Predicted and in situ performance of a solar air collector incorporating a translucent granular aerogel cover

This is the post-print version of the Article. The official published version can be accessed from the link below - Copyright @ 2012 ElsevierThere is an opportunity to improve the efficiency of flat plate solar air collectors by replacing their conventional glass covers with lightweight polycarbonate panels filled with high performance aerogel insulation. The in situ performance of a 5.4m2 solar air collector containing granular aerogel is simulated and tested. The collector is incorporated into the external insulation of a mechanically ventilated end terrace house, recently refurbished in London, UK. During the 7 day test period, peak outlet temperatures up to 45 °C are observed. Resultant supply and internal air temperatures peak at 25–30 and 21–22 °C respectively. Peak efficiencies of 22–36% are calculated based on the proposed design across a range of cover types. Measured outlet temperatures are validated to within 5% of their predicted values. Estimated outputs range from 118 to 166 kWh/m2/year for collectors with different thickness granular aerogel covers, compared to 110 kWh/m2/year for a single glazed collector, 140 k h/m2/year for a double glazed collector and 202 kWh/m2/year for a collector incorporating high performance monolithic aerogel. Payback periods of 9–16 years are calculated across all cover types. An efficiency up to 60% and a payback period as low as 4.5 years is possible with an optimised collector incorporating a 10 mm thick granular aerogel cover.This work is supported by the EPSRC, Brunel University, Buro Happold Ltd. and the Technology Strategy Board

Lipase immobilised on silica monoliths as continuous-flow microreactors for triglyceride transesterification

Lipase immobilised on silica monoliths has been prepared and applied as biocatalytic continuous-flow microreactors for the transesterification of tributyrin as a model bio-oil component. Candida antarctica lipase was trapped within the pores of silica monoliths, and its successful immobilisation was demonstrated by the hydrolysis of 4-nitrophenyl butyrate to 4-nitrophenol. Lipase immobilised on silica monoliths was active for the transesterification of tributyrin at ambient temperature, with reactivity as a function of the methanol : tributyrin ratio, flow rate, temperature, and textural properties. Monoliths with a high surface area and large meso- and macropore channels enhanced the transesterification activity through improved molecule diffusion. The optimum immobilised lipase microreactor exhibited almost quantitative ester production for >100 h at 30 °C without deactivatio

Effect of aging on silica aerogel properties

Silica aerogels’ unique physical and chemical properties make them fascinating materials for a wide variety of applications. In addition to hydrophobization by silylation, aging is very important in the synthesis of silica aerogel by ambient pressure drying. Here we systematically study the effect of aging on the physico-chemical properties of silica aerogel with emphasis on ambient dried materials. Silica gels were aged for different times and at different temperatures in their gelation liquid (without solvent exchange), hydrophobized in hexamethyldisiloxane and subsequently dried either at ambient pressure or from supercritical CO2. Dynamic oscillatory rheological measurements demonstrate that aging reinforces the alcogels, particularly at high strain. The specific surface area decreases with increasing aging time and temperature as a consequence of Ostwald ripening processes during aging. With increasing aging time and temperature, the linear shrinkage and bulk density decrease and the pore size and pore volume increase for the ambient dried gels, but remain nearly constant for supercritically dried gels. Small-Angle X-ray scattering does not detect significant structural changes at length scales smaller than about hundred nanometers, but hints at systematic variations at larger length scales. The findings of this study highlight the importance of aging to increase the ability of the gel particle network to withstand irreversible pore collapse during ambient pressure drying

Crystallographic studies of gas sorption in metal-organic frameworks.

Metal-organic frameworks (MOFs) are a class of porous crystalline materials of modular design. One of the primary applications of these materials is in the adsorption and separation of gases, with potential benefits to the energy, transport and medical sectors. In situ crystallography of MOFs under gas atmospheres has enabled the behaviour of the frameworks under gas loading to be investigated and has established the precise location of adsorbed gas molecules in a significant number of MOFs. This article reviews progress in such crystallographic studies, which has taken place over the past decade, but has its origins in earlier studies of zeolites, clathrates etc. The review considers studies by single-crystal or powder diffraction using either X-rays or neutrons. Features of MOFs that strongly affect gas sorption behaviour are discussed in the context of in situ crystallographic studies, specifically framework flexibility, and the presence of (organic) functional groups and unsaturated (open) metal sites within pores that can form specific interactions with gas molecules

Oxidation-nitridation of chromium at high temperatures and its mitigation by alloying

This work investigates oxidation and nitridation in the binary Cr-Si system. Kinetics and thermodynamics of oxidation in this alloy system have been investigated with a systematic approach. Oxidation and nitridation behavior of each individual phase (solid-solution phase, Crss and silicide phase, A15) were studied separately by investigating the oxidation and nitridation kinetics for short and long term exposures, thermodynamic stability, and post-exposure characterization of the scale, and subscale morphologies. Results revealed that in both phases chromium is the predominant element in the oxidation process which made such alloys major chromia formers. It was found, however, that the role of localized SiO2 formation in reducing oxidation kinetics is significant. The oxidation rate of chromium was reduced by more than an order of magnitude upon the addition of only 3 at.% Si. The impact of SiO2 increased further with increasing Si content. The A15 Cr3Si silicide phase produced a continuous SiO2 layer at long term exposures. When combined in a two-phase eutectic alloy, both solid-solution and A15 silicide phases oxidized cooperatively via primary depletion of chromium which led to the dissolution of the Crss phase and formation of an A15 layer at the alloy-scale interface. The influence of nitrogen as an important oxidant in air was investigated for pure chromium and the binary Cr-Si alloys and it was shown that a chromium subnitride layer exclusively grew via inward diffusion of nitrogen. The positive role of Si in hindering nitridation was significant. It was revealed that the A15 silicide is stable at any nitrogen pressure at high temperatures and showed almost no solubility for nitrogen. As a continuous barrier in the subsurface region of the two phase eutectic Cr-Cr3Si alloy, it offered a self-protecting character against internal nitridation. Further development of this alloy system was conducted after exploring the ternary Cr-Ge-Si system in the high chromium range (Cr > 80 at.%). Si and Ge showed interchangeable solubility in both solid-solution Crss and A15 phases, and addition of Ge stabilized the A15 phase by supporting the peritectic reaction in the Cr-Cr3Ge system. The microstructure of the eutectic alloy remained fine-lamellar when up to 2 at.% Ge was added. Using this approach, nitridation, as the most important challenge in the development of chromium alloys, was significantly improved as the alloy microstructure remained unaffected from internal nitridation for a period of 1000 hours oxidation at 1200°C in air. Finally, the optimized oxidation behavior of binary and ternary alloys was discussed with regards to the morphology and adhesion of the chromia scale

Data for: From eutectic to peritectic: the effect of Ge on morphology, structure, and coarsening of Cr-Cr3Si alloys

The macro "Quantitative Metallography" enables ImageJ users to automatically analize single or multiple binary microstructural images. It delivers quantitative stereological parameters such as phase fraction and surface to volume ratio which is used for microstructural analysis and microstructural coarsening studies. The attached suplementary data file contains a short description about the developed macro and the user instruction