Revisión de modelos y procedimientos para la medida de la reflectancia en plantas CSP

Equation Chapter 1 Section 1 Trabajo Fin de Grado Grado en Ingeniería Química Review of models and procedures for the measurement of reflectance in CSP plants Autor: Alejandro Álvaro Llorente Carrión Tutores: Manuel Silva Pérez, Dr. Aránzazu Fernández García Departamento de Ingeniería Energética Escuela Técnica Superior de Ingeniería Universidad de Sevilla Sevilla, 2020 ii Trabajo Fin de Grado Grado en Ingeniería Química Revisión de modelos y procedimientos para la medida de la reflectancia en plantas CSP Autor: Alejandro Álvaro Llorente Carrión Tutor: Manuel Silva Pérez Profesor titular Dr. Aránzazu Fernández García CIEMAT – Plataforma Solar de Almería Dpto. de Ingeniería Energética Escuela Técnica Superior de Ingeniería Universidad de Sevilla Sevilla, 2020 iii Trabajo Fin de Grado: Review of models and procedures for the measurement of reflectance in CSP plants Autor: Alejandro Álvaro Llorente Carrión Tutores: Manuel Silva Pérez Dr. Aránzazu Fernández García El tribunal nombrado para juzgar el Trabajo Fin de Grado arriba indicado, compuesto por los siguientes miembros: Presidente: Vocales: Secretario: Acuerdan otorgarle la calificación de: Sevilla, 2020 El Secretario del Tribunal iv A mi Familia A mi Gente A mis compañeros de este tan largo y divertido viaje A ti v Agradecimientos En primer lugar, quisiera agradecer a los tutores de este trabajo de fin de grado, D. Manuel Silva Pérez y Dña. Arantxa Fernández García su tiempo, predisposición y confianza depositada en mi persona desde el primer momento. Sin apenas conocerme de nada, me han dado total libertad para la confección de este trabajo. Por circunstancias personales, el desarrollo de este se tuvo que dejar a mitad de camino, y se retomó dos años después con todo el apoyo de ambos tutores. Personalmente, situaciones como esta dejan entrever la calidad profesional y humana de las personas. Mis más sinceras gracias. Ha sido un largo camino, desde el año 2003, en el que empiezo la licenciatura en Ingeniería Química, hasta el año 2020 en el que se culmina el camino, muchas personas, parejas, experiencias, trabajos, prácticas, profesores y proyectos han pasado por delante de mí. Tantos compañeros de estudio he visto pasar, siempre me quedo con lo mejor de haber estudiado con personas de mi generación y de 10 años atrás, una experiencia muy divertida y enriquecedora. No es mi carrera académica un buen ejemplo ni tampoco sabría decir cómo he podido (y puedo actualmente) desarrollar una carrera profesional completa sin haber cerrado un estudio superior, sobre todo en España. Si algo puedo sacar en claro después de tanto tiempo, es que mi decisión de irme fuera de España, a través de la beca Erasmus, y aprovecharla lo máximo me proporcionó herramientas que fueron y siguen siendo muy necesarias para el ingeniero de hoy día, los idiomas, la empatía hacia culturas diferentes y la curiosidad por lo desconocido. Mi agradecimiento a todas las personas que me apoyaron siempre para poder seguir mi camino académico tan desviado del itinerario estándar. Mis padres, que sin ellos ninguna aventura extranjera o no extranjera hubiera sido posible. Que tan importante es tener una familia que apoye todas tus ideas, aunque vayan contracorriente. A mis amigos, que siempre estuvieron ahí en las buenas y en las malas en todos los momentos vividos. Mi agradecimiento obviamente hacia todos mis mentores, grandes olvidados en el desarrollo de los profesionales de hoy día y que yo tuve la suerte de tener, sin ellos no sería el ingeniero que me considero que soy hoy, además de mucha mejor persona, mentores a los que tanto les agradeceré siempre, a Luis, Pablo, Inma, Manuel, Jordi, Jorge, Gema, Mónica, Matthias, Marion, Pachi. Siempre he tenido la suerte de poder elegir un trabajo en el que las personas que me rodean son excepcionales. Y mi agradecimiento a mi mujer, Belén, que justo nos conocimos empezando yo este trabajo fin de grado y justo ahora que lo termino, somos una familia. Que gran paciencia y confianza ciega deposita ella en mí, nunca habrá alguien más agradecido que yo de tenerla en mi vida, una persona que no se cansa de luchar nunca, cuanta gente así hace falta en los tiempos que vivimos. Sin más, muchas gracias a todos los que me habéis acompañado de un modo u otro. Se os quiere, vi Alejandro Llorente Carrión Sevilla, 2020 vii Resumen El objetivo de este trabajo fin de grado es resumir los procedimientos, métodos y equipos de última generación que se utilizan hoy en día para medir la reflectancia de un espejo en un campo solar de una planta de generación eléctrica. Aunque esa parte sólo cubre aproximadamente el 20% del trabajo, creo firmemente que, para poder hacer un resumen de un estado del arte, es necesario también revisar las bases de este. Por esa razón, el trabajo de investigación ha sido extenso y trata de cubrir la mayoría de los documentos e informes publicados desde la década de 1970 hasta el momento de publicación de este. Se han revisado casi 80 referencias y tal vez algunas de las conclusiones y declaraciones del trabajo podrían entrar en conflicto con el paradigma actual. Este trabajo fin de grado está dirigido a los operadores de plantas que deseen tener una comprensión más profunda de todos los aspectos relacionados con la medición de la reflectancia de los espejos en un campo solar. El trabajo de investigación ha sido extenso y trata de cubrir la mayoría de los documentos e informes publicados desde la década de 1970 hasta el momento de la redacción. Además, los fabricantes de reflectores y sus nuevos productos han sido revisados en esta tesis para dar una visión actual del estado del arte de esta tecnología. Tuve la oportunidad de trabajar fabricando absorbedores solares para plantas con colectores cilindro-parabólicos en una empresa alemana desde 2009 hasta 2015. Por esa razón, tengo un profundo conocimiento de la tecnología de la energía solar de concentración y mi experiencia en un entorno de fabricación intenta dar al trabajo una visión práctica de cómo implementar correctamente la medición de la reflectancia en una planta de energía solar de concentración. Esta tesis de licenciatura ha sido escrita por un novato para principiantes en esta materia con la intención de aprender (yo primero) más sobre cómo la radiación solar llega a la Tierra, cómo se puede "cultivar" para generar electricidad y cómo todos estos pequeños defectos en los espejos, en el colector, en los cimientos, en los sensores, etc. influyen en el rendimiento de las plantas de energía solar térmica de concentración. Este trabajo está organizado de la siguiente manera. En primer lugar, se presentan y explican los principales conceptos y terminología relacionados con la radiación solar y la medición de la reflectancia. A continuación, se ofrece una descripción detallada de los espejos utilizados en las tecnologías solares de concentración, junto con los principales factores que afectan a la pérdida de reflectancia en un espejo. A continuación, se presentarán las técnicas y dispositivos más comunes utilizados para medir la reflectancia solar tanto en el laboratorio como en el campo solar. En ese capítulo, también se centrará en cómo calcular la reflectancia solar media de un campo solar. viii Como conclusión, se da una lista de mejores prácticas y de la documentación de referencia para medir correctamente la reflectancia de un espejo en una planta solar concentrada. Este trabajo fin de grado está escrito en inglés en su totalidad y ha sido terminado en diciembre de 2020. ix Abstract The goal of this Bachelor’s thesis is to resume the state-of-the-art procedures, methods, and equipment used nowadays to measure the reflectance of a mirror in a solar field. Although that part only covers roughly 20% of the work, I strongly think that for giving a picture of how it is best to do something, lying the foundations is very important. For that reason, the research work has been extensive and tries to cover most of the papers and reports published from the 1970s to the time of writing. Around 90 references have been revised and maybe some of the conclusions and statements in the work could conflict with the current paradigm. I have tried to state in the work the most updated research and developments. This Bachelor’s thesis is addressed to plant operators who are willing to have a deeper understanding of all aspects related to the measurement of the reflectance in a solar field. Besides, the manufacturers of reflectometers and their new products have been reviewed in this thesis to give a current vision of the state of the art of this technology. I have been manufacturing solar absorbers for parabolic troughs in a German company from 2009 to 2015. For that reason, I have a piece of deep knowledge in Concentrating Solar Power technology and my background in a manufacturing environment gives the thesis a practical vision of how to correctly implement the measurement of reflectance in a concentrated solar energy plant. This Bachelor’s thesis has been written by a newbie for newbies on such matter with the intention of learning (myself first) more about how the solar radiation reaches the Earth, how it can be “farmed” to generate electricity, and how all these tiny littles defects on the mirrors, on the collector, on the foundation, on the sensors, etc. influence the performance of the Solar Thermal Energy plants. This work is organized as follows. First, the main concepts and terminology related to the reflectance measurement are presented and explained. Next, a detailed description of the mirrors used in concentrated solar technologies is given together along with the main factors affecting the loss of reflectance in a mirror. Following that, it will be presented the most common techniques and devices used to measure solar reflectance at the laboratory as well as at the solar field. In that chapter, it will be also focused on how to calculate the mean solar reflectance of a solar field. As a conclusion, a list of best practices and state-of-the-art references are given for measuring properly the reflectance of a mirror in a concentrated solar plant. This Bachelor’s thesis has been redacted in English and it has been finished in December 2020. xvi 17 1 INTRODUCTION irst model for prediction of the oil production peak was published by Marion King Hubbert in 1956 [1]. After almost 65 years after the publication of Hubbert’s work, the fact that the peak of the world oil production has already happened is a proven fact, supported by several publications and public data [2]. One of the main facts that support the peak oil theory is that the expected investment in fossil fuel energy in 2020 in comparison with 2019 is going to be about 18% less, according to the last World Energy Outlook 2019 [3] and if compared with 2014 data from the same agency, the drop in investment is close to 50%. Figure 1-1 World Energy Outlook 2019, IEA, Key estimated energy demand F The violence that exists in the human heart is also manifest in the symptoms of illness that we see in the Earth, the water, the air and in living things - Pope Francisco - Introduction 18 18 Therefore, the race to replace fossil technology with greener ones has taken a large share of global infrastructure and R&D investment in the last years. One of the main participants in this race is solar technology. Solar energy is called to be a substitute for fossil fuels in the next years. More energy from sunlight strikes the Earth in 1 h (6.3×1020 J) than all the energy consumed on the planet in a year (4.727×1020 J in 2008 [4]). Although around 50% of this energy reaches the Earth’s surface [5], it is only technological constraints that prevent us from exploiting its full potential. Among this technology, Concentrated Solar Power (CSP) plays a big role in this change. Despite annual CSP capacity addition in the world in 2019 is only 0.60% of the total PV capacity addition [6], in the last 9 years, the concentrated solar technology has reduced its Levelized Cost of Electricity (LCOE) by almost a 50% according to the International Renewable Energy Agency (IRENA) last published data. (From an average 0.3460 $/kWh in 2010 to 0.1820 $/kWh in 2019. Lazard asset management firm established the 2020 CSP averaged LCOE in the range 0.126 - 0.156 $/kWh 1 2 ). See Figure 1-2. 1 Data in the IRENA Auction and PPA Database shows a weighted-average price of electricity of 0.075 $/kWh for CSP projects to be commissioned in 2021. This represents a reduction of 59% when compared to the global weighted-average project LCOE in 2019. https://www.irena.org/Statistics/View-Data-by-Topic/Costs/Global-LCOE-and-Auction-values 2 Morocco’s 800 MW CSP-PV Noor Midelt breaks last year’s auction price record of 7.3 cents set by DEWA in the UAE, with winning bid at USD 7 cents/kWh. So, the LCOE of such CSP installations is less than the last averaged range 0.126 - 0.156 $/kWh stated by Lazard’s. Source: https://www.solarpaces.org/morocco-breaks-new-record-with-800-mw-midelt-1-csp-pv-at-7-cents 19 19 Review of models and procedures for the measurement of reflectance in CSP plants Figure 1-2 IRENA 2020, CSP, Renewable Power Generation Costs in 2019 The CSP technology offers a better Capacity Factor than Solar PV, 45% versus 18% of the latter [7] [8], with only 1% of the global installed capacity concerning solar PV [9]. Then offering a more stable and reliable energy generation, making this type of technology like that generated by fossil fuels, dispatchable, and predictable. It is common to compare generation technologies using as Key Performance Indicators (KPI) the LCOE, or the capital costs [$-€/MW], but the dispatchability and predictability of a generation technology are far more important in countries like Spain, where the generation curve is not flat but has two pikes during the day, at dawn and evening. In such countries, fast-ramping generation technologies like the CSP with energy storage gives this technology an advantage against its greener competitors, solar PV, and wind. While CSP technology is still expensive in capital costs and operating costs than the solar PV or wind technology, when talking to system level, the value of the overall investment shall be considered: dispatchability, accountability, components performance degradation, grid stability, distributed vs centralized generation or maturity of the technology are, among other important factors, indicators to better assess the suitability and value of renewable generation technology. Nor the power installed, or the energy generated by different sources have the same value in Spain or Germany, or California, it is dependent on the grid and electrical system in which this energy is feeding in. Two terms describe better the value of the generated electricity: operational value mean and capacity value mean. The operational value represents the avoided costs of conventional generation at their respective dispatching times along with related ancillary services costs, such as spinning reserve, etc. Savings on emission costs are also accounted for. Capacity value reflects the ability to avoid the costs of building new conventional generation in Introduction 20 20 response to growing energy demands or plant retirements. One of the most accurate studies about the value of CSP was done by NREL [10] demonstrating that the value of CSP plants to the electrical system in California is double that of PV in a scenario of 33% penetration of renewables when operational and capacity value for the system of new plants are duly considered. The value will be triple in a 40% renewable penetration scenario. As an example of an advantage of having connected a CSP plant into the main grid, they can store energy in Thermal Energy Systems (TES), feeding this stored energy to the main grid when the electricity is needed due to demand. CSP plant developers were the pioneers to increase the dispatchability of such Solar Thermal Energy (STE) plants. Beginning 2000s, Battery Energy Storage System (BESS) were still too expensive for use in large scale utility plants (CAPEX BESS 1500 $/kWh vs TES 50 $/kWh), so CSP contractors industrialize the energy storage using molten salt as a medium in where to save process heat. Using solar energy as a resource, the CSP plants were the only ones able to produce energy once the sun had gone down, saving heat in molten salt tanks in the daylight and using it during the night to maintain nominal steam temperatures for at least 7.5 hours, as it is in the Andasol Solar Power Station, first parabolic trough power plant in Europe [11]. In any solar concentrating technology, from the most maturity ones like parabolic-trough collectors (PTC) or solar tower (ST) with molten salt TES to the newest concepts like fallen particle receivers, molten salt direct storage, or solar fuels, mirrors are present to focus the direct solar radiation into a receiver and increase the solar flux. In a typical solar power plant of 50 MWe like the PTC plant of Andasol in Spain, mirrors are responsible for almost 8% of the total investment in a concentrated solar power plant. The solar field itself represents the bigger part of the CAPEX in a PTC or ST plant, see Table 1-1. Table 1-1 CSP plant cost breakdown, 2019 (Crespo, 2021) 150MW with 9h Storage Parabolic-Trough (%) Solar Tower (%) Solar field 42 30 Tower and receiver 12 HTF system 10 7 Storage system 13 10 Power block 17 19 Control system 4 5 Electrical system 5 6 Auxiliary systems 5 5 21 21 Review of models and procedures for the measurement of reflectance in CSP plants 150MW with 9h Storage Parabolic-Trough (%) Solar Tower (%) Site and project development 4 6 As stated before, for this technology to match costs and benefits with the other greener competitors even with the fossil fuel ones, it needs to increase efficiency and reduce the technology costs, specifically mirrors. For that reason, having well-proven protocols, accurate plant models and reliable measurement equipment is critical for reaching a low OPEX in such STE plants. The key functionality of a mirror is to reflect the light as specular and efficient as possible to the solar collector. From the point of view of the operator, to measure the performance of a solar field, a standard procedure that minimizes the time consumed in such tasks will help optimizing plant costs. Such a procedure must reduce uncertainty and complexity in characterizing a mirror reflecting beam and help to predict its behaviour over time. On the other hand, portable or in real-time measurement devices must be developed in a cost-efficient way to make state-of-the-art technology to be in use in the field. Therefore, to cut capital and operational costs of a solar field, developing operational-proven characterizing standards and accurate measuring devices will help to better assess the performance of a mirror and will give the operator information valuable information about its cleanliness, aging status, or structural condition. In this Bachelor thesis, both procedures and equipment that can measure reflectance are discussed, to get an overview of the differences and challenges that exist between the laboratory and the field. As a simple approach, a 5 Ws and How (5W1H) problem-solving procedure could be used to help a CSP operator in the task of measuring the mean reflectance of a solar field. Who and Why can be neglected from the equation, but the other whys and how questions must be answered to make better decisions for the optimal operations of the plant: What properties must be measured? When must they be measured? Which mirror or location of the mirror must be measured and most important, how must they be measured? This thesis will try to answer these questions or at least, will point to the reference in which to find them. Studies about the characterization of the solar reflectance spectrum date back to 1976 with Richard B. Pettit and its work about “Characterization of the reflected solar beam profile of solar mirror materials” [12]. The work done by Pettit et.al. is still valid today and should be taken into consideration by any plant operator because it found the influence of dust over the mirror surface and the scattering producing by it [13]. The knowledge of reflectance measurement and all its involved factors has been always a matter of study since the 1980s. However, there are still challenges about how to measure the solar field mean reflectance with low uncertainty and without the need to measure it locally on the field by an operator. There are international standards and guidelines establishing the state-of-the-art procedures to evaluate the reflectance properties of a reflective material such as the published from the Task III group within the Solar PACES organization [14] or the UNE Standard 206016:2018. [15]. Introduction 22 22 The SolarPACES, founded in 1977, is an “international network of researchers, under the IEA Technology Collaboration Programme, leading the technology development, market deployment and energy partnerships for sustainable, reliable, efficient and cost-competitive concentrating solar technologies” [16]. Currently, as far as concentrating solar power technology is concerned, the SolarPACES task groups are the reference for developments and research activities related to concentrating solar power. To this work, the SolarPACES task III: Solar Technology and Advanced Applications is the reference. The mirror reflectance could be influenced by many factors, many of them not measurable by a single piece of equipment. The data calculated by a device must be treated accordingly to the case of the study (mirror type, concentrator, site location, etc). Due to the variety of locations of concentrated solar energy plants worldwide, it might be that a standard or procedure valid for a plant located in Spain could not be valid or give inaccurate information for a plant located in Morocco [17]. This is true because the soiling type and rate are different and, consequently, the frequency and number of measurements required could vary [18]. Thus, the information needed to obtain a reliable measure of the mirror reflectance value could be greater or lesser depending on where the solar field is located on the Earth. This thesis will collect most of the current procedures on how to calculate the solar mean reflectance of a solar field in a CSP plant and the most common and last measurement devices used in present plants. 23 2 CONCENTRATED SOLAR POWER. WORLDWIDE AND SPANISH CURRENT STATUS Concentrating solar power (CSP) systems use combinations of mirrors or lenses to concentrate direct beam solar radiation to produce forms of useful energy such as heat, electricity, or fuels by various downstream technologies. Within the concentrating solar power plants, there are several categories to describe them, as can be seen in Table 2-1: Table 2-1 Concentrating Solar Power Plants Categories Based on the heat transfer fluid: • Water • Air • Oil • Molten salt • Dispersed particles Based on the geometrical focus: • line focus (1D) • point focus (0D) Based on the technology • Linear Fresnel Reflector • Parabolic-Trough Collector • Solar Tower • Parabolic Dish • Solar Furnace Based on its thermodynamic cycle: • Stirling • Rankine • Brayton Based on the receiver type • Fixed • Mobile Based on how the reflector concentrates the reflected light: • Imaging concentrator • Non-Imaging concentrator Despite the number of different technologies and classifications, the PTC with oil as heat transfer fluid is the most used by far for utility-scale generation plants [19], see Table 2-2. Concentrated solar power. worldwide and spanish current status 24 24 Table 2-2 World CSP installed electrical capacity (Protermosolar, 2020) Type of Technology World CSP Installed Capacity (%) World CSP Installed Capacity (MWe) PTC 74.80% 4604.1 ST-Steam 9.41% 579.01 ST-Molten Salt 8.12% 499.9 PTC-ISCC 2.88% 177 LFR 2.28% 140.4 PTC-Molten Salt 1.71% 105 LFR-Molten Salt 0.81% 50 In the coming future, Central Receiver Systems (tower systems mostly) seems to be taking the advantage as the leading technology in this field, as its capacity under development for new projects double the developed for PTCs, see Figure 2-1. Figure 2-1 Global Future CSP Capacity: PT vs CRS (Protermosolar, 2020 3 ) At the beginning of 2020, there were 6155 MWe installed in the world, most of them installed in Spain (2304 MWe) and the USA (1694 MWe). However, the economic crisis of 2012 in Spain brought the investment in solar thermal energy plants to an abrupt halt. Feed-in-tariff incentives were withdrawn for this kind of technology and the installation of new renewable capacity stopped. No STE plants have been connected to the grid since 2013 due to this situation. 2019 CSP quota concerning all renewable energies in Spanish installed capacity is 3 Dataset from the website https://www.protermosolar.com/proyectos-termosolares/proyectos-en-el-exterior. Recently the SolarPACES organization has published a database with metadata of concentrating solar power plants of the world for energy modellers and analysts, https://csp.guru/ 814 225 410 510 0100 200 300 400 500 600 700 800 900 Under Construction Under Development Capacity (MW) Global Future CSP Capacity: PTC vs ST ST PTC 25 25 Review of models and procedures for the measurement of reflectance in CSP plants 5.3 %. Figure 2-2 Cumulative renewables installed capacity in Spain from 2010-2019 (REE, 2020) Figure 2-3 Renewable installed capacity distribution in 2019 (REE, 2020) CSP market for new installations has moved to Morocco, South Africa, and Saudi Arabia mostly, with China and Chile participating actively. China and Chile will be the only countries to connect CSP STE plants to their grids in 2020 (Protermoslar, NREL), see Figure 2-4. 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 RE Rest 782 820 886 972 948 1.054 872 1.044 1.048 1.248 Solar Thermoelectric 532 999 1.950 2.299 2.299 2.304 2.304 2.304 2.304 2.304 Solar PV 3.829 4.233 4.532 4.638 4.646 4.678 4.683 4.685 4.712 8.913 Wind 18.861 19.707 21.167 22.758 23.009 23.028 23.004 23.050 23.130 25.799 Hydro 14.656 14.668 14.887 14.890 14.897 17.026 17.030 17.028 17.046 17.085 0 10.000 20.000 30.000 40.000 50.000 60.000 Installed Capacity [MWe] Hydro Wind Solar PV Solar Thermoelectric RE Rest Hydro 25,2% Wind 55,4% Solar PV 9,4% Solar Thermoelectric 5,3% RE Others 3,7% Hydro+Wrind 0,02% RE residue 0,9% Solar reflectance. theoretical background 32 32 Figure 3-4 Zenith angle definition Figure 3-5 Air Mass examples (Oriel Corporation, 1994) The sunlight passing throughout the atmosphere is attenuated by aerosol scattering and by absorption caused by aerosols and gasses such as CO2, H2O (attenuates in the IR-range), and O3 (attenuates in the UV-range) mainly, see Table 3-1. Table 3-1 Absorption and scattering for typical clear-sky conditions (Vignola et al., 2012) Factor % absorbed % scattered % of total passing through the atmosphere Ozone 2 0 Water vapour 8 4 Dry air 2 7 Aerosol 2 3 Total not absorbed or scattered ≈ 87 ≈ 87 ≈ 76 The attenuation of solar radiation by the atmosphere is not the same for all wavelengths, hence the importance 33 33 Review of models and procedures for the measurement of reflectance in CSP plants of using the correct AM spectrum when calculating reflectance using a solar spectral irradiance standard like the ASTM G173-03 (2020) [25]. For applications using direct solar radiation, the AM 1.5D spectrum is used to find the specular solar reflectance, which is the solar radiation solar concentrators used in power generation. 3.1.3.1 Clearness Index (KT) To identify the feasibility of solar energy utilization and predict the solar average irradiation there are several parameters in the bibliography to be taken into account, but the clearness index (see equation (3-4)) gives a fair estimation about the relationship between terrestrial and extraterrestrial insolation over a horizontal surface: 𝐾𝑇=𝐻ℎ 𝐻𝑜 (3-4) Where Hh is the horizontal global irradiation and Ho is the extraterrestrial irradiation on a horizontal surface. The clearness index can also be defined using irradiance instead of irradiation. On cloudy days KT may be as low as 0.05 - 0.1 while on clear days it is in the range of 0.7 - 0.8. Monthly averages of KT range from 0.4 for very cloudy climates to 0.7 for very sunny climates. As shown by Angstrom [26], the monthly average clearness index KT is closely correlated with the monthly average number of sunshine hours. For a more accurate definition of this term, the reader can be directed to [26]. The monthly average clearness index for various locations around the world can be found in the Homer Energy website [27] or the NASA prediction of worldwide energy resources [28]. The clearness index, KT, is not to be confused with the cleanliness factor, FC, used in the STE plants to measure the soiling ratio in the heliostat field. This will be described in chapter 5.2. 3.1.4 Global Horizontal Irradiance, Diffuse Horizontal Irradiance, and Direct Normal Irradiance The Global Horizontal Irradiance (GHI) is the sum of direct and diffuse radiation received per unit from a solid angle 2π stereoradian (sr) on a horizontal plane. GHI is the reference radiation for the comparison of climatic zones. The Diffuse Horizontal Irradiance (DHI) is the amount of radiation received per unit area by a surface (not subject to any shade or shadow) that does not arrive on a direct path from the sun but has been scattered by molecules and particles in the atmosphere and comes from all directions. The Direct Normal Irradiance (DNI) is the amount of solar radiation received per unit area by a surface that is always held perpendicular (or normal) to the rays that come in a straight line from the direction of the sun at its current position in the sky. The maximum terrestrial DNI varies significantly with location and weather conditions but is often taken as 1000 W/m2. DNI data are available in different databases typically with hourly resolution [29]. Being the θz the zenith angle (see 3.1.3), these three concepts are related as described in the equation (3-5): Solar reflectance. theoretical background 34 34 𝐺𝐻𝐼=𝐷𝐻𝐼+𝐷𝑁𝐼⋅𝑐𝑜𝑠(𝜃𝑍) (3-5) As non-concentrating photovoltaic (PV) can also utilize a substantial amount of diffuse irradiance, GHI is closely related to the assessment of PV energy yields, while DNI is applied for the estimation of energy yields from CSP and CPV (concentrating PV) plants. There are some theoretical models in which they calculate the DNI through the AM coefficient and extraterrestrial radiation, see [30] for further information. 3.1.5 Spectral solar irradiance Gb(λ) The irradiance at a point of a surface is the radiant instantaneous power of all wavelengths incident from all upward directions falling on a unit area per unit time, SI unit: W/m2. The symbol G is used to describe the solar irradiance (insolation, radiant flux, and flux density are also synonyms in engineering terms, but they are used in different contexts). The spectral irradiance, denoted as Gb(λ), is the irradiance at a given wavelength per unit wavelength interval. SI unit: W/m2/nm or W/m2/µm. Then, the total radiant power Φ is denoted as in equation (3-6): 𝛷=𝛷𝜆=∫𝛷𝜆𝑑𝜆 ∞ 0 SI-unit [W/m2] (3-6) The solar irradiance spectra could be divided into 3 regions: ultraviolet, visible, and near-infrared. As can be seen in figure 3-6, the visible range of the solar flux contains the most powerful energy wavelengths of the sun, so the ideal mirror must reflect primarily above the 400 nm range. Figure 3-6 Spectral solar irradiance distribution for AM1GH spectrum 35 35 Review of models and procedures for the measurement of reflectance in CSP plants The knowledge of the relative amount of energy contained in sunlight at different wavelengths allows the engineer to evaluate the impact of wavelength phenomena on the total energy collection. 3.2 Reflectance definition and the law of reflection The National Institute of Standards and Technology (NIST) in the United States of America (USA) defines reflectivity as an intensive property of a material, just as resistivity, thermal conductivity, etc. On the other hand, the reflectance is defined as an extensive property of a material. When talking about the optical properties of a material in solar applications, the ending “-ance” must be used, because such materials can be degraded by thermal processes, soiling, corrosion, etc. and that has an effect on their intrinsic properties [31]. From now on as far as mirrors are concerned, only the term reflectance will be used. A mirror could reflect a light beam in different ways. It is important to define the characteristics of a reflected beam and standardize the symbols and parameters which define the reflected spectra. Hereunder a standard definition of the reflectance concept will be presented along with its classification. The reflectivity (in this case is reflectivity because is only one layer) of a surface, ρ, according to UNE-EN ISO 9488 [32], is the “ratio of the energy flux reflected, Φreflected, by a surface to the radiation incident on it, Φincident”, see equation (3-7). 𝜌=𝛷𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑 𝛷𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 (3-7) In any layer of a mirror setup must be met that the sum of absorptance, transmittance, and reflectance is equal to 1. The reflectance of a mirror is a dimensionless magnitude ranging from 0 to 1, which depends on wavelength, λ, the direction of the incident radiation or incidence angle, θi, direction of the reflecting radiation, θr, the reflecting light acceptance surface size S(Ω) and the temperature of the surface Ts. In this work, 1 reflectance units will be the same as 100 percentage points (ppt) and will be the same as 100% absolute reflectance of a reflector. The temperature of the surface (Ts) will be considered constant along in a reflectance measurement and its effects on the reflected beam will be omitted. When a light beam impinges a material, the first law of reflection states that the incident ray, the reflected ray, and the normal to the reflective surface at the point of incidence lie in the same plane. The second law of reflection specifies that the angle of incidence, θi, is equal to the angle of reflection, θr. This last statement is true for an ideal reflector. In that case, if all the incoming light beams are reflected according to the laws of reflection, the reflectance of the mirror is called specular, ρspecular or ρφ. However, imperfections over the surface of a layer like roughness, particles, scratches, inhomogeneities in the chemical composition, etc. cause that the direction of the reflected irradiance is not always specular. Many light beams are scattered around the supposed ideal spatial direction, creating more diffuse (or scattered) reflectance, ρdiffuse or ρd. Therefore, in general terms, we can define the total reflectance as, ρtotal, see equation (3-8): Solar reflectance. theoretical background 36 36 𝜌𝑡𝑜𝑡𝑎𝑙 =𝜌𝑠𝑝𝑒𝑐𝑢𝑙𝑎𝑟+𝜌𝑑𝑖𝑓𝑓𝑢𝑠𝑒 (3-8) The total reflectance is also known as hemispherical reflectance, ρhemispherical or ρh, since this one measures the reflected light regardless of its directionality. As for CSP purposes, the correct addressing of the measurement of both specular and diffuse reflectance is a key task in a solar power plant to have an overview of the general performance of a solar field and its aging condition. The solar reflectance model will be described deeper in the next sections. 3.3 Characterization of the solar reflectance: specular, diffuse, and hemispherical reflectance The solar reflectance concept, according to UNE 206009:2013 [31], is defined as “the reflectance weighted in the solar spectrum”. Since the visible range of the solar flux contains the most powerful energy wavelengths of the sun, measuring the reflectance according to its wavelength makes sense for solar concentrating applications. In the nomenclature, the solar reflectance is indicated with the subscript s after the reflectance symbol,  s, while the reflectance given just at a certain λ is named as monochromatic reflectance and its symbol is  λ. Normally, the temperature Ts is the ambient temperature (25 ºC ± 2 ºC). The effect of this parameter in the reflectance calculation will be omitted as has been mentioned before.  s expressed in general terms can be described as in equation (3-9): 𝜌𝑠([𝜆𝑎,𝜆𝑏],𝜃𝑖)=∫𝜌𝜆(𝜆,𝜃𝑖)⋅𝐺𝑏(𝜆)𝑑𝜆 𝜆𝑏 𝜆𝑎∫𝐺𝑏(𝜆)𝑑𝜆 𝜆𝑏 𝜆𝑎 (3-9) This definition is also known as solar-weighted reflectance. The values to be used for the solar spectral irradiance term Gb(λ), are recommended to be taken from reference standards as the ASTM G173-03 (2012) [25], spectrum AM 1.5. Solar-weighted specular reflectance is the most relevant parameter to assess the quality and status of the reflection by mirrors since gives not only information about which portion of the solar irradiation is being reflected but also if it is being reflected properly towards the receiver. However, as Aránzazu et al. [14] stated in the SolarPACES Reflectance Guidelines 3rd edition, it is not currently possible to measure the specular reflectance spectrum at a specified angle of acceptance and within the whole solar spectrum with commercial portable devices, therefore, the solar-weighted hemispherical reflectance and the monochromatic specular reflectance are the relevant and almost unique parameters to evaluate the quantity of solar irradiance reflected 37 37 Review of models and procedures for the measurement of reflectance in CSP plants in the collector. Then, hemispherical, and specular reflectances are the most representative values of a reflector, so in the next subsections, the definition and standard nomenclature for these concepts will be presented. 3.3.1 Hemispherical reflectance According to UNE 206009:2013 [31], the hemispherical reflectance is “the ratio of the energy flux reflected by a surface within the complete hemisphere over that surface to the radiation incident on it.” As it is depicted in Figure 3-7, the hemispherical reflectance is the collection of all reflected beams into the hemisphere above the reflecting surface of a mirror. Figure 3-7 Geometry of an incident and reflected beam in a reflective material (A. Heimsath et al., 2011) Therefore, the hemispherical reflectance is the sum of the specular reflectance and the diffuse reflectance, ρh. The solar hemispherical reflectance is indicated with the s (for solar) as the first subscript and the h (for hemispherical) as the second subscript, and it is defined in equation (3-10): 𝜌𝑠,ℎ([𝜆𝑎,𝜆𝑏],𝜃𝑖,ℎ]=∫𝜌𝜆,𝜑(𝜆,𝜃𝑖,ℎ)⋅𝐺𝑏(𝜆)𝑑𝜆 𝜆𝑏 𝜆𝑎∫𝐺𝑏(𝜆)𝑑𝜆 𝜆𝑏 𝜆𝑎 (3-10) being [λa,λb] the wavelength range and θi the incidence angle. The spectral or monochromatic hemispherical reflectance is defined as ρλ,h(λ,θi,h), being λ the first subscript, in this case, to indicate that the measurement is done just at a certain wavelength. The direct measurement of the hemispherical reflectance is most commonly carried out by devices like the integrating spheres, attached to a spectrophotometer. These devices will be explained in Chapter 6.3. 3.3.2 Specular reflectance The standard definition for the specular reflectance according to UNE 206009:2013 [31] is “the ratio of the energy flux reflected by a surface in the specular direction to the radiation incident on it. The specular direction Solar reflectance. theoretical background 38 38 is the one forming an angle with the normal to the surface equal to the angle of incidence of the incident radiation. The specular direction is on the same plane as the incident radiation and the normal to the surface and opposite to the direction of incidence. A certain angle of acceptance of specular reflectance, φ must be defined.” The solar near-specular reflectance is represented as  s,φ ([  a,  b],θi,φ), being [  a,  b] the wavelength range, θi the incidence angle, φ the acceptance angle. As in the previous section, the first subscript is used to indicate that the reflectance is measured in the whole solar spectrum, while the second subscript is reserve to indicate that in this case, the reflectance is specular, measured with a certain φ. The solar-weighted specular reflectance is described in equation (3-11): 𝜌𝑠,𝜑([𝜆𝑎,𝜆𝑏],𝜃𝑖,𝜑]=∫𝜌𝜆,𝜑(𝜆,𝜃𝑖,𝜑)⋅𝐺𝑏(𝜆)𝑑𝜆 𝜆𝑏 𝜆𝑎∫𝐺𝑏(𝜆)𝑑𝜆 𝜆𝑏 𝜆𝑎 (3-11) In the case of concentrating applications, it is necessary to set an angle of acceptance (see 3.3.2.2), so, the reflectance of a mirror is defined as the ratio of incident power reflected by a mirror into the acceptance halfangle of the receiver φ. That reflected flux is denominated in that case specular reflectance (see Figure 3-8). Strictly speaking, the ideal specular reflectance is related to φ = 0, whereas the measurement at any small offset φ is more appropriately termed near-specular reflectance. Because of the angular radius of the sun, any element of an ideal mirror surface in a concentrator system will effectively reflect a cone of radiation with the same angular spread. Therefore, for an ideal reflector, the specular reflectance should be measured with a φ ≥ 4.70 mrad. Figure 3-8 Rays reflected by a mirror in a PTC form a cone with a designated half-angle depending on the established specular limit A real reflector will reflect a cone of solar radiation with an angular spread larger than the sun’s due to imperfections like concentrator waviness, surface roughness, collector tracking tolerances, etc. (see Figure 3-9). The source of optical errors will be explained in detail in chapter 5.2. 39 39 Review of models and procedures for the measurement of reflectance in CSP plants Figure 3-9 Rays reflected in a real mirror in a PTC Therefore, all the solar radiation out of this established half-angle cone is denominated diffuse or scattered reflectance. Each solar receiver has a defined acceptance half-angle cone; Thus, each reflector can have different values of specular or diffuse reflectance depending on the absorber selected, since the acceptance angle will be different for each setup, for that purpose, the use of the term specularity gives more accurate information about the characterisation of a specular condition on a mirror, refer to 3.3.2.1. The reflected power of a surface, more precisely the specular reflectance, is strongly related to the wavelength of the incoming wavelength solar radiation [33], as can be seen in Figure 3-10, mainly because the solar irradiance distribution is wavelength dependant. Figure 3-10 Specular reflectance for continuously varying acceptance angles for an aluminium reflector sample, measured at 3 wavelengths, angle of incidence 8° (A. Heimsath et al., 2015) Due to that, loss of specular reflectance in wavelengths where the irradiance is high (Visible: λ ≈ 400 – 700 nm) must be avoided. The specular reflectance is usually measured at an acceptance half-angle of φ ≤ 20 mrad for CSP applications, so all the radiation within this half-angle cone can be considered near-specular [14]. Although for very specular materials, the difference between hemispherical and specular reflectance is small [34]. Solar reflectance. theoretical background 40 40 3.3.2.1 Specularity (near-specular reflectance) For a solar mirror, it is not only important to reflect all the amount of incoming radiation possible but also to do so in a designated direction. The parameter known as specularity characterizes the capacity of a mirror to focus all the reflected light into the solar collector, or in other words, the specularity defines what percentage of the incoming radiation is reflected into the specular direction within the defined half-angle cone, φ. As stated before, usually for CSP applications the half-angle cone is φ ≤ 20 mrad. Therefore, the specular reflectance is strictly defined as the reflected radiation with a φ = 0 mrad. Specularity can be defined as in equations (3-12) and (3-13): 1. As solar specularity 𝑠𝑝𝑒𝑐𝑢𝑙𝑎𝑟𝑖𝑡𝑦(𝑠,𝜑)=𝜌𝑠,𝜑([𝜆𝑎,𝜆𝑏],𝜃𝑖,𝜑) 𝜌𝑠,ℎ([𝜆𝑎,𝜆𝑏],𝜃𝑖,ℎ) (3-12) 2. As a spectral response specularity 𝑠𝑝𝑒𝑐𝑢𝑙𝑎𝑟𝑖𝑡𝑦(𝜆,𝜑)=𝜌𝜆,𝜑(𝜆,𝜃𝑖,𝜑) 𝜌𝜆,ℎ(𝜆,𝜃𝑖,ℎ) (3-13) Since no mirror has an ideal specular behaviour, meaning, scattering of the outcoming radiation is usually expected, specular reflectance is always mentioned at a half-angle cone. Depending on the angle of acceptance φ, the specularity could be different, so to compare values of different mirrors, the same measuring parameters and conditions must be compared in general terms. The specularity is an important parameter to be characterised in a mirror since gives important information about the optical quality of a reflector or its condition. In a raw state, cleaned and out-of-the-production-line, the specularity gives information about the optical performance of a mirror configuration. Calculated in the solar field, gives information about the condition of the surface and its aging status. Specularity can also be defined as the Root Mean Square (RMS) value of the beam widening, in milliradians, instead of percentage points [35] [36]. A perfect mirror would have a RMS value of 0 milliradians (excluding the solar broadening). The better the specularity, the smaller will be the RMS value of the beam spread from the reflective surface. If the specularity is described as RMS mrads, is giving information about the overall optical loss of specular reflection due to the mirror, in other words, the sum of all errors from the mirror which diminish the specular reflection. Specularity does not give any specific information about the soiling or aging of the mirror if not comparable with other data collected when the mirror is in a cleaned state. 41 41 Review of models and procedures for the measurement of reflectance in CSP plants Specularity is strongly influenced by the roughness of the substrate and the dust buildup over the first surface. Carlos Alcañiz et al. and the Photonics Technology Group (PTG) at the University of Zaragoza [37] did an experimental study to demonstrate the influence of the roughness of the substrate on the mirror specularity. They used Front Surface Mirrors (FSM) made by stainless steel AISI 304 coated with a silver layer through a Physical Vapour Deposition (PVD) process. The results of the hemispherical and the specular reflectance measure in the PTG laboratory are presented in Table 3-2: Table 3-2 Influence of the roughness of the substrate in the solar hemispherical and solar spectral reflectance (C. Alcañiz et al., 2013) Arithmetical mean roughness (Ra) (µm) ρs,h ([300-2500], θi, h) ρs,φ ([300-2500], θi, 30⁰) 0.00319 96.9% 96.9% 0.00623 96.6% 94.6% 0.03000 96.5% 91.6% 0.08520 93.9% 57.6% 0.10200 93.8% 44.7% 0.59800 89.5% 16.2% 0.71600 89.1% 4.5% 1.34000 89.0% 1.4% The loss of intrinsic specularity in the mirror is originated by the roughness of the reflective surface or by the substrate where the reflector is deposited. In the case of study, the bigger the roughness of the steel substrate the bigger the scattering produced by the mirror, so a less specular light is reflected. Then, the roughness of a substrate is a key parameter to manufacture high-quality mirrors for concentrated solar plants. A simple sketch of how the roughness affects the specular beam profile can be seen in Figure 3-11. The scattering theory will be explained in chapter 3.3.3. Figure 3-11 Transition from specular reflection to diffuse scattering due to the roughness of the surface (Peverini, 2005) Mirrors 48 48 Figure 4-2 Concentrator classification categories Any layer configuration can be used as a point focus concentrator or as a line focus concentrator. Concentrators can be also classified as imaging or non-imaging concentrators and as continuous or segmented surface concentrators. Regarding the geometrically focus configuration, the concentrator can focus the solar radiation in two different patterns, based on the shape of the reflector: over a single point or a line, see Figure 4-3. Using one or another shape depends on the collector used, i.e., PTC and LFR are line-focusing concentrators, but heliostats or parabolic dishes are point-focusing concentrators. Figure 4-3 Types of concentrators according to its geometrical focus. Line focus: PTC and LFR. Point focus: parabolic dishes and ST Concentrator clasification acc. to its geometrical focus Point focus two-axis tracking Line focus one-axis tracking acc. to its layer configuration Second surface mirror Silvered thick glass Silvered thin glass Silvered double glass laminated Front surface mirror Aluminum mirrors Silvered polymer film 49 49 Review of models and procedures for the measurement of reflectance in CSP plants The manufactured shape quality of the concentrator plays a key role in its optical efficiency. Irregularities in the shape quality are defined as slope deviations. The variations over the nominal shape can influence the reflected beam and provoke a deviation of the reflected ray by twice the slope deviation angle of the respective mirror surface (see chapter 5.2 for more information). Given the purpose of this work, it is decided to outline only the classification according to its layer configuration. There are a lot of materials which combines a series of properties making them an ideal candidate to be used in a mirror. For instance, the next Figure 4-4 shows a comparison of the reflectance spectrum of several reflective materials. Figure 4-4 Hemispherical reflectance, incidence angle 8°, for candidate CPV optical component materials. The normalized direct solar spectral irradiance (AM1.5 in ASTM G173) is provided for reference (Miller, 2009) But for better performance in STE plants, reflective materials must be attached to other layer systems to form a proper solar reflector and to protect the reflective layer from the environment to avoid fast aging. In the next subsections, the most proven mirror configurations will be described to give an overview of what to expect in the solar field when measuring reflectance on site. Next, a detailed description will be presented to review the main materials and configurations suitable for concentrating solar thermal technologies [49]. 4.1 Second surface mirrors The second surface mirror (SSM) configuration is named so because the reflective layer (usually of a silveredbase material) is deposited on the back of a transparent layer or added onto a back structure. This transparent layer is also the substrate which protects the reflective layer. These types of mirrors are also known as silveredglass mirrors because the substrate is normally a glass layer. Silvered-glass mirrors are the most employed in STE plants given its demonstrated stability over the life cycle of a plant and almost neglectable loss of optical properties. Depending on the thickness of the glass and the layer setup, it can be categorized into three types. Mirrors 50 50 4.1.1 Silvered thick glass mirrors A thick glass layer could be defined as a layer with a thickness range between 3-5 mm, being 4 mm the standard commercial thickness. Monolithic glass mirrors or floated glass mirrors are also definitions for such configuration. Figure 4-5 Thick glass mirror installed in a Eurotrough Parabolic Trough Collector. An advantage of thicker glass mirrors (> 3 mm) over thinner mirrors is that their mechanical strength enables them to withstand wind loads without requiring special backing structures to avoid breakage and maintain their shape. Usually, they are fixed to the collector through bonding clamps (typically named pads), as it can be seen in Figure 4-5. The glass layer in this setup is used as a top protective layer, therefore it must be very transparent to solar radiation. That is the reason why the glass used in STE plants is called “water-white” or “extra-clear” glass. The water-white is a silica-based glass with less than a 0.02 % w/w of ferric oxide (typically 0.01 % w/w) [26]. Is well known that ferric oxide absorbs light radiation in the infrared spectrum (around 1000 nm – 1100 nm), as can be seen in Figure 4-6. 51 51 Review of models and procedures for the measurement of reflectance in CSP plants Figure 4-6 Spectral transmittance of 6-mm-thick-glass with various iron oxide contents. A high concentration of Fe2+ in the glass leads to the appearance of an absorption band in the glass centred at approximately 1100nm (Dietz, 1954) In this type of mirror, it has been noticed that the loss of specular reflectance overtime was not increasing as rapidly as expected. On the contrary, the specular reflectance was increasing and not due to the aging of the reflective layer but because of the change in the transmittance and absorption spectrum of the glass layer. This phenomenon is called “glass solarisation” [50] [13]. As can be seen in Figure 4-7 and Figure 4-8, both hemispheric and specular reflectance in the range of 900 nm to 1200 nm increase over time. Figure 4-7 Hemispherical reflectance of two aged silvered glass mirrors and a control sample (Pettit and Freese, 1980) Mirrors 52 52 Figure 4-8 Specular reflectance at wavelengths of 500 nm and 900 nm for a silvered glass mirror cleaned every two days over 66 days of outdoor exposure (Pettit and Freese, 1980) It seems to be that the long-term exposure reduces the Fe2+ to Fe3+, which has no absorption band at 1100 nm (see Figure 4-9). There are some studies about the root cause of the solarization [51], which at the beginning of the mirror life cycle, this phenomenon increases lightly the reflectance of the glass (A standard low-iron-glass has a luminous transmittance around 91-92 %), since due to the UV exposure, Fe2+ is reduced to Fe3+. Figure 4-9 Absorption coefficients of Fe2+ and Fe3+ in Soda-Lime Glass (Coyle, 1981) Silver is typically used as a reflective layer. Silver has a hemispherical reflectivity of 97% and is known to be one of the best solar reflectors. However, it may corrode under atmospheric conditions, consequently, it must be protected [52]. The first mirror generation used a back protective layer system comprised of two materials. First, a copper layer over the silver is applied to slow down the silver corrosion (copper acts as a sacrificial anode as it has a lower standard electrode potential) and to prevent transmitted UV-light through the silver layer to damage back protecting coats. Over the copper, lead-based protective paint is applied to prevent diffusion to the silver layer and protection against the UV radiation. A first-generation mirror could be portrayed as depicted in Figure 4-10: 53 53 Review of models and procedures for the measurement of reflectance in CSP plants Thick Glass (3 - 5 mm) Silver Layer (≈ 150 nm) Copper Layer (≈ 150 nm) Lead-Based protective layer (≈ 60 µm) UV-resistant top coating (≈ 40 µm) Figure 4-10 Monolithic silvered glass mirror layer setup State-of-the-art 4 mm silvered-glass mirror reflectance achieves solar-weighted hemispherical reflectance values around 94.7 %. Although this kind of material and layer configuration has given a proven performance in a solar field for many years, for SSMs, the copper layer might be replaced by a 100 Å SnO2 to cut production costs and comply with environmental regulations. Also, because of the environmental and health issues, lead content in the protective layer has been reduced from 20 % w/w to less than 1 % w/w 6 , or, in the best cases, the lead-based paints have been replaced by lead-free paints or antioxidant pigments with similar proven durability results [53]. 4.1.2 Silvered thin glass mirrors This kind of mirror setup uses the same layer configuration as the above described, but instead using a thick glass layer of around 3 mm - 5 mm, a thin layer of glass around 1 mm performs as the main substrate. Figure 4-11 Thin glass mirror installed in a parabolic trough collector, Acurex 3001 Light passing through an absorbing material is attenuated as a function of the absorption coefficient and the thickness of the material [45]. Since the glass substrate is 80 % thinner than the thicker ones, it absorbs less light and the radiation transmission to the reflective layer is higher. Therefore, the reflectance of this kind of setup is higher. The layer configuration is the same as for the thick glass (see Figure 4-10). 6 In the US, since 2009, the lead allowable in most paints is now 0.009% w/w (90 ppm) [https://www.atsdr.cdc.gov/csem/csem.asp?csem=34&po=8] while in the EU, the REACH regulation stablishes a limit in less than 0.1 % w/w (1000 ppm) weight by weight. [https://europa.eu/capacity4dev/unep/documents/global-report-status-legal-limits-lead-paint]. Mirrors 54 54 The main disadvantage of using thinner glass is that the mechanical strength is not high enough to be a selfsupporting structure and it makes necessary to glue them to metal or composite structures as a substrate (see Figure 4-11), making it more expensive. Besides, this configuration usually supports fewer wind loads than the thicker glass ones, depending on the substrate. Both thicker and thinner glass reflectors have passed several years of Accelerated Exposure Testings (AET) and Outdoor Exposure Testings (OET). Less than 4 percentage point loss of hemispherical reflectance was reported for both setups [54]. In the case of thick glass reflectors, the OPAC group did a study exposing several samples of thick glass mirrors to AETs and OETs. For an AET, the highest decrease in solar-weighted hemispherical reflectance reached 3.3 percentage points. For the case of the OET, the maximum change in reflectance was 4.3 percentage points [55]. Also, the choice of the adhesive between the steel substrate and the mirror has been observed to affect the performance of them in outside exposure and corrosion phenomena have been detected [53]. For comparison, the next figure includes a reflectance spectrum of a 1 mm glass mirror sample and one with 4 mm. Figure 4-12 Comparison of hemispherical and specular (φ = 12.3 mrad) reflectance spectrum of a 1 mm and 4 mm silvered glass reflector (Sutter et al., 2016) 4.1.3 Silvered double glass laminated The double glass laminated mirror just uses as back protective coating another layer of shaped glass, serving as an anticorrosion layer. This kind of setup has been used in the automotive industry for a while, so its production quality is well established, but installing in a solar field is still ongoing since the costs are much higher than the other alternatives. A pilot plant installed in the Plataforma Solar de Almería (PSA) uses this kind of mirror configuration for testing purposes 7 but even the original equipment manufacturer, Guardian, does not sell this type of mirrors anymore. 7 https://sfera3.sollab.eu/access/#infrastructures 55 55 Review of models and procedures for the measurement of reflectance in CSP plants 4.2 First surface mirrors The first surface mirror (FSM) configuration is named so because the substrate is an opaque material, like polished aluminium or a polymeric substrate, and the reflective surface is placed upon it. Besides, the top protective layer covering the reflective surface usually has a thickness of around 3 - 5 µm. So, the FSM are usually thinner than the SSM. There are two available types of FSM, aluminium mirrors, and silvered polymer mirrors. 4.2.1 Aluminium mirrors The most extended aluminium reflectors are composed of a polished aluminium sheet as the substrate, an enhanced aluminium as the reflective material and a top nano-composite protective coating like the one used by Alanod GmbH [56]. A typical aluminium mirror configuration could be pictured as follows: Nano-composite protective layer (SiO2/Al2O3) (≈ 3 µm) PVD Aluminium reflective layer (≈ 200 nm) Anodizing layer Al2O3 (≈ 3 µm) Aluminium substrate (≈ 0.5 mm) Figure 4-13 Aluminium mirror setup Aluminium has a hemispherical reflectance ρλ,h of around 88% - 91% in the range 250 - 2500 nm [57]. Aluminium reflectors are too thin and need a support structure that might increase the cost of the concentrator. Besides, its manufacturing process creates a non-smooth surface that leads to high scattering of the reflected rays and the specular reflectance shows a significant decrease when the acceptance angle is reduced, as can be seen in Figure 4-15. That great decreasing in the specular reflectance makes this kind of mirrors only suitable for low-temperature concentrating applications [53] [58]. The loss of specularity in the FSM could be explained by the spectral characterization comparison between a monolithic glass mirror (named as “floated glass mirror”) and an aluminium mirror (named “commercial product 1”) in the next Figure 4-14 [37]. Mirrors 56 56 Figure 4-14 Front Surface Mirror vs Reference (Alcañiz et al., 2015) In the figure above, the loss of reflectance in the visible range is caused by the protective layers above the silver coating. All silicon oxides exhibit stronger absorption bands below 300 nm. Also, if Aluminium is used as a reflective material, the reflectance will be lower due to the absorption band of the Aluminium in the visible range (interband transition of aluminium at 860 nm). As can be seen in Figure 4-15, the Aluminium reflectors present a high scattering profile according to its specular reflectance spectra. The lower the acceptance angle, the higher the specular reflectance. The scattering losses peak in the λ ≈ 400 nm is approximately 15 percentage points. The valleys (up and downs) in the spectra are originated by interference effects. These interference patterns are produced by the inhomogeneities of thickness and quality in the dielectric and protective layers, diminishing the total solar reflectance [59]. For this reason, its high scattering added to its low hemispherical reflectance make them not adequate for solar applications with high concentration ratio. Figure 4-15 Specular reflectance spectra of an enhanced aluminium mirror plotted for different acceptance angles at θi = 8° incidence angle (Sutter et al., 2016) The optical durability in the solar field has been proved to be poorly, losing specular reflectance after AET [58]. For instance, after a damp-heat cycling test, Uppsala University measured a decrease in the solar-weighted specular reflectance around 72 percentage points (From 79 % to 7%) over a polymer-protected evaporated 57 57 Review of models and procedures for the measurement of reflectance in CSP plants aluminium reflector [60]. Nevertheless, a top coating such as a SiO2 sol-gel deposited layer has shown improved results after a 5 years’ OET at the Tabernas Desert (PSA). The maximum lost in monochromatic specular reflectance was 3.7 percentage points [61]. A summary of accelerated tests done in this kind of reflectors can be found in [55]. Another study done by Sutter et al. [62] with a typical aluminium reflector configuration revealed the specular reflectance losses after 10 years of outdoor exposure. The specular losses caused by corrosion phenomena are expected to be in the range of 0.5 - 3.0 ± 1.4 percentage points depending on the outdoor exposure site. In the same exposure period, specular reflectance losses caused by scattering are expected to be in the range of 6.6 ± 1.4 percentage points. The reflectance losses caused by scattering are almost independent of the exposure site, consequently, they are inherent to the aluminium setup itself. Besides, although manufacturers like Alanod GbmH have carried out AETs on its products for solar applications, they still must improve its durability but are suitable for some applications (intermediate-temperatures) and several facilities of “small-sized” collectors. 4.2.2 Silvered polymer film mirrors This configuration uses a polymer film as a substrate and as a top protective coating. Silver is used as a reflective material. Anti-soiling and protective layer UV screening superstrate Bonding layer Reflective layer (Ag) Flexible polymer substrate (Al) Figure 4-16 Standard silvered polymer mirror configuration These mirrors are more flexible than other commercially available and have the potential to adapt to any desired shape. Albeit due to the temperature limits of the polymer film, this setup cannot withstand temperatures greater than 80 ⁰C, which limits its use in some operations. Manufacturers like Skyfuel Inc. 8 claims that the specular reflectance of this setup (ReflecTech® PLUS) reaches values over 91% at 7.5 mrads and has been tested outdoors for several years in the Solar Electric Generating Station (SEGS) without any sign of degradation. One major disadvantage of the polymer reflectors is that they degraded rapidly after several brush washing cycles. Samson et al. published a study where highlights the effects of contact cleaning methods on this kind of setup. After 100 brush cycles (almost two years of simulated 8 Skyfuel Inc. filed for bankruptcy in 2019. Performance in a solar concentrator 64 64 Figure 5-3 Description of potential geometrical and optical errors in a PTC (Güven and Bannerot, 1986) There is a relationship between the source of the potential error and its dependency with the optical performance, see Figure 5-4. The specular reflectance is mainly lost due to issues with the reflective material, like aging and due to problems associated with the surface of the mirror and its intrinsic properties: roughness and soiling. The phenomena involved in the aging of a mirror could be listed as chemical corrosion, erosion (scratches due to dust), abrasion (scratches due to washing), solarization, delamination (layer bonding issues), UV degradation, layer cracking Specular reflectance Soiling Aging Mirror roughness Intercept Factor Sunshape (Circumsolar radiation) Mirror contouring (slope) See figure 3-3 Receiver alignment Reflector profile error Solar tracking (sensor & drive) Shadowing and blocking effects (can be included in IAM) Transmittance Soiling of the receiver AR Coating Aging Absorptance Loss of vacuum Selective Coating Aging Figure 5-4 Root causes of loss of optical performance in a concentrator applied to a PTC 65 65 Review of models and procedures for the measurement of reflectance in CSP plants (due to thermal stress) and tarnishing (degradation of the silver due to high temperatures). Those are factors affecting the durability of the reflector. The intercept factor is influenced by quality problems in the manufacturing and assembly of the concentrator, involves all geometrical problems associated with the collector. They include reflector profile errors, consistent misalignment of the collector with the sun, and misalignment of the receiver with the effective focus of the sun (this problem is neglectable for a solar tower since the deviations in the design can be compensated by an offset in the heliostats), all problems which can be addressed implementing appropriate quality controls on site. The operation of the plant can influence both specular reflectance and intercept factor. The quality of the tracking instruments installed, and the experience of skilled workers play a significant role. The atmospheric conditions influence all parameters equally. For instance, high atmospheric humidity will absorb DNI (GSC) and also reduces the reflectance of the mirrors due to condensation over the first layer. A sandstorm will difficult the tracking of the collector and soil the surface of the reflectors. In general terms, there are two mechanisms of losing reflectance in a mirror: by absorption of the radiation or by scattering (dispersion) [42]. For instance, mirror soiling leads to a loss of optical performance due to absorption and dispersion at the same time. The effect of mirror soiling must be well studied and known by the plant operators to better predict the yield of a solar field and be able to schedule efficiently the cleaning and washing operations of the mirrors. It has been reported a daily loss of 0.5% hemispherical reflectance on outdoorexposed mirrors in commercial plants in operation [73]. The loss of specular reflectance due to soiling in a reflector could range from 14 % to 26 % after a few months and it varies considerably depending on the location of the plant. The scattering produced by dust on the mirrors is strongly related to the location of the plant, therefore dust characterization is a mandatory task for the correct measurement of solar reflectance in the field [74]. For modelling the soiling in a solar plant (CSP or PV), please refer to Bellmann et al., 2020 [75]. As a key performance indicator in a CSP plant, some authors defined the cleanliness factor, FC as in equation (5-7): 𝐹𝐶=𝜌𝑠𝑜𝑖𝑙𝑒𝑑,𝑠,𝜑(𝜆,𝜑,𝜃𝑖) 𝜌𝑐𝑙𝑒𝑎𝑛,𝑠,𝜑(𝜆,𝜑,𝜃𝑖) (5-7) For using this concept in the solar field, for instance, for the TraCS instrument described in chapter 6.4.3.1, the cleanliness factor of a mirror can also be defined as in equation (5-8) [76]: 𝐹𝐶 (𝑡)= 𝐷𝑁𝐼𝑟𝑒𝑓𝑙𝑒𝑐𝑡𝑒𝑑,𝑠𝑜𝑖𝑙𝑒𝑑(𝑡) 𝑐𝑎𝑙𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡∙ 𝐷𝑁𝐼𝑠𝑢𝑛(𝑡) (5-8) Being the calibration constant, the averaged clean reflected DNI of a mirror in comparison with the DNI of the sun reaching the collector. The change of cleanliness over a defined time interval ∆t is defined as the soiling rate rs,dt. The soiling rate is always related to a specific reported time interval, being hourly and daily the typical Performance in a solar concentrator 66 66 values. For a more graphical description about how soiling affects the reflectance in SSMs, see Figure 5-5. A Pfahl et al. did resume a list of soiling-rate measurement campaigns over the world, refer to [47]. Figure 5-5 Interaction of irradiance with soiling particles on the surfaces of CSP mirrors (P. Bellmann, et al. 2020) Regarding its randomness, errors can be categorised as random errors and non-random errors. Random errors are defined as those errors that are truly random in nature and can therefore be represented by normal (probability) distributions. These errors are identified as apparent changes in the width of the sun, scattering of defects associated with the optical material used in the reflector, and scattering effects caused by random local slope errors. Non-random errors are deterministic in nature. They account for the gross errors in manufacture/assembly and/or in operation. The non-random errors are identified as: concentrator profile errors, consistent misalignment of the trough with the sun, and misalignment of the receiver with the effective focus of the trough. The non-random errors can only be characterised to a Normal distribution if averaged over a large array of collectors or heliostats, or to include these errors as a factor to calculate a global intercept factor for the collector. One of the advantages to define the reflectance as a sum of Gaussian functions is to be able to use these parameters such as standard deviation, in ray tracing models which calculate the intercept factor of an absorber or the performance of the plant giving the standard deviation of the collector or heliostat [77]. Then, optical errors can also be represented as a normal probability distribution. The total error in a concentrator is given as the sum of individual errors occurring in the system, as described in equation (5-9) [35]: 𝜎𝑡𝑜𝑡𝑎𝑙 2=𝜎𝑠𝑢𝑛 2+(2𝜎)𝑠𝑙𝑜𝑝𝑒 2+𝜎𝑡𝑟𝑎𝑐𝑘𝑖𝑛𝑔 2+𝜎𝑚𝑖𝑟𝑟𝑜𝑟 2 (5-9) Where σsun represents the standard deviation of the sun’s energy distribution, σslope is the slope error distribution, σtracking is the tracking error distribution and σmirror is the specularity error distribution. According to Carlos Alcañiz et al. [37], the reflected profile (characterised as normal distributions) of the parabolic through configuration is different from a tower configuration and the tower setup is more sensible to non-specular behaviour in the reflectors than the PTC. 67 6 SOLAR REFLECTANCE MEASUREMENT 6.1 Characterization of the reflected beam profile of a mirror. Models for measuring the specular reflectance Richard B. Pettit [12] [78] was one of the first and leading researchers to characterise the beam profile of reflected irradiation from a mirror for concentrated solar thermal purposes. His goal was to develop a method to characterise the properties of a mirror material independent of the characteristics of the measurement technique. He concluded that the radiation reflected would scatter the beam according to a normal distribution 10 . In later research, as previously mentioned, Richard B. Petit and J.M. Freese pointed out the wavelength dependence in the scattering phenomena. 6.1.1 Solar-weighted specular reflectance model This model was developed by Pettit to ease the measurement of the averaged specular reflectance of a solar field, using portable available devices. Pettit proposed two assumptions: 1. The ratio between specular and hemispherical is wavelength-independent 2. If the first assumption is false, the second assumption is that the above-mentioned approximation leads to a small error compared with the averaged loss of reflectance in a mirror. In that case, the following equation shall be used: 𝜌𝑠,𝜑([𝜆𝑎,𝜆𝑏],𝜃𝑖,𝜑]=𝜌𝑠,ℎ([𝜆𝑎,𝜆𝑏],𝜃𝑖,ℎ)∙𝜌𝜆,𝜑(𝜆𝑐,𝜃𝑖,𝜑) 𝜌𝜆,ℎ(𝜆𝑐,𝜃𝑖,ℎ) In this equation, 𝜌𝜆,𝜑(𝜆𝑐,𝜃𝑖,𝜑) is measured using a portable reflectometer and the solar-weighted hemispherical reflectance and the monochromatic hemispherical reflectance are measured by laboratory devices (spectrophotometer) before the installation of the mirror on the field. This model might be used for highly specular reflectors as the silvered second surface mirrors. The solar-weighted specular reflectance can be calculated by multiplying ρs,h ([280–2500] nm, 8°, h) by the ratio of ρλ,h (660 nm, 8°, h) / ρλ,φ (660 nm, 8°, 12.5 mrad), according to Pettit equation proposed in [12]. This calculation for determining the solar-weighted specular reflectance, which assumes that the ratio of specular to hemispherical reflectance at a certain λ (in this case at 660 nm) is constant over the whole solar spectrum, is widely accepted [41]. However, this assumption is only valid for highly specular reflectors such as silvered glass. Single wavelength measurements in multilayer mirrors might lead to large deviations of the calculated solarweighted specular reflectance [37] [59]. As for the SSM, it can be assumed that scattering is essentially 10 The relationship between surface roughness, which is present in all the reflectance cases, and the scattering of the reflected light was first described by H. Davies, 1954 in his work “The reflection of electromagnetic waves from a rough surface”. Although Richard B. Pettit et al. were the ones to develop techniques to apply such research to the solar field. Solar reflectance measurement 68 68 independent of wavelength over the whole solar spectrum and that the beam has a Gaussian shape. However, it is well-known that these two conditions are not fulfilled after the aging tests due to scattering and absorption effects on the corroded reflector surface. Therefore, degradation of the reflective layer cannot be monitored by using this equation. 6.1.2 Spectral specular reflectance model with one and two Gaussian The beam spread of a solar reflector (beam scattering) can be described by a model function with one Gaussian [12], see equation (6-1): 𝜌𝜆,𝜑(𝜆,𝜃𝑖,𝜑]∝exp(−∆𝜑2 2𝜎2) (6-1) Where ∆θ is the deviation of the reflected beam from the specular direction and σ is the standard deviation of the distribution. It is assumed that the average surface height and slope distribution can be characterised by a normal distribution. σ is wavelength dependant. This model is validated for material with low scattering and no dust over the tested surface. However, for some materials like the roll-polished aluminium, the reflectance profile does not suit well by a single normal distribution. The reflectance profile of such materials can be characterised by the sum of two normal distributions, see equation (6-2) : 𝜌𝜆,𝜑(λ,𝜃𝑖,𝜑]∝𝜌1exp(−∆𝜑2 2𝜎12)+𝜌2exp(−∆𝜑2 2𝜎22) (6-2) For instance, for laminated plastic films, one distribution will refer to the intrinsic properties of metallized plastic film and the other one will refer to the scattering due to the bonding interface. With these two models, it can be covered most of the commercial reflective surfaces. There are more reflectance models in the bibliography, please refer to [76] for a detailed list. There are more techniques used for the characterization of reflective surfaces of solar concentrators, please refer to Arancibia-Bulnes et al. [77] for a more detailed view of the methods developed to characterise the peculiarities of reflectance profile of a mirror. 69 69 Review of models and procedures for the measurement of reflectance in CSP plants 6.2 Equipment to measure solar radiation The instruments for measuring solar radiation are generally of two types: pyranometers, see Figure 6-1, which accept radiation from the entire hemisphere, and pyrheliometers, see Figure 6-2, which accept radiation from only one direction (more precisely from a cone of 2.8° half-angle). The pyranometer measures hemispherical insolation (GHI), while the pyrheliometer measures the beam component of the insolation (DNI or GSC). Both pyranometer and pyrheliometer operate on the principle of measuring the temperature rise of a black absorber as it is heated by the sun. These devices with adaptations can be used in the field to evaluate the dust accumulation on the mirrors or the current soiling rate. Figure 6-1 CMP3 second class pyranometer - Kipp & Zonen Figure 6-2 DN5 First Class Pyrheliometer - Middleton Solar 6.3 Equipment to measure solar reflectance The difficulty in measuring solar reflectance lies in the broad parameter range influencing the reflectance of a mirror employed in CSP. The evaluation of the quality of a reflector involves most of the solar spectral wavelength range, an incidence angle from near normal up to 70º and a very narrow acceptance angle from near specular to around 20 mrad. To measure reflectance spectrophotometers and reflectometers are usually used. Solar reflectance measurement 70 70 6.3.1 Spectrophotometers A spectrophotometer is a photometer (a device for measuring light intensity) that can measure intensity as a function of the light source wavelength λ. The main advantage of a spectrophotometer for solar applications is the fact that the reflectance can be measured over the entire solar spectral range, UV, VIS and NIR, with the use of one or dual light sources and monochromators. See Figure 6-3 for a schematic plan of a typical spectrophotometer. Figure 6-3 Schematic plan of a typical spectrophotometer (Ross, 2004) For the case of solar applications, it is extensively used in laboratories with integrating spheres (see Figure 6-4) for evaluation of the hemispherical, diffuse and (as a difference of both) specular reflectance. Diffuse reflectance is measured by directing the specular reflected beam through a sphere opening (light trap). Thus, in the sphere, only the diffusely reflected light is detected. Figure 6-4 Type I illumination system with an integrating sphere from a Cary Spectrophotometer Model 1711 (Murray, 1998) The major disadvantage of this kind of equipment is that it is not portable. So, its use is restricted to laboratories only. Besides, in general, the adjustment of the acceptance angle for the measurement of specular reflectance is not possible. Additionally, the usual angle of incidence of these devices ranges from 8º < θi < 15º and measuring curved mirrors leads to errors in the measured value. For CSP purposes, the are several accessories to help to characterize a mirror. For instance, the Universal Reflectance Accessory (URA) by Perkin Elmer [76] is used to measure specular reflectance at larger incidence angles than in an integrating sphere, being able to detect scattering of the material due to degradation. However, as the acceptance angle is larger than the required in concentrating solar thermal technologies, its use is not widely extended. To meet this need, Montecchi proposed 71 71 Review of models and procedures for the measurement of reflectance in CSP plants a single experimental device disposition using though an integrating sphere for measuring near-specular and hemispherical reflectance at semi-acceptance-angles suitable for CSP applications [79]. As a list of spectrophotometers used for solar applications, there are many: • Lambda 950/1050 by Perkin Elmer • Cary 5000 by Agilent (formerly Varian) • OL 750 by Gooch and Housego (formerly Optronic laboratories) • V670 by Jasco A more detailed description of these devices and its characteristics can be found in [76]. The influencing factors for measuring solar reflectance using spectrophotometers with integrating spheres can be listed in: • The spectral bandwidth of the light source • Accuracy of measurement • The mirror used as a reference standard (or coupon) • Size and opening of integrating sphere • Size of the reflected beam • Size of the light trap for diffuse measurements 6.3.2 Reflectometers Reflectometers measure the intensity of the light source after reflection on a sample. They are equipped with light sources that radiate only one or a few narrow wavelength lines or bands, so they give less information about the broad spreading of the reflected light than spectrophotometers. These lines or bands should at least be near the solar energy peak between λ = 500 nm and λ = 660 nm (see Figure 3-6). They must always be calibrated with a known reference standard, since the device measures the flux in the sample and compare it with the calibrated one, hence giving a relative reflectance value. These are used in the field to quantify the cleanliness factor (see chapters 5.2 and 6.4.3). The beam alignment for each measurement should be adjustable to compensate for any surface curvatures. The positioning adjustment should be stable without damaging the mirror. It has been told several times during this work that the scattering phenomenon is wavelength-dependent and that relationship is stronger if the quality of the mirror material is poor and there is dust over the sample surface, therefore, the reflectometers designed to be used in the field must have as a feature the possibility to measure at different wavelengths, as broad as possible is desirable, but in the range of the VIS is recommended. The lamps used for the generation of the sample light is never monochromatic, it has to pass through several filters to modify its wave spectral distribution, so the quality of the lamps used is also a critical point in the design of such devices. The near-specular reflectance is only interesting to calculate for CSP purposes (see chapter 3.3.2), so at least the acceptance angle should have a range between 7.5 mrads and 23 mrads. Solar reflectance measurement 72 72 Both types of equipment (spectrophotometers and reflectometers) use detectors to measure the reflected beam from the sample. Detectors are devices which convert the useful information carried in electromagnetic waves into forms which can be more easily handled and interpreted. The ideal detector properties can be summarized as follows [80]: • Wide wavelength range • Highly sensitive • High signal-to-noise ratio • Reacts immediately to changes in the incoming signal • Produces a linear response which is independent of wavelength • Durable and stable over time For example, a list of detectors and its useful detection range is presented in Table 6-1. Table 6-1 UV-VIS, NIR, and IR Detectors [81] Useful detection range Detector type μm Silicon 0.30 - 1.10 PbS 1.1 - 3.0 InAs 1.7 - 5.7 InGaAs 0.90 - 1.7 Ge : X 2 – 40 Ge : Au 2 – 9 Ge : Cd 2 – 24 PbSe 1.7 – 5.5 Ge : Zn 2 – 40 InSb 1.8 – 6.8 73 73 Review of models and procedures for the measurement of reflectance in CSP plants Useful detection range Detector type μm PbTe 1.5 – 4.5 MCT (photovoltaic) 1 – 17 TGS (triglycine sulfate) 10 – 120 HgCdTe 1.5 – 16 PLT (pyroelectric lithium tantalate) 1.5 – 30 For a better understanding of the characteristics and limits of the equipment used to measure reflectance, the parameters which measure the performance of a detector can be condensed in: 1. Responsivity (as photosensitivity) 2. Spectral response 3. Response time 4. Time-constant 5. Cut-off frequency 6. Noise Equivalent Power 7. Detectivity 8. Dynamic Range 6.4 Reflectance measurement in the field 11 6.4.1 Methods and procedures The main goal to measure reflectance for an O&M department is to evaluate the aging of the reflector material 12 and optimize the mirror washing schedules in the plant. Soiling induces losses 8 -14 times greater for CSP plants than in a PV plant [75] since the scattered light can not be used in STE plants. It is recommended to create and validate a model to find a correlation between cleanliness or soiling rate and plant energy yield, to separate the 11 For the standard equipment used in laboratory to characterize the reflectance spectra of mirrors, please refer to [74]. 12 The reflectance measurement also gives valuable information about the quality of the reflector material and its construction, in case of multilayer reflectors. But this characterisation must be performed either by the manufacturer or by the constructor or owner before the installation of the mirror in the field. Solar reflectance measurement 80 80 6.4.4 Determination of the mean reflectance of a solar field in operation For any proposed model to calculate straight-forward the current mean reflectance of a solar field, the number of measurements to be taken and the validation of the method are two fundamental steps to give credibility to the proposed model. The goal of statistical analysis is to identify some criteria for finding the mean reflectance of a heliostat solar field and its possible source of variations. According to Fernández-Reche [87], three criteria can be identified: 1. the position of the measurement in the facet 2. the facet position in the heliostat 3. the focal group in the solar field. The statistical analysis (it can be applied for CSP reflectors too, please refer to Fernández García [88]) will conclude if it could be measured in any position of the facet or some particular position reveals more information about reflectance than others. And the same goes for the other possible sources. For the first two sources of variations, the statistical analysis is performed through an ANOVA analysis of variance. Before doing that, one of the assumptions in the ANOVA is that the probability distribution of the responses must be normal or fit in a normal distribution. So, a test of normality must be performed to perform the ANOVA with the raw data. Otherwise, the data would have to be normalized. All the ANOVA analysis must be performed with a significance level of 95%. To determine if there are one single group of heliostats or more in the solar field, without considering its focal length, a cluster analysis is more appropriated. In the case that the field heliostats would be clusterised, it will require to measure representative heliostats for each cluster, making the task more time-consuming. One reason why this can happen is that there are heliostats that get dirtier than others in the solar field. In the study of Fernández-Reche, the statistical analysis shows that there is no significant variation in the position of the measurement on the facet, neither was a significant difference between the facets in the heliostats. From the cluster analysis, two groups were defined, but it was related to the dirtiness of two heliostats which could not be washed while the others were, so if the soiling factor is homogeneous in all the field, all the heliostats can be grouped regardless of where they are located in the solar field. Therefore, the plant operator could select the easiest point in the facet to perform the measurement and the more accessible facet to do it. To select a representative sample, Fernández-Reche uses the following formula: 𝑛= 1 𝐸2 𝑘2∙𝑆2+1 𝑁 Where n is the size of the sample, N the sample population, E the error associated with the process, k a factor associated with the significance level and S2 is the sample variance. The parameters in this equation which are variables are n and E. So, if an error is defined, several heliostats have to be sampled to obtain this error. The 81 81 Review of models and procedures for the measurement of reflectance in CSP plants total error of the measurement then is the sum of the statistical error of the process and the error associated with the measurement device. In the case that the null hypothesis of the ANOVA analysis cannot be accepted, other post hoc tests might be performed. Fernández-Reche recommends Tukey test [89]. For the validation of this method, a measurement campaign has to be carried out after agreeing on the measurement procedure, as said, position in the facet, facet position in the heliostat and heliostat clusters. The campaign has to be long enough in time to cover all the environmental and maintenance situations given in real life. At the same time, sufficient measurement data must be collected to ensure that the total procedural error is less than 0,3%. The collected data from the campaign could be used in a heliostat field simulation software, to obtain a total power coming from the field. That value could be then compared with solar flux measurements done in the receptor itself through a flux mapping system like the proHERMES developed by the DLR. As an example, the data measured with the proHERMES-2A in the receptor and the given by a heliostat simulation software differs between 1.9% and 2.2%. Having both methods almost same error in calculating the total power in kW. 6.5 Ishikawa Diagram: Reflectance measurement To summarize all the factors involved in the measurement of the reflectance in solar thermal power plants, an Ishikawa diagram or Fishbone diagram has been created. The problem here describe is the reflectance measurement in a mirror. The Ishikawa diagram categorizes the sources of errors in 6 categories: Materials, Method - Procedure, Environmental - Location, Operator, Measurement and Machine. Solar reflectance measurement 82 82 Figure 6-10 Ishikawa diagram: reflectance measurement Reflectance measurement ENVIROMENTAL - LOCATION Solar Irradiance( kWh/m2/year) Air Mass used for calculations Clearness Index Kt TMY CCP collector type PTC Tower Stirling dish Mirror height on site working and standby position Tilt and Azimut angle film build-up over the mirror surface Humidity METHOD - PROCEDURE Reference mirror state Calibration record Calibration range of acceptance and Cleanliness MATERIALS Mirror coating type Front Surface Mirror: Al-Si polymer Second Surface Mirror: Silvered glass Microscopic surface irregularities (loss of specularity due to Angular distribution) Flatness-Bent Roughness Microcracks Haze Aging Lamination configuration (scattering) trapped dust between layers adhesive between reflective layer and substrate Hemispherical reflectance Directionality Solar weighted reflectance Measurement beam characteristics Surface normal Beam Alignment Beam Spot size Type of Collector PTC Tower Dish Dust particle size Aerosols (i.e Sahara dust) Gusts of wind MACHINE Operating temperature weight, size, battery life half acceptance angle of the detector aperture MEASUREMENT Hemispherical reflectance wavelenght dependence normal vs incoming light angle Reflectivity range (min-max) Linearity of the measurement Specularity of the mirror aperture size incidence angle - Correct incidence angle selection OPERATOR Spot measurement accuracy sample mounting Misshandling of the operator poor training long campaigns Scattering distribution function wavelenght Acceptance angle Dust inhomogenity Temperature dependance Background light distortion 83 7 CONCLUSIONS The characterization of the reflectance beam profile on any material has been researched during the past 70 years, reaching a very mature status about the procedures, equipment, and knowledge about how to measure reflectance in a reflective surface and how to characterise its sources of errors. However, the measuring of the condition of the solar field (aging, soiling), regarding mirrors, is still a very timeconsuming and expensive task for any plant operators. The portable reflectance devices have been developed over the past 40 years without many notable advances. The list of portable devices manufacturers is short, and no device stands out from the rest, meaning that the measurement of reflectance in the field by portable devices could not be the solution to measure the overall condition of a heliostat/collector field regularly. These campaigns are expensive in manpower and in time, and the data collected is not always accurate since the portable devices are not user friendly. Any model proposed to measure in near-real-time the average reflectance value of a plant should not include such continuous campaigns where the operators, which should always be properly trained for that task, measure reflectance with portable devices in the field. More research effort is needed to eliminate portable equipment and that these are only used for specific measurements to validate the results of other more advanced and less labour-intensive methods: the idea of installing sensors in the mirror such as the SMARTMIRROR, the idea of the AVUS, using devices with IoT capabilities or drones (there are research centres such as DLR, CIEMAT and ENEA working on this issue) should be the lines of action of research centres. Sadly, most of the advances and research in the reflectance field have come the abovementioned research centres. Initiatives like the SOLWARIS project [86], funded by the European Union, help to maintain the progress in solving the biggest problems STE plants are facing. Although this project aims to reduce the water consumption in a CSP plant, measure reflectance as a relative measure when measuring soiling could help to develop the equipment and procedures to assess the overall condition of the solar field in an agile manner. In the past years, the bankruptcy of several mirror manufacturers did not help in the search for better mirror setups. AET and OET performed in the most common used mirror setups have shown that there is still work to do to achieve a high reflectance profile over the lifetime of an STE plant. As well, it seems that all the progress from new mirror materials and configuration is coming from the research centres and the private sector is no investing more funds in such tasks. The low integration of STE plants in the grid systems all over the world might induce the constructors not to invest money in technologies with high LCOE. However, it has been demonstrated that in order to have a high integration of renewable systems into the electricity grid, it is necessary to integrate systems with high availability and high-capacity factors into the grid, and in this case, CSP technology is the only mature technology that offers dispatchability of power generation at low risk. Finally, the new era of the 5G communications is going to ease the use of drones and the implementation of IoT devices in the energy plants. A revision of how these new devices can help to reduce the LCOE of a CSP plant might be helpful in the coming future. 84 CONCLUSIONES La caracterización del perfil de reflectancia de los materiales con características reflectantes se lleva investigando desde hace más de 60 años, habiendo alcanzado este aspecto una madurez en cuanto a los procedimientos, los equipos de medida y el progreso sobre cómo medir la reflectancia en una superficie reflectante y cómo caracterizar sus errores. Sin embargo, la medición del estado del campo solar (envejecimiento, suciedad), en lo que respecta a los espejos, sigue siendo una tarea muy lenta y costosa para cualquier operador de planta. Los dispositivos portátiles de reflectancia se han desarrollado en los últimos 40 años sin muchos avances notables. La lista de fabricantes de dispositivos portátiles es corta y ningún dispositivo se destaca del resto, lo que significa que la medición de la reflectancia en el campo mediante dispositivos portátiles no podría ser la solución para medir el estado general de un campo de helióstatos/colectores con regularidad. Estas campañas de testeo son costosas en mano de obra y en tiempo, y los datos recogidos no siempre son precisos, ya que los dispositivos portátiles no son fáciles de usar. Cualquier modelo propuesto para medir en tiempo casi real el valor medio de reflectancia de una planta no debería incluir esas campañas continuas en las que los operadores, que siempre deberían estar debidamente capacitados para esa tarea, miden la reflectancia con dispositivos portátiles sobre el terreno. Es necesario un mayor esfuerzo de investigación para eliminar los equipos portátiles y que éstos sólo se utilicen para mediciones específicas para validar los resultados de otros métodos más avanzados y menos intensivos en mano de obra: la idea de instalar sensores en el espejo como el SMARTMIRROR, la idea del dispositivo AVUS, utilizar dispositivos con capacidad de IoT o drones (hay centros de investigación como el DLR, el CIEMAT y el ENEA que trabajan en este tema) deberían ser las líneas de acción de los centros de investigación. Lamentablemente, la mayoría de los avances e investigaciones en el campo de la reflectancia han venido de los centros de investigación mencionados. Iniciativas como el proyecto SOLWARIS [86], financiado por la Unión Europea, ayudan a reducir los costes medioambientales y operativos de las plantas de energía de concentración. Aunque este proyecto tiene como objetivo reducir el consumo de agua en una planta de CSP, la medición de la reflectancia como medida relativa al medir la suciedad podría ayudar a desarrollar el equipo y los procedimientos para evaluar el estado general del campo solar de una manera ágil. En los últimos años, la quiebra de varios fabricantes de espejos no ha ayudado en el desarrollo de mejores configuraciones de espejos o nuevos materiales. Los AET y OET realizados en las configuraciones de espejos más comunes han demostrado que todavía queda trabajo por hacer para lograr una configuración que mantenga una reflectancia especular alta durante la vida útil de la planta. Además, parece que todo el progreso de los nuevos materiales y configuraciones de espejos proviene de los centros de investigación y el sector privado no está invirtiendo más fondos en esas tareas. La escasa integración de las plantas CSP en los sistemas de red de todo el mundo podría inducir a los promotores de tales plantas a no invertir dinero en tecnologías de alto LCOE. No obstante, se ha demostrado que, para tener una alta integración de sistemas renovables en la red eléctrica, es necesario integrar en la red sistemas con alta disponibilidad y factores de capacidad altos, y en ese caso, la 85 85 Review of models and procedures for the measurement of reflectance in CSP plants tecnología CSP es la única tecnología madura que ofrece gestionabilidad de la generación de la energía a bajo riesgo. Finalmente, la nueva era de las comunicaciones 5G va a facilitar el uso de drones y la implementación de dispositivos de IoT en las plantas de energía. Una revisión de cómo estos nuevos dispositivos pueden ayudar a reducir el LCOE de una planta CSP podría ser útil en un futuro próximo. 86 NOMENCLATURE AET Accelerated Exposure Testing AM Air Mass AU Astronomical Unit: is a unit of the averaged distance between the Sun and the Earth. Is defined as exactly 149.597.870.700 metres (≈ 150 Million kilometres). BESS Battery Energy Storage System CAPEX Capital Expenditure CSIRO Commonwealth Scientific and Industrial Research Organisation CSP Concentrating Solar Power CSTP Concentrated Solar Thermal Power DLR Deutsches Zentrum für Luft- und Raumfahrt DNI Direct Normal Irradiance FRES Fluctuating Renewable Energy Sources. Also called Variable Renewable Energy Sources FSM Front Surface Mirror G Irradiance: Radiant flux incident on the sample surface per unit area. SI-Unit: W/m2; The spectral solar irradiance or direct solar irradiance is the same concept but takin into account the dependence with the wavelength, is denoted as Gb(λ). SI unit: W/m2/nm. 1. The subscript b designates beam insolation; the surface orientation is normal to the sun unless otherwise specified by a further subscript. 2. The subscript d designates diffuse insolation; the surface orientation is horizontal unless otherwise specified by a further subscript. 3. The subscript h designates hemispherical insolation; the surface orientation is horizontal unless otherwise specified by a further subscript. 4. The subscript o designates extraterrestrial insolation. H Irradiation: incident energy per unit of a surface on a given plane, obtained by integration of irradiance G during a given time interval, usually an hour or a day. SI Unit: MJ/m2 for a given interval. 3.6 MJ/m2 = 1 kWh/m2. HCE Heat Collector Element HTF Heat Transfer Fluid I Radiant Intensity: radiant power per unit solid angle of a light source in any given 87 87 Review of models and procedures for the measurement of reflectance in CSP plants direction. SI-Unit: W/sr IAM Incidence Angle Modifier IC Intercept Factor IoT Internet of Things IRENA International Renewable Energy Agency ISCC Integrated Solar Combined Cycle KPI Key Performance Indicator L Radiance: is the emitted radiant intensity by a surface unit of a light source in a direction given by an angle θ. SI Unit: W/steradian/m2 LCOE Levelized Cost of Electricity LED Light Emitting Diode LFR Linear Fresnel Reflectors M Radiant Emittance or radiant exitance NIR Near-Infrared NIST National Institute of Standards and Technology NREL National Renewable Energy Laboratory O&M Operation & Maintenance OET Outdoor Exposure Testing PPT Percentage Points PSA Plataforma Solar de Almería PTC Parabolic-Trough Collector PTG Photonics Technology Group at the University of Zaragoza PV Photovoltaic (plants) PVD Physical Vapour Deposition Ra Arithmetical Mean Roughness RMS Root Mean Square RU Reflectance Units SEGS Solar Electric Generating Station sr Steradian: is the SI-Unit of the solid angle Ω. Is a dimensionless unit. Is the analogue in three dimensions to the radian. The solid angle is related to the area of the spherical surface that it spans. A solid angle of one steradian (sr), is defined as the solid angle that delineates an area on the surface of a reference sphere equal to the radius-squared of that sphere. There are 2π sr in a Nomenclature 88 88 hemisphere. SSM Second Surface Mirror ST Solar Tower STE Solar Thermal Energy TES Thermal Energy Storage TIS Total Integrated Scattering TSI Total Solar Irradiance USA United States of America UV Ultraviolet γs Solar azimuth angle θ Incidence angle, Acceptance half-angle, half-cone angle ρ Reflectivity, an intrinsic property of a material. Reflectance is defined as an extensive property of a material σ Standard deviation φ Acceptance angle ϕ Angular spread, for solar concentrating purpose, angular spread of the concentrated radiation incident on the receiver 89 TERMS AND DEFINITIONS Capacity Factor [Cp] Power output during time t (MWh) / time * rated power (MW) Concentrator Reflector surface that focuses sunlight into a receiver Solar collector A solar collector is the sum of the receiver, reflector surface, the supporting structure, and its foundation References 96 96 [76] A. Fernández García, F. Sutter, L. Martínez-Arcos, C. Sansom, F. Wolfertstetter and C. Delord, “Equipment and methods for measuring reflectance of concentrating solar reflector materials,” Solar Energy Materials and Solar Cells, vol. 167, no. August, pp. 28-52, 2017. [77] C. A. Arancibia-Bulnes, M. I. Peña-Cruz, A. Mutuberría, R. Díaz-Uribe and M. 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