parametric study on the thermal performance of the solar air heater with energy storage

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 1, January- April (2012), © IAEME

90

PARAMETRIC STUDY ON THE THERMAL PERFORMANCE

OF THE SOLAR AIR HEATER WITH ENERGY STORAGE

Yogesh C. Dhote 1

, Dr. S.B. Thombre 2

1

Department of Mechanical Engineering, Hindustan College of Science & Technology, Farah, Mathura - 281 122 (U.P.) India

Email: [email protected] 2

Department of Mechanical Engineering,

Visvesvaraya National Institute of Technology, Nagpur – 440 011 (M.S.) India,

ABSTRACT

Under investigation solar air heater is tested analytically under this exercise using

prepared mathematical model using simulation tool as MATLAB 7 for the available

solar radiation data on a particular day of the year. The computations are carried out

using available solar radiation data for the month of March at Nagpur (21o06’ N,

79o03’ E). The purpose of this parametric study is to analyze the performance of the

under investigation solar air heater having thermal storage with the help of calculated

data and corresponding graphs. More appropriate heat transfer correlations suggested

by the different investigators are used for the calculations of heat transfer coefficients

at different surfaces. With standard and very less assumptions made while

calculations, it is expected that the mathematical model developed is reliable and

provides more accurate results.

Keywords: Solar Air heater, Heat transfer coefficient, Thermal reservoir, Useful heat gain

1. INTRODUCTION

Drying is one of the most practical methods of preserving the quality of

agricultural products [2]. Direct sun drying has been practiced since ancient times.

However, it is not hygienic for some products which are easily contaminated in the

open air [4]. In addition it depends upon weather conditions because there is no

shelter to protect the product in the event of rain. As a result, new drying methods

with conventional heat sources have been widely developed and used in order to solve

these problems. Because of the energy crisis and intensive energy consumption in the

drying process, solar drying has been studied widely in many countries in order to

reduce cost and substitute conventional energy.

One of the possible areas of immediate intervention in developing countries

like India appears to be the solar drying of cash crops such as tobacco, tea, coffee,

INTERNATIONAL JOURNAL OF MECHANICAL

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ISSN 0976 – 6340 (Print)

ISSN 0976 – 6359 (Online)

Volume 3, Issue 1, January- April (2012), pp. 90-99

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91

grapes raisin, chili, coriander seeds, ginger, turmeric, black pepper, onion flakes and

garlic flakes, timber etc. where solar energy is available in most of the region

throughout the year. A solar air heater also finds applications in air-conditioning for

space heating purpose. Though lot of research is taken place in storage type of solar

air heater mostly the research took place with latent heat storage. Still there is scope

for thorough analysis of solar air heater with liquid sensible heat storage medium.

2. DESCRIPTION OF THE PROPOSED SOLAR AIR HEATER

A schematic diagram of the proposed double flow solar air heater with thermal

energy storage is as shown in Fig.1. The solar radiation is transmitted from the glass

covers and is absorbed by the absorber plate and below, the storage material, where it

is heated. Double flow operation of the collector may lead to the further improvement

of the efficiency. The different parameters are assigned the values as given in Table 1.

.

Fig.1 Under-Investigation Double Flow Solar Air Heater with Thermal Storage

Table 1 Parameters used for analysis

Length of the collector 2 m

Width of the collector 1 m

Side insulation thickness 0.05 m

Back insulation thickness 0.05 m

Absorber plate thickness 0.001m

Glass cover thickness 0.004 m

Collector tilt 36 o

3. COMPUTATIONAL MODEL

As mid of month March is the harvesting period at area surrounding nearby

Nagpur as well as for similar climatic conditions at different part of the state; solar

radiation data at Nagpur for the month of March is used for calculation purpose.

Simple energy balance equations written for proposed arrangement of solar air heater

are being used to get various simultaneous equations in terms of different temperature

variables. For simplifying the calculations standard assumptions have been assumed

and a self reliant mathematical model is developed in MATLAB 7. A forced

convection varying air flow rate corresponding to the free wind velocity of air is

assumed through the ducts for this analysis.



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4. HEAT TRANSFER CORRELATIONS USED

Most appropriate available correlations devised by various investigators have

been selected for calculations of heat transfer coefficients at different locations of the

proposed system under investigation as given below.

a) Convective heat transfer from various surfaces to the duct air (W. M. Kays):

Nu = 0.0158 x (Re)0.8 ..........

(i)

b) Convective heat transfer from the absorber plate to the liquid sensible heat storage

medium (Buchberg et.al equation):

Nu = 1, for Raδcosβ < 1708 ..........

(ii)

= 1 + 1.446{1 – 1708/(Raδcosβ)}, for 1708 < Raδcosβ <5900

..........

(iii)

= 0.229 x (Raδcosβ)0.252

, for 5900 < Raδcosβ < 9.23 x 104

......... (iv)

= 0.157 x (Raδcosβ)0.285

for 9.23 x 104 < Raδcosβ < 10

6

......... (v)

c) Convective heat transfer from the glass cover surface to the ambient air (Sparrow):

hc = j.ρ.cp.v∞.Pr-2/3

.........

(vi)

d) Radiative heat transfer from one surface to another surface (Duffie and Beckman):

hr = σεeff(T12 + T2

2) (T1 + T2)

......... (vii)

e) Bottom Loss Coefficient:

Ub = ki/δbi ......... (viii)

5. RESULTS AND DISCUSSION

The proposed solar air heater is tested analytically using developed

mathematical model under following conditions.

i) Different mass of storage material (Unused Engine Oil)

ii) Different plate spacing

For the given set of parameters the outcome results of the analysis are presented as

below followed by the graphs where the variation of different properties can be seen.

a) Maximum temperature difference throughout the day for Stream-1 (∆Tf1) =

25.50 oC at δpc = δsb = 0.01 m and moil = 88.25 kg, while maximum rise in

temperature of the inlet ambient air is found to be 12.4209 oC, again at δpc =

δsb = 0.01 m and moil = 88.25 kg.

b) Maximum temperature difference throughout the day for Stream-2 (∆Tf2) =

14.17 oC at moil = 88.25 kg and remains constant for all assumed plate

spacing while maximum rise in temperature of the inlet ambient air is found to

be 0.0977 oC at δpc = δsb = 0.01 m and moil = 88.25 kg,

c) Maximum rise in storage oil temperature (∆Ts) = 14.17 oC for the plate

spacing of 0.01 m and almost same values are obtained for different oil masses

at the same spacing.

d) Maximum average instantaneous collection efficiency of 67.58% is observed

corresponding to the maximum average useful heat gain of 787.76 W for plate

spacing δpc = δsb = 0.05 m and storage oil mass of 88.24 kg. It has been noticed

that maximum values of (qu)av and (ηi)av are observed at this condition due to

higher mass flow rate of air through the duct. In fact the rise in temperature of

air is considerable less than the cases of lower plate spacing.

e) It is clearly observed that for less spacing between the plates there will be

more rise in outlet temperature of Stream-1 (Tf1).



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f) In this double flow arrangement, though oil temperature initially increases

gradually it seems that this temperature is always adjacent to the ambient

temperature during the sunshine hours.

As compared to the outlet temperature of Stream-1 (Tf1) there is negligible or

no increment in outlet temperature of Stream-2 (Tf2) takes place during day time,

hence it is concluded that there is no need of Air-Stream-2.



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95



96



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Further results obtained by varying parameters are tabulated as shown in Table 2 .

Table 2 Consolidate performance of under investigation storage type solar air heater

δpc = δsb (m)

Average Useful Heat gain /

Average Collection

Efficiency

moil (kg)

22.06 44.12 88.25

0.01 qu(av) (W) 578.45 610.33 633.26

ηi(av) (%) 49.42 52.10 54.05

0.02 qu(av) (W) 685.07 716.48 740.32

ηi(av) (%) 58.57 61.30 63.34

0.05 qu(av) (W) 732.30 763.83 787.76

ηi(av) (%) 62.77 65.52 67.58

Table 3 Maximum temperatures obtained

Temperatures

Plate

spacing

δpc = δsb (m)

Oil mass moil (kg)

22.06 44.12 88.24

Tf1 (K)

0.01 317.60 318.20 318.61

0.02 313.20 313.50 313.70

0.05 309.90 310.00 310.10

Tf2 (K)

0.01 307.60 307.60 307.60

0.02 307.63 307.63 307.63

0.05 307.70 307.70 307.70

Toil (K)

0.01 307.50 307.50 307.50

0.02 307.51 307.51 307.51

0.05 307.52 307.52 307.72

6. CONCLUSION

As per the results obtained for the mathematical model it has been concluded

that in under investigation storage type solar air heater with double flow arrangement,

though oil temperature gradually increases it always remains adjacent to the ambient

temperature. Further there is no or negligible increment in the temperature (Tf2) of air

passing through Stream-2 and hence it is concluded that there is no need of Air-

Stream-2. Higher values of outlet temperature (Tf1) of air passing through Stream-1



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are observed for smaller plate-cover spacing and for larger mass of heat storage

medium (oil). Average useful heat gain and corresponding average instantaneous

collection efficiency also increases with increased plate spacing and larger mass of

storage medium (oil) although it has been observed that with increased plate spacing

there is further decrement in the temperature Tf1.

7. SCOPE FOR FUTURE WORK

The analysis shows that with the use of sensible heat storage medium, the

efficiency of the air heater increases. In fact, it will be more realistic to calculate the

overall efficiency for the period of operation in place of instantaneous collection

efficiency due to the presence of thermal energy storage element. The analysis can be

extended to optimize the size of the storage mass for maximum nocturnal heat gain.

NOMENCLATURE

c specific heat, J/kg-K

h heat transfer coefficient, W/m2-K;

j j-factor

m mass flow rate, kg/s

Nu Nusselt number

Pr Prandtl number

Ra Rayleigh number

Re Reynolds number

T temperature, K

U loss coefficient, W/m2-K

V velocity, m/s

Greek Letters:

β collector tilt angle (Slope), degrees

δ thickness, spacing, m

ε emissivity

η efficiency

ρ density, kg/m3

σ Stefan-Boltzmann const, W/m2-K

4

Subscripts:

a air

b back

c convective

f fluid (air) stream

i insulation

m mean fluid

g glass cover

i instantaneous, initial

l loss

p absorber plate, constant pressure

r radiative

s storage medium (oil)

∞ free stream

1 stream-1, surface-1

2 stream-2, surface-2

REFERENCES

[1] Enibe, S.O., 2003, Thermal analysis of a natural circulation solar air heater

with phase change material energy storage, Renewable Energy, Vol. 28, Issue

14, pp. 2269-2299.

[2] Hassan, Fath, E.S., 1995, Thermal performance of a simple design solar air

heater with built-in thermal energy storage system, Renewable Energy, Vol. 6,

Issue 8, pp. 1033-1039.

[3] Hassan, Fath, E.S., 1995, Transient analysis of thermosyphon solar air heater

with built-in latent heat thermal energy storage system, Renewable

Energy, Vol. 6, Issue 2, pp. 119-124.

[4] Soponronnarit, S., 1995, Solar Drying in Thailand, Energy for Sustainable

Development, Vol. 2, Issue 2, pp. 19-25.



[5] Sukhatme, S.P., 2005. Solar Energy Principles of Thermal Collection and

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[6] Guyer, Eric C., Hand Book of Applied Thermal Design, book published by

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[7] Mani, A., 1980. Hand Book of Solar Radiation Data for India, Allied

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[8] Thombre, S.B., 2000. A Data Book on Thermal Engineering, Green Brains

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[9] William, J., Palm III., 2008. Introduction to MATLAB ® 7.4, Tata McGraw-

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[10] Ozisik, Necati,M., 1988. Heat Transfer, Tata McGraw-Hill, International

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[11] Holman, J.P., 2008. Heat Transfer, Tata McGraw-Hill Publishing Company

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parametric study on the thermal performance of the solar air heater with energy storage

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