Solar air heater

Proceedings of the Global Engineering, Science and Technology Conference 2012
28-29 December 2012, Dhaka, Bangladesh
Design, Fabrication and Performance Study of a Solar Air
Collector for Room Heating By Forced Draught System in
Bangladesh
Asma-Ul-Husna, Md.Harun Or Roshid, Mir Masud Rana, and
Md.Rakibuzzaman
Bangladesh is a developing country but it has huge amount of natural resources and
renewable energies in which solar energy is one of the most available and enormous
source of energy. In winter season some region of Bangladesh is very cold. Middle class
people cannot afford to buy air conditioning system for comfort. For this reason this thesis
work is about to design and construct a simple way to make winter sunlight adequate
to provide a significant portion of the heat for shelter. This work is done and observed within
one year. The design of the solar air heating systems for a particular application
demands some factors to be considered such as Absorber plate (48 in ×36 in),
Absorber coating (generally black), Glazing or Cover plate, Insulation. The sizes of
collector is (49 in × 38 in) =1.25 m × 0.984 m =1.23m²,square duct is 2 in=0.051m,
absorber plate is (48 in ×36 in)=(1.21m×0.984m), room inside volume is (70×35×34)
in
3
=83300 in
3
= 1.365m³.The related data are presented in detail for designing the
solar air collector from where the duration of solar intensity, sunshine hour, and
density of air, and wind velocity, initial temperature, and final temperature, heat loss at
top, bottom and edges are considered. Heat required for the room are estimated.
Then the solar air heater ducts are designed for the required heat output. The collector
thermal efficiency and the material required for this work are estimated. The solar air
heating system developed in this study is a free convection system, movement of
heated air inside a room thus this system can be used for several areas of the country
like Bangladesh. The room required to be heated will get a direct heating from
9:00A.M and it will continue up to 4 P.M. Therefore, the air heater has a direct
effectiveness of 8 hours of a day. The maximum output temperature reached by the
setup with glass as cover is 47
0
C, but there some loss in the system. The
performance of air collector i.e. maximum efficiency was 32.79% (03-05-09) where the
inlet temperature is 35
0
C and the outlet temperature is 45
0
C.
Field of Research: Mechanical Engineering.
1. INTRODUCTION
The available energy sources can be broadly classified in two types: conventional
(nonrenewable) and non-conventional (renewable). Conventional energy sources are
polluting and are leading to global pollution.
_________________________________
Asma-Ul-Husna, Lecturer, Department of Mechanical Engineering, Rajshahi University of Engineering
and Technology Bangladesh. E-mail:evu_me04@yahoo.com
Md. Harun Or Roshid, M.Sc. Student, Department of Mechanical Engineering, Rajshahi University of
Engineering and Technology, Bangladesh. E-mail: harun_me04@yahoo.com
Mir Masud Rana,Shift In-charge, Venture Energy Resources Ltd. (SINHA Group) Bangladesh. E-mail:
masud_04ruet@yahoo.com
Md.Rakibuzzaman, Executive (Sales & Service), Karnaphuli Industries Ltd. Piaggio Division, Bangladesh.
E-mail: rakib04me@yahoo.com

2
There is an increasing trend towards the using of non-conventional resources, which
are non-polluting and lead to healthier environment. Solar energy, which is a non-
conventional energy resource, can be converted into thermal energy or directly into
electricity. The thermal energy conversion, divided into three categories, can be
grouped as: (a) low (<10°C), which deals with solar cooling, (b) medium (10-150°C),
which includes flat plate collectors, air heaters, solar distillation systems, solar water
heating systems, passive heating systems, swimming pool and greenhouse heating etc.
(c) high (>150°C) which includes the concentrator, used in power generation. The
important areas of applications of solar air heater are heating the buildings, heating of
greenhouses, industrial processes such as drying of agricultural crops and timbers.
Thus air heater can be designed using cheaper as well as lesser amount of material. A
conventional solar air heater is essentially a flat plate collector with an absorber plate, a
transparent cover system at the top and insulation at the bottom and on sides. The
working fluid air, through the passage for its flow varies according to the type of the air
heater. Materials for construction of air heater are similar to those liquid flat plate
collectors. Selective coatings on the absorber plate can be used to improve the collector
efficiency but cost effectiveness criterion should be kept in mind. Depending on type of
the absorber plate the air heater can be non-porous and porous.
The geographical position of Bangladesh between 20°34' and 26°38' North latitude is very
favorable in respect of solar energy. In Bangladesh the solar energy that reaches 6414 square
kilometers in one hour is equal to the total yearly energy consumption of the country.
2. PREVIOUS STUDIES
In the following studies, some of the systems that were designed as air heater are
enlisted.
Satcunanathan and Deonarine (Satcunanathan and Deonarine 1973) have suggested
the use of two pass solar air heater. This reduces the heat loss. In a single pass solar
air heater, air flows either in the duct between the absorber plate and rear plate or
between the transparent cover and the absorbing plate. Hence the thermal losses are
higher and consequently the lower efficiency. It is known that an increase in number of
air passes from one to two reduces the net thermal loss from collector and thereby
increases the total energy collected. In this type of air enters the flow channel formed by
the absorber plate and another plate below it. The inlet air removes heat from the
covers so that they are kept cooled and the top heat loss reduced. This was later

3
considered by couris et al. wijeysundera et al. introduced a two pass air heater where
air flows first through the absorber plate and the inner cover and then through the
absorber plate and another plate below it.
Whillier (Whillier 1964) carried out experiment and analyze the conventional air heater.
It consists of an absorbing plate, a rear plate, insulation below the rear plate,
transparent cover on exposed side, and the air flows between the absorbing plate and
rear plate. He considered a steady state one dimensional analysis. The performance
analyses were modified by Sukhatme and Garg. Here the air to be heated flows in a
parallel passage below the absorber. Ranjan et al [3] refers to an air heater with flow
above the absorber. It consists of an absorber plate with a transparent cover at the top
and insulation at the bottom. The cover and the plate provide the passage for the air.
Solar radiation, after transmission through the cover, is absorbed by the absorber plate.
Part of the absorbed energy is convected to air and heated air moves upwards, and the
rest is lost to the ambient through the cover and bottom insulation. An air gap is
introduced between absorber plate and the insulation with the help of a reflecting sheet.
This helps is reducing the bottom loss.
Sodha et el. (Sodha, Bansal, Singh 1982) have studied an air heater with flow on both
sides of the absorber. As compared to the flow over the absorber, this arrangement
provides an increased contact area between the absorber and the air stream. Here it is
assumed that equal flow occurs both above and below the absorber plate. And the heat
transfer coefficient between the absorber plate and the air stream on either side is the
same. The analysis being very approximate as it dose not take into account the
interaction of cover and between the absorber plate and the bottom plate have not been
considered.
Malik and Buelow (Malik and Buelow, 1975) have work on air heater with finned
absorber. In order to improve the heat transfer from plate air stream, and hence the
efficiency of the heater, fins are added to the rear side of absorber. This, however,
introduces some extra pressure drop. Also the number of fins and their depth cannot be
increased beyond a limit as in the case fan power requirements increase. Kuegy et al.
also worked on this. The heat transfer model of such air heater can be easily developed
since heat transfer from finned surface is very common in convective heat transfer
problem. However the limitation of this model is that here are hardly and appropriate
correlations for heat transfer coefficients corresponding to situations encountered.
Sulcuk has summarized the works of above two. Test results of air heaters with
staggered galvanized fins and U-shaped staggered aluminium fins attached to the rear
side of the absorber plate have been reported. The efficiencies with fins are
substantially higher than conventional air heaters.

4
Hollands (Hollands, 1963) studied the directional selectivity, emittance and an
absorptance property of Vee-corrugated absorber in place of a flat absorber provides a
large surface area for heat transfer to the air stream. The convective heat transfer from
plate to cover increases in this case but the loss is largely compensated by the
increased heat transfer to the following air. Selective coating on the absorber with
acceptance angle of 55º in the Vee-groove is suggested. Air heaters with Vee-
corrugated absorbers of copper foil and which are selectively painted are used. Here
flow can be on either sides or one side only.
2.1. WORLD ENERGY SUPPLY
Energy supply of the world is combined resourced by which the nations of the world
attempt to meet their energy need. Energy is the basis of industrial civilization; without
energy, modern life would cease to exist. The current energy consumption in the world
is given below.
Fig. 2.1: World energy consumption by region
Much of the world’s energy comes from nonrenewable resources, such as oil, coal, and
natural gas. Although these resources are distributed over a large geographical region,
consumption of energy centers in industrialized nations. This table shows the percent of
the world’s total energy supply used by each region. Oil is not the only resource
involved in the comparison; oil equivalents are provided to give perspective to the
percentages. The different sources of energy are petroleum and natural gas, coal,
synthetic fuels, nuclear energy, solar energy, geothermal energy etc. A short description

5
of each is given in the following sections. Although there is increasing interest in
alternate energy sources such a solar power almost two-thirds of the world energy
comes from oil and natural gas. The world energy production in the world is given
below.
Fig 2.2: World energy production by source
3. METHODOLOGY
At first we have made the two pass flat plate collector and placed the absorber plate
between the two passes with insulation at the bottom. Then we painted the collector
with black paint, which helps to absorb solar radiation. Then the room was made. After
these two, we have placed the collector with the room at the proper position. A glass
cover is then placed on the collector which helps to reduce the loss from the collector.
Then a tunnel is placed at the inlet pass of the collector to suck the cold air of the room
which is at the bottom. we have use tape to cause the room air tight. A door is made at
the rear side of room which is used to enter the room and take different data.
The cold air gets heated by the solar radiation and goes to the upper part of room for its
lower density. The cold air which has higher density comes at the bottom and enters the
collector through the inlet pass. In natural circulation, the air is circulation naturally for
the density difference but flow rate is lower. In case of forced circulation, we have used
a small fan and here the flow rate is higher. After placing all in position we have taken
the data and analysis is made based on them.

6
3.1. SIZE OF THE SYSTEM
 Collector size: (49 in × 38 in) =1.25 m × 0.984 m =1.23m²
 Square duct : 2 in=0.051m
 Absorber plate size: (48 in ×36 in)=(1.21m×0.984m)
 Room inside volume: (70×35×34) in3
=83300 in3
= 1.365m³
Fig. 3.1: Model of solar air heater
3.2. DESIGN OF THE SOLAR AIR COLLECTOR
Heat to the outlet air, Q = mCp T = AVCP T … … … … (3.2)
Where, = Density of air = 1.293 kg/m3
,Cp = Specific heat at constant pressure =
1.006kJ/kg°K .

7
Let, = 20°C and T2 = 10°C, L = outlet duct size =2 = 0.051, T = - = 10°, V = wind
velocity = 170 ~ 175ft/min=170 0.3048 60m/hr = 3108.9m/hr = 0.8636m/sec. (Climatology
Department, Government of Bangladesh, 2007),Outlet duct area =A =0.002061 m2
.
From equation (3.1) Qout = 1.293 0.002601 3108.9 1.00 10 = 98.40 kJ/hr.
Now, total heat absorbed by the collector=Ac Solar intensity/sunshine hour
Where, Ac = Collector area in m2
, average daily solar intensity= 16.22 MJ/m2
day. (Climatology
department, Government of Bangladesh), average bright sunshine hour during winter (From
Sep. to Feb) = 8.42 hr.
Total heat absorbed by the collector = Ac 16.22/8.42 = kJ/hr = (80.27 Ac)
kJ/hr.
Now, Heat gain=Heat loss or, 80.27 Ac= 98.40 or, Ac= 1.23 m2
By trial and error method, Area Ac= 1.23m2
, Length L= 1.25m, Width B=0.984m
3.3. FOR ROOM DESIGN
Let, volume of air in the room = (70 35 34) in3
= 1.365m3
The amount of heat required during the sunshine hour,
Q room = m CP T = ρVroom Cp ∆T............... (3.3)

8
Where, ρ = Density of air = 1.293 kg/m3
, V = Volume of room in m3
, CP = Specific heat of air of
constant pressure = 1.006 kJ/kg-K, ∆T = Temperature difference in °C.
From equation (3.3) Qroom=1.293 1.365 1.00 10 = 17.64kJ/h
3.4. DESIGN OF FAN
Rpm of fan = 1649 rpm, Size of the fan =2×2 , Power of the fan= 6w, Current I =1
Amp, Volt = 6 v.
4. PERFORMANCE STUDY OF THE COLLECTOR
4.1. DATA COLLECTION
Air inlet and outlet temperatures were measured by placing two thermometers at inlet
and outlet holes respectively. The mass flow rate was measured from which finally heat
calculation was accomplished from the formula given in equation-
Q = Vf × Cp × T ………..(4.1)
Where, Q = heat to outlet air, kJ/hr, Vf volume flow rate, Cp= specific heat of air at
constant pressure, T = T1 – T2 = temperature difference between outlet and inlet air, T1
= outlet air temperature, T2 = inlet air temperature.
4.2. CALCULATION OF HEAT FOR OUTLET AIR
Area =L²; where, L = Duct size of the outlet pipe.

9
Volume flow rate (Vf), = Volume density, Q = Vf × Cp × T
Heat, Q = (Vf) × Cp × T = (Vf) × × Cp × T = (Vf) × × Cp × (T1 - T2) …………….. (4.1.2)
5. Results
From the graphs the maximum collection efficiency of the collector gradually increases with
increase in temperature difference and maximum value is 32.79% at 1:30 P.M. (03-05-09) and
the maximum temperature difference is 10 ° C.
Date: 03-05-09
Fig (5a1): Solar time Vs Temp. difference curve

10
Fig (5a2): Time Vs Efficiency curve
Date: 04-05-09
Fig (5b1): Solar time Vs Temp. difference curve

11
Fig (5b2): Time Vs Efficiency curve
Date: 06-05-09
Fig (5c1): Solar time Vs Temp. difference curve

12
Fig (5c2): Time Vs Efficiency curve
6. DISCUSSIONS
In this system corrosion, which can cause serious problem in solar water heater, is completely
eliminated, leakage of air from duct does not pose any major problem, freezing of fluid e.g.,
water fluid virtually does not exist. The solar air heating system developed in this study is a free
convection system, movement of heated air inside a room thus this system can be used in
different areas of the country. The room required to be heated will get a direct heating from
9:00A.M and it will continue up to 4 P.M. Therefore, the air heater has a direct effectiveness of 8
hours of a day. From the graphs (4a1-4e2) the maximum collection efficiency of the collector
gradually increases with increase in temperature difference because variation of weather
condition. The maximum temperature difference obtained in this study is 10 . It is very
interesting to note that the total cost for the system is very low i.e. a person can construct it with
a cost of Tk. 2050 including the fact of very cheap labor availability in the country and that its
operating cost is zero.

13
7. CONCLUSIONS
After performing the project, it can be concluded that it is a simple process to heat our room.
The cost is also low. In this process the unlimited free solar energy which will minimize the
energy consumption. The heated room can be modified to use for other applications like drying
crops. In winter there is proper sunshine and can be used to heat our room for comfort. The
higher efficiency can be obtained by using a device for circulating the air. The maximum output
temperature reached by the setup with glass as cover is 47 0
C, but there some loss in the
system. The performance of collector maximum efficiency 32.79% (03-05-09) where the inlet
temperature is 35 0
C and the outlet temperature is 45 0
C.
References
Hollands K.G.T.(1963), “Directional selectivity, emittance and absorptance
properties of Vee-corrugated specular surfaces”, solar energy,7pp.3.
Malik M.A.S. and Buelow F.H. 1975, “Hydrodynamic and heat transfer
characteristics of heated air duct”, Helio-Technique and Development, 2, pp. 3.
Sodha M.S., Bansal N.K. and Singh D. 1982 , “analysis of a nonporous double
flow solar air heater”, Applied energy, 12, pp. 251.
Satcunanathan S. and Deonarine S. 1973 , “A two pass solar air heater”, solar
Energy, 15, pp.41
Whillier A. 1964 , “Black-Painted solar air heaters of conventional design”, solar
energy, 8, pp. 31.

Solar air heater

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