Effects on Amorphous Silicon Photovoltaic Performance from High-temperature Annealing Pulses in Photovoltaic Thermal Hybrid Devices

There is a renewed interest in photovoltaic solar thermal (PVT) hybrid systems, which harvest solar energy for heat and electricity. Typically, a main focus of a PVT system is to cool the photovoltaic (PV) cells to improve the electrical performance, however, this causes the thermal component to under-perform compared to a solar thermal collector. The low temperature coefficients of amorphous silicon (a-Si:H) allow for the PV cells to be operated at higher temperatures and are a potential candidate for a more symbiotic PVT system. The fundamental challenge of a-Si:H PV is light-induced degradation known as the Staebler-Wronski effect (SWE). Fortunately, SWE is reversible and the a-Si:H PV efficiency can be returned to its initial state if the cell is annealed. Thus an opportunity exists to deposit a-Si:H directly on the solar thermal absorber plate where the cells could reach the high temperatures required for annealing. In this study, this opportunity is explored experimentally. First a-Si:H PV cells were annealed for 1 hour at 100\degreeC on a 12 hour cycle and for the remaining time the cells were degraded at 50\degreeC in order to simulate stagnation of a PVT system for 1 hour once a day. It was found that, when comparing the cells after stabilization at normal 50\degreeC degradation, this annealing sequence resulted in a 10.6% energy gain when compared to a cell that was only degraded at 50\degreeC

Improving the thermal performance of single-glazed windows using translucent granular aerogel

Copyright @ 2011 Taylor & FrancisCost-effective materials, products and installation methods are required to improve the energy efficiency of the UK's existing building stock. The aim of this paper is to assess the potential for high-performance translucent granular aerogel insulation to be retrofitted over single glazing to reduce heat loss without blocking out all of the useful natural light. In situ testing of a 10-mm-thick prototype panel, consisting of a clear twin-wall polycarbonate sheet filled with granular aerogel, was carried out and validated with steady-state calculations. Results demonstrate that an 80% reduction in heat loss can be achieved without detrimental reductions in light transmission. Payback calculations accounting for the inevitable thermal bridging from openable solutions such as roller shutters or pop-in secondary glazing suggest that a return on investment between 3.5 and 9.5 years is possible if products are consistently used over the heating season. Granular aerogel is a promising material for improving the thermal performance of existing windows. Future research will seek to map out different ways in which the material can be applied to the existing UK housing stock, identifying which systems offer the greatest potential for widespread CO2 savings over their life cycle.This work is funded by the EPSRC, Brunel University and Buro Happold Ltd

Streamlined life cycle assessment of transparent silica aerogel made by supercritical drying

This is the post-print version of the Article. The official published version can be accessed from the link below - Copyright @ 2011 ElsevierWhen developing sustainable building fabric technologies, it is essential that the energy use and CO2 burden arising from manufacture does not outweigh the respective in-use savings. This study investigates this paradigm by carrying out a streamlined life cycle assessment (LCA) of silica aerogel. This unique, nanoporous translucent insulation material has the lowest thermal conductivity of any solid, retaining up to four times as much heat as conventional insulation, whilst being highly transparent to light and solar radiation. Monolithic silica aerogel has been cited as the ‘holy grail’ of future glazing technology. Alternatively, translucent granular aerogel is now being produced on a commercial scale. In each case, many solvents are used in production, often accompanied by intensive drying processes, which may consume large amounts of energy and CO2. To date, there has been no peer-reviewed LCA of this material conducted to the ISO 14000 standard. Primary data for this ‘cradle-to-factory gate’ LCA is collected for silica aerogel made by low and high temperature supercritical drying. In both cases, the mass of raw materials and electricity usage for each process is monitored to determine the total energy use and CO2 burden. Findings are compared against the predicted operational savings arising from retrofitting translucent silica aerogel to a single glazed window to upgrade its thermal performance. Results should be treated as a conservative estimate as the aerogel is produced in a laboratory, which has not been developed for mass manufacture or refined to reduce its environmental impact. Furthermore, the samples are small and assumptions to upscale the manufacturing volume occur without major changes to production steps or equipment used. Despite this, parity between the CO2 burden and CO2 savings is achieved in less than 2 years, indicating that silica aerogel can provide a measurable environmental benefit.This work is funded by the EPSRC, Brunel University and Buro Happold Ltd, the University of Bath is funded by the EPSRC grant EP/F018622/1

Absorption characteristics of a quantum dot array induced intermediate band: implications for solar cell design

We present a theoretical study of the electronic and absorption properties of the intermediate band (IB) formed by a three dimensional structure of InAs/GaAs quantum dots (QDs) arranged in a periodic array. Analysis of the electronic and absorption structures suggests that the most promising design for an IB solar cell material, which will exhibit its own quasi-Fermi level, is to employ small QDs (~6–12 nm QD lateral size). The use of larger QDs leads to extension of the absorption spectra into a longer wavelength region but does not provide a separate IB in the forbidden energy gap

Circulatory responses to hypoxia in experimental myocardial infarction

Hypoxia affecting circulatory responses in dogs, such as cardiac output, left ventricular dp/dt, and stroke volum

Absorption in atomic wires

The transfer matrix formalism is implemented in the form of the multiple collision technique to account for dissipative transmission processes by using complex potentials in several models of atomic chains. The absorption term is rigorously treated to recover unitarity for the non-hermitian hamiltonians. In contrast to other models of parametrized scatterers we assemble explicit potentials profiles in the form of delta arrays, Poschl-Teller holes and complex Scarf potentials. The techniques developed provide analytical expressions for the scattering and absorption probabilities of arbitrarily long wires. The approach presented is suitable for modelling molecular aggregate potentials and also supports new models of continuous disordered systems. The results obtained also suggest the possibility of using these complex potentials within disordered wires to study the loss of coherence in the electronic localization regime due to phase-breaking inelastic processes.Comment: 14 pages, 15 figures. To appear in Phys. Rev.

Spectral determinants and zeta functions of Schr\"odinger operators on metric graphs

A derivation of the spectral determinant of the Schr\"odinger operator on a metric graph is presented where the local matching conditions at the vertices are of the general form classified according to the scheme of Kostrykin and Schrader. To formulate the spectral determinant we first derive the spectral zeta function of the Schr\"odinger operator using an appropriate secular equation. The result obtained for the spectral determinant is along the lines of the recent conjecture.Comment: 16 pages, 2 figure