Barbara Milow

Development Steps for Parabolic Trough Solar Power Technologies With Maximum Impact on Cost Reduction

Besides continuous implementation of concentrating solar power plants (CSP) in Europe, which stipulate cost reduction by mass production effects, further R&D activities are necessary to achieve the cost competitiveness to fossil power generation. The European Concentrated Solar Thermal Roadmap (ECOSTAR) study that was conducted by European research institutes in the field of CSP intends to stipulate the direction for R&D activities in the context of cost reduction. This paper gives an overview about the methodology and the results for one of the seven different CSP system concepts that are currently under promotion worldwide and considered within ECOSTAR. The technology presented here is the parabolic trough with direct steam generation (DSG), which may be considered as an evolution of the existing parabolic systems with thermal oil as heat transfer fluid. The methodology is explained using this exemplary system, and the technical improvements are evaluated according to their cost-reduction potential using a common approach, based on an annual performance model. Research priorities are given based on the results. The simultaneous implementation of three measures is required in order to achieve the cost-reduction target: Technical improvement by R&D, upscaling of the unit size, and mass production of the equipment.

IMMOBILIZED PHOTOSENSITIZERS FOR SOLAR PHOTOCHEMICAL APPLICATIONS

New hydrophilic immobilized photosensitizers (heterogeneous phase) were synthesized that overcome some disadvantages of the use of homogeneous phase sensitizers for detoxification and disinfection of water. The chosen sensitizers, based on porphyrin moieties, were bound on poly(methyl methacrylate) (PMMA). The measured production rate of singlet oxygen is significantly higher than that of the well-known rose bengal immobilized on Merrifield polymer. The sensitive polymer can be used for detoxification and disinfection of polluted water.

Synthesis of inorganic aerogels via rapid gelation using chloride precursors

This paper describes the synthesis of silica, titania, zirconia and mixed-metal-oxide aerogels by a new facile approach. The corresponding low cost and non-toxic chlorides instead of the well established but more expensive and deleterious alkoxy precursors were used as starting materials. Hydrolysis of the precursors led to wet gels. The strong exothermic reaction in the case of TiCl4 or SiCl4 as a gel precursor can be controlled by both cooling and immersing the needle of the syringe in the aqueous solution. Under optimal conditions a rapid formation of silica- and zirconia based hydrogels could be achieved even without additional amounts of an auxiliary agent. It is shown that in all other cases the hydrolysis reaction and gel formation can be controlled by adding agents with crosslinking properties like propylene oxide, dilution with alcohols or starting from water glass. Water glass not only has the function of a cheap network former but also allows the strict adjustment of the pH value via neutralization of the acid forming hydrolysis reactions. Supercritical drying of the wet alkogel in CO2 delivered aerogels with BET surfaces up to 1390 m2 g−1, as determined by nitrogen adsorption measurements. In addition, the characterization includes XRD and SEM. These findings provide new physical insights into the hydrolysis of the gel precursors and lead to a significant reduction in production costs of inorganic aerogels.

From Fragile to Resilient Insulation: Synthesis and Characterization of Aramid-Honeycomb Reinforced Silica Aerogel Composite Materials

The production of a new composite material embedding aramid honeycomb materials into nano-porous silica aerogels is studied. Our aim is to improve the poor mechanical strength of silica aerogels by aramid honeycombs without losing the amazing properties of the aerogels like little density and low thermal conductivity. The composite materials were prepared using two formulations of silica aerogels in combination with aramid honeycomb materials of different cell sizes. The silica aerogels are prepared using silicon alkoxides methyltrimethoxysilane and tetraethylorthosilicate as precursors in a two-step acid–base sol–gel process. Shortly in advance of the gelation point, the aramid honeycombs were fluted by the sol, gelation occurred and, after the aging process, the gel bodies were supercritically dried. The properties of the received composite materials are satisfying. Even the thermal conductivities and the densities are a bit higher than for pure aerogels. Most importantly, the mechanical strength is improved by a factor of 2.3 compared to aramid honeycomb materials and by a factor of 10 compared to the two silica aerogels themselves. The composite materials have a good prospective to be used as an impressive insulation material.

Zirconia-based aerogels via hydrolysis of salts and alkoxides: the influence of the synthesis procedures on the properties of the aerogels.

This contribution aims at evaluating different synthesis procedures leading to zirconia-based aerogels. A series of undoped and yttrium-doped zirconia aerogels have been prepared via hydrolysis and condensation reaction of different alkoxy- and different inorganic salt-based precursors followed by supercritical drying. Well-established but deleterious zirconium n-propoxide (TPOZ) or zirconium n-butoxide (TBOZ) were used as metal precursors in combination with acids like nitric acid and acetic acid as auxiliary agent for the generation of non-yttrium stabilized zirconia aerogels. Yttrium-stabilized zirconia aerogels as well as pure zirconia aerogels were obtained by the salt route starting from ZrCl4 and crosslinking agents like propylene oxide or acetylacetone. The characteristics of the products were analyzed by nitrogen adsorption measurements, electron microscopy, and X-ray scattering. It turned out that with respect to all relevant properties of the aerogels as well as the practicability of the synthesis procedures, approaches based on inexpensive non-toxic salt precursors are the methods of choice. The salt-based approaches allow not only for low-cost, easy-to-handle synthesis procedures with realizable gelation times of less than 60 seconds, but also delivered the products with the highest surface area (449 m(2)  g(-1) for ZrCl4) within this series of syntheses.

The effect of zinc oxide (ZnO) addition on the physical and morphological properties of cellulose aerogel beads

Microsized open porous beads of cellulose were made using the dissolution medium containing mixtures of 7 wt% NaOH and 12 wt% urea and additionally various concentrations of ZnO to study its effect on physical and morphological properties of the cellulose beads formed. It has been observed that such cellulose aerogel beads prepared with lower concentrations of ZnO show shrinkage while drying whereas beads prepared with higher concentrations of ZnO do not exhibit much shrinkage. The dried cellulose aerogel beads were spherical with diameters between 2 and 2.5 mm. The skeletal density of all dried cellulose beads was measured as 1.5 g cm−3. FT-IR spectra reveal that the structure of cellulose I transformed to cellulose II during dissolution and regeneration in a coagulation medium, which was also confirmed from XRD measurements. The beads prepared with a NaOH/urea/ZnO aqueous solution exhibit better thermal stability. We found that the addition of 0.5 wt% ZnO to the NaOH/urea mixture greatly increased the specific surface area of the cellulose beads up to 407 m2 g−1 compared to control cellulose beads (341 m2 g−1). SEM images indicate that a dense nano-fibrillar network structure was formed in the interior of the cellulose aerogel beads prepared with 0.5 wt% ZnO.

Development Steps for Concentrating Solar Power Technologies With Maximum Impact on Cost Reduction: Results of the European ECOSTAR Study

Beside continuous implementation of concentrating solar power plants (CSP) in Europe, which stipulate cost reduction by mass production effects, further R&D activities are necessary to achieve the cost competitiveness to fossil power generation. Therefore the cost range of 15–20 cents€/kWh for the currently planned CSP systems in Europe has to be decreased by a factor of 2–4. The European Concentrated Solar Thermal Roadmap (ECOSTAR) study that is conducted by leading CSP research institutes in Europe intends to stipulate the direction for R&D activities in the context of cost reduction. It uses a common methodology approach, based on an annual performance model to identify the most essential technical innovations that will reduce the cost of seven different CSP system concepts, which are currently under promotion world wide. The potential of innovative concepts for solar light weight concentrators, low-cost thermal energy storage concepts, solar receivers/absorbers and power cycles are in the main focus of interest. The results of the study include a description of the value of CSP power, the sensitivity of the electricity cost information, a list of innovations that have been investigated and recommendations for the focus of further R&D work.Copyright

The three-dimensional structure of flexible resorcinol-formaldehyde aerogels investigated by means of holotomography

Organic aerogels based on resorcinol-formaldehyde gels display remarkable properties due to their pronounced nanoporosity. Therefore, studies towards the understanding of their structure-property-relationship are of high value for the design of improved materials. X-ray tomography is a technique that has been used for the structural elucidation of porous materials, but so far no highly resolved three-dimensional structures of resorcinol-formaldehyde gels have been obtained under the classical absorption-based experimental X-ray setup. This paper reports on the successful analysis of a superflexible resorcinol-formaldehyde aerogel using zoom holotomography that yielded images with an unprecedented resolution in the sub-micrometer range. The preparation of suitable powder from monolithic superflexible resorcinol-formaldehyde, the experimental conditions for tomography, and data-processing to obtain a 3D-image of the dried gel sample are described. Macropores above ca. 75 nm could be identified and visualized. They were shown to adopt almost spherical shape and to display a low connectivity. A quantitative analysis of the pore space revealed that most of the identified pores are small macropores (diameter < 0.5 µm), yet most pore volume is located in larger macropores of 1–4 µm diameter.Graphical Abstract

Silica aerogel paper honeycomb composites for thermal insulations

Applicability of super-insulating silica aerogel materials for thermal insulation is still restricted by limited mechanical strength. We report on a systematic improvement of silica aerogels in respect to shrinkage, density and thermal conductivity by varying sol-gel parameters as the molar ratios of TEOS:EtOH:H2O or the hydrolysis time. The received silica aerogels can be combined with paper-based honeycomb structures to improve the mechanical strength. We successfully manufactured such silica aerogel paper honeycomb composites. The challenges in the preparation are the must to prevent shrinkage of the silica aerogels during synthesis, to avoid cracks by supercritical drying, and to get a perfect bonding at the paper–silica interface. The composite materials are characterized with respect to their compression modulus, thermal conductivity, and flammability.Graphical abstract

Polysaccharide-Based Aerogel Bead Production via Jet Cutting Method

The aim of this work is to develop a method to produce spherical biopolymer-based aerogel particles, which is capable for scale-up in the future. Therefore, the jet cutting method is suggested. Amidated pectin, sodium alginate, and chitosan are used as a precursor (a 1–3 wt. % solution) for particle production via jet cutting. Gelation is realized via two methods: the internal setting method (using calcium carbonate particles as cross-linkers and citric and acidic acid for pH adjustment) and the diffusion method (in calcium chloride solutions). Gel particles are subjected to solvent exchange to ethanol and consequent supercritical drying with CO2. Spherical aerogel particles with narrow particle size distributions in the range of 400 to 1500 µm and a specific surface area of around 500 m2/g are produced. Overall, it can be concluded that the jet cutting method is suitable for aerogel particle production, although the shape of the particles is not perfectly spherical in all cases. However, parameter adjustment might lead to even better shaped particles in further work. Moreover, the biopolymer-based aerogel particles synthesized in this study are tested as humidity absorbers in drying units for home appliances, particularly for dishwashers. It has been shown that for several cycles of absorption and desorption of humidity, aerogel particles are stable with an absorption capacity of around 20 wt. %.