Performance Analysis of Flat Plate Solar Air Collectors With and Without Fins

Performance Analysis of Flat Plate Solar Air Collectors With and Without Fins

PERFORMANCE ANALYSIS OF FLAT PLATE SOLAR AIR COLLECTORS WITH AND WITHOUT FINS

D. Bhandari1 , Dr. S. Singp

1M. Tech Scholar, 2Associate Professor

Department of Mechanical Engineering,

Bipin Tripathi Kumaon Institute of Technology, Dwarahat, Almora, Uttarakhand (India) 263653

Abstract

The present work involves a comparative study of performance analysis of different types of flat plate solar air heaters. A MATLAB program code is generated to carry out the whole analysis. The effect of mass flow rates, inlet temperature and intensity of solar radiation on the performance of solar air heaters is also investigated in the present study. The three types of solar air heaters used for the analysis are: Conventional solar air heater, double glazing single pass solar air heater and double pass solar air heater with internal fins.

Keywords: Conventional single pass solar air heater, Double pass solar air heater, Internal fins, Thermal efficiency, MATLAB.

1. Introduction

   Humans have always used the rays of the sun to gather their energy needs. In the energy needs of today with increasing environmental concern, alternatives to the use of non-renewable and polluting fossil fuels have to be investigated. One such possibility is solar energy, which has become increasingly popular in recent years. Various types of solar energy systems for agricultural and marine products have been reviewed [4]. One of the most important components of a solar energy system is the solar collector. Solar air collectors are simple, cheap and most widely used. Solar collectors can be used

   for drying, space heating, solar desalination, etc. Extensive investigations have been carried out on the optimum design of conventional and modified solar air heaters, in order to search for efficient and inexpensive designs suitable for mass production for different practical applications. The researchers have given their attention to the effects of design and operational parameters, type of flow passes, number of glazing and type of absorber flat, corrugated or finned, on the thermal performance of solar air heaters (Ratna et al. [10]; Ratna et al. [11]; Choudhury et al. [1]; Karim and Hawlader,

   [8] ). They concluded that for shorter duct lengths and lower air mass flow rates, the performance of the two pass air heaters with a single cover is most cost-effective as compared to the other designs. Helal et al. [6] studied energetic performances of an integrated collector storage solar water heater. The systems shows little cost, simplicity and simpler to be installed on the roof of the building. Fudholi et al.[3]

   Where,

   Solving these equations we get

   +

   (1)

   (2)

   (3)

   (4)

   conducted an experimental study on a forced- convective double-pass solar air

   collector with fins in the second channel.

   The experiments were conducted by changing the parameters that influence the

   thermal efficiency of the collector. Ho et

   al. [7] experimentally investigated a new

   (5)

   (6)

   (7)

   (8)

   device of inserting an absorbing plate to divide a flat-plate channel into two channels with fins attached and external recycling at the ends, resulting in substantially improving the heat transfer efficiency. The present work deals with comparing the performances of flat plate solar air heaters with and without fins using MATLAB computer software.

2. Mathematical Model

   1. Conventional Solar air heater

      The steady state energy balance conditions for the various parts of conventional flat plate solar air collector is given as

      + (9)

      Mean fluid temperature is calculated by using the formula

      which on substituting terms by relevant gives

      (10)

   2. Double Glazing Solar air heater

      The steady state energy balance conditions for the various parts of conventional flat plate solar air collector is given as

      +

      (1)

      (2)

      (3)

      Solving these equations we get

      (5)

      Where,

      (6)

      Solving these equations we get

      (4)

      (7)

      (8)

      (5)

      (9)

      (7)

      (8)

      (10)

      Mean fluid temperature is calculated by using the formula

      + (9)

      (10)

      which on substituting terms by relevant gives

   3. Double Pass Solar air heater with internal fins

      The steady state energy balance conditions for the various parts of conventional flat plate solar air collector is given as

      + (12)

   4. Empirical relations

      Where

      And,

      (1)

      (3)

      (4)

      For calculating the above parameters we need to find out the overall heat transfer coefficients and dimensionless numbers such a Reynolds number and Nusselt number.

      An empirical equation for the loss coefficient from the top of solar collector to the ambient Ut was developed by Klein

      Nomenclature hw Convective heat transfer coefficient between glass cover and ambient, W/m2K
      Ap	Surface area of the absorbing plate,m2	p	Convective heat transfer coefficient between the absorber plate and the air stream, W/m2
      Af	Total surface area of fins,m2	p	Convective heat transfer coefficient between the bottom plate and the air stream, W/m2
      Afb	Total cross-sectional area of fins,m2	L	Length of collector, m
      B	Width of the air tunnel in the solar collector,	I0	Intensity of solar radiation, W/m2
      	m
      Cp	Specific heat of air at constant pressure,	Ks	Thermal conductivity of material of fins,
      	J/kgK		W/mK
      De	Equivalent diameter of the air tunnel, m	Ta	Ambient temperature, K
      F	Efficiency factor of the solar air heater	m	Mass flow rate, m/s
      FR	Heat removal factor for the solar air heater	N	Total number of fins
      H	Height of the air tunnel in the solar air	Nu	Nusselt number
      	collector, m
      R	Remixing ratio	Tfm	Mean fluid temperature, K
      Re	Reynolds number	Ti	Inlet air temperature, K
      T	Thickness of fin, m	Tbm	Mean bottom plate temperature, K
      Ut	Loss coefficient from the top of absorbing plate to the atmosphere, W/m2K	Tpm	Assumed mean temperature of absorber plate, K
      w1	Distance between fins, m	T0	Fluid outlet temperature, K
      w2	Height of fins, m	Qu	Useful heat gain per unit time, W
      V	Wind velocity, m/s	p	Emissivity of absorber plate
      X	Axis along the flow direction, m	b	Emissivity of bottom plate
      	Efficiency of solar air collector	f	Fin efficiency
      	Density of air, kg/m3		Stefan Boltzman Constant
      	Transmittance of glass cover		Dimensionless quantity

      Absorptivity of the absorbing plate

      µ

      Absolute viscosity of fluid , Ns/m2

      Tpm

      Calculated value of mean absorber

      temperature, K

      plate T

      Difference between assumed and calculated

      values of mean absorber plate temperature, K

      Glass Cover Absorber

      Insulation Bottom Plate

      L

      Figure1: Conventional Solar Air Heater

      Glass Cover

      Absorber

      Insulation Bottom Plate

      Figure2: Double glazing solar air heater

      Glass Covers

      Ab

      sorber

      Insulation Recycling Channel

      Figure3: Double pass solar air heater with internal fins

      [14] following the basic procedure of Hottel and Woertz [5] ,

      The convective heat transfer coefficient hw for air flowing over the outside surface of the glass cover depends primarily on the wind velocity. V. McAdams [15] obtained experimental result as

      Where,

      (1)

      (2)

      (3)

      Reynolds number is calculated by using the following relation

      (4)

      And, Nusselt number is calculated by using following relation

3. Computational Solution

   (5)

   It provides an interactive environment with hundreds of built-in functions for technical computation, graphics and animation. It

   The whole analysis is carried out with the help of MATLAB R2009b. MATLAB is a software package for high-performance numerical computation and visualization.

   also provides easy extensibility with its own high-level programming language. The name MATLAB stands for MATrix LABoratory.

   Figure4: Schematic diagram of double pass internally finned solar air heater

   Input Parameters

   S.No. 1	Input Parameter Width of the air tunnel, m	Magnitude 0.6
   2	Length of the collector, m	0.6
   3	Thermal conductivity of fin, W/mK	386
   4	Mass flow rate, m/s	0.01, 0.015, 0.02
   5	Solar radiation intensity, W/m2	950, 1100
   6	Thickness of fin, m	0.001
   7	Emissivity of glass cover	0.94
   8	Emissivity of absorber plate	0.95
   9	Absorptivity of absorbing plate	0.95
   10	Transmittance of glass cover	0.0875
   11	Stefan Boltzmann Constant, W/m2K4	5.67×10-8
   12	Air velocity, m/s	1
   13	Height of fins, m	0.013
   14	Distance of fins, m	0.025
   15	Ambient temperature, K	283
   16	Number of fins	24

   1. Algorithms

      The MATLAB programme algorithm with respective notations and formulae for the three types of solar air heaters described above are given below. Algorithm is the

      step by step indication of execution of any program. Algorithm is very essential to understand the flowchart of any programme.

      1. Algorithm for the MATLAB programming of Conventional and Single pass double glazing solar air heaters

         1. START

         2. Assume any arbitrary value of Tpm.

         3. Enter input value of Cp, µ, K, Ti , Io, H, Ks, B, W1, W2, t, n, V, , , p , g

         4. Set initial value of m =0.01

            ,T

         5. Find out the value of required parameters ( Ut , F, FR , Qu ) and hence the value of u, v and

            pm

            T . Here u= { m,T

            o, }, v={m} and T=T

            pm-T

            pm

         6. If value of T lies in the range of {-1,1} then display the value of u as optimized otherwise display the value of v as not optimized

         7. Increase value of m by 0.005, if value of m is less than 0.03 go to STEP 4.

         8. When the whole loop is completed notice the values which are not optimised to find such values again go to step 2 and repeat the whole steps further until all of the values get optimised.

         9. END

      2. Algorithm for the MATLAB programming of Double pass internally finned solar air heater

         1. START

         2. Assume any arbitrary value of Tpm.

         3. Enter input value of Cp, µ, K, Ti , Io, H, Ks, B, W1, W2, t, n, V, , , p , g

         4. Set initial value of m =0.01

         5. Set initial value of R=1 .

            ,T

         6. Find out the value of required parameters ( Ut , F, FR , Qu ) and hence the value of u, v and

            pm

            T . Here u= { m,T

            o, }, v={m} and T=T

            pm-T

            pm

         7. If value of T lies in the range of {-1,1} then display the value of u as optimized otherwise display the value of v as not optimized

         8. Increase value of R by 1 , if value of R is less than 3 go to STEP 5 else go to NEXT STEP.

         9. Increase value of m by 0.005, if value of m is less than 0.03 go to STEP 4.

         10. When the whole loop is completed notice the values which are not optimised to find such values again go to step 2 and repeat the whole steps further until all of the values get optimised.

         11. END

             3.1 Flow charts

             Flow charts for the MATLAB programming for the three types of solar air heaters are illustrated below. The first flow chart corresponds to conventional and

             single pass double glazing solar air heater while the second one corresponds to double pass internally finned solar air heater.

             Figure5: Flow Chart for the Conventional and single pass double glazing solar air heaters

             Figure6: Flow Chart for the Double pass internally finned solar air heater

4. Results and Discussions

The effects of the air inlet temperature and incident solar radiation on the collector efficiency of the conventional, double glazing single pass and double pass solar air heaters with the fins attached have been investigated computationally. As evident from the results, it is clear that the efficiency of all the three types of solar air heaters increases with the increase in mass flow rate and decreasing fluid inlet

pressure. The remixing ratio also has a tremendous effect on the efficiency of double pass finned solar air heater and the efficiency increases with the increase in the value of remixing ratio R. Similarly it is clear from the results obtained that the value of mean absorber plate temperature Tpm and fluid outlet temperature To decreases with the increase in mass flow rate.

45

40

35

30

25

Efficiency ,

20

15

Ti=293K

Ti=288K

10

5

0

0

0.005

0.01 0.015

0.02

0.025

0.03

0.035

Mass flow rate, m'(kg/s)

Figure7: Graph of conventional solar air heater for Io=950W/m2

10

5

0

0

0.005 0.01 0.015 0.02 0.025 0.03 0.035

Mass flow rate, m'(kg/s)

Ti=293K

Ti=288K

20

15

Efficiency,

45

35

30

25

Figure8: Graph of conventional solar air heater for I-0 = 1100 W/m2

40

35

30

25

Efficiency, 20

15

Ti=293

Ti=288

10

5

0

0

0.005 0.01 0.015 0.02 0.025 0.03 0.035

Mass flow rate, m'(kg/s)

Figure9: Graph of double glazing single pass solar air heater for I0=950 W/m2

40

35

30

25

Efficiency, 20

15

Ti=293K

Ti=288K

10

5

0

0

0.005 0.01 0.015 0.02 0.025 0.03 0.035

Mass flow rate, m'(kg/s)

Figure10: Graph of double glazing single pass solar air heater for I0=1100 W/m2

70

60

50

40

Efficiency ,

30

20

R=1

R=2 R=3

10

0

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

Mass Flow rate , m' (kg/s)

Figure11: Graph of double pass finned solar air heater for Io=950W/m2 and Ti=288 K

60

50

40

10

0

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

Mass flow rate, m'(kg/s)

R=1

R=2 R=3

20

Efficiency, 30

Figure12: Graph of double pass finned solar air heater for Io= 950 W/m2 and Ti = 293K

70

60

50

40

Efficiency,

30

20

R=1

R=2 R=3

10

0

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

Mass flow rate, m' (kg/s)

Figure13: Graph of double pass finned solar air heater for Io= 1100 W/m2 and Ti = 288K

70

60

50

40

efficiency,

30

20

R=1

R=2 R=3

10

0

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

Mass flow rate, m'(kg/s)

Figure14: Graph of double pass finned solar air heater for Io=1100W/m2 and Ti=293K

The improvement of collector performance for Conventional solar air heater

The improvement of collector performance for double glazing solar air heater

The improvement of collector performance for double pass finned solar air heater for R=1

The improvement of collector performance for double pass finned solar air heater for R=2

The improvement of collector performance for double pass finned solar air heater for R=3

Conclusion

From the analysis of various types of solar air heaters viz conventional , double glazing single pass and double pass finned solar air heaters, it is concluded that for the same mass flow rate, double pass finned solar air heater is having the highest efficiency.

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