Elaboration of the physical model of an air-to-water heat exchanger under condensing and frosting conditions

Sviluppo di un modello numerico di una batteria di scambio termico aria-acqua (e glicole) in condizioni di condensazione e brinamento. Analisi critica della letteratura sullo scambio combinato di calore e massa.Scrittura del modello, validazione e successiva applicazione in simulazioni dinamiche. Analisi parametrica per evidenziare l'influenza delle condizioni ambiente e di carico sulle prestazioni termiche ed elettriche dello scambiatoreope

Advanced surface and volumetric receivers to convert concentrated solar radiation

This 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

Monitoring Of a Commercial Refrigeration CO2 System And Comparison With Simulations

The demand for natural refrigerants is growing in commercial refrigeration systems. In the recent years, carbon dioxide has increased its market share in the field of commercial refrigeration and has proven to be a viable solution for the replacement of hydrofluorocarbon (HFC) systems. This success is mainly due to the ongoing technological evolution of carbon dioxide refrigeration systems. In the present paper, a CO2 commercial refrigeration unit serving a supermarket located in Northern Italy is presented. The unit consists of a booster compressor rack with parallel compression, with ten compressors arranged to provide around 20 kW cooling capacity at low temperature and 90 kW cooling capacity at medium temperature. Four out of ten compressors, provided by Frascold®, are arranged in parallel. The installation, in addition to the cooling load, provides all the thermal functions in one unit: it integrates a heat exchanger for the air conditioning and the possibility of two stage heat recovery, for sanitary hot water production and for space heating. The refrigeration unit is equipped with pressure and temperature sensors, power consumption and load analysers for the compressors. A computer model has been developed to evaluate the acquired data of the system and to analyse the key parameters. The preliminary results from the monitoring of this unit are presented in this paper and used to calibrate the model of the system. Afterwards, simulations have been performed at variable operating conditions in a cold month to evaluate the performance of the unit. The results of the model have been compared to an independent set of monitoring data

Requirements of a Supportive Environment for People on the Autism Spectrum: A Human-Centered Design Story

People on the autism spectrum have a different perception of the environment than neurotypical people and often require support in various activities of daily living. Assistive technology can support those affected, but very few smart-home-like technologies exist. To support people on the autism spectrum in their autonomy and safety and to help caregivers, a smart home and interior design environment was developed. Requirements were gathered by employing a holistic human-centered design approach through interactive workshops and questionnaires to create a useful and user-friendly solution. From this process, requirements for a comprehensive solution (the SENSHOME environment) emerged. These requirements include a set of functionalities tailored to the needs of people on the autism spectrum, such as a crowd warning that informs when many people are in a certain area (for example, the entrance), an automatic light regulation system, or a daily life planner that supports task completion. Furthermore, inclusive furniture elements such as a refuge seat or a table with dividers can support wellbeing, autonomy, and safety. This paper demonstrates a consequent and considerable participatory research approach and the story from the target group and context of use through design requirements to the initial design solution of the SENSHOME environment

Advanced surface and volumetric receivers to convert concentrated solar radiation

This 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.La presente tesi è il risultato del lavoro svolto durante i tre anni di dottorato. presso il Dipartimento di Ingegneria Industriale dell'Università degli Studi di Padova. La conversione dell'energia solare in calore a media temperatura (tra 80 °C e 250 °C) ha recentemente riscontrato un rinnovato interesse per le applicazioni di riscaldamento e raffreddamento in settori industriali, commerciali, residenziali e dei servizi. I collettori solari termici a concentrazione per media temperatura ben si prestano per l’impiego in molte applicazioni commerciali e industriali, come per la produzione di calore di processo industriale, il solar-cooling e la desalinizzazione dell'acqua di mare. Si prevede un significativo sviluppo tecnologico per questa tipologia di collettori, a condizione che la conversione dell'energia solare diventi più efficiente ed economica. La corretta progettazione del ricevitore, considerato il cuore di ogni collettore a concentrazione, è essenziale per il futuro incremento dell'efficienza di conversione di questa tecnologia. In questo contesto, questa tesi riporta i risultati dell'applicazione di due innovativi ricevitori in un prototipo di concentratore parabolico asimmetrico installato nel Laboratorio di conversione dell'energia solare del Dipartimento di Ingegneria Industriale dell'Università degli Studi di Padova. Nel Capitolo 1 è presentato lo studio di diverse procedure per la stima dell'irradianza normale diretta, che è la risorsa solare utilizzata dai concentratori solari. Lo studio include una valutazione indiretta da misurazioni di irradianza orizzontale globale e diffusa insieme all'uso di modelli semi-fisici/empirici. Viene fornita un'analisi dettagliata della strumentazione e del metodo di misurazione utilizzati nonché dell'espressione dell'incertezza sperimentale. Nel Capitolo 2, le prestazioni ottiche del concentratore parabolico asimmetrico sono caratterizzate sperimentalmente. Un modello statistico di ray-tracing del concentratore per l'analisi delle prestazioni ottiche in diverse condizioni di lavoro è stato convalidato ed utilizzato per ottimizzare la geometria dei ricevitori proposti. Nel Capitolo 3, un innovativo ricevitore superficiale in alluminio e prodotto con la tecnologia bar-and-plate e un turbolatore al suo interno è stato testato nel collettore parabolico asimmetrico in regime di deflusso monofase e bifase con acqua e con un fluido alogenato a basso GWP. Un modello numerico per prevedere le prestazioni di tale ricevitore è stato sviluppato e validato sulla base dei dati sperimentali acquisiti. Nel Capitolo 4, questo modello è stato impiegato per valutare le prestazioni di un impianto ORC di piccola taglia alimentato da energia solare attraverso il suddetto collettore solare a concentrazione. L’analisi ha compreso la vaporizzazione diretta di un fluido alogenato a basso GWP e l’utilizzo un circuito solare intermedio per riscaldare acqua pressurizzata ed evaporare lo stesso fluido alogenato in uno scambiatore di calore. Infine, nel Capitolo 5 è stato proposto l’impiego di un nuovo ricevitore volumetrico ad assorbimento diretto per studiare la capacità di assorbimento della radiazione solare concentrata di un nanofluido. Il nanofluido è una sospensione di nano-corni di carbonio a parete singola in acqua distillata ed è il risultato di un progetto di collaborazione con la sede di Padova del Consiglio Nazionale della Ricerca. Attraverso lo sviluppo di un modello CFD tridimensionale, il ricevitore volumetrico è stato progettato per essere installato nella regione di fuoco del concentratore parabolico asimmetrico. La capacità del nanofluido di assorbire la radiazione solare a concentrata e non concentrata è stata studiata sperimentalmente. Al fine di indagare sulla stabilità nel tempo del nanofluido, delle prove sono state condotte in diverse condizioni, con e senza circolazione

Elaboration of the physical model of an air-to-water heat exchanger under condensing and frosting conditions

Sviluppo di un modello numerico di una batteria di scambio termico aria-acqua (e glicole) in condizioni di condensazione e brinamento. Analisi critica della letteratura sullo scambio combinato di calore e massa.Scrittura del modello, validazione e successiva applicazione in simulazioni dinamiche. Analisi parametrica per evidenziare l'influenza delle condizioni ambiente e di carico sulle prestazioni termiche ed elettriche dello scambiator

Modelling of a direct absorption solar receiver using carbon based nanofluids under concentrated solar radiation

The 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

Nanofluids application in direct absorption solar collectors: review and numerical model

The 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