INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET)

INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 ISSN 0976 6340 (Print) ISSN 0976 6359 (Online) Volume 3, Issue 1, January- April (2012), pp. 90-99 IAEME: www.iaeme.com/ijmet.html Journal Impact Factor (2011) - 1.2083 (Calculated by GISI) www.jifactor.com IJMET I A E M E 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: yogidhote@gmail.com 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 (21 o 06 N, 79 o 03 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, 90

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 3. COMPUTATIONAL MODEL 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 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. 91

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 (R e ) 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 Ra δ cosβ < 1708... (ii) = 1 + 1.446{1 1708/(Ra δ cosβ)}, for 1708 < Ra δ cosβ <5900... (iii) = 0.229 x (Ra δ cosβ) 0.252, for 5900 < Ra δ cosβ < 9.23 x 10 4... (iv) = 0.157 x (Ra δ cosβ) 0.285 for 9.23 x 10 4 < Ra δ cosβ < 10 6... (v) c) Convective heat transfer from the glass cover surface to the ambient air (Sparrow): -2/3... h c = j.ρ.c p.v.p r (vi) d) Radiative heat transfer from one surface to another surface (Duffie and Beckman): h r = σε eff (T 2 1 + T 2... 2 ) (T 1 + T 2 ) (vii) e) Bottom Loss Coefficient: U b = k i /δ bi 5. RESULTS AND DISCUSSION... (viii) 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 ( T f1 ) = 25.50 o C at δ pc = δ sb = 0.01 m and m oil = 88.25 kg, while maximum rise in temperature of the inlet ambient air is found to be 12.4209 o C, again at δ pc = δ sb = 0.01 m and m oil = 88.25 kg. b) Maximum temperature difference throughout the day for Stream-2 ( T f2 ) = 14.17 o C 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 o C at δ pc = δ sb = 0.01 m and m oil = 88.25 kg, c) Maximum rise in storage oil temperature ( Ts) = 14.17 o C 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 δ pc = δ sb = 0.05 m and storage oil mass of 88.24 kg. It has been noticed that maximum values of (q u ) av and (η i ) 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 (T f1 ). 92

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 (T f1 ) there is negligible or no increment in outlet temperature of Stream-2 (T f2 ) takes place during day time, hence it is concluded that there is no need of Air-Stream-2. 93

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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 δ pc = δ sb (m) Average Useful Heat gain / m oil (kg) Average Collection Efficiency 22.06 44.12 88.25 0.01 q u(av) (W) 578.45 610.33 633.26 η i(av) (%) 49.42 52.10 54.05 0.02 0.05 q u(av) (W) 685.07 716.48 740.32 η i(av) (%) 58.57 61.30 63.34 q u(av) (W) 732.30 763.83 787.76 η i(av) (%) 62.77 65.52 67.58 Table 3 Maximum temperatures obtained Plate Oil mass m oil (kg) Temperatures spacing δ pc = δ sb (m) 22.06 44.12 88.24 0.01 317.60 318.20 318.61 T f1 (K) 0.02 313.20 313.50 313.70 0.05 309.90 310.00 310.10 0.01 307.60 307.60 307.60 T f2 (K) 0.02 307.63 307.63 307.63 0.05 307.70 307.70 307.70 0.01 307.50 307.50 307.50 T oil (K) 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 (T f2 ) 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 (T f1 ) of air passing through Stream-1 97

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 T f1. 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/m 2 -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/m 2 -K V velocity, m/s Greek Letters: β collector tilt angle (Slope), degrees δ thickness, spacing, m ε emissivity η efficiency ρ density, kg/m 3 σ Stefan-Boltzmann const, W/m 2 -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. 98

[5] Sukhatme, S.P., 2005. Solar Energy Principles of Thermal Collection and Storage, book published by Tata McGraw-Hill Publishing Company Limited, New Delhi, pp. 266-268. [6] Guyer, Eric C., Hand Book of Applied Thermal Design, book published by Tata McGraw-Hill Publishing Company Limited, New York, pp.1.31-1.47. [7] Mani, A., 1980. Hand Book of Solar Radiation Data for India, Allied Publishers Private Limited, New Delhi, pp. 1-88. [8] Thombre, S.B., 2000. A Data Book on Thermal Engineering, Green Brains Publications, Nagpur, pp. 5-93. [9] William, J., Palm III., 2008. Introduction to MATLAB 7.4, Tata McGraw- Hill Publishing Company Limited, New Delhi, pp. 1-135. [10] Ozisik, Necati,M., 1988. Heat Transfer, Tata McGraw-Hill, International Edition, pp. 403-407. [11] Holman, J.P., 2008. Heat Transfer, Tata McGraw-Hill Publishing Company Limited, New Delhi, pp. 340-344. [12] Mills, A.F., 1992. Heat Transfer, Irwin Homewood Boston, pp. 301-308. 99