Applied Energy xxx (2010) xxx–xxx

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Contents lists available at ScienceDirect

Applied Energy

journal homepage: www.elsevier .com/locate /apenergy

Design and fabrication of a cost effective solar air heater for Bangladesh

M.A. Wazed a,*, Y. Nukman a,1, M.T. Islam b,2

a Department of Engineering Design and Manufacture, University of Malaya (UM), 50603 Kuala Lumpur, Malaysiab Department of Mechanical Engineering, Chittagong University of Engineering & Technology (CUET), Chittagong 4349, Bangladesh

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 July 2009Received in revised form 3 February 2010Accepted 8 February 2010Available online xxxx

Keywords:Solar energyAir heaterBangladesh

0306-2619/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.apenergy.2010.02.014

* Corresponding author. Tel.: +60 14 930 2473; faxE-mail addresses: awazed@gmail.com (M.A. Waz

Nukman), tazul2003@yahoo.com (M.T. Islam).1 Tel.: +60 12 937 3765; fax: +60 3 7967 5330.2 Tel.: +880 1713 109888; fax: +880 31 714910.

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In this research, a solar air heater is designed, fabricated and its performance is assessed in the perspec-tive of an emerging/developing country with a huge energy demand like Bangladesh. The winter season(mid-November–mid-February) of the country characterizes by low temperatures, cool air blowing fromthe west or northwest, clear sky and meager rainfall. Minimum temperature in the extreme northwest inlate December and early January sometimes reaches 3 �C and day length is about 10 h. The shortness ofwinter days can be compensated by reducing the heat loss during long nights. The solar air heater is con-structed to prevent as much heat loss as possible. In other words, the heating of air is accomplished bymaximizing light gain and minimizing heat loss. It is observed that the fabricated solar air heater is work-ing efficiently. The maximum room temperature and the temperature difference from ambient are45.5 �C and 12.25 �C for forced circulation and 41.75 �C and 8.5 �C for natural circulation respectively.The experimental outlet temperatures have been compared with that of theoretical values. Due to itslow-cost and simple technology, it is affordable in all aspects, viz. of cost, operation and maintenanceby the typical people of Bangladesh.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

People have become increasingly concerned about the rapiddepletion and uncertainty in cost of fossil fuels. There is some fearabout the possible environmental and safety risks associated withfossil fuels, such as global warming, greenhouse effect, sea levelrise, climatic change and acid rain precipitation. These concernshave focused worldwide attention on the potential of harnessingthe Sun’s power in new and varied forms to meet society’s growingenergy needs and for saving conventional energy. Although the Sunhas been a major energy source throughout the ages, technologicaladvances in several fields of science and engineering now make itpossible to accelerate the use of solar energy to meet the world’sexpanding energy requirement.

A great deal of attention has been given to solar air-heating inrecent years. Among solar air-heaters, those using flat-plate collec-tors are extensively used in low-temperature solar energy technol-ogy over years, because they are relatively simple, easy to operateand have low capital costs. The efficiency of these collectors in airheating is low due to low convective heat transfer coefficients be-tween absorber and the flowing air that increases the absorber

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: +60 3 7967 5330.ed), nukman@um.edu.my (Y.

et al. Design and fabrication

plate temperature, resulting in higher heat loses to the ambient.Solar air heaters may be used for space heating, drying and paintspraying operations [1,2]. The solar air heater occupies an impor-tant place among solar heating systems because of minimal useof materials, and the direct use of air as working substance reducesthe number of required system components [3].

In many industrial applications where recirculation of air is notpractical because of contamination, outside air is heated and useddirectly, especially for supplying fresh air to hospitals. Further,heating of ambient air is an ideal operation for a collector, as itoperates very close to ambient temperature. In addition to theessential effects of free and forced convections [4–6], considerableimprovement in collector efficiency is also accessible to increasethe transfer area by adding fins [7,5] and to create the turbulenceinside the flow channel by using baffles [8], or corrugated surfaces[9–11].

Recently, several designs of flat-plate air-heaters have been putforwarded, constructed and tested [12,13]. Improvements to suchair-heaters have extensively concentrated in increasing the num-ber of air passages [14], using selective glasses [15], contacting pat-terns between air flow and absorber and forming variously shapedabsorbing surfaces [16–18]. Further attempts have also been madeon the use of absorbers with high porosity surfaces [19] or specialcoatings, which behave as a selective surface with low diffusivityconstant for solar radiation [20].

There are also studies on improving the efficiencies of the solarcollectors. Evacuated tubes are used to reduce heat losses from the

of a cost effective solar air heater for Bangladesh. Appl Energy (2010),

Nomenclature

FR collector heat removal factorQu useful total energy gainqu useful energy gain for a specific timeTa ambient temperatureTg glass surface temperatureTi inlet temperatureT0 outlet temperatureTr room temperatureTS collector surface temperatureUb bottom heat loss coefficient

Ue edge heat loss coefficientUL overall heat loss coefficientUt top heat loss coefficientV wind velocity, m/s

Greek lettersa absorptanceg efficiency

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receiver. Sims [21] developed a solar energy receiver, which couldbe used in a high concentration ratio compound parabolic reflector.Here, the absorber is an evacuated transparent envelope with a so-lar receiver made of extruded aluminum, through which passageshave been formed. Two-stage collectors can be used to boost theconcentration ratio of solar collectors. The laws of thermodynamicslimit the efficiency of a concentrating collector which can be ap-proached with a two-stage system. In some cases, the efficiencycan be doubled [22]. Solomon et al. [23] have developed a modelof a flat-plate solar collection system. A theoretical model has beendeveloped to predict the thermal performance of double pass solarair heater with porous media [24]. Yousef and Adam [25] haveinvestigated the effect of mass flow rate, flow channel depth andcollector length on the system thermal performance and pressuredrop through the collector with and without porous medium.

A comprehensive parametric study on the thermal performanceof cross-corrugated solar air collectors has been carried out by Linet al. [26]. The collector consists of a wavelike absorbing plate anda wavelike bottom plate which is crosswise positioned to form theair flow channel. Liu et al. [27] improved the thermal performanceof a solar air collector using v-groove absorber. A comparativestudy of the thermal performances of cross-corrugated andv-groove solar air collectors is conducted by Liu et al. [28]. Thestudy concludes that cross-corrugated collectors are superior tov-groove ones for all the configurations and operating conditionsconsidered.

A conventional solar air heater is essentially a flat-plate collec-tor with an absorber plate, a transparent cover system at the topand insulation at the bottom and on the sides. The working fluidis air, though the passage for its flow varies according to the typeof the air heater. Satcunanathan and Deonarine [29] have sug-gested use of two pass solar air heater to reduce the heat loss.Whillier [30] carried out experiments and analyze the conven-tional air heater consists of an absorbing plate, a rear plate, insula-tion below the rear plate, transparent cover on exposed side, andthe air flows between the absorbing plate and rear plate. Ranjanet al. [31] refers to an air heater with flow above the absorberwhich consists of an absorber plate with a transparent cover atthe top and insulation at the bottom. The cover and the plate pro-vide the passage for the air. Solar radiation, after transmissionthrough the cover, is absorbed by the absorber plate. Sodha andBansalt [32] have studied an air heater with flow on both sidesof the absorber assuming that equal flow occurs both above andbelow the absorber plate and the heat transfer coefficient betweenthe absorber plate and the air stream on either side is the same.Malik and Buelow [33] have worked on air heater with finned ab-sorber in order to improve the heat transfer from the plate to airstream and hence the efficiency of the heater. Fins are added tothe rear side of the absorber. Hollands [34] studied the directionalselectivity, emittance and absorptance properties of Vee-corru-

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gated specular surfaces of air heater. The use of Vee-corrugated ab-sorber in place of a flat absorber provides a large surface area forheat transfer to the air stream. Löf et al. [35] was first developedthe overlapped glass plate air heater and used in his house in Col-orado, USA. It combines the advantages of low pressure drop andhigh efficiency with moderate temperature losses. It consists of aseries of overlapping parallel glass plates with each plate consist-ing of two parts, a black part and a clear part. Gupta and Garg[36] studied two types of matrix air heaters and observed an aver-age rise of temperature of 20 �C at an efficiency of 50%. In one hea-ter galvanized iron wire mesh as a matrix was employed while inthe second aluminum matrix was employed. In a matrix air heaterthe air flows through the matrix, generally made of metal of regu-lar or irregular shape, and the solar radiation is absorbed graduallyalong the depth of the matrix. The flow of air can be from top tobottom or from bottom to top. Lalude and Buchberg [37] proposedthe honeycomb porous bed air heater which consists of a specu-larly reflecting rectangular honeycomb over a black porous absor-ber to reduce convective and radiative losses. Satcunanathan [38]has used a two pass solar air heater connected to a drying chamber.The hot air passes through the crops immediately after it leavesthrough the collector. Hence insulation is not required to be usedin the air heater. Khoukhi et al. [39] have studied the flat-plate so-lar collector performance with coated and uncoated glass cover.Elminir et al. [40] provided recommendations on how the collectedenergy can be increased for solar flat-plate collector by varying thetilt angle. Linear and quadratic sets of equations are proposed forcalculating the thermal efficiency of a flat-plate solar collector hav-ing a single glass cover and an absorber by Yildiz and Gungor [41].

The devastating energy crisis and its future escalation in Ban-gladesh can presume from some reported literatures. The electricalenergy crisis in the country has become such extent that the load-shedding exceeds 4–5 h in a day [42]. There is a huge gap betweenthe current use energy and potential for generation. This gap iscompounded by the large need for investment in electricity gener-ation capacity due to increasing demand for electrification [43].Bangladesh Power System (BPS) can meet only 75% of the total de-mand. Generation shortage forces BPS for massive load shadinghindering nation’s development activities [44]. Therefore, the pur-pose of this study is to design and fabricate an efficient solar airheater using locally available cheaper as well as lesser amount ofmaterial and to analyze its performance.

2. Mathematical approach

The theoretical calculations are performed under the followingassumptions: the temperatures of the absorbing plate, bottomplate and bulk fluids are functions of the flow direction only, andboth the glass cover and the fluids do not absorb radiant energy.Further, except the glass cover, all parts of the outside surface of

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the solar air collector, as well as the flow channel, are well insu-lated thermally.

It is obvious that the overall heat loss coefficient (UL) is made upof top (Ut) bottom (Ub) and edge loss coefficient (Ue).

So; UL ¼ Ut þ Ub þ Ue

The top loss coefficient is given by

Ut ¼N1

CTp;m

ðTp;m�TaÞðN1þf Þ

n oe þ1

hw

264

375�1

þrðTp;m þ TaÞðT2

p;m þ T2aÞ

ðep þ :00591N1hwÞ�1 þ 2N1þf�1þ:133ep

eg� N1

where N1 = number of glass covers; f ¼ ð1þ :089hw�0:1166hwepÞð1þ 0:07866N1Þ; e ¼ 0:43ð1� 100

Tp;mÞ; b = collector tilt

(�); eg = emittance of glass (0.88); ep = emittance of plate; Ta = ambi-ent temperature (K); Tp,m = mean plate temperature (K); V = velocityof wind (air) in m/s; Tsky = Sky temperature = 0.0552 T1:5

a = Ta � 6;hw = wind heat transfer coefficient (W/m2 �C) = 2.8 + 3.0V;C = 520(1 � 0.000051b2) for 0� < b < 70� (for 70� < b < 90�, useb = 70�).

The bottom loss coefficient is given by Ub ¼ kL

� �b; where k and L

are the thermal conductivity and thickness of insulation respec-

tively. The edge loss coefficient is given by Ue ¼kLð ÞeAe

Ac; where

Ae = perimeter � thickness of collector and Ac = collector area. So,the losses based on inlet temperature is UL (Ti � Ta) � 3600� 10�6 MJ/m2.

Overall heat loss coefficient, collector efficiency and flow factorsare used to calculate the outlet temperature. Overall heat loss coef-ficient is assumed to be constant along the flow direction. The col-lector efficiency factor is essentially a constant for any design andfluid-flow rate. It is denoted by F 0 and mainly dependent on thegeometry of the collector surface. Collector efficiency factor is de-fined as

F 0 ¼ actual useful energy collecteduseful energy collected if the entire absorber surface is at the local fluid temperature

:

The collector flow factor is a function of the single variable, the

dimensionless collector capacitance rate _mCp

Ac ULF 0 : It can be obtained

from F 00 ¼ FRF 0 ¼

_mCp

AcULF0 1� e� Ac ULF0

_mCP

� �" #where

collector heat removal factor; FR

¼ actual useful energy collecteduseful energy collected if the entire absorber surface is at the temperature of the fluid entering the collector

¼ F 0F 00

The useful energy gain for a specified hour, per unit area of col-lector is

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qu ¼Q u

Ac¼ FR S� ULðTi � TaÞ � 3600� 10�6

h iThe collector efficiency for that hour, g ¼ Qu

AcIT¼ qu

ITand the day-

long collector efficiency, gday ¼P

quPIT

.

The total useful gain, Qu ¼ Ac �P

qu.

3. Design considerations

Several designs for solar air heaters have been proposed anddiscussed in literature. The designer and potential user of thesesystems must consider a number of factors when comparing theirmerits. These can mainly be categorized as: (i) thermal perfor-mance, (ii) cost and (iii) lifetime, durability, maintenance and easeof installation [45].

The design of the solar air heating systems for a particular appli-cation demands some factors to be considered. The different ele-ments of this system are absorber plate, absorber coating, coverplate or glazing and insulation. The design requirements for the ab-sorber plate are operating temperature range, maximum stagnanttemperature, operating flow rate and cost. The coating should ab-sorb maximum energy and emits the minimum. The primary func-tion of the cover plate or glazing is to minimize the heat losses,convection and radiation, from the collector plate.

Insulation is an important factor playing a very significant rolein minimizing the heat losses from the lower surface of the collec-tor plate and from the lateral edges of the collector. The insulationmaterial in a collector should have low thermal conductivity. Theauthor has used a two pass solar air heater for heating the room.Fig. 1 shows the flow chart of operations of the system.

4. Components of the system

A flat-plate collector (two pass) consists of cover plate of glass,battens to hold the glass in place, air as heat exchanger medium,metal plate painted black as absorber and insulation. Insulationis placed below the absorber plate to reduce the heat loss. A table

shaped as a room has used as room compartment and consists of adoor for entering and taking data. Equipments needed nails, bat-tens, glue, tape and others. Fig. 2 is showing the schematic drawingof the solar air heater.

Main features of the system includes saves energy that is usedfor heating, uses renewable energy, low-cost heating system, sim-ple to install, no moving parts, no auxiliary is needed and easy touse. When there is no solar radiation, we can cover the collector

with foam. The heat storage medium like water drum, rock, etc.can also be used. The dimension of the collector is101.6 � 76.2 cm2, absorber plate is 91.44 � 62.23 cm2 and inside

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room volume is 784063.7 cm3. Fig. 3a and b are showing photo-graphs of the system respectively.

5. Data collection

The author has set up the solar air heater on the roof of EMEbuilding of Chittagong University of Engineering and Technology(CUET), a public engineering university situated in the port cityof Bangladesh, faced to south. The collector is tilted at an angleof 35� and all the outlet air passages are sealed. In the two pass col-lector, cool air enters the collector at the bottom opening and getsheated by taking heat from the absorber surface by convection heat

Fig. 2. Drawing of the model type solar air heater from side.

Fig. 1. Flowchart of operations of the system.

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transfer. Then the hot air comes out by the upper channel. Six dif-ferent temperatures are observed for every half an hour by k-typethermocouple and thermometer. The temperatures are inlet air,outlet air, absorber surface, ambient, glass surface and room tem-perature. The cover is kept clean to get best results. Air flow rate iscalculated from its velocity measured by anemometer (Alnor�

Model RVA801). The parameters have been measured both for nat-ural draft and for forced draft. In case of natural circulation, theflow of air is due to the density difference. The hot air has lowerdensity than cold air, for this reason hot air goes upward naturallyand cold takes the position. In forced draft, a small fan forces outthe hot air. The air velocity is obtained as 0.03 m/s in case of nat-

Fig. 3. Photograph of solar air heater: (a) front view and (b) left view side.

Fig. 4. Average values of hourly solar radiation and ambient temperatures.

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ural draft. In contrary, the inlet and outlet air velocity are 0.9 m/sand 0.3 m/s, respectively, for force draft. The average outside windvelocity is observed as 2.5 m/s.

Fig. 5. Temperature variance caused by using solar air heater on (a) 24-August forforced draft; (b) 26-August for natural draft; (c) 27-August for forced draft and (d)28-August for natural draft.

6. Results and discussions

The solar radiance is strongly dependant on location and localweather. Solar radiance measurements consist of global and/or di-rect radiation measurements taken periodically throughout theday. The measurements are taken using a pyranometer (Manufac-turer: TBCL, Model: 8-48, black and white) for measuring globalradiation and a pyrheliometer (Manufacturer: TBCL, Model: NIP)for measuring direct radiation. Typical hourly values of solar radi-ation and ambient temperatures between 7 am and 5 pm areshown in Fig. 4. The solar radiance varies throughout the day from0 kW/m2 at night to a maximum of about 1 kW/m2. Direct and glo-bal solar radiation is at their highest values at noon (at about12.30 pm) as expected. The global radiation slightly decreases asthe time passes through the afternoon.

The proposed collector is tested according to the ASHARAE 93-97 standard before the performance measurements are conducted.It is observed that the heated air stream is intermittently ob-structed at the head of the absorber during free convection mea-surements. The air started to flow when the pressure increasedenough to overcome the air mass and friction resistance in thedownstream section. This oscillating behavior of the heated airnecessitated taking average air speed in all the efficiency computa-tions. The relative error in assuming constant airflow rate may beconsidered to be small due to the low differences between succes-sive temperature recordings.

We have collected the values of different parameters and calcu-lations are made. Then graphs have been plotted to show the rela-tionships/disparities in different parameters. Fig. 5a–d views thedifferences in the ambient and the room temperatures in the prag-matic days. It is pellucid from the figures that the differences aresignificant.

Fig. 5a–d shows that the temperatures increases first and thendecreases. The maximum temperatures are obtained at aroundmid noon. In forced circulation, the maximum outlet air tempera-ture obtained is 55.0 �C and the maximum difference from ambientis 21.75 �C. The values are, respectively, 54.75 �C and 19.75 �C fornatural circulation. The highest room temperatures 45.5 �C and41.75 �C and the maximum difference from ambient 12.25 �C and8.5 �C are observed for forced and natural circulation in order.We can notice that in natural circulation, the difference betweeninlet and outlet temperatures is higher as the air needs greatertime to circulate than forced circulation. The difference betweenoutlet and surface temperatures is also higher. We obtained theroom temperature is higher in forced circulation, because the totalvolume of air circulates more times in this case. The difference be-tween ambient and room temperatures is also higher here.

Temperature in all cases maintains nearly a linear relationship;if one increases then the other also increases. The certain fluctua-tions are due to decrease in solar radiation because of clouds atthe time of measurement. It increases roughly till mid noon andthen decreases (Fig. 5). From the data collected and after necessarycalculation as well as from graphs plotted (Fig. 5) the author wouldlike to summarize the different parameters of the solar air heater inTable 1. Maximum efficiency in case of forced circulation is 62.21%and for natural circulation 24.96%. The efficiency is higher in caseof forced circulation than in natural. It may because of the flow rateis higher in the first one and the air needs less time to circulate. Theday long efficiency is lower in natural circulation. In both the case,the efficiencies are lower because we have calculated with extra-terrestrial solar radiation.

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The differences between measured and theoretical outlet tem-peratures (both for the natural and forced draft) are shown in

of a cost effective solar air heater for Bangladesh. Appl Energy (2010),

Table 1Different measured parameters of the solar air heater.

24-August

26-August

27-August

28-August

Draft Forced Natural Forced NaturalInlet temperature 37.25 36.5 39.5 38.75Outlet temperature 46.25 52.0 55.0 54.75Difference between inlet and outlet

temperature9.0 15.5 16.5 16.0

Absorber surface temperature 58.75 67.0 62.5 67.75Difference between surface and

outlet temperature12.5 17.0 7.5 13.0

Ambient temperature 31.75 33.25 33.5 35Room temperature 40.75 41.75 45.5 41.75Difference between ambient and

outlet temperatures14.75 18.75 21.75 19.75

Difference between ambient androom temperatures

9.25 8.5 12.25 6.75

Maximum efficiency (%) 62.2 24.8 59.9 24.9Daily average efficiency (%) 58.9 23.2 56.5 24.0

Fig. 6. Comparison of theoretical and measured outlet temperatures: (a) naturaldraft and (b) forced draft for different days in the month of August.

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Fig. 6. The disparities are less in the case of forced draft then thenatural draft. The variations may occur for assumption of roomtemperature as inlet temperature and due to variations of massflow rate. This variation is higher in case of natural circulation.We have taken average values of air velocity for inside and outsidethe room. This may has influences on such variations.

7. Conclusions

From the collected data, tables, graphs, figures in relation withthe analysis and discussions this research investigation can be con-cluded that,

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The fabricated solar air heater is quite efficient in trappingheat. As the construction is very simple and locally availablelow-cost materials are used in construction, it could be manufac-tured in any workshop. Due to its low-cost and simple technol-ogy, it is affordable in respect to cost, operation andmaintenance by huge number of middle and lower middle classpeople of Bangladesh.

The air temperature obtain in the test room is noteworthy. Inforced circulation, the maximum room temperature obtained was45.5 �C and the maximum difference from ambient was 12.25 �C.In case of natural circulation, the maximum room temperature ob-tained was 41.75 �C and the maximum difference from ambientwas 8.5 �C.

The room temperature is higher in forced circulation, becausethe total volume of air circulates for more times in this case. Thedifference between ambient and room temperatures is also higherhere.

The efficiency is higher in case of forced circulation than that ofnatural.

The measured out temperature of the air heater is similar tothat of the theoretical values in quality.

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