A model of a storage tank with an immersed serpentine heat exchanger is described and validated against experimental data available from the literature. The tank is modelled one dimensionally using the multi-node approach corrected by an energy conservative reversion elimination algorithm to prevent inverse gradient solutions to occur. A one dimensional model in the flow direction is also used for the serpentine based on control volume techniques. The serpentine is discretized in equal sized control volumes and the energy equation is solved in each of them. The energy exchanged between the serpentine and the tank is then introduced as an internal heat source of the tank multi-node. With this model the behaviour of tanks with internal serpentines can be predicted minimising tuning parameters to be derived from previous experimental analysis of the tank. Additionally, by an appropriate formulation of the governing equations in the serpentine control volumes, it is possible to handle complex internal fluid phenomena as coupling of the tank within a thermosyphone cycle or two phase flow.
A detailed numerical model for flat-plate solar thermal collectors based on one-dimensional finite volume techniques was recently presented, see Cadafalch (2009). The model considers a solar thermal device as a pile of components represented by one or several layers characterized by thermal inertia, internal energy generation and heat transfer to neighboring layers. A multi-layer model is then used to evaluate the full flat-plate solar thermal device. The model permits to investigate any configuration and material by combining appropriate layers. Standard components as opaque insulation, absorbers, air-gaps and glasses were addressed in Cadafalch (2009).Here, a numerical model to evaluate honeycomb-like transparent insulation material in the covers as a component of the multi-layer model is discussed in detail. The honeycomb is evaluated coupling radiation, convection and conduction phenomena. The discret ordinate method is used to evaluate media participation in thermal radiation.A comparison of numerical and experimental results is presented and discussed in order to show evidence of the model credibility.
Cadafalch, J.; Consul, R.; Ruiz Mansilla, Rafael International Conference on Solar Heating, Cooling and Buildings p. 2828-2837 DOI: doi:10.1016/j.egypro.2014.10.316 Data de presentació: 2013-11-05 Presentació treball a congrés
The calculation of radiation losses on collecting surfaces due to shadows effect of surrounding objects is a key aspect in the design process of both photovoltaics and solar thermal collecting fields. This aspect is of special relevance in small size installations located in residential areas. Even the calculations procedures involved are well known, they are complicated and need skilled planners that usually cannot be afforded within the constrained budgets of this kind of projects.
The authors present a new cloud application available for free at www.omnilus.com designed with the goal of maximum user simplicity while based in detailed calculation procedures. Not only yearly shadow losses are calculated but also monthly averaged values, so that it is possible to use this value to reduce monthly averaged daily solar radiation values used for systems calculation. This work, presents an overlook of the application, details on the calculation procedure used, and one case example.
Software development for renewable heating and cooling systems has mainly been focused on design optimization and prediction. On the contrary, no much effort has been done on the development of software to support the sales process. Tools already available from standard installations and the construction sector can be used. However, they can be costly and time consuming, and out of the scope of the small SMEs usually involved in the sector. This work presents an integral tool developed by the authors to address budget technical design and communication with providers and customers. The tool is designed in order to automate the process and minimize office-time and costs. A beta version is already available at the free cloud platform www.omnilus.com. A simple example case of a domestic hot water system for a family house is used to support the explanation
Cadafalch, J.; Consul, R.; González-Valero, A.; Ruiz Mansilla, Rafael International Conference on Solar Heating, Cooling and Buildings p. 2898-2903 DOI: 10.1016/j.egypro.2014.10.324 Data de presentació: 2013-11-05 Presentació treball a congrés
With the continuous improvements in web technology, communications and computers (servers), many new low-cost and high-performance computer centers have been set-up which are able to store data and host applications that can be accessed through any-internet browser from any device like PCs, tablets or smartphones. This new approach is known as Cloud Computing or only just as the Cloud, and it has become a major innovation that is drastically changing the information and communication technologies (ICT) world.
A new web platform called OmniluS based on Cloud technology is here presented. OmniluS is designed to hold apps to support engineering in general, and the design, commercialization and maintenance of renewable heating and cooling (RHC) plants in particular. The OmniluS creators are much concerned with the need of automation and standarization of processes related to RHC engineering as a key aspect for a market break-through of this technology. Therefore, OmniluS apps are designed and distributed using a FSF approach, where FSF stands for Free, Simple and Fast.
This work presents a general overlook of the platform with some insight in the software structure, the databases, and applications already available both in stable or demo versions.
Two collector models were analyzed under thermosyphon solar thermal system conditions: an extension of the physical model described by Duffie and Beckman (1991) and a modified correlation model based on the test efficiency curve obtained from European Standards. Special attention was paid to the body forces term of the momentum equation, a key aspect for thermosyphon system calculations. The models were verified and validated using a virtual test that numerically reproduces efficiency curves according to EN12975 (2006). A virtual test generated to represent thermosyphon unsteady system conditions was used to analyze model response under transient conditions. The Extended Duffie–Beckman model was shown to perform well when submitted to strong unsteady boundary conditions such as inlet fluid temperature, irradiance and mass flow rate. The model based on the efficiency curve was shown to work well for time steps larger than the collector residence time. However, for lower time steps, the model was found to be inaccurate due to the hypothesis of a single control volume for the fluid analysis. For the same reason, besides the assumption of a first order temperature profile in the fluid flow, the model was not capable to predict a physical behavior when submitted to strong variations of the fluid inlet temperature.
An equation-fit (EF) and a refrigerant cycle (RC) based heat pump models have been implemented, validated, analyzed and compared to each other under steady state conditions for a brine to water heat
pump. Models validations have been provided through comparisons against experimental data obtained at ISFH. The advantages and disadvantages of the both models have been identified. This work provides significant inputs regarding the selection of a specific model depending on the needs. Analysis of mass flow rates and calculations far from typical catalogue data (non-standard conditions) are provided. The
main conclusions can be summarized as: i) the EF model is recommended when the boundary conditions for the estimation and prediction modes are the same and when non-standard conditions are considered;
ii) the RC model is the chosen alternative when the mass flow rates are modified from the estimation to the prediction mode.
Renewable energy technologies for heating and cooling can be often optimized using low temperatures in the
terminal heat exchangers for heating and the opposite for cooling. For example, thermal and electrical driven
heat pumps have higher coefficient of performance (COP) for low impulse heating temperature and relatively
high cooling temperature. If solar energy is directly used for heating purposes also a low temperature is
necessary. Using this range of temperatures, a large exchange area is needed in order to obtain the desired
conditions. In this sense, radiant floors and ceilings can be used with the objective to reduce the impulse
water temperature in winter and increase it in summer to obtain high energy savings. However, a careful
design and optimal control strategy are important to reach expected energy savings. Therefore, a model
capable to capture transient effects, system control strategies, and it’s coupling with building energy
simulation, is of importance.
A transient numerical model for radiant floors and ceilings is presented and validated. The model has been
implemented in RDmes online web platform and can solve both steady state and transient situations for
sizing and predicting respectively. The radiant floor model has been developed to be used in the framework
of the IEA-Task44 and the radiant ceiling has been employed in the FREDSOL project.
The model developed is based on the composite fin model for radiant floor described by Kilkis et. al.
(1994) coupled with a multi layer model and a step-by-step algorithm. The multi layer model solves the
transient one-dimensional conduction behavior of the different layers. The two models are coupled
considering the heat flow from the pipe as a sink source term in a typical transient conduction problem.
The similar concept was used in the collector model presented by Cadafalch (2009). The step-by-step
algorithm solves the fluid flow in one dimension.
In the paper, an explanation of the mathematical formulation and numerical algorithm is provided in detail. A
validation has been realized by means of experimental data comparisons from other references.
Computational results have been analyzed and compared with other models presented in the test cases
This work describes the air-conditioning solar driven system installed at the premises of the company RDmes, located in Terrassa (about 25 km inland from Barcelona). Main components that make up the system are a small-capacity adsorption machine of 8 kW, a solar field of flat plate
single glazed collectors with a small buffer tank, a cold distribution system through a radiant ceiling and liquid-to-ambient heat exchanger as auxiliary temperature source.
The engineering of the plant has been carried out by using detailed transient modelling tools developed by RDmes. Sensors and hardware for detailed measuring and control of the plant have been installed in order to analyze the actual performance.
First measurements of the plant will be obtained during summer 2010. This work addresses details on the engineering and planning process.
L'objectiu principal d'aquesta tesi es l'estudi de la transferència d'energia per radiació. Per aquest motiu, s'ha estudiat la fenomenologia bàsica de la transferencia de calor per radiació. Tenint en compte el tipus d'equació que descriu aquesta transferència d'energia, aquesta tesi esta encarada als metodes numèrics que ens permetran incorporar la radiació en els nostres càlculs. Donat que aquest és el primer treball d'aquestes característiques en el grup de recerca CTTC ("Centre Tecnològic de Transferència de Calor"), està limitat a geometries senzilles, cartesianes i cilíndriques.
En el capítol 1 s'exposa una breu introducció a la transferència d'energia per radiació, i una explicació de les equacions que la governen. Es tracta de l'equació del transport radiatiu, formulada en termes dels coeficients d'absorció i de dispersió, i l'equació de l'energia. També s'indica quan cal tenir en compte aquest fenòmen, i a més a més, es defineixen totes les magnituds i conceptes que s'han utilitzat en aquesta tesi. També es dóna una breu descripció d'algunes simplificacions que es poden fer a les equacions governants.
El mètode de les radiositats s'explica en el capítol 2. També s'hi descriu un procediment numèric que permet calcular els factors de vista en geometries amb simetria cilíndrica, i es presenten resultats obtinguts amb el mètode descrit. Tot i que aquest capítol està una mica deslligat de la resta de la tesi, l'algoritme ideat per tractar geometries tridimensionals amb un temps computacional molt proper al de geometries bidimensionals, sense un increment de memòria apreciable, dóna uns resultats prou bons com per formar part de la tesi.
El mètode de les ordenades discretes (DOM) es detalla en el capítol 3. L'aspecte més important d'aquest mètode es l'elecció del conjunt d'ordenades per integrar l'equació del transport radiatiu. S'enumeren quines propietats han d'acomplir aquests conjunts. S'hi explica amb detall la discretització de la equació del transport radiatiu, tant en coordenades cartesianes com en cilíndriques. Es presenten també alguns resultats ilustratius obtinguts amb aquest mètode.
En el moment en que es vol resoldre un problema real, cal tenir present que el coeficients d'absorció pot dependre bruscament de la longitud d'ona de la radiació. En aquesta tesi s'ha considerat aquesta dependència amb especial interés, en el capítol 4. Aquest interès ha motivat una recerca bibliogràfica sobre la modelització aquesta forta dependència espectral del coeficient d'absorció. Aquesta recerca s'ha dirigit també a l'estudi dels diferents models numèrics existents capaços d'abordar-la, i de resoldre la equació del transport radiatiu en aquestes condicions. Es descriuen diversos mètodes, i, d'aquests, se n'han implementat dos: el mètode de la suma ponderada de gasos grisos (WSGG), i el mètode de la suma de gasos grisos ponderada per línies espectrals (SLW). S'hi presenten també resultats ilustratius.
S'han realitzat multitud de proves en el codi numèric resultant de l'elaboració d'aquesta tesi. Tenint en compte els resultats obtinguts, es pot dir que els objectius proposats a l'inici de la tesi s'han acomplert. Com a demostració de la utilitat del codi resultant, aquest ha estat integrat en un codi de proposit general (DPC), resultat del treball de molts investigadors en els darrers anys.
Aquesta esmentada integració permet la resolució de problemes combinats de transferència de calor, analitzats en els capítols 5 i 6, on la radiació s'acobla amb la transferència de calor per convecció. La influència de la radiació en la transferència total de calor s'estudia en el capítol 5, publicat a la International Journal of Heat and Mass Transfer, volum 47 (núm. 2), pàg. 257-269, 2004. En el capítol 6, s'analitza l'efecte d'alguns paràmetres del mètode SLW en un problema combinat de transferència de calor. Aquest capítol s'ha enviat a la revista Journal of Quantitative Spectroscopy and Radiative Transfer, per què en consideri la publicació.
The main objective of the present thesis is to study the energy transfer by means of radiation. Therefore, the basic phenomenology of radiative heat transfer has been studied. However, considering the nature of the equation that describes such energy transfer, this work is focussed on the numerical methods which will allow us to take radiation into account, for both transparent and participating media. Being this the first effort within the CTTC ("Centre Tecnològic de Transferència de Calor") research group on this subject, it is limited to simple cartesian and cylindrical geometries.
For this purpose, chapter 1 contains an introduction to radiative energy transfer and the basic equations that govern radiative transfer are discussed. These are the radiative transfer equation, formulated in terms of the absorption and scattering coefficients, and the energy equation. It is also given a discussion on when this mode of energy transfer should be considered. In this chapter are also defined all of the magnitudes and concepts used throughout this work. It ends with a brief description of some approximate methods to take radiation into account.
The Radiosity Irradiosity Method is introduced in chapter 2. In this chapter it is also described a numerical method to calculate the view factors for axial symmetric geometries. The main results obtained in such geometries are also presented. Although a little disconnected from the rest of the present thesis, the algorithm used to handle "de facto"' three dimensional geometries with computation time just a little longer than two dimensional cases, with no additional memory consumption, is considered worthy enough to be included in this work.
In chapter 3, the Discrete Ordinates Method (DOM) is detailed. The fundamental aspect of this method is the choice of an ordinate set to integrate the radiative transfer equation. The characterization of such valuable ordinate sets is laid out properly. The discretization of the radiative transfer equation is explained in etail. The direct solution procedure is also outlined. Finally, illustrative results obtained with the DOM under several conditions are presented.
In the moment we wish to solve real problems, we face the fact that the absorption and scattering coefficients depend strongly on radiation wavelength. In the present thesis, special emphasis has been placed on studying the radiative properties of real gases in chapter 4. This interest resulted on a bibliographical research on how the wavenumber dependence of the absorption coefficient is modeled and estimated. Furthermore, this bibliographical research was focussed also on numerical models able to handle such wavenumber dependence. Several methods are discussed, and two of them, namely the Weighted Sum of Gray Gases (WSGG) and the Spectral Line Weighted sum of gray gases (SLW), have been implemented to perform non gray calculations. Some significant results are shown.
Plenty of tests have been performed to the numerical code that resulted from the elaboration of this thesis. According to the results obtained, the objectives proposed in this thesis have been satisfied. As a demonstration of the usefulness of the implemented code, it has been succesfully integrated to a general purpose computational fluid dynamics code (DPC), fruit of the effort of many researchers during many years. Results of the above integration lead to the resolution of combined heat transfer problems, that are analyzed in chapters 5 and 6, where radiative heat transfer is coupled to convection heat transfer. The effect of radiation on the total heat transfer is studied in chapter 5, which has been published as International Journal of Heat and Mass Transfer, volume 47 (issue 2), pages 257--269, year 2004. In chapter 6, the impact of some parameters of the SLW model on a combined heat transfer problem is analyzed. This chapter has been submitted for publication at the Journal of Quantitative Spectroscopy and Radiative Transfer.