EFFECTIVE UTILIZATION OF SOLAR ENERGY IN AIR DRYER

Transcription

1 International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN Vol. 3, Issue 1, Mar 2013, TJPRC Pvt Ltd. EFFECTIVE UTILIZATION OF SOLAR ENERGY IN AIR DRYER M. JOSEPH STALIN & P. BARATH Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai, India ABSTRACT Energy is a part and parcel of our life and energy crisis is the utmost problem in the future. Energy cannot be created but energy can be saved by utilizing it efficaciously by means of thermal storage. There are many researchers claiming that there are attempts to minimize the heat loss by storing it in different ways. Solar air dryer is a very common one in which lot of energy gets wasted as heat because of its less utilization and absence of thermal storage. Therefore Phase change materials are installed in solar air dryer in an eminent way to increase its efficiency which is far better than existing designs. This paper mainly focuses on the arrangement of Phase change materials in solar air dryer and its back up time for dryer without radiation, illustrating the increased efficiency of the dryer due to its storage capacity. Phase change material is installed in the path of the air in solar dryer. The design of Phase change material is that the spherical balls are shaped with cylindrical holes in the centre. Large numbers of balls are placed in the path of the air in which the air passes through them. During solar radiation, air gets heated and the remaining heat is stored by PCM which also gives heat back to air, thereby increasing the efficiency of the dryer and operating without radiation. Factors like mass flow rate of air, moisture content of product, solar irradiation and collector area are pondered and theoretically calculated. This is a very efficient method since the solar radiation is utilized in a effective way. KEYWORDS: Solar Air Dryer, Phase Change Material, Spherical Balls, Cylindrical Holes, Moisture Content, and Sun s Radiation INTRODUCTION Energy saving and heat storage is a challenging one among hefty of researchers. Nowadays majority of scientists are doing research to improve the efficiency in the field of thermal storage and recently Phase change materials are invented which is the most efficient form of thermal storage. Hence, Phase change materials are inserted inside the solar air dryer. Many researches are publishing research papers by improving efficiency and this paper is confined to design, integration and investigation of Phase change materials which increases the efficiency of the solar air dryer. By mixing aluminium powders with Phase change materials, its efficiency is further increased. Recovery of waste heat and storing it in a storage medium would satisfy the future needs. In equatorial regions, there is a lavish amount of solar radiation which could be used efficiently and effectively for profitable and beneficial work. The temperature which is obtained in the solar simulator plays a hefty role in culling of heat and for storage. Air is poor conductor on comparing with water and so lot of heat gets wasted and thermal storage is the expedient solution. The solar collector area is theoretically calculated according to the capacity of drying container. In Madurai (India) the average solar irradiation is 5.03 kw / m 2 k. The new design consists of a capacity of 50 kg of product for drying and back up time for the same capacity. Similar research is done by Alkilani. They used the PCM in the form of cylinder which is efficient than existing system. But sphere is the shape which has larger surface area than cylinder for the same volume and hence spherical balls of PCM are preferred. This paper focuses on the storage of heat which is repudiated and dumped into the environment from solar air dryer. The system is designed in such a way that the Phase change materials are in the shape of spherical balls with cylindrical hole in the centre. They have more contact area with air which would be more efficient than the existing

2 134 M. Joseph Stalin & P. Barath system. Factors like mass flow rate of air, volume of PCM, collector area is pondered and calculated. In the past years Morison and Abdel-Khalik (1978) developed a theoretical model for studying the transient behaviour of phase-change energy storage (PCES) unit and studied the performance of solar heating systems using both air and liquid as working fluid. Enaib (2003) studied the transient thermal analysis of the previous design. The heated air and glazing surface were predicted to be within 10 C. The maximum predicted airflow rate was 0.01 kg/s, corresponding to a maximum inlet velocity of 0.33 m/s. Hed and Bellander (2006), used a PCM as an air heat exchanger in a building. Eman-Bellah (2006), investigated a method of enhancing the thermal conductivity of paraffin wax by embedding aluminium powder in paraffin wax in a water base collector. In our system, the PCM are placed in shape of spherical balls with cylindrical hole in the cylinder. Sphere is the shape which has largest surface area with small volume compared to other shapes. Heat transfer is directly proportional to surface area. So by integrating this kind of designed PCM in solar air heater, a greater efficiency is achieved. DESCRIPTION The utilization of energy efficiently is always a secret of engineers. Heat is the low grade energy which is always lost during any process. The efficient usage of that heat energy is the hardest job among hefty of engineers. The easiest way is storing of that heat energy using latent storage. Therefore, Phase change materials are used for our research since it is good latent heat storage medium. A system is designed for efficient utilization of waste heat in solar air heater using phase change material. The system consists of drying chamber or incubator, air channels for air flow filled with Phase change material along with solar simulator. The drying chamber has a capacity of 25 kg of products. Phase change materials are selected according to the outside temperature and temperature required for drying. The capacity of the air compartment is 3m * 1.28m * 0.15m. The drying time for 25 kg of products and corresponding mass flow rate and other parameter is theoretically calculated. The Phase change material is designed for the backup of 3 hours of drying time. By considering the above data, the mass of PCM is calculated. The design of PCM is that a spherical ball of diameter 10cm with cylindrical hole of 3cm diameter in the centre. 144 balls of PCM are required for our setup. It is arranged in the compartment as 18 rows and 8 columns. The entire compartment is manufactured without leakage of air. The compartment is attached with drying chamber and it is assumed that the initial moisture content of the product is 50%. The mass flow rate of air is calculated for the estimation of drying time. By installing PCM in the solar air heater, the efficiency is improved. The solar simulator area is designed by pondering the solar irradiation and the amount of heat required, for the backup time of 3 hours. So by installing this system, heat is utilized very efficiently by decreasing a part of global warming. WORKING The system deals with the effective utilization of heat using Phase change materials along with the solar air heater. Solar air heater plays a significant role in the field of drying but its efficiency is always a question mark compared to other conventional dryer using other sources. By introducing PCM, we can enhance the efficiency of solar air heater to a

3 Effective Utilization of Solar Energy in Air Dryer 135 considerable amount. The selection of PCM is the major criteria and it depends on the operating temperature. The melting point of the selected PCM is 50 degree Celsius and latent heat is 173 kj / kg. The ultimate aim and principle behind this process is heating of air to certain temperature. Theoretically, the mass of air is calculated which passes through the phase change material. Fresh air which passes through the spherical balls and cylindrical hole comes into contact with large surface area of heated PCM due to solar simulator. The temperature received in the solar simulator depends on the solar irradiation and simulator area. The capacity of drying chamber is 25 kg of products which has the moisture content of 50 %. The needed final moisture content is 30 %. For the calculated mass flow rate, fresh air is allowed and it gets heated and it dries the product. The solar air heater is used even there is no solar radiation for the given backup time. If it is erected commercially there is a huge improvement in efficiency and people get benefitted. The outlet air goes into the environment from the drying chamber. This process continues and so on. This is the efficient method compared to all other ordinary solar heater. MATHEMATICAL CALCULATION OF SOLAR AIR DRYER Mass of water to be evaporated to dry products is given by: M w = M *{(M i M f ) / (100 M f )} M w = Mass of water to be evaporated to dry products (kg). M = Mass of drying products (kg). M i = Initial moisture content of products (%). M f = Final moisture content of products (%). Mass of PCM is installed to store heat from the sun during sunshine hours and to use in the night and is given by: M pcm = Q / L pcm M pcm = Mass of PCM (kg). Q = Total heat required to dry products of 10 kg (KJ). L pcm = Latent heat of PCM (KJ/kg). Volume of air required to dry M mass of products is given by: V = (M a * R * T) / P V = Volume of air needed to dry M mass of products (m 3 ). M a = Mass of dry air required to dry M mass of products (kg). R = Gas constant (0.291kpa m 3 / kg K). T = Dry bulb temperature ( C). P = Atmospheric pressure (k pa).

4 136 M. Joseph Stalin & P. Barath Volume flow rate is very important factor in dryer because it decides the time of drying products and is given by: V = V / t V = Volume flow rate (m 3 /s). t = drying time(s). Mass flow rate is calculated from volume flow rate and is given by: M = V * ρ M = Mass flow rate of air (kg/s). V = Volume flow rate of air (m 3 /s). ESTIMATION OF CONVECTIVE HEAT TRANSFER COEFFICIENT Reynolds number is used to find out whether the flow is laminar or turbulent and is given by: Re = (V * D h ) / γ Re = Reynolds number. V = Velocity of air (m/s). D h = Hydraulic diameter (m). γ= kinematic viscosity of air (m 2 /s). Hydraulic diameter for rectangular section is given by: D h = 4A / P A = Area of the rectangular section (m 2 ). P = Perimeter of the rectangular section (m). Nusselt number is used to estimate the convective heat transfer coefficient and is given by: Nu = Nusselt number. Pr = Prandl number. D = Diameter of the duct (m). Nu D Re Pr L D Re Pr / L

5 Effective Utilization of Solar Energy in Air Dryer 137 L = Length of the duct (m). Convective heat transfer coefficient is calculated by, h = (Nu * k) / L h = Convective heat transfer coefficient (W/m 2 K). k = Thermal conductivity of air (W /m K). ESTIMATION OF CHARGING TIME OF PCM Heat transferred to the PCM is calculated by: Q = h * A * dt Q = Heat transferred to the PCM (W). A = surface area of contact (m 2 ). Charging time of PCM is calculated by, T = { (M pcm * L pcm ) / Q} T = Charging time of PCM (s). RESULTS AND DISCUSSIONS By using the above calculation we are designed the various parameters of solar air dryer integrated with PCM and is given below: S.NO PARAMETERS VALUES 1. Initial moisture content of products 50% 2. Final moisture content of products 30% 3. Mass of products 25kg 4. Mass of water to be evaporated to dry products 7.14kg 5. Amount of heat required to dry products 2.5MJ 6. Latent heat of PCM 173KJ/kg 7. Mass of PCM ~100kg 8. Mass of air to dry products 246.2kg 9. Gas constant 0.291kpam 3 /kg K 10. Dry bulb temperature of air 45 C 11. Atmospheric pressure kpa 12. Volume of air needed to dry products m Drying time 3 hours 14. Volume flow rate of air m 3 /s 15. Mass flow rate of air kg/s 16. Velocity of air 0.108m/s 17. Hydraulic diameter of air 0.267m 18. Kinematic viscosity of air 19.92* Reynolds number Nusselt Number Convective heat transfer coefficient 15W/m 2 K 22. Charging time of PCM ~100 minutes

6 138 M. Joseph Stalin & P. Barath The system has been designed in such a way that the products are of initial moisture content of 50% to final moisture content of 30% and for the mass of 25kg backup without sunshine. Likewise using above mathematical calculations, design for different moisture content, for different backup products can be made. The following graph shows the temperature variation of first 10 days of November 2012 and the temperature variation at different times of the day. These are the factors which influence the efficiency of the solar collector. The following graph shows the variation of relative humidity at different times of the day. It also plays a major role in solar dryer efficiency. The difference in relative humidity in inlet and outlet condition takes the moisture away from the dryer. The following graph shows that the variation of solar irradiation with respect to the months of the year It is the input source to the solar air dryer. It is the main part for determining the overall dryer efficiency.

7 Effective Utilization of Solar Energy in Air Dryer 139 The following graph explains the variation of moisture content at respective drying time for various mass flow rates. Mass flow rate is directly proportional to drying time. The below graph clearly indicates the corresponding mass flow rate drying time and moisture content. The following graph shows the variation of drying rate with respect to the time in minutes. Drying rate is the effective parameter that is the final requirement of many people. Drying rate is also very important specifications of the solar air dryer. The following graph explains the variation of drying rate with respect to the various masss flow rates of air. Mass flow is very critical parameter which also has an adverse effect on the drying rate.

8 140 M. Joseph Stalin & P. Barath The following graph shows the variation of time with respect to the PCM temperature. PCM gives the back up if there is no sunlight and even it works in the night with better efficiency. CONCLUSIONS Solar air dryers are widely used because of its need, low cost and the availability of enormous solar power. This dryer is very effective and efficient and the main drawback of the system is it cannot work without sunlight (i.e.) during night. In order to overcome this problem, we have designed a system with thermal storage material to absorb the sun s heat during day time and using it in the night.. This system increases the efficiency of the solar air dryer. The spherical ball shape with cylindrical hole in the centre which has the larger surface area compared to other designs results with improved efficiency. Because of this system, the utilisation and the usage efficiency of renewable energy is very much improved. REFERENCES 1. Morrison, D. J. and Abdel-Khalik, S. I Effects of phase-change energy storage on the performance of air-based and liquid-based solar heating systems, Solar Energy, 20, pp Jurinak, J. J. and Abdel-Khalik, S. I., Sizing phase-change energy storage units for air based solar heating systems, Solar Energy, 22, pp Enibe, S.O., Thermal analysis of a natural circulation solar air heater with phase change material energy storage, Renewable Energy, 28, pp Hed, G. and Bellander R, Mathematical modeling of PCM air heat exchanger,, Energy and Buildings, 38, pp Fatah, H.E.S, Thermal performance of a simple design solar air heater with built-in thermal energy storage system, Energy Convers. Mgmt, 36, pp Farid M. M., Khudhair A. M., Razack S. A.and Al-Hallaj S A review on phase change energy storage: materials, Energy Conversion and Management, 45, pp Veraj R, Seeniraj RV, Hafner B, Faber C. and Schwarzer K Experimental analysis and numerical modeling of inward solidification on a platened vertical tube for a latent heat storage unit. Solar Energy, 60, pp

9 Effective Utilization of Solar Energy in Air Dryer Shatikian V., Ziskind G. and Letan R., Numerical investigation of a PCM-based heat sink with internal fins: constant heat flux International Journal of Heat and Mass Transfer 51, pp Mettawee, Eman-Bellah S. and Assassa Ghazy M.R., Thermal conductivity enhancement in a latent heat storage system, Solar Energy 81, pp Jompob W (2006). A Mathematical Modeling Study of Hot Air Drying for Some Agricultural Products. ThammasaItn t. J. Sc. Tech., pp. 1- I l. 11. J. P.Holman,Heat Transfer,McGraw-Hill book Co.,New York,NY, USA, 7th edition, ASHRAE Standard 93-77, Methods of testing to determine the thermal performance of solar collectors, Tech. Rep., American Society of Heating, Refrigerating and Air- Conditioning Engineers, Inc., New York, NY, USA, Lacroix M., Study of the heat transfer behaviour of a latent heat thermal energy unit with a platened tube, Int J Heat Mass Transfer,36,:pp Marín et al, Improvement of a thermal energy storage using plates with paraffin graphite composite, International Journal of Heat and Mass Transfer, 48 pp

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