DESIGN, FABRICATION & PERFORMANCE ANALYSIS OF SOLAR AIR HEATER

Transcription

1 DESIGN, FABRICATION & PERFORMANCE ANALYSIS OF SOLAR AIR HEATER Amrita Agrawal 1, Nalin Deshpande 2, Aakanksha Dubey 3 Department of Energy Maulana Azad National Institute of Technology (MANIT), Bhopal India Phone: , , ambilavanya@gmail.com, 2 nalin191189@yahoo.com, 3 aakanksha512@gmail.com Abstract The objective of this paper is to discuss the designing, fabrication and performance analysis of a double-pass vee corrugated Solar Air Heater. The design and fabrication section incorporates the various types of materials used for insulation, outer enclosure etc and their corresponding dimensions. The efficiencies and the observations drawn on two consecutive typical Indian summer days have been proposed taking into consideration the intermittent availability of sunlight at different times of the day and keeping the unit at an inclination of 23.5 degrees(latitude Angle of Bhopal) facing due South, using DC fans(for forced convection) of different air flow rates. The possible applications and scope of this solar air heater have been also been suggested. Keywords: 1) SAHs-Solar Air Heaters, 2) GI-Galvanised Iron 3)AHs-Air Heaters 4)APA-Absorber Plate Assembly 5)FRP-Fibre Reinforced Plastic 1. Introduction A conventional SAH generally consists of an absorber plate with a parallel plate below forming a small passage through which the air to be heated flows. As in the case of the liquid flat-plate collector a transparent cover system is provided above the absorber plate while in a sheet metal container filled with insulation is provided on the bottom and the sides. However, the value of the heat transfer coefficient between the absorber plate and the air is low and this result in lower efficiency. For this reason, the surfaces are sometimes roughened and longitudinally fins are provided in air flow passage. Another variation is to use a V-shaped absorber plate. In this SAH the face areas range from 1-2 m 2. Materials of construction and size are similar to that used to the liquid flat plate collectors. The absorber plate is a metal sheet about 1 mm in thickness usually made of GI or steel. Glass of thickness 4-5 mm is the most commonly used material. However, plastics are used in increasing numbers. Mineral wool or glass wool of thickness 5-8 cm is used for the bottom and side insulation. The whole assembly is contained in a sheet metal box and inclined at a suitable angle. SAH is a type of energy collector in which the energy from the sun is captured by an absorbing medium and used to heat air. Solar air heating is a renewable energy heating technology used to heat or condition air for buildings or process heat applications. A conventional SAH generally consists of an absorber plate with a parallel plate below forming a small passage through which the air to be heated flows. As in the case of the liquid plat-plate collector, transparent cover system is provided on the bottom and sides. It is technically feasible to use solar heated air providing energy for almost any application that uses solar-heated liquids. The important areas of applications are as follows: Heating and cooling of buildings, Heating of greenhouse, and Industrial processes such as drying of agricultural crops and timber.

2 Various types of SAHs have been designed and used in space heating and cooling. AHs are only used in actively heated or cooled buildings. AHs are also used with desiccant beds for solar air conditioning. The heat from the air heaters can also be used to heat the generator of an absorption air conditioner for cooling purpose. Drying is a promising area for the application of AHs. Hot air from SAH is circulated through the crop to reduce its moisture content. The air can be circulated either by a fan or by natural convection; correspondingly, the heaters are called active or passive dryers depending on the mode of circulating air, a number of designs are possible. One such design is two-pass air heater connected to a drying chamber. In this design, the hot air passes through the crops immediately after it leaves through the collector. The moist air escapes into the atmosphere through an opening at the bottom of the dryer. 2. Design & Fabrication Figure 1: Schematic diagram of a two-pass air heater connected to drying chamber The SAH is a combination of two pass and Vee-corrugated design. It is an assembly of the following six major units: Outer Enclosure Absorber Plate Assembly Duct Insulation Transparent Glass Cover DC Fans 2.1 Outer Enclosure It provides a casing for all the internal units and prevents heat loss Materials Fiber reinforced polymer (FRP) sheets of 3mm thickness Dimensions 1m x 1m x.2m Fabrication The FRP sheets are held together with the help of L-shaped aluminium angles. Sealing has been done with the help of white cement. 2.2 Absorber Plate Assembly Figure 2: Outer Enclosure It absorbs the incoming solar radiations and transfers the heat energy to the incoming low temperature air to raise its temperature to an optimum level. It has been painted black with selective coating to reduce reflective losses and for absorption of maximum incoming solar radiations Thermal Storage Unit Sand is used in an enclosed volume which is provided below the APA. Air cavities in

3 the sand absorb heat during peak sunshine hours ant dissipates it during low sunshine hours. Thus, increasing the efficiency of the system Materials Galvanized Iron sheets 2.2.3Dimensions:.9mx.85mx.1m(volume) Top-GI-sheetarea:.8mx.85m Vee-corrugatedsheetarea:88sq.meterss Bottom-GI-sheetarea:.9mx.85m Thermal Storage Volume:.9m x.85m x.5m Fabrication The Vee corrugated sheet is sandwiched between two flat plate absorbers and this assembly being.7m in height is held at a height of..5m from the bottom which constitutes the thermal storage unit. 2.3 Duct Figure 3: Absorber Plate Assembly A partitioned duct is provided for the inlet and outlet of air. It prevents heat loss to the surrounding and low temperature incoming air Material FRP sheet of thickness 1 mm Dimensions as shown in the figure Fabrication The above shape has been adopted for uniform pressure drop. The duct has been internally partitioned in the ratio of 3:7 of its height and all its constituents have been assembled with the help of clamps. To prevent air losses M-Seal and Araldite were used. Figure 4: Duct Dimensions, Insulation and Transparent Glass cover 3. Thermal Analysis A unified approach to specifying efficiency will make possible a comparison with various models and the determination of yield under given climatic and operating regimes. The main functions of collector testing are: To provide information for predicting the performance of a solar collection system in known meteorological conditions, To provide information to study and develop the design of solar collectors, To permit evaluation of the performance of solar collectors for their commercial comparison, To establish performance standards. The instantaneous efficiency of a solar collector can be calculated as:

4 ᶯ c = mc p (T 2 -T 1 ) (1) A c I t Where, m = mass flow rate of the heat transfer fluid, kg/s C p =specific heat of heat transfer fluid, J/kgK T 2 = temperature of the heat transfer fluid leaving the collector, K T 1 = temperature of the heat transfer fluid entering the collector, K A c = Area of collector, m 2 I t = Total solar energy incident upon the plane of the collector per unit time per unit area, W/m 2 The parameters considered for performing the thermal analysis are as follows: I g = total global radiation falling on the tilted surface per unit area per unit time, W/m 2 I d = total diffused radiations falling on a tilted surface per unit area per unit time, W/m 2 T a = ambient air temperature, C Rh a = relative humidity of ambient air, % T i = inlet air temperature, C Rh i = relative humidity of inlet air, % T o = outlet air temperature, C Rh o = relative humidity of outlet air, % v i = inlet air velocity, m/s v o = outlet air velocity, m/s T ab = absorber plate temperature, C T ig = inner glass temperature, C T og = outer glass temperature, C 4. Observations Date of Experiment Performance: Average Mass Flow Rate:.156 kg/s Average Ambient Air Temperature: C Average Ambient Air Relative Humidity: % Table 1: Table for Day1 S.No. TIME Ig Id Ta Rha Ti To Vi Vo Rhi Rho Tab Tig Tog ᶯ (W/m²) (W/m²) ( C) (%) ( C) ( C) (m/s) (m/s) (%) (%) ( C) ( C) ( C) (%) 1 11:am :pm :pm :pm :pm :pm :pm Date of Experiment Performance: Average Mass Flow Rate:.4839 kg/s Average Ambient Air Temperature: C Average Ambient Air Relative Humidity: % Table 2: Table for Day 2 S.No. TIME Ig Id Ta Rha Ti To Vi Vo Rhi Rho Tab Tig Tog ᶯ (W/m²) (W/m²) ( C) (%) ( C) ( C) (m/s) (m/s) (%) (%) ( C) ( C) ( C) (%) 1 11:am :pm :pm :pm :pm :pm :pm

5 5. Conclusions On Day1, efficiency lies between the range of 3-7% and the maximum temperature reaches approximately 85 C. On Day2, efficiency lies between the range of 22-47% and the maximum temperature attained is 78 C Due to the thermal energy storage effect, the efficiency of the air heater does not let the efficiency to drop below 2%, which is good during the evening hours and in the evening hours. It is designed to be used with the solar dryers and compare efficiency of the dryer with and without the use of heater. Efficiency (%) Instantaneous Efficiency of Solar Air Heater at different Mass Flow Rates 1 5 Figure 5: Graphical representation of the Observations Day1 Day2 The conclusions drawn from the graph are: On Day1, that is, when the mass flow rate is larger the efficiency is generally higher than that on Day2 when the mass flow rate is low. The highest observed efficiency in the 2-day experiment period is 67.56% and the lowest being 17.81%. The efficiency is generally reaches its peak value somewhere between 12pm to 1pm when the solar radiations are maximum. The morning and evening hours experience low efficiency due to less heat gain by the air and cool climatic conditions. Cloud cover and weather conditions plays a major role in determining the extent of temperature rise and thus efficiency of the air heater. Temperature ( C) Temperature v/s Time Plot (Day1) Ta 4 2 Ti To Figure 6: Temperature Vs Time Plot The above graphs show the variation of ambient, input and output temperature with the time of the day. The following conclusions can be drawn: The ambient and inlet temperatures are approximately equal. The small difference in temperature is because of the fact that the ambient temperature is recorded in shaded region whereas inlet temperature is recorded under the sun. On day1, the outlet temperature rises to its highest value at around 1pm and started to decline with time. On day2, the outlet temperature kept fluctuating, maximum being 78.1 C.

6 The Graph below that follows represents the variation between the ambient, absorber plate, inner glass and outer glass temperature with respect to time of the day. Following conclusions can be drawn: T ab, T ig and T og are significantly above the ambient air temperature. T ab and T ig are approximately at the same temperature throughout. T og lies between T ab or T ig and the ambient air temperature. Temperature ( C) Temperature v/s Time Plot (Day1) 1 Temperature v/s Time Plot (Day2) 1 Ta 5 Tab 5 Tig Tog Figure 7: Temperature Vs Time plot w.r.t inner & outer glass temperatures Temperature ( C) Ta Tab Tig Tog The graph below shows the relation between Relative Humidity and time of the day. Following are the inferences drawn from the graph: Rh a and Rh i are relatively at the same level with Rh i being lower due to higher temperature. On day1, Rh o decreases initially as temperature rises and then it increases in the evening hours. On day2, Rh o fluctuates with time due to the weather conditions. Maximum humidity being at 3pm and minimum at 12pm. Relative Humidity (%) Relative Humidity v/s Time Plot (Day1) Relative Humidity v/s Time Plot (Day2) Rha 2 Rha 1 Rhi 1 Rhi Rho Rho Figure 8: Relative Humidity Vs Time Plot Relative Humidity (%) 6. References [1]G.N.Tiwari, May 2, Solar Energy Fundamentals, Design, Modeling & Applications [2]H.P Garg, 2, Solar Energy Fundamentals &Applications [3]John.A. Duffie and William.A. Beckman, 1991, Solar Engineering of Thermal Processes [4]S.P Sukhatme and J.K Nayak, 28, Solar Energy Principles of thermal collection & Storage