Optical designs for improved solar cell performance

The solar resource is the most abundant renewable resource on earth, yet it is currently exploited with relatively low efficiencies. To make solar energy more affordable, we can either reduce the cost of the cell or increase the efficiency with a similar cost cell. In this thesis, we consider several different optical approaches to achieve these goals. First, we consider a ray optical model for light trapping in silicon microwires. With this approach, much less material can be used, allowing for a cost savings. We next focus on reducing the escape of radiatively emitted and scattered light from the solar cell. With this angle restriction approach, light can only enter and escape the cell near normal incidence, allowing for thinner cells and higher efficiencies. In Auger-limited GaAs, we find that efficiencies greater than 38% may be achievable, a significant improvement over the current world record. To experimentally validate these results, we use a Bragg stack to restrict the angles of emitted light. Our measurements show an increase in voltage and a decrease in dark current, as less radiatively emitted light escapes. While the results in GaAs are interesting as a proof of concept, GaAs solar cells are not currently made on the production scale for terrestrial photovoltaic applications. We therefore explore the application of angle restriction to silicon solar cells. While our calculations show that Auger-limited cells give efficiency increases of up to 3% absolute, we also find that current amorphous silicion-crystalline silicon heterojunction with intrinsic thin layer (HIT) cells give significant efficiency gains with angle restriction of up to 1% absolute. Thus, angle restriction has the potential for unprecedented one sun efficiencies in GaAs, but also may be applicable to current silicon solar cell technology. Finally, we consider spectrum splitting, where optics direct light in different wavelength bands to solar cells with band gaps tuned to those wavelengths. This approach has the potential for very high efficiencies, and excellent annual power production. Using a light-trapping filtered concentrator approach, we design filter elements and find an optimal design. Thus, this thesis explores silicon microwires, angle restriction, and spectral splitting as different optical approaches for improving the cost and efficiency of solar cells.

The Economic Impacts of Biodiversity Policy for Improving the Climate Regulating Services Provided by EU Natura 2000 Habitats

22 p.

Aquatic insects as target organisms for the study of effects of projected climate change in the British Isles

The chief objectives of this brief review are to collate and synthesise quantitative information on the temperature requirements of aquatic insects, and to identify species, and groups of species, that could be useful indicators of climate change and predictors of the ecological effects of change. It arose from the first phase of the Terrestrial Initiative in Global Environmental Research (TIGER), a five-year, NERC Community Programme on the role of the terrestrial biosphere in the science of global change. This phase involved the identification of criteria for selecting species suitable for the study of effects of projected climate change in the British Isles. Field and laboratory studies are reviewed, and criteria for selection of species for future research are suggested. The literature survey shows that no species of aquatic insect can be found to meet all three criteria, but information on the British stoneflies and their eggs already satisfies two of them.

Ecosystem exploitation, sustainability and biodiversity: Are they compatible?

This articles offers a basis for describing sustainability and then seeks to place this concept on an energetic basis by reference to recent advances in the understanding of patterns and processes in (mainly pelagic) fresh waters. Finally, by relating these to terrestrial ecosystems, it is shown how their sustainability may be attained through encouraging healthy fresh waters. Features of population succession are taken from observations on phytoplankton ecology.

Rapid changes in the vegetation of a shallow pond in Epping Forest, related to recent droughts

Over much of Britain, 1995 and 1996 have been perceived as drought years. To evaluate the impact that local climatic conditions are having upon successional changes in higher vegetation (macrophytes), Speakmans Pond in Epping Forest was surveyed and mapped in 1996. The results are related to previous vegetation surveys carried out in 1989 and 1991. In 1989 the dominant marginal vegetation was floating sweet-grass Glyceria fluitans, which also covered a major part of the main body of the pond. Other abundant species included soft rush Juncus effusus, reed mace Typha latifolia and yellow flag Iris pseudocorus. A small (central) area of open water contained bladderwort Utricularia vulgaris and white water-lily Nymphaea alba. A similar plant coverage was found in 1991, with a dominance of floating sweet-grass along the shallow eastern edge. A marked change in the pond was found during the 1996 survey of vegetation in July, when the pool was dry. The major plant cover now consisted of creeping bent Agrostis stolonifera, with isolated clumps of Yorkshire fog Holcus lanatus around the edges; both are terrestrial grasses found on land surrounding the pond. Rushes (Juncus) had increased their distribution round the margins of the pond, and the patch of yellow flag noted in 1989 and 1991 was not found in 1996. The deeper trenches were also dry, but a small patch of white water-lily remained adjacent to one of the trenches.

The avifauna of Laguna Llancanelo (Mendoza, Argentina), a South American wetland of international importance

Laguna Llancanelo is a large (40,000 ha), very shallow (less than 3 m deep), inland saline waterbody, located in southern Mendoza, Argentina. A survey of the avifauna was undertaken during 1983 to 1985; field trips were made mostly to the northeastern and central-western sectors of the lagoon, in the months of April, May, September, October and December. Complementary studies were made by an aerial survey and assessments of terrestrial birds in the surroundings of the lagoon. A total of 64 species in 22 families of birds usually associated with aquatic environments were recorded. The best-represented families of aquatic birds in terms of numbers of species were: Anatidae (13 swans, geese, ducks), Scolopacidae (7 sandpipers and other small waders), Ardeidae (6 herons, bitterns, egrets, Podicipedidae (4 grebes), Rallidae (4 rails, coots) and Charadriidae (4 plovers, lapwings). The most abundant nesting birds on the lagoon were: Black-necked swan Cygnus melancoryphus (824), silvery grebe Podiceps occipitalis (202) and black-crowned night heron Nycticorax nycticorax (100).

Estudio de criterios de planificación de redes DVB-T2 en la banda VHF

[ES]El estándar de radiodifusión de televisión digital terrestre DVB-T (Digital Video Broadcasting - Terrestrial) es uno de los estándares más extendidos en todo el mundo. Su sucesor, DVB-T2, ha sido desplegado en 24 países y adoptado por otros 32 a lo largo de los últimos cinco años. Este despliegue ha coincidido con el rápido desarrollo de la telefonía móvil, con la que comparte la banda UHF. Debido a las limitaciones del espectro, es necesario estudiar la posibilidad de ofrecer los servicios de DVB-T2 en la banda VHF. El objetivo de este trabajo de fin de grado es estudiar los criterios de planificación de las redes DVB-T2 en la banda VHF. Para ello se analizan las áreas afectadas por el cambio de banda, como son la estimación del campo eléctrico, los características de los componentes de transmisión y recepción de la señal y los parámetros estándar. Además, se calculan las intensidades mínimas de campo y los alcances máximos de cobertura para diferentes configuraciones.

Geochemistry and resonance ionization of platinum-group elements

Experimental studies were conducted with the goals of 1) determining the origin of Pt- group element (PGE) alloys and associated mineral assemblages in refractory inclusions from meteorites and 2) developing a new ultrasensitive method for the in situ chemical and isotopic analysis of PGE. A general review of the geochemistry and cosmochemistry of the PGE is given, and specific research contributions are presented within the context of this broad framework.

An important step toward understanding the cosmochemistry of the PGE is the determination of the origin of POE-rich metallic phases (most commonly εRu-Fe) that are found in Ca, AJ-rich refractory inclusions (CAI) in C3V meteorites. These metals occur along with γNi-Fe metals, Ni-Fe sulfides and Fe oxides in multiphase opaque assemblages. Laboratory experiments were used to show that the mineral assemblages and textures observed in opaque assemblages could be produced by sulfidation and oxidation of once homogeneous Ni-Fe-PGE metals. Phase equilibria, partitioning and diffusion kinetics were studied in the Ni-Fe-Ru system in order to quantify the conditions of opaque assemblage formation. Phase boundaries and tie lines in the Ni-Fe-Ru system were determined at 1273, 1073 and 873K using an experimental technique that allowed the investigation of a large portion of the Ni-Fe-Ru system with a single experiment at each temperature by establishing a concentration gradient within which local equilibrium between coexisting phases was maintained. A wide miscibility gap was found to be present at each temperature, separating a hexagonal close-packed εRu-Fe phase from a face-centered cubic γNi-Fe phase. Phase equilibria determined here for the Ni-Fe-Ru system, and phase equilibria from the literature for the Ni-Fe-S and Ni-Fe-O systems, were compared with analyses of minerals from opaque assemblages to estimate the temperature and chemical conditions of opaque assemblage formation. It was determined that opaque assemblages equilibrated at a temperature of ~770K, a sulfur fugacity 10 times higher than an equilibrium solar gas, and an oxygen fugacity 10⁶ times higher than an equilibrium solar gas.

Diffusion rates between -γNi-Fe and εRu-Fe metal play a critical role in determining the time (with respect to CAI petrogenesis) and duration of the opaque assemblage equilibration process. The diffusion coefficient for Ru in Ni (D^Ru_Ni) was determined as an analog for the Ni-Fe-Ru system by the thin-film diffusion method in the temperature range of 1073 to 1673K and is given by the expression:

D^Ru_Ni (cm² sec^-1) = 5.0(±0.7) x 10^-3 exp(-2.3(±0.1) x 10¹² erg mole^-1/RT) where R is the gas constant and T is the temperature in K. Based on the rates of dissolution and exsolution of metallic phases in the Ni-Fe-Ru system it is suggested that opaque assemblages equilibrated after the melting and crystallization of host CAI during a metamorphic event of ≥ 10³ years duration. It is inferred that opaque assemblages originated as immiscible metallic liquid droplets in the CAI silicate liquid. The bulk compositions of PGE in these precursor alloys reflects an early stage of condensation from the solar nebula and the partitioning of V between the precursor alloys and CAI silicate liquid reflects the reducing nebular conditions under which CAI were melted. The individual mineral phases now observed in opaque assemblages do not preserve an independent history prior to CAI melting and crystallization, but instead provide important information on the post-accretionary history of C3V meteorites and allow the quantification of the temperature, sulfur fugacity and oxygen fugacity of cooling planetary environments. This contrasts with previous models that called upon the formation of opaque assemblages by aggregation of phases that formed independently under highly variable conditions in the solar nebula prior to the crystallization of CAI.

Analytical studies were carried out on PGE-rich phases from meteorites and the products of synthetic experiments using traditional electron microprobe x-ray analytical techniques. The concentrations of PGE in common minerals from meteorites and terrestrial rocks are far below the ~100 ppm detection limit of the electron microprobe. This has limited the scope of analytical studies to the very few cases where PGE are unusually enriched. To study the distribution of PGE in common minerals will require an in situ analytical technique with much lower detection limits than any methods currently in use. To overcome this limitation, resonance ionization of sputtered atoms was investigated for use as an ultrasensitive in situ analytical technique for the analysis of PGE. The mass spectrometric analysis of Os and Re was investigated using a pulsed primary Ar⁺ ion beam to provide sputtered atoms for resonance ionization mass spectrometry. An ionization scheme for Os that utilizes three resonant energy levels (including an autoionizing energy level) was investigated and found to have superior sensitivity and selectivity compared to nonresonant and one and two energy level resonant ionization schemes. An elemental selectivity for Os over Re of ≥ 10³ was demonstrated. It was found that detuning the ionizing laser from the autoionizing energy level to an arbitrary region in the ionization continuum resulted in a five-fold decrease in signal intensity and a ten-fold decrease in elemental selectivity. Osmium concentrations in synthetic metals and iron meteorites were measured to demonstrate the analytical capabilities of the technique. A linear correlation between Os⁺ signal intensity and the known Os concentration was observed over a range of nearly 10⁴ in Os concentration with an accuracy of ~ ±10%, a millimum detection limit of 7 parts per billion atomic, and a useful yield of 1%. Resonance ionization of sputtered atoms samples the dominant neutral-fraction of sputtered atoms and utilizes multiphoton resonance ionization to achieve high sensitivity and to eliminate atomic and molecular interferences. Matrix effects should be small compared to secondary ion mass spectrometry because ionization occurs in the gas phase and is largely independent of the physical properties of the matrix material. Resonance ionization of sputtered atoms can be applied to in situ chemical analysis of most high ionization potential elements (including all of the PGE) in a wide range of natural and synthetic materials. The high useful yield and elemental selectivity of this method should eventually allow the in situ measurement of Os isotope ratios in some natural samples and in sample extracts enriched in PGE by fire assay fusion.

Phase equilibria and diffusion experiments have provided the basis for a reinterpretation of the origin of opaque assemblages in CAI and have yielded quantitative information on conditions in the primitive solar nebula and cooling planetary environments. Development of the method of resonance ionization of sputtered atoms for the analysis of Os has shown that this technique has wide applications in geochemistry and will for the first time allow in situ studies of the distribution of PGE at the low concentration levels at which they occur in common minerals.

Freshwater crabs in Africa

Freshwater crabs are a strangely neglected component of the world's inland aquatic ecosystems. Despite their wide distribution throughout the tropical and warm temperate zones of the world, and their great diversity, their role in the ecology of freshwaters is very poorly understood. This is nowhere more true than in Africa, where crabs occur in almost every freshwater system, yet even fundamentals such as their higher taxonomy are yet to be determined. This review will attempt to stimulate interest in this fascinating group of organisms by summarising what little we do know about freshwater crabs in Africa. The author describes habitat, reproduction, diet, population density, terrestrial activity and economical importance of African freshwater crabs. The article concludes with an assessment of threats and conservation. In an appendix the author describes the five families of freshwater crabs which are recognised in Africa.

Comparative studies on the structure of an upland African stream ecosystem

Upland stream systems have been extensively investigated in Europe, North America and Australasia and many of the central ideas concerning their function are based on these systems. One central paradigm, the river continuum concept is ultimately derived from those North American streams whose catchments remain forested with native vegetation. Streams of the tropics may or may not fit the model. They have been little studied. The Amani Nature Reserve in the East Usambara Mountains of north-eastern Tanzania offers an opportunity to bring these naturally forested systems to the attention of the ecological community. This article describes a comparison made between two lengths of the River Dodwe in this area. The work was carried out by a group of postgraduate students from eighteen European and African countries with advice from five staff members, as part of a course organised by the Tropical Biology Association. Rigorous efforts were made to standardise techniques, in a situation where equipment and laboratory facilities were very basic, through a management structure and deliberate allocation of work to specialists in each area.The article offers a summary of invertebrate communities found in the stream and its biomass. Crabs seem to be the key organism in both sections of the streams.

Biodiversity of macrozoobenthos in a large river, the Austrian Danube, including quantitative studies in a free-flowing stretch below Vienna: a short review

The Danube is ca. 2850 km in length and is the second largest river in Europe. The Austrian part of the Danube falls 156 metres in altitude over its 351 km length and, since the early 1950s, the river has been developed into a power-generating waterway, so that the continuity of the river is now interrupted by ten impounded areas. Only two stretches of the original free-flowing river are left, the Wachau region (above river-km 2005, west of Vienna) and the region downstream from the impoundment at Vienna (river-km 1921). Most of the recent theories and concepts related to invertebrates, in the context of the ecology of running waters, are based on studies on small streams, whereas investigations of large rivers have played a minor role for a long time, mainly due to methodological difficulties. The authors' recent detailed studies on macroinvertebrates in the free-flowing section of the Danube below Vienna, provide an excellent opportunity to survey or restate scientific hypotheses on the basis of a large river. In this review the main interest focuses on the investigation of biodiversity, i.e. the number of species and their relative proportions in the whole invertebrate community, as well as major governing environmental factors. The article summarises the species composition, the important environmental variables at the river cross-section and the effect of upstream impoundment on the riverbed and its fauna.

The ecology of Bassenthwaite Lake (English Lake District)

Bassenthwaite Lake is, in many ways, different from the other major lakes in the English Lake District: it is the most northerly, the shallowest, has the largest catchment and the shortest mean retention time. There is also considerable temporal variation in lake level. This article summarises the limnological features of Bassenthwaite Lake, the catchment and physical characteristics before describing the water chemistry, phytoplankton, macrophytes, zooplankton, invertebrates, fish, mammal and invertebrate population. The authors then describe the ecological pressures faced by Bassenthwaite Lake such as nutrients, sediments and introduced species.

Toward Understanding Astrophysical Phenomena

Fast radio bursts (FRBs), a novel type of radio pulse, whose physics is not yet understood at all. Only a handful of FRBs had been detected when we started this project. Taking account of the scant observations, we put physical constraints on FRBs. We excluded proposals of a galactic origin for their extraordinarily high dispersion measures (DM), in particular stellar coronas and HII regions. Therefore our work supports an extragalactic origin for FRBs. We show that the resolved scattering tail of FRB 110220 is unlikely to be due to propagation through the intergalactic plasma. Instead the scattering is probably caused by the interstellar medium in the FRB's host galaxy, and indicates that this burst sits in the central region of that galaxy. Pulse durations of order $\ms$ constrain source sizes of FRBs implying enormous brightness temperatures and thus coherent emission. Electric fields near FRBs at cosmological distances would be so strong that they could accelerate free electrons from rest to relativistic energies in a single wave period. When we worked on FRBs, it was unclear whether they were genuine astronomical signals as distinct from `perytons', clearly terrestrial radio bursts, sharing some common properties with FRBs. Recently, in April 2015, astronomers discovered that perytons were emitted by microwave ovens. Radio chirps similar to FRBs were emitted when their doors opened while they were still heating. Evidence for the astronomical nature of FRBs has strengthened since our paper was published. Some bursts have been found to show linear and circular polarizations and Faraday rotation of the linear polarization has also been detected. I hope to resume working on FRBs in the near future. But after we completed our FRB paper, I decided to pause this project because of the lack of observational constraints.

The pulsar triple system, J0733+1715, has its orbital parameters fitted to high accuracy owing to the precise timing of the central $\ms$ pulsar. The two orbits are highly hierarchical, namely $P_{\mathrm{orb,1}}\ll P_{\mathrm{orb,2}}$, where 1 and 2 label the inner and outer white dwarf (WD) companions respectively. Moreover, their orbital planes almost coincide, providing a unique opportunity to study secular interaction associated purely with eccentricity beyond the solar system. Secular interaction only involves effect averaged over many orbits. Thus each companion can be represented by an elliptical wire with its mass distributed inversely proportional to its local orbital speed. Generally there exists a mutual torque, which vanishes only when their apsidal lines are parallel or anti-parallel. To maintain either mode, the eccentricity ratio, $e_1/e_2$, must be of the proper value, so that both apsidal lines precess together. For J0733+1715, $e_1\ll e_2$ for the parallel mode, while $e_1\gg e_2$ for the anti-parallel one. We show that the former precesses $\sim 10$ times slower than the latter. Currently the system is dominated by the parallel mode. Although only a little anti-parallel mode survives, both eccentricities especially $e_1$ oscillate on $\sim 10^3\yr$ timescale. Detectable changes would occur within $\sim 1\yr$. We demonstrate that the anti-parallel mode gets damped $\sim 10^4$ times faster than its parallel brother by any dissipative process diminishing $e_1$. If it is the tidal damping in the inner WD, we proceed to estimate its tidal quantity parameter ($Q$) to be $\sim 10^6$, which was poorly constrained by observations. However, tidal damping may also happen during the preceding low-mass X-ray binary (LMXB) phase or hydrogen thermal nuclear flashes. But, in both cases, the inner companion fills its Roche lobe and probably suffers mass/angular momentum loss, which might cause $e_1$ to grow rather than decay.

Several pairs of solar system satellites occupy mean motion resonances (MMRs). We divide these into two groups according to their proximity to exact resonance. Proximity is measured by the existence of a separatrix in phase space. MMRs between Io-Europa, Europa-Ganymede and Enceladus-Dione are too distant from exact resonance for a separatrix to appear. A separatrix is present only in the phase spaces of the Mimas-Tethys and Titan-Hyperion MMRs and their resonant arguments are the only ones to exhibit substantial librations. When a separatrix is present, tidal damping of eccentricity or inclination excites overstable librations that can lead to passage through resonance on the damping timescale. However, after investigation, we conclude that the librations in the Mimas-Tethys and Titan-Hyperion MMRs are fossils and do not result from overstability.

Rubble piles are common in the solar system. Monolithic elements touch their neighbors in small localized areas. Voids occupy a significant fraction of the volume. In a fluid-free environment, heat cannot conduct through voids; only radiation can transfer energy across them. We model the effective thermal conductivity of a rubble pile and show that it is proportional the square root of the pressure, $P$, for $P\leq \epsy^3\mu$ where $\epsy$ is the material's yield strain and $\mu$ its shear modulus. Our model provides an excellent fit to the depth dependence of the thermal conductivity in the top $140\,\mathrm{cm}$ of the lunar regolith. It also offers an explanation for the low thermal inertias of rocky asteroids and icy satellites. Lastly, we discuss how rubble piles slow down the cooling of small bodies such as asteroids.

Electromagnetic (EM) follow-up observations of gravitational wave (GW) events will help shed light on the nature of the sources, and more can be learned if the EM follow-ups can start as soon as the GW event becomes observable. In this paper, we propose a computationally efficient time-domain algorithm capable of detecting gravitational waves (GWs) from coalescing binaries of compact objects with nearly zero time delay. In case when the signal is strong enough, our algorithm also has the flexibility to trigger EM observation {\it before} the merger. The key to the efficiency of our algorithm arises from the use of chains of so-called Infinite Impulse Response (IIR) filters, which filter time-series data recursively. Computational cost is further reduced by a template interpolation technique that requires filtering to be done only for a much coarser template bank than otherwise required to sufficiently recover optimal signal-to-noise ratio. Towards future detectors with sensitivity extending to lower frequencies, our algorithm's computational cost is shown to increase rather insignificantly compared to the conventional time-domain correlation method. Moreover, at latencies of less than hundreds to thousands of seconds, this method is expected to be computationally more efficient than the straightforward frequency-domain method.

Predation and selection in copepod populations of Alpine waters [Translation from: Bollettino di Zoologia 21 541-545, 1954]

Zooplankton was studied in four alpine lakes in Switzerland, France and Italy. The presence the presence of the invertebrate predator Heterocope in three lakes was stated. It is then discussed why in three of these four lakes, the copepod Arctodiaptomus denticornis is present in the absence of Arctodiaptomus bacillifer, and vice versa respectively in the second and first parts of the lacustrine summer.

Observations and Modeling of Tropical Planetary Atmospheres

This thesis is a comprised of three different projects within the topic of tropical atmospheric dynamics. First, I analyze observations of thermal radiation from Saturn’s atmosphere and from them, determine the latitudinal distribution of ammonia vapor near the 1.5-bar pressure level. The most prominent feature of the observations is the high brightness temperature of Saturn’s subtropical latitudes on either side of the equator. After comparing the observations to a microwave radiative transfer model, I find that these subtropical bands require very low ammonia relative humidity below the ammonia cloud layer in order to achieve the high brightness temperatures observed. We suggest that these bright subtropical bands represent dry zones created by a meridionally overturning circulation.

Second, I use a dry atmospheric general circulation model to study equatorial superrotation in terrestrial atmospheres. A wide range of atmospheres are simulated by varying three parameters: the pole-equator radiative equilibrium temperature contrast, the convective lapse rate, and the planetary rotation rate. A scaling theory is developed that establishes conditions under which superrotation occurs in terrestrial atmospheres. The scaling arguments show that superrotation is favored when the off-equatorial baroclinicity and planetary rotation rates are low. Similarly, superrotation is favored when the convective heating strengthens, which may account for the superrotation seen in extreme global-warming simulations.

Third, I use a moist slab-ocean general circulation model to study the impact of a zonally-symmetric continent on the distribution of monsoonal precipitation. I show that adding a hemispheric asymmetry in surface heat capacity is sufficient to cause symmetry breaking in both the spatial and temporal distribution of precipitation. This spatial symmetry breaking can be understood from a large-scale energetic perspective, while the temporal symmetry breaking requires consideration of the dynamical response to the heat capacity asymmetry and the seasonal cycle of insolation. Interestingly, the idealized monsoonal precipitation bears resemblance to precipitation in the Indian monsoon sector, suggesting that this work may provide insight into the causes of the temporally asymmetric distribution of precipitation over southeast Asia.