1 Evaluation of efficiency and collector time constant of a solar flat plate collector at various intensities of light and constant wind speed by using forced mode circulation of water Abhijit Devaraj 1 Abhishek Hiremath 2 Akshay R Patil 3 Krushik B N 4 Department of Mechanical Engineering, BMS College of Engineering, Bangalore, INDIA Abstract- The present attempt of the work is to calibrate the efficiency and collector time constant of a flat plate collector which is used to heat water flowing through the pipes by forced circulation at varying intensity of heat flux, when wind is blowing at a constant speed. It was observed that these factors affect the flat plate collector in a profound way. This work helps us in giving an insight on practical scenarios where solar collectors are usually placed at high elevations to receive heat as high altitudes involve flow of wind across the collector. Keywords: Flat plate collector, solar water heater, intensity of sunlight, wind flow I - INTRODUCTION Solar Energy is one of the major alternative sources of energy being used in the current world scenario. Processes of industrialization and economic development require important energy inputs. Reserves of fossil fuel are limited and their large scale use is associated with environmental deterioration.  Solar energy is considered one of the main promising alternative sources of energy to replace the dependency on other fossil fuel resources   There are adverse environmental effects caused by greenhouse gas emissions from fossil fuel combustion.  Solar energy is an ecologically clean source of energy and freely available to everyone over long time periods at all parts of the earth.  Incoming solar radiation is converted into thermal energy using black bodies which trap the excess heat emitted from the sun in the form of infrared radiations Availability of solar energy depends on day and night cycles and weather conditions hence collectors are used to trap solar energy radiated from the sun. Solar ing (SWH) is the conversion of sunlight into renewable energy for water heating using a solar thermal collector. The heat collector used here is a Flat-plate solar collector which is used to collect heat for various applications such as space heating, domestic hot water or cooling with an absorption chiller. The advantages of solar flat plate collector are that we receive hot water throughout the year, it decreases our daily fuel consumption and reduces our energy bills and also reduces carbon emissions. Valve 5 Valve 1 Pump Valve 7 II IMPLEMENTATION Hot Tank Cold Tank Flat Plate Collector Valve 3 FIGURE 1- Block Diagram of the experimental setup There are two types of solar water heating systems namely passive and active. Flat plate collectors can be either glazed or unglazed and either air or liquids can be us ed as heat transporting fluids.  This experiment involves an active water heating system where a pump is used to circulate water which allows us to have the collector tank above the collector and also use drain back tanks. 27
2 FIGURE 2 Experimental Setup of ECOSENSE water heating system based on solar flat plate collector. system. The sensors are RTD based platinum probe and work on the principle of variation of resistance with temperature. The probes are class A RTD and can measure the temperature in the range of 2 to 65. Pressure Gauge: Two pressure gauges are there in the setup. They work on the principle of generation of electric signal by semi-conductor device due to exertion of pressure. Pressure gauges can measure the pressure in the range of 11.3 to 65 KPa. flow meter: To measure the water flow rate a panel mount flow meter with a mini turbine flow sensor is connected near the collector inlet. It is a programmable meter. It can measure the flow rate in the range of.5 to 25 liters/minute. A temperature limit of meter is up to 8. Pump: We are using an AC pump to fill up the collector tank as well as to circulate the water through the collector at some regulated speed. A continuous regulator is there to maintain the flow rate. Anemometer: An anemometer is supplied with the system. This can be used to measure the air velocity and ambient air temperature. The air flow sensor is conventional angled vane arms with low friction ball bearing while the temperature sensor is a precision thermistor. The Anemometer can measure the wind velocity in the range of.5 to 45 m/s while the temperature range is 1 to 6. Fan: One AC fan is integrated with the system to generate artificial wind speed. To set the wind speed as per requirement a regulator is there in the control unit. Valve: Different valves are there to direct the water flow as per requirement.  A - Specifications The specifications of the equipment are as follows: Tank capacity: 5 litres Collector area:.716m² Tungsten halogen fixture s area:.72m² Halogen system Power: 15 watt each Radiation meter range: to 1999 w/m² pump power:.12hp flow range:.5 to 25 LPM. flow maximum pressure: 17.5 bar FIGURE 3- Panel used to display input and output parameters. The setup consists of the following components: Radiation meter: To measure the radiation level that is received by the collector a radiation meter is supplied with the system. It is a sensing based device. It can measure the radiation level in the range of to 2 W/m 2. Thermometer: Four thermometers are connected to the 28 Thermometer sensor: class A sensor Thermometer range: 2 to 65 C Anemometer range:.4 to 45 m/s Fan range: to 5 m/s 
3 B - Assumptions made in the setup 1. The collector is in steady state condition. 2. Headers cover a small area of the collector and can be neglected. 3. Headers provide uniform flow to riser tubes. 4. Flow through the back insulation is one dimensional. 5. erature Gradients around the tube are neglected. 6. Properties of materials are independent of temperature. 7. No energy is absorbed by the cover. 8. flow through the cover is one dimensional. 9. The covers are opaque to infra red radiation. 1. Same ambient temperature exists at both front and back of the collector. 11. Dust effects on the cover are negligible. 12. There is no shadowing of the absorber plate. 13. erature drop across glass tube is uniform. 14. Solar radiation transmitted through glass cover is  reflected not absorbed. A Formulae III RESULTS AND DISCUSSIONS Calculations were performed using the following formulae s: C Methodology The cold water tank 1 was filled with water at atmospheric temperature. Valve 1 and valve 7 were opened which allows flow from the cold water tank 1 to the Flat Plate Collector inlet. The pump was switched on and the regulator was set at the minimum power at which the pump can work. A suitable flow rate was set whose value can be observed on the flow meter screen. Valve 3 was opened which allows flow from the Flat plate collector outlet to the hot water tank. After waiting for some time to get a stable reading the fan regulator was adjusted to get the desired wind speed which in this case is 5 m/s. The wind speed was measured using an anemometer. Once the flow rate and the wind speed were set the initial readings of collector plate temperature, water inlet temperature, water outlet temperature and hot water temperature were noted down at time= sec. The Halogen system was then switched on and the radiation was set to desired level which in the first case is 1 W/m 2. The cold water was allowed to flow through the Flat plate Collector which absorbed the heat and was then allowed to flow into the hot water tank. The temperature readings as mentioned above were noted down for every one minute for a total duration of 1 minutes. After the experiment was completed the pump was switched off and the valve 1 was closed and valve 5 was opened which allows the water to drain from the hot water tank to the cold water tank. The water was allowed to cool for some time. The experiment was repeated two more times by following the exact same procedure but the flux rates were set at 13 and 16 W/m 2 for the next two trials respectively and the readings were tabulated. The heat supplied was obtained by multiplying the flux supplied by the collector area. The collector time constant and radiative efficiency of the collector were calculated using suitable formulae. Graphs were plotted for efficiency vs. time and collector time constant vs. time for various specific flux rates. Supplied= specific heat flux *area of collector flow rate = 2.35 Lpm = 2.35/6 =.3916 Kg/s Radiated = Q rad = Q = σa T 4 = σa(t 4 1 -T 4 )  Collector Constant = R R = [T 3 - T 3 ()] / [ T 4 - T 3 ()]  Collector Plate = ɳ = (Q rad / Q sup )*1  29
4 B - Tables Table 1: Readings for a specific flux of 1 W/m 2 Supplie d (Q in ) in Wind Velocity (V) in m/s in sec Flow Rate (ṁ) in Kg/s Plate (T 1 ) in Table 2: Readings for a specific heat flux of 13 W/m 2 Supplied (Q in ) in Wind Velocity (V) in m/s Inlet (T 2 ) in Outlet (T 3 ) in Hot (T 4 ) in Radiated (Q rad ) in Collector Constant (R) of plate ɳ (in % ) in sec Flow Rate (ṁ) in Kg/s Plate (T 1 ) in Inlet (T 2 ) in Outlet (T 3 ) in Hot (T 4 ) in Radiated (Q rad ) in Collector Constant (R) of plate ɳ (in % ) Table 3: Readings for a specific heat flux of 16 W/m 2 Supplied (Q in ) in Wind Velocity (V) in m/s in sec Flow Rate (ṁ) in Kg/s Plate (T 1 ) in Inlet (T 2 ) in Outlet (T 3 ) in Hot (T 4 ) in Radiated (Q rad ) in Collector Constant (R) of plate ɳ (in % )
5 R in % in % in % International Journal of Mechanical Civil and Control Engineering C of Collector FIGURE 4 Plot of vs. time for a flux of 1 W/m v/s time plot for 1 W/m 2 flux (seconds) v/s time plot for 13 W/m 2 flux (seconds) efficiency FIGURE 5 Plot of vs. time for a flux of 13 W/m FIGURE 6 Plot of vs. time for a flux of 16 W/m 2 The plots of efficiency v/s time for each specific flux rate (figure 4, figure 5, and figure 6) showed that the efficiency decreased as time increased. This was due to wind blowing constantly over the Flat plate Collector which reduced the collector plate temperature resulting in reduced heat radiation. This resulted in decreased efficiency. The graph after a certain time interval becomes almost linear. This was because after some amount of cooling of the Flat plate Collector had taken place, the plate attained an almost steady temperature which gave steady heat radiation and almost constant efficiency. D Collector time Constant v/s time plot for 16 W/m 2 flux (seconds) R v/s time for flux of 1 W/m 2 efficiency (seconds) 31 FIGURE 7 Plot of Collector time constant vs. time for a flux of 13 W/m 2
6 R R International Journal of Mechanical Civil and Control Engineering 1.6 R v/s time for a flux of 13 W/m 2 between the time interval of 36 and 48 seconds and hence R =.35 for figure FIGURE 8 Plot of Collector time constant vs. time for a flux of 13 W/m (seconds) R v/s time for a flux of 16 W/m 2 IV - CONCLUSIONS In the present study on Flat plate Collector s the potential barriers to using them in practical scenarios at high elevations involving wind flow was determined. From the readings obtained and the graphs plotted it was inferred that the Collector Constant R decreased as time increased. Also as time increased the temperature of Flat Plate Collector decreased due to which the heat radiated decreased. This resulted in a decrease in efficiency. The temperature drop was due to cooling of the Flat Plate Collector due to the constant wind blowing over it. Also the efficiency decreased as heat flux incident normally on the collector plate decreased. Hence in practical scenarios maximum efficiency is obtained at noon when maximum normal heat flux is incident on the Flat plate Collector. This particular study helped us understand the influence of day night cycles and wind flow velocity on flat plate collectors. It gave us estimation that for solar collectors to heat water to higher temperatures and generate more efficiency, the collectors should be kept at a high altitude to receive more sunlight and also at a location where the wind is blowing at minimum or negligible speed to avoid cooling and temperature drops (seconds) FIGURE 9 Plot of Collector time constant vs. time for a flux of 16 W/m 2 Collector time constant is required to evaluate the transient behavior of a collector. It can be calculated from the curve between R and time as shown above. The plots of Collector time constant R v/s time for each specific flux rate showed that R decreased as time increased. The graph for a flux rate of 1 W/m 2 becomes almost constant or linear between the time interval of 24 and 48 seconds and hence R =.75 for figure-7. The graph for a flux rate of 13 W/m 2 becomes almost constant or linear between the time interval of 42 and 48 seconds and hence R = 2 for figure-8. The graph for a flux rate of 16 W/m 2 becomes almost constant or linear 32 REFERENCES  Amirhossein Zamzamian, Mansoor Keyanpour Rad, Maryam Kiani Neyestani, Milad Tajik Jamal-Abad., An experimental study on the effect of Cu-synthesized/Eg nanofluid on the efficiency of flat plate collectors, Renewable Energy, vol. 71, pp , 214.  F.Cruz-Peragon, J.M.Palomar, P.J.Casanova, M.P.Dorado, F.Manzano-Agugliaro, Characterization of solar flat plate collectors, Renewable and sustainable energy reviews, vol. 16, pp , 212.  R. Manzano-Agugliaro F, Montoya FG, Gil C, Alcayde A, Gomez J. Banos, Optimization methods applied to renewable and sustainable energy: a review, Renewable & Sustainable Energy Reviews, vol. 15, pp , 211.  Ssen Z, Solar energy in progress and future research trends, Progress in energy and combustion science, vol. 3, pp , 24.  Gurveer Sandhu, Kamran Siddiqui, Alberto Garcia, Experimental study on the combined effects of inclination angle and insert devices on the performance of a flat plate solar collector, International Journal of and Mass Transfer, vol. 71, pp , 214.
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