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Advanced surface and volumetric receivers to convert concentrated solar radiation
Full text linkThis thesis is the results of the work conducted during the three years of Ph.D. at the Department of Industrial Engineering of the University of Padova.
The conversion of solar energy into heat in the medium-temperature range (between 80 °C and 250 °C) has recently encountered a renewed interest in heating and cooling applications of industrial, commercial, residential and service sectors. Concentrating solar thermal collectors at medium temperature are suitable for many commercial and industrial applications, such as industrial process heat, solar cooling and desalination of the seawater. It is expected that in the future, a significant technological development can be achieved for these collectors, provided that the conversion of solar energy becomes more efficient and cost-effective. The proper design of the receiver, which is considered the heart of any concentrating collector, is essential to the future improvement in the conversion efficiency of this technology. In this context, the present thesis investigates the application of two innovative concepts of receivers in a prototype of an asymmetrical parabolic trough concentrator installed in the Solar Energy Conversion Lab of the Industrial Engineering Department, at the University of Padova.
In Chapter 1, a study on different estimation procedures for the assessment of the direct normal irradiance, which is the solar resource utilized by solar concentrators, is presented. The study includes an indirect evaluation from measurements of global and diffuse horizontal irradiances and the use of semi-physical/empirical models. A detailed analysis of the instrumentation and of the measuring technique as well as the expression of the experimental uncertainty is provided. In Chapter 2, the optical performance of the asymmetrical parabolic trough is experimentally characterized. As a result, a statistical ray-tracing model of the concentrator for optical performance analysis in different working conditions is validated and used to optimize the design of the proposed receivers. In Chapter 3, an innovative flat aluminium absorber manufactured with the bar-and-plate technology, including an internal turbulator, is tested in the asymmetrical parabolic trough collector under single-phase and two-phase flow regimes. A numerical model to predict its performance has been developed and validated against the experimental data. In Chapter 4, this model is used to evaluate the performance of a small solar-powered ORC system by coupling the aforementioned concentrating solar system with direct vaporization of a low-GWP halogenated fluid or by using an intermediate solar circuit to heat pressurized water and evaporate the same organic working fluid in a separate heat exchanger. Finally, in Chapter 5 a new direct absorption receiver is proposed to investigate the capability of a suspension of single-wall carbon nanohorns in distilled water to absorb concentrated sunlight. The volumetric receiver has been designed through the development of a three-dimensional computational fluid dynamics model for its installation in the focus region of the asymmetrical parabolic trough. The capability of the nanofluid in collecting solar radiation when exposed to concentrated and non-concentrated solar flux are experimentally investigated thanks to the cooperation with National Council of the Research (CNR), that provided the aqueous solution. The nanofluid was tested in several conditions, with and without circulation, to investigate its stability with time
Experimental study of a parabolic trough solar collector with flat bar-and-plate absorber during direct steam generation
No full textNanofluids application in direct absorption solar collectors: review and numerical model
No full textThe application of nanofluids has the potential to solve technical key issues in many solar thermal engineering systems. Recent literature indicates that nanofluids offer unique advantages over conventional fluids. Nanofluids are made of solid nanoparticles suspended in a liquid. These particles enhance optical properties of the liquid suspension, increasing the efficiency in the conversion of solar radiation into thermal energy. This study investigates the application and challenges of nanofluids in solar energy systems. The main literature on numerical models of nanofluids in solar thermal energy is here presented. In particular, the attention has been focused on nanofluid-based direct absorption solar collectors (DASC). Based on this review, a new model of a nanofluid-based direct absorption solar receiver for a concentrating solar collector has been proposed and then applied to predict the performance of a receiver with single-wall carbon nanohorns aqueous suspension
Modelling of a direct absorption solar receiver using carbon based nanofluids under concentrated solar radiation
No full textThe addition of nanoparticles in a base fluid can enhance its optical properties, in particular its absorption properties. Thus, nanofluids can be successfully used in solar collectors to absorb the solar radiation in their volume and avoid using an absorber plate. This paper investigates the application of aqueous suspensions as volumetric absorber in a concentrating direct absorption solar collector: a suspension of single wall carbon nanohorns (SWCNHs) in water is chosen as the nanofluid. A model of a solar receiver with a planar geometry to be installed in a parabolic trough concentrator is developed: the radiative transfer equation in participating medium and the energy equation are numerically solved to predict the thermal performance of the receiver. The developed model is capable to predict the temperature distribution, heat transfer rate and penetration distance of the concentrated solar radiation inside the nanofluid volume. The simulated performance of the direct absorption receiver has been compared with calculations and experimental data of two surface absorption conventional receivers under the same operating conditions
Modelling heat and mass transfer in a membrane-based air-to-air enthalpy exchanger
Full text linkThe diffusion of total energy recovery systems could lead to a significant reduction in the energy demand for building air-conditioning. With these devices, sensible heat and humidity can be recovered in winter from the exhaust airstream, while, in summer, the incoming air stream can be cooled and dehumidified by transferring the excess heat and moisture to the exhaust air stream. Membrane based enthalpy exchangers are composed by different channels separated by semi-permeable membranes. The membrane allows moisture transfer under vapour pressure difference, or water concentration difference, between the two sides and, at the same time, it is ideally impermeable to air and other contaminants present in exhaust air. Heat transfer between the airstreams occurs through the membrane due to the temperature gradient. The aim of this work is to develop a detailed model of the coupled heat and mass transfer mechanisms through the membrane between the two airstreams. After a review of the most ..
Coupled radiative and fluid flow modelling for a direct absorption solar receiver
No full textThe application of nanofluids has the potential to solve technical issues in many solar thermal engineering systems. Recent literature indicates that nanofluids offer unique advantages over conventional fluids. Nanofluids are solid nanoparticles suspended in a liquid: the average dimensions of these particles may vary from 1 to 100 nm. The addition of particles in a base fluid can enhance its optical properties, in particular its absorption properties. Thus nanofluids can be successfully used in direct absorption solar collectors to directly absorb the solar radiation in their volume. In this kind of devices, it is possible to surpass the constraints of conventional collectors due to the absence of the absorber plate. An important advantage of direct absorption of solar radiation is to avoid the thermal resistance between the absorber surface and the heat transfer fluid. This paper investigates the application of water based nanofluids as volumetric absorber in a direct absorption solar collector: a suspension of single wall carbon nanohorns in water is chosen as nanofluid. A new model of a solar receiver with a planar geometry is developed: radiative transfer equation in participating medium and energy equation are numerically solved to predict the performance of the receiver; the optical and thermal behaviors of the nanofluid are modelled according to the properties available in the current scientific literature. Monte Carlo ray tracing is used to determine the directional and spatial distribution of the concentrated solar radiation coming from a parabolic trough concentrator. This distribution is then applied to the receiver geometry using a commercial computational fluid dynamic software to simulate the incoming solar flux. The developed model is capable to predict temperature distribution, heat transfer rate and penetration distance of the concentrated solar radiation inside the nanofluid volume. Two different configurations of the bottom surface are considered: transparent wall and reflecting wall. The effects of inlet temperature, flow rate and nanoparticle concentration on the energy efficiency of the receiver are studied. Finally, the model is applied to compare the performance of a direct absorption receiver to a conventional surface receiver under the same operating conditions
CONCENTRATED FLUX MEASUREMENT APPARATUS FOR AN ASYMMETRICAL PARABOLIC TROUGH SOLAR CONCENTRATOR
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