Performance Evaluation Of A Solar Air Heater With Double Glazing And Dual Airflow Channels

Performance Evaluation Of A Solar Air Heater With Double Glazing And Dual Airflow Channels

Som Nath Saha

Department of Mechanical Engineering, B.I.T. Sindri Dhanbad, India

Abstract – The double airflow solar air heater, in which air simultaneously passes above and below the absorber plate, demonstrates superior performance compared to conventional single-flow designs where air flows only on one side of the plate. This improvement is primarily attributed to the increased heat- transfer area available in the double-flow configuration.

In this study, an analytical model has been developed to characterize the thermal behavior of a double airflow channels solar air heater. The model is formulated using energy balance equations to predict system performance. Furthermore, the influence of mass flow rate distribution between the upper and lower air channels on the collector efficiency has also been examined.

Keywordsdouble glazing; double airflow; solar air heater

1. INTRODUCTION

   Solar energy has great potential for low temperature applications, particularly for drying of agricultural products such as crop, grain, seeds, fruits, and vegetables. Also, solar air heaters utilized as preheaters in industries and space heating in building to save energy. There are many different parameters affecting the solar air heater efficiency, e.g. collector length, collector depth, type of absorber plate, number of glass cover, mass flow rate, etc. Due to the poor thermal conductivity and low heat capacity of air, the convective heat transfer rate inside the air flow channel where the air is heated is low; many of studies have been made to increase this rate. One of the effective way to increase the convective heat transfer rate is to increase the heat transfer area or to increase turbulence inside the channel by using fins or corrugated surface [1-5].

   Several configurations of solar air heaters have been developed in literature. Various designs, with different shapes and dimensions of the air flow passage in plate type solar air collectors were tested [6-11]. The double-flow type solar air heaters have been introduced for increasing the heat-transfer area, leading to improved thermal performance [2,9]. Garg et al. [12] developed the theory of multiple pass solar air heaters. It is well known that the collector configuration will influence the fluid velocity as well as the strength of forced convection. A simple procedure for changing the fluid velocity as well as the strength of forced convection involved adjusting the aspect ratio of a rectangular flat-plate collector with constant flow rate [13]. A simple but practical procedure for increasing the heat- transfer area is the use of cooling fins. Improvement in the performance of solar air heaters could be obtained when the

   fins in the collector were provided with attached baffles to create air turbulence and an extended heat transfer area [14]. A similar study [15] was carried out in obtaining different designs with optimum flow channel depth and mass flow rate of solar air heaters. The design variations considered are flat absorber type with and without cover glazing; single, double and triple pass etc. It is found that a single glazing solar air heater operating under double flow configuration gives the best performance.

2. THEORRETICAL ANALYSIS

   A. Selecting a Template (Heading 2)

   The model of a double-flow type solar air heater may be illustrated by the schematic diagram of Fig. 1. As seen in Fig. 1, two air streams (fluid 1 and fluid 2) of different flow rates but with total flow rate fixed, are flowing steadily and simultaneously through two separate channels (above and under the absorbing plate) for heating.

   The energy balances will be taken under the following assumptions: Air flow is in steady state, one dimensional heat flow through back insulation, negligible temperature drop through the glass cover, absorber plate and bottom plate, the temperature s of the absorbing plate, bottom plate and fluids are functions of the flow direction.

   Fig. 1. Diagram of double glass double flow type solar air heater

   Energy balance equations

   The solar radiation absorbed by the absorbing plate per unit area, (W/m2),

   Sky temperature is estimated by the formulation given by [16],

   Energy balance for glass cover 2, Energy balance for glass cover 1,

   Energy balance for fluid 1, Where

   Energy balance for absorber plate,

   The radiation heat-transfer coefficient from the glass cover to sky the referred to the ambient air temperature Ta may be obtained as follows [18]

   The radiation heat-transfer coefficients between the glass cover 1 and glass cover 2 and between the absorbing plate and glass cover 1 between the absorbing plate and the bottom plate are predicted, respectively, by

   Energy balance for fluid 2, Where

   Energy balance for bottom plate,

   With the assumption of

   The conduction heat-transfer coefficient across the insulation is estimated by

   The convection heat-transfer coefficient between the glass cover 1 and the glass cover 2 is calculated by

   From Eq. (3)

   can be estimated by the following correlation [19]

   The efficiency of solar heat gain of the heater is

   where the + symbol in the superscript means that only positive values of the terms in the square brackets are to be used (i.e., use zero if the term is negative). This correlation is valid for .

   The convection heat-transfer coefficients for the fluid moving in between the absorbing plate and glass cover 1 (flow 1), and for the fluid moving in between absorbing plate and bottom plate (flow 2), are calculated by

   For turbulent flow following correlation may be derived from

   Kays [20] data with the modification of McAdams [17]

   Where

   For laminar flow, the equation presented by [21]

   Heat-transfer coefficient

   The convection heat-transfer coefficient from the glass cover due to the wind is recommended by [17] as

   Equivalent diameter of the duct

   Reynolds numbers for rectangular ducts

   Calculation method for temperatures and efficiency

   m = 0.01 kg/s m = 0.02 kg/s m = 0.03 kg/s m = 0.04 kg/s m = 0.05 kg/s

   The procedure for calculation of is first using guessed temperature to calculate the heat transfer coefficients, from Eq. (22) (25) then used to estimate new temperatures by using Eq. (13) (18). If the calculated values of temperatures are different from the assumed values continued calculation by iteration method. These new temperatures will be used as the guessed temperatures for next iteration and the process will be repeated until all the newest temperatures obtained are their respective previous values.

   Collector length (m)

3. RESULTS AND DISCUSSIONS

   The following values are used for the parameters under the typical configuration and operating conditions: I = 1000 w/m2, = 0, L = 1.25 m, W = 0.8 m, H1 = 0.02 m H2 = H3 = 0.02

   m, c = 0.875, ap = 0.96, c = 0.94, ap = 0.80, bp = 0.94, Ta = 303 K, Tin = 303 K, V = 1 m/s, = 0.01 0.05 kg/m2s, r =

   Fig. 3. Effect of L on efficiency of solar air heater with different mass flow rate.

   370

   m = 0.01 kg/s m = 0.02 kg/s

   0.2, 0.4, 0.5, 0.6 and 0.8.

   The results for the configuration and operating conditions are shown in graph (Fig. 2). It is found that all most maximum efficiency at r = 0.5 for all mass flow rate.

   Outlet Temperature (K)

   m = 0.03 kg/s m = 0.04 kg/s m = 0.05 kg/s

   m = 0.01 kg/s m = 0.02 kg/s m = 0.03 kg/s m = 0.04 kg/s m = 0.05 kg/s

   Collector length (m)

   Efficiency ()

   Fig. 4. Effect of L on outlet temperature of solar air heater with different mass flow rate.

   Mass flow rate fraction (r)

   The effect of width of range 1 m to 5 m at r = 0.5 on the efficiency, as collector width increases efficiency decreases for all mass flow rate (Fig. 5). At higher mass flow rate effect of width on outlet temperature is negligible but at lower mass flow rate outlet temperature decreases as width increases (Fig. 6).

   Fig. 2. Effect of the fraction of mass-flow rate on collector efficiency in a double flow type solar air heater.

   Effect of L, W and H

   The results showing the effect of collector length L, width W and flow channel height H on the thermal performance of double flow collector. The L changes in the range of 1 m to 5 m in configurations and operating conditions as presented above are used for the other parameters at r = 0.5.

   From the results, it is found, that the higher mass flow rate has higher efficiency and efficiency become almost constant as length increases for all mass flow rate (Fig. 3). At lower mass flow rate outlet air temperature is higher than higher mass flow rate. There is negligible effect on outlet temperature as increase in length of collector (Fig. 4).

   Efficiency ()

   m = 0.01 kg/s m = 0.02 kg/s m = 0.03 kg/s m = 0.04 kg/s m = 0.05 kg/s

   0 1 2 3 4 5 6

   Collector width (m)

   Fig. 5. Effect of W on efficiency of solar air heater with different mass flow rate.

   m = 0.01 kg/s m = 0.02 kg/s

   Outlet Temperature (K)

   m = 0.01 kg/s m = 0.02 kg/s m = 0.03 kg/s m = 0.04 kg/s m = 0.05 kg/s

   Outlet Temperature (K)

   m = 0.03 kg/s m = 0.04 kg/s m = 0.05 kg/s

   Collector width (m)

   Channel depth (m)

   Fig. 8. Effect of H on outlet temperature of solar air heater with different mass flow rate.

   Fig. 6. Effect of W on outlet temperature of solar air heater with different mass flow rate.

   The air flow channel depth for upper and lower flow is same. The range of channel depth 0.01 m to 0.05 m. From the results for r = 0.5, it is found that as air flow channel depth increases efficiency decreases for all mass flow rate (Fig. 7). At higher mass flow rate effect of air flow channel depth is negligible but at lower mass flow rate outlet temperature decreases as height increases (Fig. 8).

   0.7

   m = 0.01 kg/s m = 0.02 kg/s m = 0.03 kg/s

4. CONCLUSIONS

A parametric study has been carried out on the performance of double glazing double airflow channels solar air heater under various configurations. The double airflow type solar air heater increases the heat transfer area and double glazing reduces the top loss coefficient, leading to improve thermal performance. The effect of the fraction of mass flow rate in upper and lower airflow channel on the collector efficiency of double flow type solar air heater has been investigated. The results shows that the optimal fraction of mass flow rate is 0.5 for all mass flow rate. This means that in order to achieve the best thermal performance in a double airflow channels solar air heater, in which cross-section areas of upper and lower flow channels are same, the mass flow rates in both flow

Efficiency ()

m = 0.04 kg/s m = 0.05 kg/s

channels must be same. The thermal performance decreases where r, as well as (1-r), goes away from 0.5. The results can be summarized as follows:

The results shows that as collector system parameters changes efficiency and outlet temperature also changes. The investigated range of collector length 1 m to 5 m, collector width 1 m to 5 m and flow channel depth 0.01 m to 0.05 m. Upper and lower flow channel have same cross-section area.

0.00 0.01 0.02 0.03 0.04 0.05 0.06

Channel depth (m)

Fig. 7. Effect of H on efficiency of solar air heater with different mass flow rate.

As increases the collector length, width and airflow channel depth efficiency decreases. There is negligible effect on outlet temperature as increase collector length. There is less effect of width and flow channel depth for high mass flow rate but for low mass flow rate as width and flow channel depth increases outlet temperature decreases.

Nomenclature

L Length of collector (m).

W Width of collector (m).

H Flow channel depth of collector (m). Hc Distance between two cover (m).

I Incoming solar radiation (W/m2).

S Radiation absorb by absorber (W/m2).

Absorptivity.

Emissivity.

Transmissivity.

h Heat transfer coefficient (W/m2K). T Temperature (K).

m Mass flow rate (kg/s).

r Fraction of mass flow rate.

Cp Specific heat (J/kgK).

Dh Hydraulic diameter of duct.

k Thermal conductivity (W/mK). Re Reynolds number.

Ra Raylight number.

Air density (kg/m3).

g Acceleration due to gravity (m/s2).

Air viscosity (kg/mh).

Stefan Boltzmann constant (W/m2K4). Nu Nusselt number.

q Heat gain.

Efficiency.

Subscripts

ap Absorber plate.

bp Bottom plate.

s Sky.

a Air.

C1, C2 Cover1, Cover 2 c Convective

r Radiative

f1, f2 Fluid 1, Fluid 2

REFERENCES

1. Paisarn Naphon, On the performance and entropy generation of the double- pass solar air heater with longitudinal fins, Renewable Energy, vol. 30, pp. 1345-57, 2006.
2. H.M. Yeh, C.D. Ho, and J.Z. Hou, Collector efficiency of double-flow solar air heaters with fins attached, Energy, vol. 27, pp. 715-27, 2002.
3. H.M. Yeh, and C.D. Ho, Effect of external recycle on the performances of flat-plate solar air heaters with internal fins attached, Renewable Energy, vol. 34, pp. 1340-47, 2009.
4. S. Youcif-Ali, and J.Y. Desmons, Numerical and experimental study of a solar equipped with offset rectangular plate fin absorber plate, Renewable Energy, vol. 31, pp. 2063-2075, 2006.
5. L. Wenxian, G. Wenfeng, and L. Tao, A parametric study on the thermal performance of cross-corrugated solar air collectors, Applied Thermal Engineering, vol. 26, pp. 1043-1053, 2006.
6. K.G.T. Hollands, and E.C. Shewan, Optimization of flow passage geometry for airheating, plate-type solar collectors, Trans ASME J Solar Energy Eng, vol. 30, pp. 103-323, 1981.
7. C. Choudhury, and H.P. Garg, Design analysis of corrugated and flat plate solar air heaters, Renew Energy, vol. 1, pp. 595607, 1991.
8. A. Hachemi, Thermal performance enhancement of solar air heaters, by fan– blown absorber plate with rectangular fins, Int J Energy Res, vol. 19, pp. 567578, 1995.
9. H.M. Yeh, C.D. Ho, and J.Z. Hou, The improvement of collector efficiency in solar air heaters by simultaneously air flow over and under the absorbing plate, Energy, vol. 24, pp. 857-871, 1999.
10. A.A. Hegazy, Performance of flat plate solar air heaters with optimum channel geometry for constant/variable flow operation, Energy Convers Manag, vol. 41, pp. 401-417, 2000.
11. H.M. Yeh, C.D. Ho, and C.Y. Lin, Effect of collector aspect ratio on the collector efficiency of upward type baffled solar air heaters, Energy Convers Manag, vol. 9, pp. 971-981, 2000.
12. H.P. Garg, V.K. Sharma, and A.K. Bhargava, Theory of multiple-pass solar air heaters, Energy, vol. 10, pp. 589599, 1985.
13. H.M. Yeh, and T.T. Lin, The effect of collector aspect ratio on the collector efficiency of flat-plate solar air heaters, Energy, vol. 20, pp. 10411047, 1995.
14. F. Kreith, and J.F. Kreider, Principles of solar engineering New York:

    McGraw-Hill, 1978.

15. R. Verma, R. Chandra, and H.P. Garg, Optimization of solar air heaters of different designs, Renewable Energy, vol. 2, pp. 521531, 1992.
16. W.C. Swinbank, Long-wave radiation from clear skies, Quart J Royal

    Meteorol Soc, vol. 89, pp. 339-348, 1963.

17. W.H. McAdams, Heat transmission,. 3rd ed. New York: McGraw-Hill 1954.
18. X.Q. Zhai, Y.J. Dai, and R.Z. Wang, Comparison of heating and natural ventilation in a solar house induced by two roof solar collectors, App Therm Eng, vol. 25, pp. 741-757, 2005.
19. R.C. Weast, editor Handbook of tables for applied engineering science, Boca Raton: CRC Press, 1970.
20. W.M. Kays, Convective heat and mass transfer, New York: McGraw- Hill, 1980.
21. H.S. Heaton, W.C. Reynolds, and W.M. Kays, Heat transfer in annular passages. Simultaneous development of velocity and temperature fields in laminar flow, Int J Heat Mass Transfer, vol. 7, pp.763-781, 1964.