Performance Analysis of Solar Air Heater Using Three Wire Mesh

Transcription

1 MIT International Journal of Mechanical Engineering, Vol. 5, No. 1, January 2015, pp Performance Analysis of Solar Air Heater Using Three Wire Mesh Sumit Kumar Singh Nitin Agarwal Vineet Tirth ABSTRACT In order to produce process heat for drying of agricultural, textile, marine products, heating of buildings and re-generating dehumidify agent, solar energy is one of the promising heat sources for meeting energy demand without putting adverse impact of environment. In the present work experiments has been performed on a solar air heater using three wire mesh and the result are discussed in change in thermal performance of solar air heater. In the set up there are two absorbing material viz. sand and charcoal. Three wire mesh of aluminum is used to improvement of thermal performance and a numbers of experiments are done on that and the changes in thermal performance are recorded. The maximum temperature difference is up to 29 0 C was recorded with this solar air heater and the average temperature increased is C. NOMENCLATURE A c = surface area of absorber plate (m 2 ) Cp = specific heat of air (J/kg K) CPo = specific heat of air at the outlet (J/kg K) CPi = input specific heat of air (J/kg K) h = heat transfer coefficient (W/m 2 K) I = intensity of solar radiation (W/ m 2 ) K = thermal conductivity of air (W/m K) L = length of test section of duct or long way length of mesh (m)? = mass flow rate (kg/s) Qu = useful heat gain (W) Qu = useful heat flux (W/m 2 ) UL = overall heat loss coefficient (W/ m 2 K) T 0 = Ambient temperature ( 0 C) T 1 = First thermometer temperature ( 0 C) T 2 = Second thermometer temperature ( 0 C) T 3 = Third thermometer temperature ( 0 C) T 4 = Output temperature of air ( 0 C) INTRODUCTION Solar energy is an inexhaustible resource. The sun produces vast amounts of renewable solar energy that can be collected and converted into heat and electricity. In the application of solar energy to the heating of dwellings and other uses, the primary element in the heating system is the Collector. Solar energy is the one most abundant renewable energy source and emits energy at a rate of kw, of which, approximately kw is intercepted by the earth [1]. The solar collector converts the solar radiation to energy in the form of sensible or latent heat in a fluid (air or water) which is passed through the collecting unit [2]. One of the useful applications of solar energy is air heating. Solar air heating (SAH) is a solar thermal technology in which the energy from the sun, solar insulation, is captured by an absorbing medium and used to heat air. SAH as a renewable energy, the technology is used for air conditioning and often used for heating purposes. One of the most potential applications of solar energy is the supply of hot air for the drying of agricultural, textile, marine products, heating of buildings to maintain a comfortable environment especially in the winter season and re-generating dehumidify agent. Unlike other sources of energy, solar energy can play a significant role for air heating system because the warm air is also the final receiver of energy [3]. This energy possesses a thermal conversion mode which necessitates a simple technology which is adapted to the site and to the particular region for many applications. All these systems are based on the solar air collectors. Solar energy collectors are employed to gain useful heat energy from incident solar radiation. They can be concentrating or flat plate type different air heating system with and without storage systems and its potential applications are presented.[4] It is typically the most cost-effective solar technologies, especially in commercial and industrial applications and it addresses the largest usage of building energy in heating climates, which is space heating and Industrial process heating. The performance of solar air heaters is mainly influenced by the several parameters such

2 MIT International Journal of Mechanical Engineering, Vol. 5, No. 1, January 2015, pp as: meteorological parameters (direct and diffuse radiation, ambient temperature and wind speed), design parameters type of collector, collector materials and flow parameters (air flow rate, mode of flow). The principal requirements of these designs are a large contact area between the absorbing surface and air [5]. Solar air heaters can be used for many applications including crop drying and space heating. Solar air heaters have many attractive advantages over liquid heaters regarding the problems of corrosion, boiling, freezing and leaks. the flowing air. A hybrid system is, thus, operated solely by the solar radiation Fig. 2: Classification of solar air heater Fig. 1: Solar radiation CLASSIFICATION OF SOLAR AIR HEATERS Solar air heater is a device in which energy transfer is from a distant source of radiant energy to air. Solar air heaters can be used for many purposes, including crop drying, space heating, and for re-generating dehumidifying agents [6]. It is a difficult task to classify solar air heaters in proper manner. There are numbers of configurations and many of which are empirical constructions. They can be classified on the basis of mode such as active, hybrid and passive. (i) Active Solar air heater: Active solar heating systems use solar energy to heat a fluid either liquid or air and then transfer the solar heat directly to the interior space or to a storage system for later use. If the solar system cannot provide adequate space heating, an auxiliary or back-up system provides the additional heat. (ii) Passive Solar air heater: The passive solar design creates a natural vacuum that allows cool air to be sucked into the lower section of the solar heater where it immediately starts to warm. This type of heater is also referred to as a thermo-siphon solar air heater using the naturally occurring laws of physics, a passive solar hot air heater uses the heat from the winter sun to heat indoor living spaces. Because the sun follows a similar, predictable pattern throughout the seasons, if the solar heater is positioned in a certain place in a certain way, it will not receive any summer sun. (iii) Hybrid Solar air heater: A hybrid solar air heater system, a combination of thermal and photovoltaic systems can generate sufficient electrical energy to turn the pump. The solar cells are pasted directly over the absorber plate. The part of the solar radiation falling on the cell area is converted into electrical energy, and part is collected by The solar air heater are further classified on the basis of air channel flow configuration and air channel design: 1. Single Flow Single Pass: Single flow single pass is the most common and simplest type of solar air heater. This type of solar air heater mainly consists of an air flow channel, a transparent cover, an absorber, and insulation material in the bottom of the absorber as shown in Figure 3. Fig. 3: A schematic view of single flow single pass 2. Double Flow Single Pass: The double flow single pass solar air heater is very similar to the single flow single pass heater. The main difference between them is the number of air flow channels. In a double flow single pass solar air heater, there are two air channels as illustrated in Figure 4. Fig. 4: A schematic view of double flow single pass 3. Single Flow Double Pass: There are two overlapping air flow channels in a single flow double pass solar air heater. Air flows from the upper channel, changes direction at the channel end and enters the lower channel. It flows straight through the bottom channel. That is why this type of solar air heater is named a single flow double pass. As shown in Figure 5.

3 MIT International Journal of Mechanical Engineering, Vol. 5, No. 1, January 2015, pp Fig. 5: A schematic view of a single flow double pass 4. Single Flow Recycled double Pass: Using recycled heated air in the design of a solar air heater may improve its efficiency and adjust the air outlet temperature. The partial circulation of heated air can provide the desired air temperature at the air flow exit if the outlet temperature is different than the desired temperature. As illustrated in Figure 6. Fig. 6: A schematic view of a single flow recycled double pass. Thermal performance of solar air Heater In solar air heater air flows in a parallel plate passage between the cover and the absorber plate. The energy balances for the absorber plate, the cover plate and the air flowing in between For Absorber plate- S = h fp - T f ) + h r T c ) + U b T a ) (1) For Coverh r - T c ) = U t (T c ) + h fc (T c T f ) (2) m Cp L2 Where dtf dx hr = = h fp - T f ) + h fc T f ) (3) T T σ 4 4 ( pm ) c * (4) ( Tpm Tc) εp εc Heat and energy transfer phenomenon with in air heater is illustrated in Figure 1. Thermal performance of solar air heater can be computed with the help of Hottel Whillier Bliss equation reported by Duffie and Beckman [7]. Qu =A C F R [I(ατ) e U L (T i )] (5) Or q u = Qu/A C = F R [I(ατ) e U L (T i )] (6) The rate of energy gain by flowing air in the course of duct of a solar air heater can be intended as follows: Qu =? Cp(To Ti) = ha C (Tpm Tam) (7) The value of heat transfer coefficient (h) can be increased by applying artificial roughness on the on the surface of absorber plate. Basically it represent in nondimensional form of Nusselt number (Nu) reported by Duffie and Beckman [7]. hl Nu = K Thermal efficiency of solar air heater can be expressed by the following equation: = Qu m Cp(To -Ti) = th I Ac I Ac Experimental setup 1. Fan of capacity 40 Watt 2500 RPM (max) speed of air supplying was set at 2 m/s for forced convection. The speed of the fan can be controlled by a knob (provided with fan) which was fixed on 3 inbuilt control voice; low speed 1750 RPM, mid speed 1980RPM and high speed 2200 RPM. Even this speed can also be varied up to 2500 RPM by supplying a high voltage of current supply, through an external stabilizer. 2. Inlet duct and outlet duct. 3. Flat plate collector in which sand and charcoal is used as absorbing material and glass is used as a glazing and three wire mesh of black paint coated is used. 4. Outlet duct which is insulated by using of thermacol to ensure that the exhaust heat is not transferred to the atmosphere. 5. Plywood of 1 cm thickness was used for the fabrication of solar air heaters. 6. The specific area of absorber tray was cm 2 which has been made of by using a 0.5 mm thick 22 SWG Al sheet. The absorber tray was painted dull black to store the maximum amount of solar energy. 7. To reduce the heat losses, a 2 cm thick layer of glass-wool was placed between the absorber tray and outer cabinet. 8. A single pane transparent float glass of 0.3 cm thickness and cm 2 was used for glazing to allow the solar radiation inside the SAH. 9. Thermometers 4 Nos. glycol is used range (-10 0 C to C). 10. A layer of granular powder and silica of 5 mm was spread on the absorber tray and sealed by 2 mm float glass. 11. The system was placed towards the south direction at an angle of 43 0 C from horizontal.

4 MIT International Journal of Mechanical Engineering, Vol. 5, No. 1, January 2015, pp Series of wire mesh three in numbers was placed between the absorber tray and upper glazing. As shown in Figure 7. Fig. 8: Thermal performance of solar air heater day one (14/05/2013) Fig. 7: Solar air heater (with wire mesh) Result and discussion The reading are taken in months of May 2013 at MIT Moradabad whose coordinates are N E of Moradabad and on the experimental setup there are some results which are shown in graph here the temperature are: T 0 = Ambient temperature T 1 = First thermometer temperature T 2 = Second thermometer temperature T 3 = Third thermometer temperature T 4 =Output temperature of air Here a graph between time and temperature are plotted with readings which are taken with different time interval and readings of temperature are taken five four thermometers out of which four are used in the setup one is used for measuring the ambient temperature. A fan is used for forced convection of heat which was absorbed by wire mesh and the absorber tray and sand over the absorbing tray and readings at different time interval are taken and results are discussed below. On the first day 14/05/2013 the setup is started at 9:45 am and in beginning about first reading was taken after fifteen minutes after attaining of saturation stage and at 10 am and ambient temperature was 34 0 C and the outlet temperature is 41 0 C means 7 0 C temperature was increased and second reading was taken at 12:00 pm and at that time ambient temperature was 38 0 C and the outlet temperature is 57 0 C means 19 0 C temperature was increased and at third reading is taken at 2:00 pm and a increment of 17 0 C was toted and after than the difference was decreased to 13 0 C and then 10 0 C and so on. So the maximum temperature difference and efficiency is between 12:00 pm to 2:00 pm. S. No. Time Table 1: Readings of day one (14/05/2013) Temperature T0 T1 T2 T3 T4 Mw/ mm 2 Radiation Langley/ Hr. 1 10:00 AM :00 PM :00 PM :00 PM :00 PM On the second day 14/05/2013 same as day one maximum temperature difference is taken at 12:30 pm is 29 0 C and minimum temperature difference is 9 0 C at 9:00 am. Fig. 9: Thermal performance of solar air heater day two (15/05/2013)

5 MIT International Journal of Mechanical Engineering, Vol. 5, No. 1, January 2015, pp S. No. Table 2: Readings of day two (15/05/2013)) Time Temperature T0 T1 T2 T3 T4 Mw/ mm 2 Radiation Langley/ Hr. 1. 9:30 AM :30 AM :30 AM :30 PM :30 PM :30 PM :30 PM :30 PM Conclusion So the maximum temperature difference is up to 29 0 C was recorded with this solar air heater and the average temperature increased is C. To improve thermal performance some modifications can be done viz. using halogens, using solar radiation concentrator can be used for improving thermal performance. References 1. Thirugnanasambandam, M. Iniyan, S. Goic, R., A review of solar thermal technologies. Renewable and Sustainable Energy Reviews 2010;14: G.O.G. Lof, Thomas D. Nevens, Heating of air by solar energy, Ohio J. Sci., 53 (5) (1953) Turhan, K., Performance of various design of solar air heaters for crop drying applications. Renewable Energy, 2006;31: Tchinda, R., A review of the mathematical models for predicting solar air heaters systems. Renewable and Sustainable Energy Reviews 2009;13: Kalogirou, S., Solar energy engineering processes and systems, Elsevier, USA. 6. Kalogirou, S.A., Solar thermal collectors and applications. Progress in Energy and Combustion Science, 2004; 30: Duffie, J.A., Beckman, W.A., Solar engineering of thermal processes. New York: Wiley; Abhishek Saxena, Nitin Agarwal, Ghansyham Srivastava, Design and performance of a solar air heater with long-term heat storage.