Predicting the effects of sand erosion on collector surfaces in CSP plants

This paper presents a methodology to predict the optical performance and physical topography of the glass collector surfaces of any given CSP plant in the presence of sand and dust storms, providing that local climate conditions are known and representative sand and dust particles samples are available. Using existing meteorological data for a defined CSP plant in Egypt, plus sand and dust samples from two desert locations in Libya, we describe how to derive air speed, duration, and sand concentrations to use within the Global CSP Laboratory sand erosion simulation rig at Cranfield University. This then allows us to predict the optical performance of parabolic trough collector glass after an extended period by the use of accelerated ageing. However the behavior of particles in sandstorms is complex and has prompted a theoretical analysis of sand particle dynamics which is also described in this paper

Photogrammetry for concentrating solar collector form measurement, validated using a coordinate measuring machine

Concentrating solar power systems currently have a high capital cost when compared with other energy generating systems. The solar energy is captured in the form of thermal energy rather than direct electrical, which is attractive as thermal energy is more straightforward and currently more cost-effective to store in the amounts required for extended plant operation. It is also used directly as industrial process heat, including desalination and water purification. For the technology to compete against other generating systems, it is crucial to reduce the electrical energy cost to less than $0.10 per kilowatt-hour. One of the significant capital costs is the solar field, which contains the concentrators. Novel constructions and improvements to the durability and lifetime of the concentrators are required to reduce the cost of this field. This paper describes the development and validation of an inexpensive, highly portable photogrammetry technique, which has been used to measure the shape of large mirror facets for solar collectors. The accuracy of the technique has been validated to show a whole surface measurement capability of better than 100 mm using a large coordinate measuring machine. Qualification of facets of the MATS plant was performed during its installation phase, giving results of the shape, slope and intercept errors over each facet

Numerical simulation and design of multi-tower concentrated solar power fields

In power tower systems, the heliostat field is one of the essential subsystems in the plant due to its significant contribution to the plant’s overall power losses and total plant investment cost. The design and optimization of the heliostat field is hence an active area of research, with new field improvement processes and configurations being actively investigated. In this paper, a different configuration of a multi-tower field is explored. This involves adding an auxiliary tower to the field of a conventional power tower Concentrated Solar Power (CSP) system. The choice of the position of the auxiliary tower was based on the region in the field which has the least effective reflecting heliostats. The multi-tower configuration was initially applied to a 50MWth conventional field in the case study region of Nigeria. The results from an optimized field show a marked increase in the annual thermal energy output and mean annual efficiency of the field. The biggest improvement in the optical efficiency loss factors be seen from the cosine, which records an improvement of 6.63%. Due to the size of the field, a minimal increment of 3020 MWht in the Levelized Cost of Heat (LCOH) was, however, recorded. In much larger fields, though, a higher number of weaker heliostats were witnessed in the field. The auxiliary tower in the field provides an alternate aim point for the weaker heliostat, thereby considerably cutting down on some optical losses, which in turn gives rise to higher energy output. At 400MWth, the multi-tower field configuration provides a lower LCOH than the single conventional power tower field

Theoretical and experimental analysis of an innovative dual-axis tracking linear Fresnel lenses concentrated solar thermal collector

Linear concentrating solar thermal systems offer a promising method for harvesting solar energy. In this paper, a model for a novel linear Fresnel lens collector with dual-axis tracking capability is presented. The main objective is to determine the performance curve of this technology by means of both experiment and theoretical analysis. A mathematical model including the optical model of the concentrator and the heat transfer model of the receiver pipe was developed. This tool was validated with experimental data collected using a proof of concept prototype installed in Bourne, UK. The performance curve of the collector was derived for temperatures between 40 °C and 90 °C. The results show that the global efficiency of the collector is limited to less than 20%. The energy losses have been analysed. The optical losses in the lens system accounts for 47% of the total energy dissipated. These are due to absorption, reflection and diffraction in the Fresnel lenses. Furthermore manufacturing error in the lens fabrication has to be considered. One third of the solar radiation collected is lost due to the low solar absorptance of the receiver pipe. Thermal radiation and convection accounts for 6% of the total as relatively low temperatures (up to 90 °C) are involved. In order to increase the performance of the system, it is recommended to install an evacuated receiver and to insulate the recirculation system. Considering data from manufacturers, these improvements could increase the global efficiency up to 55%. Utilising the results from this work, there is the intention of building an improved version of this prototype and to conduct further tests

Airborne sand and dust soiling of solar collecting mirrors

The reflectance of solar collecting mirrors can be significantly reduced by sand and dust soiling, particularly in arid environments. Larger airborne sand and dust particles can also cause damage by erosion, again reducing reflectance. This work describes investigations of the airborne particle size, shape, and composition in three arid locations that are considered suitable for CSP plants, namely in Iran, Libya, and Algeria. Sand and dust has been collected at heights between 0.5 to 2.0m by a variety of techniques, but are shown not to be representative of the particle size found either in ground dust and sand, or on the solar collecting mirror facets themselves. The possible reasons for this are proposed, most notably that larger particles may rebound from the mirror surface. The implications for mirror cleaning and collector facet erosion are discusse

Reflectometer comparison for assessment of back-silvered glass solar mirrors

This paper compares the two most common reflectometers used to assess the specular reflectance of back-silvered glass mirrors for Concentrating Solar Power (CSP) applications, namely the Device and Services (D&S) 15R-USB and the Abengoa Condor SR-6.1 instruments. Comparisons are first made between the two instruments themselves using a Gage Repeatability and Reproducibility (R&R) study. Results are given for the as-cleaned collector mirrors and then as the mirrors become naturally soiled over a one month period. The results of the Gage R&R study show that for the D&S the gage itself contributes 40.97% of the variability, whilst 59.03% is due to part-to-part (location on the mirror under investigation) variability. For the Condor we show that the % Contribution from the gage is 62.18% of the total variability with only 37.82% of the contribution attributable to the location dependent reflectance. The Condor has a wider acceptance angle, and over the reflectance range of 0.91–0.95 the condor was found to measure higher than the D&S by an average of 1.53%. The differences between the soiling results obtained from the two instruments are explained, and the results are used to derive a predictive model for the soiling of solar collectors. In conclusion, both instruments have advantages and shortcomings, and the factors that influence which instrument to select are discussed

Statistical and economic analysis of solar radiation and climatic data for the development of solar PV system in Nigeria

The growth in energy demand and global concern about the environment has resulted in the drive towards alternative energy sources and consequently this research concerning solar energy harvesting of radiation received at the earth’s surface. Analysis of solar radiation data is an important tool in the accurate designing/sizing of solar Photovoltaic (PV) systems and conducting performance analysis of the system. This paper presents the statistical and economic analysis of solar radiation and climatic data for the development of Solar PV systems in Nigeria. The data for three locations, one from each of the radiation regions in Nigeria were analysed using Minitab 17. The analysis shows that Maiduguri is more viable for solar energy conversion system than the two other locations, and for the same energy demand of 1.1MWh, the peak watt (Wp) of solar PV array required are 619419.27 Wp, 821142.52 Wp and 1219489.32 Wp for Maiduguri, Minna and Port Harcourt respectively. This leads to a difference of $460,984.72 in the total project cost between Maiduguri and Minna for the same energy demand

The design of dust barriers to reduce collector mirror soiling in CSP plants

In this work we investigate, design, and evaluate a number of dust barrier designs that would be appropriate to reduce soiling of glass mirror solar collectors in the solar field of an existing CSP plant. The principal objective was to reduce the amount of soiling (and hence the amount of cleaning water consumed) by 50% in comparison with current cleaning procedures (considering particles of size >25 µm). “Fluent” CFD software was used to model of a range of potential dust barrier shapes, sizes, and porosities. Airflows and wind loadings were analyzed in this way. A number of potential designs were then taken forward for experimental validation. Initial validation involved wind tunnel evaluation of a small number of potential designs, using a new wind tunnel specifically designed and built for this project. Larger-scale outdoor validation was carried out both at Cranfield University in the UK and at CIEMAT-PSA (Plataforma Solar de Almeria) in Spain. Initial results were independent of location and barrier shape and showed that the percentage of particles that were stopped completely or travelled less than 1m beyond the barrier was in the range 45.8 ± 5%

Enhancing thermoelectric properties of NaCo2O4 ceramics through Na pre-treatment induced nano-decoration

High quality NaCo2O4 thermoelectrics are challenging to process due to the volatile nature of Na, the slow densification kinetics, and degradation of NaCo2O4 above 900–950 °C leading to the formation of Na-poor second phases. Fine grained sol-gel derived powders have been used to enhance the densification kinetics while pre-treatment of the NaCo2O4 powder with NaOH, to provide a Na rich environment, has been shown to mitigate Na loss at elevated temperatures. While insufficient to compensate for Na loss at processing temperatures of 1000 °C and above, at lower temperatures it is able to enhance densification and facilitate the formation of complex crystal structures yielding low thermal conductivity (0.66 Wm−1K−1) coupled with high electrical conductivity (3.8 × 103 Sm−1) and a Seebeck coefficient of 34.9. The resultant room temperature power factor and ZT were 6.19 × 10−6 Wm−1K−2 and 0.0026, respectively

Equipment and methods for measuring reflectance of concentrating solar reflector materials

The proper optical characterization of solar reflector materials is a challenging task. Although several commercial instruments exist to measure reflectance, they have been developed for other applications and often do not meet all the specific requirements demanded by the solar thermal industry. In particular, the characterization of solar reflectors involve the complete solar spectral wavelength range, an incidence angle range from near normal to 70° and most importantly a very narrow acceptance angle range from near specular to 20 mrad. The accurate measurement of reflectance as a function of all the previously mentioned parameters has not been commercially implemented. This paper reviews the different alternatives to measure reflector materials, describes reflectance models used to approximate the missing information and presents current research work on prototype reflectometers to fill the gap