Bioplastics for solar collector components
IEA-SHC Task 39 INFO Sheet C5
May 2015 - PDF 0.3MB
Numerous research and development activities carried out by renowned research facilities and the global players in the polymer and solar industry – presented and discussed also within the framework of IEA SHC Task 39 – demonstrate that polymers are the materials of choice for next generation solar thermal systems. In addition to classical polymers made from petrochemical resources bioplastics (i.e. polymers based on renewable resources and/or biodegradable polymers) have been introduced as a sustainable and seminal alternative.

Case study: Accelerated UV-Aging
IEA-SHC Task 39 INFO Sheet C7.1
May 2015 - PDF 0.39MB
Polymeric materials for solar thermal applications have to be tested thoroughly before they can be used for the construction of novel collectors. Used as absorbers, glazing or framing, just to mention a few applications, polymers are exposed to a broad variation of different environmental influences which may affect physical and mechanical properties of the materials and limit their service life time. Three major environmental influences, so called degradation factors are: temperature, UV radiation and humidity. One of these factors, the UV radiation, is of great impact on all organic macromolecules, like polymeric materials.

Case Study: Twin wall sheet testing
IEA-SHC Task 39 INFO Sheet C7.4
May 2015 - PDF 0.38MB
When using polymeric materials for solar thermal flat plate collectors, one distinct difference in physical properties compels us to reinvent the absorber design. Due to the low thermal conductivity of polymeric materials, the absorber, in order to prevent local overheating and to increase the collectors’ efficiency, needs water contact throughout the whole surface. In general only few absorber designs fulfill this requirement, e.g. thin plastic film absorbers, tube absorbers or twin wall sheets. The latter two are in the focus of recent development due to mechanical stability and economic efficiency. In order to investigate changes in the mechanical properties of the used materials as closely to the product as possible suitable mechanical testing methods need to be applied. For pipes, methods to test different mechanical loads already exist, but for twin wall sheets none of these can be applied.

Overheating protection thermotropic layer
IEA-SHC Task 39 INFO Sheet C3
May 2015 - PDF 0.31MB
In recent years thermotropic overheating protection glazings are in focus of interest both, in solar thermal collector technology and architectural design of buildings. A thermotropic glazing changes its light transmittance from highly transparent to light diffusing upon reaching a certain threshold temperature autonomously and reversibly. Among different thermotropic systems, especially thermotropic systems with fixed domains (TSFD) are considered to have the highest potential for practical application. In TSFD scattering domains are embedded in a polymer matrix, which exhibit a sudden change of the refractive index upon reaching the threshold temperature. At low temperatures the layer is translucent, as the refractive indices of matrix and domain are almost equal. If the difference of the refractive index of both components increases due to warming up to a determined temperature (switching threshold) the thermotropic film turns opaque

Polyamide based integrated storage collector
IEA-SHC Task 39 INFO Sheet C2
May 2015 - PDF 0.43MB
Integrated storage collectors (ISC) offer a high potential for domestic hot water preparation in tropical, dry and moderate climate zones. In commercially available pressurized single-loop ISC systems the absorber/storage-tank is made from costly metals such as stainless steel or copper pipes. A specific objective of the European Union collaborative research project SCOOP (Solar Collectors made Of Polymers) is to develop and evaluate plastics based ISC tanks manufactured by well-established, high-throughput injection moulding technologies. This info sheet describes the collector concept, most-promising engineering plastics grades, experimental tools and manufacturing processes and the performance of functional model components.

Polymeric Liner Materials for Hot Water Heat Storages
IEA-SHC Task 39 INFO Sheet C7.3
May 2015 - PDF 0.36MB
Hot water heat storages are important components of solar thermal systems. Various polyolefin-based material grades are material candidates to serve as liners in hot water heat storages. In order to fulfil the requirements, characterization of the aging behavior is indispensable for material selection.

Polypropylene absorber materials
IEA-SHC Task 39 INFO Sheet C1
May 2015 - PDF 0.4MB
Polypropylene (PP) grades offer a high potential for use in solar thermal absorbers, in particular for flat plate collectors with adequate overheating control. For unglazed swimming pool collectors blackpigmented PP grades are state of the art. PP absorber grades for glazed collectors have to be tailored according to the specific loadings conditions and performance requirements which are significantly dependent on collector and system type. In this info sheet a PP grade widely used for swimming pool absorbers and a novel PP grade investigated within the collaborative research project SolPol-2 are evaluated as to their applicability for glazed collectors with full overheating protection.

The Art of Stabilization – Analytical Evaluation of Stabilizer Systems
IEA-SHC Task 39 INFO Sheet C7.2
May 2015 - PDF 0.36MB
Without proper stabilization, polymers are susceptible to degradation caused by reactions with oxygen or UV-light, which lead to undesirable changes in the properties of the polymer. A stabilization system is normally added to the polymeric material, which is responsible for maintaining mechanical properties like strength and toughness. Unfortunately, it is not yet fully clear which combination of stabilizers provides the best performance for a specific application. Furthermore, interactions between different stabilizer classes have to be carefully investigated as they may lead to exploitable synergistic or avoidable antagonistic effects.

TISS coating as added value for polymeric solar absorber
IEA-SHC Task 39 INFO Sheet C4
May 2015 - PDF 0.35MB
Polymer based solar collectors are one of the important parts of modern architecture. Market demands are oriented to the colored absorbers to fulfil the demand of architects and integrate the collectors in new buildings. One of the options for production of colored collectors is usage of the Thickness Insensitive Spectrally Selective (TISS) paints. TISS paints are tailor-made, multifunctional materials based on a variety of organic macromolecules and functional and processing additives. The material aspects of the colored TISS paint coatings are focusing on pigments, metallic and metallized flake pigments and polymeric resin binders used for the production of solar paint coatings with the help of dispersant molecules in order to achieve uniform distribution of the finely ground pigment particles in the polymeric resin binder. An important prerequisite for the successful selection and use of pigments in solar-thermal systems is the usage of high absorptivity pigments with their high loading in combination with the low thermal emitting binder.

Tool box for basic characterization of plastics
IEA-SHC Task 39 INFO Sheet C6
May 2015 - PDF 0.51MB
Plastics are tailor-made, multifunctional materials based on a variety of organic macromolecules and functional and processing additives. An important prerequisite for the successful selection and use of plastics in solar-thermal systems is the comprehensive definition and description of application-relevant loading profiles and the deduction of property requirements. For specified plastics grades reproducible material quality has to be controlled and assured. In this info sheet a tool box for the basic characterization of plastics for solar-thermal systems is described. Relevant features and properties are exemplarily depicted for absorber materials based on polyphenyleneoxide (PPO) and polyphenylenesulfide (PPS).

Task 39 Subtask C Report
Presentation at SHC 2014 in Beijing China
October 2014 - PDF 1.68MB
Task 39 Subtask C Final Report

Thermotropic glazings for overheating protection. I. Material preselection, formulation, and light-shielding efficiency
Journal of Applied Polymer Science 131 (4), 2014, doi: 10.1002/app.39950
February 2014
Publisher: Wiley Periodicals, Inc.
--- This article presents a systematic strategy for formulation and optimization of thermotropic layers for overheating protection purposes. Specifically, thermotropic systems with fixed domains (TSFD) which consist of a thermotropic additive finely dispersed in a matrix material are considered. Based on systematic material (component) preselection regarding thermoanalytical characteristics and refractive indices, numerous thermotropic layers were formulated. [....]

Thermotropic glazings for overheating protection. II. Morphology and structure-property relationships
Journal of Applied Polymer Science 131 (4), doi: 10.1002/app.39910
February 2014
Publisher: Wiley Periodicals, Inc.
--- This article completes a systematic strategy for formulation and optimization of thermotropic systems with fixed domains (TSFDs) for overheating protection purposes. Focus was on characterization of morphology and on revealing optimization potential. A comprehensive characterization of scattering domain size and shape was done applying optical microscopy and scanning electron microscopy. In general, scattering domains exhibited inappropriate size and/or shape for optimum overheating protection performance ...

Thermotropic Overheating Protection Glazings: Effect of Functional Additives and Processing Conditions on Light-Shielding Efficiency
Journal of Polymer Engineering 2014 Vol: 34(2):161-172. DOI: 10.1515/polyeng-2013-0275
February 2014
Editor: Grizzuti, Nino
Publisher: Walter de Gruyter GmbH
ISBN: ISSN (Online) 2191-0340
--- Within this study, the effect of functional additives and processing conditions on the overheating protection performance of thermotropic systems with fixed domains (TSFD) for overheating protection purposes was evaluated. The focus was on improving the overheating protection performance of a prototype TSFD based on a UV curable acrylate resin by optimization of the material constitution (addition of functional additives like surfactants and nucleating agents) and the processing conditions (temperature conditions during manufacturing, annealing). For the evaluated system, an effect of the nucleating agent on the overheating protection performance was ascertained. Furthermore, omission of an annealing step improved the overheating protection performance slightly.

Thermotropic systems with fixed domains exhibiting enhanced overheating protection performance
Journal of Applied Polymer Science 131 (12), doi: 10.1002/app.40417
February 2014
Publisher: Wiley Periodicals, Inc.
--- Within this study, a time saving photo-initiated miniemulsion polymerization process (duration of polymerization was 15 min) was established in order to encapsulate a paraffin wax with an acrylate polymer shell. The obtained freeze-dried latex was an off-white powder exhibiting spherical particles with mean diameters around 400 nm and a concentration of paraffin wax around 56% ...

A Model of the Optical Properties of a Non-absorbing Media with Application to Thermotropic Materials for Overheat Protection,
Energy Procedia 30 (2012) 116-124
November 2012
Publisher: Elsevier Ltd.
--- Thermotropic materials offer the potential to provide overheat protection for polymer absorbers. These materials are composed of a matrix material in which a second material, referred to as the scattering domain, is dispersed. Temperature control is accomplished by a reduction in transmittance at a desired temperature corresponding to the phase change temperature of the scattering domain. The phase change is accompanied by a change in refractive index. This paper describes a numerical model to predict the transmittance and reflectance of a polymer based thermotropic material as a function of the relative index of refraction m between the matrix and scattering domains, the scattering domain size and volume fraction fv, and the sheet thickness ...

Thermotropic glazings for overheating protection
Energy Procedia 30 (2012) 471-477
November 2012
Publisher: Elsevier Ltd.
--- In this paper, the effect of material composition on the overheating protection performance of thermotropic systems with fixed domains (TSFD) is studied. Several thermotropic layers were formulated by variation of both, matrix material and thermotropic additive. Refractometry was applied in order to obtain refractive index data as a function of temperature of all material constituents and to screen wether material combinations are promising to formulate or not. Investigations concerning optical properties, switching temperature and switching process were carried out applying UV/Vis/NIR spectrometry. Morphological analysis was conducted with a Scanning Electron Microscope (SEM). Several TSFD showed reasonable light-shielding efficiency. Nevertheless further optimization of scattering domain size and shape is necessary to improve the light shielding performance.

Smart Windows - Smart Collectors, Entwicklung und Charakterisierung von Überhitzungsschutzverglasungen für Gebäudeverglasungen und thermische Solarkollektoren
BHM 156 (2011) 11, S. 429 - 433
December 2011
Publisher: Springer Science+Business Media
--- Thermotropic glazings change their light transmittance from transparent to opaque upon reaching a certain threshold temperature reversibly. Such functional glazings can provide effective overheating protection for building façades or solar thermal collectors. The overall objective of the present study is to produce tailor-made thermotropic overheating protection glazings by systematic material pre-selection and formulation ...

Coil-coated spectrally selective coatings on copper or aluminium with pigments modified by aminosilanes
WIPO Patent Application WO 2010/133693
May 2010
--- The invention relates to TSSS and TISS coatings applied by coil-coating onto substrates of copper or aluminum. Inorganic pigments functionalized by aminosilane are used for making paints, which after the coil-coating onto the substrate form the coatings. For the functionalization the pigment is dispersed in a solution of an aminosilane in a solvent or a mixture of a solvent and a binder without an addition of a non-aminosilane dispersing agent with or without subsequent grinding and use of 0.05-30 % of aminosilane on pigment. An aromatic, aliphatic, cycloaliphatic, ketone, ester, ether or alcohol compound or a mixture thereof can be used as the solvent. A silicone-polyester, polyurethane or fluoropolymer can be used as the binder.

The effect of polyhedral oligomeric silsesquioxane dispersant and low surface energy additives on spectrally selective paint coatings with self-cleaning properties.
Solar Energy Materials and Solar Cells (94) 2, pp. 232-245
February 2010
--- Thickness-Insensitive Spectrally Selective (TISS) paint coatings were made of fluoropolymer resin binder (Lumiflon (LF), Asahi Glass Co., Ltd., Japan) and their water- and oil-repellent properties were obtained by the addition of Polyhedral Oligomeric Silsesquioxane (POSS) characterized by amino (AP), isooctyl (IO) and perfluoro (PF) groups attached on the silsesquioxane cube. Paint dispersions were made by modifying of black spinel pigment with trisilanol isobutyl POSS dispersant and with a single-capped silane isobutyltrimethoxysilane (IBTMS). Infrared and 29Si NMR spectra measurements were used for the identification of the structural characteristic of the corresponding POSS compounds ...

Geeignete Polymere für Innovationen in der Solarthermie
2. Solarthermie-Technologiekonferenz, Berlin, Germany
January 2010
Presentation is available as PDF-file and online video. Image: © Creventis Services 2010

Morphology of phase-separated thermotropic layers based on UV cured acrylate resins Polymers for Advanced Technologies
Polymers for Advanced Technologies Vol. 20, Issue 12, 1163–1167, Dec. 2009
December 2009
--- In this paper, relationships between the scattering domain parameters (size and shape) and the light-shielding properties of thermotropic systems with fixed domains (TSFD) are established. Specific focus is given to the effect of additive type on the formation of scattering domain size. Various functional layers are prepared by a variation of thermotropic additives. Scattering domains are investigated applying high resolution Atomic Force Microscopy (AFM) in a phase imaging mode. Thermotropic layers formulated with additive types exhibiting a short chain length display roughly spherical scattering particles with dimensions between 0.5 and 3?µm and a moderate reduction in hemispheric solar transmittance along with a significant increase in diffuse solar transmittance above the switching threshold ...

Corrosion protection of Sunselect, a spectrally selective solar absorber coating by (3-mercaptopropyl) trimethoxysilane.
Solar Energy Materials and Solar Cells (93) 10, pp. 1733-1742.
October 2009
--- The formation of a protective layer from (3-mercaptopropyl)trimethoxysilane (MPTMS) on commercial Sunselect, cermet-based spectrally selective coating (Alanod, DE), was studied by non-electrochemical (infrared reflection–absorption—IR RA, X-ray photoelectron spectroscopy—XPS), electrochemical (cyclic voltammetry (CV) in the presence of a redox probe (Cd+2), and potentiodynamic (PD)) techniques. By simple immersion and subsequent dip coating of the Sunselect substrate in the MPTMS sol, the hydrolyzed MPTMS precursor was adsorbed on the substrate, forming a protective layer imparting corrosion stability to Sunselect in a salt spray chamber for at least 20 days, outperforming any other sol–gel coating used so far for the corrosion protection of Sunselect ...

Phase separated thermotropic layers based on UV cured acrylate resins – Effect of material formulation on overheating protection properties and application in a solar collector
Solar Energy (83) 9, pp. 1689-1697
September 2009
--- This paper focuses on the effect of material composition on the overheating protection properties of thermotropic systems with fixed domains for solar thermal collectors. Numerous functional layers were prepared by a variation of base resin (polyester-, epoxy- or urethane–acrylate) and of thermotropic additives (non-polar and polar waxes) as well as by additive concentration (5 and 7 wt%). A detailed investigation of optical properties, switching temperature and switching process was performed applying UV/Vis/NIR spectroscopy. Thermal transitions of both the thermotropic layers and the additives used were determined by Differential Scanning Calorimetry (DSC). The capability of the produced thermotropic layers to reduce stagnation temperatures in an all-polymeric flat plate collector was evaluated by theoretical modeling. The thermotropic layers showed a hemispheric solar transmittance between 76% and 87% in clear state. Above the switching threshold this transmittance changed by 1–16% to values between 62% and 85% ...

Adhesion and thermal stability of thickness insensitive spectrally selective (TISS) polyurethane-based paint coatings on copper substrates
Solar Energy Materials and Solar Cells (93) 5, pp. 630-640
May 2009
--- Thickness insensitive spectrally selective (TISS) paint coatings based on a polyurethane polymeric binder deposited on copper substrates were investigated to obtain information about their service lifetime. The degradation of TISS paint coatings was performed according to the methodology worked out within Task 10 of the IEA's Solar heating and the cooling programme. The activation energy (Ea) for the degradation process was derived from vibrational band changes of the polyurethane binder recorded in the infrared hemispherical reflectance spectra of TISS paint coatings exposed to different thermal loads. The results of the vibrational band analysis were correlated with cross-cut tests, showing that the coatings started to lose integrity at 190 °C but protected the copper substrate against oxidation perfectly even at 200 °C (15 days). An accelerated test procedure confirmed that TISS coatings could be safely used in solar collectors for at least 45 years.

Thermotropic layers for flat-plate collectors - A review of various concepts for overheating protection with polymeric materials
Solar Energy Materials and Solar Cells (93) 1, pp. 119-128
January 2009
--- Within this paper a comprehensive review of the developments of thermotropic hydrogels, thermotropic polymer blends and thermotropic systems with fixed domains for overheating protection purposes is given. In addition, performance properties for thermotropic layers to prevent overheating in solar collectors are defined. The different thermotropic material classes are discussed as to their ability to meet these requirements. The review shows that thermotropic layers developed so far need to be adapted as to switching temperature and long-term stability for applicability in solar thermal collectors.

Spectroscopic investigations of phase-separated thermotropic layers based on UV cured acrylate resins
Macromolecular Symposia (2008)
December 2008
Document Number: 265, 49-60
--- In this paper spectroscopic techniques were adapted and applied to characterize the optical and morphological properties of thermotropic resins. Thermotropic films were prepared by variation of resin base and thermotropic additives. By UV/Vis/NIR spectroscopy the solar optical properties, the switching temperature, the switching process and the residual transmittance in the opaque state were determined. To control the sample temperature from ambient to 100?°C a conventional UV/Vis/NIR spectrophotometer equipped with an Ulbricht-sphere was adapted by a heating stage. Additive content, distribution and scattering domain size were characterized by Differential Scanning Calorimetry, Attenuated Total Reflectance spectroscopy and microscopic techniques ...

Property and performance requirements for thermotropic layers to prevent overheating in an all polymeric flat-plate collector
Solar Energy Materials and Solar Cells (92), pp. 614-620
June 2008
--- Within this study the potential of thermotropic layers to prevent overheating of an all-polymeric solar collector is evaluated by theoretical modeling. The investigations show that collector stagnation temperatures can be reduced to temperatures between 80 and 90 °C by using thermotropic layers either in the glazing or on the absorber. For overheating protection to a maximum temperature of 80–90 °C a residual solar transmittance of the thermotropic layer below 0.25 in the opaque state (>0.85 in clear state) is effectual. The required switching temperature is dependent on the collector configuration.