J.M. Belman-Flores

Analysis of a variable speed vapor compression system using artificial neural networks

An artificial neural network (ANN) is a mathematical model that is inspired by the operation of biological neural networks. However, this is typically considered a computational model. An ANN can easily adapt to multiple situations and extract information that is apparently hidden in a system. An ANN can be used in three basic configurations: mapping, auto-association and classification. An ANN in a mapping configuration can be used to model a two port system with inputs and outputs. Therefore, a vapor compression system can be modeled using an ANN in a mapping configuration. That is, some parameters from the compression system can be used for input while other parameters can be used as output. The simulation experiments include the design, training and validation of a set of ANNs to find the best architecture while minimizing over-fitting. This paper presents a new method to model a variable speed vapor compression system. This method accurately estimates the number of neurons in the hidden layer of an ANN. The analysis and the experimental results provide new insights to understand the dependency between the input and output parameters in a vapor compression system, concluding that the model can predict the energetic performance of a variable speed vapor compression system. Additionally, the simulation results indicate that an ANN can extract, from the data sets, information that is implicit in the configuration of the vapor compression system.

Second Law Analysis of a Mobile Air Conditioning System with Internal Heat Exchanger Using Low GWP Refrigerants

This paper investigates the results of a Second Law analysis applied to a mobile air conditioning system (MACs) integrated with an internal heat exchanger (IHX) by considering R152a, R1234yf and R1234ze as low global warming potential (GWP) refrigerants and establishing R134a as baseline. System simulation is performed considering the maximum value of entropy generated in the IHX. The maximum entropy production occurs at an effectiveness of 66% for both R152a and R134a, whereas for the cases of R1234yf and R1234ze occurs at 55%. Sub-cooling and superheating effects are evaluated for each one of the cases. It is also found that the sub-cooling effect shows the greatest impact on the cycle efficiency. The results also show the influence of isentropic efficiency on relative exergy destruction, resulting that the most affected components are the compressor and the condenser for all of the refrigerants studied herein. It is also found that the most efficient operation of the system resulted to be when using the R1234ze refrigerant.

Experimental Evaluation of DBD Reactor Applied to Bacterial Inactivation in Water Flowing Continuously

In this paper, we present the evaluation of a DBD reactor that generates an energy per pulse of 65.3 mJ in order to inactivate E. coli ATCC 8739 at 103, 106, and 108 CFU/mL order concentrations and E. cloacae CDBB-B-963 at 103 and 107 CFU/mL order concentrations, in a volume of water flowing continuously. Particularly, experimentation is focused in 500 mL, under atmospheric pressure. The DBD reactor was coupled to a peristaltic pump type, a cooling jacket, containers for water in treatment and cooling fluid, and systems to monitor the experimental conditions. The implementation of the experiments was carried out at three different fluid flow rates: 2.0±0.1, 3.5±0.1, and 4.4±0.1 mL/s; in each of them the time required to carry out a cycle of treatment corresponded to 256±7, 143 ± 2, and 113 ± 1 s, respectively. During the DBD application, water flowed continuously at recirculation mode, promoting the continuous oxygen injection at 8.3 mL/s. In the three established fluid flow rates, the efficacy of treatment reached 100% in inactivation of E. coli bacteria at 103 and 106 order concentrations at the end of the first cycle of treatment. At 108 CFU/mL order concentration, complete inactivation of E. coli bacteria was achieved at the end of the fourth cycle of treatment. Meanwhile, E. cloacae bacteria showed an inactivation of 100% at the end of the first cycle of treatment at 103 order concentration; when the order concentration increased to 107, the efficacy of the process depended on the fluid flow rate, at 3.5 ± 0.1 and 2.0 ± 0.1 mL/s survival was 0% at the end of the first cycle of treatment; nevertheless, at the highest (4.4 ± 0.1 mL/s), an additional cycle was required.

Analysis of alternatives for a multiproduct system using geothermal energy under cascade utilization concept

Geothermal energy is one of the no fossil energy sources that has been utilized mainly for electricity generation, by using the so-called high enthalpy geothermal resource. Nevertheless, low and medium enthalpy geothermal resources are most abundant, but utilized in less extension due mainly to technological barriers or the thermal match between temperature of energy resources and the technology requirements. This work presents the analysis of alternatives for integrating a multiproduct system, producing sequentially electricity, ice and useful heating. For the purpose, the cascade utilization concept is considered for geothermal energy, utilizing low and medium enthalpy resources. To carry out the analysis, it is assumed availability of geothermal hot water with different temperatures typical of already drilled geothermal wells or studied geothermal reservoirs in Mexico. In order to produce electricity, ice and heating for further use (dehydration process or greenhouse heat supply), three cascade levels are proposed to operate sequentially and simultaneously. For electricity generation Organic Rankine Cycles are considered, and for ice production, thermally activated technologies are the best candidates. If necessary, supplementary heat is provided as a mean of geothermal energy upgrade; among the technologies to integrate are parabolic trough collectors, linear Fresnel collectors and biomass boiler. Particularly, with regard to Organic Rankine Cycles, are considered the ones that works with geothermal hot water in the range of 90 °C to 125 °C with rated power output between 25 kWe to 250 kWe. For ice production, two type of machines are under study, i.e. single-effect absorption machines with coefficient of performance around 0.6, and half-effect absorption machines with a value around 0.3 for the coefficient of performance. Absorption machines can be activated thermally with geothermal hot water with temperature in the range of 70 °C to 90 °C. Afterwards, a number of alternatives are proposed to integrate the multiproduct system, which are analyzed and compared both from the energy and economic point of view, obtaining in this way the main energy interactions of the systems, including electricity produced, amount of ice produced and heat availability. In the model, economic indicators are evaluated, obtaining for each alternative the capital cost, simple payback and net present value. Results shows quantitatively that cascade use of geothermal energy is a viable concept to increase the use of low and medium enthalpy geothermal resources with increase of energy performance and improvement of economical profit.Copyright

Transient behavior of CO2 in the internal heat exchanger during the start-up of the transcritical refrigeration system

Transient behavior of internal heat exchanger during start-up of transcritical refrigeration system is investigated in this article. The work is focused on the local transient analysis of CO2 thermophysical properties, in order to see how these transient changes affect the heat transfer rate and the effectiveness of internal heat exchanger, as well as the coefficient of performance during the start-up of the system. The study is conducted through a numerical simulation using the commercial computational fluid dynamics (CFD) software Ansys Fluent. Validation of numerical results is carried out by using seven different empirical correlations applied for the Nusselt number. It is observed that the thermophysical properties of the hot CO2 stream experience large changes during the transient period. This instability is accompanied by a decrease in the heat transfer rate. Finally, the change in the internal heat exchanger effectiveness during the start-up results in a loss of about 12% of the coefficient of performance.

Assessing the Exergy Costs of a 332-MW Pulverized Coal-Fired Boiler

In this paper, we analyze the exergy costs of a real large industrial boiler with the aim of improving efficiency. Specifically, the 350-MW front-fired, natural circulation, single reheat and balanced draft coal-fired boiler forms part of a 1050-MW conventional power plant located in Spain. We start with a diagram of the power plant, followed by a formulation of the exergy cost allocation problem to determine the exergy cost of the product of the boiler as a whole and the expenses of the individual components and energy streams. We also define a productive structure of the system. Furthermore, a proposal for including the exergy of radiation is provided in this study. Our results show that the unit exergy cost of the product of the boiler goes from 2.352 to 2.5, and that the maximum values are located in the ancillary electrical devices, such as induced-draft fans and coil heaters. Finally, radiation does not have an effect on the electricity cost, but affects at least 30% of the unit exergy cost of the boiler’s product.

Analysis of COP stability in a refrigeration system using artificial neural networks

This paper presents the application of an artificial neural network to perform an analysis of the Coefficient of Performance for a compression vapor system operating with R1234yf. A testing facility was built to measure several parameters at the input and at the output of the refrigeration system. These parameters were: the compressor rotation speed, the temperature, and the volumetric flow in the secondary fluids. An artificial neural network was trained to model the behavior of the refrigeration system. A random variable with a uniform distribution was applied to one input of the artificial neural network to measure the effect of this parameter on the Coefficient of Performance. Color plots were built to show the efficiency of the systems under different working conditions. Computer simulations using artificial neural networks were used to analyze the refrigeration system, and observe the best performance of this system. In the same way, these simulations were used to identify which parameter affect the most the coefficient of performance of the installation.

A Brief Model to Evaluate the Behaviour of R134a/R1234yf Mixtures on a Reciprocating Compressor

Transitioning from R134a refrigerant to a low global warming potential (GWP) refrigerant is a current issue of global importance. Although any refrigerant still has set; there are a few options to replace it such as the R1234yf. In this paper is presented a semi-empirical model to assess the energy performance of mixtures with R134a and its possible substitute R1234yf. The inputs variables to the computational model are: suction conditions (pressure and temperature), discharge pressure and rotation speed. With these variables the model must compute the following parameters: mass flow rate, discharge temperature and energy consumption. The model is validated with data obtained from an experimental facility; calculations are obtained within a relative error band of ±10% for mass flow rate and energy consumption, and an error of ±1 K for discharge temperature. Finally, the model is carried out to an energy simulation in order to predict the behavior of different mass fractions of R1234yf. Energy savings are found when R1234yf mass fraction is reduced from 1 to 0.9. Knowing that the mixture with y=0.9 may be used as its GWP is 150.Copyright

Technical and economic study for the integration of parabolic trough collectors coupled to a binary cycle

In this paper, the preliminary technical and economic study for the integration of parabolic trough solar collectors to a geothermal binary cycle plan is presented, in order to maximize the use of medium enthalpy geothermal resource. It is proposed the use of solar collectors to increase the geothermal brine temperature and achieve design temperature of the plant. A study was conducted to determine the main features of the solar field and its coupling with binary cycle power plant. Subsequently, the thermodynamic models were established in order to obtain the main parameters of the hybrid plant. Additionally, a study was conducted to determine the economic feasibility of the plant including the determination of the levelized cost of electricity (LCoE).

Analysis of the Manifold Area Change in a Flat Solar Collector

A numerical analysis of the characterization of the water flow through a flat solar collector is presented. The manifold area change for minimizing the water flow variation in the solar collector is analyzed. The area ratio in the inlet and outlet of the manifolds were modified in a range of Am/Ao = 1 to 4, where Am and Ao are the cross-sectional area modified and original of the manifolds, respectively. The solar collector investigated is equipped with six riser tubes, which are attached to the manifolds pipe. The numerical study was developed in a commercial Computational Fluid Dynamics (CFD) using FLUENT®. This code allows to solve the Reynolds averaged Navier-Stokes equations and the transport equations of the turbulence quantities. The results shown that increasing the inlet and outlet area of the manifolds allow a more uniform flow distribution compared to the original configuration of the solar collector. It also shows that the overall pressure drop in the solar collector is reduced.Copyright