6340(Print), ISSN (Online) TECHNOLOGY Volume 3, Issue (IJMET) 2, May-August (2012), IAEME

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1 INTERNATIONAL International Journal of JOURNAL Mechanical OF Engineering MECHANICAL and Technology ENGINEERING (IJMET), ISSN 0976 AND 6340(Print), ISSN (Online) TECHNOLOGY Volume 3, Issue (IJMET) 2, May-August (2012), IAEME ISSN (Print) ISSN (Online) Volume 3, Issue 2, May-August (2012), pp IAEME: Journal Impact Factor (2012): (Calculated by GISI) IJMET I A E M E EXPERIMENTAL INVESTIGATION OF SOLAR AIR HEATER USING POROUS MEDIUM Ajay Kumar Kapardar *; Dr. R. P. Sharma ** * Dept. of Mechanical Engineering, Birla Institute of Technology, Mesra, India ajaykapardar@gmail.com **Dept. of Mechanical Engineering, Birla Institute of Technology, Mesra, India rpsbit123@gmail.com ABSTRACT The experimental study of single pass solar air heater with different porous media has been conducted. The experiments were conducted by changing the pareter that influences the thermal efficiency of the collector. The efficiency of solar air heater has been exined by changing the mass flow rate and using different types of porous media. Porous media provided large area for the heat transfer and therefore have high heat transfer co-efficient. Their use as absorbers enhances heat transfer from the absorber to the air stre. In the present study it has been found that efficiency of solar air heater using steel wool as a porous medium is higher than glass wool (porous media) as well as conventional air heater also. Key word: Porous medium, solar air heater, thermal efficiency, mass flow rate 1.0 INTRODUCTION Solar energy collector is a special type of heat exchanger that transforms solar radiation energy into internal energy of the transport medium. The simplest method to utilize solar energy for heating applications is to convert it into thermal energy by using solar collectors. Basically, there are two types of flat-plate solar heating collectors; water heating collectors and air heating collectors. The pace of development of air heating collector is slow compared to water heating collector mainly due to lower thermal efficiency. Solar air heaters are considered to be compact and less complicated as compared to solar water heaters. These are also free from corrosion and freezing problems. Solar air heater can be fabricated using cheaper as well as lesser ount of material and is simpler to use than solar water heater. Solar air collectors are widely used for low moderate temperature applications like space heating, crop drying, timber seasoning and other industrial applications. Conventional solar air collectors have poor thermal efficiency principally due to high heat loses and low convective heat transfer coefficient between the absorber plate and flowing air stre. Many researches have attempted to improve the 387

2 thermal performance of conventional solar air collectors by employing various design and flow arrangements. Wao etal.[1] used the sine wave absorber in place of plane absorber plate and numerically studied the natural convection inside the channel between cover and the sine wave absorber by single pass flat plate solar air collector. Garg etal.[2]used an absorber with fin attached in order to improve the thermal performance of single pass solar air collector. Yeh[3] studied the effect of parallel barriers on the collector efficiency of single pass flat plate solar collector. Sodha etal.[4] presented the performance of solar air heater with matrix. Ahmad etal.[5] investigate the effect of absorber matrix, mass flow rates and solar energy flux on a packed bed single pass solar air collector. Metwally etal.[6] experimentally investigate the thermal performance of corrugated duct solar air collector. Dhiman and Tiwari [7] studied performance of a two channel suspended flat-plate solar air heater. Sahu and Bhugoria [8] reported the augmentation of heat transfer coefficient by using transverse ribs on absorber plate of solar air collector. Karwa etal.[9] presented the performance of solar air heaters having integral chfered rib roughness on absorber plate. Mohad [10] analyzed the performance of counter flow solar air heater with a porous matrix. Naphon etal.[11] applied the mathematical models for predicting heat transfer characteristics and performance of various configuration of flat-plate solar air collector. The thermal performance of a packed bed double pass solar air heater was presented by Radan etal. [12]. Sopain.K. [13] carried out the simulation study and the thermal performance of double pass solar heater with and without porous media. The use of porous packing material for improving the performance of solar air collector has been proposed by many investigators including Kolb etal. [14]. Varshney and Saini [15] & Thakur etal. [16] investigated this kind of arrangement for single pass solar collectors. Paisarn Niphan [17] has presented the mathematical model for predicting the heat transfer characteristics, the performance, and heat generation of double-pass solar air heater with longitudinal fins. It was found that the thermal efficiency increases with increasing the height and number of fins. Rani etal. [18] has developed a mathematical model for predicting the thermal performance of a double pass solar air collector with porous media and reveals that thermal efficiency of double pass solar collector with porous media is 20-25% higher. 2.0 POROUS MEDIA A porous medium (or a porous material) is a material containing pores (voids). The skeletal portion of the material is often called the "matrix" or "fre". The pores are typically filled with a fluid (liquid or gas). The skeletal material is usually a solid, but structures like fos are often also usefully analyzed using concept of porous media. A porous medium is most often characterized by its porosity. Other properties of the medium (e.g., permeability, tensile strength, electrical conductivity) can sometimes be derived from the respective properties of its constituents (solid matrix and fluid) and the media porosity and pores structure, but such a derivation is usually complex. Even the concept of porosity is only straightforward for a poro-elastic medium. Often both the solid matrix and the pore network (also known as the pore space) are continuous, so as to form two interpenetrating continua such as in a sponge. However, there is also a concept of closed porosity and effective porosity, i.e., the pore space accessible to flow. The concept of porous media is used in many areas of applied science and engineering: filtration, mechanics (acoustics, geomechanics, soil mechanics, rock mechanics), engineering (petroleum engineering, bio-remediation, construction engineering), 388

3 geosciences (hydrogeology, petroleum geology, geophysics), biology and biophysics, material science, etc. Fluid flow through porous media is a subject of most common interest and has emerged a separate field of study. The study of more general behaviour of porous media involving deformation of the solid fre is called poromechanics. 3.0 EXPERIMENTAL SETUP The experimental setup is shown in figure.1 for the estimation of mass flow rate and efficiency of the flat plate air heater under varying conditions. Solar air heater Thermometer Manometer Figure1: Working model of the flat plate single pass solar air heater. The fre of the solar collector was cuboidal in shape and made of plywood of 10mm thickness. The internal dimension was 1m 0.5m 0.15m. The top surface of the collector was left open for glass cover. Aluminium channels were nailed onto the top of the fre to secure the glass cover. The inlet was a 10 cm hole drilled on the side surface near the bottom. For the outlet section 3 holes each of 1 inch dieter were drilled on the adjacent surface near top. Angle of the stand was set at angle of 28.A glazed glass sheet measuring 1.02m x 0.52m x 5mm was used as the single glass cover for the apparatus. The characteristics of the glass were: Reflectivity 0.1, Transmittivity A thermocol sheet measuring 0.9m x 0.5m x 2.5cm was secured to the bottom surface of the wooden fre by nails and glue. It was used to minimize heat loss from the absorber to the surroundings. Multiple aluminium sheets were used to cover the entire floor area of the collector. To increase absorptivity of heat the aluminium sheets were painted black.3 pipes of dieter 1 inch were attached to the outlet holes in the collector fre. The three parallel pipes fed a larger outlet pipe of 1.5 inch dieter through T joint couplers. The outlet section was insulated with fo wherever possible to minimize or eliminate heat exchange with the surroundings. To measure the different flow rates achieved while conducting the experiment, an orifice section from an existing pipe (from scrap) was sawed off and attached to the end of the outlet section via a 1.5 x 1 reducer. The orifice was of 8mm dieter and the pipe dieter was of 1. There were two holes on either side of the orifice which were attached to a U-tube manometer for flow measurement. The outlet from the orifice led directly to the DC fan creating an induced draught. Glass wool and steel wool were used as porous medium for experiment.

4 Glass Wool Glass wool (UK) or fiberglass insulation (US) is an insulating material made from fiberglass, arranged into a texture similar to wool. Glass wool is produced in rolls or in slabs, with different thermal and mechanical properties. Steel wool Steel wool, also known as wire wool, is a bundle of strands of very fine soft steel filents, used in finishing and repairing work to polish wood or metal objects, for cleaning household cookware and light painting. Steel wool is made from low-carbon steel (low enough to be close to plain iron). U-tube manometer was used to measure the differential pressure between two sections in terms of difference of height of liquid column in both limbs of the manometer. In this case the two different limbs were connected to either side of the orifice for flow measurement. The liquid used was coloured water (blue). The manometer also had a graduated scale (1mm Least Count) for measuring the difference in liquid levels. Q = CA c 2g(P P ) Where C= ( ) Where: Q = volumetric flow rate (at any cross section) C d = coefficient of discharge C = orifice flow coefficient d 1 = dieter of the pipe, m P 1 = fluid upstre pressure P 2 = fluid downstre pressure ρ = fluid density Following are the experimental graphs, which are plotted after calculation. 390

5 EXPERIMENTAL DATA FOR THE PERFORMANCE OF THE FLAT PLATE AIR HEATER UNDER VARYING CONDITIONS WITHOUT POROUS MEDIA (15 TH MARCH) Performance of Solar Collector without porous media on Thursday (15th Mar. 2012) Time (11:15 a.m. - 01:15 ) 12:06 12:14 12:36 12:48 01:00 01:05 01:15 Intensity of Solar Radiation I (W/sq. m) Ambient Temperature (deg. C) Outlet Temperature (deg C) T (Outlet - Ambient) (deg C) p (in mm of H2O) Density (kg/cu. Metre) Volume flow rate (litr./min.) Mass flow rate (in kg/hr.) Efficiency (percent) E f f i c e n c y Mass Flow Rate vs. Efficiency Mass Flow Rate vs. Efficiency Poly. (Mass Flow Rate vs. Efficiency) Mass Flow rate Temp. difference Fig. 2 Mass Flow Rate vs. Temp. difference Mass Flow Rate Mass Flow Rate vs. Temp. differe nce 391

6 WITH STEEL WOOL AS POROUS MEDIA (20 TH MARCH) PACKING DENSITY 400 GRAMS IN CUBIC METER OF COLLECTOR SPACE Performance of Solar Collector with steel as porous media (20 th Mar. 2012) Time (11:10 a.m. 01:40 ) 11:10 a.m. 11:30 a.m. 11:45 a.m. 12:05 12:20 12:40 01:00 Intensity of Solar Radiation I (W/sq. m) Ambient Temp. (deg. C) Outlet Temp. (deg C) T (Outlet Ambient) (deg C) p (in mm of H2O) Density (kg/cu. m)(average) Vol. flow rate (litr./min.) Mass flow rate (in kg/hr.) Efficiency (percent) E f f i c e n c y Mass Flow Rate vs. Efficiency Mass Flow rate Mass Flow Rate vs. Efficien cy Temp. difference Fig. 3 Mass Flow Rate vs. Temp. difference Mass Flow Rate Mass Flow Rate vs. Temp. difference Poly. (Mass Flow Rate vs. Temp. difference) 392

7 WITH GLASS WOOL AS POROUS MEDIA (26 MARCH) PACKING DENSITY 578 GRAMS IN CUBIC METER OF COLLECTOR SPACE Performance of Solar Collector with glass wool porous media (26 th Mar. 2012) Time (10.00 a.m ) Avg.Intensity of Solar Radiation I (W/sq. m) Ambient Temp. (deg. C) Outlet Temp. (deg C) T (Outlet Ambient) (deg C) p (in mm of H2O) Density (kg/cu. m)(average) Vol. flow rate (litr./min.) Mass flow rate (in kg/hr.) Efficiency (percent) pm Mass Flow Rate vs. Efficiency Mass Flow Rate vs. Temp. difference 50 E3.5 f 3 f2.5 i 2 c 1.5 e 1 n c0.5 y Mass Flow rate Mass Flow Rate vs. Efficiency Temp. difference Mass Flow Rate Mass Flow Rate vs. Temp. differenc e Fig.4 393

8 RESULT AND DISCUSSION The useful gain by the solar collector to solar radiation with measured values of fluid inlet (T i ) and outlet temperature (T o ) and the fluid mass flow rate (m) is given as follows Q u = mc P (T o - T i ) Where C P is the specific heat of the fluid. The density of the outlet air which was varying with the outlet temperature and ranged from 1-1.1kg/m 3 was assumed constant at 1.05 kg/m 3. The efficiency of the collector is given by η = where A c is the area of the collector, S is the solar radiation incident on the collector. The air was heated up relatively quickly with time. However the maximum temperature achieved was o C. The heat retentiveness of the collector was relatively poor as compared to the collector having porous media. Inconsistencies and undulations in the curves of mass flow rate vs temperature differences occurred due to the low mass flow rate obtained. However it was generally observed that the efficiency increased with the increasing mass flow of air and the temperature difference between inlet and outlet was inversely proportional to the mass flow rate although the nature of the curve was not determined over the small range of mass flow. The methods of conducting the experiment for both porous and non-porous media were almost se. The outlet temperatures obtained for steel wool as heat transfer media were higher (max 69 0 C) than those for glass wool (max 62 0 C). The efficiency curves were also of a similar nature. However, the heat retentivity of the collector with glass wool was observed to be better than the corresponding characteristic for steel wool although comparative tables for similar times of exposure without sunlight are not tabulated. It means that when the collector is used at night, utilizing the stored solar energy, the collector having glass wool would supply warmer air for longer time than steel wool or without porous media. The comparative graphs for the performances of the 3 designs are as shown: From fig.5 efficiency of the collector is strongly dependent on mass flow rate. The collector efficiencies increase with flow rate. Efficiency increase is about 39.29% at mass flow rate of ( ) kg/hr when steel wool was used as porous media where efficiency increased by at se conditions when glass wool was used with respect to the conventional solar air collector. 394

9 From fig.6 by using porous media that is steel wool highest temperature was noted i.e 69 0 C when steel wool was used as porous media as compared to 62 0 C when glass wool was used. Fig. 5 CONCLUSION Fig. 6 Performances curves of single pass solar air collector with steel wool and glass wool was used as porous media and conventional solar air collector have been obtained. These include the effects of mass flow rate and solar radiation on efficiency of solar collector. The efficiency of the collector is strongly dependent on the flow rate. It increases with flow rate. The solar collector with porous media as steel wool has efficiency about 26% more than the conventional one whereas it is about 10% more when glass wool is used. REFERENCES 1. Wao, Wenfeng et. al., (2000), Numerical study on natural convection inside the channel between the flat-plate cover and sine-wave absorber of a cross-corrugated solar air heater. Energy Conversion and Management 41, pp Garg, H.P. et al., (1989), Performance studies on finned air heater. Energy14 (2), pp

10 3. Yeh, H.M., (2000), Effect of collector aspect ratio on the collector efficiency of upward type baffled solar air heater. Energy Conversion and Management 41, pp Sodha, M.S., Bansal, N.K., Singh, D., (1982). Analysis of a non porous doubleflow solar air heater. Applied Energy 12 (4), pp Ahmad A., Saini J.S., Solanki S.C., (1995). Studies on the performance of packed bed solar air heaters. Ph. D. Thesis, department of Mechanical & Industrial Engineering, IIT Roorkee. 6. Metwally et al., (1997). Performance of advanced corrugated-duct solar air collector compared with five conventional designs. Renewable Energy 10 (4), pp Dhiman, N.K., Tiwari, G.N., (1984). Performance of a two channel suspended flat plate solar air heater. Energy Conversion and Management 24 (4), pp Sahu, M.M., Bhagoria, J.L., (2005). Augmentation of heat transfer coefficient by 90 broken transverse ribs on absorber plate of solar air collector. Renewable Energy 30, pp Karwa et al., (2001). Thermo-hydraulic performance of solar air heaters having integral chfered ribs roughness on absorber plate. Energy 26, pp Mohad, A.A., (1997). High efficiency solar air heater. Solar Energy 60 (2), pp Naphon, P.etal, (2003). Theoretical study on heat transfer characteristics and performance of the flat-plate solar air heaters. International Communications in Heat and Mass Transfer 30 (8), pp Radan, M.R.I. et al., (2007). Thermal performance of a packed bed double pass solar air heater. Energy 32, pp Sopain, K., (2000). Performance of a double pass photovoltaic thermal solar collector suitable for solar drying systems. Energy Conversion and Management 41, pp Kolb, A. et al, (1999). Experimental studies on solar air collector with metal matrix absorber. Solar Energy 65 (2), pp Varshney, L. et al, (1998). Heat transfer and friction factor correlations for rectangular solar air heater duct packed with wire mesh screen matrices. Solar Energy 62 (4), pp Thakur, N.S. et al., (2003). Heat transfer and friction factor correlations for packed bed solar air heater for a low porosity system. Solar Energy 74, pp Naphon.P., (2004). On the performance and entropy generation of the double-pass solar air heater with longitudinal fins. Renewable Energy 30 (2005) pp Rani.B.M. et al., (2010). Performance of a double pass solar air collector. Solar Energy 84 (2010) pp