Browsing by Subject "Irradiance"
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Item Coupling photovoltaics and grid-scale energy storage : performance and sitability(2015-05) Stoll, Brady Leigh; Deinert, Mark; Baldick, Ross; Edgar, Thomas; Howell, John; Shi, Li; Webber, MichaelThe Fifth Assessment of the International Panel on Climate Change has called for a four fold increase in the use of low-carbon sources of electricity to help stabilize climate change by mid century. Many people look to solar power systems to help reduce carbon intensity, but cost and variability have been significant obstacles to their widespread deployment. However, the cost of photovoltaics has dropped significantly in recent years, and grid-scale energy storage technologies are available to allow for production of dispatchable electricity from photovoltaics. In particular, compressed-air energy storage is both low-cost and can be built in a wide variety of geologies as well as above ground. I show that coupling large-scale photovoltaic arrays and grid-scale storage allows for dispatchable electricity production at costs that are comparable to other low carbon electricity sources. I examine four load curves: base-load generation, on-peak generation, and averaged load curves for the Electric Reliability Council of Texas (ERCOT) and PJM Independent System Operators. I found that on-peak and ERCOT loads typically required the lowest amount of storage, up to 2000 MWh [subscript e] less than that for base-load generation. However, in some regions, and for some storage amounts, baseload output actually provided the lowest cost of electricity. I also show that such coupled systems could provide base-load electricity for ≤ 0.08/kWh [subscript e] on more than 40% of global land surface, with a capacity factor equivalent to that of the US nuclear fleet. Importantly, this is below the projected cost of electricity from new nuclear power systems. While cost is a major factor, also of importance is where systems of photovoltaics and grid-scale storage would provide the most benefit. Locations expected to provide energy at the lowest cost do not necessarily correspond to load and population centers, where the electricity is most needed. I use multi-criteria decision analysis techniques to perform a global study of the optimal locations for siting these coupled systems to maximize their social benefit. I found that the most ideal locations are generally located in Africa, Iraq, and southeast Asia, as these locations have both high irradiance levels as well as expanding populations and low grid connectivity.Item Developing Optimal Growth Parameters for the Green Microalgae Nannochloris oculata and the Diatom Nitzschia sp. for Large scale Raceway Production(2011-10-21) Luedecke, Phillip RyanMicroalgae produce large quantities of lipids that can be used for biofuel feedstock. The goal of this project was to determine the effect of several engineering and management parameters on the productivity of microalgae cultivated in large, outdoor facilities. The specific objectives were focused on the effects of inoculation ratios; the effects of light, temperature, and culture depth on growth; and the minimum circulation velocity necessary to maintain growth and minimize settling in open ponds. Microalgae must first be cultured in smaller quantities before the raceway is inoculated for optimized growth. Concentration ratios are defined as the ratio of the volume of microalgae inoculum to the volume of new growth media. The microalgae species used was Nannochloris oculata (UTEX #LB 1998). Inoculation ratios studied varied from 1:1 to 1:32 and were grown in 500 mL Erlenmeyer flasks. The study found that 1:16 and 1:32 were too dilute, while the 1:8 concentration had the largest growth rate. Determination of the effects of temperature, light intensity, and cultivation depth is critical to maintaining healthy cultures. Excess light intensity can result in photoinhibition and temperatures above the maximum growing tolerance can have detrimental effects. These factors can affect growth and evidence suggests an interaction that exacerbates these effects. In an outdoor culture there are few practical control variables other than pond depth. As cultivation depth increases, the algae undergo "selfshading" and the increased cultivation volume hinders temperature changes. Scaled raceway ponds were maintained at 10.16 cm (4 inch) and 13.97 cm (5.5 inch) depths. The species used was Nannochloris oculata and it was found to grow best at 785 micromol m?? s??m^-2 s^-1, 20?C, and 10.16 cm. Diatoms are attractive because of high growth rates, faster lipid production, and greater cell density. The latter promotes rapid settling once mixing has stopped. Because of the silica cell wall composition, diatoms are believed to be more susceptible to shear forces which can result in lysis. Determining the natural settling rate to the minimum channel velocity relationship in cultivation ponds was the objective. No flocculants/coagulants were added which created a case of "natural" settling. Four pennate Nitzschia sp. and one centric diatom were tested in a jar tester. There was no significant difference in settling times between the species. The mean settling time was 4.55 cm min?? and the minimum channel velocity was determined to be 10.12 cm min??.Item Temperature and irradiance dependence of dye-sensitized solar cell performance(2010-12) Peng, Edwin, 1989-; Meyers, Jeremy P.; Berberoglu, HalilDye-sensitized solar cells (DSSCs) are photoelectrochemical cells that offer efficient and potentially economical alternative to conventional solar electricity production technologies. DSSCs belong to the third generation of solar cells and offer several advantages over the solid-state junction solar cells. They utilize materials, such as titanium dioxide that are inexpensive and abundant relative to those used in conventional solar cells. Moreover, DSSCs can be fabricated with simple and scalable manufacturing processes. Finally, in DSSCs, photon absorption and charge-carrier transport are undertaken by different materials, namely molecular dyes and wide band gap semiconductors, respectively. Unlike conventional solar cells, no compromise is necessary between decreasing the band gap for visible light absorption and increasing the band gap to resist photocorrosion. For successful commercialization, a photovoltaic system incorporating DSSCs must operate reliably under a wide range of solar irradiance and operating temperatures. This experimental study reports the fabrication and characterization of the performance of a DSSC as a function of irradiance and operating temperature. The prototyped DSSCs had (i) nanocrystalline titanium(IV) dioxide, TiO₂, photoanode, (ii) platinum thin film cathode, and (iii) acetonitrile based liquid electrolyte. The photoanodes were sensitized with N-749 dye. The current-voltage characteristics of the DSSCs were measured at operating temperatures from 5 to 50° C and under 500, 1000, and 1500 W m⁻² irradiance. The open circuit voltage, V[subscript oc], decreased linearly with increasing temperature and had positive, logarithmic relation with irradiance. At temperatures lower than 15° C and 1500 W m⁻² irradiance, short circuit current density, J[subscript sc], was limited by the diffusion of I₃ in the electrolyte and increased with increasing temperature. At temperatures lower than 15° C and lower irradiance, J[subscript sc] increased with increasing temperature due to electron density limited recombination of electrons injected into the TiO₂ conduction band. At higher temperatures, the recombination was dominant over diffusion and J[scubscript sc] decreased with increasing temperature. Moreover, J[subscript sc] increased linearly with increasing irradiance. The DSSC photoconversion efficiency did not vary appreciably at temperatures lower than 15° C but decreased with increasing temperature. Finally, the DSSC efficiency increased with increasing irradiance. There was no indication of significant coupling effect of irradiance and temperature on DSSC efficiency. This study reports for the first time the coupling between irradiance and thermal effects on the operation of DSSCs. The results reported in this study can be used in recovering kinetic and transport properties that can be used in modeling and optimization of DSSCs.