R.N.N. Koury

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.

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.

Energy efficiency labeling: Study about the influence of schedule definitions

Technical literature shows that there is a performance gap between a simulated design and the actual building performance, especially given the differences between the post-occupation routines and those provided in the design phase. This paper aims to analyze the differences between the electric energy consumption prediction of a call center building located in Belo Horizonte, Brazil, using (a) a schedule based on routines provided by the air conditioning designer made in order to establish the system loads and to define the system equipment and (b) a schedule based in the employees routines provided by the owners of the building. First, the simulation results were compared to the mean measured building energy consumption and, differently from the first schedule, when the second schedule was used, a Normalized Mean Bias Error (NMBE) of 5% was met which is considered a prediction acceptable by the ASHRAE Guideline 14-2002 [Measurement of Energy and Demand Savings (American Society of Heating, Refrigerating and Air-conditioning Engineers, 2002)]. Afterwards, it was verified how the two occupation schedules influenced the building classification according to the Brazilian Governmental Building Energy Efficiency Labeling. In this case, the two different schedules resulted in a difference in electric energy consumption of 10.9%, leading to a difference of one level in the classification. This fact clearly demonstrates the importance of a correct definition of occupation and equipment usage in the energy performance prediction of a building.Technical literature shows that there is a performance gap between a simulated design and the actual building performance, especially given the differences between the post-occupation routines and those provided in the design phase. This paper aims to analyze the differences between the electric energy consumption prediction of a call center building located in Belo Horizonte, Brazil, using (a) a schedule based on routines provided by the air conditioning designer made in order to establish the system loads and to define the system equipment and (b) a schedule based in the employees routines provided by the owners of the building. First, the simulation results were compared to the mean measured building energy consumption and, differently from the first schedule, when the second schedule was used, a Normalized Mean Bias Error (NMBE) of 5% was met which is considered a prediction acceptable by the ASHRAE Guideline 14-2002 [Measurement of Energy and Demand Savings (American Society of Heating, Refrigeratin...

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.

Dynamic model and experimental validation for a gas cooler of a CO2 heat pump for heating residential water

This work presents a dynamic model for a CO2 heat pump gas cooler. The gas cooler model is based on the conservation equations of energy, mass, and momentum. Due to the importance of an expansion device and a compressor for heat exchanger modeling, detailed mathematical treatment is also given to those devices. The spatial variations of temperature, pressure, specific volume, and mass flow rate are predicted at each time step. The model is validated by comparing simulation results and experimental data for transient and steady conditions. It observes a reasonably good agreement between these results, with a maximum deviation of 2°C (3.6°F) between the mathematical model and experimental data, with the test data exhibiting a fairly similar trend. The developed model is therefore presented as a useful tool for analyzing the dynamic behavior for the gas cooler of a heat pump operating with CO2.