Numerical Analysis of Heat Loss From a Parabolic Trough Absorber Tube With Active Vacuum System

2011 ◽  
Author(s):  
Matthew Roesle ◽  
Volkan Coskun ◽  
Aldo Steinfeld
Keyword(s):  
Heat Loss ◽  
Power Plants ◽  
Rarefied Gas ◽  
Radiation Heat Transfer ◽  
Vacuum System ◽  
Transfer Model ◽  
Parabolic Trough ◽  
Annular Gap ◽  
Continuous Vacuum ◽  
Vacuum Jacket

In current designs of parabolic trough collectors for concentrating solar power plants, the absorber tube is manufactured in segments that are individually insulated with glass vacuum jackets. During the lifetime of a power plant, some segments lose vacuum and thereafter suffer from significant convective heat loss. An alternative to this design is to use a vacuum pump to actively maintain low pressure in a long section of absorber with a continuous vacuum jacket. A detailed thermal model of such a configuration is needed to inform design efforts for such a receiver. This paper describes a combined conduction, convection, and radiation heat transfer model for a receiver that includes the effects of nonuniform solar flux on the absorber tube and vacuum jacket as well as detailed analysis of conduction through the rarefied gas in the annular gap inside the vacuum jacket. The model is implemented in commercial CFD software coupled to a Monte Carlo ray-tracing code. The results of simulations performed for a two-dimensional cross-section of a receiver are reported for various conditions. The parameters for the model are chosen to match the current generation of parabolic trough receivers and the simulation results correspond well with experimental measurements.

Numerical Analysis of Heat Loss From a Parabolic Trough Absorber Tube With Active Vacuum System

2011 ◽  
Vol 133 (3) ◽  
Author(s):  
Matthew Roesle ◽  
Volkan Coskun ◽  
Aldo Steinfeld
Keyword(s):  
Heat Loss ◽  
Power Plants ◽  
Rarefied Gas ◽  
Radiation Heat Transfer ◽  
Vacuum System ◽  
Transfer Model ◽  
Parabolic Trough ◽  
Annular Gap ◽  
Continuous Vacuum ◽  
Vacuum Jacket

In current designs of parabolic trough collectors for concentrating solar power plants, the absorber tube is manufactured in segments that are individually insulated with glass vacuum jackets. During the lifetime of a power plant, some segments lose vacuum and thereafter suffer from significant convective heat loss. An alternative to this design is to use a vacuum pump to actively maintain low pressure in a long section of absorber with a continuous vacuum jacket. A detailed thermal model of such a configuration is needed to inform design efforts for such a receiver. This paper describes a combined conduction, convection, and radiation heat transfer model for a receiver that includes the effects of nonuniform solar flux on the absorber tube and vacuum jacket as well as detailed analysis of conduction through the rarefied gas in the annular gap inside the vacuum jacket. The model is implemented in commercial CFD software coupled to a Monte Carlo ray-tracing code. The results of simulations performed for a two-dimensional cross-section of a receiver are reported for various conditions. The parameters for the model are chosen to match the current generation of parabolic trough receivers, and the simulation results correspond well with experimental measurements.

Heat Efficiency of Trough Solar Vacuum Receiver

2014 ◽  
Vol 521 ◽  
pp. 23-27
Author(s):  
Jun Ming Liang ◽  
Jian Feng Lu ◽  
Jing Ding ◽  
Jian Ping Yang
Keyword(s):  
Heat Transfer ◽  
Solar Radiation ◽  
Heat Loss ◽  
Wall Temperature ◽  
Radiation Flux ◽  
Heat Transfer Model ◽  
Transfer Model ◽  
Parabolic Trough ◽  
Heat Efficiency ◽  
Solar Radiation Flux

The heat loss and thermal performance of solar parabolic trough vacuum receiver were experimentally measured and analyzed by heat transfer model. According to the present experiments, the heat loss of solar parabolic trough vacuum receiver has good agreement with the heat loss of vacuum receiver from Solel company. As the wall temperature increase from 108°C to 158°C, the heat loss of solar parabolic trough vacuum receiver remarkably increases from 35 Wm-2to 57 Wm-2. The heat transfer model of parabolic trough solar receiver is then theoretically investigated due to the energy balances between the heat transfer fluid, absorber tube, glass envelope and surroundings. When solar radiation flux is constant, the heat efficiency of solar parabolic trough system decreases with the wall temperature and oil temperature. When solar radiation flux or solar concentration ratio increases, the heat efficiency of solar parabolic trough system increases.

Heat Loss Experiments on a Non-Evacuated Parabolic Trough Receiver Employing a Thermally Insulating Layer in the Annular Gap

2013 ◽  
Author(s):  
Hany Al-Ansary ◽  
Obida Zeitoun
Keyword(s):  
Heat Loss ◽  
Low Cost ◽  
Power Level ◽  
Power Input ◽  
Parabolic Trough ◽  
Insulating Layer ◽  
Direct Normal Irradiance ◽  
Annular Gap ◽  
Collector Efficiency ◽  
Performance Results

Parabolic trough collectors are economically and technically attractive options for process heat applications that require temperatures in excess of 200 °C. One of the reasons is that low-cost non-evacuated receivers are used in this type of application. However, at higher temperatures, the performance of non-evacuated receivers deteriorates considerably due to excessive radiation and natural convection losses. A new idea had been preliminarily investigated by the authors both numerically and experimentally. The idea was to introduce a thermally insulating layer to the part of the receiver’s annular gap that does not receive concentrated sunlight from the parabolic mirrors, and the results had been quite promising. This paper presents additional, more extensive experiments on this concept. In these experiments, a cartridge heater is inserted along the axis of the receiver tube of a non-evacuated receiver. The heater is surrounded by a conductive material to ensure uniform heating of the receiver tube. A number of thermocouples are affixed near the inner surface of the receiver as well as on the outer surface of the glass envelope to monitor temperature uniformity. Two sets of experiments are then conducted, one with the insulating layer, and the other without. In each set, the power input is set to a certain level and the receiver temperature is measured once steady state conditions are attained. The power level is then increased, and the measurements are repeated. The heat loss values from each set are compared to determine whether adding the insulating layer enhances receiver performance. Results show that a reduction in heat loss of as much as 15% can be achieved using this design, and collector efficiency can increase by up to about 6%. However, it was also found that the extent of improvement in collector efficiency depends on the operating temperature and direct normal irradiance, with the improvement being more significant at higher temperature applications and at low direct normal irradiance.

Microcontroller Based Single Axis Intelligent Control Sun Tracker for Parabolic Trough Collectors

2012 ◽  
Vol 562-564 ◽  
pp. 1772-1775
Author(s):  
Shakeel Akram ◽  
Farhan Hameed Malik ◽  
Rui Jin Liao ◽  
Bin Liu ◽  
Tariq Nazir
Keyword(s):  
Energy Efficiency ◽  
Embedded System ◽  
Alternative Energy ◽  
Power Plants ◽  
Tracking System ◽  
Appropriate Technology ◽  
Working Fluid ◽  
The Sun ◽  
Solar Collectors ◽  
Parabolic Trough

Due to the complex design and high costs of production, solar thermal systems have fallen behind in the world of alternative energy systems. Different mechanisms are applied to increase the efficiency of the solar collectors and to reduce the cost. Solar tracking system is the most appropriate technology to increase the efficiency of solar collectors as well as solar power plants by tracking the sun timely. In order to maximize the efficiency of collectors, one needs to keep the reflecting surface of parabolic trough collectors perpendicular to the sun rays. For this purpose microcontroller based real time sun tracker is designed which is controlled by an intelligent algorithm using shadow technique. The aim of the research project is to test the solar-to-thermal energy efficiency by tracking parabolic trough collector (PTC). The energy efficiency is determined by measuring the temperature rise of working fluid as it flows through the receiver of the collector when it is properly focused. The design tracker is also simulated to check its accuracy. The main purpose to design this embedded system is to increase the efficiency and reliability of solar plants by reducing size, complexity and cost of product.

Comparison of Two Linear Collectors in Solar Thermal Plants: Parabolic Trough vs Fresnel

2011 ◽  
Author(s):  
A. Giostri ◽  
M. Binotti ◽  
P. Silva ◽  
E. Macchi ◽  
G. Manzolini
Keyword(s):  
Power Plants ◽  
Thermal Power ◽  
Solar Thermal ◽  
Heat Transfer Fluid ◽  
Steam Generation ◽  
Parabolic Trough ◽  
Research Activity ◽  
Optical Efficiency ◽  
Synthetic Oil ◽  
Yearly Average

Parabolic trough can be considered the state of the art for solar thermal power plants thanks to the almost 30 years experience gained in SEGS and, recently, Nevada Solar One plants in US and Andasol plants in Spain. One of the major issues that limits the wide diffusion of this technology is the high investment cost of the solar field and, particularly, of the solar collector. For this reason, since several years research activity has been trying to develop new solutions with the aim of cost reduction. This work compares commercial Fresnel technology with conventional parabolic trough plant based on synthetic oil as heat transfer fluid at nominal conditions and evaluates yearly average performances. In both technologies, no thermal storage system is considered. In addition, for Fresnel, a Direct Steam Generation (DSG) case is investigated. Performances are calculated by a commercial code, Thermoflex®, with dedicated component to evaluate solar plant. Results will show that, at nominal conditions, Fresnel technology have an optical efficiency of 67% which is lower than 75% of parabolic trough. Calculated net electric efficiency is about 19.25%, while parabolic trough technology achieves 23.6%. In off-design conditions, the gap between Fresnel and parabolic trough increases because the former is significantly affected by high radiation incident angles. The calculated sun-to-electric annual average efficiency for Fresnel plant is 10.2%, consequence of the average optical efficiency of 38.8%, while parabolic trough achieve an overall efficiency of 16%, with an optical one of 52.7%. An additional case with Fresnel collector and synthetic oil outlines differences among investigated cases. Finally, because part of performance difference between PT and Fresnel is simple due to different definitions, additional indexes are introduced in order to make a consistent comparison.

Design Point for Predicting Year-Round Performance of Solar Parabolic Trough Concentrator Systems

2013 ◽  
Author(s):  
Men Wirz ◽  
Matthew Roesle ◽  
Aldo Steinfeld
Keyword(s):  
Operating Conditions ◽  
Transfer Model ◽  
Parabolic Trough ◽  
Specific System ◽  
Simple Method ◽  
Operating Condition ◽  
Average Efficiency ◽  
Design Point ◽  
Yearly Average ◽  
Solar Field

Thermal efficiencies of the solar field of two different parabolic trough concentrator (PTC) systems are evaluated for a variety of operating conditions and geographical locations, using a detailed 3D heat transfer model. Results calculated at specific design points are compared to yearly average efficiencies determined using measured direct normal solar irradiance (DNI) data as well as an empirical correlation for DNI. It is shown that the most common choices of operating conditions at which solar field performance is evaluated, such as the equinox or the summer solstice, are inadequate for predicting the yearly average efficiency of the solar field. For a specific system and location, the different design point efficiencies vary significantly and differ by as much as 11.5% from the actual yearly average values. An alternative simple method is presented of determining a representative operating condition for solar fields through weighted averages of the incident solar radiation. For all tested PTC systems and locations, the efficiency of the solar field at the representative operating condition lies within 0.3% of the yearly average efficiency. Thus, with this procedure, it is possible to accurately predict year-round performance of PTC systems using a single design point, while saving computational effort. The importance of the design point is illustrated by an optimization study of the absorber tube diameter, where different choices of operating conditions result in different predicted optimum absorber diameters.

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