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

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

O.V. Ekechukwua, *, B. Nortonb

aEnergy Research Centre, University of Nigeria, Nsukka, NigeriabPROBE, 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 ofthe wide variety of practically-realised designs of solar-energy drying systems reported previously ispresented. A systematic approach for the classi®cation of solar-energy dryers has been evolved. Twogeneric groups of solar-energy dryers can be identi®ed, viz passive or natural-circulation solar-energydryers 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 byrural farmers in developing countries is discussed. Some very recent developments in solar dryingtechnology 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 electricityand supplies of other non-renewable sources of energy are either unavailable, unreliable or, formany farmers, too expensive. Thus, in such areas, crop drying systems that employ motorisedfans and/or electrical heating are inappropriate. The large initial and running costs of fossilfuel powered dryers present such barriers that they are rarely adopted by small scale farmers.

Energy Conversion & Management 40 (1999) 615±655

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

PERGAMON

* Corresponding author. Tel. (Home): 00 234 42 771702; Fax: 00 234 42 771 855; E-mail: [email protected].

The traditional open sun drying utilised widely by rural farmers has inherent limitations: highcrop losses ensue from inadequate drying, fungal attacks, insects, birds and rodentsencroachment, 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 thecrop, 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 alsorequires an undesirably long period to reach this equilibrium moisture content. In hot and humidclimates, crop deterioration is obviously worse, as both warmth and high moisture contentspromote the growth of fungi, bacteria, mites and insects in crops. Warmth and siccity arepropitious conditions for natural open sun drying. Unfortunately, the tropics are characterisedby hot damp climates. If the relative humidity of ambient air is too high to facilitate drying inthe ®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 storageperiod'' [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 surfaceto the surrounding air.

Drying involves the extraction of moisture from the product by heating and the passage of airmass around it to carry away the released vapour. Under ambient conditions, these processescontinue until the vapour pressure of the moisture held in the product equals that held in theatmosphere [1, 2]. Thus, the rate of moisture desorption from the product to the environmentand absorption from the environment are in equilibrium, and the crop moisture content at thiscondition is known as the equilibrium moisture content. Under ambient conditions, the dryingprocess is slow, and in environments of high relative humidity, the equilibrium moisturecontent is insu�ciently low for safe storage [1, 2]. The objective of a dryer is to supply theproduct with more heat than is available under ambient conditions, thereby increasingsu�ciently the vapour pressure of the moisture held within the crop and decreasingsigni®cantly the relative humidity of the drying air and thereby increasing its moisture carryingcapacity 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. Theheating procedure could involve the passage of preheated air through the product or bydirectly exposing the product to solar radiation or a combination of both [2]. The majorrequirement is the transfer of heat to the moist product by convection and conduction fromthe surrounding air mass at temperatures above that of the product or by radiation, mainlyfrom the sun and to a little extent from surrounding hot surfaces, or conduction from heatedsurfaces in contact with the product [2]. Absorption of heat by the product supplies the energy

O.V. Ekechukwu, B. Norton / Energy Conversion & Management 40 (1999) 615±655616

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 tovapourise from the surface of the moist product when the absorbed energy has increased itstemperature su�ciently for the water vapour pressure of the product moisture to exceed thevapour pressure of the surrounding air. Moisture replenishment to the surface is by di�usionfrom the interior, and this process depends on the nature of the product and its moisturecontent. If the di�usion rate is slow, it becomes the limiting factor in the drying process, but ifit 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 productitself, thereby generating heat in the interior of the product as well as at its surface, andthereby enhancing heat transfer. The solar radiation absorptance of the product is animportant factor in direct solar drying. Fortunately, most agricultural materials have relativelyhigh absorptances (between 0.67 and 0.90) [3] which may increase or decrease as the dryingprogresses. The thermal conductivity of the crop is also important, particularly if the dryinglayer 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 likegerminability) must be avoided when drying certain crops. During drying, some crops have atendency to form dry surface layers which are impervious to subsequent moisture transfer ifthe drying rate is very rapid. To avoid this requires e�ective control of the drying process. Theheat transfer and evaporation rates must be closely controlled to guarantee optimum dryingrates. 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 controlmechanisms are inappropriate. The best approach is to incorporate into the design of thedryers such structural features that would guarantee that extreme conditions to not prevail inthe 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 temperatureranges 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 fueldryers (more commonly known as conventional dryers) and solar-energy dryers. Strictly, allpractically-realised designs of high temperature dryers are fossil fuel powered, while the lowtemperature 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 usuallyemployed when the products require a short exposure to the drying air. Their operating

O.V. Ekechukwu, B. Norton / Energy Conversion & Management 40 (1999) 615±655 617

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

2.2. Low temperature dryers

In low temperature drying systems, the moisture content of the product is usually brought inequilibrium with the drying air by constant ventilation. Thus, they do tolerate intermittent orvariable heat input. Low temperature drying enables crops to be dried in bulk and is mostsuited also for long term storage systems. Thus, they are usually known as bulk or storagedryers [1]. Their ability to accommodate intermittent heat input makes low temperature dryingmost appropriate for solar-energy applications. Thus, some conventional dryers and mostpractically-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 andthe group lineage of solar drying systems. Solar-energy drying systems are classi®ed primarilyaccording 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 solardryers); and

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

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 ofutilisation 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 illustratedin Fig. 2.


Fig.1.Classi®cationofdryersanddryingmodes.


3. Passive solar drying systems

3.1. Open-to-sun drying

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

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

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

Fig. 2. Typical solar energy dryer designs.


Despite the rudimentary nature of the processes involved, such techniques still remain incommon use. Because the power requirements (i.e. from the solar radiation and the air'senthalpy) are readily available in the ambient environment, and as little or no capital cost isrequired and running costs low (often labour only), these are frequently the only commerciallyviable methods in which to dry agricultural produce in developing countries. Though utilisedwidely, natural open-to-sun drying techniques have inherent limitations: high crop losses ensuefrom inadequate drying, fungal and insect infestation, birds and rodent encroachment andweathering e�ects. The process is intermittent, being a�ected by cloudiness and unexpectedrain. 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. Thequality of the dried product is often degraded seriously, sometimes beyond edibility. Thus, atpresent, 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 methodsare 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 aresult 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 airthrough the crop. The latter are termed ``active'' solar dryers. Natural-circulation solar-energydryers appear the most attractive option for use in remote rural locations. They are superioroperationally and competitive economically to natural open-to-sun drying. The advantages ofnatural-circulation solar-energy tropical dryers that enable them to compete economically withtraditional drying techniques are:

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

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

. the drying period is shortened compared with open air drying, thus attaining higher rates ofproduct 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 useof readily available indigenous labour and materials for construction.


Three generic types of natural-circulation solar-energy dryers have evolved and both retainmany 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 dryersThese 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 ¯owthrough a low pressure drop thermosyphonic solar collector [6]. Because solar radiation is notincident 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 theirvitamin content are reduced considerably by direct exposure to sunlight and for colourretention in some highly pigmented commodities that are also very adversely a�ected by directexposure 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 relativelydeep layer drying [8]. Their shortcomings, however, are the ¯uctuations in temperatures of theair leaving the air heaters, thereby making it di�cult to maintain constant operating conditionswithin the drying chamber, and the operational di�culties of loading and unloading the traysand occasional stirring of the product [8]. Distributed-type dryers, though, have an inherenttendency towards greater e�ciency, as the component units can be designed for optimale�ciency of their respective functions [7]. They are, however, relatively elaborate structuresrequiring 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 welldesigned forced-convection distributed solar dryers are more e�ective and more controllablethan the natural-circulation types [7]. Thus, most practically-realised distributed solar dryersare of the active (forced-convection) type. Of the natural-circulation types built, most are ofthe mixed-mode design (which retain most of the features of distributed dryers). Thus, fewpractically-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 absorberand an air gap of 5 cm from the glazing. The air heater was connected to an insulated drying


bin equipped with a chimney. The entire dryer assembly was made from hardboard. Toimprove e�ciency, the air heater was modi®ed with a wider air gap (15 cm) to accommodatethree layers of wire-mesh absorber between the glazing and the ¯at-plate absorber. The dryerwas capable of drying 90 kg of wet maize from a moisture content of about 20% wet basis to12% 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 designinclude absorbers equipped with thermal storage, either of rock bed [14] or water [15, 16], and

Fig. 3. Features of a typical distributed-type (indirect) natural-circulation solar-energy dryer.


the ventilated greenhouse solar air heaters made of clear plastic covers and black plasticabsorbers [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 wiremesh and local bush stems as the drying chamber) attached to a solar air heater. Moist air exitwas via the numerous vents of the mesh work. Performance studies of other practically-realiseddesigns 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 tothe e�cient design of the drying chambers. Drying chambers are usually constructed fromwooden materials (sometimes without additional insulation) and thus are highly susceptible todamage under harsh weathering e�ects. Drying chambers should be insulated properly tominimise heat losses and made durable (within economically justi®able limits). Constructionfrom metal sheets or water resistant cladding is recommended.

3.2.2. Integral-type natural-circulation solar-energy dryersIn 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 necessaryfor the drying process to be transmitted. Thus, solar radiation impinges directly on theproduct. The heat extracts the moisture from the crop and concomitantly lowers the relativehumidity of the resident air, thereby increasing its moisture carrying capability. In addition, itexpands the air in the chamber, generating its circulation and the subsequent removal ofmoisture along with the warm air. The features of a typical integral passive solar dryer areillustrated in Fig. 5.

Fig. 4. A distributed-type natural-circulation solar maize dryer.


Direct exposure to sunlight enhances the proper colour ripening of greenish fruits byallowing, 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 thedevelopment of the required colour in the dried product, and for arabica co�ee, a period ofexposure to sunlight is considered inviolable for the development of full ¯avour in the roastedbean [2, 7, 30].

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

Fig. 5. Features of a typical integral-type (direct) natural-circulation solar-energy dryer.


no elaborate structures, such as separate air-heating collectors and ducting. However, thepotential drawback of the former are the liability to over heat locally (thereby causing cropdamage) 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 thebouyancy force imposed on the air stream, to provide a greater air ¯ow velocity and, thus, amore rapid rate of moisture removal. Two generic types of the integral system can beidenti®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. Thesolar radiation necessary for the drying process is transmitted through the cover and isabsorbed on blackened interior surfaces as well as on the product. Air circulation is providedby the warm moist air leaving via the upper apertures under the action of bouyancy forceswhile 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 Institutesolar cabinet dryer [39]. The dryer consists of a container, insulated at both its base and sides

Fig. 6. A typical natural-circulation solar-energy cabinet dryer.


and covered with a double-layered transparent roof. Drying temperatures in excess of about808C 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�ectsof the side panels;

. an optimal angle of slope for the glazing as a function of the local latitude (applicable tosites 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 reasonablelevel of air circulation under and around the product;

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

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

Several other passive cabinet solar dryers similar in con®guration to the Brace ResearchInstitute 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) wasequipped with a wooden plenum to guide the air inlet and a long plywood chimney to enhancenatural-circulation. This dryer was reported to have accelerated the drying rate about ®ve timesover open sun drying.

Fig. 7. Optimal tilt angle of a solar cabinet dryer as a function of local latitude.


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 wasconstructed from three vertical wooden poles with an asbestos sheet mounted on the back sideand a black PVC foil absorber at the south facing front side. Test results indicated that abetter drying e�ciency was obtained compared with the traditional passive cabinet dryerwithout 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 reducesdrying rates and the very high internal temperatures with the likelihood of over heating theproduct. Drying air temperatures as high as 708C±1008C, reported widely for thesedryers [34, 35, 41±43, 45], are excessive for most products, particularly perishables for which thepassive cabinet dryers are intended. Larger air inlets and improved low cost solar chimneys arerecommended for enhanced air ¯ow rates, thus minimising excessive internal temperatures andimproving drying rates.

3.2.2.2. Natural-circulation greenhouse dryers. Often called tent dryers, these are essentiallymodi®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 panelsmay be drawn over the glazing at night to reduce heat losses and heat storage facilities may beprovided. 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 drying.

Fig. 8. A modi®ed natural-circulation solar-energy cabinet dryer.


The earliest form of practically-realised natural-circulation solar greenhouse dryers reportedwas 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 wiremesh surface laid over wooden beams. A ®xed slanted glass roof over the platform allowedsolar radiation over the product. The dryer, aligned lengthwise in the north±south axis, hadblack coated internal walls for improved absorption of solar radiation. A ridge cap made offolded zinc sheet over the roof provides an air exit vent. Shutters at the outer sides of theplatforms 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 ridgedbamboo 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 toimprove absorption of solar radiation. The cladding at one end was arranged to allow accessinto the drying chamber. The clear plastic cladding at the bottom edge of the front side wasrolled 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.

Fig. 9. A natural-circulation solar-energy cabinet dryer with chimney.


The original greenhouse con®guration was modi®ed to allow for a black galvanised iron sheetabsorber at the ¯oor, inlet vents along the full length of both sides of the base and exit ventsalong the top of the dome, with both exits equipped with ®ne plastic netting to keep outinsects 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 etal. [68] Yang [69], Price et al. [70], Bailey [71] and Ghosh [72]. Yang's design [69] was a passivesolar kiln for lumber drying. Drying rates of over 2 times and 9 times faster than open airdrying in winter and summer, respectively, were reported. Average daily maximum kilntemperature in July (tests were conducted in Ontario, Canada) was 49.38C against the averageambient temperature of 20.28C.

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

Fig. 11. Natural-circulation polythene-tent dryer.


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 ofparabolic (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. Amajor problem is that of uneven heating of the crops due to tracking problems of the directsolar 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'' forair 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 varyingheight) 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 galvanisedsteel 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. Thesurrounding air is, in turn, heated. As this heated air rises and ¯ows up the chimney to theoutside of the dryer, fresh replenishing air is drawn in from the other end of the dryer. Thissimple operation, involving no additional power source, provides the circulation of air throughthe dryer.

Fig. 12. Natural-circulation solar dome dryer.


Apart from the obvious advantages of passive solar-energy dryers over the active types (forapplications in rural farm locations in developing countries), the advantages of the natural-circulation solar-energy ``ventilated greenhouse dryer'' over other passive solar-energy dryerdesigns 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 aconcise 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. Acomprehensive overview of their design, construction and performance evaluation are discussedelsewhere [77]. We have also presented previously, details of design, construction and

Fig. 13. A greenhouse-type natural-circulation solar-energy dryer.


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

3.2.3. Mixed-mode natural-circulation solar-energy dryersThese 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 radiationincident directly on the product to be dried and pre-heated in a solar air heater furnishes thenecessary 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 thedistributed-type (i.e. a solar air heater, a separate drying chamber and a chimney), but inaddition, the walls of the drying chamber are glazed so that the solar radiation impingesdirectly 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 theAsian Institute of Technology [7, 30, 51, 54, 59, 61, 79±85]. Considerable research on the designand application of these dryers has been conducted. Fig. 15 [79] illustrates a typical design ofthe solar rice dryer. The unit consists of a solar air heater, a cabinet for the rice bed and achimney which provides a tall column of warm air to increase buoyancy. The air heater'sabsorber consists of a thick layer of burnt rice husks covered by a clear plastic sheet on aninclined bamboo framework. The drying chamber is a shallow wooden box with a base madeof 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 tocrop

Radiation (ie. By direct absorption of solar

radiation) and convection (ie. from heatedsurrounding 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 costÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ4Construction,operation and

maintenance

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

little maintenance.

Consists of comparatively elaboratestructures, thus requires more capital

investment in materials and large runningcosts. More operational di�culties of loadingand occasional stirring of the crop (since

crops are usually dried in relatively deeplayers).

E�ciency Little information on comparison of

performance with distributed-type dryers.Likely to operate at lower e�ciencies due toits simplicity and less controllability of dryingoperations.

Have a tendency to higher e�ciency since

individual components can be designed tooptimal performance.


covered with a nylon netting to prevent the rice grains from falling through. A clear plasticsheet covering the rice bed allows the direct heating of the rice (by direct absorption of solarradiation) while protecting it against rain. The chimney consists of a bamboo framework cladwith 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 byExell [80]. Several modi®cations to the design described above have also beenreported [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 thicklayer of straw. A single layer of glass was used as a glazing. The drying chamber, made ofplywood sides with a glazed top, held 3 layers of wire mesh (for the products) within it. Accessto the chamber was via removable panels at the rear. The cylindrical chimney of 30 cm

Fig. 14. Features of a typical mixed-mode natural-circulation solar-energy dryer.


diameter and 1.9 m height (above the chamber) was made from matt black painted galvanizediron sheets ®tted with a metal cap at the top to keep out rain. All wooden components of theunit were treated with wood preservatives to prevent termite and fungi damage with theinternal 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 ResearchInstitute, 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 metalpanel) [36] or corrugated galvanized iron sheet (painted dull black) [51] with eitherhardboard [36] or thermopile insulation [51] and a multi-stacked drying chamber glazed on thefront side and at the top. The air exit is via rear side vents, thus the dryer is not equipped witha chimney. However, the tall column of the drying chamber (about 1.27 m) [36] was expectedto generate the necessary bouyant head for the natural convective air ¯ow. Loading andunloading of the dryer is accomplished via a wooden access door at the rear. The glazed frontis oriented appropriately, depending on the location of the dryer. The multi-stacked design ofthe 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].

Fig. 15. A mixed-mode natural-circulation solar rice dryer.


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-mode dryer,illustrated in Fig. 18. The design is being discussed in detail to illustrate the principles ofimproved air ¯ow by the use of ventilators which depend entirely on the wind e�ect. Designedand built by the Brace Research Institute, Canada [35, 36, 38, 45], the solar wind-ventilateddryer 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 arecontrolled by a damper. The dryer consists of a drying chamber through which warm airheated 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 ofinsulating the ducting. Additional heating is obtained from direct absorption of solar radiationthrough 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

Fig. 16. A mixed-mode natural-circulation solar-energy dryer with thermal storage.


bottom (horizontal) panels of the dryer are blackened hardboard, which is insulated to reduceheat 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 chimneye�ect and ``catch'' more wind. As the rotor spins in the wind, it expels air from the ventilatorstack. The rotor is mounted on a ball bearing suspension with low friction. The momentumkeeps the head spinning even in sporadic winds. Tests indicated that the rotary ventilator keepsspinning 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 lowpressure area is created on the leeward side into which the exhaust air is drawn, acceleratingthe ejector action. The ventilators are designed to operate regardless of the wind direction. Itslimitation is that it can only follow the wind pattern and is essentially inoperative betweenwind peaks and has periods of complete inactivity during lulls. Air ¯ows are critical factors in

Fig. 17. A multi-stacked mixed-mode natural-circulation solar-energy dryer.


natural-circulation solar drying, thus the greater and more dependable air ¯ow capacities of therotary ventilators are desired. Stationary ventilators are simpler to construct in developingcountries and might be just as practicable to use, especially for areas with relatively highaverage 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 ofthe most recent mixed-mode natural-circulation solar dryers was reported by Iloeje, Ekechukwuand Ezeike [89]. The two tonne per batch capacity mixed-mode natural-circulation solar ricedryer (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 isheated by direct absorption of solar radiation and an elevated rack dryer compartment where

Fig. 18. Mixed-mode wind-ventilated solar dryer.


the rice is heated on top by direct absorption of solar radiation and from below by pre-heatedair from the ``solar collector''. A thin layer of the wet rice spread on the ¯oor dryer trays actsas a collector absorber plate while being dried simultaneously as in open-sun drying. Theremaining portion of the rice is packed in deeper layers in the rack compartment. These twocompartments are arranged on both sides of a central gangway running through the entirelength 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 whichburns rice husk to provide supplemental heating during periods of low insolation and/or highhumidity. Heat from the burner exhaust is transferred to the rice through heat exchangerslocated below the dryer trays by natural convection. Air enters the dryer through louvredopenings (for ¯ow control) on either side, passes over the ¯oor trays and is forced to ¯owthrough 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 liftthe trays with relative convenience. Axially, there are 8 tray compartments on each side of thegangway, 2.2 m long and separated by support columns. With this, small farmers may use oneor more compartments, simultaneously, to suit their individual sizes of operation. The entireglazing and rack support structure is of ``Iroko'' hardwood. The glazing is of corrugatedtransparent PVC sheets. The tray base is of expanded metal and wire mesh. The dryer, alignedin the north±south axis, is glazed on all surfaces except for the 1 m high dwarf walls and theentrance doors at the ends. The side glazing slope is 78. During construction, the ¯oor areawas specially prepared with stone and concrete overlay for strength and to prevent moisture

Fig. 19. Two tonne capacity mixed-mode natural-circulation tropical solar rice dryer.


migration from the ground. The foundations for the walls and columns were treated withchemicals 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-energyand/or electrical or fossil-fuel based heating systems and motorised fans and/or pumps for aircirculation. All active solar dryers are, thus, by their application, forced-convection dryers. Atypical active solar dryer depends solely on solar-energy as the heat source but employsmotorised fans and/or pumps for forced circulation of the drying air. Other major applicationsof active solar dryers are in large-scale commercial drying operations in which air heatingsolar-energy collectors supplement conventional fossil-fuel ®red dehydrators [90±107], thusreducing the overall conventional energy consumption, while maintaining control of the dryingconditions. 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 tothe required level (for example during night time drying operations or periods of low insolationlevels), thus avoiding the e�ects of ¯uctuating energy output from the solar collector, since thefossil-fuel system can be controlled automatically to provide the required optimum dryingconditions. These active solar dryer types that incorporate dehydrators for supplementalheating are commonly known as ``hybrid solar dryers''.

Fig. 20. A forced-convection greenhouse dryer.


Since high temperature drying requires high air ¯ow rates (due to the requirement of limitedexposure of the product to the very hot drying air), all high temperature solar dryingapplications would, of necessity, employ active solar dryers (requiring forced circulation of airby fans and/or pumps). Thus, all practically-realised designs of continuous-¯ow solar-energydrying systems [99, 108, 109] are of the active type. A variety of active solar-energy dryers existwhich 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 ofthe entire system, thus, no special ducting to channel the drying air to a separate dryingchamber is required. Three distinct designs of integral-type active solar dryers can be identi®ed.

4.1.1. Direct absorption dryersIn 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 commercialforced-convection greenhouse dryers, illustrated in Fig. 20 (as in some solar kilns for timber

Fig. 21. A forced-convection transparent-roof solar barn.


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

Fig. 23. A collector-roof active solar-energy storage dryer.


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 dryersThese 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 solarcollector 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 dryersThese 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 designshown in Fig. 25 which has been reported extensively by Huang et al. [92±97] consists of asemi-cylindrical structure made of tedlarTM (a polyvinyl¯uoride glazing material) coated clearcorrugated ®bre-glass (draped over a pipe frame support) and an internal drying chamberconsisting of rotary or stationary drums with a black-painted outer surface to e�ect solarabsorption.

Fig. 24. A collector wall active solar-energy storage dryer.


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], hasa single glazing of horticultural-grade polythene with an internal back-painted corrugatedmetal 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 chamberare 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, thusencouraging drying at temperatures as high as the product can withstand. However, fordistributed-type active solar dryers, the maximum allowable temperature may not yield an

Fig. 25. Interior-drum-absorber greenhouse active solar dryer.


optimal dryer design, as the e�ciencies of solar collectors decrease with higher outlettemperatures. Thus, a critical decision in the design of distributed active solar dryers would beeither to choose high drying air temperatures and, consequently, accommodating lower air-¯owrates (implying the use of smaller fans and requiring high levels of insulated ducting) or toemploy low temperature drying, thus minimising the cost of insulation, since heat losses arelow. However, the e�ciency of high-temperature distributed active solar dryers is signi®cantlyimproved by high air-¯ow rates, thus a balance has to be made between the size of fans usedand 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 thefollowing features:

4.2.1. The solar air heatersMost 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 dryerconstruction [121±123]. A particular design uses a layer of granulated charcoal over a sandlayer (in a shallow excavation) as an inexpensive solar-energy absorbing surface [124].

Fig. 26. Interior-plastic-absorber greenhouse active solar dryer.


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

Fig. 28. Features of a typical distributed-type active solar energy dryer.


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

4.2.2. Air re-circulationThe 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 increasinge�ciency. In non-recirculation drying, the existing air may still be containing someconsiderable heat. Re-circulation of the drying air implies a higher total temperature and thatthe warm air is not discarded until it carries an appreciable quantity of moisture, therebyensuring an e�cient use of energy. Fig. 29 [122] illustrates a design with a polyethylene-tubesolar collector employing partial air re-circulation. The polyethylene tube collectorcon®guration consists of a black solar absorber tube inside a larger diameter clear tube actingas the glazing.

Fig. 29. A distributed-type active solar dehydrator with partial air re-circulation.


4.2.3. Fan/pump locationThis 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 collectorunder negative pressure, ensuring that all air leakages and the additional heat generated by thepump is into the system. Locating a pump and/or fan at the air inlet of the collector involvesless elaborate construction details and ensures that each component can be easily de-coupledfrom the system for maintenance and repair. For systems employing air recirculation, thepump 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 featuresof a typical continuous-¯ow active solar dryer. The design consists of a vertical bin in whichthe grain is continuously dried by the ¯ow of hot air at right angles to the vertical bed of grainmoving downwards under gravity.

4.3. Mixed-mode active solar-energy dryers

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

Fig. 30. A continuous-¯ow active grain dryer.


4.4. Other design features of active solar-energy dryers

To achieve more e�cient energy use, some active solar dryers are equipped with thermalstorage devices, mostly rock bed or gravel storage [98, 128±131]. This improves drying duringnight 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 moisturecarrying capacity. The use of desiccants would only be appropriate for forced-convectionsystems, as their incorporation into the system increases the resistance to air ¯ow. Finally, asindicated earlier, large-scale commercial active solar dryers employ mostly air-heating solarcollectors as supplements to electricity or fossil-fuel ®red dehydrators to reduce the overallconventional energy consumption. Practically-realised designs of these ``hybrid systems'' havebeen 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 andoperational costs prohibitive for small scale farming operations. Clearly, they are inappropriatefor remote rural village farm application in most developing countries.

5. Conclusion

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

Fig. 31. Features of a typical mixed-mode active solar -energy dryer.


drying systems. We have also evolved a systematic classi®cation of solar-energy dryers. Thisclassi®cation illustrates clearly how these solar dryer designs can be grouped systematicallyaccording to either their operating temperature ranges, heating sources and heating modes,operational modes or structural modes. Two broad groups of solar-energy dryers can beidenti®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 directmode 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, therequirement 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 inmost developing countries and makes both their capital, maintenance and operational costsprohibitive 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 tropicaldryers and the grant from the Third World Academy of Sciences (TWAS) for the comparativestudy and optimization of generic passive solar dryers for tropical rural applications. Libraryfacilities 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 ofthe Energy Research Centre, University of Nigeria, Nsukka, Nigeria, and the JuniorFellowship of the International Centre for Theoretical Physics awarded Dr Ekechukwu are alsoacknowledged.

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