Trace metals in fresh water plankton

The uptake of Cu, Zn, and Cd by fresh water plankton was studied by analyzing samples of water and plankton from six lakes in southern California. Co, Pb, Mn, Fe, Na, K, Mg, Ca, Sr, Ba, and Al were also determined in the plankton samples. Special precautions were taken during sampling and analysis to avoid metal contamination.

The relation between aqueous metal concentrations and the concentrations of metals in plankton was studied by plotting aqueous and plankton metal concentrations vs time and comparing the plots. No plankton metal plot showed the same changes as its corresponding aqueous metal plot, though long-term trends were similar. Thus, passive sorption did not completely explain plankton metal uptake.

The fractions of Cu, Zn, and Cd in lake water which were associated with plankton were calculated and these fractions were less than 1% in every case.

To see whether or not plankton metal uptake could deplete aqueous metal concentrations by measurable amounts (e.g. 20%) in short periods (e.g. less than six days), three integrated rate equations were used as models of plankton metal sorption. Parameters for the equations were taken from actual field measurements. Measurable reductions in concentration within short times were predicted by all three equations when the concentration factor was greater than 10^5. All Cu concentration factors were less than 10^5.

The role of plankton was regulating metal concentrations considered in the context of a model of trace metal chemistry in lakes. The model assumes that all particles can be represented by a single solid phase and that the solid phase controls aqueous metal concentrations. A term for the rate of in situ production of particulate matter is included and primary productivity was used for this parameter. In San Vicente Reservoir, the test case, the rate of in situ production of particulate matter was of the same order of magnitude as the rate of introduction of particulate matter by the influent stream.

Design strategies for ultra-high efficiency photovoltaics

While concentrator photovoltaic cells have shown significant improvements in efficiency in the past ten years, once these cells are integrated into concentrating optics, connected to a power conditioning system and deployed in the field, the overall module efficiency drops to only 34 to 36%. This efficiency is impressive compared to conventional flat plate modules, but it is far short of the theoretical limits for solar energy conversion. Designing a system capable of achieving ultra high efficiency of 50% or greater cannot be achieved by refinement and iteration of current design approaches.

This thesis takes a systems approach to designing a photovoltaic system capable of 50% efficient performance using conventional diode-based solar cells. The effort began with an exploration of the limiting efficiency of spectrum splitting ensembles with 2 to 20 sub cells in different electrical configurations. Incorporating realistic non-ideal performance with the computationally simple detailed balance approach resulted in practical limits that are useful to identify specific cell performance requirements. This effort quantified the relative benefit of additional cells and concentration for system efficiency, which will help in designing practical optical systems.

Efforts to improve the quality of the solar cells themselves focused on the development of tunable lattice constant epitaxial templates. Initially intended to enable lattice matched multijunction solar cells, these templates would enable increased flexibility in band gap selection for spectrum splitting ensembles and enhanced radiative quality relative to metamorphic growth. The III-V material family is commonly used for multijunction solar cells both for its high radiative quality and for the ease of integrating multiple band gaps into one monolithic growth. The band gap flexibility is limited by the lattice constant of available growth templates. The virtual substrate consists of a thin III-V film with the desired lattice constant. The film is grown strained on an available wafer substrate, but the thickness is below the dislocation nucleation threshold. By removing the film from the growth substrate, allowing the strain to relax elastically, and bonding it to a supportive handle, a template with the desired lattice constant is formed. Experimental efforts towards this structure and initial proof of concept are presented.

Cells with high radiative quality present the opportunity to recover a large amount of their radiative losses if they are incorporated in an ensemble that couples emission from one cell to another. This effect is well known, but has been explored previously in the context of sub cells that independently operate at their maximum power point. This analysis explicitly accounts for the system interaction and identifies ways to enhance overall performance by operating some cells in an ensemble at voltages that reduce the power converted in the individual cell. Series connected multijunctions, which by their nature facilitate strong optical coupling between sub-cells, are reoptimized with substantial performance benefit.

Photovoltaic efficiency is usually measured relative to a standard incident spectrum to allow comparison between systems. Deployed in the field systems may differ in energy production due to sensitivity to changes in the spectrum. The series connection constraint in particular causes system efficiency to decrease as the incident spectrum deviates from the standard spectral composition. This thesis performs a case study comparing performance of systems over a year at a particular location to identify the energy production penalty caused by series connection relative to independent electrical connection.