Solar Energy

Solar Air Heater

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Solar air heater and It’s performance analysis

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 Solar Air Heater

 Solar air heater is a thermal device in which the

energy from the sun, is captured by an
absorbing medium and is used to heat air.
 Solar air heating is a renewable energy heating
technology used to heat or condition air for
buildings or process heat applications.
Other applications
 Drying of agricultural
products
 Industrial process heat
 Space heating

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 Solar Air Heaters
Air out
Cover
 Eliminate the need to transfer heat from
one fluid to another.
 Consists of an absorber plate with a
parallel plate below forming a small h
passage through which the air to be L1
L2
heated.
Air in
 A transparent cover system is provided
Absorber
above the absorber plate.
 A sheet metal container filled with Insulation
insulation on the bottom and sides.

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 Solar Air Heaters
 Face areas of solar air heater range from 1 to 2
m2.
 MOC and sizes are similar to those used in
Liquid flat plate collectors.
 Absorber plate is a metal sheet of thickness 1
mm usually made of GI or steel.
 Glass thickness : 4-5 mm, plastics are also used.
 Mineral wool or glass wool of thickness 5 to 8
cm is used for the bottom and side insulation.
 Whole assembly is contained in a sheet metal
box and inclined at a suitable angle. 5
 Advantages and Disadvantages
Advantages:

• Corrosion and leakage problems are less compared to LFPC

• Does not require any special attention at temperature below
0 ᵒC
Disadvantages:
 Heat transfer coefficient between the absorber
plate and the air is low – results in a lower Roughened
efficiency. surface or
longitudinal fins
 Large volume of fluid have to be handled –
are provided in
electrical power required is high if the pressure the air flow
drop is not kept within prescribed limits. passes to increase
the HTC 6
Various types of Solar Air Heaters

 Air flows between the cover

and the absorber plate
instead of through the
separate passage.
 Air flows between the cover
and the absorber plate as
well as through the passage
below the absorber plate.

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Performance analysis of SAH

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 Performance analysis of solar air heater

Considering a slice of the solar air heater, width L2 and thickness dx

at a distance x from inlet
# Assumptions:
i. Temperature changes from Tf to (Tf + df) in distance dx
ii. ṁ = Air mass flow rate
iii. Tpm = Absorber plate temperature, Tbm = Bottom plate
temperature
iv. Variations of Tpm & Tbm neglected
v. Side losses neglected
Now, Energy balance for Absorber plate: 𝜎 𝐿2 𝑑𝑥
𝑆 𝐿2 𝑑𝑥=𝑈 𝑡 𝐿2 𝑑𝑥 ( 𝑇 𝑝𝑚 − 𝑇 𝑎 )+h 𝑓𝑝 𝐿2 𝑑𝑥 (𝑇 𝑝𝑚 − 𝑇 𝑓 )+

(1)
( 1
+
1
𝜀𝑝 𝜀𝑏
9
−1
)
 Energy balance for Bottom plate:
𝜎 𝐿2 𝑑𝑥
( 𝑇 4𝑝𝑚 −𝑇 4𝑏𝑚 ) =h 𝑓𝑏 𝐿2 𝑑𝑥 ( 𝑇 𝑏𝑚 − 𝑇 𝑓 ) +𝑈 𝑏 𝐿2 𝑑𝑥 ( 𝑇 𝑏𝑚 −
( 1
+
𝜀𝑝 𝜀𝑏
1
−1
) (2)
Energy balance for Air stream:
˙ 𝑝 𝑑𝑇 𝑓 =h 𝑓𝑝 𝐿2 𝑑𝑥 ( 𝑇 𝑝𝑚 − 𝑇 𝑓 ) +h 𝑓𝑏 𝐿2 𝑑𝑥 ( 𝑇 𝑏𝑚 −(3)
𝑚𝐶 𝑇𝑓 )
Again, assuming negligible difference in heat transfer coefficients of both plates
h 𝑓𝑝=h 𝑓𝑏
Introducing equivalent radiative heat transfer coefficient hr ,
𝜎
h 𝑟 ( 𝑇 𝑝𝑚 −𝑇 𝑏𝑚 ) = ( 𝑇 4𝑝𝑚 − 𝑇 4𝑏𝑚 ) (4)
( 1
+
𝜀𝑝 𝜀𝑏
1
−1
)
For small temperature difference between absorber and bottom plates, can be
approximated as , where Tav is the average of two plate temperatures.

Then, eq. (4) becomes:

3
4 𝜎 𝑇 𝑎𝑣
h𝑟 =
1
(+
𝜀𝑝 𝜀𝑏
1
−1
) (5)
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Assuming Ub<<Ut , equations (1), (2) and (3) reduces to:

¿ (6)
¿ (7)
𝑚
˙ 𝐶𝑝 𝑑 𝑇 𝑓
=h 𝑓𝑝 (𝑇 𝑝𝑚 − 𝑇 𝑓 )+h 𝑓𝑏 ( 𝑇 𝑏𝑚 −𝑇 𝑓 ) (8)
𝐿2 𝑑𝑥
From eq. (7),
h 𝑟 𝑇 𝑝𝑚 +h 𝑓𝑏 𝑇 𝑓
𝑇 𝑏𝑚= (9)
h𝑟 + h 𝑓𝑏
Substituting (9) in (6),
𝑆+𝑈 𝑙 𝑇 𝑎+ h𝑒 𝑇 𝑓 (10)
𝑇 𝑝𝑚=
𝑈 𝑙 + h𝑒
where, he is effective heat transfer coefficient between absorber plate and
air given by:

[
h 𝑒 = h 𝑓𝑝 +
h𝑟 h 𝑓𝑏
h𝑟 +h 𝑓𝑏 ] (11)

Hence,
( 𝑇 𝑝𝑚 −𝑇 𝑎 ) =¿ ¿ (12)
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From eq. (6) to (8), we have:
𝑚
˙ 𝐶𝑝 𝑑 𝑇 𝑓
=𝑆 − 𝑈 𝑙 ( 𝑇 𝑝𝑚 − 𝑇 𝑎) (13)
𝐿2 𝑑𝑥
Substituting (12) in (13),
𝑚
˙ 𝐶𝑝 𝑑 𝑇 𝑓 1
= { 𝑆 −𝑈 𝑙 ( 𝑇 𝑓 − 𝑇 𝑎) } (14)
( )
𝐿2 𝑑𝑥 𝑈
1+ 𝑙
h𝑒
Now, defining collector efficiency factor F´as:

( )
−1
′ 𝑈𝑙 (15)
𝐹 = 1+
h𝑒
Thus, eq. (14) becomes the following differential equation:
𝑚
˙ 𝐶𝑝 𝑑 𝑇 𝑓
=𝐹 { 𝑆 −𝑈 𝑙 ( 𝑇 𝑓 − 𝑇 𝑎) } (16)
′
𝐿2 𝑑𝑥
Integrating eq. (16) and applying boundary conditions Tf = Tfi at x = 0,

( 𝑆
)
+𝑇 𝑎 − 𝑇 𝑓

[
𝑈𝑙
𝑆
𝑈𝑙 ]
+ 𝑇 𝑎 − 𝑇 𝑓𝑖
=exp −
[
𝐿2 𝐹 ′ 𝑈 𝑙 𝑥
𝑚˙ 𝐶𝑝 ] (17)

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Similarly, useful heat gain rate from the collector can be obtained as:
𝑞𝑢 =𝐹 𝑅 𝐴𝑝 [ 𝑆 −𝑈 𝑙 ( 𝑇 𝑓𝑖 −𝑇 𝑓𝑜 ) ] (18)
where, FR is the collector heat removal factor given as:

[ { }]
′
𝑚˙ 𝐶𝑝 𝐹 𝑈 𝑙 𝐴𝑝
𝐹 𝑅= 1− exp − (19)
𝑈 𝑙 𝐴𝑝 𝑚˙ 𝐶𝑝
If the assumption Ub<<Ut was not considered, eq. (16) would have been:
𝑚˙ 𝐶𝑝 𝑑 𝑇 𝑓 (20)
=𝐹 { 𝑆 −𝑈 𝑙 ( 𝑇 𝑓 −𝑇 𝑎 ) }
′ ″
𝐿2 𝑑𝑥
where, is the equivalent overall loss coefficient and & are given as:

( )
−1 ′
′ 𝑈𝑙 (21)
𝐹 = 1+
h𝑒
𝑈 𝑏 h 𝑓𝑏
𝑈 𝑙″ =𝑈 𝑙′ + ′ (22)
𝐹 ( h𝑟 +h 𝑓𝑏+𝑈 𝑏 )
where,
′ h𝑟 𝑈 𝑏
𝑈 𝑙 =𝑈 𝑡 + (23)
( h𝑟 +h 𝑓𝑏+𝑈 𝑏 )
and,
h𝑟 h 𝑓𝑏 (24)
h 𝑒=h 𝑓𝑝+
( h𝑟 +h 𝑓𝑏 +𝑈 𝑏 )
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The useful heat gain for the collector is then given by:
𝑞𝑢 =𝐹 𝑅 𝐴 𝑝 [ 𝑆 −𝑈 𝑙 ( 𝑇 𝑓𝑖 −𝑇 𝑓𝑜 ) ]
″
(25)
and FR here is given as:

[ { }]
′ ″
𝑚
˙ 𝐶𝑝 𝐹 𝑈 𝑙 𝐴𝑝 (26)
𝐹 𝑅= 1 −exp −
″
𝑈 𝑙 𝐴𝑝 𝑚˙ 𝐶𝑝

Heat Transfer and Pressure Drop in parallel duct

Considering fully developed flow when length to equivalent diameter

ratio exceeds 30 and surfaces
𝑁𝑢 = 0.0158 𝑅 𝑒 to be smooth, the correlations are: (27)
0.8
0.75(Kays)
0.01344 𝑅𝑒
𝑁𝑢= (28)
1−1.586 𝑅𝑒− 0.125
(Malik & Buelow)

The equivalent diameter to be used in above equations is given by:

de = (4 × Cross-sectional area of duct)/Wetted Perimeter
Nu values calculated from eq. (27) and (28) agree within 10% for Re =
10,000 to 20,000 − 0.25
𝑓 =0.079 𝑅𝑒 (29)
The dimensionless pressure drop in duct is given by Blasius equation
as: 14
• If Reynolds No. is 5515, f = 0.009167

4 fL V 2
• Pressure drop = 2d e

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 Two-pass air heater
• Outer glass cover temperature lowered by 2 to 5 oC
• Efficiency of this type of collector is measured to be 10-15%
higher than a conventional air heaters.

Better up to an inlet
air temperature
difference of 20 oC

Better up to an
inlet air
temperature
difference of 50 oC
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Two-pass air heater with porous medium

• Solar radiation and width of the flow channels

are varied. Efficiency: 70% (10-20 % higher than
that for the collector without porous medium )
• An outlet temperature of 90 oC at a solar
radiation of 900-1000 W/m2 can be achieved at
a flow rate of 0.0995 kg/s.

Steel
wool

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 Other SAH design

 Collector with porous

• The matrix air heater absorber
• The plastic air heater  Yield higher
efficiencies than
• Transpired air heater conventional design
 Because of large flow
areas these designs
have small pressure
drops

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 Matrix air heater
• Top loss is reduced Cover
• Matrix provide large heat transfer
area to volume ratios
• Higher HTC DUE TO INCREASED
TURBULENCE of air flowing through Matrix
the matrix
• Higher collector efficiency
• With an air inlet temperature of 21
o
C, the efficiency is reported to be 75  Matrix is made by stacking
%. wire screen meshes or slit–
expanded metallic foils.
 Low cost material like glass
bids, crushed glass wool etc.
are used.

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Two-pass air heater with matrix

 Much higher efficiency than conventional

air heater
 Pressure drop is high compared to
conventional air heaters

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 Plastic air heater
• Flexible plastic sheet is used
• 10 m2 – 20 m2, 1 m wide
• Best 6 cm thick polyethylene
• Efficiency of 67.9 %
• 770 m3/h, 759 w/m2

 Absorber: Porous black textile of polyester

 Transparent sheet: Polyvinyl chloride
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Plastic air heater connected to green house

• 5 m long , 0.36 m dia

• Suitable for agricultural
drying operations
• Collector is divided into
two halves (1st half only
polyethylene sheet, 2nd
half another layer of
plastic wrapping film
with air bubble to
reduce the convection
heat loss )
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Inflatable-tunnel plastic solar air heater
• Absorber is a layer of pebble
(5.5 cm thick) and is painted
black
• The cover is a UV stabilized
polyethylene sheet (178 µm
thick)
• After an exposure of 6.5 h,
Outlet temperature of 53 oC
has been achieved at an air
flow of 4.36 kg/s, and I=850
W/sq.m.
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 Unglazed transpired collector on a vertical
wall

• Cold countries for space

heating applications Perforated
absorber
• Crop drying plate

• Porosity is about 0.5% Ambient

air wal
• Mass flow rate : 0.01- l
0.05 kg/s-sq. m
• Planum depth range: 5-
Plenum
30 cm

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 SAH System Operation

1. Dark perforated absorber

captures solar energy
3
2. Fan draws air through
collector & canopy 7 2 4
4
3. Controls regulate 6
temperature 6
 Dampers
5
 Auxiliary heating
1
4. Air is distributed through
building
5. Wall heat loss recovered

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© Minister of Natural Resources Canada 2001 – 2004.
Ex.1: The temperature rise T of air through a vertical south-facing unglazed
transpired collector (UTC) is found to satisfy the following empirical relation:
T  0.03IT  3.0
IT
For an air flowrate
T of 36 m3/h-m2 of UTC. is the total solar radiation incident on
UTC in W/m2; is in oC. Assuming this relation to be valid, calculate the efficiency of
a vertical south facing UTC for the following data:
• Location: 28o35/N, 77o12/ E;
• Date: December 10;
• Hour angle: 15o

• Air flow rate: 36 m3/h-m2 of UTC;

• Global solar radiation on horizontal surface: 543 W/m2;
• Diffuse solar radiation on horizontal surface: 144 W/m2;
• Reflectivity of the surrounding surface: 0.2.

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Thank you

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