Evaluación de un sistema de energía solar térmica basado en colectores de tubos de vacío para suministro de agua a alta temperatura

Esta tesis pretende contribuir al fomento y utilización de la energía solar como alternativa para la producción de agua caliente en el sector agroindustrial. La demanda de agua caliente es un aspecto clave en un gran número de agroindustrias y explotaciones agrarias. Esta demanda presenta una gran variabilidad, tanto en los horarios en que se solicita como en la temperatura del agua del depósito requerida (TADr), difiriendo del perfil de demanda habitual para uso doméstico. Existe una necesidad de profundizar en la influencia que tiene la variación de la TADr en la eficiencia y viabilidad de estos sistemas. El objetivo principal de esta tesis es caracterizar el funcionamiento de un sistema solar térmico (SST) con captador de tubos de vacío (CTV) para producir agua a temperaturas superiores a las habituales en estos sistemas. Se pretende determinar la influencia que la TADr tiene sobre la eficiencia energética del sistema, cuantificar el volumen de agua caliente que es capaz de suministrar en función de la TADr y determinar la rentabilidad del SST como sistema complementario de suministro. Para ello, se ha diseñado, instalado y puesto a punto un sistema experimental de calentamiento de agua, monitorizando su funcionamiento a diferentes TADr bajo condiciones ambientales reales. Los resultados cuantifican cómo el aumento de la TADr provoca una disminución de la energía suministrada al depósito, pudiendo superar diferencias de 1000 Wh m-2 d-1 entre 40 ºC y 80 ºC, para valores de irradiación solar próximos a 8000 Wh m-2 d-1 (la eficiencia del sistema oscila entre 73% y 56%). Esta reducción es consecuencia de la disminución de la eficiencia del captador y del aumento de las pérdidas de calor en las tuberías del circuito. En cuanto al agua suministrada, cuanto mayor es la TADr, mayor es la irradiación solar requerida para que tenga lugar la primera descarga de agua, aumentando el tiempo entre descargas y disminuyendo el número de éstas a lo largo del día. A medida que se incrementa la TADr, se produce una reducción del volumen de agua suministrado a la TADr, por factores como la pérdida de eficiencia del captador, las pérdidas en las tuberías, la energía acumulada en el agua que no alcanza la TADr y la mayor energía extraída del sistema en el agua producida. Para una TADr de 80 ºC, una parte importante de la energía permanece acumulada en el depósito sin alcanzar la TADr al final del día. Para aprovechar esta energía sería necesario disponer de un sistema complementario de suministro, ya que las pérdidas de calor nocturnas en el depósito pueden reducir considerablemente la energía útil disponible al día siguiente. La utilización del sistema solar como sistema único de suministro es inviable en la mayoría de los casos, especialmente a TADr elevadas, al no ajustarse la demanda de agua caliente a la estacionalidad de la producción del sistema solar, y al existir muchos días sin producción de agua caliente por la ausencia de irradiación mínima. Por el contrario, la inversión del sistema solar como sistema complementario para suministrar parte de la demanda térmica de una instalación es altamente recomendable. La energía útil anual del sistema solar estimada oscila entre 1322 kWh m-2 y 1084 kWh m-2. La mayor rentabilidad se obtendría suponiendo la existencia de una caldera eléctrica, donde la inversión se recuperaría en pocos años -entre 5.7 años a 40 ºC y 7.2 años a 80 ºC -. La rentabilidad también es elevada suponiendo la existencia de una caldera de gasóleo, con periodos de recuperación inferiores a 10 años. En una industria ficticia con demanda de 100 kWh d-1 y caldera de gasóleo existente, la inversión en una instalación solar optimizada sería rentable a cualquier TADr, con valores de VAN cercanos a la inversión realizada -12000 € a 80 ºC y 15000€ a 40 ºC- y un plazo de recuperación de la inversión entre 8 y 10 años. Los resultados de este estudio pueden ser de gran utilidad a la hora de determinar la viabilidad de utilización de sistemas similares para suministrar la demanda de agua caliente de agroindustrias y explotaciones agropecuarias, o para otras aplicaciones en las que se demande agua a temperaturas distintas de la habitual en uso doméstico (60 ºC). En cada caso, los rendimientos y la rentabilidad vendrán determinados por la irradiación de la zona, la temperatura del agua requerida y la curva de demanda de los procesos específicos. ABSTRACT The aim of this thesis is to contribute to the development and use of solar energy as an alternative for producing hot water in the agribusiness sector. Hot water supply is a key issue for a great many agribusinesses and agricultural holdings. Both hot water demand times and required tank water temperature (rTWT) are highly variable, where the demand profile tends to differ from domestic use. Further research is needed on how differences in rTWT influence the performance and feasibility of these systems. The main objective of this thesis is to characterize the performance and test the feasibility of an evacuated tube collector (ETC) solar water heating (SWH) system providing water at a higher temperature than is usual for such systems. The aim is to determine what influence the rTWT has on the system’s energy efficiency, quantify the volume of hot water that the system is capable of supplying at the respective rTWT and establish whether SWH is feasible as a booster supply system for the different analysed rTWTs. To do this, a prototype water heating system has been designed, installed and commissioned and its performance monitored at different rTWTs under real operating conditions. The quantitative results show that a higher rTWT results in a lower energy supply to the tank, where the differences may be greater than 1000 Wh m-2 d-1 from 40 ºC to 80 ºC for insolation values of around 8000 Wh m-2 d-1 (system efficiency ranges from 73% to 56%). The drop in supply is due to lower collector efficiency and greater heat losses from the pipe system. As regards water supplied at the rTWT, the insolation required for the first withdrawal of water to take place is greater at higher rTWTs, where the time between withdrawals increases and the number of withdrawals decreases throughout the day. As rTWT increases, the volume of water supplied at the rTWT decreases due to factors such as lower collector efficiency, pipe system heat losses, energy stored in the water at below the rTWT and more energy being extracted from the system by water heating. For a rTWT of 80 ºC, much of the energy is stored in the tank at below the rTWT at the end of the day. A booster supply system would be required to take advantage of this energy, as overnight tank heat losses may significantly reduce the usable energy available on the following day. It is often not feasible to use the solar system as a single supply system, especially at high rTWTs, as, unlike the supply from the solar heating system which does not produce hot water on many days of the year because insolation is below the required minimum, hot water demand is not seasonal. On the other hand, investment in a solar system as a booster system to meet part of a plant’s heat energy demand is highly recommended. The solar system’s estimated annual usable energy ranges from 1322 kWh m-2 to 1084 kWh m-2. Cost efficiency would be greatest if there were an existing electric boiler, where the payback period would be just a few years —from 5.7 years at 40 ºC to 7.2 years at 80 ºC—. Cost efficiency is also high if there is an existing diesel boiler with payback periods of under 10 years. In a fictitious industry with a demand of 100 kWh day-1 and an existing diesel boiler, the investment in the solar plant would be highly recommended at any rTWT, with a net present value similar to investment costs —12000 € at 80 ºC and 15000 € at 40 ºC— and a payback period of 10 years. The results of this study are potentially very useful for determining the feasibility of using similar systems for meeting the hot water demand of agribusinesses and arable and livestock farms or for other applications demanding water at temperatures not typical of domestic demand (60ºC). Performance and cost efficiency will be determined by the regional insolation, the required water temperature and the demand curve of the specific processes in each case.

Ultra high temperature latent heat energy storage and thermophotovoltaic energy conversion

A conceptual energy storage system design that utilizes ultra high temperature phase change materials is presented. In this system, the energy is stored in the form of latent heat and converted to electricity upon demand by TPV (thermophotovoltaic) cells. Silicon is considered in this study as PCM (phase change material) due to its extremely high latent heat (1800 J/g or 500 Wh/kg), melting point (1410 C), thermal conductivity (~25 W/mK), low cost (less than $2/kg or $4/kWh) and abundance on earth. The proposed system enables an enormous thermal energy storage density of ~1 MWh/m3, which is 10e20 times higher than that of lead-acid batteries, 2e6 times than that of Li-ion batteries and 5e10 times than that of the current state of the art LHTES systems utilized in CSP (concentrated solar power) applications. The discharge efficiency of the system is ultimately determined by the TPV converter, which theoretically can exceed 50%. However, realistic discharge efficiencies utilizing single junction TPV cells are in the range of 20e45%, depending on the semiconductor bandgap and quality, and the photon recycling efficiency. This concept has the potential to achieve output electric energy densities in the range of 200-450 kWhe/m3, which is comparable to the best performing state of the art Lithium-ion batteries.

Circulation of core collection monographs in an academic medical library

Academic medical librarians responsible for monograph acquisition face a challenging task. From the plethora of medical monographs published each year, academic medical librarians must select those most useful to their patrons. Unfortunately, none of the selection tools available to medical librarians are specifically intended to assist academic librarians with medical monograph selection. The few short core collection lists that are available are intended for use in the small hospital or internal medicine department library. As these are the only selection tools available, however, many academic medical librarians spend considerable time reviewing these collection lists and place heavy emphasis on the acquisition of listed books. The study reported here was initiated to determine whether the circulation of listed books in an academic library justified the emphasis placed on the acquisition of these books. Circulation statistics for “listed” and “nonlisted” books in the hematology (WH) section of Indiana University School of Medicine's Ruth Lilly Medical Library were studied. The average circulation figures for listed books were nearly two times as high as the corresponding figures for the WH books in general. These data support the policies of those academic medical libraries that place a high priority on collection of listed books.

Investigação do mecanismo cinético da reação de redução de oxigênio em solventes não aquosos

O aumento no consumo energético e a crescente preocupação ambiental frente à emissão de gases poluentes criam um apelo mundial favorável para pesquisas de novas tecnologias não poluentes de fontes de energia. Baterias recarregáveis de lítio-ar em solventes não aquosos possuem uma alta densidade de energia teórica (5200 Wh kg-1), o que as tornam promissoras para aplicação em dispositivos estacionários e em veículos elétricos. Entretanto, muitos problemas relacionados ao cátodo necessitam ser contornados para permitir a aplicação desta tecnologia, por exemplo, a baixa reversibilidade das reações, baixa potência e instabilidades dos materiais empregados nos eletrodos e dos solventes eletrolíticos. Assim, neste trabalho um modelo cinético foi empregado para os dados experimentais de espectroscopia de impedância eletroquímica, para a obtenção das constantes cinéticas das etapas elementares do mecanismo da reação de redução de oxigênio (RRO), o que permitiu investigar a influência de parâmetros como o tipo e tamanho de partícula do eletrocatalisador, o papel do solvente utilizado na RRO e compreender melhor as reações ocorridas no cátodo dessa bateria. A investigação inicial se deu com a utilização de sistemas menos complexos como uma folha de platina ou eletrodo de carbono vítreo como eletrodos de trabalho em 1,2-dimetoxietano (DME)/perclorato de lítio (LiClO4). A seguir, sistemas complexos com a presença de nanopartículas de carbono favoreceu o processo de adsorção das moléculas de oxigênio e aumentou ligeiramente (uma ordem de magnitude) a etapa de formação de superóxido de lítio (etapa determinante de reação) quando comparada com os eletrodos de platina e carbono vítreo, atribuída à presença dos grupos laterais mediando à transferência eletrônica para as moléculas de oxigênio. No entanto, foi observada uma rápida passivação da superfície eletrocatalítica através da formação de filmes finos de Li2O2 e Li2CO3 aumentando o sobrepotencial da bateria durante a carga (diferença de potencial entre a carga e descarga > 1 V). Adicionalmente, a incorporação das nanopartículas de platina (Ptnp), ao invés da folha de platina, resultou no aumento da constante cinética da etapa determinante da reação em duas ordens de magnitude, o qual pode ser atribuído a uma mudança das propriedades eletrônicas na banda d metálica em função do tamanho nanométrico das partículas, e estas modificações contribuíram para uma melhor eficiência energética quando comparado ao sistema sem a presença de eletrocatalisador. Entretanto, as Ptnp se mostraram não específicas para a RRO, catalisando as reações de degradação do solvente eletrolítico e diminuindo rapidamente a eficiência energética do dispositivo prático, devido ao acúmulo de material no eletrodo. O emprego de líquido iônico como solvente eletrolítico, ao invés de DME, promoveu uma maior estabilização do intermediário superóxido formado na primeira etapa de transferência eletrônica, devido à interação com os cátions do líquido iônico em solução, o qual resultou em um valor de constante cinética da formação do superóxido de três ordens de magnitude maior que o obtido com o mesmo eletrodo de carbono vítreo em DME, além de diminuir as reações de degradação do solvente. Estes fatores podem contribuir para uma maior potência e ciclabilidade da bateria de lítio-ar operando com líquidos iônicos.

Dense carbon monoliths for supercapacitors with outstanding volumetric capacitances

A commercially available dense carbon monolith (CM) and four carbon monoliths obtained from it have been studied as electrochemical capacitor electrodes in a two-electrode cell. CM has: (i) very high density (1.17 g cm−3), (ii) high electrical conductivity (9.3 S cm−1), (iii) well-compacted and interconnected carbon spheres, (iv) homogeneous microporous structure and (v) apparent BET surface area of 957 m2g−1. It presents interesting electrochemical behaviors (e.g., excellent gravimetric capacitance and outstanding volumetric capacitance). The textural characteristics of CM (porosity and surface chemistry) have been modified by means of different treatments. The electrochemical performances of the starting and treated monoliths have been analyzed as a function of their porous textures and surface chemistry, both on gravimetric and volumetric basis. The monoliths present high specific and volumetric capacitances (292 F g−1 and 342 F cm−3), high energy densities (38 Wh kg−1 and 44 Wh L−1), and high power densities (176 W kg−1 and 183 W L−1). The specific and volumetric capacitances, especially the volumetric capacitance, are the highest ever reported for carbon monoliths. The high values are achieved due to a suitable combination of density, electrical conductivity, porosity and oxygen surface content.

Pliocene-Pleistocene stack of globally distributed benthic stable oxygen isotope records

We present a 5.3-Myr stack (the ''LR04'' stack) of benthic d18O records from 57 globally distributed sites aligned by an automated graphic correlation algorithm. This is the first benthic delta18O stack composed of more than three records to extend beyond 850 ka, and we use its improved signal quality to identify 24 new marine isotope stages in the early Pliocene. We also present a new LR04 age model for the Pliocene-Pleistocene derived from tuning the delta18O stack to a simple ice model based on 21 June insolation at 65 N. Stacked sedimentation rates provide additional age model constraints to prevent overtuning. Despite a conservative tuning strategy, the LR04 benthic stack exhibits significant coherency with insolation in the obliquity band throughout the entire 5.3 Myr and in the precession band for more than half of the record. The LR04 stack contains significantly more variance in benthic delta18O than previously published stacks of the late Pleistocene as the result of higher resolution records, a better alignment technique, and a greater percentage of records from the Atlantic. Finally, the relative phases of the stack's 41- and 23-kyr components suggest that the precession component of delta18O from 2.7-1.6 Ma is primarily a deep-water temperature signal and that the phase of d18O precession response changed suddenly at 1.6 Ma.

(Table 1) Stratigraphic distribution and abundances of Cretaceous radiolarians from ODP Holes 130-803D and 130-807C

Among the five sites drilled during Ocean Drilling Program Leg 130, two deep holes (8O3D and 807C) penetrated Cretaceous sediments overlying the basaltic pillows, flows, and possibly basement rocks. Abundant, poorly preserved radiolarians with limited diversity were recovered from a few horizons within the sediments proximal to the basalt. At Site 803, three thin layers of radiolarites interbedded with claystone and clayey siltstone yielded radiolarian assemblages of late Albian age. At Site 807, several layers of radiolarian siltstones were recovered proximal to the basalt. Among them the most significant radiolarian assemblage is an Aptian fauna, located approximately 7 m above the basaltic flows. The Aptian radiolarian age for Site 807 is at least in accord with those suggested by planktonic foraminifer and paleomagnetic evidence. These Cretaceous radiolarians are the oldest assemblages recorded from the Ontong Java Plateau region.

Stable isotope composition of Neogene planktonic foraminifera from ODP Leg 130 sites

We produced a preliminary record for shallow-dwelling planktonic foraminifer d18O at Site 807 for the late Pleistocene, early Pliocene, and early Miocene. Site 807 d18O values between 4 and 5 Ma average 0.75 per mil more than Holocene values and show an average variation of 0.5 per mil. For the early Pliocene, peak maximum d18O at Site 807 attain values equivalent with the last glacial maximum whereas peak minimum d18O were never less than Holocene d18O. Shallow-dwelling planktonic d18O at Site 807 between 16 and 24 Ma average more than 1.0 per mil more positive than Holocene d18O and exhibit 0.5 per mil average amplitude. Assuming that the global ice budget for the early Pliocene and early Miocene was restricted to Antarctica, it is difficult to attribute the very positive Site 807 d18O for these intervals to ice on Antarctica. Site 807 d18O for these intervals more likely reflect sea-surface temperatures cooler than at present, sea-surface salinity greater than at present, increased dissolution, or some combination of these changes.

(Table 1) Consolidation test for sediments of ODP Leg 130 sites

Consolidation tests were performed on 19 samples of calcareous ooze from the Ontong Java Plateau, obtained during Ocean Drilling Program Leg 130. Rebound curves from consolidation tests on Ontong Java Plateau samples yield porosity rebounds of 1%-4% for these sediments at equivalent depths up to 1200 mbsf. The exception is a radiolarian-rich sample that has 6% rebound. A rebound correction derived from the porosity rebound vs. depth data has been combined with a correction for pore-water expansion to correct the shipboard laboratory porosity data to in-situ values. Comparison of the laboratory porosity data corrected in this manner with the downhole log data shows good agreement.

(Table 1) Abundances of Discoaster species in ODP Hole 130-806C

Members of the calcareous nannofossil genus Discoaster have been used extensively to subdivide Tertiary deep-sea sediments into biostratigraphic zones or subzones (e.g., Martini, 1971; Bukry, 1973). Haq and Lohmann (1976) mapped biogeographic migrations of this group through time and over latitude. They suggested that expansions and contractions of Discoaster-dominated assemblages across latitudes reflect sea-surface temperature changes. Subsequently, late Pliocene Discoaster species were counted at closely spaced sample intervals from various Atlantic sites (Backman et al., 1986; Backman and Pestiaux, 1987; Chepstow-Lusty et al., 1989, 1991), and Indian Ocean as well as Pacific Ocean sites (Chepstow-Lusty, 1990). In addition to the biostratigraphic information revealing positions and the precision by which the different late Pliocene Discoaster species can be determined, these studies also demonstrated that discoasters strongly fluctuate in abundance as a function of time. These abundance variations occur in equatorial as well as temperate temperature regimes, and show periodicities that reflect orbital frequencies. Chepstow-Lusty et al. (1989, 1991) also suggested that the oscillating abundances partly represent productivity pressure, because discoasters tend to show low abundances under high productivity conditions and vice versa. In the Pacific Ocean, counts showing late Pliocene Discoaster abundances exist from three sites, namely Ocean Drilling Program (ODP) Site 677 in the eastern equatorial upwelling region, Core V28-179 from the central equatorial region, and Core V32-127 from the mid-latitude Hess Rise. The two Vema cores are condensed and show sedimentation rates below 0.5 cm/1000 yr, thus offering a poorly resolved stratigraphy. Hole 806C from the Ontong Java Plateau provided an opportunity to establish a highly resolved Discoaster record from the western extreme of the equatorial Pacific under an environmental setting that differed from ODP Site 677 by being less influenced by intense upwelling. The Discoaster counting technique is described by Backman and Shackleton (1983).