Performance tests on helical Savonius rotors

Conventional Savonius rotors have high coefficient of static torque at certain rotor angles and a negative coefficient of static torque from 135 degrees to 165 degrees and from 315 degrees to 345 degrees in one cycle of 360 degrees. In order to decrease this variation in static torque from 0 degrees to 360 degrees, a helical Savonius rotor with a twist of 90 degrees is proposed. In this study, tests on helical Savonius rotors are conducted in an open jet wind tunnel. Coefficient of static torque, coefficient of torque and coefficient of power for each helical Savonius rotor are measured. The performance of helical rotor with shaft between the end plates and helical rotor without shaft between the end plates at different overlap ratios namely 0.0, 0.1 and 0.16 is compared. Helical Savonius rotor without shaft is also compared with the performance of the conventional Savonius rotor. The results indicate that all the helical Savonius rotors have positive coefficient of static torque at all the rotor angles. The helical rotors with shaft have lower coefficient of power than the helical rotors without shaft. Helical rotor without shaft at an overlap ratio of 0.0 and an aspect ratio of 0.88 is found to have almost the same coefficient of power when compared with the conventional Savonius rotor. Correlation for coefficient of torque and power is developed for helical Savonius rotor for a range of Reynolds numbers studied. (C) 200

Shading and available energy in a parabolic trough concentrator field

Shadow cast by one collector row on its adjacent one in a solar collector field affects the energy collection. Due to continuous change in sun's position, shadow and its effect on adjacent row also change with time. For any orientation of collector field, extent of shading depends upon the gap between the rows. It is necessary to have enough gap between the rows to avoid shading for maximum time of the day resulting in the highest energy collection. However, the availability of open land puts constraints on the gap. The present work aims at finding the energy available for a collector field for any pitch, any orientation and any latitude considering shading. A factor called annual energy availability factor has been defined to quantify the annual energy availability at the aperture of a collector field. It is found that this factor increases significantly with increase in pitch initially and later saturates with no substantial improvement in energy collection. Performances of collector field for different orientations have also been compared. It is found that N-S mounted collector field is the best in terms of energy availability considering shading. This orientation also leads to the least cost of electricity generation. From this work, one can predict the energy availability and cost of energy production for any location, orientation and pitch. One can also choose the minimum pitch required to meet a particular energy availability requirement for given location and orientation. (C) 2013 Elsevier Ltd. All rights reserved

Cyclic analysis and optimization of design parameters for Beta-configuration Stirling engine using rhombic drive

Design and development of Beta configuration Stirling engine is a complex combination of thermodynamic analysis and mechanical design of components. The present work uses the second order cyclic analysis for the thermodynamic investigations and includes the effect of the overlapping volume displaced by piston and displacer within cylinder. It determines the ideal heat input, ideal power output and related losses separately. This provides estimate of net heat input, net brake power and cooling duty. The sensitivity analysis of various design and operating parameters is carried out to decide the order in which these should be modified and includes heater tube assembly parameters; annular regenerator assembly parameters; and cooler assembly parameters, using univariate method. The optimization considers the combined effect of the net power output and the efficiency instead of a single Optimization Parameter. The present work thus presents the cyclic analysis and optimization of design parameters of a Beta configuration Stirling Engine of 1.5 kWe using rhombic drive. It was observed that the power output exceeds the requirement of 1.5 kWe. Instead of carrying out the optimization process all over again, the operating parameter p(mean) is adjusted to 24 bar to obtain the desired output of 1.5 kWe. (C) 2017 Elsevier Ltd. All rights reserved

Experimental investigations on single stage, two stage and three stage conventional Savonius rotor

The performance of single stage (rotor aspect ratio of 1.0), two stage Savonius rotor aspect ratios of 1.0 and 2.0 (stage aspect ratios of 0.50 and 1.0) and three stage Savonius rotor with rotor aspect ratios of 1.0 and 3.0 (stage aspect ratios of 0.33 and 1.0) are studied at different Reynolds numbers and compared at the same Reynolds number. The results show that the coefficient of power and the coefficient of torque increase with the increase in the Reynolds numbers for all the rotors tested. The coefficient of static torque is independent of the Reynolds number for all the rotors tested. The performance of two stage and three stage rotors remains the same even after increasing the stage aspect ratio and the rotor aspect ratio by a factor of two and three, respectively. For the same rotor aspect ratio of 1.0, by increasing the number of stages (stage aspect ratio decreases), the performance deteriorates in terms Cp and Ct. However, at the same stage aspect ratio of 1.0 and same Reynolds number, two and three stage rotors show the same performance in terms of coefficient of power and coefficient of torque. The variation in coefficient of static torque is lower for a three stage rotor when compared with the variation of coefficient of static torque for two stage or single stage rotor. Copyright (C) 2008 , Ltd

Effects of shading and blocking in linear Fresnel reflector field

In a linear Fresnel reflector field, parallel rows of reflectors in a collector direct the incident sun rays towards a common linear receiver. Some portion of reflector aperture remains unused due to end effect, inter-row shading and blocking. In addition to these factors, cosine effect, cleanliness factor, reflectivity of reflectors, intercept factor, transmissivity of receiver cover, reflectivity of secondary reflector, absorptivity of absorber tube and thermal losses are other factors that contribute to energy losses and thus affect net energy collection by the heat transfer fluid in the absorber, electricity generation and cost of electricity. Conventionally, the collectors are oriented either along North South or East West directions in most cases. However the energy collection, electricity generation and cost of electricity need to be found out for all possible collector-orientations lying between North South and East West. This can be used in designing collectors for places where the available land strip does not align with any of these two directions. In this work, explicit analytical expressions for energy losses due to cosine effect, end effect, shading and blocking are derived for any desired time interval as functions of length (L) and width (w) of aperture of reflector-row, spacing between adjacent reflector-rows (p), number of reflector-rows in a collector (n), height of receiver (H), collector-orientation angle (52) and location. The expressions for the net energy collected by the working fluid, electricity generated by a collector and the cost of electricity are presented. The effects of L, w, p, n, H, Omega and location on energy losses, net energy collection by fluid, electricity generation and cost of electricity are studied. The minimum cost of electricity is found out for different collector-orientations at various locations and relative comparisons have been made. The corresponding collector parameters, annual energy collection by fluid and the annual electricity generation are also found out. (C) 2014 Elsevier Ltd. All rights reserved

Analytical expression for circumferential and axial distribution of absorbed flux on a bent absorber tube of solar parabolic trough concentrator

A parabolic trough has a property to concentrate the incident rays at its focal line, when tracked appropriately. The flux distribution on the absorber tube is non-uniform. Part of the absorber's periphery facing the sun receives direct incident rays where as part of the other side receives concentrated rays resulting in circumferential non-uniform flux distribution. The intensity of flux also varies along the length of the absorber tube, especially at the sun facing end, depending upon the incidence angle of the sun rays and rim angle of the parabolic cylinder. Such non-uniformity in the flux distribution on the absorber tube leads to non-uniform temperature distribution. Thus the absorber experiences thermal stresses which may lead to bending of the tube thereby creating risk of glass cover damage. In order to estimate the extent of bending, study of the flux distribution is needed. In the present work, expression for the absorbed flux on a bent absorber tube accounting circumferential and axial variations is analytically derived. Optical errors and Gaussian sun shape have also been incorporated. Results have been plotted to study the effect of bending, optical errors and rim angle of the trough on flux distribution. (C) 2013 Elsevier Ltd. All rights reserved

Optimum sizing of wind-battery systems incorporating resource uncertainty

The inherent uncertainty of the wind is a major impediment for successful implementation of wind based power generation technology A methodology has been proposed in this paper to incorporate wind speed uncertainty in sizing wind-battery system for isolated applications. The uncertainty associated with the wind speed is incorporated using chance constraint programming approach. For a pre-specified reliability requirement, a deterministic equivalent energy balance equation may be derived from the chance constraint that allows time series simulation of the entire system. This results in a generation of the entire set of feasible design options, satisfying different system level constraints, on a battery capacity vs. generator rating diagram, also known as the design space The proposed methodology highlights the tradeoffs between the wind turbine rating, rotor diameter and the battery size for a given reliability of power supply The optimum configuration is chosen on the basis of the minimum cost of energy (US$/kWh) It is shown with the help of illustrative examples that the proposed methodology is generic and flexible to incorporate alternate sub-component models (C) 2010 Elsevier Ltd

Experimental Investigation on Heat Losses From Differentially Heated Cylindrical Cavity Receiver Used in Paraboloid Concentrator

In the present study, an experimental testing facility is created to analyze the heat losses from the cylindrical solar cavity. Tests are carried out under the temperature range from 225 degrees C to 425 degrees C for a cavity inclination from theta = 0-90 deg in steps of 30 deg. It is observed that for off-flux investigation of solar cavity receiver, near isothermal wall temperature condition can be realized with the differential heating arrangement. The total loss is found to be the highest when the cavity aperture is positioned at sideways (theta = 0 deg). It decreases by 43-51% when the cavity is inclined (theta = 90 deg). The conduction loss is found to be accounted for up to 32-34% of the total heat loss, whereas the cavity radiative loss is estimated to be 13%, 16%, and 20% of the total heat loss, respectively, for cavity wall temperature 225 degrees C, 325 degrees C, and 425 degrees C. The investigation of convective losses showed significant change with cavity tilt angles. It is 46-54% of the total heat loss when the cavity aperture is facing sideways (theta = 0 deg), whereas its value reduces up to 4% of the total heat loss when the cavity aperture is facing downward (theta = 90 deg). A Nusselt number correlation has been developed for predicting the convective heat loss from a open cavity. The Nusselt number correlation correlates 100% of data within +/- 20% deviation

Design of wind power generation systems for industrial application incorporating resource uncertainty

The technical challenge in designing an onsite power generation system incorporating renewable energy technology for overall sustainability is the inherent unpredictability of the renewable resource. Integration of a battery bank as energy storage can alleviate the mismatch between the load and power generation. By accounting for the system operating and geometrical constraints, the entire set of feasible design options can be identified on a plot of battery capacity vs. rated power of wind generator. Such a diagram is known as the design space. The present study illustrates the generation of design space for different reliability levels for wind-battery systems using chance constrained programming. The proposed approach can generate and evaluate a range of possible design alternatives which can speed up the decision making process and also provide a clear understanding of the system design limitations