Reset control for DC-DC converters: an experimental application

© 2019 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Power converters in grid connected systems are required to have fast response to ensure the stability of the system. The standard PI controllers used in most power converters are capable of fast response but with significant overshoot. In this paper a hybrid control technique for power converter using a reset PI + CI controller is proposed. The PI + CI controller can overcome the limitation of its linear counterpart (PI) and ensure a fast flat response for power converter. The design, stability and cost of feedback analysis for a DC-DC boost converter employing a PI + CI controller is explored in this work. The simulation and experimental results which confirm the fast, flat response will be presented and discussed.Peer ReviewedPostprint (published version

Hydrogen-methane fuel control systems for turbojet engines

Design, development, and test of a fuel conditioning and control system utilizing liquid methane (natural gas) and liquid hydrogen fuels for operation of a J85 jet engine were performed. The experimental program evaluated the stability and response of an engine fuel control employing liquid pumping of cryogenic fuels, gasification of the fuels at supercritical pressure, and gaseous metering and control. Acceptably stable and responsive control of the engine was demonstrated throughout the sea level power range for liquid gas fuel and up to 88 percent engine speed using liquid hydrogen fuel

Tuning of Constant in gain Lead in phase (CgLp) Reset Controller using higher-order sinusoidal input describing function (HOSIDF)

Due to development of technology, linear controllers cannot satisfy requirements of high-tech industry. One solution is using nonlinear controllers such as reset elements to overcome this big barrier. In literature, the Constant in gain Lead in phase (CgLp) compensator is a novel reset element developed to overcome the inherent linear controller limitations. However, a tuning guideline for these controllers has not been proposed so far. In this paper, a recently developed method named higher-order sinusoidal input describing function (HOSIDF), which gives deeper insight into the frequency behaviour of non-linear controllers compared to sinusoidal input describing function (DF), is used to obtain a straight-forward tuning method for CgLp compensators. In this respect, comparative analyses on tracking performance of these compensators are carried out. Based on these analyses, tuning guidelines for CgLp compensators are developed and validated on a high-tech precision positioning stage. The results show the effectiveness of the developed tuning method

Multivariable robust control of a simulated hybrid solid oxide fuel cell gas turbine plant

This work presents a systematic approach to the multivariable robust control of a hybrid fuel cell gas turbine plant. The hybrid configuration under investigation built by the National Energy Technology Laboratory comprises a physical simulation of a 300kW fuel cell coupled to a 120kW auxiliary power unit single spool gas turbine. The public facility provides for the testing and simulation of different fuel cell models that in turn help identify the key difficulties encountered in the transient operation of such systems. An empirical model of the built facility comprising a simulated fuel cell cathode volume and balance of plant components is derived via frequency response data. Through the modulation of various airflow bypass valves within the hybrid configuration, Bode plots are used to derive key input/output interactions in transfer function format. A multivariate system is then built from individual transfer functions, creating a matrix that serves as the nominal plant in an Hinfinity robust control algorithm. The controller\u27s main objective is to track and maintain hybrid operational constraints in the fuel cell\u27s cathode airflow, and the turbo machinery states of temperature and speed, under transient disturbances. This algorithm is then tested on a Simulink/MatLab platform for various perturbations of load and fuel cell heat effluence.;As a complementary tool to the aforementioned empirical plant, a nonlinear analytical model faithful to the existing process and instrumentation arrangement is evaluated and designed in the Simulink environment. This parallel task intends to serve as a building block to scalable hybrid configurations that might require a more detailed nonlinear representation for a wide variety of controller schemes and hardware implementations

Increasing the reactant conversion through induced oscillations in a continuous stirred tank reactor by using PI control

We report a strategy to increase the reactant conversion in a continuous stirred tank reactor (CSTR) to produce propylene glycol through induced oscillations generated by two controllers PI1 and PI2 that manipulate the reactor outlet flow and the coolant flow rate respectively. It is shown that an adequate parameter choice for the PI controllers allows one to derive sustained oscillations in the concentrations and reactor temperature, which in turn allows increasing the propylene glycol production. For a suitable choice of the PI1 and PI2 controller parameters, we use a complete reactor model that provides with physically feasible parameters. The issues of external disturbance rejection, self-oscillations and stability have also been discussed. The analytical calculations are verified by means of full numerical simulations

Drainage control and diffusion resistance in dropwise condensation in a compact heat exchanger

Condensation of a vapor in the presence of non-condensable gas occurs frequently in process industry. For example in compact condensers for heat recovery, in extraction of toxic components from exhaust gases, in cooling systems of nuclear power plants, seawater desalination systems, air conditioning and petrochemical industry. It is well known that even small concentrations of non-condensable gas have a detrimental effect on condensation heat transfer rates. The key difference between the condensation of pure vapor and vapor in the presence of non-condensable gas compounds is that mass diffusion in the gas phase instead of heat diffusion in the condensate layer dominates heat transfer in the heat exchanger in the latter case. In condensation heat transfer with non-condensable gases, vapor-sided heat and mass transfer are essential. If possible, the diffusion resistance in the gas-liquid boundary layer should be reduced in order to enhance heat and mass transfer. A way to augment heat transfer is by introducing dropwise condensation instead of filmwise condensation. Heat transfer by dropwise condensation is possibly a factor 8 to 10 higher than filmwise condensation [115]. The exact reasons for this difference are still not fully understood. Where filmwise condensation is characteristic of metal heat exchangers with clean uncontaminated surfaces, dropwise condensation is, for example, achieved by applying a fluoropolymer heat exchanging surface. This study aims to elucidate the importance of the initial phase of dropwise condensation after drainage on heat transfer, when diffusion is not yet limiting. The effects of growth, coalescence and drainage of droplets with surface refreshing on airsteam condensation heat transfer enhancement are quantified. For this reason, a new small scale condenser setup is designed and applied. To supply a gas flow at well defined conditions, existing infrastructure is combined with an acoustic relative humidity sensor tailored to the required flow conditions. Also other measures are taken to increase accuracy of the heat exchanger test rig. An apparatus with controlled removal of condensate droplets from the condenser plates is designed and applied. The dropwise condensation process is frequently interrupted upon which nucleation restarts upon each sweep. Condensate growth and surface temperatures are assessed by simultaneous video and infrared recordings. Software is developed to automatically extract the positions and radii of condensate droplets from images quickly and reliably. Cold wakes downstream of big drops on the condenser plate were observed. A single controlled droplet removal action enables a ’reset’ of the condenser surface. This allows measurement of droplet growth histories. It is found that droplet growth follows a power law with the exponent increasing with increasing inlet vapor mass fraction. Direct contact condensation on drops at condenser plate dominates drop growth. The main finding is that that the total heat transfer resistance decreases with increasing droplet removal frequency, while two measures for mass transfer simultaneously increase. Increasing diffusion limitation is one explanation for the observed decreasing mass transfer rate with time. After initial fast growth of drops, the slight increase in interfacial temperature observed offers another explanation. Furthermore, the effect of a structured heterogeneous plate surface on droplet drainage and heat transfer in dropwise condensation is investigated. A structured coating of the condenser plates is applied to create two coexisting dropwise condensation patterns. The structured coating constrains drainage and introduces directed surface energy gradients. The condenser with the structured coating is compared with two equally sized condensers: a non-coated pvdf and a fully coated pvdf condenser. It is found that drop drainage is promoted by oriented Ti-coated tracks to such a degree that the maximum obtainable heat transfer performance is practically reached. Design recommendations are given

MODELING, SIMULATION AND OPTIMIZATION OF INDUSTRIAL HEAT EXCHANGER NETWORK FOR OPTIMAL CLEANING SCHEDULE

Sustaining the thermal and hydraulic performances of heat exchanger network (HEN) for crude oil preheating is one of the major concerns in refining industry. Virtually, the overall economy of the refineries revolves around the performance of crude preheat train (CPT). Fouling in the heat exchangers deteriorates the thermal performance of the CPT leading to an increase in energy consumption and hence giving rise to economic losses. Normally the energy consumption is compensated by additional fuel gas in the fired heater. Thus, increase of energy consumption causes an increase in carbon dioxide emission and contributes to green house effect. Due to these factors, heat exchanger cleaning is performed on a regular basis either by chemical or mechanical cleanings. The disadvantage of these cleanings is the potential environmental problem through the application, handling, storage and disposal of cleaning effluents. Nevertheless, the loss of production caused by plant downtime for cleaning is often more significant than the cost of cleaning itself, particularly in refineries. Thus, it is essential to optimize the cleaning schedule of heat exchangers in the HEN of CPT. The present research focuses on the analysis of the effects of fouling on heat transfer performance and optimization of the cleaning schedule for the CPT. The study involves collection and analysis of plant historical operating data from a Malaysian refinery processing sweet crude oils. A simulation model of the CPT comprising 7 shell and tube heat exchangers post desalter with different mechanical designs and physical arrangements was developed under Petro-SIM™ environment to perform the studies. In the analysis of effects of fouling on heat transfer performance in CPT, the simulation model was integrated with threshold fouling models that are unique to each heat exchanger. The fouling model parameters are estimated from the historical data. The simulation study was performed for 300 days and the analysis indicated that the position of heat exchangers has a dominant role in the heat transfer performance of CPT under fouled conditions. It is observed from this simulation study, fouling of upstream heat exchangers of the CPT will have higher impact to overall heat transfer performance of the CPT. For the downstream heat exchangers, the decline in their heat transfer performances due to fouling can be compensated by the log-mean temperature difference (LMTD) effect, which will reduce or even increase the heat transfer performance of these heat exchangers. An optimization problem for the cleaning schedule of the CPT was formulated and solved. The optimization problem considered un-recovered energy cost and cleaning cost of the heat exchangers in the objective function. Optimization of the cleaning schedule was illustrated with a case study of simulation over a period of two years. Constant fouling rates that are extracted from the historical data are used to estimate the fouling characteristics of each heat exchanger in the CPT. For the purpose of comparison, a base case was developed based on the assumption that the heat exchangers will be cleaned when the maximum allowable fouling resistance was reached. mixed integer programming approach was used to optimize the cleaning schedule of heat exchangers. An optimized cleaning schedule with significant cost savings has been determined and reported over a period of two years