Luiz Machado

Numerical simulation of a variable speed refrigeration system

Abstract This work presents two numerical models to simulate the transient and steady state behavior of a vapor compression refrigeration system. The condenser and the evaporator were divided into a number of control volumes. Time dependent partial differential equations system was obtained from the mass, energy and momentum balances for each control volume. As the expansion valve and the compressor both have very small thermal inertia, the steady state models were applied for these components. Transient and steady state models numerical predictions were compared and good agreement was found. Further simulations were performed with the objective of verifying the possibility of controlling the refrigeration system and the superheating of the refrigerant in the evaporator outlet by varying the compressor speed and the throttling valve sectional area. The results indicate that the proposed models can be used to formulate an algorithm for controlling a refrigeration system.

USING A HEAT PUMP AS AN ALTERNATIVE TO SUPPORT SOLAR COLLECTOR FOR WATER HEATING IN BRAZIL

With related greenhouse effect environmental issues linked to the constant problems of the fluctuations in oil prices, the use of solar energy is an important renewable energy source. Brazil is a country which is privileged considering the high rates of solar irradiation present throughout almost the entire national territory. Nevertheless, during certain times of the year, there is a solar energy deficit, which leads solar systems to require electrical resistance support at these times. The use of electrical resistance represents 23.5% of electric energy consumption and it presents a low residential energy efficiency. The purpose of this work is an alternative technical design for reduction of electric energy consumption through the use of a solar energy system together with a generating heat pump for water heaters for households, as well as the financial feasibility study on the use of this system. One such heat pump has been designed, constructed and tested experimentally. The average performance coefficient is equal to 2.10, a low value due to the use of a hermetic reciprocating compressor. Despite this low moderate price coefficient of acquisition and installation of a heat pump, one can allow a return on investment in from 2.1 to 3.3 years, whereas the equipment has a useful life of about 20 years, this period of return on investment is interesting.

Digestibilidade aparente dos nutrientes de dietas simplificadas baseadas em forragens para coelhos em crescimento

The effect of simplified diets based on forages on the apparent digestibility in white New Zealand rabbits was evaluated. The treatments were based on the following diets: reference (REF), hay of alfalfa (FAL), hay of rami leaves (FRA), hay of mulberry leaves (FAM) and hay of upper to 1/3 aereal part of cassava (FMA). The type of food affected the digestibility of the diets. The FMA diet showed low coefficients of digestibility in comparison to the other diets for all the analyzed nutrients. For the FAM diet the coefficients of apparent digestibility of the nutrients had higher values (P 0.05). The estimated values of digestible energy (kcal DE/kg DM) and digestible protein (%DP/DM) were, respectively, 2285.27 and 16.04 for alfalfa hay, 1857.88 and 16.37 for hay of rami leaves, 2838.48 and 15.12 for hay of mulberry leaves and 2155.55 and 10.57 for hay of upper to 1/3 aereal part of cassava.

Superheating control using an adaptive PID controller

Electronic expansion valves have been used to replace conventional expansion devices in many refrigeration systems. Electronically controlled valves respond more rapidly to changes in operating conditions and improve the steady-state superheating. These valves are usually used with an automatic controller that regulates the superheating at the evaporator outlet. The controller gains (Kp, Ti, and Td) must be properly tuned for efficient operation. However, these controllers can result in poor performance because they have been poorly tuned or put into operation using factory tuning. For refrigeration systems that are subject to large changes in operating conditions, the controller gains should be adjusted for each change to improve the system performance. Within this context, we developed an adaptive Proportional-Integral-Derivative controller (PID controller) in this study to regulate the degree of superheating. A dynamic model obtained from experimental tests was used in the controller design. The controller effectiveness was evaluated using computer simulations and experimental tests. In comparison to a nonadaptive PID controller, the adaptive controller provided better disturbance rejection and set-point tracking and was able to control the superheating more efficiently, demanding less servomotor effort.

Distributed and Nonsteady-State Model of an Air Cooler Working with R22 and R410A

The restrictions imposed by Montreal Protocol for use of CFCs fluids and Kyoto Protocol to HCFCs have motivated researchers and the industry to seek new alternatives. Within this context, R410A has emerged as one of the most likely replacement of R22. The purpose of this work is to develop a numerical model of an air cooler to simulate its behavior operating under dynamic conditions loaded with R22 or R410A refrigerant. The model divides the air cooler in volumes control in which mass, energy, and momentum balance equations are applied and solved. Theoretical data obtained by model simulations repeated tendencies observed in experimental data taken from literature. Model simulations have also shown that for a step change in the inlet refrigerant mass flow, the superheating response of air cooler is almost the same when it is working with R22 or R410A refrigerant.

Adiabatic Capillary Tube Model for a Carbon Dioxide Transcritical Cycle

This paper presents a mathematical model for a capillary tube using CO2 as fluid in steady flow transcritical cycle. The capillary tube is divided into N volumes controls and the model is based on applying the equations of conservation of energy, mass and momentum in the fluid in each of these volumes controls. The model calculates the mass flow of the CO2 in the capillary tube as a function of CO2 pressures at the inlet and outlet of the capillary and the temperature of CO2 at the input of this device. The capillary tube is considered to be adiabatic, and the limit of operation due to blocked flow condition is also considered in the model. The validation of the model was performed with experimental data and the results showed that the model is capable of predicting the mass flow in the capillary tube with errors less than 10%. The model was also used to determine the minimum diameter of the capillary tube for various conditions of CO2 transcritical cycle.

Numerical model of an inflatable solar collector

ABSTRACT The main objective of this paper is to present a mathematical model to simulate the operation of an inflatable solar air heat collector for grain dryers. The solar collector exhibits a semicylindrical shape when air is injected into the confined space between the walls of the two covers. The mathematical model of the collector is obtained by applying the law of conservation of energy on the four main parts of the collector, absorber plate, two covers, and air that flows inside the equipment. To improve the accuracy of the model, the solar collector was divided into N control volumes, with each one containing the four parts, in such a way that one system with 4N differential equations is obtained. The unknown quantities of this system are the temperatures of the absorber plate, air, and walls of both covers in each control volume. The results allow us to conclude that the collector performance strongly depends on several entrance variables of the model (the air flow and air temperature at the collector entrance) and geometric equipment parameters (the collector length, internal radiation at the inner surface of the cover, and space between both covers). Therefore, the model can be used for optimizing the solar collector.

Estudo experimental e modelo de simulação para a determinação do inventário em sistemas de refrigeração carregados com os fluidos R134a e R1234yf

O presente trabalho e um estudo teorico/experimental sobre o inventario em sistemas de refrigeracao carregados com os fluidos R-134a e HFO-1234yf. Embora o R-134a seja um hidrofluorcarboneto (HFC) livre de cloro, ou seja, com potencial de destruicao da camada de ozonio (ODP) igual a zero, o seu potencial de aquecimento global (GWP) e de 1430, valor considerado alto. Ja o fluido HFO-1234yf pertence a uma nova familia de refrigerantes, os hidrofluorolefinas (HFO), com propriedades termodinâmicas proximas as do R-134a. Da mesma forma, ele apresenta um ODP igual a zero, porem, com um GWP reduzido, igual a 4, sendo considerado um substituto do R-134a para novas aplicacoes e com potencial para substitui-lo nos climatizadores automotivos dos carros atuais. Neste trabalho, a revisao bibliografica abordou diversos temas referentes aos escoamentos monofasico e bifasico no interior de tubulacoes, como padroes de escoamento, fracao de vazio e correlacoes empregadas na sua determinacao, calculo do coeficiente de transferencia de calor e metodos para estimativa da perda de pressao. Resultados experimentais foram obtidos utilizando os fluidos R-134a e HFO-1234yf, com velocidades massicas de 300kg/s-m2 e 400kg/s-m2, temperaturas de ebulicao de 25°C e 30°C e uma ampla faixa de titulos na secao de testes do banco experimental. Estes resultados foram analisados e comparados com um modelo de simulacao utilizando oito diferentes correlacoes para determinacao da fracao de vazio. Os melhores resultados para previsao da massa foram obtidos pelas correlacoes de Hughmark (1962) e Zivi (1964), usando as correlacoes de Gnielinski (1976) e Shah (2013) para a determinacao dos coeficientes de transferencia de calor das regioes monofasica e bifasica do condensador, respectivamente. Nessa situacao, o modelo alcancou um indice de acerto de aproximadamente 78% na previsao da massa total de fluido.

The effect of energy efficiency increase on a PV plant with a non-conventional energy storage system income

This Photovoltaic (PV) plants are widely used to produce power in either large or small scales all around the world. In addition, compressed air energy storage (CAES) system has attracted considerable attention as one of the most efficient candidates for large scales energy storage applications in the recent years. In this work, detailed energy and exergy analysis of a 100 MWp grid connected PV plant equipped with a CAES system is carried out. The PV plant is assumed to be located in Brazil. The formulations related to the first and the second laws of thermodynamic for all components as well as detailed solar engineering formulations for both the PV farm and the solar heating unit are presented. The performance of the power plant is comprehensively investigated for one entire year in real circumstances. The results revealed that the annual average exergy and energy efficiencies of the power plant are 17.9% and 16.2%, respectively and for 1% energy efficiency enhancement the power plant, the annual income increases almost 4 million USD.

Stabilizing a photovoltaic plant power output by employing an auxiliary power source

The use of compressed air energy storage (CAES) systems instead of conventional energy storage systems in large scale grid connected photovoltaic (PV) plans has already been proposed and investigated thermo-economically, resulting to very satisfactory outcomes. On the other hand, city gate stations (CGS), in which high pressure natural gas is expanded to much lower presser levels, has been proved to be a very suitable place for producing free electricity by employing turbo-expanders instead of conventional throttling valves. In this work, the feasibility of employing a CGS power output for improving the performance of a grid connected PV plant accompanied with a CAES system and enhancing its power output stability is studied. Comprehensive energy analysis and economic assessment on the proposed configuration is carried out and the results are discussed thoroughly. Finally, the performance of this hybrid configuration is compared with the PV plant and the CGS station while working individually. Internal rate of return (IRR) method as an authentic economic evaluation approach is used for comparing the considered systems economically.