Gary Jorgensen

Alternating current impedance and Raman spectroscopic study on electrochromic a-WO3 films

The chemical diffusion of lithium ions in a-LixWO3 films is investigated using alternating current impedance spectroscopy and Raman scattering measurements. The diffusion coefficients increase with increasing x in a-LixWO3 up to x=0.072 and then decrease. Raman measurements show that the W6+=O/O–W6+–O ratio also increases at the early stage of lithium insertion and then decreases with further lithium insertion. We conclude that the diffusion kinetics of lithium ions in a-LixWO3 films is very closely related to the W6+=O/O–W6+–O ratio.

Durability issues and service lifetime prediction of electrochromic windows for buildings applications

Abstract In general, the purposes of this paper are to elucidate the crucial importance of durability and service lifetime prediction (SLP) for electrochromic windows (ECWs) and to present an outline for developing a SLP methodology for ECWs. The specific objectives are (a) to illustrate the generic nature of SLP for several types of solar energy conversion or energy conservation devices, (b) to summarize the major durability issues associated with ECWs, (c) to justify using SLP in the triad of cost, performance, and durability rather than just durability, (d) to define and explain the seven major elements that constitute a generic SLP methodology, (e) to provide background for implementing the SLP methodology for ECWs, including the complexity of the potential degradation mechanisms, and (f) to provide an outline of studies using ECWs for improving the durability of ECW materials and predicting a service lifetime for ECWs using the SLP methodology outlined in objective (d). Our major conclusions are that substantial R&Dis necessary to understand the factors that limit ECW durability, and that it is possible to predict the service lifetime of ECWs.

Comparative assessment of solar concentrator materials

Abstract This paper reports results from long-term durability tests of reflector materials to be used for solar concentrating systems. The studies have been conducted under the auspices of an IEA–SolarPACES collaboration between the National Renewable Energy Laboratory (NREL, USA), the Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT, Spain) and Deutsches Zentrum fur Luft- und Raumfahrt (DLR, Germany). In this co-operative effort, accelerated ageing tests as well as outdoor exposures at a number of test sites having various climatic conditions have been carried out since 1995. In addition to materials already in use at solar power stations, newer materials offering the chance of a significant cost reduction in solar electricity and process heat generation are being investigated. Comparative optical tests are carried out to assess the efficiency as a function of exposure/service time in a solar concentrator. Among the materials showing promise for long-term outdoor applications are various silvered glass mirrors, a silvered polymer film, and an anodized sheet aluminium having an additional protective polymer coating. In addition to durability tests of reflector material samples, practical results are also reported for experiences with field applications of silvered thin glass and anodized sheet aluminium mirrors.

Applicability of highly reflective aluminium coil for solar concentrators

Abstract Because of their manufacturing flexibility and their low costs, mirrors based on anodized or coated sheet aluminium are a promising alternative as primary or secondary concentrators in a number of solar energy applications. They offer solar weighted reflectances of 88–91%, good mechanical properties and are easy to recycle. However, problems occur due to their limited corrosion resistance. Therefore, prior to application, lifetime tests including outdoor and accelerated ageing tests are necessary to prove their optical durability in terms of achieving a 10-year service lifetime. In this study the optical properties of a number of different aluminized reflector materials after accelerated and outdoor exposure tests have been investigated. Optical testing has been performed by measuring the spectral hemispherical reflectance of exposed samples and calculating the solar weighted value. Additionally, specular reflectance has been measured with a simple mobile reflectometer. Materials involved are standard commercial anodized sheet aluminium with layers of different thicknesses and standard high specular aluminium with a metaloxide layer system plus an anti-oxidation polymer coating. Results show that optical degradation is strongly dependent on climatic conditions. Non-organic coatings involved are primarily attacked by humid climates with higher amounts of atmospheric pollution. Standard anodized materials withstand outdoor and accelerated weathering. However reflectance tends to become less specular, which limits their application in concentrating technologies. Finally, small scale application tests have been performed to demonstrate the applicability concerning handling and mechanical connection with support structures. By measuring power density in the focus of a test collector, minimum specular reflectance requirements for trough systems can be defined.

A Review of Polymer Materials for Solar Water Heating Systems

This paper summarizes current research aimed at using polymer materials for glazing and heat exchanger components in solar water heating systems. Functional requirements, relevant polymer properties and an approach for selecting polymers are described for each of these components. Glazing must have high transmittance across the solar spectrum and withstand long term exposure to ultraviolet (UV) light. Candidate glazing materials were tested outdoors for one year in Golden, Phoenix and Miami, as well as exposed for over 300 days in an accelerated testing facility at a concentration ratio of two at the National Renewable Energy Laboratory. Measurements of hemispherical transmittance indicate that a 3.35 mm polycarbonate sheet with a thin film acrylic UV screen provides good transmittance without excessive degradation. The primary challenge to designing a polymer heat exchanger is selecting a polymer that is compatible with potable water and capable of withstanding the high pressure and temperature requirements of domestic hot water systems. Polymers certified for hot water applications by the National Sanitation Foundation or currently used in heat exchangers and exhibit good high temperature characteristics were compared on the basis of a merit value (thermal conductance per unit area per dollar) and manufacturers recommendations. High temperature nylon (HTN), polypropylene (PP) and cross linked polypropylene (PEX) are recommended for tube components. For structural components (i.e. headers), glass reinforced high temperature nylon (HTN), polyphthalamide (PPA), polyphenylene sulphide (PPS) and polypropylene (PP) are recommended.

Silvered-PMMA reflectors

Abstract Metallized, flexible polymeric reflector materials are much lighter and potentially less expensive than the conventional glass/metal mirrors often used in solar thermal concentrators such as heliostats and parabolic dishes and troughs. Unweathered silvered-PMMA reflectors have a solar reflectance at least as high as glass reflectors, but their environmental durability needs to be demonstrated. ECP-305, a silvered-PMMA film available commercially from the 3M Company and developed in collaboration with the National Renewable Energy Laboratory, is the current state of the art. Important progress has been made in overcoming the three primary mechanisms causing ECP-305 to lose reflectance. These mechanisms are: (1) photon-induced silver corrosion, (2) surface soiling, and (3) a form of delamination called tunneling. Given the progress in resolving these performance lifetime issues, silvered-PMMA films should meet the reflectance and durability goals.

Influence of microstructure on the chemical diffusion of lithium ions in amorphous lithiated tungsten oxide films

We describe how the chemical diffusion of lithium ions is affected by microstructural changes in amorphous lithiated tungsten oxide (a-LixWO3) films as a function of lithium ion concentration. The chemical diffusion of lithium ions in a-LixWO3 films is investigated using ac impedance spectroscopy and Raman scattering measurements. Two different a-WO3 films deposited by thermal evaporation at different partial pressures of nitrogen are used for these measurements. Based on the results of ac impedance spectroscopy and Raman scattering measurements, we conclude that the mechanism of these diffusion phenomena is attributed to an increase or decrease of the W 6 + O/OW 6 + O ratio with lithium ion insertion.

Durability studies of solar reflector materials exposed to environmental stresses

Methods that allow accurate prediction of service lifetimes need to be developed and demonstrated to spur the commercialization of solar reflector materials and systems. Without confident knowledge of service lifetimes, uncertainties in life cycle cost and warranty projections will significantly limit the economic viability of solar concentrators. To assure general and robust usefulness, methods should be based on correlations between accelerated, laboratory-controlled environmental stress factors and actual deployment conditions for a variety of applicable geographical locations. Accelerated experiments have identified the key stress factors of UV radiation, temperature, and relative humidity, and their effect on loss of performance, for a particular class of reflector materials. A model is proposed that allows prediction of the time dependent degradation of such materials as an analytic function of combined applied stresses. The model is shown to provide excellent agreement with measured performance data and provides a strong basis for future comparisons with data collected in field tests.

Ethylene-Vinyl Acetate Potential Problems for Photovoltaic Packaging

Photovoltaic (PV) devices are typically encapsulated using ethylene-vinyl acetate (EVA) to provide mechanical support, optical coupling, electrical isolation, and protection against environmental exposure. Under exposure to atmospheric water and/or ultraviolet radiation, EVA will decompose to produce acetic acid, lowering the pH and increasing the surface corrosion rates of embedded devices. Even though acetic acid is produced at a very slow rate, it may not take much to catalyze reactions that lead to rapid module deterioration. Another consideration is that the glass transition of EVA, as measured using dynamic mechanical analysis, begins at temperatures of about -15 degC. Temperatures lower than this can be reached for extended periods of time in some climates. Because of increased moduli below the glass transition temperature, a module may be more vulnerable to damage if a mechanical load is applied by snow or wind at low temperatures. Modules using EVA should not be rated for use at such low temperatures without additional low-temperature mechanical testing beyond the scope of UL1703

REFLECTECH ® MIRROR FILM ATTRIBUTES AND DURABILITY FOR CSP APPLICATIONS

Reflectors are an essential part of parabolic trough solar electric and other concentrating solar power (CSP) systems. Reflectors in CSP systems require a high reflectance over the solar wavelength spectrum and they must be durable to outdoor exposure and resist all forms of degradation over time. All utility-scale CSP systems installed to date use glass reflectors. Glass mirrors have maintained their reflectance very well in CSP environments, but they are susceptible to windrelated breakage and are expensive to transport and install. Alternative lower-cost reflectors are needed to reduce the cost of CSP systems [1]. ReflecTech ® Mirror Film is a highly reflective polymer-based film co-developed with the National Renewable Energy Laboratory (NREL) for concentrating solar energy applications. The attributes of ReflecTech ® Mirror Film and test results for weatherability are described herein. This paper discusses field and lab test results and properties of ReflecTech ® Mirror Film, specifically: 1. Stability under ultraviolet (UV) light through accelerated testing and outdoor real-time testing . 2. Mechanical stability and resistance to moisture through water immersion tests for delamination and “tunneling”. 3. Mechanical resistance to high wind events common in utility-scale concentrating solar power applications. 4. Lighter weight and resistance to breakage that reduces transportation and installation costs, and allows greater design flexibility of concentrator geometries. 5. Lower initial cost compared with curved glass mirrors. To test for weatherability, reflector samples were subjected to controlled conditions more extreme than actual outdoor environments. NREL maintains a world-class testing capability for solar reflectors that includes a Solar Simulator (SS), QUV (an accelerated exposure chamber manufactured by Q-Lab Corp. that subjects materials to alternating cycles of light and condensation at elevated temperatures), and several WeatherOmeter ® (WOM) exposure chambers that allow accelerated testing of reflector samples. In addition, samples of ReflecTech