Comparative Analysis of Performance of Two Scheffler Solar Concentrators Having Different Concentration Ratios

Two different Scheffler concentrators with concentration ratio 48 and 17 were installed and experimentation was carried out to improve the performance of the Scheffler solar concentrator system. Receiver is the heart of the system. Parameters used for experimentation are related to receiver only. Shape of receiver, position of receiver, inlet temperature of water of receiver, covering of receiver with glass to reduce convection losses. Concentration ratio is the ratio of aperture area to receiver area both Scheffler concentrators have receiver of same size and volume. scheffler with concentration ratio 48 generates steam up to 3 bar gauge pressure of steam and heat gain rate is 2 kw, and scheffler with concentration ratio 17 develops up to 1 bar gauge pressure of steam and heat gain rate is 0.6 kw. If temperature of water at inlet is 50 degree instead of 30 degree, performance of Schefflers improves due to reduction in thermal inertia. Conical receiver due to more surface area for heat transfer during tilt gives better performance than cylindrical receiver to generate steam. As solar radiations increases efficiency of Scheffler solar concentrators decreases. If wind speed increases from 1 to 3 m/s efficiency decreases. Keywords—scheffler concentrator, solar radiations, thermal efficiency, concentration ratio, optimization, receiver.

concentrator rotates. During the day the concentrated light will only rotate around its own centre but not move sideways in any direction so focus is fixed III. Experimental Set Up Fig.1 and Fig.2 shows the experimental setups of Scheffler concentrator systems [8]. with area 2.7 m 2 and 9.2 m 2 and concentration ratio 17 and 48 respectively, cylindrical and conical receivers are used for experimentation. Diameter and volume of both the receivers are designed equal.  An anemometer is used to measure wind speed and ambient temperature is measured by temperature sensor. All instruments such as pyrometer, anemometer and temperature sensor are coupled with data logger. Data logger with its various instruments is mounted at weather station available at experimental field. The data logger is a microchip equipped tool that helps in measuring analogue data collected by the instruments and presents and stores them in digital format. K-type thermocouples are used for measuring the surface temperature of receiver. Tilting arrangement is provided for both receivers and measurement of titling angle has been done using an inclinometer.
For big Scheffler, receiver and storage tank are built separately, while for small Scheffler, receiver cum storage tank is used. Experimentation is done as per Bureau of Indian Standards (BIS) procedure [1][2][3]. For both big and small Scheffler, working fluid is water. Mass flow rate of steam is measured and thermal Efficiency is calculated. Readings include focus Temperature, receiver surface temperature, Air temperature, Global radiation, Diffused radiation, wind speed, mass of steam generated per hour and pressure. More than fifty readings were recorded during month of March to May 2016 for different cases.
Thermal efficiency of receiver is calculated on the basis of parameters like(1) shapes of receiver(2) initial heating of inlet temperature to 50ᵒC(3) tilting of the receiver with horizontal at 30,45,60 degree and(4) receiver with glass cover. The thermal efficiency experimentally is calculated with equation: Thermal efficiency (η) = x 100 (1) Where, M s -Mass flow rate of Steam (kg/hr),h fg -latent heat(kJ/kg) I bn -Direct Solar Radiation (W/m 2 ),A p -aperture area T -3600/1000(conversion factor)

IV. Result and Discussion
In the present study parametric analysis has been done for improvement in thermal Efficiency of Scheffler concentrator systems used for steam generation. For small Scheffler steam generation rate is 0.5 kg/hr at 1 bar gauge steam pressure and 1.5 kg/hr at 3 bar gauge steam pressure at normal condition. For big Scheffler steam generation rate is 1.5 kg/h at 1bar gauge steam pressure and 3.5 kg/hr at 3bar gauge steam pressure at normal condition . Table 1 shows values of thermal efficiency for different conditions of two Scheffler concentrators by experimentation.

Condition
Thermal Efficiency (%) Low CR (17) p-up to 3 bar absolute  Fig.3 and Fig.4 shows thermal efficiencies and mass flow rate during tilt of receiver in upward direction to horizontal, For big scheffler and small scheffler respectively. For conical receiver system efficiency and mass flow rate increases with tilt angle up to 45 o tilt of receiver with horizontal for small Scheffler. Tilt up to 60 degree it is out of focus for small scheffler. For big Scheffler mass flow rate and efficiency seems to increase up to 30 o and saturate beyond 30 o tilt of conical receiver. Tilt up to 60 degree is out of focus for big scheffler. Focal area were measured for all these conditions on receiver surface .At 45 0 tilt area is0.015m 2 for small scheffler. It shows for big scheffler that focal area on receiver surface does not increase significantly beyond 30 o tilt of receiver and focal area at 30 degree tilt is 0.042m 2  fig.5& fig. 6. Focal area where solar rays get concentrated remains approximately same for tilt and without tilt conditions of cylindrical receiver. Hence it is not a good option for thermal efficiency improvement. Increase in steam pressure leads to increased efficiency and mass flow rate in big Scheffler concentrator while small Scheffler concentrator shows decrease in efficiency and mass flow rate with increase steam pressure above 2 bar gauge in both cases of conical and cylindrical receiver. Initial heating is the best option for cylindrical receiver. Tilting up to 30 degree for big scheffler and tilting up to 45 o for small scheffler with initial heating of inlet water of receiver up to 50 degree is best option for conical receiver.

V. Conclusion
An experimental results shows that cylindrical receiver has maximum efficiency in case of initial heating of water (50 o C) for small and big scheffler, as initial heating of inlet water reduces the thermal inertia and this result in increase in mass flow rate of steam, hence thermal efficiency gets improved. Conical receiver gives maximum efficiency for tilt of receiver 45 o for small Scheffler because in tilting condition it has maximum focus area (0.015m 2 ) which increase the heat input to the water .Conical receiver gives maximum efficiency For tilt of conical receiver 30 o for big scheffler.. Among different parameter tested, general condition and initial heating is found good operating condition for cylindrical receiver. Conical receiver performed well for initial heating and tilting condition. Thermal efficiency does not improve for glass cover condition. There is good agreement between analytical and experimental value of thermal efficiency with maximum deviation 20% for cylindrical receiver and 14 % for conical receiver.