheets or water resistant cladding is recommended.

3.2.2. Integral-type natural-circulation solar-energy dryers

In integral-type natural-circulation solar-energy dryers (often termed direct solar dryers), the crop is placed in a drying chamber with transparent walls that allow the insolation necessary for the drying process to be transmitted. Thus, solar radiation impinges directly on the product. The heat extracts the moisture from the crop and concomitantly lowers the relative humidity of the resident air, thereby increasing its moisture carrying capability. In addition, it expands the air in the chamber, generating its circulation and the subsequent removal of moisture along with the warm air. The features of a typical integral passive solar dryer are illustrated in Fig. 5.

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Direct exposure to sunlight enhances the proper colour ripening of greenish fruits by allowing, during dehydration, the decomposition of the residual chlorophyll in the tissue [2, 7]. For certain varieties of grapes and dates, exposure to sunlight is considered essential for the development of the required colour in the dried product, and for arabica co€ee, a period of exposure to sunlight is considered inviolable for the development of full ¯avour in the roasted bean [2, 7, 30].

Integral-type natural-circulation solar-energy dryers are both simpler and cheaper to construct than those of the distributed-type for the same loading capacity [31, 32]. They require

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no elaborate structures, such as separate air-heating collectors and ducting. However, the potential drawback of the former are the liability to over heat locally (thereby causing crop damage) and the relatively slow overall drying rates achieved due to poor vapour removal [31± 33]. To overcome these limitations, a solar chimney can be employed, which increases the bouyancy force imposed on the air stream, to provide a greater air ¯ow velocity and, thus, a more rapid rate of moisture removal. Two generic types of the integral system can be identi®ed:

3.2.2.1. Passive solar cabinet dryers. These are usually relatively small units used to preserve ``household'' quantities of fruit, vegetables, ®sh and meat. They are usually single or double-glazed insulated hot boxes with holes at the base and upper parts of the cabinet's walls. The solar radiation necessary for the drying process is transmitted through the cover and is absorbed on blackened interior surfaces as well as on the product. Air circulation is provided by the warm moist air leaving via the upper apertures under the action of bouyancy forces while replenishing fresh air is drawn from the base.

Pioneering works on solar cabinet dryers were reported by the Brace Research Institute, Canada [8, 34±39]. Fig. 6 illustrates the fundamental features of the standard Brace Institute solar cabinet dryer [39]. The dryer consists of a container, insulated at both its base and sides

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and covered with a double-layered transparent roof. Drying temperatures in excess of about 808C were reported for the dryer [34, 35].

Standard guidelines recommended by the Brace Research Institute for the construction of solar cabinet dryers include:

. the length of the cabinet should be at least three times its width to minimise shading e€ects of the side panels;

. an optimal angle of slope for the glazing as a function of the local latitude (applicable to sites both north and south of the equator). Fig. 7 gives this slope as a function of latitude;

. the interior walls should be painted black;

. the crop trays should be placed reasonably above the cabinet ¯oor to ensure a reasonable level of air circulation under and around the product;

. the top cover glazing, double preferably, should be treated against degradation under UV radiation; and

. the choice of construction materials should be determined by local availability and the desired level of dryer sophistication.

Several other passive cabinet solar dryers similar in con®guration to the Brace Research Institute dryer have been built and tested for a variety of crops and locations [27, 28, 40±60]. Ezekwe [41] reported a modi®cation of the typical design. This cabinet dryer (Fig. 8) was equipped with a wooden plenum to guide the air inlet and a long plywood chimney to enhance natural-circulation. This dryer was reported to have accelerated the drying rate about ®ve times over open sun drying.

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The design reported by Gustafsson, tested in Nicaragua (see Fig. 9), [55] had a mesh work ¯oor to allow for air inlet and a chimney at the north end of the cabinet. The chimney was constructed from three vertical wooden poles with an asbestos sheet mounted on the back side and a black PVC foil absorber at the south facing front side. Test results indicated that a better drying eciency was obtained compared with the traditional passive cabinet dryer without chimney and four times better drying rate than open sun drying.

Passive solar cabinet dryers have the advantage of cheap and easy construction from locally available materials. Their major drawbacks are the poor moist air removal which reduces drying rates and the very high internal temperatures with the likelihood of over heating the product. Drying air temperatures as high as 708C±1008C, reported widely for these dryers [34, 35, 41±43, 45], are excessive for most products, particularly perishables for which the passive cabinet dryers are intended. Larger air inlets and improved low cost solar chimneys are recommended for enhanced air ¯ow rates, thus minimising excessive internal temperatures and improving drying rates.

3.2.2.2. Natural-circulation greenhouse dryers. Often called tent dryers, these are essentially modi®ed greenhouses. They are equipped with vents sized and positioned appropriately to con-trol the air ¯ow. They are characterized by extensive glazing on their sides. Insulant panels may be drawn over the glazing at night to reduce heat losses and heat storage facilities may be provided. Designed properly, greenhouse dryers allow a greater degree of control over the dry-ing process than the cabinet dryers [8] and are more appropriate for large scale drydry-ing.

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The earliest form of practically-realised natural-circulation solar greenhouse dryers reported was the Brace Research Institute glass-roof solar dryer [36, 38, 45]. The dryer (see Fig. 10) consisted of two parallel rows of drying platforms (along the long side) of galvanised iron wire mesh surface laid over wooden beams. A ®xed slanted glass roof over the platform allowed solar radiation over the product. The dryer, aligned lengthwise in the north±south axis, had black coated internal walls for improved absorption of solar radiation. A ridge cap made of folded zinc sheet over the roof provides an air exit vent. Shutters at the outer sides of the platforms regulated the air inlet.

Later designs of typical passive greenhouse dryers include the widely reported polythene-tent dryer by Doe et al. [7, 30, 54, 61±64]. The dryer (illustrated in Fig. 11) consists of a ridged bamboo framework clad with clear polyethylene sheet on the sun facing side and at the ends. The rear side was clad with black polyethylene sheet which was also spread on the ¯oor to improve absorption of solar radiation. The cladding at one end was arranged to allow access into the drying chamber. The clear plastic cladding at the bottom edge of the front side was rolled around a bamboo pole which could be adjusted to control air ¯ow into the chamber, while the vents at the top of the ends served as the exit for the moist exhaust air.

The solar dome dryer (see Fig. 12) reported by Sachithananthan et al. [65] was essentially a horticultural greenhouse of clear plastic sheet cladding over a dome shaped metal framework.

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The original greenhouse con®guration was modi®ed to allow for a black galvanised iron sheet absorber at the ¯oor, inlet vents along the full length of both sides of the base and exit vents along the top of the dome, with both exits equipped with ®ne plastic netting to keep out insects and dust.

Other practically-realised designs of the greenhouse-type natural-circulation solar-energy dryers have been reported by Sadykov and Khairoddinov [66], Shaw [67], Muthuveerappan et al. [68] Yang [69], Price et al. [70], Bailey [71] and Ghosh [72]. Yang's design [69] was a passive solar kiln for lumber drying. Drying rates of over 2 times and 9 times faster than open air drying in winter and summer, respectively, were reported. Average daily maximum kiln temperature in July (tests were conducted in Ontario, Canada) was 49.38C against the average ambient temperature of 20.28C.

Fig. 10. Natural-circulation glass-roof solar-energy dryer.

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Wagner et al. [46] and Bolin et al. [73] reported the construction of integral-type natural-circulation trough solar dryers. These unusual designs of integral-type dryers make use of parabolic (or half cylindrical) concentrators which concentrate the solar radiation on the crop. The trough solar dryer requires very strict design speci®cations for an e€ective performance. A major problem is that of uneven heating of the crops due to tracking problems of the direct solar radiation.

3.2.2.2.1. Large scale integral-type passive tropical solar dryer.A simpli®ed design of the typi-cal greenhouse-type natural-circulation solar dryer reported by Ekechukwu and Norton [31± 33, 74±76] consists of a transparent semi-cylindrical drying chamber with an attached cylindrical ``chimney'', rising vertically out of one end, while the other end is equipped with a ``door'' for air inlet and access to the drying chamber (see Fig. 13). The drying chamber measures approxi-mately 6.67 m long by 3.0 m wide by 2.3 m high. The chimney (designed to allow for a varying height) has a maximum possible height of 3.0 m above the chamber and a diameter of 1.64 m. The drying chamber was a modi®ed and augmented version of a commercially-available ``poly-tunnel'' type greenhouse. Both the chamber and the chimney, constructed from a galvanised steel framework clad in transparent polyethylene sheet (which had been treated against degra-dation under exposure to ultra-violet radiation) were supplied by the manufacturer.

The dryer operates by the action of solar-energy impinging directly on the crop within the dryer, and no auxiliary power source is required to operate it. The crop and a vertically-hung, black absorbing curtain within the chimney absorb the solar radiation and are warmed. The surrounding air is, in turn, heated. As this heated air rises and ¯ows up the chimney to the outside of the dryer, fresh replenishing air is drawn in from the other end of the dryer. This simple operation, involving no additional power source, provides the circulation of air through the dryer.

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Apart from the obvious advantages of passive solar-energy dryers over the active types (for applications in rural farm locations in developing countries), the advantages of the natural-circulation solar-energy ``ventilated greenhouse dryer'' over other passive solar-energy dryer designs include its low cost and its simplicity in both on-the-site construction and operation. Its major drawback is its susceptibility to damage under very high wind speeds. Table 1 gives a concise comparison of the integral and distributed natural-circulation solar-energy dryers.

The design and performance of solar air heaters are critical to the overall performance of both passive and active forms of the distributed and mixed-mode types of solar dryers. A comprehensive overview of their design, construction and performance evaluation are discussed elsewhere [77]. We have also presented previously, details of design, construction and

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performance of solar chimneys [78] as the requirement of air circulation by natural convection makes the incorporation of solar chimneys critical features for all types of passive solar dryers.

3.2.3. Mixed-mode natural-circulation solar-energy dryers

These dryers combine the features of the integral (direct) type and the distributed (indirect) type natural-circulation solar-energy dryers. Here the combined action of solar radiation incident directly on the product to be dried and pre-heated in a solar air heater furnishes the necessary heat required for the drying process [31±33]. A typical mixed-mode natural-circulation solar-energy dryer (see Fig. 14) would have the same structural features as the distributed-type (i.e. a solar air heater, a separate drying chamber and a chimney), but in addition, the walls of the drying chamber are glazed so that the solar radiation impinges directly on the product as in the integral-type dryers [31±33].

Typical examples of practically-realised designs of the mixed-mode natural-circulation solar-energy dryers include the widely-reported solar rice dryers developed by Exell et al. at the Asian Institute of Technology [7, 30, 51, 54, 59, 61, 79±85]. Considerable research on the design and application of these dryers has been conducted. Fig. 15 [79] illustrates a typical design of the solar rice dryer. The unit consists of a solar air heater, a cabinet for the rice bed and a chimney which provides a tall column of warm air to increase buoyancy. The air heater's absorber consists of a thick layer of burnt rice husks covered by a clear plastic sheet on an inclined bamboo framework. The drying chamber is a shallow wooden box with a base made of bamboo mat with a fairly open structure to allow for an easy ¯ow of the drying air. It is

Table 1. Comparisons of natural-circulation solar-energy dryers Type

Integral Distributed

Principal modes of heat transfer to crop

Radiation (ie. By direct absorption of solar radiation) and convection (ie. from heated surrounding air).

Convection from pre-heated air in an air-heating solar-energy collector.

Components Glazed drying chamber and chimney. Air-heating solar-energy collector, ducting, drying chamber and chimney.

Initial cost Increasing costÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ4

Construction, operation and maintenance

Simplicity in both construction (ie. On-the-site construction) and operation. Requires little maintenance.

Consists of comparatively elaborate structures, thus requires more capital investment in materials and large running costs. More operational diculties of loading and occasional stirring of the crop (since crops are usually dried in relatively deep layers).

Eciency Little information on comparison of performance with distributed-type dryers. Likely to operate at lower eciencies due to its simplicity and less controllability of drying operations.

Have a tendency to higher eciency since individual components can be designed to optimal performance.

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covered with a nylon netting to prevent the rice grains from falling through. A clear plastic sheet covering the rice bed allows the direct heating of the rice (by direct absorption of solar radiation) while protecting it against rain. The chimney consists of a bamboo framework clad with dark plastic sheet (which absorbs solar radiation, thus keeping the chimney inside warm). A detailed design theory, sizing and construction speci®cation of this dryer is reported by Exell [80]. Several modi®cations to the design described above have also been reported [81, 82, 84±86].

The design by Ayensu and Asiedu-Bondizie [86] (see Fig. 16) consists of an air heater with a pile of granite (as absorber cum heat storage) insulated from the base ground by a 5 cm thick layer of straw. A single layer of glass was used as a glazing. The drying chamber, made of plywood sides with a glazed top, held 3 layers of wire mesh (for the products) within it. Access to the chamber was via removable panels at the rear. The cylindrical chimney of 30 cm

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diameter and 1.9 m height (above the chamber) was made from matt black painted galvanized iron sheets ®tted with a metal cap at the top to keep out rain. All wooden components of the unit were treated with wood preservatives to prevent termite and fungi damage with the internal sections insulated with polystyrene.

Another mixed-mode design, distinct from that by Exell et al. [79±85], is the multi-stacked dryer. This design, ®rst reported by Sauliner [38], then Lawand [36] (at the Brace Research Institute, Canada), has also been built and tested by Sharma et al. [51, 59]. The dryer (Fig. 17) consists of a bare-plate air-heating solar-energy collector (made from a black painted metal panel) [36] or corrugated galvanized iron sheet (painted dull black) [51] with either hardboard [36] or thermopile insulation [51] and a multi-stacked drying chamber glazed on the front side and at the top. The air exit is via rear side vents, thus the dryer is not equipped with a chimney. However, the tall column of the drying chamber (about 1.27 m) [36] was expected to generate the necessary bouyant head for the natural convective air ¯ow. Loading and unloading of the dryer is accomplished via a wooden access door at the rear. The glazed front is oriented appropriately, depending on the location of the dryer. The multi-stacked design of the dryer enables the simultaneous drying of a variety of materials.

The design and construction of several other designs of the mixed-mode natural-circulation solar-energy dryer have been reported [25, 28, 52, 54, 59, 87, 88].

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3.2.3.1. The wind-ventilated mixed-mode solar dryer. One of the earliest designs of a mixed-mode natural-circulation solar-energy dryer is the solar and wind-ventilated mixed-mixed-mode dryer, illustrated in Fig. 18. The design is being discussed in detail to illustrate the principles of improved air ¯ow by the use of ventilators which depend entirely on the wind e€ect. Designed and built by the Brace Research Institute, Canada [35, 36, 38, 45], the solar wind-ventilated dryer retains the main features of a mixed-mode natural-circulation solar-energy dryer, the dis-tinctive feature being its air circulation system. Air is drawn through the dryer by wind-pow-ered rotary vanes located on top of the dryer chimney. Temperature and air-¯ow rates are controlled by a damper. The dryer consists of a drying chamber through which warm air heated in a solar air heater is drawn by means of the rotary wind ventilator.

The warm air outlet of the air heater is connected to the base of the drying chamber. No special ducting was required, thus minimising heat losses and avoiding the extra cost of insulating the ducting. Additional heating is obtained from direct absorption of solar radiation through transparent sheets which cover the south, east and west sides of the drying chamber (for a location with a south facing collector orientation). The rear vertical (north side) and

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bottom (horizontal) panels of the dryer are blackened hardboard, which is insulated to reduce heat losses.

The rotary wind ventilator, made of a moving corrugated vane rotor, is placed on top of a stack above the drying chamber. The stack requires an appropriate length to achieve a chimney e€ect and ``catch'' more wind. As the rotor spins in the wind, it expels air from the ventilator stack. The rotor is mounted on a ball bearing suspension with low friction. The momentum keeps the head spinning even in sporadic winds. Tests indicated that the rotary ventilator keeps spinning between gusts, yielding a high, constant exhaust irrespective of intermittent winds.

A stationary ventilator could also be used. This has no moving parts and operates with an ejector action which draws air through the stack from the area being ventilated below. A low pressure area is created on the leeward side into which the exhaust air is drawn, accelerating the ejector action. The ventilators are designed to operate regardless of the wind direction. Its limitation is that it can only follow the wind pattern and is essentially inoperative between wind peaks and has periods of complete inactivity during lulls. Air ¯ows are critical factors in

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natural-circulation solar drying, thus the greater and more dependable air ¯ow capacities of the rotary ventilators are desired. Stationary ventilators are simpler to construct in developing countries and might be just as practicable to use, especially for areas with relatively high average wind speed. Preliminary tests indicated a reduction in drying time of over 25% compared with traditional sun drying techniques.

3.2.3.2. Large-scale mixed-mode tropical solar rice dryer.The design and construction of one of the most recent mixed-mode natural-circulation solar dryers was reported by Iloeje, Ekechukwu and Ezeike [89]. The two tonne per batch capacity mixed-mode natural-circulation solar rice dryer (see Fig. 19) consists of two separate drying compartments, namely, a ¯oor dryer com-partment (which also acts as the air-heating solar collector of the dryer), where the rice is heated by direct absorption of solar radiation and an elevated rack dryer compartment where

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the rice is heated on top by direct absorption of solar radiation and from below by pre-heated air from the ``solar collector''. A thin layer of the wet rice spread on the ¯oor dryer trays acts as a collector absorber plate while being dried simultaneously as in open-sun drying. The remaining portion of the rice is packed in deeper layers in the rack compartment. These two compartments are arranged on both sides of a central gangway running through the entire length of the dryer.

The walls and roof of the dryer are glazed extensively. The chimney is of the ridged design, also running through the entire length of the dryer. Provision is made for an incinerator which burns rice husk to provide supplemental heating during periods of low insolation and/or high humidity. Heat from the burner exhaust is transferred to the rice through heat exchangers located below the dryer trays by natural convection. Air enters the dryer through louvred openings (for ¯ow control) on either side, passes over the ¯oor trays and is forced to ¯ow through the rack trays to the chimney by a transparent ceiling and a wall over the gangway. There are three layers of rack trays of 50 mm (0.05 m) depth each. A heated space, 0.7 m wide, separates the ¯oor trays from the outside to protect the former from splashes of rain water. The 1.5 m height above the ¯oor trays was chosen to allow an operator to stir the rice and lift the trays with relative convenience. Axially, there are 8 tray compartments on each side of the gangway, 2.2 m long and separated by support columns. With this, small farmers may use one or more compartments, simultaneously, to suit their individual sizes of operation. The entire glazing and rack support structure is of ``Iroko'' hardwood. The glazing is of corrugated transparent PVC sheets. The tray base is of expanded metal and wire mesh. The dryer, aligned in the north±south axis, is glazed on all surfaces except for the 1 m high dwarf walls and the entrance doors at the ends. The side glazing slope is 78. During construction, the ¯oor area was specially prepared with stone and concrete overlay for strength and to prevent moisture

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migration from the ground. The foundations for the walls and columns were treated with chemicals to prevent destruction of the woodwork by ants.

4. Active solar drying systems

Active solar drying systems depend only partly on solar-energy. They employ solar-energy and/or electrical or fossil-fuel based heating systems and motorised fans and/or pumps for air circulation. All active solar dryers are, thus, by their application, forced-convection dryers. A typical active solar dryer depends solely on solar-energy as the heat source but employs motorised fans and/or pumps for forced circulation of the drying air. Other major applications of active solar dryers are in large-scale commercial drying operations in which air heating solar-energy collectors supplement conventional fossil-fuel ®red dehydrators [90±107], thus reducing the overall conventional energy consumption, while maintaining control of the drying conditions. If warm enough, the solar-heated air could be used directly for the drying process, otherwise the fossil-fuel ®red dehydrator would be used to raise the drying air temperature to the required level (for example during night time drying operations or periods of low insolation levels), thus avoiding the e€ects of ¯uctuating energy output from the solar collector, since the fossil-fuel system can be controlled automatically to provide the required optimum drying conditions. These active solar dryer types that incorporate dehydrators for supplemental heating are commonly known as ``hybrid solar dryers''.

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Since high temperature drying requires high air ¯ow rates (due to the requirement of limited exposure of the product to the very hot drying air), all high temperature solar drying applications would, of necessity, employ active solar dryers (requiring forced circulation of air by fans and/or pumps). Thus, all practically-realised designs of continuous-¯ow solar-energy drying systems [99, 108, 109] are of the active type. A variety of active solar-energy dryers exist which could be classi®ed into either the integral-type, distributed-type or mixed-mode dryers.

4.1. Integral-type active solar-energy drying systems

These are solar drying designs in which the solar-energy collection unit is an integral part of the entire system, thus, no special ducting to channel the drying air to a separate drying chamber is required. Three distinct designs of integral-type active solar dryers can be identi®ed.

4.1.1. Direct absorption dryers

In this design of active solar dryers, the product absorbs solar radiation directly, thus no separate solar collectors are required. Practically-realised designs include large-scale commercial forced-convection greenhouse dryers, illustrated in Fig. 20 (as in some solar kilns for timber

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Fig. 22. Features of a typical active solar-energy cabinet dryer.

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drying [110], transparent roof solar barns [111] (see Fig. 21) and some small scale forced-convection dryers [40, 49, 91], often equipped with auxiliary heating [91] (see Fig. 22).

4.1.2. Solar collector-roof/collector-wall dryers

These are storage-type dryers usually. In these designs, the solar collector forms an integral part of the roof and/or wall of the drying/storage chamber. Fig. 23 [25] illustrates a solar collector roof dryer. In a typical solar collector-wall dryer design [112] (Fig. 24), a black-painted and glazed concrete wall forms the solar collector and also serves as thermal storage. Other built designs of the solar collector-roof/wall dryers have been reported in Refs. [90, 113± 118].

4.1.3. Internal-absorber-chamber greenhouse dryers

These consist basically of a transparent exterior (or greenhouse outer shell) which acts as the solar collector glazing and an inner drying chamber which is also the absorber. A design shown in Fig. 25 which has been reported extensively by Huang et al. [92±97] consists of a semi-cylindrical structure made of tedlarTM (a polyvinyl¯uoride glazing material) coated clear

corrugated ®bre-glass (draped over a pipe frame support) and an internal drying chamber consisting of rotary or stationary drums with a black-painted outer surface to e€ect solar absorption.

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Another design (Fig. 26) [119] has a clear plastic outer skin and a black plastic interior, while the one illustrated in Fig. 27, which is a solar kiln design for timber drying [13, 120], has a single glazing of horticultural-grade polythene with an internal back-painted corrugated metal absorber over the timber stack.

4.2. Distributed-type active solar-energy drying systems

A distributed-type active solar dryer is one in which the solar collector and drying chamber are separate units. A typical design (Fig. 28) would be comprised of four basic components, namely;

. the drying chamber;

. the solar air heater;

. the fan and/or pump; and

. the ducting.

For conventional drying systems, drying eciencies increase with temperature, thus encouraging drying at temperatures as high as the product can withstand. However, for distributed-type active solar dryers, the maximum allowable temperature may not yield an

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optimal dryer design, as the eciencies of solar collectors decrease with higher outlet temperatures. Thus, a critical decision in the design of distributed active solar dryers would be either to choose high drying air temperatures and, consequently, accommodating lower air-¯ow rates (implying the use of smaller fans and requiring high levels of insulated ducting) or to employ low temperature drying, thus minimising the cost of insulation, since heat losses are low. However, the eciency of high-temperature distributed active solar dryers is signi®cantly improved by high air-¯ow rates, thus a balance has to be made between the size of fans used and the level of insulation for a cost e€ective design.

Most distributed-type active solar drying systems have similar structural designs comprising the basic components. Modi®cations to the typical design have tended to be based on the following features:

4.2.1. The solar air heaters

Most air heaters make use of metal or wood absorbers (with appropriate surface treatment). A few designs employ black polythene absorbers to minimise the overall cost of dryer construction [121±123]. A particular design uses a layer of granulated charcoal over a sand layer (in a shallow excavation) as an inexpensive solar-energy absorbing surface [124].

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Fig. 27. Internal-absorber greenhouse active solar timber kiln.

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Moreover, charcoal, being a good insulator, reduces ground heat losses, but since the collector is ®xed to the ground, appropriate collector orientation is compromised.

4.2.2. Air re-circulation

The re-circulation of the drying air employed in some known designs [105, 108, 122] is another distinguishing feature. This ensures a low exhaust air temperature, thereby increasing eciency. In non-recirculation drying, the existing air may still be containing some considerable heat. Re-circulation of the drying air implies a higher total temperature and that the warm air is not discarded until it carries an appreciable quantity of moisture, thereby ensuring an ecient use of energy. Fig. 29 [122] illustrates a design with a polyethylene-tube solar collector employing partial air re-circulation. The polyethylene tube collector con®guration consists of a black solar absorber tube inside a larger diameter clear tube acting as the glazing.

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4.2.3. Fan/pump location

This is not very critical in dryer designs. Some designs have tended to locate the pump inside the dryer (between the air heater and the drying chamber) [108, 124]. This keeps the collector under negative pressure, ensuring that all air leakages and the additional heat generated by the pump is into the system. Locating a pump and/or fan at the air inlet of the collector involves less elaborate construction details and ensures that each component can be easily de-coupled from the system for maintenance and repair. For systems employing air recirculation, the pump should be located appropriately.

Conventional distributed active solar-energy dryers are batch-type designs mostly. However, some continuous-¯ow designs have been built [99, 108, 109, 125]. Fig. 30 illustrates the features of a typical continuous-¯ow active solar dryer. The design consists of a vertical bin in which the grain is continuously dried by the ¯ow of hot air at right angles to the vertical bed of grain moving downwards under gravity.

4.3. Mixed-mode active solar-energy dryers

These are rather uncommon designs of active solar dryers. Mixed-mode designs combine some features of the integral and distributed-types. Typical designs [126, 127] would comprise the following components: a solar air heater, air ducting, a separate drying chamber and a fan and/or pump as in a distributed-type dryer. However, the drying chamber is glazed so that the product absorbs solar radiation directly as in direct absorption integral designs. Features of an active mixed-mode solar dryer are illustrated in Fig. 31.

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4.4. Other design features of active solar-energy dryers

To achieve more ecient energy use, some active solar dryers are equipped with thermal storage devices, mostly rock bed or gravel storage [98, 128±131]. This improves drying during night time or periods of low insolution levels. Desiccants are incorporated in some designs [132± 136] to further reduce the relative humidity of the drying air so as to improve its moisture carrying capacity. The use of desiccants would only be appropriate for forced-convection systems, as their incorporation into the system increases the resistance to air ¯ow. Finally, as indicated earlier, large-scale commercial active solar dryers employ mostly air-heating solar collectors as supplements to electricity or fossil-fuel ®red dehydrators to reduce the overall conventional energy consumption. Practically-realised designs of these ``hybrid systems'' have been reported widely [90±107].

The requirement of fossil-fuel driven fans and/or the use of auxiliary heating sources improves the eciency of these dryers, but it renders both their capital, maintenance and operational costs prohibitive for small scale farming operations. Clearly, they are inappropriate for remote rural village farm application in most developing countries.

5. Conclusion

We have presented a comprehensive review of the various designs, details of construction and operational principles of the wide variety of practically-realised designs of solar-energy

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drying systems. We have also evolved a systematic classi®cation of solar-energy dryers. This classi®cation illustrates clearly how these solar dryer designs can be grouped systematically according to either their operating temperature ranges, heating sources and heating modes, operational modes or structural modes. Two broad groups of solar-energy dryers can be identi®ed, viz, passive or natural-circulation solar-energy dryers and active or forced-convection solar-energy dryers (often called hybrid solar dryers). Three sub-groups of these, which di€er mainly on their structural arrangement, can also be identi®ed, viz integral or direct mode solar dryers, distributed or indirect-modes and the mixed-modes.

Though properly designed forced-convection (active) solar dryers are agreed generally to be more e€ective and more controllable than the natural-circulation (passive) types, the requirement of electricity or fossil-fuel driven fans and/or the use of auxiliary heating sources, however, renders the former clearly inappropriate for remote rural village farm application in most developing countries and makes both their capital, maintenance and operational costs prohibitive for small scale farming operations. For large scale applications in rural locations, the ``ventilated greenhouse dryer'' has the advantage of low cost and simplicity in both on-the-site construction and operation.

Acknowledgements

The authors wish to acknowledge the grant from the Commission of European Communities (CEC) for the study on optimization of integral-type natural-circulation solar-energy tropical dryers and the grant from the Third World Academy of Sciences (TWAS) for the comparative study and optimization of generic passive solar dryers for tropical rural applications. Library facilities provided by the Cran®eld Institute of Technology (now Cran®eld University), U.K. and the International Centre for Theoretical Physics, Trieste, Italy, the computing facilities of the Energy Research Centre, University of Nigeria, Nsukka, Nigeria, and the Junior Fellowship of the International Centre for Theoretical Physics awarded Dr Ekechukwu are also acknowledged.

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References

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