Volume 13, Issue 1 -

Vanadium redox flow battery (VRFB) is one of the promising technologies for large scale renewable energy storage due to its long life time and flexible design. Electrolyte flow rate, electrolyte concentration, electrode porosity, temperature and applied magnitude of current have significant effect on the efficiency of VRFB. In the present work, a simplified dynamic lumped parameter model is developed in MATLAB to provide accurate results at unit cell level which can be used to control and optimize the parameters. The proposed model is based on the mass balance and charge conservation of various vanadium species and hydrogen ion concentration in the electrolyte. Major losses such as ohmic losses and activation losses are included in the cell potential. Recirculation of the electrolyte through external reservoirs is also considered. Numerical simulation demonstrates the effect of change in initial vanadium concentration and operating temperature on unit cell performance. It is shown that variations in electrolyte flow rate, magnitude of applied current substantially alter the charge/discharge characteristics and efficiency.

Davison (1976) proposed a method to design controllers for multivariable systems using the knowledge of only the Steady-State Gain Matrix (SSGM) of the system. In the present work, Davison's method is suitably modified to design controllers for unstable non-square multivariable systems. A single-stage multivariable PI controller is designed using the SSGM. Simulation results show that the overshoots of the closed loop responses are larger, hence a two-stage P-PI control system is proposed. For the proposed two- stage design (i) the system is first stabilized by a simple proportional controller matrix designed based on SSGM using modified Davison's method (1976). (ii) Diagonal PI controllers are designed for this stabilized system based on gain matrix. Simulation studies are carried out to compare the closed loop performance of the single stage multivariable PI control system with that of the two-stage control system (inner loop centralized P controllers and outer loop diagonal PI controllers). A method to identify the steady state gain matrix of a non-square multivariable (SSGM) unstable system under closed loop control is presented. The effect of disturbances and measurement noise on the identification of SSGM is also discussed.

A novel Proportional-Integral-Derivative (PID) controller is proposed for stable and unstable first order processes with time delay. The controller is cascaded in series with a second order filter. Polynomial approach is employed to derive the controller and filter parameters. Simple tuning rules are derived by analysing the maximum sensitivity of the control loop. Formulae are provided for initial guess of tuning parameter. The range of tuning parameter around the initial guess and the corresponding range of maximum sensitivity is specified based on time delay to time constant ratio. Promising results are obtained with the proposed method is compared against recently proposed methods in the literature. The comparison is made in terms of various performance indices for servo and regulatory responses separately. The proposed method is implemented for an isothermal chemical reactor at an unstable equilibrium point.

The gas diffusion layer (GDL) is one of the key components in a polymer electrolyte membrane (PEM) fuel cell. Generally it is a carbon-based fibrous medium that allows for the transport of electrons through the fibers and distributes the reactants through the void space to the catalyst layer in a PEM fuel cell. In the present work, a microstructure study of reactant transport is carried out by reconstructing the typical fibrous microstructure of the GDL and investigating the transport characteristics of the porous medium using computational fluid dynamics (CFD) simulations. The results confirm the applicability of Darcy’s law formulation for permeability determination and Bruggemann correction for calculation of effective diffusivity for typical conditions encountered in PEM fuel cells. Macroscopic material properties such as through-plane and in-plane permeabilities and effective diffusion coefficient are determined and compared against experimental values reported in the literature.

This study discusses the modeling and linear quadratic regulator (LQR) controller based closed loop control of a three tank hybrid (TTH) process. A pseudo random binary signal (PRBS) based excitation data obtained from a real time TTH setup is utilized in validating its first principle model (FPM). Based on top and bottom interactions, various modes prevalent are considered based on steady state physical reachability analysis of the TTH for a given input range for controller design. The FPM is linearized using nominal values of process parameters using Jacobians from each existing mode. LQR controllers are designed for each mode. A supervisory structure is designed for selecting estimation model and controller for each appropriate mode. Results from real time servo tracking and disturbance rejection experiments are discussed.

Using the phase change materials (PCMs) can help to store thermal energy as latent heat method in the multi PCMs storage system. In this research study, an analytical model has been developed for solar thermal storage system. An especial shell and tube heat exchanger is considered to heat exchange purpose. Double pipes have been used as tubes and PCMs allocated on the shell side of the double pipes for the heat exchanger. The hot heat transfer fluid (HTF) flows across the tube banks to charge the thermal energy storage system during the charging cycle. The thermal energy absorbed in PCMs while the HFT temperature reduced in the charging process. The heat exchanger tube length is function of HTF residence time then the equivalent length is defined in the presented model. The validity of the proposed model has been checked by comparison between the analytical results and numerical data obtained before using CFD simulation. The result shows that required times to melt of the PCMs for the presented study has the consistency of about 0.928 with CFD results in different PCMs melting percentages. Also the required time to melt of the PCMs for the presented study has the consistency of 0.915 with CFD results in different air inlet temperatures. Therefore the presented model can predict the system behavior also can apply for cascaded PCMs storage systems for exergy analysis.