The effects of precipitation variability on C4 photosynthesis, net primary production and soil respiration in a Chihuahuan Desert grassland

Although the Earths climate system has always been inherently variable, the magnitude and rate of anthropogenic climate change is subjecting ecosystems and the populations that they contain to novel environmental conditions. Because water is the most limiting resource, arid-semiarid ecosystems are likely to be highly responsive to future climate variability. The goal of my dissertation is to understand how precipitation variability affects primary productivity and soil respiration in desert grassland ecosystems. Initially, I reviewed the literature to understand how climate change affects net ecosystem exchange (NEE) across the warm deserts of North America (Chapter 2). Next, I examined the effects of precipitation frequency and intensity on soil moisture (θ), leaf-level photosynthesis (Anet), predawn leaf water potential (ψpd), aboveground net primary productivity (ANPP), and soil respiration (Rs) (Chapter 3). Last, I studied how large (10 mm) and extreme (30 mm) rainfall events with extended dry periods affected the ecophysiological response of two co-occurring dominant perennial C4 grasses, Bouteloua eriopoda and B. gracilis across an arid-semiarid ecotone (Chapter 4)

Thermodynamic Model of a Solar Receiver for Superheating of Sulfur Trioxide and Steam at Pilot Plant Scale

Within the European research project SOL2HY2, key components for a solar hybrid sulfur cycle are being developed and demonstrated at pilot scale in a real environment. Regarding the thermal portion, a plant for solar sulfuric acid decomposition is set up and initially operated at the research platform of the DLR Solar Tower in Jülich, Germany. One major component is the directly irradiated volumetric receiver, superheating steam and SO3 coming from a tube-type evaporator to above 1000 °C. At the design flow rate of sulfuric acid (50%-wt.) of 1 l/min, a nominal solar power of 57 kW is required at the receiver. With a flat ceramic absorber made from SiC and a flat quartz glass window, the design is based on lab scale reactors successfully demonstrated at the solar furnace of the German Aerospace Centre (DLR) in Cologne, Germany. A flexible lumped thermodynamic tool representing the receiver, compiled to assess different configurations, is presented in detail. An additional raytracing model has been established to provide the irradiation boundaries and support the design of a conical secondary concentrator with an aperture diameter of 0.6 m. A comparison with first experimental data (up to 65% nominal power), obtained during initial operation, indicates the models to be viable tools for design and operational forecast of such systems. With a provisional method to account for the efficiency of the secondary concentrator, measured fluid outlet temperatures (up to 1000 °C) are predicted with deviations of ±60 °C. Respective absorber front temperatures (up to 1200 °C) are under-predicted by 100-200 °C, with lower deviations at higher mass flows. The measured window temperature (up to 700 °C) mainly depends on the absorber front temperature level, which is well predicted by the model

Integration assessment of the hybrid sulphur cycle with a copper production plant

Copper is the third-most widely-used metal worldwide. However, copper processing is an energy-intensive process consuming large quantities of fossil fuels, both as the reducing agent and for energy which contributes significantly to anthropogenic carbon dioxide emissions. The hybrid sulphur cycle combines concentrating solar thermal energy with electrolysis to offer strong potential for low-cost green hydrogen production. A preliminary evaluation is reported of the techno-economic potential of this cycle to displace current fossil-based energy sources in an integrated copper mine and refinery (cradle-to-gate approach) at a remote location in Australia with an excellent solar resource. The effect of ore composition on the integration of the hybrid sulphur cycle and copper processing plant is evaluated using models developed in Aspen Plus. The evaluations show that sizing the hybrid sulphur cycle cycle to meet the oxygen demand of the copper refining process is more technoeconomically viable than sizing the hybrid sulphur cycle to meet the hydrogen required to replace the fossil fuel demand of the copper processing process. Moreover, it has been found that the integration of the hybrid sulphur cycle with the copper process plant has the potential to decrease both the carbon dixoide emissions and the operational expenditure of copper refineries for ores with a sufficiently high sulphide content (~50% mass fraction)

Differential effects of extreme drought on production and respiration: synthesis and modeling analysis

This is the published version, also available here: http://dx.doi.org/10.5194/bg-11-621-2014, 2014.Extremes in climate may severely impact ecosystem structure and function, with both the magnitude and rate of response differing among ecosystem types and processes. We conducted a modeling analysis of the effects of extreme drought on two key ecosystem processes, production and respiration, and, to provide a broader context, we complemented this with a synthesis of published results that cover a wide variety of ecosystems. The synthesis indicated that across a broad range of biomes, gross primary production (GPP) was generally more sensitive to extreme drought (defined as proportional reduction relative to average rainfall periods) than was ecosystem respiration (ER). Furthermore, this differential sensitivity between production and respiration increased as drought severity increased; it occurred only in grassland ecosystems, and not in evergreen needle-leaf and broad-leaf forests or woody savannahs. The modeling analysis was designed to enable a better understanding of the mechanisms underlying this pattern, and focused on four grassland sites arrayed across the Great Plains, USA. Model results consistently showed that net primary productivity (NPP) was reduced more than heterotrophic respiration (Rh) by extreme drought (i.e., 67% reduction in annual ambient rainfall) at all four study sites. The sensitivity of NPP to drought was directly attributable to rainfall amount, whereas the sensitivity of Rh to drought was driven by soil drying, reduced carbon (C) input and a drought-induced reduction in soil C content – a much slower process. However, differences in reductions in NPP and Rh diminished as extreme drought continued, due to a gradual decline in the soil C pool leading to further reductions in Rh. We also varied the way in which drought was imposed in the modeling analysis; it was either imposed by simulating reductions in rainfall event size (ESR) or by reducing rainfall event number (REN). Modeled NPP and Rh decreased more by ESR than REN at the two relatively mesic sites but less so at the two xeric sites. Our findings suggest that responses of production and respiration differ in magnitude, occur on different timescales, and are affected by different mechanisms under extreme, prolonged drought

Experimental Investigation of a System of two Vacuum Solar Receivers for the Continuous Reduction of Ceria Particles

A solar receiver for the continuous reduction of redox particles under vacuum conditions has been developed previously as part of a system to produce hydrogen from solar energy. Here, we report about a joint effort of Sandia and DLR to improve the receivers design and to demonstrate a system of two receivers with different vacuum pressures at DLR’s solar simulator Synlight. We focus on the design and experimental investigation of three components: the novel beam down mirror, the novel secondary concentrator and the improved version of the particle conveying plate in the receiver. Irradiation test results and heat transfer analyses of the beam-down mirror and the secondary concentrator are being presented. The motion of the conveyor plate was improved by a MATLAB model, which predicts the transport speed of the particles on the conveyor

Solar thermochemical energy storage in elemental sulphur: design, development and con-struction of a lab-scale sulphuric acid splitting reactor powered by hot ceramic particles

A proof of concept sulphuric acid splitting/decomposition prototype driven by hot bauxite particles is designed and developed. The lab-scale test reactor is a novel counter-current flow shell-and-tube heat exchanger with particles on the shell side and sulphuric acid on the tube side with mass flow rates of 10 kg/h and 2 kg/h, respectively. A one-dimensional heat transfer model was developed based on correlations of the flow boiling heat transfer coefficient and particle bed heat transfer coefficient for sizing the shell-and-tube heat exchanger. A detailed study was carried out in order to choose suitable materials especially in the sulphuric acid inlet and evaporation section. A new concept of an electrically heated, continuously operated particle heating system was designed and developed to provide the splitting reactor with hot particles. Different cases were studied using a finite element method (FEM) analysis to qualify the particle heater and examine its thermo-mechanical stabilit