DYNAMIC ANALYSIS OF SOLAR HOT WATER SYSTEMS: THE CASE STUDY OF A DWELLING LOCATED IN ITALY

Abstract The present paper is aimed at the energetic analysis of a solar hot water system performed on a dwelling located in different cities in Italy, precisely, from north to south: Bolzano, Turin, Bologna, Rome, Brindisi and Trapani. The study is focused on a group of apartments, where the domestic hot water is provided by a solar system coupled with a storage tank and a recirculation loop; it is worth to mention that the recirculation loop is obligatory on this kind of plants. Therefore, a dynamic energetic study is carried out, assigned the hot water load profiles and the set parameters of the solar collectors with reference to the configurations here tested (flat plane and evacuated tube). Thus, dynamic simulations have been performed by means of TRNSYS simulations tool, setting different parameters: collector configuration and mass flow rate. Several ideas and results emerged from this study, such as the sizing process of the whole system, according to the variation of the solar fraction for all the analyzed scenarios. A digression with simulation results regarding a specific case concludes the paper. More specifically, the study of water temperature oscillations in winter and summer time with reference to the climatic data of Bologna has been carried out under particular operating conditions: i) the pump of the recirculation loop is powered all time (24 hours a day) ii) the pump is switched off during the night, except when the water temperature in the loop falls below a certain threshold temperature

Theoretical and Numerical Study on Buongiorno’s Model with a Couette Flow of a Nanofluid in a Channel with an Embedded Cavity

In the present paper, the fluid flow and heat transfer of a nanofluid are numerically investigated. More specifically, reference is made to a nanofluid, described by means of Buongiorno’s model, subjected to Couette flow. The considered domain consists of a channel that displays a cavity shortly after the inlet section. The transport model for the nanofluid, that is the mass conservation, momentum, and nanoparticles equation, is written in a dimensionless form and solved by employing the software package Comsol Multiphysics. Many ideas emerged from this work: the visualization of the velocity stream function, the dimensionless temperature, and nanoparticle concentration fields are provided, as a function of the governing parameters: Reynolds, Peclet, Lewis, Brownian diffusivity number, and thermophoretic diffusivity number. Concerning the nanofluid typical effects, the thermophoretic diffusion seems to affect the solution much more than the Brownian diffusion. The Nusselt number on the upper wall is calculated as well, and the results show that it proves to be, in most of the considered cases, an increasing function of the Reynolds number. Moreover, concerning the Nusselt number, the Brownian diffusion effects are shown to be negligible

Constructal Design of an Elliptical Cavity into a Solid Conducting Wall

Abstract This work reports, according to Bejan's Constructal theory, the geometric optimization of an elliptical cavity that intrudes into a solid conducting wall. The objective is to minimize the global thermal resistance between the solid and the cavity. There is uniform heat generation on the solid wall. The cavity is isothermal and the solid conducting wall is isolated on the external perimeter. The total volume and the elliptical cavity volume are fixed while the geometry of the cavity is free to vary. The cavity shape is optimal when penetrates the conducting wall completely

Fixed Grid Numerical Models for Solidification and Melting of Phase Change Materials (PCMs)

Phase change materials (PCMs) are classified according to their phase change process, temperature, and composition. The utilization of PCMs lies mainly in the field of solar energy and building applications as well as in industrial processes. The main advantage of such materials is the use of latent heat, which allows the storage of a large amount of thermal energy with small temperature variation, improving the energy eciency of the system. The study of PCMs using computational fluid dynamics (CFD) is widespread and has been documented in several papers, following the tendency that CFD nowadays tends to become increasingly widespread. Numerical studies of solidification and melting processes use a combination of formulations to describe the physical phenomena related to such processes, these being mainly the latent heat and the velocity transition between the liquid and the solid phases. The methods used to describe the latent heat are divided into three main groups: source term methods (E-STM), enthalpy methods (E-EM), and temperature-transforming models (E-TTM). The description of the velocity transition is, in turn, divided into three main groups: switch-o methods (SOM), source term methods (STM), and variable viscosity methods (VVM). Since a full numerical model uses a combination of at least one of the methods for each phenomenon, several combinations are possible. The main objective of the present paper was to review the numerical approaches used to describe solidification and melting processes in fixed grid models. In the first part of the present review, we focus on the PCM classification and applications, as well as analyze the main features of solidification and melting processes in dierent container shapes and boundary conditions. Regarding numerical models adopted in phase-change processes, the review is focused on the fixed grid methods used to describe both latent heat and velocity transition between the phases. Additionally, we discuss the most common simplifications and boundary conditions used when studying solidification and melting processes, as well as the impact of such simplifications on computational cost. Afterwards, we compare the combinations of formulations used in numerical studies of solidification and melting processes, concluding that “enthalpy–porosity” is the most widespread numerical model used in PCM studies. Moreover, several combinations of formulations are barely explored. Regarding the simulation performance, we also show a new basic method that can be employed to evaluate the computing performance in transient numerical simulations

A Simple Transient Poiseuille-Based Approach to Mimic the Womersley Function and to Model Pulsatile Blood Flow

As it is known, the Womersley function models velocity as a function of radius and time. It has been widely used to simulate the pulsatile blood flow through circular ducts. In this context, the present study is focused on the introduction of a simple function as an approximation of theWomersley function in order to evaluate its accuracy. This approximation consists of a simple quadratic function, suitable to be implemented in most commercial and non-commercial computational fluid dynamics codes, without the aid of external mathematical libraries. The Womersley function and the new function have been implemented here as boundary conditions in OpenFOAM ESI software (v.1906). The discrepancy between the obtained results proved to be within 0.7%, which fully validates the calculation approach implemented here. This approach is valid when a simplified analysis of the system is pointed out, in which flow reversals are not contemplated

Implementation of a Floating Head Pressure Condensation Control to Reduce Electrical Energy Consumption in an Industrial Refrigeration System

The growing global demand for energy and the costly taxes on electric energy demonstrate the importance of seeking new techniques to improve energy efficiency in industrial facilities. Refrigeration units demand a large amount of electricity due to the high power needs of the components of the system. One strategy to reduce the electric energy consumption in these facilities is pressure condensation control. The objective here was to develop a logical control model where the physical quantities in the thermodynamic process can be monitored and used to determine the optimum point of the condensation pressure and the mass flow rate of the air in the evaporative condenser. The algorithm developed was validated through experiments and was posteriorly implemented in an ammonia industrial system of refrigeration over a period of sixteen months (480 days). The results showed that the operation of the evaporative condenser with a controlled air mass flow rate by logical modeling achieved a reduction of 7.5% in the consumption of electric energy, leading to a significant reduction in the operational cost of the refrigeration plant