Scheffler Concentrator Research Articles

Parametric analysis for exergetic optimisation of a solar shell-and-tube latent heat storage unit for agricultural applications

Thermal energy storage by employing phase change materials (PCMs) has gained huge attention in many fields but despite great potential, a broader implementation in agricultural processes is yet to be achieved. Therefore, a standalone system for solar thermal energy storage using a Scheffler concentrator was developed for variety of agricultural applications. This study is aimed at the exergetic optimisation of the system by assessing the impact of various intensities of solar thermal input and flow rates on the exergetic performance and exergetic distribution inside an inversely arranged shell and tube heat storage unit. The experimental setup consisted of a cylindrical heat receiver, an inversely arranged shell and tube PCM tank and a flat plate resistive electric heater for solar irradiation simulation. The tank filled with 4 kg of RT70HC PCM was subject to various charging cycles under 2.5 LPM, 5 LPM, 7.5 LPM and 10 LPM flow rates and 450 W/m2, 575 W/m2, 700 W/m2 and 825 W/m2 of simulated solar intensities. The results indicate that increasing the mass flow rate enhances the heating uniformity within the tank but also leads to higher total exergy losses. However, the difference in the rate of exergy stored in the PCM was mostly negligible. Conversely, increasing the exergetic thermal input had a significantly positive impact on the rate of exergy stored in the PCM.. The exergetic efficiency of the PCM tank varied from 17.5% to 9.2%, with its peak value recorded at a 2.5 LPM flow rate and 1070 W of exergetic input.

Performance enhancement of Scheffler concentrator receiver using Conical type Secondary Reflector (CSR) fitted with toughened glass cover

Fixed focus type Scheffler Concentrators and convex receivers are popularly employed for several medium temperature process heat applications. The excessive convection heat loss from the receiver surface and the improper distribution of solar flux at the receiver are few key issues commonly observed for the Scheffler concentrators. The excessive heat lost is attributed to the convection losses at the receiver convex surface due to the wind. Moreover, experimental studies carried out on the receiver of 16 m2 Scheffler concentrator revealed that about 15–20 % of the total radiations received at the receiver are lost due to the focusing and tracking errors of the Scheffler reflector. In order to minimize the heat losses from the receiver convex surface, a Conical type Secondary Reflector (CSR) made up of highly polished mirror finished stainless steel material, in the form of frustum of a cone, and covered with toughened glass was designed and developed. The semi-cone angle of 36° has been estimated for the CSR based on the ray trace diagram analysis. The CSR, fixed over the periphery of the convex type receiver refocuses the lost solar radiations over the receiver surface and enhances the heat flux received at the receiver, while improving the overall efficiency of the concentrator. The toughened glass cover over the CSR, shields the receiver from the wind and thus reduces the overall convection losses due to wind. Experimental analysis carried with the installation of CSR revealed that the convective heat transfer coefficient has been reduced by 10–12 % on average while the overall system efficiency has been increased by at least 3–5 %. The increased heat flux received at the receiver has not only reduced the convection losses but also reduced the initial idle or lag time for the system by almost 15 min based on the solar insolation for the day.

Thermal analysis of a solar latent heat storage system using Scheffler concentrator for agricultural applications

Latent heat thermal storage (LHTS) using phase change materials (PCMs) has been successfully adapted in various fields but its use in agriculture has remained very limited mainly involving solar dryers and greenhouses. This study aims to develop a standalone application flexible LHTS system capable of storing and supplying solar thermal energy to various suitable agricultural processes under 100 °C. The system consisted of a 2.5 m2 laying type Scheffler concentrator, a heat receiver, a PCM tank, and an oil tank. The system was subject to various flow rates and solar irradiance to evaluate its performance as a thermal backup for milk pasteurization. During charging, the heat receiver showed a 67.9 % improvement in thermal efficiency while the PCM tank registered a 16.7 % decline against an increase in flow rate from 2.5 LPM to 15 LPM. The liquefaction rate of the PCM was noticeably influenced by the HTF inlet temperature rise as the melting time under 1.14 °C was 95 min followed by 108 min, 113 min, and 117 min against temperature rise of 1.05 °C, 0.88 °C, and 0.86 °C respectively. The total heat content of the PCM tank against various flow rates varied from 1.07 kWh to 1.14 kWh. During discharging, the rate of heat extraction was strongly influenced by the thermal gradient between the HTF and the PCM resulting in a 1.26 times higher PCM tank efficiency compared to charging. The latent heat released by the PCM was enough to theoretically pasteurize 5 kg of raw milk in a setup having 50 % heat transfer efficiency.

Design aspects of a fixed focus type Scheffler concentrator and its receiver for its utilization in thermal processing units

Fixed focus type Scheffler concentrators introduced by Wolfgang Scheffler are popularly used for different low to medium industrial process heat applications. These Scheffler concentrators are available in the standard sizes of 8 m2 and 16 m2. The present paper provides a detailed stepwise procedure for the design of a standing type 16 m2 Scheffler concentrator and its fixed focus receiver, intended to be utilized in a solar thermal processing unit for essential oil distillation at our institute. The dome type receivers of boiler steel grade material, SS-304, are selected for the present application. The diameter of the receiver is estimated based on the analysis of the image formed at the receiver of the Scheffler concentrator. The crossbar equations, seasonal parabola calculations and aperture area calculations for the Scheffler concentrator have also been presented in the paper. The geometrical concentration ratio for the Scheffler reflector dish and receiver system are also evaluated for any given day of the year. The design procedure presented in the paper can be used for designing any suitable size Scheffler concentrator and its receiver as per the energy requirement of the application.

A comprehensive review of design parameters, thermal performance assessment, and medium temperature solar thermal applications of Scheffler concentrator

Decentralized renewable energy sources such as solar thermal energy with diversified applications help to mitigate fossil fuel requirements and trigger new technology development. Fixed focus Scheffler concentrator offers thermal applications which are presently being met by conventional heaters powered by grid or steam generated using fossil fuel-fired boilers. The Scheffler concentrator developed by the German scientist Wolfgang Scheffler is attractive because of various advantages like simplicity of construction and easy access to hardware materials to build the system. Overall, it is an open-source model. This paper provides a comprehensive review of the use of such a solar concentrator. The first part describes the complete working principle, design aspects, and the tracking mechanism. The concentrator, a small lateral section of a large paraboloid, has a simple mechanical tracking system to rotate the disc in the direction of the sun’s movement. The study next assesses the application of the concentrator for medium temperature requirements up to 150–300 °C, which includes community cooking, laundry, desalination, brick & POP making, water heating, steam generation, extraction of essential oil, and even cremation, amongst many others. The performance assessment of the concentrator carried out through modeling, and experimental investigation is discussed subsequently. There are, nevertheless, several limitations of the Scheffler concentrator technology. These include vast space requirements, high cosine losses, difficulty in reaching temperatures >300 °C, single-axis tracking, and high maintenance cost. These gaps can open up new research areas for the spread of this environmentally benign technology, particularly in sunny regions of the world.

Optical Raytracing Analysis of a Scheffler Type Concentrator

The Scheffler type concentrator is a curved metal reflector particularly suitable for solar thermal systems with a receiver fixed to the ground. Its operating principle is to deform the reflector throughout the year to optimize its performance in collecting sunlight. This study analyses the optical performance of a Scheffler reflector during the year. A CAD software tool is utilized to reproduce the mechanical deformations of a real Scheffler concentrator and the shape of the light spot on the receiver is analyzed by means of raytracing simulations. The starting configuration is the equinoctial paraboloid, which produces a point-like spot on the two equinox days only. On all other days of the year, this paraboloid is deformed in a suitable way in order to keep the spot as small as possible, but, even so, it is no longer a point-like spot. In the present work the simulated light distributions on the receiver, generated by the paraboloids (deformed or original), are compared. The results confirm the working principle of the Scheffler type concentrator and allow correctly sizing the receiver.

Performance analysis of an Injera baking machine by using solar energy

The majority of Ethiopia's population, primarily women, is susceptible to smoke inhalation during cooking, and they spend the majority of their time striving to collect firewood. Biomass deforestation for cooking harms the rural household economy and contributes to national and global environmental change, such as pollution and health issues. The effort should aim to integrate the Scheffler concentrator with a thermal storage system via the secondary solar cooking reflector in order to stimulate indoor cooking while avoiding concerns associated by carbon inhalation and deforestation. As a result, the Injera bakery system powered by solar thermal energy is born. There is a primary (Scheffler), secondary (reflective), and thermal storage device incorporated, as well as a cooking (pan) mounts. Solid works software was utilized for Scheffler focus and secondary reflector modeling and simulation. The thermal retention capacity of a storage device is determined using heating and cooling curves generated by software modeling and simulation. The findings gained would aid in the development of any comfortable indoor cooking system specifically for rural areas without a grid connection, and as a result of this study, carbon inhalation and deforestation difficulties would be mitigated. Through automation, the output rate would also increase the amount and efficiency of injera. This analysis focuses on the Injera baker concept for domestic applications.

Experimental investigation on solar-powered ejector refrigeration system integrated with different concentrators.

The objective of the work is to analyse and to improve the efficiency of solar-powered ejector refrigeration system integrated with flat-plate collector and Scheffler concentrator. The Scheffler concentrator of 2.7m2 and flat-plate collector of 5m2 collecting area are coupled with the storage tank of 15l capacity. The developed system was designed for a potential replacement of conventional 1-ton room air conditioner with much reduced electrical energy consumption. The system was built based on two key subsystems namely 'Scheffler concentrator-based vapour system' and 'ejector-based cooling system'. The pilot effort showed promising results with the probability of energy-saving potential as near 70 to 80% over conventional air conditioners.

Integrated system of flat plate collector and Scheffler solar concentrator for enhancing thermal efficiency and steam generation rate

ABSTRACT Scheffler concentrators are used for heating water and producing steam. Flat plate collectors are normally used for low-temperature water heating. In this research work, the performances of a flat plate collector and a Scheffler solar concentrator were studied individually. After careful study of the results, an integrated system was developed to enhance the efficiency. Also, the experimental results were checked and validated through CFD. Flat plate collector shows higher efficiency in heating water up to 50∼60°C and hence this feature is used for pre-heating the water fed to the Scheffler concentrator. As a result, Scheffler concentrator’s thermal efficiency is improved significantly by about 6%. The steam generation rate of integrated system is also increased when compared to that of individual Scheffler concentrator. This study uses experimental works along with numerical methods to substantiate the results providing a good visualisation of the receiver’s performance.

Studies on Scheffler solar concentrator to optimise thermal efficiency

Parametric analysis of Scheffler solar concentrator system is performed for improvement in thermal efficiency. A Scheffler concentrator of size 2.7 m2 has been used for experimentation. Cylindrical and conical shape receivers each with 8-liter capacity was used for experimentation which serve a dual purpose of absorber as well as steam storage device. The thermal efficiencies with different receivers were calculated on the basis of parameters like shape of receiver, initial heating of inlet water, tilting angle of receiver and receiver with the glass cover for steam pressures up to 3 bars. Cylindrical receiver has the maximum efficiency of 56.64% in case of initial heating of water (50°C) and conical receiver has the maximum efficiency of 76.04% in case of tilting position of receiver (45°) at 1 bar. Response surface method was used to optimise the thermal efficiency of receiver. A general mathematical model for obtaining thermal efficiency has been developed for both types of receivers in terms of incident average solar radiation and mass flow rate of steam. The optimum predicted value of thermal efficiency is calculated from the general mathematical model and there is a good agreement found between the experimental and predicted results.

Thermal performance of cubical receiver with trapezoidal ice-pot for melting of ice at high altitude regions based on Scheffler solar concentrator

ABSTRACT The thermal performance of cubical receiver with a trapezoidal ice-pot has been investigated with the aim to generate hot water by melting of ice at high altitude regions (Leh Ladakh). In the experimental set-up, one hollow mild steel cubical receiver (filled with heat transfer fluid) with the ice-pot and four different types of solid cubical receivers with the ice-pot are used to melt the ice with direct heating in minimum time. The radiations are focused on receiver with the help of a 1 m2 Scheffler concentrator that is exposed to the atmospheric situations of NIT Kurukshetra. The results from the designed set-up are obtained that the frozen ice at −4°C melts completely, converting into water and the maximum temperature of the water attained in hollow cubical receiver, solid receiver (granite), solid receiver (dholpur stone), solid receiver (concrete) and solid receiver (mild steel) is 42.5°C, 32.9°C, 49.5°C, 30.2°C and 52.7°C, respectively.

Performance Analysis of Scheffler Solar Concentrator for Turmeric Blanching

Blanching is an important stage of post harvesting processing of turmeric, which destroys the viability of the fresh rhizomes and eliminates the raw odor. Large amount of crop residue and wood is consumed during conventional turmeric boiling process. Solar-based turmeric blanching system had been developed to show the viability of concentrated solar power for agricultural produce processing and contribute to CO2 mitigation. Experiments were conducted for several days in the month of April 2019, to study the performance of 16-m2 Scheffler concentrator for turmeric blanching. Turmeric was blanched in batches of 10 kg. Average 110 kg turmeric was successfully processed in a day. From the experimental data analysis, it is perceived that the thermal performance of the system directly depends on beam radiation. Average efficiency of 19.35 % was achieved with average beam radiation 721 W/m2. Minimum blanching time was found as 27 minutes with an average steam flow rate 5.30 kg/hr. Power available at reflector, receiver, and blanching vessel is presented to identify energy losses at each component of blanching system.

Technical analysis of an oven with coupled receiver for scheffler solar concentrator tested under cloudy weather conditions

Most of the solar collectors experiments are carried out under clear-sky conditions to evaluate the maximum performance of collectors, even though this condition is not critical for some uses, such as cooking. The optical and thermal performance of a solar oven heated by Scheffler concentrator is here analyzed in more adverse weather conditions. The receiver for conversion and heat transfer of the concentrated solar energy is coupled to an oven specially developed for this work. The Scheffler concentrator geometry is a lateral cut angled 43.23° of a paraboloid matrix, and it works in a two-axis tracking system, to always maintain its focal image at the stationary receiver with the progression of the Earth rotation and solar declination movements. A model for distributing the daily radiation over the hours is used to compare the results. The time-constant experimental method is considered. The heating and cooling tests were carried out at the official local time. The maximum temperature achieved by the absorber was 328°C, and the maximum average temperature in the oven was 150°C. The results for heat loss factor were evaluated, and the trends for thermal efficiency and optical efficiency factor were analyzed for the system considered

Parametric analysis of Scheffler concentrator with convex receiver for direct steam generation

In this work, Scheffler dish with improved design of the receiver is used for direct steam generation. Experiments were conducted from February to May 2019, for monthly performance assessment of 16 m2 Scheffler concentrator. Effect of varying aperture area, solar beam radiation, ambient temperature and wind speed on the thermal efficiency of the system is investigated. The average receiver temperature at focus observed to be 323°C with average beam radiation 656 W/m2. From the results, the thermal efficiency of the system is reduced by 2.36% with a decrease in aperture area from 12.82 m2 to 9.59 m2. The average thermal efficiency of the system decreased linearly from 57.29% to 39.99% with an increase in wind speed from 0.5 m/s to 6 m/s where as it reduced by 1.13% with an increase in ambient temperature from 24°C to 46°C.

Parametric analysis of Scheffler concentrator with convex receiver for direct steam generation

Parametric analysis of Scheffler concentrator with convex receiver for direct steam generation

Experimental investigation of 16 square meter Scheffler concentrator system and its performance assessments for various regions of India

An experimental investigation of 16 square meter Scheffler concentrator system with automatic polar axis tracking has been carried out. Various test sequences were performed under quasi-steady-state to analyze various parameters like optical efficiency and thermal characteristics of the system. The paper mainly focuses on the experimental analysis of 16 square meter Scheffler concentrator system and its performance assessment for various regions of India such as New Delhi, Leh-Ladhak, Madurai, and Mumbai. It is studied that, the optical efficiency of the system is achieved up to 59% and the convection heat losses (W/m2K), radiation heat losses coefficient (W/m2K2) are 2.2 and 0.001 respectively. The performance of the system was also studied based on various reflectivity (i.e. 84% and 95%) of the solar-grade mirror and various selective coating (i.e. Silicon based high-temperature coating and black paint). The performance characteristic of the system is also investigated on a seasonal basis i.e. equinox, winter and summer solstice.

Decentralized stand-alone Multi Effect Desalination (MED) system using fixed focus type Scheffler concentrator for the remote and arid rural regions of Sultanate of Oman

Rising population and industrialization made desalination of prime importance in physically water scarce Sultanate of Oman for fulfilling the gap between the rising demand and supply of fresh water. Almost 80-85% of the installed and planned desalination plants in the Sultanate are based on Combined Cycle Gas Turbine (CCGT) power plants while remaining are standalone which includes about 5-7% of the installed plants for rural arid and dry regions. All the installed and planned desalination plants utilises fossil fuels for their operation and are based on Reverse Osmosis (almost 90-95% of the plants) and Multi Stage Flash technologies. Sultanate of Oman is a tropical country with most of the regions being arid and dry receiving solar energy most abundantly. But the utilisation of solar energy in the country is mostly limited to installations based on photovoltaic systems. Only recently solar thermal Enhanced Oil Recovery plant with 1 GWth power output has been undertaken in the country at Amal oil field. A pilot standalone Multi Effect Desalination (MED) plant using fixed focus type Scheffler concentrator for remote and arid rural regions of country has been discussed in this paper to address this gap. This pilot plant has been designed for producing 100 kg/day output based on three stage cross flow type multi effect desalination technology with two 16 m2 Scheffler concentrators and operates in the temperature limits of 170 – 90°C. Preliminary testing carried out on the system during summer and winter period has shown that with available insolation above 700 W/m2, steam at a pressure of 8 to 8.5 bar could be generated in a batch experiment after 2-3 hours from starting the operation for 42 to 55 kg of water in the header of the system. This steam generated is further utilised for desalination in three stages. The initial lag period for the system is measured to be 35-50 minutes depending on the quantity of water in the header, wind speed and solar insolation. System comes out of the lag period when solar insolation reaches just above 650-700 W/m2. The desalination output for the system is measured between 60-65 litres per day for the summer period. Batch type intermittent operation, tracking errors, optical concentration losses, receiver convection losses, brine heat losses are few of the reasons observed based on the analysis for the lower output from the system. Experimentation has given a direction to make operation of the system from batch to continuous production while reducing optical and convection losses through appropriate rectifications for increasing overall yield of the system.

Brine heat recovery unit for Multi Effect Desalination (MED) system based on Scheffler Concentrator using shell and coil heat exchanger

Three stage standalone solar thermal Multi Effect Desalination (MED) pilot plant for remote and arid rural regions of Sultanate of Oman using fixed focus type Scheffler concentrator producing 100 litres per day has been set up at author’s institute. Preliminary tests carried out on this plant yielded lower outputs of 60-65 litres per day. Heat loss through high temperature brine discharged at 85°C was observed as one of the reasons for lower yield. Literature reviewed and analysis shown that desalination outputs can be increased with increased feed water inlet temperature. To recover this heat and preheat the feed water supplied, brine heat recovery unit using counter flow type shell and coil heat exchanger has been proposed in this paper. Helically coiled heat exchanger has been selected for its compactness, higher heat transfer area per unit volume, higher inside heat transfer coefficient and lower fouling rates due to induced turbulence. The operating parameters of this heat exchanger will handle hot brine at 80-85°C and 1 bar pressure. In the present work, heat load, number of turns, coil length and pressure drop calculations are done for estimating the theoretical values based on heat transfer & fluid mechanics principles using numerical correlations from the literature reviewed. The steady state analysis has shown that that coil pitch, inside tube diameter, mass flow rate through the coil and shell affects the heat transfer rates. Practical dimensional constraints have been taken into consideration while finalising the dimensions of the heat exchanger. Overall effectiveness of 84.6% has been calculated for the proposed heat exchanger.

Effect of Wind Flow on Convective Heat Losses from Scheffler Solar Concentrator Receivers

Receiver is an important element of solar concentrator system. In a Scheffler concentrator, solar rays get concentrated at focus of parabolic dish. While radiation losses are more predictable and calculable since strongly related to receiver temperature, convective looses are difficult to estimate in view of additional factors such as wind flow direction, speed, receiver geometry, prior to current work. Experimental investigation was carried out on two geometries of receiver namely cylindrical and conical with 2.7 m2 Scheffler to find optimum condition of tilt to provide best efficiency. Experimental results showed that as compared to cylindrical receiver, conical receiver gave maximum efficiency at 45° tilt angle. However effect of additional factors like wind speed, wind direction on especially convective losses could not be separately seen. The current work was undertaken to investigate further the same two geometries using computation fluid dynamics using FLUENT to compute convective losses considering all variables such at tilt angle of receiver, wind velocity and wind direction. For cylindrical receiver, directional heat transfer coefficient (HTC) is remarkably high to tilt condition meaning this geometry is critical to tilt leading to higher convective heat losses. For conical receiver, directional average HTC is remarkably less to tilt condition leading to lower convective heat loss.

Numerical and Experimental Analysis of Scheffler Concentrator Receiver for Steam Generation Using Phase Change Material

This paper proposes a design of receiver of a scheffler dish solar concentrator which uses the phase change material to store the thermal energy. For this, water is used for direct steam generation from the receiver. The proposed model consists of a phase change material (PCM) present in between the inner and outer cylinder of the copper receiver. A binary mixture commonly known as solar salt (60% NaNO 3 -40% KNO 3 ) is used as PCM for analysis. It helps to get rid of fluctuations due to sudden weather changes. Scheffler solar concentrator of 16 m 2 area with a fixed focus is used to concentrate the solar radiations to the receiver. The purpose of this study is to see the effect of PCM on steam generation and compare it with the results obtained without the use of solar salt in the receiver experimentally as well as numerically using Finite Element Method. Numerical analysis was performed using Transient thermal analysis in Ansys-16 and experimental investigations were performed. Results showed that the use of PCM in the receiver not only increases the mass flow rate of the steam but also the duration of steam generation by storing the thermal energy.